The reactions of hydrogen peroxide (H2O2) and free available chlorine (FAC) with various non-ferrous metal(loid) oxides (MeOx), including amorphous zirconium oxide (a-ZrO2), monoclinic zirconium oxide (m-ZrO2), cerium oxide (CeO2), titanium oxide (TiO2), aluminum oxide (Al2O3) and silicon oxide (SiO2) were investigated. This work focused on the kinetics of H2O2 and FAC decomposition and generation of reactive species in suspensions of each MeOx. Furfuryl alcohol (FFA) was used as a 1O2 probe, and methanol (MeOH), isopropanol (iPOH) and tert-butanol (tBuOH) were used as radical/reactive species probes and scavengers, which yield formaldehyde (in cases of MeOH and tBuOH) and acetone (in case of iPOH) upon reaction with HO● and certain other oxidants. Reactions were conducted by dosing 1-100mM H2O2 or 0.1-1mM FAC into 0.01-0.1 g/mL suspensions of MeOx solids (nominal dia. 5 µm). FFA showed significant loss in the presence of H2O2 with a-ZrO2, m-ZrO2 and CeO2, and in the presence of FAC with m-ZrO2, CeO2, TiO2, Al2O3 and SiO2 over 10 min of reaction (but no loss in the absence of H2O2, FAC, or MeOx), implying 1O2 generation. Formaldehyde (≥100 µM) and acetone (≥30 µM) were observed over 6h of reaction in systems of m-ZrO2+H2O2, m-ZrO2+FAC, CeO2+FAC and TiO2+FAC containing MeOH and iPOH, respectively (but not in the absence of H2O2, FAC, or MeOx). However, lower levels of formaldehyde formation (≤5 µM) were observed when using tBuOH as a scavenger, indicating that other weaker reactive oxygen or chlorine species may be responsible for formaldehyde and acetone formation in the presence of MeOH and iPOH (which are less selective for HO●). Use of tetranitromethane, a selective probe for O2●−, also confirmed O2●− production in MeOx+H2O2 systems. Activities of the MeOx systems are sustained in repeated use, as re-dosing of H2O2 or FAC yielded comparable performance in FFA degradation and formaldehyde formation over at least five treatment cycles. The reactions prevailing in these systems, although not fully elucidated, appear to involve formation of 1O2 and other as yet unconfirmed reactive species, potentially including HO● (free or surface-bound), ClO●, and O2●−, at the MeOx surface. These observations highlight the potential for use of these in some cases underexplored metal(loid) oxides in driving novel advanced oxidation processes for organic contaminant degradation, as well as for applications requiring removal of residual oxidants in (waste)water treatment.

Reactive species production during decomposition of aqueous hydrogen peroxide and free available chlorine at non-ferrous metal(loid) oxide surfaces

ACS Spring 2022

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