| Permeable reactive barrier (PRB) is fast emerging as an alternative to traditional pump-and-treat and dig-and-treat methods for in situ remediation of inorganic and organic groundwater contaminants. A basic model has been firstly presented for describing migration and transformation of contaminants in groundwater, and then using immobilized biological two-layer PRB developed by author, the removal of methyl tert-butyl ether (MTBE) from groundwater considering process parameters, mass transfer between solid and liquid phases, biodegradation and hydromechanics has been investigated.The results of batch experiment indicated that linear equilibrium adsorption equation can well describe the adsorption behavior of MTBE in different clayey soils. A soil column experiment was performed to calculate the dispersive coefficient and retardation factor of the test medium. The retardation factor is only 1.004, which means the soil used in this study has hardly the retarding and purification capabilities for MTBE.Permeability of expanded perlite used as carrier for immobilizing the microbes was measured by TST-70 soil penetrate apparatus. The static adsorption experiment was conducted to investigate the performance of granular expanded perlite for the removal of MTBE in solution and thermodynamic characteristic of adsorption. The results showed that the adsorption of MTBE in expanded perlite follow Freundlich equation, and the adsorption amount of equilibrium increases as the operational temperature increases. Furthermore, the thermodynamic analyze on the experimental data meant that the adsorption of MTBE is an endothermic process, and can occur automatically. The degradation performance of MTBE using mixed aerobic microbe enriched and acclimated in laboratory was studied. The optimum conditions for MTBE degradation is as follows: 20～25℃, pH8.0 and 1:10 (V/V) of inoculation amount of microbes.Calcium peroxide (CaO2) was used as source of supplying oxygen for microbes immobilized in two-layer PRB system, and its oxygen-releasing characteristic and the regulation of high pH were explored. As useful nutrient components in medium, KH2PO4 and (NH4)2SO4 at a certain ratio can regulate pH caused by CaO2 from 12.1 to the range of 6.5-8.5, which is suitable for microbial growth. Although the effect of tourmaline and saturated soil on the pH is weaker than the former, it can help to decrease excessive use of nitrogen and phosphorus and preserve groundwater from new pollution. In addition, microbial growth curve suggested that the metabolism of mixed aerobic microbes could be meet by CaO2 and relevant medium.The removal process of MTBE in two-layer PRB system was investigated in laboratory by using two stainless columns. Based on the results obtained from the column experiment, we concluded that the oxygen-releasing material layer could supply sufficient oxygen and suitable pH environment for the microbes immobilized in the second layer. In the passive system, occurrence of aerobic degradation can be verified by the consumption of MTBE, the decrease in DO levels in the biodegradation column effluent compared to the influent. As the MTBE byproduct, tert-butyl alcohol (TBA) can also be biodegraded after the transient accumulative period. According to the basic control equation, a one-dimension mass transfer model under the condition of this study was presented, and the result of simulation was in good agreement with the experimental data.By using finite element approach, hydraulic capture zone width, which refers to the width of the zone of groundwater that passes through PRB, and the distribution of MTBE concentration in PRB and adjacent area were simulated based on the continuity equation, momentum transfer, migration and transformation model of MTBE. The simulated results are important for design, construction of PRB as well as appropriately evaluating effectiveness of groundwater remediation.
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