Mechanistic Investigation On Bioremediation Of TNT-contaminated Soil By Bioslurry Reactor

TNT, a pollutant with toxic, carcinogenic, and mutagenic, has been list in a category of priority pollutants in China and USA. Since it has been used widely in military activities, the TNT pollution was serious over the world. Thus, the TNT bioremediation attracted more and more attention. In this paper, the method for TNT extraction from soil was optimized and the potential of TNT bioavailability was analyzed. Also, the toxicity of TNT on the methane-producing systems was evaluated. Moreover, the mechanism of the rapid disappearance of TNT under anaerobic condition was analyzed. Finally, a safe and high efficient bioslurry reactor technology has been.developed to remove TNT in contaminated soil, with the aim to meet the remediation standard of 17.2mg TNT/kg soil. The results of this study would fill a vacancy of remediation of heavy TNT-contaminated soil. It is also of referential value for the remediation of nitroaromatic compounds in similar media. Major research results are summarized as follows:(1) The extraction method of TNT from soilWhen extraction solvent was acetonitrile, the optimum extraction condition of TNT from soil was:20 mL acetonitrile:1 g soil,12 h vibration-5 h sonication. The average of TNT recovery rates from clay soil, sandy soil, and corresponding soil slurry (2:1 water/soil ratio) were higher than 95%. It suggested that the type of soil had no effect on TNT recovery. However, effect of TNT concentration on TNT average recovery rate was significant. When TNT concentration of soil was higher than 500 mg TNT/kg (soil), the average recovery rate of TNT from soil was between 95% and 105%; when TNT concentration of soil and corresponding soil slurry was 10 mg TNT/kg (soil), the average recovery of TNT was between 70% and 130%. The coefficient of variability is less than 10%. In all, both the accuracy and degree of precision of TNT recovery met the analysis requirement.(2) The sorption/desorption behavior of TNT and the potential of TNT bioavailabilityUnder the condition of the liquid to soil ratio of 2:1, the sorption of TNT on soil reached the equilibrium state within 24 h and the sorption kenitics fitting Elovich equation reached the best, with the correlation coefficient of 0.962, which indicated that the sorption was a non-homogeneous diffusion process The sorption/desorption isotherm fitted Freundlich equation very well, of which the constant 1/n indicated existence of hysteresis (the calculated index of hysteresis was 3.2). The free energy of sorption suggested the physical sorption was dominant for TNT sorption on soil. The desorption of TNT from contaminated soil was fast and increased with the increase of pollution level of soil, indicating that the potential of TNT bioavailability was high.(3) The methane-producing toxicity of TNTThe methane-producing toxicity of TNT increased with the increase of TNT concentration. The relative activity was. more than 77.62% when TNT concentration was lower than 100 mg/L, suggesting the inhibition was light. Relative activity was between 75% and 40% when TNT concentration was higher than 100 mg/L, suggesting the inhibition was middle. Moreover, the rapid conversion of TNT eliminated the threat of lyses to methanogenic bacteria under anaerobic condition. It could be concluded that the toxicity of TNT to microbiologic population wasn't large and the technology of anaerobic bioremediation of TNT was feasible.(4) The mechanism of the rapid disappearance of TNT in an anaerobic solutionUnder Simulated Anaerobic Conditions at pH 6-7, TNT could be reduced by both Na2S and Zn0 which could produce nascent hydrogen, and both of TNT reduction kinetics followed the pseudo-first order rate law. The reductant concentration played a very important role in reduction of TNT, and a 5-fold increase in reductant concentration above the theoretical stoichiometric concentration was optimum. The reaction time for the conversion of TNT in system B which both controlled pH and simulated components of an anaerobic solution was much shorter than that in system A which only controled pH. About 90% TNT in system B could be reduced by both Na2S and Zn0 within 2h at pH 6-7, when the reductants concentration was increased 5-fold. It could be concluded that abiotic reductants including sulfide and nascent hydrogen, which were produced by biological action, played a very important role in the rapid disappearance of TNT under anaerobic condition, and the conversion of TNT increased with reductants concentration increasing.The LC/MS results of intermediates indicated that, both nitroreduction and denitration of TNT could be initiated by sulfide simultaneously, but the former was dominant over the latter. However, only denitration could be initiated by Zn0. Moreover, intermediates of both pathways may form polymer. It could be concluded that TNT could be transformed into many products by reductants under anaerobic condition and the reduction pathways varied with different reductants.(5) The high efficient bioslurry reactor technology for remediation of heavy TNT-contaminated soilBased on the results of the batch static experiments for remediation of 1000 mg TNT/kg (soil) TNT-contaminated soil, the optimum condition were obtained as follows:4 of mCOD/mTNT, 2 of the ratio of liquid to solid,5% of inoculum,1.5 fold CMC of Tween 80,5 d anaerobic treatment and 2 d aerobic operation. Under the optimal conditions, 1000mg TNT/kg (soil) TNT-contaminated soil reached the remediation standard (17.2 mg TNT/kg (soil)) after 5 d anaerobic treatment and the volume loading was 100 g/(m3-d);the subsequent 2 d aerobic treatment could further realized the stabilization and humification; the whole remediation period was shorter than that of literature. Also, under the optimal conditions, all polluted soil, with volume loading less than 100 g/(m3·-d)and mCOD/mTNT more than 4, could be remediated after 5 d anaerobic treatment; while the polluted soil with volume loading more than 100 g/(m3-d) could reach the remediation standard by controlling volume loading and mCOD/mTNTThe results from dynamic operation of sequence boi-slurry reactor suggested that TNT concentration of 1000 mg TNT/kg (soil) soil met the remediation standard (17.2 mgTNT/kg (soil)) after 5 d anaerobic treatment-2 d aerobic treatment during four operation periods on the basis of sufficient stir (200 rpm) and under the above optimum condition. Moreover it realized stabilization and humification of contaminated soil.The operation performance of bio-slurry reactor was steady and remediation period was shorter than that of literature.