Adsorption Behaviour of Delta-Manganese Dioxide in Relation to its Use as a Resin in Trace Metal Speciation Studies

Abstract

<p>The accuracy of the δ-MnO₂ method in determining Conditional Stability Constants (CSC's) for trace metal - organic complexes depends on the precise modeling of trace metal uptake by δ-MnO₂. Accordingly, characteristics of δ-MnO₂ and its adsorption behaviour for Cu were studied.</p> <p>Laboratory preparation of δ-MnO₂ is influenced by pH. A neutral redox process and an acid reduction of Mn⁺⁷ yield δ-MnO₂, but an alkaline oxidation of Mn²⁺ does not produce δ-MnO₂. Positive identification of MnOᵪ as δ-MnO₂ includes confirmation of characteristics such as x > 1.9, an adsorption capacity (ɼmax) ≥ 0.25 mol Cu/mol MnO₂, an amorphous XRD pattern, a specific TEM morphology and 1 < pHzpc < 2.</p> <p>Natural and simulated aging of δ-MnO₂ depends on temperature, pH and K content of the solid, and causes reduction of surface area and adsorption capacity by a factor 1.5 (natural) to 7 (simulated).</p> <p>Adsorption of trace metals onto δ-MnO₂ is described almost exclusively by a linearized Single Langmuir isotherm in the literature. This study found that Cu adsorption on δ-MnO₂ deviates from Langmuir linearity at low surface coverages. Explanations include the existence of two adsorption sites (Double Langmuir model) and non-constant activity of surface groups. The Implicit Langmuir expression is derived from surface complexation theory to model the latter, and has the form:</p> <p>[equation removed]</p> <p>where ɼads (= mol Cu adsorbed/mol MnO₂) and the free Cu²⁺ concentration in solution are measured. r ,B and n are the adsorption parameters. This model predicts the observed deviation from linearity. The bindings energy depends on the pH, the H⁺/Cu²⁺ exchange ratio (n), the surface coverage (ɼads/ɼmax) and a constant B.</p> <p>Adsorption isotherms were obtained over a pH range of 6 to 8.5, and a Cutotal range of 1 - 40 μM with glycine added to prevent precipitation of Cu. Adsorption results were fitted to Single, Double and Implicit Langmuir models, and compared. The Implicit Langmuir model describes Cu uptake by δ-MnO₂ most accurately, and CSC's for Cu-NTA and Cu-glycine complexes were readily determined using this model.</p> <p>¹⁴C studies indicated that NTA, glycine and aspartic acid do not adsorb on δ-MnO₂.</p>