pH-dependent transport of neonicotinoid pesticides in saturated soil: single and combined functions of rhamnolipid and biochar colloids

Abstract

Applying biochar as an amendment for the remediation of neonicotinoid pesticide-contaminated soils is a promising way to reduce the environmental risks posed by these pollutants. Rhamnolipid, a widespread glycolipid biosurfactant in soils, may regulate the transport of biochar colloids and/or pesticides in soil–water environments. Currently, critical knowledge gaps remain regarding how biosurfactant/biochar affects neonicotinoid pesticide mobility. Herein, rhamnolipid was employed to explore its influences on neonicotinoid pesticide (acetamiprid or nitenpyram) mobility and biochar colloid-affected mobility of pesticides at variable solution pH levels (5.0–9.0). In the binary system, rhamnolipid restrained pesticide transport owing to the biosurfactants' bridging effects, forming soil–biosurfactant–pesticide ternary complexes; similarly, biochar colloids also inhibited pesticide mobility because of colloid–pesticide complex deposited on soil surfaces. Notably, the degree of the inhibiting impacts of biochar/biosurfactant varied with pesticide types (acetamiprid > nitenpyram), which was attributed to differences in the chemical features of pesticides (e.g., hydrophobicity). Interestingly, the pH-dependent inhibition effects followed the order pH 5.0 > pH 7.0 > pH 9.0, which were ascribed to the different deposition amounts of biosurfactant molecules or biochar colloids. Surprisingly, in the ternary system, adding biosurfactant weakened the repressive influences of biochar colloids on pesticide migration over a broad pH range of 5.0 to 9.0 because of the reduced retention of colloid-associated pesticides and the great mobility of free neonicotinoid pesticides. Additionally, the degree of rhamnolipid's suppressive effects declined as the pH value increased. These findings provide critical insights into the environmental behaviors and fate of neonicotinoid pesticides influenced by ubiquitous biosurfactants in biochar-amended soils.