Microbial Passivation Analysis And Electrochemical Depassivation Of Iron In ZVI PRB

Permeable reactive barriers (PRBs) composed of zero-valent iron (ZVI) aresusceptible to passivation, resulting in substantially decreased rates of chlorinatedsolvent removal over time. For contaminated ground water treatment, ZVI PRB is astandard rather than a sustainable technology. As a key problem during the ZVIPRB application, investigation of the passivation, catalyzing and restoration of ZVIis very useful and important. Both biological and chemical processes can potentiallypassivate ZVI, although the understanding of biological passivation is limited.Moreover, the eoectron induced reduction technology (EIR) can supply electron byits cathode and helpful to the removal of nitrate (NO₃^-) in lower permeabilityaquifer. The application of EIR in ZVI PRB maybe catalyzes and restores thepassivated ZVI.This study was conducted in bench-scale reactors packed with fresh ZVI orZVI pre-exposed to NO₃^-and in the presence or absence of a denitrifying bacterialenrichment (DNBE). The first-order removal coefficients (k) for NO₃^-reduction byZVI in the presence and absence of DNBE were 0.20 and 0.09 h^-1, respectively,suggesting that DNBE is helpful to NO₃^-removal. The first-order removalcoefficients (k) of TCE by ZVI was reduced 49% with DNBE existed, and DNBEcould inhibit the removal of TCE in ZVI PRB. For the reactor with DNBE only, theremoval of NO₃^-by denitrifying process was very lower (only 13%) when TCEpresented, TCE has a biotoxic effect on DNBE; however, when ZVI added, thefirst-order removal coefficients (k) of nitrate by ZVI and DNBE was similar (0.18VS 0.20 h^-1), ZVI can inhibit the biotoxic effect of TCE to DNBE.In this study, the application of low electrical direct current (DC) to restore thereductive capacity of passivated ZVI was also examined. Electrical current wasapplied to a laboratory column reactor filled with a mixture of pre-passivated ZVIand sand. Variable voltage settings (3^-12 V) were applied through two stainless steel electrodes placed at the ends of the reactor. After 12 V electricity added, thefirst order degradation rate (k) of TCE was increased about 4 times compared to thek of the ZVI passivated by NO₃^-with no electricity added (0.655 VS 0.147 h^-1), EIRcould catalyzes the TCE removal in ZVI PRB and the increasing of removal ratewere roughly proportional to the voltage level. And after restored by cathode, thefirst order degradation rate (k) of ZVI was about 3 times higher than the the k of theZVI passivated by NO₃^-with no electricity added (0.536 h^-1). Although differenceswere observed in the rates and extent of TCE reduction within the column, it isnoteworthy that TCE reduction was not restricted to only that region of the columnwhere the electrons entered (i.e., at the cathode). While complete "depassivation" ofZVI may be difficult to achieve in practice, the application of DC demonstratedobservable restoration of reactivity of the passivated ZVI, but the negative effect ofthe anode should be carefully treated.Above all, such conclusions were obtained: a. In ZVI PRB, the presence ofDNBE is helpful to the removal of NO₃^-; b. DNBE could accelerate the passivationof ZVI, and inhibit TCE removal in PRB; c. TCE has biotoxicity to DNBE,however, when ZVI added, ZVI can reduce the biotoxic effect of TCE to DNBE; d.With EIR loaded, higher rates of TCE removal were always obtained when currentwas applied and were roughly proportional to the voltage level; e. After theapplication of EIR, observable restoration of reactivity of the passivated ZVI wasobtained; f. For field application of EIR in ZVI PRB, the negative effect of theanode should be carefully treated.