The adsorption of divalent metal ions Cu(2+), Pb(2+), Zn(2+), and Cd(2+) on coffee grounds as a function of coffee grounds concentration was studied in which adsorption density decreased as the concentration of coffee grounds (C(s)) increased. Adsorption studies were conducted by equilibrating aqueous solutions of each metal ion at concentrations in the range 19-291 mu mol L(-1) with coffee suspensions in the concentration range 0.971-8.738 g L(-1), with the initial pH adjusted to 5.0 +/- 0.1 using NaOH or HNO(3), Metastable Equilibrium Adsorption theory did not adequately explain the adsorption phenomenon, except at low concentrations of coffee grounds and trace metal ions. Instead the results indicated that flocculation might reduce the surface availability thus reducing the adsorption density. The flocculation theory was confirmed after a further experiment adding dispersant sodium hexa-meta-phosphate (NaHMP) to the suspension. (C) 2009 Elsevier Ltd. All rights reserved.