In-line TiO2-assisted photodigestion of organic matter in aqueous solution for voltammetric flow analysis of heavy metals in water samples Original Research Article

A novel enhanced photocatalytic digestion method for water samples, based on the addition, to the sample, of a suspension of TiO2 (anatase) and aiming at voltammetric heavy metal determination, is presented and evaluated. The quelate Cd(II)–EDTA acted as a model system in this investigation. The voltammetric wave of Cd(II) is effectively suppressed in the presence of EDTA in excess, but it experiences recovery as the coordinating capacity of the ligand is destroyed. It was observed that the presence of a TiO2 suspension 0.1% w/w neither affects the polarographic wave of the metal ion nor hinders its stripping voltammetry, although some catalyst adheres to the mercury drop. The retention of TiO2 on the electrode is further minimised by operating in the flow injection mode with renewable Hg drop electrodes. The voltammetry of samples of 1 mmol l−1 EDTA solutions, buffered with AcH/Ac− to pH 4 and containing 0.1 mmol l−1 Cd(II), collected during irradiation, revealed significant acceleration of the recovery of the Cd(II) signal in the presence of TiO2. It was demonstrated that the method is perfectly fit for flow analysis through a system that integrates the following stages: injection of sample, monosegmented with air bubbles; addition of TiO2 to the sample; in-line photodigestion; debubbling; degassing (dissolved O2 removal); accumulation of Cd(Hg) at the mercury electrode; and voltammetric stripping. Complete recovery of the 0.01 mmol l−1 Cd(II) signal was obtained for 150 μl sample plugs containing 0.1 mmol l−1 EDTA, after a transit time of 155 s in the irradiation chamber, at 30°C. Degradation of EDTA in real matrixes was also carried out.