Colloids and Surfaces A: Physicochemical and Engineering Aspects 203 (2002) 105 – 116 Complexation of heavy metals by humic acids: analysis of voltammetric data by polyelectrolyte theory Marc A.G.T. van den Hoop a, *, Rodolfo D. Porasso b , Julio C. Benegas b a National Institute of Public Health and Enironment, PO Box 1, 3720 BA Bilthoen, The Netherlands b Department of Physics -IMASL, National Uniersity of San Luis, 5700 San Luis, Argentina Received 26 March 2001; accepted 8 October 2001 Abstract The complexation of heavy metals by humic acids has been studied by voltammetry for a number of different cases, including (a) different metal ions; (b) various concentrations of the supporting salt; (c) different total metal concentrations and (d) different charge densities of the humic acid. The voltammetric speciation data are compared with predictions of a theoretical model which, besides polyelectrolytic interactions, takes into account: (i) chemical binding of metal ions; (ii) ionic strength effects; (iii) entropic effects; and (iv) competitive interactions between counterions of different valences. All experimentally obtained speciation data agree well with theoretical predictions by considering a single value of the intrinsic free energy of binding, which appeared to be the same for the Cd and Zn metal ions. This means that under the presently applied experimental conditions the interactions between the humate ion and the metal ions is similar. It is furthermore shown that with the present experimental-theoretical procedure a very consistent and precise behavior of the stability constant of the complex is reached in the wide range of the physico-chemical solution variables reported in this work. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Humic acid; Polyelectrolyte theory; Voltammetric data www.elsevier.com/locate/colsurfa 1. Introduction Knowledge of the complexation of heavy metals with humic acids is of great importance in the understanding of metal (bio)availability and mobility in natural aquatic systems. Studying these systems should result in obtaining the neces- sary information about the distribution of the metal ions over different physico-chemical forms, i.e. their speciation. Although nowadays metal association by simple inorganic and organic molecules can be fairly well described, for the case of heavy metal/humic acid systems, the physico- chemical processes involved are still not well un- derstood. From an experimental point of view, voltammetry has been shown a valuable analytical technique to determine the actual metal speciation in natural aquatic solutions, especially at low metal concentrations. In general, voltammetry is a technique that measures the diffusion properties * Corresponding author. Tel.: +31-30-274-4013; fax: +31- 30-274-4411. E-mail address: magt.van.den.hoop@rivm.nl (M.A.G.T. van den Hoop). 0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0927-7757(01)01090-1