Multi-metal oxide incorporated microcapsules for efficient As(III) and As(V) removal from water

Abstract

Arsenic contamination in ground water is a major problem in various countries, and causes serious health issues, such as cancer and skin diseases. Among the many different methods of arsenic removal, extraction using functional adsorbents has been employed owing to its low cost, simplicity, and fast regeneration. In this paper, preparation and characterization of mixed metal oxide loaded polycaprolactone microcapsules and their application as adsorbents for both As(III) and As(V) from water is demonstrated. The presence of CeO₂ nanoparticles on microcapsules enhance the affinity towards As(III), while magnetic Fe₃O₄ nanoparticles help to bind As(V) and facilitate the removal of capsules from the medium under an applied magnetic field. Polyvinylpyrrolidone was used as a surface modifier to prepare microcapsules with high surface roughness and large surface area. Efficient removal of As(III) and As(V) can be achieved using the modified microcapsules within 15 minutes. The arsenic extraction was insensitive to changes in pH within a range of 3–11. The microcapsules were recycled using 1 M NaOH solution and reused 3 times without losing significant extraction efficiency. The microcapsules showed a maximum Langmuir adsorption capacity of 32 and 28 mg g⁻¹ for As(III) and As(V), respectively. In summary, our metal oxide incorporated polycaprolactone microcapsules can be used as efficient and readily removable adsorbents for arsenic removal from ground water.

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Introduction

Arsenic is widely known for its toxicity and exists mainly in its inorganic form as trivalent As(III) and pentavalent As(V) in groundwater.¹ According to the speciation diagram described by Lombi et al. (1999), As(III) is mainly neutral at pH < 9 while As(V) is both mono and di-anionic at pH < 12.² Serious health problems such as skin,³ cardiovascular,⁴ renal,⁵ hematological⁶ and respiratory disorders⁷ are observed as a result of long time exposure to high concentrations of arsenic in drinking water. The World Health Organization (WHO) has set provisional guidelines for arsenic content in drinking water to be below 10 ppb, but the developing countries affected with contaminated groundwater are still struggling to maintain arsenic concentration in the acceptable range.⁸

All known arsenic removal processes can be divided into four categories: ion exchange, membrane process, chemical precipitation, or adsorption. Ion-exchange treatments are very limited for the removal of arsenic owing to the interference from other anions found in groundwater.^9–12 on the other hand, membrane processes are effective in removing arsenic from groundwater, but the high cost of installing and maintaining such systems limit wide usage, especially in underdeveloped countries.^13–22 Chemical precipitation may seem to be a cost-effective process, however it suffers from generation of residual metal sludge.²³ As(III) is more toxic and usually more difficult to remove than As(V),²⁴ therefore, adsorption is being explored as alternative methods for arsenic removal from ground water.²³ A few adsorbents such as metal oxide loaded MCM-41 were used for removing arsenic from ground water, but high cost and difficulty in getting these adsorbents in large quantities hinder the field implementation of such technologies.²⁵ Metal oxides such as TiO₂, Fe₃O₄, CeO₂, and ZrO₂ nanoparticles have been employed for arsenic removal,^26,27 and showed high efficiency of extraction owing to their larger surface area and enhanced surface charges.²⁸ Desired characteristics of a suitable adsorbents include large surface area, high stability and no release of contaminants into water, recyclable, and inexpensive.

In water purification, spherical microcapsules offer a few advantages such as high surface area, high velocity in a moving fluid due to their low drag coefficient,²⁹ the potential to be packed efficiently in a column, and ease of removal through simple filtration. Here, we report the use of multimetal oxide incorporated on the surface of microcapsules for efficient arsenic removal from water. CeO₂ and Fe₃O₄ nanoparticles were selected for this study to enhance arsenic adsorption while magnetic property of Fe₃O₄ was used to remove the adsorbent from the medium.³⁰ Magnetic Fe₃O₄ nanoparticles were synthesized and loaded onto microcapsules, followed by the CeO₂ nanoparticles using a reported procedure.²⁷ The incorporation of CeO₂ enhances the extraction efficiency of As(III) and reduces the interference from phosphate anions.²⁷ Fe₃O₄ nanoparticles showed greater affinity towards As(V) anions, exhibit superparamagnetic properties and stable at a wide pH range of 3–8.^31,32

Combining CeO₂ and Fe₃O₄ in a capsule allows us to extract both As(III) and As(V) within a wide range of pH. Polycaprolactone (PCL) and the porogen³³ polyvinylpyrrolidone (PVP) were used to prepare microcapsules with large surface area and high surface roughness. Eventhough, microcapsules are used for various applications, modification of the microcapsule surface with metal oxide nanoparticles enhanced the extraction efficiencies for As(III) and As(V) from water. Microcapsules were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), inductively coupled plasma atomic emission spectroscopy (ICP-OES), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) techniques and used for the extraction of As(III) and As(V) from water. Influence of variables such as concentration, time, pH, ionic strength, and presence of interfering anions on the extraction process was established. Optimization of such factors is essential for developing new adsorbents with different functionalities and morphologies for removing arsenic from water (Fig. 1).

Fig. 1 Extraction of As(III) and As(V) on the surface of microcapsule incorporated with metal oxides.

Experimental

Materials

Polyvinyl alcohol (88% hydrolyzed, M_W = 100 KDa), polycaprolactone (PCL, M_W = 80 KDa), polyvinylpyrrolidone (PVP10, M_W = 10 KDa), sodium dodecyl sulfate (SDS), cerium ammonium nitrate (CAN), iron(III) acetylacetonate (Fe(acac)₃), triethylene glycol (TREG) were purchased from Sigma Aldrich Pte Ltd. Chloroform (CHCl₃, analytical grade), ethanol (95%, technical grade), ethyl acetate (technical grade) and ammonia solution (25%) were purchased from Merck. Stock solutions (1000 ppm) of As(III) and As(V) were prepared using sodium (meta)arsenite (NaAsO₂; >99% purity, Fluka) and sodium arsenate heptahydrate (NaH₂AsO₄·7H₂O; >98% purity, Fluka), respectively. Sodium sulphate (Na₂SO₄), potassium nitrate (KNO₃), and trisodium phosphate (Na₃PO₄·12H₂O) were purchased from Sigma Aldrich Pte Ltd. and used as such to prepare stock solutions (1000 ppm) of interfering anions. Appropriate amounts of the stock solutions were diluted further to get the desired concentrations for extraction studies.

Synthesis of Fe₃O₄ nanoparticles

According to a similar procedure, Fe(acac)₃ (1.7 g, 4.81 mmol) and triethyleneglycol (60 mL) were mixed and stirred under nitrogen, slowly heated to 120 °C for 15 min, 180 °C for 30 min and at 270 °C for 2 hours, using a sand bath.³⁴ After cooling to room temperature, ethanol (95%, 200 mL) and ethyl acetate (300 mL) were added to the reaction mixture, which led to the separation of nanoparticles. After magnetic separation, the nanoparticles were washed with ethanol and ethyl acetate mixture twice to remove all impurities. The resulting black precipitate was dispersed in ethanol and used for characterization (Fig. S1†).

Preparation of microcapsules

PVA solution (700 mL, 5 mg mL⁻¹) was prepared in water and mixed with ethanolic solution of Fe₃O₄ (19 mL, 13 mg mL⁻¹), and stirred for 30 minutes. Solution of PCL and SDS (80 mL, 8 mg mL⁻¹ for each component) in chloroform was prepared, PVA solution was added dropwise and stirred for 24 h at room temperature for complete evaporation of chloroform in accordance to a similar procedure for microcapsule synthesis.³⁵ For preparing porous microcapsules, PVP10 (0.128 g, 1.6 mg mL⁻¹), PCL (0.512 g, 6.4 mg mL⁻¹), and SDS (0.64 g, 8 mg mL⁻¹) were added to the chloroform solution and the above mentioned steps were repeated. The formed microcapsules were separated from solution, redispersed in water (30 mL), and cerium ammonium nitrate (2 : 1 w/w ratio of cerium ammonium nitrate to PCL) was added to the solution. The solution was stirred for 5 hours, capsules were separated by centrifugation and exposed to ammonia vapor for 1.5 hours to hydrolyze the cerium salt into CeO₂ nanoparticles. The microcapsules were washed with deionized water (200 mL), separated by centrifugation (using Eppendorf 5810R centrifuge at 18 g centrifugal force), dried in vacuum for 24 hours at room temperature, before using them for extraction studies. Hereafter, microcapsules without PVP10 are designated as MC-1 and microcapsules with PVP10 as MC-2. MC-1 and MC-2 preparation was repeated for 5–6 times with high reproducibility (±0.5–2% metal content).

Characterization of microcapsules

X-ray diffraction (XRD) patterns of the microcapsules were recorded using Bruker-AXS: D8 DISCOVER with GADDS Powder X-ray diffractometer with Cu-Kα (λ = 1.54 Å) at 40 kV and 40 mA over a range of 2Θ angle 10° to 90° with a step size of 1°. The morphologies of the microcapsules were examined using JEOL JSM-6701F field emission scanning electron microscope (FESEM). Energy dispersive X-ray spectroscopy (EDS) was used for the identification of elements on the surface of microcapsules. Percentage of Ce and Fe incorporated in the microcapsules were measured using Dual-view Optima 5300 DV Inductively coupled plasma-optical emission spectroscopy (ICP-OES) system. Zero point surface charge of the microcapsules was determined using Malvern Multi Purpose Titrator and Zetasizer. Elemental (CHNS) analysis was done using Elementar Vario Micro Cube CHNS Analyzer to estimate the amount of N from leftover PVP10, if any. A vibrating sample magnetometer with 10 000 Oersteds applied field was used to characterize the magnetic properties of MC-2 at room temperature. Surface chemical compositions of MC-2 before and after extraction of arsenic pollutants were studied using X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra^DLD using Mono Al Kα as photoexcitation source). C 1s (C–C bond) was calibrated at 285 eV. Samples were prepared by dispersing microcapsules (0.5 mg mL⁻¹) in As(III) or As(V) solution (100 ppm) for 24 h, microcapsules were filtered, washed with water and dried in vacuum. Higher concentration of arsenic was used for XPS studies to collect high quality data for characterization.

Time-dependent studies

To determine the maximum extraction efficiency, microcapsules (30 mg) were dispersed in As(III) or As(V) solution (15 mL, 10 ppm) and put on an orbital shaker at 250 rpm. Samples were collected at time intervals of 5, 10, 15, 30, 60, 120, 240, 480 minutes and after 24 h, diluted to 7 mL, filtered and the arsenic content in the solution was determined using ICP-OES. To determine the maximum amount of arsenic adsorbed, microcapsules (15 mg) were dispersed in the arsenic salt solution (15 mL, 80 ppm) at pH 7. Samples were placed on a shaker, small samples (1.5 mL) were collected periodically, diluted to 7 mL, filtered and the arsenic content the solution was determined using ICP-OES. All extraction experiments were carried out in triplicate with <10% standard deviation.

Concentration-dependent studies

As(III) and As(V) salt solutions (10 mL, 100 ppm) were adjusted to pH 7 using 0.1 M HCl and appropriate amounts were diluted to obtain solutions with 10, 20, 30, 40, 50, 60, and 80 ppm concentrations. Microcapsules (10 mg) were dispersed in the arsenic salt solutions and kept on a shaker for 24 hours. Samples (1.5 mL) were collected, diluted to 7 mL, filtered and analyzed using ICP-OES.

Effect of pH, ionic strength, and interfering anions

As(III) and As(V) salt solutions (5 mL, 10 ppm) were adjusted to a pH of 3, 5, 7, 9, and 11 using either 0.1 M NaOH or HCl solutions. Microcapsules (10 mg) were dispersed in arsenic solution (5 mL, 10 ppm) at different pH and kept on a shaker for 24 hours for understanding the effect of pH on extraction efficiencies. Samples (1.5 mL) were collected after the extraction, diluted to 7 mL, filtered and analyzed using ICP-OES. For understanding the effect of ionic strength on extraction efficiencies, microcapsules (10 mg) were dispersed in arsenic solution (5 mL, 10 ppm) at pH 7 and kept on a shaker for 24 hours. Samples (1.5 mL) were collected, diluted to 7 mL and filtered prior to ICP-OES analysis.

In order to understand the effect of interfering anions, microcapsules (10 mg) were dispersed in As(III) or As(V) salt solution (5 mL, 10 ppm) and stirred with sulphate, nitrate, or phosphate salt solutions with concentrations ranging from 10 to 20 ppm. Samples (1.5 mL) collected were diluted to 7 mL, filtered and analyzed using ICP-OES instrument.

Desorption studies

The arsenic adsorbed on the capsules was desorbed by dispersing the microcapsules in NaOH solution (10 mL, 1 M) and stirring for 4 h. Samples (1.5 mL) were collected, diluted to 7 mL and analyzed to calculate the percentage of arsenic desorbed. The microcapsules were again separated, washed with deionized water and followed by a second cycle of adsorption studies using fresh As(III) or As(V) solution (10 ppm). The desorption and adsorption procedures were repeated.

Results and discussions

Microcapsule characterization

Significant differences were observed between the surface of MC-1 and MC-2 without any metal oxide incorporated (Fig. 2A and B). Low magnification images of MC-1 and MC-2 having metal oxide showed similar morphology (Fig. 2C and D). High magnification images of both PCL microcapsules showed that they were covered with CeO₂ and Fe₃O₄ nanoparticles of size range 10–30 nm. Similar results were reported on synthesis of surfactant-templated bimodal mesoporous carbon with Fe₂O₃ particles confined inside the mesopores.³⁶

Fig. 2 SEM micrographs of microcapsule without metal oxide, MC-1 (A) and MC-2 (B), and with metal oxide, MC-1 (C) and MC-2 (D). Images shown in inset represent magnified image of the surface of respective capsule.

The adsorbent material with uniformly-dispersed and spatially-separated Fe₂O₃ was reported to possess higher arsenic extraction capacities as compared to neat Fe₂O₃ without the mesoporous carbon. Smaller nanoparticles are more efficient in arsenic adsorption owing to larger surface area.³⁷ In addition, this simple and cost-effective method to prepare multi-metal oxide loaded microcapsules in large scale could be used for environmental applications.

Majority of peaks in the XRD patterns of the metal oxide nanoparticles on the microcapsules are indexed based on the reported diffraction patterns of CeO₂ and Fe₃O₄ lattice (Fig. 3).^27,31 The X-ray diffraction pattern of CeO₂ on MC-2 showed amorphous peaks while those from MC-1 were more crystalline. Amorphous nanoparticles are known to have enhanced activity as they have higher surface defect density than their crystalline counterpart. EDS-SEM data (Fig. 4) showed the presence of Ce and Fe atoms on inner and outer surfaces of MC-1 and MC-2. Pt was also present in the EDS-SEM data as it was used for coating the sample to improve conductivity. This indicates uniform distribution of Ce and Fe oxide nanoparticles on the microcapsule surface. From the ICP-OES data, both MC-1 and MC-2 have relatively similar percentage of Fe (6 ± 1% and 8 ± 2%, respectively), however MC-1 has higher amounts of Ce (7 ± 1%) as compared to MC-2 (3 ± 0.5%). The percentage loading of Fe is similar to that of waste metal hydroxides (10%) entrapped into calcium alginate beads previously reported for the removal of arsenic.³⁸ Zero point surface charges of MC-1 and MC-2 were determined to be at 9.9 and 10.5, respectively. CeO₂ nanoparticles are known to have zero point surface charge of 7.9–10.5 while that of Fe₃O₄ is in the range of 6.8–7.9.^39–42 It is interesting to note that despite having higher zero point surface charge, the arsenic extraction efficiencies of MC-2 are better than that of MC-1. The extraction studies can be found in the following sections.

Fig. 3 XRD spectrum of MC-1 (A) and MC-2 (B). Red labels denotes CeO₂ and black labels denotes Fe₃O₄.

Fig. 4 EDS data of MC-1 inside (A) and outside (B) surfaces; MC-2 inside (C) and outside (D) surfaces.

Time-dependent studies

Time-dependent arsenic removal studies were performed using arsenic solution (10 ppm) to illustrate the extraction efficiency of microcapsules for As(III) and As(V) from water (Fig. 5A). Based on the detection limit of ICP-OES, a concentration of 10 ppm was used in the experiments to standardize our studies. MC-2 showed significantly higher extraction efficiency (∼95% in 4 hours) for As(III) and As(V) removal from water. After 15 minutes (i.e. before the steady-state), As(III) and As(V) removal efficiency was at ∼90%.

Fig. 5 Time-dependent arsenic extraction using arsenic salt solutions of concentrations 10 ppm (A), and 80 ppm (B). Experiments were done at neutral pH and in room temperature. MC-1-As(III), MC-1-As(V), MC-2-As(III), MC-2-As(V).

Time-dependent studies were performed at a higher concentration (80 ppm) to determine the amount of arsenic adsorbed at equilibrium (q_e, Fig. 5B). MC-2 showed higher q_e values and As(III) was more efficiently removed than As(V), which is expected owing to the binding selectivity of metal oxide nanoparticles.

Information on the kinetic parameters of arsenic extraction (i.e. mechanism of adsorption and potential rate-controlling steps such as mass transport and chemical reaction processes) is required to select the optimum conditions for a full-scale batch water purification system.³⁷ The pseudo-first and pseudo-second order kinetic models are the most widely used models for studying the kinetic parameters of heavy metal extraction by solid adsorbents and quantifying the various parameters. The pseudo-first order model was plotted as follows,⁴³

log(q_e − q_t) = log q_e − (k₁/2.303)t (1)

where q_e and q_t describe the amount of arsenic adsorbed (mg g⁻¹) at equilibrium and at a given time t, while k₁ (min⁻¹) is the pseudo first-order rate constant of arsenic adsorption. The plots obtained from pseudo-first order kinetics model have low R² values and theoretical q_e values lower than the experimental q_e. This indicates that the pseudo-first order kinetics model is not suitable to describe the kinetics of arsenic adsorption on MC-1 and MC-2.

The pseudo-second order model was plotted as follows,⁴⁴

t/q_t = t/q_e + 1/(k₂q_e²) (2)

where q_e and q_t describe the amount of arsenic adsorbed (mg g⁻¹) at equilibrium and at a given time, t, respectively while k₂ (g mg⁻¹ min⁻¹) is the pseudo-second order rate constant of arsenic adsorption. The pseudo-second order rate constant (k₂) and maximum amount of arsenic adsorbed at equilibrium (q_e) were determined by plotting t/q_t against t (Fig. 6).

Fig. 6 Pseudo-second (B) order kinetic plots for arsenite and arsenate adsorption on MC-1 and MC-2. MC-1-As(III), MC-1-As(V), MC-2-As(III), and MC-2-As(V).

All data collected using the microcapsules fit pseudo-second order kinetics better than pseudo-first order kinetics, with predicted q_e values matching the experimental q_e values with R² value of 0.99 (Fig. 6 and Table 1). Therefore, chemisorption is proposed to be the rate determining mechanism for arsenic binding.⁴⁵ Predicted q_e values for first order kinetics were lower than experimental q_e, because the first order kinetics takes into account external mass transfer resistance controlling initial adsorption.⁴⁶

Table 1 Pseudo-second order kinetic parameters for the As(III) and As(V) anion extraction studies

Kinetic parameters	Capsules
	MC-1		MC-2
	As(III)	As(V)	As(III)	As(V)
qe (mg g^−1)	19.46	11.93	26.95	21.98
k2 (g mg^−1 min^−1)	0.019	0.011	0.008	0.003
R^2	0.9999	0.9996	0.9988	0.9998

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Concentration-dependent studies

The Langmuir equation describes the coverage of adsorbent molecules on a solid surface related to the concentration of the same molecules in a medium above the solid surface at a fixed temperature.⁴⁷

Three assumptions govern the Langmuir isotherm model, which include adsorption is limited to monolayer coverage, all surface sites are the same and limited to binding one atom, and lastly that the binding of a molecule by a given site is independent of its neighboring sites' occupancy.⁴⁸

The following linear form⁴⁹ is used to plot the Langmuir isotherm (Fig. 7A):

C_e/Q_e = 1/(K_LQ_m) + (C_e/Q_m) (3)

whereby Q_e = amount of adsorbed adsorbate in mg g⁻¹, Q_m = maximum adsorption capacity for monolayer coverage in mg g⁻¹, K_L = Langmuir adsorption constant related to heat of adsorption, C_e = equilibrium concentration of adsorbate in solution.

Fig. 7 Langmuir (A) and Freundlich (B) isotherm plots. The experiments were done at adsorbent concentration (1 mg mL⁻¹), adsorbate concentrations from 10–80 ppm within a constant period of 24 h, neutral pH and in room temperature. MC-1-As(III), MC-1-As(V), MC-2-As(III), and MC-2-As(V).

The measure of how favorable the adsorption process is described by the separation factor (R_L),^50,51 whereby:

R_L = 1/(1 + K_LC₀) (4)

C₀ is the initial adsorbate concentration (mg L⁻¹), R_L > 1 indicates unfavorable adsorption; R_L = 1 corresponds to a linear adsorption process; 0 < R_L < 1 indicates favorable adsorption and R_L = 0 means irreversible adsorption.

The Freundlich isotherm describes a reversible adsorption, not restricted to the formation of monolayers.⁵⁰ This empirical model is often used to study multilayer adsorption on heterogeneous surfaces having non-uniform distribution of adsorption heat and affinities.⁵² The following linearized form of Freundlich equation was used,^53,54

ln Q_e = 1/n ln C_e + ln K_F (5)

where K_F and n are the Freundlich constant and adsorption intensity, respectively.

Non-linear regression analysis (NLLS) can be used to calculate more accurate Langmuir and Freundlich parameters if large analytical data is available.^55,56 In this paper, linear regression was used instead of non-linear regression analysis (NLLS) as a narrow arsenic equilibrium concentration range was used. All concentration-dependent studies were done at pH 7 to mimic the neutral pH of natural water. Langmuir and Freundlich parameters for MC-1 and MC-2 were tabulated in Table 2. Q_m values for As(III) and As(V) were higher for MC-2. These Q_m values were relatively close to the experimental q_e obtained from time-dependent studies (As(III) – 26 mg g⁻¹ and As(V) – 21 mg g⁻¹), indicating that monolayer surface coverage is relatively close to 1. The Q_m value for As(III) was slightly higher than As(V), which is expected owing to the strong interaction between CeO₂ and As(III).²⁷ Similarly, the Fe₃O₄ content enhances the extraction of As(V), resulting in a Q_m value at 28 mg g⁻¹.³³ It is also noted that despite the higher percentage of Ce found in MC-1, the arsenic binding ability was lower than MC-2. This demonstrates that the morphology, distribution and size of nanoparticles play a significant role in the extraction of arsenic.³⁷

Table 2 Langmuir and Freundlich parameters for MC-1 and MC-2 capsules. Experiments were done at ambient conditions

		MC-1		MC-2
		As(III)	As(V)	As(III)	As(V)
	Qm (mg g^−1)	20	18	32	28
Langmuir	KL	0.281	0.051	0.074	0.034
	RL	0.043	0.196	0.14	0.27
	R^2	0.9563	0.9157	0.9829	0.9027
Freundlich	KF (mg g^−1)	2.20	1.63	4.30	2.38
	1/n	0.5489	0.5099	0.2142	0.2008
	R^2	0.9777	0.9278	0.9982	0.9602

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Both microcapsules have slightly higher R² values for Freundlich isotherms of As(III) and As(V) anion adsorptions. It could be due to the presence of both CeO₂ and Fe₃O₄ on the surface of microcapsules, resulting in heterogeneous surface. Microcapsules with 1/n values below 1 shows chemisorption and heterogeneous binding sites.⁵⁷ Lower 1/n values for PVP10-modified microcapsules indicate stronger binding between adsorbent and adsorbate.⁵⁸ R_L values for all microcapsules were between 0 to 1, which indicates that the adsorption processes were favorable.

Several bimetal oxides were used for extraction of arsenic, but most of them release toxic metal nanoparticles into water during the extraction, which have to be filtered.^36,59–65 Microcapsules prepared in this study did not release detectable amounts of Ce or Fe oxide nanoparticles into water during the extraction process. Concentration of Ce and Fe detected by ICP in the arsenic solution after extraction was <0.1 ppm, which was below the detection limit of the instrument. Strong interaction between the carbonyl groups of PCL and surface of nanoparticles was used to immobilize nanoparticles on the surface of microcapsules.⁶⁶ Upon comparison with several other adsorbents in Table 3, MC-2 showed higher extraction efficiencies towards both As(III) and As(V) at neutral pH.^{36,60,61,65,67} The observed superior extraction efficiencies were due to the large surface area of amorphous nanoparticles incorporated on MC-2.

Table 3 Comparison of maximum As(III) and As(V) adsorption capacities of different adsorbents

Adsorbent	Maximum adsorption capacity (mg g^−1)
	As(III)	As(V)
Iron oxide incorporated mesoporous carbon^36	29.4	17.9
Granular Fe–Ce oxide adsorbent^60	—	18.2
Ce–Fe mixed oxide decorated multiwalled carbon nanotubes^61	29	31 (at pH 4)
Magnetite-reduced graphene oxide composites^65	13.10	5.83
Electrospun chitosan-PVA nanofiber loaded with cerium^67	18.0	—
MC-2 (this study)	32	28

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pH, ionic strength, and interfering anions effect

During the investigation of pH sensitivity towards extraction, MC-1 and MC-2 were able to remove ∼95% As(III) and As(V) from water in the pH range of 3–9 (Fig. 8A). This was as expected because when the pH is below the zero point surface charge of the adsorbent, the adsorbent surface has a net positive charge. Such pH stability in arsenic extraction is desirable for practical applications.⁵⁹

Fig. 8 Effect of pH (A), phosphate (B), sulphate (C), nitrate (D) anion concentration, and ionic strength by addition of NaCl salt (E) on arsenite and arsenate anion removal using MC-1 and MC-2. Extractions were done at ambient conditions. MC-1-As(III), MC-1-As(V), MC-2-As(III), MC-2-As(V).

Ionic strength up to 0.1 M NaCl concentration did not influence the arsenic extraction efficiencies of MC-2, however those of MC-1 decreased to about 40–50%. This shows that the interaction of CeO₂ on MC-2 with As(III) and As(V) was stronger with inner-sphere surface complex formation as the binding mechanism.^68,69

Presence of phosphate anions reduced the extraction efficiencies of As(V) by more than 50%, while extraction of As(III) remains same (Fig. 8B). Such interference is expected from phosphate anions owing to similar structure and charge.⁷⁰ Based on our experimental data, sulphates and nitrate anions showed no interference on extraction of As(III) and As(V) using MC-2 (Fig. 8C and D). However the addition of sulphate anions reduced the As(III) and As(V) extraction efficiencies of MC-1 to below 50%.

X-ray photoelectron spectroscopy

Binding energies of As(III) and As(V) adsorbed on MC-2 after calibration based on C 1s peak (Fig. 9) were 44.25 and 44.9 eV, respectively.⁷¹ This confirms the presence of arsenic on the microcapsule surface after extraction.

Fig. 9 XPS spectra of As(III) (A) and As(V) (B) adsorbed on MC-2. The extractions were done in ambient conditions.

A decrease in the percentage atomic ratio of Fe and Ce was expected after extraction owing to the strong affinity of microcapsules towards arsenic.⁶² Percentage atomic ratio of Ce decreased after the extraction of both As(III) and As(V) (Table 4). However, no increase in Ce or Fe content in the aqueous solution was detected, eliminating the leaching of these nanoparticles from capsules. This shows that CeO₂ on the microcapsule surface is active in the extraction of both As(III) and As(V) while Fe₃O₄ helps to adsorb mostly As(V). Since the XPS only detect the surface elements, after adsorption of arsenic, a decrease in surface concentration of Ce and Fe is expected along with an increasing concentration of arsenic element.

Table 4 Surface chemical compositions of MC-2 before and after As(III) and As(V) extraction. Percentage atomic ratios were calculated based on Ce 3d (RSF = 8.808), Fe 2p (RSF = 2.957), C 1s (RSF = 0.278), and As 3d (RSF = 0.677) transition peaks

Atomic ratios (%)	Ce	Fe	As
MC-2	7.04	2.75	—
MC-2-As(III)	6.03	2.67	2.20
MC-2-As(V)	6.21	1.44	1.33

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Arsenic desorption studies

The adsorption–desorption studies on MC-2 (Table S1†) were performed twice to confirm reproducibility of the experiments. More than 80% of both As(III) and As(V) were successfully desorbed from the microcapsule surface by stirring in NaOH solution (10 mL, 1 M) for 4 hours. Such desorption method is simple compared to the reported harsh regenerating conditions such as treatment with H₂O₂ solution at pH 11 used for CeO₂-loaded silica monoliths.⁷² The method was also more cost-effective than the desorption method used on adsorbent such as aluminum loaded shirazu-zeolite.⁷³ The microcapsules can be used and re-used for arsenic removal for about 3 to 5 cycles with good efficiency.

Magnetic properties of microcapsules

The hysteresis plot (Fig. 10A) of the magnetic properties of microcapsules showed paramagnetic property of Fe₃O₄ nanoparticles on the surface.³⁵ MC-2 has a relatively weak saturation value of 2.0 emu g⁻¹, which is similar to the reported mesoporous Fe₂O₃-loaded carbon adsorbents (1.6 emu g⁻¹).³⁶ MC-2 capsules were removed quantitatively from water using magnetic separation (Fig. 10B) after extraction of arsenic pollutants.

Fig. 10 Hysteresis data on the capsule (A) and magnetic separation of MC-2 (B) after extraction of arsenic. The inset in (B) shows the distribution of capsules before applying the magnetic field.

Conclusion

Mixed metal oxides (CeO₂ and Fe₃O₄) loaded PCL microcapsules (MC-1 and MC-2) were synthesized and characterized. Both MC-1 and MC-2 were used for the extraction of As(III) and As(V) under a wide range of pH and high ionic strength. The Q_m values determined from the Langmuir isotherm of MC-2 were 32 and 28 mg g⁻¹, while those for MC-1 were 20 and 18 mg g⁻¹ for As(III) and As(V), respectively. The amorphous nanoparticles adsorbed on the surface of MC-2 were the main reason for the enhanced arsenic extraction capacities. In this study, large surface area and morphology of the nanoparticles and high surface charges of the adsorbent have important roles in arsenic extraction. Compared to other reported Ce or Fe based adsorbents, the microcapsules investigated here are stable over a wide pH range of 3–9 with higher As(III) and As(V) anion extraction capacities. The extraction kinetics followed pseudo-second order with high R² values, indicating cooperative extraction. Extraction of As(III) using MC-2 was not effected by the presence of interfering phosphate anions in solution. Metal oxide incorporated microcapsules with magnetic properties and recyclability are useful adsorbents for arsenic removal from contaminated water.

Acknowledgements

The authors thank the Environment and Water Industry Programme Office (EWI) under the National Research Foundation of Singapore (PUBPP 21100/36/2, NUS WBS no. R-706-002-013-290, R-143-000-458-750, R-143-000-458-731) for the funding of the work. They also thank Department of Chemistry and NUS-Environmental Research Institute, National University of Singapore for all technical support. DS gratefully acknowledges EWI for a PhD scholarship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09030f

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