The migration and immobilization for heavy metal chromium ions in the hydration products of calcium sulfoaluminate cement and their leaching behavior

Abstract

Calcium sulfoaluminate cement (CSA) produced from the solid wastes is an effective way to realize the resource utilization of solid waste. Aiming at the key issue that limits this technique, the negative effects of the heavy metals introduced from solid waste on the environment. In this research, Cr(VI) ions, as the heavy metal with great environmental toxicity, are introduced into the calcium sulfoaluminate cement, and their influence on the setting time, compressive strength are explored. The thermal stability, phase structure, micromorphology , and chemical composition of the Cr(VI)-containing hydration products of cement were studied by thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), scanning electron microscopy and X-ray energy spectroscopy (SEM–EDS). After hydration for 7 days, chromium ion leaching experiments were performed in different solutions, including deionized water , sodium sulfate solution , sodium carbonate solution, acetic acid solution, NaCl solution, and 70 °C deionized water. As a result, the introduction of 3% sodium chromate and potassium dichromate increased the setting time of CSA by 47.2% and 117.9%, and the compressive strength was reduced by 15.3% and 8.1%, respectively. The leaching concentration of chromium ions gradually increases with the oscillation time and remains basically unchanged after 12 h. For the above solutions, in NaCl solution, the immobilization of Cr(VI) in CSA cement is the highest (20.3 mg/g) but the lowest that in NaCO 3 solution (14.2 mg/g). The research results confirmed that the chloride ions in the NaCl solution promote the crystal growth of ettringite , thus facilitating the immobilization of Cr(VI) ions in their lattice, while in NaCO 3 solution, Cr(VI) ions are easily released due to CO 3 2− ions binding calcium ions in ettringite crystals but still maintain a high curing amount. Therefore, this work provides a theoretical criterion for toxicity leaching and safety application in different environments for calcium sulfoaluminate cement from solid waste. The main hydration product of sulphoaluminate cement is rod-like ettringite. The columnar structure of ettringite can stably solidify heavy metal Cr ions through physical adsorption and chemical exchange. By studying Cr(VI) leaching in different environments, it can be found that carbonate and acidic conditions are easy to destroy the structure of ettringite and increase the leaching concentration of Cr(VI). NaCl does not destroy the structure of ettringite and can stabilize Cr(VI), with the lowest leaching rate. • Heavy metal Cr (VI) ions enter the lattice of ettringite crystal – one of the hydrated products, by ionic substitution. • Cr(VI) contained ettringite crystals can exist stably in NaCl solution. • The leach of Cr(VI) resulted from the ettringite structure destroyed in acid environment.