Study on influencing factors and mechanism of removal of Cr(VI) from soil suspended liquid by bentonite-supported nanoscale zero-valent iron

Abstract

In order to clarify the mechanism and effect of bentonite-supported nanoscale zero-valent iron (nZVI@Bent) on Cr(VI) removal in soil suspended liquid, nZVI@Bent was prepared by liquid-phase reduction method in this research. A number of factors, including the mass ratio of Fe 2+ to bentonite during preparation of nZVI@Bent, nZVI@Bent dosage, soil suspended liquid pH value and reaction temperature were assessed to determine their impact on the reduction of Cr(VI) in soil suspended liquid. The nZVI@Bent was characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to analyze the mechanism of removal of Cr(VI) from the soil. The results showed that the temperature of soil suspended liquid had a significant effect on the removal efficiency. Calculated by the Arrhenius formula, nZVI@Bent removes Cr(VI) from the soil suspended liquid as an endothermic reaction with a reaction activation energy of 47.02 kJ/mol, showed that the reaction occurred easily. The removal of mechanism Cr(VI) from the soil by nZVI@Bent included adsorption and reduction, moreover, the reduction process can be divided into direct reduction and indirect reduction. According to XPS spectrogram analysis, the content of Cr(III) in the reaction product was 2.1 times of Cr(VI), indicated that the reduction effect was greater than the adsorption effect in the process of Cr(VI) removal. The experiment proved that nZVI@Bent can effectively remove Cr(VI) from soil suspension, and can provide technical support for repairing Cr(VI)-polluted paddy fields. Chromium (Cr) is a heavy metal element that exists on the earth, mostly in the two valence states of Cr(III) and Cr(VI). Under certain conditions, Cr(VI) will be reduced to Cr(III) by reducing substances in the soil. Different environmental pollution effects of chromium in different valence states. Cr(VI) can be inhaled with dust to cause respiratory diseases. Oral administration can also cause digestive tract corrosion. After long-term exposure to the high concentration of Cr(VI), gene expression may be changed by destroying DNA, protein and lipid, which can cause cancer in severe cases 1,2. However, the toxicity of Cr(III) is much lower than that of Cr(VI), moreover, Cr(III) was an essential trace element in biological activities, and low concentration of Cr(III) was beneficial to organisms 3-6. Compared with Cr(VI), Cr(III) is more stable in the environment and difficult to migrate. Many industries containing chromium pollution such as metallurgy, tannery and electroplating and so on had sprung up in recent years. The discharge of wastewater in these industries lead to increasing the concentration of chromium in the cultivated soil environment, meanwhile, the land near factories was also polluted by a large accumulation of chromium slag and chromium dust 7-9. It is almost impossible to remove Cr from the soil, so it is the

Application of nanoscale zero-valent iron in hexavalent chromium-contaminated soil: A review

“…Various tests were performed to evaluate the remediation effect of Cr(VI)-contaminated soil from different aspects. The Cr(VI) will adsorb on nZVI, through the surface pores and oxygen-containing functional groups, and then react directly with the surface to reduce to Cr(III), whereas nZVI is oxidized to Fe 2+ and indirectly reduced with Cr(VI) [66]. For example, toxicity characteristic leaching procedure is used to examine the leaching ability of Cr, and the immobilization and bioaccumulation of Cr in the improved soil are discussed through in vitro and plant experiments [67].…”

“…In the experiment of nZVI@Bent materials mentioned above, it is found that with the increasing reaction temperature, the removal rate of Cr(VI) in soil solution by nZVI@Bent presents an ascending trend. The specific reason is that after the temperature rises, the energy in the reaction system increases correspondingly; thus, the number of reaction molecules increases and the reaction rate accelerates [66].…”

“…Under high Cr(VI) concentration, the immobilization efficiency of Cr (III) in the reduced soil liquid phase is low, and the surface area of nZVI is limited. In this case, there isn't enough position for excess Cr(III), which leads to the slow removal rate of Cr(VI) [66].…”

“…According to the theme of this paper, the process of Cr(VI) reduction in soil can be classified into direct reduction and indirect reduction. The final products from reduction are Cr(OH) 3 and Fe(III)-Cr(III) complexes [66].…”

“…First, in the process of direct reduction, Cr(VI) in soil adsorbs on the curved surface of nZVI and reacts directly with the surface containing nZVI to form Cr(III) [87]; second, in the indirect reduction process, nZVI will be oxidized to Fe 2+ first [88], and to Fe 3+ subsequently [89], both with the reduction from Cr(VI) to Cr(III). Cr(III) will combine with OHand Fe 3+ , to form Cr(OH) 3 and Fe(III)-Cr(III) complexes [66].…”