Total removal of oxalic acid via synergistic parameter interaction in montmorillonite catalyzed ozonation Dariush Shahidi, Rene ´ Roy, Abdelkrim Azzouz * Nanoqam, Department of Chemistry, University of Quebec at Montreal, Succursale centre-ville, Montreal (QC), P.O Box 8888, Canada H3C3P8 Introduction Chemical, petrochemical, pharmaceutical and food industries, along with agriculture and forestry are major sources of organic pollutant in waste-waters [1–3]. Primary treatments are not sufficiently effective because of residual traces of pollutants [4,5]. Complete decomposition of organic pollutants requires powerful oxidative treatments to avoid the persistent presence of refractory short chain intermediates like oxalic acid (OA). The latter is known to display high chemical stability even in the so-called Advanced Oxidation Processes (AOPs). The main issue to be addressed is that the formation of OA is often the final stage in most oxidation methods [6–9]. Besides, OA is a hazardous compound [10], and shows even more toxicity than most parent pollutants [11]. Notwithstanding that some plants like rhubarb and sorrel contain OA, the latter is recognized as being quite poisonous to the kidneys (nephrotoxic), more particularly when present in drinking water. OA precipitation in the presence of calcium produces kidney stones, which contain calcium oxalate in a proportion of 80%. OA may even be fatal around a median lethal dose (LD50) of 375–380 mg/kg body weight for humans [12–16]. In human and animal metabolism, OA may also form from excessive use of vitamin C, which is readily oxidized owing to its powerful antioxidizing capacity. This often results in gut lining irritation, rheumatoid arthritis, certain forms of chronic vulvar pains (vulvodynia) and other diseases. However, being quite harmless in low doses, OA is rather regarded as being a precise indicator of incomplete oxidation of organic matter. For this reason, total mineralization of organic pollutants into carbon dioxide (CO 2 ) without generating any traces of OA or any other short chain compounds has become an essential requirement for effective oxidative water treatments. So far, attempts to OA oxidative mineralization into CO 2 have received fairly good attention in the published literature, and many advances have been made in this regard [9]. Photocatalysis and photoelectrocatalysis have long been considered promising approaches, but the incomplete decomposition of organic pollu- tants and high operating costs were still major obstacles for commercial applications [1,3]. AOPs result from continuous improvements of conventional oxidative methods [2]. One of these, namely ozonation, more particularly at elevated pH, has focused interest. Nonetheless, no prospects can be envisaged as long as the issues related to the low solubility of ozone in the liquid media and its weak reactivity as compared to radical species still remain to be addressed. The use of metal cations produce higher effectiveness as compared to the non-catalytic routes, but water contamination by metals turned out to be a major drawback [17– 19]. Significant improvements were registered in the presence of Journal of Environmental Chemical Engineering 2 (2014) 20–30 A R T I C L E I N F O Article history: Received 2 September 2013 Accepted 21 November 2013 Keywords: Heterogeneous ozonation Montmorillonite Oxalic acid Adsorption Parameter interactions A B S T R A C T Oxalic acid ozonation in water at room temperature produced decomposition yields of 50–100% in the presence of Na + , Fe 2+ , Fe 3+ , Co 2+ , Ni 2+ and Cu 2+ ion-exchanged montmorillonite as catalysts. Among these, Co(II)Mt and Fe(II)Mt displayed the highest performances. The appreciable enhancement of oxalic acid removal as compared to homogenous ozonation and the ozonation yield decay upon heating suggest a significant contribution of adsorption. The initial pH of the reaction mixture, ozonation time and catalyst concentration showed strong influence. A 3 k factorial design with 27 ozonation attempts for each catalyst produced total removal of oxalic acid after 15 min at pH 2.87 with 1.88 g/L of Co(II)Mt, and pH 2.88 with 1.91 g/L of Fe(II)Mt. Because oxalic acid is well known to be a quite refractory intermediate in most oxidation attempts, these results suggest that total mineralization of any hazardous organic pollutants from aqueous media is possible under similar optimum conditions. Higher pH and catalyst amounts were detrimental, presumably due clay compaction, which is supposed to reduce the cation mobility and adsorption contribution. The initial pH was found to act also via synergistic interaction with the other parameters, presumably by enhancing clay exfoliation and adsorption of oxalate anion and ozone. ß 2013 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +1 514 987 4119; fax: +1 514 987 4054. E-mail address: azzouz.a@uqam.ca (A. Azzouz). Contents lists available at ScienceDirect Journal of Environmental Chemical Engineering jou r n al h o mep ag e: w ww .elsevier .co m /loc ate/jec e 2213-3437/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.11.020