Unexpected significance of magnesium ion in the coupling effect of mixed divalent cations on polysaccharide membrane fouling

•Cations can regulate the aggregation of polysaccharide chains to affect fouling.•The gel layer homogeneity and permeability are mainly related to the TEP type.•Amorphous-TEP form a compact fouling layer with high filtration resistance.•Particulate-TEP form a porous gel layer, pre-filtrate foulants and alleviate fouling.•The resistance of fouling layer to cleaning is also subject to the Ca2+/Mg2+ ratio.