Lab-scale experimental study on induced co-crystallization removal of iron， manganese， and hardness from groundwater in a chemical crystallization cyclic granulation fluidized bed

Groundwater and certain industrial water sources commonly exhibit elevated hardness and excessive iron/manganese concentrations. In this study， an induced co-crystallization process was employed to address these water quality issues， and dynamic experiments were conducted in a chemical crystallization cyclic granulation fluidized bed. The effects of seed crystal type， static bed height， upflow velocity， and NaOH dosage on treatment efficiency were systematically investigated. The morphological and compositional changes of the seed crystals before and after the reaction were characterized using scanning electron microscopy-energy dispersive X-ray spectroscopy （SEM-EDS）， X-ray diffraction （XRD）， and Fourier transform infrared spectroscopy （FTIR）. The results indicated that calcite seed crystals achieved the optimal induced co-crystallization performance. The optimal parameters were determined as a static bed height of 70 cm， an upflow velocity of 35 m/h， and an NaOH dosage of 200 mg/L. Under these conditions， the removal efficiencies reached 69.40% for Ca²⁺， 64.50% for Fe²⁺， and 72.82% for Mn²⁺. Characterization results revealed that the initially smooth seed crystal surfaces became rough with substantial precipitate formation. EDS analysis detected significant iron enrichment on the crystal surfaces， while XRD and FTIR analysis further confirmed the presence of composite co-deposition layers containing Ca， Fe， and Mn， revealing a synergistic mechanism involving adsorption， co-precipitation， and heterogeneous nucleation. This study demonstrates that the induced co-crystallization process can serve as an efficient treatment technology， achieving simultaneous water softening and iron/manganese removal， thereby providing a theoretical basis and technical reference for treating groundwater with similarly complex water quality.