Carboxylic ligands, which mainly originate from root exudates and plant debris, are ubiquitously distributed in the environment. Although the treatment of polychlorinated compounds such as trichloroethylene (TCE) with zerovalent iron (ZVI) has been studied well, the mechanisms of the combination of organic ligands and ZVI relevant to the removal of chlorinated compounds is not clear. The degradation of TCE by ZVI in the presence of carboxylic ligands (formic acid (FA), oxalic acid (OA), and citric acid (CA)) was investigated. The pH, reduction potentials, and morphology of ZVIs during the reactions were studied. The selected organic ligands enhanced the dechlorination rates in the order of 10 mM CA < 15 mM OA  CA > OA. And among these three ligands, no formation of Fe(II)-FA complex suggests that FA can provide proton ions to promote the dissolution of passive layers on ZVI. With the strong complexing ability, OA and CA could complex with ferrous or ferric ions originally deposited on ZVI surface, thus maintain the exposure of active sites on ZVI surface. And these complexes could also provide their possible ability to remove TCE. The removal efficiencies of TCE with 50 mM Fe(II) in 18 hr were 50 and 30 % in the presence of OA and CA, respectively. The results indicated that strong complexion of OA and CA inhibited the formation of passive layers on ZVI surface, and subsequently the reduction of TCE by Fe(II) complexes with organic ligands. These better understandings facilitate the design and prediction of treatment of polychlorinated compounds with ZVIs.

Carboxylic ligand-enhanced degradation of polychlorinated compounds with zerovalent iron

248th National Meeting (2014)

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