Acid–base and copper(II) sorption properties of a natural lake sediment: Potentiometric and atomic absorption spectrometric characterisation

The interactions of the sediment surface with proton and copper(II) ions were investigated by means of alkalimetric titrations performed on suspensions containing different sediment–solution ratios at ionic strengths of 0.1 and 1.0 mol dm−3. As the equilibrium conditions are critical both in acid and in base media for the considered lake sediment, a batch-titration procedure was adopted, which permitted to fully overcome the data uncertainty related to signal drift. Potentiometric data were computed for the refinement of protonation and complexation constants by means of a program, which minimises the difference between calculated and experimental potential values during the refinement. In the elaboration of the potentiometric data some relevant aspects related to the batch-titration procedure were taken into account. As regards the influence of the ionic medium and ionic strength, the experimental data could be refined by assuming that the surface-active centers act independently of one another, without any appreciable effect due to the intervention of the surface charge. The computation showed that all the independent data were consistent and it was possible to refine three protonation constant values, one referring to groups titratable in the acid region and two to groups titratable in the base one. After the determination of protonation constants, copper(II)–sediment interaction was considered: the stability-constant values were calculated by elaborating pH-metric data recorded on the sediment–copper(II) system; copper(II) not adsorbed on the sediment was determined by atomic absorption spectrometry for each titration point; the good agreement between pCu calculated from potentiometric data and measured pCu, by supposing copper(II) mainly bonded to titratable groups, can be considered as a validation of the method.