Coupling Ion Exchange and Biosorption for Copper(II) Removal From

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Coupling Ion Exchange and Biosorption for Copper(II) Removal From Wastewaters Anna Gorka,† Justyna Zamorska,‡ and Dorota Antos*,† †

Department of Chemical and Process Engineering and ‡Department of Water Purification and Protection, Rzeszow University of Technology, al. Powsta ncow Warszawy 6, 35-959 Rzeszow, Poland, Pl ABSTRACT: A coupled process of ion exchange and biosorption for recovery of copper(II) from wastewater has been proposed. In this process a cation exchanger has been used as a carrier for the biofilm formation. Two different types of microorganisms capable for copper biosorption have been selected for immobilization, i.e, effective microorganisms and activated sludge microorganisms. As a regenerating agent solutions of sodium bicarbonate with aqueous ammonia as an additive have been used, which allowed operating the column under mild pH conditions. The activity of biofilm was preserved after the cyclic process of ion exchange and regeneration. The effect of biofilm on the mass transfer kinetics and the ion exchange equilibrium was quantified by comparing the efficiency of ion exchange on the fresh bed and that covered by biofilm. The linear driving force model was used for simulations of the process dynamics.

1. INTRODUCTION The contamination of the environment by heavy metals is of growing concern because their discharge into receiving waters causes detrimental effects on human health and the environment.1 The term “heavy metals” applies to the group of metals with high atomic density (>6 g/cm3) and includes Cd, Cr, Cu, Hg, Ni, Pb, Zn, which are commonly associated with pollution and toxicity problems. Common sources of polluted wastes include electroplating plants and metal finishing operations as well as a number of mining and electronics industries. Copper intake from drinking water can be around 0.05-1.4 mg per day2 where the guideline for the maximum acceptable level of copper concentration should be less than 3 mg/L.3 It is one of the most toxic metals to the plant and microorganisms and exhibits very high mammalian toxicity.4 Nevertheless, copper besides posing environmental threads belongs to strategic and precious metals that are targets for concentration and effective recovery. For this reason, there is a constant need to remove and recover copper from industrial effluents and wastewaters. Commonly employed methods of treating heavy metalpolluted wastes include chemical precipitation, electro-flotation, reverse osmosis, adsorption on activated carbon, and ion exchange.5 Although some of these methods may be under given conditions more effective than ion exchange, the latter process is considered attractive because of the relative simplicity of application6 and in many cases is proven to be economic and effective technique to remove ions of heavy metals from wastewaters.7 An alternative treatment method is biosorption, which is one of the most promising technologies exploited for removal of toxic metals from industrial waste streams and natural waters. Biosorption involves the accumulation of heavy metals by biological materials either by metabolically dependent mechanisms or by purely physicochemical means.8 Many microorganisms (algae, bacteria, fungi, and yeasts) have been reported to be able to accumulate copper and other heavy metals from solutions.8-30 However, the process strongly depends on screening and domestication of heavy metal-resistant bacteria.15,16 r 2011 American Chemical Society

The biological removal of heavy metals from wastewaters can be achieved using suspended biomass, nevertheless, it is known that for industrial application freely suspended biomass may cause several undesired effects such as compaction, clogging and washout from the system.28 These disadvantages can be overcome if biomass is immobilized on a carrier. Microorganisms can be immobilized in a polymeric matrix or in other supports and packed into columns, which improves the biomass performance and allows its use in many subsequent cycles in the usual unit processes characteristic of chemical engineering.13,17,22,23,30 Alternatively, biofilm reactors can be used, wherein high biomass concentrations can easily be maintained, although in many cases mass transfer of substrate within biofilm cell clusters is the rate-limiting process. Processes using thin biofilms (8 allowed reducing dissociation of ammonia to the ionic form participating in ion exchange. In the last stage of regeneration the column was flushed out with water (more precisely with 0.0017 mol/L NaCl) to remove the excess of Naþ ions. To maintain the biofilm activity the column regeneration was occasionally preceded by feeding nutrient solutions into the column. Peptone Aminobak and glucose were added as the biofilm nutrients (see section 3.2.2). 3.2.6. Copper Recovery. Copper was recovered from the concentrated postregeneration solutions by precipitation, which could be easily performed by slight pH decrease resulting in decomposition of the ammonia complex ½CuðNH3 Þ4 ðOHÞ2 þ 4HCl f CuðOHÞ2V þ 4NH4 Cl ð14Þ Note that the solubility of copper hydroxide is very low, its solubility product, I, for diluted solutions is I = CCu2þ 3 (COH-)2 = 2.12 10-20. Low limit of the Cu(II) solubility depends strongly on pH value, i.e. CCu2þ ¼ I = ðCOH- Þ2

ð15Þ

4. RESULTS AND DISCUSSION Biofilm formed on the bed particles was found to affect the ion exchange process; it influenced on the bed void volumes, the pH profile, and the ion exchange thermodynamics as well as on the kinetics of mass transfer. All these factors will be described below. 4.1. Influence of Biofilm on the Bed Porosity. Table 3 presents the values of porosity measured for beds before and after the biofilm formation. It can be observed that the bed porosity was very weakly affected by the biofilm with low thickness such as that cultivated with ACMs and EMs within 2-weeks cultivation period (EMs I; see Figure 1b). Marked changes of the column void volumes were recorded for higher biofilm thickness achieved after a longer cultivation period (EMs II, see Figure 1c). A large amount of biomass in the column limited accessibility of ions to the bed void volumes. 4.2. Influence of Biofilm on the Process Performance. To analyze the process dynamics the breakthrough experiments were performed for the fresh zeolite and that covered by both

εt

εe

beds with fresh zeolite

0.79-0.80

0.57-0.59

bed with biofilm Ems Ia

0.82

0.64

bed with biofilm Ems IIb

0.70

0.39

bed with biofilm ACMs

0.77

0.53

Shorter time of cultivation. b Longer time of cultivation.

Figure 2. Typical sequence of the breakthrough curves within the concentration range of Cu2þ investigated: 1) fresh zeolite; 2) zeolite with the biofilm of EMs I (Figure 1b); 3) zeolite with the biofilm of ASMs; 4) zeolite with the biofilm EMs II (Figure 1c). Lines - guide to eye.

types of biofilm. Within the whole concentration range investigated the following sequence of the breakthrough retention was observed: the concentration fronts of Cu2þ propagated fastest for the bed covered by biofilm with the highest thickness (EMs II), while the highest retention was recorded for the zeolite with the immobilized biofilm of ACMs. Typical retention sequence is shown in Figure 2. Because the process performance was always the worst for the biofilm of EMs II that case was excluded from further investigations. 4.3. Influence of Biofilm on the Ion Exchange Equilibrium. For each breakthrough curve the equilibrium concentration in the solid phase was calculated according to eq 11. The results obtained for the fresh zeolite and that covered by the biofilm are presented in Figure 3 in the form of the isotherm curves. Note very favorable shape of the isotherms for all the presented cases which corresponded to very long retention of breakthrough profiles at low copper concentration. To compare the isotherms quantitatively the parameters of eqs 7-9 (i.e., the coefficient K and the ion exchange capacity Γ) were estimated on the basis of the experimental curves. To evaluate the activity coefficients in the mobile phase (eq 7) the BromleyPitzer method was used implemented in ASPEN ver. 11.1. The concentration dependence of the coefficient K (eq 7) was neglected to simplify the isotherm model; K should be considered as a lumped value that averages possible concentration depens 2 s þ) /γCu2þ which cannot be experimentally dency of the term (γNa determined. The isotherm coefficients are summarized in Table 4. The values estimated were verified by comparison of the simulated and experimental breakthrough profiles, as described below. 3498

dx.doi.org/10.1021/ie101019s |Ind. Eng. Chem. Res. 2011, 50, 3494–3502

Industrial & Engineering Chemistry Research

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Figure 3. Isotherm courses for Cu2þ ion exchange on the zeolite. Symbols - experiment data, lines simulations according to eqs 7-89: (1) fresh zeolite; (2) zeolite covered by the biofilm of EMs I; (3) zeolite covered by the biofilm of ASMs.

Table 4. Isotherm Parameters for Ion Exchange on the Fresh Zeolite and Zeolite Covered by Biofilm Γ (eq/Lsolid)

K [-]

retention factor k (eq 16)

fresh zeolite

6.94

0.0025

1.04 104

biofilm EMs I

5.52

0.0106

2.45 104

biofilm ACMs

6.50

0.0046

2.01 104

isotherm coefficients

Analyzing the contents of Table 4 it can be observed that the presence of the biofilm results in a decrease of the ion exchange capacity Γ. This phenomenon is more pronounced for the biofilm of EMs due to a relatively high amount of biomass immobilized in the biofilm for this type of microorganisms. The uniform structure of biofilm may limit the access of ion to the surface of zeolite. Moreover, metabolism products of microorganisms may affect the electrostatic potential of the surface. It is evident that increase of the adsorption capacity due to the biofilm presence was too weak to counterbalance decrease of the ion exchange capacity. Nevertheless, the retention factor, k, correlated with the slope of the linear part of the isotherm * 2þ, Γ) (see eqs 7-9 at CCu2þf0, qCu KΓ2 ð16Þ k ¼ F3 2 CNaCl markedly increased in the biofilm presence due to increase of the coefficient K, and it was the highest for the thickest biofilm of immobilized EMs. Such a retention increase originated from interactions between the copper ions and the immobilized microorganisms. Summarizing, the biofilm presence resulted in two contradictory effects: (1) decrease of the process efficiency at copper concentrations corresponding to strongly nonlinear range of isotherm within which the ion exchange capacity determines retention. Note that for favorable nonlinear isotherms the value of the solid phase concentration tends to the ion exchange capacity, q* f Γ, at sufficiently high mobile phase concentration, C. In such a case the velocity of the breakthrough front propagation is practically independent of the coefficient K.

Figure 4. Influence of biofilm on the process performance; symbols experimental data, lines the LDF model simulations; (1) fresh zeolite; (2) zeolite covered by the biofilm of EMs; (3) biofilm of ACMs; the effect of the feed concentration of Cu2þ: (a) 0.0063 mol/L (400 mg/L); (b) 0.0034 mol/L; (c) 0.0004 mol/L.

(2) increase of the process efficiency at low concentration related to linear or weakly nonlinear range of the isotherm within which velocity of the breakthrough front propagation is determined by the retention factor and, therefore, depends on both the isotherm coefficients: K and Γ. Note that the ion exchange processes are generally efficient at relatively low concentration corresponding to linear or weakly nonlinear isotherm range. The effects discussed above were illustrated in Figure 4a-c. 3499



Figure 5. Typical pH transition profiles (3) and solubility curves (2), corresponding to a breakthrough curve (1), for the fresh zeolite (a) and the zeolite with the biofilm of ACMs (b).

The data presented in Figure 4 indicate that within the concentration range investigated the best performance of the copper ion removal was achieved for the bed covered by the biofilm of ACMs, for which marked delay of the breakthrough profiles was recorded compared to those registered on the fresh bed. The improvement of the efficiency enhanced with decrease of the feed concentration according to the change of the isotherm range. The efficiency of ion exchange of zeolite covered by EMs biofilm was lower compared to that of the zeolite covered by ACMs as well as to that of the fresh zeolite. It can be expected that the presence of the EMs biofilm may be beneficial only for very low feed concentration corresponding to the linear isotherm range, for which the propagation velocity of the concentration fronts is governed by the retention factor (i.e., isotherm slope). 4.4. Influence of Biofilm on pH Profile. As mentioned above the solubility of copper in aqueous solutions is strongly pH dependent. Sorption of Hþ ion on the cation exchanger can result in a pH change of the mobile phase. Due to low concentration of hydrogen ions the pH drop was small and the effects of the competitive adsorption could be neglected; however, it might influence on the local solubility of copper. This phenomenon can be observed in Figure 5 which presents the pH and solubility changes accompanying a breakthrough profile. The solubility curve was calculated on the basis of the experimental pH profile in the effluent stream using eq 15. As is can be observed the solubility limits may be exceeded and precipitation within the column is possible. Decrease of the concentration of copper in the ionic form might have an influence on the process thermodynamics as well as on the mass transport mechanism. Because the solution acidification is characteristic for the biosorption mechanism8 a pH drop is expected for beds covered by biofilm. Typical comparison between pH profiles registered for the fresh and biofilm covered zeolite is depicted in Figure 5. This is evident that the probability of precipitation is lower for the biofilm covered bed. This factor might have an influence on the process performance. 4.5. Mass Transport Kinetics. To quantify the mass transport kinetics the LDF model was used (eqs 1 and 2). The model was solved for each breakthrough profile with simultaneous estimation of the lumped mass transport coefficient km. The estimation was performed by minimizing the sum of squares of the differences between the measured data and the model simulations.

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Figure 6. Plot 1/R = f(Δq*/ΔC). Symbols - data obtained by estimation of km: (1) fresh zeolite; (2) zeolite with the biofilm of EMs; (3) zeolite with the biofilm of ASMs.

Table 5. Mass Transport Coefficients for Ion Exchange on the Zeolite before and after the Biofilm Formation mass transport coefficient fresh zeolite biofilm EMs biofilm ACMs

keff [m/s]

Dseff [m2/s]

1.14 10-4

1.14 10-11

-5

8.95 10-12

-4

1.32 10-11

5.82 10 1.63 10

The values of km were used to construct the plot 1/R = f(Δq*/ ΔC) according to eqs 3 and 4 as it is shown in Figure 6. The data obtained were approximated by a linear dependence which was exploited to evaluate values of the mass transport coefficients. The contribution of the pore diffusion to the overall mass transport mechanism was evaluated to be negligible compared to that of the surface diffusion (details of this procedure can be found elsewhere39). The values of the external mass transport and surface diffusion coefficients were determined by the linear regression and reported in Table 5. These values were used for the back simulations of breakthrough curves. The agreement between the simulations and the experimental profiles was found to be satisfactory in the whole concentration range (see Figure 4a-c). The data reported in Table 5 indicate that in the presence of EMs the values of the kinetic rate coefficients decreased, particularly those for the external mass transport kinetics. This can be attributed to the reduced mobility of ions in the biofilm accumulated on the external particle surface. The decrease in the rate coefficients caused broadening of the breakthrough curves and the reduction of the ion exchange efficiency. Surprisingly, for the zeolite covered by ACMs increase of the mass transfer rate was reported. Because the biofilm was heterogeneous with relatively low thickness, which did not exceed the laminar sublayer thickness, the rate of the mass transport was expected to remain unchanged.36 The origin of this phenomenon is not clear; however, it might result from the pH and solubility changes indicated above. 4.6. Influence of Biofilm on Bed Regeneration. Aqueous solutions of 3-5% NaCl or NaHCO3 were too weak as regenerating agents. The regeneration was not efficient and the ion exchange capacity of the exchanger decreased systematically after 3500



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As it can be observed, to achieve complete bed regeneration in the first stage relatively long time was necessary. However, because the regenerant could be recycled to the process, the optimization of the regeneration conditions was not performed. At the end of series of breakthrough and regeneration experiments the column effluents were withdrawn to examine the biological activity of microorganisms. The samples were diluted in Ringer’s solutions and transferred on the nutrient agar plate for the cultivation. High CFU confirmed the activity of microorganisms in both types of biofilm investigated.

Figure 7. Bed regeneration. (a) first stage of regeneration with 5% NaHCO3 and 0.05 mol/L NH3 3 H2O. Before the regeneration, the bed was equilibrated with the solution of CCu2þ = 0.0063 at 0.0017 mol/L of NaCl; (b) second stage of the regeneration with 3% NaHCO3.

each subsequent breakthrough experiments resulting in a reduction of the process efficiency. Small addition of aqueous ammonia allowed complete column regeneration at relatively low salt concentration and mild pH. The regeneration course for the bed covered by the EMs biofilm is shown in Figure 7a and 7b. The efficiency of regeneration of the fresh zeolite as well as of that covered by biofilm was found to be practically the same. To recover copper after the ion exchange process the effluent collected in the first stage of regeneration was titrated by 0.1 N HCl until pH 6 was reached. Such pH conditions allowed decomposition of the ammonia complexes (eq 14) due to dissociation of aqueous ammonia. The reaction underwent with the color transition - the blue color characteristic for the ammonia complex disappeared, which was accompanied by precipitation of the copper salt. The regenerants after filtering out the precipitate contained 3.5 10-5-5.5 10-5 mol/L Cu2þ (2-3.5 ppm). After adjusting the ammonia concentration and the solution pH by titration with 0.1 N NaOH, the exhausted regenerant could be efficiently used in a subsequent regeneration process. Small amount of the fresh regenerant might be added at the end of the regeneration to complete removal of Cu2þ from the solid phase. The effluent of the second regeneration stage can also be reused to supplement ammonia in the regenerating solution of the first stage.

5. CONCLUSIONS The present study examined the possibility of coupling ion exchange and biosorption for removal and recovery of copper(II) from aqueous solutions. Two types of microorganisms capable for copper biosorption were selected for the preliminary investigations - microorganisms isolated from the active sludge and from the preparation of effective microorganisms. The attempts were made to immobilize biofilm on a nonporous strong cation resin and a porous synthetic zeolite. The cultivation was successful only on the porous zeolite; the microorganisms were not able to adhere to the nonporous surface of resin. Therefore, a porous exchanger can be suggested as a preferable carrier for the biofilm immobilization. As a regenerating agent solutions of sodium bicarbonate with ammonia additive were used which assured mild operating conditions. Moreover, employing such a regenerant allowed the use of standard equipments, nonresistant to acids and concentrated solutions of chloride ions, and operating the cation exchanger in the sodium form characterized by very favorable adsorption toward divalent copper ions. The presence of biofilm was found to affect the ion exchange thermodynamics and the mass transfer kinetics. The ion exchange capacity of the zeolite covered by the biofilm was found to be lower, while values of the isotherm constant higher compared to those of the fresh zeolite. As a result an increase of the process efficiency was registered at low copper concentration, while practically no improvement was achieved at high copper concentration. Nevertheless, range of low concentration is typical for ion exchange processes. The efficiency improvement was noticeable for the zeolite covered by the active sludge microorganisms. The biofilm of the effective microorganisms was homogeneous and thicker compared to that of the active sludge microorganisms. For the former case the drop of the ion exchange capacity was significant and could not be compensated by increase of the equilibrium constant. The presence of a higher amount of biomass in the column also resulted in worsening the mass transfer kinetics. Therefore, the heterogeneous biofilm of the active sludge microorganisms was found to be more suitable for copper removal within the concentration range investigated. The presence of the ACMs biofilm allowed the improvement of the efficiency of the coupled process compared to the single process of ion exchange. This proved that combination of biosorption and ion exchange is possible and might be beneficial, provided proper selection of microorganisms that are able to form heterogeneous biofilm with relatively low thickness that does not exceed the laminar sublayer thickness. Nevertheless, for general conclusions a continuation of the preliminary research presented here is necessary. 3501



’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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Coupling Ion Exchange and Biosorption for Copper(II) Removal From

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