Application of surface complexation models to describe the K+, NO3-, Ca2+,SO42-, F-, Co2+ and Cr3+ ions adsorption onto the surface of silica gel

The surface complex formation of K+, NO3-, SO42-, Ca2+, F-, Co2+, Cr3+ ions was examined on the surface of silica gel. Experimental data obtained by a cid-base titration of a solution containing adsorbent were calculated using the Fiteql fitting program applying a triple layer model. The value the of the deprotonation constant is log K=7.6+/-0.2. The value of the protonatio n constant of silica gel is log K+ = 2.2+/-0.6, depending strongly on the m odel used. The ion-pair formation constants of K+, NO3-, Ca2+, SO42- are: l og K-ipK = 0.5+/-0.3; log K(ipNO3)similar to 0 (it is not measurable), log K-ipCa: 0.7+/-0.3 and logK(ipSO4): 0.3+/-0.2. These values are around zero, therefore these ions can be considered as non-specifically binding ions in the investigated pH range of pH= 2.6-9. The determined surface complex for mation constant of F is logK SiF = 4+/-0.4. The ion-pair formation constant s of Co2+, Cr3+ ions are: logK(ipSOCo[+]) = 4.35+/-0.36, logK(ipSOCr(OH)2) = 3.6+/-0.36. It is shown that the goodness of optimization itself, i. e. WSOS/DF < 20, i s not a good test for the applied equilibrium model. The results are affect ed by the relatively large error of the values of ion-pair formation consta nts used for inert ions. This uncertainty can be reduced using the modified triple layer model.