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 Cd 2 + 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 Cd 2 + sorption onto ferrihydrite by forming soluble complexes with the phthalate in solution. In binary systems, Cd 2 + and phthalic acid

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... orption 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 Cd 2 + sorption onto ferrihydrite in the presence of phthalic was poor and achieving a good fit required the inclusion of two additional ternary complexes.