Molecular Study of Thermal Immobilization of Chromium(VI) with Clay

Abstract

Clay that contains kaolinite has been used extensively as a raw material for manufacturing of bricks and china at 900 -1100°C. This study used clay to stabilize the contaminant chromium(VI) [Cr(VI)] through a heating process at 500 -1100°C. X-ray absorption spectroscopic results indicated that the 500 -900°C heating process transformed hazardous Cr(VI) to nontoxic Cr(III); Cr 2 O 3 was the species detected as most abundant. The 1100°C heating process caused the formation of Cr 2 SiO 5 , which was not detected in the samples heated at 500 -900°C. Fourier transformed extended X-ray absorption fine structure spectra were fitted by use of WinXAS software. Phase shifts and backscatter(ing) amplitudes for specific atom pairs, based on the crystallographic data for CrO 3 and Cr 2 O 3 , were theoretically calculated with the FEFF software. The processed XAS data show that the first shell coordination numbers were similar to each other as the temperature was increased from 500 to 900°C and 1100°C, implying that their Cr(III) crystallite size was relatively similar. The interatomic distance between the target center element and the first shell for the 500 -1100°C samples was 1.98Å. The Debye-Waller factor for the 1100°C sample was increased compared with the 500 and 900°C samples and probably indicates the formation of Cr 2 SiO 5 .

Effect of Humic Substance on Thermal Treatment of Chromium(VI)-Containing Latosol Soil

“…8 Cr(VI) is thermally reduced to Cr 2 O 3 , which then partially reacts with kaolin and a clay to form amorphous Cr silicate (ie, Cr 2 O 3 ⅐SiO 2 ) as determined via synchrotron radiationbased X-ray absorption spectroscopy (XAS). 4,8 The kaolin and clay reported in the previous studies are organic free and are mined and/or refined for the porcelain-production sector by the manufacturer. The effect of organics on sintering of kaolin and clay was not investigated.…”

“…This influence has not been investigated previously. 4,8 Based on the above, this work aims to investigate how the release of LOI from latosol during thermal treatment affects both Cr speciation and sintering of latosol, both of which eventually impact the extent of Cr stabilization. To do so, we accumulate and compare the information, including XAS spectra, surface morphology, surface area, thermogravimetric analyzer/differential scanning calorimeter (TGA/DSC) results, and 13 C-nuclear magnetic resonance (NMR) spectra of Cr-contaminated latosol and LOI-free latosol samples; both samples were heated previously at various temperatures.…”

“…The effect of organics on sintering of kaolin and clay was not investigated. 4,8 Latosol is one of the primary soils in tropical and semitropical zones. Latosol mainly consists of humic substance, kaolinite, micas, feldspar, gibbsite, vermiculite, and quartz, consequently having the potential to serve as an important raw material in the brick and porcelain sectors; these sectors usually fire the raw materials near 1100°C for Ն4 hr to enable sintering reaction of the raw material.…”

Thermal treatment of solid waste in view of recycling: Chromate and molybdate formation and leaching behaviour

“…Some authors describe a similar decrease in Cr leaching at elevated temperatures and several explanations were reported for this. Some suggest that Cr(VI) can be reduced to Cr(III) by more reducing conditions at higher temperatures (Sorensen et al, 2000; Wei et al, 2005). However, their heating conditions differed much from the conditions in this article as they heated their samples in covered crucibles, which promotes reduction owing to lack of oxygen.…”

Inhibition of Cr(VI) in the Al2O3-CaO-Cr2O3 castables by using (Al1-x,Crx)2O3 solid solution

“…For the Cr 2 O 3 containing refractory linings, since the operation conditions and service atmosphere in a given furnace can hardly be changed in practical production, most of the related work have focused on Cr(VI) minimization using some additives at high temperatures. And the results suggested that acidic components such as SiO 2 , TiO 2 , Fe 2 O 3 and P 2 O 5 can effectively suppress the Cr(III) oxidation during thermal treatment of Cr 2 O 3 containing refractories [4,[23][24][25][26][27][28][29]. However, these oxide additives usually result in formation of low melting point phases in the matrix, which would obviously deteriorate the thermo-mechanical properties or slag corrosion resistance of the refractories [30][31][32].…”