Probing the structural role of Cr in stabilized tannery wastes with X ray absorption fine structure spectroscopy

Abstract

The effective stabilization of tannery sludge wastes is explored using X Ray Absorption Fine Structure XAFS spectroscopies. Solidification of the Cr rich waste was realized via vitrification of the incinerated sludge with silica and flux agents. It is demonstrated that the effective reduction of Cr VI and the structural role of Cr are strongly modulated by the chemical composition of the waste. Eskolaite microcrystallites are embedded in the silica matrix of all vitrified samples and the extent of microcrystalline formation is strongly related to the glass basicity. Both Cr VI and Cr III species are identified, corresponding to Cr VI O4 glass formers and Cr III O6 network modifiers. The toxic Cr VI prevails only in the glasses with the highest basicity index and lowest waste content, nevertheless it is safely incorporated and immobilized in the silica matrix. However, the detected abundance of Cr VI increases glass basicity and as a result, glass polymerization is hindered. Thermal treatment, a process that leads to glass ceramics transforms almost all Cr VI to Cr III , while eskolaite formation is promoted concurrently. Nevertheless, microcrystalline growth proceeds mainly via depletion of Cr III from the silica matrix and not from the reduced Cr VI ; yet, Cr removal from the glass matrix does not impair the chemical stability of the devitrified product