Adsorption of Pesticides with Different Chemical Properties to a Wood Biochar Treated with Heat and Iron

Biochars are known to affect the environmental fate of pesticides when used as soil amendments and have been suggested to be useful as cheap adsorbents of organic contaminants. We studied the ability of a wood-based biochar produced by slow pyrolysis from a mixture of about 80 % hardwood (Betula sp.) and 20 % softwood (Picea abies) to adsorb pesticides in order to assess its potential use as a filter material to prevent point source pollution in agriculture. The pesticides bentazone, chlorpyrifos, diuron, glyphosate and (4-chloro-2-methylphenoxy)acetic acid (MCPA) were used as model compounds. Their adsorption and desorption to the biochar were tested before and after it had been subjected to treatments with heat and/or iron intended to enhance its adsorptive properties. The adsorption affinity of the native biochar, as indicated by the Freundlich K F value, varied greatly and decreased in the order diuron > chlorpyrifos > MCPA > bentazone > glyphosate. Activation with heat (t = 450 °C) increased the specific surface area and the wettability of the biochar, measured by a water drop penetration time assay, and increased the adsorption of bentazone and MCPA. Treatment with iron salts, which partially coated the biochar with an iron oxide identified as magnetite, decreased the specific surface area but increased the adsorption of glyphosate. Mixing biochar fractions subjected to different treatments was a successful approach for optimising the adsorption of all model compounds and could be a viable path for creating a versatile yet comparably cheap filter material.