Integrated evaluation of five coagulants and precipitants for fluoride and turbidity removal from steel industry wastewater: Removal performance, sedimentation, and life cycle assessment

Effective removal of fluoride and turbidity from steel industry wastewater is essential for environmental protection and water reuse. This study evaluates five industrial-grade coagulants and precipitants, including alum ( Al ₂ ( SO ₄ ) ₃ ), poly aluminium chloride ( PACl ), sodium ferrate solution( Na 2 FeO 4 : 0.50 %, sodium hydroxide ( NaOH ): 49.5 %, H 2 O : 50 %), calcium oxide ( CaO ), and ferric chloride ( FeCl ₃ ), under acidic (pH 4), neutral (pH 7), and alkaline (pH 10) conditions using real steel effluent. Treatment performance was assessed in terms of fluoride and turbidity removal efficiency, sedimentation behaviour, sludge production, and life cycle carbon emissions. Aluminium-based coagulants ( Al ₂ ( SO ₄ ) ₃ and PACl ) showed the highest fluoride removal efficiency, reducing fluoride concentrations from about 16 mg / L to below 2 mg / L under all the pH tested at optimal dosages ( 125 − 275 mg / L ). Fluoride removal mainly occurred through adsorption and surface complexation onto Al ( OH ) ₃ flocs. Sodium ferrate solution achieved moderate fluoride removal (final concentration 7.5 − 9 mg / L ) via adsorption onto Fe ( OH ) ₃ formed after ferrate decomposition. For turbidity, sodium ferrate solution performed best, decreasing turbidity from above 12 NTU to below 1 NTU under all pH conditions, mainly through oxidation, charge neutralization, and sweep flocculation. It also produced the fastest settling flocs and the lowest wet sludge volume (about 2 % of treated volume). Life cycle assessment (LCA) indicated that sodium ferrate solution had the lowest CO ₂ emission (0.089 kg CO ₂ / m³ ), attributed to low chemical dosage and minimal sludge generation. Although aluminium coagulants achieved superior fluoride removal, their higher sludge production increased the environmental burden.