Biosorption of copper(II) and cobalt(II) from aqueous solutions by crab shell particles

Bioresource Technology 97 (2006) 1411–1419

Biosorption of copper(II) and cobalt(II) from aqueous solutions by crab shell particles K. Vijayaraghavan a, K. Palanivelu b, M. Velan a

a,*

Department of Chemical Engineering, Anna University, Chennai 600 025, India b Centre for Environmental Studies, Anna University, Chennai 600 025, India

Received 4 December 2004; received in revised form 24 June 2005; accepted 1 July 2005 Available online 19 August 2005

Abstract Biosorption of each of the heavy metals, copper(II) and cobalt(II) by crab shell was investigated in this study. The biosorption capacities of crab shell for copper and cobalt were studied at different particle sizes (0.456–1.117 mm), biosorbent dosages (1–10 g/l), initial metal concentrations (500–2000 mg/l) and solution pH values (3.5–6) in batch mode. At optimum particle size (0.767 mm), biosorbent dosage (5 g/l) and initial solution pH (pH 6); crab shell recorded maximum copper and cobalt uptakes of 243.9 and 322.6 mg/g, respectively, according to Langmuir model. The kinetic data obtained at different initial metal concentrations indicated that biosorption rate was fast and most of the process was completed within 2 h, followed by slow attainment of equilibrium. Pseudo-second order model fitted the data well with very high correlation coefficients (>0.998). The presence of light and heavy metal ions influenced the copper and cobalt uptake potential of crab shell. Among several eluting agents, EDTA (pH 3.5, in HCl) performed well and also caused low biosorbent damage. The biosorbent was successfully regenerated and reused for five cycles.  2005 Elsevier Ltd. All rights reserved. Keywords: Wastewater treatment; Elution; Regeneration; Crab shell; Kinetics

1. Introduction Heavy metals are a group of pollutants of much concern due to their almost indefinite persistence in the environment (Reed, 1998). Conventional methods for heavy metal removal from aqueous solution include chemical precipitation, electrolytic recovery, ion exchange/chelation and solvent extraction/liquid membrane separation (Rorrer, 1998). Application of these methods, however, is sometimes restricted because of technical or economical constraints (Puranik and Paknikar, 1999). Recently, biological methods of heavy metal removal have gained considerable momentum due to their high efficiency, low operating cost and simplicity. *

Corresponding author. Tel.: +91 44 2220 3506; fax: +91 44 2235 2642. E-mail address: [email protected] (M. Velan). 0960-8524/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.07.001

One such method is biosorption, in which inactive biological materials are utilized to sequester toxic heavy metals and is particularly useful for the removal of contaminants from industrial effluents (Volesky and Holan, 1995; Kratochvil and Volesky, 1998). The biological materials that have been investigated for heavy metals uptake include bacteria (Brierley, 1990), fungi (Tsezos and Volesky, 1981), yeast (Volesky et al., 1993), microalgae (Darnall et al., 1986) and macroalgae (Kuyucak and Volesky, 1989a). All these biomaterials have shown adequate heavy metal sorption capacity to be considered for process applications. Biosorption of heavy metals by these biological materials involves several mechanisms that differ qualitatively and quantitatively depending on the species used, the origin of biomass and its processing procedure (Volesky and Holan, 1995). In recent years, another biomaterial crab shell performed well in the removal of lead (Lee et al., 1997),

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cadmium (Evans et al., 2002) and nickel (Vijayaraghavan et al., 2004). Also, crab shell known to possess rigid structure, excellent mechanical strength and ability to withstand extreme conditions employed during regeneration process (Lee et al., 1997; Vijayaraghavan et al., 2005). Crab shell comprises of mainly calcium carbonate, chitin along with some proteins. Chitin and its deacetylated form (chitosan) have been recognized as effective biosorbents for metal removal (da Silva and da Silva, 2004; Kartal and Imamura, 2005). Chitin and chitosan are long linear polymeric molecules of b(1 ! 4)-linked glycans. The repeating unit in chitin is 2-acetamido-2-deoxy-D-glucose-(N-acetylglucosamine), while chitosan comprises a non-homogenous mixture with the deacetylated form (glucosamine) (Onsoyen and Skaugrud, 1990). Copper and cobalt are the focus of this study. Copper, one of the most important heavy metal used in electroplating industries, in high doses causes serious toxicological concerns; it is known to deposit in brain, skin, liver, pancreas and myocardium (Davis et al., 2000). The extraordinary usage of cobalt in many important industrial applications (e.g., in the production of satellite alloys) and occurrence of the main cobalt resources in politically unstable areas of the world make it one of the most important and strategic metals (Kuyucak and Volesky, 1989a). The aim of the work was to determine the ability of crab shell to remove copper(II) and cobalt(II) separately from aqueous solutions. First, the effects of particle size, biosorbent dosage and initial metal concentration were studied. Further, the influence of solution pH on metal uptake was studied and the data were fitted using Langmuir and Freundlich model. Desorption studies using different elutants at different concentrations and solid-to-liquid ratios were also investigated.

2. Methods Waste shells of Portunus sanguinolentus commonly known as three spot crabs were collected from Marina beach, India and were sun dried and crushed to desired sizes using ball mill. The pretreatment of crab shells was carried out by washing with 0.1 M HCl for 4 h to remove CaCO3 (Vijayaraghavan et al., 2004). The treated shells were then washed with de-ionized water and dried naturally and the weight loss was found to be approximately 50%. The pretreated crab shell particles obtained are referred as ‘‘CSP’’ in this paper. Analytical grades of HCl, HNO3, H2SO4, NaOH, NaCl, KCl, CaCl2 Æ 2H2O, MgCl2 Æ 6H2O, CuSO4 Æ 5H2O, CoSO4 Æ 7H2O, 3CdSO4 Æ 8H2O, NiSO4 Æ 6H2O, ZnSO4 Æ 7H2O, NH4OH, NH4Cl and EDTA (Na) were purchased from Ranbaxy Fine Chemicals Ltd., India. For biosorption studies, desired quantity of dried CSP was contacted with 100 ml of copper or cobalt solu-

tion in 250 ml Erlenmeyer flasks. The flasks were then placed on a rotary shaker at 150 rpm and samples were taken at regular time intervals. The samples were then centrifuged at 3000 rpm for 10 min. The metal content in the supernatant was determined using flame Atomic Absorption Spectrophotometer (AAS 6VARIO; Analytik Jena, Germany). Metal uptake was determined using the following equation: Q ¼ ðC 0  C f Þ  V =M

ð1Þ

where Q is the metal uptake (mg/g); C0 and Cf are the initial and equilibrium metal concentrations in the solution (mg/l), respectively; V is the solution volume (l); and M is the mass of biosorbent (g). The pH of the solution was adjusted by using 0.1 M HCl or 0.1 M NaOH. The Langmuir sorption model (Langmuir, 1918) was chosen for the estimation of maximum metal biosorption by the biosorbent. The Langmuir isotherm can be expressed as Q¼

Qmax bC f 1 þ bC f

ð2Þ

where Qmax is the maximum metal uptake (mg/g) and b is the Langmuir equilibrium constant (l/mg). For fitting the experimental data, the Langmuir model was linearized as follows: 1=Q ¼ 1=Qmax þ 1=bQmax C f

ð3Þ

The Freundlich model (Freundlich, 1907) is represented by the equation 1=n

Q ¼ KC f

ð4Þ

where K and n are constants. The metal-loaded CSP after biosorption was contacted with different elutants at desired concentrations for 3 h on a rotary shaker (150 rpm) to study the elution of biosorbed metal ions. The remaining procedure was the same as that in the sorption equilibrium experiments. After desorption, CSP was washed with de-ionized water, then filtered and finally dried overnight at 60 C. The loss in biosorbent weight was calculated and the biosorbent was subsequently used for re-sorption studies.

3. Results and discussion 3.1. Effect of CSP size In the first stage of batch biosorption experiments on CSP, the effect of particle sizes on metal biosorption by CSP was examined. Four different particle sizes were used: size-1 (0.456 mm), size-2 (0.598 mm), size-3 (0.767 mm) and size-4 (1.117 mm). For both the metal ions, significant variations in uptake capacity and removal efficiency were observed at different particles sizes

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(Fig. 1). In general, smaller particles (0.456–0.767 mm) provided larger surface area and resulted in high metal uptake capacity and removal efficiency. In all cases, the largest CSP size (1.117 mm) resulted in very low biosorption performance. Although the smallest particle size (0.456 mm) resulted in slightly better biosorption performance compared to 0.598 and 0.767 mm CSP sizes, its increase in uptake and removal efficiency were within 5% compared to other two CSP sizes. It is always preferable to use rigid and slightly large particles especially in sorption column processes so that it can withstand extreme conditions employed in regeneration processes (Volesky, 2001). Since the CSP size 0.767 mm performed very closely to CSP size 0.456 mm (considering that the difference was within 5%), it was subsequently used in all biosorption experiments. 3.2. Effect of CSP dosage The influence of CSP dosage on copper and cobalt biosorption was examined by varying dosages from 1 to 10 g/l. Fig. 2 presents typical set of results obtained by varying biosorbent dosages during copper and cobalt

Fig. 2. Influence of biosorbent dosage on uptake and removal efficiency of CSP for copper and cobalt (CSP size = 0.767 mm; pH = 6; initial metal concentration = 2000 mg/l; agitation speed = 150 rpm; equilibrium time = 6 h).

Fig. 1. Influence of particle size on uptake and removal efficiency of CSP for copper and cobalt (biosorbent dosage = 5 g/l; pH = 6; initial metal concentration = 2000 mg/l; agitation speed = 150 rpm; equilibrium time = 6 h).

biosorption. From the analysis of experimental data obtained for two metal ions, it was observed that the removal efficiency increased with increase in biosorbent dosage. An increase in biomass concentration generally increases the biosorbed metal ions because of an increase in surface area of the biosorbent, which in turn increases the binding sites (Esposito et al., 2001). Whereas the metal uptake decreases by increasing the biosorbent dosage, this may be due to complex interactions of several factors. The important factors being at high sorbent dosages the available metal ions are insufficient to cover all the exchangeable sites on the biosorbent, usually resulting in low metal uptake. Several investigators observed this trend during their biosorption experiments. For instance, Tangaromsuk et al. (2002) observed that cadmium removal efficiency of Sphingomonas paucimobilis biomass increased with increasing biomass concentration, whereas cadmium uptake capacity decreased at higher biomass concentrations. In the present study, the lowest biosorbent dosage (1 g/l) resulted in highest metal uptake capacity and lowest removal efficiency. The metal uptake capacity and removal efficiency are equally important in sorption experiments as both usually take part in deciding the sorption performance of a given biosorbent. Taking

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this into consideration, the biosorbent dosage 5 g/l was selected for further studies as it showed more than four times enhancement in removal efficiency and only 21% less uptake capacity compared to 1 g/l biosorbent dosage for both metal ions. 3.3. Effect of initial metal concentration The experimental results of biosorption of copper and cobalt ions onto CSP at various initial metal concentrations are shown in Fig. 3. For both metal ions, the biosorption rate was fast and most of the process was completed within 2 h, followed by slow attainment of equilibrium. Also, the equilibrium time attainment increased with increasing concentration of metal ions. On increasing the initial solute concentrations, the total metal uptake increased and the total percent removal decreased. For instance, on changing initial copper concentration from 500 to 2000 mg/l, the amount sorbed increased from 75.4 to 197.7 mg/g at pH 6. But the removal efficiency of copper decreased from 75.4% to 49.4% as the copper concentration increased from 500

Copper concentr ation (mg/l)

2000 Copper 1500

1000

500

0 0

100

200

300

400

500

600

Time (min) 2000

Cobalt concentr ation (mg/l)

Cobalt 1500

1000

500

0 0

100

200

300

400

500

600

Time (min) Fig. 3. Concentration–time profile during copper and cobalt biosorption at different initial metal concentrations (CSP size = 0.767 mm; biosorbent dosage = 5 g/l; pH = 6; agitation speed = 150 rpm). Initial metal concentration: () 500 mg/l; (j) 1000 mg/l; (m) 1500 mg/l; (·) 2000 mg/l.

to 2000 mg/l. This was because at lower concentration, the ratio of the initial moles of metal ions to the available surface area was low and subsequently, the fractional sorption became independent of initial concentration. However, at higher concentration the available sites of sorption become fewer compared to the moles of metal ions present and hence, the percentage removal of metal would be dependent upon the initial metal ion concentration. 3.4. Kinetic models To evaluate the differences in the biosorption kinetic rates and the metal recoveries, the kinetics of metal uptake were described with pseudo-first and pseudo-second order models. The linearized form of pseudo-first and pseudo-second order model (Ho and McKay, 1998) are shown below as Eqs. (5) and (6), respectively: logðQe  Qt Þ ¼ logðQe Þ  t 1 1 ¼ þ t Qt K 2 Q2e Qe

K1 t 2.303

ð5Þ ð6Þ

where Qe is the amount of metal sorbed at equilibrium (mg/g); Qt is the amount of metal sorbed at time t (mg/g) and K1 is the first order equilibrium rate constant (1/min); K2 is the second order equilibrium rate constant (g/mg min). Initially, the validity of the two models was checked by studying the kinetics under different initial metal concentrations. In order to obtain the rate constants and equilibrium metal uptake, the straight-line plots of log (Qe  Qt) against t of Eq. (5) were made for CSP at different initial metal concentrations (figure not presented). The intercept of the above plot should equal to log Qe. However, if the intercept does not equal to the experimental equilibrium metal uptake then the reaction is not likely to be first order even this plot has high correlation coefficient with the experimental data (Ho and McKay, 1998). The rate constants, predicted equilibrium uptakes and the corresponding correlation coefficients for all concentrations tested have been calculated and summarized in Table 1. For both metal ions, correlation coefficients were found to be above 0.895, but the calculated Qe is not equal to experimental Qe, suggesting the insufficiency of pseudo-first-order model to fit the kinetic data for the initial concentrations examined. The reason for these differences in the Qe values is that there is a time lag, possibly due to a boundary layer or external resistance controlling at the beginning of the sorption process (McKay et al., 1999). In most cases in the literature, the pseudo-first order model does not fit the kinetic data well for the whole range of contact time, and generally underestimate the Qe values (Ho and McKay, 1998; Reddad et al., 2002).

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Table 1 Kinetic parameters for the metal biosorption onto CSP at different initial metal concentrations Initial concentration (mg/l)

(Qe)exp (mg/g)

Pseudo-first order K1 (1/min)

Qe (mg/g)

R

Copper

500 1000 1500 2000

75.4 147.9 181.4 197.7

0.034 0.038 0.030 0.031

36.8 100.9 130.9 162.2

Cobalt

500 1000 1500 2000

72.3 131.7 205.5 259.4

0.033 0.032 0.033 0.023

50.3 93.4 122.0 162.5

Metal

K2 (g/mg min)

Qe (mg/g)

R2 a

0.920 0.958 0.946 0.991

0.0035 0.0012 0.0006 0.0005

75.8 149.3 185.2 200.0

0.999 0.999 0.999 0.998

0.895 0.950 0.935 0.923

0.0017 0.0009 0.0008 0.0004

72.9 133.3 208.3 263.2

0.999 0.999 0.999 0.999

Correlation coefficient.

3.5. Effect of pH The pH of the metal solution played an important role in the biosorption of metals by CSP. The experimental results of metal biosorption by CSP at different initial pH conditions are shown in Fig. 4. CSP dose (5 g/l) and agitation speed (150 rpm) were kept constant. The metal uptake by CSP was sensitive to pH variations in the examined range 3.5–6. Working at over pH 6 was avoided to prevent precipitation of metals (Klimmek et al., 2001). For both metal ions at all pH conditions, as the metal concentration increases the uptake also increased and reached a plateau at higher concentrations resulted in a favorable sorption isotherm. Also, metal uptake increased with increasing pH and reached maximum in the pH range of 4.5–6 for both metal ions. This was not only because of the hydrogen ion competition at low pH, but also may be due to sorbate lyophobic behavior (Volesky and Schiewer, 1999). Since the solubility of many metal complexes in solution decreases with increasing pH, the sorption increases with increasing pH. The optimum pH for copper and cobalt biosorption by CSP was found to be 6.0.

200

Copper uptake (mg/g)

The pseudo-second order model is based on the sorption capacity on the solid phase. Contrary to other wellestablished models, it predicts the behavior over the whole range of studies and it is in agreement with a chemisorption mechanism being the rate-controlling step (McKay et al., 1999). This was consistent with the better results obtained with the pseudo-second order model (Table 1). Correlation coefficients were always greater than 0.998, and the lowest correlation coefficient in this case was better than the first order model correlation coefficients. The values of predicted equilibrium sorption capacities showed good agreement with the experimental equilibrium uptake values. The very fast sorption kinetics observed with CSP represents an advantageous aspect when water treatment systems are designed. The implication is that the material could be suitable for a continuous flow system.

150

100

50 Copper 0 0

200

400

600

800

1000

1200

1400

Final copper concentration (mg/l) 300 250

Cobalt uptake (mg/g)

a

Pseudo-second order 2a

200 150 100 50 Cobalt 0 0

200

400

600

800

1000

1200

1400

Final cobalt concentration (mg/l) Fig. 4. Biosorption isotherms at different pH conditions for copper and cobalt (CSP size = 0.767 mm; biosorbent dosage = 5 g/l; agitation speed = 150 rpm). Solution pH: () pH 3.5; (j) pH 4.0; (m) pH 4.5; (·) pH 5.0; ( ) pH 5.5; (d) pH 6.0.

The equilibrium biosorption data were modeled using both Langmuir and Freundlich adsorption models. Table 2 shows the model constants along with correlation coefficients for biosorption of metal ions onto CSP. Langmuir sorption model served to estimate the maximum metal uptake values where they could not be reached in the experiments. The constant b represents affinity between the sorbent and sorbate. The Langmuir model parameters were largely dependent on the initial

K. Vijayaraghavan et al. / Bioresource Technology 97 (2006) 1411–1419

Metal

Copper

Cobalt

a

pH

3.5 4.0 4.5 5.0 5.5 6.0 3.5 4.0 4.5 5.0 5.5 6.0

Langmuir parameters

Freundlich parameters

Qmax (mg/g)

b (l/mg)

R2 a

K (l/g)

n

R2 a

163.9 188.7 204.1 208.3 222.2 243.9

0.0022 0.0023 0.0026 0.0039 0.0054 0.0055

0.980 0.986 0.988 0.977 0.979 0.985

2.05 2.53 3.27 6.29 8.75 9.13

1.68 1.72 1.77 2.05 2.16 2.20

0.979 0.970 0.963 0.889 0.895 0.897

212.8 232.5 270.3 285.7 303.0 322.6

0.0015 0.0016 0.0017 0.0018 0.0021 0.0025

0.975 0.977 0.973 0.964 0.968 0.972

0.92 1.11 1.33 1.40 1.67 1.69

1.37 1.37 1.38 1.39 1.40 1.41

0.986 0.987 0.980 0.967 0.976 0.979

Correlation coefficient.

solution pH. Both Qmax and b increases with increasing initial solution pH. High values of b are reflected in the steep initial slope of a sorption isotherm, indicating desirable high affinity. Thus, for good biosorbents in general, high Qmax and a steep initial isotherm slope (i.e., high b) are desirable (Davis et al., 2003). Among the two metal ions, highest Qmax of 322.6 mg/g was observed for cobalt. In the case of b, copper recorded 0.0055 l/mg compared to 0.0025 l/mg for cobalt. Even though, CSP showed high affinity towards copper, the binding sites may be higher for cobalt also reflected well by Qmax value. It is worth noting both K and n values also reached their maximum values at this optimum pH value, this implies that the binding capacity reaches the highest value and the affinity between the CSP and metal ions was also higher than other pH values investigated. 3.6. Effect of co-ions Industrial effluents often contain more than one metal ion. Consequently, biosorption become competitive, in which several metal ions compete for a limited number of binding sites (Kratochvil and Volesky, 1998). Light metal ions, such as Na+, K+ and Mg2+ are almost common in all industrial effluents. Other heavy metal ions, such as Ni2+, Cd2+ and Zn2+, which are commonly found in electroplating industrial effluents, were also tested in this study. Fig. 5 shows the influence of co-ions on copper and cobalt uptake by CSP. Among light metal ions, Mg2+ had an appreciable effect on the metal uptake. It is interesting to note that a positive effect was observed in the case of Na+ and K+ ions, when used separately as competing ions in the metal solutions. Similarly Kuyucak and Volesky (1989a) observed an enhanced cobalt uptake by Ascophyllum nodosum, when K+ was used as competing ion. The possible reason

110

Change in copper uptake (%)

Table 2 Langmuir and Freundlich model parameters at different pH conditions

100

90 80

70 Copper 60 0

250

500

750

1000

1250

1500

1750

2000

Co-ion concentration (mg/l) 110

Change in cobalt uptake (%)

1416

100

90 80

70 Cobalt

60 0

250

500

750

1000

1250

1500

1750

2000

Co-ion concentration (mg/l) Fig. 5. Influence of co-ions on copper and cobalt uptake by CSP (CSP size = 0.767 mm; biosorbent dosage = 5 g/l; pH = 6; initial metal concentration = 2000 mg/l; agitation speed = 150 rpm). Co-ions: () Na+; (j) K+; (m) Mg2+; () Cu2+; (+) Co2+; ( ) Ni2+; (d) Cd2+; (·) Zn2+.

for this increase might be due to chloride ion in NaCl or KCl combines with metal ion and gets deposited on the surface of CSP. Among two metal ions (Cu2+ and Co2+), the presence of Co2+ in copper solution severely affected the copper uptake potential of CSP. The existence of Ni2+, Cd2+ and Zn2+ also significantly affected the metal uptake capacity of CSP. The suppression was well pronounced in the case copper than that of cobalt. 3.7. Desorption In initial part of desorption experiments, different elutants (HCl, H2SO4, HNO3, CaCl2, NaCl, KCl, NH4Cl, NaOH, NH4OH and EDTA (Na)) were screened for their potential to desorb copper and cobalt ions from metal-loaded CSP (Table 3). The optimal elutant must be effective, non-damaging to the biomass, non-polluting and cheap. Elution efficiency was determined by the ratio of the metal mass in the solution after desorption to the metal mass initially bound to the biosorbent (Davis et al., 2000). The mineral acids (0.1 M HCl, 0.1 M H2SO4 and 0.1 M HNO3) wash of metal-laden CSP released all the metal ions. De-ionized water

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Initial pH

Copper elution efficiency (%)

Cobalt elution efficiency (%)

De-ionized water Boiled water 0.1 M HCl 0.1 M H2SO4 0.1 M HNO3 0.1 M CaCl2 0.1 M CaCl2/HCl 0.1 M NaCl 0.1 M KCl 0.1 M NH4Cl Conc. NH4OH 2 M NH4OH 0.1 M NaOH 0.01 M EDTA (Na) 0.01 M EDTA (Na)/HCl

Unadjusted Unadjusted 1.2 1.0 1.2 Unadjusted 2.0 Unadjusted Unadjusted Unadjusted Unadjusted Unadjusted Unadjusted Unadjusted 3.5

1.5 2.3 100.0 99.8 99.9 13.4 25.3 3.8 3.0 6.7 57.8 49.7 5.3 92.7 99.3

1.3 2.5 100.0 99.9 100.0 10.4 14.2 3.0 3.4 5.7 99.9 88.3 4.7 94.7 99.7

Conditions: copper loading on CSP was 192.8 mg/g; cobalt loading on CSP was 259.4 mg/g.

and boiled water were not able to elute biosorbed metal ions, indicating strong affinity that CSP possess towards both metal ions. Furthermore the metal-loaded CSP was contacted with 0.1 M solutions of NaCl, KCl, CaCl2 and NH4Cl, expecting the potential involvement of the ion-exchange process in the biosorption phenomenon. But these elutants were able to elute biosorbed metal ions only to a limited extent, indicating the absence of ion exchange as a major mechanism. The elution efficiency of CaCl2 was improved slightly at pH 2. For both metal ions, the 0.1 M NaOH-eluting solution showed a very low desorption capacity. The conc. NH4OH wash of metal-laden CSP performed well for cobalt; however dilution of NH4OH by 2 M resulted in considerable reduction in elution efficiency. In the case of copper, the exposure of concentrated and diluted NH4OH to copper-loaded CSP turned the shell particles into dark blue. This may be due to complex formation of ammonia with copper and settled on the surface of CSP. EDTA, which is a strong complexing agent, could assist in the elution of metal ions sequestered by the CSP. The solution of 0.01 M EDTA performed reasonably well and the elution efficiency of EDTA solution was enhanced by decreasing the pH (using HCl). The cations physicochemically sequestered to the cell surface are easily coupled to complexing agents, whereas intracellularly accumulated cations could not be so easily desorbed from biosorbent materials (Kuyucak and Volesky, 1989b). An important parameter for metal biosorption is the solid-to-liquid ratio (S/L) defined as the mass of metalladen biosorbent to the volume of the elutant (Davis et al., 2000). Upon elution of the metal from the biosorbent, it is desirable to use the smallest possible eluting

3.8. Regeneration If the biosorption process were to be used as an alternative in the wastewater treatment scheme, regeneration of the biosorbent may be crucially important for keeping the process costs down and opening the possibility

100

Elution efficiency (%)

Elutant

volume so as to contain the highest concentration of the metal. At the same time, the volume of the solution should be enough to provide maximum solubility for the metal desorbed. Fig. 6 illustrates the effect of S/L ratio on the copper and cobalt elution efficiencies of the selected elutants examined. The elution efficiencies of all the mineral acids examined are nearly independent of the S/L ratio up to the examined value of 10 g/l. Similarly, Davis et al. (2000) observed elution efficiency of HCl from copper-loaded Sargassum filipendula remains unaltered by varying S/L ratios. In the case of cobalt, elution efficiency of conc. NH4OH almost remains unaltered by variation of S/L. However for both metal ions, elution efficiency decreased with increasing S/L for EDTA solution, whereas EDTA (pH 3.5, HCl) solution almost retains its elution efficiency up to S/L 10 g/l.

95

90

85 Copper 80 0

2

4

6

8

10

8

10

Solid-to-Liquid ratio (g/l) 100

Elution efficiency (%)

Table 3 The elution of biosorbed copper and cobalt by various chemical agents at 1 g/l S/L ratio

1417

95

90

85 Cobalt 80 0

2

4

6

Solid-to-Liquid ratio (g/l) Fig. 6. Influence of solid-to-liquid ratio on copper and cobalt elution efficiencies of different elutants (copper loading on CSP = 192.8 mg/g; cobalt loading on CSP = 259.4 mg/g). Elutant: () HCl; (j) HNO3; (m) H2SO4; (d) NH4OH; (·) EDTA (Na); ( ) EDTA (Na)/HCl.

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References

Fig. 7. Reuse of regenerated CSP for copper and cobalt biosorption (elutant = 0.01 M EDTA (Na)/HCl at pH 3.5; S/L = 10 g/l).

of recovering the metal ions extracted from the liquid phase (Volesky, 2001). In this part, CSP was reused for five biosorption–elution cycles to study the changes in metal biosorption with subsequent usage. A preliminary examination on elutants revealed that the application of mineral acids and conc. NH4OH resulted in damaging the macroscopic appearance of CSP and the weight loss was greater than 60% for both metal systems. Furthermore, EDTA solution was inefficient at high S/L; which could result in decreased next-cycle metal uptake. These aspects limited the elutants examined to EDTA (pH 3.5, HCl) for both copper and cobalt. Fig. 7 showed that the CSP was subsequently used for the sorption of copper and cobalt in five cycles and regenerated using EDTA (pH 3.5, HCl) at S/L 10 g/l. The regenerated CSP maintained high metal uptake capacity for both metal ions in all five cycles examined. In both cases, the loss in the dry weight of CSP was less than 10% after five cycles. These observations indicated that 0.01 M EDTA (pH 3.5, HCl) appeared as the most efficient and practical eluting agent releasing both copper and cobalt sequestered on the CSP.

4. Conclusions This work indicated that the crab shell could be used as an effective biosorbent material for the treatment of copper and cobalt bearing wastewater streams. The particle size, biosorbent dosage and initial metal concentration influenced metal uptake by CSP. The biosorption capacity was dependent on solution pH and maximum copper and cobalt uptakes of 243.9 and 322.6 mg/g, respectively, were observed at pH 6. The presence of Mg2+, Ni2+, Cd2+ and Zn2+ significantly interfere with the binding of both copper and cobalt ions onto CSP. Metal-loaded CSP was successfully regenerated using EDTA (pH 3.5, in HCl) and reused for five cycles.

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