Study on the influence mechanism of underground mineral element Fe(II) on Cr(VI) transformation under subsurface and groundwater interaction zones

Abstract

Background: With the acceleration of urbanization, heavy metal contamination in subsurface water and groundwater is becoming more and more serious, and the interaction between surface water and underground water is an important factor that cannot be ignored. Based on the change of oxygen content in surface water and groundwater environment, this research mainly studied the influencing factors and mechanism of underground mineral element Fe(II) on the morphological transformation of heavy metal Cr(VI) under the condition of surface–groundwater interaction, and simulates the interaction process of subsurface–groundwater by changing the dissolved oxygen content in the redox reaction system of dissolved Fe(II) and Cr(VI). The influence mechanism of pH, Fe(II):Cr(VI), groundwater hardness ion and humic acid on the morphological transformation of Cr(VI) was investigated. Results: The results showed that the content of DO did affect the conversion of Cr(VI) by Fe(II). pH had a great influence on the redox reaction: while pH = 3–5, the reaction rate of Fe(II) and Cr(VI) and the removal rate of Cr(VI) decreased with the increase of pH; while pH = 6–8, the reaction rate and the removal of Cr(VI) increased with the increase of pH. When Fe(II):Cr(VI) = 3:1, the reaction of Fe(II) and Cr(VI) is affected by DO; when Fe(II):Cr(VI) is 1:1, 2:1, 4:1 and 5:1, DO has no obvious effect on the reaction. Groundwater hardness ions can greatly promote the redox reaction. When humic acid exists in the reaction system, the Fe(III) formed by the oxidation of Fe(II) by Cr(VI) will once again be converted into soluble Fe(II), the reduction of humic acid further promotes the redox reaction between Fe(II) and Cr(VI), and promotes the conversion of Cr(VI) by Fe(II) more effectively. Moreover, the higher the concentration of humic acid, the higher the removal rate of Cr(VI). Conclusions: The research results have practical guiding significance for groundwater contamination prediction, pollution control and remediation, integrated water resources management and the construction of sponge city in China.