Impact of particle size and field aging on biochar sorption capacity for the herbicides terbuthylazine, fluometuron, and 2-methyl-4-chlorophenoxyacetic acid

The use of biochar as a soil amendment is considered an effective agricultural practice for retaining pesticides, thereby preventing their dissipation into the environment. The particle size of the biochar and the changes it undergoes in soil over time are key factors in its sorption capacity. In this study, the sorption of three herbicides of different nature, terbuthylazine (TBA), fluometuron (FM) and 2-methyl-4-chlorophenoxyacetic acid (MCPA), was evaluated on a biochar with two particle sizes: 0.063-1 mm and 1-2 mm. Additionally, the effect of field aging on the sorption capacity of the biochar at both particle sizes for these herbicides was also evaluated. The results show that the sorption of the herbicides on fresh biochar was similar regardless of particle sizes. However, after aging, herbicide sorption depended on particle size, with lower sorption observed on biochar with smaller particles ( q e = 22.4 and 30.3 mg/kg for TBA and FM, respectively, on 30-month-aged biochar) than on biochar with larger particles ( q e = 41.6, 74.4 and 14.5 mg/kg for TBA, FM and MCPA, respectively, on 30-month-aged biochar). The decrease in herbicide sorption was attributed to changes in aromaticity and polarity of biochar, and to the physical blockage of its pores by soil components. Amending a soil with the biochar of smaller particle size led to an increase in herbicide sorption. However, herbicide sorption on the soil amended with aged biochar of the same smaller particle size was considerably reduced, likely due to a decrease in the C content as a result of aging.