Solvation and electrostatic model for specific electrolyte adsorptio

Experimental evidence indicates that adsorption of symmetrical electrolytes depends on the specific electrolyte and on the mineral. The triple-layer model allows for specific electrolyte adsorption but requires numerous parameters: surface protonation constants $rm (Ksb{s,1}, Ksb{s,2}),$ electrolyte adsorption constants $rm(Ksb{s,Msp+}, Ksb{s,Lsp-}),$ capacitances $rm(Csb1, Csb2),$ and site-density $rm(Nsb{s}).$ The applicability of the triple-layer model would be facilitated by reducing the number of parameters obtained by fitting experimental data. Furthermore, existing surface speciation codes are restricted to low ionic strengths ($<$0.5 M), limiting their use in many geochemical systems.;The proposed solvation and electrostatic model relates triple-layer model parameters to physical and chemical properties of the system. Surface protonation constants $rm(Ksb{s,1}, Ksb{s,2})$ depend on the inverse dielectric constant of the k-th mineral $(1/rmvarepsilonsb{k})$ and on the Pauling bond strength per bond-length $rm(s/rsb{>S{-}OH}).$ It is proposed that electrolyte adsorption constants $rm (Ksb{s,Msp+}, Ksb{s,Lsp-})$ are a sum of contributions from an ion-intrinsic and a solvation component. According to solvation theory. $rm Ksb{s,Msp+}$ and $rm Ksb{s,Lsp-}$ representing the equilibria$$rm{>}SOsp-+Msbsp{aq}{+} = {>}SOsp{-}-Msp+$$and$$rm{>}SOHsbsp{2}{+}+Lsbsp{aq}{-}= {>}SOHsbsp{2}{+}-Lsp-$$can be linearly correlated with the inverse dielectric constant of the mineral resulting in$$eqalign{rm log Ksb{s,Msp+} &=rm -{DeltaOmegasb{Msp+}over2.3RT} left({1overvarepsilonsb{k}}right)+log Ksbsp{ii,Msp+}{primeprime} andcrrm log Ksb{s,Lsp-} &=rm {-}{DeltaOmegasb{Lsp-}over2.3RT} left({1overvarepsilonsb{k}}right)+log Ksbsp{ii,Lsp-}{primeprime}}$$The ion-intrinsic part $rm(log Ksp{primeprime}sb{ii})$ and the interfacial solvation coefficient $(DeltaOmegarmsb{j}),$ respectively, are inversely related to the electrostatic radius of the aqueous and the adsorbed ion. As a first approximation, capacitance (C$sb1$) is related to the electrostatic radius and the solvation coefficient of the aqueous electrolyte. Model results are consistent with $rm Csb2 = 0.2$ Farads/m$sp2$ and N$rmsb{s}$ from isotopic-exchange experiments.;The model is calibrated on twenty-seven sets of experimental surface titration data for nine minerals (anatase, rutile, hematitie, goethite, magnetite, amorphous silica, $delta$-MnO$sb2$, corundum, $gamma$-alumina), in ten electrolytes (LiCl, NaCl, KCl, CsCl, LiNO$sb3,$ NaNO$sb3,$ KNO$sb3,$ LiNO$sb3$ NaI and NaClO$sb4)$ over a range of ionic strengths (0.001-2.9M). The date were analyzed using the program GEOSURF, based on HYDRAQL and MINEQL. GEOSURF uses the extended Debye-Huckel Equation for activity coefficients which is valid to ionic strengths $>$0.5 M.;The proposed model permits prediction of surface protonation and electrolyte adsorption for a variety of oxides and electrolytes, accounting for both mineral and electrolyte.