GENERALIZED TWO-LAYER MODEL

The central component of the GTLM is the Two-Layer (or Diffuse Layer) Model proposed by Stumm and co-workers (Stumm et al., 1970; Huang and Stumm, 1973). This basic model was expanded in order to fit inorganic ion sorption data at moderate-to-high sorbate/sorbent ratios. In particular, the model includes two site types rather than one for cation binding, and surface precipitation (Farley et al., 1985) at high cation and anion concentrations.

The GTLM was originally implemented in MICROQL (Westall, 1979a, 1979b).  The modified MICROQL code for the Two-Layer Model (does not include surface precipitation) and for the GTLM (includes surface precipitation) can be obtained by downloading the files below.  The .doc files included with each set of files explains the various files and how they should be linked.  If the download feature below does not work with your web browser, contact David Dzombak by email (dzombak@cmu.edu) and the files will be emailed to you.

Two-Layer Model (double click on each file to download via ftp) 

Generalized Two-Layer Model (double click on each file to download via ftp)

The GTLM without the surface precipitation component, which is needed only for very high concentrations of inorganic ions, has been implemented in the widely-used general chemical equilibrium programs MINTEQA2 (available from the USEPA Center for Exposure Assessment Modeling (CEAM) in Athens, GA) and MINEQL+ (available from Environmental Research Software). The GTLM database for inorganic ion sorption on hydrous ferric oxide has been included in these programs as well. Both programs have been developed for use on an IBM PC or compatible microcomputer.

MINEQL+, which has a very nice Windows-based user interface, can be obtained from Environmental Research Software.  The complete MINEQL+ package includes the most recent version of the software and a detailed manual on its use (Schecher and McAvoy, 1998). The manual contains technical guidance on the use of MINEQL+ and example problems, including example applications of the GTLM prepared by David Dzombak.

REFERENCES

Dzombak, D.A. and Morel, F.M.M. (1990), Surface Complexation Modeling: Hydrous Ferric Oxide, Wiley-Interscience, New York.

Farley, K.J., Dzombak, D.A., and Morel, F.M.M. (1985), "A Surface Precipitation Model for the Sorption of Cations on Metal Oxides," J. Colloid Interface Sci., 106:226-242.

Huang, C.P. and Stumm, W. (1973), "Specific Adsorption of Cations on Hydrous -Al2O3 ," J. Colloid Interface Sci., 43:409-420.

Schecher, W.D. and McAvoy, D.C. (1998), MINEQL+: A Chemical Equilibrium Modeling System, User's Manual, Version 4.0, Environmental Research Software, Hallowell, ME 04347.

Stumm, W., Huang, C.P., and Jenkins, S.R. (1970), "Specific Chemical Interactions Affecting the Stability of Dispersed Systems," Croat. Chem. Acta, 42:223-244.

Westall, J.C. (1979a), "MICROQL. I. A Chemical Equilibrium Program in BASIC," Technical Report, Swiss Federal Institute of Technology, EAWAG, Dubendorf, Switzerland.

Westall, J.C. (1979b), "MICROQL. II. Computation of Adsorption Equilibria in BASIC," Technical Report, Swiss Federal Institute of Technology, EAWAG, Dubendorf, Switzerland.