EVALUATION OF THE EFFECT OF ALTERNATING POLARITY EXCHANGE ON CRUDE OIL REMOVAL FROM SOIL BY BIOELECTROKINETIC METHOD

Removal of petroleum hydrocarbons from contaminated soil using electrokinetic method or biological processes has been considered in recent century. The most limiting factors in the electrokinetic process are extreme changes in pH around the electrodes and non-polarity of some pollutants. On the other hand, the key factor of biological treatment is simultaneous presence of microorganisms, pollutants (carbon source of microorganisms), electron acceptors, and essential nutrients for microorganisms' growth. But in fine-grained soils with low permeability, it is difficult to uniformly distribute bacteria, electron acceptors and nutrients, or making available pollutants for microorganisms. To solve these problems, bioelectrokinetic method is used to eliminate the limitations of both biological and electrokinetic processes. But the main limitation of this hybrid method is reduction of decomposition rate due to pH extreme changes caused by electrolysis reactions in electrodes. This study investigates the influence of alternating polarity exchange on soil conditions, bacteria pollution and thus removal efficiency of crude oil from kaolin by bioelectrokinetic method in the presence of Pseudomonas Putida strain. Effects of initial concentration of crude oil and voltage gradient on soil treatment were also evaluated. In this study each test was conducted in cylindrical cells made of Plexiglas with the length and diameter equal to 55 and 5 cm respectively, for 35 days. Based on the results, in experiments without polarity exchange, due to extreme changes in soil pH and therefore effect on the electroosmosis flow and bacteria activity, overall crude oil degradation rate was decreased. However, applying alternative polarity exchange, soil pH was retained in the neutral range. Then, better activity of bacteria caused the treatment efficiency improvement. According to the results, during a 35-day process, switching polarity in 1-hour periods increased pollutants removal efficiency from soil contaminated with 6000 mg/kg of crude oil up to 20 percent. In this process the soil moisture content variations were controlled well, and the microbial population growth was observed. In addition, increasing the initial pollutant concentration and also applied voltage gradient led to the improvement of the decomposition and thus enhancement of contaminant removal efficiency.