Synthesis of high quality zeolites from coal fly ash: Mobility of hazardous elements and environmental applications

Converting fly ash into zeolite is an alternative way to reduce the waste landfills while also producing high value-added products. However, hydrothermal synthesis of zeolite from fly ash in alkaline media could also induce mobilization of toxic heavy metals, possibly causing environmental contamination. Systematic research into this subject is rare in the open literature and the mobility of heavy metals from fly ash derived zeolites is yet to be understood. In the present contribution, we investigated the migration of heavy metals and quantified their distribution from fly ash to product zeolites and waste water during the synthesis of high-quality type A zeolites (471 m²/g surface area). High conversions of major elements (98.2% aluminium and 96.5% silicon) were achieved, with zero secondary solid waste. Metalloid elements including arsenic and selenium, and those with strong amphoteric properties e.g. molybdenum were found highly mobile and mostly presented in the wastewater. In comparison, less than 20% of the heavy metals with weak amphoteric nature including copper, chromium and lead from the fly ash went to the wastewater; the rest along with almost all cadmium, iron and nickel were fixed into the product zeolites. Despite the existence of heavy metal elements in zeolites, there was no noticeable leaching under various pH conditions, hence deemed safe for applications. Furthermore, the effective removal of trace strontium and caesium cations from contaminated water using such zeolites was demonstrated.