Hydrolysis and biodegradation of sulfonylurea herbicides in aqueous buffers and anaerobic water-sediment systems: Assessing fate pathways using molecular descriptors

Abstract

Abstract The degradation of 12 sulfonylurea herbicides in buffers at different pH levels and in anaerobic sediments was investigated in laboratory studies. Reaction products, which are not commercially available, were synthesized in order to follow degradation kinetics. Different hydrolysis processes were shown to occur under acidic, neutral, and alkaline conditions. At 40°C, pseudo-first-order reaction rate constants between pH 4 and 10 covered two orders of magnitude. Activation energies of the acid-catalyzed hydrolysis were about 100 kJ/mol. Reaction rate-pH profiles were different for all compounds. In native sediments at neutral pH, microbial degradation was prevalent. At lower pH, chemical hydrolysis became more important. Pseudo-first-order dissipation rate constants between 12.50 × 10−2 d−1 and 0.19 × 10−2 d−1 were determined. Quantum-chemical molecular descriptors were calculated with MOPAC, and steric, electronic, and geometrical descriptors with Molecular Modeling Pro. Quantitative structure-reactivity relationships were found between bond orders, atomic charges, lowest unoccupied molecular orbitals, and pKa values on one hand and rate constants on the other. Disappearance rates in native sediments could be assessed with a multiple correlation considering calculated octanol water partition coefficients, parachors, and the atomic charges at the main reactive site of the molecules.