Aggregation Of Low-Concentration Rhamnolipid Biosurfactant And The Solubilization Of Dodecane In Porous Medium

Subsurface contamination by hydrophobic organic compounds(HOCs), such as solvents or gasoline products, represents a serious environmental problem. When released in subsurface, these HOCs may form a separate phase commonly labeled Non-Aqueous Phase Liquid(NAPL), which may lead to long-term threaten for human’s health. Rhamnolipid, as a amphipathic compound, have received much attention in remediating HOCs pollution in bioremediation field. One of the most outstanding characteristic of biosurfactant in this process is promoting the ehhcement of HOCs’ solubity, which is related to the aggregation formation of biosurfactant in solution. What’s more, the high cost of biosurfactant have limited the wide application of biosurfactant in bioremediation field. Therefore, we investigated the self-aggragation behaviour of dirhamnolipid at concentration lower than surface-tension-based critical micelle concentration(CMCst). Furthermore, we also studied the effect of the flushing of low concentration rhamnolipid on dodecane phase existed in porous medium by one-dimension column experiment.The dirhamnolipid critical micelle concentration(CMC) values are 62, 78, 82, 83 and 82 μM for pH of 6.0, 6.5, 7.0, 7.5 and 8.0, respectively. Dynamic light scattering(DLS) and cryo-transmission electron microscopy(cryo-TEM) tests demonstrated aggregate formation for dirhamnolipid biosurfactant(diRL) at concentrations lower than surface-tension-based critical micelle concentration(CMCst). An increase of diRL concentration and solution pH results in a decrease of the aggregate size at diRL concentrations below CMCst, whereas it has no influence at diRL concentrations above CMCst. The cryo-TEM micrographs show spherical morphology of the aggregates, and the logarithm of aggregate size follows Gaussian distribution. The aggregates are negatively charged. The zeta potential of the aggregates decreases with an increase of diRL concentration to CMCst, and stabilizes at diRL concentrations higher than CMCst. An increase of the solution pH causes a decrease of the zeta potential. A transitional state assumption is raised for the interpretation of the diRL aggregation behavior. The results demonstrate formation of aggregates at signifi- cantly low diRL concentrations, which is of importance for the costeffective application of rhamnolipid biosurfactants.The result obtained from column experioment showed that the flushing of rhamnolipid solution facilitated the removal of dodecane in saturated porous medium. What’s more, flushing the column with 500 μM rhamnolipid achieved excellent effect, which resulted in the concentration of dodecane in effluent reaching 150 μM. Based on the analysis for aggragate size, aggregate surface zeta potential and the breakthrough curve of rhamnolipid,we concluded that enhencenent for dodecane removal caused by the flushing of rhamnolipid is based on the formation of rhamnolipid aggragate.