X-ray absorption spectroscopy study on zinc immobilization in metakaolin-based potassium-geopolymer

Prihutami, P.*; Chaerun, R. I.; Oya, Yusuke*; Toda, Kanako*; Saito, Takumi*; Kikuchi, Ryosuke*; Otake, Tsubasa*; Sato, Tsutomu*

Untreated waste from anthropogenic activities increases the possibility of zinc contamination in the environment. This metal is commonly found in solid and liquid waste. Due to its toxic behavior at high concentration, preventing zinc release to the environment is crucial to protect human health and the environment. Zinc-containing waste needs to be treated with a binder for its solidification and stabilization. Geopolymer has been an attractive binder for hazardous waste due to its ability to contain heavy metals, low CO emission upon production, and durability. Understanding the immobilization form of zinc in the geopolymer matrix becomes essential to aim for the long-term stability of the treated waste. Enhance stability can be achieved by using a potassium activator since it maintains the amorphousness of geopolymer. This study aims to clarify the immobilization mechanisms of zinc in geopolymer. In this study, 1 wt.% of zinc was incorporated into geopolymer by mixing a certain amount of metal solution, metakaolin, and potassium activator to have Al2O3:K2O ratio of 1:1. Test results identify zinc to be well-immobilized in potassium geopolymer with 99.86% retention after 90 days of leaching. Zinc is found to be retained not in its crystalline form, but in the geopolymer matrices. The incorporation does not change the short-ranging order of geopolymer as supported by selected area electron diffraction (SAED) pattern showing a diffuse ring from the randomly scattered electrons. Zinc K-edge X-ray absorption spectroscopy (XAS) suggests that zinc bonds to aluminosilicate chain. The XAS results (Figure 1) also exhibit a change in the local structure after leaching. Initially, zinc largely takes tetrahedral coordination, which changes to octahedral coordination upon leaching. This study shows that zinc changes its coordination environment from tetrahedral to octahedral during its leaching, which could explain the immobilization mechanisms of zinc with geopolymer.