Washing Single Or Combined Polycyclic Aromatic Hydrocarbons/Heavy Metals Contaminated Loess By Surfactants And Chelating Agents

In recent years, with the rapid development of industrial and agricultural production, thepollution of soil is increasingly serious, and the humans health are threatened greatly. It isurgent to explore and find safe and effective soil pollution remediation technology. Chemicalremediation technologies have been paid widespread attention from governments and scholarsbecause of its advantages of short cycle, easy operation and wide range.It is well known that polycyclic aromatic hydrocarbons (PAHs) are oftenco-contaminants with heavy metals in soils. Removal of PAHs and heavy metals are verydifficult because of their low bioavailability and potential toxicity. It is effective to removePAHs and heavy metals by surfactants and chelating agents since surfacta nts can enhance theclean up of soils contaminated with PAHs by solubilization and chelating agents can desorbheavy metals from soil solid phases by forming strong water-soluble coordinationcompounds.Based on review of the principles, the current research results and the main problems ofsoil washing, the batch equilibrium washing of single or combined phenanthrene and pyrenecontaminated loess by four surfactants, ethoxylated anhydrosorbitol esters (TW80),octylphenoxypolyethoxyethanol (TX100), sodium dodecyl sulfate (SDS) and sodiumdodecylbenzyl sulfonate (SDBS), single or combined Cd(II) and Pb(II) contaminated loess byfour chelating agents, citric acid (CA), disodium ethylenediamine tetraacerare (EDTA-2Na),ammonium thiocyanate (NH4SCN) and nitrilotriacetic acid (NTA), phenanthrene-Cd(II) andphenanthrene-pyrene-Cd(II)-Pb(II) contaminated loess by TW80and EDTA-2Na are studiedand compared. The influences of washing time, temperature, the ratio of soil to water, agentconcentrations, contaminating order of PAHs and heavy metals, adding order of agents,inorganic salts and pH value on the removal efficiencies of contamiants are investigated. Thepaper attempts to provide some technical reference for soil washing remediation. The majorfindings are observed as following:⑴For single or combined phenanthrene and pyrene contaminated loess, the results areas following. When the washing time of phenanthrene reaches4h and pyrene reaches1h thewashing efficiency achieves the maximum. The dates give a good fit by employing thepseudo-second order models. The smaller the ratios of soil to water are, the higher thewashing efficiencies will be. It is shown that the washing efficiency of phenanthrene andpyrene increased sharply with the increase of surfactant concentrations, and the removal rateof phenanthrene is higher. For single contaminated soils, the maximum washing efficiency ofphenanthrene can be found following the order of TW80＞TX100＞SDS＞SDBS. Given thesurfactant concentrations, the washing efficiency of1000mg/kg of phenanthrene is lower than that of100mg/kg. For single pyrene, when surfactant concentrations are lower than5000mg/L, the efficiency by TW80and TX100are higher than that by SDS and SDBS, but whensurfactant concentrations are higher than5000mg/L, the result is opposite. For Mixedcontaminated soil, the washing efficiency of phenanthrene was obviously much higher, butfor pyrene, it is relatively lower.⑵For single or combined Cd(II) and Pb(II) contaminated loess, the results are asfollowing. There is little effect of washing time on removal of Cd(II)and Pb(II). At relativelylow concentration, CA, EDTA-2Na, and NTA have well washing performance of Cd(II) andPb(II), and NH4SCN has well washing performance of Cd(II). However, among all chelatingagents, EDTA-2Na has relatively stable effect. At beginning, there is little difference betweenthe washing effiency of Cd(II) and Pb(II). But with increasing of chelant concentrations,removal rate of Cd(II) gradually decreases, and removal rate of Pb(II) unchanges or graduallyincreases. Chelating agents have effects on Cd(II) and Pb(II) in a wide pH range, but theeffects are different. As pH value increasing, the effect of NTA is enhanced, and the effect ofCA and EDTA-2Na is weakened. There are greater changes for Cd(II). The pH value ofsolutions after reaction are larger than the ones before reaction.⑶For phenanthrene-Cd(II) and phenanthrene-pyrene-Cd(II)-Pb(II) contaminated loess,the results are as follows. There is little effect of washing time on removal of Cd(II) andPb(II), but the efficiency of phenanthrene and pyrene increases with increasing of washingtime. The equilibrium washing time of phenanthrene in phenanthrene-Cd(II) contaminatedsoil is8h, and in phenanthrene-pyrene-Cd(II)-Pb(II) contaminated soil, the equilibrium timeof phenanthrene and pyrene are16h and8h, and the time is more longer. It is found thatpseudo-second order reaction model is more appropriate for washing of phenanthrene andpyrene by the TW80and EDTA-2Na. For phenanthrene and pyrene, higher temperature ishelpful to improve the washing efficiency, but have little effect on Cd(II) and Pb(II). Giventhe TW80(or EDTA-2Na) concentrations, increasing of EDTA-2Na (or TW80)concentrations can inhibit the effect of TW80(or EDTA-2Na) on phenanthrene and pyrene(or Cd(II) and Pb(II)), but the inhibition can be controlled when choose the suitableconcentrations. The washing efficiency indicates to be: for Cd(II) and Pb(II), PAHs and heavymetals contaminated at the same time> PAHs contaminated first> heavy metal contaminatedfirst, for phenanthrene and pyrene, the order was opposite. The results are best when both ofTW80and EDTA are added at the same time. For phenanthrene and pyrene, the addition ofthree inorganic salts have no regular or obvious influence, but show a little suppression on thewashing efficiency of Cd(II) and Pb(II). As pH value increasing, the washing efficiency ofCd(II) performes to decrease, but for Pb(II), the washing efficiency performes like a parabola, of which the maximum value is obtained when pH value is5.46. The pH value of solutionsafter reaction are larger than the ones before reaction.