3.3.2.4 Other Chlorinated Compounds
The degradation of polychlorinated biphenyl (PCB) has been investigated by various researchers using various iron and iron bimetallic nanomaterials. When Fe/Pd bimetallic nanomaterials were impregnated onto a mesoporous granular activated carbon, PCBs were adsorbed on the surface of carbon and subsequently dechlorinated by the bimetallic nanoparticles [93]. The system was able to dechlorinate 2-chlorobiphenyl by 90% after 2 d. The remaining 10% was adsorbed on the activated carbon together with the biphenyl dechlorination product. Similarly, Fe–Pd bimetallic nanoparticles stabilized by a water-soluble starch were able to remove over 80% of PCB in less than 100 h, whereas the unstarched particles removed only 24% of PCB [24]. Using nZVI immobilized on a cation-exchange resin, the dechlorination of decachlorobiphenyl was successfully achieved after 10 d (Li et al., 2007). However, when particles were suspended in solution for the same duration, 21% of the parent compound remained.
nZVI was used to reduce eight chlorinated ethanes [102]. Most ethane molecules were dechlorinated, except 1,2-dichloroethane. The reactivity of the nZVI was dependent on the degree of chlorination and the location of chlorine atoms on the molecule. A higher rate of dechlorination was observed with highly chlorinated ethanes and when the chlorine atoms were localized on a single carbon atom. Dechlorination of hexachloroethane was most efficient and followed a reductive β-elimination (two chlorine atoms from adjacent carbons were removed followed by double bond formation) pathway forming PCE (tetrachloroethylene) as the major product.
Comparing biodegradation of monochloroethane with nZVI-assisted dechlorination of the compound, there was a significant difference in the degradation of the products formed [94]. The products of abiotic dichloro-elimination degraded 10% more than the products of biodegradation. This suggests that the degradation of chlorinated ethane by nZVI is superior to biodegradation since the products of the former degraded faster than those from the latter reaction.
Removal of 1,1,1,-trichloroethane using a series of bimetallic iron nanomaterials (Au, Cu, Ni, Pd, and Pt) did not show any periodic trend [95]. However, the first order rate constant for the reactions correlated with the solubility of atomic hydrogen on the second metal. This suggested that the enhanced activity of bimetallic iron nanomaterials was due to absorbed atomic hydrogen and not by galvanic corrosion processes.
Freshly prepared Pd/Fe nanoparticles were used to dechlorinate mono-, di-, and 1,2,4-trichlorobenzene [96]. Successful dechlorination to form benzene as the final product was achieved. The reaction followed a pseudo first-order kinetics with more chlorinated products having higher reaction rates than the less chlorinated compounds. Pd/Fe nanomaterial was active since the unpalladized iron showed very minimal reactivity. The aged bimetallic particles had less activity presumably due to the dislodgment of Pd and Pd encapsulation by the iron oxides formed during aging.
Further investigation on the dechlorination of trichlorobenzene by Pd/Fe particles revealed that the presence of surfactants influenced the reaction rate [97]. When the concentrations of surfactants were below the critical micelle concentration (CMC) values of cationic (cetyltrimethylammonium bromide), anionic (sodium dodecyl sulfate), and nonionic (nonylphenol ethoxylate and octylphenol polyethylene glycol ether) surfactants, the rate constants increased as compared to pure water. However, above the CMC values, the rate decreased. The presence of natural organic matter (NOM) also reduced the catalytic dechlorination of 1,2,4-trichloro benzene by the particles due to competition with NOM as a H2 acceptor.
Other aromatic chlorinated compounds investigated are chlorophenols. Hydrodechlorination of 2,4-dichlorophenol by nano Pd/Fe bimetallic particles was investigated [98]. Phenol was found to be the major product; however, trace levels of 2-chlorophenol and 4-chlorophenol were also formed. Optimum conditions such as high Pd loading, higher reaction temperatures, and weak acid conditions favored the catalytic dechlorination of these compound.
Dror et al. deposited nZVI and cyanocobalamine on a diatomite matrix and used the composite to study the degradation of PCE and tribromoneopentyl alcohol [99]. The resulting material was superior than nZVI material since it prevents agglomeration making the surface area larger. Laboratory experiments showed rapid first-order decay of the contaminants containing the composite material, whereas the concentration in the control remained the same. In addition, various chlorinated compounds in well water were passed through a column containing the composite for 30 min. The concentrations of contaminants were reduced considerably but the degradation rate was slower for the less chlorinated compounds. Field experiments were also conducted and well water was made to pass through a 50 kg of the composite material in a column. Inlet contained significant amounts of TCE, PCE, cis-DCE, carbon tetrachloride, and chloroform. These contaminants were reduced substantially in the outlet. Specifically, the concentrations of TCE and PCE were reduced to a third of inlet values. Other compounds degraded include nitrate ions and pesticides such as bromacyl and prometryn.