Fractionation and release behaviors of metals (In, Mo, Sr) from industrial sludge

• Fractionation of In, Mo, and Sr in an industrial sludge to assess their mobility.
• We examined the effects of Eh/pH on release of In, Mo, and Sr.
• Limited mobility of In under acidic and oxidizing conditions.
• Significant release of Mo under alkaline conditions and increased with decreasing Eh.
• Strontium was mobile under acidic conditions, while immobile under alkaline conditions.

Little is known on the fate of rare metals in the environment and the associated risks. The fractionation and release of three metals of an industrial sludge were assessed. The average concentration ranges from 39.3 to 41.5 mg/kg for indium (In), 43.1–77.8 mg/kg for molybdenum (Mo), and 131.1–376.4 mg/kg for strontium (Sr). Sequential extraction results implied that In was mobile, while Mo and Sr were immobile. However, experimental results from effects of Eh/pH revealed that In was slightly mobile under acidic (pH 4.5) in Eh range of 210–260 mV, and immobile under alkaline conditions (9.0) in Eh range of −250 to 125 mV. The release of Mo was slightly mobile under acidic conditions. However, Mo was very mobile under alkaline conditions and it increased with decreasing Eh. The release of Sr was significant under acidic conditions; however, it was immobile under alkaline conditions. Solubility and adsorption as affected by pH, and speciation could explain their release behaviors. Discrepancy in predictions from sequential extraction and actual observation from Eh/pH experiments was discussed.

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