Insight into the inhibitory role of rhamnolipid in the adsorption of neonicotinoid pesticides onto biochars

Biochar, an economical porous carbonaceous material, exhibits considerable potential for diverse applications, particularly in soil remediation and wastewater treatment. Rhamnolipid—a ubiquitous biosurfactant in natural environments—may significantly influence the interaction between biochar and neonicotinoid pesticides. To evaluate this, the adsorption of two representative neonicotinoid pesticides (nitenpyram and dinotefuran) on biochars produced at 300 °C and 750 °C (BC_300 and BC_750) was investigated in the presence of rhamnolipid. The adsorption kinetics were well described by the pseudo-second-order model ( R 2  = 0.994–0.999), and the isotherms by the Langmuir model ( R 2  = 0.951–0.997). In the absence of biosurfactants, BC_750 exhibited substantially higher adsorption capacity for both neonicotinoid pesticides compared to BC_300. Furthermore, nitenpyram demonstrated greater adsorption affinity to the biochars than dinotefuran. These differences are primarily attributed to variations in the physicochemical properties of the biochars, including surface area, aromaticity, and surface functional groups, as well as the molecular structures of the pesticides, particularly the presence of nitrogen-containing heterocyclic moieties. More importantly, rhamnolipid suppressed the adsorption of neonicotinoid pesticides onto both biochars, primarily through pore blockage, competitive adsorption, and steric hindrance. For instance, the maximum adsorption capacity ( q max ) of nitenpyram onto BC_300 declined from 7.3 mg/g (without rhamnolipid) to 2.8 mg/g (with 30 mg/L rhamnolipid). Additionally, for a given biochar, the inhibitory effect of rhamnolipid on nitenpyram adsorption was more sensitive to variations in biosurfactant concentration compared to dinotefuran. This enhanced inhibition stemmed from the disruption of π–π interactions between nitenpyram and the biochar. Additionally, the inhibitory effect of rhamnolipid on pesticide adsorption decreased progressively from pH 3.0 to 10.0 due to the pH-dependent adsorption behavior of the biosurfactant on the biochars. These findings highlight the necessity of considering the influence of coexisting biosurfactants when employing biochar as an amendment in soil/water systems.