Abstract: The smelting industries of nonferrous metals are suffering from the emission of large number of hazardous wastes and high cost of environmental protection during the disposal of waste acid. We proposed to remove arsenic from waste acid using Ca/Fe-enriched coal slag to produce arsenic-stabilized precipitates. In combination with materials' characterization, the reaction behaviors (dosage of coal slag, reaction time and initial pH) between coal slag and waste acid have been investigated, and the reaction mechanism was also explored. The results shows that coal slag mainly composes of Ca and Fe oxides and exhibits high activity for arsenic removal from waste acid. At a coal slag-to-waste acid ratio of 100 g/L, the arsenic concentration decreases from 7 g/L to 118.2 mg/L and its arsenic removal capacity reaches 68.82 mg/g. The stability of arsenic-enriched precipitates increases with the increasing reaction time. Highest arsenic removal efficiency was obtained at an initial pH of 0.98 and it deceases with the increasing pHs. It is found that Fe oxides in coal slag dissolve in waste acid and then Fe³⁺ precipitates arsenate to form ferric arsenate. Under the protection of Si, Al and Ca oxides, the ferric arsenate is fixed in precipitates and results a lower arsenic leaching concentration in leaching testes.