Treatment of a model HA compound (resorcinol) by potassium manganate

Abstract

Current work is exploring the feasibility of using potassium manganate (K2MnO4) in drinking water treatment. The manganate species (MnO42−) can act as both an oxidant and a coagulant/adsorbent arising from the formation of insoluble manganese dioxide from the chemical reduction of Mn(VI) to Mn(IV). Extensive tests have been undertaken to investigate the treatability of humic acids by manganate, with and without the co-addition of ferrous sulphate. In these tests solutions of commercial humic acid (HA) have been reacted at varying concentrations of K2MnO4 and FeSO4 and pH. A high degree of HA removal has been demonstrated, particularly at low pH (pH 5), as expected from conventional theories of charge interaction and solid phase adsorption. In attempting to understand the mechanisms of HA reaction and separation by manganate, given the heterogeneous nature of HA, laboratory tests have been undertaken also with resorcinol as a model compound of HA. Since manganate disproportionates during its aqueous reaction to permanganate and MnO2, the comparative reactivity and separation of resorcinol by manganate (Mn(VI)) and permanganate (Mn(VII)) have been considered. During the chemical reduction of potassium manganate and potassium permanganate insoluble manganese dioxide (Mn(IV)) is produced which can adsorb un-reacted resorcinol and reaction products. The adsorption of resorcinol by MnO2 was studied by pre-forming MnO2 by the reaction of the Mn(VI) and Mn(VII) with sodium thiosulphate, prior to the addition of the resorcinol. The results at pH 5 have shown that resorcinol can be removed to a moderate degree by MnO2 adsorption. In reaction with both Mn oxidants, the ratio of oxidant to resorcinol had a significant effect on compound removal, with the removal increasing substantially at higher ratios of oxidant to resorcinol. During the chemical oxidation and MnO2 adsorption experiments with the two Mn oxidants, varying the respective periods of oxidation and adsorption had only a moderate effect on the compound removal efficiency. Comparatively, the treatment performance of potassium manganate was superior to that of potassium permanganate.