Insights into the Biogeochemical Cycling of Cobalt: Precipitation and Transformation of Cobalt Sulfide Nanoparticles under Low-Temperature Aqueous Conditions.

Cobalt sulfide precipitates, key phases in the natural biogeochemistry of cobalt and in relevant remediation and resource recovery processes, are poorly defined under low-temperature aqueous conditions. Here, we systematically studied Co (Fe) sulfides precipitated and aged in environmentally relevant solutions, defined by different combinations of pH, initial cobalt to iron ratios ([Co]aq/[Fe]aq), with/without S0, and the presence/absence of sulfate-reducing bacteria. The initial abiogenic precipitates were composed exclusively of amorphous Co sulfide nanoparticles (CoS·xH2O) that were stable in anoxic solution for 2 months, with estimated log K* values 1-5 orders of magnitude higher than that previously reported for Co sulfides. The addition of S0, in combination with acidic pH and elevated temperature (60 °C), resulted in recrystallization of the amorphous precipitates into nanocrystalline jaipurite (hexagonal CoS) within 1 month. In the presence of Fe(II)aq, the abiogenic precipitates were composed of more crystalline Co sulfides and/or Co-rich mackinawite, the exact phase being dependent on the [Co]aq/[Fe]aq value. The biogenic precipitates displayed higher crystallinity for Co sulfides (up to the formation of nanocrystalline cobalt pentlandite, Co9S8) and lower crystallinity for Co-rich mackinawite, suggestive of mineral-specific bacterial interaction. The revealed precipitation and transformation pathways of Co (Fe) sulfides in this study allows for a better constraint of Co biogeochemistry in various natural and engineered environments.

Stormwater green infrastructures retain high concentrations of TiO2 engineered (nano)-particles.

Stormwater conveys natural and engineered (nano)-particles, like any other pollutants, from urban areas to water resources. Thus, the use of stormwater green infrastructures (SGI), which infiltrate and treat stormwater, can potentially limit the spread of engineered (nano)-particles in the environment. However, the concentration of engineered (nano)-particles in soil or biofilter media used in SGI has not been measured due to difficulties in distinguishing natural vs. engineered (nano)-particles. This study reports, for the first time, the concentration and size distribution of TiO2 engineered (nano)-particles in soils collected from SGI. The concentrations of TiO2 engineered (nano)-particles were determined by mass balance calculations based on shifts in elemental concentration ratios, i.e., Ti to Nb, Ti to Ta, and Ti to Al in SGI soils relative to natural background elemental ratios. The concentrations of TiO2 engineered (nano)-particles in SGI soils varied between 550 ± 13 and 1800 ± 200 mg kg-1. A small fraction of TiO2 engineered (nano)-particles could be extracted by ultrapure water (UPW) and Na4P2O7; however, the concentration of TiO2 engineered (nano)-particles was higher in the Na4P2O7-extracted suspensions than in UPW-extracted suspensions. The concentration of TiO2 in the nanosize range increased with the increase in extractant (Na4P2O7) volume to soil mass ratio due to the increased disaggregation of soil heteroaggregates. The size distribution of TiO2 engineered (nano)-particles in the < 450 nm Na4P2O7-extracted suspension from one of the SGI soils was determined by asymmetrical flow-field flow fractionation coupled to inductively coupled plasma-mass spectrometer, and was found to vary in the range of 25-200 nm with a modal size of 50 nm. These results demonstrated that the increase in the Ti to natural tracers (e.g., Nb, Ta, and Al) elemental ratios in the SGI soil relative to bulk soil can be used to estimate the concentration of TiO2 engineered (nano)-particles in SGI.

Detection and quantification of engineered particles in urban runoff.

Urban runoff conveys contaminants including titanium dioxide (TiO2), widely used as engineered nanoparticles (e.g., 1-100 nm) and pigments (e.g., 100-300 nm) in the urban environment, to receiving surface waters. Yet, the concentrations of TiO2 engineered particles (e.g., engineered nanoparticles and pigments) in urban runoff has not been determined due to difficulties in distinguishing natural from engineered TiO2 particles in environmental matrices. The present study examines the occurrence and estimates the concentrations of TiO2 engineered particles in urban runoff under wet- and dry-weather conditions. Urban runoff was collected from two bridges in Columbia, South Carolina, USA under wet-weather conditions and from the Ballona Creek and Los Angeles (LA) River in Los Angeles, California, USA under dry-weather conditions. The concentrations of TiO2 engineered particles were determined by mass balance calculations based on shifts in elemental concentration ratios in urban runoff relative to natural background elemental ratios. Elemental ratios of Ti to Nb in urban runoff were higher than the natural background ratios, indicating Ti contamination. The occurrence of TiO2 engineered particles was further confirmed by transmission electron microscopy coupled with energy dispersive spectroscopy. The concentration of TiO2 engineered particles in urban runoff was estimated to be in the range of 5-150 µg L-1. Therefore, this study identifies urban runoff as a previously unaccounted source of TiO2 engineered particle release to the environment, which should be included in engineered nanoparticle fate modeling studies and in estimating environmental release of engineered nanoparticles.