Efficiency and mechanism of stabilization/solidification of Pb(II), Cd(II), Cu(II), Th(IV) and U(VI) in metakaolin based geopolymers

Abstract This work deals with stabilization/solidification of Pb(II), Cd(II), Cu(II), Th(IV) and U(VI) in metakaolin (MK) based geopolymer (GP) prepared using one activator (sodium silicate) or two activators (sodium silicate and sodium hydroxide). Different leaching solutions with varying pH and ionic strength were used: deionized water, 0.1 M HCl, 0.1 M NaCl, 1.0 M NaCl and 0.1 M NaOH to evaluate the efficiency of stabilization. Effective stabilization was achieved regardless of using one or two activators. The efficiency of stabilization was found to increase with increasing the ionic size of heavy metal which was explained by ion-pair theory. The leaching of total ions from the GP matrix was studied by conductivity measurements which indicated that heavy metal ions in the GP matrix resulted in reduction of the leaching of the alkali ions. This excluded the mechanism that heavy metal ions can be exchanged with the alkali ions balancing the negatively charged Al tetrahedral in GP. Heavy metals were proposed to form coordination bonds between the nonbridging Si O − and Al O − of GP framework (crosslinkers) leading to more effective stabilization of total ions in the GP matrix. The FTIR study suggested formation of heavy metal hydrates, carbonates and silicates. The XRD patterns revealed an amorphous structure, and the SEM analysis indicated almost condensed homogenous surface of GP.