Clément Trellu

Removal of hydrophobic organic pollutants from soil washing/flushing solutions: A critical review

The release of hydrophobic organoxenobiotics such as polycyclic aromatic hydrocarbons, petroleum hydrocarbons or polychlorobiphenyls results in long-term contamination of soils and groundwaters. This constitutes a common concern as these compounds have high potential toxicological impact. Therefore, the development of cost-effective processes with high pollutant removal efficiency is a major challenge for researchers and soil remediation companies. Soil washing (SW) and soil flushing (SF) processes enhanced by the use of extracting agents (surfactants, biosurfactants, cyclodextrins etc.) are conceivable and efficient approaches. However, this generates high strength effluents containing large amount of extracting agent. For the treatment of these SW/SF solutions, the goal is to remove target pollutants and to recover extracting agents for further SW/SF steps. Heterogeneous photocatalysis, technologies based on Fenton reaction chemistry (including homogeneous photocatalysis such as photo-Fenton), ozonation, electrochemical processes and biological treatments have been investigated. Main advantages and drawbacks as well as target pollutant removal mechanisms are reviewed and compared. Promising integrated treatments, particularly the use of a selective adsorption step of target pollutants and the combination of advanced oxidation processes with biological treatments, are also discussed.

Characteristics of PAH tar oil contaminated soils—Black particles, resins and implications for treatment strategies

Tar oil contamination is a major environmental concern due to health impacts of polycyclic aromatic hydrocarbons (PAH) and the difficulty of reaching acceptable remediation end-points. Six tar oil-contaminated soils with different industrial histories were compared to investigate contamination characteristics by black particles. Here we provide a simple method tested on 6 soils to visualize and identify large amounts of black particles (BP) as either solid aggregates of resinified and weathered tar oil or various wood/coke/coal-like materials derived from the contamination history. These materials contain 2-10 times higher PAH concentrations than the average soil and were dominantly found in the sand fraction containing 42-86% of the total PAH. The PAH contamination in the different granulometric fractions was directly proportional to the respective total organic carbon content, since the PAH were associated to the carbonaceous particulate materials. Significantly lower (bio)availability of PAH associated to these carbonaceous phases is widely recognized, thus limiting the efficiency of remediation techniques. We provide a conceptual model of the limited mass transfer of PAH from resinated tar oil phases to the water phase and emphasize the options to physically separate BP based on their lower bulk density and slower settling velocity.

Bio-electro-Fenton: A New Combined Process – Principles and Applications

Biological treatments show insufficient removal efficiency in the case of recalcitrant organic compounds. Therefore, the necessity of upgrading wastewater treatment plants (WWTPs) with advanced treatment steps is unequivocal. Advanced oxidation processes (AOPs) are the most effective technologies for the removal of a large range of organic pollutants from water due to the generation of strong oxidizing species like hydroxyl radicals (•OH). However, AOPs often involve high energy and/or reagent consumption and are considered as less cost-effective than biological processes. Hence, the combination of AOPs and biological treatments has been implemented aiming at maximizing efficient removal of recalcitrant organic pollutants while minimizing treatment costs. Among AOPs, electrochemical advanced oxidation processes (EAOPs) have been widely explored during coupled processes, since they possess remarkable advantages, such as high efficiencies, operability at mild conditions, economic feasibility, ease of automation, as well as eco-friendly character. The electro-Fenton process (EF) stands out as one of the most applied EAOPs and the present chapter is devoted to the advances and applications of EF process as a treatment step coupled with biological methods: the so-called bio-electro-Fenton (Bio-EF) process, which brings together the high oxidation power of EF and cost-effectiveness of biological methods.

Soil Remediation by Electro-Fenton Process

Soil remediation by electro-Fenton (EF) process has been recently proposed in literature. Being applied for solution treatment, EF is mainly combined with soil washing (SW)/soil flushing (SF) separation techniques to remove the organic pollutants. The main criteria influencing the combined process have been identified as (1) operating parameters (electrode materials, current density, and catalyst (Fe2+) concentration), (2) the matrix composition (nature and dose of extracting agent, pH, complexity of SW/SF solutions), and (3) the environmental impact (acute ecotoxicity and biodegradability of effluent as well as impact on soil microbial activity). The influence of these parameters on the SW/EF and SF/EF integrated processes has been reviewed. Energy consumption calculations have been finally considered as it constitutes the main source of operating cost in EF process.

Regeneration of Activated Carbon Fiber by Electro-Fenton Process

An electro-Fenton (EF) based technology using activated carbon (AC) fiber as cathode and BDD as anode has been investigated for both regeneration of AC and mineralization of organic pollutants. The large specific surface area and low intraparticle diffusion resistance of AC tissue resulted in high maximum adsorption capacity of phenol (PH) (3.7 mmol g-1) and fast adsorption kinetics. Spent AC tissue was subsequently used as the cathode during the EF process. After 6 h of treatment at 300 mA, 70% of PH was removed from the AC surface. The effectiveness of the process is ascribed to (i) direct oxidation of adsorbed PH by generated hydroxyl radicals, (ii) continuous shift of adsorption equilibrium due to oxidation of organic compounds in the bulk, and (iii) local pH change leading to electrostatic repulsive interactions. Moreover, 91% of PH removed from AC was completely mineralized, thus avoiding adsorption of degradation byproducts and accumulation of toxic compounds such as benzoquinone. Morphological and chemical characteristics of AC were not affected due to the effect of cathodic polarization protection. AC tissue was successfully reused during 10 cycles of adsorption/regeneration with regeneration efficiency ranging from 65 to 78%, in accordance with the amount of PH removed from the AC surface.