Ozonation of oxalic acid catalyzed by ion-exchanged montmorillonite in moderately acidic media

Abstract

Ion-exchanged montmorillonite-rich materials (ca. 96% purity) like NaMt, Fe(II)Mt, Co(II)Mt, Ni(II)Mt and Cu(II)Mt showed catalytic activity in the ozonation of oxalic acid in water at room temperature, in the pH range 3.4-6.0. The conversion of oxalic acid exceeds 95% after 180 min of ozone bubbling in the presence of Fe(II)Mt. The oxalic acid removal efficiency was found to increase swiftly with the acid character of the clay surface up to a certain level, but decreases gently with excessive surface acidity. The pH exerts a strong influence on the catalyst efficiency, because it induces changes in the composition of both the liquid media and catalyst. The synergic action of ozone and clay catalysts at acidic pH seems to involve ozone adsorption and interaction between cation and adsorbed oxalate. The negative effect of increasing pH between 3.44 and 6.0 is discussed in terms of a decrease in the amount and mobility of the cation in the vicinity of the clay surface, and of a decay in the clay surface area available to ozonation.

Catalytic ozonation of Orange-G through highly interactive contributions of hematite and SBA-16 – To better understand azo-dye oxidation in nature

“…No OG was detected after a 4 min ozonation with 2 g.L À1 of 1:1 HS or 3 min ozonation with 1 g.L À1 . This beneficial effect of low catalyst concentration in correlation with acidic pH (1.8) was already reported for clay catalysts (Shahidi et al, 2015a(Shahidi et al, , 2015b(Shahidi et al, , 2014Lanzl et al, 2012;Sulpizi et al, 2012;Araujo et al, 2011;Azzouz et al, 2010;Xu and Li, 2010;Deng et al, 2008;Xu and Gao, 2008;Kim et al, 2005;Okawa et al, 2005;Bodo et al, 2004). Here, a combination of hematite dissolution with silanol protonation must be involved.…”

“…2-b), must correspond to an induction period due to the low ozone dissolution in water. This plateau totally disappeared in the presence of hematite, SBA-16 or their mixtures, suggesting a beneficial role of a solid surface in promoting surface processes (Azzouz, 2012;Azzouz et al, 2010). Triplicate of ozonation tests revealed unequivocally the occurrence of curve shoulders in the presence of each catalyst after 1e3 min ozonation ( Fig.…”

“…Here, a combined effect of hematite acting as a Fe 3þ cation reservoir and partial ion-exchange of silanol groups should promote simultaneous adsorption and ozonation of both dispersed and adsorbed species (Azzouz et al, 2010). The first traces of oxalic acid are supposedly dissociated into HC 2 O 4 À anions at higher pH than pK a1 1.27, generating ferrioxalate anions ([Fe(C 2 O 4 ) 3 ] 3e ).…”

Total mineralization of sulfamethoxazole and aromatic pollutants through Fe2+-montmorillonite catalyzed ozonation

“…This finding is of great importance, because it demonstrates that the reactivity towards ozone increases with increasing molecular complexity, but decreases with increasing oxidation states. The consecutive decrease of (A/A o ) indicates a quick decay of the catalyst activity, partly due to a pH increase, which is supposed to induce change in the mechanism pathways [14]. An additional explanation involves a temporary catalyst poisoning due the adsorption of base-like intermediates such as hydroquinone (pka 1 =9.9), catechol (pka=9.48), resorcinol (pka=9.15) and others.…”

“…Among these, ozonation and catalytic ozonation are particularly suitable for eco-friendly water treatment than can lead to total mineralization of organic pollutants [13][14][15][16][17].…”