Study On Remediation Of Heavy Metal Conta Minated Riverine Sediment By Biosurfactant Washing

Heavy metal conta minated sediments posed a serious threat to both human being and environment.Recently,the remediation of heavy metal conta minated sediment has become a prevailing topic.The sediment washing process with surfactant could remove heavy metal in sediment effectively.As an environmental friendly surfactant,biosurfactants have some advantages for sediment remediation over synthetic surfactant.In this study,a biosurfactant,rhamnolipid,was employed as a washing reagent to remove heavy metals in river sediment.In batch experiments,the effects of rhamnolipid concentration,p H,contact time,L/S(the ratio of washing solution volume to solid mass),washing times on heavy metals removal were investigated.Continuous flow column tests were used to simulate a full-scale flow-through remediation technique.Moreover,the heavy metals toxicity leaching characteristic and speciation in sediment were also analyzed before and after washing test.The increase of rhamnolipid concentration is in favor of heavy metals removal from sediment.The maximum removal rate was 21.86% of Cu,18.90% of Cd,2.54% of Pb and 0.66% of Cr when rhamnolipid concentration is of 3.0%.As rhamnolipid concentration increasing,heavy metals toxicity leaching characteristic were decreased with TCLP leaching rate decreasing by 71.11%,5.51%,77.52%,and 58.70% for Cu,Cd,Pb and Cr,respectively.The amount of heavy metals in carbanate-bound and Fe-Mn oxide-bound was decreased.In the washing test of the rest,0.8% rhamnolipid solution was used.The change of p H had different effect on washing removal rate for different heavy metals.Removal rate of Cu was improved quickly as p H value increasing,while that of Cd was declined continuously,and Pb and Cr stayed stably.Therefore,basic condition could help remove Cu from sediment by rhamnolipid while acidic condition was fovor for Cd removal.The change of p H has little impact on heavy metals toxicity leaching characteristic but could transfer heavy metals in stable fractions to mobile fractions,since high p H was contributed to rhamnolipid molecule ionization.As contact time went on,heavy metals removal rates were totally on the rise,toxicity leaching characteristic on the decrease and speciation of Cu and Pb had remarkable change with the amount of carbanate bound metal drop off while Cd speciation had handly any change.However,after 120 min's contact,the complexation between rhamnolipid and heavy metals tended to equilibrium,which resulted to unobvious increase of removal rate.In this study,the effect of L/S on heavy metal removal efficiency was also investigated.Keeping the same sediment mass,Cu,Cd,Pb and Cr removal rates were gradually increasing as the L/S enlarged with the maximum of 14.63%,19.84%,2.08% and 0.84%,respectively.After washing sediment for many times,single removal rate of Cu and Cd was on the decline whilie that of Pb and Cr on the rise.The accumulative removal rates of Cu,Cd,Pb and Cr were separately 48.21%,55.21%,7.89% and 12.20%.After washing 8 times,Cu,Pb and Cr toxicity leaching characteristic were highly declined.Continuous flow column tests found that rhamnolipid biosurfactant could greatly remove heavy metals from conta minated sediment.After contimuous washing with 100 h by 0.225% rhamnolipid solution,the accumulative removal rate was 46.45% for Cu,9.37% for Cd,3.16% for Pb and 15.19% for Cr.Heavy metals toxicity leaching characteristic had no significant difference everywhere in sediment column.Compared with before,TCLP leaching rate of Cu,Cd,Pb and Cr was respectively decreased by 60.14%,30.51%,45.77% and 18.97% on average.In every layer of the sediment column,all heavy metals speciation changed with carbonate-bound and Fe-Mn oxide-bound changing more.