No abstract

In this paper, well-defined vaterite hollow spheres and amorphous barium carbonate microrods are synthesized in Proteus mirabilis/urea solution. The urease-generated bacterium Proteus mirabilis is able to convert urea to ammonia and CO2, thereby leading to the precipitation of metal carbonate in Proteus mirabilis/urea solution containing Ca2+ or Ba2+ ions. It is found that the vaterite hollow spheres are so-called mesocrystals because they have identified primary particles and single-crystalline nature. Crystallization of CaCO3 using Proteus mirabilis and other two bacteria Bacillus subtilis and Aerobacter aerogenes in dilute ammonia aqueous solutions (pH 8.5) is also investigated, suggesting that the products are all CaCO3 mesocrystals. Therefore, we speculate that bacteria promoting formation of CaCO3 mesocrystals may be a common phenomenon. In addition, marked morphological changes and structural transition in the CaCO3 particles from amorphous calcium carbonate irregular aggregates to vaterite hollow spheres to a mixture of calcite and vaterite hollow discs and polyhedrons in Proteus mirabilis/urea solution are observed depending on the reaction time. BaCO3 particles change from oval to rod in morphology within 7 days of reaction, but the structure of them is still amorphous even after a month. The biomolecules mainly proteins secreted by the bacteria are probably responsible for the morphologies and structures of metal carbonate minerals by first stabilizing their nanosized precursors, which then transform into mesocrystals or amorphous aggregates via oriented or nonoriented aggregation of nanoparticles. This provides a novel and facile way for the study of biomineralization mechanisms and crystal growth modification.

A substantial cost of granular iron permeable reactive barriers is that of the granular iron itself. Cutting the iron with sand can reduce costs, but several performance issues arise. In particular, reaction rates are expected to decline as the percentage of iron in the blend is diminished. This might occur simply as a function of iron content, or mass transfer effects may play a role in a much less predictable fashion. Column experiments were conducted to investigate the performance consequences of mixing Connelly granular iron with sand using the reduction kinetics of trichloroethylene (TCE) to quantify the changes. Five mixing ratios (i.e., 100%, 85%, 75%, 50%, and 25% of iron by weight) were studied. The experimental data showed that there is a noticeable decrease in the reaction rate when the content of sand is 25% by weight (iron mass to pore volume ratio, Fe/Vp = 3548 g/L) or greater. An analysis of the reaction kinetics, using the Langmuir‐Hinshelwood rate equation, indicated that mass transfer became an apparent cause of rate loss when the iron content fell below 50% by weight (Fe/Vp = 2223 g/L). Paradoxically, there were tentative indications that TCE removal rates were higher in a 15% sand + 85% iron mixture (Fe/Vp = 4416 g/L) than they were in 100% iron (Fe/Vp = 4577 g/L). This subtle improvement in performance might be due to an increase of iron surface available for contact with TCE, due to grain packing in the sand‐iron mixture.