Study On Permeable Reactive Barrier For Remediation Of Chromate Contaminated Groundwater

Chromium, which is a world-recognized toxic pollutant, its compounds have been used for the tanning of leather, galvanic industries and wood impregnation. Chromium is often found in soil and groundwater of former industrial sites, which now requires remediation measures according to environmental regulations. Chromium exists in various oxidation states, but only the trivalent and hexavalent oxidation states are of major environmental concern due to their stability in the natural environment, where Cr(VI) is very toxic due to its high solubility and mobility which expand the mutagenic and carcinogenic influence.Compared with traditional pump-and-treat techniques (treatment of contaminated water pumped to the surface), permeable reactive barrier (PRB) is considered to be one of the new passive remediation method that has great potential in dealing with low concentration contaminated groundwater.The selection of reactive material is an important content of the PRB designing and is strictly related with the specific pollutant to be treated.In this study, a series of laboratory batch experiments and column tests are conducted to evaluate the potential for remediation of chromate contaminated groundwater using a permeable reactive barrier composed of mixtures of scrap iron and coconut shell charcoal.The removal of Cr(VI) from low concentration contaminated groundwater has been investigated in batch experiment with various operating parameters such as initial pH, agitation time and adsorbent dosage. It is evident from the batch studies that the coconut shell charcoal and scrap iron as PRB medium for treating Cr(VI) contaminated groundwater whose pH tends to be neutral appears to be technically feasible. However, due to the limited adsorption capacity of coconut shell charcoal and the precipitation produced in the iron corrosion process, the reaction capacity and permeability of PRB medium can be greatly influenced in the field applications, indicating that scrap iron and coconut shell charcoal should not be used individually in the PRB technology.Simulation experiment indicates that when scrap iron and coconut shell charcoal act in combination, dissolved chromate concentrations are reduced to less than 0.05 mg/L as a result of the reduction of the scrap iron and the adsorption effect of coconut shell charcoal, the proportioning of the two affects the removal efficiency of Cr(VI) and iron-carbon galvanic cell reaction occurred in the process. When the volume fraction of scrap iron, coconut shell charcoal and quartz sand are 25%,25% and 50% respectively, the mass concentration of Cr(VI) can be reduced from 5mg/L to 0.05mg/L or below, which meets the requirement of Groundwater Quality Standard GB/T14848-9 â…¢. All of which makes the mixture of scrap iron and coconut shell charcoal a potential and applicable PRB reactive medium for remediation of chromate contaminated groundwater.