Developing Slow-Release Reductant for Treating Herbicide-Contaminated Groundwater

Herbicide use in agriculture is important to increase crop yield; however, it has caused groundwater contamination in some countries. Our objective was to develop a slow-release reductant to treat herbicide-contaminated groundwater. The tested herbicides were alachlor (ALC), metolachlor (MTC), 2,4-dichlorophenoxyacetic acid (2,4-D), and atrazine (ATZ). Among these, ALC degraded the best, reaching a degradation plateau with a small level of Fe-0 (2.5%, w/v). Under aerobic conditions, both reductive reactions for electron transfer and oxidative reactions for generating active radicals are complex reactions responsible for herbicide degradation. By using slow-release Fe-0 (SRZ), we found that the degradation was prolonged and the plateau disappeared, indicating the greatest slow-release benefit. Fe-0 and SRZ removed ALC at 15% and 28% faster rates, respectively, under anaerobic conditions. At pH 3, the oxide layers on Fe-0 were removed facilitating better electron transfer for ALC degradation. Several reductants were tested in this study, but we mainly used Fe-0 as the main slow-release ingredient that was prepared by mixing Fe-0 with heated paraffin in a 4.7:1 ratio (w/w) and molding it into a 0.6 cm null x 2.5 cm length cylindrical shape. We stacked slow-release Fe-0 in a 2D ALC-saturated sand tank (70 x 30 x 3 cm), spatially collected samples along with the tank sampling array, and quantified the changes in ALC concentration. The results showed that the ALC concentration decreased 20-35% faster compared to DI flushing. These results supported the use of SRZ as an alternative approach for treating herbicide-contaminated groundwater.

Automated summary

Herbicide use is crucial for crop yield increase, but it has led to groundwater contamination in some countries. This study aimed to develop a slow-release reductant for treating herbicide-contaminated groundwater. We found that slow-release Fe-0 extended degradation time and improved herbicide removal, especially for the herbicide alachlor.