Heavy metal stabilization in MSWI fly ash using an additive-assisted microwave hydrothermal method

Abstract

The dioxins and leachable heavy metals in municipal solid waste incineration (MSWI) fly ash made it a hazardous waste. Microwave hydrothermal with four common additives, Na2S, Na2SiO3, Na2B4O7 or H3BO3, was researched at different temperatures, L/S ratios, time and additive concentrations by single factor and orthogonal methods to achieve effective heavy metal stabilization and waste reuse of MSWI fly ash. The samples were investigated by XRD, BET, SEM, ICP-MS to determine the influence of additive-assisted microwave hydrothermal on heavy metals stabilizing. According to the results of the orthogonal study, the degree of influence of each parameter heavy metals stabilization was in the order of additive, temperature, additive concentration, time and L/S ratio. Single-factor experiments suggested that the conditions of 125 °C, 10 min, 3 mol/L Na2S and 2 mL/g were enough for the leaching concentration of heavy metals meeting the specified limitation of China. The results of analyses confirmed that microwave hydrothermal treatment with Na2S could enhance heavy metals stabilizing by forming zeolites for adsorption, precipitation and convert heavy metals to a more stable state. Microwave hydrothermal with Na2S was a promising method to manage MSWI fly ash, which greatly decrease the reaction time and realize wastes utilization.