Redox Reactants Research Articles

Analytical theoretical framework for closed bipolar electrochemical cells under homogeneous molecular catalysis: Implications for electrochemiluminescence applications

A novel analytical framework has been formulated to describe the current-potential-time response in systems with two polarized interfaces where one charge transfer process follows a catalytic mechanism, as found in co-reactant electrochemiluminescence (ECL) systems. The developed theory provides mathematical expressions for electrochemical and ECL signals, concentrations, and interfacial potentials upon the application of a constant potential pulse. Theoretical analysis reveals that the chemical regeneration of the redox reactant within the catalytic cycle has a remarkable impact on the chronoamperometric, voltammetric, and ECL responses, thereby providing means to estimate the catalytic rate constant and to optimize light emission. The system's behaviors are rationalized through the influence of catalysis on the interfacial concentrations and potentials. Also, the primary theoretical predictions have been experimentally validated using the [Ru(bpy)₃]²⁺/oxalate ECL system.

Coupling Electron Transfer Reactions of Quinones with Grotthuss Proton Transport of Concentrated Hydrogen-Bonded Electrolytes

Concentrated Hydrogen Bonded Electrolytes (CoHBEs) such as deep eutectic solvents and ionic liquids have been studied for redox flow batteries. The CoHBEs approach has advantages for large-scale energy storage in flow batteries due to its potentially high redox reactant solubility, large electrochemical stability window, low volatility, and the ability to be made of non-toxic and non-corrosive materials. However, they are often viscous and thus transport of ions and reactants are hindered. Protic Concentrated Hydrogen Bonded Electrolytes (P-CoHBEs) are structured electrolytes that facilitate ionic conductivity through Grotthuss type proton transport, and thus decoupling ionic conductivity from electrolyte viscosity. Imidazoles protonated with an acid is an example of a P-CoHBEs. Ideally, coupling a proton coupled electron transfer redox couple with a P-CoHBE will be useful for storing energy in a flow battery. In this research, we studied the charge transfer mechanism and kinetics of 9,10-anthraquinone (AQ) in a P-CoHBE composed of levulinic acid (LA) and 1H-Imidazole (Im) by electroanalytical techniques. The details of the experiments, the measurement results, and interpretation of data will be described in this presentation. The electrochemical behavior of AQ was found to be sensitive to the LA:Im composition where the hydrogen bonding and speciation were effectively varied, thus directly impacting the diffusivity of AQ. The La:Im composition also affected the proton-coupled electron transfer mechanism and under certain conditions the 2-electron transfer of AQ⇌AQ-⇌ AQ2- was observed to occur at a single potential at -0.91 V vs. Fc/Fc+. This suggests tunability of the hydrogen network for efficient and reversible PCET reactions based on P-CoHBEs’ component pKa values and Kamlet Taft parameters. By innovatively decoupling proton conductivity from viscosity through the potential exploitation of the Grotthuss mechanism, our research opens new avenues for enhancing ion transport efficiency in flow batteries and lays the groundwork for designing protic P-CoHBEs that optimize the hydrogen network for quick and reversible PCET reactions on quinones.

Synthesis of NO by rotating sliding arc discharge reactor with conical-spiral electrodes

The present work investigates the potential applications of nitrogen oxides (NO x ), particularly nitric oxide (NO) and nitrogen dioxide (NO2), generated through discharge plasma in diverse sectors such as medicine, nitrogen fixation, energy, and environmental protection. In this study, a rotating sliding arc discharge reactor was initially employed to produce high concentrations of gaseous NO x , followed by the utilization of a molybdenum wire redox reactor for NO2-to-NO conversion. The outcomes reveal that the discharge states and generations of NO x are affected by varying parameters, including the applied energies, frequencies and airflow states (1.3–2.6 m/s are the laminar flow, 2.6–5.2 m/s are the transition state, 5.2–6.5 m/s are the turbulent flow), and the concentrations of NO x within the arc discharge are higher than that in the spark discharge. Moreover, the concentrations of NO, NO2 and NO x gradually increased, and the concentration ratios of NO/NO2 and NO x /NO2 decreased with increasing the applied energy for one cycle from 14.8 mJ to 24.3 mJ. Meanwhile, the concentrations of NO, NO2 and NO x gradually decreased, and the concentration ratios of NO/NO2 and NO x /NO2 first decreased and then increased with increasing the applied frequencies from 5.0 kHz to 9.0 kHz. Further, the concentrations of NO, NO2 and NO x gradually decreased, and the concentration ratios of NO/NO2 and NO x /NO2 first increased and then decreased with increasing the air flow speeds from 1.3 m/s to 6.5 m/s. Lastly, the concentrations of NO increased and NO2 decreased with increasing temperature from 25 °C to 400 °C using molybdenum converted. These findings provide experimental support for the application of plasma in the fields of medicine, nitrogen fixation, energy and environmental protection.

Oxygen Carrier Circulation Rate for Novel Cold Flow Chemical Looping Reactors

To achieve net-zero emissions by the year 2050, carbon capture, utilization, and storage technologies must be implemented to decarbonize sectors with hard-to-abate emissions. Pressurized chemical looping (PCL) with a novel reactor design called a plug flow with internal recirculation (PFIR) fluidized bed is proposed as an attractive carbon capture technology to decarbonize small- and medium-scale emitters. The objective of this work is to examine the solid circulation rate between redox reactors in a cold flow chemical looping facility using an energy balance approach. The effects of static bed height, weir opening height, purge configuration, and gas flow rate on solid circulation rate were investigated. It was determined that parameters that greatly affected the total gas momentum, such as the fluidization ratio or number of purge rows, tended to also have a large effect on solid circulation rate. Parameters that had a small effect on total gas momentum, such as bed height, did not have a measurable effect on solid circulation rate. It was noted that parameters that posed a restriction to solids flow, such as a vertical purge jet or the weir itself, decreased the solid circulation rate compared to similar tests without restrictions.

Corals and sponges are hotspots of reactive oxygen species in the deep sea.

Reactive oxygen species (ROS) are central to diverse biological processes through which organisms respond to and interact with their surroundings. Yet, a lack of direct measurements limits our understanding of the distribution of ROS in the ocean. Using a recently developed in situ sensor, we show that deep-sea corals and sponges produce the ROS superoxide, revealing that benthic organisms can be sources and hotspots of ROS production in these environments. These findings confirm previous contentions that extracellular superoxide production by corals can be independent of the activity of photosynthetic symbionts. The discovery of deep-sea corals and sponges as sources of ROS has implications for the physiology and ecology of benthic organisms and introduces a previously overlooked suite of redox reactants at depth.

Experimental demonstration of high-temperature (>1000 °C) heat extraction from a moving-bed oxidation reactor for thermochemical energy storage

Previously developed reduction-oxidation (redox) thermochemical energy storage technologies must store their products at high temperatures, complicating handling and transportation. This work describes a countercurrent, tubular, moving bed oxidation reactor at laboratory scale that produces high grade heat and allows solids to enter and exit the system at ambient temperatures. The particles implemented in the system consist of a novel magnesium‑manganese-oxide material well-suited for thermochemical energy storage. Output heat is obtained via a separate extraction gas flow, which exits from the middle portion of the main reactor tube. With this design, reactor temperatures in excess of 1000 °C and extraction temperatures above 950 °C were achieved. Deviation between the two measurements is a result of extraction thermocouple placement and losses in the reactor extraction arm; improvements to these parameters would bring the extraction temperature closer to the bed temperature. The reactor produces enough energy via oxidation to sustain both heat extraction and continued chemical reaction. During one representative steady state experiment at a particle flow rate of 1.5 g/s, an average of 447 W was extracted from the reactor out of an estimated 1083 W of released chemical energy for a duration of 70 min. Among the four experiments, the maximum bench-scale oxidation reactor energy efficiency of 36.2% and corresponding round-trip efficiency of 13.7% considering both redox reactors were demonstrated. Characteristics of an ideal system are considered and future improvements are proposed.

Technical, economic and environmental analysis of solar thermochemical production of drop-in fuels

This study analyzes the technical performance, costs and life-cycle greenhouse gas (GHG) emissions of the production of various fuels using air-captured water and CO2, and concentrated solar energy as the source of high-temperature process heat. The solar thermochemical fuel production pathway utilizes a ceria-based redox cycle for splitting water and CO2 to syngas – a tailored mixture of H2 and CO – which in turn is further converted to liquid hydrocarbon fuels. The cycle is driven by concentrated solar heat and supplemented by a high-temperature thermal energy storage for round-the-clock continuous operation. The study examines three locations with high direct normal irradiation using a baseline heliostat field reflective area of 1 km2 for the production of six fuels, i.e. jet fuel and diesel via Fischer-Tropsch, methanol, gasoline via methanol, dimethyl ether, and hydrogen. Two scenarios are considered: near-term future by the year 2030 and long-term future beyond 2030.In the near-term future in Sierra Gorda (Chile), the minimum fuel selling price is estimated at around 76 €/GJ (2.5 €/L) for jet fuel and diesel, 65 €/GJ for methanol, gasoline (via methanol) and dimethyl ether (DME), and 42 €/GJ for hydrogen (excluding liquefaction). In the long-term future, with advancements in solar receiver, redox reactor, high-temperature heat recovery and direct air capture technologies, the industrial-scale plant could achieve a solar-to-fuel efficiency up to 13–19 %, depending on the target fuel, resulting in a minimum fuel selling price of 16–38 €/GJ (0.6–1.3 €/L) for jet fuel and diesel, and 14–32 €/GJ for methanol, gasoline, and DME, making these fuels synthesized from sunlight and air cost-competitive vis-à-vis e-fuels. To produce the same fuels in Tabernas (Spain) and Ouarzazate (Morocco) as in Sierra Gorda, the production cost would increase by 22–33 %. Greenhouse gas savings can be over 80 % already in the near-term future.

Redox-zoning in high-energy subterranean estuaries as a function of storm floods, temperatures, seasonal groundwater recharge and morphodynamics

Redox conditions are a major control for the concentrations and mass fluxes of water constituents, e.g., nutrients, metals and organic molecules within subterranean estuaries (STE). Due to the transient flow and transport processes in STEs as well as the variable temperatures and input of redox reactants, redox zoning in the STE is believed to be highly dynamic. In the present study we analyzed the individual and combined effects of storm floods, seasonal changes of temperatures and groundwater recharge rates, as well as beach morphodynamics on the redox zoning in the STE. For this purpose, a 2D cross-shore density-dependent flow and reactive transport model was set up representing a beach aquifer exposed to high-mesotidal and medium-to high-energy wave conditions, as well as to storm floods. The simulation results show that under the given conditions, redox dynamics may occur down to a depth of 20 m. Morphodynamics appear to be the most important factor for the transience of redox zones compared to the other factors. Seasonal changes in meteoric groundwater recharge rates appear to be least relevant for the redox dynamics in STEs.

Size and Charge Effects on Crossover of Flow Battery Reactants Evaluated by Quinone Permeabilities Through Nafion

Organic reactants are promising candidates for long-lifetime redox flow batteries, and synthetic chemistry unlocks a wide design space for new molecules. Minimizing crossover of these molecules through ion exchange membranes is one important design consideration, but the ways in which the crossover rate depends on the structure of the crossing species remain unclear. Here, we contribute a systematic evaluation of size- and charge-based effects on dilute-solution small molecule permeability through the Nafion NR212 cation exchange membrane. We found that increasing the magnitude of charge number z with the same sign as membrane fixed charges, achieved here by successive sulfonation of quinone redox cores, results in more than an order of magnitude permeability reduction per sulfonate. Size-based effects, understood by comparing the Stokes radii of the quinones studied, also reduces permeability with increasing effective molecule size, but doubling the effective size of the redox reactants resulted in a permeability decrease of less than a factor of three.

Intensification of the reverse water–gas shift process using a countercurrent chemical looping regenerative reactor

Chemical reactions have thermodynamic limits on species conversion which can negatively impact process design. Unconverted feedstock often needs to be separated and recycled, increasing energy demand, process complexity and cost. Just as is the case for heat exchangers, countercurrent reactor systems can improve the thermodynamic limits on species exchange and conversion. This work describes and demonstrates a countercurrent redox reactor system, which can be realised in a packed-bed chemical-looping reactor by storing the favourable oxygen chemical potential inclines using the unique properties of non-stoichiometric oxides. The concept is analogous to a regenerative heat exchanger, but for oxygen exchange and storage, and so we use the term regenerative reactor. We apply this approach to the reverse water–gas shift reaction, which is a critical step in the processing of synthetic e-fuels. The concept is modelled and experimentally validated via a lab-scale demonstration performed with CeO2 at 1073 K, which resulted in a CO2-to-CO molar conversion of 88 %, compared to a thermodynamic limit of 58 % for the conventional process at the same conditions. The modelling results indicate that the ideal thermodynamic conversion can be approximately doubled via this method. Furthermore, the CO is formed separately from the H2 flow, allowing for the syngas composition to be finely tuned for downstream processing.

Assessing Mediated Redox Flow Battery Reaction Progression

One potential next generation battery system is mediated redox flow batteries (RFBs). With mediated RFBs, soluble electroactive species (redox shuttles) deliver power via electrochemical reactions in a flow-through stack reactor much like a conventional RFB. However, the redox shuttles then undergo chemical redox in a coupled chemical reactor system with solid electroactive materials. The use of solid electroactive material for chemical energy storage results in substantial increases in volumetric energy density for the system.Mediated RFBs with different configurations for the chemical redox reactor have previously been reported. In this work, a packed bed reactor system will be described, where the impact of varying different experimental parameters on the chemical redox progression will be highlighted. Generally, the progression of the chemical redox can be tracked in the lab by taking advantage of electrochemical analytical techniques. However, additional assessment of the reaction progression using neutron imaging will also be discussed. Neutron imaging offered a unique opportunity to access the spatial progression of the chemical redox in the packed bed reactor. The combination of techniques for assessing the chemical redox progression provides insights into the limiting processes in the reactor systems.

Screening Ruddlesden-Popper (n=1) Oxide Materials for Thermochemical Water Splitting By Density Functional Theory

Thermochemical redox reactors store concentrated solar power by thermally inducing an oxygen deficiency within a metal oxide structure. The metal oxide’s affinity for reoxidation allows it to facilitate the splitting of H2O or CO2 to produce H2 or CO for syngas formation.[1] Typically, high temperatures (>1400 °C) are used to drive the reduction of the benchmark material, CeO2, [2] however there is motivation to lower this temperature and investigate new metal oxides capable of larger fuel productions. Perovskite materials have been thoroughly investigated due to their crystallographic stability and ability to accommodate relatively large oxygen deficiencies. Emery et al. [3] conducted a wide computational screening of this family of materials based on a few simple thermodynamic parameters originally proposed by Meredig and Wolverton. [4]Herein, we extend these previous studies and investigate the A2BO4 Ruddlesden-Popper (RP) oxides family [5], layered perovskites, that have previously demonstrated fast redox kinetics and large oxygen storage as solid oxide fuel cell cathodes.[6] A combination of screening parameters based on charge neutrality, Goldschmidt tolerance and computed defect formation energy, identified 38 possible RP candidate materials. One of which - Ca2MnO4-δ – was taken forward to experimental testing due to its Earth abundant elements. The powder was synthesized via a modified Pechini method and thermal analysis experiments demonstrated thermally driven oxygen evolution from 800 to 1200 °C equating to a non-stoichiometry of δ=0.18. High-temperature X-ray diffraction alluded to the formation of a CaMn2O4 secondary phase above 1075 °C, therefore consequent thermochemical water splitting cycles were carried out at a maximum of 1000 °C. Oxidation under steam at 800 °C demonstrated good hydrogen production volumes, although gas production on further cycles was limited by particle sintering observed by scanning electron microscopy. Further investigations aim to understand and improve the cyclability and kinetics under different reaction temperatures and humidity. This study outlines, with experimental validation, how computational screening can be used to find future suitable RP candidates for thermochemical water splitting whose performance can be further improved with doping strategies and morphological adaptation.

Water Splitting by MnOx/Na2CO3 Reversible Redox Reactions

Thermal water splitting by redox reactants could contribute to a hydrogen-based energy economy. The authors previously assessed and classified these thermo-chemical water splitting redox reactions. The Mn3O4/MnO/NaMnO2 multi-step redox cycles were demonstrated to have high potential. The present research experimentally investigated the MnOx/Na2CO3 redox water splitting system both in an electric furnace and in a concentrated solar furnace at 775 and 825 °C, respectively, using 10 to 250 g of redox reactants. The characteristics of all reactants were determined by particle size distribution, porosity, XRD and SEM. With milled particle and grain sizes below 1 µm, the reactants offer a large surface area for the heterogeneous gas/solid reaction. Up to 10 complete cycles (oxidation/reduction) were assessed in the electric furnace. After 10 cycles, an equilibrium yield appeared to be reached. The milled Mn3O4/Na2CO3 cycle showed an efficiency of 78% at 825 °C. After 10 redox cycles, the efficiency was still close to 60%. At 775 °C, the milled MnO/Na2CO3 cycles showed an 80% conversion during cycle 1, which decreased to 77% after cycle 10. Other reactant compounds achieved a significantly lower conversion yield. In the solar furnace, the highest conversion (>95%) was obtained with the Mn3O4/Na2CO3 system at 775 °C. A final assessment of the process economics revealed that at least 30 to 40 cycles would be needed to produce H2 at the price of 4 €/kg H2. To meet competitive prices below 2 €/kg H2, over 80 cycles should be achieved. The experimental and economic results stress the importance of improving the reverse cycles of the redox system.

Non-Uniform Porous Structures and Cycling Control for Optimized Fixed-Bed Solar Thermochemical Water Splitting

Abstract Solar thermochemical redox cycles provide a sustainable pathway for solar fuel processing. If done in porous (ceria) structures, they can profit from faster reaction rates owed to the enhanced heat and mass transport characteristics. However, the exact porous structure and operating conditions significantly affect the performance. We present a transient volume-averaged fixed-bed model of a thermochemical redox reactor utilizing macroporous ceria. We studied the porosity-dependent (ɛ = 0.4–0.9) and operating condition-dependent (solar concentration ratio, ratio of oxygen partial pressure to total pressure, and gas flowrate) performance of the fixed-bed ceria redox cycle. Structures with large porosity (ɛ = 0.9) showed better performance than low-porosity structures, owning to the enhanced heat absorption and resulting higher temperatures. We show that the cycle duration requires optimization according to the porosity of the structure. Two hours of operation for a structure with ɛ = 0.75 resulted in the largest hydrogen production (115.78mLgceria−1) if the single cycle duration was 240 s (i.e., 30 cycles in 2 h), while nearly five times less was produced for a 15 times longer single cycle duration (i.e., two cycles in 2 h). We subsequently introduced porous structures with different types of non-uniform porosity distributions. For an average porosity of ɛ = 0.75, the most favorable non-uniform porosity media exhibited higher porosity at the boundaries and a denser core. The fuel production of the best non-uniform porous structure was six times larger compared to a uniform porous structure. Adjusting on top of this the cycling conditions, a 14.6 times production gain was achieved. This work suggests that under non-isothermal operation condition for macroporous ceria redox fixed-bed cycling, non-uniform porous structure with higher porosity boundaries and a dense core benefit fuel production and porosity-dependent cycle duration modulation can be used to increase performance.

Performance assessment of thermochemical CO2/H2O splitting in moving bed and fluidized bed reactors

In this paper, we present the assessment of moving bed reactors and fluidized bed reactors operating in different fluidizing regimes for solar thermochemical redox cycles (STRC) for syngas production. The reduction reactor with a moving bed (MBRED) while the oxidation reactor (OXI) is either a moving bed reactor (MBOXI) or bubbling bed (BBOXI) yields higher performance. It was observed that only water splitting is suitable at 1400 °C and 10−3 bar reduction conditions. The higher reduction temperature and pressure improved the efficiency of the CO2/H2O splitting unit. The requirement of the H2/CO ratio drives the gas feed (CO2/H2O) into OXI. To achieve an H2/CO ratio of 1, MBOXI and BBOXI require an equimolar mixture of CO2 and H2O at 1600 °C. However, to achieve a similar H2/CO ratio at a lower temperature of 1500 °C, the gas feed of the CO2/H2O ratio required is 3. A similar H2/CO ratio is achieved for OXI operating in a turbulent and fast fluidizing, but the selectivity is lower due to lower reaction rates. OXI as a transport bed is least suited based on solid conversion (XOXI), H2/CO, or efficiency. The results are useful in designing the redox reactors for syngas.

Sensitivity analysis and optimization of geometric and operational parameters in a thermochemical heat storage redox reactor used for concentrated solar power plants

Sensitivity analysis and optimization of geometric and operational parameters in a thermochemical heat storage redox reactor used for concentrated solar power plants

Designing Stimuli-Responsive Upconversion Nanoparticles that Exploit the Tumor Microenvironment.

Tailoring personalized cancer nanomedicines demands detailed understanding of the tumor microenvironment. In recent years, smart upconversion nanoparticles with the ability to exploit the unique characteristics of the tumor microenvironment for precise targeting have been designed. To activate upconversion nanoparticles, various bio-physicochemical characteristics of the tumor microenvironment, namely, acidic pH, redox reactants, and hypoxia, are exploited. Stimuli-responsive upconversion nanoparticles also utilize the excessive presence of adenosine triphosphate (ATP), riboflavin, and Zn2+ in tumors. An overview of the design of stimulus-responsive upconversion nanoparticles that precisely target and respond to tumors via targeting the tumor microenvironment and intracellular signals is provided. Detailed understanding of the tumor microenvironment and the personalized design of upconversion nanoparticles will result in more effective clinical translation.

Highly dispersed metal nanoparticles on TiO2 acted as nano redox reactor and its synergistic catalysis on the hydrogen storage properties of magnesium hydride

In this paper, we have synthesized highly dispersed Co metal nanoparticles with the particle size about 5–10 nm on TiO2 (25–50 nm) for the first time through an extremely facile solvothermal method. It is supposed that the synthesized Co/TiO2 composite can combine the catalytic advantage of both Co and TiO2, exhibiting the superior catalytic effect on the hydrogen de/absorption properties of MgH2. The experimental data confirmed the above supposition and demonstrated that Co/TiO2 additive highly enhances the hydrogen de/absorption kinetics of MgH2 as compared to separate Co or TiO2 additive. Specifically, the MgH2Co/TiO2 composite begins to desorb hydrogen at about 190 °C with a low apparent activation energy of 77 kJ/mol. Besides, the MgH2Co/TiO2 composite has a desorption peak temperature of 235.2 °C, which is 53.2, 94.2 and 132.2 °C lower than that of MgH2TiO2 (288.4 °C), MgH2Co (329.4 °C) and ball-milled MgH2 (367.4 °C). Moreover, MgH2Co/TiO2 composite also exhibits low temperature rehydrogenation properties, which can absorb 6.07, 5.56 and 4.24 wt% H2 within 10 min at the temperature of 165, 130 and 100 °C, respectively. It is supposed that such excellent hydrogen desorption properties and low desorption energy barrier of MgH2Co/TiO2 composite are mainly ascribed to the novel synergistic catalytic effects of Co and TiO2. Herein, we propose a novel catalytic mechanism and think that Co/TiO2 acts as “nano redox reactor”, which can facilitate the dissociation and recombination process of hydrogen, thus reducing the reaction energy barrier and enhancing the de/rehydrogenation of MgH2.

Electrochemical Molecular Switch for the Selective Profiling of Cysteine in Live Cells and Whole Blood and for the Quantification of Aminoacylase-1.

A first-of-a-kind latent electrochemical redox probe, ferrocene carbamate phenyl acrylate (FCPA), was developed for the selective detection of cysteine (Cys) and aminoacylase (ACY-1). The electrochemical signal generated by this probe was shown to be highly specific to Cys and insensitive to other amino acids and biological redox reactants. The FCPA-incorporated electrochemical sensor exhibited a broad dynamic range of 0.25-100 μM toward Cys. This probe also proficiently monitored the ACY-1-catalyzed biochemical transformation of N-acetylcysteine (NAC) into Cys, and this proficiency was used to develop an electrochemical assay for quantifying active ACY-1, which it did so in a dynamic range of 10-200 pM (0.1-2 mU/cm3) with a detection limit of 1 pM (0.01 mU/cm3). Furthermore, the probe was utilized in real-time tracking and quantification of cellular Cys production, specifically in Escherichia coli W3110, along with a whole blood assay to determine levels of Cys and spiked ACY-1 in blood with a reliable analytical performance.

Speciation and Mobility of Mercury in Soils Contaminated by Legacy Emissions from a Chemical Factory in the Rhône Valley in Canton of Valais, Switzerland

Legacy contamination of soils and sediments with mercury (Hg) can pose serious threats to the environment and to human health. Assessing risks and possible remediation strategies must consider the chemical forms of Hg, as different Hg species exhibit vastly different environmental behaviors and toxicities. Here, we present a study on Hg speciation and potential mobility in sediments from a chemical factory site, and soils from nearby settlement areas in the canton of Valais, Switzerland. Total Hg ranged from 0.5 to 28.4 mg/kg in the soils, and 3.5 to 174.7 mg/kg in the sediments, respectively. Elemental Hg(0) was not detectable in the soils by thermal desorption analysis. Methylmercury, the most toxic form of Hg, was present at low levels in all soils (<0.010 mg/kg; <0.8% of total Hg). Sequential extractions and thermal desorption analyses suggested that most of the Hg in the soils was present as “matrix-bound Hg(II)”, most likely associated with soil organic matter. For factory sediments, which contained less organic matter, the results suggested a higher fraction of sulfide-bound Hg. Batch extractions in different CaCl2 solutions revealed that Hg solubility was low overall, and there was no Hg-mobilizing effects of Ca2+ or Cl− in solution. Only in some of the factory sediments did high CaCl2 concentrations result in increased extractability of Hg, due to the formation of Hg-chloride complexes. Additional experiments with soil redox reactors showed that even mildly reducing conditions led to a sharp release of Hg into solution, which may be highly relevant in soils that are prone to periodic water saturation of flooding.