In order to develop a biotechnological solution to the disposal of waste pigment dispersions, the ability of azo dye-reducing Shewanella strain J18 143 cells to reduce model pigment dispersions was assessed. Resting cells were able to couple the oxidation of formate to the reduction of the azo/ketohydrazone chromophore in virtually insoluble pigmentary species. Pigments from the azoacetoacetanilide range and the azonaphthol range were used to investigate the effects of process parameters such as dispersion quality, addition of biocides, temperature and use of exogenous extracellular redox mediators. The physical impact of bioreduction on the pigment structure was also assessed. Pigments present in industrially manufactured dispersions were more readily reduced than those in powder form and the presence of a biocide in the dispersion did not affect the activity of cells of Shewanella strain J18 143. The initial pigment reduction rate increased with temperature up to 50 °C, but the higher temperatures had a detrimental effect on the long term activity of Shewanella strain J18 143. The reduction of the pigment dispersions was stimulated by the addition of the soluble electron shuttle anthraquinone-2,6-disulphonate. Particle sizing and environmental scanning electron microscopy showed that Shewanella strain J18 143 was able to degrade large pigment aggregates to produce individual pigment particles. GemSperse Orange EX5 was the pigment dispersion reduced to the corresponding amines most efficiently by Shewanella strain J18 143.

Sediments with basaltic provenance, such as those at the Hanford nuclear reservation, Washington, USA, are rich in Fe-bearing minerals of mixed valence. These minerals are redox reactive with aqueous O2 or Fe(II), and have the potential to react with important environmental contaminants including Tc. Here we isolate, identify and characterize natural Fe(II)/Fe(III)-bearing microparticles from Hanford sediments, develop synthetic analogues and investigate their batch redox reactivity with aqueous Tc(VII). Natural Fe-rich mineral samples were isolated by magnetic separation from sediments collected at several locations on Hanford’s central plateau. This magnetic mineral fraction was found to represent up to 1 wt% of the total sediment, and be composed of 90% magnetite with minor ilmenite and hematite, as determined by X-ray diffraction. The magnetite contained variable amounts of transition metals consistent with alio- and isovalent metal substitutions for Fe. X-ray microprobe analysis showed that Ti was the most significant substituent, and that these grains could be described with the titanomagnetite formula Fe3−xTixO4, which falls between endmember magnetite (x = 0) and ulvöspinel (x = 1). The dominant composition was determined to be x = 0.15 by chemical analysis and electron probe microanalysis in the bulk, and by L-edge X-ray absorption spectroscopy and X-ray photoelectron spectroscopy at the surface.Site-level characterization of the titanomagnetites by X-ray magnetic circular dichroism showed that despite native oxidation, octahedral Fe(II) was detectable within 5 nm of the mineral surface. By testing the effect of contact with oxic Hanford and Ringold groundwaters to reduced Ringold groundwater, it was found that the concentration of this near-surface structural Fe(II) was strongly dependent on aqueous redox condition. This highlights the potential for restoring reducing equivalents and thus reduction capacity to oxidized Fe-mineral surfaces through redox cycling in the natural environment. Reaction of these magnetically-separated natural phases from Hanford sediments with a solution containing 10 μM Tc(VII) showed that they were able to reductively immobilize Tc(VII) with concurrent oxidation of Fe(II) to Fe(III) at the mineral surface, as were synthetic x = 0.15 microparticle and nanoparticle analogue phases. When differences in the particle surface area to solution volume ratio were taken into consideration, measured Tc(VII) reduction rates for Fe3−xTixO4 (x = 0.15) natural material, synthetic bulk powder and nanoparticles scaled systematically, suggesting possible utility for comprehensive batch and flow reactivity studies.