Column Studies On Effects Of Benzene, Toluene On Reductive Dechlorination Of TCE, PCE By Granular Iron

Groundwater pollution is becoming more and more complicated gradually. Particularly, the multi-components plume in groundwater which blends chlorinated solvents with petroleum hydrocarbons has recently attracted great attention because of its well-known carcinogenicity in humans and pronounced difficulty to remedy simultaneously. Sequential treatment technology which includes Fe-PRBs and biodegradation is essential for removal of BTEX and chlorinated hydrocarbons. This paper presents some column experiments which were conducted to evaluate the effects of benzene, toluene on reductive dechlorination of trichloroethene (TCE) and perchloroethene (PCE) by granular iron, and to discuss the competition law for reductive dechlorination of TCE and PCE in this column system.The results showed that the kinetics of TCE/PCE or their mixture (at the initial concentration of 2.00 mg/L more or less) reduction was accorded with pseudo first-order even in the presence of benzene or toluene (at about 2.00 mg/L, respectively). When environment temperature was stable, the kobs value would reduce gradually with the reaction time through the experimental process. After benzene or toluene got adsorption equilibrium in the reaction columns, the presence of benzene increased PCE degradation by 13.45% averagely, while PCE reduction rate averagely decreased by 13.79% in the existence of toluene. Nevertheless, the presence of benzene or toluene both inhibited the removal rates of TCE by 19.11% and 25.33% respectively. When TCE and PCE were mixed, the results suggested that benzene would be a potential promoter to the dechlorination of the mixture, increased the TCE or PCE degradation by 8.97% and 14.56% in average; but toluene suppressed the removal of the mixture, averagely declined the removal rates by 12.88% and 22.26% respectively. Moreover, interspecies competition presents in the mixture of TCE and PCE, the existence of PCE would inhibit the removal rate of TCE by 13.70% in average; on the contrary, the inhibitory effect of TCE on the PCE needs further verification. At the same time, these data also proved that the active sites of TCE and PCE in the surface of granular iron were different. Furthermore, the dechlorinated daughter products in the presence/absence of benzene or toluene were identical. Trichloroethylene (TCE), perchloroethene (PCE), cis-1,2-dichloroethene (cis-1,2-DCE), 1,1-dichloroethene (1,1-DCE) and vinyl chloride (VC), except for trans-1,2-dichloroethene (trans-1,2-DCE), were observed as the qualitative scan results of the target pollutants and chlorinated intermediates. The carbon mass balance results indicated that ethyne, ethylene and ethane were the major non-chlorinated end products of the dechlorination process. The accumulation of chlorinated intermediates was consistent with the removal ratios of TCE/PCE in the existence of benzene or toluene. In addition, the investigated changing of groundwater chemistry showed noticeable disappearance of calcium (II), bicarbonate and sulfate occurred. However, in the presence of benzene or toluene, slightly various among these changes were observed. After a long-term operation, effluent of each column had a lower value of the DO, below 1.00 mg/L, and a higher value of pH which could reach 9.60 at most, also companied with a large amount of precipitation of calcium carbonate in the columns. The temperature had a great impact on the reaction rate. With the temperature growth, the kobs increased significantly. The effect of temperature on PCE in the mixed pollutants was less significant than that on TCE in the mixture.