The Influence Of Cations On The Distribution Of Triclosan In Sediment-water-rhamnolipid System

As a broad spectrum antibacterial agent,triclosan（TCS）is one of the emerging environmental pollutants which belong to PPCPs（phamaceuticals and personal care products）.Due to its high hydrophobicity,TCS tend to be absorbed onto particulate materials.On one hand,the biodegradation of TCS on sediment is restricted because of the anaerobic condition,while TCS in water can be removed/transformed by aerobic biodegradation.On the other hand,TCS in water can be transformed into potentially more toxic and persistent compounds like chlorinated dibenzodioxins after photooxidation.TCS is expected to be bound to sediment by this time.So,the directed migration is an important foundation for its stabilization/biodegradation.As one of the most representative biosurfactant,rhamnolipid（RL）could regulate the distribution of hydrophobic organic compounds（HOCs）in sediment/soil-water system is focused on two mechanisms:（i）the micellar solubilization which results in the facilitated transport of HOCs to water;（ii）the adsorption of biosurfactant onto sediment/soil which leads to HOCs partitioning onto sediment.Cations,especially Na⁺/K⁺/Ca²⁺/Mg²⁺,are ubiquitous in surface water.In order to illustrate the effect and mechanism of cation type and cationic strength on the distribution of TCS in sediment-water-RL systems,the self-assembly of RL and the solubility of TCS in RL solutions were tested to explore the interaction among cations,TCS and RL.The influence of cations on the self-assembly of RL was monitored in terms of CMC,micellar size and zeta potential of RL solutions.The experimental data provided meaningful information for the effectively controlled migration of TCS by RL based on different ambient conditions and provided preliminary guide to the subsequent RL-enhanced in situ and ex situ remediation of TCS in sediment-water system.Cations,especially Ca²⁺/Mg²⁺,reduced the critical micelle concentration,micellar size and zeta potential of RL solutions.With the increase of ionic strength,the micellar size of RL first increased and then decreased in Na⁺ solutions and kept decreasing in Ca²⁺ solutions,and the zeta potential of RL solutions decreased in both Na⁺/Ca²⁺ solutions,indicating that the stability of RL solutions decreased.The molar solubilization ratio（MSR）of TCS in RL solutions increased with the addition of Na⁺/K⁺ whereas decreased with Ca²⁺/Mg²⁺.With the increase of ionic strength,the solubility of TCS in RL solutions first increased and then decreased in Na⁺ solutions.The highest solubility of TCS was detected with 30 mM Na⁺ and decreased sharply on the cationic strength over 50 mM.The solubility of TCS kept decreasing with the increase of cationic strength of Ca²⁺,especially when the cationic strength was over 1mM.With the increasing of RL concentration,TCS was slightly solubilized from sediment to water by RL in the presence of Na⁺/K⁺.While in the presence of Ca²⁺/Mg²⁺,with the increase of RL concentration,and the distribution of TCS in sediment was enhanced.With the increase of cationic strength of Na⁺,the distribution of TCS in sediment first decreased and then increased.In the presence of Ca²⁺ over 1mM,the distribution of TCS in sediment kept increasing with the increase of cationic strength.There was little variation between the residual RL concentration and the initial RL concentration in solutions of Na⁺ below 50 mM.The residual RL concentration decreased sharply in solutions of Ca²⁺ over 1mM.According to the experimental data,the intensity of the salt bridge between cations and RL monomers influenced the self-assembly of RL micelles,and caused the different residual RL concentrations in water.This in turn affected the water solubility of TCS and led to the different distuibution of TCS in sediment-water-RL system.