Solar irradiation combined with chlorine can detoxify herbicides

The solar/chlorine process is an energy-efficient advanced oxidation process that can produce reactive species such as hydroxyl radical, reactive chlorine species and ozone. This study investigated the process' ability to detoxify the typical herbicides atrazine and mecoprop (methylchlorophenoxypropionic acid). Both herbicides are resistant to direct solar photolysis or chlorination alone, but they can be degraded by the solar/chlorine process effectively. Atrazine inhibited the development of Arabidopsis thaliana, but such inhibition was negligible after solar/chlorine treatment of an atrazine solution. The transformation of atrazine in the process was shown to be through hydroxylation, hydrogen abstraction and dechlorination but did not involve chlorine substitution or addition. Cl^• reacts with atrazine and mecoprop with rate constants of 6.87 × 10⁹ M^-1s^-1 and 1.08 × 10¹⁰ M^-1s^-1, respectively, while ClO^• reacts with mecoprop with a rate constant of 1.11 × 10⁸ M^-1s^-1. The degradation kinetics of atrazine and mecoprop by solar/chlorine was simulated by modeling, which fitted the experimental results well. Hydroxyl radicals (HO^•) mainly contributed to the degradation of atrazine by solar/chlorine at pH 7 with the contribution of 65%, whereas ClO^• and O₃ were main species responsible for the degradation of mecoprop with the contribution of 72% and 17%, respectively. The pseudo-first-order rate constants (k's) of the two degradations increased substantially (by 28.8% for atrazine and by 198% for mecoprop) when the chlorine dosage was increased from 50 μM to 200 μM. The k's decreased with increasing pH. The presence of natural organic matter inhibited the degradation of both herbicides, while the presence of bromide enhanced their degradation. This work reveals a feasible method for the detoxifying herbicides by combining chlorine with solar radiation.