Modelling and spectroscopic investigation of 2,4-D adsorption in soil amended with pine sawdust, paunch grass and sewage sludge biochars

Abstract

2,4-Dichlorophenoxyacetic acid (2,4-D) is a widely used herbicide possessing high herbicidal activity; however, it is potentially toxic to humans and quite persistent in the environment. We investigated the adsorption of 2,4-D in two pasture soils amended with pine sawdust (PSD), paunch grass (PG) and sewage sludge (SS) biochars using batch studies. The results showed that PSD biochar produced at 700 oC exhibited the highest adsorption capacity for 2,4-D among the waste-derived biochars tested, which is 200-fold greater than the control and other biochar-amended soils. In general, the sorption affinity of 2,4-D for biochar amended soils followed an order: PSD > > PG > SS. The high adsorption capacity of the PSD may be attributed to its significantly higher specific surface area of 795 m2·g-1 as compared to other soils and biochars. Moreover, the results of the physicochemical characterization showed no difference in surface oxygen functional groups among the PSD, PG and SS. These findings indicate that oxygen-containing surface functional groups have a negligible role in 2,4-D adsorption, as evident from the spectroscopic investigation, thus emphasizing the role of surface area in the 2,4-D adsorption. Lastly, pore intrusion/filling, hydrogen bonding and π-π EDA interactions are postulated to be the plausible adsorption mechanisms for 2,4-D onto biochar amended soils.