This study investigated the remediation of clay soil polluted with low-solubility organics (diesel hydrocarbons) using a combined electrochemical-biological technology. Electrokinetic soil flushing was used, coupled with a biological permeable reactive barrier, which was placed into soil away from electrodes to prevent the negative effects of the acidic and basic fronts on the viability of microorganisms. Three two-week long batch experiments were performed in a lab-scale installation specifically designed to evaluate the remediation of polluted soil. The primary variable under study was the electric field applied to the soil (0.5, 1.0 and 1.5 V•cm-1), and the experimental procedure included daily polarity reversal of the electrodes and the addition of anionic surfactant in the electrode wells. The polarity reversal was observed to be a key strategy, which allowed adequate experimental conditions to be maintained in the soil (especially temperature and pH) for the success of the organic biodegradation process. The surfactant was evenly distributed across the soil, which helped the pollutant be transported. The biological activity was not limited to the barrier area but extended to the entire soil portion due to the microbial transport and growth far from the central barrier position. The voltage gradient did not have a strong influence on the measured experimental conditions (soil temperature, pH and moisture) but affected the electroosmotic flow. A higher diesel removal efficiency (36%) was observed when using the higher voltage gradient (1.5 V•cm-1) after two weeks, which demonstrates a promising performance of the studied technology for a future real in-situ application.

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Removal of diesel from spiked kaolin has been studied in the laboratory using coupled electrokinetic soil flushing (EKSF) and bioremediation through an innovative biological permeable reactive barriers (Bio-PRBs) positioned between electrode wells. The results show that this technology is efficient in the removal of pollutants and allows the soil to maintain the appropriate conditions for microorganism growth in terms of pH, temperature, and nutrients. At the same time, EKSF was demonstrated to be a very interesting technology for transporting pollutants, microorganisms and nutrients, although results indicate that careful management is necessary to avoid the depletion of nutrients, which are effectively transported by electro-migration. After two weeks of operation, 30% of pollutants are removed and energy consumption is under 70 kWh m(-3). Main fluxes (electroosmosis and evaporation) and changes in the most relevant parameters (nutrients, diesel, microorganisms, surfactants, moisture conductivity and pH) during treatment and in a complete post-study analysis are studied to give a comprehensive description of the most relevant processes occurring in the soil (pollutant transport and biodegradation).

In this work, it was carried out the study of the electro-bioremediation of soil polluted with diesel by means of combined electrokinetic soil flushing and biobarriers, using bench scale setups and fourteen days-long tests. Two different biobarriers were evaluated: one of them was developed in the laboratory, with a culture of diesel-degrading microorganisms supported on gravel particles (BB1); the other was obtained by mixing directly clay soil with activated sludge obtained in a municipal wastewater treatment plant (BB2). Biobarriers were placed in a central point of the soil section to be treated, to prevent the negative consequences of pH fronts on microorganisms viability. Potential difference of 0.5 and 1.0 V cm-1 were applied. For the promotion in the transport of the diesel to the biobarrier, a surfactant solution was used as electrolyte (flushing fluid). Results confirm that negative influence of the extreme pH fronts on the microbial viability can be prevented using this electro-bioremediation approach. In the same way, uniform diesel removal was obtained all over the soil by using the surfactant solution. After fourteen days of treatment, in the experiments using the BB1, 19.36% and 27.36% of the total amount of diesel present in the soil were removed applying 0.5 and 1.0 V cm-1 , respectively. In the experiments using BB2, 23.33% and 29.10% referred to the total amount of diesel were removed after the fourteen days tests at 0.5 and 1.0 V cm-1 , respectively, indicating that the non-specific barrier is slightly more efficient despite not containing an acclimated culture.

Sewage sludge used as soil organic amendment can positively affect soil physical, chemical, and biological properties. This organic material may be a solution for improving the low organic matter content of Mediterranean soils. However, the addition of sewage sludge to soils have not always shown positive effects, probably due to the particular characteristics of the residues concerned: high heavy metal or phytotoxic substance content, excess of labile organic matter, pathogenic microorganisms, etc. Composting process can be a useful tool to eliminate some of these problems. In the present work, a sewage sludge mixed with wood shaving as bulking agent has been composted. During this process, a decrease of organic matter fractions due to organic matter mineralization was observed. Fresh sewage sludge contained high numbers of pathogen microorganisms but when this material was submitted to composting process microorganisms decreased to suitable level. Organic compounds 957 such as pesticide residues can also be partially or totally degraded by microorganisms during composting process, prometrine and terbutrine being the most recalcitrant compounds. Composting process eliminated phytotoxic compounds contained in sewage sludge, permitting an adequate germination index.