Efficient removal of malachite green from aqueous solution using flue gas desulfurization gypsum-loaded ferrous sulfide composites: Synthesis, mechanism, and behavior

Malachite green (MG) is a cationic dye that has been widely used in the dyeing of leather, silk, paper, and textiles. However, MG is difficult to adsorb and separate from treated water, leading to environmental pollution and posing a significant threat to living organisms and the natural environment. Herein, a composite material (FeS@FGD) loaded with ferrous sulfide (FeS) was synthesized from flue gas desulfurization gypsum (FGD) to adsorb MG from aquatic environments. The adsorption mechanism was analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and zeta potential measurements. Furthermore, the effects of pH, adsorbent mass, initial MG concentration, temperature, and adsorption time on the MG adsorption capacity of FeS@FGD were investigated. The results indicate that:(1) The adsorption mechanism of FeS@FGD for MG involves electrostatic interactions and hydrogen bonding. (2) At pH=10, with an FeS@FGD mass of 30 mg, an initial MG concentration of 100 mg/L, and a temperature of 313 K, FeS@FGD achieves optimal adsorption, with a removal rate of 96.9% and an adsorption capacity of 153.6 mg/g. (3) Elevated temperatures promote the adsorption of MG by FeS@FGD, indicating an endothermal reaction, with the adsorption behavior more accurately described by the Freundlich isotherm model. Adsorption equilibrium is reached at 480 min and aligns more closely with pseudo-second-order kinetic equations. (4) FeS@FGD maintains 76.4% efficiency after four regeneration cycles, demonstrating excellent reusability. In conclusion, the composite material derived from solid waste FGD and loaded with FeS shows promise as a low-cost and efficient adsorbent for the removal of MG from aqueous solutions, offering a valuable reference for addressing cationic dye pollution in the natural environment.