Silica Gel Loaded with o-Dihydroxybenzene: Design, Metal Sorption Equilibrium Studies and Application to Metal Enrichment Prior to Determination by Flame Atomic Absorption Spectrometry

Abstract.

The aminopropyl silica gel (APSG) prepared by reaction of activated silica gel with 3-aminopropyltriethoxysilane on reaction with 3,4-dihydroxybenzaldehyde has resulted in a new chelating matrix, o-dihydroxybenzene (DHB) anchored silica gel, which is characterized by IR, TGA and elemental analyses. APSG is characterized with ¹³C CPMAS NMR spectroscopy. DHB anchored silica gel sorbs quantitatively (97.4–99.2% recovery) Cu(II), Pb(II), Fe(III), Zn(II), Co(II), Ni(II) and Cd(II) at pH 6.0–7.5, 5.0–7.0, 5.5–7.0, 6.0–8.0, 6.5–8.0, 5.5–7.0 and 6.5–7.5, respectively. The sorption capacity varies from 32 to 348 µmol g⁻¹ and is highest for copper. Desorption was found to be quantitative with 1.0–3.0 mol L⁻¹ HCl/HNO₃ (for Pb). The optimum flow rate of the solution for quantitative sorption of metal ions on a column (10 cm × 10 mm) packed with 1 g of the modified silica gel is 1.0–4.0 mL min⁻¹, whereas for desorption it is 2.0 −4.0 mL min⁻¹. The tolerance limits for NaCl, NaBr, NaI, NaNO₃, Na₂SO₄, Na₃PO₄, humic acid, EDTA, ascorbic acid, citric acid, sodium tartrate, Ca(II) and Mg(II) in the sorption of all the seven metal ions are reported. The preconcentration factors are between 100 and 300 and t_1/2 values ≤17 min. The present matrix coupled with FAAS has been used to enrich and determine the seven metal ions in river and tap water samples (RSD 1.4–7.0%) and in synthetic certified water samples SLRS-4 (NRC, Canada) with an RSD of 2.73–2.83%. The cobalt present in pharmaceutical vitamin tablets and Zn in milk powder was preconcentrated on DHB anchored silica gel and determined by FAAS (RSD of 2.00 to 2.72%).