Beat Müller

Adsorption of lead(II) on the goethite surface: Voltammetric evaluation of surface complexation parameters

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++H+, log KS1= -0.52, 2 FeOOH + Pb+2 ⇄(FeOO)2Pb+2H+, log βS2= -6.27 kgdm3 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.

Interaction of trace metals with natural particle surfaces: Comparison between adsorption experiments and field measurements

The distribution of the metal ions Zn and Pb between particulate and dissolved phase in river Glatt was studied by field measurements and compared with calculated simulations, using parameters obtained by adsorption experiments with natural suspended particulate material. Differences in distribution coefficients obtained from field data are observed in function of the sampling locations and of the composition of the particulate matter.Experiments in which metal ion solutions are titrated with a suspension of natural particles and analyzed by anodic stripping voltammetry, are interpreted in terms of binding capacities and conditional stability constants of Zn and Pb with the surface sites. Binding constants of a particular metal ion varied very little for all samples. We obtained mean values for the conditional average complex formation constants at pH 8 of: logcondKPb = 9.44 ± 0.18 and logcondKZn = 8.17 ± 0.20. At this pH, binding capacities of 5 ⋅ 10−3 – 1.7 ⋅ 10−2 mol/kg of particles were obtained for samples collected at different locations and times; organic material, iron and manganese oxides are considered to be the main components that control the adsorption to the particles.Distribution coefficients are calculated from the experimentally obtained binding capacities and conditional stability constants. Calculated distribution coefficients for Zn agree with those obtained from the field data and are not very sensitive to changes in the composition of the solution. Good agreement was obtained for lead as well; for some samples it was important to take two types of sites with different affinity into consideration.

Amperometric detection of gaseous ethanol and acetaldehyde at low concentrations on an Au–Nafion electrode

An amperometric sensor capable of detecting mixtures of ethanol and acetaldehyde in the low ppb range without the need for prior separation is described. The electrochemical cell was based on a Au sensing electrode chemically deposited onto one side of a Nafion membrane with 1 M NaOH internal electrolyte solution. The detection was achieved by applying two potentials, –450 and –290 mV vs. a saturated mercury–mercurous sulfate electrode (MSE), at which ethanol and acetaldehyde react at different rates. Under the conditions investigated, acetaldehyde oxidation was mass transport limited at both potentials, whereas the anodic current due to the oxidation of ethanol was 40% lower at the more cathodic potential. Detection limits of 2 and 1 ppb (S/N = 3) were determined for ethanol and acetaldehyde respectively when the analyte species were detected individually. Poisoning of the working electrode was not observed for concentrations of ethanol in the ppb range. Acetaldehyde oxidation was found not to affect the sensing electrode condition, even at concentrations of several tens of ppm.