Abstract
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.
Highlights
The precipitation of solid solutions from aqueous solutions has been studied to a great extent due to the prevalence of solid solutions in nature and their importance in engineered systems[1,2,3,4,5,6,7,8,9]
Nanoscale compositional zonation in particles precipitated from aqueous solutions at high salinity
As described in the Methods section, it was not possible to quantify in absolute terms of the moles of each element, but it is possible to quantify the relevant ratios, and because both Sr and Ba are strongly detected at the Hard X-ray Nanoprobe (HXN) beamline, the ratio in equation 1 is directly quantified from the relevant X-ray fluorescence intensities
Summary
The precipitation of solid solutions from aqueous solutions has been studied to a great extent due to the prevalence of solid solutions in nature and their importance in engineered systems[1,2,3,4,5,6,7,8,9]. Pina and Putnis[28] have developed a model to predict trace element incorporation and compositional zonation, incorporating theory for the nucleation and growth of solid solutions Their modeling results have been compared to counterdiffusion experiments, in which solutes in two aqueous solutions diffuse through a porous silica hydrogel before reacting to form zoned particles. When Ba2+ is sufficiently depleted at the mineral-fluid interface due to incorporation into the solid phase as BaSO4, Sr2+ can be preferentially incorporated until an influx of diffusing Ba2+ ions can replenish the supply for further BaSO4 precipitation In counterdiffusion experiments, these effects produce >40 μm sized particles with ~5 to ~10 μm wide bands of high Sr2+ content after reacting for ~1 month[29,30,36]
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