Removal Of Sulfonylurea Herbicides In Drinking Water Treatment Processes

Sulfonylurea herbicides (SUs) are used to control broadleaf weeds and some annual grasses. The widely application of these herbicides coupled with their resistance to photolysis, hydrolysis, and microbial degradation causes serious concern. SUs are highly mobile in the environment and tend to be leached to surface and ground waters. Their occurrence in rivers, streams, and ground waters has been reported, Since SUs are widely presented in natural water, a question raised that whether current water treatment processes can provide sufficient protection for people from exposure to these herbicides when the contaminated water were used as drinking water sources.The removal of 5 representative sulfonylurea herbicides (chlorsulfuron, metsulfuron-methyl, chlorimuron-ethyl, halosulfuron-methyl, triflusulfuron-methyl) in simulated water treatment processes including coagulation, activated carbon adsorption, chlorination, and ozonation disinfection was systematically investigated. Results show that coagulation/sedimentation had little effect on the removal of the herbicides, with the average removal rates less than 10%. Powder activated carbon adsorption was apparently more effective with removal rates of 50-70%. Nonetheless, this process could not completely eliminate the herbicides from water even the dosage of PAC amount to 20 mg·L-1. Sulfonylurea herbicides were nearly completely removed during chlorination process and the extent of removal varied among different species. Chlosufuron and metsulfuron-methyl were less reluctant. SUs were limitedly removed after contacting ozone.The removal of the 5 SUs was further explored in combinations of 2 or 3 treatment processes. Compared with the removal of chlorination process, a coagulation process preceding significantly improves the overall removal in synthetic water samples. However, fore the real surface water samples, the improvement was apparently less. Complete removal was only achieved when a PAC adsorption process was inserted between coagulation and chlorination.Although majority of the SUs was removed by chlorination, it was found that this process generated stable products which may related the heterocyclic moieties (triazine or pyrimidine) of sulfonylurea molecules (i.e. triazine). The generation of these products depends on the intial concentration of parent herbicides. These products did not have any further degradation as the increase amount of chlorine and reaction time. The toxicity of these triazine and pyrimidine products are not available. However, species with similar structural such as atrazine are well recognized as carcinogens and endocrine disruptors.In summary, SUs are possibly being completely removed in the traditional drinking water treatment processes involving chlorination. However, their chlorinated products are not likely being degraded completely. Thus, current drinking water treatment process can not provide enough protection for human population from exposure to sulfonylurea contamination which may be a ubiquitous problem. This research complements the studies on the fate of SUs in environment and provides data for comprehensive evaluation of the potential risks associated with SUs application.