Metal Ion Adsorption to Complexes of Humic Acid and Metal Oxides: Deviations from the Additivity Rule ARNOLDUS W. P. VERMEER,* ,²,‡ JENNY K. MCCULLOCH, ² WILLEM H. VAN RIEMSDIJK, § AND LUUK K. KOOPAL ² Laboratory of Physical Chemistry & Colloid Science, Wageningen Agricultural University, Wageningen, P.O. Box 8038, 6700 EK Wageningen, The Netherlands, and Department of Soil Science & Plant Nutrition, Wageningen Agricultural University, Wageningen, P.O. Box 8005, 6700 EC Wageningen, The Netherlands The adsorption of cadmium ions to a mixture of Aldrich humic acid and hematite is investigated. The actual adsorption to the humic acid-hematite complex is compared with the sum of the cadmium ion adsorptivities to each of the isolated components. It is shown that the sum of the cadmium ion adsorptivities is not equal to the adsorption to the complex. In general, the adsorption of a specific metal ion to the complex can be understood and qualitatively predicted using the adsorptivities to each of the pure components and taking into account the effect of the pH on the interaction between humic acid and iron oxide on the metal ion adsorption. Due to the interaction between the negatively charged humic acid and the positively charged iron oxide, the adsorption of metal ions on the mineral oxide in the complex will increase as compared to that on the isolated oxide, whereas the adsorption to the humic acid will decrease as compared to that on the isolated humic acid. As a result, the overall adsorption of a specific metal ion to the complex will be smaller than predicted by the additivity rule when this metal ion has a more pronounced affinity for the humic acid than for the mineral oxide, whereas it will be larger than predicted by the additivity rule when the metal ion has a higher affinity for the oxide than for the humic acid. Introduction The speciation of heavy metal ions has been studied in natural soil systems (1, 2) as well as in model systems (3, 4). Knowledge of the binding of the metal species to soil systems is of importance for the prediction of the movement of these species and for the way in which they are distributed in the environment. McLaren et al. (2) and Bibak (5) have found that metal ion adsorption characteristics of complete soils are controlled to a large extent by their organic matter and metal oxide content. To explain the overall binding, Bibak (5) has compared the adsorption of different metal ions to an entire soil with their adsorption to the single components in this soil system. The calculated adsorptivities, assuming additivity, were only about half the measured values for the entire soil. Although Bibak concludes that the additivity can only be applied under restricted conditions, an alternative procedure is not given. Several authors have simplified the situation by studying the binding of metal ions to well-defined model systems. For instance, the adsorption of metal ions to humic substances (6-12) and to iron oxides (14-22), two important compo- nents of many types of soils, has been investigated in detail. Robertson (4, 23) measured copper binding to a goethite- humic acid mixture and reported a significantly lower adsorption than the simple sum of the adsorptivities of the noninteracting components, similar to what Bibak found for the entire soil. On the other hand, there are also studies that indicate that the adsorption of metal ions onto metal oxide- humic acid mixtures is increased due to the interactions between these two components (1, 3, 4, 24-28). However, these studies mainly compared the metal binding to the mixture with that to the bare oxide only, neglecting the binding to the pure humic. It is clear that, even for simple mineral oxide-humic acid complexes, the additivity of adsorptivities does not apply under all circumstances and that metal ion binding to these complexes is not yet fully understood. Vermeer et al. (12, 29) have studied the overall proton adsorption to a complex of humic acid and hematite. It has been shown that the proton adsorption is clearly affected by the interaction between humic acid and hematite. At low pH, a decreased proton adsorption is observed as compared to the sum of the adsorptivities due to a loss of binding sites for the protons on the humic acid. At relatively high pH, an increased proton adsorptivity occurs at the metal oxide due to the fact that the adsorbed humic acid screens the positive electric field of the metal oxide very effectively. It seems most probably that the adsorption of metal ions will be affected in a similar way. The aim of the present work is therefore to determine at different pH values the binding of heavy metal ions and cadmium ions in particular to iron oxide and humic acid individually and to the complex of these components. To analyze the observations, the sum of the adsorptivities to the individual components will be calculated (the additivity rule), and the result will be compared with the measured adsorption to the complex. Deviations from the additivity rule will be discussed in view of what is known about the effect that the adsorption of humic acid onto the oxide has on the proton adsorptivity. To enable the comparison with the binding to the individual components, we will start with a short description of the metal ion adsorption in the isolated systems (humic acid or hematite) as a function of pH and then consider the adsorption characteristics of cadmium to the complex (humic acid adsorbed to hematite). After the discussion of the cadmium results, literature data on copper adsorption and the differences in adsorption behavior between cadmium and copper will be discussed. Experimental Procedures All experiments have been performed in a thermostated room at 21 ( 1 °C, and the water is purified by percolating it through a mixed-bed ion exchange column and a commercial water purification unit (Elgastat UHP-3 (Elga)). Other chemicals (cadmium nitrate, potassium nitrate, hydrochloric acid, and potassium hydroxide) were obtained from Fluka (p.a. quality) and are used without further purification. * Corresponding author phone: (49) 214 3023988; fax: (49) 214 3050698; e-mail: Ronald.Vermeer.RV@Bayer-AG.de. ² Laboratory of Physical Chemistry & Colloid Science. ‡ Present address: Bayer AG, ZF Biophysik, Building E 41, D-51368 Leverkusen, Germany. § Department of Soil Science & Plant Nutrition. Environ. Sci. Technol. 1999, 33, 3892-3897 3892 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 21, 1999 10.1021/es990260k CCC: $18.00  1999 American Chemical Society Published on Web 09/24/1999