Novel hydroxyapatite (HAP)-assisted hydrothermal solidification of heavy metals in fly ash from MSW incineration: Effect of HAP liquid-precursor and HAP seed crystal derived from eggshell waste

Abstract

Novel hydroxyapatite (HAP)-assisted hydrothermal methods were applied to solidify heavy metals in fly ash (FA) from solid waste incineration to create hydrothermally-treated (HT) products that meet landfilling standards. The leaching behavior, speciation distribution of heavy metals, and characteristics of HT products were analyzed by XRD, FT-IR, SEM-EDX, and XPS to explore the solidification mechanism. Large amounts of Ca2+, HPO42−, and PO43− in the HAP liquid-precursor (LP-HAP) directly formed hydrothermally-synthesized HAP and reacted with heavy metal ions to form stable compounds. Adsorption and ion exchange occurred between HAP and metals (i.e., Pb2+, Cd2+, and Zn2+), in which heavy metals occupied the Ca2+ sites. For one-step hydrothermal method with LP-HAP, a lower temperature of 35 °C resulted in better solidification efficiency than a subcritical temperature of 140 °C. By adding hydroxyapatite seed crystal (C-HAP) during the second step of two-step hydrothermal method, solidification was improved because more HAP or metal-exchanged HAP was formed via seed-induced mechanism. Under hydrothermal conditions, LP-HAP and C-HAP additives reduced the leaching toxicity but also inhibited the transfer of heavy metals from FA to the liquid phase. The overall pollution toxicity index of HT products from the optimized two-step hydrothermal method with LP-HAP and C-HAP was reduced by 96.3%.