In situ monitoring of copper(II) fixation on silica gel in aqueous ammonia by means of dielectric measurements and quantitative analysis of adsorbed species

The accumulation of copper(II) on silica surfaces, following the general reaction 2(≡SiOH)+Cu(NH3)2+4⇌(≡SiO)2Cu(NH3)2+2 NH+4 has been monitored from aqueous suspensions of silica gel by high frequency impedance measurements. Experiments were carried out either in the static (batch) mode from decanted silica gel or in the dynamic mode by means of a flow through cell mounted on a silica column. Samples with various copper(II) loadings were prepared and the amount of adsorbed species was monitored by chemical analysis, X-rays photoelectron spectrometry, voltammetry at the silica modified electrode and dielectric experiments. The advantages of each method were discussed in terms of sensitivity and possibility of in situ studies. Dielectric measurements performed from both silica gel precipitates and silica columns were found to provide useful information very rapidly and under non-destructive conditions on the adsorption isotherm for copper(II) on silica gel (maximum loadings, breakthrough curves). They also support the assumption of the covalent character of the SiO–Cu bond.