Uranium theoretical speciation for drinking water from private drilled wells in Sweden – Implications for choice of removal method

Highlights: • Neutral charge uranium complexes dominated in the pH range 6.7–7.8. • The Ca concentration influence which calcium-UO₂ carbonate complexes was formed. • In the acidic pH range several different U complexes can comprise a large fraction of total complexes. • It is crucial to include all relevant chemical compounds in the model. • Before removal method is selected, some crucial parameters should be measured. - Abstract: Elevated concentrations of uranium (U) from natural sources have been measured in drinking water from private drilled wells in Sweden and many other countries world-wide. Although U is a radioactive element, radioactivity is not the main concern, but rather chemical toxicity, e.g. kidney damage. Uranium chemistry is complex and U in water has a very high tendency to form complexes with other compounds. Since speciation is crucial for the properties of U, and therefore the removal efficiency, this study determined theoretical U species in drinking water from private drilled wells using the geochemical model Visual MINTEQ. The drinking water samples used in modelling were from two datasets: (1) 76 water samples selected from a previous survey of 722 wells; and (2) samples of drinking water from 21 private wells sampled in May 2013. The results showed that neutrally charged U complexes dominated in the pH range 6.7–7.8, which is common in private drilled wells. This has important implications for removal method, since charge is an important factor for U removal efficiency. In the alkaline pH range, one of two calcium-UO₂ carbonate complexes dominated and calcium (Ca) concentration proved to be a key factor determining the Ca-UO₂ carbonate complex formed: the neutral Ca₂UO₂(CO₃)₃⁰(aq) or the negative CaUO₂(CO₃)₃²⁻. Complexes with organic carbon (C) varied greatly in the acidic range, indicating that it is crucial to measure organic C content in the water since it is critical for the dissolved organic matter (DOM)-UO₂ complex formation. Therefore before U removal method is selected, some crucial parameters for complex formation should be measured. Based on our results, such measurements should include pH, Ca, alkalinity and organic C concentration, as these determine the type of complexes formed and their charge