Removal of Heavy Metals and PAH from Contaminated Soil by Chemical Enhanced Soil Flushing/Washing and Advanced Oxidation Processes

As a consequence of the increasing concern about soil contamination, research on soil remediation technology has grown rapidly. The effects of the chelant dosage, solution pH, soil-to-solution ratio and the dissolved/soil organic matter on metal extraction with the application of ethylene-diamine-tetraacetic acid (EDTA) and [S,S]-stereoisomer of ethylenediaminedisuccinic acid (EDDS) mixture (including individual additions) were investigated through batch and column studies. In addition, soil contaminated with both heavy metals and organic pollutants are commonly found and often require extra effort for remediation. The effectiveness and mechanisms of heavy metal and naphthalene co-removal from artificially contaminated soil by FeEDTA/FeEDDS-activated persulfate were investigated, again through batch studies.;Compared with Cu extraction, Pb and Zn extraction exhibit different kinetic behaviors at varying EDDS-to-metal molar ratios (under EDDS deficiency conditions) within two days. The extracted Pb and Zn were readily re-adsorbed onto the soil surfaces over time until reaching an apparent equilibrium between extraction and re-adsorption. In contrast, no readsorption occurred for Cu due to its high preference for forming complexes with EDDS. An alkaline pH range was preferable for Pb extraction when the EDDS was deficient, whereas the influence of varying the soil-to-solution ratio under EDDS deficiency was marginal compared with that of the EDDS-to-metal molar ratio and solution pH.;Higher Pb extraction by EDDS and EDTA mixture was found compared with individual additions of EDDS or EDTA under chelant deficiency conditions. It appears that the EDDS and EDTA mixture can be utilized for metal extraction in a more efficient way according to their respective affinity towards particular metals, in which Pb is primarily complexed with EDTA, while Cu and Zn bind with both EDTA and EDDS. The synergistic performance of the EDDS and EDTA mixture probably resulted from the change of chemical speciation and thus provided less competition among the Cu, Zn and Pb for the chelants.;With an excess amount of EDDS, a higher efficiency of heavy metal extraction was achieved compared to that with a deficiency of EDDS. No re-adsorption and competition among the heavy metals for EDDS was observed when a sufficient amount of EDDS was provided. In addition, the extraction of Cu, Zn and Pb by EDDS was enhanced in the presence of dissolved organic matter. The enhancement was probably due to the formation of metal-humate complexes and the soil disruption due to the humic acid enhanced Al and Fe dissolution, which induced more metals to dissolve from the soil. However, re-adsorption of the metal-humate complexes onto the soil surfaces occurred before the soil was saturated with organic matter, resulting in minor enhancement of the metal extraction.;EDDS and EDTA were found to be effective in extracting heavy metals, primarily from the exchangeable and carbonate fractions of soil (i.e., weakly sorbed fractions). To enhance the metal extraction, removal of heavy metals from the strongly sorbed fraction is necessary. Na4P 2O7 was employed after the application of EDTA and EDDS which significantly enhanced the removal of heavy metals. It promoted mineral dissolution, thereby enhancing the metal extraction as a result of soil disruption. In addition, the order of metal extraction by Na4P2O 7 was found to be Ni > Cr > Cu, probably due to the different affinities between the metals and P2O74--.;In most cases, heavy metals and polycyclic aromatic hydrocarbons in soil are removed with different individual treatment processes. Using FeEDTA/FeEDDS-activated persulfate, both types of contaminants were successfully removed from soil in one single treatment process. Heavy metals were removed due to (1) the formation of metal-chelant complexes with free EDDS/EDTA which dissociated from FeEDDS/FeEDTA; (2) metal leaching under acidic environments; (3) metal dissolution caused by soil disruption; and (4) metal exchange of sorbed metals on the soil surfaces with FeEDTA/FeEDDS. FeEDTA-activated persulfate resulted in higher naphthalene removal from the soil, compared with FeEDDS-activated persulfate. The removal was mainly via the dissolution of the naphthalene partitioned on mineral surfaces, followed by activated persulfate oxidation. Although EDDS is advantageous over EDTA in terms of biodegradability, it is not recommended for iron chelate-activated persulfate oxidation since persulfate is consumed to oxidize EDDS, resulting in a persulfate inadequacy for naphthalene oxidation.