Elsevier

Environmental Pollution

Volume 227, August 2017, Pages 444-450
Environmental Pollution

Effect of pH, temperature, humic acid and coexisting anions on reduction of Cr(Ⅵ) in the soil leachate by nZVI/Ni bimetal material

https://doi.org/10.1016/j.envpol.2017.04.074Get rights and content

Highlights

  • nZVI/Ni was used to remediate Cr(Ⅵ) in the soil leachate and has good reduction rate.

  • The reduction rate of Cr(Ⅵ) exceeded 99% at pH 5 and temperature 313 K.

  • SO42-, HCO3 and CO32- could inhibit the reaction rate between nZVI/Ni and Cr(Ⅵ).

  • The reduction process followed Langumir-Hinshelwood first order kinetic model.

Abstract

Nano zero valent iron/Ni bimetal materials (nZVI/Ni) were prepared by borohydride reduction method to remediate toxic Cr(Ⅵ) contaminated in soil leachate. nZVI/Ni was characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). Different factors including pH value of soil leachate, reaction time, temperature, humic acid and coexisting anions (SO42-, NO3, HCO3, CO32-) were studied to analyze the reduction rate. Results showed that the reduction rate of Cr(Ⅵ) could reach 99.84% under the condition of pH of 5 and temperature of 303 K. pH values and temperature of soil leachate had a significant effect on the reduction efficiency, while humic acid had inhibition effect for the reduction reaction. SO42-, HCO3 and CO32- had inhibition effect for reduction rate, while NO3 barely influenced the reduction process of nZVI/Ni. Moreover, Langumir-Hinshelwood first order kinetic model was studied and could describe the reduction process well. The thermodynamic studies indicated that the reaction process was endothermic and spontaneous. Activation energy was 143.80 kJ mol−1, showing that the reaction occurred easily. Therefore, the study provides an idea for nZVI/Ni further research and practical application of nZVI/Ni in soil remediation.

Introduction

Chromium is applied extensively in the industry of electroplating, leather industries, metal cleaning and so on. Hexavalent chromium exited in chromium slag could lead to the pollution of groundwater and soil contamination. It is strongly toxic to humans as it may cause problems to the digestive tract, respiratory tract, skin and mucous membrane. Trivalent chromium, which is one of the essential element of the human body, has less toxicity. The conventional treatment methods for Cr(Ⅵ) waste water are adsorption, electrochemical method, oxidation method, reduction method, ionic exchange and membrane separation. The reduction method is one of the effective methods to solve the toxicity of hexavalent chromium (Mystrioti et al., 2015).

Nano zero valent iron (nZVI) is well known for its efficiency for the treatment of various kinds of organic pollutants and heavy metals in water and soil (Wen et al., 2014, Katsenovich and Miralles-Wilhelm, 2009, Huang et al., 2013). Cr(Ⅵ) could be reduced to less toxic Cr(Ⅲ) by nZVI due to its strong reduction ability, superior adsorptive property, large specific area and high reduction activation(Shi et al., 2011, Gheju, 2011). Quan et al. (Quan et al., 2014) applied nZVI grafted on acid-activated attapulgite clay to remove Cr(Ⅵ) from aqueous solution. Results showed that removal efficiency of Cr(Ⅵ) was 98.73% within 60 min. Di Palma et al. (2015) found that nZVI is more effective than ferrous sulfate to remove Cr(Ⅵ) in contaminated soil. However, the shortcomings of nZVI are agglomerate rapidly and oxidized easily when exposed to air as the oxidation layer formed on the surface could slow down the reaction rate (Phenrat et al., 2007, Li et al., 2006).

In order to prevent the aggregation and oxidization of nZVI, a second catalytic metal, such as Cu, Ni, Au and Pd (Fang et al., 2011, Su et al., 2014, Wu et al., 2014; Zhu et al., 2016) is added in the synthetic process of nZVI to form bimetallic nanoparticles suitable for the treatment of pollutants. Adding a second catalytic metal to the synthetic process of nZVI could decrease the activation energy of the reaction process and accelerates the reduction rate (Huguet and Marshall, 2009). The price of nickel is cheaper than that of Au and Pd. In addition, Fe/Ni bimetal has good removal effect for pollutants and has been used to treat kinds of organic pollutant (Barnes et al., 2010, Kang et al., 2012). For instance, Tian et al. (Tian et al., 2009) compared the degradation rate of DDT using nZVI and nZVI/Ni bimetallic materials and found that over 90% DDT was degraded in 4 h under neutral condition by Ni/Fe. Moreover, nZVI/Ni bimetallic materials were also used to remove heavy metal ions (Tang et al., 2016). Kadu et al. (Kadu et al., 2011) found that Cr(Ⅵ) in simulated water streams was reduced completely by nZVI/Ni bimetallic materials in 10 min and the reduction process followed the rule of first order reaction kinetics. Zhou et al. (Zhou et al., 2014) used nZVI/Ni bimetallic materials to remove Cr(Ⅵ) from aqueous solutions. Results showed that acidic condition was favor to remove Cr(Ⅵ) by nZVI/Ni bimetallic materials. SO42- and PO42- could impede the adsorption of Cr(Ⅵ). But the research referring to the remediation of Cr(Ⅵ) contaminated soil using nZVI/Ni bimetallic materials has not been reported yet.

In this work, nZVI/Ni bimetallic materials were prepared byborohydride reduction method and characterized by SEM, XRD and XPS. nZVI/Ni bimetallic materials were used to remediate Cr(Ⅵ) contaminated soil leachate. The effect of pH values, temperature, the concentration of humic acid and coexisting anions on the reduction rate of Cr(Ⅵ) were investigated. The Langumir-Hinshelwood first order kinetic model and Arrhenius equation were used to analyze the reduction process and mechanism.

Section snippets

Materials

Ferric trichloride (FeCl3·6H2O), nickel sulfate (NiSO4·7H2O), sodium borohydride (NaBH4), sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), sodium nitrate (NaNO3), sodium sulfate (Na2SO4), anhydrous ethyl alcohol, potassium dichromate (K2Cr2O7), glacial acetic acid, hydrochloric acid (HCl) and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co. Ltd. China. Diphenylcarbazide (C13H14N4O), phosphoric acid(H3PO4) and sulfuric acid(H2SO4) were purchased from Tianjin

Characterization

The morphology of nZVI/Ni bimetallic materials was characterized by scanning electron microscope(SEM), as shown in Fig. 1. The SEM image showed that the morphology of nZVI/Ni bimetallic materials was mainly spherical, and Fe-Ni presents with chain structure distribution. The diameter size of nZVI/Ni bimetallic materials was about 20 nm. The agglomeration phenomenon existed among nanoparticles. High void fraction of agglomerates had little effect on the surface area and reduction rate of nZVI/Ni

Conclusions

In the present work, nZVI/Ni bimetallic materials were synthesized by liquid phase reduction method and it has high reduction efficiency for hexavalent chromium in the soil leachate. The reduction rate of nZVI/Ni bimetallic materials were investigated on the different influence factors including pH value, temperature, humic acid and coexisting anions. The pH value of soil leachate had the most significant effect on the reduction rate. Reducing pH value and improving the reaction temperature

Acknowledgements

This work was sponsored by the National Natural Science Foundation of China (Project No. 21276174) and Natural Science Foundation of Shanxi province (Project No. 2013011040-1).

References (21)

There are more references available in the full text version of this article.

Cited by (0)

This paper has been recommended for acceptance by Dr. J. Rinklebe.

View full text