Cr(VI) removal by combined redox reactions and adsorption using pectin-stabilized nanoscale zero-valent iron for simulated chromium contaminated water

Abstract

The synthetic pectin-stabilized nanoscale zero-valent iron was used to remove Cr(VI) from simulated chromium contaminated water. The Cr(VI) removal data were well fitted with the pseudo-first order kinetic equation. The observed pseudo-first order rate constant for Cr(VI) removal decreased from 0.0781 to 0.0413 min⁻¹ when the pH increased from 3 to 9. When the initial Cr(VI) concentration increased from 20 to 80 mg L⁻¹, the observed pseudo-first order rate constant decreased from 0.0645 to 0.0366 min⁻¹. The Cr(VI) removal efficiency had obviously increased as the dose of pectin-stabilized nZVI increased from 0.02 to 0.10 g, and it increased from 0.0276 to 0.1159 min⁻¹ as the temperature increased from 15 to 35 °C. The scanning electron microscopy (SEM) images proved that the presence of pectin successfully stabilized the nZVI particles and thus increased the BET specific surface area of nZVI. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses demonstrated that the mechanisms of Cr(VI) removal by pectin-stabilized nZVI were a combined process of redox reactions and adsorption.

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1. Introduction

Since Cr(VI) is carcinogenic, toxic and harmful to the environment and human health when present in groundwater and surface water, many approaches are carried out to convert Cr(VI) into immobile, insoluble and less harmful Cr(III).^1–6 The maximum concentration of chromium in drinking water is 0.1 mg L⁻¹, but the detected Cr(VI) in many wastewaters can reach 50–100 mg L⁻¹, or even higher.⁷ Therefore, it is essential to develop new and effective technologies for Cr(VI)-contained wastewaters.

The reported Cr(VI) removal methods include bioremediation technology, electrokinetic remediation, chemical precipitation processes, adsorption approaches, membrane separation technology, and chemical redox reactions.^8–10 However, the most convenient and conventional method is chemical redox. Dittert and coworkers¹¹ used Laminaria digitata macro-algae to reduce and adsorb Cr(VI), and reported that the maximum Cr(VI) reduction capacity was 2.1 mmol g⁻¹ algae. In the study of Mo et al.,¹² a novel adsorption-electroreduction system in which reticulated vitreous carbon electrodes was modified with sulfuric acid-glycine co-doped polyaniline was used for the reduction of Cr(VI), and achieved satisfied effects. Zhang et al.¹³ studied the combined pillared bentonite and zero-valent iron (ZVI) for enhanced Cr(VI) removal and extended longevity of reaction medium. In the research of Singh and coworkers,⁷ starch was used to functionalize iron oxide nanoparticles for removal of Cr(VI), and the highest monolayer saturation adsorption capacity reached 9.02 mg g⁻¹. Li and coworkers¹⁴ used nanoscale zero-valent iron combined pillared bentonite to achieve enhanced Cr(VI) removal. Huang et al.¹⁵ made nanoscale zero-valent iron load onto activated carbon fiber felt as a new material for Cr(VI) removal and attained excellent efficiency. In the study of Alidokht and coworkers,⁸ starch was used to stabilize Fe⁰ nanoparticles and Fe₃O₄ particles for reduction of Cr(VI), and this combined method was proven as an efficient approach for Cr(VI) removal.

In this study, pectin was used to stabilize nanoscale zero-valent iron (nZVI), and this new combined material was investigated to removal Cr(VI) from simulated chromium contaminated water. The novelty of this paper is that pectin as a stabilizer to modify nZVI through covalent bond. The combined method can change the surface characteristic of nZVI and thus enhance the removal efficiency. The effects of pH, initial Cr(VI) concentration, adsorbent dose, and temperature on this reduction process were also investigated. SEM, FTIR, XPS analyses were carried out to deeply reveal the mechanisms of Cr(VI) removal by this synthesized pectin-stabilized nZVI particles.

2. Materials and methods

2.1. Pectin-stabilized nZVI synthesis

The nZVI particles were synthesized by the reaction of FeCl₃·6H₂O solution and KBH₄. The reduction reaction is as following:

4Fe³⁺ + 3BH₄⁻ + 9H₂O → 4Fe⁰ + 3H₂BO₃⁻ + 12H⁺ + 6H₂ (1)

Ferric chloride hydrate was dissolved into deoxygenated de-ionized (DI) and ethanol mixed liquor (solution A), and potassium borohydride was dissolved into deoxygenated DI water in another beaker (solution B). Then solution B was added drop wise to solution A under continuously stirring conditions (using a mechanical stirrer). The reaction lasted 30 minutes under vigorously mixed and no oxygen condition. 10.0 wt% pectin (10.0 wt% pectin coated on nZVI achieved better removal efficiency than higher or lower contents) was added into the reaction solution when the reaction run 10 minutes in order to coated pectin on the nZVI. After reaction, the particles were washed by anhydrous ethanol several times in order to prevent oxidation, then the synthetic PNZVI particles were dried under vacuum environment.

2.2. Batch Cr(VI) removal experiments

In order to investigate the parameters pH, initial Cr(VI) concentration, adsorbent dose, and temperature on initial Cr(VI) removal by this synthesized pectin-stabilized nZVI, batch experiments were carried out in 100 mL centrifuge tubes in shaking table. The pH was severally adjusted to 3, 4, 5, 6, 7, 8, and 9 under initial Cr(VI) concentration 20 mg L⁻¹, adsorbent dose 0.06 g, and temperature 25 °C condition. The initial Cr(VI) concentration was severally adjusted to 20 mg L⁻¹, 40 mg L⁻¹, 60 mg L⁻¹, and 80 mg L⁻¹ under adsorbent dose 0.06 g, temperature 25 °C, and pH 5 condition. The adsorbent dose was severally adjusted to 0.02 g, 0.04 g, 0.06 g, 0.08 g, and 0.10 g under initial Cr(VI) concentration 20 mg L⁻¹, pH 5, and temperature 25 °C condition. The temperature was severally adjusted to 15 °C, 25 °C, and 35 °C under initial Cr(VI) concentration 20 mg L⁻¹, adsorbent dose 0.06 g, and pH 5 condition.

2.3. Analytical methods

Cr(VI) solutions used in batch experiments were prepared by dissolving K₂Cr₂O₇. The pH was adjusted by adding NaOH and HCl solution during the experiments.

The suspension samples during the experiments were filtered by 0.30 μm membrane by a suction filter machine. Then the concentrations of Cr(VI), total Cr, Fe(III) in the samples were determined in accordance with the Standard Methods for the Examination of Water and Wastewater.¹⁶ The SEM images were taken on S-4700, Hitachi. FTIR spectroscopy spectra of the samples were taken on Nicolet 5700 spectrometer using KBr pellets in the range of 4000–400 cm⁻¹. XPS was taken on Thermo Fisher ESCALAB 250Xi. Specific surface area was determined by BET specific surface area by the TriStarII3020 V1.03.

3. Results and discussion

3.1. Effects of pH on Cr(VI) removal efficiency

Fig. 1 shows the changes of Cr(VI) removal efficiency under different pH values. The Cr(VI) reduction rate decreased with pH increased from 3 to 9. The data of Cr(VI) removal rates were fitted by the pseudo-first order kinetic equation, eqn (2)

ln(C_Cr(VI)/C⁰_[Cr(VI)]) = −k_obst (2)

where k_obs is the observed pseudo-first order rate constant (min⁻¹), t is the reaction time (min), C_Cr(VI) is the instantaneous concentration of Cr(VI), C⁰_[Cr(VI)] is the initial concentration of Cr(VI).

Fig. 1 Cr(VI) removal efficiency under different pH values.

It can be seen from Table 1 that Cr(VI) removal rates decreased from 0.0781 to 0.0413 min⁻¹ when pH increased from 3 to 9. When pH was lower than 6.0, the predominant composition of Cr(VI) was HCrO₄⁻, while the main composition of Cr(VI) was CrO₄²⁻ at pH higher than 6.0. The redox reactions between pectin-stabilized nZVI and Cr(VI) are as following:

Table 1 Cr(VI) removal rates under different pH condition

pH	3	4	5	6	7	8	9
kobs (min^−1)	0.0781	0.0699	0.0647	0.0567	0.0554	0.0514	0.0413
R^2	0.9985	0.9911	0.9846	0.9940	0.9792	0.9814	0.9940

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Eqn (3) presents the redox reactions between pectin-stabilized nZVI and Cr(VI) under acid environment, and eqn (4) presents the redox reactions under alkaline environment. Table 2 shows the concentrations of Cr(III) and Fe(III) after reaction, which demonstrated that Cr(III) and Fe(III) were the main products during the redox reactions. However, not all Cr(VI) were reduced to Cr(III), meaning that redox reaction was not the only reaction during the removal process. Under acid environment, large numbers of H⁺ would accelerate the reduction process for HCrO₄⁻ to Cr(III) (eqn (3)), so that Cr(VI) removal rates increased with decreasing pH. However, large amounts of OH⁻ would inhibit the reduction process under alkaline condition (eqn (4)), resulting in decreased Cr(VI) removal rates. On the other hand, the surface of pectin-stabilized Fe⁰ often carried positive charges under acid environment,¹⁷ so that pectin-stabilized Fe⁰ could easily adsorb negative ion HCrO₄⁻ and CrO₄²⁻.

Table 2 Concentrations of Cr(III) and Fe(III) after reaction

pH	3	4	5	6	7	8	9
Cr(III) (mg L^−1)	13.69	12.83	13.25	11.89	12.65	10.03	9.56
Fe(III) (mg L^−1)	14.88	15.52	14.39	13.87	13.43	12.76	10.52
Initial Cr(VI) (mg L^−1)	20	40	60	80
Cr(III) (mg L^−1)	11.98	28.91	48.26	60.02
Fe(III) (mg L^−1)	13.32	29.53	53.18	74.21
Adsorbent dose (g)	0.02	0.04	0.06	0.08	0.10
Cr(III) (mg L^−1)	10.92	11.28	13.03	12.12	14.59
Fe(III) (mg L^−1)	12.20	14.14	14.56	18.35	18.97
Temperature (°C)	15	25	35
Cr(III) (mg L^−1)	12.14	12.69	13.27
Fe(III) (mg L^−1)	18.34	18.96	19.38

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3.2. Effects of initial Cr(VI) concentration on Cr(VI) removal efficiency

As shown in Fig. 2, Cr(VI) removal efficiency decreased with initial Cr(VI) concentration increased from 20 to 80 mg L⁻¹. Table 3 also shows that the observed pseudo-first order rate constant of Cr(VI) removal by pectin-stabilized nZVI decreased from 0.0645 to 0.0366 min⁻¹ when initial Cr(VI) concentration increased from 20 to 80 mg L⁻¹. These results demonstrated that Cr(VI) removal rate decreased with increasing initial Cr(VI) concentration using pectin-stabilized nZVI. The reason for this phenomenon might be that a passivation layer¹⁸ was formed on the surface of pectin-stabilized nZVI under high and instantaneous initial Cr(VI) concentration condition, preventing the electronic transmission between Cr(VI) and pectin-stabilized nZVI, so that the redox reaction rates were decreased. This result was in accordance with Dittert et al.¹¹ and Alidokht et al.⁸

Fig. 2 Cr(VI) removal efficiency under different initial Cr(VI) concentrations.

Table 3 Cr(VI) removal rates under different initial Cr(VI) concentration condition

Initial Cr(VI) concentration	20 mg L^−1	40 mg L^−1	60 mg L^−1	80 mg L^−1
kobs (min^−1)	0.0645	0.0505	0.0472	0.0366
R^2	0.9572	0.9515	0.9592	0.9906

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3.3. Effects of adsorbent dose on Cr(VI) removal efficiency

As shown in Fig. 3, when dose of pectin-stabilized nZVI increased from 0.02 to 0.10 g L⁻¹, the Cr(VI) removal efficiency had obvious increases. It can be seen from Table 4 that the observed pseudo-first order rate constant of Cr(VI) removal by pectin-stabilized nZVI increased from 0.0386 to 0.1203 min⁻¹ with adsorbent dose increased from 0.02 to 0.10 g. This result indicated that Cr(VI) removal rate was increased at larger pectin-stabilized nZVI dose condition, and also demonstrated that the pectin-stabilized nZVI exhibited high reactivity for Cr(VI) removal.

Fig. 3 Cr(VI) removal efficiency under different adsorbent doses.

Table 4 Cr(VI) removal rates under different adsorbent dose condition

Adsorbent dose	0.02 g	0.04 g	0.06 g	0.08 g	0.10 g
kobs (min^−1)	0.0386	0.0567	0.0716	0.0738	0.1203
R^2	0.9793	0.9940	0.9848	0.9678	0.9853

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3.4. Effects of temperature on Cr(VI) removal efficiency

Fig. 4 shows that Cr(VI) removal efficiency increased with temperature increased from 15 to 35 °C, and the observed pseudo-first order rate constant also increased from 0.0276 to 0.1159 min⁻¹ with increasing temperature (Table 5), suggesting that reduction process of Cr(VI) by pectin-stabilized nZVI was attributing to an endothermic process. This result was in accordance with the researches of Dubey et al.,³ Dittert et al.,¹¹ and Cissoko et al.¹⁹

Fig. 4 Cr(VI) removal efficiency under different temperatures.

Table 5 Cr(VI) removal rates under different temperature condition

Temperature	15 °C	25 °C	35 °C
kobs (min^−1)	0.0276	0.0716	0.1159
R^2	0.9478	0.9939	0.9918

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The effects of temperature on Cr(VI) removal can be described by Arrhenius equation eqn (5)

where k_obs is the observed pseudo-first order rate constant (min⁻¹), E_a is the activation energy (kJ mol⁻¹), R is the gas constant (0.008314 kJ mol⁻¹ K⁻¹), T is the Kelvin temperature (K), k is the kinetic constant (min⁻¹).

The calculated activation energy for Cr(VI) removal by pectin-stabilized nZVI was 53.14 kJ mol⁻¹. Park and coworkers²⁰ reported the activation energy for Cr(VI) removal was 32.24 kJ mol⁻¹, and Dittert et al.¹¹ reported the activation energy for Cr(VI) removal was 45 kJ mol⁻¹. The activation energy in this study was in the same order of magnitude as these studies and higher than these values, meaning that this reduction process for Cr(VI) removal by pectin-stabilized nZVI was mainly controlled by chemical reaction process, rather than physical reaction process.

3.5. Removal mechanisms

Fig. 5 shows the SEM images of nZVI (a) and pectin-stabilized nZVI (b). The pectin-stabilized nZVI particles were more scattered, while the pure nZVI particles presented obvious agglomeration phenomenon, indicating that the presence of pectin successfully stabilized the nZVI particles. It also demonstrated that the reactivity of pectin-stabilized nZVI was enhanced because pectin can scatter nZVI through covalent bond effect. The pectin-stabilized nZVI exhibited larger BET specific surface area 43.98 m² g⁻¹ than pure nZVI 19.53 m² g⁻¹, which also illustrated that pectin-stabilized nZVI had higher reactivity.

Fig. 5 SEM images of nZVI (a) and pectin-stabilized nZVI (b).

FTIR spectra of pectin-stabilized nZVI (a) and reacted pectin-stabilized nZVI (b) are shown in Fig. 6. The obvious peaks appeared at 3340 cm⁻¹ and 3376 cm⁻¹ were due to the –OH stretching vibrations. Water molecules were observed at 1600 cm⁻¹ and 1633 cm⁻¹. The peaks observed at 1334 cm⁻¹ and 1375 cm⁻¹ were attributing to the C–O–H bending. C–C bending⁷ was appeared at 1002 cm⁻¹ and 1054 cm⁻¹. The appearance of C–O–H and C–C bending proved the presence of pectin, which was consisted of polysaccharide backbone. The new peak compared to pure pectin-stabilized nZVI appeared at 538 cm⁻¹ after reaction was attributing to Fe–O bond,^7,15 suggesting that the iron oxides FeOOH and Fe₂O₃ (ref. 21) formed onto the surface of pectin-stabilized nZVI after reaction.

Fig. 6 FTIR spectra of pectin-stabilized nZVI (a) and reacted pectin-stabilized nZVI (b).

Fig. 7 shows the XPS spectra of pectin-stabilized nZVI before (a) and after reaction (b). The main elements of pure pectin-stabilized nZVI were Fe 2p, O 1s, and C 1s, while the main elements of reacted pectin-stabilized nZVI were Fe 2p, Cr 2p, O 1s, and C 1s. The obvious peak Cr 2p presented on the reacted pectin-stabilized nZVI survey, meaning that pectin-stabilized nZVI carried large amounts of Cr 2p after the reduction reaction. The detailed survey of Cr 2p was shown in Fig. 7(c). The two obvious peaks appeared at 576.7 eV and 587.4 eV were due to Cr(III) 2p_3/2 and Cr(III) 2p_1/2,²² indicating that Cr(VI) was reduced to Cr(III) by pectin-stabilized nZVI. The small peaks at 579.4 eV and 583.1 eV were attributing to Cr(VI) 2p_3/2 and Cr(VI) 2p_1/2,¹⁵ suggesting that adsorption process was also present in the Cr(VI) removal process by pectin-stabilized nZVI. It can be seen from Table 2 that Cr(VI) was not completely reduced to Cr(III), which also illustrated that redox was not the only reaction occurred in the process. The reason for adsorption process was that the generated Cr(III) would form a membrane with FeOOH²³ on the surface of pectin-stabilized nZVI, preventing the electronic transmission between Cr(VI) and pectin-stabilized nZVI, so that adsorption would occur. Therefore, the mechanisms of Cr(VI) removal by pectin-stabilized nZVI were the combined process of redox reactions and adsorption.

Fig. 7 XPS spectra of pectin-stabilized nZVI before (a) and after reaction (b), and detailed survey of Cr 2p (c).

4. Conclusion

The pectin-stabilized nanoscale zero-valent iron was proved to exhibit effective Cr(VI) removal performance. The Cr(VI) reduction rate decreased with increasing pH and increasing initial Cr(VI) concentration. Cr(VI) removal rate was increased at larger pectin-stabilized nZVI dose condition, and the reduction process of Cr(VI) by pectin-stabilized nZVI was attributing to an endothermic process. The SEM images indicated that the presence of pectin successfully stabilized the nZVI particles. The FTIR analysis also proved the presence of pectin on this synthetic material. The XPS spectra demonstrated that Cr(III) and Cr(VI) were both observed onto pectin-stabilized nZVI after reaction, suggesting that the redox reactions and adsorption were both occurred in the Cr(VI) removal process.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 51378400) and the National Science and Technology Pillar Program (2014BAL04B04).

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