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Jan 9, 2013 - [47] S. S. Ahluwalia, D. Goyal, Microbial and Plant Derived Biomass for. Removal of Heavy Metals from Wastewater – Review, Bioresour.
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Sibel Yalc¸ın Faculty of Engineering, Chemistry Department, Istanbul University, Avcilar, Istanbul, Turkey

Research Article The Mechanism of Heavy Metal Biosorption on Green Marine Macroalga Enteromorpha linza The biosorption mechanism of divalent Ni(II), Cd(II), and Pb(II) ions onto calcium treated Entemorpha linza was investigated as a function of pH, contact time, biomass dose, and temperature. The experimental data were evaluated by Langmuir, Freundlich, and Dubinin–Radushkevich isotherm models. The uptake capacity of the tested metal ions was markedly influenced by pH in the range of 2–3.5 and maximum rates were observed at pH 5–5.5. The kinetics of the metal ions adsorption were rather rapid, with 90% of adsorption occurring within 10 min. In addition to batch sorption tests, the functional groups on the cell wall matrix of the biomass were revealed by potentiometric titration data and Fourier transform infrared analysis. The relative contribution of the chemical groups involved in metal biosorption such as carboxyl, amino, sulfonate was evaluated to characterize their binding mechanisms using these instrumental techniques. The density of strong and weak acidic functional groups in the biomass was found to be 0.25 and 0.95 mmol g1 biomass, respectively. In conclusion, the present work showed that the marine algae E. linza could be used as a potentially cost-effective biosorbent for the treatment of complex wastewater containing heavy metals. Keywords: Biomass; Pollution; Thermodynamics; Wastewater Received: September 17, 2012; revised: January 9, 2013; accepted: January 11, 2013 DOI: 10.1002/clen.201200500

1 Introduction Today, one of the major environmental problems is heavy metal pollution. The main source of heavy metal pollution in aquatic environment derived from mostly human activities such as disposing sewage effluents, mining, processing of metal ores, the finishing and plating of metals, battery production, etc. are domestic and industrial wastewaters and their associated solid wastes. Therefore, these pollutants should be removed from wastewaters before discharging into waterways. The use of many types of non-living biomass for the removal of toxic metals from aqueous solutions has increased during recent years because of the good performance (i.e., high effluent quality, minimization of chemical and low biological sludge, regeneration of biosorbent, possibility of metal recovery) and low cost of these sorbent materials. These biomass types include fungi [1, 2], bacteria [3, 4], and algae [5–8]. Among them, marine algae divided into three broad groups as green, red, and brown are considered to be the highest potential for heavy metal removal due to their high uptake capacities [9–12]. The removal of heavy metals such as nickel, cadmium, and lead from aqueous solution has been achieved effectively by many investigators using various brown seaweeds [13–17]. Thus, we also investigated the mechanisms of nickel, cadmium, and lead biosorption

Correspondence: Dr. S. Yalc¸ın, Faculty of Engineering, Chemistry Department, Istanbul University, Avcilar 34320 Istanbul, Turkey E-mail: [email protected] Abbreviations: D-R, Dubinin–Radushkevich; FTIR, Fourier transform infrared

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using brown marine algae Cystoseira barbata from aqueous solution in our previous work [18]. Up to now, because of having high metal uptake ability, brown algae have been used extensively for removing heavy metals when compared to the others. Nevertheless, limited studies in the literature [19–23] have been reported on biosorption of metal ions using green marine algae. Consequently, additional investigations are required to examine the use of non-living green marine algae for biosorption of heavy metals. The cell walls of marine algae, more commonly known as seaweeds, contain various functionalities such as carboxyl, sulfate, hydroxyl, and amino which can play an important role in metal binding with different mechanisms. It was revealed that biosorption of metal ions by non-living algae comprises a number of multiple sequestering mechanisms, such as ion exchange, complexation, adsorption, electrostatic interaction, chelation, and micro-precipitation [24, 25]. The functional groups of different algae species involved in the biosorption process have been clearly identified by using various techniques such as potentiometric titration, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron (XPS) spectroscopy [17, 26–28]. Due to being indicators of heavy metal contamination [29, 30], the widespread green alga, Entemorpha linza in the macro marine algae was chosen based on its relatively high capacity, reusability, and ready availability in large quantities for the biosorption of Ni(II), Cd(II), and Pb(II) ions in this study. The objectives of the present work are also to identify the functional groups and determine corresponding pKa values of the acidic sites on dried green marine algae E. linza using FTIR and potentiometric titrations, and to explain the biosorption characteristics of biomass towards divalent nickel, cadmium, and lead ions as a function of pH, contact time, biomass dose, and temperature.

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2 Materials and methods

2.5 Batch biosorption procedure

2.1 Materials All chemicals were purchased from E. Merck, Darmstadt, and were analytical reagent grade. Ni(II), Cd(II), and Pb(II) stock solutions-each at 2000 mg L1 concentration were prepared by dissolving the appropriate amounts of Ni(NO3)2, Cd(NO3)2, and Pb(NO3)2 in deionize water and adding HNO3 until final concentration of the acid was 0.1 M. The concentration of each working metal ion solution before and after adsorption was measured using a Varian SpectrAAFS-220 atomic absorption spectrophotometer with an air-acetylene flame for atomization.

Biosorption onto 0.1 g of dried alga was carried out in 250-mL conical flasks containing 50 mL solutions of varying initial metal concentrations (10–1000 ppm). The alga–metal mixture solutions were shaken at 150 rpm for different pH values (2–5.5) and contact times (2–80 min) in a temperature-controlled incubator at 258C. After equilibration of biosorption, the biosorbent was filtered through a 0.45 mm membrane filter, filtrate acidified, and analyzed for residual metal concentration using flame-atomic absorption spectrometry. All experiments were carried out in duplicates with the average presented in the results. Biosorption of the metal ions in the sorption system were determined using the following mass balance equation: qe ¼

2.2 Biomass preparation Samples of the green marine macroalga E. linza were collected from the West Black Sea Coast of Turkey. The raw alga was rinsed several times with deionize water, dried under sunlight, and then sieved to a size range 600–1000 mm. Since CaCl2 pre-treatment agent is one of the most suitable and economic one for the activation of algal biomass, it was preferentially chosen for this study. Therefore, in order to conduct pre-treatment process, a sample of biomass was soaked in 0.2 M CaCl2 solution for 24 h under slow stirring [31]. The solution pH was fixed at pH 5.0 using 0.1 M HNO3 and 0.1 M NaOH solutions. The calcium-treated biomass was washed several times using deionized water to remove excess Ca2þ from the biomass. The biomass was then dried in an oven at 608C for 24 h and stored in polyethylene bottles for biosorption experiments.

2.3 Titration of the protonated biomass A known amount of dried algae was protonated by soaking in 0.1 M HCl with shaking in a rotary shaker at 150 rpm for 3 h. After the protonated process, the biomass was washed thoroughly with deionized water until pH 4.5 was obtained and then dried in an oven at 608C. For each titration, ca. 0.2 g of protonated E. linza biomass was dispersed in 50 mL of 1 mM sodium chloride solution to keep ionic strength constant. Titration was performed by the gradual addition of 0.1 mL of 0.1 M NaOH whilst the suspension was stirred under nitrogen atmosphere. After each addition of NaOH, the solution was ensured to equilibrate and corresponding changes in pH were noted. All pH measurements were recorded using Metrohm Herisau-E-512 pH meter equipped with a glass electrode. It was calibrated against buffer solutions of pH 4 and 7 prior to use.

VðCi  Ce Þ W

where qe is the amount of metal taken up by the biomass, V is the solution volume, W is the amount of biomass, and Ci and Ce are the initial and equilibrium metal concentrations, respectively.

3 Results and discussion 3.1 Potentiometric titration of the biomass samples The protonated biomass was potentiometrically titrated with 0.1 M NaOH. Based on the acid–base titration results shown in Fig. 1, inflection points representing pKa values of acidic groups were evaluated as suggested by Murphy et al. [27]. In other words, the number of dissociable protons in the biomass was estimated based on the number of plateaus on the titration curve. Each plateau was corresponded to a buffer system which gave the pKa value. However, the inflection points were more accurately determined using first derivative plots of average pH titration data. The first derivative plots of midpoint of successive quantities of NaOH added (x-axis) against dpH/dV (y-axis) enabled to read the position of each apparent peak on the x-axis which gave the value of acidic groups on the biomass. The first peak in Fig. 2 gave the density of strong acidic groups

2.4 Fourier transform infra-red spectroscopy The chemical functional groups of algae samples (protonated, Ni-, Cd-, Pb-loaded) were analyzed in KBr tablets with the aid of a Mattson 1000 FT-IR spectrometer. Since the raw alga is actually loaded with alkali and alkaline earth ions (Naþ, Kþ, Mg2þ, and Ca2þ) originally present in sea water, it exhibited nearly the same spectrum as heavy metal-loaded alga. Therefore, the spectrum of raw alga was not presented in the infrared spectra.

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Figure 1. Potentiometric titration curve of protonated biomass.

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protonated biomass with pKa ¼ 3.82 was attributed mainly to carboxyl groups of alginic acid. In the pH interval 3.5–5.5, carboxyl groups on the biosorbent produce a negatively charged surface which is responsible for metal sorption by electrostatic interaction. It was reported that the carboxylic acid dissociation constants of alginic acid called mannuronic (M) and glucuronic (G) acids were determined as pKa ¼ 3.38 and 3.65, respectively [34]. As previously mentioned, because of having the highest percentage of titratable sites in dried algal biomasses, carboxylic acid groups play a key role in the complexation of heavy metals [18, 25, 26]. At lower pH, sulfonic acid groups can generally become dominant in metal uptake and their characteristic pKa values are in the range of 1.0– 2.5 [35]. An obvious pKa value in this range was not measured by titration, but the existence of sulfonate groups on the surface of biomass was later indicated by FTIR analysis. According to the acid– base titration curve of the biomass, the pKa values at 6.63 possibly indicated the presence of protonated amino groups.

3.2 Fourier transform infrared spectroscopy Figure 2. First derivative potentiometric titration curve of protonated biomass.

(0.25 mmol g1) and the highest final peak gave the total density of acidic groups (0.95 mmol g1). Hence, the density of weak acidic groups was found by difference. The pKa values, which also showed the amount of acidic groups on the biomass surface, were then concluded from the titration curve (Fig. 1) as 3.82 and 6.63, respectively. It was revealed that protonated E. linza contained a number of acidic functionalities such as carboxyl groups and some free amino acids (aspartic acid, glutamic acid, alanine, serine, etc.) like all other green marine algae [32]. Taking into consideration of this complicated structure of the biomass, the results were also evaluated by Lodeiro et al. [33]. The peaks observed in the range of pH 3–6 gave the total amount of total carboxyl groups considered to be main responsible for metal binding process. The detected weak acidic group in

FTIR analysis was performed with protonated and metal-loaded E. linza to assign groups responsible for the biosorption. The spectra (Fig. 3) show a number of absorptions, indicating the complex nature of the examined biomass. The broad band observed around at 3417 cm1 represented —OH and —NH groups. A weaker band at about 2926 cm1 could be related to the —CH2 groups. Protonated E. linza gave rise to absorbance bands at 1729 cm1 (free C — — O), 1 1650 cm1 (asymmetric C — — O), 1257 cm indicating carboxyl functionalities [11, 22]. The spectra of protonated biomaterials typically exhibit an absorbance peak at 1740 cm1 representing the stretching of the carbon–oxygen bond in the carboxyl functional [26]. Compared with the spectrum of the protonated species, considerable changes were observed in the spectra of metal-loaded E. linza resulting from divalent metal ion–biomass interactions. After contact with metal ion solutions, no band was observed at 1729 cm1 while the loaded biomass showed a remarkable increase in the intensity of the bands at 1648–1647 cm1. The disappearance of the band at 1729 cm1 was typical of complexation of the carbonyl group by

Figure 3. FTIR spectra of E. linza biomass before and after adsorption (a) acid treated (b) Ni-loaded (c) Cd-loaded (d) Pb-loaded.

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dative coordination with metal ions [26]. Although high level amount of alginate content is known as a characteristic specificity for brown algae, it has also previously been reported to be abundantly found on the surface of green marine algae E. linza [36]. It was obvious that the bands about at 1648 and 1419 cm1 were in coherence with the observation made on metal-alginate-based resin. The presence of these two bands indicated that the algal biomass contained significant amount of alginate [35]. The existence of carboxyl, amino, and sulfate groups on the cell wall polymers of green algae was also notified by other investigators [37]. The band observed at about 1534 cm1 indicated to —NH stretching of amide group representing seaweed protein. After the biosorption process, there were also a clear band shifts and intensity decrease of the —NH band at 1534 cm1. However, the new peak occurred at about 1419 cm1 indicated to C—O stretching in carboxylic acid group which was involved with metal ions sorption [35]. Absorbance peaks at around 1378, 1156, and 1052 cm1 were attributed to asymmetric and symmetric stretching of —SO3 bonds in sulfonic acid and to —C—O stretching of alcoholic groups, respectively. The band observed at about 1325 cm1 was assigned to the C—N stretching of amine group. Absorbance peaks at around 846 cm1 may correspond to S — — O bands [38]. The assignments of FTIR bands observed by various authors and their corresponding wavenumbers are listed in Table 1 [26, 27, 35, 38, 39].

3.3 Effect of pH Figure 4 shows the effect of pH on the biosorption of nickel, cadmium, and lead ions onto E. linza biomass from aqueous solution. Experiments were carried out up to pH 5.5, since possibly some precipitation of metal could have occurred at higher pH values in the light of solubility equilibria calculations. As can be seen from the figure, the yield of biosorption was strongly affected by changes in the solution pH, probably due to the cell wall of E. linza which contains high level of carboxyl groups from mannuronic and glucuronic acids. Since carboxylic protons essentially dissociate in the pH range 3.0–5.0, these functionalities are expected to play a major role in metal ion uptake at relatively higher pH. Thus, metal sorption percentage initially showed a steep rise in the pH range of 3.0–4.5, and almost achieved the maximal value for Ni(II), Cd(II), and

Table 1. Characteristic infrared stretching frequencies of seaweed

Wavenumber (cm1) 3280 2920 2854 1740 1630 1530 1450 1371 1237 1160 1117 1033 817 a) b) c) d)

Assignment Bonded —OH, —NH stretchinga) Asymmetric stretch of aliphatic chains (—CH)b) Symmetric stretch of aliphatic chains (—CH)b) c) C— — O stretch of COOH c) Asymmetric C — —O Amide IIa) c) Symmetric C — —O Asymmetric —SO3 stretchingd) C—O stretch of COOHc) Symmetric —SO3 stretchingd) C—O (ether)a) C—O (alcohol)a) d) S— — O stretch

[35]. [39]. [26]. [38].

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Figure 4. Effect of pH on the biosorption of Ni(II), Cd(II), and Pb(II) by E. linza biomass (metal concentration: 25 mg L1, respectively).

Pb(II) ions in the pH 5.0–5.5 range. At low pH values, the decrease in biosorption levels could be explained by an increase of competition between hydrogen ions and metal cations for binding active sites of biomass. In this case, the biomass surface was more positively charged and biosorption rates decreased. On the contrary, when the pH increased, since biomass surface became more negatively charged, optimal metal uptake rates were acquired at around pH 5. On the other hand, sulfonate-donating groups may have played an important role especially in the sorption of Pb(II) and Cd(II) metals at lower pH (2) [35].

3.4 Effect of contact time The effect of contact time on the adsorption of Ni(II), Cd(II), and Pb(II) was performed at different time intervals in the range of 2–80 min using 60, 120, 220 mg L1 working solutions for nickel, cadmium, and lead, respectively. After they were filtered, the solutions were taken for metal ion analysis using atomic absorption spectrometry. As shown in Fig. 5, the equilibrium adsorption was established rapidly within 5– 10 min, indicating that the initial adsorption was very fast and maximum uptake was reached within 10 min for nickel, cadmium, and lead. This rapid binding of metal ions by biomass can be explained as a result of the formation of exterior surface complexes neglecting intraparticle diffusion as described by Hatje et al. [40]. Since biotechnological processes are required based considerably on chemisorption with very fast kinetic mechanism for wastewater treatment, these high rapid rates of biosorption are very advantageous. In order to make sure that equilibrium conditions were attained, contact time was chosen as 20 min for further experiments.

3.5 Effect of biomass dose The effect of biosorbent dose for Ni(II), Cd(II), and Pb(II) biosorption on Ca-pre-treated E. linza was investigated using various biomass dose (0.05–0.35 g) at pH 5 and 100 mg L1 initial metal ion concen-

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Figure 7. Effect of pre-treatment on Ni(II), Cd(II), and Pb(II) biosorption by E. linza (metal concentration: 60, 120, 220 mg L1, respectively).

Figure 5. Effect of contact time on the biosorption of Ni(II), Cd(II), and Pb(II) by E. linza biomass (metal concentration: 60, 120, 220 mg L1, respectively).

trations. Experimental results indicated that biosorption efficiency increased with increasing biomass dose. However, biosorption capacity per unit mass decreased with increasing biomass dose, as can be seen in Fig. 6. This most probably resulted from unsaturated adsorption sites through the sorption process. Biosorption capacity decreased from 20.8, 60.5, and 76.0 mg g1 to 7.9, 14.2, and 21.0 mg g1 for nickel, cadmium and lead, respectively. Therefore, optimum biomass dose was chosen as 0.1 g for all the biosorption experiments.

dehyde are applied to increase the stability and adsorption capacity of biomass as well as eliminate available cations. In this study, calcium-treated biomass, compared to the natural (raw) and acidtreated biomass, exhibited higher biosorption efficiency rates approximately 10 and 15% in an average removal percentage for all the tested metal ions, respectively, as in Fig. 7. In the case of acidtreatment, since the activated binding sites were occupied by hydrogen ions, metal ions were exposed to compete with Hþ ions for binding the same sites even at relatively high pH (4–5) values. Therefore, the decrease of metal binding efficiency can be explained in terms of ion-exchange concept suggested by Davis et al. [25]. According to this, protonated acidic functional groups on the surface of biomass release Hþ to the solution by the replacement of metal ions during the ion exchange process. In addition, as CaCl2 treatment agent has an ability to form cross-link with carboxyl groups of alginate which are present on the biomass cell wall, calcium treated E. linza exhibited a higher metal binding capacity in comparison with raw biomass.

3.6 Effect of calcium pre-treatment Various pre-treatment processes such as protonation (HCl), alkalinization (NaOH), and chemical cross-linking with CaCl2 or formal-

3.7 Biosorption equilibrium The experimental data were evaluated according to the Langmuir, Freundlich, and Dubinin–Radushkevich isotherm (D–R) models [41]. The Langmuir model is: qe ¼ qm

Figure 6. Effect of biomass dose on biosorption of Ni(II), Cd(II), and Pb(II).

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bCe 1 þ bCe

where qm (mg g1) is the monolayer biosorption capacity of the biomass, and b (L mg1) is a constant related to heat of adsorption and affinity to binding site, respectively. Figure 8a–c demonstrates the Langmuir adsorption isotherm of nickel, cadmium, and lead ions on E. linza biomass (0.1 g) at pH 5 for 20 min, respectively. In order to avoid some precipitation of metal ions (especially Pb(II)), the experiments were performed at pH 5.0. As shown in Fig. 8a–c and Table 2, the individual metal binding capacities of the biomass (on a mass basis) increased in the following order: Pb > Cd > Ni. However, the same order on a molar basis was Pb ¼ Cd > Ni, with capacities laying between 0.95 (Pb, Cd) and 0.70 mmol g1 (Ni). Cadmium, and with a greater affinity lead ions, may be assumed to bind to carboxyl groups providing negatively charged electrostatic and coordinative interactions (bidentate complex formation) at pH 3–5 [26]. The sorption isotherm for Pb(II) was considerably steeper (i.e., showed a higher rate of increase of qe with Ce) than for Cd(II) and Ni(II), indicative of

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the stronger affinity of carboxylic groups towards Pb(II) ions. The adsorption process was favorable for all the tested metal cations, showing Langmuir character. The preferential sorption order of Pb > Cd > Ni onto E. linza could be explained by some metal properties, such as ionic radii, electronegativity, and softness. This was also supported by Bakir et al. [9] that algal polysaccharides with carboxylate groups show preferential binding of cations with large ionic radii. The Langmuir parameters qm (maximum sorption capacity, mg g1) and b (Langmuir intensity constant, L mg1) for Ni, Cd, and Pb are presented in Table 2). The Langmuir constants for biosorption of all the tested metal ions by different green marine algae are also listed in Table 3. As distinct from similar green marine algae, E. linza exhibited relatively higher metal uptake rates. On the other hand, as can be seen from Fig. 8a–c, a decrease in the biosorption capacity of the biomass was observed from 41.14 to 40.04 mg g1 for Ni(II) ion, from 107.09 to 100.58 mg g1 for Cd(II) ion, and from 197.78 to 193.54 mg g1 for Pb(II) with increasing temperature from 25 to 458C. This can be attributed to an increasing tendency to desorb metal ions from the interface to the solution at higher temperatures [23]. Therefore, all the biosorption experiments were conducted at 258C. The Freundlich isotherm model was also applied to explain biosorption equilibrium data. The linearized Freundlich model is: 1 log qe ¼ log KF þ log Ce n where KF (L g1) is a constant related to extent of metal ion removal and n is an empirical parameter which gives biosorption intensity. The constants KF and n for the tested metal ions are presented in Table 4. Values of n > 1.0 indicated the intensive metal biosorption by E. linza. However, Langmuir isotherm model fitted the experiment data better than the Freundlich isotherm model for Cd(II) and Pb(II) ions considering correlation coefficients (R2). In the case of nickel, both of isotherm models were well-fitted the equilibrium data, as can be seen from Tables 2 and 4. D–R isotherm was also applied to determine the mean free energy of biosorption (E) which indicates biosorption processes to be either physical (E < 8 kJ mol1) or chemical (E ¼ 8–16 kJ mol1). The linearized form D–R isotherm is represented as follows [41]: ln qe ¼ ln qm  b"2 where b is a constant which gives the mean biosorption energy (mol2 J2), qm is the theoretical saturation capacity (mol g1), and Table 2. Langmuir isotherm parameters for nickel, cadmium, and lead biosorption onto E. linza

Temperature (8C)

Figure 8. Langmuir isotherms for the biosorption of Ni(II), Cd(II), and Pb(II) onto E. linza biomass (a: Ni(II), b: Cd(II), c: Pb(II) divalent cations, pH 5 (constant); 50 mL solution contacted with 0.1 g biomass for 60 min, at 25, 35, 458C, respectively).

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Ni(II) 25 35 45 Cd(II) 25 35 45 Pb(II) 25 35 45

qm (mg g1)

b (L mg1)

R2

41.14 40.50 40.04

0.024 0.017 0.013

0.998 0.998 0.996

107.09 104.49 100.58

0.031 0.022 0.018

0.996 0.995 0.996

197.78 194.94 193.54

0.039 0.027 0.020

0.997 0.999 0.999

Incubation time 20 min, pH 5.

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Table 3. Comparison of Langmuir parameters for Ni(II), Cd(II), and Pb(II) biosorption on some reported green marine macroalgae

Metal

qm

Alga mg g

Ni(II)

Cd(II)

Pb(II)

a)

Codium vermilara Ulva reticulataa) Ulva sp. Codium taylori Codium vermilara Ulva sp. Ulva lactuca Chaetomorpha linum Caulerpa lentillifera Codium vermilara Ulva sp. Ulva lactuca Caulerpa lentillifera Cladophora fascicularis Valoniopsis pachynema Cladophora glomerata

1

b mmol g

13.20 46.51 17.02 5.80 21.80 65.19 29.20 53.95 4.70 63.30 302.51 34.70 28.72 198.50 83.33 73.5

1

0.22 0.79 0.29 0.10 0.19 0.58 0.26 0.48 0.04 0.30 1.46 0.17 0.14 0.96 0.40 0.35

1

L mg

0.09 – 0.03 0.05 0.10 0.01 0.07 0.01 0.07 0.11 – 0.05 0.07 0.04 0.19 0.22

Refs. 1

L mmol 5.34 – 1.58 29.06 11.15 1.45 1.12 1.43 8.35 23.45 1.11 10.36 14.87 8.29 39.57 44.75

[12] [20] [35] [42] [12] [32] [23] [10] [43] [12] [35] [23] [43] [21] [19] [13]

Column process.

e is the Polanyi potential (e ¼ RT ln(1 þ 1/Ce) related to the equilibrium concentration. b and qm values were determined from the slope and intercept of the straight line plots of ln qe against e2, respectively and are presented in Table 4. The mean of free energy (E) was evaluated using the relation given as below: 1 E ¼ pffiffiffiffiffiffiffiffiffi 2b According to D–R isotherm, the values of E were found to be 12.1 and 12.3 kJ mol1 for cadmium and lead, respectively corresponding to chemisorption whereas it was found to be 3.2 kJ mol1 for nickel representing physical nature of the biosorption process. These values were correlated with thermodynamic parameter (DH0) except for nickel. Biosorption of nickel was evaluated as the physisorption together with chemisorption based on the value of DH0 (24.41 kJ mol1) [21]. D–R isotherm is often used to find out the mean biosorption energy which presents indicative information on type of biosorption mechanism, physical or chemical, rather than biosorption capacity of biomass. However, in the case of nickel, the equilibrium data were better fitted into the Freundlich and Langmuir isotherm models due to the higher R2 values compared to D–R isotherm, as can be seen in Tables 2 and 4. According to the hard and soft acid and base theory (HSAB), metal ions (as Lewis acids) are classified as hard and soft, like Lewis base ligands. Hard ions are characterized by mainly electrostatic interactions with oxygen-containing ligands, and soft metals are charac-

terized by mainly covalent or coordinative interactions with sulfur or nitrogen-containing ligands [17]. Cadmium and lead are soft and intermediate to soft (borderline) metals, respectively, whereas nickel is an intermediate to hard metal. Since lead has a higher affinity than cadmium for harder ligands containing sulfur or nitrogen atoms, it may be more sensitive to low concentration of sulfonate groups. These properties may account for sulfonate contribution to metal (essentially lead) binding, which is small but could nevertheless be significant at low pH [26]. Sulfonic acids, being strong acids, are completely ionized at low pH, enabling primarily ionexchange binding of metal cations via electrostatic interaction. The enthalpy value of adsorption (DH0) was calculated from (ln b) ¨ tem et al. versus (1/T) curves (from data in Table 2), as described by Tu [44] and the values were 24.41, 22.53, and 25.66 kJ mol1 for nickel, cadmium, and lead, respectively. The negative values of DH0 indicated that the biosorption process was exothermic. This was also confirmed by the decrease of biosorption capacity with the increasing temperature. However, it should be noted that the rate of decrease in biosorption capacity was quite low for all the tested metal ions, as given in Table 2. The magnitude of biosorption heat value gives indicative information on the type of biosorption, which can be either physical or chemical. It is said that the enthalpy for physisorption is between 2.1 and 20.9 kJ mol1, the physisorption together with chemisorption is at the range of 20.9–80 kJ mol1 and chemisorption is at the range of 80–418.4 kJ mol1 [21, 45]. Based on the values of DH0 given above, it can be deduced that the biosorption process took place both physically and chemically for Ni, Cd and Pb. Gibbs free energy change (DG0) was found 17.96, 20.28, and

Table 4. Freundlich and D–R isotherm parameters for nickel, cadmium and lead biosorption onto E. linza

Metal

Ni(II) Cd(II) Pb(II)

Freundlich constants

D–R constants

KF (L g1)

n

RF2

qm (mg g1)

b (mol2 kJ2)

R2DR

E (kJ mol1)

2.678 10.970 21.262

2.011 2.563 2.687

0.994 0.964 0.954

29.38 96.91 436.67

4.85361E-08 3.42115E-09 3.31713E-09

0.876 0.984 0.985

3.2 12.1 12.3

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Table 5. Reusability efficiency of E. linza biomass

Cycle number

2 4 6 8

% Biosorption Ni(II)

Cd(II)

Pb(II)

97 96 96 94

98 97 95 96

97 98 96 96

Operational capacity was measured under the experimental conditions without reaching saturation.

22.28 kJ mol1 for the biosorption of Ni(II), Cd(II), and Pb(II) at 258C, respectively. The negative (DG) values of biosorption revealed that the process was feasible and spontaneous for all tested metal ions. Entropy change (DS0) was calculated to be 21.45, 7.74, and 11.47 J mol1 K1 for the biosorption of Ni(II), Cd(II), and Pb(II) at 258C, respectively. The negative DS0 shows a decrease in the randomness at the solid/solution interface during the biosorption process. The adsorption process is favored by the negative free energy changes (DG < 0) accompanying the release of coordinated water molecules from the primary coordination sphere of adsorbed metal cation into aqueous solution. As for the prospects of using E. linza in industrial wastewater treatment, it can be mentioned that Ni(II), Cd(II), and Pb(II) are three important toxic heavy metal ions that are frequently encountered together in wastewater of fine chemical processes or battery industries [46]. It was shown in this study that E. linza had a higher Pb retention capacity than most microbial and plant derived biomass used in similar studies [47]. As opposed to the simple ion-exchanger sorbents that may show fouling and oversaturation in the treatment of high metal-load wastewaters, non-living biomass as ion exchangers may fairly rapidly retain metals (within minutes to hours) from both dilute and concentrated solutions [47]. In the case of this study, as the biosorption kinetics were very fast, 15 min of solution–sorbent contact time was sufficient to reach equilibrium for all the tested metal ions. The Langmuir isotherm adsorption parameters allow to reach quite low solution concentrations of metal ions in effluents of wastewater treatment plants.

3.8 Reusability of E. linza biomass Following biosorption process with 25, 100, and 100 mg L1 concentration for Ni(II), Cd(II), and Pb(II), respectively, elution was achieved using 15 mL of 0.1 M HNO3 for 15 min, with success of approximately 98%. After elution of bound metal ions, biomass was washed with water repeatedly and then regenerated with 0.2 M CaCl2 (15 min contact time) before metal loading. The change in operational batch capacity of the E. linza over repeated cycles of biosorption and elution was within experimental error (see Table 5), meaning that sorption capacity loss could be negligible with extended use [13].

4 Conclusions The functional groups and the corresponding pKa values of the acidic sites on dried green algae E. linza were found using FTIR and potentiometric titrations, and its biosorption ability towards divalent nickel, cadmium, and lead ions was investigated. Adsorption was studied as a function of solution pH and contact time, and exper-

ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

imental data were modeled with the aid of Langmuir, Freundlich, and D–R isotherms. The capacity of the biomass was found 41.14, 107.09, 197.78 mg g1 for nickel, cadmium, lead, respectively. Regarding the results of potentiometric titration, carboxyl groups which were equal to 0.7 mmol g1 biomass played a key role in binding of the heavy metals by biomass. Thermodynamic parameters obtained from Langmuir constant showed that nature of the biosorption process was feasible, exothermic, and spontaneous. In addition, since enthalpy values were found as 24.41, 22.53, and 25.66 kJ mol1 for Ni(II), Cd(II), and Pb(II), respectively, biosorption was caused by both physical and chemical forces for all the tested metal ions. This was also supported by D–R isotherm results for particularly Cd(II) and Pb(II) with values of 12.1 and 12.3 kJ mol1, respectively. Considering its extremely high abundance and low cost, E. linza may be potentially important in metal ion removal from contaminated water and industrial effluents. The favorability of the Langmuir isotherm adsorption parameters for all the tested metal cations, the high capacity of the biomass for metal sorption, combined with fast kinetics of sorption offer good prospects for the use of E. linza biomass for industrial wastewater treatment.

Acknowledgments I would like to thank MSc Chemist Semih Sezer for laboratory ¨ nlu ¨ for the collection and assistance and Assoc. Prof. Dr. Selma U classification of the biomass. This research was supported by Istanbul University Scientific Research Fund (Project grant No: UDP-24290/2012 and 14447/2011). The author has declared no conflict of interest.

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