Degradation of polychlorinated biphenyls using mesoporous iron-based spinels

Abstract

h i g h l i g h t s• The NiFe 2 O 4 had the highest activity in degradation of CB-209, followed by Fe 3 O 4 .• Hydroxyl species, organic acids, PCBs and chlorobenzenes were identified as products.• Three degradation reactions and one combination reaction competitively occurred.• Hydrodechlorination of CB-209 was more favored over Fe 3 O 4 than NiFe 2 O 4 .• Oxidation reaction of CB-209 was more favored over t r a c tA series of mesoporous iron-based spinel materials were synthesized to degrade polychlorinated biphenyls (PCBs), with CB-209 being used as a model compound. The materials were characterized by X-ray powder diffraction (XRD), pore structure analysis, and X-ray photoelectron spectroscopy (XPS). A comparison of the dechlorination efficiencies (DEs) of the materials revealed that NiFe 2 O 4 had the highest DE, followed by Fe 3 O 4 . Newly produced polychlorinated biphenyls, chlorinated benzenes, hydroxyl species and organic acids were detected by gas chromatography-mass spectrometry, high performance liquid chromatography-mass spectrometry and ion chromatograph. Identification of the intermediate products indicates that three degradation pathways, hydrodechlorination, the breakage of C C bridge bond and oxidative reaction, accompanied by one combination reaction, are competitively occurring over the iron-based spinels. The relative amounts of produced three NoCB isomers were illustrated by the C Cl BDEs of CB-209 at meta-, para-and ortho-positions, and their energy gap between HOMO and LUMO. The consumption of the reactive oxygen species caused by the transformation of Fe 3 O 4 into Fe 2 O 3 in the Fe 3 O 4 reaction system, and the existence of the highly reactive O 2 −• species in the NiFe 2 O 4 reaction system, could provide a reason why the oxidation reaction was more favored over NiFe 2 O 4 than Fe 3 O 4 .

Thermal dechlorination of PCB-209 over Ca species-doped Fe2O3

“…The above results meant that chlorines at ortho-positions were the most stable, while those at meta-and para-positions were relatively easer to drop during catalytic degradation of PCB-209 at 300°C. The dechlorination pattern in our study was similar with that of PCB dechlorination over iron-based spinel at 300°C (Huang et al, 2013) and microbial reductive dechlorination of PCBs in soils and aquatic sediments under anaerobic conditions (Wiegel and Wu, 2000). The dechlorination pattern described above matches PCB dechlorination Process N, which defined microbiologically mediated dechlorination of PCBs which typically removes meta and/or para chlorines to generate primarily ortho-substituted lower chlorinated biphenyls (Wiegel and Wu, 2000;LaRoe et al, 2014).…”

“…The detection of newly formed lower chlorinated biphenyls homologues and their time-dependent distribution pattern indicated the occurrence of successive reductive hydrodechlorination reactions during the degradation of PCB-209 over the prepared 1%Ca-Fe 2 O 3 . Similar successive reductive hydrodechlorination of PCB-209 over as-prepared Fe 3 O 4 was also observed with identification of dechlorinated products from NonaCBs to DiCBs (Huang et al, 2013). Furthermore, it is noted that the sum of the residue PCB-209 and the new formed PCBs took 97.6%, 42.6%, 12.3% and 1.55% of the origin PCB-209 at 10, 20, 30, and 60 min, respectively.…”

“…The amount of intermediate product was determined by both its formation and further degradation rates. The experimental data demonstrated that the stabilities of chlorine atoms substituted on the PCBs were in the order of ortho >> para > meta Huang et al, 2013). The reason was that the chlorines on ortho-position had a major influence on the torsional angles and rotational barriers of PCBs, while the effect of adjacent meta chlorine atoms was much smaller (Dorofeeva et al, 2004).…”

“…The material's reactivity was primarily related to the number of available surface active sites, and this in turn depended on factors such as the material's surface area, pore volume, and phase composition. The increased surface area would increase the number of active sites, resulting in an increase in the degradation activity (Fan et al, 2011;Huang et al, 2013 phase (Chen et al, 2006;Wang et al, 2014). α-Fe 2 O 3 also promoted the catalytic activity of Ca 2 Fe 2 O 5 , owing to the interaction between supporter and active component (Tauster et al, 1981).…”

“…However, there are considerable problems, including the high cost and the possible generation of more toxic polychlorinated dibenzodioxins/polychlorinated dibenzofurans during incineration (Addink and Olie, 1995). Catalytic degradation at lower temperature involving metal oxides as a safe alternative technology is thought as the most promising method, due to its relatively low costs, high levels of activity (Subbanna et al, 1988;Tanaka et al, 2005;Huang et al, 2013). Numerous metal oxides exhibited high activity in the decomposition of PCBs, including Cr 2 O 3 , Co 3 O 4 , CuO, Al 2 O 3 , La 2 O 3 (Subbanna et al, 1988;Tanaka et al, 2005).…”