Pure Appl. Chem., Vol. 73, No. 12, pp. 2005–2016, 2001. © 2001 IUPAC 2005 Humic matter and contaminants. General aspects and modeling metal ion binding* Luuk K. Koopal 1,‡ , Willem H. van Riemsdijk 2 , and David G. Kinniburgh 3 1 Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands; 2 Sub-department of Soil Science and Plant Nutrition, Wageningen University, Dreijenplein 10, 6703 HB Wageningen, The Netherlands; 3 British Geological Survey, Wallingford, Oxon OX10 8BB, UK Abstract: Humic substances are soil and fresh-water components that play an important role in the binding and transport of both organic and inorganic contaminants. Transport of the contaminants due to ground- and fresh-water dynamics is directly related to the risks associ- ated with contaminations. The mobility of soluble humic substances is related to their inter- action with soil mineral particles. Some key references for the binding of organic and inor- ganic contaminants and for the binding of humics to mineral particles are presented. Humic substances also play a role in the analysis of the contaminants in natural waters and with remediation of water or soil polluted with pesticides, heavy metal ions, and radionuclides. These aspects are illustrated with some examples. The problems that are encountered with the modeling of the binding of contaminants to humics and of heavy metal ions in particular are illustrated by considering the nonideal competitive adsorption model (NICA) extended with electrostatic interactions. The NICA-Donnan model gives quite good results for the description of metal ion binding, as is illustrated for metal ion binding to purified peat humic acid (PPHA). Finally, some remarks are made with respect to the use of the NICA-Donnan model in general purpose speciation programs and of simplified versions of the model for predictions under restricted environmental conditions. INTRODUCTION Humic substances are formed in both terrestrial and aquatic systems. They can be insoluble compo- nents that are parts of soil and/or sediment, or be dissolved in ground- or surface water. The soluble humics can also be bound to the other soil/sediment components. The humic matter composition of ecosystems is mainly controlled by a net balance of formation, degradation, and transfer [1–5]. The diagram in Fig. 1 shows the occurrence and flow paths of dissolved humic substances in the natural environment. Owing to the widespread presence of humics, humic matter plays an important role in binding, transport, analysis, and remediation of pollutants in natural environments. In the first part of the paper, a brief overview will be given of humics and contaminant binding, and some references will be pro- vided on the role of humics in monitoring pollutants and in water and soil or sediment remediation. Modeling of contaminant binding to humics and metal ion binding, in particular, will be discussed in *An issue of reviews and research papers based on presentations made at the IUPAC/ICSU Workshop on Electrochemistry and Interfacial Chemistry in Environmental Clean-up and Green Chemical Processes, Coimbra, Portugal, 6–7 April, 2001. ‡ Corresponding author: E-mail: koopal@fenk.wag-ur.nl