Degradation of dicamba – A persistent herbicide – By combined application of formic acid and UV as an advanced reduction process

The degradation of dicamba as a persistent herbicide was studied with the combined application of UV and formic acid (FA) as a novel advanced reduction process (ARP). The effects of key parameters of FA concentration, dissolved organic matter, and inorganic anions were studied. A 97 % degradation and 94 % dechlorination of dicamba were obtained through the combination of UV and FA (UV-FA) at a dicamba concentration of 0.023 mM and FA concentration of 0.123 M. With respect to the dechlorination, at a dicamba concentration of 0.23 mM, FA concentration of 0.123 M, and pH of 2, chloride concentration of 12.4 mg/L and 5.2 mg/L was obtained for ARP (UV-FA) and sole UV in acidic condition, respectively. Scavenging test using Methyl viologen (MV^2 +) as a scavenger for reductive radicals including carboxyl anion radicals (CO₂^•¯) led to a decrease in the chloride concentration to 1.7 mg/L, revealing the importance of this radical in the dechlorination of dicamba. Inorganic anions (CO₃²¯ and SO₄²¯) had a slightly positive effect on the degradation of dicamba and led to an increase in degradation to 99 %, while they had a negative effect on the dechlorination by 7 % and 30 %, respectively. Due to the turbidity induced by dissolved organic matters (DOM), a moderate decrease in degradation by 39 % and dechlorination by 30 % was observed. The existence of five intermediates identified by GC-MS technique confirmed the proposed mechanism of dicamba degradation via ARP. Reductive degradation of dicamba mainly consists of processes based on CO₂^•¯, including single electron transfer process and radical-nucleophilic aromatic substitution (SRN) reactions, demonstrating the capability of this ARP for the effective degradation of dicamba.