Dynamic hydrothermal synthesis of Al-substituted 11 Å tobermorite from solid waste fly ash residue-extracted Al2O3

Abstract

Fly ash residue (hereafter, FAR) is the by-product of pulverized coal combustion fly ash-extracted Al2O3. It results in a hazardous industrial solid waste if it does not have appropriate treatment and utilization. Al-substituted 11 A tobermorite is successfully synthesized from FAR/SiO2 mixture by dynamic hydrothermal treatment at 220 °C for 6 h. FAR and its hydrothermal reaction products are studied by chemical analysis, XRD, SEM, FTIR, and BET methods. The XRD results show that dicalcium silicate existing in FAR has been converted into tobermorite after the hydrothermal treatment. The main crystalline phases of product are Al-substituted 11 A tobermorite and minor calcite. SEM results show that the hydrothermal products of the FAR/SiO2 mixture consist of many tiny needlelike and platy crystals which form micro-porous spherical particles, ranging in size from a few microns to dozens of microns, and have a specific surface area of 49.004 m2/g. The Al-substituted tobermorite-bearing products have a high performance of exclusion of Cr3+ from acidified aqueous media, and the adsorption efficiency of Cr3+ is 98 %. The exclusion reaction proceeds rapidly, reaching equilibria within 1 h. The results show that this product has a potential to be used in industrial processes for adsorption of heavy metal cations from wastewater.