In-situ fabrication of metal oxide nanocaps based on biphasic reactions with surface nanodroplets

Surface-bound nanomaterials are widely used in clean energy techniques from lithium batteries, solar-driven evaporation in desalination to hydrogen production by photocatalytic electrolysis. Reactive surface nanodroplets may potentially streamline the process of fabrication of a range of surface-bound nanomaterials invoking biphasic reactions at interfaces. In this work, we demonstrate the feasibility of reactive surface nanodroplets for in-situ synthesis and anchoring of nanocaps of metal oxides with tailored porous structures. Spatial arrangement and surface coverage of nanocaps are predetermined during the formation of reactive nanodroplets, while the crystalline structures of metal oxides can be controlled by thermal treatment of organometallic nanodroplets produced from the biphasic reactions. Notably, tuning the ratio of reactive and non-reactive components in surface nanodroplets enables the formation of porous nanocaps that can double photocatalytic efficiency in the degradation of organic contaminants in water, compared to smooth nanocaps. In total, we demonstrate in-situ fabrication of four types of metal oxides in the shape of nanocaps. Our work shows that reactive surface nanodroplets may open a door to a general, fast and tuneable route for preparing surface-bound metal oxides. This fabrication approach may help develop new nanomaterials needed for photocatalytic reactions, wastewater treatment, optical focusing, solar energy conversion and other clean energy techniques.

Mercury isotopes reveal atmospheric gaseous mercury deposition directly to the Arctic coastal snowpack

Springtime atmospheric mercury depletion events (AMDEs) lead to snow with elevated mercury concentrations (>200 ng Hg/L) in the Arctic and Antarctic. During AMDEs gaseous elemental mercury (GEM) is photochemically oxidized by halogens to reactive gaseous mercury which is deposited to the snowpack. This reactive mercury is either photochemically reduced back to GEM and reemitted to the atmosphere or remains in the snowpack until spring snowmelt. GEM is also deposited to the snowpack and tundra vegetation by reactive surface uptake (dry deposition) from the atmosphere. There is little consensus on the proportion of AMDE-sourced Hg versus Hg from dry deposition that is released in spring runoff. We used mercury stable isotope measurements of GEM, snowfall, snowpack, snowmelt, surface water, vegetation, and peat from a northern Alaska coastal watershed to quantify Hg sources. Although high Hg concentrations are deposited to the snowpack during AMDEs, we estimate that ∼76 to 91% is released back to the atmosphere prior to snowmelt. Mercury deposited to the snowpack as GEM comprises the majority of snowmelt Hg and has a Hg stable isotope composition similar to Hg deposited by reactive surface uptake of GEM into the leaves of trees in temperate forests. This GEM-sourced Hg is the dominant Hg we measured in the spring snowpack and in tundra peat permafrost deposits.

Impact of Mineral Reactive Surface Area on Forecasting Geological Carbon Sequestration in a CO2-EOR Field

Mineral reactive surface area (RSA) is one of the key factors that control mineral reactions, as it describes how much mineral is accessible and can participate in reactions. This work aims to evaluate the impact of mineral RSA on numerical simulations for CO2 storage at depleted oil fields. The Farnsworth Unit (FWU) in northern Texas was chosen as a case study. A simplified model was used to screen representative cases from 87 RSA combinations to reduce the computational cost. Three selected cases with low, mid, and high RSA values were used for the FWU model. Results suggest that the impact of RSA values on CO2 mineral trapping is more complex than it is on individual reactions. While the low RSA case predicted negligible porosity change and an insignificant amount of CO2 mineral trapping for the FWU model, the mid and high RSA cases forecasted up to 1.19% and 5.04% of porosity reduction due to mineral reactions, and 2.46% and 9.44% of total CO2 trapped in minerals by the end of the 600-year simulation, respectively. The presence of hydrocarbons affects geochemical reactions and can lead to net CO2 mineral trapping, whereas mineral dissolution is forecasted when hydrocarbons are removed from the system.

Reactive transport model of kinetically controlled celestite to barite replacement

<p>Barite formation is of concern for many sustainable utilisations of the geological subsurface, ranging from oil and gas extraction to geothermal reservoirs, and also acts as a scavenger mineral for the retention of radium for nuclear waste disposal. The surface reaction-controlled nature of its formation in these dynamic systems entails a strong sensitivity of the host rock's permeability towards heterogeneities and boundary conditions. The impact of precipitation on effective flow properties can vary by many orders of magnitude as shown by barite scale formation and injectivity loss models for geothermal systems [1], emphasising the need for robust prediction models.</p><p>A relevant example case is the replacement of celestite (SrSO4) with barite (BaSO4), which was investigated for various barite supersaturations with flow-through experiments on the core-scale [2]. Three distinct cases were observed for supersaturations from high to low: (1) quick overgrowth and passivation of soluble celestite grains, (2) partial replacement of celestite with barite, (3) slow moving reaction front with complete mineral replacement. The authors presented heuristic approaches that include linking reactive surface area development to molar fractions to model the results. We provide a comprehensive, full-physics geochemical modelling approach using precipitation and dissolution kinetics as well as nucleation and crystal growth [3] for a more flexible representation of the problem. Additionally, the generation of a digital rock representation based on CT-scans of the granular sample is utilised to derive its inner surface area [4]. The experiments were modelled using core-scale reactive transport simulations. The three observed cases at varying supersaturations were reproduced with regard to evolution of sample rock composition and porosity.</p><p>In a next step, the characteristic values taken from the calibrated reactive transport models can be further integrated into the existing digital rock physics model [4], thus enabling the development of up-scaled relationships such as reactive surface area as a function of mineral fractions and porosity. The resulting models can then be applied to reservoir-scale simulations for various applications related to subsurface utilisation. </p><p>---</p><p>[1] Tranter, M., De Lucia, M., Wolfgramm, M., Kühn, M., 2020. Barite Scale Formation and Injectivity Loss Models for Geothermal Systems. Water 12, 3078. https://doi.org/10/ghntzk<br>[2] Poonoosamy, J., Klinkenberg, M., Deissmann, G., Brandt, F., Bosbach, D., Mäder, U., Kosakowski, G., 2020. Effects of solution supersaturation on barite precipitation in porous media and consequences on permeability: Experiments and modelling. Geochimica et Cosmochimica Acta 270, 43–60. https://doi.org/10/ghntxn<br>[3] Tranter, M., De Lucia, M., Kühn, M., 2021. Numerical investigation of barite scaling kinetics in fractures. Geothermics 91, 102027. https://doi.org/10/ghr89n<br>[4] Wetzel, M., Kempka, T., Kühn, M., 2020. Hydraulic and Mechanical Impacts of Pore Space Alterations within a Sandstone Quantified by a Flow Velocity-Dependent Precipitation Approach. Materials 13, 3100. https://doi.org/10/ghsp42</p>

Reactive transport of micropollutants in laboratory aquifers: combining Compound-Specific Isotope Analysis (CSIA) and biomolecular approaches

<p>Groundwater quality is of increasing concern due to the ubiquitous release of micropollutants, often originating from surface water. Micropollutants comprise a wide range of substances such as pesticides, pharmaceuticals and personal care products (PCPs), and pose risks to groundwater contamination due to their high persistence and toxicity. Although biodegradation is a major process for the removal of organic contaminants in aquifers, the interplay of hydrogeochemical conditions, microbial diversity and micropollutant dissipation at low concentrations remains yet poorly understood. We developed here an integrative approach to understand and predict the factors affecting micropollutant dissipation within the surface-/ground-water transition zone. Compound-specific Isotope Analysis (CSIA) was used to evaluate micropollutant transformation based on changes in the ratio of stable isotopes (i.e., <sup>13</sup>C/<sup>12</sup>C and <sup>15</sup>N/<sup>14</sup>N). The responses of aquifer microbes – the key players during contaminant transformation – to micropollutant exposures was examined through biomolecular approaches, proving advantageous in combination with CSIA.</p><p> </p><p>We examined the dissipation of a micropollutant mixture in two lab-scale aquifer systems fed with river water collected from an agricultural area, thus representing the highly reactive surface-/ground-water transition zone. The micropollutant mixture included legacy and currently used pesticides such as atrazine, terbutryn, S-metolachlor and metalaxyl. Caffeine and metformin were also examined as anthropogenic compounds with physico-chemical properties close to currently used pesticides. Changes in bacterial diversity was examined in both aquifer systems during variations of micropollutant exposures under static hydrological conditions. It is hypothesized that such variations may induce bacterial changes and thus alter micropollutant transformation pathways. To this end, three periods of micropollutant injections during 140 d were induced as follow: (i) a first pulse (about 25 µM) to identify dissipation processes and bacterial adaptation to micropollutants, (ii) a constant injection (2 pore volumes) at 10 fold lower concentrations (chronic exposure phase), and (iii) a second pulse injection (25 µM) to examine whether transformation of micropollutants was enhanced. Concentration breakthrough curves (BTCs) of atrazine, terbutryn and metaxyl showed sorption as the main dissipation process for the three periods, whereas both sorption and degradation were observed for caffeine and S-metolachlor. Carbon and nitrogen CSIA further supported the <em>in situ</em> transformation of caffeine and S-metolachlor (Δδ<sup>13</sup>C of ≥ 4‰ and ≥ 2‰, respectively), while no significant enrichment of <sup>13</sup>C and <sup>15</sup>N were observed for atrazine, terbutryn and metalaxyl (Δδ<sup>13</sup>C < 2‰). In parallel, surface-water microcosm experiments showed half-life times of atrazine, terbutryn and metalaxyl of >200 days. Microbial diversity is currently examined in pore water and sand samples. A numerical model is under development to improve the interpretation of micropollutant dissipation in the highly reactive surface-/ground-water transition zone based on concentrations, CSIA and bacteria diversity data obtained in this study. Altogether, our results demonstrated degradation capacity within the laboratory systems, mainly for caffeine and S-metolachlor, and highlight the persistence and risk to long-term groundwater contamination of both legacy and currently used pesticides.</p>