Modeling of copper(II) and lead(II) adsorption on kaolinite-based clay minerals individually and in the presence of humic acid

The aim of this study is to explain how clay minerals adsorb heavy metals individually and in the presence of humic acid, and to model heavy metal adsorption specifically based on surface–metal binary and surface–metal–ligand ternary complexation. The adsorption of Cu(II) and Pb(II) on kaolinite-based clay minerals has been modeled by the aid of the FITEQL3.2 computer program using single- and double-site binding models of the Langmuir approach. Potentiometric titrations and adsorption capacity experiments were carried out in solutions containing different concentrations of the inert electrolyte NaClO4; however, the modeling of binary and ternary surface complexation was deliberately done at high ionic strength (0.1 M electrolyte) for eliminating adsorption onto the permanent negatively charged sites of kaolinite. A “two-site, two pKaKa” model was adapted, and as for the two surface sites responsible for adsorption, it may be arbitrarily assigned that S1OH sites represent silanol and organic functional groups such as carboxyl having pKaKa values close to that of silanol, and S2OH sites represent aluminol and organic functional groups such as phenolics whose pKaKa values are close to that of aluminol, as all the studied clays contained organic carbon. Copper(II) showed a higher adsorption capacity and higher binding constants, while lead(II), being a softer cation (in respect to HSAB theory) preferred the softer basic sites with aluminol–phenol functional groups. Heavy metal cations are assumed to bind to the clay surface as the sole (unhydrolyzed) M(II) ion and form monodentate surface complexes. Cu(II) and Pb(II) adsorption in the presence of humic acid was modeled using a double-site binding model by the aid of FITEQL3.2, and then the whole system including binary surface–metal and surface–ligand and ternary surface–metal–ligand complexes was resolved with respect to species distributions and relevant stability constants. Electrostatic effects were accounted for using a diffuse layer model (DLM) requiring minimum number of adjustable parameters. Metal adsorption onto clay at low pH increased in the presence of humic acid, and the metal adsorption vs pH curves of metal–kaolinite–humic acid suspensions were much steeper (and distinctly S shaped) compared to the wider pH-gradient curves observed in binary clay–metal systems. The clay mineral in the presence of humic acid probably behaved more like a chelating ion-exchanger sorbent for heavy metals rather than being a simple inorganic ion exchanger.

Graphical abstractThe figure was drawn as a result of modeling of Cu(II) and humic acid adsorption on kaolinite-based clay by the aid of FITEQL program. It describes metal adsorption (as experimental and model curves) in the ternary Cu(II)–humic acid–clay system as mol adsorbed/L liquid solution vs pH (for 50 g/L solid-to-liquid ratio, 0.004% humic acid, 10 ppm metal nitrate in 0.1 M NaClO4 solution). Various surface species are symbolized.Figure optionsDownload full-size imageDownload as PowerPoint slide