| Chlorinated organic compounds (COCs) have been extensively used as synthetic intermediates and solvents in industrial applications, such as manufacture of chemicals, pharmaceuticals, leatherwares, electro-products, and pesticides. Due to leaks, spills, and releases from industrial sources, they inevitably contaminate the environment. COCs are ubiquitous and hazardous pollutants found in atmosphere, soil, surface water and groundwater. They are highly toxic and poorly biodegradable, and present a serious threat to the environment and human health. Reductive dechlorination of COCs by zerovalent iron (ZVI) has gained acceptance as a promising treatment technology. Compared with other treatment methods, ZVI technology has obvious advantages due the lowcost and nontoxicity of iron. Dechlorination of COCs can be conducted at ambient temperature and atmospheric pressure by relatively simple technique and equipment. The dechlorination reaction rate can be significantly increased by using iron nanoparticles post-coated with palladium. The enhanced reactivity of Pd/Fe bimetallic nanoparticles is attributed to the increased surface area and the deposition of the second metal Pd as a catalyst onto the surface of iron.Pd/Fe bimetallic nanoparticles were prepared by the liquid phase method, a simple two-step synthetic method where Pd was deposited onto the Fe surface. An aqueous solution of potassium hexachloropalladate and an ethanol solution of palladium acetate were used as palladization solution, respectively. Pd/Fe bimetallic nanoparticles were dried in an oven under vacuum, and extensively characterized in terms of BET-N2, SEM, TEM, XRD, XRF and EDS to identify the specific surface area, morphology, particle shape, size, crystal structure, Pd content, and surface elemental composition. The results suggest that Pd/Fe bimetallic nanoparticles palladized by using an ethanol solution of palladium acetate had smaller aggregation tendency and were more evenly sized than Pd/Fe bimetallic nanoparticles palladized by using an aqueous solution of potassium hexachloropalladate with a diameter in the range of 20?50 nm and a surface area of 51.4 m2/g. The crystal structure of iron was a regular bccα-Fe crystalline state.Dechlorination efficiencies of chlorinated methanes (including tetrachlorinated methane (CT), trichlorinated methane (CF), and dichlorinated methane (DCM)) by Pd/Fe bimetallic nanoparticles were higher than that by iron nanoparticles or microscale iron powders. The effects of some essential parameters, such as Pd loading, Pd/Fe nanoparticle addition, initial pH value of reaction system, initial concentration of target pollutants, on the dechlorination efficiency of chlorinated methanes were systematically studied. Dechlorination efficiency increased with increasing nanoparticle addition. For chlorinated methane solution with initial concentration of 100 mg/L, the optimal dechlorination efficiency was obtained at Pd/Fe nanoparticle addition of 10 g/L, Pd loading of 0.20 wt%, and initial pH value of 7.Effects of Pd loading, nanoparticle addition, initial pH value of reaction system on dechlorination efficiency of chloroacetic acids (including trichloroacedic acid (TCAA), dichloroacetic acid (DCAA), and monochloroacetic acid (MCAA)) were investigated. Dechlorination efficiency of chloroacetic acids increased with the increase of Pd loading at Pd loading < 0.10 wt%, while it was found to decrease at Pd loading > 0.10 wt%. Increasing Pd/Fe nanoparticle addition resulted in the increase of dechlorination efficiency. Dechlorination of chloroacetic acid by Pd/Fe bimetallic nanoparticles can be conducted in a wide range of pH value. Dechlorination efficiency of chloroacetic acid with low-chlorinated chloroacetic acid was greater than that of high-chlorinated chloroacetic acid.PVDF microfiltration (MF) membrane was hydrophilized by coating with a mixture of polyvinyl alcohol (PVA), glutaraldehyde, and polyethylene glycol (PEG). Polyacrylic acid (PAA) was used as chelating reagent to functionalize hydrophilized PVDF MF membrane. Pd/Fe bimetallic nanoparticles were immobilized in modified PAA/PVDF MF membrane matrix. Pd/Fe composite membrane system showed high reactivity in catalytic dechlorination of TCAA. The amount and dispersal of Pd/Fe nanoparticles immobilized in PAA/PVDF MF membrane were largely influenced by hydrophilization and PAA crosslinking treatment of the pristine membrane. The metal content, morphology, and surface chemical structure of composite membrane were measured by using ICP-OES, SEM, and FT-IR, respectively. No obvious aggregation of Pd/Fe bimetallic nanoparticles were found in hydrophilized and PAA crosslinked PVDF MF membrane. Compared with unmodified Pd/Fe composite membrane, the hydrophilized and crosslinked PAA functionalized Pd/Fe composite membrane showed higher reactivity for catalytic dechlorination of TCAA.Removal efficiency of TCAA by Pd/Fe PVDF composite membrane was greater than that by nonimmobilized Pd/Fe bimetallic nanoparticles. Some important parameters, such as Pd loading and Pd/Fe nanoparticles addition contained in the composite membrane, and initial concentration of target pollutant, were found to have effects on the removal efficiency of TCAA. The highest removal efficiency of TCAA with initial concentration of 5 mg/L was obtained at Pd loading of 0.534 wt% and Pd/Fe nanoparticles addition of 0.391 g/L. Removal efficiency decreased with the increase of initial concentration of TCAA. The stability and longevity of Pd/Fe membrane system were preliminarily studied by a multi-spiking batch experiment where complete removal of TCAA was achieved for four times although the aging of the composite membrane was found. With ferrous chelating by PAA, the secondary pollution caused by the dissolved metal could be avoided. Water plays a key role in dechlorination reaction process. No removal of TCAA was detected in an ethanol solution of TCAA, indicating that no dechlorination reaction took place in the absence of water, which is the donor of hydrogen from the Fe corrosion reaction, and the hydrodechlorination reaction requires the presence of some water to facilitate the corrosion. Being stored in pure ethanol solution without water and dissolved oxygen would benefit the maintenance of catalytic dechlorination activity of Pd/Fe composite membrane.
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