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Glyphosate sorption/desorption on biochars – interactions of physical and chemical processes

Hall, Kathleen E, Spokas, Kurt A, Gamiz, Beatriz, Cox, Lucia, Papiernik, Sharon K, Koskinen, William C
Pest management science 2018 v.74 no.5 pp. 1206-1212
biochar, biomass, calcium chloride, copper, desorption, dipotassium hydrogen phosphate, feedstocks, glyphosate, hardwood, iron, micropores, phosphorus fertilizers, pyrolysis, risk, temperature, water pollution
BACKGROUND: Biochar, a carbon‐rich product of biomass pyrolysis, could limit glyphosate transport in soil and remediate contaminated water. The present study investigates the sorption/desorption behavior of glyphosate on biochars prepared from different hardwoods at temperatures ranging from 350 to 900 °C to elucidate fundamental mechanisms. RESULTS: Glyphosate (1 mg L⁻¹) sorption on biochars increased with pyrolysis temperature and was highest on 900 °C biochars; however, total sorption was low on a mass basis (<0.1 mg g⁻¹). Sorption varied across feedstock materials, and isotherms indicated concentration dependence. Biochars with a greater fraction of micropores exhibited lower sorption capacities, and specific surface groups were also found to be influential. Prepyrolysis treatments with iron and copper, which complex glyphosate in soils, did not alter biochar sorption capacities. Glyphosate did not desorb from biochar with CaCl₂ solution; however, up to 86% of the bound glyphosate was released with a K₂HPO₄ solution. CONCLUSION: Results from this study suggest a combined impact of surface chemistry and physical constraints on glyphosate sorption/desorption on biochar. Based on the observed phosphate‐induced desorption of glyphosate, the addition of P‐fertilizer to biochar‐amended soils can remobilize the herbicide and damage non‐target plants; therefore, improved understanding of this risk is necessary. © 2017 Society of Chemical Industry