Improper management of wood impregnation chemicals and treated wood has led to soil contamination at many wood treatment sites, particularly with toxic substances like creosote oil and chromated copper arsenate (CCA). The simultaneous presence of these pollutants complicates the choice of soil remediation technologies, especially if they are to be applied in situ. In this laboratory study, we attempted to immobilise arsenic (As) and simultaneously degrade polycyclic aromatic hydrocarbons (PAHs) (constituents of creosote oil) by applying a modified electrochemical oxidation method. The supply of iron (Fe) amendments in contaminated soil was done using corroding Fe electrodes as an Fe source and applying an alternating polarity electrical current. Soil with a large fraction of organic matter (25%) and containing 505 mg kg−1 As and 5160 mg kg−1 16-PAHs was placed in Plexiglas cells equipped with porewater samplers and an iron electrode pair connected to a power supply unit. The porewater and percolating solution were periodically sampled and analysed over an 8-week period. The modified electrochemical soil treatment led to a decrease in the total concentration of 16-PAHs in soil by 56–68%. The amount of poorly crystalline Fe oxides in the soil substantially increased, especially close to the electrodes, enabling 76–89% of As to be bound to this most reactive Fe fraction. Nevertheless, over 10% of soil As remained in the most soluble and available fraction (exchangeable), most likely due to the decline in soil redox potential over time. This study suggests that electrochemical oxidation of organic soil with mixed contaminants could be used for in situ soil remediation but needs further improvement to achieve more efficient As immobilisation.

Although landfilling is environmentally and economically unsustainable, it is the dominant soil remediation method in EU member states. This paper describes part of a study on mixed contaminants that investigated the stabilisation of arsenic (As) in contaminated soil in an outdoor box experiment with electrokinetic treatment (EK). The experiment was conducted in two 1 m3 boxes, each containing a 20 cm bottom layer of sand, overlaid with 20 cm of peat. In EK, a pulsating, low-voltage current was applied with the intention of corroding the zerovalent iron (Fe) electrodes, migrating ionic Fe species, and forming secondary iron minerals, thereby immobilizing As. Porewater samples were collected over two seasons to determine whether the treatment decreased the concentration of dissolved As. Sequential extraction was performed on the soil samples to determine whether the fraction of Fe-bound As increased. Reed canary grass was planted in one of the boxes during the second season and analysed for As uptake. The results showed that the treatment decreased the porewater As concentration in sand by 50–54 %, while the concentration of Fe increased. The sequential extraction of sand showed that the fraction of As bound to poorly crystalline Fe oxides increased during this time. This treatment effect was less visible in the peat. Moreover, the exchangeable As fraction increased in both peat and sand, most likely because of the decrease in redox potential at the end of the experiment. The plants grown in treated soil accumulated less As than those grown in untreated soil, indicating that the phytoavailable As fraction decreased. This study showed that EK remediation can be a suitable in situ remediation technique, mostly in sand. Future research should focus on redox control to further optimise EK remediation and ensure long-term As stability in treated soils.

The wood preservation industry has contaminated numerous sites with polycyclic aromatic hydrocarbons (PAH). The aim of this study was to investigate the use of electrochemical oxidation (EO), without chemical additives, for in situ degradation of PAHs in soil co-contaminated with arsenic. Two 1 m3 boxes, each containing a 20 cm layer of contaminated sand overlaid with 20 cm of peat, were equipped with iron electrodes and placed outdoors. EO was applied using a pulsating direct current with alternating polarity, and its impact on PAH concentrations in soil and soil solution, as well as on the associated microbial community was assessed. Soil solution was sampled from the boxes over two seasons and analysed for PAH16 concentrations, showing an average decrease of 82 % by the end of the second season. This reduction was mainly observed in the medium and high molecular weight PAH fractions, suggesting that EO can effectively degrade more recalcitrant PAH compounds. The least reduction was seen in the low molecular weight PAH16, likely due to the replenishment from PAHs sorbed to the soil. Soil samples were taken from 12 different locations within the boxes, showing that PAH16 concentrations significantly decreased in 10 out of 12 sampling points, with a greater average reduction in sand (84 % decrease) compared to peat (69 % decrease). Microbial analyses of the soil samples revealed no significant changes in DNA concentrations across all taxa over time, with Gammaproteobacteria remaining the most abundant microorganisms in all samples, suggesting its ability to persist complex contamination.