Laboratory simulation of a glucose-induced redox treatment for in-situ remediation of groundwater polluted by hexavalent chromium

The problem of pollution by hexavalent chromium has recently assumed great importance. Cr(VI) is considered one of the 20 most dangerous pollutants over the past 15 years; it is highly toxic to living organisms and if inhaled or ingested is a potent carcinogen. In Italy, the contamination of soil and water by Cr(VI) is a serious and widespread problem; indeed numerous cases of polluted soils have been identified, which led to a content of Cr(VI) in groundwater much higher than the limit of 5 g / L imposed by Italian law. Cr(VI) released in soil can be reduced to Cr(III); however, if the reducing ability of the soil is not sufficient to transform all of the introduced Cr(VI), it may persist in the soil and, because of its high mobility, can be easily leached and cause contamination of surface water and groundwater. There is therefore the need to develop techniques for remediation, in particular of groundwater. At the present time the techniques for the removal of Cr from water can be divided in three categories: containment, ex-situ treatment and in-situ treatment. Beside the absolute necessity of being efficient, in order to be proposed for the application on a large scale, a decontamination process should be of relatively simple implementation and present reduced costs. At this purpose a promising approach is represented by bioremediation technique with organic nutrients, an in-situ process involving the addition of sugar solution to groundwater, as carbohydrate source, to stimulate microbial activity. Indeed it has been reported that Cr(VI) reduction in sediments occurs through a complex network of mechanisms where the microbial biomass present seems to be of great relevance. The reduction of Cr(VI) may occur through direct bacteria assimilation and enzymatic depletion, or indirectly if the bacteria metabolism modifies the local environment and makes it suitable to Cr(VI) reduction. The present research aims to simulate at a laboratory scale, with a simple and cheap experimental set-up, an in-situ glucose-mediated bioremediation treatment involving a water/sediment suspension contaminated by Cr(VI). The monitored parameters have been: pH, dissolved oxygen, content of Cr (VI), Fe, Ni and Mn. The effect on the process of experimental parameters such as contact time, glucose concentration and aerobic/anaerobic condition has been examined. The obtained results show the progressive Cr (VI) depletion when increasing time of contact and glucose concentration. Nevertheless, after long contact time and at high glucose concentration the contemporary release in solution of Fe, Ni and Mn has been observed.Further investigation is therefore necessary to unravel the complex redox mechanism involving Cr(VI) and other metal ions of environmental relevance.