As the increase of pH (>8), the functional groups of Fe3O4@LDHs composites might be progressively deprotonated and formed negative charge.
The electrostatic repulsion between the negatively charged surface sites and electronegative phosphate species HPO42 hardly led to the formation of metal–ligand magnetic composite complexes.
Additionally, it was known that carbonate was highly stable at alkaline pH conditions and the electrostatic interaction between the layer and interlayer was strong under those conditions.
The HPO42 was hard to be adsorbed onto the surfaces of Fe3O4@LDHs composites and the ion exchangeability between HPO42 and interlayer CO32 was lowered. Meanwhile,the concentration of hydroxyl was increased with further pH increase.
The competition for the adsorption sites between OH groups and HPO42 was increasingly fierce. As a result, the alkaline
condition contributed to the decrease of phosphorus adsorption capacity.