Chemical evaluation of immobilization of wastes containing Pb, Cd, Cu and Zn in alkali-activated materials: A critical review

Alkali activated materials, known as geopolymers (GPs), have been shown to be potential hosts for immobilization of heavy metal wastes. Upon incorporation of heavy metals into the GP matrix, metal hydroxides, carbonates, silicates, oxides and sulfides are expected to be formed. Since experimental study of these species is very difficult, the present work is devoted to use the available thermodynamic data to calculate the solubility of metal species in leaching solutions. Metal hydroxides of Pb, Cd, Cu and Zn have water-solubility (0.034, 1.4, 0.011 and 0.13 mg/L, respectively) lower than the recommended limits (5, 1, 15, 100 mg/L, respectively), however at pH 6, the solubility is much higher (43.3, 5.38 x 10(6), 27.96 and 2.09 x 10(4) mg/L, respectively). Thermodynamic calculations show that metal hydroxides (M(OH)(2)) have high solubility in 6.0 M NaOH (7.2 x 10(3), 15, 2.9 x 10(3), 9.8 x 10(4) mgL, respectively) as oxoanionic species (M(OH)(3)(-) and M(OH)(4)(2-)) which could not be stabilized by attachment to the negatively charged Al of GP matrix. Physical encapsulation of heavy metal species is an irrefutable mechanism. The water solubility of Cd species follows the order: CdS (10 x 10(-9)) < CdSiO3 (0.0683) < CdCO3 (0.11) < Cd(OH)(2) (2.0 mg/L), which suggests that the availability of small amounts of sulfide and increasing the dose of sodium silicate have positive effect on immobilization of Cd. Future work must focus on the role of following factors on immobilization efficiency: the nature and dose alkaline activator, the stage at which the waste is introduced in GP and the effect of small amounts of sulfide and calcium.