Coprecipitation can be an effective treatment method for the removal of environmentally relevant metals from industrial wastewaters such as produced waters from the oil and gas industry. The precipitation of barite, BaSO 4 , through the addition of sulfate removes barium while coprecipitating strontium and other alkaline earth metals even when these are present at concentrations below their solubility limit. Among other analytical methods, X-ray fluorescence (XRF) nanospectroscopy at the Hard X-ray Nanoprobe (HXN) beamline at the National Synchrotron Light Source II (NSLS-II) was used to quantify Sr incorporation into barite. Thermodynamic modeling of (Ba,Sr)SO 4 solid solutions was done using solid solution-aqueous solution (SS-AS) theory. The quantitative, high-resolution nano-XRF data show clearly that the Sr content in (Ba,Sr)SO 4 solid solutions varies widely among particles and even within a single particle. We observed substantial Sr incorporation that is far larger than thermodynamic models predict, likely indicating the formation of metastable solid solutions. We also observed that increasing barite supersaturation of the aqueous phase led to increased Sr incorporation, as predicted by available kinetic models. These results suggest that coprecipitation offers significant potential for designing treatment systems for aqueous metals' removal in desired metastable compositions. Solution conditions may be optimized to enhance the incorporation of Sr by increasing sulfate addition such that the barite saturation index remains above *3 or by increasing the aqueous Sr to Ba ratio.

Scientists have long suspected that compositionally zoned particles can form under far-from equilibrium precipitation conditions, but their inferences have been based on bulk solid and solution measurements. We are the first to directly observe nanoscale trace element compositional zonation in <10 µm-sized particles using X-ray fluorescence nanospectroscopy at the Hard X-ray Nanoprobe (HXN) Beamline at National Synchrotron Light Source II (NSLS-II). Through high-resolution images, compositional zonation was observed in barite (BaSO4) particles precipitated from aqueous solution, in which Sr2+ cations as well as HAsO42− anions were co-precipitated into (Ba,Sr)SO4 or Ba(SO4,HAsO4) solid solutions. Under high salinity conditions (NaCl ≥ 1.0 M), bands contained ~3.5 to ~5 times more trace element compared to the center of the particle formed in early stages of particle growth. Quantitative analysis of Sr and As fractional substitution allowed us to determine that different crystallographic growth directions incorporated trace elements to different extents. These findings provide supporting evidence that barite solid solutions have great potential for trace element incorporation; this has significant implications for environmental and engineered systems that remove hazardous substances from water.

Water that comes into contact with coal fly ash contains toxic elements, including heavy metals. CO 2 gas can be used for treatment by inducing precipitation of carbonates with alkaline earth metals, thereby coprecipitating toxic elements and neutralizing the pH. This process has the added benefit of sequestering CO 2 and contributing to negative emissions of greenhouse gases. This investigation used a novel synchrotronbased approach with X-ray fluorescence nanospectroscopy at the NSLS-II HXN beamline. Multielement incorporation of As, Ba, Cr, Cu, and Zn into calcium carbonate was measured for a simulated fly ash leachate. We discovered that the extent of trace element incorporation was consistently higher than predicted by a thermodynamic model of solid solution coprecipitation, with incorporation of Cr being 3000 times more than predicted, and incorporation of Zn being 3 times more. Enhancement of trace element incorporation was correlated to the solubility of the endmembers (CaCrO 4 > CaHAsO 4 > BaCO 3 > CuCO 3 > ZnCO 3 ). Iron inhibited trace element coprecipitation because the dominant process was surface adsorption. These results suggest that mineralization of heavy metals via coprecipitation in metastable carbonate phases is a more effective strategy for immobilizing toxic elements than solubility would suggest and is a promising environmental remediation strategy.

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