The mechanisms of ·OH formation in MnO 2 and oxalate system: Implication for ATZ removal

Manganese oxides (MnO 2 ) are commonly prevalent in groundwater, sediment and soil. In this study, we found that oxalate (H 2 C 2 O 4 ) dissolved MnO 2, leading to the formation of Mn(II)/(III), CO 2 (aq) and reactive oxygen species (·CO 2 - /O 2 ·- /H 2 O 2 /·OH). Notably, CO 2 (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 CO 2 (aq) concentration was 11.2 mM, 4.7 mM, and 2.8 mM, respectively. The proposed reaction pathway was that CO 2 (aq) participated in the formation of a ternary complex [C 2 O 4 -Mn(II)-HCO 4 ·3 H 2 O ]-, which converted to a transition state (TS) as [C 2 O 4 -Mn(II)-CO 3 -OH·3 H 2 O ]-, then decomposed to a complex radical [C 2 O 4 -Mn(II)-CO 3 ·3 H 2 O] ·- and ·OH after electron transfer within TS. It was novel to discover the role of CO 2 (aq) for ·OH yielding during MnO 2 dissolution by H 2 C 2 O 4. This finding helps revealing the overlooked processes that CO 2 (aq) influenced the fate of ATZ or other organic compounds in environment and providing us ideas for new technique development in contaminant remediation. Manganese oxides and oxalate are common in soil, sediment and water. Their interactions could induce the formation of Mn(II)/(III), CO 2 (aq) and ·CO 2 - /O 2 · - /H 2 O 2. This study found that atrazine could be effectively removed due to ·OH radicals under condition of high CO 2 (aq) concentration. The concentrations of Mn (0.0002–8.34 mg·L −1 ) and CO 2 (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.