Remediation And Mechanism Study Of Chlorinated Organic Compounds Contaminated Soils By Fenton-like System

With accelerated development of urbanization and implementation of Stockholm Convention in China, a great number of contaminated sites are left over due to the relocation or closedown of chemical enterprises. The majority of these sites are contaminated by chlorinated organic compounds (COCs), posing tremendous risk to public health and environmental safety. Therefore, developing remediation technologies of high efficiency is great urgency. H2O2based Fenton and Fenton-like reactions have been applied successfully to the remediation of COCs contaminated soils. However, large consumption and instability of H2O2, result in high cost and storage-transport difficulties. Moreover, when encountering with minerals and organic matters in soil, H2O2could be decomposed easily, which limits the practical applications. Fortunately, zero valent iron (ZVI) can activate oxygen for air (ZVI/Air) to generate H2O2in-situ, and the addition of ligands to ZVI/Air could obviously promote the generation of reactive oxygen species (ROS). The O2based Fenton-like system consisting of ZVI, EDTA ligand and Air (ZEA), could efficiently remediate COCs contaminated soils, but also avoid the H2O2application problems, possessing wide application prospect.In the present study,2,4-dichlorophenol (2,4-DCP) and hexachlorobenzene (HCB) were employed as representative water-soluble and hydrophobic COCs, respectively. We systematically investigated the remediation of COCs contaminated soils with ZEA and modified ZEA systems. In general, the main contents and conclusions can be summarized as follows:(1)2,4-DCP in soil could be effectively decomposed by ZEA system under a broad range of pH. The oxidation ability of ZVI/Air system could be enhanced by the addition of EDTA, but restrained by EDDS, probably due to their different degradable properties and different complexing modes with ZVI. With2.5g ZVI and1.2mmol/L EDTA, the degradation rate of2,4-DCP achieved96%in45min and EDTA was simultaneously degraded by63%. The coaction of EDTA and ZVI influenced the degradation of2,4-DCP. When the molar concentration ratio of EDTA to dissolved iron was above1,2,4-DCP degradation was inhibited. Soil properties also affect the degradation process, and better degradations were observed with sandy soil and less amount of organic matters. Reaction in ZEA system was proved to be heterogeneous Fenton and the main ROS were determined as O2-·/HO2·and FeⅣ.(2)2,4-DCP contaminated soil could be remediated rapidly and effectively by MW-SiCZEA system. With1g SiC/nZVI and1.2mmol/L EDTA,2,4-DCP was removed by94%under540W MW irradiation for20min. By characterization, the SiC/nZVI catalyst, with loaded iron of138mg/kg, was observed as sphere nanoparticles with diameter of50-100run and was partially oxidized superficially. SiC/nZVI possesses favorable ability for O2activation. MW greatly enhanced the degradation efficiency via both thermal effect and non-thermal effect. pH and MW power were the dominant factors influencing the removal of2,4-DCP. With the performance of·OH and O2-·/HO2· as main ROS,2,4-DCP could be completely dechlorinated.(3) The combination of biosurfactant with ZEA could remediate HCB contaminated soil to great extent. Biosurfactant alkyl glycoside0814(APG0814) showed good solubilization effect on HCB with its molar solubilization ratio (MSR)7.7×10-4and desorption of HCB from soil. With APG0814concentration increased from1000to3000mg/L, HCB desprtion rate increased from20.6%to35%. HCB in desorption solution could be removed further by ZEA system, and HCB was removed by76%in2h. APG0814enhanced ZEA system was superior for the remediation of HCB contaminated soil. With100mg/L APG0814, the enhancement was observed better than other APG0814concentrations, and HCB was removed by68%in8h. APG0814enhanced HCB desorption through its hydrophobic group and adsorbed HCB to ZVI surface, and therefore accelerated HCB degradation by ROS generated on the ZVI surface.