P.C.C.Siu ,L.F.Koong ,J.Saleem ,J.Barford ,G.McKay ,2,*

1 Department of Chemical and Biomolecular Engineering,The Hong Kong University of Science and Technology,Clear Water Bay,Kowloon,Hong Kong,China

2 Division of Sustainable Development,College of Science,Engineering and Technology,Hamad Bin Khalifa University,Qatar Foundation,Doha,Qatar

1.Introduction

Copper is a w ell-know n metal in human history and it has been in use for over 10000 years.Nowadays,the use of copper is still widespread in different industries,for instance,the manufacture of alloys and the production of printed circuit boards(PCBs).The human body,a daily micro-to milligram scale intake of copper is required.It plays an essential role in the development and performance of the human nervous and cardiovascular systems[1].Ingestion of gram quantities of copper salt daily may,how ever,cause harmful effects to the human body,although copper is not classified as a carcinogenic material by US EPA[2].The Guidelines for Drinking Water Quality(GDWQ)of World Health Organisation(WHO)recommended that the daily average copper requirements are 12.5 μg·kg-1of body w eight for adults and about 50μg·kg-1of body weight for infants[3].The Institute of Medicine(IOM)advocated 10 mg·d-1as a tolerable upper intake level for adults from foods and supplements[4].

Now adays,numerous methods have been developed for treating metal ion containing waste solution.The basic principles of treating waste solution are the removal of ions from the waste solution or concentrating the ions in the waste solution in the same phase or different phases.In chemical precipitation,copper ions react with hydroxide ions and form an insoluble copper precipitate at alkaline(high p H)condition.It is,however,sensitive to p H,ion concentration and composition,and non-compatible with any soaps or surfactants[5].Evaporation may be used to produce a more concentrated solution which can sometimes be reused in the electroplating industry or reprocessed to produce a metal salt.The evaporated water can be condensed for rinsing purposes.This is one of the suitable waste treatment methods for the electroplating industry.In membrane filtration processes cross flow filtration method is used.Depending on the specific requirement micro filtration(MF),ultra filtration(UF),nano filtration(NF),or reverse osmosis(RO)membranes can be used.Membranes separate the wastewater into two streams.One(the concentrate)contains contaminants and the other,cleaned water.Electrochemical coagulation has been investigated[6]but produces a waste floc.

Ion exchange is an effective technology to remove heavy metal ions from water.Natural ion exchangers,such as peat[7-9],have found some success for the removal of copper and other heavy metals.Other biomass,sources have also the exchange/chelating capacity to exchange with copper ions in solution[10-12].Ion exchange materials can be found naturally or artificially synthesized[13].Synthetic ion exchange materials can be divided into two categories:inorganic material and organic material.Inorganic ion exchange materials can be classified by their ow n chemical characteristics and include:hydrous oxides,acidic salts of multivalent metals,salts of heteropolyacids,insoluble ferrocyanides and aluminosilicates.Most inorganic ion exchange materials have high selectivity to alkali metals[14].Synthetic ion exchange resins are high molecular w eight polymer carrying ionic groups as integral parts of the polymer structure.Cation exchange resin attracts cations since its ionic group is phenolic,sulfonic,carboxylic or phosphoric acid group[15].Cation resins can be further classified into strong acid resin and weak acid resin depending on which ionic group it contains.The strong acid resins carry sulfonic acid functional group,correspondingly,weak acid resin has carboxylic acid functional groups and slightly dissociate in solution.Weak acid resins are highly affected by the p H value of the solution[16].

The present study investigates the ion exchange capacity and the rate of ion exchange of an iminodiacetate weak acid resin to remove copper ions from wastewater.Equilibrium isotherms are determined and the reaction kinetics are analyzed.The results are useful for industrial adsorber design.

2.Materials and Methodology

2.1.Materials

2.1.1.Ion exchange resin

The ion exchange resin is a synthetic organic material with polystyrene polymeric base.The chelating resin(D401)used in experiments was supplied by the Suqing Group Limited,Beijing,China.This chelating resin is the sodium form of a weak acid cation resin,but it has high degree of selectivity for heavy metal cations because it forms stable complexes with heavy metal ions due to its functional iminodiacetate group.

Fig.1 show s the functional group and ion-exchange process and Table 1 lists the physical and chemical properties of D401 resin,respectively[17,18].The uptake mechanism of heavy metals is:

2.1.2.Adsorbate

Analytical grade copper(II)chloride(Cu Cl2·2H2O)used in the experiments was supplied by Acros Chemicals,Geel,Belgium.Stock solutions of metal ions were prepared using deionized(DI)water.The p H values of the copper solutions were adjusted to 3.5 at the commencement of each experiment since PCB effluents are acidic in nature.

2.2.Experimental set up

2.2.1.Pre-treatment of resin

The chelating resin supplied was in the sodium form.How ever,the sodium content of the resin may not be consistent.Pre-treatment of resin,therefore,was required to ensure consistent experimental results.The pre-treatment procedure was similar to the regeneration of exhausted resin.

First the resins were immersed into DI water for more than 30 min and then the resin was immersed in 8 w t%hydrochloric acid with a portion of 180 ml HCl·L-1resin for more than 45 min to replace all the sodium ions by protons.The acid treated resin was washed with DI water to remove the acid residue until the solution p H was above 2.Then the resin was immersed in 4 wt%sodium hydroxidesolution for 45-min to replace the protons by sodium ions with the sodium content being 3 mmol·g-1of theres in.There sins were then washed with DI water and dried at 100-110°C for 3 days before allow ed to cool to ambient temperature in a desiccator.The resins used are 710-1000 μm in particle sizes.

2.2.2.Contact agitation time for sorption isotherm studies

The equilibrium time for the sorption isotherm must be established before any other experiments are conducted.The highest concentration of Cu ions solution,which was to be used in the equilibrium sorption isotherms,was used in these experiments.A fixed mass of 0.100 g resin was added to 50 ml of copper metal ion solution,transferred into a screw cap jar and shaken at a speed of 200 r·min-1at(25 ±2)°C.Each jar was removed from the shaker at a pre-specified time and the copper ion solution was sampled and its copper content was measured utilizing an Inductively Coupled Plasma-Atomic Emission Spectrophotometer(ICP-AES),a Perkin Elmer Optima Model 3000XL.The removal percentages can then be calculated as a function of time.

Although the equilibrium was considered reached after 72 h,the agitation time of 96 h was adopted in all further isotherm experiments to ensure all isotherms achieved equilibrium.

2.2.3.Equilibrium sorption isotherms

The sorption capacity(qe)of the resin was calculated from the mass balance:

w here

C0=initial concentration of metal ion solution(mmol·L-1)

Ce=equilibrium concentration of metal ion solution(mmol·L-1)

m=mass of resin(g)

qe=equilibrium metal ion concentration on resin(mmol·g-1)

V=volume of metal ion solution(L)

Since fresh resin was used in the experiments,the initial metal ion concentration on resin(mmol·g-1),q0,is zero.

2.2.4.Batch kinetic studies

Batch kinetic experiments were conducted to investigate the influence of different parameters on the adsorption rate.A standard configuration of adsorption vessel and stirrer developed by Furusaw a and Smith[17]was employed in all of the experiments.The dimension of each component was designed based on the inner diameter of the tank Di.

Fig.1.Functional group of D401 resin(Na+form).

Table 1 Physical and chemical properties of D401 resin

Fig.2.Standard tank con figuration adsorber for batch kinetic studies.

Fig.2 shows the experimental vessel setup.A 2-L plastic vessel was used as the adsorption tank with the inner diameter Di=0.13 m to hold 1.70 Lmetal ion solution for the kinetic experiment.A flat plastic impeller with six blades provided the mixing effect to the solution.The diameter of the impeller was0.065 m and the height of all the blades was 0.013 m.An overhead stirrer was used to drive the impeller using a 0.005 m diameter plastic shaft.Six plastic baffles were located around the circumference of the vessel with a position at 60°intervals and held securely in place on the top of the vessel.Each baffle was located slightly away from the wall of the vessel to prevent solid accumulation.The alignment of the impeller could prevent power loss due to air entrainment.

2.2.4.1.Effect of initial concentration of metal ion solution.The effect of copper ion concentration was first investigated at a fixed mass of resin,1.3 g.The initial pH value of solutions was adjusted to 3.50.Five different initial concentrations of solution,1.7,2.0,2.3,2.6 and 2.9 mmol·L-1,were examined.The agitation speed of impeller was adjusted to 400 r·min-1.

2.2.4.2.Effect of mass of resin.The effect of mass of resin was investigated under the same concentration of metal ion solution 2.6 mmol·L-1.Five different values of resin mass were studied,0.9,1.1,1.3,1.5 and 1.7 g with the particle size ranging from 710 to 1000 μm.

3.Results and Discussion

3.1.Equilibrium isotherm studies

The purpose of adsorption equilibrium studies is to find the maximum adsorption capacity at different ion concentration and also to find the best- fit isotherm model to represent these results.Initially,we investigated the effect of solution p H on the copper uptake capacity.The results are shown in Fig.3 and for final p H values above 3.5 up to 6.0,the uptake capacity is fairly constant between 2.20 and 2.30 mmol copper per g of resin.There are several equilibrium isotherm models,each predicts the experimental results based on different the oriesand assumptions.Many authors preferred to apply three models,the Langmuir,Freundlich and Redlich-Peterson to correlate the experimental data but w e have analyzed seven different isotherms.The detailed equations are listed in Table 2 together with the best fitted values of the parameters.

Fig.3.Effect of equilibrium p H on the sorption of copper.

In equilibrium isotherm modeling,no single isotherm can usually fit all the experimental data practically.The model predicted adsorption capacity,qecal,therefore,is compared with the experimental adsorption capacity,qe,for each isotherm model in terms of the Sum of the Squares of the Errors(E):

The isotherm parameters were determined by minimizing the corresponding error across the concentration range studies using the solver add-in with Microsoft Excel spreadsheet.

The Langmuir model data in Table 2 indicate that the maximum monolayer copper ion exchange capacity of the ion exchange resin is 2212/970=2.28 mmol copper ion per gram of resin.This model gives the second least overall error between the model prediction and the experimental results.The best fitting model is the Sips or Langmuir-Freundlich isotherm which has three parameters compared to the two parameter model of Langmuir isotherm.Fig.4 show s the fitting of the model isotherm curves to the experimental data.The To th isotherm reduced to Langmuir isotherm because the third parameter ntis 1.

It is interesting to compare the present findings with the ion exchange capacities and the best fit isotherm models of other studies.McKay and Porter[26,27]applied three models,namely,Langmuir,Freundlich and Redlich-Peterson,to compare the best- fitted model for the sorption of copper ions onto the naturally occurring humic acid ion exchanger peat.The best correlation of the sorption was obtained using the Langmuir isotherm equation,consistent with the present work,in which the Langmuir correlation was excellent and just slightly better was the Langmuir-Freundlich.The capacity of peat for copper was show n to be 12.60 mg·g-1.Cheung and co-workers[28,29]studied the sorption capacity of the calcium based ion exchanger bone char for copper with capacity 47.7 mg·g-1and the Langmuir-Freundlich isotherm provided the best correlation.Veli and Pekey[30]used tw o strong cationic resins,Dow ex HCR S/S and Dow ex Marathon C,and the copper adsorption capacities were 26.27 mg·g-1and 46.55 mg·g-1,respectively.Demirbas and co-workers[31]investigated the ion exchange capacity of Amberlite IR-120 synthetic resin to copper and they found that the adsorption capacity for copper was 50.9 mg·g-1.The capacity of our Suqing resin for copper is 2.28 mmol·g-1or 145 mg·g-1,which is equivalent to or better than any of the values stated.

Table 2 Summary of correlated constants for different isotherms

The Langmuir isotherm is commonly applicable to model ion exchange equilibrium data as the exchange process happens at the monolayer active sites of the resin.How ever,the error value of Langmuir-Freundlich isotherm is less than that of the Langmuir isotherm.In addition to the one more fitting parameter,the adsorption energy pro file of the ion exchange process may be another reason.One of the assumptions of the Langmuir isotherm is that all sites are identical with the same energy level.This small effect may be due to the swelling of the resin particle in water with time.

The effect of varying the ion exchange resin particle size is shown in Fig.5 and the results show that there is only a small or negligible effect on capacity found despite the wide range in particle size tested from 250-450 μm to 710-1000 μm.The Langmuir isotherm capacities are(2.1±0.2)mmol Cu·g-1for all three particle sizes regardless of particle size,with external surface area playing an insignificant role.

3.2.Ion exchange kinetics

Kinetic models for ion exchangeby resin have been used in the study of the adsorption rate for the batch liquid-solid phase ion exchange sorption systems.The mechanism of sorption and potential rate controlling steps,like chemical reaction processes and mass transport processes,can also be investigated through kinetic studies.It is important to consider a series of kinetic models to determine the best fit model for chemisorption of the batch ion exchange study.In the present study,four kinetic models were tested.Since fresh adsorbents were used in all the tests,the kinetic models are subjected to the following initial condition:qt=0 at t=0.

Fig.4.Equilibrium isotherms for copper ion exchange system.

3.2.1.Pseudo- first order equation

The pseudo- first order chemisorption kinetic rate equation[32,33]is expressed by the following equation:

w here

k1=pseudo- first order sorption constant(min-1)

qe=sorption capacity at equilibrium(mmol·g-1)

qt=sorption capacity load at time t(mmol·g-1)

t=adsorption time(min)

The parameter k(qe-qt)represents the number of available sites in the adsorbent.The integrated form of Eq.(3)becomes:

3.2.2.Pseudo-second order equation

The pseudo-second order chemisorption kinetic rate equation[34,35]is as follows:

Fig.5.Langmuir isotherm for copper ion exchange system with different particle sizes.

w ith k2being the pseudo-second order sorption constant(g·mmol-1·min-1).

The integration of Eq.(5)gives:

3.2.3.Elovich equation

The Elovich chemisorption kinetic rate equation[36,37]is different in form from those show n above.It depicts an exponential decay of the adsorption rate with time as:

where α is the initial sorption rate(mmol·g-1·min-1)and β is the desorption constant(g·mmol-1).

Eq.(7)can be integrated to give:

When αβt? 1,Eq.(8)becomes:

Eq.(9)assumes that the activation energy for chemisorption increases with the energy of adsorption.This assumption is incompatible with the situation w hen the energy of chemisorption decreases with coverage.

3.2.4.Ritchie equation

The Ritchie chemisorption kinetic rate equation[38,39]is expressed as:

with k3being the sorption constant(min-1).Eq.(10)assumes that the rate of adsorption depends only on the fraction of sites unoccupied.

3.2.5.Model fitting methodology

The values of the model parameters were determined in a similar manner to that of the isotherm fitting.The SSE(the square of the summation of the difference between qtmeasured and qtcalculated)was minimized for all the time values using the “Solver”in Excel.The maximum values of qewere set as constraints according to the predicted qefrom the Langmuir-Freundlich isotherm.

3.3.Results of kinetic modeling

The best fit kinetic modeling results are presented in this section showing the fitting of the different curves of the four models in comparison with experimental results.

The effect of the changes of solution concentration/resin mass/resin particle size for each kinetic model are also compared on the same figure to determine how these changes would affect the adsorbate uptake with time.

3.3.1.The effect of initial copper ion concentration

To compare how the changes in concentration affect the performance of exchange chemisorption by the resin,the concentration decay curves are plotted for each kinetic model on the same graph.For example,Fig.6 show s a comparison of q of copper ion system under different concentrations,namely,1.7,2.0,2.3,2.6 and 2.9 mmol·L-1for the pseudo-second order model.The pseudo-second order model gives a good correlation to the experimental data curves.

Fig.6.Comparison of qt of acopper ion system for different concentrations using the pseudo-second order modeling.

The best fit values of all four model parameters,pseudo- first order,pseudo-second order,the Elovich model and the Ritchie model,are show n in Tables 3 to 6 with the errors in the best fit model parameters.In the case of the pseudo- first order model the error values are muchhigher than those of the second order model.In addition,the model qevalues are much closer to the experimental qevalues in the case of the pseudo-second order model.For the Elovich results in Table 3,the data reveal that the best fit values of α vary widely,indicating the inadequacy of the Elovich model for the copper removal.Table 6 show s the data for the Ritchie nth order model and the best fit data are for the second order model thus resulting in the same low error values as the pseudo-second order model.All the best fit n values fall within n=2.0±0.3,thus con firming a second order mechanism.These data enable the best fit model to the copper removal process and the likely mechanism to be determined as pseudo-second order.Regarding the mechanism for instance,if low initial p H values are used,the excessive hydrogen ions in the solution may compete with the metal cations for the active sites(i.e.-COO-)and thus a decrease in metal ion uptake would result as indicated in Fig.3.Many literature studies also state that maximum sorption is also favored at high p H values especially in the case that the sorption rate is highly controlled by ion exchange process rather than complexation process.

Table 3 Values of kinetic model parameters and SSE at different initial concentrations of copper ions

Table 4 Values of calculated variables and SSE at different concentrations using the pseudo-second order of a copper system

Table 5 Values of calculated variables and SSE at different concentrations using the Elovich equation of a copper system

Table 6 Values of calculated variables and SSE at different concentrations using the Ritchie equation of a copper system

Even though the most suitable initial p H is used before the start of the experiment,the p H changes throughout the w hole experiment and is very difficult to be controlled and monitored.As the resin used consisted of two-COONa functional groups,the difference in p H might affect the sorption rate which means the effect of changes in p H on sorption during the experiment is more dominant than the effect of the changes in concentration on the pattern of the model curve.How ever,all the final p H values were in the range of 4.5 to 5.5.This explanation does not explain the low decay curve and the low rate constant for the 1.7 mmol concentration value but at the low concentration there may be a diffusion rate limitation due to the low concentration of copper ions in the bulk solution to the surface of the resin,so that for low copper concentrations the mechanism may no longer be reaction kinetic controlled.

3.3.2.Effect of resin mass

Fig.7.Comparison of qt of copper ion system for different masses using the pseudo-second order model.

The effect of resin mass on the uptake of Cu(II)ions onto the resin was also studied.Fig.7 show sthechangesin qtcalwith time under different resin mass.How ever,the model curves for different resin masses are very close to each other.In other words,the dosage of resin seems to have very little effect on adsorption rate for the Cu(II)ion systems.This would again suggest that the mechanism is kinetically controlled rather than by diffusion mass transfer.Theoretically,the increase in the resin dosage can increase the surface area of resin and the availability of adsorption sites to increase the adsorption rate.The surface metal ion concentration on the surface of resin and the solution metal ion concentration have reached the equilibrium towards 1440 min and thus the removal efficiency is approximately the same even though different absorbent dosages have been used.Consequently,it is recommended that a relatively much larger or smaller amount of resin should be used to study the optimum resin dosage because a more distinct difference in the model curvesmay be obtained.The model parameter values are presented in Tables7 to 8 and support the findings of the effect of initial concentration with the pseudo-second order model having the low est error values and providing the best correlation of the experimental data.The data for the effect of mass variation using pseudofirst order gave a fairly constant value for k1,0.0087±0.02 but the SSE value of 0.385 is high.The Elovich model for mass also gave relatively constant values of,α =1.303 × 10-5and β =24.5,how ever the model has a very high SSE of 3.69.Again the data for the Ritchie pow er model produced a relatively constant k3=0.0112 and a very good SSE of 0.168;this being due to the fact that as the Ritchie equation exponent approaches n=2,it approximates to the pseudo-second order model.Each mass parameter for the pseudo-second order model is presented in Table 7 because this is the best fit model based on the error values even though the k2values are again constant.

Table 7 Values of calculated variables and SSE at different masses using the pseudo-second order of a copper system

Table 8 Values of calculated variables and SSE at different particle sizes using the pseudo-second order of a copper system

3.3.3.The effect of resin particle size

Fig.8 shows the comparisons of model curves of q against t for Cu(II)ion systems,respectively,w ith three different resin particle sizes,namely,450-600 μm,600-710 μm and 710-1000 μm.

How ever,for columnar operation it is essential to consider the relative pressure drops between the particle sizes,as smaller particle sizes have higher pressure drops and therefore increase pumping costs.

Fig.8.Comparison of qt of copper ion system for different particle sizes using the pseudo second order model.

In the case of the effect of particle size range of the ion exchange resin only the data for the pseudo-second order model are considered because again it has the low est error value.Table 8 show s the parameters for the three particle size ranges and although the error values and parameter differences are larger than in the two previous cases,initial copper concentration and resin mass,the parameter values are essentially constant over such a wide particle size range.It con firms the previous conclusions that the ion exchange reaction,over most of the parameter ranges studies,is reaction kinetics controlled and diffusional mass transfer does not play a significant part in this study.

4.Conclusions

The equilibrium copper exchange capacities at different p H values and different resin particle size ranges;for all final p H values above 4.0 the copper uptake capacities were 2.1±0.2.The best fit isotherms for the copper ion equilibrium studies are given by the Langmuir and the Langmuir-Freundlich equations with the Langmuir-Freundlich giving slightly better fits.For straight forward ion-exchange we would expect the Langmuir to give the best fit for the copper ion and equal energy exchange on the sites,but there is a p H shift during the ion exchange and this will create a change in the chemical potential at the surface of the ion exchange resin during the exchange and the Langmuir-Freundlich equation allows for this potential change.

It can be concluded from the experimental study that the rate of sorption of Cu(II)is generally very fast and the concentration decay curve decreases steadily with time until equilibrium is reached.The adsorption kinetics were analyzed using four kinetic models:the pseudo- first order,pseudo-second order,the Elovich model and the Ritchie model.The data were found to follow the pseudo-second order or Ritchie equation model for n=2,effectively the pseudo-second order model.It is common to find the metal ion exchange by resin fits the pseudo-second order kinetics in many literature studies.

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