In situ arsenic immobilization by chemical oxidation: Field trial and reactive transport modelling

In situ chemical oxidation (ISCO) can be used as a technique for in situ arsenic (As) removal from groundwater. NaClO as an oxidant was intermittently injected into an anoxic aquifer with high aqueous As and Fe concentrations which is located on the Jianghan Plain (JHP). The aim was to promote the formation of hydrous ferric oxide (HFO) and oxidize arsenite to arsenate, thus immobilization aqueous arsenate via adsorption and/or co-precipitation. A three-dimensional reactive transport model was developed to determine the hydrogeochemical processes responsible for As (im)mobilization during the field trial. The modeling results indicate that the variation of AsO₃³⁻ concentration can be divided into adsorption non-equilibrium and adsorption equilibrium stage. In adsorption non-equilibrium stage, oxidation and accompanying adsorption onto the surface of HFO causes the rapid decrease of AsO₃³⁻ concentration. However, the modeled AsO₃³⁻ concentrations were lower than measured concentrations in adsorption equilibrium stage, which may be due to AsO₃³⁻ desorption from the surface of HFO as advantages of other competitive ions. Comparison of field data and modeling results suggest the product of the oxidative hydrolysis of Fe²⁺(aq) in the field experiment was ferrihydrite. The concentration of total As was mainly affected by the HFO content and concentration of competitive adsorption ions in groundwater. The natural groundwater flow field led to a difference in the concentration of HFO upstream and downstream of the injection well, where the downstream well had a higher HFO content and lower total As concentration. HCO₃– was the dominant surface species affecting the adsorption of AsO₄³⁻ by HFO. There is more sorption site on HFO surface due to the oxidation of adsorbed Fe²⁺, and then resulted in an increase in adsorbed H₂SiO₃(aq) and Mg²⁺(aq) content on the surface of the HFO over the course of the experiment. The reactive transport modeling (RTM) results reasonably match with observed concentrations of hydrochemical ions, indicating RTM can explain the crucial chemical processes in this in situ As remediation trial, providing important technical guidance for similar remediation operations.