The effect of biochar, hydrochar particles and dissolved organic matter on the photodegradation of metribuzin herbicide in aquatic media

Biochar and hydrochar particles and/or dissolved organic matter (DOM) may participate in several decomposition and transport processes of organic pollutants, thus their effect on aqueous photolysis of metribuzin (MBZ), selected as a potential pesticide pollutant of water systems, has been studied. Chars have been characterized by several techniques in order to correlate physicochemical characteristics with observed photodegradation rates. Biochar presented a micro-mesoporous graphitic structure with high surface area, and lower amounts of oxygen functional groups while hydrochar presented coral like structure, low surface area, incomplete carbonization and higher amounts of oxygen functional groups. MBZ photolysis was enhanced in the presence of low concentration levels (50 mgL(-1)) of char particles and DOM while an inhibitory effect was observed for higher concentrations (100-400 mgL(-1)). DOM from hydrochar present a stronger optical filter effect than biochar DOM while the biochar matrix and DOM present the higher ability to generate hydroxyl radicals (.OH) and singlet oxygen (O-1(2)), respectively. The participation of different ROS in photodegradation pathways for each matrix was studied also by scavenging experiments. The overall effect on photodegradation rate was the result of counterplay between the decrease of direct photolysis due to the light attenuation and the occurrence of indirect photolysis reaction paths via the generated ROS depending on the role of char matrix and DOM to act as source or sink for the ROS. The results demonstrate the significant and dissimilar role of char matrices and dissolved organic matter on the photo degradation rate of organic pollutants in aquatic matrices.

Automated summary

Biochar and hydrochar particles and/or dissolved organic matter (DOM) can enhance photolysis of organic pollutants at low concentrations, but inhibit the process at higher concentrations. Different types of char matrices and DOM play significant and distinct roles in the photodegradation rate of organic pollutants in aquatic systems.