The ozonation kinetics of oxalic acid in water in the presence of an activated carbon has been investigated at acid pH. The presence of the activated carbon significantly enhances the degradation rate of oxalic acid if compared to single ozonation and single adsorption. According to total organic carbon measurements, nearly complete mineralization of oxalic acid can be achieved depending on the experimental conditions. The presence of tert-butyl alcohol, which scarcely adsorbs on the carbon surface at the conditions investigated, led to a significant reduction of the oxalic acid removal rate. Consequently, experimental results suggest that the reaction proceeds in the water phase between oxalic acid and oxidant species, likely hydroxyl radicals, coming from the ozone decomposition on the carbon surface. The proposed mechanism yielded a first-order kinetics with respect to ozone, close to the 0.8 order experimentally observed. Also, the energy of activation was found to be approximately 15 kcal mol-1.

Oxalic acid in water has been treated with ozone in the presence of a Co(II) salt at acidic pH. The influence of different variables, including the initial oxalic acid, cobalt(II), and ozone gas concentrations and the temperature, has been investigated. The experimental stoichiometric ratio varied between 0.7 and 1.4 mol of ozone consumed per mole of oxalic acid consumed, while the ozone efficiency reached values as high as 25%. At any conditions applied, nearly total mineralization was achieved. The process developed between the slow and moderate kinetic regimes of ozone absorption. Experiments in the slow kinetic regime allowed for the experimental determination of the reaction kinetics, which was found to be first order with respect to oxalic acid and catalyst. For the case of ozone, nearly one-half-order kinetics was found. A mechanism that involves the participation of cobalt-oxalate complexes is also discussed.

Ozone and a Co 3 O 4 /Al 2 O 3 catalyst have been used to remove oxalic acid from water at acidic pH. The influence of different variables, including the initial oxalic acid and ozone gas concentrations, the catalyst mass, and the temperature, has been investigated under conditions of chemical control. The ozone efficiency was found to reach values of up to 40%, which are higher than those found in homogeneous catalysis (see also the first part of this work, which immediately precedes this paper in this issue), and the mean stoichiometry was determined to be 0.8 mol of ozone consumed per mole of oxalic acid consumed. Metal leaching was also followed to check the stability of the catalyst. As a result, ozonation of oxalic acid was found to be due to both heterogeneous and homogeneous catalytic ozonation. The average contribution of heterogeneous catalytic ozonation for the removal of oxalic acid was found to be 75%. A kinetic study taking into account both catalytic contributions is also presented. The experimental heterogeneous catalysis was found to be first-, one-half-, and zeroth-order with respect to catalyst, dissolved ozone, and oxalic acid, respectively. A mechanism is also proposed to account for the kinetic results.

The adsorption of the herbicide 4-chloro-2-methylphenoxyacetic acid (CAS 94-74-6) has been studied using four commercial activated carbons (Norit 0.8, Aquacarb 207C, Aquacarb 208A, and Aquacarb 208EA). Adsorption equilibrium isotherms were obtained in the 293-358 K temperature range. The trend of adsorption for MCPA onto the four activated carbon was in the order Norit 0.8 > Aquacarb 207C > Aquacarb 208A > Aquacarb 208EA. Among various adsorption isotherm models, the Freundlich equation best fit the experimental data. Experiments conducted at different temperatures allowed for the calculation of the isosteric heat of adsorption, revealing that the adsorption process was exothermic for three of the four adsorbents studied. The distribution of the herbicide into the pores of activated carbon Norit 0.8 was studied by considering the Dubinin-Radushkevich isotherm. Also, the adsorption kinetics were assessed by means of a simplistic mechanism based on the shrinking-core mass-transfer model. Finally, as a preliminary step in the study of the potential use of wet air oxidation (WAO) for the regeneration of exhausted activated carbon, the reversibility of the adsorption process was tested. Only partial desorption of MCPA was achieved under WAO conditions, which involves a likely double route of activated carbon regeneration, i.e., liquid and surface contaminant oxidation.