The mutual effects of Cu(II) and phosphate on their interaction with gamma-Al2O3 are investigated by using batch experiments combined with density functional theory (DFT) calculations. The results of batch experiments show that coexisting phosphate promotes the retention of Cu(II) on gamma-Al2O3, whereas phosphate retention is not affected by coexisting Cu(II) at low initial phosphate concentrations (<= 3.6 mg P/L). Cu-phosphate aqueous complexes control Cu(II) retention through the formation of type B ternary surface complexes (where phosphate bridges gamma-Al2O3 and Cu(II)) at pH 5.5. This deduction is further supported by the results of DFT calculations. More specifically, the DFT calculation results indicate that the type B ternary surface complexes prefer to form outer-sphere or monodentate inner-sphere binding mode under our experimental conditions. The enhancement of phosphate retention on gamma-Al2O3 in the presence of Cu(II) at high initial phosphate concentrations (>3.6 mg P/L) may be attributed to the formation of 1:2 Cu(II)-phosphate species and/or surface precipitates. Understanding the mutual effects of phosphate and Cu(II) on their mobility and transport in mineral/water environments is more realistic to design effective remediation strategies for reducing their negative impacts on aquatic/terrestrial environments. (C) 2012 Elsevier B.V. All rights reserved.