Discussion on the applicability and mechanism of phosphate-based geopolymers used for cadmium and cadmium-lead heavy metals solidification/stabilization
Shaoyun Pu, Zewei Shen, Wei Duan, Lei Lang, Yang Liu, Bomin Xu, Huiran Yao, Guoxiong Mei
Abstract
Lead (Pb 2+ ) and cadmium (Cd 2+ ) 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 Pb 2+ , 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 Cd 2+ and Pb 2+ on the compressive strength of PGEOs was inconsistent, as the Cd 2+ 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 Pb 2+ was used to replace Cd 2+ . 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 Cd 2+ . Moreover, the PGEOs presents an excellent stabilization effect on Pb 2+ , but poor effect on Cd 2+ . The Pb 2+ leaching concentration meet the hazardous solid waste identification criteria (<5 mg/L), but the Cd 2+ 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 Pb 2+ , Cd 2+ includes physical adsorption, encapsulation, and chemical precipitation . However, Pb 2+ involved in the geopolymerization process, while Cd 2+ 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 Cd 2+ contaminated soil.