Biostimulation versus bioaugmentation for the electro-bioremediation of 2,4-dichlorophenoxyacetic acid polluted soils

The aim of this work is to compare three biological strategies for the in situ remediation of a 2,4-dichlorophenoxyacetic acid (2,4-D) polluted clayey soil by coupling electrokinetics (EK) and bioremediation (technology named as electrobioremediation, EBR). The first option (i) is EK-biostimulation, in which the activity of microorganisms already present in soil is enhanced by EK phenomena. The second and third options are EK-bioaugmentation, which consist of addition of microorganisms to soil through the inclusion of permeable biological barriers: (ii) using a microbial fixed biofilm reactor as biobarrier (BB1), and (iii) using a mixture of clean soil and a microbial suspension as biobarrier (BB2). Thus, three batch experiments at bench scale were conducted under a constant electric field of 1 V cm^-1, and electrode polarity was periodically reversed every 12 h (2 d^-1). The duration of each test was 10 days. Two additional tests using only biodegradation or only EK were performed as auxiliary reference tests. A microbial consortium acclimated to 2,4-D biodegradation was employed. Results showed that EK-biostimulation strategy offered the best pollutant removal efficiency (reaching up almost 100%) while biobarriers offered pollutant removal rates between 75 and 85%. Permeable biobarriers allowed the introduction of microorganism but caused a decrease in the electro-osmotic flow which, in turn, reduced the mobilization and contact between microorganisms and pollutants. These results can contribute to the knowledge and understanding of electrobioremediation of polluted soil and to the feasibility of delivering microorganism to the soil by using biobarriers. Despite biostimulation was found to be the best option, results show that permeable reactive biobarriers may result in a successful alternative for in-situ EK-bioaugmentation when acclimated microbial population is not already present in soil.