Municipal Solid Waste Fly Ash-Derived Zeolites as Adsorbents for the Recovery of Nutrients and Heavy Metals—A Review

Abstract

Though fly ash from the incineration of municipal solid waste (MSW-FA) is considered hazardous waste, its huge and increasing volumes and potential value due to high concentrations of salts and heavy metals such as copper and zinc have attracted commercial interest in mining it for resources. The associated treatments used for extracting these resources may make it economically feasible to extract other constituents from the residuals, adding another potential pathway towards a zero-waste society. This review assesses the feasibility of using zeolites, synthesized from precursors extracted from MSW-FA, as sorbents for the recovery of nutrients (ammonium, nitrate, and phosphate) and heavy metals. It is possible to tailor the properties of the zeolites; however, the large variability in reported adsorption capacities and specificities, as well as the inherent heterogenic nature and variable composition and concentrations of most nutrient- or heavy metal-rich waste streams, make such tailoring challenging. A remaining important issue is the transfer of unwanted micropollutants from the MSW-FA or waste stream to the final products and the loss of surfactants from surfactant-modified zeolites during adsorption and/or desorption of nitrate and/or phosphate. Nonetheless, the recovery process is benefited by high concentrations of the target compounds and low concentrations of competing ions.