Wet oxidation of coke oven wastewaters and landfill leachates

Abstract

In the present work, it is analysed the use of wet oxidation as a technique for the treatment of industrial effluents, which contain compounds characterised by their toxicity and low biodegradability. Specifically, the work has focused on coke oven wastewaters (real and synthetic) and landfill leachates, paying special attention to the effect of the operating conditions. Besides, the main reaction products and intermediates have been identified. Based on such results, the kinetic models were obtained and reaction pathways were also proposed. As a starting point for studying the wet oxidation of coke oven effluents, cyanide and thiocyanate were selected as model cyanide compounds. Hence, individual degradations of each pollutant (cyanide or thiocyanate) and mixtures of both pollutants (cyanide and thiocyanate) were performed with the aim of knowing the existence of synergistic and inhibitory phenomena between these two compounds. The processes carried out with real and synthetic coke oven effluents, containing only thiocyanate or mixtures of cyanide and thiocyanate, have been significantly affected by the operating pressure and temperature. Nevertheless, the pressure had no effect in the case of the synthetic coke oven effluents which contained only cyanide. This suggested the existence of that hydrolysis mechanism for this contaminant. A comparison between the wet oxidation of thiocyanate in real and synthetic coke effluents showed a positive effect of the matrix of the real coke oven effluent on the oxidation of such pollutant. Additionally, the use of transition metal salts as homogeneous catalyst was studied in order to improve the wet oxidation of thiocyanate. Iron(II) was selected as catalyst due to its low toxicity and cost. This metal showed a high effectiveness for thiocyanate wet oxidation in acid medium. Besides, a reaction mechanism was proposed for the degradation of thiocyanate in presence of iron and the kinetic model was also determined. Regarding landfill leachates, various types of real leachates with different characteristics depending on their age and previous treatment were employed during the wet oxidation of these effluents. The evolution of mineralization (COD), average oxidation state (AOS), biodegradability (BOD5/COD), toxicity, pH and color reduction was analysed. It was observed that the operating pressure and temperature markedly enhanced the degradation and mineralization rate of these leachates. On the other hand, the biodegradability, toxicity or colour of the treated effluent was not affected by the pressure. It was observed that in this case pH was a key parameter, achieving the best results when the landfill leachate was treated without any modification of its initial pH. It was also studied the effect of the composition of the leachate, mainly humic and fulvic acids. In order to improve the wet oxidation process the use of iron oxides and hydroxides as heterogeneous catalysts was tested. Besides, the influence of the preparation conditions on the morphological and textural properties of iron catalyst was analysed as well as the effect on their catalytic activity. Such catalysts were moderately effective for the degradation of landfill leachates.