Unravelling 2,4-D -- biochar interactions by molecular dynamics: adsorption modes and surface functionalities

We report a molecular dynamics investigation of 2,4-dichlorophenoxyacetic acid (2,4-D) adsorption at the aqueous-biochar interface using experimentally constrained woody biochar models representative of softwood-derived biochars produced at 400, 600 and 800 ${\deg}$C. The models reproduce experimental descriptors (H/C, O/C, aromaticity, true density, and surface functionality) of their experimental counterparts, and simulations enable calculation of adsorption isotherms that align with available experimental measurements. Our results reveal that 2,4-D$^{-}$ uptake is governed by a synergy of three interaction classes: (i) ${\pi}$-${\pi}$ and ${\pi}$-Cl contacts with graphitic domains with either parallel or perpendicular alignments, (ii) polar interactions including H-bonding to surface -OH and other oxygen-containing groups, and (iii) Na$^{+}$-mediated cation bridging that links 2,4-D$^{-}$ anion to surface oxygens, that would have an increasing relevance for biochars near or above the pH at point of zero charge. Notably, we found that low-temperature produced biochars, which retain higher densities of surface O functionalities, exhibit higher adsorption per unit surface area due to cooperative polar interactions alongside ${\pi}$-${\pi}$ binding, whereas medium-to-high temperature biochars rely more on ${\pi}$-${\pi}$ and cation-bridging mechanisms. The distinct adsorption distances measured emphasize surface heterogeneity and porosity. Taken together, these atomistic insights corroborate experimental observations and yield actionable guidance for the rational design of biochars for remediation of anionic herbicides, highlighting how surface functionality and solution chemistry can be tuned to optimize sorption. Our approach provides a general framework to interrogate pollutant-biochar interactions and to inform remediation strategies.