Insight into the effects of rhamnolipid biosurfactant on biochar colloid-mediated transport of dinotefuran in saturated soil porous media

As an effective soil amendment, biochar can reduce the adverse effects and environmental impacts of neonicotinoid pesticides. Rhamnolipid, an active and common biosurfactant in soil systems, may alter the co-transport behaviors of biochar colloids and pesticides. Nevertheless, biosurfactant-mediated changes in pesticide mobility in soils affected by biochar colloids are currently lacking. Herein, the single or combined effects of biochar colloids and rhamnolipid on the transport properties of dinotefuran (DTF, a typical neonicotinoid pesticide) in soil matrix were investigated. When the influents contained biochar colloids, the DTF transport was reduced, primarily via colloid-associated DTF deposition and pesticide bound to the deposited colloids on porous media surfaces. Meanwhile, rhamnolipid also restrained DTF mobility because of its bridging effect (i.e., this biosurfactant mediated DTF-soil binding through molecular bridging). However, rhamnolipid notably diminished the suppressed influences of biochar colloids on DTF transport because of the decrease of colloid-associated DTF retention and the high mobility of free DTF. Moreover, the degree of rhamnolipid's weakened effects was related to the solution pH values (i.e., the influences declined as the pH values increased). Because rhamnolipid exhibited greater inhibition effects on the binding abilities of colloids to pesticide molecules under lower pH conditions (pH 5.0), leading to more free DTF mobility. The results underscore the critical roles of ubiquitous biosurfactants in the fate and risks of pesticides when biochar is used as an amendment for contaminated soil.