The ability of biochars to bind various organic compounds has been widely studied due to the potential effects on pesticide fate in soil and interest in the adoption of biochar as a “low-cost” filter material.  However, the sorptive behaviors of biochars are extremely variable and much of the reported data is limited to specific biochar-chemical interactions. The lack of knowledge regarding biochar sorption mechanisms limits our current ability to predict and optimize biochar’s use. This work unveils mechanistic drivers of organic pesticide sorption on biochars through targeted alteration of biochar surface chemistry. Changes in the quantity and type of functional groups on biochars and other black carbon materials were achieved through treatments with HCl, H2SO4, H3PO4, HNO3, H2O2, and CO2, and characterized using Fourier transform infrared spectroscopy and scanning electron microscope (SEM/EDX). The sorption capacities of these treated biochars were subsequently measured to evaluate the effects of different surface moieties on the binding of a target herbicide {cyhalofop; (R)-2-[4-(4-cyano-2-fluorophenoxy)phenoxy]propionic acid}. Results offer insight into the mechanisms of sorption and provide the basis to simultaneously evaluate the overall effectiveness of surface activation treatments.