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Abstract This work compares three strategies for the in-situ remediation of oxyfluorfen polluted clay soil, based on the combination of biological processes and electrokinetics (EK): (i) EK-biostimulation, in which the action of microorganisms contained in soil is promoted without the use of biological permeable barriers, (ii) EK-bioaugmentation with fixed culture of microorganisms (BB1), which enhanced the natural bioremediation by including a microbial fixed biofilm reactor as permeable biobarrier, and (iii) EK-bioaugmentation with suspended culture of microorganisms (BB2), which improves the natural remediation by including a mixture of soil and a microbial suspension as permeable biobarrier. To do this, two-week batch electro-bioremediation tests were carried out using bench scale set-ups. In each case, a constant electric field (1 V cm−1) was applied, and electrode polarity was reversed periodically, every 12 h. In addition, a microbial consortium acclimated to oxyfluorfen biodegradation was used. The best oxyfluorfen removal efficiency (15%) was obtained using the option (i). Permeable biobarriers allowed the introduction of microorganism and nutrients into the soil, and the biological activity reached the whole soil positions. However, the insertion of these biobarriers caused an excessive decrease in the electro-osmotic flow (approximately 80%) which, in turn reduced the mobilization of the pollutants. Despite of that, results show that permeable reactive biobarriers may result in a successful alternative for in-situ EK-bioaugmentation, although higher retention times would be needed to obtain higher removal efficiencies. Graphical abstract Image 1 Highlights • Electrobioremediation of oxyfluorfen polluted soil was studied in laboratory tests. • The influence of using or not biological barriers was evaluated. • Bio-barriers successfully included microorganisms but decreased electroosmotic flow. • The best result, without bio-barriers, obtained 15% removal efficiency in 14 days. [ABSTRACT FROM AUTHOR]

Abstract Oxyfluorfen is a non-polar herbicide that may cause severe soil pollution. The present work studies the possible improvement due to surfactant addition in the efficiency of electro-bioremediation of a clay soil polluted which such a non-polar, low-mobility pollutant. Two-week-long batch electro-bioremediation experiments were performed in a bench-scale device. Oxyfluorfen-polluted soil (20 mg kg−1) was inoculated with an acclimated microbial culture, and several experiments were performed using different surfactant concentrations in the electrode wells (0.0, 2.5, 5.0, 10.0 and 20.0 g L−1 of SDS, sodium dodecyl sulphate). Experiments were performed under 1.0 V cm−1 and electrode polarity reversal. It was observed that the electro-osmotic flow (EOF) increased with SDS concentration and that SDS was successfully distributed across the soil, probably improving the oxyfluorfen mobility. Additionally, microbiological activity was fully maintained during the experiments. Electro-bioremediation without SDS removed 14% of the oxyfluorfen, while under 2.5 g L−1 SDS, the efficiency increased to 22% because of an expected improvement in the contact between the different species in the soil. However, higher SDS concentrations (between 10.0 and 20.0 g L−1) caused a decrease in the oxyfluorfen removal efficiency, as SDS is an easily biodegradable compound and was preferably used as substrate by the microbial culture instead of oxyfluorfen. Additionally, the use of high concentrations of SDS was clearly inefficient, as high amounts of the surfactant were lost through the EOF, and even low amounts of oxyfluorfen were removed to the electrode wells, which means that ex situ treatment of the polluted water would be needed. Graphical abstract Unlabelled Image Highlights • Oxyfluorfen polluted clay soil was treated through electro-bioremediation. • Surfactant (SDS) addition improved electro-osmotic flow and oxyfluorfen mobility. • Low SDS dosage increased oxyfluorfen removal efficiencies up to 22% in 2 weeks. • High SDS dosage washed out surfactant and pollutant and biodegradation rate decreased. [ABSTRACT FROM AUTHOR]

This work studies the feasibility of the periodic polarity reversal strategy (PRS) in a combined electrokinetic-biological process for the remediation of clayey soil polluted with a herbicide. Five two-weeks duration electrobioremediation batch experiments were performed in a bench scale set-up using spiked clay soil polluted with oxyfluorfen (20 mg kg −1 ) under potentiostatic conditions applying an electric field between the electrodes of 1.0 V cm −1 (20.0 V) and using PRS with five frequencies ( f ) ranging from 0 to 6 d −1 . Additionally, two complementary reference tests were done: single bioremediation and single electrokinetic. The microbial consortium used was obtained from an oil refinery wastewater treatment plant and acclimated to oxyfluorfen degradation. Main soil conditions (temperature, pH, moisture and conductivity) were correctly controlled using PRS. On the contrary, the electroosmotic flow clearly decreased as f increased. The uniform soil microbial distribution at the end of the experiments indicated that the microbial activity remained in every parts of the soil after two weeks when applying PRS. Despite the adapted microbial culture was capable of degrade 100% of oxyfluorfen in water, the remediation efficiency in soil in a reference test, without the application of electric current, was negligible. However, under the low voltage gradients and polarity reversal, removal efficiencies between 5% and 15% were obtained, and it suggested that oxyfluorfen had difficulties to interact with the microbial culture or nutrients and that PRS promoted transport of species, which caused a positive influence on remediation. An optimal f value was observed between 2 and 3 d −1 . [ABSTRACT FROM AUTHOR]

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. Image 1 • Electrobioremediation of 2,4-D polluted soil was studied at bench scale set-up. • The influence of using or not biological barriers was evaluated. • EK-biostimulation allowed 100% pollutant removal in 10 d. • EK-bioaugmentation by bio-barriers allowed 75–85% pollutant removal in 10 d. • Biobarriers successfully included microorganisms but decreased electroosmotic flow. [ABSTRACT FROM AUTHOR]