| Abstract: |
The adsorption of Pb(II) to the goethite surface was investigated in dilute solutions in the pH range 6.6–8.2. Increments of goethite suspension were added to dilute (2.41 × 10−7M) Pb(II) solutions at constant pH in a titration procedure. Differential pulse anodic stripping voltammetry (DPASV) was used to determine the concentration of free, nonadsorbed Pb(II) in goethite suspensions without prior separation of the solution and solid phases. This procedure permits achievement of high adsorption density at relatively low total metal concentrations and correction for adsorption losses to the reaction vessel. Surface reaction parameters were obtained using different methods of interpretation of the experimental data. Surface complex formation constants for monodentate and bidentate surface complexes are
≡FeOOH + Pb+2 ↔ ≡FeOOPb+, log K1S = −0.52,
2 ≡FeOOH + Pb+2 ↔ ≡(FeOO)Pb + 2H+, log β2S = −6.27 kg/dm3.
Intrinsic acidity constants obtained from acid-base titrations are, respectively, log Kal5 = −6.7, and log Ka25 = −9.0; the exchangeable H+ of the goethite surface was determined as 0.135 mole/kg. The Pb(II) adsorption capacity of the goethite is 5.6 × 10−2 mole/kg (3.8 × 10−6 mole/m2). The average ligand number was found to be 1.25 and constant throughout the concentration range of adsorption. The determined surface reaction parameters describe the effect of changes in pH and solution composition on Pb(II) binding and give a consistent and comprehensive thermodynamic description of the adsorption process within variations of Pb(II) concentration and particles by factors of 3000 and 1000, respectively. The methods developed here can readily be applied to the determination of adsorption parameters of natural particulate matter. |
