Effects of a wood-based biochar on the leaching of pesticides chlorpyrifos, diuron, glyphosate and MCPA

- The adsorption kinetics of four pesticides was controlled by their hydrophobicity.
- Biochar reduced the leaching of diuron and MCPA.
- Biochar did not reduce leaching of chlorpyrifos or glyphosate.
- When the pesticides had been pre-sorbed by the biochar leaching was low in all cases.
- Using an adsorptive biochar-layer on the soil surface may effectively reduce leaching.

We studied the ability of a wood-based biochar to reduce the leaching of the pesticides chlorpyrifos, diuron, glyphosate and MCPA in a sand column test system. In addition, time-dependent adsorption of the pesticides to the biochar and to the sand used in the columns was determined. The sorption kinetics was shown to be controlled by the log Kow-values of the pesticides and sorption rates varied in the order: chlorpyrifos (log Kow = 4.7) > diuron (log Kow = 2.87) > MCPA (log Kow = −0.8) > glyphosate (log Kow = −3.2). Glyphosate sorbed very weakly to the biochar but strongly to the sand. Biochar was most effective at retaining the pesticides if applied as a distinct layer rather than mixed with the sand. Leaching of diuron and MCPA was reduced by biochar application, and the retention was linearly related to the thickness of the biochar layers. However, leaching of chlorpyrifos and glyphosate was not affected by biochar addition. Leaching was low for all pesticides when the pesticides were added directly to biochar that was then added to the column. Together, our results suggest that a viable strategy for using biochar as a means to mitigate leaching of pesticides may be to use it as an adsorptive layer directly on or close to the soil surface. This would be especially useful in areas where pesticides are routinely handled and potentially spilled.