Redox potential model for guiding moderate oxidation of polycyclic aromatic hydrocarbons in soils

In-situ chemical oxidation is an important approach to remediate soils contaminated with persistent organic pollutants, e.g., polycyclic aromatic hydrocarbons (PAHs). However, massive oxidants are added into soils without an explicit model for predicting the redox potential (E_h) during soil remediation, and overdosed oxidants would pose secondary damage by disturbing soil organic matter and acidity. Here, a soil redox potential (E_h) model was first established to quantify the relationship among oxidation parameters, crucial soil properties, and pollutant elimination. The impacts of oxidant types and doses, soil pH, and soil organic carbon contents on soil E_h were systematically clarified in four commonly used oxidation systems (i.e., KMnO₄, H₂O₂, fenton, and persulfate). The relative error of preliminary E_h model was increased from 48-62% to 4-16% after being modified with the soil texture and dissolved organic carbon, and this high accuracy was verified by 12 actual PAHs contaminated soils. Combining the discovered critical oxidation potential (COP) of PAHs, the moderate oxidation process could be regulated by the guidance of the soil E_h model in different soil conditions. Moreover, the product analysis revealed that the hydroxylation of PAHs occurred most frequently when the soil E_h reached their COP, providing a foundation for further microorganism remediation. These results provide a feasible strategy for selecting oxidants and controlling their doses toward moderate oxidation of contaminated soils, which will reduce the consumption of soil organic matter and protect the main structure and function of soil for future utilization. ENVIRONMENTAL IMPLICATIONS: This study provides a novel insight into the moderate chemical oxidation by the E_h model and largely reduces the secondary risks of excessive oxidation and oxidant residual in ISCO. The moderate oxidation of PAHs could be a first step to decrease their toxicity and increase their bioaccessibility, favoring the microbial degradation of PAHs. Controlling the soil E_h with the established model here could be a promising approach to couple moderate oxidation of organic contaminants with microbial degradation. Such an effective and green soil remediation will largely preserve the soil's functional structure and favor the subsequent utilization of remediated soil.