Dataset of surface reactions for gwb programs Dataset format: sep22 Surface type: IonEx Model type: ion-exchange gaines-thomas Surface potential: n/a Surface capacitance: n/a Thermo dataset: thermo.tdat * * Example dataset for setting ion exchange reactions. * * Follow this format to set ion exchange in a GWB run. Note that the * user sets the exchange capacity (CEC) from the command line with the * "exchange_capacity" command. * * * STEP 1: Set a basis species. The "sorbed species" must be a basis entry in * the current thermo dataset. The programs will converge most readily, * especially when using the Vanselow convention, if you set as the * basis species a surface complex present in high concentration. * * Be sure that your selectivity constants correspond to the * reaction specified below. Selectivity coefficients reported in * the literature may be normalized to a unit charge equivalent of * exchange sites. -end- 1 basis species >X:Na charge= 0 mole wt.= 22.9898 g sorbed species= Na+ 1 elements in species 1.000 Na -end- * STEP 2: Set the exchange reaction(s). 2 surface species >X2:Ca charge= 0 mole wt.= 40.0800 g 3 species in reaction 2.000 >X:Na 1.000 Ca++ -2.000 Na+ Selectivity coefficient= .0300 >X:K charge= 0 mole wt.= 39.0983 g 3 species in reaction 1.000 >X:Na 1.000 K+ -1.000 Na+ Selectivity coefficient= .0800 -end- * * IMPORTANT NOTES: * * Set a single basis species, which is the surface complex for one * of the exchanging ions. It is customary to choose a monovalent ion * for the basis species. * * All species, basis and secondary, should be uncharged. * * The surface species are formed by ion exchange with the basis species. * * Enter selectivity coefficients as linear, not logarithmic values * (they are converted to log values internally in the program). * * The selectivity coefficients here refer to mass action expressions for * the exchange reaction written in terms of the ion activities, rather * than molalities. To be precise, therefore, data from the literature * presented in the molal convention should be corrected according to * the ions' activity coefficients. * * The GWB recognizes three activity conventions for the sorbed sites: * Gaines-Thomas (the default), Vanselow, and Gapon. You set the convention * in the fourth line of this dataset, and balance reactions accordingly. * * The Gaines-Thomas convention holds that the activity of the sorbed ion * is its equivalent fraction of the sorbing sites. You balance reactions * on the exchanging cations: * >X:Ca + 2 Na+ <-> 2 >X:Na + Ca++ * * The Vanselow convention is the same, except that the sorbed ion activity * is the molar fraction of the sorbing sites. * * In the Gapon convention, you balance reactions on the sites rather than * ions: * >X:Ca(.5) + Na+ <-> >X:Na + 1/2 Ca++ * The sorbed ion activities can be expressed as either equivalent or * molar fractions, since the corresponding reaction coefficients are the same * on both sides of the reaction.