The oxidative degradation of organic contaminants using a manganese oxide-containing mine waste

Abstract

Manganese oxide tailings material, a waste product generated during Mn ore extraction processes in South Africa, has been assessed in terms of its potential to oxidatively breakdown organic contaminants. Azo dyes and polycyclic aromatic hydrocarbons (anthracene) show oxidative interactions with the tailings, resulting in the formation of products which are more environmentally favourable than the parent compound. Tailings samples from five mines were characterised to establish the redox reactivity of the material. Based on chemical and mineralogical data the tailings were grouped into the carbonate-rich Mamatwan type (MT) tailings (Mamatwan and Gloria mines), the Mn oxide- enriched Wessels type (WT) tailings (Wessels and Nchwaning mines) and the Mn oxide enriched Hotazel type (HT) tailings (Hotazel mine). The tailings are net-alkaline and non acid generating with a point of zero charge below pH 4. The average Mn oxidation state of the three tailings types ranges from 1.2 to 1.5 in the order HT>WT>MT. Despite a low surface area (1.5 to 6.4 m(^2) .g(^-1)) the tailings show a substantial (0.5 to 3.0%) 'easily' reducible, reactive Mn phase as well as a large pool of more recalcitrant dithionite-extractable Mn. Thus the tailings material displays both 'quick and slow release' oxidative capacity. The oxidative decolorisation of acid azo dyes acid orange 7 (AO 7) and acid yellow 36 (AY 36) by the Mn tailings is highly pH dependent, with increased oxidation occurring at lower pH. The reaction mechanism for the oxidation of AO 7 by the tailings has many similarities to enzymatic degradation of the dye observed with white rot fungi. The reaction, initiated on the phenolic group, occurs via successive one electron transfers from the dye molecule to the Mn oxide. A series of radical reactions occur resulting m the asymmetrical cleavage of the azo bond and the generation of terminal reaction products 1,2 naphthoquinone and 4- hydroxybenzenesulfonate. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) demonstrated that initial sorption of AO 7 is pH dependent and outer-sphere. A pronounced lag phase exists between the initial sorption of the dye to a Mn oxide surface and the initiation of oxidation. This lag phase can indicate that either the transfer of the initial electron is rate limiting or that correct orientation followed by inner- sphere complexation is necessary before oxidation can take place. The reaction mechanism proposed for the oxidation of AY 36 is initiated at the amino moiety and proceeds via successive, one electron transfers from the dye to the Mn tailings. The reaction pathway involves the formation of a number of colourless intermediate products, some of which hydrolyse in a Mn oxide-independent step. The terminal oxidation products were observed to bep-benzoquinone and 3-hydroxybenzenesulfonate.Light, both UV and ambient, and auxiliary compounds such as acetate buffer and salts did not reduce the decolorisation capacity of the tailings. Increased buffer strength enhanced decolorisation and addition of Na(_2)S0(_4) in the presence of buffer increased the initial oxidation of AO 7. The decolorisation capacity of the Mn tailings showed durability with 90% colour removal observed 60 days after daily dye replenishment. Drying anthracene-spiked Mn tailings, synthetic Mn oxide and calcite water slurries resulted in anthracene oxidation to anthraquinone (6-30% oxidation). Small but significant (4%) anthracene oxidation was also observed when anthracene spiked water was evaporated from quartz and a clean glass surface. No anthracene oxidation was apparent without the evaporation of water at pH > 5. The HT tailings oxidised up to 30% anthracene when dried, the most substantial oxidation took place below 5% gravimetric water content. Evaporation of anthracene-spiked cyclohexane slurries resulted in the same observed oxidation from both Mn tailings and calcite. It could not be established whether elecfron transfer was occurring between the Mn oxide phase of the tailings and the anthracene or whether the transformation was solely a surface mediated phenomenon with oxygen being used as the elecfron acceptor. Under fully hydrated conditions the Mn oxide tailings oxidised 75% of anthracene to anthraquinone at pH values less than 4.5. This would suggest that the Mn tailings can oxidise anthracene and sufficient mineral-contaminant contact can be achieved despite the low water solubility of the compound.