Transformation of Atrazine on δ-MnO surfaces under dry conditions: Hydrolysis and dealkylation.

Atrazine (ATZ) is a frequently detected herbicide in corn fields, and manganese oxides (MnO) are reactive soil components. Reportedly, the sole use of MnO cannot efficiently achieve ATZ removal in aqueous solution. However, this study found that 90.2 % of ATZ could be degraded on δ-MnO surfaces under dry conditions (mineral moisture content < 3 %, relative humidity (RH) ≤ 97.30 %). Atrazine-desisopropyl-2-hydroxy and 2-hydroxyatrazine were obtained as the main products through the dealkylation and hydrolysis of ATZ, and the intermediates exhibited low ecotoxicity toward aquatic organisms. The experiments were conducted at 23°C (RH: 6.40-97.3 %). Based on quenching experiments, Mn(IV) contributed approximately 88.0 % to ATZ degradation by accepting electrons from Cl and N in ATZ, forming Mn-Cl-C and Mn-N-H bonds, and reducing the activation energy required for hydrolysis based on DFT calculation. The oxygen vacancy in δ-MnO provided no considerable contribution to ATZ removal. RH notably influenced the processes, and the optimal RH value was 22.5 %. Similar mechanisms were observed for simazine, simetone, propazine, and ametryn on δ-MnO surfaces. In agricultural soil featuring low Mn contents, ATZ degradation was limited (< 1.0 %), however, the ATZ transformation efficiency increased to 31.2 % and 63.0 % after the addition of 1 % and 5 % δ-MnO particles. The effectiveness of δ-MnO was significantly correlated with the soil texture (R = 0.81-0.91, p < 0.05). These findings provide insights into the abiotic degradation mechanisms of triazine herbicides in dry soils, presenting strategies to remove triazine herbicide residues from soil.