Heavy metal removal from water by red mud and coal fly ash: an integrated adsorption-solidification/stabilization process

The use of low-cost unconventional sorbents for the removal of heavy metals from aqueous solution has been extensively studied by the analytical chemistry research group of Istanbul University. The selectivity order of metal ion uptake was highest for Cu, medium for Pb and lowest for Cd, in accord with the order of insolubility of their corresponding metal hydroxides. The metal-loaded red muds and fly ashes were successfully solidified by adding cement, sand and water, producing both physically and chemically resistant durable concrete blocks as confirmed by chemical leaching and compressive strength tests. The metals were essentially held irreversibly, and would not leach out into carbonic acid or bicarbonate buffer and humic acid solutions. The metal-loaded solid wastes were solidified into an environmentally safe form by adding stabilization-solidification agents, thereby serving the doublefold aim of water treatment and solid waste disposal. Since lead (II) caused the major hardening and strengthening problems in the solidification-stabilization process, sodium aluminate or triethanolamine was added as a stabilization/promotion agent.