Le péroxyde d'hydrogène en désodorisation physico-chimique : Rôle de la composition de la solution de lavage dans le mécanisme d'action Hydrogen Peroxide in Physico-Chemical Deodorizaton: The Role of Wash Solution Composition in H2O2 Action

Résumé

La désodorisation physico-chimique en stations d'épuration s'effectue généralement par lavage basique oxydant pour piéger les espèces soufrées réduites telles que H₂ S ou CH₃ SH. L'utilisation du peroxyde d'hydrogène n'est pas encore répandue en comparaison de celle du chlore. Cette étude a été menée afin de déterminer le comportement de H₂O₂ en fonction de la composition de l'eau de lavage. L'influence des paramètres : concentration en métaux (fer, manganèse, cuivre et zinc), pH, [H₂O₂], [CO₃^2-], [HS^-] a été étudiée en utilisant un plan d'expériences. La décomposition de H₂O₂ et la concentration de radicaux libres ont été mesurées pour chaque expérience. En présence de métaux, un pH élevé et une forte concentration en peroxyde sont les deux paramètres principalement responsables d'une forte décomposition. Cette décomposition serait accompagnée d'une production de radicaux avec [HO°]_max =10^-13 M. Cette valeur mesurée de radicaux dans le milieu n'explique qu'une petite part de la décomposition de peroxyde observée. Par conséquent, la majorité de la décomposition est due à des réactions soit à la surface des oxydes, soit en solution avec les cations dissous. Le mélange de métaux et de carbonates à pH 10,5 présente un effet de synergie sur la décomposition de H₂O₂. Ces résultats démontrent que malgré le pouvoir oxydant des radicaux HO° formés, l'utilisation de H₂O₂ en désodorisation ne sera possible qu'avec l'ajout de stabilisant.

Mots-clés:

• Désodorisation,
• H2O2,
• péroxyde d'hydrogène,
• radicaux libres,
• décomposition,
• milieu alcalin

Abstract

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), [CO₃^2-] (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 C_t18 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] - k_2[ Mⁿ⁺][H₂O₂] - k₃ [HO°][H₂O₂], with [metal]_Tot =[Mⁿ⁺] + [oxide] and, in the case of Cu and Fe, k₁ [oxide][H₂O₂] << k_2[ Mⁿ⁺][H₂O₂].

To conclude, the addition of four metals with [CO₃^2-]=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.

Keywords:

• Deodorization,
• H2O2,
• hydrogen peroxide,
• free radicals,
• decomposition,
• alkaline medium