Barium (Ba) leaching from soils and certified reference materials

Despite the position of Ba as the 14th most abundant element on Earth, relatively few studies have examined the occurrence of Ba in soil. Information on how changing environmental conditions can potentially affect Ba mobility, which is obtained from laboratory or field investigations, is scarcely available. In the present study, the release of Ba from soil samples that have varying organic matter, iron, calcium, and sulfur contents and a wide range of total Ba concentrations was investigated using commonly applied single and sequential extractions, pHstat leaching tests and column tests. Additionally, certified reference materials (soil and sediment) were analyzed to provide data for Ba that are not yet available. Despite relatively high pseudo-total concentrations in soil (69–709 mg/kg) compared to commonly investigated trace elements, such as Cd, Zn, Pb, Cu and Ni, Ba showed low mobility in the soils investigated in the present study, indicating that only a very small proportion of Ba is readily available for leaching or uptake by plants. Ammonium-EDTA only extracted small amounts of Ba from the soils (<3% of the total content, except in one sample). Acidification of the soils (below pH 4) caused a substantial release of Ba (between 3 and 14% of its pseudo-total content). The investigation of Ba leaching as a function of time in combination with geochemical modeling showed that unintended side effects, such as precipitation of BaSO4 during leaching tests, can influence the results. Leaching tests and extractions that have been developed to evaluate heavy metal mobility in soils require careful interpretation when Ba is considered and are likely to not always be suited for Ba because of unintended side effects, such as precipitation reactions. Single extractions with ammonium-EDTA are not suitable for determining the ‘potentially mobile pool’ of Ba in soils because of the low stability constant of Ba-EDTA complexes. Single extractions with 0.01 mol/L CaCl2 and 0.43 mol/L acetic acid solutions can offer a fast preliminary screening of Ba mobility. Investigating the kinetics of Ba release, for example, in pHstat leaching tests, can provide useful information about reactions that occur during leaching tests.