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Microbial selenium (Se) bioremediation is based on conversion of water soluble, toxic Se oxyanions to water insoluble, elemental Se. Formed biogenic elemental Se is of nanometer size, hampering straightforward separation from the aqueous phase. This study represents the first systematic investigation on colloidal properties of pure biogenic Se suspensions, linking electrophoretic mobility (ζ-potential) to column settling behavior. It was demonstrated that circumneutral pH, commonly applied in bioremediation, is not appropriate for gravitational separation due to the negative ζ-potential preventing agglomeration. Mono/di/trivalent counter cations and acidity (protons) were used to screen efficiently the intrinsic negative charge of biogenic Se suspensions at circumneutral pH. Fast settling was induced by La(3+) addition in the micromolar range (86.2 ± 3.5% within 0.5 h), whereas considerably higher concentrations were needed when Ca(2+) or Na(+) was used. Colloidal stability was furthermore studied in different model waters. It was demonstrated that surface waters as such represent a fragile system regarding colloidal stability of biogenic Se suspensions (ζ-potential ∼ -30 mV), whereas dissolved organic matter increases colloidal stability. In marine waters, biogenic Se is colloidally destabilized and is thus expected to settle, representing a potential sink for Se during transport in the aquatic environment.

Phytotechnologies are often shown as an emerging tool to remediate contaminated soils. Research in this field has resulted in many important findings relating to plant and soil sciences. However, there have been scant private and public investments and little commercial success with this technology. Here, we investigate the barriers to the adoption of phytotechnologies and determine whether it is still a fertile area for future research. The terminology used in phytotechnologies includes a confusing mish-mash of terms relating to concepts and processes increasing the difficulty of developing a unique commercial image. We argue that the commercial success of phytotechnologies depends on the generation of valuable biomass on contaminated land, rather than a pure remediation technique that may not compare favourably with the costs of inaction or alternative technologies. Valuable biomass includes timber, bioenergy, feedstock for pyrolosis, biofortified products, or ecologically important species.