BACKGROUND: To elucidate the process and mechanism of Cu(II) biosorption onto aerobic granules, the influence of pH and ionic strength (IS) on the Cu(II) biosorption capacity and biosorption mechanism was studied. RESULTS: The biosorption of Cu(II) onto aerobic granules under different conditions of pH (3, 4 and 5), IS (0, 0.1 and 0.5mol L-1) and Cu(II) concentration (25-250 mg L-1) was investigated. The correlation coefficients of the pseudo-second-order kinetic model were R-2(2) > 0.99, while those of the Langmuir and Freundlich models were R-1(2) > 0.975 and R-2(2) > 0.955 respectively. The biosorption of Cu(II) increased with increasing pH, while the effect of IS on the biosorption was complicated, which could be explained by the competition among different metallic ions, colloidal chemistry theory or Derjaguin, Landau, verwey and Overbeek (DLVO) theory. About 70% of the solid phase Cu(II) was exchanged by Na(I), Ca(II) and Mg(II) released from the aerobic granules at pH 4 and 5. The results revealed that ion exchange is the most important biosorption mechanism but that other mechanisms also play a part. CONCLUSION: The sorption performance can be optimised by adjusting the pH and IS. Aerobic granules can be used as an alternative effective, economical and practical biosorbent for heavy metal removal. (c) 2008 Society of Chemical Industry.