Leaching behavior of aluminum, copper, iron and zinc from cement activated fly ash and slag stabilized soils

The use of industrial by-products such as fly ash and slag have become very prevalent in soil stabilization owing to its suitable physical and mechanical properties, and economical advantages. However, fly ash and slag have been identified as the potential source of toxic substances, and may pose environmental risk by leaching heavy and trace metals into soil, surface and groundwater. Toxicity characteristic leaching procedure (TCLP) tests were conducted to investigate the environmental hazards associated with the leaching of aluminum (Al), copper (Cu), iron (Fe) and zinc (Zn) from fly ashes, slag, type I/II cement and cement activated fly ash and slag stabilized soils. Sulfate (SO₄), dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) concentrations were also quantified to evaluate their influence on metal leaching. To understand the effect of pH on the leaching behavior, pH-dependent leach tests were conducted at the pH ranges of 2–14. Results indicated that an increase in fly ash or slag content may not necessarily increase the effluent metal concentrations. Al, Cu, Zn and DOC followed an amphoteric leaching pattern where concentrations increased in both acidic and basic conditions. In contrast, maximum DIC concentrations occurred at neutral or near neutral pH values. Fe and SO₄ showed cationic leaching behavior where concentrations decreased with an increase in effluent pH. Additionally, the leaching controlling mechanisms of the metals were identified by implementing geochemical modeling program Visual MINTEQ. The geochemical analyses indicated that the solubility of Al³⁺ and Fe³⁺ were controlled by precipitation/dissolution reactions of oxide/hydroxide minerals at all pH values. Leaching of Cu²⁺ was only solubility controlled at pH higher than 7, whereas Zn²⁺ leaching was solubility controlled in the pH range of 8–12.