Comparing Methods for Measuring Dissolved and Particulate Selenium in Water

Determining the concentrations of dissolved and particulate selenium is of great importance, because it affects the toxicity, removal, recovery and reuse of selenium. For measurement, particulate selenium is usually separated from dissolved selenium by centrifugation or filtration. The reported relative centrifugation forces (rcf) used in separation are inconsistent and vary widely from 500 to 41,000 g. The 0.45 µm pore size filter is the most frequently used for the filtration. We systematically studied the effects of rcf and filter pore size on separation for typical environmental samples. We found that rcf ≥ 20,000 g and filter pore size ≤ 20 nm led to almost complete separation. The minimum rcf needed for separation could be predicted by Stokes’ law when the particulate selenium concentration was low (0.05 mg-Se/L), but could not be predicted when the particulate selenium concentration increased to ≥ 0.5 mg-Se/L probably due to aggregation of selenium nanoparticles at high concentrations. The presence of other particles (e.g., bacteria) also made the minimum rcf not predictable by Stokes’ law due to attachment of particulate selenium to bacteria. Therefore, the presence of other particles and the concentration of particulate selenium should be considered while choosing the appropriate centrifugation condition.