Selenite and Selenate Sequestration during Coprecipitation with Barite: Insights from Mineralization Processes of Adsorption, Nucleation, and Growth

Heterogeneous Coprecipitation of Nanocrystals with Metals on Substrates

ConspectusThe heterogeneous coprecipitation of nanocrystals with metals on substrates plays a significant role in both natural and engineered systems. Due to the small dimensions and thereby the large specific surface area, nanocrystal coprecipitation with metals, which is ubiquitous in natural settings, exerts drastic effects on the biogeochemical cycling of metals on the earth's crust. Meanwhile, the controlled synthesis of nanocrystals with metal doping to achieve tunable size/composition enables their broad applications as adsorbents and catalysts in many engineered settings. Despite their importance, complex interactions among aqueous ions/polymers, nanocrystals, substrates, and metals are far from being well-understood, leaving the controlling mechanisms for nanocrystal formation with metals on substrates uncovered.In this Account, we discuss our systematic investigation over the past 10 years of the heterogeneous formation of representative nanocrystals with metals on typical substrates. We chose Fe(OH)3 and BaSO4 as representative nanocrystals. Mechanisms for varied metal coprecipitation were also investigated for both types of nanocrystals (i.e., Fe, Al, Cr, Cu, and Pb)(OH)3 and (Ba, Sr)(SO4, SeO4, and SeO3)). Bare SiO2 and Al2O3, as well as those coated with varied organics, were selected as geologically or synthetically representative substrates. Through the integration of state-of-the-art nanoscale interfacial characterization techniques with theoretical calculations, the complex interactions during nanocrystal formation at interfaces were probed and the controlling mechanisms were identified.For BaSO4 and Fe(OH)3 formation on substrates, the local supersaturation levels near substrates were controlled by Ba2+ adsorption and the electrostatic attraction of Fe(OH)3 monomer/polymer to substrates, respectively. Meanwhile, substrate hydrophobicity controlled the interfacial energy for the nucleation of both nanocrystals on (in)organic substrates. Metal ions' (i.e., Cr/Al/Cu/Pb) hydrolysis constants and substrates' dielectric constants controlled metal ion adsorption onto substrates, which altered the surface charges of substrates, thus controlling heterogeneous Fe(OH)3 nanocrystal formation on substrates by electrostatic interactions. The sizes and compositions of heterogeneous (Fe, Cr)(OH)3 and (Ba, Sr)(SO4, SeO4, SeO3) formed on substrates were found to be distinct from those of homogeneous precipitates formed in solution. The substrate (de)protonation could alter the local solution's pH and the substrates' surface charge; substrates could also adsorb cations, affecting local Fe/Cr/Ba/Sr ion concentrations at solid-water interfaces, thus controlling the amount/size/composition of nanocrystals by tuning their nucleation/growth/deposition on substrates. From slightly supersaturated solution, homogeneous coprecipitates of microsized (Ba, Sr)(SO4, SeO4, SeO3) formed through growth, with little Sr/Se(VI) incorporation due to higher solubilities of SrSO4 and BaSeO4 over BaSO4. While cation enrichment near substrates made the local solution highly supersaturated, nanosized coprecipitates formed on substrates through nucleation, with more Sr/Se(VI) incorporation due to lower interfacial energies of SrSO4 and BaSeO4 over BaSO4. The new insights gained advanced our understanding of the biogeochemical cycling of varied elements at solid-water interfaces and of the controlled synthesis of functional nanocrystals.

Enhanced extraction of heavy metals from gypsum-based hazardous waste by nanoscale sulfuric acid film at ambient conditions

Low-cost, low-energy extraction of heavy metal(loid)s (HMs) from hazardous gypsum cake is the goal of the metallurgical industry to mitigate environmental risks and carbon emissions. However, current extracting routes of hydrometallurgy often suffer from great energy inputs and substantial chemical inputs. Here, we report a novel solid-like approach with low energy consumption and chemical input to extract HMs by thin films under ambient conditions. Through constructing a nanoscale sulfuric acid film (NSF) of ∼50 nm thickness on the surface of arsenic-bearing gypsum (ABG), 99.6% of arsenic can be removed, surpassing the 50.3% removal in bulk solution. In-situ X-ray diffraction, infrared spectral, and ab initio molecular dynamics (AIMD) simulations demonstrate that NSF plays a dual role in promoting the phase transformation from gypsum to anhydrite and in changing the ionic species to prevent re-doping in anhydrite, which is not occurred in bulk solutions. The potential of the NSF is further validated in extracting other heavy metal(loid)s (e.g., Cu, Zn, and Cr) from synthetic and actual gypsum cake. With energy consumption and costs at 1/200 and 1/10 of traditional hydrometallurgy separately, this method offers an efficient and economical pathway for extracting HMs from heavy metal-bearing waste and recycling industrial solid waste.

Near Quantitative Removal of Selenate and Sulfate Anions from Wastewaters by Cocrystallization with Chelating Hydrogen-Bonding Guanidinium Ligands

Selenium (Se) has become an environmental contaminant of aquatic ecosystems as a result of human activities, particularly mining, fossil fuel combustion, and agricultural activities. By leveraging the high sulfate concentrations relative to Se oxyanions (i.e., SeO _n ^2-, n = 3, 4) present in some wastewaters, we have developed an efficient approach to Se-oxyanion removal by cocrystallization with bisiminoguanidinium (BIG) ligands that form crystalline sulfate/selenate solid solutions. The crystallization of the sulfate, selenate and selenite, oxyanions and of sulfate/selenate mixtures with five candidate BIG ligands are reported along with the thermodynamics of crystallization and aqueous solubilities. Oxyanion removal experiments with the top two performing candidate ligands show a near quantitative removal (>99%) of sulfate or selenate from solution. When both sulfate and selenate are present, there is near quantitative removal (>99%) of selenate, down to sub-ppb Se levels, with no discrimination between the two oxyanions during cocrystallization. Reducing the selenate concentrations by 3 orders of magnitude or more relative to sulfate, as found in many wastewaters, led to no measurable loss in Se removal efficiencies. This work offers a simple and effective alternative to selective separation of trace amounts of highly toxic selenate oxyanions from wastewaters, to meet stringent regulatory discharge limits.