Hydrogen Peroxide in Physico-Chemical Deodorizaton: The Role of Wash Solution Composition in H2O2 Action
Deodorization of wastewater treatment plants involves the elimination of molecules such as NH3, amines and sulphur compounds like H2 S and mercaptans. In classical physico-chemical processes, NH3 and amines are trapped in acid solution by washing air in a scrubbing tower, while sulphides are eliminated in basic oxidising solutions. The oxidant usually used is sodium hypochlorite. Elimination of sulphides and organosulphides generally demands two scrubbers: one at pH 9 and the other at pH 11. Because chlorine in deodorization generates the formation of organochlorinated species, it should soon become necessary to replace this oxidant in order to avoid the formation of such compounds. The present study follows the behaviour in wash conditions not of chlorine, but hydrogen peroxide, in order to discover the deodorization capacity of this molecule. The kinetics of H2 S oxidation by H2O2 are well known; the constant is given by log k = 12.04 - (2641/T) - 0.186 x pH (Millero et al., 1989). Unfortunately H2O2 shows strong decomposition in alkaline medium, due to the presence of metals and carbonates in the solution. Initiating a homolytic reaction results in the decomposition of peroxide. However, increasing the concentration of free radicals may improve H2 S oxidation and consequently, the efficiency of the process. To better understand the behaviour of H2O2 in wash conditions, various parameters were studied, namely pH (9 and 10.5), [H2O2] (1 and 5 g L-1), metal concentrations (iron, manganese, copper and zinc) (20 and 200 μg L-1), [CO32-] (0 and 100 mg L-1) and [HS-] (0 and 2 mg L-1). Four experimental designs, one for each metal, were employed to reduce the number of experiments and benefit from statistical laws. H2O2 decomposition and HO° concentration were measured and empirical equations established. All experiments were performed in closed-batch reactors with ultra-pure reactants and water. Measurements of HO° concentrations necessitated the addition of atrazine to the solution. The oxidation of this pesticide by HO° is well known. Using atrazine concentrations measured through time, the HO° concentrations were calculated according to the equation ln ([Atz]0/[Atz]) = k[HO∘]t with k=2.1 × 109 M-1 s-1 (De Laat et al., 1997). Oxidation of atrazine was halted by extraction onto a Ct18 Sep-Pack resin and samples were analysed by liquid chromatography. The results showed that in the presence of metals H2O2 decomposition was maximal at high pH and with high peroxide concentrations. The decomposition was accompanied by HO° production. However, the presence of metals generated the decomposition of H2O2 with a reduced production of free radicals compared with ultra-pure water, which indicates that metal oxides were not only decomposition catalysts, but also radical inhibitors. Comparison of simplified radical decomposition, calculated according to the equation ([H2O2]/[H2O2]0)=e-k[HO∘]t, and observed decomposition showed that under these conditions H2O2 consumption was mainly due to metal reactivity. Nonetheless, increasing iron and copper concentrations from 20 to 200 μg L-1 did not modify the decomposition rate of H2O2. For this reason we postulate a Fenton-like reaction between H2O2 and dissolved metals in which concentrations are determined by solubility products. It follows that the kinetics of H2O2 decomposition can be summarised by r=-k1 [oxide][H2O2] - k2[ Mn+][H2O2] - k3 [HO°][H2O2], with [metal]Tot =[Mn+] + [oxide] and, in the case of Cu and Fe, k1 [oxide][H2O2] << k2[ Mn+][H2O2]. To conclude, the addition of four metals with [CO32-]=1 g L-1 at pH 10.5 produces a synergetic effect, resulting in a much faster decomposition. These conditions, unfortunately, resemble deodorization conditions. The use of a stabiliser that inhibits not only free radicals but also decomposition catalysts is therefore necessary for deodorization.