Abstract

In this work, montmorillonite (Mt) was modified by environmentally friendly arginine (Arg) and lysine (Lys) amino acids with di-cationic groups for arsenic removal from contaminated water. The modified Mts were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, zeta potential and thermal analysis. The adsorption of As(V) onto modified Mts as a function of initial As(V) concentration, contact time and solution pH was investigated. The removal efficiency was increased with increasing the As(V) concentration and contact time; however, it was decreased with increasing solution pH. The maximum As(V) adsorption capacities of Mt-Arg and Mt-Lys were 11.5 and 11 mg/g, respectively, which were five times larger than pristine Mt. The high adsorption capacity makes them promising candidates for arsenic removal from contaminated water. The regeneration studies were carried out up to 10 cycles for both modified Mts. The obtained results confirmed that the modified adsorbents could also be effectively used for As(V) removal from water for multiple adsorption – desorption cycles.

INTRODUCTION

The presence of arsenic with elevated concentrations in the environment, especially in water, is a worldwide health concern (Chutia et al. 2009; Bagherifam et al. 2014b). Arsenic contaminated water may cause numerous skin, lung, liver diseases and lymphatic cancer (Smedley & Kinniburgh 2002). In natural water resources, arsenic generally exists in two inorganic oxidation states including arsenate, As(V), and arsenite, As(III). Arsenate is the dominant species in surface water due to the high redox potential while arsenite is usually found in anaerobic groundwater conditions (Ren et al. 2014). Due to the health hazards associated with arsenic contaminated water, the World Health Organization (WHO) has set a maximum contaminant level of 10 ppb for arsenic in drinking water. Therefore, the removal of arsenic from contaminated waters intended for drinking water is of great significance (Sogaard 2014).

Several techniques, such as precipitation (Li et al. 2011; Battaglia-Brunet et al. 2012), coagulation (Balasubramanian & Madhavan 2001; Parga et al. 2005), ion exchange (Pakzadeh & Batista 2011), membrane filtration (Sato et al. 2002; Uddin et al. 2007) and adsorption have been extensively studied for arsenic removal. Nevertheless, all these methods have their own drawbacks and have been found to be limited by cost, complexity and efficiency (Fu et al. 2016). Among these methods, the adsorption process still receives more attention for the removal of arsenic from water owing to its low cost, high efficiency, easy operation and flexibility (Bagherifam et al. 2014a; Yassine et al. 2016). Various adsorbents, such as iron oxide (Elizalde et al. 2001; Guo et al. 2015; Mangwandi et al. 2016), activated alumina (Hao et al. 2009), activated carbon (Karmacharya et al. 2016), zeolites (Camacho et al. 2011), titanium oxide (Pena et al. 2006) and many natural and synthetic media (Elizalde et al. 2001; Kofa et al. 2015) have been used to remove arsenic. However, problems still exist for the current adsorbents, including relatively low adsorption capacity and efficiency, and regeneration capability. To address these limitations, more researches are needed to develop new low cost, efficient, robust and regenerable adsorbents (Li et al. 2012b).

As one of the widely available adsorbents, clay mineral has been extensively studied due to its low cost, large surface areas, cation exchange capacity (CEC) and other advantages (Bae et al. 2000; Naseem & Tahir 2001; Liu & Zhang 2007; Unuabonah et al. 2012; Bagherifam et al. 2014a; Shokri et al. 2016). Unmodified clay minerals display relatively low adsorption capacity for anionic pollutants such as arsenate. This is mainly due to the existence of net negative charge on their surface (Dousová et al. 2009; Yassine et al. 2016). Extensive studies have shown that the adsorption capacity of clay minerals such as montmorillonite, denoted by Mt, can be improved by replacing the natural inorganic interlayer cations with suitable quaternary amine cations or other surfactants (Li & Bowman 2001; Baskaralingam et al. 2006; Ozcan et al. 2007). At present, the prevailing mechanism of adsorption of anions on organically modified Mt is believed to be the binding between anions and the positively charged surface of organoclays (Pulikesi et al. 2006; Zhu & Ma 2008; Yu et al. 2010). In order to create a positive surface charge and provide suitable anionic adsorbents, different surfactants have been employed to modify Mt (Baskaralingam et al. 2006; Ozcan et al. 2007; Shen et al. 2009). Among modified clay minerals, amino-organoclay is considered as one of the most effective adsorbent for anionic pollutants due to its net positive surface charge. In this regard, Lee et al. (Lee et al. 2011) used amino-organoclay for anionic metals removal and showed that the electrostatic interaction between anionic pollutants and protonated amino groups gives rise to rapid adsorption. Recently, Pan et al. utilized amino-functionalized alkaline clay as adsorbent for the removal of Cr(VI) from aqueous solution (Pan et al. 2016). It is therefore concluded that amino functionalization can convert the negative charge of clay to positive and make it a promising adsorbent for the removal of anionic pollutants such as arsenic from contaminated water.

Nevertheless, there are certain disadvantages in the present reported adsorbents, such as the use of environmentally unfriendly solvents or surfactants in the synthetic process, as well as the complicated preparation methods. Therefore, it is vital to functionalize clay minerals with biodegradable and low toxic surfactants by using a simple method. Among the various surfactants with amine functional groups, amino acids are promising modifiers and have been used to modify other adsorbents for pollutant removal. For instance, modified Arg- Fe3O4 and Lys- Fe3O4 were synthesized and showed promising arsenic removal efficiency from water (Zhang et al. 2014a). Zhang et al. used lysine to modify Fe3O4 nanoparticles to enhance the maximum adsorption capacity for anionic dyes (Zhang et al. 2014b). Dalvand et al. recently functionalized Fe3O4 with lysine to enhance the adsorption capacity of pristine Fe3O4 for dye removal from colored textile wastewater (Dalvand et al. 2016).

By taking these points into consideration, arginine; Arg, and lysine; Lys with two positive amine groups in pH lower than their isoelectric pH value; pHPI, can modify clay mineral with a simple cation exchange method and adsorb anionic pollutants with other positive amine group.

Preparation of regenerable adsorbents is another important feature for newly synthesized efficient adsorbents and conducting research on recovery and regeneration is the area of investigation which needs to be explored (Liang-hsing et al. 2006; Kullkarni & Kaware 2014).

Modified clays with Arg and Lys offer additional opportunities for regeneration capability. It can be expected that clay minerals modified with Arg or Lys would have repulsion with arsenate anions at high pH due to zwitterionic properties and negative side chains.

This study consists of detailed characterization of modified Mts with Arg and Lys and their application to arsenic removal. Batch adsorption characteristics and the mechanisms of As(V) adsorption of adsorbents were analyzed by fitting the experimental data to Langmuir and Freundlich isotherms, as well as pseudo-first- and second-order kinetics models. In order to determine the feasibility of proposed regeneration method for the utilized adsorbent, to be used in multiple cycles of operation, regeneration experiments were also conducted to identify the applicability of the modified Mts for As(V) removal from surface water.

MATERIALS AND METHODS

Materials

Natural montmorillonite (Cloisite Na+) with CEC of 92.6 meq/100 g was obtained from Southern Clay Products Inc., USA. l-Arginine (C6H14N4O2, 174.20 g/mol, 99%, Merck) and l-lysine monohydrochloride (C6H14N2O2.HCl, 182.65 g/mol, 99%, Merck), were used as surfactants to modify the utilized montmorillonite. Their chemical structure is shown in Figure 1. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were purchased from Merck Chemical Co. As(V) stock solutions were prepared by dissolving Na2HAsO4.7H2O (Sigma-Aldrich) in deionized water. All chemicals and reagents were analytical grade.

Figure 1

Chemical structure of (a) arginine, (b) lysine.

Figure 1

Chemical structure of (a) arginine, (b) lysine.

Synthesis of modified Mts

Cation exchange method (Jones 1983) was used to modify the natural clay mineral, Mt, by displacement of the sodium cations of Mt with the protonated amino acids. Typically, 0.2 g of Mt was dispersed in distilled water (25 mL) using a magnetic stirrer for 12 h at 30 °C to swell the montmorillonite. The measured amount of amino acid was separately dissolved in 25 mL distillated water, while pH was adjusted to 3 by adding of 0.1 M HCl, dropwise. The amount of amino acid was equivalent to three times the CEC value of Mt. Then, it was added to the Mt solution and stirred for 4 h at 70 °C. After precipitation, the modified Mt was centrifuged and washed with distilled water for several times. Rinsing with distilled water was repeated thrice and finally the modified Mt was dried at 60 °C for 12 h.

Characterization methods

X-ray diffraction (XRD) patterns of samples were obtained by D500/Siemens diffractometer equipped with monochromatic Cu-Kα radiation (λ =0.154 nm) under a voltage of 35 kV and a current of 30 mA. All samples were analyzed in continuous scan mode with the 2θ ranging from 2° to 50°. The Bragg equation, 2dsinθ = nλ, was used to calculate basal spacing (d-value) of mineral clays. Fourier transform infrared (FT-IR) spectra for samples were recorded for wavenumbers ranging from 4,000 to 400 cm−1 using a VERTEX 70 FT-IR spectrometer (Bruker, Germany). The zeta potentials of adsorbents were measured by Nano ZS (red badge) ZEN 3600. The pH dependencies of zeta potentials for Mts were measured in aqueous solutions at different pH values, adjusted by NaOH and HCl aqueous solutions. Thermo gravimetric analysis (TGA) was performed by using a Perkin Elmer Pyris Diamond TGA system. Samples were heated up to 800 °C under the nitrogen-flow and a heating rate of 10 °C/min.

As(V) adsorption experiments

Adsorption isotherms

The adsorption isotherm experiments were conducted for different As(V) concentrations ranging from 5 to 20 ppm. For all tests, 100 mL of As(V) solution containing 0.1 g adsorbent was poured into volumetric flask and stirred for 24 h at 25 °C, while pH of the solution was adjusted to 7 ± 0.5 in accordance with previously published works (Gohari et al. 2013a, 2013b, 2015). After reaching an equilibrium condition, the solution was centrifuged at 10,000 rpm for 20 min. The residual concentration of As(V) in the solution was determined by an atomic absorption spectrometer (Varian 220-Graphite Furnace, Shimadzu AA-670). The equilibrium adsorption capacity and removal efficiency of As(V) were calculated as follows:  
formula
(1)
 
formula
(2)
where C0 (mg/L), Ce (mg/L) and Ct (mg/L) are concentrations at the initial, equilibrium and time t (min) in the solution, respectively, V is the total volume (L) of the arsenic solution and M is the mass (g) of dry adsorbent used in the adsorption study.

Adsorption kinetics

Adsorption kinetic analysis was carried out by addition of 0.1 g of adsorbent into the 100 ml sealed beaker containing 100 ppb of As(V) at 25 °C. The removal efficiency and residual concentration of As(V) as a function of time were studied for 24 h at room temperature for specific time intervals. In these series of experiments, the pH of the As(V) solution was adjusted to 7 ± 0.5. In order to determine the kinetic mechanism of the As(V) adsorption, the experimental data were analyzed using pseudo-first-order and pseudo-second-order kinetic models.

Effects of initial solution pH

In order to identify the effects of initial solution pH on As(V) adsorption, 0.1 g of adsorbent was added to the 100 mL solution containing 100 ppb As(V) at different pH ranging between 3 and 9. The pH of solution was adjusted by adding 0.1 M HCl or 0.1 M NaOH. Under continuous shaking for 24 h at 25 °C, the concentration of As(V) of each pH solution was determined.

Desorption and regeneration experiments

Adsorption–desorption cycles, as one of the most interesting feathers of the current work, were conducted to investigate the reusability of modified adsorbents. Each cycle consisted of loading of the adsorbent (0.1 g) into the 100 mL of aqueous solution containing 100 ppb of As(V) at 25 °C. After 24 h of adsorption test, the solution was centrifuged and the adsorbents were separated. The used adsorbents were immersed in 100 mL water and the pH of solution was adjusted according to the results of the previous section and then the adsorption test was repeated. In order to determine the optimum contact time to achieve a complete regeneration of the adsorbents, various time intervals consisting of 15, 30, 60, 90 min were selected for the regeneration period. The optimum time was fixed and the process was continued up to the 10th cycle.

Adsorbents performance in arsenic removal from surface water

Performance of modified Mts in arsenic removal from surface water was also investigated. River water was taken from Zarrineh River near the Orumieh Salt Lake, Iran, and physico-chemical characteristics are listed in Table 1. Due to the lower concentration of arsenic in the sample; 23 ppb, additional Na2HAsO4.7H2O was added to the sample to increase the arsenate concentration to 100 ppb.

Table 1

Characterization of surface water sample

pHEC (μS/cm)SO4 (mg/L)CO32− (mg/L)HCO3 (mg/L)NO3 (mg/L)NO2 (mg/L)PO43− (mg/L)As(V) (μg/L)
7.8 383 37 2.3 155 4.3 0.021 0.1 23 
pHEC (μS/cm)SO4 (mg/L)CO32− (mg/L)HCO3 (mg/L)NO3 (mg/L)NO2 (mg/L)PO43− (mg/L)As(V) (μg/L)
7.8 383 37 2.3 155 4.3 0.021 0.1 23 

RESULTS AND DISCUSSION

Characterizations of unmodified and modified Mts

XRD analysis

XRD analysis is capable of measuring the interlayer space of clay mineral. Figure 2 exhibits the XRD results of pristine and modified montmorillonites. According to the XRD pattern of pristine Mt, the typical reflection at 2θ = 8.84° related to the basal spacing of 0.98 nm. For modified Mts with arginine and lysine denoted by Mt-Arg and Mt-Lys, respectively, the reflection at 8.84° shifts to 6.95° which corresponds to 1.3 nm basal spacing. The increase in the interlayer space is attributed to the successful intercalation of amino acid molecules in the interlayer of Mt. In order to identify the arrangement of amino acid molecules, dimensions of the amino acid molecules are necessary. According to the interlayer space increment (0.32 nm) of modified Mts and width of Arg and Lys molecules (∼0.3 nm), the interlayered amino acid molecules have a monolayer arrangement (Parbhakar et al. 2007).

Figure 2

XRD patterns of Mt, Mt-Arg and Mt-Lys.

Figure 2

XRD patterns of Mt, Mt-Arg and Mt-Lys.

FT-IR analysis

In order to clarify the interaction between amino acids and Mt, FT-IR spectroscopy analysis was carried out. Figure 3 shows the FT-IR spectra of Mt (a), Mt-Arg (b) and Mt-Lys (c). In the FT-IR spectra of Mt, the bands at 3,660 and 1,660 cm−1 are attributed to the O–H stretching and H–O–H bending of H2O, respectively. An intense peak at 1,100 cm− 1 is ascribed to the Si–O stretching. Finally, the Si–O and Al–O bending vibrations were observed around 400–1,000 cm−1. In the FT-IR spectra of Mt-Arg and Mt-Lys, the same peaks were observed with some additional peaks. It means that incorporation of Arg and Lys in the interlayer space of Mt just increases the interlayer space and does not induce changes in the crystal structure of Mt. These additional peaks correspond to the functional groups of Arg and Lys, which confirms the modification of Mt with these amino acids. The bands around 1,515 and 1,540 are attributed to the NH3+ symmetric deformation of α-amino group and side-chain amino group in the amino acid, respectively. The peak assignments at 1,759 cm−1 is related to the C = O stretching bands of the carboxylic group. In addition to these bands, the observed bands around 3,300 cm−1 are attributed to the NH3+ stretching (Parbhakar et al. 2007). The obtained results confirm the protonated state for two amino and carboxylic groups of amino acids.

Figure 3

FT-IR spectra of (a) Mt, (b) Mt-Arg and (c) Mt-Lys.

Figure 3

FT-IR spectra of (a) Mt, (b) Mt-Arg and (c) Mt-Lys.

Zeta potential

The zeta potential was measured for Mt, Mt-Arg and Mt-Lys versus pH and the obtained results are shown in Figure 4. The zeta potential of Mt was negative, which was similar with other reports (Avena et al. 1990). Positive zeta potentials were observed for modified Mts; Mt-Arg and Mt-Lys in lower pH values. The zeta potential was transformed to negative values by increasing the pH value. It is obvious that the isoelectric points of Mt-Arg and Mt-Lys are about 8, therefore, the positive zeta-potentials of modified Mts at neutral pH are due to the protonation of the NH2 group of amino acids, while the negative values at higher pH values are attributed to the deprotonation of the carboxyl group of Arg and Lys. Positive zeta potential indicates that the modified Mts have positive surface charge, which would increase the electrostatic attraction between the adsorbent surface and the As(V) anions. This provided further evidence to demonstrate that Arg and Lys have been intercalated between Mt layers.

Figure 4

Zeta potential of Mt, Mt-Arg and Mt-Lys.

Figure 4

Zeta potential of Mt, Mt-Arg and Mt-Lys.

TGA analysis

TGA analysis was carried out to determine both the thermal behavior of adsorbents and the quantity of intercalated surfactant molecules. The TGA curves for Mt, Mt-Arg and Mt-Lys are shown in Figure 5. According to the results, in case of modified Mts, the main weight loss had occurred in the three temperature domains of 30–200 °C, 200–550 °C and 550–800 °C. In case of pristine Mt, the main weight loss occurred in only two temperature domains including 30–200 °C and 550–800 °C. At the first temperature domain; 30–200 °C, a weight loss about 6.6% and 3.5% for pristine and modified Mts, respectively, can be mainly attributed to the loss of water. In comparison with pristine Mt, lower weight loss of modified Mts confirms the hydrophobic property of these samples. The second weigh loss for Mt-Arg and Mt-Lys could be corresponding to the decomposition of amino acids (Ghadiri et al. 2014). The amounts of amino acid intercalated the Mt were found to be 8.5%. Furthermore, the final weight loss between 550 and 800 °C was attributed to the dehydroxylation of the silicate structure (He et al. 2006). These results also confirm the successful modification of Mt with amino acids.

Figure 5

TGA analysis of Mt, Mt-Arg and Mt-Lys.

Figure 5

TGA analysis of Mt, Mt-Arg and Mt-Lys.

Adsorption of arsenic on synthetic Mts

Adsorption isotherms

The relationship between the equilibrium adsorption capacity of pristine and modified Mts versus the equilibrium concentrations of the As(V) solutions is shown in Figure 6(a). For all samples, the adsorption capacity of As(V); denoted by qe, would increase as the initial concentration of As(V) increased. Furthermore, there are clear differences between the pristine and modified Mts. The As(V) adsorption capacity of pristine Mt was very low due to the negative charge of the surface. However, the adsorption capacity for Mt-Arg and Mt-Lys was about five times as large as pristine Mt. These results implied that the modified Mts could attract As(V) anions due to their positive surface charge. Table 2 summarizes the arsenic adsorption capacities of the modified adsorbents examined in this study, as well as those of various adsorbents reported in the literature. The data show that Mt-Arg and Mt-Lys possessed high adsorption capacity toward As(V).

Table 2

Maximum As(V) adsorption capacities of different adsorbents

AdsorbentInitial concentration of As(V) (mg/L)Adsorption capacity (mg/g)Reference
Unmodified Mt 10–200 0.64 Mohapatra et al. (2007)  
Mt-Fex(OH)y 0–60 Zhang et al. (2010)  
Mt-Fe 10–80 15.15 Ren et al. (2014)  
Mt-TiO2 4.86 Li et al. (2012c)  
Mt-Zerovalent iron 2–200 45.5 Tandon et al. (2013)  
Mt-Arg 5–20 11.5 This study 
Mt-Lys 5–20 11 This study 
AdsorbentInitial concentration of As(V) (mg/L)Adsorption capacity (mg/g)Reference
Unmodified Mt 10–200 0.64 Mohapatra et al. (2007)  
Mt-Fex(OH)y 0–60 Zhang et al. (2010)  
Mt-Fe 10–80 15.15 Ren et al. (2014)  
Mt-TiO2 4.86 Li et al. (2012c)  
Mt-Zerovalent iron 2–200 45.5 Tandon et al. (2013)  
Mt-Arg 5–20 11.5 This study 
Mt-Lys 5–20 11 This study 
Figure 6

(a) Adsorption isotherms and (b) Langmuir isotherm model for the adsorption of As(V) by pristine and modified Mts (adsorbent dosage: 1 g/L, pH: 7, T: 25 °C).

Figure 6

(a) Adsorption isotherms and (b) Langmuir isotherm model for the adsorption of As(V) by pristine and modified Mts (adsorbent dosage: 1 g/L, pH: 7, T: 25 °C).

In order to correlate the equilibrium data, two main isothermal models including Langmuir and Freundlich were utilized as shown in Equations (3) and (4), respectively:  
formula
(3)
where KL is the Langmuir adsorption constant (L/mg), Ce is the As(V) concentration in solution (mg/L) and qmax is maximal adsorption capacity (mg/g). Note that KL and qmax can be determined from the slope and intercept of a linear plot of 1/q against 1/Ce, respectively,  
formula
(4)
where KF is the Freundlich constant and n is the heterogeneity factor. The constants for the Langmuir and Freundlich isotherms were calculated by the linear regression of isotherms and the obtained results are shown in Table 3. From R2 values, it can be concluded that the Langmuir adsorption isotherm is more suitable to represent the adsorption isotherm of As(V) for modified adsorbents. This finding was also confirmed by the linear plot of 1/qe versus 1/Ce shown in Figure 6(b). Based on Langmuir model, the adsorption takes place at specific homogeneous sites within the adsorbent and molecule occupies a single site (Li et al. 2012a). Since Arg and Lys exhibit cation exchange capability with Mt, a homogenous distribution of amino acids among the Mt layers was anticipated (Mallakpour & Dinar 2013).
Table 3

Langmuir and Freundlich isotherm parameters for As(V) removal on Mt-Arg and Mt-Lys

AdsorbentLangmuir model
Freundlich model
KL(L/mg)qmax(mg/g)R2KF(mg/g)1/nR2
Mt-Arg 1.07 13.02 0.9944 6.291 0.311 0.895 
Mt-Lys 0.81 12.8 0.9961 5.558 0.348 0.923 
AdsorbentLangmuir model
Freundlich model
KL(L/mg)qmax(mg/g)R2KF(mg/g)1/nR2
Mt-Arg 1.07 13.02 0.9944 6.291 0.311 0.895 
Mt-Lys 0.81 12.8 0.9961 5.558 0.348 0.923 

Investigation of adsorption kinetic

In order to investigate the adsorption kinetic of As(V) on the modified Mts, the removal efficiencies and residual As(V) concentrations of modified adsorbents as a function of time were investigated and the obtained results are shown in Figure 7. The trend of adsorption process was sharp in the first 30 min and adsorption occurred rapidly in this time interval for both adsorbents. The obtained results revealed that about 90% of As(V) was removed, while the residual concentration of As(V) reached to 10 ppb. Thereafter, As(V) adsorption continued at a slower rate to finally reach to the maximum removal efficiency after 120 min. The rapid adsorption is attributed to the fact that numerous vacant adsorption sites of adsorbents were available at initial times. The number of available adsorption sites decreased with an increasing in the contact time and led to a decline in adsorption.

Figure 7

The effect of contact time on the removal efficiency and residual concentration of As(V) (As(V) concentration: 100 ppb, adsorbent dosage: 1 g/L, pH: 7, T: 25 °C).

Figure 7

The effect of contact time on the removal efficiency and residual concentration of As(V) (As(V) concentration: 100 ppb, adsorbent dosage: 1 g/L, pH: 7, T: 25 °C).

In order to better understand the adsorption kinetic mechanism of the As(V), the experimental data were analyzed using pseudo-first and pseudo-second order kinetic models, shown in Equations (5) and (6), respectively:  
formula
(5)
 
formula
(6)
where qe and qt are the adsorption capacity of As(V) at equilibrium and time t, k1 and k2 are the first and second order rate constants, respectively. The obtained results shown in Table 4 revealed that the removal of As(V) using modified Mts could be more accurately described by the pseudo-second order kinetic model with the correlation coefficient of 0.992 and 0.994 for Mt-Arg and Mt-Lys, respectively. These results indicated that chemical adsorption might be the rate-limiting step for both adsorbents.
Table 4

The rate constants from the first- and second-order rate equations for As(V) adsorption on Mt-Arg and Mt-Lys

AdsorbentPseudo-first-order
Pseudo-second-order
Experimental qe (μg/g)
K1 (min−1)R2K2 (g/μg·min)qe (μg/g)R2
Mt-Arg 0.034 0.812 0.001 104 0.994 98 
Mt-Lys 0.038 0.883 0.0009 101 0.992 95 
AdsorbentPseudo-first-order
Pseudo-second-order
Experimental qe (μg/g)
K1 (min−1)R2K2 (g/μg·min)qe (μg/g)R2
Mt-Arg 0.034 0.812 0.001 104 0.994 98 
Mt-Lys 0.038 0.883 0.0009 101 0.992 95 

Effects of initial solution pH on arsenic removal

The solution pH is the most prominent parameter in As(V) adsorption, because it seriously affects the surface charge of adsorbent and degree of ionization of the As(V) oxyanions. Depending upon the solution pH, As(V) exists in various forms like H3AsO4, H2AsO4, HAsO42− and AsO43−. Figure 8 illustrates the adsorption behaviors of As(V) on Mt-Arg and Mt-Lys as a function of pH. At pH = 3.0, nearly 100% of As(V) was adsorbed onto both adsorbents. It is due to the positive charge of modified Mts in acidic condition and thus modified Mts were capable of removing As(V) anions. A slight decrease in As(V) adsorption was observed following an increase in the pH from 3 to 5 and 7. A slight decrease in As(V) adsorption is more pronounced for Mt-Lys. This is due to the higher zeta potential of Mt-Arg in comparison with Mt-Lys under acidic condition; therefore, Mt-Arg provide more adsorption sites for the As(V) anions.

Figure 8

The effect of pH variation on the As(V) removal efficiency of modified Mts (As(V) concentration: 100 ppb, adsorbent dosage: 1 g/L, T: 25 °C).

Figure 8

The effect of pH variation on the As(V) removal efficiency of modified Mts (As(V) concentration: 100 ppb, adsorbent dosage: 1 g/L, T: 25 °C).

By increasing pH over pHPI of adsorbents, repulsion between the negative side chains of amino acids and As(V) anions cause a decrease in adsorption. Higher concentration of competitive anions (OH) in alkaline medium also has a negative impact on the removal of As(V) anions from water (Gohari et al. 2013b). According to the Dzomback and Morel (Dzombak & Morel 1990), most oxyanions including As(V), tends to become less strongly adsorbed as the pH increases. The solution pH and redox potential are the most influential factors on arsenic speciation. By increasing pH to 9, the absorption efficiency dramatically decreases to 45% and 50% for Mt-Arg and Mt-Lys, respectively. Despite the lower As(V) adsorption at higher pH, this particular behavior can be potentially used in the regeneration process of modified Mts, since the adsorbed As(V) is easily dissociated from the adsorbent.

Regeneration studies

For the regeneration of adsorbents, the important aspect is to determine the optimum time to reach maximum removal efficiency. The regeneration efficiency, R, can be calculated using Equation (7):  
formula
(7)
where qr and qe (mg/g) are the adsorption capacity of regenerated and original adsorbent, respectively.

The obtained results regarding the regeneration efficiencies of modified adsorbents in various regeneration time cycles using alkaline solution with pH = 9 are shown in Figure 9(a). The obtained results exhibited 99% and 98.5% of regeneration for Mt-Arg and Mt-Lys after 60 min, respectively. Any further increase in the regeneration time cycle – up to 90 min – showed less impact on the degree of regeneration. Therefore, 60 min regeneration with the highest R value was selected as an optimum regeneration time cycle for both adsorbents.

Figure 9

(a) The effect of desorption time on the regeneration efficiency of As(V) adsorbed on modified Mts. (b) As(V) removal efficiency for modified Mts from repeated adsorption/desorption cycles (As(V) concentration:100 ppb, adsorbent dosage:1 g/L, pH: 7, T: 25°C, contact time: 60 min).

Figure 9

(a) The effect of desorption time on the regeneration efficiency of As(V) adsorbed on modified Mts. (b) As(V) removal efficiency for modified Mts from repeated adsorption/desorption cycles (As(V) concentration:100 ppb, adsorbent dosage:1 g/L, pH: 7, T: 25°C, contact time: 60 min).

Adsorption–desorption trials of modified Mts are shown in Figure 9(b). The obtained results indicated that As(V) removal efficiency for Mt-Arg was more than 90% after nine cycles. When adsorption–desorption cycle was increased to 10, the removal efficiencies of Mt-Arg and Mt-Lys were decreased from 98% and 96% to 89.5.5% and 83.7%, respectively. This finding indicates that the regeneration efficiency of Mt-Arg is higher than Mt-Lys. It is mainly due to the fact that the zeta potential of Mt-Arg was more negative than Mt-Lys in alkaline pH, which results in a higher regeneration efficiency of Mt-Arg in comparison with Mt-Lys. A very high regeneration efficiency of modified Mts proves that As(V) anions are bound through reversible interaction.

Mechanism of adsorption and desorption

According to the obtained results from several analyses, the mechanism of As(V) adsorption–desorption on modified adsorbents is proposed in Figure 10. Electrostatic interaction of As(V) anions and positive charge of amino acids in pH < pHPI give rise to a high adsorption capacity for modified adsorbents. Since the regeneration was carried out in pH = 9 which is higher than isoelectric points of modified Mts, there is repulsion between negative charged chains and As(V) anions.

Figure 10

Schematic adsorption and desorption of As(V) anions on modified Mt with amino acid.

Figure 10

Schematic adsorption and desorption of As(V) anions on modified Mt with amino acid.

Application of modified adsorbents to surface water

In order to determine the applicability of adsorbents in arsenic removal from surface water, Zarrineh River sample was selected. Existence of other pollutants in real conditions affects the As(V) removal efficiency of modified Mts. These results showed that the removal of As(V) decreased from 96% to 92% for Mt-Arg and from 94% to 88% for Mt-Lys. As can be seen in Figure 11, the As(V) removal efficiency is up 90% after five and four cycles for Mt-Arg and Mt-Lys, respectively. The results of this experiment indicated that fabricated Mts could be effective for As(V) removal from contaminated surface water.

Figure 11

As(V) removal efficiency in surface water in 5 adsorption/desorption cycles for modified Mts (As(V) concentration: 100 ppb, adsorbent dosage: 1 g/L, pH: 7, T: 25 °C, contact time: 60 min).

Figure 11

As(V) removal efficiency in surface water in 5 adsorption/desorption cycles for modified Mts (As(V) concentration: 100 ppb, adsorbent dosage: 1 g/L, pH: 7, T: 25 °C, contact time: 60 min).

CONCLUSIONS

Clay mineral; Mt was modified with arginine and lysine amino acids to obtain efficient, robust, regenerable and novel adsorbent for arsenic removal from water resources. Pristine and modified Mts were structurally and operationally characterized to explore the feasibility of using these adsorbents for As(V) removal from surface water. Due to the positive surface charges of prepared adsorbents, they showed promising performance in arsenic anions removal via electrostatic interaction. The equilibrium adsorption analysis were well fitted to the Langmuir isotherm model and kinetic investigations confirmed that the pseudo-second-order adsorption kinetic was predominated for the adsorption of As(V).

Regenerability of the prepared adsorbents as a very important aspect of the adsorption process from economy and environmental point of view was also investigated. The obtained results revealed that the adsorbents activities could be regenerated after As(V) adsorption for several adsorption–desorption cycles.

ACKNOWLEDGEMENTS

The authors would like to thank Iranian Nanotechnology Initiative Council (Grant no. 79312) for the financially supporting this project. In addition, the authors would like to express their appreciation to water and wastewater company's laboratory of East Azarbaijan.

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