Rapid dechlorination of polychlorinated dibenzo-p-dioxins by bimetallic and nanosized zerovalent iron

Polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs), especially the 2,3,7,8-substituted congeners, are extremely toxic, persistent, and recalcitrant to remediation. Dechlorination of PCDD/Fs by zerovalent iron (ZVI) is thermodynamically feasible, but useful rates of reaction have not been previously reported. Here we show that ZVI (both micro- and nanosized ZVI, without palladization) dechlorinates PCDD congeners with four or more chlorines in aqueous systems, but the reaction is too slow to achieve complete dechlorination within a practical period of time. In contrast, palladized nanosized ZVI (Pd/nFe) rapidly dechlorinates PCDDs, including the mono- to tetra-chlorinated congeners. The rate of 1,2,3,4-tetrachloro dibenzo-p-dioxin (1,2 3,4-TeCDD) degradation using Pd/nFe was about 3 orders of magnitude faster than 1,2,3,4-TeCDD degradation using unpalladized ZVI. The distribution of products obtained from dechlorination of 1,2,3,4-TeCDD suggests that palladization shifts the pathways of contaminant degradation toward a greater role of H atom transfer rather than electron transfer.