Effect of Carbonaceous Soil Amendments on Potential Mobility of Weak Acid Herbicides in Soil

Abstract

Use of carbonaceous amendments in soil has been proposed to decrease potential off-site transport of weak acid herbicides and metabolites by increasing their sorption to soil. The effects of organic olive mill waste, biochars from different feed stocks, and humic acid bound to clay on sorption of 4-chloro-2-methylphenoxyacetic acid (MCPA), aminocyclopyrachlor, or indaziflam acid metabolite to soils with varying physical and chemical properties were determined. At natural agricultural pH soil levels, these chemicals are anionic and weakly sorbed to soils; sorption of the three weak acids on soil was in the order MCPA (K _f = 0.1) < aminocyclopyrachlor (K _f = 0.3) < indaziflam acid (K _f = 1.6). Results indicate that not all carbonaceous amendments, including biochars, increase sorption, thereby decreasing the potential for off-site transport. The amount and composition of the organic amendment, along with pH of the system, affect sorption and potential off-site transport. For instance, depending on the feedstock of the biochar and the method of production, changes in sorption ranged from no effect, to a decrease by a factor of 3.3×, to an increase of 383× upon addition of amendment to soil. In contrast, activated charcoal increased sorption by >10,000×. While generalizations have been made that soil sorption of nonpolar, neutral, or weakly basic chemicals increases by the addition of different carbonaceous amendments, no such generalizations can be made for weak acids. More work on properties of these amendments, biochars in particular, and how they affect weak acid sorption is required.