Discussion on the applicability and mechanism of phosphate-based geopolymers used for cadmium and cadmium-lead heavy metals solidification/stabilization

Lead (Pb2+) and cadmium (Cd2+) are common heavy metal pollution, which is harmful to humans and animals. However, the diffusion rate of heavy metal ions moves faster in acidic environment, there are a large number of acidic heavy metal polluted soil in China, but the conventional binders are not effective to deal with them. Phosphate-based geopolymers (PGEOs) are acidic materials, and have excellence mechanical properties in acidic environment and solidification/stabilization (S/S) effect on Pb2+, but the applicability of PGEOs to Cd, Pb and Cd composite pollution is not clear. Therefore, this study discusses the applicability of PGEOs fabricated from fly ash (FA) and aluminum dihydrogen phosphate (ADP) as activator for S/S treatment of Cd, Pb-Cd composite pollution. Results show that the influence of Cd2+ and Pb2+ on the compressive strength of PGEOs was inconsistent, as the Cd2+ content raised, the compressive strength reduced slightly at 7 and 14 days, but it did not change basically at 28 days. However, the compressive strength rose firstly and then declined with an increase in Pb-Cd content when partial Pb2+ was used to replace Cd2+. The compressive strength of PGEOs with a Pb-Cd content of 0.6 % reached maximum (11.45 MPa), exceeding the compressive strength of PGEOs containing Cd2+. Moreover, the PGEOs presents an excellent stabilization effect on Pb2+, but poor effect on Cd2+. The Pb2+ leaching concentration meet the hazardous solid waste identification criteria (<5 mg/L), but the Cd2+ leaching is high and is more than 1 mg/L. Furthermore, the compressive strength of PGEOs had a good correlation with pH and EC. The S/S mechanism of PGEOs for Pb2+, Cd2+ includes physical adsorption, encapsulation, and chemical precipitation. However, Pb2+ involved in the geopolymerization process, while Cd2+ can not participate in the structure formation, so the PGEOs have a poor S/S effect. Therefore, it needs to be further improved when using PGEOs to stabilize acidic Cd2+ contaminated soil.