90

CHAPTER 3

MATERIALS AND METHODS

3.1 INTRODUCTION

The following chapter details the materials and methods used in the

present investigation. Ion exchange resin used modification of ion exchange

resin, SEPRW solution preparation, analysis of metal concentration and batch,

column procedure of removal of heavy metals discussed. The mathematical

model used for batch, column and EIX process has also beed described.

3.2 ION EXCHANGE RESIN

The Ceralite IR120 ion exchange resin used in the experiments was

supplied by CDH, India. It is a copolymer of styrene and divinyl benzene.

Table 3.1 shows the physical properties of the resin as given by the supplier.

The resin is a strong acidic cationic exchange resin.

Table 3.1 Physical properties of cation exchange resin Ceralite IR 120

Parameter Ceralite IR 120

Manufacturer CDH, New Delhi

Ionic group H+ strongly cation exchange resin

Particle size 0.045-0.06 (effective size) cm

Density 770 g L1

Exchange capacity (fresh) 4.5 meq g1

Maximum temperature withstanding 120ºC

pH range 1-14

Cross linking 8% DVB

Porosity 0.42

91

It has fairly good capacity and appreciable stability at elevated

temperatures. Before using the resins they were soaked in double distilled

water for 12h and then washed several times with triple distilled water.

3.3 POLYETHYLENEIMINE FOR MODIFICATION OF

CERALITE IR 120

Polyethyleneimine (PEI) is an Aldrich product (Mn–60,000, MW-

7, 50,000, 50% aqueous solution) purchased from Sigma Aldrich Chem. Co

(USA).

PEI also known as poly Aziridination propane (C6H21N5)n is a

typical polymeric amine having chelating properties with various metal ions.

It is known to exist as a linear structure or a branched structure. A colourless

or pale yellow and thick fluid, hygroscopicity, soluble in water, ethanol, is not

soluble in benzene. Commercial branched PEI was employed in the present

work depending on the polymerization process; it contains primary, secondary

and tertiary amino groups in a ratio of approximately 1/4, 1/2 and 1/4,

respectively (Bahulekar et al 1991). PEI possesses quite number of

advantages as polymer chelating agent, such as good water solubility and

suitable molecular weights (Amara and Kerdjoudj 2003).

3.4 ELECTROCHEMICAL TREATMENT ACCESSORIES

The dimensionally stable anodes (DSA) are promising materials for

many electro-organic applications and have been classified as ‘active’ or

‘non-active’, depending on its chemical nature (Simond et al 1997; Malpass

and Motheo 2001).

Active electrodes mediate the oxidation of organic species via the

formation of higher oxidation states oxides of the metal (MOx+1), whenever a

92

higher oxidation state can be reached by the metal oxide (e.g., RuO2) - leading

to selective oxidation (Subbiah et al 1990).

Titanium substrate insoluble anodes with RuO2 coating anodes

were employed in the present investigation since these electrodes are

commercially available and are well known anode material for use in neutral

chloride media. Hence, for this present study stainless steel was selected as

cathode and three DSA (Titanium (Ti), Ti/RuO2, Ti/PbO2) anodes were

selected as anode material.

Stainless steel cathode, Titanium (Ti), Ti/RuO2, Ti/PbO2 anodes

were purchased from Tianno, Ti anode Fabricators Pvt. Ltd, Chennai.

Anion exchange membranes (AEM), cation exchange membranes

(CEM) used in the experiment were NEOSEPTA and NAFION 117

membranes were purchased from Tokuyama Corp., Japan.

3.4.1 Electrochemical Ion Exchange (EIX) Part Setup

Schematic diagram of the electrochemical ion exchange part is

shown in the Figure 3.1(a). It consist of four compartments separated by two

CEM, one AEM membranes and had an effective electrode area of 4.9 dm2

(0.7×0.7 dm). The geometric dimensions of each of the compartments were

0.7×0.7×0.15 dm. Stainless steel plate of size 0.7×0.7 dm was used as a

cathode and metal oxide (RuO2 or PbO2) coated on expanded Ti mesh of area

4.9 dm2 (0.7×0.7 dm) embedded in PVC frame was used as an anode. The

process compartment was filled with 40g of PEI modified Ceralite IR 120

resin (PMR). Provisions are made for electrical connections so as to constitute

an electrolytic cell. The cell had one inlet at the bottom and one outlet at the

top cover for solution runoff.

93

Figure 3.1a Experimental setup of the Electrochemical Ion exchange part

3.4.2 Electrochemical Oxidation (EO) Part Setup

Schematic diagram of the Electrooxidation setup is shown in the

Figure 3.1(b). It is a filter press type electrochemical cell consists of stainless

steel cathode and RuO2 coated Titanium mesh as anode had an effective

electrode area of 4.9 dm2 (0.7×0.7 dm). Provisions are made for electrical

connections so as to constitute an electrolytic cell and the two terminals were

connected to a power supplier.

Figure 3.1b Experimental setup of the Electrooxidation part

94

3.5 PREPARATION OF SYNTHETIC ELECTROPLATING

RINSE WATER (SEPRW)

All the reagents and chemicals used were of analytical grade. Based

on the practical data obtained from different electroplating industries, the

synthetic rinse water of Cu(II), Ni(II) and Zn(II) ions of appropriate

concentration (M2+

=200 mg L1) were prepared by diluting the electrolytic

plating baths (a, b and c) of the following composition: (a) 200 g L1 of

CuSO4, 75 g L1 of H2SO4 ; (b) 200 g L

1 of NiSO4, 40 g L

1 of NiCl2, 40 g

L1 of Boric acid; (c) 240 g L

1 of ZnSO4, 30 g L

1 of sodium acetate, 30 g

L1 of Al2SO4. All the solutions were prepared with double distilled water.

3.6 COLLECTION OF INDUSTRIAL ELECTROPLATING

RINSE WATER

Industrial electroplating rinse water (IEPRW) sample prior to any

wastewater treatment was collected from a small scale electroplating industry

located at Balaji Nagar, Padi, Chennai. This unit is a metal finishing industry

doing copper, nickel and zinc electroplating for the base metallic material.

Typically during processing, this industry produces effluent around 5,200 L

per day containing heavy metals and some organic constituents. Samples of

IEPRW were collected directly from the first rinse tank of the electro plating

unit prior to any treatment.

3.7 CHARACTERIZATION OF IEPRW

The IEPRW samples were diluted and analyzed for different waste

water parameters. The metal ions presence was analyzed in Varian spectraa

220 Atomic adsorption spectrophotometer (AAS). Chemical oxygen demand

(COD) was analyzed by reflux condensation method in Lovibond ET 125

95

COD digester. The presence of sulphate was analyzed by Spectrophotometery

in Elico Ltd, SL 160 spectrophotometer.

3.8 MODIFICATION OF CERALITE IR 120 RESIN

Five grams of resin was immersed in 0.02 L of 0.05M (2/5water,

3/5 alcohol) PEI solution and agitated constantly using a glass agitator

connected with an electric driving force for 7 days at constant room

temperature 32°C (Bahulekar et al 1991).

The hydrophobic property of the resin increased after introducing

these amorphous branched structure with a distribution of primary, secondary,

and tertiary amino groups in the ratio 1:2:1 (Amara and Kerdjoudj 2004),

adsorbed and/or ion exchanged with H+ counter-ion on the surface (Deng and

Ting 2005). The structure of PEI is shown in Figure 3.2.

Figure 3.2 Structure of PEI

96

Figure 3.3 Reactions behind in the modification of UMR using PEI

nano cluster

The resin thus obtained had the following principal characteristics:

PMR has a preferential selectivity towards heavy metals and particularly for

the Cu(II) ion. However, the PEI molecules would desorb from the resin

surface during sorption and regeneration processes. To fix the PEI onto the

surface, the modified ion exchange resin was soaked in 1% glutaraldehyde in

isoproponal as the cross linking agent for 20min (Ghoul et al 2003). PMR was

washed with distilled water and soaked in 0.5N NaOH solution for 3h to

change them to Na+ form (Figure 3.3).

97

3.9 ANALYTICAL MEASUREMENTS

3.9.1 Analysis of Metal Ions Concentration

Atomic absorption spectrometry (AAS) is a widely used and

accepted technique capable of determining trace (µg mL1) and ultra trace

(sub-µg mL1) levels of elements or metals in a wide variety of samples,

including biological, clinical, environmental, food, and geological samples,

with good accuracy and acceptable precision. It is arguably the predominant

technique in elemental analysis, although it does have some limitations (Settle

1997). Atomic absorption spectrometry uses the absorption of radiation by

free gaseous atoms in order to achieve qualitative detection and quantitative

determination of elements (Welz and Sperling 1998).

There are two main components in an atomic spectrometer: atom

cell which creates atoms at the free gaseous ground state, optical system to

measure the signal. Atom cell dissolvates the liquid sample and dissociates

analyte elements into their free gaseous ground state form in which the atoms

are available to absorb radiation coming from light source and create a

measurable signal which is proportional to concentration (Tyson and Haswell

1991). The atomizer (in which the analyte is atomized) is flame, graphite tube

or quartz tube. In flame atomization fixed aliquot of measurement solution is

converted into an aerosol in nebulizer and transported into the flame which

must has enough energy both vaporize and atomize the sample. Properties of

flame types are presented in Table 3.2 (Welz and Sperling 1998).

98

Table 3.2 Spectroscopic flames for AAS with their properties (Tyson and

Haswell 1991)

Oxidant Fuel Gas

Max. Flame

Temperature

(ºC)

Max. Burning

Velocity

(cm s1)

Remarks

Air Acetylene 2250 158 Most commonly used

flame

Nitrous

Oxide

Acetylene 2700 160 For difficulty volatilized

and atomized substances

Air Hydrogen 2050 310 Flame of high

transparency; for easily

ionized elements

Air Propane/

Butane

1920 82 Flame of ionized

elements

The filtrate solutions obtained in the adsorption process were

analyzed using flame AAS. The instrument used was a VARIAN MODEL

SPECTRAA 220 atomic absorption spectrometer. Element content of aqueous

solutions was determined with hallow cathode lamps. Table 3.3 shows that

specific wavelengths and analysis conditions for used metal ions in our study.

Table 3.3 Specific wavelengths and AAS analysis conditions used for the

metal ions

ElementWavelength

(nm)Flame Type

Hallow-Cathode

Lamp

Copper (Cu2+

) 324.8 Air/acetylene Copper

Nickel (Ni2+

) 232.0 Air/acetylene Nickel

Zinc (Zn2+

) 213.8 Air/acetylene Zinc

99

3.9.2 Analysis of Chemical Oxygen Demand (COD)

Chemical oxygen demand (COD) is the measure of oxygen

consumed during the oxidation of the oxidizable organic matter by a strong

oxidizing agent. Chemical oxygen demand (COD) test is commonly used to

indirectly measure the amount of organic compounds in water. The basis for

the COD test is that nearly all organic compounds can be fully oxidized to

carbon dioxide with a strong oxidizing agent under acidic conditions

(Manivasakam 1984).

Lovibond ET 125 model COD digester was used for the digestion

of samples at 120ºC for 2h. The organic matters present in the IEPRW

samples were oxidised by Potassium dichromate (K2Cr2O7) in the presence of

H2SO4 in the digester. After digestion the remaining unreduced K2Cr2O7 is

titrated with standard ferrous ammonium sulphate using Ferroin indicator.

The amount of oxidizable matter (COD) is calculated in terms of oxygen

equivalent.

3.9.3 Analysis of Sulphate ( 2

4SO ) Ions Concentration

The amount of sulphate present in the IEPRW was analyzed by

spectrophotometric method using barium chloride (BaCl2). The analysis for

sulphate was done by taking appropriate (0.005 L) volume of sample in a

volumetric flask, and diluting to 0.1 L using distilled water. From that 0.01 L

of sample was taken in a conical flask and 0.005 L conditional reagents

(conditional reagent was prepared by mixing 0.05 L glycerol with a solution

containing 0.03 L conc. HCl, 0.3 L of distilled water, 0.1 L of 95% ethanol

and 75 g NaCl), and a spoon full BaCl2 crystals (about 0.2-0.3 g) and mixed

well. Sulphate ions present in the IEPRW samples was converted to BaSO4

precipitate hence the solution becomes turbid. Immediately after the stirring

period the solution was poured into quartz cuvette and the absorbance was

100

measured at 420 nm by Elico Ltd, SL 160 spectrophotometer (Manivasakam

1984).

3.9.4 Analysis of Available Hypochlorite (OCl )

The available hypochlorite ions in the samples were analyzed by

iodometric method. The starch iodide titration method is one of the oldest

methods for determining chlorine, is very non specific for oxidants and

generally is used for total chlorine testing at levels above 1 mg L1 Cl2

(Manivasakam 1984). The method is based on reaction with thiosulfate

solution:

2OCl Cl2 + O2

Cl2 + 3KI I3 + 3K+ + 2Cl

I3 + 2Na2S2O3 3I + 4Na+ + S4O6

2

The end point of the titration is indicated by the disappearance of

the blue colored starch iodide complex.

3.9.5 FTIR Analysis

Fourier transform Infrared (IR) spectroscopy measures the

absorption of IR radiation by materials as the atoms vibrate about their bonds.

It is primarily used to identify bond types, structures and functional groups in

organic and inorganic compounds. IR sensitive vibrations are associated with

changes in dipole moments. IR spectroscopy measures vibrational energy

levels in molecules. It can be used for both qualitative and quantitative

analysis, to identify molecules and compounds, and to determine the presence

or absence of certain types of bonds and functional groups.

101

When functional groups can be bonded at different locations on

molecules, IR spectroscopy can frequently identify the positions at which the

functional groups are attached. The reason is that vibrational frequencies

differ when functional groups are attached at different sides in molecules.

When illuminated by IR radiation of the appropriate frequencies, atoms, ions,

and functional groups in molecules will vibrate about their bonds and energy

will be absorbed. Each bending and stretching vibrational mode of a molecule

or functional group will absorb at a particular frequency. When exposed to

appropriate IR frequencies, energy will be absorbed from the incident

radiation as vibrational intensities increase. Many IR frequencies have no

effect at all and will not be absorbed (Dinger 2005).

IR characterization of raw UMR, PMR and metal ion adsorbed

UMR, PMR were performed with PERKLIN–ELMER SPECTRUM RX1

instrument with a frequency range of 4000 - 400 cm1. The resin samples

were prepared in the form of potassium bromide (KBr) pellets.

Approximately 40-60 mg of resin powder and 120 mg of KBr was blended

and powdered with the help of mortar and pestle for 20 min. Approximately

40 mg of the mixture were compacted using an IR hydraulic press at the

pressure of 8 tons for 60 sec. The pellets were conditioned in a desiccator

placed in an oven at 80°C for 8h before analysis. The spectra of resin samples

were obtained with a frequency range of 4000-400 cm1.

3.9.6 SEM Analysis

Scanning Electron Microscope (SEM) is commonly used to study

surfaces, structures, morphologies, and forms of materials. The images

viewed using SEM are created by detecting secondary electrons ejected from

samples as they are bombarded by focused, high energy electron beams. SEM

can achieve higher magnifications than optical microscopes. When samples

are probed with focused electron beams, a variety of signals can be collected

102

and displayed on the view screen. In addition to secondary electron signals,

X-rays characteristic of the elemental composition of the sample can be

mapped to sample images, and back-scattered electrons can also be collected

and displayed. SEM analyses are conducted in vacuum environments (Dinger

2005).

SEM characterization of the raw UMR, PMR and metal ion

adsorbed UMR, PMR were taken using a Hitachi S-3400N Magnification 5X

3,00,000X type instruments in vacuum environment. Prior to analysis the

samples were sprinkled onto carbon tapes which are adhesive and supported

on metallic disks and coated with gold. Images of the sample surfaces were

recorded at different areas and different magnifications.

3.9.7 EDX Analysis

Energy-dispersive X-ray spectroscopy (EDS or EDX or EDAX) is

an analytical technique used for the elemental analysis or chemical

characterization of a sample. It relies on the investigation of an interaction of

some source of X-ray excitation and a sample. Its characterization capabilities

are due in large part to the fundamental principle that each element has a

unique atomic structure allowing X-rays that are characteristic of an element's

atomic structure to be identified uniquely from one another. EDX analysis of

the raw UMR, PMR and metal ion adsorbed UMR, PMR were taken were

carried out in Hitachi S-3400N Magnification 5 X 3,00,000 X.

3.10 BATCH ADSORPTION STUDIES

The adsorption capability of PMR and UMR towards metal ions at

room temperature (35°C) was investigated using SEPRW having metal ion

concentration of 200 mg L1. The extent of metal ion removal was calculated

from the residual concentration of sorbate in equilibrated solution.

103

The effect of pH of the medium on the adsorption process was

studied at the pH values of 2 to 8 with constant dosage. The optimum pH

value obtained from this study was fixed for further experiments. The

minimum contact time, adsorbent dosage required for maximum metal ion

uptake was studied by varying the contact time from 1 min to 60min and

adsorbent dosage from 0.125 to 2.0 g L1. In addition the influence of other

experimental parameters such as presence of chelating agent (EDTA), co-ions

(Ni, Zn) on the adsorption process was also studied.

Metal ion uptake (mg g1) at equilibrium conditions was calculated from

(C - C )Vo eQ =

e m (3.1)

where V is the volume of sample in litre. C0 and Ce are concentration of

metal ion in mg L1 and W is amount of resin in grams.

3.10.1 Regeneration of Resin

An amount of 0.1 g of metal ion saturated resin was immersed in

0.1 L of different acid solutions (0.5M HNO3, 0.5M, HCl and 0.5M H2SO4),

and the systems were agitated for 2.0 h, then filtered. The filtrate was

analyzed for metal ion concentration by AAS and the percentage of elution (E

%) was calculated from the equation:

100eluted%

fixed

nM

nM

E

(3.2)

104

3.11 ADSORPTION COLUMN STUDIES

A glass column of 2.0 dm height and 0.25 dm outer diameter and

0.20 dm inner diameter was fabricated with bottom and top provisions for

inlet and outlet of solution flow. The column was equipped with a bottom

filtration device to prevent the escape of fine resin beads during processing.

The column was then loaded with 40 g of PMR to get 1.65 dm height of

packed bed. The experimental arrangement is shown in Figure 3.4. Peristaltic

pumps were used for the upward flow of process streams at particular

constant flow rate.

3.11.1 Batch Recirculation Column Studies

The initial pH of the SEPRW having 500 mg L1 of Cu(II) ions was

adjusted to the range 4 – 6 and allowed to pass through the column in batch

recirculation mode using peristaltic pump. Experiments were conducted at

different flow rates (0.005, 0.010, 0.015 L min1), different initial Cu(II) ion

concentration of 300, 400 and 500 mg L1 and at different packed PMR bed

heights 0.55, 1.1 and 1.65 dm. Samples were collected at regular time

intervals and were analyzed for Cu(II) ion using AAS.

3.11.2 Column Breakthrough Studies

The experimental arrangement for continuous column studies is

shown in Figure 3.4; it consist a glass column of similar dimension and

capacity of the packing of PMR as like in batch recirculation process. Initial

pH of the feed solution was fixed at optimum value obtained from batch

study. The SEPRW having metal concentration of 500 mg L1 was allowed to

pass through the ion exchange column in continuous pass mode at constant

flow rate of 0.015 L min1 using peristaltic pump. The operation is shown in

Figure 3.4.

105

Figure 3.4 Experimental setup for column studies

3.11.3 Column Regeneration Studies

When ion exchange process was carried out continuously with

initial metal concentration of 500 mg L1 at fixed flow rate it was observed

that the column was exhausted after several hours of treatment process. After

exhaustion the rinse water present in the voids of resin beads were washed out

from the column using double distilled water. Regeneration of the resin

column was studied by taking the copper ion loaded resin column as an

example. Regeneration was done by passing 0.5 L of different molarities (0.5

to 1.25 M) of sulphuric acid as eluant at constant flow rate (0.015 L min1)

through the bed in the upward direction to reservoir under batch recirculation

mode (Figure 3.4). The reservoir volume fixed for this mode of operation

was 0.5 L. The concentration built up of Cu(II) ions in the reservoir was

monitored at different time intervals. The desorbed column was washed with

distilled water several times and reactivated with 0.05 M NaOH. The

regenerated column was reused for second cycle of adsorption-desorption of

Cu(II) keeping the other parameters same as above, for first cycle.

106

Elution Efficiency (E%) = (Amount of metal ions eluted / Amount of metal

ions adsorbed) X 100

3.12 EIX-EO PROCESS

3.12.1 Batch Recirculation Process

A schematic view of the batch recirculation system is shown in

Figure 3.5. The setup consists of a reservoir, a peristaltic pump, an

Electrochemical ion exchange cell (EIX part) and an Electrooxidation cell

(EO part) connected to an electrical circuit consists of 5A, 30V DC regulated

power supply, an ammeter and a voltmeter. Since membranes divide the EIX

cell, it comprise of four parts of experimental solutions anolyte, process

stream, concentrated and catholyte. The solutions taken in the reservoirs and

their concentrations are given in Table.3.4.

Table 3.4 Type of solution for the different compartments of EIX-EO

reactor

ParameterProcess

stream (P)Anolyte (A)

Catholyte

(C)Receiver (R)

Type of

Solution

Electro plating

rinse water

Sodium

chloride

Sulphuric

acid

Distilled water

Concentration 500 mg L1

2 to 8 g L1 0.05 M -

Volume 1 L 1 L 0.5 L 0.2 L

Solutions in the four reservoirs were allowed to flow in batch

recirculation mode as shown in Figure 3.5 through the respective

compartments of the reactor using peristaltic pump at particular flow rate.

Experiments were conducted at various applied current densities specifically

at 0.5, 1.0, 1.5, 2.0, 2.5 Amp dm2, at various flow rates specifically at 0.005,

0.010, 0.015, 0.020 and 0.025 L min1, at various Cu(II), COD concentration

and at various NaCl concentration in the anolyte of EIX.

107

Initially, the Cu(II) ions present in the rinse water gets adsorbed by

the PMR and then migrated to the receiver compartment with the influence of

applied current density. The organic matter present in the rinse water gets

oxidized to oxygen and carbon dioxide at the anode surface of the EO part.

At regular time interval samples were collected, diluted to appropriate

volume and analyzed for Cu(II) in atomic absorption spectrophotometer

(AAS), organic matter present (COD) by refluxed COD digester and sulphate

by spectrophotometer.

Figure 3.5 Experimental setup of EIX-EO reactor and their processes

3.12.2 Continuous Flow EIX Process

An experimental study was carried out to determine the break-even

point of hybrid EIX-EO reactor. It is very much important to study the

operational capacity of the process. This experiment was conducted at

optimum flow rate (0.020 L min1) and at applied current density of 2.0 Amp

108

dm2. Catholyte and anolyte solutions were re-circulated separately; the

process stream (test solution) was continuously passed through the central

compartment of EIX part and the EO part of the reactor. The outlet was

collected in the separate container as shown in the Figure 3.5. Samples were

collected from the outlet at regular time interval and analyzed for Cu(II). The

break point of the ion exchange process alone was studied by conducting the

same experiment without applying external DC current through the reactor.

3.13 THEORITICAL DESCRIPTION

3.13.1 Modeling of Batch Data

3.13.1.1 Ion Exchange - Isotherm Studies

Ion exchange isotherms are expressed in terms of relationship

between the concentration of adsorbate in the liquid and the amount of

adsorbate adsorbed by the unit mass of adsorbent at a constant temperature.

Langmuir (Langmuir 1918), Freundlich (Freundlich 1906), Redlich Peterson

(Redlich and Peterson 1959) and Toth (Toth 1971) isotherm models were

used to describe the equilibrium data.

The general expressions for sorption models are

Model General equation Equation number

Langmuir model Q b Cmax elQ =e 1+ b Cel

(3.3)

Freundlich model 1 / nQ = K C

e l e

(3.4)

Redlich–Peterson K CerQ =e

1+ a Cer

(3.5)

Toth model Q b Cp t e

Q =e 1 / n n

t t[1+ (b C ) ]t e

(3.6)

109

where Qe represents the equilibrium adsorption capacity (mg g1), Ce is the

equilibrium metal concentration (mg L1). b1 is the Langmuir isotherm

constant (L g1), Qmax is the maximum metal sorption (mg g

1), Kl represents

the capacity of adsorption (L g1), n represents the intensity of adsorption, Kr

(L g1), ar (L mg

1) and ß represents the Redlich–Peterson constants, Qp (mg

g1), bt and nt represents the Toth isotherm constants. The model parameters

were estimated using Microsoft EXCEL and MATLAB.

3.13.1.2 Modeling of Batch Ion Exchange - Kinetic Studies

The design of most equipment requires data on the amount of ions

exchanged between solid phase and liquid phase in a given contact time. The

ion exchange kinetic studies describe the solute uptake rate which in turn

governs residence time of an ion exchange reaction. It is one of the important

characteristics for defining the efficiency. The sorption kinetics of a sorbent

depends mainly on the property of the sorbate. Hence, in this part of the

present study, the kinetics for Cu(II) ions removal by PMR has been verified

by pseudo first (Lagergren 1988) and second (Ho and Mckay 2000) order

kinetics to understand the behaviour of the resin.

The pseudo-first order equation

The pseudo-first order reaction equation for solid–liquid sorption

system was proposed by Lagergren in 1898 (Lagergren 1898). This equation

has been traditionally the most conventional model used for liquid sorption

modelling.

Q = Q 1 - exp -k tft e

(3.7)

where kf is the pseudo-first order equation rate constant (min1).

110

The pseudo-second order equation

A second order equation is based on the amount of sorbate which

was sorbed onto the Sorbent proposed by Ho and McKay (2000). The second

order equation can be derived from chemical equations. The expression is

2k Q t

s eQ =t 1+ k Q t

s e

(3.8)

where kS is Pseudo second order rate constant (g mg1 min

1).

3.13.2 Modeling of Column Ion Exchange

The dynamic behaviour of the column can be predicted by various

simple mathematical models. In this study Adams–Bohart and Yoon-Nelson

models were used to predict the performance of the column ion exchange

studies. The effect of flow rate on Cu(II) ions removal in column operation

was investigated.

3.13.2.1 Adams–Bohart model

Bohart and Adams (Aksu and Gonen 2004; Han et al 2008)

established the fundamental equations that described the relationship between

C/C0 and t in a flowing system for the adsorption of chlorine on charcoal. The

model proposed assumes that the adsorption rate is proportional to the

residual capacity of the activated carbon and to the concentration of the

sorbing species. The Adams–Bohart model provides a simple and

comprehensive approach to running and evaluating sorption-column tests and

valid to the range of conditions used. The model expression is given by

111

Z= exp K C t - K N

ab 0 ab 0 U0

C

C0

(3.9)

where Kab is the kinetic constant (L mg1

min1), U0 is the superficial velocity

calculated by dividing the flow rate by the column section area (cm min1)

and Z is the bed depth of column (cm). C0 is the influent concentration (mg

L1); C is the effluent concentration (mg L

1) at time t. Values describing the

characteristic operational parameters of the column can be determined from

the plot of ln C/C0 versus t at a given bed height and flow rate.

3.13.2.2 Yoon-Nelson model

Yoon and Nelson (Yoon and Nelson 1984) developed a model

based on the assumption that the rate of decrease in the probability of

adsorption of adsorbate molecule is proportional to the probability of the

adsorbate adsorption and the adsorbate breakthrough on the adsorbent. The

linearized Yoon-Nelson model for a single component system is expressed as:

expC

C0

k t kYN YNC

(3.10)

where kYN (min1) is the rate velocity constant, (min) is the time required for

50% adsorbate breakthrough. From a linear plot of ln[C/ (C0-C)] against

sampling time (t), values of kYN and were determined from the intercept and

slope of the plot ln (C/C0-C) versus t.

3.13.2.3 BDST model

The BDST model is based on physically measuring the capacity of

the bed at different breakthrough values. The BDST model works well and

provides useful modelling equations for the changes of system parameters

112

(Ko et al 2000). BDST column model is the simplest model which gives the

useful information about the relationship between bed height (Z) and service

time (t) of a column.

A modified form of the equation is given below (Goel et al 2005):

10 0ln 1

0 0

N Z Ct

C K C Ca b

(3.11)

where Cb is the breakthrough metal ion concentration (mg L1); N0 the

sorption capacity of bed per unit volume; the linear velocity (cm h1) and Ka

the rate constant (mg1

h1).

where 0ln 1

0

CZ

k N Ca b

(3.12)

A simplified form of the BDST model is: t = aZ + (- b)

where,

0

0

Na

C F

1 0ln 1

0

Cb

K C Ca t

3.14 ERROR ANALYSIS

The least sum of the squares (SS), of the differences between the

three experimental data obtained from the experiments (by calculation), could

be computed. If all three data from the experiments are similar, SS will be a

113

small number; if they are different, SS will be a large number. In order to

point out the actual data, the average between three values is used in the

graphs and their deviation is given in the form of standard error (Equation

3.13).

standard deviation of three experimental data/

Standard error (SE) =

Square root of total number of experiments

SE = STDEV(range of values)/SQRT (number) (3.13)

3.15 REACTION MECHANISMS IN EIX-EO REACTOR

3.15.1 Reaction Mechanisms in EIX Part

The IEPRW containing copper and sulphate in the reactor initially

gets ionized and the anions specifically sulphate, cations Cu(II) due to their

affinity move towards anode, cathode respectively. Cationic membrane

hinders the movement of sulphate ions into the anode compartment. Whereas

the Cu(II) ions can enter the receiver compartment and cannot enter into the

cathode compartment so as to avoid the precipitation of Cu(II) ions as

hydroxide.

Rinse water contains copper sulphate initially gets ionized with the

passage of current according to the Equation (3.14).

CuSO4 Cu2+

+ SO42

(3.14)

The PMR adsorbs this cation. The cations i.e.) Cu(II), due to their affinity

towards cathode, passes through membrane and gets concentrated in the

receiver compartment.

114

Anode Compartment

The general anode reaction for pH<7 takes place at the vicinity of

anode is given in Equation (3.15) which results the liberation of oxygen (O2).

e4H4OOH2 22 (3.15)

The general reactions at the bulk of the anode compartment are

given in Equations (3.16) – (3.18).

NaOClClNaOHNaCl 22 (3.16)

OClNaNaOCl (3.17)

eClOOCl 22 22 (3.18)

Cathode Compartment

The general reaction takes place in the cathode is by the following

Equation (3.19)

24242 22 SOHeSOH (3.19)

3.15.2 Reaction Mechanism in EO Part

In the EO part of the reactor the high molecular weight aromatic

compounds and aliphatic chains are broken to intermediate products for

further processing. In electrochemical combustion the organics are completely

oxidized to CO2 and H2O. The oxidation of organics takes place always with

simultaneous oxygen evolution. In our group a mechanism has been proposed

for the electrochemical oxidation of organics at different electrode material

with simultaneous oxygen evolution. The mechanism of electrochemical

oxidation of wastewater is a complex phenomenon involving coupling of

electron transfer reaction with a dissociate chemisorptions step. Basically two

115

different processes occur at the anode; on anode having high electro -

catalytic activity, oxidation occurs at the electrode surface (direct

electrolysis); on metal oxide electrode, oxidation occurs via surface mediator

on the anodic surface, where they are generated continuously (indirect

electrolysis).

In direct electrolysis, the rate of oxidation is depending on

electrode activity, pollutants diffusion rate and current density. A generalized

scheme of the electrochemical conversion/combustion of organics on noble

oxide coated catalytic anode (MOx) (Panizza and Cerisola 2004).

In the first step, H2O is discharged at the anode to produce adsorbed

hydroxyl radicals according to the reaction

eHOHMOOHMOxx

)(2 (3.20)

In the second step, generally the adsorbed hydroxyl radicals may

interact with the oxygen already present in the oxide anode with possible

transition of oxygen from the adsorbed hydroxyl radical according to the

Equation (3.21) and forming the higher oxide MOx+1.

eHMOOHMOxx 1)(

(3.21)

At the anode surface the active oxygen can be present in two states.

Either as physisorbed (adsorbed hydroxyl radicals (•OH) or/and as

chemisorbed (oxygen in the lattice, MOx+1). In the absence of oxidisable

organics, the active oxygen produces dioxygen according to the following

Equations (3.22) and (3.23).

eHOMOOHMOxx 22

1)( (3.22)

221

1 OMOMOxx (3.23)

116

At cathode

As the concentration of sulphate is much higher, the sulphate

present in the effluent predominantly gets reduced to HS at the cathode. This

reaction is represented as

OHHSeHSO 2

2

4 489 (3.24)

Some of the Cu(II) ions escaped from the EIX part get reduced at

the cathode of EO part to metallic copper by the following reaction

02 2 CueCu (3.25)

222 HeH (3.26)

At the Bulk

The HS- anions formed at cathode surface rejected by the cathode

and come into the bulk. In the bulk electrolysis of HS ions takes place by the

following reaction

2SHHS (3.27)

At Anode

The S2-

anion thus formed attracted by the anode where it gets

oxidized to sulphur by losing two electrons to the cathode by the following

reaction.

eSS 202

(3.28)

117

3.15.3 Modeling for Removal of Copper in EIX Reactor

In electrochemical treatment of IEPRW containing copper together

with COD, several possible reactions can occur at the electrodes; however the

scope of this paper is limited to discussions with respect to copper and

COD removal only. The mode of operation of EIX/EO system is depicted in

Figure 3.5 involves the recirculation of the IEPRW. There is a gradual

depletion of the concentration of the Cu(II) ions, COD and 2

4SO in the

reservoir.

In order to design the plant for treatment processes, the

development of the model is essential which permits the computation of

variation in concentration of the Cu(II), COD with time in the reservoir. The

basic assumptions involved in the ensuing derivation may be outlined as

follows: Back-mix flow exists in the process compartment reservoir, the

anodic compartment, and the cathodic compartment in the present reactor

system; that was arrived at based on a tracer experiment. Hence the

concentration of reactive species is the same throughout the compartment and

equal to the exit concentration.

An approximate model which represents the given EIX is

developed based on a continuous stirred tank flow reactor in which the

reactions and physical phenomena take place. A dynamic material balance to

each of component or species of every compartment can be written as,

[Rate of change mass in the system] = [rate of mass input] - [rate of mass

output] – [rate of mass disappeared

or generated due to physicochemical

phenomena] (3.29)

118

All the reservoirs are perfectly back-mix systems. The

concentration variation of Cu(II) ion in the process stream (central

compartment) of EIX is written as

kdC po = QC - QC - 6(1- ) C + IA CR M oR odt d

po

(3.30)

The left-hand side (LHS) represents the rate of change of mass of Cu(II) in

the pore volume of the central compartment. The first two terms of the right-

hand side (RHS) are the rate of mass of Cu(II) ion entering and leaving the

central compartment. The driving force is the concentration difference

through the diffusion film, and the expression should be [6(1 - )/dp]VRkp(Co -

Co*). Assuming phase equilibrium, the concentration in the solution near the

interface, Co*, is zero, which leads to [6(1 - )/dp]VRkpCo. The next dominant

transport process is ionic migration, which accounts for the transfer of nickel

ion through liquid toward the interface of membrane that passes through the

cation exchange membrane to the liquid bulk in the cathode compartment.

The reservoir (rinse water tank) is always a perfectly back-mix system.

Hence the mass balance equation for the rinse water reservoir tank is,

dCV = QC - QC

rw 0dt (3.31)

Under steady-state conditions as dCo/dt= 0, and Equation (3.30) is rewritten

as,

C0 1=

C 6(1 - )k

IAR pdp M1+ +Q Q

(3.32)

119

Assume,6 1 -

a =dp

,A

ma =m

R

,I A

mk =m Q

, RR

Q

C0 1=

C1+ k a + k a

p m m R

(3.33)

where a is the specific surface area [6(1 - )/dp] of the ion exchange resin, am

is the specific area (Am R) of the ion exchange membrane, km is the

migration mass transfer coefficient, and ôR is the residence time of Cu(II) ion

R/Q) in the middle compartment. The mass balance Equation (3.31) is

solved after substitution of the expression for C0 from Equation (3.32),

knowing the initial concentration of Cu(II) ion, C = C0 at t = 0 in the

reservoir. Then the following resultant equation gives the variation of

concentration of Cu(II) in the effluent reservoir. Where C0 is the initial

concentration of Cu(II) and ô is the residence time of Cu(II) ion (Vrw/Q) in the

effluent reservoir, respectively. In accordance with Equation (3.33), the slope

of the plot of ln(C/C0) versus t or ln (1 - X) versus t is (1/ô) {[(kpa + kmam)

ôR]/ [1 + (kpa + kmam) ôR]}, from which the value of (kpa + kmam), the total

transfer coefficient is computed.

3.15.3.1 Model for Build up of Cu(II) Ion in receiver Reservoir during

Continuous Operation of Middle Compartment

Since there is no accumulation of the Cu(II) ion in the liquid phase

of the middle compartment of the EIX reactor, it can also be assumed that the

reactor is under steady-state conditions as

120

(dCo/dt ) = 0 and Equation (3.32) is rewritten as

Co 1=

C1+ ak + k a

p R m m R

(3.34)

Co 1=

C1+ ak + k a

p m m R

(3.35)

CC =o

1+ ak + k ap m m R

(3.36)

The mass balance Equation (3.30) is solved after substitution of the

expression for Co from Equation (3.36), knowing the initial concentration of

Cu(II) ion, C=Co at t=0 in the reservoir. Then the following resultant equation

gives the variation of concentration of Cu(II) in the effluent reservoir.

dCC - C Q = V

o rw dt (3.37)

dCCQ - C = Vrw dt

1+ ak + k ap m m R

(3.38)

dC Q= dt

Vrw

1C - 1

1+ k a + k ap m m R

(3.39)

121

C t 1= exp - - 1+ k a + k a

p m m RC1+ k a + k ao

p m m R

(3.40)

k a + k ap m m RC t

= exp -C

1+ k a + k aop m m R

(3.41)

In accordance with Equation (3.35), the slope of the plot of ln(C/Co) versus t

is (1/ ) {[(kpa + kmam) R ]/ [1 + (kpa + kmam) R ]}, from which the value of

(kpa + kmam), the total transfer coefficient is computed.