Oxidative mineral growth of goethite (α-FeOOH) on hematite (α-Fe2O3) nanoparticles during the oxidation of adsorbed Fe­(II) is thermodynamically controlled by mineral surface characteristics and solution conditions. Here, the impact of added organic carbon (OC) on reactivity of ultrafine mineral particles is evaluated. For batch reactors using 0.007 m2/mL hematite, the observed rate constant of 4-chloronitrobenzene reduction to 4-chloroaniline decreases by 4× with the addition of 5 ppm of OC from Suwanee River natural organic matter (SRNOM) and 5× with 10 μM catechol (0.72 ppm of C). Both goethite and hematite are produced, and the fraction of Fe­(II) converted to goethite decreases with the addition of SRNOM or catechol. In the absence of added OC, postreaction solids are 17 ± 3 mass % goethite, which decreased to 11 ± 2 mass % and 4 ± 2 mass % with 20 ppm of OC as SRNOM and 20 μM catechol, respectively, and substantial changes in morphology of the goethite product were observed. In the absence of added OC, goethite rods formed at the acute tips of hematite rhombohedra as singular rods ∼50–80 nm long and 10 nm wide. Goethite crystals formed in the presence of 10 μM catechol occurred as 5 to 8 parallel growths measuring 10–50 nm long and 5 nm wide. Batch reactors containing SRNOM had similar results, although the goethite morphology was more irregular. Low-temperature magnetic measurements show that experiments conducted using 20 ppm of SRNOM produced finer grained nano-hematite and goethite than formed in the presence of 20 μM catechol. This study highlights the need for improved methods for characterizing ultrafine mineral phases and demonstrates that organic matter changes the microstructure and morphology of materials formed by oxidative mineral growth and thus how reactive surface area evolves with the extent of reaction.

Microbial precipitation of calcium carbonate is a widespread environmental phenomenon that has diverse engineering applications, from building and soil restoration to carbon sequestration. Urease-mediated ureolysis and CO 2 (de)hydration by carbonic anhydrase (CA) are known for their potential to precipitate carbonate minerals, yet many environmental microbial community studies rely on marker gene or metagenomic approaches that are unable to determine in situ activity. Here, we developed fast and cost-effective tests for the field detection of urease and CA activity using pH-sensitive strips inside microcentrifuge tubes that change colour in response to the reaction products of urease (NH 3) and CA (CO 2). The urease assay proved sensitive and useful in the field to detect in situ activity in biofilms from a saline lake, a series of calcareous fens, and ferrous springs, finding relatively high urease activity in lake samples. Incubations of lake microbes with urea resulted in significantly higher CaCO 3 precipitation compared to incubations with a urease inhibitor, showing that the rapid assay indicated an on-site active metabolism potentially mediating carbonate precipitation. The CA assay, however, showed less sensitivity compared to the urease test. While its sensitivity limits its utility, the assay may still be useful as a preliminary indicator given the paucity of other means for detecting CA activity in the field. Field urease, and potentially CA, activity assays complement molecular approaches and facilitate the search for carbonate-precipitating microbes and their in situ activity, which could be applied toward agriculture, engineering and carbon sequestration technologies.

No abstract

Microbial precipitation of calcium carbonate has diverse engineering applications, from building and soil restoration, to carbon sequestration. Urease-mediated ureolysis and CO2 (de)hydration by carbonic anhydrase (CA) are known for their potential to precipitate carbonate minerals, yet many microbial community studies rely on marker gene or metagenomic approaches that are unable to determine in situ activity. Here, we developed fast and cost-effective tests for the field detection of urease and CA activity using pH-sensitive strips inside microcentrifuge tubes that change color in response to the reaction products of urease (NH3) and CA (CO2). Samples from a saline lake, a series of calcareous fens, and ferrous springs were assayed in the field, finding relatively high urease activity in lake samples, whereas CA activity was only detected in a ferrous spring. Incubations of lake microbes with urea resulted in significantly higher CaCO3 precipitation compared to incubations with a urease inhibitor. Therefore, the rapid assay indicated an on-site active metabolism potentially mediating carbonate mineralization. Field urease and CA activity assays complement molecular approaches and facilitate the search for carbonate-precipitating microbes and their in situ activity, which could be applied toward agriculture, engineering and carbon sequestration technologies.

No abstract