Cadmium sorption by ferrihydrite in the presence of phthalic acid was examined over a range of pH (4.0 ∼ 8.5) conditions and sorbate/sorbent ratios. The presence of phthalic acid enhanced Cd2 +  sorption by forming ternary complexes on ferrihydrite surface, especially at low pH, but for high pH and high total organic ligand/Fe(mmol/mol) ratios, it decreased Cd2 +  sorption onto ferrihydrite by forming soluble complexes with the phthalate in solution. In binary systems, Cd2 +  and phthalic acid sorption by ferrihydrite was well reproduced using the diffuse layer model with sorption constants derived from the experimental data. Prediction using the optimised binary sorption constants for Cd2 +  sorption onto ferrihydrite in the presence of phthalic was poor and achieving a good fit required the inclusion of two additional ternary complexes.

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