Sulphur compounds removal from an industrial landfill leachate by catalytic oxidation and chemical precipitation: From a hazardous effluent to a value-added product

Abstract

This study focused on the removal of sulphur compounds from a high-strength leachate of a hazardous industrial waste landfill. Firstly, sulphides (0.5 g L−1) and sulphites (2.5 g L−1) were catalytic oxidised at natural pH (8.7). Air or H2O2 were applied as oxidants and metals present in the leachate were used as catalysts. Distinct air flow rates and H2O2:sulphur molar ratios were tested. Concentrations of sulphide and sulphite lower than 1.0 mg L−1 (emission limit value - ELV) were obtained after 5-h oxygenation or 1-min peroxidation under the best conditions, i.e. air flow rate of 1 Lair Lleachate−1 min−1 and H2O2:sulphur stoichiometric ratio. Aeration was considered unsafe since >33 volatile organic compounds (VOCs) and hydrogen sulphide (H2S) were released to the atmosphere. Thus, only the H2O2-oxidised leachate pursued treatment. Sulphates (13 g L−1) were removed by chemical precipitation as ettringite or barite applying different reactants contents and pH values. Without pH correction, sulphate contents below 2.0 g L−1 (ELV) were achieved using a [Ca2+]:[Al3+]:[SO42−] molar ratio of 12:4:3 (2-fold stoichiometry) and a [Ba2+]:[SO42−] molar ratio of 1.0:1.0 (1-fold stoichiometry). The analysis of precipitates by X-ray diffraction (XRD) showed a three-phase ettringite (only 67% corresponding to ettringite itself) and single-phase barite. Barite precipitation proved to be more appealing since a value-added product was obtained and, furthermore, less reactants were required. After sulphur compounds removal using H2O2-driven catalytic oxidation and chemical precipitation through barite, the leachate was suitable for biological treatment, despite the high salinity, and a high fraction of the organic load (46%) could be biologically oxidised.