AbstractThe aim of this work is to investigate and model the simultaneous adsorption of Cu(II), Cd(II), and Pb(II) in the presence and absence of humic acid on kaolinite‐based clays. The preliminary capacity estimation of clays for metal was made with the use of a modified Langmuir approach, and adsorption data collected at various pH were processed using the FITEQL 3.2 computer program to establish the model. The three types of surface sites responsible for adsorption were considered to be the permanent charge sites X, and variable charge sites comprised of S1OH silanol groups and S2OH aluminol groups of kaolinite‐based clays. Heavy metal cations were assumed to bind to the surface in the form of outer sphere and inner sphere monodentate complexes. When humic acid was added, divalent metal ion adsorption was modeled using a multisite binding model by the aid of FITEQL 3.2. Since the stability of the ternary surface complexes in the presence of humic acid was higher than that of the corresponding binary heavy metal cation complexes, the adsorption versus pH curves were steeper (and distinctly S‐shaped) compared with the tailed curves observed in binary clay‐metal ion systems, probably due to the fact that humic acid‐coated kaolinite essentially constituted the active surface for metal sorption. Although competitive metal adsorption from (metal ions mixture+humate) solutions was generally lower than those from individual metal ion solutions, Cd2+, being the metal ion with the highest affinity toward permanent charge sites, was the least affected ion at relatively low pH from competitive adsorption. © 2008 American Institute of Chemical Engineers Environ Prog, 2009