Stockholm Humic Model

The affinity part of the SHM proposed by Gustafsson (2001) is similar to the WHAM case, but simpler, with less parameters. The proton-binding part is given by Equations 13.2 and 13.3, whereas the monodentate metal binding is allowed only 

FIGURE 13.10 Cadmium binding by HA. The data are from Benedetti et al. (1995). Lines are fits with WHAM VI (4 adjustable parameters). (O) pH = 4 ( ) pH = 6 ( ) pH = 8. (Reprinted with kind permission from Springer Science -I- Business Media Aquatic Geochemistry, Tipping E., Humic ion-binding model VI An improved description of the interactions of protons and metal ions with humic substances, 4,1998, 3-48, Copyright 1998.). 

In the electrostatic part, instead, the SHM uses an impermeable sphere model, applying the basic Stern model (BSM) (Section 12.2.2) with some concepts borrowed from the CD-MUSIC model (Section 12.3). In the SHM, the bulk of the HSs is considered to form gels, modeled using the BSM. However, depending on their configuration and size, some humic molecules may have certain groups outside the gel that behave like normal monomeric molecules. Electrostatic interactions within this part of the HSs are considered to be negligible. This has some similarities with the EPN discussed above (Section 13.3.2), but here, the gel is actually an impermeable sphere. 

In the BSM, the surface charge is related to the surface potential y/o and the diffuse layer potential j/2 by Equation 12.13  

The charge-potential relationship for the spherical geometry assumed is given approximately for a 1 1 electrolyte of concentration c by (Ohshima, Healy, and White 1982) 

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Gustafsson, J.P, Modehng the acid-base properties and metal complexation of humic substances with the Stockholm humic model, J. Colloid Interf. Sci., 244,102, 2001. 

FIGURE 13.1 Scheme of the proton affinity spectra in Tipping Models V and VI and in Gnstaffson Stockholm humic model. 

FIGURE 13.2 Schemes of metal cation affinity spectra according to Tipping s Model VI and Gustafsson s Stockholm humic model. 

Gustafsson, J. R, and D. B. Kleja. 2005. Modehng salt-dependent proton binding by organic soils with the NICA-Donnan and Stockholm humic models. Environmental Science Technology 39, no. 14 5372-5377. 

Gustafsson and coworkers (Khai et al. 2008) applied a similar approach in a study of metal binding in selected Vietnamese soils. The multisurface models considered metal binding to three types of surfaces iron (oxo)hydroxides (modeled using the DLM, Section 12.2.2), organic matter (using either the Stockholm Humic model or... 

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