Surface Modification of Commercial Zero Valent Iron (ZVI) for Degradation of Chlorinated Ethenes

It has been shown in several recent studies that sulfidation enhances the reactivity of nanoscale zero-valent iron (nZVI) for dechlorination of chlorinated ethenes. The majority of these studies have been carried out using lab synthesized nZVI particles via borohydride reduction method. Sulfidation of commercially available zero valent iron (ZVI) has scarcely been studied. Given the widespread application of commercial ZVI products (e.g., ZVI granules, filings or powder) in remediation field installations, the use of sulfidation to enhance the reactivity of commercial ZVI in degrading chlorinated contaminants is of growing interest to remediation researchers and practitioners. To address this necessity, the effects of sulfidation on dechlorination performance of five different commercial ZVI particles were assessed using trichloroethene (TCE), tetrachloroethene (PCE), and cis-1,2-dicchloroethene (cis-DCE) as model contaminants. Alfa Aesar iron powder (spherical, <10 micron, 99.9+% (metals basis)), BASF Carbonyl Iron Powder (CIP) OM and CC grade, HEPURE Ferox-PRBTM zero valent cast iron powder, and Peerless zero valent cast iron were employed in this study. They were referred to as AA, B-OM, B-CC, HPR, and PLES, respectively. Sulfidation of ZVI was done either by soaking particles in sulfur precursor solutions (referred to as “direct sulfidation”), or by pre-washing the particles in diluted hydrochloric or acetic acid before mixing with sulfur precursor solutions (referred to as “sulfidation”). Sodium thiosulfate was used to prepare sulfur precursor solutions, and S/Fe molar ratio was fixed at 0.05 for all the batch experiments. As a control, as-received ZVI materials or those washed in acid solutions without further exposure to sulfur amendments were prepared as well. The chemically treated ZVI particles were reacted with model contaminants in batch reactors with headspace. The headspace of the batch reactors was sampled periodically to quantify parent compounds, reaction intermediates, and products. Results of the batch experiments indicated that sulfidation of commercial ZVI significantly enhanced TCE reduction rates compared to the respective as-received ZVI products (i.e., no acid washing and/or sulfidation treatments). The apparent mass normalized pseudo-first-order TCE degradation rate constants of sulfided AA, B-CC, HPR, and PLES ZVI particles were 1.77, 1.02, 0.682, and 6.29 x 10-5 L/g-min respectively, which were 53.2, 54.3, 4.6, and 118 folds higher respectively than that of the untreated particles. TCE reduction rate constant for B-OM was 2.39 10-5 L/g-min, which was 1.9 folds higher compared to direct sulfidation. TCE degradation by all five ZVI yielded similar products including ethene and ethane as dominant products. No chlorinated intermediate was detected, except that very small amount of cis-DCE was detected during TCE degradation by sulfided and direct sulfided B-OM, and sulfided AA particles. Compared to significant improvements in TCE dechlorination rates, PCE and cis-DCE dechlorination rates were enhanced to smaller extents by the sulfidation of commercial ZVI used in this study. Mass normalized pseudo-first-order PCE degradation rate constants of sulfided AA, B-OM, and HPR ZVI particles were 2.74, 1.65, and 2.46 x 10-6 L/g-min, respectively, and cis-DCE degradation rate constant of sulfided AA, B-OM, and HPR ZVI particles were 1.33, 0.76, and 0.21 x 10-6 L/g-min, respectively. In the last part of this study, the effect of metal impurities on ZVI reactivity for the degradation of PCE and cis-DCE was also evaluated. Experimental results showed that copper amended particles enabled higher degradation rates for both PCE and cis-DCE compare to unamended ZVI. Nickel amended ZVI increased PCE removal rate, but was ineffective for cis-DCE. Manganese amendment did not show any improvement in removal of either PCE or cis-DCE. Lastly, sulfidation was conducted on Cu-amended B-OM ZVI for PCE degradation. Data revealed that sulfur is able to poison the catalytic effect of Cu and significantly slow down PCE reduction. Therefore, the effect of sulfidation on reactivity of commercial ZVI is selective to the type of contaminants and importantly, depends on their chemical compositions and nature of impurities, which are strongly affected by their manufacturing routes.