Influence of dissolved Al(III) on electrochemical iron removal from coal fly ash hydrochloric acid leachate

Efficient iron removal is essential for the high-value utilization of coal fly ash. Electrochemical treatment of hydrochloric acid leachate provides a promising alternative to conventional ion-exchange processes. However, the influence of high-concentration Al(III) commonly present in fly ash leachates remains unclear. This study systematically investigated the influence of AlCl₃ over a wide concentration range on electrochemical iron removal. Metal speciation, iron removal efficiency, Faradaic efficiency, and deposit characteristics were analyzed. Under strongly acidic and high-Al conditions, both Fe(III) and Al(III) predominantly existed as [MCl₂]⁺ complexes. Increasing AlCl₃ concentrations stabilized the solution at low pH, suppressed hydroxide precipitation, and improved process stability. Al(III) inhibited the direct reduction of Fe(III). Meanwhile, it suppressed the competing hydrogen evolution reaction and reduced bubble-induced disturbances. This promoted Fe²⁺ reduction and enhanced iron deposition efficiency. During the first hour of electrolysis, iron removal efficiency increased with AlCl₃ concentration, reaching 51% at 40 g/L Al. After 6 h of electrolysis, iron removal reached 93% was achieved. AlCl₃ also refined iron grain size, while excessive concentrations induced structural defects such as cracks. This work clarifies the influence of dissolved Al(III) in electrochemical iron removal and provides practical guidance for coal fly ash leachate treatment.