Abstract

Nanoscale zero-valent iron (nZVI) and sulfides have been confirmed to be effective in arsenic sequestration from aqueous solution. In this study, attapulgite supported and sulfide-modified nanoscale zero-valent iron (S-nZVI@ATP) are synthesized to realize the superposition effect of enhanced arsenic sequestration. The results indicated that nZVI clusters were well disaggregated and the BET specific surface area increased from 19.61 m2·g−1 to 46.04 m2·g−1 of S-nZVI@ATP, resulting in an enhanced removal efficiency of arsenic from 51.4% to 65.1% at 20 min. The sulfides in S-nZVI@ATP mainly exist as mackinawite (FeS) and this causes the spherical nanoparticles to exhibit a larger average particle size (94.6 nm) compared to bare nZVI (66.0 nm). In addition, S-nZVI@ATP exhibited a prominent ability for arsenic sequestration over a wide pH range of 3.0–6.0. The presence of anions SO42− and Cl can enhance the arsenic removal whereas HCO3 inhibited it. The arsenic adsorption by S-nZVI@ATP could be explained by the pseudo-second-order kinetic model and the Langmuir model, with the maximum adsorption capacity of 193.8 mg·g−1. The mechanism of As(III) sequestration by S-nZVI@ATP involved multiple processes, mainly including precipitation conversion from FeS to As2S3, surface-complexation adsorption and co-precipitation.

HIGHLIGHTS

  • S-nZVI@ATP was synthesized to superimpose the performance of nZVI and sulfides on arsenic removal.

  • The distribution of sulfides in S-nZVI@ATP and its role for As(III) removal were investigated.

  • S-nZVI@ATP showed an enlarged specific surface area and an enhanced arsenic removal efficiency.

  • The maximum adsorption capacity for arsenic was 193.8 mg·g−1.

  • The mechanism involved the combined action of Fe(0) core and sulfide shell.

Graphical Abstract

Graphical Abstract
Graphical Abstract

ABBREVIATIONS/NOTATIONS

     
  • nZVI

    Nanoscale zero-valent iron

  •  
  • ATP

    Attapulgite

  •  
  • nZVI@ATP

    Attapulgite supported nanoscale zero-valent iron

  •  
  • S-nZVI@ATP

    Attapulgite supported and sulfide-modified nanoscale zero-valent iron

INTRODUCTION

Arsenic in the environment poses a health threat in many countries all over the world. In order to reduce the risk of arsenic to human life, the allowable concentration of arsenic in drinking water is expected to be less than 10 μg·L−1 as proposed by the World Health Organization (WHO) in 2001 (Kapaj et al. 2006). Arsenic in an aquatic system is primarily derived from natural and anthropogenic sources. The former originates from the dissolution of arsenic minerals which exist as solids in the earth's crust, while the latter results from industrial production such as metal smelting, acid production from pyrite and the use of arsenic-containing chemicals (Smedley & Kinniburgh 2002; Sharma & Sohn 2009). Arsenic exists in two oxidation states as As(III) (H3AsO3, H2AsO3) and As(V) (H2AsO4, HAsO42−), and the toxicity of As(III) is about 25–60 times that of As(V) with stronger mobility (Dixit & Hering 2003; Huang et al. 2018). Thus, the treatment of As(III) contamination is particularly important.

Various conventional technologies have been used for the treatment of As(III) contamination including co-precipitation, reverse osmosis and anion exchange (Segura et al. 2002; Baskan & Pala 2010; Akin et al. 2011). Recently, much attention was put on nanoscale zero-valent iron (nZVI) due to its strong reducibility, adsorption capacity and environmental safety (Shi et al. 2013; Gisi et al. 2017). It has been employed to decontaminate environmental media (water, soil and sediment) polluted with heavy metals (Gisi et al. 2017; Chen et al. 2020), dyes (Dutta et al. 2016; Hamdy et al. 2018), chloro-organic compounds (Wang et al. 2011; Singh & Bose 2017) and pesticides (Šimkovič et al. 2015). Actually, nZVI technology has great potential for arsenic removal because of its large active surface and adsorption capacity (Yan et al. 2012a, 2012b).

However, nZVI particles are easy to aggregate due to their magnetism and large specific surface energy, which significantly inhibits reactivity (Mukherjee et al. 2015). One effective strategy for addressing this shortcoming is to load nZVI onto porous materials such as montmorillonite, graphene and resin (Du et al. 2013; Bhowmick et al. 2014; Wang et al. 2014). Compared with these materials, attapulgite (ATP), as a natural clay mineral with large specific surface area and unique nanometer crystal morphology, has the advantages of abundant reserves, low cost and ready availability, and is an ideal carrier for nZVI (Yin et al. 2017; Ding et al. 2019).

Another challenging limitation for the application of nZVI technology is its poor chemical stability (Wu et al. 2018). The oxidation reaction between nZVI and water/dissolved oxygen results in the passivation and diminished reactive life of nZVI as well as low selectivity of target pollutants (Fan et al. 2016; Jia et al. 2019). Sulfide-modification of nZVI is a recently developed countermeasure to enhance the stability of nZVI. Generally, sulfide-modification of nZVI refers to coating nZVI with iron sulfides by using various reducing sulfides such as Na2S, Na2S2O3 and Na2S2O4 (Han & Yan 2016; Tang et al. 2016). Recent studies have pointed out that sulfidation is effective for adjusting the surface reactivity of nZVI, so as to improve the selectivity for organic contaminants and accelerate the removal efficiency and rate of refractory contaminants (Li et al. 2016, 2017; Cao et al. 2017). Additionally, the surface modification of nZVI with sulfides can reduce the passivation of nZVI, because Fe2+ produced from ZVI corrosion has a stronger affinity for S2− than for O2− (Song et al. 2017), and the formation of FeS can weaken the magnetic attraction between ZVI nanoparticles and thus alleviate their agglomeration (Su et al. 2019).

Nonetheless, limited study exists on the sulfide-modification of nZVI for As(III) removal. In fact, the S2− produced from FeS dissociation could combine with As(III) to form sulfide precipitation such as orpiment (As2S3) or arsenopyrite (FeAsS), which would result in the sequestration of As(III) from aqueous solution (Bostick & Fendorf 2003; Han et al. 2011). For instance, Han et al. (2013, 2015) reported that both nano-particulate FeS and FeS-coated sand were effective for arsenic removal.

In this work, a novelty composite – attapulgite supported and sulfide-modified nZVI (S-nZVI@ATP) is synthesized to superposition the enhanced effect of loading and sulfide-modification on the removal of arsenic by nZVI. The objectives of this work are to investigate: (1) the changes on SEM and TEM morphology, as well as specific surface area and pore parameters of nZVI before and after its loading and sulfidation; (2) the enhanced sequestration performance of S-nZVI@ATP towards As(III) compared with nZVI and nZVI loaded on ATP (nZVI@ATP); (3) the effects of solution pH and coexisting anions on As(III) sequestration; (4) the kinetics, adsorption isotherms and mechanism underlying the As(III) sequestration by S-nZVI@ATP.

MATERIALS AND METHODS

Materials and chemicals

Attapulgite (≤ 0.0385 mm) with the BET specific surface area of 110.16 m2 g−1 was obtained from Anhui Mingmei mineral chemical Co., Ltd, China. Ferrous sulfate heptahydrate (FeSO4·7H2O), sodium borohydride (NaBH4), sodium thiosulfate (Na2S2O3) and sodium arsenite (NaAsO2) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. All chemicals used in this study were of analytical grade and used as received. As(III) stock solutions (1 g·L−1) were prepared using sodium arsenite.

Preparation of materials

Preparation of nZVI: nZVI was prepared by liquid-phase reduction method under N2 atmosphere (Mukherjee et al. 2015; Zou et al. 2016). 20 mL of FeSO4 solution (1.0 M) was added in a three-necked flask, followed by adding 80 mL of NaBH4 solution (0.5 M) dropwise under mechanical stirring (150 rpm, 25 ± 1°C). The as-prepared nZVI was separated from the mixture by vacuum filtration and washed three times with deionized water. The obtained particles were vacuum-dried at −30°C for 12 h in a freeze dryer.

Preparation of nZVI@ATP: 0.5584 g of attapulgite and 20 mL of FeSO4 solution (1.0 M) were mixed in three-necked flask for 30 min under stirring (150 rpm, 25 ± 1 °C). The subsequent procedure was the same as the preparation of nZVI.

Preparation of S-nZVI@ATP: The mixing of attapulgite and FeSO4 solution and subsequent reduction of FeSO4 by NaBH4 were the same as the preparation of nZVI@ATP. 2.5 mL of Na2S2O3 solution (1.0 M) was then added to the suspension and stirred for 30 min under ultrasonic assistance (KQ-300E ultrasonicator, 40 kHz). The subsequent procedure was the same as the preparation of n-ZVI. The composite as-prepared consisted of a 1:2 mass ratio of attapulgite and iron, and a 1:4 molar ratio of sulfur and iron.

Batch experiments

To investigate As(III) removal efficiency by different materials, ATP, nZVI, nZVI@ATP and S-nZVI@ATP at a dosage of 0.2 g·L−1 were separately added to 200 mL As(III) solutions with the concentration of 0.269 mM and the initial pH of 5.6 ± 0.1 (adjusted by dilute sulphuric acid before batch experiments). The batch experiments were performed in a shaking incubator (200 rpm) at 25 ± 1°C. Aliquots of samples were taken at selected time intervals and filtered through a 0.45 μm membrane filter for arsenic analyses. All batch experiments were performed in duplicate, and the mean values were used for the analyses.

The effects of operating parameters including solution pH and coexisting anions on As(III) sequestration were investigated in the above conditions. The effect of solution pH on As(III) removal was conducted within a pH range of 2.0 to 11.0 by adding 0.3 g·L−1 of S-nZVI@ATP to 200 mL As(III) solutions with the concentration of 0.269 mM for 1 h (Wang et al. 2014; Wu et al. 2018). The effect of coexisting anions (SO42−, Cl and HCO3) under different anion concentrations (0, 1, 10, 50, 100 and 250 mM) on As(III) removal were examined by adding 0.5 g·L−1 of S-nZVI@ATP to 200 mL As(III) solutions with the concentration of 2 mM for 1 h.

The kinetics of As(III) sequestration by S-nZVI@ATP were obtained by adding 0.5 g·L−1 of S-nZVI@ATP to 200 mL As(III) solutions with different concentrations (0.5, 1.0 and 1.5 mM). Samples were taken at selected time intervals from 0 to 24 h. Adsorption isotherms were obtained by adding 0.5 g·L−1 of S-nZVI@ATP to As(III) solutions with a concentration range of 0.5–3.0 mM for 12 h reaction (Bhowmick et al. 2014).

Analytical methods

The initial concentration (C0, mg·L−1) and residual concentration at time t (Ct, mg·L−1) of arsenic in solution were determined on an atomic fluorescence spectrophotometer (NOVAA-350, Analytic Jena, Germany) coupled with a hydride generator (Wang et al. 2014). The removal efficiencies of As(III) by different materials were calculated using Equation (1), and the specific amount of arsenic adsorbed (Qe, mg·g−1) was calculated using Equation (2).
formula
(1)
formula
(2)
where V is the solution volume (L), and W is the mass of the adsorbent (g).

Material characterizations

The morphologies of the materials were characterized by scanning electron microscopy (SEM) (Nova 400 Nano, FEI, USA). TEM images and elemental mapping were obtained by transmission electron microscopy (TecnaiG2 F30 S-Twin, FEI, USA) integrated with X-ray energy-dispersive spectroscopy (EDS). The specific surface areas and pore parameters of materials were determined using the N2 adsorption-desorption method by a specific surface area analyzer (Micromeritics ASAP 2020, USA). The zeta potential was measured by Zetasizer Nano ZS90 Zeta Potential Analyzer. XRD patterns were performed using an X-ray diffractometer (D/MAX-2500, Rigaku Co., Japan) with Mo Kα operated at 45 kV and 250 mA. Continuous scans from 10° to 100° at 2θ were collected with a step size of 0.02° and a count time of 0.6 s per step. XPS spectra were recorded using an X-ray photoelectron spectrometer (Escalab 250Xi, Thermo Fisher Scientific, USA) with a monochromatic Al Kα X-ray source (hν = 1486.6 eV), where the binding energies (BEs) of the samples were calibrated using the C1 s peak at 284.8 eV.

RESULTS AND DISCUSSION

Characterizations of different materials

SEM analyses

The surface morphology of ATP, nZVI, nZVI@ATP and S-nZVI@ATP are presented in Figure 1(a)–1(d) respectively. The ATP, as shown in Figure 1(a), is characterized by closely packed fibers that appear to be interwoven, with differing length and thicknesses (Neaman & Singer 2000). The fibers of ATP are straight-shaped and randomly oriented, as identified by Frost et al. (2010). The nZVI shows a chain-like morphology and flocculated agglomeration of particles due to its magnetic effect (Figure 1(b)) (He et al. 2007). However, the nZVI loaded on ATP (nZVI@ATP) presents a unique morphology, as the agglomeration of nZVI is greatly reduced due to the steric hindrance of ATP (Figure 1(c)). The sulfide-modification of nZVI@ATP attributes more stability to the nZVI, which may result from the sulfidation weakening the magnetic effect (Figure 1(d)).

Figure 1

SEM images of (a) ATP, (b) nZVI, (c) nZVI@ATP and (d) S-nZVI@ATP.

Figure 1

SEM images of (a) ATP, (b) nZVI, (c) nZVI@ATP and (d) S-nZVI@ATP.

TEM-EDS analysis

Further characterization to examine the structural morphology and elemental mapping of nZVI and S-nZVI@ATP with the help of TEM and EDS are shown in Figure 2. The chain-like morphology of nZVI coupled with dark spherical structures that depict the Fe(0) core are observed in Figure 2(a). The thin oxide shell (thickness = 3–5 nm) circled by the yellow ring in Figure 2(a) inset is similar to that reported in previous studies focused on the core-shell structure of nZVI (Liu et al. 2018). It was found that the S-nZVI particles in S-nZVI@ATP were significantly dispersed and the entire mixture appeared to be interwoven and held together as a unified composition due to loading on ATP (Figure 2(c)). Moreover, the Fe(0) shell disappeared, which may be attributed to the sulfidation (Figure 2(c) inset). The iron sulfides formed on the surface of nZVI by sulfidation occupied the position of some iron oxides and further inhibited the oxidation of nZVI, thus yielding an incomplete formation of the oxide layer. The average particle size of nZVI particles was 66.0 nm, while that of S-nZVI particles in S-nZVI@ATP was 94.6 nm as measured and counted by the Image J software (Figure 3). Increase in the S-nZVI@ATP particle size was realized due to the coating of iron sulfide on nZVI (Song et al. 2017). However, the sulfide shell, characterized by a flake-like morphology as reported in previous studies, is not identified in this work (Su et al. 2015; Su et al. 2019).

Figure 2

(a) TEM image and (b) EDS spectra of nZVI, (c) TEM image and (d) EDS spectra of S-nZVI@ATP, and (e) TEM image as well as elemental mapping of (f) Fe, and (g) S in S-nZVI@ATP.

Figure 2

(a) TEM image and (b) EDS spectra of nZVI, (c) TEM image and (d) EDS spectra of S-nZVI@ATP, and (e) TEM image as well as elemental mapping of (f) Fe, and (g) S in S-nZVI@ATP.

Figure 3

Particle size distribution of (a) nZVI particles and (b) S-nZVI particles in S-nZVI@ATP.

Figure 3

Particle size distribution of (a) nZVI particles and (b) S-nZVI particles in S-nZVI@ATP.

The EDS spectra of nZVI particles (Figure 2(b)) show peaks for only Fe and O while that of S-nZVI@ATP particles (Figure 2(d)) exhibit peaks for Fe, O and S; hence, S is staunchly present in S-nZVI@ATP as much as Fe and O are present in both nZVI and S-nZVI@ATP. As seen in the elemental mapping of S-nZVI particles in S-nZVI@ATP (Figure 2(f) and 2(g)), Fe elements were mostly distributed in the sphere of nZVI particles and a few in other regions, while sparse dispersion of S was distributed on the nZVI particles. Both EDS spectra and elemental mapping show that the content of S in S-nZVI@ATP was relatively low.

Analysis of BET specific surface area and pore parameters

The BET specific surface area (SBET) and pore volume of nZVI, nZVI@ATP and S-nZVI@ATP are presented in Figure 4. The SBET of nZVI, nZVI@ATP and S-nZVI@ATP were 19.61 m2·g−1, 38.44 m2·g−1 and 46.04 m2·g−1 respectively. The pore volumes of the three materials were 0.0962 cm3·g−1, 0.1621 cm3·g−1 and 0.1962 cm3·g−1, respectively. The increase of SBET and pore volume indicated that both the loading on ATP and sulfide modification effectively inhibited the aggregation of nZVI particles. The great SBET of S-nZVI@ATP provided more reaction sites and the high pore volume was beneficial to the rapid diffusion of pollutants, which would enhance the removal of As(III).

Figure 4

SBET and pore volume of nZVI, nZVI@ATP and S-nZVI@ATP.

Figure 4

SBET and pore volume of nZVI, nZVI@ATP and S-nZVI@ATP.

Enhanced sequestration of As(III) by S-nZVI@ATP

The removal efficiencies of As(III) by ATP, nZVI, nZVI@ATP and S-nZVI@ATP are shown in Figure 5. It is observed that ATP exhibited a negligible removal efficiency for As(III). On the contrary, nZVI, nZVI@ATP and S-nZVI@ATP proved to be better in removing As(III) as the removal efficiencies of these three materials reached 51.4%, 57.5% and 65.1% respectively at 20 min. The highest removal efficiency was reached by S-nZVI@ATP, although its theoretical iron content was about two thirds of that in nZVI, which was attributed to the loading of nZVI on attapulgite and subsequent sulfidation.

Figure 5

Removal efficiencies of As(III) by ATP, nZVI, nZVI@ATP and S-nZVI@ATP (reaction conditions: As(III) concentration = 0.269 mM, dosage = 0.2 g·L−1, initial pH = 5.6 ± 0.1, temperature = 25 ± 1 °C).

Figure 5

Removal efficiencies of As(III) by ATP, nZVI, nZVI@ATP and S-nZVI@ATP (reaction conditions: As(III) concentration = 0.269 mM, dosage = 0.2 g·L−1, initial pH = 5.6 ± 0.1, temperature = 25 ± 1 °C).

Effect of operating parameters on As(III) sequestration

Solution pH

The effect of solution pH on As(III) sequestration by S-nZVI@ATP over 1 h is shown in Figure 6. The removal efficiency of As(III) by S-nZVI@ATP was highest (about 77%) when the pH ranged from 3.0 to 6.0, but declined to about 60% when the pH ranged from 7.0 to 9.0. Additionally, the removal capacity of S-nZVI@ATP for As(III) declined sharply in a strong acid (pH = 2.0) or strong alkali (pH = 10.0, 11.0) environment, such that 39.3% As(III) was removed at pH = 2.0, whereas only 7.0% was removed at pH = 11.0.

Figure 6

Effect of initial pH on As(III) sequestration by S-NZVI@ATP (reaction conditions: As(III) concentration = 0.269 mM, dosage = 0.3 g·L−1, reaction time = 1 h, temperature = 25 ± 1 °C).

Figure 6

Effect of initial pH on As(III) sequestration by S-NZVI@ATP (reaction conditions: As(III) concentration = 0.269 mM, dosage = 0.3 g·L−1, reaction time = 1 h, temperature = 25 ± 1 °C).

Measurements of the zeta potentials at various pH levels demonstrate that the pH point of zero charge (pHPZC) of S-nZVI@ATP was about 3.8, which means that S-nZVI@ATP was positively charged at pH <3.8, and negatively charged at pH >3.8 (Babaee et al. 2018). Moreover, the As(III) species mainly exist in the form of neutral H3AsO3 (pH <9) and anionic H2AsO3 (9< pH <12) according to the dissociation constants of aqueous As(III) (Zhu et al. 2009). Hence, the removal of As(III) by S-nZVI@ATP at pH< 9 was not due to electrostatic factors, and the decrease in the removal efficiency of As(III) at pH >9 was partly due to electrostatic repulsion (Wang et al. 2014).

Coexisting anions

The effects of individual anions (SO42−, Cl and HCO3) on the removal efficiency of As(III) are illustrated in Figure 7. The SO42− and Cl anions positively affected the removal of As(III) by S-nZVI@ATP, and the higher the concentration of coexisting anions, the more obvious the effect. The removal efficiency of As(III) was 59.8% in the absence of coexisting anions. When the concentration of Cl and SO42− was 250 mM, the removal efficiency of As(III) was 82.7% and 88.7%, respectively. The reason for the promotion of As(III) removal may be that the Cl in solution could induce pitting corrosion of the nZVI surface, while the SO42− may destroy the passivation layer of S-nZVI@ATP and promote Fe0 corrosion (Choe et al. 2004; Devlin & Allin 2005). Actually, Fe0 corrosion could result in several forms of iron (oxy)hydroxides, which served as good sites for arsenic adsorption.

Figure 7

Effect of coexisting anions on As(III) sequestration by S-NZVI@ATP (reaction conditions: As(III) concentration = 2 mM, dosage = 0.5 g·L−1, concentration of coexisting anions = 0, 1, 10, 50, 100, 250 mM, reaction time = 1 h, temperature = 25 ± 1 °C).

Figure 7

Effect of coexisting anions on As(III) sequestration by S-NZVI@ATP (reaction conditions: As(III) concentration = 2 mM, dosage = 0.5 g·L−1, concentration of coexisting anions = 0, 1, 10, 50, 100, 250 mM, reaction time = 1 h, temperature = 25 ± 1 °C).

On the contrary, the As(III) removal was inhibited in the presence of HCO3, and the removal efficiency dropped sharply to 34.3% when the concentration of HCO3 was 250 mM. HCO3 could form surface complexes with iron (oxy)hydroxides and form iron carbonate with generated Fe2+, inhibiting the adsorption of arsenic by S-nZVI@ATP (Tanboonchuy et al. 2012).

Kinetics of As(III) sequestration by S-nZVI@ATP

The kinetics of S-nZVI@ATP for As(III) removal were studied by batch experiments with varied As(III) concentrations. Two kinetic models of the pseudo-first-order (Equation (3)) and pseudo-second-order (Equation (4)) were employed and the equations are displayed as follows (Azizian 2004; Tran et al. 2017):
formula
(3)
formula
(4)
where Qe (mg·g−1) and Qt (mg·g−1) are the amounts adsorbed at equilibrium and at any time t (min), respectively; k1 (min−1) and k2 (g·mg−1·min−1) are the rate constants of the pseudo first-order and pseudo-second-order equations, correspondingly.

The related parameters of two models are given in Table 1. The R2 values obtained in the pseudo-second-order model were higher than those of the pseudo-first-order model under different initial concentrations (0.5, 1.0 and 1.5 mM). Similarly, the Chi-squared (χ2) values were smaller in the pseudo-second-order model, suggesting that the data observed at each initial concentration in the pseudo-second-order model fitted the experimental data (Qe (exp)) well. The relationship was statistically significant since p < 0.05. Similarly, the t-values obtained for both pseudo-first-order and pseudo-second-order models proved the magnitude of the relationship, suggesting that As(III) sequestration by S-nZVI@ATP could be more accurately explained using the pseudo-second-order kinetic model. Consequently, chemisorption rather than diffusion was the rate-limiting step to adsorption (Pang et al. 2019).

Table 1

Parameters of the pseudo-first-order and pseudo-second-order models for As(III) sequestration by S-nZVI@ATP at pH 5.6 (±0.1) and 25 (±1) °C

Initial C(As(III))Qe (exp)Pseudo-first-order kinetic model
Pseudo-second-order kinetic model
k1R2Qe (cal)χ2t-valuep valuek2R2Qe (cal)χ2t-valuep value
0.5 74.91 0.1247 0.9034 70.01 46.53 32.70 7.15(−14) 0.00256 0.9694 73.46 14.75 6.70 1.47(−5) 
1.0 128.51 0.1936 0.8441 114.50 192.93 27.08 8.05(−13) 0.00225 0.9298 120.15 86.81 4.37 7.49(−4) 
1.5 160.91 0.1892 0.8582 151.95 303.29 28.61 3.98(−13) 0.00172 0.9409 159.09 126.47 4.80 3.49(−4) 
Initial C(As(III))Qe (exp)Pseudo-first-order kinetic model
Pseudo-second-order kinetic model
k1R2Qe (cal)χ2t-valuep valuek2R2Qe (cal)χ2t-valuep value
0.5 74.91 0.1247 0.9034 70.01 46.53 32.70 7.15(−14) 0.00256 0.9694 73.46 14.75 6.70 1.47(−5) 
1.0 128.51 0.1936 0.8441 114.50 192.93 27.08 8.05(−13) 0.00225 0.9298 120.15 86.81 4.37 7.49(−4) 
1.5 160.91 0.1892 0.8582 151.95 303.29 28.61 3.98(−13) 0.00172 0.9409 159.09 126.47 4.80 3.49(−4) 

*Units:C(As(III)) (mM), Qe (mg·g−1), k1 (min−1), Qe (mg·g−1), k2 (g·mg−1·min−1).

Moreover, Table 1 shows that Qe increased as the initial As(III) concentrations increased, indicating that the maximum adsorption capacity of S-nZVI@ATP had not been reached at the time of adsorption equilibrium in As(III) solution of 1.5 mM. However, k2 decreased as the initial concentrations of As(III) increased, mainly because the adsorption sites were gradually occupied by As(III), which restrained the removal of As(III).

Adsorption isotherms of As(III) removal by S-nZVI@ATP

In order to explore the relationship between adsorbate and adsorbent, adsorption isotherms of the Langmuir and Freundlich were applied to simulate As(III) adsorption on S-nZVI@ATP. The related parameters of two models are given in Table 2. The Langmuir and Freundlich model can be expressed as:
formula
(5)
formula
(6)
where Ce is the equilibrium concentration (mg·L−1), Qe and Qmax are the amounts (mg·g−1) adsorbed at equilibrium and theoretical maximum capacity correspondingly, KL (L·mg−1) and KF (mg1−n·Ln·g−1) are constants associated with affinity and adsorption capacities of Langmuir and Freundlich models respectively.
Table 2

Parameters of Langmuir and Freundlich models for As(III) adsorption on S-nZVI@ATP at pH 5.6 (±0.1) and 25 (±1) °C

LangmuirQmax (mg·g−1)KL (L·mg−1)R2χ2 (χ2)t-valuep value (χ2)
 193.83 0.1819 0.9545 26.26 47.65 2.03(−5) 
FreundlichKF (mg1−n·Ln·g−1)1/nR2χ2 (χ2)t-valuep value (χ2)
 98.31 0.1362 0.8237 102.75 7.34 0.005 
LangmuirQmax (mg·g−1)KL (L·mg−1)R2χ2 (χ2)t-valuep value (χ2)
 193.83 0.1819 0.9545 26.26 47.65 2.03(−5) 
FreundlichKF (mg1−n·Ln·g−1)1/nR2χ2 (χ2)t-valuep value (χ2)
 98.31 0.1362 0.8237 102.75 7.34 0.005 

As illustrated in Table 2, the Langmuir model exhibited a higher R-squared (R2 = 0.9545) and a smaller Chi-Squared (χ2 = 26.26) as compared to the Freundlich model (R2 = 0. 0.8237, χ2 = 102.75). Similarly, the t value obtained for the Langmuir model (47.65) was higher than that of the Freundlich (7.34), and both were statistically significant (p value <0.05). This suggests that the Langmuir model is a better fit in explaining the adsorption of As(III) by S-nZVI@ATP since the t value depicted a greater magnitude and evidence, illustrating that a monolayer reaction dominated the adsorption processes rather than a heterogeneous reaction (Yadav et al. 2016). The theoretical maximum adsorption capacity of S-nZVI@ATP for As(III) was 193.8 mg·g−1. Compared with the adsorption capacity of other iron-based materials (Table 3), it is clear that S-nZVI@ATP is an effective composite material for As(III) removal.

Table 3

Comparison between some iron-based materials for As(III) adsorption capacity

MaterialpHIsotherm modelAdsorption capacity Qmax (mg·g−1)References
Bare nZVI 7.0 Langmuir 55.0 Giasuddin et al. (2007)  
Montmorillonite supported nZVI (Mt-nZVI) 7.0 Langmuir 59.9 Bhowmick et al. (2014)  
nZVI-reduced graphite oxide modified composites (nZVI-RGO) 7.0 Langmuir 35.8 Wang et al. (2014)  
Bifunctional resin-ZVI (N-S-ZVI) 6.5 Langmuir 121.0 Du et al. (2013)  
Activated carbon supported nZVI (nZVI/AC) 6.5 Langmuir 18.2 Zhu et al. (2009)  
Salicylic acid -TiO2@SiO2@Ni/nZVI (SA-NFST) 7.0 Langmuir 73.9 Huang et al. (2018)  
MaterialpHIsotherm modelAdsorption capacity Qmax (mg·g−1)References
Bare nZVI 7.0 Langmuir 55.0 Giasuddin et al. (2007)  
Montmorillonite supported nZVI (Mt-nZVI) 7.0 Langmuir 59.9 Bhowmick et al. (2014)  
nZVI-reduced graphite oxide modified composites (nZVI-RGO) 7.0 Langmuir 35.8 Wang et al. (2014)  
Bifunctional resin-ZVI (N-S-ZVI) 6.5 Langmuir 121.0 Du et al. (2013)  
Activated carbon supported nZVI (nZVI/AC) 6.5 Langmuir 18.2 Zhu et al. (2009)  
Salicylic acid -TiO2@SiO2@Ni/nZVI (SA-NFST) 7.0 Langmuir 73.9 Huang et al. (2018)  

Mechanism on sequestration of As(III) by S-nZVI@ATP

XRD analyses

The XRD patterns of S-nZVI@ATP before and after the removal for As(III) are presented in Figure 8. The peaks at 16.5°, 19.9°, 21.4°, 27.7° and 34.7° are characteristic of attapulgite while those at 44.8°, 65.0° and 82.6° are reflective of Fe(0) (Giasuddin et al. 2007; Zhang et al. 2019), suggesting that nZVI combined with ATP. It is evident that hematite (Fe2O3, characteristic peaks at 24.1°, 49.5° and 54.3°) and ferrihydrite (Fe5O3(OH)9, characteristic peaks at 35.6°, 40.6°, 61.4° and 62.7°) formed before the As(III) removal by S-nZVI@ATP. In addition, the diffraction peaks at 17.7°, 30.1°, 38.9° and 50.4° could be attributed to mackinawite (FeS), indicating that the S-nZVI shell in S-nZVI@ATP was composed of iron oxides and iron sulfides. The XRD pattern of S-nZVI@ATP after the removal for As(III) shows that the peak intensity at 44.8° corresponding to Fe(0) reduced significantly and that at 35.6° and around 62° corresponding to ferrihydrite increased, suggesting that Fe(0) may be transformed into iron oxides. The presence of arsenic compounds cannot be determined, possibly due to their low content and poor crystallinity.

Figure 8

XRD patterns of S-nZVI@ATP before and after the removal for As(III).

Figure 8

XRD patterns of S-nZVI@ATP before and after the removal for As(III).

XPS analyses

XPS helps to examine the composition of S-nZVI@ATP before and after the removal for As(III). The wide scan spectra of pristine S-nZVI@ATP (Figure 9(a)) denotes the co-existence of Fe 2p, O 1s and S 2p at binding energies of ∼711 eV, ∼532 eV and ∼162 eV, respectively. The Fe 2p spectra of S-nZVI@ATP before the removal for As(III) (Figure 9(b)) was deconvoluted into peaks of Fe(0) (707.3 eV), Fe2+ (710.3 eV and 723.6 eV) and Fe3+ (712.9 eV and 726.2 eV), further confirming the existence of Fe(0) along with iron oxides/hydroxides as discussed in XRD analysis. Moreover, it was observed that the peak of Fe(0) almost disappeared and the peak intensity of Fe2+/Fe3+ species increased in the Fe 2p spectra of S-nZVI@ATP after the removal for As(III), showing the corrosion of Fe(0) and its subsequent conversion into iron oxides/hydroxides. In addition, it was found that new spectra of As 3d emerged at the binding energy of ∼45 eV after the As(III) removal by S-nZVI@ATP (Figure 9(a)), indicating that As(III) was incorporated into S-nZVI@ATP. The XPS results were consistent with the XRD analysis on the transformation of Fe (0) into iron oxides such as ferrihydrite. Hence, As(III) may be trapped on the growing corrosion products via precipitation/co-precipitation (Kanel et al. 2005; Noubactep 2008; Bhowmick et al. 2014).

Figure 9

(a) XPS wide scan spectra and narrow scan spectra of (b) Fe 2p, (c) S 2p and (d) As 3d of S-nZVI@ATP before and after the removal for As(III).

Figure 9

(a) XPS wide scan spectra and narrow scan spectra of (b) Fe 2p, (c) S 2p and (d) As 3d of S-nZVI@ATP before and after the removal for As(III).

Based on the surface complexation model proposed by Stumm (Stumm 1992), iron (hydr)oxides can be complexed with H2O to undergo hydroxylation, resulting in the formation of ≡FeOH. In addition, previous studies (Manning et al. 2002; Kanel et al. 2005; Zhu et al. 2009; Bhowmick et al. 2014) reported that As(III) are capable of forming surface complexes with iron (hydr)oxides that exist independently or on the surface of nZVI-based materials. Therefore, As(III) can be removed by forming a surface complex with ≡FeOH, and the specific equation is expressed as (As(III) mainly exists as H3AsO3 at pH< 9):
formula
(7)
Figure 9(c) shows the S 2p spectra of S-nZVI@ATP before and after the removal for As(III). The peaks at 161.2 eV and 161.9 eV were attributed to the peaks of S2− species, which corresponds to mackinawite (FeS) as identified in the XRD pattern. It was reported that mackinawite (FeS) exhibited a high capacity for As(III) removal in an acidic environment due to the surface complexation and the formation of As2S3 precipitation. Niazi and Burton (Niazi & Burton 2016) indicated that an As2S3-like species formed at pH = 6 during As(III) removal by nanoparticulate mackinawite. Farquhar et al. (Farquhar et al. 2002) investigated the sorption of As(III) onto mackinawite in the high concentration of As(III) (initial pH = 5.5–6.5, As(III) concentration = 0.2 mM), and found that As(III) formed surface complexes on the crystalline FeS, with an As2S3-like species developing. Additionally, Han et al. (Han et al. 2011) reported that As(III) could form solid phase orpiment (As2S3) in the FeS-coated sand system at pH 5. The situation of FeS in the shell of S-nZVI was similar to that of FeS in the sand coating, and the initial pH of As(III) solution in this study is 5.6; hence, it can be inferred that As(III) may be adsorbed on the interface of S-nZVI particles firstly and then removed by precipitation conversion from FeS to As2S3:
formula
(8)
formula
(9)
The peak at 162.6 eV was attributed to S22−, probably due to the existence of disulphide. The peak of S 2p3/2 moved to higher binding energy after the As(III) removal, indicating the presence of electron loss process. In addition, a new peak at 164.0 eV corresponding to S0 emerged and the peak area of S2− decreased. With E0(Fe3+/Fe2+) = 0.77 V and E0(S/S2−) = −0.48 V being derived from the Table of Standard Electrode Potentials, it can be speculated that S0 stems from the loss electron of S2− and the gain electron of Fe3+. The specific reaction is expressed as:
formula
(10)
The narrow scan spectra of As 3d (Figure 9(d)) shows the characteristic peaks at 44.5 eV and 45.5 eV, which represent As(III) and As(V), respectively, indicating the partial oxidation of As(III) to As(V) during As(III) removal by S-nZVI@ATP. Similar results in open atmospheric condition were also found by other researchers (Ramos et al. 2009; Yan et al. 2012a; Bhowmick et al. 2014; Zou et al. 2016). As(III) is oxidized to As(V), mainly because n-ZVI (Yan et al. 2012a) or S-nZVI particles (Wu et al. 2018) can react with dissolved oxygen to produce oxidizing intermediates (e.g. •OH radical and H2O2) through a double electron transfer on the interface (Equations (10) and (11)):
formula
(11)
formula
(12)

Schematic of As(III) sequestration by S-nZVI@ATP

Based on the experimental results and theoretical analysis, the schematic of the mechanism of As(III) sequestration by S-nZVI@ATP is summarized in Figure 10. Fe (0) coupled with the oxide/sulfide shell in S-nZVI@ATP pose as the reactive areas during the sequestration process. Here, As(III) in aqueous solution reached the surface of S-nZVI@ATP through liquid phase diffusion, and was later sequestrated through the following processes: (1) precipitation conversion – reacting with FeS to form As2S3 precipitation; (2) co-precipitation – co-precipitating with Fe2+/Fe3+ produced by corrosion of nZVI; (3) surface-complexation adsorption – complexing with the iron oxides (≡FeOH) on the surface of S-nZVI particles. Besides, partial oxidation of As(III) to As(V) occurred on the interface of S-nZVI particles.

Figure 10

A schematic showing the possible reaction mechanism of As(III) sequestration by S-nZVI@ATP.

Figure 10

A schematic showing the possible reaction mechanism of As(III) sequestration by S-nZVI@ATP.

CONCLUSION

This study investigated the characterizations, performance, kinetics and mechanism of S-nZVI@ATP for As(III) sequestration. The results showed that nZVI particles were well dispersed owing to the loading by attapulgite, and the spherical nanoparticles in S-nZVI@ATP exhibited a larger average particle size (94.6 nm) compared to bare nZVI (66.0 nm). S-nZVI@ATP exhibited a higher removal efficiency for arsenic (51.4%) compared to bare nZVI (65.1%) at the reaction time of 20 min, and a prominent ability for As(III) sequestration over a wide pH range of 3.0–6.0. The removal efficiency of arsenic was markedly decreased in the presence of HCO3, while it was enhanced in the presence of SO42− and Cl. The kinetics of As(III) sequestration by S-nZVI@ATP conformed to the pseudo-second-order model, and the adsorption isotherm fitted the Langmuir model, with the maximum adsorption capacity of 193.8 mg·g−1. XPS analyses depicted the occurrence of As(III) → As(V) oxidation. Moreover, As(III) sequestration by S-nZVI@ATP involves multiple processes: reacting with FeS to form As2S3 precipitation, co-precipitating with Fe2+/Fe3+ produced by corrosion of Fe(0) core and complexing with the iron oxides (≡FeOH) on the surface of S-nZVI. The characterization, kinetics and mechanism investigated in this study help to curve complete comprehension of As(III) sequestration, and are important in the removal of arsenic pollutants in wastewater.

ACKNOWLEDGEMENTS

This work was supported by the Key Project of Natural Science Foundation of China (Grant No. 41230638) and the Open Fund of Hubei Key Laboratory for Efficient Utilization and Agglomeration of Metallurgic Mineral Resources from Wuhan University of Science and Technology (2017zy008).

DECLARATION OF COMPETING INTERESTS

The authors declare no competing financial interest.

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

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