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Arsenic removal using biosorption with Chitosan
Evaluating the extraction and adsorption performance of Chitosan from shrimp
shell waste
A Minor Field Study
MSc. Thesis
Robin Westergren
2006
2
Arsenic removal using biosorption with Chitosan
Evaluating the extraction and sorption performance of Chitosan from shrimp
shell waste
Robin Westergren
2006
Supervisors
Lic. Martha Benavente Assoc. Prof. Olle Wahlberg
Facultad de Ingeniería Quimica Department of Inorganic Chemistry
Universidad Nacional de Ingeniería (UNI) Royal Institute of Technology (KTH)
Managua, Nicaragua Stockholm, Sweden
Assoc. Prof. Joquín Martínez
Department of Chemical Engineering
Royal Institute of Technology (KTH)
Stockholm, Sweden
Examiner
Prof. Lars Kloo
Department of Inorganic Chemistry
Royal Institute of Technology (KTH)
Stockholm, Sweden
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Preface
This study has been carried out within the framework of the Minor Field Studies Scholarship
Programme, MFS, which is funded by the Swedish International Development Cooperation
Agency, Sida /Asdi.
The MFS Scholarship Programme offers Swedish university students an opportunity to carry out two months’ field work, usually the student’s final degree project, in a Third World country. The results of the work are presented in an MFS report which is also the student’s Master of Science Thesis. Minor Field Studies are primarily conducted within subject areas of importance from a development perspective and in a country where Swedish international cooperation is ongoing.
The main purpose of the MFS Programme is to enhance Swedish university students’ knowledge and understanding of these countries and their problems and opportunities. MFS should provide the student with initial experience of conditions in such a country. The overall goals are to widen the Swedish human resources cadre for engagement in international development cooperation as well as to promote scientific exchange between unversities, research institutes and similar authorities as well as NGOs in developing countries and in Sweden.
The International Office at KTH, the Royal Institute of Technology, Stockholm, administers
the MFS Programme for the faculties of engineering and natural sciences in Sweden.
Sigrun Santesson
Programme Officer
MFS Programme
International Office, MFS
KTH Drottning Kristinas väg 6, SE-100 44 Stockholm. Phone: +46 8 790 6000. Fax: +46 8 790 8192. E-mail: [email protected]
www.kth.se/student/utlandsstudier/examensarbete/mfs
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Summary
Nicaragua is a country in which the toxic metal contamination of freshwater resources has become an
increasingly important problem in certain regions posing a threat to the environment as well as to human health.
Among the metals found in the waters of Nicaragua, arsenic is one of the most problematic since its long time
consumption is connected to serious health problems such as cancer and neurological disorders. The arsenic
contamination of water recourses in Nicaragua is mostly attributable natural factors, even though anthropogenic
activities including gold mining may be a contributing factor.
In this work the biopolymer Chitosan was studied as a potential adsorption material for the removal of arsenic
from aqueous solutions for water treatment design purposes.
The Chitosan used in this study was extracted from shrimp shells with an overall yield of 40% and a
deacetylation grade of 59%. The maximum adsorption capacity was determined to 20.9 mg As/g at a controlled
pH of 5.5 using the Langmuir isotherm. The adsorption was found to be strongly pH dependant with a fourfold
increase in adsorption capacity when pH was well under the pKa of Chitosan. The pH dependence indicates that
ionic exchange was the most important mechanism. No difference in adsorption capacity with respect to the
initial pH of the solution was detected in the pH range 3-7. This was attributed to the ability of Chitosan to act
as a weak base in water solutions.
The arsenic was desorbed from Chitosan using NaOH, (NH4) 2SO 4 and NaCl, with a 1M NaOH solution being
the most efficient displaying a concentration ratio of 1.08. The NaOH and (NH4) 2SO 4 solutions displayed a
steep desorption curvature with a large fraction of the arsenic being easily desorbed. The arsenic was, however,
not completely desorbed from the Chitosan implying that the adsorption capacity would decrease for the
coming cycles. Being a biopolymer the Chitosan is quite easily degraded in acid and alkali solutions, which
might be a limiting step for the process applicability.
Key Words: Adsorption; arsenic; Chitosan; ion-exchange; isotherm;
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Resumen
En Nicaragua, la contaminación de los recursos hídricos con metales tóxicos se ha convertido en un importante
problema, el cual se ha ido incrementando en ciertas regiones planteando una amenaza al medio ambiente y a la
salud humana. Entre los metales encontrados en las aguas de Nicaragua, el arsénico es uno de los más
perjudiciales ya que su consumo por un tiempo relativamente largo esta relacionado a serios problemas de salud
tales como cáncer y desordenes neurológicos. La contaminación de arsénico, en los recursos acuáticos en
Nicaragua, se atribuye mayormente a factores naturales, aunque las actividades antropogénicas, incluyendo la
minería para la extracción de oro, puede ser un factor que contribuya a incrementar los niveles de
contaminación.
En este trabajo, se estudió el biopolímero quitosana como un potencial material adsorbente para la remoción de
arsénico de soluciones acuosas para propósitos de diseño en el tratamiento de agua.
La quitosana usada en este estudio fue extraída del caparazón de camarón con un rendimiento global del 40% y
un grado de desacetilación del 59%. Utilizando la isoterma de Langmuir, se determinó que la máxima capacidad
de adsorción fue de 20.9 mg As/g a un pH controlado de 5.5. Se encontró que la adsorción depende fuertemente
del pH con un aumento cuatro veces mayor en la capacidad de adsorción cuando el pH esta por debajo del pKa
de la quitosana. La dependencia del pH indica que el intercambio iónico es el mecanismo más importante.
Además, se detectó que no hay diferencia en la capacidad de adsorción con respecto al pH inicial de la solución
en un rango de pH de 3-7. Esto fue atribuido a la habilidad de la quitosana de actuar como una base débil en
soluciones acuosas.
El arsénico fue des-adsorbido de la quitosana usando NaOH 1M, NaOH 0.1M, (NH4)2SO4 1M y NaCl 1M. Los
resultados mostraron que con la solución de NaOH 1M, la des-adsorción fue más eficiente, con una relación de
concentración de 1.08. Así mismo, con las soluciones de NaOH y (NH4) 2SO4 se obtuvieron curvas de
desorción más inclinadas, con respecto a la curva obtenida con la solución de NaCl, indicando que el arsénico
es más fácilmente des-adsorbido. Sin embargo, el arsénico no fue completamente des-adsorbido de la quitosana
implicando que la capacidad de adsorción decrecerá en los siguientes ciclos.
Ya que la quitosana es un biopolímero, el cual es fácilmente degradado en soluciones ácidas y alcalinas, puede
ser un paso limitante para su aplicabilidad en los procesos.
Palabras Claves: Adsorción; arsénico; quitosana; intercambio iónico; isoterma;
6
Sammanfattning
Nicaragua är ett land där förekomsten av toxiska metaller i yt- och grundvatten har blivit ett problem i vissa
regioner, då de utgör ett alvarligt miljöhot såväl som ett hot mot människors hälsa. Bland de metaller som
påträffats i Nicaraguas vattenresurser så utgör arsenik en av de mest problematiska, då dess konsumtion är
förknippad med livshotande sjukdomar som cancer och störningar på centrala nervsystemet.
Arsenikföroreningen i Nicaragua är främst av naturligt ursprung även om mänskliga aktiviteter såsom gruvdrift
tros bidra till de förhöjda halterna.
I detta arbete studerades biopolymeren Chitosan som ett potentiellt adsorptionsmaterial för rening av
arsenikförorenat vatten.
Det Chitosan som användes i denna studie utvanns ur räkskal med ett utbyte på 40% och en deacetyleringsgrad
på 59%. Den maximala adsorptionskapaciteten för Chitosan bestämdes till 20.9 mg As/g genom approximering
till Langmuir isotermen vid ett kontrollerat pH värde av 5.5. Adsorptionskapaciteten fanns vara beroende av pH
med en fyrfaldig ökning då pH var under pKa värdet för Chitosan. pH beroendet indikerar att adsorptionen sker
företrädesvis med en jonbytes mekanism. Inga skillnader i adsorptionskapacitet kunde påvisas till följd av
skillnader i vattenlösningars initial pH i området 3-7. Detta tillskrivs Chitosanets förmåga att agera som en svag
bas i en vattenlösning.
Arseniken kunde desorberas från Chitosan med hjälp av NaOH, (NH4) 2SO 4 och NaCl lösningar. 1M NaOH
fanns vara den mest effektiva med ett koncentrations förhållande (CR) på 1.08. NaOH och (NH4) 2SO 4
lösningarna uppvisade branta desorptionskurvor, vilket innebär att en stor del av arseniken lätt kan desorberas.
Arseniken kunde dock inte i något försök desorberas fullständigt vilket skulle ge upphov till en försämrad
adsorptions kapacitet för de kommande cyklerna. Då Chitosan är en biopolymer bryts den lätt ner under sura
och basiska förhållanden, vilket skulle kunna vara en begränsning för processens tillämplighet.
Nyckelord: Adsorption; arsenik; Chitosan; Jonbyte; Isoterm.
7
Table of contents
1 Introduction.................................................................................................................................................. 8
1.1 Background ............................................................................................................................................. 8
1.2 Objectives................................................................................................................................................ 8
2 Arsenic water contamination – A global problem...................................................................................... 10
2.1 Arsenic in natural waters ....................................................................................................................... 10
2.2 Aqueous speciation of arsenic ............................................................................................................... 10
2.3 Arsenic mobilisation ............................................................................................................................. 12
2.4 Arsenic toxicity ..................................................................................................................................... 13
2.5 Arsenic water contamination in Nicaragua ........................................................................................... 14
2.6 Arsenic removal methods...................................................................................................................... 14
3 The Adsorption Process ............................................................................................................................. 15
3.1 Adsorption mechanisms and materials .................................................................................................. 15
3.2 Diffusion and mass transfer................................................................................................................... 15
3.3 Adsorption equilibrium and isotherms .................................................................................................. 16
3.4 Adsorption in a continuous systems ...................................................................................................... 17
3.5 Regeneration of the sorbent................................................................................................................... 17
4 Chitosan biosorption .................................................................................................................................. 18
4.1 Chitin and Chitosan production............................................................................................................. 18
4.2 The structure and chemical properties of Chitosan ............................................................................... 19
4.3 Biosorption mechanisms on Chitosan ................................................................................................... 20
5 Materials and methods ............................................................................................................................... 20
5.1 Material ................................................................................................................................................. 20
5.2 Experimental procedure of arsenic adsorption ...................................................................................... 21
5.3 Experimental design .............................................................................................................................. 21
5.4 Desorption experiments......................................................................................................................... 21
5.5 Arsenic analysis and equipment ............................................................................................................ 22
6 Results and discussion ............................................................................................................................... 23
6.1 Chitosan yield and quality..................................................................................................................... 23
6.2 Adsorption kinetics ............................................................................................................................... 24
6.3 Adsorption isotherms ............................................................................................................................ 25
6.4 Desorption and sorbent recycling.......................................................................................................... 29
6.5 Process applicability.............................................................................................................................. 32
7 Conclusions................................................................................................................................................ 34
8 Acknowledgments...................................................................................................................................... 35
9 References.................................................................................................................................................. 36
8
1 Introduction
1.1 Background
Despite being a country with ample supply of fresh water resources, shortage of safe drinking water has become
an increasingly important problem in certain regions of Nicaragua. The increasing contamination of toxic
metals have caused a serious degradation of rivers, lakes and ground water reserves and has been recognized as
a threat to the environment as well as a threat to the population of affected regions (Benavente et al 2005) .
Among the metals found in the natural waters of Nicaragua arsenic is one of the most problematic due to its
documented toxic and carcinogenic effects at low concentrations (Espinoza 2005).
A study monitoring the waters of the Zapoyol community in central Nicaragua revealed that people for some
time had been consuming ground water contaminated with arsenic (Espinoza 2005). The concentrations in some
cases exceeded 100 µg/l, which is ten times the permissible limit stated by the World Health Organization
guidelines for drinking water quality. The occurrence of arsenic in Nicaraguan waters can be attributed to both
natural and anthropogenic factors and the sources differ between different regions. Groundwater interactions
with arsenic rich rocks and geothermal activities often combine with arsenic rich discharges from gold mining
activities to complicate remediation strategies (Espinoza 2005).
There exists a number of treatment processes to remove arsenic from water effluents including sulphide
precipitation, co-precipitation with iron and metal hydroxides and coagulation processes. The standard methods
for removal of arsenic from industrial effluents are, however, often expensive or fail to concentrate arsenic in
small waste volumes (Dambies et al. 2002). Thus, there is an urgent need to develop a cost efficient treatment
technology capable of separating arsenic from both drinking water and industrial effluents. One method that
recently has gained some of attention is biosorption, in which dead biomass is used to concentrate the metals.
Chitin and Chitosan are two biopolymers that can be derived from shrimp shells that previously have displayed
a high capacity to fix a great variety of metals (Guibal 1999). Since Nicaragua produces about 5.5 thousand tons
of shrimps per year, of which the residual shells constitute about 20% of the production volume, there is a
plentiful supply of raw material for the production of these polymers (Benavente 2001). This study was
therefore initiated to evaluate weather biosorption with locally produced Chitosan could be used to treat arsenic
contaminated water in Nicaragua.
1.2 Objectives
The general objective of this thesis was to gain a deeper understanding about arsenic adsorption on Chitosan for
future design of a water treatment technology. Special attention was given to determining the adsorption
mechanism and the applicability of quantitative descriptions of the adsorption process. The thesis, however,
also encompasses a screening for potential regeneration agents of spent Chitosan and a broader discussion
regarding the advantages and limitations of the technology.
9
The specific objectives were to:
Produce the Chitosan that was to be used in adsorption experiments from dried shrimp shells at laboratory scale,
determine the overall yield and the grade of deacetylation.
Determine the isotherms for arsenic adsorption on Chitosan with special reference to the influence of solution
pH.
Screen for effective desorption agents by determining the eluation curves for the different solutions and
comparing their overall efficiency.
Use adsorption and desorption data to establish the chemical mechanisms for adsorption.
Discuss the process applicability and the potential of Chitosan biosorption to improve the water situation in
Nicaragua.
10
2 Arsenic water contamination – A global problem
The occurrence of Arsenic in surface and ground waters can be attributed to a number of different sources of
both natural and anthropogenic origin. Due to the chronic toxicological effects of arsenic, increased levels of
arsenic have become a problem in several countries among which India, Bangladesh are the most critical with
over 40 million persons affected (Smedley and Kinnburgh 2002). It is, thus, an environmental problem of global
concern and indeed a very active field of research. Other countries which suffer from elevated levels of arsenic
include Argentine, Chile, Mexico, China, Hungary and as will be discussed more thoroughly Nicaragua.
2.1 Arsenic in natural waters
The range of As concentrations found in natural waters is large, ranging from less than 0.5 µg/l to more than
5000 µg/l. Typical concentrations in freshwater are less than 10 µg and often lower than 1µg/l (van Loon and
Duffy 2005). Higher concentrations are rarely found in ground waters. In certain high-As regions, however, up
to 90 % of the drinking water wells show concentrations exceeding 50µg/l. These large-scale As ground water
problems are often found in two types of environment. Firstly, inland or closed basins in arid or semi-arid areas,
secondly, in strongly reducing aquifers. Both environments tend to contain geologically young sediments and to
be in flat low-lying areas where ground water flow is slow and sluggish (Smedly et al. 2002). Arsenic rich
ground waters can also found in areas of geothermal activity and on a more localised scale, in areas of mining
activity and in areas where oxidation of sulphide minerals have occurred. Common for most arsenic rich
environments is that there is great variability in the As concentrations found within an affected area.
2.2 Aqueous speciation of arsenic
Arsenic can occur in several oxidation states (-3, 0, +3 or +5) but is found mostly as trivalent arsenite [As(III)]
or pentavalent arsenate[As(V)] in natural waters. The most abundant arsenic species found in aqueous solutions
are inorganic oxyanions.
11
2 4 6 8 10 12
-10
0
10
pe
pH
H3AsO3
AsH3
AsO43−
H2AsO3−
H2AsO4−
H3AsO4
HAsO42−
As ( c )
[H3AsO3]TOT
= 10.00 mM
t= 25°C
Figure 2.2.1. pe-pH diagram for aqueous As species in the system As- O2- H2O at 25°C and 1bar total pressure
(Smedly et al. 2002).
The redox potential and pH are the variables that determine the speciation of arsenic in a water body. Figure
2.2.1. illustrates that under oxidising conditions H2AsO4- is the dominant species at pH values under 6.9, whilst
at higher pH, H2AsO32- becomes dominant. H3AsO4 and AsO4
3- may be present in extremely acidic and alkaline
conditions respectively. Under reducing conditions and pH less than 9.2 the uncharged arsenite species H3AsO30
predominate.
2 4 6 8 10 12
0. 0
0. 2
0. 4
0. 6
0. 8
1. 0
Fractio
n
pH
H3AsO3
H2AsO3−
H4AsO3+
HAsO32−
[H3AsO3]TOT = 1.00 µM
Figure 2.2.2. Arsenite speciation as a function of pH at the redox conditions, where the chosen species
dominates.
12
2 4 6 8 10 12
0. 0
0. 2
0. 4
0. 6
0. 8
1. 0
Fractio
n
pH
AsO43−
H2AsO4−
H3AsO4
HAsO42−
[AsO43−]TOT = 1.00 µM
Figure 2.2.3. Arsenate speciation as a function of pH at the redox conditions, where the chosen species
dominates.
In Figure 2.2.2. and 2.2.3.the single variable diagrams for Arsenite and Arsenate respectively can be seen. In
natural waters organic As can be formed in areas of high biological activity and may be quantitatively important
in areas of industrial pollution (Smedley et al. 2002). In the presence of high concentrations of reduced S,
dissolved As-sulphide species can be significant.
2.3 Arsenic mobilisation
Compared to other heavy metalloids and oxyanion forming elements such as Se, Sb, Mo, Cr, U and Re, arsenic
displays a rather exceptional sensitivity to mobilisation at the pH values found in ground waters (pH 6.5- 8.5)
under both reducing and oxidizing conditions (Smedley et al. 2002).
Most toxic trace metals occur in solution as cations (e.g. Pb2+, Cu2+, Ni2+, Cd2+, Co2+ and Zn2+), which become
increasingly insoluble as the pH increases. At the near neutral pH of most groundwaters the mobility of these
metal cations is severely limited by precipitation or co-precipitation, or by their strong adsorption to metal
hydroxides, clay or organic material. Oxyanions, on the contrary, tend to be less strongly adsorbed as the pH
increases (Smedley et al. 2002). Under some conditions these anions can persist in solution at relatively high
concentrations even at neutral pH conditions. The oxyanion forming elements Cr, As, U and Se are therefore the
most common trace contaminants in groundwaters (Smedley et al. 2002). Relative to these other oxyanion
forming elements arsenic is the most problematic contaminant due to its relative mobility over a wide range of
redox conditions. Selenium is mobile as the selenate (SeO42-) oxyanion under oxidising conditions but is
immobilised under reducing conditions due to the stronger adsorption of its reduced form or (SeO32-), or due to
its reduction to the metal. In a similar way Chromium is mobilised under oxidising conditions as stable Cr(VI)
13
oxyanion species while its reduced Cr(III) form behaves like other cations at near neutral conditions. Other
oxyanions may form insoluble suphides in S-rich reducing conditions. Arsenic is in this way distinctive in being
mobile under both reducing and oxidising conditions (Smedley et al. 2002).
Two different triggers have been identified to be responsible for the release of arsenic in groundwaters. The first
trigger is the development of a pH>8.5, which is often an effect of high evaporation and weathering rates in arid
or semi-arid environments. This elevation of pH enhances the desorption of As from mineral oxides, especially
Fe oxides, or prevents As adsorption. The second trigger is the development of strongly reducing conditions at
near neutral pH conditions, leading to the desorption of As from mineral oxides and to the reductive dissolution
of Fe and Mn oxides, also leading to As release. Thus, in high As areas there is often a strong correlation
between high concentrations of As(III) and Fe(II) and typically low sulphate concentrations (van Loon and
Duffy 2005). Large concentrations of phosphate, bicarbonate silicate and and possibly organic material can
enhance the desorption of arsenic due to the competition for adsorption sites.
2.4 Arsenic toxicity
The valency state of arsenic has an important role for its behaviour in the environment with respect to transport
and accumulation. The chemical form in which arsenic is present, however, also is a key factor in assessing its
toxicity. Changes in the degree of oxidation are recognized to have an important effect on the degree of
bioavailability and its magnification of arsenic in food chains (Jain and Ali 2000).
When considering the toxicity of metals in general, simple hydrated metal ions posses the highest toxicity while
strong complexes and species associated with colloidal particles are considered less toxic. Trivalent arsenic
species are considered to be more toxic than the pentavalent form. Studies of the toxicity to humans are rather
limited but Arsenite is about 60 times more toxic than the oxidised arsenate (Jain and Ali 2000).
Organometallic compounds of tin, mercury and lead are often more toxic than the corresponding inorganic
species. This is particularly true for simple methylated species. Organoarsenic compounds, however, pose an
exception to this rule since they reduce their toxicity when methylated (Jain and Ali 2000). The organoarsenic
compounds are about 100 times less toxic than inorganic arsenic compounds. Methylation of inorganic arsenic
has actually been described as the most important detoxification process in the human body since it reduces the
affinity of the compound for tissue (Vahter and Marafante 1988).
The solubility of the arsenic compound is another factor that strongly correlates to its bioavailability and
toxicity to plants. It has been demonstrated that crops display a low uptake capacity of arsenic in soils with high
arsenic concentrations when the speciation is unfavourable with respect to its solubility (Thornton 1995).
The maximum acceptable level of arsenic in potable water set by the World Health Organisation is 10µg/l.
Higher levels have been known to cause dermatitis, anaemia, nerve condition abnormalities, hyperpigmentation
and circulatory disorders (Lorenzen et al. 1995). The consumption of arsenic containing water can cause acute
or chronic poisoning increase the risk of various types of cancer. The lethal dosage for man is 0.2-0.3g and for
freshwater life concentrations in the range 1-45 mg/l are considered lethal (Lorenzen Et al. 1995).
14
2.5 Arsenic water contamination in Nicaragua
The number of studies previously conducted to determine the occurrence of arsenic in Nicaraguan ground and
surface waters are quite few there is little information in literature covering the topic. The knowledge about
sources of arsenic as well as natural release mechanisms is therefore quite sparse. A geo physical survey
performed by CIRA in the (Centro para la Investigación en Recursos Acuáticos de Nicaragua), however,
established that Sebaco-Matagalpa region display increased arsenic groundwater concentrations with severe
effects for both the environment and its population (Espinoza 2005).
Of the 57 water samples included in the survey, 21 showed total arsenic concentrations between 10 and 122
µg/l, which is above the guidelines established for drinking water. The highest concentrations of arsenic where
found in the community of El Zapote. The total arsenic concentration in rocks and soil were 14.98 µg g-1 and
57.19 µg g-1 respectively, water concentrations were found to be 122.15 µg l-1. In this community cases of
hydroarsenicism were reported in the year 1996 from people drinking water from a well for a period of 6
months with an arsenic concentration of 1,320 µg l-1. People from this community were affected by irreversible
health problems, which forced the populations to change its water supply. The dug well used by the community
today displays an arsenic concentration of 122.15 µg l-1, which is still above the guideline for drinking water
(Espinoza 2005). The elevated arsenic concentrations of the Sebaco-Matagalpa region associates with the
hydrothermal activity originated from tertiary volcanism and are thought to spread in a NW-SE path (Espinoza
2005). The area affected by arsenic contamination might therefore be larger than the area covered by the study.
Except from the natural release of arsenic, there is also an anthropogenic input of arsenic from the gold mining
industry in certain regions. Since the gold ore findings occur together with arsenopyrite large quantities of
arsenic is displaced in the ore tailing dumps (Smedley et. al 2002). These tailing dumps are thereafter subject to
subsequent leakage of various metals including arsenic (Bennavente 2005). As higher grade ores are depleted
more complex sulphide ores and concentrates are being processed. The processing of more complex materials
results in higher levels of impurities such as arsenic in the process streams, which will make this an increasingly
important issue in the future (Lorenzen et al. 1995).
2.6 Arsenic removal methods
As discussed earlier the chemistry of arsenic is quite different from other toxic elements which are usually
found as cations in natural water. Thus normal precipitation is insufficient for the removal of arsenic and other
methods have been developed for arsenic. Sulphide and co-precipitation with iron hydroxides are often used for
as an alternative waste water treatment (Lorenzen et al. 1995).
It is especially difficult to concentrate arsenic in small waste volumes from dilute media. Coagulation and
electro coagulation are often used for this purpose (Dambies et al. 2002). In order to reduce arsenic levels to
admissible drinking water levels adsorption processes are viewed by many as a feasible option.
15
3 The Adsorption Process
In terms of process engineering, adsorption is a chemical separation technique involving a fluid flowing over a
solid. The strength of adsorption is its capacity to remove mere traces of solutes, making this method especially
useful for pollution control of dilute solutions. Compared to other chemical separation methods the adsorption
process depends to great extent on experimental data of the solution-solid interaction for design purposes.
3.1 Adsorption mechanisms and materials
A common characteristic of adsorbent materials is a high porosity and highly irregular geometries which result
in a large specific area. Since molecules can adsorb to some extent on all surfaces the amount adsorbed shows
proportionality to the surface area of the sorbent. There are, however, several types of adsorption phenomena
which all require individual explanation.
Electrostatic attraction to a charged surface is perhaps the most common and occurs on a large variety of
materials including clays metal oxides as well as organic materials. The surface charge is often dependent on
the properties of the surrounding solution. Iron and aluminium oxides are examples of inorganic sorbent
materials where the surface charge depends on whether they occur in their protonated (positive), neutral or
deprotonated (negative) state. Organic material, analogously, often display a variable surface charge with the
deprotonation of carboxyl groups or protonation of amino groups. The electrostatic attraction is therefore
strongly dependant of the solution pH.
The electrostatic interactions between sorbent and sorbate are more correctly described in terms of an ion
exchange process where co-ions may replace each other. The equilibrium position of the reaction depends on
the nature of the sorbent and the nature and concentration of the dissolved species adjacent to the sorbent (van
Loon and Duffy 2005).
In addition to electrostatic interactions molecules may also be adsorbed by covalent binding. Such chemical
processes are often referred to as specific adsorption and may be irreversible. Here the adsorption depends on a
chemical, as opposed to, electrostatic affinity between the sorbent and the species in solute. In adsorption
processes involving covalent bounding there is a weaker dependence of surface charge and thus also of the
solution pH. A combination of electrostatic and covalent bounds often occurs to complicate the picture (van
Loon and Duffy 2005).
3.2 Diffusion and mass transfer
The overall mechanism for the adsorption process can be described as the succession of the following steps: (i)
Solute transfer from the bulk solution to the boundary film; (ii) solute transport from the boundary film to the
surface of the sorbent; (iii) transfer from the sorbent surface to the intraparticular active sites; (iv) uptake of the
solute on the active sites, via ion exchange mechanism or specific adsorption (Guibal et al. 1995). With
sufficient mixing due to flow or agitation the bulk diffusion of the first two steps are often negligible. The
16
controlling steps involved in the adsorption process are therefore mainly intraparticle diffusion and the actual
sorption through electrostatic interactions or covalent binding.
Since adsorption by electrostatic interactions is more or less an instantaneous process, while complexation and
chelation often occur with a lower rate some conclusions about the sorption mechanism can be drawn from the
kinetics of the reaction (Guibal 1995).
3.3 Adsorption equilibrium and isotherms
In order to design an adsorption process the isotherms for each solute-sorbent system need to be determined
experimentally. The isotherms describe the chemical equilibrium for the specific conditions of temperature, pH,
co-ion existence etc. and thus represent the maximum achievable adsorption capacity for a given system. The
plotted isotherm shows the amount adsorbed as a function of the solution equilibrium concentration. A
downward curvature is often referred to as a favourable sorption characteristic since the sorption capacity
increases rapidly for low equilibrium concentrations. This terminology implies that adsorption is commonly
used to capture small amounts of solutes from dilute solutions.
To quantitatively describe the adsorption, mathematical descriptions are often used to withdraw information
from the adsorption data. Three commonly cited isotherm models are the linear, Freundlich and Langmuir
which implicitly hold information about the sorption mechanism. The Linear isotherm, which is rarely occurs
but is sometimes assumed for its simplicity, is given as.
eqKCq = (Eq. 3.3.1)
Where q is the concentration in the adsorbent and Ceq is the concentration in solution. The linear isotherm
simply states that the amount adsorbed is proportional to the concentration of solute. A fact that is often true in
a certain range of the isotherm but usually does not fit adsorption data over an entire isotherm.
Compared to the linear isotherm the Langmuir model uses some theoretical basis to better describe the
adsorption process. By assuming that there are a limited number of sites on the adsorbent and a monolayer
adsorption a mass balance can be stated.
+
=
sites
empty
sites
filled
sites
total (Eq. 3.3.2)
The sites are further subject to a chemical equilibrium
+
site
empty
solute
bulk
site
filled (Eq. 3.3.3)
In quantitative terms introducing the equilibrium constant
=
solute
bulk
site
emptyK
site
filled (Eq. 3.3.4)
17
Combining Equation 3.3.2 and 3.3.4 gives the expression
+
=
solute
bulkK
solute
bulk
sites
totalK
site
filled
1
(Eq. 3.3.5)
Or
eq
eq
mKC
KCqq
+=
1 (Eq. 3.3.6)
Were qm is the monolayer saturation capacity (mg/g sorbent) and K the equilibrium constant. By testing this
plot with adsorption data we can get some information about the sorption mechanism.
The third of the common isotherms called the Freundlich isotherm is given by
nKyq = (Eq. 3.3.7)
Were both K and n are Freundlich constants. The Freundlich relation is differs from that of Langmuir in that it
does not consider all the sites on the adsorbent to be equal, but rather that adsorption becomes increasingly
difficult as the sorbent becomes more saturated. Furthermore, the Freundlich assumes that there can be
interactions between the adsorbed molecules and that multilayer adsorption is possible. Thus the equation does
not propose any particular mechanism of adsorption but assumes several different interactions between the
sorbent and the adsorbate. The Freundlich model is strictly empirical, but has been found to fit adsorption data
of small molecules in small concentrations (van Loon and Duffy 2005).
3.4 Adsorption in a continuous systems
The isotherm gives, as stated above, the maximum achievable adsorption capacity for a given solute sorbent
system. In a system with a continuous flow of solute equilibrium is rarely the case and a lower adsorption
capacity is therefore to be expected. When using a packed bed adsorption system for water treatment purposes
there are, however, some advantages compared to a stirred tank.
The compact packed bed permits a faster mass transfer from bulk solution to the sorbent than any stirred tank
analogue (Cussler 1997). Although, the main advantage of using a packed bed is the effect of a counter current
flow with a more effective treatment as result. In the case of the tank reactor the equilibrium is reached with the
average depleted solution, while the packed bed approaches equilibrium with the concentrated feed solution
(Cussler 1997).
3.5 Regeneration of the sorbent
In order to obtain an efficient treatment method the regeneration of the spent sorbent is a crucial step. In order
to recover the metals in a high concentration for safe disposal as well as keeping the process costs down the
possibility of desorption is in many cases as important as the adsorption. The overall concentration effectiveness
18
for the sorption-desorption process can be defined as a concentration ratio between regeneration fluid and the
incoming water for a packed bed system (Volesky 1999).
][
][
Feed
EluateCR = (Eq. 3.5.1)
Another important parameter that often determines the efficiency of the regeneration that can be used in batch
experiments is the solid to liquid ratio (S/L ratio) (Jeon and Ha Park 2005). Combining the S/L ratio with the
desorption ratio gives an expression of the concentration ratio of a batch experiment similar to that stated above.
Generally as the S/L ratio increases as the desorption ratio decreases, however complete desorption is not
always the desirable case, but rather to find an optimum high the concentration ratio while maintaining an
acceptable desorption ratio (Jeon and Ha Park 2005).
Some qualitative information about the desorption process is given by the adsorption isotherm. An isotherm
which is strongly favourable for adsorption will be unfavourably when it comes to regenerating the sorbent.
Unfavourable desorption reflects that it will be difficult to regenerate the sorbent completely which leads to a
decreased adsorption capacity for the following cycles (Cussler 1997).
4 Chitosan biosorption
Biosorption is defined as an adsorption process in which biomass is used to concentrate solutes. In the search
for cheap and effective adsorbents containing natural polymers Chitin and its derivate Chitosan are very
interesting since they hold many of the desired properties for sorbent materials such as being biodegradable and
cost efficient (Guibal et al. 1999). One advantage of using biosorption is simply that the sorbent is a waste
product that can be recycled.
4.1 Chitin and Chitosan production
Chitin is the second most abundant polymer found in nature after cellulose and is found in high concentrations
in the shells of crustaceans (Crini 2005). Chitin constitutes about 20 % of raw shrimp shells and may be
regarded as cellulose with a hydroxyl at the second carbon position replaced by an acetamido group. To obtain
Chitosan from Chitin the acetamido group is deacetylated using a concentrated NaOH solution.
19
NH
O
O
O
OH
OH
CH3
CH3
CH3
O
Picture 4.1.1 Chitin structure showing the hydroxyl and acetamido groups on the glucose unit.
However, Chitin in biomass is closely associated with proteins, lipids, minerals that have to be removed
quantitatively in order to obtain a high quality sorbent. The major inorganic components are magnesium and
carbonates (Percot et al. 2002).
4.2 The structure and chemical properties of Chitosan
Chitosan is a linear polymer, chemically described as a poly(N-glucosamine) with hydroxyl and amine groups
present at the 2,3- and 5- position in the glucose unit respectively. The novel adsorption capacity of Chitosan
can be attributed to its functional groups. The hydroxyl groups increases the hydrophilicity of the polymer,
which enables diffusion into to the polymer network allows adsorption from aquatic solutions. The hydroxyl-
and amino groups also have a high reactivity and can react with solutes in a number of different ways (Crini
2005). Since the amino group is perhaps the most important when it comes to adsorption capacity, the degree of
deacetylation is an important parameter to asses the quality of the Chitosan.
NH2
O
O
O
OH
OH
CH3
CH3
Picture 4.2.1. Chitosan structure showing the amine- and hydroxyl groups on the glucose unit
20
In spite of these favourable properties some problems can occur in chemical process applications. For example
Chitosan is soluble in acidic media and, therefore, Can not be used as an insoluble sorbent under such
conditions (Varma et al. 2004). The stability, acid and alkali resistance of Chitosan may be enhanced by
crosslinking reactions although this procedure leads to a loss in adsorption capacity. Another problem occurs
when Chitosan in the form of flakes or powder is used in adsorption columns. Due to the low porosity around
0.85 of pure Chitosan this procedure causes a significant pressure drop when used in a sorption column. To
avoid this problem Chitosan beds have been developed (Crini 2005).
4.3 Biosorption mechanisms on Chitosan
The high Nitrogen content of Chitosan makes up a large number of active sites that are subject to different
chemical interactions in water solutions. The amine groups of the Chitosan polymer are weak bases that will
deprotonate water according to equation 6.1.1.
−+
←+− →+− OHNHChitosanOHNHChitosan 322 (Eq. 4.3.1.)
According to Elson et al. the pKa for Chitosan is 6.3 Thus when Chitosan is slurried in water it will slightly
increase pH of the solution (Elson et al. 1980). The direct consequence of this acid-base reaction is that the
adsorption on Chitosan will be dependant on pH. The amine sites in their deprotonated form may bind metals
through chelation mechanisms (Crini 2005). In its protonated form, on the other hand, the Chitosan possesses
electrostatic properties. Thus, it is possible to adsorb metals through anion exchange mechanisms according to
equation 5.3.2.
−−+
←
−−+ +− →+− XYNHChitosanYXNHChitosan 33 (Eq. 4.3.2)
5 Materials and methods
5.1 Material
The Chitosan used in the adsorption experiments was obtained at laboratory scale from shrimp shells provided
by CAMANICA S.A. Deproteinization of the shrimp shells was made using a 0.5% NaOH solution under
boiling temperature for 30 minutes with constant agitation. The liquid was thereafter separated by filtration and
shrimp shells were transferred to a new beaker. The residual shrimp shells were boiled for 10 minutes in a 3%
NaOH solution. This procedure was repeated for 3 times. The shells were thereafter agitated with a NaClO
solution for 30 minutes in order to remove all pigments.
The separated shells were then demineralised with a 1.25M HCl in room temperature for 30 minutes. The solid
phase was separated and dried at 60°C for at least 24 hours. The product was thereafter weighed to determine
the chitin content of the shrimp shells.
100)(
)(Pr% ×=
gShell
goductChitin (Eq. 5.1.1)
21
The Chitin was deacetylated using a 50% NaOH solution. The reaction rate was increased using a boiling water
bath for 30 minutes. The Chitosan product was obtained by floatation in the beaker. The Chitosan was washed
in distilled water until pH reached 7. The product was thereafter weighed and the Chitosan yield was
determined.
100)(
)(Pr% ×=
gChitin
goductChitosan (Eq 5.1.2)
The dried Chitosan was grinded and passed through a size excluding mesh to separate particles with a diameter
above 0.05 mm.
The Chitosan quality was analysed using IR spectrometry according to the following procedure: 0,1g of
Chitosan was grinded, mixed with 0.2 g KBr and left to dry for 3 hours at 100°C. The sample was pressed into a
pastille and an IR spectra was produced using a Magna 550 IR spectrometer. The deacetylation percentage was
calculated as the relationship between the characteristic Chitin and Chitosan peaks.
5.2 Experimental procedure of arsenic adsorption
A certified arsenic standard solution was used to prepare solutions with initial concentrations raging between 5-
500 mg/l. To achieve the desired pH for the sorption experiments, micro volumes of nitric acid and Sodium
hydroxide solutions were added to the solutions. Measurements of pH were performed with a model 410A
ORION pH meter. Adsorption experiments where carried out by putting 0.5 g of Chitosan (dry sorbent mass) in
contact of 20 ml of arsenic solution. The samples were collected and separated after 1 hour of agitation at 200
rpm. The final concentration of Arsenic in the solution was determined by Atomic Absorption spectrometry.
The Arsenic content, q (mg As/g Chitosan) was determined by a mass balance between the solid and the liquid
phase. Kinetic experiments were performed as described above while collecting and separating samples over
time.
5.3 Experimental design
The experiments to determine the isotherms were designed to study the influence of solution pH on the sorption
capacity of Chitosan. All experiments were carried out in duplicate accepting a 5 % deviation between the
replicates for calculations of a mean value.
5.4 Desorption experiments
For desorption studies the eluation curves were determined using Chitosan with a known mean concentration of
Arsenic. The Chitosan containing arsenic was grinded and mixed to receive an even distribution of arsenic in
the adsorbent. The desorption experiments were conducted by putting known amounts of adsorbent (dry mass
varying between 0.4 and 2 g) in contact with potential stripping solutions of varying concentrations. The
desorption was performed in batch experiments where the efficiency of Sodium chloride, Sodium hydroxide
and ammonium sulphate was determined.
22
5.5 Arsenic analysis and equipment
The Arsenic concentrations in the water samples were determined using a HG3000 atomic adsorption
spectrometry apparatus of model GBC 932 plus. Arsenic was analysed at wavelength 193.7 nm using a 25.0 mA
photron super lamp with 2.0 nm bandpass. Arsenic concentrations above 1 ppm were analysed using an
air/acetylene flame generation, while concentrations under 1ppm were analysed using hydride generation. Since
the hydrogenation process is valence state dependent the samples analysed with hydride generation were
therefore reduced prior to analysis using 20% v/v HNO3 solution. Calibration curves were made daily with
arsenic standard solutions in HNO3.
23
6 Results and discussion
6.1 Chitosan yield and quality
Chitin was extracted from the dried shrimp shells with an average yield of 25%. The Chitosan was obtained
with a yield of 40% after deacetylation and washing of the Chitin. The actual washing is thought to represent a
significant loss of polymer, which could easily be diminished with a more sophisticated washing technique and
a finer mesh filter.
In accordance with the method used by Moore and Domzy, the degree of deacetylation was calculated as the
fraction between the characteristic peaks of the carboxyl oxygen of the amid, and the nitrogen- hydrogen stretch
of the deacetylated Chitosan (Moore and Dumzy 2000).
Figure 6.1.1. IR spectra for the produced Chitosan.
In figure 6.1.1. we ca see the IR spectra for the produced Chitosan. The peak found at 3409 cm-1is typically
belongs to that of N-H stretching. The broadness of the peak may indicate tat there is both symmetric and
asymmetric stretching belonging to both amine and amid groups. At 1654 cm-1 there is a sharp peak, which is
typical for the carbonyl stretching of an amide group.
)33.1/100)(/(% 34091654 AAacetylN =−
24
And the deacetylation percentage was thereafter determined as
AcetylNionDeacetylat −−= %100%
Where the absorbance is equal to the minus logarithm of the transmittance on the IR spectra
TA log−=
Thus
%03.5997.40100%
%97,4033.1
100
556.0
303.0%
=−=
=
=−
ionDeacetylat
acetylN
Compared to commercially produced Chitosan a 59% grade of deacetylation is quite a poor result.
Commercially produced Chitosan normally holds a degree of deacetylation above 85% (Elson et al. 1980),
which implies that the produced Chitosan in this study would have a lower adsorption capacity compared to a
commercial analogue.
6.2 Adsorption kinetics
In order to establish the proper conditions for the equilibrium experiments, the study of the isotherms were
preceded by a brief study of the sorption kinetics. At the point where no further adsorption occurs the Chitosan
has reached equilibrium with the solute.
Adsorption kinetics
0
0,2
0,4
0,6
0,8
1
1,2
0 1000 2000 3000 4000 5000
Time (min)
C(t)/C
o
150 mg/l
500 mg/l
Figure 6.2.1. describes the sorption ratio as a function of time, with samples taken at 30 min, 60 min, 12 h, 24h,
48 h and 72 h.
As seen in Figure 6.2.1 the adsorption of arsenic on Chitosan displayed very fast kinetics with concentration
ratio C(t)/Co changing only during the first 60 min. Guibal and co-workers have previously shown that particle
size is an important factor in determining the rate of adsorption with the time required to reach equilibrium
increasing from 12 hours to 7 days with decreasing particle size for different anionic species (Guibal et al. 1995,
Dambies et al. 2002). The fast kinetics in these experiments were, therefore, probably a result of the very fine
25
Chitosan flakes used (d<0,05mm), which reduced time required for intra particulate diffusion of Arsenic to the
adsorption site.
Figure 6.3.1. also displays that the kinetics were not affected by the initial concentration, since equilibrium was
reached within 60 minutes in both cases. This was confirmed by all concentrations included in the isotherms,
which were established to be at equilibrium after 60 minutes.
6.3 Adsorption isotherms
Figure 6.3.1, 6.3.2 and 6.3.3 shows experimental adsorption data for different values of initial pH in the solution
along with the approximated Langmuir and Freundlich models.
Isotherm pH 3
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
0 100 200 300 400 500
Ceq (mg/l)
q (
mg
As/g
Ch
ito
san
)
Experimental data
Langmuir
Freundlich
Figure 6.3.1. Arsenic adsorption isotherm at pH 7. Experimental data and modelled curves illustrating
adsorption capacity as a function of equilibrium concentration.
26
Isotherm pH 5
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
0 200 400 600
Ceq (mg/l)
q (m
g A
s/g
Chitosan)
Experimental
dataLangmuir
Freundlich
Figure 6.3.2. Arsenic adsorption isotherm at pH 5. Experimental data and modelled curves illustrating
adsorption capacity as a function of equilibrium concentration.
Isoterm pH 7
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
0 200 400 600
Ceq (mg/l)
q (
mg
As
/g C
hit
os
an
)
Experimental data
Langmuir
Freundlich
Figure 6.3.3. Arsenic adsorption isotherm at pH 7. Experimental data and modelled curves illustrating
adsorption capacity as a function of equilibrium concentration.
Since the principal mechanism for adsorption of anionic species on Chitosan is thought to be that of ionic
exchange, an increase in adsorption capacity was expected with decreasing pH.
27
Initial pH Langmuir model Freundlich model
qm (mg/g) b (l/mg) K (l/g) n
pH 3 5,1 0,0075 0,35 0,4
pH 5 5,05 0,0065 0,26 0,44
pH 7 5,05 0,006 0,3 0,42
Table 6.3.1. Approximated Langmuir and Freundlich constants for the experimental data.
The results from these experiments concluded in table 6.4.1. does not, however, support the hypothesis that a
decrease in solution pH would increase the adsorption capacity. The maximum adsorption capacities were
estimated to 5.1, 5.05 and 5.05 mg As/g Chitosan respectively using the Langmuir model. There was thus no
significant difference in adsorption capacity with respect to initial pH of the solution.
Isotherms
0
0,5
1
1,5
2
2,5
3
3,5
4
0 100 200 300 400 500
Ceq (mg/l)
q (
mg
As/g
Ch
ito
san
)
pH 3
pH 5
pH 7
Figure 6.3.4. Adsorption isotherms illustrating the effect of initial pH on.
The Similarity between the experiments can also be illustrated graphically, as in figure 6.4.4. The results can,
however, be explained by the ability of Chitosan to act as a weak base in aquatic solution. When the final pH of
the solutions was measured they were all found to be slightly above neutral pH regardless of the initial pH
which means that the Chitosan was only partly protonated and cationic.
In order to gain a better understanding of the pH dependence and the sorption mechanism, the sorption
experiments were remade with controlled pH during the adsorption.
28
Adsorption isotherm
0
2
4
6
8
10
12
14
16
18
20
0 200 400 600
Ceq (mg/l)
q (m
g A
s/g
Ch
itosan)
pH>pKa
pH<pKa
Langmuir
Langmuir
Figure 6.3.5. Arsenic adsorption isotherms for controlled pH<pKa and pH>pKa respectively.
Isotherm pH<pKa
0
2
4
6
8
10
12
14
16
18
0 50 100 150 200 250
Ceq (mg/l)
q (
mg
As
/g C
hit
os
an
)
Experimental data
Langmuir
Freundlich
Figure 6.3.5.Arsenic adsorption isotherm for a controlled pH<pKa.
Langmuir model Freundlich model
qm (mg/g) b (l/mg) K (l/g) n
pH<pKa 20,9 0,012 0,9 0,55
pH>pKa 5,1 0,0075 0,35 0,4
Table 6.3.2. Adsorption coefficients for the modelled Langmuir and Freundlich curves.
When the pH was stabilised at 5.5 (well under the pKa for Chitosan) the adsorption capacity increased fivefold
to 20.9 mg As/g Chitosan (Figure 6.4.5. and table 6.4.2.). Thus, the protonation of the amine groups
29
significantly increased the adsorption capacity of Chitosan. This implies that ionic exchange was quantitatively
the most important adsorption mechanism.
The adsorption capacity calculated with the Langmuir model (Table 6.4.2.) also differs from those previously
cited in literature. Guibal et al. estimated the maximum sorption capacity of Chitosan to be 7 mg As/g (Guibal
1995). Elson et al. calculated the column capacity of Chitosan to be 224.76 mg As/g for acidic conditions
(Elson et al. 1980) and Benavente et al. calculated the column capacity to 38.7 mg As/g (Benavente et al. 2006).
These big differences in sorption capacity can not be explained by different grade of deacetylation, but that
some other phenomenon is present. Studies of the uranium sorption on Chitosan showed that the sorption
capacity is a function of the particle size (Guibal et al. 1995). A plausible explanation could be that the
adsorption capacity is dependant of the diffusion into the polymer network. Intraparticular diffusion has already
been established to affect the kinetics of the adsorption may also be the case of the arsenic adsorption and could
explain the discrepancies in adsorption capacities in previous studies.
One other factor that contributed to the increase in adsorption with decreasing pH was the form in which the
arsenic was present. The speciation of arsenic suggests that arsenates were present as H2AsO4- and HAsO4
-2
(pK2 6.8), which both could be adsorbed through ion exchange to the cationic Chitosan. As pH decreases below
3 the speciation favours the uncharged H3AsO4 specie. A preliminary study confirmed that the adsorption
capacity of uncharged arsenite species was significantly lower than that of arsenates. The adsorption capacity in
Figure 6.4.5 was therefore close to an optimum favoured by the protonation of the amine groups as well as
arsenic speciation. This conclusion is also supported by Dambies et al. (Dambies et al. 2002).
A problem of decreasing the final pH of the Chitosan-solute mixture was that the Chitosan started to dissolve.
Although the dissolution of Chitosan did not adversely affect the adsorption performance, it would be a serious
disadvantage in a continuous column system. The dissolution of sorbent would not only lead to a loss of
sorbent, but would also create a large pressure drop over the column by reducing the particle size.
6.4 Desorption and sorbent recycling
The desorption of Chitosan is a topic that has been sparsely examined compared to the adsorption of various
metals. An initial screening for an efficient desorbing solution was therefore conducted.
Since the adsorption process was shown to be strongly pH dependant, an alkali desorption solution was tested.
Figure 6.6.1. shows the desorption curve for arsenic containing Chitosan with a solid to liquid (S/L) ratio of 42.
30
NaOH desorption S/L=42
0
5
10
15
20
25
30
35
0 10 20 30 40
Eluation volume (ml)
As c
on
cen
trati
on
(p
pm
)
NaOH 1M
Figure 6.6.1. Desorption of arsenic containing Chitosan with a 1M sodium hydroxide solution.
The first desorption step resulted in a 50.2% recovery of the adsorbed arsenic and a concentration ratio of 0.82.
This implies that even though the NaOH enables the desorption of arsenic, the eluated liquid held a lower
concentration than the incoming feed solution and that all adsorbed arsenic was not stripped.
NaOH desorption S/L=100g/l
0
5
10
15
20
25
30
35
40
45
0 20 40 60 80 100
Eluated volume (ml)
Ars
enic
concentr
ation (ppm
)
NaOH 1M
NaOH 0,1M
Figure 6.6.3 Desorption of arsenic containing Chitosan with NaOH solutions of 1M and 0.1M.
When increasing the S/L ratio to 100g/l the concentration ratio was increased to 1.08 which represents a small
improvement of the desorption process. When a 0.1M NaOH solution was used the concentration ratio slightly
decreased to 0.80 indicating that the concentrated NaOH solution was more effective.
31
Studying the curvature of figure 6.6.2. and 6.6.3. one can see that the desorption was increasingly difficult for
each step. Since the adsorption of arsenic was a favourable process it was expected that desorption would be
unfavourable, since small amounts of arsenic were strongly adsorbed to the Chitosan. This means that the
performance in terms of concentration ratio was lowered for each desorption step. The steepness of the curve,
however, indicates that most arsenic is desorbed rapidly even though it is not desorbed completely.
In the search for other desorption agents, NaCl was chosen to test weather the presence of competing anion
could strip the arsenic by reversing the ion- exchange equilibrium. Due to its low cost NaCl was an especially
interesting desorption agent to study. Figure 6.6.3. shows the desorption curve using a 1M NaCl solution. Even
though chloride has been showed to have a low affinity for binding to Chitosan (Vold et al. 2003) the
concentrated NaCl solution was able to desorb the arsenic. The concentration ratio for the first desorption step
is, still, only 0.38, which is less than half compared to desorption with NaOH.
NaCl desorption S/L=50g/l
0
2
4
6
8
10
12
14
16
0 50 100 150 200 250 300
Eluated volume (ml)
Ars
en
ic c
on
cetr
ati
on
(p
pm
)
NaCl 1M
Figure 6.6.3. Desorption of arsenic containing Chitosan with a 1M sodium chloride solution.
Since sulphate has been reported to form coordinate linkages with Chitosan (Elson et al. 1980) a 1M
Amoniumsulphate solution was tested as a potential desorption solution. The first desorption with ammonium
sulphate was comparable to that of NaOH with a concentration ratio of 0.73, but than the desorption efficiency
rapidly decreases with a steep curvature. The expectation that covalently bound arsenic would be desorbed was,
thus, not confirmed by the experiment.
32
Amoniumsulphate desorption S/L=50
0
5
10
15
20
25
30
0 50 100 150 200 250
Volume of eluent (ml)
Ars
en
ic c
on
cen
trati
on
(p
pm
)
Amoniumsulphate
1M
Figure 6.6.4. Desorption of arsenic containing Chitosan with a 1M ammonium sulphate solution.
The poor concentration ratios attained in this study are probably due to the concentrated feed solutions that were
used in the adsorption studies (up to 500ppm). The concentration ratios displayed is, therefore, an indicator of
the desorption solution’s relative efficiency. When the adsorption process is used for more dilute solutions the
efficiency of the desorption is expected to increase in terms of concentration ratio.
It has previously been demonstrated by Guibal et al. that Chitosan can be successfully desorbed and reused 10
times with unaffected sorption capacity for the oxyanions Molybdate and Vanadate using a 1M NaOH solution
(Guibal 2004). This is a strong indication that arsenic could be desorbed with good results in a column system
since the oxyanions share a similar sorption mechanism.
6.5 Process applicability
When considering the biosorption process with Chitosan from a broader perspective it is important to identify
the limiting parameters of its applicability.
Being a biopolymer, Chitosan is rather easily degraded. The poor stability of the Chitosan polymer is a property
that has serious drawbacks for its use in adsorption columns. As stated above it was noticed that the Chitosan
started to dissolve during pH controlled experiments when acid was added. This was also observed during
desorption experiments using strong alkali solutions. The degradation of the polymer seems to be limiting for
the reuse and, therefore, the overall the cost-effectiveness of the treatment method.
The fact that the adsorption with Chitosan proceeds most effectively under acidic conditions may limit its
possibilities of being a cheap and simple method to purify potable water in affected regions of Nicaragua. The
adsorption capacity of arsenic is also quite low compared to other toxic metals, such as Mercury (Sjörén 2006).
The treatment of mining effluents with Chitosan adsorption would still be one plausible mode of application.
33
The success of such operation will, however, be limited by to what degree the effluents can traced back to a
point source.
34
7 Conclusions
The degree of deacetylation in the produced Chitosan was significantly lower than that of commercial
equivalents and contained impurities. The fact that adsorption capacity should be proportional to the grade of
deacetylation was, however, not reflected in the calculated adsorption capacity.
The maximum theoretical capacity of Chitosan to adsorb arsenic was calculated to 21 mg As/ g Chitosan at final
a pH of 5.5. This is comparable to the adsorption capacity of arsenic on activated carbon from coconut shells
(Lorenzen et al. 1995) and about 10 times smaller than that of commercial ion-exchangers (Elson et al. 19809.
Compared to other adsorption studies carried out on Chitosan, Arsenic is one of the metals that display the
lowest adsorption capacity.
The capacity of Chitosan to adsorb Arsenic was strongly pH dependant with a fourfold increase in adsorption
capacity when the pH was well under the pKa of Chitosan. The pH dependence supports the hypothesis that ion-
exchange was the most important mechanism. Though, it can not be excluded that surface sorption was present.
Desorption studies also suggests that some arsenic was bounded covalently to the Chitosan.
The pH of the initial solution had no significant effect on the adsorption capacity in the pH range 3-7. When
optimizing the adsorption process with respect to pH it was important measure the final pH of the solution,
since the Chitosan increased the pH when it was slurried in water.
Arsenic desorption was achieved using NaOH, NaCl or (NH4)2SO4, with NaOH of 1M being the most effective.
The desorption had an unfavourable curvature that made it difficult to desorb the Chitosan, while maintaining a
high concentration ratio.
The possibility to regenerate the sorbent and the fact that the sorption proceeds more effectively under acidic
conditions direct the biosorption process to treatment of industrial raw water and mining effluents.
35
8 Acknowledgments
This study would not have been realised without the support and assistance of a number of persons to whom I
wish to express my gratitude.
First and foremost, I would like to thank Martha Benavente for leading the experimental part of this work and
for taking such good care of me and Anna in Nicaragua.
Thank you, Olle Wahlberg, for being a mentor in the field of environmental chemistry as well as being the
principal supervisor and examiner of this thesis.
I also want to thank Joaquín Martínez for invaluable support, for putting this project together and for soccer
games in Managua.
The financial support from the Swedish International Development Cooperation Agency (Sida/Sarec) is
gratefully acknowledged. I would like to thank Sigrun Santesson in particular for her help with administration
of the scholarship.
36
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