Chemofiltration of mercury water samples through zinc sulfide layer and determination by wavelength-dispersive X-ray fluorescence spectrometry

Abstract

Chemofiltration through zinc sulfide collected on a membrane filter was used for mercury preconcentration from water samples. Measurements of mercury radiation from the front and back sides of the sample indicate that the ion exchange proceeds on the surface and consequently mercury is collected as a thin layer of HgS on the ZnS. Therefore, filtration of mercury solutions through other sulfides, e.g., CuS, MnS and As₂S₃ will give the same radiation intensity. The loading inhomogeneity was examined using a mapping sample with 1 mm spot size and WDXRF measurements. The mercury recovery was investigated for zinc mass per unit area from 20 to 1000 μg cm⁻². Samples with mass per unit area of 200 μg cm⁻² of zinc were used because of their good recovery (ca. 100%), the fact that different flow rates between 10 mL min⁻¹ and 25 mL min⁻¹ do not influence the quantity of recovered mercury, the good precision and the ruggedness of the method. The detection limit for 1000 mL of filtered water, 100 s counting time and a molybdenum target X-ray tube operated at 50 kV and 40 mA is equal to 0.09 and 0.17 ng mL⁻¹ for Hg Lα₁ and Hg Lβ₁, respectively. The RSD characterizing the sample preparation is ca. 3.5%. The calibration was performed using standard samples in two Hg mass per unit area ranges: 0.5–20 and 0.5–3 μg cm⁻². R² and residual errors RMS of 0.9998, 0.091 μg cm⁻² and 0.9997, 0.024 μg cm⁻² were obtained for the two calibrations, respectively. The selectivity was investigated by determination of mercury in test mixtures to which selected compounds such as alkali and heavy metals, phenol and bromides had been deliberately introduced.