Facet-Dependent Photoinduced Transformation of Cadmium Sulfide (CdS) Nanoparticles

Immobilization of cadmium in paddy soil using a novel active silicon-potassium amendment: a field experimental study

The rapid development of industrialization and agriculture has led to extensive environmental issues worldwide such as cadmium (Cd) pollution of paddy soils, posing a potential threat to environmental safety and food health. Therefore, there is an urgent need to reduce the Cd contents in paddy soils. In this study, a newly active silicon-potassium amendment was first prepared from potassium hydroxide-assisted potassium feldspar at a low temperature, and then was used to remediate a contaminated paddy soil by Cd over a long period. The obtained results demonstrated the effectiveness of the applied active silicon-potassium in promoting rice growth in the experimental field. In addition, soil pH values increased to 6.89-7.03, thus decreasing the bioavailability of Cd bioavailability by 8.61-13.7%. The soil enzyme activities and available nutrients (Si, Ca, Mg, N, and P) were also significantly increased. In particular, the Cd contents in the rice grains decreased from 0.279 to 0.179-0.194 mg/kg following the application of the active silicon-potassium amendment, reaching the food crop standard level of China (< 0.2 mg/kg). The detailed remediation mechanisms of the Cd-contaminated paddy soil involved several processes, including ion exchange, ligand complexation, electrostatic attraction, and precipitation. Overall, the active silicon-potassium material is a promising amendment for achieving effective control of Cd-contaminated paddy soils.

Dissolution Potential of Elemental Mercury in the Presence of Bisulfide and Implications for Mobilization

Liquid elemental mercury (Hg0L) pollution can remain in soils for decades and, over time, will undergo corrosion, a process in which the droplet surface oxidizes soil constituents to form more reactive phases, such as mercury oxide (HgO). While these reactive coatings may enhance Hg migration in the subsurface, little is known about the transformation potential of corroded Hg0L in the presence of reduced inorganic sulfur species to form sparingly soluble HgS particles, a process that enables the long-term sequestration of mercury in soils and generally reduces its mobility and bioavailability. In this study, we investigated the dissolution of corroded Hg0L in the presence of sulfide by quantifying rates of aqueous Hg release from corroded Hg0L droplets under different sulfide concentrations (expressed as the S:Hg molar ratio). For droplets corroded in ambient air, no differences in soluble Hg release were observed among all sulfide exposure levels (S:Hg mole ratios ranging from 10-4 to 10). However, for droplets oxidized in the presence of a more reactive oxidant (hydrogen peroxide, H2O2), we observed a 10- to 25-fold increase in dissolved Hg when the oxidized droplets were exposed to low sulfide concentrations (S:Hg ratios from 10-4 to 10-1) relative to droplets exposed to high sulfide concentrations. These results suggest two critical factors that dictate the release of soluble Hg from Hg0L in the presence of sulfide: the extent of surface corrosion of the Hg0L droplet and sufficient sulfide concentration for the formation of HgS solids. The mobilization of Hg0L in porous media, therefore, largely depends on aging conditions in the subsurface and chemical reactivity at the Hg0L droplet interface.

Sunlight-Driven Production of Reactive Oxygen Species from Natural Iron Minerals: Quantum Yield and Wavelength Dependence

Photochemically generated reactive oxygen species (ROS) play numerous key roles in earth's surface biogeochemical processes and pollutant dynamics. ROS production has historically been linked to the photosensitization of natural organic matter. Here, we report the photochemical ROS production from three naturally abundant iron minerals. All investigated iron minerals are photoactive toward sunlight irradiation, with photogenerated currents linearly correlated with incident light intensity. Hydroxyl radicals (^•OH) and hydrogen peroxide (H₂O₂) are identified as the major ROS species, with apparent quantum yields ranging from 1.4 × 10^-8 to 3.9 × 10^-8 and 5.8 × 10^-8 to 2.5 × 10^-6, respectively. Photochemical ROS production exhibits high wavelength dependence, for instance, the ^•OH quantum yield decreases with the increase of light wavelength from 375 to 425 nm, and above 425 nm it sharply decreases to zero. The temperature shows a positive impact on ^•OH production, with apparent activation energies ranging from 8.0 to 17.8 kJ/mol. Interestingly, natural iron minerals with impurities exhibit higher ROS production than their pure crystal counterparts. Compared with organic photosensitizers, iron minerals exhibit higher wavelength dependence, higher selectivity, lower efficiency, and long-term stability in photochemical ROS production. Our study highlights natural inorganic iron mineral photochemistry as a ubiquitous yet previously overlooked source of ROS.

Synergistic Effect of the Sulfur Vacancy and Schottky Heterojunction on Photocatalytic Uranium Immobilization: The Thermodynamics and Kinetics

Not only a critical matter in the nuclear fuel cycle but uranium is also a global contaminant with both radioactive and chemical toxicity. Reducing soluble hexavalent uranium [U(VI)] to relatively nonimmigrated tetravalent uranium [U(IV)] by photocatalytic technologies is recognized as a highly promising strategy for avoiding environmental pollution and re-extracting uranium resources from nuclear wastewater. Herein, we have designed a heterojunction photocatalyst constructed from the carbon aerogels (CA) and the CdS nanoflowers with an S-vacancy (CA@CdS-SV). With the S-vacancy and heterojunction being synergized, the U(VI) removal rate exceeded 97% in 40 min without the addition of any sacrificial agents. As impacted by the synergistic effects of the S-vacancy and heterojunction, thermodynamics and kinetics revealed that photogenerated electrons were first captured via shallow traps generated by vacancies on CdS-SV and then transferred to the CA surfaces through the heterojunction to realize the spatial separation of carriers, thereby achieving a satisfactory performance. This work is considered to underpin the improvement of U(VI) immobilization by exploiting the synergistic effect of vacancy engineering and the Schottky heterojunction from the perspective of thermodynamics and kinetics.