Selective sequestration of strontium in desmid green algae by biogenic co-precipitation with barite.

The generation of radioactive waste and/or environmental radioactive contamination is a common side effect of activities such as nuclear power generation, medical use of radioisotopes, nuclear weapons testing, and occasionally disasters such as Chernobyl. The subsequent decontamination of waste or the environment requires the nontrivial ability to selectively separate and remove harmful radioisotopes such as Sr, a product of nuclear fission with a half-life of approximately 30 years. In the case of Sr, the chemical similarity of Ca, Sr, and Ba presents a challenge for even the most advanced ion-exchange materials. While phytoremediation approaches utilizing the accumulation of environmental contaminants by green plants are becoming increasingly popular, the effectiveness of such approaches for Sr sequestration are drastically reduced in the presence of Ca, due to the indiscriminate transport of Ca, Sr, and Ba exhibited by most organisms. Surprisingly, there are a small number of organisms that selectively sequester Sr and/or Ba in biominerals. For example, the marine radiolarian acantharea builds an endoskeleton from celestite (SrSO4), and the desmid [5] and stonewort green algae deposit barite (BaSO4) in vacuoles. Accumulation of Sr and Ba in the presence of up to five orders of magnitude excess Ca emphasizes that to address the selectivity problem, there is much to be learned and possibly gained from the strategies these organisms have evolved. Here, we quantitatively demonstrate the incorporation of up to 45 mol% Sr in barite crystals deposited by desmid green algae. The unicellular desmid green algae are ubiquitous in fresh water habitats and robust in culture, and as such are particularly suitable as a model system for Sr/Ba biomineralization and as a potential candidate for phytoremediation. In the desmid Closterium moniliferum, BaSO4 crystals are found in small terminal vacuoles at the tips of the crescent-shaped cells (Figure 1). Scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS) analysis of BaSO4 crystals in ashed cells reveals clusters of mixed rhombic and hexagonal crystals of 0.5–1 mm diameter with 0.1–0.5 mm thickness (Figure 1), which exhibit strong Ba and S signals and little Ca (Supporting Information), consistent with previous descriptions.