Mechanistic Insights into Ferric Chloride Effects on the Modified Induced-Crystallization Softening (ICS) Process

Elevated total hardness in drinking water promotes scaling, reduces detergent efficiency, and poses health risks. Pellet fluidized bed crystallization offers an economical and environmentally sustainable softening strategy. However, its performance is constrained by water quality fluctuations, limited utilization of induced-crystallization seeding grains (ICSG), and poor reactor stability. Ferric chloride (FeCl3), which is a widely used coagulant, facilitates particle aggregation, interfacial binding, and heterogeneous crystallization. This study therefore investigates the enhancing effect of FeCl3 on the modified induced-crystallization softening (ICS) process and elucidates the underlying molecular mechanisms using density functional theory. Results demonstrate that FeCl3 effectively reduces effluent magnesium hardness, turbidity, and pH through coagulation and hydrolysis and improves ICSG softening capacity by 25–40%. Lower Gibbs free energies enhanced the heterogeneous crystallization of CaCO3 and Mg(OH)2 on the ICSG surfaces by 67%. FeCl3 also suppressed homogeneous CaCO3 crystallization while promoting heterogeneous crystallization, thereby reducing interparticle adhesion forces and hydraulic head loss by 15% and 18%, respectively. These combined effects enable energy-efficient modified ICS operation with a lower head loss and inlet pressure requirements. The optimal FeCl3-to-softening agent (NaOH) dosage ratio was determined to be 1:40–1:20, ensuring compliance with drinking water safety standards.