In Focus: Oxidation Editorial
Many industrial, domestic and natural activities collectively result in the production of vast quantities of hazardous wastes, of which wastewaters comprise about 90%. The treatment and safe disposal of hazardous organic waste material in an environmentally acceptable manner and at reasonable cost is a topic of great universal importance. There is little doubt that biological processes will continue to be employed as a baseline treatment process for most wastewaters as such processes seem to fulfill the above two requirements. Nonetheless, biological processes do not always give satisfactory results, especially when applied to the treatment of industrial wastewaters, because many organic substances produced by the chemical and related industries are inhibitory, toxic and/or resistant to biological treatment.
In view of these challenges, advances in chemical water and wastewater treatment have led to the development of the so-called advanced oxidation processes (AOPs) or technologies (AOTs), including UV irradiation, semiconductor photocatalysis, ozonation, electrochemical oxidation, Fenton's reagent, sonolysis and wet air oxidation. These processes can be broadly defined as aqueous phase oxidation methods based primarily on the intermediacy of hydroxyl radicals in the mechanisms leading to the destruction of the target compound and which can be used either separately or in various combinations. The role of chemical oxidation depends on the treatment objectives and may vary from partial decontamination to complete mineralization.
The first of the In Focus papers, by Ca Γ±izares et al., reports the use of electrochemical oxidation for the complete treatment of an olive mill effluent already pre-treated by Fenton oxidation. Electrolysis over a borondoped diamond anode, a highly efficient and stable electrode material, was capable of completely mineralizing the effluent with reasonable energy consumption through both direct anodic oxidation and indirect oxidation in the liquid bulk, with the latter being induced by the electrochemical generation of primary and secondary oxidants. This work indirectly emphasizes the need for process combination, with effective treatment of this heavily polluted agro-industrial effluent requiring oxidation by H 2 O 2 /Fe 2+ followed by electrochemical polishing.
In addition to industrial effluent treatment, several AOPs are successfully being employed for the removal of micropollutants from waters and wastewaters. Over the past few years, particular attention has been paid to the environmental monitoring and fate of the so-called endocrine disruptors, i.e. persistent natural or synthetic molecules with various applications in the pharmaceutical sector that often are only partially removed from water in domestic wastewater treatment plants and consequently enter the environment. The paper by Pauwels et al. deals with the electrochemical degradation of 17Ξ±-ethinylestradiol, a synthetic estrogen, over a titanium anode. The authors studied the effect of changing current intensity and salinity on estrogen removal from tap water as well as from a partially biologically treated hospital effluent, both of which were spiked with the estrogen at the low mg/L concentration level. Optimal conditions led to quantitative estrogen degradation in both water matrices and also achieved satisfactory disinfection for the medical effluent. The encouraging results reported by the authors may, to some extent at least, be counterbalanced by the relatively high cost associated with electrolysis and the likely formation of organochlorinated reaction by-products.
Environmental biotechnology can certainly offer simple and effective solutions for wastewater treatment. This is highlighted in the work of Kim and Nicell, who studied the laccase-catalyzed oxidation of triclosan in water. Like estrogens, triclosan, a commercially important antimicrobial agent used in personal care and pharmaceutical products, can enter the water cycle due to its incomplete removal from conventional treatment plants. The authors comprehensively investigated the key operating parameters affecting both substrate degradation and enzyme activity, namely solution pH, reaction temperature, enzyme concentration and the water matrix (e.g. presence of dissolved ions, enzyme mediators and additives) and determined optimal conditions for quantitative degradation; under these conditions, oxidation by-products were substantially less ecotoxic than triclosan itself.
Despite the large volume of research on water and wastewater treatment by advanced oxidation, substantially less work on soil remediation or air pollution abatement is being carried out. A paper published in the April issue (Vol. 81, No. 4, pp. 598-607, doi. 10.1002/jctb.1476) 'Treatment of PAH-contaminated soil by combination of Fenton's reaction and biodegradation', by Palmroth et al., is a step in this direction, describing the removal of polycyclic aromatic hydrocarbons (PAHs) from an iron-rich soil contaminated with creosote oil by means of H 2 O 2 oxidation. This Fenton-like process led to significant PAH degradation, avoiding the need for supplementary iron use or pH adjustment. More importantly, chemical pre-oxidation enhanced soil biodegradability despite the presence of residual, unreacted hydrogen peroxide in the soil. The concept of coupling chemical pre-oxidation with biological post-treatment is in principle beneficial as it can lead to increased overall treatment efficiencies compared to the efficiency of each individual stage.
In this issue we have brought three additional papers In Focus to demonstrate the environmental applications of oxidation processes. A sequence of unit processes, including chemical and biological oxidation, to treat such