New Process of Catalytic Oxidation of Organic Pollutants from Water in Soft Conditions in Presence of Sulfonated Cobalt Phthalocyanines Supported on Activated Carbon

Abstract

In the field of water treatment, catalytic oxidation has been developed around the Fenton reagent (hydrogen peroxide coupled with ferrous iron salt) but this oxidative system has drawbacks. In effect, this system only works in acidic pH. In addition, the Fenton reagent used at room temperature generally offers a partial oxidation of pollutants and is not accompanied by a significant removal of total organic carbon.

The WPO process (Wet Peroxide Oxidation) that uses the catalytic activation of peroxide hydrogen at high temperature accelerates the kinetic of degradation reactions. But this method poses problems of cost due to the use of high temperatures reaching 140°C. In clean water, the oxygen is found mainly in the processes WAO (Wet Air Oxidation) which generates a huge investment because of high temperature and pressure (250 to 320 ° C - 50 to 150 bar). This represents a major drawback in the industry.

The catalytic oxidation reactions in the presence of métallophthalocyanines described in the literature involve KHSO5 or H2O2 which presents a disadvantage in a field of water depollution. Indeed, for the oxidation of trichlorophenol, adding an organic co-solvent is necessary: The reactions present the inconvenient to be conducted in a mixture acetonitrile / buffer (1 / 3, V / V) and not in an aqueous medium .Therefore, it appears that finding a method for oxidation catalysis involving transition metals and using oxygen under mild conditions of temperature and pressure is a particularly important challenge.

During this work, we showed that the synthesis of a catalyst: phthaolocyanine cobalt sulfonated fixed on activated carbon is a simple process that takes place at room temperature. The action of these catalysts has been studied in the oxidation of hexanoic acid by oxygen in normal conditions (20 ° C, 1 atm.).

We studied the reaction using two catalysts supported on activated carbon as grains, containing
respectively 12 and 50 μmol / g cobalt. We showed that:
• the acid mineralization increases with the degree of grafting. In all cases, 97% of hexanoic acid
is removed after 6h.
• the behavior of catalysts has been studied through recycling. The catalyst with the lowest content
in cobalt showed an increase of activity during the first four cycles. Such activity is stabilised with
apparent values of the fall of total organic carbon (TOC) and chemical demand of oxygen (COD) of
about 90 %. The increase of cobalt concentration enhances the rate of mineralization (expressed in
relation to TOC eliminate d) from 45 % in the first cycle to 62 % at the third cycle.

The loss of cobalt is very low. It is below 0,57 % after six cycles. We have also tested the catalytic action of this system in the succinic acid oxidation, oxalic acid and trichlorophenol. We obtain a quasi total fall of TOC and COD and an important rate of mineralization.All these consistent results, can validate this catalytic system. Especially since the catalyst is inexpensive.