thesis amicucci chiara (ripristinato) (1)...(1) the tanning process can be carried out in various...
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Innovative solutions for the treatment of tannery waste
Chiara Amicucci
Thesis to obtain the Master of Science Degree in
Chemistry
Supervisors: Profª. Silvia Zamponi; Profª. M. Joana Neiva Correia
Examination Committee
Chairperson: Profª. Isabel M. Marrucho
Supervisors: Profª. Maria Joana Neiva Correia
Members of the Committee: Prof. João A. Vieira Canário
December 2017
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ABSTRACT
Recently, geopolymers have been studied as a cheaper alternative to organic polymers and inorganic
cements in diverse applications. Additionally, they can be made from a great number of minerals and
industrial by-products. During the last decade, geopolymerisation has emerged as a possible
technological solution for the effective stabilisation and immobilisation of toxic metals. In this thesis it
was investigated the application of geopolymerisation to the treatment of some wastes, in particular to
the immobilisation of chromium present in tannery wastewaters. Geopolymers showed very good results
because in the studied conditions they allowed to reduce the concentration of chlorides from 14832
mg/L to 9 m/L, sulphates from 10970 mg/L to 22 mg/L, in addition to the cut-down of the chromium
from 2260,16 mg/L to 0,06 mg/L.
Key words. Geopolymers, metal immobilisation, tannery waste treatment.
RESUMO
Recentemente, os geopolímeros têm sido estudados como uma alternativa mais barata aos polímeros
orgânicos e cimentos inorgânicos para diversas aplicações . Estes materiais podem também ser
preparados a partir de um grande número de minerais e subprodutos industriais. Durante a última
década, a geopolimerização surgiu como uma possível solução tecnológica para uma efetiva
estabilização e imobilização de metais tóxicos. Nesta tese foi investigada a aplicação da
geopolimerização para o tratamento de alguns efluentes, em particular para a imobilização de crómio
presente nas águas residuais das indústrias de curtumes. Os geopolímeros conduziram a bons
resultados tendo permitido reduzir, , nas condições estudadas, a concentração de cloretos de 14832
mg/L to 9 mg/L, dos sulfatos de 10970 mg/L to 22 mg/L, além de terem permitido a redução do crómio
de 2260,16 mg/L to 0,06 mg/L.
Palavras-chave. Geopolímeros, imobilização de metais, tratamento de resíduos de curtumes.
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INDEX
ACKNOWLEDGMENTS ................................................................................................................ 4
1. INTRODUCTION ....................................................................................................................... 5
1.1 Leather tanning process ........................................................................................................ 5
1.2 Chromium as tanning agent ................................................................................................. 6
2. ENVIRONMENTAL CONCERNS IN TANNERY INDUSTRY ................................................... 8
2.1 Water management .............................................................................................................. 8
2.2 Waste production .................................................................................................................. 8
2.3 Atmospheric emissions.......................................................................................................... 9
2.4 Process approach of in-put/out-put analysis ......................................................................... 9
3. CURRENT DEPURATION SYSTEM OF TANNERY WASTE ................................................ 15
3.1 Chrome precipitation ........................................................................................................... 16
4. INNOVATIVE SOLUTIONS FOR TANNERY WASTE DISPOSAL ......................................... 17
4.1 Geopolimers ........................................................................................................................ 17
4.1.1 Raw materials used to make geopolymer ...................................................................... 19
4.1.2 Geopolymers characterizzation ...................................................................................... 21
4.1.3 Potential application in waste treatment ......................................................................... 23
4.2 Molecular Reactants® ......................................................................................................... 24
5. EXPERIMENTAL PART .......................................................................................................... 24
5.1 Instruments .......................................................................................................................... 24
5.1.1 UV-Vis Spettroscopy ...................................................................................................... 24
5.1.2 ICP-AES ......................................................................................................................... 25
5.1.3 GF-AAS .......................................................................................................................... 26
5.2 Analysis of waste water principal components .................................................................... 28
5.2.1 Methods .......................................................................................................................... 28
5.3 Treatment of waste water with molecular reactants ............................................................ 31
5.4 Geopolymer synthesis ......................................................................................................... 38
6. RESULT AND DISCUSSION .................................................................................................. 41
7. CONCLUSIONS ...................................................................................................................... 41
BIBLIOGRAFY ............................................................................................................................. 43
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INDEX OF FIGURES
Figure 1 Chrome-olo formation reaction ................................................................................................ 7
Figure 2 Oxo-Chrome formation reaction .............................................................................................. 7
Figure 3 Cross-linking reaction between collagen chains ..................................................................... 7
Figure 4 Pollutants generated in leather processing ............................................................................ 10
Figure 5 Streams organization: chrome recycling and silphide oxydation .......................................... 16
Figure 6 Chrome recovery plant .......................................................................................................... 17
Figure 7 Pourbaif diagrams ofchromium .............................................................................................. 17
Figure 8 silico-aluminate 3-D structures of geopolymers .................................................................... 18
Figure 9 Schematic rapresentation of geopolimerization process ...................................................... 19
Figure 10 XRD patternsof (a( fly-ash and (b) metakaolin .................................................................... 21
Figure 11 SEM analysis microstructure of geopolymers ....................................................................... 22
Figure 12 SEM analysis microstructure of geopolymers ....................................................................... 22
Figure 13 General scheme of UV-Vis spectrophotometer ................................................................... 25
Figure 14 Schematic illustration of plasma torch ................................................................................. 26
Figure 15 Graphite furnace cross section ............................................................................................ 27
Figure 16 Scheme of reactors plant ..................................................................................................... 32
Figure 17 Pictures of reactors plant during the experiment .................................................................. 32
Figure 18 Solid residue of R3C2 sample .............................................................................................. 36
Figure 19 R3C2 sample with H2SO4 ...................................................................................................... 36
Figure 20 R3C2 sample with water ....................................................................................................... 36
Figure 21 R3C3 sample with water ..................................................................................................... 37
Figure 22 R3C3 sample with H2SO4 ...................................................................................................... 37
Figure 23 R4C2 sample ......................................................................................................................... 37
Figure 24 mashed geopolymer .............................................................................................................. 40
Figura 28 filtration geopolymer after 24 h ............................................................................................. 40
INDEX OF TABLES
Table1 operative condition of instrument .............................................................................................. 29
Table2 calibration settings ..................................................................................................................... 30
Tabella 3 Analytical results of tannery waste water .............................................................................. 31
Table4 Cycles duration time .................................................................................................................. 32
Table5 Samples collected for analysis .................................................................................................. 34
Table6 Analytical results of the sample collected ................................................................................ 34
Tabella 7 Average values for each cycles ............................................................................................. 35
Tabella 8 Analytical results of leaching test .......................................................................................... 38
Table 9 Reagent for geopolymer synthesis .......................................................................................... 38
Tabella 10 Analytical results after leaching test of geopolymers .......................................................... 40
Tabella 11 Law limits of discharge of pollutants .................................................................................... 41
Tabella 12 Results of the analysis, before and after the treatments ..................................................... 41
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ACKNOWLEDGMENTS
At the end of a path you look back and see everything with different eyes. You realize that all the
difficulties and disappointments are the most precious experiences,because they determine the value
of your path and how much it has really enriched you.
For these reasons I need to thanks all the professors but especially the ones that told me “ Not good,
try again!”.
I need to thank also the university of Camerino that really support students through scholarship, mobility
programs and so on.
Obviously I’m grateful to my family and to my husband for having borne me, in the most stressful and
depressing moment.
Finally I thank those who allowed the achievement of this thesis including the JH-CTC that hosted me
in this period, professors Silvia Zamponi and Mario Berrettoni who supervised the whole work, the doctor
Giacomo Boldrini who was always very helpful and polite and Mauro who helped me to carry out many
experiments.
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1. INTRODUCTION
Leather is usually made fromthe hides and skins of animals, large animals such as cattle have hides,
small animals such as sheep have skins. The skin of any animal is largely composed of the protein
collagen, so it is the chemistry of this fibrous protein and the properties it confers to the skin with which
the tanner is most concerned.
In addition, other components of the skin impact on processing, impact on the chemistry of the material and
impact on the properties of the product, leather. Therefore, it is useful to understand the relationships between
skin structure at the molecular and macro levels, the changes imposed by modifying the chemistry of the
material and the eventual properties of the leather. (1)
The tanning process can be carried out in various ways depending on the type of raw skin and the type of
product you want to make.In this brief introduction we will refer to the production process of leather used for
shoes industry.
1.1 Leather tanning process
The whole process of the tanning process is generally grouped into four phases:
1. Preparatory stages: first chemical and mechanical operationsto which raw skin is subjected with the
purpose of preparing it for tanning.
2. Tanning: a set of operations that enable stable crosslinking of the dermis collagen fibers.
3. Crusting: chemical treatments that improve the aesthetic and merchandise characteristics of the skin
4.Surface coating: Workgroup on dry skin with the purpose of protecting the surface and improving
appearance.
Each group includes a number of operationthat are summarized below:
Preparatory stages
• Soaking: it consists of the removal of the salt used for storage, at this stage it releases dirt and reassembles
the water lost in the storage.
• Liming and Unhairing:they are performed simultaneously, depilation removes the hair and the epidermis
while the calcine relaxes the dermal tissue in order to increase the reactivity and the absorption capacity of
the tanning products.
• Fleshing: it is a mechanical operation by which the meat residues and the adipose tissue of the subcutaneous
layer are eliminated.
• Deliming: removal of previously used lime with acid or acid salts.
• Bating:introduction of proteolitic enzymes to relax furthe the tissues.
Tanning
• Pickel: lowering of the pH to help the penetration of tanners in the next step.
• Tanning: formation of croslinking between collagen chains by tanning agent.
Crusting
• Retanning: furthe treatment with tanning agents.
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• Dyeing: it gives to the skin the required color. Usually water-soluble or dispersed dyes are used and the
process is dependent on the tanning type.
• Fatliquoring:it gives the leather the characteristics of softness and hydrophobicity, and also improves
mechanical properties.
Surface coating
• Oiling: it is a process whereby leather is hand coated with either a raw oil or a combination of raw oil, blended
with emulsified oils and a penetrating aid.
• Brushing
• Coating
• Glazing
From a chemical point of view the most interesting step is the tanning because it can be performed by very
different chemicals acordind to different mechanisms. Tanning is defined as the process that transforms
skin into leather, or, in other words transforms a fermentable product into a non-degradable product for
organic-bacterial effect.Tanning agents are classified acording to the kind of tannery or the type of bond
they are able to establish with collagen:
• Inorganic tanners: Cr, Al, Fe, Zr, Ti
• Organic tanners: tannins, oil, aldehydes, chlorosulphonate paraffins
The possible collagen-tanning links that are established in tanning may be of different chemical nature,
such as a covalent bond, divalent covalent bondor dipolar interaction. The former is generally obtained
by the use of oils and aldehydes,it is a strong bond that confers good resistance to chemical agent.The
divalent covalent bond is less strong than the covalent one, it is generally formed by mineral tanners, it
allows to obtain a product with discrete resistance to chemical agents. Dipolar interaction is very weak
interaction and it is not able to give water and chemical agent resistance.
1.2 Chromium as tanning agent
Chromium tanning is one of the most widely used tannery mainly because of the ability to create very
stable cross-linked trivalent chromium (Cr3+) bonds with the collagen structure of the skins. Chromium
is added as chromium sulphate,which derives from chromite (Cr2O3-FeO), mineral which is relatively
abundant in the earth's crust.Chromium sulphate is a water soluble green color salt which release
chromium ions in solution.
Cr2(SO4)3� 2Cr3+ + 3SO42-
Chromium ions can form complex with water, the most stable complex formed is the esaaquochromium
ion with six molecules of water. This complex is moderately strong and can generate three dissociation
equilibria.
2Cr3+ + 6H2O �[Cr(H2O)6]3+
[Cr(H2O)6]3+ �[CrOH(H2O)5]2+ +H+
[CrOH(H2O)5]2+�[Cr(OH)2(H2O)4]+ +H+
[Cr(OH)2(H2O)4]+� Cr(OH)3 +H+ + 3 H2O
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This makes us understand that pH conditions are very important, becauseat different pH we will have
different chemical species in solution.
Monobasic chromium [CrOH(H2O)5]2+ is a key species in tannery chemistry, indeed it changes over
time forming species that are stable to acids.
Figure 1 Chrome-olo formation reaction
As shown in the Figure, the bridge is represented by a hydroxyl group which forms a covalent bond with
a Cr atom and a coordinative bond with another Cr atom. When the monobasic chrome undergoesthis
reaction it is no longer able to come back to its original state. The amount and size of the chrome-olo
that is formed are a function of time, pH, concentration and temperature. The higher the number of
cross-linked Cr complexes, the greater the molecular size. In thereticulation process increased
molecules size, increases the probability of attachment to polypeptide chains, because they can reach
the useful distance, but at the same time makes it more difficult to penetrate the product through the
skin section. So we should optimize time and pH in order to have small molecules at the beginning of
the process, and as soon as the section pass is completed, we will proceed to make molecules bigger
favouring the cross-linking.The chrome-olo will give then a further reaction called oxalation:
Figure 2 Oxo-Chrome formation reaction
The oxo-chrome species penetrates into collagen fibers, once inside it can react with the functional
groups of aminoacids the make up collagene. This phase is called cross-linking reaction, it is
supposed that consist in a nucleophilic attach of the carboxylic groups to chrome, generally also the
amino groups act as nucleophiles, but in this case we can exclude that because they are in the
protonated form, so they can’t form anothe bond with chrome.
Figure 3 Cross-linking reaction between collagen chains
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2. ENVIRONMENTAL CONCERNS IN TANNERY INDUSTRY
In the tanning industry, a culture of sustainability is spreading, which results in actions to reduce
pollutants and prevention initiatives.
The "environment" asset is also gradually becoming an important factor in competitiveness, an
additional intangible added value for Italian productions, especially in a period of economic crisis such
as the present one. Technological innovation joint to environmental issues is, in fact, a reasonable bet
for mature entrepreneurship and is an element that contributes to increasing the perception of quality
that is usually associated with the productions of Italian manufacturing. However, even if environmental
care is beginning to spread to some tanneries, it is also true that much remains to be done to ensure
that green investments result in a return in economic profit.
In general, from 2002 to 2010, the impact of environmental costs on sales has increased, in fact, from
1.9% in 2002 to 4% in 2010, confirming the growing environmental commitment of tanneries. (2)
The stages of the trenching process that are the most critical are: water management, waste production
and atmospheric emissions.
2.1 Water management
Concerning in water management, it should be remembered that leather processing passes through a
series of steps conducted in an aqueous medium. Water is used as a tool for the chemical transformation
of leather through the use of products dissolved in solution, this implies that the consumption and
purification of the waste water represent the most important environmental aspects in tannery. This
problem was partially faced by the creation of consortia born to meet the needs of local businesses; the
purification processes are extremely efficient for almost all pollutants. Many parameters levels present
a lowering, except for chlorides and sulphates for whichsome treatment problems are not still resolved;
furthermore the problem of purification costs remains.A possible solution is to move the attencionto the
upstream of the problem; in other words to move from the simple reduction of the effects on the
environment, to the use of new technologies that directly affect the causes that determine the
environmental impact during the production processes, and to the decrease in water consumption.
2.2 Waste production
Only 20-25% of the input raw material, used to make leather, becomes a finished product. The rest 75-
80% joined to the chemicals used, becomes waste. The leather maufacturing process produces waste
of a different nature depending on the production phase: animal by-products (carcass, hair, scouring,
waste and scrap) account for over 48.4% of the total, sewage sludges account for about 21.7% and
tanning liquids for 20.9%. (3)
The pollution of tannery waste water is mainly due to chemical substances such as: sulfides, sulfates,
chlorides, surfactants, ammoniacal salts, chromium salts or other minerals, phenols; and also due to the
presence of suspended solids and high organic load.
The recovery and reuse of waste produced by the tanning industry start from thedifferentiated collection
and storage procedures that avoid mixing For example, during the unhairing operations, the hairs can
be retrieved in special grids and then destined for re-use as a felt, while in the finishing stage, leather
trimmings are collected separately and reused for the production of small leather articles or regenerated
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in leather fiber. Also the animal residue, produced during the stage of flashing, can be trasformed
through a particular process into different products, such as soaps, sewage for agriculture. The recovery
of this waste is of vital importance to the tanning industry, as it eliminates the problem of its disposal,
thus reducing the environmental impact and all the problems associated with its downstream production.
2.3 Atmospheric emissions
The main parameters affecting air quality are: volatile organic compounds (VOCs), powders and
hydrogen sulphide. The atmospheric emissions related to these parameters are produced in different
stages of the tannery processes,for exsample VOCs are emitted during finishing, while powders are
generally produced in some mechanical operations such as shaving and grinding.Enterprises have
committed themselves to reducing emissions through the development of low-pollution processes. They
include the purchase of high efficiency machinery and the use of less polluting products; for example
were developedinnovative oxidation-depilation process that uses hydrogen peroxide in aqueous solution
instead of other oxidants those must then be reduced by sodium sulphide before disposal.
2.4 Process approach of in-put/out-put analysis
The tanning of 1 t of crude leather involves the consumption of 350-400kg of chemicals, from 15 to 30
m3 of water and from 9.3 to 42 GJ of energy (4). We will see more in detail the environmental aspects
by analyzing phase by phase of the resources used and the relative in-put and out-put of the following
phases: leather reception,soaking,liming and unhairing, fleshing, deliming and bating, degreasing,
pickling, tanning.
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Figure 4 Pollutants generated in leather processing
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Leather reception
In the warehouse of a tanner comes leathers of different nature and origin divided into two big families:
wet skins and non-moist skins. The former can be further divided into: Raw skins (which must be
submitted to the entire working cycle); Pikled skins (the process of which starts with degreasing); Tanned
skins, which after a preparation phase are retanned. Non-moist skins are of two type: to be crusted or
to be coated.
If there are wet skins in the warehouse, you may have accidentally spilled liquids high organic content,
Biological oxygen demand (BOD), chimica oxygen demand (COD), Suspended Solids(SS), Chlorides,
Nitrogen Composites, Biocides, from collect and send to the purification plant.
INPUT PROCESS OUTPUT
wet skin
leather reception
BOD ,COD
Suspended Solids,
Chlorides, Nitrogen
Biocides
Soaking
It is done to restore the natural hydration to the skin; in this phase, salt residues are eliminated, and they
cleaned the impurities present as dung, blood, and other foreign matter. In the operation large amounts
of water are consumed and the water discharged is full of dissolved substances that affect parameters
such as COD suspended solids, chlorides and nitrogen.
INPUT
PROCESS
OUTPUT
water and skins
surfactants
biocides
enzymatic products
soaking
COD
Suspended Solids,
Chlorides, Nitrogen
Liming and unhairing
The function of liming and unhairing is to remove the hair, the interfibrillary components and the
epidermis, and open the fibrous interstice of the dermis for tanning. Hair removal is achieved by chemical
and mechanical means. The keratinous material (hairs, hair roots, epidermis) and a part of the fat are
eliminated from skins mainly by means of sulphides and lime. Alternative to inorganic sulphides are
organic compounds, mercaptans or sodium thioglycolates (sulfur based), in combination with strong
alkali and amine compounds. Sometimes enzymatic preparations are added to improve the performance
of the process.
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INPUT
PROCESS
OUTPUT
Water, skins and energy
Inorganic sulphide
lime, thioglycolates
enzymes
liming and unhairing
COD
Suspended Solids,
Chlorides, organic
nitrogen,sulphide, high pH, hair,
muds,
bad smell
Fleshing
During the fleshing is removed the excess organic matter from the dermis.At this stage, a high-organic
waste is produced, said flashng waste, useful for the production of both food and cosmetic jelly.
INPUT
PROCESS
OUTPUT
Skins
Fleshing
Fleshing waste
Liming waste
Deliming and bating
The purpose of deliming is to remove from the skins the residues of the chemicals used in the phase of
liming, in particular lime, and bring them to optimum conditions for the next stage (bating). It is carried
in bottles containing an aqueous solution (weakly acidic) in which organic acids lower the pH, this make
leather softer, pasty, with fine flore and elastic. Waste water from deliming affects water discharge
parameters such as COD and ammoniacal nitrogen. The bating is carried out in the same deliming bath,
with the aid of macerants enzymes that degrade the dermal substance, especially globular proteins, in
controller way, in order to make it even more absorbent for the next tanning step. The enzymatic and
chemical products used during the bating phase affect the waste water by modifying the COD, ammonia
andhydrogen sulphidethat is a toxic gas for humans with the characteristic smell of eggs.
INPUT
PROCESS
OUTPUT
Salts
organic acids
macerants enzymatic
Deliming and bating
chlorides, ammonia, COD,
sulphides
hydrogen sulphide
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Degreasing
Natural fats should be removed from the skins in order to avoid the formation of insoluble chromium
soaps and fat efflorescences which on the one hand diminish the organoleptic qualities of the finished
product and, on the other hand, result in not homogeneous distribution of the tanning agents and
colorants.The three different methods commonly used for degreasing are:
• Degreasing in aqueous medium with anionic, cationic, non-ionic surfactants or anophytes;
• Degreasing in aqueous medium with anionic or non-ionic solvents and surfactants
• Degreasing with solvents
Usually the most used compounds in the degreasing stages of the skins are surfactants because
solvents require a big investment for their recovery as they cannot be discharged to the environment for
obvious ecological reasons, furthermore they do not offer satisfactory results.The water discharges from
the degreasing, affect parameters such as COD, phenol derivatives, present in some surfactants, inhibit
the development of microorganisms. The environmental impact of surfactants is mainly linked to their
elimination in water treatment; Especially non-ionic ones are more difficult to eliminate than ionic ones,
having a lower biodegradability rate. Surfactants, in addition to having a direct toxicity on a variety of
living species, create foams with the consequence that water encounters more difficulty in
reoxygenating. The effect of surfactants, if added to that of COD, would make living conditions difficult
for aquatic organisms.
INPUT
PROCESS
OUTPUT
Skins, water
Surfactants / solvents
degreasing
COD, BOD, surfactants
Hydrocarbons
(chlorinated or non-chlorinated)
VOC ( if solvents are used)
Pickling
Picklingis done to reduce the pH of the skin in tripe before mineral tanning and, in some cases, before
some types of organic tanning. In addition to have very acidic pH, the waste water contain high amounts
of chlorides and sulphates. Sulphates have a toxic effect comparable to that of the chlorides by affecting
the ionic concentration.
INPUT
PROCESS
OUTPUT
Skins, water
Salts,acids
pickling
COD, BOD, sulphates, chloride,
low pH
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Tanning (chromium)
The purpose of the tanning process is to penetrate and fix the tanning material to obtain the stabilization
of the dermal tissue and its imputresibility. Also tanned leathers increase their dimensional stability,
resistance to mechanical actions,chemical agents and heat. The water discharges of the chrome tanning
operation contain trivalent chromium Cr (III), chlorides and sulphates. Trivalent chromium compounds
are mostly insoluble: their fate, if released in the environment, would then be to deposit on the bottom
of the water bodies as a sediment where they would remain for an indefinite period. Another matter is
the action of hexavalent chromium which is highly toxic; it is not used as such in the tanning process
and can be derived from trivalent chromium. However this process doesn’t usually occurs naturally (only
in the rare mineral crocoite) instead it’s produced by antropogenic sources such as the strong oxidative
enviroment used in certain tanning steps.(27)
INPUT
PROCESS
OUTPUT
Skins, waterand energy
Chromium salts,Basifying
agents,
masking
tanning
COD, BOD, sulphates, chloride,
low pH
Chrome(III),
damaged leathers
Summarizing: approximately 60% of the total chloride comes from the salt used for conservation and
released into the effluents during the soaking step, the remainder deriving from the pickling and to some
extent from tanning and dyeing processes. About 75% of the BOD and COD load is produced in the
preparatory stages. The greater the result of hair removal that does not use a technique to save the
hair, a significant portion of the COD load (about 45%) and BOD (about 50%) comes from unhairing
/liming steps. Unhairing /liming steps are also the main responsible for the production of suspended
solids (about 60%). Overall, the discharge parameters of the preparatory stages account for 90% of the
total suspended solids. Most of the total nitrogenous substances come from the liming process.
Approximately 65-70% of total chromium in effluents comes from tanning; only a modest share is derived
from the processes behind wet doughing, dripping and winding. The drainage water come from the
processes of the Riviera department (rinse, sculpting, depilation and calcination) and its rinsing contains
substances released from skins, dirt, blood, dung (high levels of BOD and suspended solids), residual
lime and sulphides. This water also has a high salt content and high alkalinity. The waters used for
deliming and maceration contain sulphides, ammonium salts and calcium and have a low alkalinity. After
the pickling and tanning process, the main pollutants of the waste water are determined by the tanning
techniques adopted. For tanning in chromium, these are chromium salts and low pH, about 4. (5)
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3. CURRENT DEPURATION SYSTEM OF TANNERY WASTE
The technologies available in the industry allow to recover, through physical and chemical treatments,
the chromium contained in tanning liquids, reducing its input concentration into the purifying systems; is
recovered as chromium sulphate and sometimes recycled in their production cycle. All with benefits of
different nature. There is first of all an energy saving, as the recovery process takes place cold, without
heat input; there is also an economic savings for companies that can re-use the recovered chromium.
There is finally a benefit to the environment due to the removal of chromium from the sludge resulting
from the purification and to a lesser exploitation of the metal in nature.
What we are going to deal with is a general purification system, but we have to keep in mind that each
plant differs according to different parameters.
Another important concept is that pollutants can not disappear, they are only transformed into a form
that can be less dangerous or easier to dispose of.
First, it is necessary to determine whether it is convenient to keep separate discharges from those that
require more thorough treatment or mix all discharges before purification. (6)
Wastewater treatment is a multi-stage process to purify wastewater before it enters a body of natural
water, or it is applied to the land, or it is reused. The goal is to reduce or remove organic matter, solids,
nutrients, Cr and other pollutants since each receiving body of water can only receive certain amounts
of pollutants without suffering degradation.
Therefore, each effluent treatment plant must adhere to discharge standards limits usually promulgated
by the relevant environmental authority as allowable levels of pollutants, for
practical reasons expressed as BOD5, COD, suspended solids, Cr(III) and Cr(VI)
If the tannery decide to separate the discharge,the waste water can be divide in three main lines:
• Effluents emanating from the beam-house, liming, deliming/bating, they contain sulphides, their pH is
high, but they are chrome-free.
• Effluents emanating from the tanning process including tanning and re-tanning,which contain high Cr
concentration, and acidic pH.
• Soaking and other general effluents, mainly from post-tanning operations (fat-liquoring, dyeing) which
have low Cr content. (7)
By separating these streams it is possible to avoid safety risks such as the formation of deadly, poison
gas hydrogen sulphide and to reduce the cost of treatment and sludge disposal to avoid contamination
of sludge with Cr.
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Figure 5 Streams organization: chrome recycling and silphide oxydation
Each stream will follow several treatment which we can resume in five groups:
Mechanical pre-treatment: consists of a dewatering, coarse and fine grinding; the purpose is to
eliminate the coarse matter, remove the major part of suspended solids, and considerably reduce the
BOD and COD content.
Physical-chemical treatment: includes oxidation, precipitation, sedimentation, flotation, compensation
flows and neutralization. It is mainly carried out to remove most of the organic substances, sulphide from
the wastewater of primary steps, chromium from tanning andpost-tanning operations, and also other
inorganic compounds.
Organic treatment: consists in reducing further organic content. A nitrification / denitrification phase is
possible if lower nitrogen content is desired. Sometimes, during nitrification, biological oxidation of
sulphides occurs.
Sedimentation: It consists of sedimentation of all sludge in a special tank
Dehydration: it isoften done to reduce the volume of sludge to be disposed of. This operation, largely
carried out by mechanical equipment that eliminates water through pressure, it is sometimes followed
by a drying process. Innovative solutions for disposal of tannery waste.
3.1 Chrome precipitation
The tanning liquor is treated with a base that allow the precipitation of chromium hydroxide. The
precipitate is filtrated and then the basic solution neutralized with an acid.
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Figure 6 Chrome recovery plant
Each metal, in the form of hydroxide, has a
characteristic solubility in function of the solution
pH. From their intersections with the solubility
curves, we can know the theoretical limit values of
pH ranges that must be maintained to obtain a
good precipitation. Precipitation of hydroxides is
carried out mainly with lime or caustic soda.Lime
has the defect of generating more sludge
quantities, but the advantage of increasing the
sedimentation efficiency, thus reducing the
volumes of the sludges produced and improving
thefilterability.The efficiency of chromium
precipitation is also increased inthe case of
segregated streams.
4. INNOVATIVE SOLUTIONS FOR TANNERY WASTE DISPOSAL
4.1 Geopolimers
The term “geopolymer” was created in 1979 by Davidovits, a french scientist who intended to represent
a kind of inorganic polymer with SiO4and AlO4 tetrahedra being the structural units (8).
In general, geopolymers are formed by reaction between an alkaline solution, for example sodium
hydroxide and sodium silicate solution, and an aluminosilicate source. Recently, geopolymer studies
catches attention because they may be used as cheaper alternative to organic polymers and inorganic
Figure 7 Pourbaif diagrams ofchromium
18
cements in diverse applications, such as aircraft (9), high-tech ceramics (10), thermal insulating foams
(11), fire-proof building materials (12)and hybrid inorganic-organic composites (13). This interest is also
due to their exceptionally high thermal and chemical stability, excellent mechanical strength, adhesive
behavior and long-term durability. Additionally they can be made from a great number of minerals and
industrial by-products; moreover, they are environmentally friendly materials from the point of view of
reducing green house effects caused by CO2 emission from the manufacturing of Portland cement (14).
The chemical composition of geopolymer material is similar to natural zeolitic materials, but differs
because the former show amorphous microstructure, while the latteris crystalline.More precisely
geopolymers possess amorphous to semi-crystalline three dimensional silico-aluminate structures
consisting of linked SiO4 and AlO4 tetrahedra by sharing all the oxygen atoms, which can be designated
as poly-sialate, poly-sialate-siloxo, poly-sialate-disiloxo and sialate links.
Figure 8 silico-aluminate 3-D structures of geopolymers
The sialate is an abbreviation for silicon-oxo-aluminate anion. To compesate the charge are always
present cations in the structure. The empirical formula for a generic geopolymer matrix is:
M+n{-(SiO2)z-AlO2-}n
Where M+is an alkaly cation (K+, Na+) for balancing the negative charge of Al (III) in IV-fold
coordination, nis the degree of polymerization and z is the Si/Al ratio.
By varying the Si/Al ratios geopolymers exhibit different properties, for example low ratios ( Si/Al ≤ 3)
result in three-dimensional cross-linked rigid networks and stiff and brittle properties; high ratios (Si/Al
>3) results in 2-D networks and linearly linked polymeric structures with adhesive and rubbery
properties, respectively. In general, geopolymerization is a complex multiphase process, comprising a
series of dissolution-reorientation-solidification reactions.
Summarizing we can divide the process in three steps
• The generation of reactive species or alkali activation, which is the dissolution of amorphous phases
(e.g., aluminosilicates) by alkali to produce small reactive silica and alumina;
• Reorientation, which is the transportation or orientation or condensation of precursor ions into oligomers;
19
• The actual setting reaction, which is the polycondensation process leading to the formation of
amorphous to semi-crystalline aluminosilicate polymers.
However, these three steps can overlap each other and occur almost simultaneously, thus making it
difficult to isolate and examine each of them separately.The schematic formation of geopolymer
materials can be shown as illustrated by the following two reactions (15):
Figure 9 Schematic rapresentation of geopolimerization process
It appears that an alkali metal salt and/or hydroxide is required for dissolution of silica and alumina to
form Si(OH)4 and Al(OH)4 monomers or short chain polymers, as well as for the catalysis of the
condensation reaction to form rigid chains. The tetrahedral monomers can be formed since in
aluminosilicate
structures silicon is always IV-fold co-ordinated, while aluminium ions can be IV- or VI-fold coordinated.
These monomer molecules tend to poly-condense, or polymerize, to form rigid chains or nets of oxygen
bonded tetrahedra with a final structural network consisting of amorphous to semicrystalline three
dimensional silicoaluminate. This frame work must also contain sufficient alkali cations to balance the
negative charge derived from the IV-fold coordination of Al3+in a Si4+ (16)network.
4.1.1 Raw materials used to make geopolymer
The general scheme showed above suggest us that is sufficient a starting material that contains mostly
silica and alumina in amorphous form is a possible source for the production of geopolymers; this is a
key concept that make possible the use of raw materials or mineral sources. The most common raw
materials used to make geopolymers are metakaolin, red mud, fly ash.
Metakaolin (Kaolinite)
Kaolinite is a 1:1 clay mineral with the chemical composition Al2Si2O5(OH)4, which means each particle
has one tetrahedral silica layer and one octahedral alumina layer.Individual particles of kaolinite form
stacks with hydrogen bonds and van der Waals forces holding together successive particles (17).The
strength of these bonds prevents water from entering the interlayer spaces and causing swelling. It is a
soft, earthy, usually white mineral, produced by the chemical weathering of aluminum silicate minerals
like feldspar. Rocks that are rich in kaolinite are known as china clay, white clay, or kaolin. Kaolin is a
fine, white, clay mineral that has been traditionally used in the manufacture of porcelain. Metakaolin is
a dehydroxylated form of the clay mineral kaolinite associated with the reaction Al2Si2O5(OH)4 Al2O3SiO2
20
+ 2 H2O. Between 100-200°C, kaolinites lose most of their adsorbed water. In the range of 500-800°C,
kaolinites become calcined by losing water through dehydroxilization. The dehydroxilization of kaolinite
to metakaolin is an endothermic process due to the large amount of energy required to remove the
chemically bonded hydroxyl ions, which breaks down the crystal structure producing a transition phase
(silica and amorphous alumina in reactive form) with high surface area. Metakaolin is a highly pozzolanic
and reactive material. Burning at higher temperature will cause recrystallization into quartz and mullite.
Red Mud
Red mud is the major industrial waste produced by the process for the extraction of alumina from bauxite
ores, one of the oldest large-scale industries in the world.
Red mud is characterized by strong alkalinity even with a high water content (up to 95%), owing to the
presence of an excessive amount of dissolved sodium hydroxide used to extract silicates and alumina.
Although red mud varies in physical, chemical and mineralogical properties due to differing mineral
sources and refining processes adopted, rust hue is an intrinsic property of all red mud, which is caused
by the oxidized iron present in the mud. In addition, solid constituents of red mud include mainly iron
oxides (mostly hematite), alumina, and some toxic heavy metals.It also can be slightly radioactive if the
original bauxite contained radioactive minerals.Strong alkalinity and high water content are the two major
environmental concerns for the safe and economical disposal of red mud. Thus, its treatment and
disposal are a major difficulty to alumina refineries. Although intense research work on utilization of red
mud was conducted during previous decades, a widely accepted technology that can be employed to
recycle red mud is not available at present. In the past, red mud was disposed from the plant site mainly
two ways, including dumping it directly into the sea or onto the land creating huge ponds. Due to the
intrinsic properties (e.g., high pH, heavy metals, radioactivity), the above disposal methods caused
significantly environmental problems to the surrounded communities. Therefore, new technologies
utilizing red mud as a raw material for manufacturing high added-value products are urgently needed.
Fly Ash
Since the fly ash particles solidify while suspended in the exhaust gases, they are generally spherical in
shape and range in size from 0.5 μm to 100 μm. They consist mostly of SiO2, which is present in two
forms: amorphous, which is rounded and smooth, and crystalline, which is sharp, pointed and
hazardous, Al2O3, and Fe2O3. The utilization of fly ash, especially in concrete production, has significant
environmental benefits, improved concrete durability, reduced use of energy, diminished greenhouse
gas production, and reduced amount of fly ash that must be disposed in landfills, and saving of the other
natural resources and materials. The major influence on the fly ash chemical composition comes from
the type of coal. The combustion of sub-bituminous coal contains more calcium and less iron than fly
ash from bituminous coal. The physical and chemical characteristics depend on the combustion
methods, coal source and particle shape.
21
4.1.2 Geopolymers characterizzation
The main techniques used to investigate the geopolymer structure are: X-ray Diffraction (XRD),
Scanning Electron Microscopy (SEM), and FT-IR spettroscopy.
X-ray Diffraction is the most important technique used to investigate the crystalline and quasi-crystalline
materials, it can provide information about basic crystal dimensions, chemical composition, crystallite
sizes and stacking sequences. XRD techniques are also used to quantify mineral abundance in clay-
rich materials.The analysis is based on the Bragg’s law that relates the wave lenght of X-ray radiation
with the distance between parallel atomic planes.The values of d and the number and types of atoms in
each plane are unique for every mineral. Since each mineral has its specific spacings of interatomic
planes in three dimensions, the angles at which diffraction occurs can be used for identification.
nλ=2d sinθ (Bragg’s law)
Geopolymers in contrast to zeolite are ‘X-ray amorphous’, since the major feature of powder X-ray
diffraction (XRD) patterns is a ‘featureless hump’. The XRD diffractograms of geopolymers appear
almost identical to those of many predominantly amorphous materials, so the major information that we
can recive from diffraction studies are comparisons the between different starting materials and final
product.
For example we can compare fly ash-based geopolymers and metakaolin-based geopolimer compering
the XRD patterns
Figure 10 XRD patternsof (a( fly-ash and (b) metakaolin
22
Geopolymer’s microstructure can be characterized by the SEM analysis, we can obtain information
about morphology.
The geopolymer matrix appears to exhibit an inhomogeneous structure at the microscale, as reflected
by the presence of randomly distributed micropores and microcracks, it shows a typical semi-spherical
pore in diameter on the fractured surface, surrounded by geopolymer matrix with microcracks. The pores
are likely caused by two reasons: the residual air bubbles that are introduced into thegeopolymer
precursor through mixing or trapped inside the geopolymer when pouring into the mold; and the space
that is previously occupied by water but becomes a cavity after water evaporates.
Another tool to investigate geopolymer microstructure and formation is the FT-IR transmittance
spectra of raw material metakaolin and final product.
Figure 12 SEM analysis microstructure of geopolymers
Figure 11 SEM analysis microstructure of geopolymers
23
Due to the presence of water in metakaolin and thegeopolymers, the strong characteristic peaks
approximately 3450 cm-1 and 1650 cm-1 were attributed to stretching and bending vibrations of
hydroxyl, respectively (18).
After geopolymerization, the chemical environment around regular arranged chain structures of the Si-
O bond altered, along with the formation of Al-O-Si bonds. Subsequently, the strong asymmetrical
stretching vibration peak of the Si-O bond in metakaolin (1100 cm-1) all shifted to a lower wavenumber
(to 1010 cm-1) for all the curing temperatures (19).
This indicated that the solidification process of the geopolymer is also a chemical reaction, with the
generation of a new substances. In all the spectra of geopolymers, the bands at approximately 600
cm-1 were due to Al-O-Si stretching vibrations. Si-O-Si bending vibration at approximately 450 cm-1
was also present (20). It was also found that the broad and strong peak at approximately 830 cm-1,
which belongs to the stretching vibration of hexa-coordinate Al(VI)-OH and Al(VI)-O in metakaolin,
almost disappeared after geopolymerization. A new peak at approximately 710 cm-1 from the
bendingvibration of tetra-coordinated Al(IV)-O-Si in a cyclic structure emerged on the FT-IR spectra of
thegeopolymers. This phenomenon signified the formation of aluminosilicate networks with the
transitinofrom hexa-coordinated Al(VI) to tetra-coordinated Al(IV) during the geopolymerization
process, asobserved by Sitarz et al (21). However, the transition was incomplete after 24 h of curing at
20°C;therefore, the peak was not as sharp as in the higher curing temperature condition. In addition,
aftergeopolymerization, all FT-IR spectra showed a shoulder at approximately 920 cm-1, due to Al(VI)-
Oresidues, which proved that, after 24 h, the geopolymerization was incomplete, regardless of the
curingtemperature (22).
4.1.3 Potential application in waste treatment
During the last decade geopolymerisation has emerged as a possible technological solution for the
effective stabilisation and immobilisation of toxic materials. Despite the fact that this technology is based
on a very old principle, surprisingly little is know about the nature of these reactions or their products. It
is only in the last fifteen years that it has been rediscovered and attention has drawn to its useful chimica
and physical properties to investigate the application of geopolymerisation to various waste forms. It is
evident from the literature that factor governing the formation of geoolymers are still not understood,
although the physical and chimical properties suggest that these advantages can only be applied
optimally once all relevant interactions regarding the formation of geopolymers from waste materials
need to be stabilised. It must also be acknowledged that these advantages can only be applied optimally
once all relevant interactions regarding the formation of geopolymers from waste materials have been
quantified scientifically. Hence, further research is required regarding their application in industry (23).
Heavy metal encapsulation in geopolymeric structures is not thugh to be caused by physical
encapsulation alone, but also through adsorption of metal ions into the structure (24); phisical
encapsulation consist in it consists in the occupation by the metals of the cavities formed by the skeleton
of the geopolymer, while adsorption consists in the formation of bonds of various nature between the
metal and the structure of the geopolymer.
24
4.2 Molecular Reactants®
Molecolar reactants are a class of commercial produducts used for the treatment of various waste
waters. Since they are protected by an industrial secret, we couldn’t know the exact composition of the
mixture in the first phase of testing, which only involved the analysis and study of the process. The
reason why we were approached was because the tannery i was working with for my thesis had
incentives to fund reaserch to find non conventional ways to treat wastes The only information we have
is that the main component of the mixture is Ca(OH)2, or in other words it is the basic medium to which
are joint the other unknow substances. These additives can be different and in different concentration
according to the nature of waste. I choose to deal with molecolar reactants because during the period
of thesis in the tannery of Chienti a factory propose to the tannery a progect for innovative treatment of
tannery waste water.
The progect consist in use a prototype plant with molecular reactants to treat the tannery waste water.
The apparatus consisting of four cylinders connected to each other. Each of the cylinders contains 500g
of "molecular reagents" (probably with different formulations for different reactors) and is equipped with
a oxygenation / aeration system that favors the reaction and ensures proper agitation.
5. EXPERIMENTAL PART
The experimental part of this thesis has the aim of investigate geopolymers and molecular rectants
efficiency in the treatment of the tannery waste water taking into account the most important parameters
in terms of envoirmental impact.
We decide to investigate Cl- and SO42- ions which are present in high concentration in the waste water,
and exceed a lot the limits of discharge, and Cr which is the most important pollutant from tannery
industries and finally pH.
So we analyse the tannery waste water before any treatment and we compare the results with the
analitical results after the geopolymer treatment and molecular reactants treatment.
5.1 Instruments
5.1.1 UV-Vis Spettroscopy
Molecular spectroscopy studies the absorption or emission of electromagnetic radiation by molecules.
The experimental data obtained, called respectively absorption or emission spectrum, correlates the
intensity of the absorbed or emitted radiation at the frequency or wavelength variation. From these
spectra we obtain information on the nature of the molecules and in many cases it is possible to
determine quantitatively different properties. The relative ease of data acquisition and the unreasonable
cost of much of the equipment needed for this kind of analysis, allow a widespread use of spectroscopic
techniques in chemical diagnostics. There is a linear correlation between the energy of the absorbed
radiation and the concentration of the analyte in analysis dictated by the law of Lambert-Beer; the
equation describes a direct proportionality:
25
A = ε • l • C
(A = Absorbance, ε= Molar absorption coefficient,l = Length of the optical path C = Concentration of the
analyte)
It is necessary to describe a concentration range in which direct proportionality is maintained because
there are some phenomena that deviate from the linearity of the law. Unlike an atom, there are not only
energy levels associated with electronic transition in a molecule, but vibrational and rotational sub-levels
are also involved. This involves a continuous absorption spectrum with characteristic peaks.
Figure 13 General scheme of UV-Vis spectrophotometer
Source: It is a normal tungsten filament lamp (continuous source) that emits electromagnetic radiation
on a wide spectrum of UV / Visible.
Monochromator: It allows us to select the appropriate wavelength to proceed with the analysis, it
should be chosen based on the spectrum of the analyte choosing an appropriate absorption peak and
evaluating the possibility of interfering in the matrix that can absorb a wavelength to distort the result of
analysis.
Sample: The sample is inserted into the instrument within a suitable cuvette, glass cuvettes are chosen,
since the glass is a transparent visible radiation material and therefore does not interfere with the
analysis.
Detector: It is a photomultiplier tube system transforms the electrical input pulse light signal by the
photoelectric effect proportionally to the energy of the incident radiation and amplifies the signal.
5.1.2 ICP-AES
ICP-AESis the acronym of Inductively coupled plasma atomic emission spectroscopy, it is an
analytical technique used for the detection of trace metals. The main characteristics concern in the
atomization system that is the inductively coupled plasma to produce excited atoms and ions that
emit electromagnetic radiation at wavelengths characteristic of a particular elements. The ICP-AES is
composed of two parts: the ICP torch and the optical spectrometer. The ICP torch consists of three
concentric quartz glass tube. Coil of radio frequencies (RF) generator surrounds part of this quartz
torch. Typically to create the plasma is used an inert gas like Argon.When the torch is turned on, the
electromagnetic field is created . The argon gas flowing through the torch is ignited with a brief
26
discharge to initiate the ionization process. The argon gas is ionized in the intense electromagnetic
field and flows in a particular rotationally way, enerating stable high temperature of about 7000 K is
then generated as the result of the inelastic collisions created between the neutral argon atoms and
the charged particles.
The sample is nebulized inside the torch immediately collides with the electrons and charged ions in the
plasma and is itself broken down into charged ions emitting radiation at the characteristic wavelength of
the elements involved. Sometimes the emitted radiationscan be measured simultaneously, allowing the
instrument to perform multi-elements analysis.
Figure 14 Schematic illustration of plasma torch
5.1.3 GF-AAS
The spectrophotometry of atomic absorption is an instrumental technique that allows us to determine
metals concentration in our sample. The principle of the method is based on the fact that the atomized
specimen containing the analyte, if hit by a suitable radiadation (hv) characteristic of the desired metal,
causes partial absorption of the initial radiation. There is a linear relationship between the absorbance
detected and the concentration of the analyte described by the law of Lambert-Beer. The tool we use is
the graphite furnace, a flameless atomization system, totally automated. It allows you to significantly
lower detection limits and work on very small sample aliases. The chart of the graphite cooker is as
follows:
27
Figure 15 Graphite furnace cross section
A small sample volume is introduced into the graphite tube, placed on the optical path of the radiation
emitted by the source. An inert gas flows through the tube, which expels the air making the atmosphere
non-oxidizing. The tube (being graphite, i.e. an inert material with high conductivity) is electrically heated
according to a three-stage program, conducted at increasing temperatures, which carry in succession
to:
• evaporation of the solvent (heating, to dry the drop, is slow in order to avoid convective motions and
maintain reproducibility);
• incineration of the sample (it is made to move the organic matrix so that the atom in question is in free
form);
• atomization (heats up to the atomization temperature of the sample so that the atomic number is
formed).
The absorption measure is made on atomic vapors that freeze rapidly in the final heating stage.The
signal that is obtained is a peak whose area is directly proportional to the concentration of the present
atomic atom in the graphite tube.The areaof the peak represents the entire population of atoms in the
sample present and therefore does not depend on the time it takes for complete atomization, the height
of the peak depends on the maximum concentration of atoms present in the furnace during the
atomization stage and therefore also depends on the matrix itself. Generally, it is chosen to examine the
peak area.
The advantages associated with the use of this technique are that we can analyse very small ammount
of sample because the detection limits are really reduced.
28
5.2 Analysis of waste water principal components
Acording to the description of the industrial processes dealt in the former chapters we can design which
are the main substances we want to quantify in the complex waste matrix, furthermore we can use these
informations to prevent inteference errors and to choose the optimal analytical tools.
For the present study, waste water samples were collected from JH-CTC tannery in Tolentino- Italy. The
effluent samples werecollected from the stages of tanningand retanning processing.
The samples were collected in polythene containersof 20 litres capacity and were brought to the
laboratory withdue care and was stored at 20°C for further analysis.
The physical and chemical characteristics of tanneryeffluents parameters are: pH, redox
potential,chlorides and sulphates, total surfactants, anionic surfactants, non ionic surfactants and
metals, were analysed following the standard procedures.
5.2.1 Methods
Determination of pH
The pH is determined by measurement of the electromotive force (emf) of a cell comprising of an
indicator electrode (an electrode responsive to hydrogen ions such as glass electrode) immersed in the
test solution and a reference electrode (usually a calomel electrode). Contact is achieved by means of
a liquid junction, which forms a part of the reference electrode. The emf of this cell is measured with pH
meter.
Cl- determination
The analytical procedure is based on the determination of chloride ions by titration with silver nitrate
solutions AgNO3 (0,0889 N) in neutral or slightly alkaline solution using potassium chromium K2CrO4 as
indicator.Once prelevate an aliquot of sample, it was added distilled water an NaOH solution until pH 9,
in such conditions, the chloride ion precipitates quantitatively as silver chloride (AgCl) of color white and
then when chloride anions are all precipitated we note the formation of red solution due to the formation
of silver chromate (Ag2CrO4) that correspond to the combination between the excess of Ag+ and the
indicator. The analysis was repeated three time for each sample.
SO42- determination
The sulphate ion is reacted with barium chloride in acidic environment for acetic acid, so to obtain
uniform-sized BaSO4 crystals. The turbidity of the BaSO4 suspension, is directly proportional to the
sulfate ion concentration in the sample, and it is measured by a photometer.The sample was filtered
(0,2 micronn filter) was performed a screening to verify that the color of the solution did not cause
interference at the specific wavelenght of the analysis.An aliquot of sample and of each standard is
taken and put in beakers with a magnetic stirrer; were added10 ml of 1: 1 glycerol / water solution and
5 ml of NaCl solution.Sequentially were added 0.3 g of barium chloride and leaving to stir for 2 minutes;
the stirring is stopped for 5 min, then is restarted for 15 seconds and immediately the sample is analysed.
29
The unknown concentration of sulfate ion is determined by a calibration curve previously obtained from
sodium sulphate four standards (10,20,40,80 ppm).
Cr (tot) GF-AAS determination
Graphite furnace AAS is often employed for trace metal determination. It can detect concentrations
about ppb one thousend times more sensible than flame atomizzation system. The instrument posses
an autosampler in wich are inserted the samples the standard and the diluent. In our case the diluent
was a solution of HNO3.
Table1 operative condition of instrument
Wavelenght 357,9 nm Slit 0,2 nm
Lamp type HCL Lamp current 5,0mA
Integr. mode Peak area Integr. Time 5,0 s
Clean temp. 2500°C ramp 100°C/s
Hold time 5,0 s Injec. Volume 18 μL
y = 0,0064x - 0,0241R² = 0,9958
0
0,1
0,2
0,3
0,4
0,5
0,6
0 20 40 60 80 100
Ab
s
ppm
Calibration curves of sulphates
Serie1
Lineare (Serie1)
30
Table2 calibration settings
Calib. method Addition calib. Calib. unit μg/L
Diluent solution HNO3 0,65% conc. Range 10-20 μg/L
Cr(VI) UV-Vis determination
The analysis is followed by the method specified in the international standard EN ISO 17075 of 2007
for the determination of Cr (VI) in extraction solutions obtained from leather under specified conditions.
The method described is suitable for quantifying the chromium (VI) content in the skin, with a lower limit
of 3 mg / kg, applicable to all types of leather. Changes were made to the equipment available in the
laboratory.
Soluble chromium (VI) is extracted from the sample in phosphate buffer at a pH value of between 7,5
and 8,0 and the substances that influence the detection are removed, if necessary, by solid phase
extraction. Chromium (VI) in solution oxidizes the 1,5-diphenylcarbazide, obtaining 1,5
diphenylcarbazone to provide a red / purple color complex with chromium which can be quantified by
photometric at 540nm. All the sample result with a concentration below the limit of quantification.
y = 8E-05x + 0,0455R² = 0,9795
0
0,02
0,04
0,06
0,08
0,1
0,12
0 200 400 600 800 1000
Ab
s
micrograms added
standard addition method
Serie1
Lineare (Serie1)
31
Tabella 3 Analytical results of tannery waste water
Analyte Units Tannery waste water Error Method
1 pH pH units 3,6 +/- 0,1 pHmeter
2 Sulphates (SO42-) mg/L 10970 +/- 5 UV-Vis S
3 Chloride (Cl-) mg/L 14832 +/- 3 titration
4 total Cr mg/L 2260,16 +/- 0,01 GF-AAS
5 Cr(VI) mg/L < LQ UV-Vis S
5.3 Treatment of waste water with molecular reactants
The treatment is carried out following the instruction of the seller, in the apparatus consisting of four
cylinders connected to each other,(showed below). Each of the cylinders contains 500g of "molecular
reagents" and is equipped with a oxygenation / aeration system that favors the reaction and ensures
proper agitation. The "molecular reactants" in the four reactors were wet with 4L of tanning water and
then start the real treatment. So each sample of waste water undergoes a treatment in each reactor,
starting from reactor 1 until reactor 4. In order to move the solution from a reactor to the other, we wait
a certain time for the stratification of the solid molecular reactants at the bottom of the reactor, and then
we use a pump to move the solution without molecular reactants.
The main information we wanted to obtain from this preliminary study are:
• the output pH and concentration of Chlorides, Sulphates and Chromium;
• wether is necessary repeat the treatment four times (in the four reactors) or not;
• wether after a certain number of treatment the molecular reactants decrease their efficiency.
So we decide to prelevate the samples from the reactors R3 and R4 to check if there are any difference
in the results of the analysis; if we will observe very similar results we can suppose that to use four
reactors is unuseful.
In order to check if the efficiency decrease after a number of treatment, we performer three cycle
sequentially; also in this case we will obtain a specifi trend of the data if there is any dipendence with
the usury, or quite similar data with a variance due to the experimental error. We also change for some
cycle the duration time to see if there is any relevant changement in the results.
The treatment was performed sequentially for 3L per times of tannery water,for three cycles.
32
Table4 Cycles duration time
Duration timefor each step Abbrevation
Cycle 1 15 min C 1
Cycle 2 20 min C 2
Cycle 3 20 min C 3
The four cilinders were called R1, R2, R3,R4 respectively to the order of treatment,the sampling was
performed only in the reactors R3 and R4 (except for the cycle 3 during which was collected only the
sample in reactor 3 for tecnical problems).
Figure 17 Pictures of reactors plant during the experiment
Figure 16 Scheme of reactors plant
R1 R2
R4 R3
aeration system
33
Figure 18 Reactor 2 during the treatment
Figure 19 Reactors 1 and 2 during the treatmet
34
Table5 Samples collected for analysis
Samples collected Abbreviation Duration time
Cycle 1- reactor 3 C1 R3 15 minuts
Cycle 1- reactor 4 C1 R4 15 minuts
Cycle 2- reactor 3 C2 R3 20 minuts
Cycle 2- reactor 4 C2 R4 20 minuts
Cycle 3- reactor 3 C3 R3 20 minuts
The samples, containig the treated solution and solid molecular reactive, were filtered and the solution
were analysed acording to the same parameters of the pre-treatment tannery wasre water in orther to
determine the efficacy of the treatment, which for a visual point of view showed a changement of colour
from dark purple to quite transparent.
Data Analysis
Table6 Analytical results of the sample collected
Analyte Units C1 R3 C1 R4 C2 R3 C2 R4 C3 R3 Error Method
1 pH pH
units 13,1 13,2 13,4 13,2 13,2 +/- 0,1 pHmeter
2 Sulphates (SO42-) mg/L 25326 20952 16032 17424 20770 +/- 5 UV-Vis S
3 Chloride (Cl-) mg/L 23426 15710 18937 15289 19357 +/- 3 titration
4 total Cr mg/L 1,02 1,36 1,68 6,29 1,41 +/-
0,01 GF-AAS
5 Cr(VI) mg/L < LQ < LQ < LQ < LQ < LQ UV-Vis S
35
Tabella 7 Average values for each cycles
For what concern pH we have a random distribution which means that this parameter isn’t affect by the
number of cycle and reactor position variables; for what concer Cr (VI) we don’t have any values.
Comparing the average value of the different cycles for Sulphates Chloride and total Cr; we don’t obseve
that the values decrease increasing the number of cycles, so we can suppose that for three cycles there
isn’t any usury.
Comparing the results for sample collected in reactor three and reactor four we observe in same case
(for ex. cycle 1 sulphate and chloride) that the concentration decrease from R3 to R4, but we have
some exceptions (for ex. Total Cr in Cycle 1 and Cycle 2; chloride Cycle 2), so with this result we
cannot deduce anything about the dipendence of requirement o fuse four reactor, because is needed
an analysis of variance (ANOVA) with an higher number of values and replicas.
Leachig test of molecular reactants solid residues
Informations about the constituents of molecular reactants formulation are not available so the leaching
test has the main goal to investigate if the waste material obtained from the purification treatment show
stability in water and in low pH solution.The solid residues were first put in demineralized water, for all
the samples no color change is observed, and the solution remains limpid. Once the solutions were treat
with some drop of concentrated sulphuric acid, a gradual change of color is observed.The pH variation
was continuously measuring during the experiment, but sometimes after a few minutes further pH
changes were observed.
All the blank experiments don’t show any variations of colours.
Analyte Units C1 C2 C3
Sulphates (SO42-) mg/L 23139 16728 20770
Chloride (Cl-) mg/L 19568 17113 19357
total Cr mg/L 1,19 3,99 1,41
36
R3C2
• Light gray semi-compact solid
• Weighed 5,004 g
• 60 ml of demineralized water was added
• Neutral pH
• H2SO4 added up to pH 3,7
• It notes the development of gas
• Color change around pH5
R3C3
Figure 18 Solid residue of R3C2 sample
Figure 20 R3C2 sample with water Figure 19 R3C2 sample with H2SO4
37
R3C3
• Light gray semi-compact solid
• Weighed 5,040 g
• 60 ml of demineralized water was added
• Neutral pH
• H2SO4 added up to pH 2,9
• It notes the development of gas
• Color change around pH5
R4C2
• mild gray / pink solid
• Weighed 16.82 g
• 150 ml of demineralized water added
• Neutral pH
• Add H2SO4 up to pH 2
• It notes the development of gas
• Color change is lighter than the others
Figure 21 R3C3 sample with H2SO4 Figure 22 R3C3 sample with water
Figure 24 R4C2 sample with H2SO4 Figure 23 R4C2 sample
38
R4C1
• Mild gray / pink solid
• Weighed 12,23 g
• 150 ml of demineralized water added
• Neutral pH
• H2SO4 added to pH 1
• We do not notice the development of gas
• Color change is lighter than other colors
We analyse one of the sample after the acidic treatment.
Tabella 8 Analytical results of leaching test
Analyte Units of
measurements R4C1 SAMPLE
R4C1- LEACHING
TEST
1 pH pH units 12,3 1,5
3 Sulphates (SO42-) mg/L 20790 39720
4 Chloride (Cl-) mg/L 22905 24798
11 total Cr mg/L 2,0 120,19
5.4 Geopolymer synthesis
Gepolymer was synthesize starting from three commercially available components, commercial kaolin,
sodium silicate solution with SiO2/Na2O in 3:1 ratio, sodium hydroxide and our tannery waste water. The
optimized formulation is showed in the table, it have been chosen with Si/Al ratio equal to 1.8 and Na/Al
to 1.0 referred to the total mass.
These ratios are close to what indicated in literature Si/Al near 2 for geopolymers that are cement-like
materials or waste encapsulating medium (26). This value of Si/Al ratio confers the geopolymer a rigid
3D network.
Table 9 Reagent for geopolymer synthesis
Kaolin (g)
Silicate (g)
NaOH (g)
Waste water (mL)
51
52,7
7
20
39
The NaOH solution is used as activation solution, the OH- ion contributes to the dissolution step of
Si4+and Al3+, while the Na+ ion contributes to balancing the negative charge generated by tetra-
coordinatedaluminium, in our case the metal contents of waste water also contribute to the charge
balanching during the geopolymer reticulation, specially chromium ions that is the most concentrated
metal.
The geopolymers were prepared according to the followingsteps:
• To dissolve sodium hydroxide pellets in a sodium silicate solution As the dissolution process is
highly exothermic,the solution is allowed to cool down to room temperature before the
successive steps.
• addition of metakaolin and waste water, intensive stirring until aomogeneous and fluid paste is
formed; the paste is poured into different kind of moulds.
• The paste was left to solidify at room temperature this imply a slow evaporation of the water
content that confers peculiar porosity to the final material.
We can determine if the geopolymer is formed measuring structural integrity test after 24h and 72h. Our
geopolymer it showed stability both in the tests (24h, 72h).
The solid was submers in bidistilled water at room temperature with a solid/liquid ratio of 1/100.
If the samples show undamaged surface, it means that the geopolymer is cured. Also water absorption
and weight loss were measured after 24 and 72 h.
Another important test consist in the Characterisation of waste-Leaching-Compliance, that give us
information about the stability and indirectly evaluate ions release of the material four week after the
synthesis. The test was performed acording to the european norm EN 12457 :
• The dry sample was mashed and weighed (12g);
• The granular material was poured into bi-distilled water in a flask, with solid/liquid ratio 1/10
(120g of water);
• The eterogeneus solution is left to stirr for 24h
• The solid was filtered and the soluion analysed
40
Figura 27 filtration geopolymer after 24 h
Tabella 10 Analytical results after leaching test of geopolymers
Analyte Units Leaching test
1 pH pH units 7,3
6 Sulphates (SO42-) mg/L 22
14 Chloride (Cl-) mg/L 9
15 total Cr mg/L 0,06
Figure 25 mashed geopolymer Figure 26 leching test (stirring)
41
RESULTS AND CONCLUSIONS
As I previously explained the aim of the work is to investigate geopolymers and molecular rectants
efficiency in the treatment of the tannery waste water; considering also the limt of discharge in natural
stream and sewerage (Table11)
Tabella 11 Law limits of discharge of pollutants
In the table12 are reported the results of the analysis, before and after the treatments, of the most
important parameters Cl- SO42- Cr(tot) and pH; the underlined values are those that exceed the
discharge limits in natural streams.
Tabella 12 Results of the analysis, before and after the treatments
(*) After the M.R. treatment were analysed different samples (C1R3, C1R4, C2R3, C2R4, C3R3). As
explanined in the previous chapter (pg 40), since the variability of the data is not due to the differentiation
Analyte Units discharge limit in
natural stream
discharge limit in
sewerage
pH pH
units 5,5 - 9,5 5,5 - 9,5
Sulphates (SO42-) mg/L ≤ 1000 ≤ 1000
Chloride (Cl-) mg/L ≤ 1200 ≤ 1200
total Cr mg/L ≤ 2 ≤ 4
Analyte Units Before
treatment
* M.R.
treatment
Geopolymers
treatment Error Method
pH pH
units 3,6 13,2 7,3 +/- 0,1 pHmeter
Sulphates (SO42-) mg/L 10970 20101 22 +/- 5 UV-Vis S
Chloride (Cl-) mg/L 14832 18544 9 +/- 3 titration
total Cr mg/L 2260,16 2,35 0,16 +/-
0,01 GF-AAS
42
of the samples, but rather to an internal variability, we choose to use the average values of the samples
collected toh ave a more clear comparison with the waste waster and geopolimers results.
This work represents a preliminary study of two new solution that can repleace the actual depuration
systems in tannery industry. The main aspects to consider are: the efficiency of the purifying system,
the classification of outgoing waste and the economic side.
We can simplify the depurative efficiency of these systems evaluating four fundamental parameters:
Chromium, Chlorides, pH, Sulphates. The current purification system fulfills the chromium removal, but
Sulphates and Chlorides even if not dangerous in themselves, exceed the limits for discharge in water
streams. In fact the method used from the tannery of Chienti consist in diluite the solution, that come
from chromium precipitation plant, usinig a large amount of water to decrease the concentration levels
of both.
The results in the table 12, show that molecular reactants treatment does not fulfill the requirements for
replacing the old purification plant because we are in the same situation, where chromium is almost
completely cut down, but chlorides and sulphates concentration don’t decrease . The pH passes from
slightly acid to very basic so we do not have any improvement because we still exceed the limits of
discharge.
Geopolymers, on the other hand, show greater efficiency as they also reduce the concentration of
chlorides and sulphates in addition to cut down the chromium.
With regard to the classification of outcoming waste, in some regions are present recovery consortia
connected with a number of tannery in wich the chromium precipitated is trasformed in chromium
sulphates and reused for the tanning process.
In our case, there isn’t this possibility so we have to dispose off a chimica waste, using the current
depuration system.
In the case of molecular reactors, we can not make a real classification of the outgoing waste, because
we don’y know the substances present, being protected by a commercial secret; so we do not have the
necessary information. However, considering the leaching tests carried out in the acidic environment,
we can say that we don’t have an inert waste but probably a chimica waste.
In the case of geopolymers, instead, after the treatment we obtain a solid inert waste, according to the
result of the leaching test.
Finally we want to consider the economic aspect, of course, a more specific cost analysis is needed,
both in terms of plants, as well as material needed per output volume and waste disposal costs. The
only issue I would like to point out, with regard to geopolymers, is their possible use without dilute the
outflow, obtaining therefore a large saving in water resources. The disadvantage is a large amount of
solid waste, which may however, be recovered as building material for construction industry.
Based on these aspects there are good expectations about the possible use of geopolymer in the
treatment of tannery waste, but further studies on this specific field are needed.
43
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