Copper-pillared clays (Cu-PILC) for agro-food wastewater purification with H2O2

Copper-pillared clays (Cu-PILC) are effective and stable catalysts for the wet hydrogen peroxide catalytic oxidation (WHPCO) of waste in water. They are studied both in the conversion of model molecules (p-coumaric acid, p-hydroxybenzoic acid) and in the treatment of various real wastewaters from agro-food production: (i) deriving from citrus juice production, (ii) extracted concentrated polyphenolics fraction from olive oil milling (OMW) and (iii) OMW derived from three different sources. In the latter cases, tests were made both in a lab-scale reactor and in a larger volume (about 10 l) reactor. The results showed that Cu-PILC layered materials might be used to treat real wastewater from agro-food production, and not only simple model chemicals as typically made in the literature. In all cases, using a semi-batch slurry-type reactor with a continuous feed of H2O2, the behaviour both in TOC (total organic carbon) and in polyphenols abatement may be described using pseudo-first-order reaction rates. Using real wastewater the rate constants are one–two-orders of magnitude lower than using model molecules, and a decrease in the ratio between rate constant of phenols conversion and rate constant of TOC abatement is observed. However, this ratio maintains over one in all cases. A typical value is around two, but the composition of wastewater and reaction conditions influences this ratio. Scaling-up to a larger volume semi-continuous slurry-type reactor causes a further lowering of one-order of magnitude in the rate constants of TOC and polyphenols depletion, due to fouling of the catalyst related to the preferential coupling of the organic radicals and deposition over the catalyst with respect to their further degradation by hydroxyl radicals generated from H2O2 activation on the copper ions of the catalyst. The use of a different reactor to overcome this problem is suggested.