Removal of arsenic from water using synthetic Fe7S8 nanoparticles

• As(III) and As(V) binding was pH independent.
• As(III) and As(V) adsorption was zero order and exothermic.
• Fe7S8 had a lower binding capacity for As(III) than As(V).
• Activation energy of the binding process was higher for As(III) than As(V).
• PO43− interfered with As binding whereas Cl−, NO3−, and SO42− had no effect.

In the present study, pyrrhotite was used to remove arsenite and arsenate from aqueous solutions. The Fe7S8 was synthesized using a solvothermal synthetic method and it was characterized using XRD and SEM micrographs. Furthermore, the particle size for the nanomaterial Fe7S8 was determined to be 29.86 ± 0.87 nm using Scherer’s equation. During the pH profile studies, the optimum pH for the binding of As(III) and As(V) was determined to be pH 4. Batch isotherm studies were performed to determine the binding capacity of As(III) and As(V), which was determined to be 14.3 mg/g and 31.3 mg/g, respectively for 25 °C. The thermodynamic studies indicated that the ΔG for the sorption of As(III) and As(V) ranged from −115.5 kJ/mol to −0.96 kJ/mol, indicating a spontaneous process was occurring. The enthalpy indicated that an exothermic reaction was occurring during the adsorption in which the ΔH was −53.69 kJ/mol and −32.51 kJ/mol for As(III) and As(V), respectively. In addition, ΔS values for the reaction had negative values of −160.46 J/K and −99.77 J/K for the adsorption of As(III) and As(V), respectively which indicated that the reaction was spontaneous at low temperatures. Furthermore, the sorption for As(III) and As(V) was determined to follow the second order kinetics adsorption model.