Mechanism and Prediction of Cr and Zn Immobilization from Industrial Residues by Glass-Ceramics
Lina Liu, Jing Dai, Zhikun Zhang, Shiyun Liu, Yifei Sun
Abstract
Controllable preparation of a waste-derived glass-ceramics provides a promising and environmentally safe strategy for industrial residue recycling by immobilizing hazardous heavy metals in its amorphous-crystal multiphase structure. A modified two-step method was developed for treatment of Zn- and Cr-containing wastes to enhance the economic feasibility and reduce the risk of heavy metal evaporation. ZnO and Cr2O3 with various Zn or Cr contents (1, 5, and 10 wt %) were added after vitrifying the CaO(25 wt %)-Al2O3(20 wt %)-SiO2(55 wt %) system, followed by crystallization at 1000 and 1100 °C, respectively. Various qualitative and quantitative characterizations including TG-DSC, XRD, Rietveld refinement analysis of XRD patterns, XPS, and HRTEM were performed to demonstrate the immobilization mechanism of Zn and Cr species. Meanwhile, an artificial neural network (ANN) model was trained and optimized for simulation and prediction of heavy metal immobilization in glass-ceramics. The results demonstrated that the predicted results obtained from the well-trained ANN model fitted well with the experimental results. Both Zn and Cr were immobilized in glass-ceramics efficiently with immobilization efficiencies of 86.5–97.6% for Zn and 99.3–99.9% for Cr. Zn preferred to be immobilized in glass-ceramics by chemical stabilization. Around 50 wt % of Zn was incorporated in Ca2ZnSi2O7 crystals, while the rest was present in the glass matrix as dissolved Zn2+ ions. However, the most dominant immobilization mechanism of Cr was physical encapsulation, with 82 wt % of Cr embedded in the glass matrix of glass-ceramics as Cr2O3 crystals. The sensitive analysis suggests that the heavy metal content played the most important role, while crystallization temperature contributed the least for heavy metal immobilization. Among various inherent natures of heavy metals, the atomic radius was the most critical in determining immobilization efficiency of heavy metals. The results provide a comprehensive guidance for the preparation of heavy-metal-incorporated glass-ceramics and further enhancement of heavy metal immobilization effects.