Display omitted •SFR variation of alginate complexes with PACl dosage showed a unimodal pattern.•Alginate gel at PACl dosage of 100 mg/L had an ultra-high value of 1.40 × 1015 m−1 kg−1.•Unimodal pattern of SFR was caused by morphology transition from gel to floc.•Morphology transition was caused by the coordination order of Al3+with COOH.•Ultra-high gel SFR could be interpreted by Flory–Huggins lattice theory. While polyaluminum chloride (PACl) coagulation- microfiltration (MF) process is one of the most popular methods for surface water treatment, the fouling mechanism underlying this process has not well explored. In this study, sodium alginate (SA) was used as the model organic matter. It was interestingly found that, specific filtration resistance (SFR) of alginate-aluminum complexes initially increased to an ultra-high value of 1.40 × 1015 m−1 kg−1, and then sharply declined to rather low level of 3.85 × 1012 m−1 kg−1 with the increase in PACl dosage from 0 to 500 mg/L. A series of characterizations of the foulant samples showed that the alginate-aluminum complexes suffered a morphology transition from gel form to floc/cake form with the PACl dosage increase. Density functional theory (DFT) calculation indicated the preferential coordination of aluminum ions with the terminal carboxyl groups of the alginate chains at low PACl level, facilitating homogeneous gel formation. The ultra-high SFR of gel was interpreted by the Flory-Huggins lattice theory. High PACl level resulted in the coordination of aluminum ions with the non-terminal carboxyl groups and reduced the surface charge, and thus, caused gel collapse and floc formation, corresponding to rather low SFR values. Preferential coordination combined with the Flory-Huggins lattice theory revealed in this study offered a molecular level fouling mechanism in the coagulation- microfiltration (MF) process for water treatment.