The mechanisms of ·OH formation in MnO₂ and oxalate system: Implication for ATZ removal

Manganese oxides (MnO₂) are commonly prevalent in groundwater, sediment and soil. In this study, we found that oxalate (H₂C₂O₄) dissolved MnO₂, leading to the formation of Mn(II)/(III), CO₂(aq) and reactive oxygen species (·CO₂^-/O₂^·-/H₂O₂/·OH). Notably, CO₂(aq) played a crucial role in ·OH formation, contributing to the degradation of atrazine (ATZ). To elucidate underneath mechanisms, a series of reactions with different gas-liquid ratios (GLR) were conducted. At the GLR of 0.3, 3.76, and + ∞ 79.4 %, 5.32 %, and 5.28 % of ATZ were eliminated, in which the cumulative ·OH concentration was 39.6 μM, 8.11 μM, and 7.39 μM and the cumulative CO₂(aq) concentration was 11.2 mM, 4.7 mM, and 2.8 mM, respectively. The proposed reaction pathway was that CO₂(aq) participated in the formation of a ternary complex [C₂O₄-Mn(II)-HCO₄·3 H₂O^]-, which converted to a transition state (TS) as [C₂O₄-Mn(II)-CO₃-OH·3 H₂O^]-, then decomposed to a complex radical [C₂O₄-Mn(II)-CO₃·3 H₂O]^·- and ·OH after electron transfer within TS. It was novel to discover the role of CO₂(aq) for ·OH yielding during MnO₂ dissolution by H₂C₂O₄. This finding helps revealing the overlooked processes that CO₂(aq) influenced the fate of ATZ or other organic compounds in environment and providing us ideas for new technique development in contaminant remediation. ENVIRONMENTAL IMPLICATION: Manganese oxides and oxalate are common in soil, sediment and water. Their interactions could induce the formation of Mn(II)/(III), CO₂(aq) and ·CO₂^-/O₂·^-/H₂O₂. This study found that atrazine could be effectively removed due to ·OH radicals under condition of high CO₂(aq) concentration. The concentrations of Mn (0.0002-8.34 mg·L^-1) and CO₂(aq) (15-40 mg·L^-1) were high in groundwater, and the surface water or rainfall seeps into groundwater and bring organic acids, which might promote the ·OH formation. The results might explain the missing steps of herbicides transformation in these environments and be helpful in developing new techniques in remediation in future.