Chemical-enhanced washing for remediation of heavy metal- and petroleum hydrocarbon-contaminated soils

EDTA-enhanced washing was effective for Pb and Zn removal from soils, and the washing with SDS could enhance marine diesel fuel (MDF) removal to a significant extent, both of which depended on the operating conditions and contaminant behaviors in soils. EDTA in washing effluent for Pb-contaminated soils could be recovered and reused by a two-step precipitation method without losing much of its chelating capacity, while there was a loss of 15% of its chelating capacity in the first cycle of reuse for Zn-contaminated soil washing due to the presence of Zn residual in the recovered solution. The recovered SDS via lowering the temperature of the solution could be reused, but its extraction capability was noticeably reduced after each cycle.; MDF in soils reduced Pb removal by EDTA, mainly due to physical isolation arising from the free phase of MDF adhering on the soil surface. Although SDS enhanced Pb removal to some extent in the absence or at low concentration of EDTA, it reduced Pb removal at high EDTA concentration, because of the formation of complexes between some strongly-bound Pb and SDS, which is harder to be removed. Therefore, EDTA washing followed by SDS washing achieved the highest Pb removal. MDF removal by SDS was significantly hindered by coexisting Pb, since the formation of Pb-DS complexes not only decreased the amount of SDS available in solution but also favored re-adsorption of the released MDF onto the hydrophobic tails of sorbed SDS on soil surface. EDTA alone or with SDS could enhance MDF removal, but the remaining MDF in the EDTA-washed soil became more resistant to SDS. Therefore, SDS washing followed by EDTA washing is considered as the optimal washing sequence for MDF removal. In view of the contrasting optimal sequences for respective removal of Pb and MDF, an optimal washing sequence for removing both Pb and MDF depends on their corresponding initial levels of contaminations and desired cleanup goals.