<p>This study investigates the synergistic oxidation and precipitation behavior of iron-arsenic interactions in the Fe(II)/Fe(III)–As(III)–SO₄^2– system. It also defines the boundaries of arsenic immobilization technology by evaluating the impact of various process conditions on arsenic fixation. A key focus is comparing the distinct pathways driven by Fe(II) and Fe(III) in arsenic phase transformation and discussing the underlying mechanisms. Results show significant differences in the products obtained with Fe(II) and Fe(III) as the iron source. The Fe(II)-based system yields stable scorodite, while the Fe(III)-based system undergoes a phase transition from tooeleite to scorodite and ultimately to a Fe₄(AsO₄)₃(SO₄)y(OH)x phase (referred to as BFAS, where x + 2y = 3). Both the BFAS and scorodite phases exhibit distinct formation conditions, but scorodite synthesis is more cost-effective and environmentally friendly. This technology is highly adaptable to arsenic-containing wastewater from non-ferrous metal smelting processes, demonstrating high arsenic fixation efficiency. It offers significant potential for broad application in environmental treatment. This research presents a comprehensive and efficient strategy for the environmentally friendly treatment of arsenic-containing wastewater from non-ferrous smelting processes.</p>