Electro-bioremediation of diesel polluted soils

Resumen

Soil pollution is a topic of the major significance all around the World. This environmental problem has increased in the last two centuries, because the Industrial Revolution marked the starting point for the development and intensification of the different industrial activities. Many of them involves the manipulation of multitude substances, hazardous for the environment and, consequently, also for the human health. Among the most widely extended pollutants, heavy metals, fossil fuels and pesticides should be highlighted. Fuel and its derivates are the pollutants most typically present in soil. Their presence is due to accidental spills that usually occur during the transport and storage tasks. Their degradation rates are very low and have toxic effect on the environment. Soil remediation techniques are based on the destruction, extraction or immobilization of the pollutants. Bioremediation is one of the most frequently used destruction technologies. These processes use microbial population previously acclimated for the biological degradation of the pollutants. These microorganisms can be naturally present in the soil or they can be specifically added for the treatment. Successful bioremediation requires the simultaneous coexistence of microorganisms, contaminants, electron acceptors, and essential nutrients. In the ex-situ bioremediation treatments, excavation and mixing tasks are required to make easier the contact among all these species. However, the in situ treatments have important limitations for the transport of these elements, being these limitations of special importance in fine-grained soils with low hydraulic conductivity. On the other hand, the electrokinetic remediation technology is a physic-chemical treatment based on the application of low current intensity electric fields among electrodes adequately distributed in the soil. As a consequence of the electric field applied different physical and chemical processes occur (electrophoresis, electro-osmosis, electrolysis and electrokinetic heating) that can lead on the separation and extraction of the pollutants contained in the soil. Electrokinetic enhanced bioremediation or electro-bioremediation treatment, in which the present Ph.D. is focused, is the technology that aims to couple bioremediation with electrochemical remediation. Its main objective is to take advantage of the electrokinetic transport processes to overcome the most important limitation of the in situ biological processes: the low possibility of interactions among the different species taking part in the biological degradation process, caused by their limited mobility throughout the soil matrix. The E3L-TEQUIMA research group of the U.C.L.M., in which this Ph.D. Thesis has been carried out, has been working in the development of soil remediation technologies since 2007. Two previous doctoral research works about soil remediation technologies have been carried out focused on the electrokinetic treatment of hydrophobic organics-polluted low permeability soils and on the bioremediation process of diesel-polluted soils. This Ph.D. Thesis, focused on the study of the combination of the electrokinetic and biological remediation processes, can be understood as the continuation of this research. In this context, the principal goal of this Ph.D. Thesis was the study of the electro-bioremediation technology for the treatment of polluted soils and the application of the technology for the remediation of diesel polluted soils. As a first hypothesis, it was considered that, by the application of the electrokinetic processes that occur in the soil as a consequence of the application of low current intensity electric fields, an increase in the interactions among the different elements taking part in the bioremediation process may occur, and it should have a positive effect on the rate of the pollutant biodegradation. This combination of technologies was intended to be carried out using different strategies (direct electro-bioremediation, electro-bioremediation with periodic polarity reversal or with biobarriers), trying to clarify the effect of several important parameters on the efficiency of the treatments (electric field and soil texture). Moreover, it was also consider as necessary to evaluate the electrokinetic transport throughout the soil of the different species involved in the biological process: electron acceptors, nutrients, microorganisms, pollutants and surfactants required for the mobilization of nonpolar pollutants. The first set of experiments was carried out in order to study the electrokinetic mobility of nitrates, oxygen and microorganisms in soils. The influence of two different variables was studied: soil texture (sandy, silty or clay) and electric field applied. Two different experimental setups were used: one bench scale setup was used to carry out the tests related to the transport of nutrients. In this case, it was studied the variation of the oxygen and nitrates concentration in different positions of the soil; the other setup was a laboratory scale plant used to carry out the experiments about the electrokinetic transport of microorganisms. In this case, it was studied the transport of microorganisms throughout a small soil column which was placed between the anodic and cathodic compartments. In the anodic compartment a highly concentrated suspension of microorganisms was placed and in the cathodic compartment a sterile electrolyte solution was added (bicarbonate buffer). Graphite electrodes were used in these experiments. Regarding the experiments about the study of the electrokinetic transport of oxygen, it was concluded that the permeability of the soil was the most influential parameter. The transport was favored in high permeability soils, although the efficiency in the dosing of oxygen was below the 1% of the oxygen generated on the anodic surface. Anyhow, although the electrokinetic oxygen transport rates were very low, values of dissolved oxygen were high enough for the success of in situ bioremediation aerobic treatments, in particular in the sandy and silty-textured soils. In the experiments for the evaluation of the nitrates electromigration, a decrease in the nitrate concentration available for the biological process was detected in all the tests carried out. When the sandy soil was used, contrary to what occurred in the experiments using the clay soil, strong concentration gradients were not detected. Using the real silty soil, low nitrate concentrations were always detected and this was attributed to the occurrence of adsorption or ionic exchange processes. Concerning the influence of the electric field, in general, the electromigration process caused higher removal of nitrates from the soil when higher electric fields were applied. It was then concluded that the influence of the electrokinetic mass transport processes must be accounted in order to avoid the depletion of nitrates from the soil on remediation tests. In the experiments carried out for the study of the electrokinetic transport of microorganisms, it was determined that their superficial charge was positive. Because of this, the electrokinetic transport of the microbial cells occurs toward the cathode due both to the electrophoresis and electro-osmotic dragging processes. The size of the soil particles has a determinant influence on the transport rate. Values obtained (expressed as Log10 (C.F.U. mL-1) h-1) were 1.53, 0.38 and 5.85 for the experiments using the silty soil at 0.0, 2.0 and 4.0 V cm-1, respectively, 0.00, 0.50 and 0.54 for the clay soil at 0.0, 2.0 and 4.0 V cm-1, respectively, and, finally, for the sandy soil, 12.81, 13.18 and 14.51 at 0.0, 2.0 and 4.0 V cm-1, respectively. It was then concluded that the application of electric fields for the transport of microorganisms was only recommended for small particles soils because for higher pore size is not interesting the application of electric field. Another important point was the assessment of the effect of the electric field on the viability of the diesel-degrading microorganisms, for which different laboratory scale experiments were carried out. The influence of the electric field (within the range 0.0 - 4.0 V cm-1) on the viability of these microorganisms was tested by using two different setups. The first one has two electrodic compartments connected by a small soil column. In the anodic compartment a highly concentrated microbial suspension was added and in the cathode a sterile bicarbonate buffer solution was added as electrolyte. In this case, graphite rods were used as anode and cathode. The second installation was a batch bioreactor with biomass suspended in nutrient liquid medium in which the electric field was applied using inert titanium plates as electrodes. The most important conclusion drawn from the results obtained was that the decay in the concentration of microorganisms present in a liquid culture media exposed to an electric field was related to the pH changes produced in the surroundings of the active electrodes, as a consequence of the water oxidation and reduction electrochemical reactions. An increase in the electric field applied was observed to lead to increases in the value of the endogenous decay constant. By avoiding great pH variations with inert electrodes, short periods of exposure at low electric fields were found to improve the biological assimilation of the organic pollutant, probably due to an increase in the biodegradability of the lowest biodegradable fractions of the diesel. However, the application of electric field has not advantages for the degradation of highly degradable organic substrates. Once all the previous aspects were studied, it was carried out the study of the electro-bioremediation treatment for the remediation of diesel pollution in soils, using bench scale setups and fourteen days-long tests. Considering results obtained in the previous stages of the research, the most important parameter studied in these experiments was the electric field, which was modified in the range 0.0 - 1.5 V cm-1. Clay soil was the only type of soil used, because the application of the electrokinetic treatment does not improve significantly the transport of species throughout high permeability soils and, in addition, it caused an important increase in the operation costs. Firstly, the direct electro-bioremediation process was studied. A synthetic clay soil was used, that previously was prepared in the laboratory in order to fix its initial experimental conditions (diesel concentration of 10 mg gSoil-1, moisture of 40% and microbial concentration of 106 C.F.U. gSoil-1). Following, soil was placed in the experimental setup and electric fields (0.0 - 1.5 V cm-1) were applied using graphite electrodes. A bicarbonate buffer solution was used as electrolyte (flushing fluid) trying to help in the regulation of pH. At the end of the fourteen-days tests, soil underwent a post mortem analysis. In these experiments, it was concluded that pH has a determinant influence on the efficiency of the process. The bicarbonate buffer solution was not adequate for the correct regulation of the pH of the soil because at the end of the tests this parameter showed extreme values in a very important part of the soil. Moreover, it contributed to produce a significant soil heating. On the other hand, the application of high electric fields resulted in the transport of high volumes of water by electro-osmotic flow and this transport contributed to the exhaustion of nutrients. Consequently, microbial culture was not able to survive and the biological process did not work as initially expected. Diesel removal obtained was attributed to the volatilization of the pollutant, rather than to the biological degradation process. After fourteen days 1,168, 1,766 and 2,501 mg of diesel were removed (corresponding to 4.24%, 6.70% and 9.52% of the total amount of diesel present in the soil) after applying the direct electro-bioremediation technique at 0.5, 1.0 and 1.5 V cm-1, respectively. After that, the study of the electro-bioremediation treatment with the periodic polarity reversal strategy was carried out. The main goal was to avoid extreme pH variations and to control soil temperature, being these the most important adverse effects observed in the direct electro-bioremediation treatment. In this case, the same experimental procedure that in the direct electro-bioremediation tests was used, but in this case buffer bicarbonate was not used as electrolyte and daily the polarity of the electric field applied was manually changed. In comparing results obtained, it was concluded that the periodic change of the polarity of the electric field correctly regulated the changes in the pH produced on the electrodes. In these experiments, the highest pollutant removal efficiency was obtained in the experiment carried out at the highest value of electric field (1.5 V cm-1), but in this case it was also obtained the highest value of electrical consumption. 6,364, 5,389 and 9,742 mg of diesel were removed (corresponding to 22.35%, 19.18% and 35.40% of the total amount of diesel present in the treated soil) after applying the electro-bioremediation technique with periodic polarity reversal at 0.5, 1.0 and 1.5 V cm-1, respectively, during fourteen days. Following, electro-bioremediation experiments with biobarriers were carried out. The main goal was to evaluate the possibility of combining the biobarrier technology with the electrokinetic processes for the transport of the pollutants. In these experiments two different biobarriers were used: one of them was developed in the laboratory, with the specific diesel-degrading microorganisms supported on gravel particles (B.B.1); the other biobarrier was obtained by mixing directly clean clay soil with activated sludge raw material (B.B.2). The preparation of the soil was the same that in the previous experiments, but in this case microorganisms were not added to the soil. Biobarrier was placed in the middle point of the soil column to be treated, that was the area with the lowest influence of the extreme pH conditions. In these experiments, electric fields of 0.5 and 1.0 V cm-1 were applied. Moreover, for the transport of the diesel to the degradation area of the biobarrier a surfactant solution was used as electrolyte. Results confirm that the negative influence of the extreme pH fronts on the microbial viability was avoided by placing the biobarrier in the middle point of the soil column. In the same way, by using a surfactant solution an uniform diesel removal was obtained all over the soil. Results also show that when B.B.2 was used, a much better pollutant removal efficiency was obtained. After fourteen days of treatment, in the experiments using the B.B.1, 4,850 and 7,023 mg of diesel were removed (corresponding to 19.36% and 27.36% of the total amount of diesel present in the soil) applying 0.5 and 1.0 V cm-1, respectively. However, in the experiments using B.B.2, 5,613 and 7,461 mg of diesel (23.33% and 29.10% referred to the total amount) in the experiments at 0.5 and 1.0 V cm-1, respectively, were obtained. Finally, in the development of the B.B.2 a previous acclimation stage was not necessary which becomes also an advantage that must be added to the highest efficiency obtained in the experiments using this biobarrier, with the consequent advantageous economic repercussion. Taking into consideration the previous results, a technology that combines the advantages of the two previously studied techniques, i.e., the treatment of a soil with (a) periodic change of the polarity of the electric field and (b) using a biobarrier was studied. The experimental procedure was the same that the previously mentioned for the electro-bioremediation tests with biobarriers and applying the daily manual change of the polarity of the electric field applied. In these experiments only the B.B.2 was used, taking into account that the best results were obtained with this type of biobarrier, and electric fields in the range 0.5 - 1.5 V cm-1 were applied. In these experiments low current densities were obtained and, consequently, pH did not undergo extreme variations. In the same way, electro-osmosis process did not transport high volumes of water that could result in an important removal of water soluble species from the system. In this case, the use of a surfactant solution as electrolyte caused an uniform removal of pollutant throughout all the soil column treated. Comparing all results obtained by applying the different electro-bioremediation technologies, this last technique offered the best results applying 1.5 V cm-1, obtaining an electrical consumption below 350 Wh kgSoil-1. In this way, in these experiments 1,483, 2,261 and 9,244 mg of diesel were removed from the soil after applying 0.5, 1.0 and 1.5 V cm-1, respectively (5.94%, 8.96% and 36.61% of the total amount of diesel). Finally, it was carried out the study of the application of the electro-bioremediation technology for the treatment of a real soil. In this case the same bench scale setup was used that in the previous experiments and also a pilot plant installation, with a volume of 0.5 m3, approximately. Two experiments were carried out working with both experimental scales, one of them in galvanostatic conditions (0.76 A m-2) and the other one in potentiostatic conditions (1.5 V cm-1). In this scale-up stage, a S.W.O.T. analysis was carried out for confirming the selection of the best electro-bioremediation technique taking into consideration results obtained in the previous stages of the research. In this case, the electro-bioremediation technique with biobarriers and periodic polarity reversal was selected as it was deduced from the previous research carried out. The experimental procedure followed in this stage was the same that in the previous experiments, but in this case a real clay soil was used. In these experiments, although pH, temperature, moisture and nutrients concentrations (that are the most influential variables on the bioremediation process) were perfectly controlled, diesel removal efficiency obtained was below the value obtained in the single bioremediation test using the synthetic clay soil (below 10%). The most important conclusion drawn for these experiments was that results obtained in the treatment of a synthetic soil were different to what were obtained in the treatment of a real soil. In the same way, the use of higher size experimental installations has as a consequence the occurrence of higher size operative problems. Using higher scale it was necessary to apply higher electrical power that caused the formation of fissures, important soil heating and loss of moisture in some areas of the soil, and these processes caused the obtaining of lower efficiency values than the obtained in the bench scale tests. On the other hand, results obtained in the potentiostatic and in the galvanostatic experiments were very similar. Finally, results obtained about electrical consumption referred to the amount of soil treated were lower using the pilot plant scale installation. In the experiments carried out using the bench scale, electrical consumption of 492 and 515 Wh kgSoil-1 in the potentiostatic and galvanostatic tests was obtained, respectively. However, in the pilot plant scale tests, 299 and 255 Wh kgSoil-1 in the potentiostatic and galvanostatic experiments was obtained, respectively. In view of the results obtained, it can be concluded that the electro-bioremediation technology can be applied for the treatment of diesel polluted clay soils, but some aspects need to be improved. As it was obtained in this research, the efficiency of the treatment is very influenced by the correct control of the pH, temperature and availability of nutrients, and also by the size of the soil particles, due to the great complexity of the treatment and to the very different processes involved. At the beginning of this Ph.D. research work, according to most of the references found in the scientific literature (most of them corresponding with laboratory scale studies), the electro-bioremediation technology was presented as an alternative almost about to be applied at the full scale. Results presented in the present Ph.D. opposite this premise and show that the electro-bioremediation is, at the same time, a very promising and complex technology, which still requires an in-depth research effort, both at scientific and at technological level to be fully understood. In this context, this Ph.D. can be considered as a first contribution to the development of the electro-bioremediation and establishes new research topics related to this interesting and innovative environmental technology.