ACID-BASE AND COPPER(II) SORPTION PROPERTIES OF A NATURAL LAKE SEDIMENT - POTENTIOMETRIC AND ATOMIC-ABSORPTION SPECTROMETRIC CHARACTERIZATION

The interactions of the sediment surface with proton and copper(II) io ns were investigated by means of alkalimetric titrations performed on suspensions containing different sediment-solution ratios at ionic str engths of 0.1 and 1.0 mol dm(-3). As the equilibrium conditions are cr itical both in acid and in base media for the considered lake sediment , a batch-titration procedure was adopted, which permitted to fully ov ercome the data uncertainty related to signal drift. Potentiometric da ta were computed for the refinement of protonation and complexation co nstants 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 rela ted to the batch-titration procedure were taken into account. As regar ds the influence of the ionic medium and ionic strength, the experimen tal data could be refined by assuming that the surface active centers act independently of one another, without any appreciable effect due t o 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. Afte r the determination of protonation constants, copper(II)-sediment inte raction was considered: the stability-constant values were calculated by elaborating pH-metric data recorded on the sediment-copper(II) syst em; copper(II) not adsorbed on the sediment was determined by atomic a bsorption spectrometry for each titration point; the good agreement be tween pCu calculated from potentiometric data and measured pCu, by sup posing copper(II) mainly bonded to titratable groups, can be considere d as a validation of the method. (C) 1998 Elsevier Science B.V.