Microwave-induced nanoscale zero-valent iron degradation of perchloroethylene and pentachlorophenol.

Unlabelled: Microwave (MW) is applied to enhance perchloroethylene (PCE) or pentachlorophenol (PCP) removal using zero-valent iron (ZVI; Fe(0)) as the dielectric medium. ZVI has a much higher dielectric loss factor (39.5) than other media; it is capable of absorbing MW radiation rapidly to speed up the release of electrons, leading to rises of the ZVI particle surface temperature. If the MW power is continued, excessive electricity will accumulated inside ZVI particles, resulting in sparks. The results show that during the initial 5 sec (700 W), the linear aliphatic PCE has a faster decomposing rate than the ringed PCP (82.0% vs. 4.8%) because less energy is required for decomposing the linear-chlorine bond (90 kcal mol(-1)) than ring-chlorine bonds (95 kcal mol(-1)). Later the removal rate for either PCE or PCP remains the same when the exposure time is between 5 and 60 sec. Without MW irradiation, linear PCE molecules have larger surface area to contact ZVI, and hence they have better removal efficiencies than PCP molecules. Using Fe(0) as a microwave dielectric medium to treat PCE or PCP is a new and worthwhile treatment technology; it is environmentally friendly, and its use will eliminate the secondary pollution.

Implications: Nanoscale iron particles are characterized by high surface-area-to-volume ratios, high specific surface area, and high surface reactivity. With a much higher dielectric loss factor, it is capable of absorbing MW radiation rapidly to speed up the release of electrons, leading to rise in temperature. The time needed to achieve a satisfactory treatment is also reduced, leading to significant saving of energy consumption to make this method cost-effective and also environmentally friendly for the industry to pursuit sustainable development.