We present a comparative study on the efficacy of TiO2 nanoparticles for arsenate ion removal after modification with CTAB (N-cetyl-N,N,N-trimethyl ammonium bromide) followed by coating with starch biopolymer. The prepared nanoparticles were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), thermogravimetry, scanning electron microscopy (SEM) and electron dispersive X-ray analysis (EDX). The removal efficiency was studied as a function of contact time, material dose and initial As(V) concentration. CTAB-modified TiO2 showed the highest arsenate ion removal rate (∼99% from 400 μg/L). Starch-coated CTAB-modified TiO2 was found to be best for regeneration. For a targeted solution of 400 μg/L, a material dose of 2 g/L was found to be sufficient to reduce the As(V) concentration below 10 μg/L. Equilibrium was established within 90 minutes of treatment. The sorption pattern followed a Langmuir monolayer pattern, and the maximum sorption capacity was found to be 1.024 mg/g and 1.423 mg/g after starch coating and after CTAB modification, respectively. The sorption mechanisms were governed by pseudo second order kinetics.

ABBREVIATIONS

  • ATC

    arsenic-treated SPC

  • CMT

    CTAB-modified TiO2

  • CTAB

    N-cetyl-N,N,N-trimethyl ammonium bromide

  • SPC

    starch-coated CMT

  • UTi

    unmodified TiO2

INTRODUCTION

TiO2 is currently considered as the most versatile nanomaterial. It has a wide range of applications in different fields (Xiaobo & Samuel 2007). These are attributed to its low cost, low toxicity, photostability in solution, redox selectivity and strong oxidizing power of holes. The application of TiO2 at the nanoscale further improves efficiency due to the high surface area, large pore volume and presence of high affinity surface hydroxyl groups (Daimei et al. 2006; Gupta et al. 2012). TiO2 and its different modified forms have also drawn the attention of researchers in the field of the removal of contaminants from water. It is capable of removing heavy metals as well as arsenite and arsenate from contaminated water (Gautham et al. 2010).

Gupta & Tripathi (2011) reviewed the use of TiO2 for environmental clean-up of organic pollutants through photo-oxidation. Xu et al. (2007) studied the photocatalytic degradation of organic arsenic to inorganic form by TiO2 and successful sorption on its surface.

Debnath & Ghosh (2009) applied nanostructured hydrous titanium oxide to adsorb Ni(II) from natural and industrial wastewater. The sorption of Se(IV) on TiO2 (anatase) was investigated by Keliang et al. (2009). Ghosh et al. (2003) and Barakat (2005) studied the use hydrous titanium oxide for adsorption of Cr(VI) and Cu(II), respectively.

Xu et al. (2010) prepared hydrous titanium dioxide (TiO2. xH2O) by one-step hydrolysis. The adsorption capacity of sorption of As(III) was found to be very high and was pH dependent. The sorption of As(III) and As(V) on commercially available TiO2 was also found to be dependent on pH as well as initial ion concentration (Dutta et al. 2004). Bang et al. (2005) experimented with a novel granular TiO2 for removal of groundwater arsenic and found that more arsenate was adsorbed than arsenite on TiO2 at pH 7.0. Nakajima et al. (2005) investigated the use of TiO2 photocatalyst and adsorbent for removal of As(V) under photo-irradiation.

Barakat (2005) studied photocatalytic degradation using UV-irradiated TiO2 suspension for destroying complex cyanide with a simultaneous removal of copper. Kajitvichyanukul et al. (2005) prepared TiO2 thin films immobilized on glass plates for removal of Cr(VI).

Heterogeneous photocatalytic oxidation of arsenite to arsenate in aqueous TiO2 suspensions has also been proved recently to be an effective and environmentally acceptable technique for the remediation of arsenite-contaminated water. Zhang & Itoh (2006) performed a process using an adsorbent developed by loading iron oxide and TiO2 on municipal solid waste melted slag for oxidation of arsenite to arsenate.

However, the dispersibility, reusability and leaching of particles are a major concern for the use of TiO2 in water purification. The nanoparticles have very high surface energy. Therefore, they tend to agglomerate so that they can reduce their surface energy by decreasing the surface area and thus stabilize (Gupta et al. 2012). Certain modifications are necessary to overcome these limitations. Adequate surface functionalization and coating with polymeric materials, for example, are some of the promising aspects regarding selectivity and aqueous stability of this material.

Use of surfactant-like CTAB increases the stability of TiO2 nanoparticles in water by forming micelles around them and thus can enhance the dispersibility of the particles in aqueous medium (Deka & Maji 2011). CTAB also contains a hydrophilic end (Miao et al. 2014), which might enhance the solubility of modified TiO2 particles in aqueous medium. Thus some particles may get leached in to water during wastewater treatment. This difficulty may be avoided by the use of supporting materials or by coating the nanoparticles with polymeric materials.

Table 1

Pseudo first order kinetic parameters for As(V) sorption

  CMT
 
SPC
 
Co (mg/L)a M (g)b Kad (min−1qe (mg/g) R2 Kad (min−1qe (mg/g) R2 
0.1 1.86 × 10−02 0.142 0.949 2.05 × 10−02 0.108 0.606 
0.2 9.17 × 10−03 0.202 0.780 3.22 × 10−02 0.127 0.882 
0.3 2.12 × 10−02 0.335 0.923 3.05 × 10−02 0.298 0.932 
0.4 1.78 × 10−02 0.450 0.953 2.70 × 10−02 0.324 0.935 
0.5 1.19 × 10−02 0.487 0.924 1.69 × 10−02 0.455 0.851 
0.6 8.04 × 10−03 0.528 0.869 2.18 × 10−02 0.483 0.909 
0.7 4.93 × 10−03 0.630 0.773 5.51 × 10−02 0.541 0.768 
  CMT
 
SPC
 
Co (mg/L)a M (g)b Kad (min−1qe (mg/g) R2 Kad (min−1qe (mg/g) R2 
0.1 1.86 × 10−02 0.142 0.949 2.05 × 10−02 0.108 0.606 
0.2 9.17 × 10−03 0.202 0.780 3.22 × 10−02 0.127 0.882 
0.3 2.12 × 10−02 0.335 0.923 3.05 × 10−02 0.298 0.932 
0.4 1.78 × 10−02 0.450 0.953 2.70 × 10−02 0.324 0.935 
0.5 1.19 × 10−02 0.487 0.924 1.69 × 10−02 0.455 0.851 
0.6 8.04 × 10−03 0.528 0.869 2.18 × 10−02 0.483 0.909 
0.7 4.93 × 10−03 0.630 0.773 5.51 × 10−02 0.541 0.768 

aCo: Initial concentration of As(V) solution.

bM: Mass of CMT and SPC (adsorbent).

Starch, one of the cheapest and nontoxic polymers, is abundantly available in nature. Starch and starched-based materials used as bio-compatible materials have been cited in the literature (Marques et al. 2002; Saikia et al. 2010). He & Zhao (2005) have reported the advantage of coating starch onto bimetallic nanoparticles (Fe-Pd) for degradation of chlorinated hydrocarbons in water. A significant improvement in arsenic removal rate from brine by using starch-coated magnetite nanoparticles has been reported (An et al. 2011). Stabilization with starch causes less agglomeration and improves removal rate compared to the bare nanoparticles. The processing of starch for stabilizing nanomaterials is quite easy. Starch can be easily solubilized in hot water and re-precipitated by simply adding acetone or ethanol. The coating of the nanomaterial by starch can be done in aqueous medium without using any other toxic chemicals.

The aim of the present work is to prepare CTAB-modified TiO2 nanoparticles and stabilize them by coating with starch, and finally use the particles in the treatment of As(V)-contaminated water. A comparative study was made using unmodified TiO2 (UTi), CTAB-modified TiO2 (CMT) and starch-coated CMT (SPC). Low concentrations of arsenic solutions were used in the experiments considering the practical applicability of the material.

MATERIALS AND METHODS

Analytical grade nanosized TiO2 (mixture of rutile and anatase), potassium sulfate (K2SO4), and sodium arsenate dibasic heptahydrate (HAsNa2O4.7H2O) were purchased from Sigma-Aldrich. Starch polymer was purchased from SRL, India. Gluteraldehyde solution (25%), used as cross-linking agent, and di-sodium hydrogen phophate (Na2HPO4) were purchased from E-Merck, India. CTAB (N-cetyl-N,N,N-trimethyl ammonium bromide) was purchased from LOBA Chemie, India. Ethanol was supplied by Jiangsu Huaxi International Trade Co. Ltd, China. Deionized water was obtained from Milli-Q water purification system (Millipores.A.S.67 120 Molsheim, France). All other reagents and chemicals used were of analytical grade.

Methods of preparation of starch-coated TiO2 nanocomposite

Coating of starch over TiO2 particles was carried out in two steps following the method reported by Deka & Maji (2011). Initially TiO2 was modified with CTAB and then was coated with starch.

Preparation of CTAB-modified TiO2 nanoparticles

One gram of TiO2 was taken in a round-bottomed flask containing 1:1 ethanol–water mixture. The flask was fitted with a spiral condenser and kept at 80 °C for 12 h under magnetic stirring. In a similar set up, 1.2 g of CTAB was refluxed with 1:1 ethanol–water mixture at 80 °C for 3 h. This mixture was poured into the TiO2 solution mixture and was stirred again for another 6 h at 80 °C. It was then filtered and washed with lukewarm deionized water several times and oven-dried at 45 °C. Finally, the material was ground and stored in a desiccator to avoid moisture absorption.

Preparation of starch-coated CMT nanocomposite

One gram of CMT was dispersed in a 250 mL beaker under continuous magnetic stirring for 1 h and then sonicated for 30 min to completely disperse the CMT nanoparticles in the solvent.

In the meantime, a solution of starch was prepared by dissolving 0.5 g starch in 50 mL of hot water (80 °C). The CMT solution was then mixed with the starch solution and stirred for 1 h. Starch molecules typically have many –OH groups on them, which are polar. These –OH groups interact with the polar ends of CTAB and thus form a coating around them. The mixture was then poured slowly into acetone under stirring to precipitate the SPC particles.

The temperature of the beaker was brought down below room temperature, then glutaraldehyde solution (5% w/w) was added to the mixture containing SPC particles under magnetic stirring. The temperature was again raised gradually to 50 °C and the reaction was continued for another 2 h under continuous stirring condition. The temperature was then decreased to room temperature. The nanoparticles thus formed were filtered, washed with deionized water, dried and kept ready for subsequent applications.

FT-IR analysis

Fourier transform infrared spectrophotometry (FT-IR) (NICOLET Impact I-410) was used to study the probable interaction between different component materials of the prepared nanoparticles as well as As(V) binding to the adsorbent. The study was carried out in the range 500–4,000 cm−1.

Thermogravimetry

Thermogravimetric analysis (TGA) was carried out for starch, UTi, CMT and SPC at a heating rate of 20 °C/min up to 500 °C under N2 atmosphere using a thermogravimetric analyser (Metler TA 4000). The polymer content incorporated could be calculated using the following equation (Unsoy et al. 2012): 
formula
1
where Ws is the weight loss (%) from 100 °C to 500 °C for SPC and Wn is the weight loss (%) for bare CMT nanoparticles.

X-ray diffractometry

The distribution of the clay minerals within the polymer matrix in the nanoparticle was characterized by powder X-ray diffractometry (XRD) using a Miniflex instrument (Rigaku, UK). The measurement conditions were: anode material = Cu; K-alpha, λ = 1.54 Å and scanning rate 2°/min with an angle ranging from 2° to 80°.

Scanning electron microscopy-electron dispersive X-ray analysis

A scanning electron microscope (SEM) (JEOL 6390LV) was used to study the presence of elements as well as morphology of the samples before and after As(V) treatment.

pH measurement

The pH of the arsenic solutions was adjusted using NaOH (0.1 M) and/or HCl (0.1 M) solutions as required and measured by Cyberscan pH 510 (Eutech) instrument.

Atomic absorption spectroscopic analysis

The As(V) concentration was measured by using an Analyst 200 Atomic Absorption Spectrophotometer (Perkin Elmer). All the measurements were based on integrated absorbance and performed at 193.7 nm by using a quartz tube analyser (Perkin Elmer) at an atomization temperature of 2,000 K.

Batch adsorption experiments

A stock solution of 1,000 mg/L As(V) was prepared by dissolving the requisite amount of sodium arsenate (Na3AsO4) in 1,000 mL deionized water. The stock solution was diluted in steps to obtain standard solutions containing 0.1–0.7 mg/L of arsenic solution. The material dose and time for removal of As(V) was optimized by using the solution containing 400 μg As(V)/L. Batch adsorption studies were carried out with 2 g of adsorbent per litre of solution (as determined previously) of desired concentration at neutral pH in 100 mL conical flasks, which were agitated at 160 rpm for predetermined time intervals at room temperature on a mechanical shaker. After agitation, the suspension was centrifuged and filtered, and the arsenic concentration left in the aliquot was determined by atomic absorption spectrophotometry. A comparative study on the efficacy of UTi, CMT and SPC for As(V) removal was done.

The kinetic study for the adsorption of As(V) was conducted at neutral pH. In order to investigate the sorption mechanism for the removal process, pseudo first and second order kinetic models were used in different experimental conditions. The adsorption of As(V) was carried out for 90 min with initial As(V) concentration 0.1–0.7 mg/L for a material dose of 2 g/L. The kinetic parameters for sorption of As(V) for both CMT and SPC were calculated by using Lagergren's equations (Bhatt et al. 2012).

A simple pseudo first order kinetic model is represented as: 
formula
2
where, qt is the amount of As(V) adsorbed in mg/g at time t, qe is the amount of As(V) adsorbed (mg/g) at equilibrium and Kad is the rate constant of adsorption (min−1).
The simplest and the most popular linear form pseudo second order kinetic models is represented by the following equation: 
formula
3
where qt=qe2kt/(1 +qtkt), the amount (mg/g) of As(V) adsorbed on the surface of material at any time t, k being second order rate constant (g/mg min), qe is the amount adsorbed at equilibrium, and h=kqe2 (mg/g min) is the initial sorption rate.

Freundlich, Langmuir and Temkin isotherm models were applied for the study of equilibrium adsorption of As(V) on CMT and SPC.

The Langmuir isotherm is represented by the following equation (Ho 2006): 
formula
4
where, qe is the equilibrium quantity adsorbed (mg/g), Ce is the equilibrium concentration (mg/L), qm is the maximum adsorption capacity (mg/g) and b is the Langmuir constant.
The separation factor, RL an important parameter indicating the favourability of the adsorption based on the Langmuir equation is calculated using Equation (5). 
formula
5
where b and C0 have their common meaning. The value of RL indicates the type of adsorption either to be unfavourable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0).
The linear form of Freundlich adsorption equation is represented by the equation (Ho 2006): 
formula
6
where qe is the adsorbed amount (mg/g), Ce is the equilibrium arsenic concentration (mg/L), Kf (mg/g) is the Freundlich constant related to adsorption capacity and n is a constant related to energy of intensity of adsorption.
The Temkin model takes adsorbent–adsorbates interactions into account. By ignoring the extremely low and large value of concentrations, the model assumes that heat of adsorption (function of temperature) of all molecules in the layer would decrease linearly rather than logarithmically with coverage (Dada et al. 2012). The Temkin isotherm model is represented by the equation: 
formula
7
where AT = Temkin isotherm equilibrium binding constant (L/g), b = Temkin isotherm constant, R = universal gas constant (8.314 J/mol/K) and T = temperature at 298 K; qe and Ce have their common meaning.

REUSABILITY STUDY

The reusability test was performed for CMT and SPC four times. An As(V) solution with an initial concentration of 400 μg/L was treated for 90 min with 2 g/L of CMT and SPC, respectively. It was then filtered and the concentration of As(V) in the filtrate was measured to determine the removal efficiency of the materials (CMT or SPC). The material left as residue was washed with dilute HCl acid for desorption of the As(V) ions adsorbed on its surface. It was then dried, weighed and used for further study to access the removal efficiency against the As(V) solution with same initial concentration. The process was repeated under the same conditions.

RESULTS AND DISCUSSION

Characterization

FT-IR analysis

FT-IR spectra of CTAB, UTi and CMT are shown in Figure 1(A). The FT-IR spectrum of UTi nanoparticles is shown in Figure 1(A(a)). The strong absorbance near 500 cm−1 is attributable to the Ti–O–Ti stretching of TiO2. The absorbance at 3,436–3,490 (–OH stretching) and 1,636 cm−1 (–OH bending) are attributable to the surface hydroxyl groups of TiO2 (Xu et al. 2010).
Figure 1

FT-IR spectra. (A) (a) UTi, (b) CTAB, (c) CMT. (B) (a) starch, (b) SPC, (c) ATC.

Figure 1

FT-IR spectra. (A) (a) UTi, (b) CTAB, (c) CMT. (B) (a) starch, (b) SPC, (c) ATC.

In the spectrum of CTAB, shown in Figure 1(A(b)), the absorption bands at 2,916 and 2,848 cm−1, respectively, are attributable to asymmetric and symmetric modes of –CH2 stretching in the methylene chains (Kungt & Hayes 1993). Band near 1,474 cm−1 may be due to symmetric stretching of CH3+-N bond (Thakur 2015).

Bands near 2,916 cm−1, 2,850 cm−1 and 1,463 cm−1 (corresponding to CTAB) along with bands at 1,636 cm−1 and near 530 cm−1 (corresponding to TiO2) in the CMT spectrum indicate the probable interactions between the individual components.

Figure 1(B) shows the FT-IR spectra of starch, SPC and arsenic-treated SPC (ATC). The spectrum for starch (Figure 1(B(a))) shows a broad band near 3,440 cm−1 for –OH stretching, 2,914 cm−1 for –CH stretching and 1,656 cm−1 for –OH bending. The two peaks in the region 1,183–966 cm−1 are attributable to C–O bond stretching of C–O–H group (Kousalya et al. 2010). Incorporation of CMT into starch can be well predicted from the curve (b). Increase in intensity of bands corresponding to –OH stretching and –OH bending as well as appearance of a new band near 1,410 cm−1 (CH3–N+ of CMT) is an indication for interaction between the two components.

In the FT-IR spectrum of ATC (Figure 1(B(c))), it was observed that the intensity of bands assigned to the hydroxyl groups and C–N stretching in SPC decreased prominently. This might be due to the participation of those groups in arsenic binding. The band appearing near 632 cm−1 is attributable to the adsorbed arsenate ions on to the material (Sham et al. 1998).

SEM analysis

Figure 2(a)2(d) show the SEM micrographs of UTi (a), CMT (b), SPC (c) and ATC (d). Sorption and desorption of arsenic for both SPC and CMT were done for four times using the same nanoparticles and then micrographs were taken. Figure 2(e) and 2(f) are micrographs of the used SPC and CMT.
Figure 2

SEM images. (a) UTi, (b) CMT, (c) SPC, (d) ATC, (e) SPC/4 (used 4 times), (f) CMT/4 (used 4 times).

Figure 2

SEM images. (a) UTi, (b) CMT, (c) SPC, (d) ATC, (e) SPC/4 (used 4 times), (f) CMT/4 (used 4 times).

CMT nanoparticles (Figure 2(b)) became less agglomerated than those of unmodified TiO2 (Figure 2(a)) due to the presence of long hydrocarbon chains of CTAB which prevented them from agglomerating. Jiang et al. (2013) studied the morphology of CTAB-modified Ni nanoparticles and observed that modification with CTAB resulted in irregular sphere-like structures which were less aggregated than Ni-only nanoparticles. ATC (Figure 2(d)) composite appeared slightly rough compared to SPC (Figure 2(c)) composites. This might be due to sorbed arsenic on the composite structure. The surface of arsenic desorbed (i.e. reused) composite (Figure 2(e)) was rougher than SPC (Figure 2(c)). This might be attributed to the disintegration of the surface of the composite caused by the repeated washing with acid during sorption-desorption treatment. Similarly the shape of CMT nanoparticles (Figure 2(f)) became irregular and rough due to the acid treatment.

EDX study

Figure 3(a)3(e) show the electron dispersive X-ray analysis (EDX) spectra of UTi, CTAB, CMT, SPC, arsenic treated CMT (AMT) and ATC, respectively. EDX spectra of UTi (Figure 3(a)) and CTAB (Figure 3(b)) show the presence of their corresponding elements in their structure. The spectrum shown in Figure 3(c) indicates the incorporation of bromine, carbon and nitrogen to the CMT particles. This confirms the successful modification of TiO2 nanoparticles by CTAB.
Figure 3

EDX spectra. (a) UTi, (b) CTAB, (c) CMT, (d) SPC, (e) AMT, (f) ATC.

Figure 3

EDX spectra. (a) UTi, (b) CTAB, (c) CMT, (d) SPC, (e) AMT, (f) ATC.

The relative content of carbon and oxygen was greater in SPC (Figure 3(d)) than CMT (Figure 3(c)) suggesting the participation of starch polymer into the system. Presence of arsenic in the spectra of both arsenic adsorbed CMT and SPC confirmed the successful sorption of As(V) onto the systems. This further supports the findings of FT-IR data as discussed earlier (Figure 1(d)).

TGA study

The thermograms for starch, UMT, CMT and SPC are shown in Figure 4(A). All four samples exhibited their own distinctive TGA curves. The initial weight loss observed in starch (Figure 4(A(a))) and SPC (Figure 4(A(d))) at around 100 °C was due to the evaporation of the small amount of moisture present in the samples.
Figure 4

(A) TGA curves. (a) starch, (b) UTi, (c) CMT, (d) SPC. (B) XRD spectra. (a) starch, (b) UTi, (c) CMT, (d)SPC.

Figure 4

(A) TGA curves. (a) starch, (b) UTi, (c) CMT, (d) SPC. (B) XRD spectra. (a) starch, (b) UTi, (c) CMT, (d)SPC.

The TGA curve for CMT (Figure 4(A(c))) shows some weight loss in the temperature range ∼220–320 °C; UTi (Figure 4(A(b))) shows hardly any weight loss in that region. The weight loss in CMT is due to the incorporated CTAB on the surface of TiO2 nanoparticles. Coating of CMT with starch in SPC showed much more weight loss as compared to CMT. But at the same time the weight loss was less than the pristine polymer. This indicated that interaction had taken place between CMT and starch, and starch had successfully coated the surface of CMT.

No separate degradation patterns were observed for the TGA curve of SPC indicating that the mixing of starch and CMT nanoparticles did not form a blend. Rather, starch properly interacted with CMT nanoparticles.

XRD study

The XRD spectra of UTi, CMT and SPC are shown in Figure 4(B). The XRD patterns of UTi (Figure 4(B(b))) exhibit strong diffraction peaks at approximately 2θ = 25 ° (101), 37 ° (111), 47 ° (210), 53 ° (211) and 55 ° (220). The peaks occur due to the crystalline portion of the TiO2. Similar findings were reported by Juang & Shiau (2000) and Wang et al. (2008). The diffraction peak at 21° is for crystallinity of starch.

In the diffractogram of CMT (Figure 4(B(c))) and SPC (Figure 4(B(d))), no remarkable change in either peak positions/or peak intensity for TiO2 crystals can be observed. This suggests that the starch coating process did not result in any phase change of TiO2 particles.

BATCH ADSORPTION EXPERIMENT FOR AS(V) REMOVAL

Comparison of effectiveness of UTi, CMT and SPC towards As(V) removal

The effectiveness of UTi, starch, CMT and SPC materials for As(V) removal from contaminated water was tested. A targeted solution of 400 μg/L and material dose of 2 g/L were used. Starch alone did not produce any significant result. UTi, CMT and SPC showed good results as shown in Figure 5.
Figure 5

Efficacy of UTi, CMT and SPC for As(V) removal.

Figure 5

Efficacy of UTi, CMT and SPC for As(V) removal.

CMT particles exhibited better removal efficiency than UTi particles and brought down the arsenic concentration below 10 μg/L. CTAB prevented the aggregation of UTi particles and thus exposing more surfaces for interaction with arsenate ions in solution. However, part of the CMT entered into the aqueous phase during the experiment causing a significant loss of the material. This could be reduced by coating with crosslinked starch (SPC). Hence both CMT and SPC were used in the subsequent experiments.

Effect of material dosage on percentage removal of As(V)

The effect of material dose was studied by treating a laboratory-prepared arsenic(V) solution of known concentration with varying doses of CMT and SPC for about 5 h. Results are shown in Figure 6(a). Percentage removal of As(V) was plotted as a function of material dosage. On increasing material dosage of CMT and SPC, percentage removal of As(V) was observed to be increased.
Figure 6

Effect of (a) material dose, (b) treatment time, (c) initial ion concentration on As(V) removal (%).

Figure 6

Effect of (a) material dose, (b) treatment time, (c) initial ion concentration on As(V) removal (%).

It was found that for a solution of concentration 400 μg/L, a dose of 2 g/L of CMT was sufficient to bring down the As(V) concentration below 10 μg/L. No prominent change was observed with the increase of material dosage thereafter. Therefore, the material dose was fixed at 2 g/L for further study. Figure 6(a) shows that CMT has better efficiency compared to SPC for As(V) removal. However, reusability of CMT is a major problem as discussed later whereas starch coating is a good alternative.

Influence of agitation time on arsenic removal

Figure 6(b) shows the effect of agitation time on percentage removal of the As(V) ion. With increase in treatment time, the percentage removal of As(V) reached maximum (99% for CMT and 83% for SPC) after 90 min treatment and after that remained almost constant throughout the time period studied. This might be due to the rapid achievement of equilibrium between the arsenic adsorbed on the surface of sorbent and arsenic present in solution. Therefore, the agitation time was kept fixed at 90 min for the subsequent batch experiments.

Influence of initial As(V) concentration

The effect of initial arsenic(V) ion concentration was studied in the concentration range 0.1 to 0.7 mg/L. Material dose and agitation time were kept fixed as optimized before. Figure 6(c) shows the effect of As(V) concentration on percentage removal of the ion. With the increase in metal ion concentration, the amount of metal ion uptake per unit weight of adsorbent (mg/g) had also increased. It was observed that the percentage removal of As(V) decreased with the increase in initial metal ion concentration. This might be attributed to a delayed establishment of equilibrium between the adsorbent sites and the As(V) ions. The saturation of sorbent sites with the increase in initial ion concentration might also be the reason for diminished removal rate.

ADSORPTION KINETICS

Figure 7(A) and 7(B) show the variation of adsorbed amount (mg/g) of arsenate ions over time for different initial ion concentrations for SPC and CMT, respectively. Additional plots of log (qe−qt) vs t (Equation (2)) for the SPC and CMT gives straight lines from which the different pseudo first order kinetic parameters were evaluated (Table 1).
Figure 7

Time evalution of qt for different initial As(V) concentrations for (A) SPC and (B) CMT.

Figure 7

Time evalution of qt for different initial As(V) concentrations for (A) SPC and (B) CMT.

Similarly the different pseudo second order kinetics parameters were determined from the linear plot of t/qt vs t. The correlation coefficients (R2) were computed and all the values are shown in Table 2. The qe values were found to increase with the increase in the initial As(V) concentrations. The curve for pseudo second order kinetics exhibited higher correlation co-efficient compared to pseudo first order model.

Table 2

Values of different second order kinetic parameters at different initial As(V) concentrations

  CMT
 
SPC
 
C0 (mg/L)a Mass (g)b qe (mg/g) K (g/mg min) h (mg/g min) R2 qe (mg/g) K (g/mg min) H (mg/g min) R2 
0.1 0.196 0.633 0.025 0.995 0.174 6.221 0.189 0.999 
0.2 0.354 0.568 0.071 0.998 0.362 0.271 0.035 0.990 
0.3 0.597 0.154 0.055 0.992 0.538 0.148 0.043 0.988 
0.4 0.767 0.112 0.066 0.988 0.707 0.094 0.047 0.980 
0.5 0.868 0.148 0.112 0.995 0.809 0.085 0.056 0.971 
0.6 0.962 0.197 0.182 0.997 0.889 0.076 0.059 0.968 
0.7 1.010 0.297 0.304 0.998 1.091 0.052 0.062 0.945 
  CMT
 
SPC
 
C0 (mg/L)a Mass (g)b qe (mg/g) K (g/mg min) h (mg/g min) R2 qe (mg/g) K (g/mg min) H (mg/g min) R2 
0.1 0.196 0.633 0.025 0.995 0.174 6.221 0.189 0.999 
0.2 0.354 0.568 0.071 0.998 0.362 0.271 0.035 0.990 
0.3 0.597 0.154 0.055 0.992 0.538 0.148 0.043 0.988 
0.4 0.767 0.112 0.066 0.988 0.707 0.094 0.047 0.980 
0.5 0.868 0.148 0.112 0.995 0.809 0.085 0.056 0.971 
0.6 0.962 0.197 0.182 0.997 0.889 0.076 0.059 0.968 
0.7 1.010 0.297 0.304 0.998 1.091 0.052 0.062 0.945 

aCo: Initial concentration of As(V) solution.

bMass of CMT and SPC (adsorbent).

EQUILIBRIUM ADSORPTION STUDY

Figure 8(a) and 8(b) show the growth of adsorbed amount (mg/g) at equilibrium over the surface of SPC and CMT, respectively, against increase in equilibrium concentrations. To establish the influence of As(V) concentration on the adsorption process, the equilibrium data has been analysed by linear forms of the Freundlich, Langmuir and Temkin models using Equations (4)(6), respectively, and is shown in Table 3.
Table 3

Sorption parameters of different isotherm model

Model SPC CMT 
Langmuir qm (mg/g) 1.024 qm (mg/g) 1.423 
 b (L/mg) 131 B (L/mg) 10.82 
 R2 0.989 R2 0.998 
 RL 0.019 RL 0.187 
Freundlich Kf (mg/g) 2.854 Kf (mg/g) 2.751 
 2.557 1.666 
 R2 0.931 R2 0.944 
Temkin AT (L/g) 1.28 × 103 AT (L/g) 1.28 × 102 
 B (J g/mg21.13 × 104 B (J g/mg28.62 × 103 
 R2 0.987 R2 0.974 
Model SPC CMT 
Langmuir qm (mg/g) 1.024 qm (mg/g) 1.423 
 b (L/mg) 131 B (L/mg) 10.82 
 R2 0.989 R2 0.998 
 RL 0.019 RL 0.187 
Freundlich Kf (mg/g) 2.854 Kf (mg/g) 2.751 
 2.557 1.666 
 R2 0.931 R2 0.944 
Temkin AT (L/g) 1.28 × 103 AT (L/g) 1.28 × 102 
 B (J g/mg21.13 × 104 B (J g/mg28.62 × 103 
 R2 0.987 R2 0.974 
Figure 8

Variation of qe with Ce for (a) SPC and (b) CMT.

Figure 8

Variation of qe with Ce for (a) SPC and (b) CMT.

The Langmuir parameters viz. qm, b and R2 values were found to be 1.024 mg/g, 1.31 × 102 and 0.989, respectively, for SPC and 1.423 mg/g, 10.82 and 0.998, respectively, for CMT.

In Table 3, it can be seen that the sorption was favourable for both materials as RL values (0.0187 for SPC and 0.187 for CMT) occurred between 0 and 1 for each case. Also qe vs Ce showed a linear relationship.

Freundlich parameters R2 and n values, as shown in Table 3, also indicated good fitting of sorption data with the model. The Temkin parameters were also derived and the correlation coefficient (R2 ∼ 0.987 and 0.984) values showed that Temkin model could well explain the sorption pattern.

It was observed that the R2 values for all the models showed good correlations of the data. Thus the sorption could be reasonably explained by all the models under consideration. The preferential order for sorption, among these models is as follows: Langmuir > Temkin > Freundlich.

A comparison of the maximum adsorption capacity (qo) calculated from the Langmuir model with some other established materials is shown in Table 4. Ferruginous manganese ore has a high adsorption capacity as shown in Table 4, but arsenic does not get desorbed easily even on varying the pH (Chakravarty et al. 2002). The material under present investigation showed comparable results.

Table 4

Comparison of adsorption capacity with other established material

Material Arsenic species qo (mg/g) 
Fe3O4-NPs (Akin et al. 2012As(V) 0.400 
Chitosan-coated bentonite (Arida et al. 2015As(V) 0.011 
Ferruginous manganese ore (Chakravarty et al. 2002As(V)
As(III) 
15.38
0.537 
SPC
CMT 
As(V)
As(V) 
1.024
1.423 
Material Arsenic species qo (mg/g) 
Fe3O4-NPs (Akin et al. 2012As(V) 0.400 
Chitosan-coated bentonite (Arida et al. 2015As(V) 0.011 
Ferruginous manganese ore (Chakravarty et al. 2002As(V)
As(III) 
15.38
0.537 
SPC
CMT 
As(V)
As(V) 
1.024
1.423 

RESUSABILITY TEST

Reusability experiments were carried out four times for both CMT and SPC. Each time the samples were treated with 0.1 M HCl solution for desorption of the adsorbed As(V). The results are shown in Figure 9(a). As shown in the figure, CMT showed better As(V) removal efficiency than that of SPC under identical conditions.
Figure 9

Comparison of (a) reusability, (b) weight remaining of CMT and SPC after reuse.

Figure 9

Comparison of (a) reusability, (b) weight remaining of CMT and SPC after reuse.

The weight loss after each time of reuse was also calculated for both CMT and SPC. It was found that the weight loss (%) of material was low (Figure 9(b)) for SPC compared to CMT. This implied the successful stabilization of CMT particles inside the starch matrix resulting in a decreased leaching rate. Use of crosslinking agent also might have some contribution for observing lower weight loss in the case of SPC.

Effect of competitive ions on removal of arsenate

The effect of competitive ions, i.e., phosphate and sulfate, on arsenate removal by CMT in synthetic water was investigated. Figure 10 shows the variation of the sorption rate of arsenate ions with the elevated concentration of phosphate and sulfate ions. The experiments were carried out with prepared solutions containing arsenate/phosphate and arsenate/sulfate ions at neutral pH. The change in removal rate was monitored with the elevated concentration of phosphate and sulfate ions in two separate experiments.
Figure 10

Effect of competition ions on removal As(V) by CMT.

Figure 10

Effect of competition ions on removal As(V) by CMT.

With the increase in their respective concentrations, both phosphate and sulfate ions were found to notably reduce the removal rate of arsenate. This might be due to the similarity in chemistry of As(V) with phosphate and sulfate in aqueous solution at neutral pH. The effect of phosphate is more pronounced (Figure 10) compared to that of sulfate. The resemblance of ionization constants and ionic structure of phosphate with arsenate is responsible for this anomaly (Guan et al. 2012).

CONCLUSION

FT-IR analysis indicated successful modification of TiO2 particles with CTAB surfactant. Good interaction of the modified form with starch polymer was also observed. These findings were further supported by TGA, SEM and EDX analysis. XRD data showed that the phase of TiO2 remained unaltered during the modification or coating process. Successful sorption of As(V) was confirmed by FT-IR, SEM and EDX analysis. Both CMT and SPC were capable of reducing the As(V) concentration from 400 μg/L to less than 10 μg/L.

The equilibrium data were fitted to Langmuir, Freundlich and Temkin adsorption isotherm models. The Langmuir isotherm model was found to be better fitted than the other two models as judged by their equilibrium data. The pseudo second order kinetic model showed better correlation than pseudo first order model for sorption of As(V) on CMT. Competitive ions like phosphate and sulfate reduce the arsenate removal rate by CMT.

Sorption of As(V) occurred in all the materials, i.e., UTi, CMT and SPC. SPC showed the best result when efficiency, reusability and integrity all are taken under consideration.

ACKNOWLEDGEMENTS

The authors thank DRL, Tezpur for providing instrumental facilities.

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