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Removal of anionic arsenate by a PEI-coated bacterial biosorbent prepared from fermentation biowaste
Chemosphere 226 (2019) 67e74
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Removal of anionic arsenate by a PEI-coated bacterial biosorbent prepared from fermentation biowaste Namgyu Kim a, Munsik Park a, Yeoung-Sang Yun b, Donghee Park a, * a
Department of Environmental Engineering, Yonsei University, 1 Yonseidae-gil, Wonju, 26493, Republic of Korea Division of Chemical Engineering, Nanomaterials Processing Research Center, Chonbuk National University, 567 Baekje-daero, Jeounju, 54896, Republic of Korea b
h i g h l i g h t s Anion biosorbent prepared from fermentation biowaste was used for As(V) removal. The biosorbent had 62.99 mg/g of As(V) uptake, which were higher than others. Solution pH was the most important parameter which affected As(V) removal. 0.01 M NaOH solution was chosen as As(V)-desorbing eluant.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2019 Received in revised form 14 March 2019 Accepted 16 March 2019 Available online 19 March 2019
As a problematic element in water systems, arsenic exists as As(III) and As(V). Adsorption techniques can be used to remove anionic As(V) as it is present as a polyatomic anion. In the case of As(III) which exists in zero-valent state under neutral pH, it can be also removed by adsorption after being converted into As(V). Many inorganic and organic materials have been examined as potential adsorbents for anionic As(V) removal. However, most exhibit relatively low adsorption capacities (<10 mg/g). The objective of this study is to examine As(V)-removal mechanism and practical potential of a PEI-coated bacterial biosorbent prepared from fermentation biowaste. The maximum As(V) uptake of the biosorbent was determined to be 62.99 mg/g by Langmuir model. The effects of various parameters including pH, biosorbent dosage, ionic strength and temperature were also examined. Kinetic and equilibrium models were used to interpret the experimental data mathematically. A 0.01 M NaOH solution was chosen as an effective As(V)-desorbing eluent for biosorbent regeneration. The adsorption capacity of the biosorbent remained above 85% over three successive cycles of adsorption and desorption. In conclusion, the biowaste-driven biosorbent is a promising anion adsorbent for treatment of As(V)-contaminated wasters. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Y Yeomin Yoon Keywords: arsenate Bacterial biosorbent Biosorption Chitosan Polyenylenimine
1. Introduction Arsenic is a toxic element in water systems. Some environmental arsenic problems originate from metallurgical industries, alloy materials and pesticides manufacture process. Arsenic is also the 20th most widely distributed element in the Earth's surface d it occurs naturally and is widely distributed in mineral rock sediments. Unlike most other cationic metals, arsenic is present in water systems as a polyatomic anion containing oxygen (Sharma and Sohn, 2009). In aquatic systems, arsenic exists primarily as two oxidation-state
* Corresponding author. E-mail address: [email protected] (D. Park). https://doi.org/10.1016/j.chemosphere.2019.03.113 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
species, arsenite As(III) and arsenate As(V). The mobility and toxicity of arsenic are determined by its oxidation state. Arsenite predominates in moderately reducing anaerobic environments such as groundwater, whereas arsenate generally predominates in the oxidizing conditions typical of seawater, lakes, and rivers. Arsenite is more mobile and toxic than arsenate (Smedley and Kinniburgh, 2002; Bandpei et al., 2017; Tiwari et al., 2017). Arsenic dissolved in water is acutely toxic and can lead to a number of health problems. Short-term exposure can cause stomach pain, nausea, vomiting, and diarrhea. Long-term exposure to arsenic can cause skin, lung, bladder, and kidney cancer (Sharma et al., 2014). As a result, regulatory authorities such as the World Health Organization and the US Environmental Protection Agency have tightened the maximum contamination level of total arsenic in
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drinking water from 50 to 10 mg/L (Smith et al., 2003). In many countries, including Bangladesh, China, Vietnam, and the US, arsenic exposure through drinking water is a major environmental issue, and researchers have been looking for efficient methods of treating arsenic-contaminated waters (Mueller, 2017; Rahman et al., 2018). Conventional techniques capable of removing heavy metals that can be used to treat arsenic-contaminated waters include coagulation, precipitation, adsorption, and electrodialysis (Choong et al., 2007; Verma and Sharma, 2017). Among them, adsorption is getting increasingly popular for arsenic removal owing to simple process, ease of operation and handling, adsorbent regeneration and no sludge generation (Park et al., 2010; Choi et al., 2019). Using adsorption technique, anionic As(V) can be easily removed from aqueous solutions compared to As(III) which exists in zero-valent state under neutral pH condition. As(III) can be removed through adsorption technique after being converted into anionic As(V) by pre-oxidation. In treating arsenic-contaminated waters, therefore, it is important to develop or find anion adsorbents that are inexpensive and commercially available. Table 1 shows adsorption capacities of various (in)organic adsorbents reported recently. Most of the adsorbents have a less than 10 mg/g capacity for anionic As(V), which is low compared with those for other cationic metals. Some of the highest adsorption capacities for cadmium, lead and mercury were reported as follows: 215 mg Cd/g seaweed, 558 mg Cd/g chitosan, 1587 mg Pb/g lignin and 1123 mg Hg/g chitosan (Bailey et al., 1999). Recently, a few researchers have reported higher As(V)-adsorption capacities using nano-scale inorganic adsorbents and functionalized (in) organic adsorbents (Table 1). Wang et al. (2014) synthesized a nano-scale adsorbent by hybridizing zero-valent iron and reduced graphite oxide. The inorganic adsorbent exhibited an adsorption capacity of 29.04 mg/g for As(V). Anirudhan and Unnithan (2007) modified coconut coir pith into an anion exchanger with an amine group and reported 13.57 mg/g of maximum uptake of As(V). Recently, Kwak et al. (2013) developed a commercially available polyethylenimine (PEI)-modified bacterial biosorbent in the form of a fiber with promising potential for binding anionic metals such as ruthenium and gold. The maximum adsorption capacities of these anions were 110.5 mg/g and 421.1 mg/g, respectively (Kwak et al., 2013; Park et al., 2012). As there is no report on As(V) removal by this biosorbent, we studied the As(V) removal potential of the anion biosorbent under various conditions. Kinetic and equilibrium experiments were conducted to elucidate the As(V)removal mechanism of the biosorbent, and various models were used to fit the kinetic and equilibrium data. Desorption and regeneration studies were also performed to assess the reusability of the biosorbent. 2. Experimental 2.1. Preparation of the biosorbent Fermentation biowaste containing Corynebacterium glutamicum
was supplied by Daesang Corporation (Gunsan, Korea) in a slurry form with a moisture content of 85%. To manufacture a fiber biosorbent, chitosan was used as a binder to immobilize the microbial biomass. PEI was used to enhance the cationic functionality of the biosorbent. The detailed procedure can be reviewed in Kwak et al. (2013). Finally, PEI-coated biosorbent fibers 0.2e0.5 mm thick and 5e10 mm long were used for As(V) removal. 2.2. Reagents Pure analytical-grade arsenate solution was prepared by dissolving Na2HAsO4$7H2O (Sigma-Aldrich Corp, USA) in distilled water. All chemicals used in this study, including chitosan (molecular weight: 3.0 105 g/mol) and PEI (molecular weight ¼ 70,000 g/L), were of analytical grade. 2.3. Batch adsorption experiments The kinetics of As(V) biosorption was investigated at pH values from 2.0 to 6.0. The solution pH was adjusted with diluted HCl, H2SO4, and NaOH solutions. The biosorbent dosage was 1.0 g/L: 0.2 g of biosorbent was added to a plastic bottle containing 200 mL of arsenate solution (50 mg-As/L) with its respective pH value. The bottle was horizontally agitated on a shaker at 200 rpm for 2 h at room temperature (20e25 C). In batch experiments to examine the effects of pH and biosorbent dosage, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 g/L of each biosorbent were added to 50 mg/L of As(V) solution at pH 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8, and 4.0. The effect of competing anions (Cl and SO2 4 ) on As(V) adsorption on the biosorbent was investigated with solutions containing NaCl and Na2SO4 at concentrations from 1 to 5 mM. The effect of temperature was studied from 5 to 35 C. Except in the pH and dosage experiments, the initial pH of the As(V) solution was 3.0. The procedure for the equilibrium studies was essentially the same as that for the kinetic studies, with 0.2 g of biosorbent added to 200 mL As(V) solution in the range of 50e500 mg/L. The solution was maintained at pH 4.0 using HCl during the batch equilibrium experiment; an equilibrium state was reached in 1 h. Samples were intermittently removed from the bottles to analyze As(V) concentration, following appropriate dilution with deionizededistilled water. It was confirmed from three independent replicates that the batch experiments were producible within at most 5% error in this study. 2.4. Desorption experiment of arsenate loaded on the biosorbent The As(V)-laded biosorbent was produced through contact with 50 mg/L As(V) solution at pH 3.0 for 1 h. After adsorption, the As(V)loaded biosorbent was separated and washed lightly with deionized water. The 0.01 M NaCl, Na2SO4, and NaOH solutions were used as eluents to desorb As(V) from the biosorbent. The regenerated biosorbent was resuspended in 50 mg/L As(V) solution for the next
Table 1 Maximum adsorption capacity (qmax) of anionic As(V) by various adsorbents. Adsorbents
pH
Temp. (oC)
Maximum capacity (mg-As/g)
Reference
Iron oxide coated sand Activated carbon produced from oat hulls Methylated biomass Functionalized coconut coir pith Coconut shell carbon Nanoscale zero valent iron PEI coated biosorbent fiber
7.6 5.0 6.5 7.0 5.0 7.0 4.0
22 25 30 20 25 25 20
0.034 3.08 3.75 13.57 2.4 29.04 62.99
Thirunavukkarasu et al. (2003) Chuang et al. (2005) Seki et al. (2005) Anirudhan and Unnithan (2007) Lorenzen et al. (1995) Wang et al. (2014) This Study
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adsorption run. The adsorption/desorption steps described above were repeated three times. 2.5. Analytical methods The arsenate concentration of each sample was determined by inductively coupled plasma-atomic emission spectrometry (ICP/ IRIS, Thermo Jarrell Ash Co., USA) after being filtered through a 0.20 mm membrane. 3. Results 3.1. Kinetic study of As(V) biosorption at various pHs Contact time, dosage, solution pH, temperature, and ionic strength affect adsorption rate, uptake amount, and even selectivity for target removal (Park et al., 2010). Among these, solution pH is the most important factor for As(V) removal as it determines the species of the As(V) in aqueous solution. The distribution of As(V) species as a function of pH is given by the following equations (Smedley and Kinniburgh, 2002): þ H3AsO4 4 H2AsO 4 þ H3O , pK1 ¼ 2.19
(1)
2 þ H2AsO 4 4 HAsO4 þ H3O , pK2 ¼ 6.94
(2)
3 þ HAsO2 4 4AsO4 þ H3O , pK3 ¼ 11.50
(3)
2 Based on its pKa values, the As(V) species are AsO3 4 , HAsO4 , H2AsO and H AsO . As(V) usually exists as anions with a charge 4 3 4 of 1 and 2 in As(V)-contaminated waters. To examine the effect of pH on As(V) biosorption, batch kinetic experiments were conducted at various pH conditions. Kinetic studies can supply detailed information on the adsorption behavior of adsorbates and adsorption mechanisms and can be useful in determining the optimal contact time for successful batch biosorption processes (Park et al., 2007; Choi and Yu, 2019). Fig. 1(a) and (b) show the removal behavior of As(V) at pH values from 2.0 to 6.0. As shown in Fig. 1(a), the biosorbent significantly removed anionic As(V) from the aqueous solution. The removal behavior of As(V) differed according to pH. The maximum removal of As(V) was obtained at constant pH 6.0, where equilibrium was reached within 2 h. The biosorbent has abundant amine and hydroxyl groups owing to the presence of chitosan and PEI, which were used for its manufacture (Park et al., 2012). Amine groups having pKa values of 6.5 are effective at binding anionic metals (Reddad et al., 2002; Dotto et al., 2017). In this study, the biosorbent effectively adsorbed anionic As(V) in HAsO2 4 and H2AsO4 . As the solution pH was decreased, As(V)-removal efficiency of the biosorbent decreased and it was hardly removed at pH 2 (Fig. 1(a)). In other research on As(V) biosorption, the adsorption rate and uptake amount increased with decreasing solution pH (Lin and Wu, 2001; Dotto et al., 2017). Solution pH typically affects the surface charge of adsorbent, the degree of ionization, and the speciation of surface functional groups. Because amine and hydroxyl groups of adsorbents can be effectively protonated with positive charges at low pH and can attract anionic metals in aqueous solutions, low pH is known to favor anionic As(V) removal. Genç-Fuhrman et al. (2005) used an adsorbent derived from red mud to remove As(V) and reported an increase in As(V) uptake with decreasing pH; from 1.64 mg/g at pH 7.1 to 3.32 mg/g at pH 4.5. When activated alumina grains was used as adsorbents, As(V) uptake increased from 9.93 mg/g at pH 7.2e12.34 mg/g at pH 2.6 (Lin and Wu, 2001). In addition to the contrary As(V)-removal behavior of our
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biosorbent at low pH to those of other adsorbents reported in the literature, an unanticipated phenomenon was evident at low pH. At a constant pH of 4.0, the concentration of As(V) in aqueous phase sharply decreased for 0.5 h and then gradually increased, indicating that As(V) adsorbed on the biosorbent was released into the aqueous phase. The unexpected release of As(V) from the biosorbent was due to breakdown of the biosorbent itself at a low pH. In particular, the biosorbent was broken down rapidly at pH 2 within several minutes. It is possible this is because chitosan polymers exhibit poor acid resistance in even dilute HCl, HNO3, and H3PO4 solutions (Dash et al., 2011; Wan Ngah et al., 2011). The structural breakdown of the biosorbent containing chitosan caused the release of adsorbed As(V) as well as the poor As(V) uptake at low pH (<4). As a result, an optimal pH was reached in the pH-stat kinetic experiment, in which HCl was used to maintain the solution pH at a fixed value. Although the final efficiency of As(V) removal peaked at pH 6, the initial removal rate at pH 6 was lower than that at pH 4, indicating that pH 4 would be more favorable for As(V) removal than pH 6 if not for the structural breakdown of the biosorbent. It is therefore important to control the solution pH to optimize both adsorption rate and amount. To examine the relationship between solution pH and As(V) removal depending on structural stability of the biosorbent, a pHshift kinetic study was conducted without pH maintenance during the batch experiment, in which the HCl solution was only used to adjust the initial pH to desired values (2.0, 3.0, 4.0, and 6.0). As in Fig. 1(b), As(V) removal proceeded up to approximately 95% within 1 h, and an equilibrium state was reached in 2 h of contact time. The solution pH increased during As(V) removal by the biosorbent: from pH 2.00 to 3.52 and from 6.00 to 8.19. Solution pH increased due to the release of hydroxyl ions during anionic-As(V) adsorption onto amine groups of the adsorbent according to the following reactions (Awual and Jyo, 2009; Kwok et al., 2014): ReNH2 þ H2O 4 ReNHþ 3 þ OH
(4)
þ ReNH2 þ H2AsO 4 þ H2O 4 ReNH3 … H2AsO4 þ OH
(5)
þ 2 2ReNH2 þ HAsO2 (6) 4 þ 2H2O 4 (ReNH3 )2 … HAsO4 þ 2OH
In addition, the biosorbent itself had alkaline buffer capacity resulted by chemical manufacture procedure of it (Eq. (4)). These reasons caused the significant increase of solution pH during As(V) biosorption under the pH-shift condition. The adsorption rate and uptake amount of As(V) increased with decreasing initial pH from 6.0 to 3.0. The positive effect of pH on As(V) removal was as reported elsewhere (Kwok et al., 2014). The solution pH in batch experiments conducted at an initial pH of 3.0 increased up to pH 6.75. Contrary to the pH 3 stat experiment, no breakdown of biosorbent was observed in the initial pH 3 experiment. However, a slight breakdown of the biosorbent in the pHshift experiment was observed at an initial pH of 2.0. In addition to the structural stability of the biosorbent at low pH, solution pH affects the properties of the functional groups of the biosorbent as well as the chemical speciation of As(V) in solution. As can be expected from a pKa1 value of 2.19 (Eq. (1)), As(V) can be present in the neutral form of H3AsO4, and its proportion increases with decreasing solution pH (
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Fig. 1. Kinetics of As(V) removal by the biosorbent at various pHs. 0.1 M HCl or H2SO4 solutions were used to adjust and/or control the solution pH.
various pH conditions as adjusted by H2SO4, which is known to be less destructive to chitosan polymers (Dash et al., 2011; Wan Ngah et al., 2011). As expected, the destructive effect of low pH was reduced in comparison with that of HCl, but the As(V)-removal rate and amount of biosorbent also decreased. The low As(V)-removal rate was due to the competitive inhibition of sulfate ions on the anionic adsorption of As(V) ions onto the functional groups of the biosorbent (Jeon et al., 2009). Regardless of the destructive effect of Cl on the structure of chitosan polymer, finally HCl solution was chosen as acid solution to adjust and/or control the pH in this study. It can be noted that careful addition of weak HCl solution could minimize the destruction of the biosorption. To quantitatively describe the behavior of As(V) removal by the biosorbent, various well-known equations were used for kinetic modeling. Of those, the pseudo-second-order equation showed a good fit for the kinetic data obtained from pH-shift experiments at various pH. Pseudo-second-order kinetics usually applies when the rate of direct adsorption/desorption controls the overall sorption kinetics (Hasani et al., 2017). The pseudo-second-order equation can be expressed as
t 1 t ¼ þ qt k$qe 2 qe
(7)
than was H2SO4. The adsorption rate and amount of As(V) at pHi 3.0 were higher than those at other pH. As the initial pH decreased from 6.0 to 3.0, equilibrium uptake of As(V) increased from 14.65 mg/g to 35.67 mg/g. The initial sorption rate of As(V) was more than 10 times faster at pHi 3.0 than at pHi 6. Through kinetic modeling work, the optimal initial pH for As(V) removal by the biosorbent used in this study was determined to be 3.0. Another widely used equation to describe the kinetics of chemisorption was proposed by Elovich. It has also been applied to describe the process of adsorption of metals from aqueous solutions (Tran et al., 2017).
q¼
1 1 lnðabÞ þ ln t b b
(8)
where a (mg/g,min) is the parameter a represents the rate of chemisorption at zero coverage, and b (g/mg) is related to the extent of surface coverage and to the activation energy for the adsorption. These constants were calculated from the slope and intercept of the plots of q vs ln t. The Elovich equation did not fit well with the experimental data as expected (Table 2). This suggests that chemisorption could not explain simply arsenic removal under this experiment condition. 3.2. Effects of various parameters on As(V) biosorption
where k (g/mg,h) and qe (mg/g) are the rate constant and the equilibrium uptake for the pseudo-second-order equation, respectively. The calculated values of the variables and initial sorption rate (h; mg/g,h) are showed in Table 2. As can be seen, HCl was a more effective pH-adjustment solution for As(V) removal
As shown in the pH-stat and pH-shift experiments, solution pH was the most important parameter affecting As(V) removal by the biosorbent, and pH-shift using HCl was more favorable than pHstat. Due to the multiple effects of pH, there was an optimal
N. Kim et al. / Chemosphere 226 (2019) 67e74
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Table 2 Kinetic parameters of pseudo-second-order model and Elovich model of As(V) at various initial pHs. pH condition
P
a
Eb
a b
HCl
k (g/mg, h) qe (mg/g) h (mg/g, h) R2 a (mg/(g, min)) b (g/mg) R2
H2SO4
pHi 2
pHi 3
pHi 4
pHi 6
pHi 2
pHi 3
pHi 4
pHi 6
4.77 5.16 126.94 0.9983 2582 4.49 0.2451
0.68 35.67 869.45 0.9999 1549 0.30 0.8734
0.29 23.34 159.24 0.9976 8.59 0.24 0.9241
0.39 14.65 84.13 0.9755 3.03 0.31 0.7763
0.32 4.45 6.40 0.5938 0.61 1.41 0.5292
0.15 18.43 49.99 0.8012 3.28 0.25 0.6194
1.65 13.28 290.47 0.9987 29.30 0.54 0.8018
2.38 10.25 249.87 0.9967 3.02 0.31 0.4603
Pseudo-second-order model. Elovich model.
initial pH for As(V) removal. In the previous pH-shift experiment, it was 3.0. Here, the optimal initial pH can depend on the biosorbent dosage, as the biosorbent itself has an alkaline buffer capacity as a result of the chemical manufacturing process. It is therefore necessary to examine both the single and interactive effects of biosorbent dosage and initial solution pH. Fig. 2 shows three-dimensional surfaces for evaluation of As(V)removal performance at various biosorbent dosages (0.5e3.0 g/L) and initial pH (2.6e3.8). As shown in Fig. 2(a), the solution pH increased during As(V) biosorption, and the final pH was dependent on both the initial pH and biosorbent dosage. As biosorbent dosage increased, the final pH also increased due to the alkaline buffer capacity of the biosorbent itself. As a result of the pH change, As(V)-removal efficiency also depended on both parameters, as
show in Fig. 2(b). As biosorbent dosage increased, the removal efficiency of As(V) increased as more functional groups took part in As(V) adsorption. At a given biosorbent dosage, there was an optimal initial pH for As(V) biosorption. The optimal initial pH decreased from 3.2 to 2.6 with the increase of biosorbent dosage from 0.5 to 3.0 g/L. A maximum value of 61.7% for As(V)-removal efficiency was obtained at an initial pH of 2.6 and biosorbent dosage of 3.0 g/L. When considering As(V)-removal performance from the viewpoint of uptake amount (mg of As per g of biosorbent), a different condition was chosen as optimal for As(V) removal by the biosorbent: i.e., 24.4 mg/g of As(V) uptake at an initial pH 3.2 and 0.5 g of biosorbent dosage. Weak acid conditions (pH 3e6) were favorable for As(V) removal by the biosorbent, but the optimal pH differed according to
Fig. 2. Effect of initial pH and biosorbent dosage on As(V) biosorption: (a) final pH, (b) As(V)-removal efficiency and (c) As(V)-uptake.
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the experimental conditions (Figs. 1 and 2). Because As(V) exits as HAsO2 4 and H2AsO4 in the pH range of 3e6, other co-existing anions, including chloride and sulfate ions, can inhibit the adsorption reaction between the anionic metal ions and positivelycharged functional groups of the biosorbent (Park et al., 2005). Fig. 3 shows the effect of ionic strength of these anions on As(V) uptake. As the ionic strength of each anion increased from 0 to 5 mM, As(V) uptake decreased remarkably: from 32.5 to 18.2 mg/g with respect to chloride ions and from 32.5 to 0 mg/g with respect to sulfate ions. The competitive inhibition of divalent sulfate ion was more serious than that of monovalent chloride ions. As(V) was not significantly removed by the biosorbent when 5 mM of sulfate co-existed in the solution. The ionic strength of other anions should therefore be considered for efficient removal of As(V) by the biosorbent. Fig. 4 shows the effect of temperature on As(V) biosorption. As the temperature increased, As(V) uptake decreased gradually. Compared with the value at 5 C, As(V) uptake at 35 C was reduced to 35.6%, indicating that As(V) removal by the biosorbent was exothermic. Other researchers have also reported exothermic adsorption of As(V) (Ranjan et al., 2009; Markovski et al., 2014). In their reports, the decrease of As(V) removal with increasing temperature was assumed to be due to weakening from physical forces and destabilization of adsorbent surfaces at higher temperature. However, there are contrary reports in the literature. Amer and Awwad (2018), for example, reported that As(V) removal by nanocrystalline kaolinite was endothermic. 3.3. Equilibrium study of As(V) biosorption Isotherm analysis can help explain adsorption mechanisms and develop an equilibrium equation that can be used to design adsorption process (Limousin et al., 2007). Among various isotherm equations reported in the literature, Langmuir and Freundlich equations have been most frequently used for isotherm studies of As(V) biosorption. A general form of the Langmuir model is
qe ¼
qmax bCe 1 þ bCe
(9)
where qmax is the maximum adsorbate uptake (mg/g), Ce is the equilibrium concentration of adsorbate in the bulk solution (mg/L), and b is a constant related to the free energy of adsorption (L/mg).
Fig. 4. Effect of temperature on the As(V) removal by the biosorbent.
The Langmuir equation assumes that adsorption is limited to monolayer coverage, all surface sites can accommodate a single adsorbed atom, and the ability of a molecule to be adsorbed at a given site is independent of its neighboring site occupancy. Another widely used model for describing biosorption is the empirical Freundlich equation, which can describe the equilibrium behavior of non-ideal adsorption involving heterogeneous adsorption. The Freundlich equation has been successfully used to describe the adsorption of organic compounds on a wide variety of adsorbents, including biosorbents (Olufemi and Otolorin, 2017). Equation (10) is a general form of the Freundlich model: 1=n
qe ¼ KF C e
where KF is a constant indicating the relative adsorption capacity of adsorbent (mg/g), and 1/n is the intensity of the adsorption. The constants and correlation coefficients of the isotherm models are listed in Table 3. As can be seen with the coefficient of determinations (R2) and Fig. 5, the best fit for the equilibrium data of As(V) biosorption was the Langmuir isotherm. This indicates that monolayer adsorption corresponded to the As(V) removal by the biosorbent in this study. The Langmuir parameter b can be used to predict the affinity between adsorbate and adsorbent using a dimensionless separation factor, RL, defined by Eq. (11) (Gupta et al., 2010):
RL ¼
Fig. 3. Effect of competitive anions of chloride (Cl) and sulfate (SO2 4 ) on the As(V) removal by the biosorbent at different ionic strength.
(10)
1 ð1 þ bC0 Þ
(11)
where C0 is the initial concentration of adsorbate (mg/L), and b is the Langmuir constant related to the free energy of adsorption (L/ mg). The values (0.02e0.19) of RL for As(V)-adsorption by the biosorbent were less than 1 and greater than 0, indicating favorable adsorption of As(V) by the biosorbent. Through the isotherm study, it can be concluded that the removal mechanism of As(V) by the biosorbent was a highly favorable adsorption reaction (Xiong et al., 2018). Meanwhile, the Langmuir constant qmax is interpreted as the total number of binding sites of adsorbent and thus can be used to compare the adsorption capacity of adsorbents. Table 1 shows the maximum adsorption capacity of various (in)organic adsorbents, including the biosorbent used in this study. Compared with those of other adsorbents reported in the literature, the high value
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Table 3 Isothermal parameters of Langmuir and Freundlich models for As(V) biosorption by the biosorbent. Langmuir qmax (mg/g) b (L/mg) R2
Freundlich 62.99 0.10 0.9928
Fig. 5. Biosorption isotherms of As(V) onto the biosorbent and the non-linear adjustments of Freundlich and Langmuir models.
KF (L/mg) n R2
27.73 6.93 0.7960
Fig. 6. Desorption efficiency of As(V) by different eluants: 0.1 M NaCl, 0.1 M Na2SO4 and 0.1 M NaOH.
(62.99 mg/g) of our commercially available biosorbent indicated its high potential as adsorbent for As(V) removal.
3.4. Desorption and recycling study To apply an adsorbent to real wastewater, the desorption process of adsorbed metal must be considered carefully. The desorption process also includes recycling of the adsorbent. From a business point of view, the recyclability of adsorbent is of crucial importance when removing metals from wastewater. Successive adsorption and desorption experiments have therefore been conducted to evaluate the suitability and stability of new materials as candidate adsorbents (Park et al., 2010). Considering the adsorption mechanism of As(V) by the biosorbent, desorption using chemical desorbents containing competitive anions such as chloride, sulfate, and hydroxyl ions was suitable for this study. Fig. 6 shows the As(V)-desorption efficiency of 0.01 M NaCl, Na2SO4, and NaOH solutions. Due to the competitive interaction between anionic As(V) and the anions, As(V) could be desorbed by the three desorbing eluents. NaOH was ultimately chosen because it is a strong chelating agent for many heavy metals and is relatively inexpensive (Park et al., 2010). A single hour of contact time was enough for complete desorption of As(V) from the biosorbent (data not shown), indicating a rapid desorption process. Fig. 7 shows successive adsorption and desorption performance of the biosorbent. The adsorption capacity of the regenerated biosorbent was maintained at 85%, even through the third cycle. In addition, no structural damage was observed during the repeated adsorption and desorption experiment. The reason for successful adsorption and desorption is that the As(V) removal mechanism of this adsorbent is an electrostatic attraction between the biosorbent and As(V). The biosorbent have studied that other anionic metal ion is removed from aqueous solution through an adsorption mechanism whereby anionic ion binds to the positively charged groups of
Fig. 7. Successive adsorption and desorption performance of the biosorbent. After desorbing adsorbed-As(V) with 0.1 M NaOH solution for 1 h, the regenerated biosorbent was further used to adsorb As(V) again.
the biosorbent in previous studies (Park et al., 2012). The above results show that As(V) is removed by the same mechanism. 4. Conclusions A PEI-coated bacterial biosorbent prepared from fermentation biowaste was used successfully to remove anionic As(V) from an aqueous solution. Solution pH was the most important variable affecting the As(V)-adsorption capacity of the biosorbent. The optimal pH for As(V) adsorption was dependent on the biosorbent dosage due to its alkaline buffer capacity. Co-existing anions competitively inhibited the anionic adsorption of As(V) onto the positively charged groups of the biosorbent. A Langmuir model fit
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the equilibrium isotherm data better than a Freundlich model. The maximum uptake of As(V) was determined to be 62.99 mg/g at pH 4.0, which was higher than those of other (in)organic adsorbents reported in the literature. Considering regeneration of the biosorbent, 0.01 M NaOH solution was chosen as an As(V)-desorbing eluent. Successive sorption-desorption experiments showed the maintenance of adsorption capacity at more than 85%. In conclusion, the biowaste-driven biosorbent is a promising candidate for anion absorbance and can be used to treat As(V)-contaminated wasters. Acknowledgements This work was supported by Korea Ministry of Environment through grants from the KEITI (the Eco-Innovation Project, 2012000150005). References Amer, M.W., Awwad, A.M., 2018. Removal of As(V) from aqueous solution by adsorption onto nanocrystalline kaolinite: equilibrium and thermodynamic aspects of adsorption. Environ. Nanotechnol. Monit. Manag. 9, 37e41. Anirudhan, T.S., Unnithan, M.R., 2007. Arsenic(V) removal from aqueous solutions using an anion exchanger derived from coconut coir pith and its recovery. Chemosphere 66, 60e66. Awual, M.R., Jyo, A., 2009. Rapid column-mode removal of arsenate from water by crosslinked poly(allylamine) resin. Water Res. 43, 1229e1236. Bailey, S.E., Olin, T.J., Bricka, R.M., Adrian, D.D., 1999. A review of potentially low-cost sorbents for heavy metals. Water Res. 33, 2469e2479. Bandpei, A.M., Mohseni, S.M., Sheikhmohammadi, A., Sardar, M., Sarkhosh, M., Almasian, M., Avazpour, M., Mosallanejad, Z., Atafar, Z., Nazari, S., Rezaei, S., 2017. Optimization of arsenite removal by adsorption onto organically modified montmorillonite clay: experimental & theoretical approaches. Kor. J. Chem. Eng. 34, 376e383. Choi, H.-J., Yu, S.-W., 2019. Biosorption of methylene blue from aqueous solution by agricultural bioadsorbent corncob. Environ. Eng. Res. 24, 99e106. Choi, Y.-K., Jang, H.M., Kan, E., Wallace, A.R., Sun, W., 2019. Adsorption of phosphate in water on a novel calcium hydroxide-coated dairy manure-derived biochar. Environ. Eng. Res. 24, 434e442. Choong, T.S.Y., Chuah, T.G., Robiah, Y., Koay, F.L.G., Azni, I., 2007. Arsenic toxicity, health hazards and removal techniques from water: an overview. Desalination 217, 139e166. Chuang, C.L., Fan, M., Xu, M., Brown, R.C., Sung, S., Saha, B., Huang, C.P., 2005. Adsorption of arsenic(V) by activated carbon prepared from oat hulls. Chemosphere 61, 478e483. Dash, M., Chiellini, F., Ottenbrite, R.M., Chiellini, E., 2011. Chitosan - a versatile semisynthetic polymer in biomedical applications. Prog. Polym. Sci. 36, 981e1014. Dotto, G.L., Campana, S., Pinto, L., 2017. Frontiers in Biomaterials: Chitosan Based Materials and its Applications. Bentham Science Publishers. Genç-Fuhrman, H., Bregnhøj, H., McConchie, D., 2005. Arsenate removal from water using sandered mud columns. Water Res. 39, 2944e2954. Gupta, V.K., Rastogi, A., Nayak, A., 2010. Biosorption of nickel onto treated alga (Oedogonium hatei): Application of isotherm and kinetic models. J. Colloid Interface Sci. 342, 533e539. Hasani, S., Ardejani, F.D., Olya, M.E., 2017. Equilibrium and kinetic studies of azo dye (Basic Red 18) adsorption onto montmorillonite: Numerical simulation and laboratory experiments. Kor. J. Chem. Eng. 34, 2265e2274. Jeon, C.S., Baek, K., Park, J.K., Oh, Y.K., Lee, S.D., 2009. Adsorption characteristics of As(V) on iron-coated zeolite. J. Hazard Mater. 163, 804e808. Kwak, I.S., Won, S.W., Chung, Y.S., Yun, Y.S., 2013. Ruthenium recovery from acetic acid waste water through sorption with bacterial biosorbent fibers. Bioresour. Technol. 128, 30e35. Kwok, K.C.M., Koong, L.F., Chen, G., McKay, G., 2014. Mechanism of arsenic removal
using chitosan and nanochitosan. J. Colloid Interface Sci. 416, 1e10. Limousin, G., Gaudet, J.P., Charlet, L., Szenknect, S., Barthes, V., Krimissa, M., 2007. Sorption isotherms: a review on physical bases, modeling and measurement. Appl. Geochem. 22, 249e275. Lin, T.-F., Wu, J.-K., 2001. Adsorption of arsenite and arsenate within activated alumina grains: equilibrium and kinetics. Water Res. 35, 2049e2057. Lorenzen, L., van Deventer, J.S.J., Landi, W.M., 1995. Factors affecting the mechanism of the adsorption of arsenic species on activated carbon. Miner. Eng. 8, 557e569. Markovski, J.S., Markovi c, D.D., Ðoki c, V.R., Mitri c, M., Risti c, M.Ð., Onjia, A.E., Marinkovi c, A.D., 2014. Arsenate adsorption on waste eggshell modified by goethite, a-MnO2 and goethite/a-MnO2. Chem. Eng. J. 237, 430e442. Mueller, B., 2017. Arsenic in groundwater in the southern lowlands of Nepal and its mitigation options: a review. Environ. Rev. 25, 296e305. Olufemi, B.A., Otolorin, F., 2017. Comparative adsorption of crude oil using mango (Mangnifera indica) shell and mango shell activated carbon. Environ. Eng. Res. 22, 384e392. Park, D., Yun, Y.S., Jo, J.H., Park, J.M., 2005. Effects of ionic strength, background electrolytes, heavy metals, and redox-active species on the reduction of hexavalent chromium by Ecklonia biomass. J. Microbiol. Biotechnol. 15, 780e786. Park, D., Yun, Y.S., Ahn, C.K., Park, J.M., 2007. Kinetics of the reduction of hexavalent chromium with the brown seaweed Ecklonia biomass. Chemosphere 66, 939e946. Park, D., Yun, Y.S., Park, J.M., 2010. The past, present, and future trends of biosorption. Biotechnol. Bioproc. Eng. 15, 86e102. Park, S.I., Kwak, I.S., Bae, M.A., Mao, J., Won, S.W., Han, D.H., Chung, Y.S., Yun, Y.S., 2012. Recovery of gold as a type of porous fiber by using biosorption followed by incineration. Bioresour. Technol. 104, 208e214. Rahman, M.A., Rahman, A., Khan, M.Z.K., Renzaho, A.M.N., 2018. Human health risks and socio-economic perspectives of arsenic exposure in Bangladesh: a scoping review. Ecotoxicol. Environ. Saf. 150, 335e343. Ranjan, D., Talat, M., Hasan, S.H., 2009. Biosorption of arsenic from aqueous solution using agricultural residue ‘rice polish’. J. Hazard Mater. 166, 1050e1059. Reddad, Z., Gerente, C., Andres, Y., Le Cloirec, P., 2002. Adsorption of several metal ions onto a low-cost biosorbent: kinetic and equilibrium studies. Environ. Sci. Technol. 36, 2067e2073. Seki, H., Suzuki, A., Maruyama, H., 2005. Biosorption of chromium(VI) and arsenic(V) onto methylated yeast biomass. J. Colloid Interface Sci. 281, 261e266. Sharma, V.K., Sohn, M., 2009. Aquatic arsenic: toxicity, speciation, transformations, and remediation. Environ. Int. 35, 743e759. Sharma, A.K., Tjell, J.C., Sloth, J.J., Holm, P.E., 2014. Review of arsenic contamination, exposure through water and food and low cost mitigation options for rural areas. Appl. Geochem. 41, 11e33. Smedley, P.L., Kinniburgh, D.G., 2002. A review of the source, behaviour and distribution of arsenic in natural waters. Appl. Geochem. 17, 517e568. Smith, A.H., Bates, M.N., Steinmaus, C.M., von Ehrenstein, O., 2003. Arsenic epidemiology and drinking water standards. Toxicology 191, 5-5. Thirunavukkarasu, O.S., Viraraghavan, T., Subramanian, K.S., 2003. Arsenic removal from drinking water using iron oxide-coated sand. Water Air Soil Pollut. 142, 95e111. Tiwari, D., Jamsheera, A., Zirlianngura, Lee, S.M., 2017. Use of hybrid materials in the trace determination of As(V) from aqueous solutions: an electrochemical study. Environ. Eng. Res. 22, 186e192. Tran, H.N., You, S.-J., Hosseini-Bandegharaei, A., Chao, H.-P., 2017. Mistakes and inconsistencies regarding adsorption of contaminants from aqueous solutions: a critical review. Water Res. 120, 88e116. Verma, N., Sharma, R., 2017. Bioremediation of toxic heavy metals: a patent review. Recent Pat. Biotechnol. 11, 171e187. Wan Ngah, W.S., Teong, L.C., Hanafiah, M.A.K.M., 2011. Adsorption of dyes and heavy metal ions by chitosan composites: a review. Carbohydr. Polym. 83, 1446e1456. Wang, C., Luo, H., Zhang, Z., Wu, Y., Zhang, J., Chen, S., 2014. Removal of As(III) and As(V) from aqueous solutions using nanoscale zero valent iron-reduced graphite oxide modified composites. J. Hazard Mater. 268, 124e131. Xiong, W., Zeng, Z., Li, X., Zeng, G., Xiao, R., Yang, Z., Zhou, Y., Zhang, C., Cheng, M., Hu, L., Zhou, C., Qin, L., Xu, R., Zhang, Y., 2018. Multi-walled carbon nanotube/ amino-functionalized MIL-53(Fe) composites: Remarkable adsorptive removal of antibiotics from aqueous solutions. Chemosphere 210, 1061e1069.
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Removal of anionic arsenate by a PEI-coated bacterial biosorbent prepared from fermentation biowaste Namgyu Kim a, Munsik Park a, Yeoung-Sang Yun b, Donghee Park a, * a
Department of Environmental Engineering, Yonsei University, 1 Yonseidae-gil, Wonju, 26493, Republic of Korea Division of Chemical Engineering, Nanomaterials Processing Research Center, Chonbuk National University, 567 Baekje-daero, Jeounju, 54896, Republic of Korea b
h i g h l i g h t s Anion biosorbent prepared from fermentation biowaste was used for As(V) removal. The biosorbent had 62.99 mg/g of As(V) uptake, which were higher than others. Solution pH was the most important parameter which affected As(V) removal. 0.01 M NaOH solution was chosen as As(V)-desorbing eluant.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2019 Received in revised form 14 March 2019 Accepted 16 March 2019 Available online 19 March 2019
As a problematic element in water systems, arsenic exists as As(III) and As(V). Adsorption techniques can be used to remove anionic As(V) as it is present as a polyatomic anion. In the case of As(III) which exists in zero-valent state under neutral pH, it can be also removed by adsorption after being converted into As(V). Many inorganic and organic materials have been examined as potential adsorbents for anionic As(V) removal. However, most exhibit relatively low adsorption capacities (<10 mg/g). The objective of this study is to examine As(V)-removal mechanism and practical potential of a PEI-coated bacterial biosorbent prepared from fermentation biowaste. The maximum As(V) uptake of the biosorbent was determined to be 62.99 mg/g by Langmuir model. The effects of various parameters including pH, biosorbent dosage, ionic strength and temperature were also examined. Kinetic and equilibrium models were used to interpret the experimental data mathematically. A 0.01 M NaOH solution was chosen as an effective As(V)-desorbing eluent for biosorbent regeneration. The adsorption capacity of the biosorbent remained above 85% over three successive cycles of adsorption and desorption. In conclusion, the biowaste-driven biosorbent is a promising anion adsorbent for treatment of As(V)-contaminated wasters. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Y Yeomin Yoon Keywords: arsenate Bacterial biosorbent Biosorption Chitosan Polyenylenimine
1. Introduction Arsenic is a toxic element in water systems. Some environmental arsenic problems originate from metallurgical industries, alloy materials and pesticides manufacture process. Arsenic is also the 20th most widely distributed element in the Earth's surface d it occurs naturally and is widely distributed in mineral rock sediments. Unlike most other cationic metals, arsenic is present in water systems as a polyatomic anion containing oxygen (Sharma and Sohn, 2009). In aquatic systems, arsenic exists primarily as two oxidation-state
* Corresponding author. E-mail address: [email protected] (D. Park). https://doi.org/10.1016/j.chemosphere.2019.03.113 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
species, arsenite As(III) and arsenate As(V). The mobility and toxicity of arsenic are determined by its oxidation state. Arsenite predominates in moderately reducing anaerobic environments such as groundwater, whereas arsenate generally predominates in the oxidizing conditions typical of seawater, lakes, and rivers. Arsenite is more mobile and toxic than arsenate (Smedley and Kinniburgh, 2002; Bandpei et al., 2017; Tiwari et al., 2017). Arsenic dissolved in water is acutely toxic and can lead to a number of health problems. Short-term exposure can cause stomach pain, nausea, vomiting, and diarrhea. Long-term exposure to arsenic can cause skin, lung, bladder, and kidney cancer (Sharma et al., 2014). As a result, regulatory authorities such as the World Health Organization and the US Environmental Protection Agency have tightened the maximum contamination level of total arsenic in
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N. Kim et al. / Chemosphere 226 (2019) 67e74
drinking water from 50 to 10 mg/L (Smith et al., 2003). In many countries, including Bangladesh, China, Vietnam, and the US, arsenic exposure through drinking water is a major environmental issue, and researchers have been looking for efficient methods of treating arsenic-contaminated waters (Mueller, 2017; Rahman et al., 2018). Conventional techniques capable of removing heavy metals that can be used to treat arsenic-contaminated waters include coagulation, precipitation, adsorption, and electrodialysis (Choong et al., 2007; Verma and Sharma, 2017). Among them, adsorption is getting increasingly popular for arsenic removal owing to simple process, ease of operation and handling, adsorbent regeneration and no sludge generation (Park et al., 2010; Choi et al., 2019). Using adsorption technique, anionic As(V) can be easily removed from aqueous solutions compared to As(III) which exists in zero-valent state under neutral pH condition. As(III) can be removed through adsorption technique after being converted into anionic As(V) by pre-oxidation. In treating arsenic-contaminated waters, therefore, it is important to develop or find anion adsorbents that are inexpensive and commercially available. Table 1 shows adsorption capacities of various (in)organic adsorbents reported recently. Most of the adsorbents have a less than 10 mg/g capacity for anionic As(V), which is low compared with those for other cationic metals. Some of the highest adsorption capacities for cadmium, lead and mercury were reported as follows: 215 mg Cd/g seaweed, 558 mg Cd/g chitosan, 1587 mg Pb/g lignin and 1123 mg Hg/g chitosan (Bailey et al., 1999). Recently, a few researchers have reported higher As(V)-adsorption capacities using nano-scale inorganic adsorbents and functionalized (in) organic adsorbents (Table 1). Wang et al. (2014) synthesized a nano-scale adsorbent by hybridizing zero-valent iron and reduced graphite oxide. The inorganic adsorbent exhibited an adsorption capacity of 29.04 mg/g for As(V). Anirudhan and Unnithan (2007) modified coconut coir pith into an anion exchanger with an amine group and reported 13.57 mg/g of maximum uptake of As(V). Recently, Kwak et al. (2013) developed a commercially available polyethylenimine (PEI)-modified bacterial biosorbent in the form of a fiber with promising potential for binding anionic metals such as ruthenium and gold. The maximum adsorption capacities of these anions were 110.5 mg/g and 421.1 mg/g, respectively (Kwak et al., 2013; Park et al., 2012). As there is no report on As(V) removal by this biosorbent, we studied the As(V) removal potential of the anion biosorbent under various conditions. Kinetic and equilibrium experiments were conducted to elucidate the As(V)removal mechanism of the biosorbent, and various models were used to fit the kinetic and equilibrium data. Desorption and regeneration studies were also performed to assess the reusability of the biosorbent. 2. Experimental 2.1. Preparation of the biosorbent Fermentation biowaste containing Corynebacterium glutamicum
was supplied by Daesang Corporation (Gunsan, Korea) in a slurry form with a moisture content of 85%. To manufacture a fiber biosorbent, chitosan was used as a binder to immobilize the microbial biomass. PEI was used to enhance the cationic functionality of the biosorbent. The detailed procedure can be reviewed in Kwak et al. (2013). Finally, PEI-coated biosorbent fibers 0.2e0.5 mm thick and 5e10 mm long were used for As(V) removal. 2.2. Reagents Pure analytical-grade arsenate solution was prepared by dissolving Na2HAsO4$7H2O (Sigma-Aldrich Corp, USA) in distilled water. All chemicals used in this study, including chitosan (molecular weight: 3.0 105 g/mol) and PEI (molecular weight ¼ 70,000 g/L), were of analytical grade. 2.3. Batch adsorption experiments The kinetics of As(V) biosorption was investigated at pH values from 2.0 to 6.0. The solution pH was adjusted with diluted HCl, H2SO4, and NaOH solutions. The biosorbent dosage was 1.0 g/L: 0.2 g of biosorbent was added to a plastic bottle containing 200 mL of arsenate solution (50 mg-As/L) with its respective pH value. The bottle was horizontally agitated on a shaker at 200 rpm for 2 h at room temperature (20e25 C). In batch experiments to examine the effects of pH and biosorbent dosage, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 g/L of each biosorbent were added to 50 mg/L of As(V) solution at pH 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, 3.8, and 4.0. The effect of competing anions (Cl and SO2 4 ) on As(V) adsorption on the biosorbent was investigated with solutions containing NaCl and Na2SO4 at concentrations from 1 to 5 mM. The effect of temperature was studied from 5 to 35 C. Except in the pH and dosage experiments, the initial pH of the As(V) solution was 3.0. The procedure for the equilibrium studies was essentially the same as that for the kinetic studies, with 0.2 g of biosorbent added to 200 mL As(V) solution in the range of 50e500 mg/L. The solution was maintained at pH 4.0 using HCl during the batch equilibrium experiment; an equilibrium state was reached in 1 h. Samples were intermittently removed from the bottles to analyze As(V) concentration, following appropriate dilution with deionizededistilled water. It was confirmed from three independent replicates that the batch experiments were producible within at most 5% error in this study. 2.4. Desorption experiment of arsenate loaded on the biosorbent The As(V)-laded biosorbent was produced through contact with 50 mg/L As(V) solution at pH 3.0 for 1 h. After adsorption, the As(V)loaded biosorbent was separated and washed lightly with deionized water. The 0.01 M NaCl, Na2SO4, and NaOH solutions were used as eluents to desorb As(V) from the biosorbent. The regenerated biosorbent was resuspended in 50 mg/L As(V) solution for the next
Table 1 Maximum adsorption capacity (qmax) of anionic As(V) by various adsorbents. Adsorbents
pH
Temp. (oC)
Maximum capacity (mg-As/g)
Reference
Iron oxide coated sand Activated carbon produced from oat hulls Methylated biomass Functionalized coconut coir pith Coconut shell carbon Nanoscale zero valent iron PEI coated biosorbent fiber
7.6 5.0 6.5 7.0 5.0 7.0 4.0
22 25 30 20 25 25 20
0.034 3.08 3.75 13.57 2.4 29.04 62.99
Thirunavukkarasu et al. (2003) Chuang et al. (2005) Seki et al. (2005) Anirudhan and Unnithan (2007) Lorenzen et al. (1995) Wang et al. (2014) This Study
N. Kim et al. / Chemosphere 226 (2019) 67e74
adsorption run. The adsorption/desorption steps described above were repeated three times. 2.5. Analytical methods The arsenate concentration of each sample was determined by inductively coupled plasma-atomic emission spectrometry (ICP/ IRIS, Thermo Jarrell Ash Co., USA) after being filtered through a 0.20 mm membrane. 3. Results 3.1. Kinetic study of As(V) biosorption at various pHs Contact time, dosage, solution pH, temperature, and ionic strength affect adsorption rate, uptake amount, and even selectivity for target removal (Park et al., 2010). Among these, solution pH is the most important factor for As(V) removal as it determines the species of the As(V) in aqueous solution. The distribution of As(V) species as a function of pH is given by the following equations (Smedley and Kinniburgh, 2002): þ H3AsO4 4 H2AsO 4 þ H3O , pK1 ¼ 2.19
(1)
2 þ H2AsO 4 4 HAsO4 þ H3O , pK2 ¼ 6.94
(2)
3 þ HAsO2 4 4AsO4 þ H3O , pK3 ¼ 11.50
(3)
2 Based on its pKa values, the As(V) species are AsO3 4 , HAsO4 , H2AsO and H AsO . As(V) usually exists as anions with a charge 4 3 4 of 1 and 2 in As(V)-contaminated waters. To examine the effect of pH on As(V) biosorption, batch kinetic experiments were conducted at various pH conditions. Kinetic studies can supply detailed information on the adsorption behavior of adsorbates and adsorption mechanisms and can be useful in determining the optimal contact time for successful batch biosorption processes (Park et al., 2007; Choi and Yu, 2019). Fig. 1(a) and (b) show the removal behavior of As(V) at pH values from 2.0 to 6.0. As shown in Fig. 1(a), the biosorbent significantly removed anionic As(V) from the aqueous solution. The removal behavior of As(V) differed according to pH. The maximum removal of As(V) was obtained at constant pH 6.0, where equilibrium was reached within 2 h. The biosorbent has abundant amine and hydroxyl groups owing to the presence of chitosan and PEI, which were used for its manufacture (Park et al., 2012). Amine groups having pKa values of 6.5 are effective at binding anionic metals (Reddad et al., 2002; Dotto et al., 2017). In this study, the biosorbent effectively adsorbed anionic As(V) in HAsO2 4 and H2AsO4 . As the solution pH was decreased, As(V)-removal efficiency of the biosorbent decreased and it was hardly removed at pH 2 (Fig. 1(a)). In other research on As(V) biosorption, the adsorption rate and uptake amount increased with decreasing solution pH (Lin and Wu, 2001; Dotto et al., 2017). Solution pH typically affects the surface charge of adsorbent, the degree of ionization, and the speciation of surface functional groups. Because amine and hydroxyl groups of adsorbents can be effectively protonated with positive charges at low pH and can attract anionic metals in aqueous solutions, low pH is known to favor anionic As(V) removal. Genç-Fuhrman et al. (2005) used an adsorbent derived from red mud to remove As(V) and reported an increase in As(V) uptake with decreasing pH; from 1.64 mg/g at pH 7.1 to 3.32 mg/g at pH 4.5. When activated alumina grains was used as adsorbents, As(V) uptake increased from 9.93 mg/g at pH 7.2e12.34 mg/g at pH 2.6 (Lin and Wu, 2001). In addition to the contrary As(V)-removal behavior of our
69
biosorbent at low pH to those of other adsorbents reported in the literature, an unanticipated phenomenon was evident at low pH. At a constant pH of 4.0, the concentration of As(V) in aqueous phase sharply decreased for 0.5 h and then gradually increased, indicating that As(V) adsorbed on the biosorbent was released into the aqueous phase. The unexpected release of As(V) from the biosorbent was due to breakdown of the biosorbent itself at a low pH. In particular, the biosorbent was broken down rapidly at pH 2 within several minutes. It is possible this is because chitosan polymers exhibit poor acid resistance in even dilute HCl, HNO3, and H3PO4 solutions (Dash et al., 2011; Wan Ngah et al., 2011). The structural breakdown of the biosorbent containing chitosan caused the release of adsorbed As(V) as well as the poor As(V) uptake at low pH (<4). As a result, an optimal pH was reached in the pH-stat kinetic experiment, in which HCl was used to maintain the solution pH at a fixed value. Although the final efficiency of As(V) removal peaked at pH 6, the initial removal rate at pH 6 was lower than that at pH 4, indicating that pH 4 would be more favorable for As(V) removal than pH 6 if not for the structural breakdown of the biosorbent. It is therefore important to control the solution pH to optimize both adsorption rate and amount. To examine the relationship between solution pH and As(V) removal depending on structural stability of the biosorbent, a pHshift kinetic study was conducted without pH maintenance during the batch experiment, in which the HCl solution was only used to adjust the initial pH to desired values (2.0, 3.0, 4.0, and 6.0). As in Fig. 1(b), As(V) removal proceeded up to approximately 95% within 1 h, and an equilibrium state was reached in 2 h of contact time. The solution pH increased during As(V) removal by the biosorbent: from pH 2.00 to 3.52 and from 6.00 to 8.19. Solution pH increased due to the release of hydroxyl ions during anionic-As(V) adsorption onto amine groups of the adsorbent according to the following reactions (Awual and Jyo, 2009; Kwok et al., 2014): ReNH2 þ H2O 4 ReNHþ 3 þ OH
(4)
þ ReNH2 þ H2AsO 4 þ H2O 4 ReNH3 … H2AsO4 þ OH
(5)
þ 2 2ReNH2 þ HAsO2 (6) 4 þ 2H2O 4 (ReNH3 )2 … HAsO4 þ 2OH
In addition, the biosorbent itself had alkaline buffer capacity resulted by chemical manufacture procedure of it (Eq. (4)). These reasons caused the significant increase of solution pH during As(V) biosorption under the pH-shift condition. The adsorption rate and uptake amount of As(V) increased with decreasing initial pH from 6.0 to 3.0. The positive effect of pH on As(V) removal was as reported elsewhere (Kwok et al., 2014). The solution pH in batch experiments conducted at an initial pH of 3.0 increased up to pH 6.75. Contrary to the pH 3 stat experiment, no breakdown of biosorbent was observed in the initial pH 3 experiment. However, a slight breakdown of the biosorbent in the pHshift experiment was observed at an initial pH of 2.0. In addition to the structural stability of the biosorbent at low pH, solution pH affects the properties of the functional groups of the biosorbent as well as the chemical speciation of As(V) in solution. As can be expected from a pKa1 value of 2.19 (Eq. (1)), As(V) can be present in the neutral form of H3AsO4, and its proportion increases with decreasing solution pH (
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N. Kim et al. / Chemosphere 226 (2019) 67e74
Fig. 1. Kinetics of As(V) removal by the biosorbent at various pHs. 0.1 M HCl or H2SO4 solutions were used to adjust and/or control the solution pH.
various pH conditions as adjusted by H2SO4, which is known to be less destructive to chitosan polymers (Dash et al., 2011; Wan Ngah et al., 2011). As expected, the destructive effect of low pH was reduced in comparison with that of HCl, but the As(V)-removal rate and amount of biosorbent also decreased. The low As(V)-removal rate was due to the competitive inhibition of sulfate ions on the anionic adsorption of As(V) ions onto the functional groups of the biosorbent (Jeon et al., 2009). Regardless of the destructive effect of Cl on the structure of chitosan polymer, finally HCl solution was chosen as acid solution to adjust and/or control the pH in this study. It can be noted that careful addition of weak HCl solution could minimize the destruction of the biosorption. To quantitatively describe the behavior of As(V) removal by the biosorbent, various well-known equations were used for kinetic modeling. Of those, the pseudo-second-order equation showed a good fit for the kinetic data obtained from pH-shift experiments at various pH. Pseudo-second-order kinetics usually applies when the rate of direct adsorption/desorption controls the overall sorption kinetics (Hasani et al., 2017). The pseudo-second-order equation can be expressed as
t 1 t ¼ þ qt k$qe 2 qe
(7)
than was H2SO4. The adsorption rate and amount of As(V) at pHi 3.0 were higher than those at other pH. As the initial pH decreased from 6.0 to 3.0, equilibrium uptake of As(V) increased from 14.65 mg/g to 35.67 mg/g. The initial sorption rate of As(V) was more than 10 times faster at pHi 3.0 than at pHi 6. Through kinetic modeling work, the optimal initial pH for As(V) removal by the biosorbent used in this study was determined to be 3.0. Another widely used equation to describe the kinetics of chemisorption was proposed by Elovich. It has also been applied to describe the process of adsorption of metals from aqueous solutions (Tran et al., 2017).
q¼
1 1 lnðabÞ þ ln t b b
(8)
where a (mg/g,min) is the parameter a represents the rate of chemisorption at zero coverage, and b (g/mg) is related to the extent of surface coverage and to the activation energy for the adsorption. These constants were calculated from the slope and intercept of the plots of q vs ln t. The Elovich equation did not fit well with the experimental data as expected (Table 2). This suggests that chemisorption could not explain simply arsenic removal under this experiment condition. 3.2. Effects of various parameters on As(V) biosorption
where k (g/mg,h) and qe (mg/g) are the rate constant and the equilibrium uptake for the pseudo-second-order equation, respectively. The calculated values of the variables and initial sorption rate (h; mg/g,h) are showed in Table 2. As can be seen, HCl was a more effective pH-adjustment solution for As(V) removal
As shown in the pH-stat and pH-shift experiments, solution pH was the most important parameter affecting As(V) removal by the biosorbent, and pH-shift using HCl was more favorable than pHstat. Due to the multiple effects of pH, there was an optimal
N. Kim et al. / Chemosphere 226 (2019) 67e74
71
Table 2 Kinetic parameters of pseudo-second-order model and Elovich model of As(V) at various initial pHs. pH condition
P
a
Eb
a b
HCl
k (g/mg, h) qe (mg/g) h (mg/g, h) R2 a (mg/(g, min)) b (g/mg) R2
H2SO4
pHi 2
pHi 3
pHi 4
pHi 6
pHi 2
pHi 3
pHi 4
pHi 6
4.77 5.16 126.94 0.9983 2582 4.49 0.2451
0.68 35.67 869.45 0.9999 1549 0.30 0.8734
0.29 23.34 159.24 0.9976 8.59 0.24 0.9241
0.39 14.65 84.13 0.9755 3.03 0.31 0.7763
0.32 4.45 6.40 0.5938 0.61 1.41 0.5292
0.15 18.43 49.99 0.8012 3.28 0.25 0.6194
1.65 13.28 290.47 0.9987 29.30 0.54 0.8018
2.38 10.25 249.87 0.9967 3.02 0.31 0.4603
Pseudo-second-order model. Elovich model.
initial pH for As(V) removal. In the previous pH-shift experiment, it was 3.0. Here, the optimal initial pH can depend on the biosorbent dosage, as the biosorbent itself has an alkaline buffer capacity as a result of the chemical manufacturing process. It is therefore necessary to examine both the single and interactive effects of biosorbent dosage and initial solution pH. Fig. 2 shows three-dimensional surfaces for evaluation of As(V)removal performance at various biosorbent dosages (0.5e3.0 g/L) and initial pH (2.6e3.8). As shown in Fig. 2(a), the solution pH increased during As(V) biosorption, and the final pH was dependent on both the initial pH and biosorbent dosage. As biosorbent dosage increased, the final pH also increased due to the alkaline buffer capacity of the biosorbent itself. As a result of the pH change, As(V)-removal efficiency also depended on both parameters, as
show in Fig. 2(b). As biosorbent dosage increased, the removal efficiency of As(V) increased as more functional groups took part in As(V) adsorption. At a given biosorbent dosage, there was an optimal initial pH for As(V) biosorption. The optimal initial pH decreased from 3.2 to 2.6 with the increase of biosorbent dosage from 0.5 to 3.0 g/L. A maximum value of 61.7% for As(V)-removal efficiency was obtained at an initial pH of 2.6 and biosorbent dosage of 3.0 g/L. When considering As(V)-removal performance from the viewpoint of uptake amount (mg of As per g of biosorbent), a different condition was chosen as optimal for As(V) removal by the biosorbent: i.e., 24.4 mg/g of As(V) uptake at an initial pH 3.2 and 0.5 g of biosorbent dosage. Weak acid conditions (pH 3e6) were favorable for As(V) removal by the biosorbent, but the optimal pH differed according to
Fig. 2. Effect of initial pH and biosorbent dosage on As(V) biosorption: (a) final pH, (b) As(V)-removal efficiency and (c) As(V)-uptake.
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the experimental conditions (Figs. 1 and 2). Because As(V) exits as HAsO2 4 and H2AsO4 in the pH range of 3e6, other co-existing anions, including chloride and sulfate ions, can inhibit the adsorption reaction between the anionic metal ions and positivelycharged functional groups of the biosorbent (Park et al., 2005). Fig. 3 shows the effect of ionic strength of these anions on As(V) uptake. As the ionic strength of each anion increased from 0 to 5 mM, As(V) uptake decreased remarkably: from 32.5 to 18.2 mg/g with respect to chloride ions and from 32.5 to 0 mg/g with respect to sulfate ions. The competitive inhibition of divalent sulfate ion was more serious than that of monovalent chloride ions. As(V) was not significantly removed by the biosorbent when 5 mM of sulfate co-existed in the solution. The ionic strength of other anions should therefore be considered for efficient removal of As(V) by the biosorbent. Fig. 4 shows the effect of temperature on As(V) biosorption. As the temperature increased, As(V) uptake decreased gradually. Compared with the value at 5 C, As(V) uptake at 35 C was reduced to 35.6%, indicating that As(V) removal by the biosorbent was exothermic. Other researchers have also reported exothermic adsorption of As(V) (Ranjan et al., 2009; Markovski et al., 2014). In their reports, the decrease of As(V) removal with increasing temperature was assumed to be due to weakening from physical forces and destabilization of adsorbent surfaces at higher temperature. However, there are contrary reports in the literature. Amer and Awwad (2018), for example, reported that As(V) removal by nanocrystalline kaolinite was endothermic. 3.3. Equilibrium study of As(V) biosorption Isotherm analysis can help explain adsorption mechanisms and develop an equilibrium equation that can be used to design adsorption process (Limousin et al., 2007). Among various isotherm equations reported in the literature, Langmuir and Freundlich equations have been most frequently used for isotherm studies of As(V) biosorption. A general form of the Langmuir model is
qe ¼
qmax bCe 1 þ bCe
(9)
where qmax is the maximum adsorbate uptake (mg/g), Ce is the equilibrium concentration of adsorbate in the bulk solution (mg/L), and b is a constant related to the free energy of adsorption (L/mg).
Fig. 4. Effect of temperature on the As(V) removal by the biosorbent.
The Langmuir equation assumes that adsorption is limited to monolayer coverage, all surface sites can accommodate a single adsorbed atom, and the ability of a molecule to be adsorbed at a given site is independent of its neighboring site occupancy. Another widely used model for describing biosorption is the empirical Freundlich equation, which can describe the equilibrium behavior of non-ideal adsorption involving heterogeneous adsorption. The Freundlich equation has been successfully used to describe the adsorption of organic compounds on a wide variety of adsorbents, including biosorbents (Olufemi and Otolorin, 2017). Equation (10) is a general form of the Freundlich model: 1=n
qe ¼ KF C e
where KF is a constant indicating the relative adsorption capacity of adsorbent (mg/g), and 1/n is the intensity of the adsorption. The constants and correlation coefficients of the isotherm models are listed in Table 3. As can be seen with the coefficient of determinations (R2) and Fig. 5, the best fit for the equilibrium data of As(V) biosorption was the Langmuir isotherm. This indicates that monolayer adsorption corresponded to the As(V) removal by the biosorbent in this study. The Langmuir parameter b can be used to predict the affinity between adsorbate and adsorbent using a dimensionless separation factor, RL, defined by Eq. (11) (Gupta et al., 2010):
RL ¼
Fig. 3. Effect of competitive anions of chloride (Cl) and sulfate (SO2 4 ) on the As(V) removal by the biosorbent at different ionic strength.
(10)
1 ð1 þ bC0 Þ
(11)
where C0 is the initial concentration of adsorbate (mg/L), and b is the Langmuir constant related to the free energy of adsorption (L/ mg). The values (0.02e0.19) of RL for As(V)-adsorption by the biosorbent were less than 1 and greater than 0, indicating favorable adsorption of As(V) by the biosorbent. Through the isotherm study, it can be concluded that the removal mechanism of As(V) by the biosorbent was a highly favorable adsorption reaction (Xiong et al., 2018). Meanwhile, the Langmuir constant qmax is interpreted as the total number of binding sites of adsorbent and thus can be used to compare the adsorption capacity of adsorbents. Table 1 shows the maximum adsorption capacity of various (in)organic adsorbents, including the biosorbent used in this study. Compared with those of other adsorbents reported in the literature, the high value
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Table 3 Isothermal parameters of Langmuir and Freundlich models for As(V) biosorption by the biosorbent. Langmuir qmax (mg/g) b (L/mg) R2
Freundlich 62.99 0.10 0.9928
Fig. 5. Biosorption isotherms of As(V) onto the biosorbent and the non-linear adjustments of Freundlich and Langmuir models.
KF (L/mg) n R2
27.73 6.93 0.7960
Fig. 6. Desorption efficiency of As(V) by different eluants: 0.1 M NaCl, 0.1 M Na2SO4 and 0.1 M NaOH.
(62.99 mg/g) of our commercially available biosorbent indicated its high potential as adsorbent for As(V) removal.
3.4. Desorption and recycling study To apply an adsorbent to real wastewater, the desorption process of adsorbed metal must be considered carefully. The desorption process also includes recycling of the adsorbent. From a business point of view, the recyclability of adsorbent is of crucial importance when removing metals from wastewater. Successive adsorption and desorption experiments have therefore been conducted to evaluate the suitability and stability of new materials as candidate adsorbents (Park et al., 2010). Considering the adsorption mechanism of As(V) by the biosorbent, desorption using chemical desorbents containing competitive anions such as chloride, sulfate, and hydroxyl ions was suitable for this study. Fig. 6 shows the As(V)-desorption efficiency of 0.01 M NaCl, Na2SO4, and NaOH solutions. Due to the competitive interaction between anionic As(V) and the anions, As(V) could be desorbed by the three desorbing eluents. NaOH was ultimately chosen because it is a strong chelating agent for many heavy metals and is relatively inexpensive (Park et al., 2010). A single hour of contact time was enough for complete desorption of As(V) from the biosorbent (data not shown), indicating a rapid desorption process. Fig. 7 shows successive adsorption and desorption performance of the biosorbent. The adsorption capacity of the regenerated biosorbent was maintained at 85%, even through the third cycle. In addition, no structural damage was observed during the repeated adsorption and desorption experiment. The reason for successful adsorption and desorption is that the As(V) removal mechanism of this adsorbent is an electrostatic attraction between the biosorbent and As(V). The biosorbent have studied that other anionic metal ion is removed from aqueous solution through an adsorption mechanism whereby anionic ion binds to the positively charged groups of
Fig. 7. Successive adsorption and desorption performance of the biosorbent. After desorbing adsorbed-As(V) with 0.1 M NaOH solution for 1 h, the regenerated biosorbent was further used to adsorb As(V) again.
the biosorbent in previous studies (Park et al., 2012). The above results show that As(V) is removed by the same mechanism. 4. Conclusions A PEI-coated bacterial biosorbent prepared from fermentation biowaste was used successfully to remove anionic As(V) from an aqueous solution. Solution pH was the most important variable affecting the As(V)-adsorption capacity of the biosorbent. The optimal pH for As(V) adsorption was dependent on the biosorbent dosage due to its alkaline buffer capacity. Co-existing anions competitively inhibited the anionic adsorption of As(V) onto the positively charged groups of the biosorbent. A Langmuir model fit
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the equilibrium isotherm data better than a Freundlich model. The maximum uptake of As(V) was determined to be 62.99 mg/g at pH 4.0, which was higher than those of other (in)organic adsorbents reported in the literature. Considering regeneration of the biosorbent, 0.01 M NaOH solution was chosen as an As(V)-desorbing eluent. Successive sorption-desorption experiments showed the maintenance of adsorption capacity at more than 85%. In conclusion, the biowaste-driven biosorbent is a promising candidate for anion absorbance and can be used to treat As(V)-contaminated wasters. Acknowledgements This work was supported by Korea Ministry of Environment through grants from the KEITI (the Eco-Innovation Project, 2012000150005). References Amer, M.W., Awwad, A.M., 2018. Removal of As(V) from aqueous solution by adsorption onto nanocrystalline kaolinite: equilibrium and thermodynamic aspects of adsorption. Environ. Nanotechnol. Monit. Manag. 9, 37e41. Anirudhan, T.S., Unnithan, M.R., 2007. Arsenic(V) removal from aqueous solutions using an anion exchanger derived from coconut coir pith and its recovery. Chemosphere 66, 60e66. Awual, M.R., Jyo, A., 2009. Rapid column-mode removal of arsenate from water by crosslinked poly(allylamine) resin. Water Res. 43, 1229e1236. Bailey, S.E., Olin, T.J., Bricka, R.M., Adrian, D.D., 1999. A review of potentially low-cost sorbents for heavy metals. Water Res. 33, 2469e2479. Bandpei, A.M., Mohseni, S.M., Sheikhmohammadi, A., Sardar, M., Sarkhosh, M., Almasian, M., Avazpour, M., Mosallanejad, Z., Atafar, Z., Nazari, S., Rezaei, S., 2017. Optimization of arsenite removal by adsorption onto organically modified montmorillonite clay: experimental & theoretical approaches. Kor. J. Chem. Eng. 34, 376e383. Choi, H.-J., Yu, S.-W., 2019. Biosorption of methylene blue from aqueous solution by agricultural bioadsorbent corncob. Environ. Eng. Res. 24, 99e106. Choi, Y.-K., Jang, H.M., Kan, E., Wallace, A.R., Sun, W., 2019. Adsorption of phosphate in water on a novel calcium hydroxide-coated dairy manure-derived biochar. Environ. Eng. Res. 24, 434e442. Choong, T.S.Y., Chuah, T.G., Robiah, Y., Koay, F.L.G., Azni, I., 2007. Arsenic toxicity, health hazards and removal techniques from water: an overview. Desalination 217, 139e166. Chuang, C.L., Fan, M., Xu, M., Brown, R.C., Sung, S., Saha, B., Huang, C.P., 2005. Adsorption of arsenic(V) by activated carbon prepared from oat hulls. Chemosphere 61, 478e483. Dash, M., Chiellini, F., Ottenbrite, R.M., Chiellini, E., 2011. Chitosan - a versatile semisynthetic polymer in biomedical applications. Prog. Polym. Sci. 36, 981e1014. Dotto, G.L., Campana, S., Pinto, L., 2017. Frontiers in Biomaterials: Chitosan Based Materials and its Applications. Bentham Science Publishers. Genç-Fuhrman, H., Bregnhøj, H., McConchie, D., 2005. Arsenate removal from water using sandered mud columns. Water Res. 39, 2944e2954. Gupta, V.K., Rastogi, A., Nayak, A., 2010. Biosorption of nickel onto treated alga (Oedogonium hatei): Application of isotherm and kinetic models. J. Colloid Interface Sci. 342, 533e539. Hasani, S., Ardejani, F.D., Olya, M.E., 2017. Equilibrium and kinetic studies of azo dye (Basic Red 18) adsorption onto montmorillonite: Numerical simulation and laboratory experiments. Kor. J. Chem. Eng. 34, 2265e2274. Jeon, C.S., Baek, K., Park, J.K., Oh, Y.K., Lee, S.D., 2009. Adsorption characteristics of As(V) on iron-coated zeolite. J. Hazard Mater. 163, 804e808. Kwak, I.S., Won, S.W., Chung, Y.S., Yun, Y.S., 2013. Ruthenium recovery from acetic acid waste water through sorption with bacterial biosorbent fibers. Bioresour. Technol. 128, 30e35. Kwok, K.C.M., Koong, L.F., Chen, G., McKay, G., 2014. Mechanism of arsenic removal
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