Trace-Element Fractionation and Aqueous Release in Serbian FGD Gypsum: A Comparative Study of Three Materials

Flue gas desulfurization (FGD) gypsum is increasingly considered a secondary raw material, but gypsum purity and bulk mineralogy alone do not fully describe the environmental behavior of associated trace elements. Three Serbian FGD gypsum materials were investigated: FGD-A and FGD-B from two wet-FGD operating streams of the Nikola Tesla A Thermal Power Plant and FGD-C from Kostolac B. Bulk characterization, X-ray diffraction, sequential extraction, and aqueous leaching were combined. FGD-A and FGD-B contained >98.6 wt.% CaSO4·2H2O, whereas FGD-C contained 87.47–92.06 wt.%; gypsum nevertheless remained the predominant crystalline phase in all three materials. Operationally defined non-residual fractions (F1–F3) accounted for 74.4%–82.8% of As, 91.9%–95.4% of Cd, and 89.9%–96.8% of V, whereas 81.5%–88.3% of Sb was retained in the residual fraction. Aqueous release was generally low but element- and material-dependent: Zn reached 5.25 µg L−1 in FGD-B, V reached 0.98 µg L−1 in FGD-C, and Pb remained below the validated detection limit in all leachates. The non-residual fraction was not directly proportional to aqueous release, showing that sequential fractionation and water leaching provide complementary information. The results demonstrate that FGD gypsum materials with broadly similar gypsum-rich characteristics can nevertheless exhibit distinct trace-element partitioning and short-term aqueous release.