Transformation of Atrazine on δ-MnO2 Surfaces under Dry Conditions: Hydrolysis and Dealkylation

Abstract

Atrazine (ATZ) is a frequently detected herbicide in corn fields, and manganese oxides (MnO₂) are reactive soil components. Reportedly, the sole use of MnO₂ cannot efficiently achieve ATZ removal in aqueous solution. However, this study found that 90.2% of ATZ could be degraded on δ-MnO₂ 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: 6.40-97.30%). Based on quenching experiments, Mn(IV) contributed approximately 88% 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-MnO₂ 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-MnO₂ surfaces. In agricultural soil featuring low Mn contents, ATZ degradation was limited (< 1%), however, the ATZ transformation efficiency increased to 31.2% and 63.0% after the addition of 1% and 5% δ-MnO₂ particles. The effectiveness of δ-MnO₂ 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.