Synchrotron XANES and EXAFS evidences for Cr+6 and V+5 reduction within the oil shale ashes through mixing with natural additives and hydration process

Tayel El-Hasan; Messaoud Harfouche; Allayth Aldrabee; Nafeth Abdelhadi; Nizar Abu-Jaber; Giuliana Aquilanti

doi:10.1016/j.heliyon.2021.e06769

Synchrotron XANES and EXAFS evidences for Cr⁺⁶ and V⁺⁵ reduction within the oil shale ashes through mixing with natural additives and hydration process

Heliyon. 2021 Apr 14;7(4):e06769. doi: 10.1016/j.heliyon.2021.e06769. eCollection 2021 Apr.

Authors

Tayel El-Hasan¹, Messaoud Harfouche², Allayth Aldrabee³, Nafeth Abdelhadi⁴, Nizar Abu-Jaber⁵, Giuliana Aquilanti⁶

Affiliations

¹ Department of Chemistry, Mutah University, 61710, Mutah, Jordan.
² SESAME Synchrotron, Allan, Al-Balq'a, P.O. Box 7, 19252, Jordan.
³ Jordan Atomic Energy Commission, Shafa Badran, Amman, Jordan.
⁴ Faculty of Engineering and Technology, Al-Balqa Technical University, Amman, Jordan.
⁵ Dept. of Civil and Environmental Engineering, German Jordanian University, Naour, Jordan.
⁶ Elettra - Sincrotrone Trieste, Area Science Park, 34149 Basovizza, Trieste, Italy.

Abstract

Solid friable residues (i.e. Ash) from combusted oil shale are a major environmental issue because they are highly enriched with toxic elements following combustion. The synchrotron based techniques X-ray Absorption Fine Structure (XAFS) were used for determining the changes in speciation of Chromium (Cr) and Vanadium (V) in the ash and its mixtures with Red soil and Phosphogypsum as additives, through one-year period of hydration process. The X-ray Absorption Near Edge Structure (XANES) qualitative results indicate that all mixtures exhibits similar patterns showing that Vanadium has remain as pentavalent state, on the contrary Chromium has dramatic decreased from hexavalent to trivalent. This change in Cr speciation became clearer with increasing hydration period. Therefore, the results confirmed the advantage of the hydration process in the Cr(VI) reduction which might be due the domination of carbonate phase within all mixtures, thus hydration caused carbonate dissolution that increase the pH toward more alkaline which caused the Cr(IV) reduction into less-harmful and less mobile Cr(III). This increase in pH was not in favor of changing the V(V) into V(IV) due to its large stability field V(V). The Extend X-ray Absorption Fine Structure (EXAFS) analysis showed that Cr exhibiting a coordination shell of C-atoms as first nearest neighbors backscattering atoms around Cr, and at C-atoms backscattering at medium range order. This confirmed the domination of carbonate media through the best fitting of Cr-C. Which might be attributed to the more alkaline conditions developed during saturation of water (hydration), that accelerates of the reduction of Cr(VI) into Cr(III). This means simply that hydration of the ash can reduce the presence of harmful Cr(VI) in these ash tailings.

Keywords: Additives; Cr(IV) reduction; EXAFS; Hydration process; Oil shale ash; XAFS; XANES.