Effects Of Solution Chemistry Research Articles

Effect of solution chemistry on aqueous As(III) removal by titanium salts coagulation.

Solution chemistry is of great importance to the removal of arsenic by coagulation through influencing the speciation of arsenic, the in situ precipitation of metal salts coupled with the adsorption and coprecipitation behavior of arsenic during coagulation. While the researches on the influence of solution chemistry in As(III) removal by titanium salts, a promising candidate for drinking water treatment was still deficient. Batch tests were performed to evaluate the removal of As(III) by titanium salts coagulation under solution chemistry influences. The results indicated that As(III) removal by Ti(SO4)2 and TiCl4 increased first and then decreased with the rising of solution pH from 4 to 10. TiCl4 preformed better in As(III) removal than Ti(SO4)2 at pH4-8, but the opposite trends were observed at pH9-10. XPS analysis indicated that the involvement of surface hydroxyl groups was primarily responsible for As(III) adsorption on Ti(IV) precipitates. As(III) removal was inhibited in the presence of SO42- mainly by competitive adsorption, especially at elevated SO42- concentration under acidic and alkaline conditions. F- exerted a greater suppressive effect than SO42- via indirectly hindering Ti(IV) precipitate formation, and through direct competitive adsorption with H3AsO3, the inhibitive effect increased as F- concentration increased and depended highly on solution pH. As(III) removal was promoted by co-existing Fe(II) primarily through the facilitation of Ti(IV) precipitation, especially under neutral and alkaline conditions, while it was inhibited to a different extent by the presence of high-concentration Mn(II) possibly via competitive adsorption. The presence of Ca2+ and Mg2+ enhanced the removal of As(III), but the positive effect did not increase as ionic concentration elevated.

The inactivation of bacteriophage MS2 by sodium hypochlorite in the presence of particles

The inactivation of bacteriophage MS2 by sodium hypochlorite was investigated to understand the effect of solution chemistry on the disinfection efficacy in the presence of particles. Kaolinite and Microcystis aeruginosa (M. aeruginosa) were used as the models of inorganic and organic particles to simulate high turbidity and algal cells, respectively, in drinking water sources. In both particle-containing solutions, lower pH, the presence of cations (di-valent Ca2+) and natural organic matters (NOM) were regarded as the main factors to influence the aggregation and inactivation of MS2. The results showed that MS2 aggregated in all solutions at pH 3.0, protecting the inner viruses. At pH 7.0, the presence of Na+ cations (0–200 mmol/L) did not affect the inactivation efficacy of MS2, which always followed the order of particles-free ≈ kaolinite > M. aeruginosa. The inactivation efficacy of MS2 in the presence of Ca2+ cations followed the order of kaolinite > particles-free > M. aeruginosa at 0–50 mmol/L Ca2+ cations, while the inactivation efficacy remained almost constant in the range of 100–200 mmol/L Ca2+ cations. By contrast, kaolinite offered not enough protection to adsorbed MS2, but MS2 aggregation decreased disinfection efficacy at a high concentration of Ca2+ cations. Moreover, the presence of humic acid as NOM decreased the inactivation of MS2 more significantly than M. aeruginosa due to the more consumption of free chlorine from humic acids. Therefore, the co-existence of NOM and di-valent Ca2+ cations are potential challenges for the inactivation of viruses by sodium hypochlorite in safe drinking water.

(Corrosion Division Morris Cohen Graduate Student Award Address) A Systematic Study of Critical Parameters Affecting the Galvanic Corrosion between Al Alloy and Stainless Steel in Atmospheric Condition

Connections between dissimilar materials are frequently encountered in complex structures involved in the aerospace industry. In a typical aircraft structure, one of the most common couples encountered is that of an aluminum-alloy-based airframe component and noble fasteners such as stainless steel. When this type of galvanic couple is exposed to a atmospheric marine environment, it can develop cracks that pose increased risk and require costly maintenance and repair. In this work, a complementary modeling and experimental approach is applied to systematically investigate the effect of electrolyte layer thickness, solution chemistry and materials surface properties on the electrochemical and corrosion distributions in the galvanic coupling between AA7050 and SS316L, and capture the underlying mechanism of the dependence of corrosion distributions on each external variable.To validate the robustness of our modeling approach, a Laplace-Equation based modeling coupled using mathematically-fitted electrochemical kinetics from experiment as boundary conditions was applied to simulate galvanic corrosion between Zn plate and SS 316 rods under thin layer electrolyte conditions. The modeling results were compared with experimental results from a four-day modified B117 test. It shows that: electrolyte layer thickness (WL)=3,500~4, 000 \U0001d707m would be the appropriate WL range formed on the sample surface during modified B117 salt fog testing; the robustness of Laplace-Equation based modeling is highly dependent on the boundary conditions, experimentally-determined electrochemical kinetics as boundary conditions can truly reflect the electrochemical response of anode/cathode to the surrounding solution environment, which in turn yields the simulated results mostly close to experimental measurement.In the study of the effect of electrolyte layer thickness, the total cathode current capacity of a surface on the electrolyte film thickness and cathode size in a galvanic couple to support the corrosion of AA7050 was accessed. The total cathode current capacity was studied over the range of WL thickness from full immersion conditions (where the total cathode current scales directly with cathode size) to the thin film regime in which the WL is the diffusion boundary layer thickness. In order to fully assess the transition from thin film to thick film condition, an understanding of the natural convection layer thickness is required. For the conditions studied here, the natural convection boundary layer was found to be close to 800 um. The WL thickness, cathode length and electrochemical kinetics interact to create four regimes of total cathode current as a function of WL thickness.In the study of the solution chemistry, the effect of Al3+ on the electrochemical and galvanic corrosion distributions between SS316L and AA7050was emphasized as it represents the major dissolved metal species from AA7050. It is found that Al3+ accelerates the hydrogen evolution reaction (HER) kinetics due to enhanced proton diffusivity, with the effect being observed not only on Al alloys, but also on Pt and stainless steel. The cation study for the galvanic couple indicates that Al3+ is also galvanic corrosion accelerator.The effect of oxide film properties on the cathodic kinetics of stainless steel were investigated because cathodic kinetics can significantly affect the degree of galvanic corrosion based on mixed potential theory. The study focused on the oxygen reduction reaction (ORR) and was studied by electrochemical (potentiodynamic polarization, cyclic voltammetry, potentiostatic test, and electrochemical impedance) techniques and x-ray photoelectron spectroscopy surface analysis. This study shows that the kinetics of ORR on stainless steels is determined by a combined effect of oxide film composition, film thickness, and resistivity.This work has improved the fundamental understanding of the effect of several pertinent variables on the galvanic-coupling induced localized corrosion of Al alloy, provided an approachable modeling means to predict corrosion distribution at given conditions in a timely fashion as well as develop corresponding corrosion mitigation strategies, and complemented continuum/meso-scale atmospheric corrosion modeling studies in this field. Acknowledgements The financial support from the Office of Naval Research (ONR) via Grants No. N00014-14-1-0012 and No. N00014-17-1-2033, the Sea-Based Aviation Program, and William Nickerson, Program Manager is gratefully acknowledged. The assistance of my advisor, Prof. R. G. Kelly is gratefully acknowledged. Helpful technical discussions with Profs. S. J. McDonnell, J. R. Scully, and J. T. Burns (University of Virginia), Dr. Jayendran Srinivasan (the Ohio State University), Dr. Piyush Khullar (Medtronic), Dr. Veronica Rafla (SpaceX) and Mr. Victor Yang (University of Virginia) are also acknowledged.

The Interaction between Cations in Saline Water and Calcium Bentonite in Copper Flotation

In this study, the effect of cations, Na+ and Mg2+, in saline water on copper flotation in the presence of calcium bentonite was investigated. It was found that as the concentration of NaCl was increased, the copper recovery initially decreased and then increased to above 99%, while the copper grade initially increased and then decreased. In contrast, as the concentration of MgCl2 was increased, the copper recovery continuously increased, while the copper grade continuously decreased. The variation of copper recovery and grade was mainly attributed to the synergistic effect of pulp rheology and flotation solution chemistry. The underpinning mechanisms are the compressing electrical double layer and ion-exchange interaction between saline water and bentonite, which occur at the same time. Using saline water slightly affected the pulp rheology with a decrease in copper recovery at low NaCl concentration. Increasing the concentration of cations in saline water might even facilitate the flowing out of exchangeable cations, especially divalent cations, from the interlayer space of bentonite to the solution, resulting in the decrease of copper grade at high salt concentration.

Effects of solution chemistry on conformation of self-aggregated tannic acid revealed by laser light scattering

Inorganic soil solution constituents can alter the charge, size, and conformation of dissolved organic molecules, thus affecting their environmental behavior. Here, we investigated how pH, cation valence and activities induce conformational changes and aggregation-sedimentation reactions of organic polyelectrolytes. For that we determined the hydrodynamic diameter of the model compound tannic acid by laser light scattering at concentrations of 1–30 g L−1 in the pH range from 3 to 10 and with electrolyte additions of CaCl2 and hydroxyl-Al cations. Charge properties were quantified by polyelectrolyte titration and zeta potential measurements. After dispersion by sonication, aggregation was determined in time sequences up to 60 min and suspension stability was traced in sedimentation experiments. Tannic acid was present in ultrapure water in a self-aggregated state. At pH <3 as well as >7.5, its hydrodynamic diameter increased. Whereas at high pH this behavior could be assigned to unfolding of molecular conformations, at low pH it is likely that charge neutralization decreased repulsive forces and facilitated aggregation. At pH 5 and ionic strengths of up to 5 mM, CaCl2 did not affect aggregation state of tannic acid and results resembled those obtained in ultrapure water. Addition of hydroxyl-Al cations broke-up the self-aggregated tannic acid structures under formation of Al-organic coprecipitates. Strong aggregation only occurred at mixing ratios where opposite surface charges were completely balanced. Under natural conditions, self-aggregation of tannic acid can be expected only at higher solution concentrations. However, at acidic pH, hydroxyl-Al cations and tannic acid may form discrete colloidal particles already at low tannic acid concentrations, resulting in the destabilization of suspensions. Our data emphasize that the soil solution composition strongly modifies the physical state of tannic acid, and likely also of other biopolymers, and thus their interactions within environmental matrices.

Colloid retention and mobilization mechanisms under different physicochemical conditions in porous media: A micromodel study

Clear understanding of pore-scale mechanisms that control transport and retention of colloids in porous media at different physicochemical conditions is critical to improve design and efficient cleanup methodologies of filter beds. The objective of this work was to investigate the impact of hydrophobicity, solution ionic strength, and pH on colloid retention mechanisms in single-phase and two-phase flow in porous media systems. A series of experiments were conducted using a geometrically representative micromodel. Hydrophilic and hydrophobic colloids were dispersed in water at different solution ionic strength and pH conditions. Findings indicate that hydrophilic colloids exhibit high filtration efficiency as the colloids interact attractively with other colloids and solid-water-interface irrespective of the solution chemistry. However, for hydrophobic colloids, changes in solution chemistry significantly increase colloid retention where the colloid interaction become attractive with the increase in ionic strength and decrease in pH values. Colloids attached to the collector surfaces mobilized by the strong capillary forces induced by the moving gas-water interface and transported along with the interface. However, hydrophilic colloids redeposited on gas-water-solid interfaces or thin water films because of their greater capillary potential. Therefore, greater filtration efficiency is achieved with the hydrophilic colloids compared to the hydrophobic colloids for which the efficiency can be improved by changing the solution chemistry. Moreover, the removal efficiency by the moving gas-water interface was observed to be more for hydrophobic colloids compared to hydrophilic colloids for which the efficiency can be improved by lowering the ionic strength or increasing the pH value. This study indicates that the coupled effects of solution chemistry and colloid hydrophobicity should be taken into account while investigating efficient filtration and cleaning practices for the filter beds.

Surface properties and suspension stability of low-temperature pyrolyzed biochar nanoparticles: Effects of solution chemistry and feedstock sources

Intensive application of biochar requires better understanding of their environmental behaviors such as stability, fate, and mobility. The release of bulk biochar into biochar nanoparticles (NPs) may bring risks because of their potential flowing into downstream water bodies with nutrients/containments attached. Low-temperature pyrolyzed biochars, namely fruit tree branch biochar of 350/450/550 °C (FB350, FB450 and FB550), corn straw biochar of 350 °C (CB350) and peanut straw biochar of 350 °C (PB350), were produced, and their NPs were extracted. The yield, elemental composition, mineral composition, surface functional groups and zeta potential of biochar NPs were characterized. Subsequently their suspension stability was evaluated in NaCl and CaCl2 solutions by dynamic light scattering technique. The Hamaker constants and particle interaction energy of the biochar NPs were calculated by adopting Derjaguin–Landau–Verwey–Overbeek theory. For biochar NPs of same feedstock, the stability of FB350/450/550-NPs could be predicted well by their zeta potential values. The types of their surface functional groups were the same while their adsorption intensity differed. The scenarios for biochar NPs of different feedstock sources were different, that is, inconsistent variation was observed between their zeta potential and suspension stability, which were rooted in the variable type and quantity of surface functional groups. In conclusion, feedstock was the most significant factor that influenced the suspension stability of biochar NPs, followed by the pyrolysis temperature and solution chemistry, which were highly dependent on surface potential. The findings provide references for the environmental risk evaluation of biochar NPs and reasonable application of biochar in field.

Electrochemical Destruction of per- and Polyfluoroalkyl Substances (PFAS) in Complex Solution Matrices

Per- and Polyfluoroalkyl substances (PFAS) are a group of manmade chemicals which are persistent in the environment and have shown to be toxic to human health. They have accumulated in drinking water, groundwater, and wastewater around the globe for over six decades. Although the production of some chain lengths has ceased, remediation is critical. Current remediation techniques include adsorbents such as granular activated carbon (GAC) and Ion exchange (IX) columns. GAC is merely used to adsorb the PFAS, but struggles with capturing shorter chains and requires regeneration with incineration.1 IX has shown improvement in its ability to capture all PFAS chain lengths, and can be regenerated with a solution comprised of a salt or base and an alcohol.1 This regenerate solution desorbs all of the PFAS from the resin into a smaller concentrated solution of roughly a few hundred gallons. This solution is typically comprised of PFAS in the parts per million (ppm) range.2 These concentrated PFAS solutions can lead to further contamination of the environment through discharge of landfill leachate to wastewater treatment plants from landfill disposal. Due to the high salinity, low volume, and high concentration of PFAS in IX regenerate solutions, electrochemical oxidation is a promising treatment technology to destroy the PFAS and remove the chance of future human exposure.This study employs a boron-doped diamond (BDD) on niobium (Nb) electrode stack to investigate the role that current density has in the electrochemical destruction of PFAS in a complex solution matrix. The electrode stack was comprised of two anode plates sandwiched in between three cathode plates. This entire stack was then placed into a constantly stirred reactor comprised of 750 ml of solution. A constant current density of 50 mA/cm2 was investigated initially for a total of 12 hours. In order to save on energy consumption, further studies used a combined current density approach. This involved applying 50 mA/cm2 for only the first hour of the test before lowering the current density to 5 mA/cm2 for the remaining 11 hours. Multiple literature IX regenerate solutions were simulated and spiked with perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS).3–6 Multiple samples were taken over the course of the oxidation experiments and sent for PFAS analysis using the Environmental Protection Agency’s (EPA) modified 537 method. Additional process parameters were monitored over time including the pH, voltage, temperature, and the reduction in chemical oxygen demand (COD) and ammonia depending on the simulated regenerate solution used. Further analysis showed a significant reduction in energy with a minimal additional amount of PFAS remaining in the initial combined current density study compared to the traditional constant current density approach. 1. Woodard, S., Berry, J. & Newman, B. Ion exchange resin for PFAS removal and pilot test comparison to GAC. Remediation 27, 19–27 (2017).2. Liang, S., Pierce, R. “David”, Lin, H., Chiang, S. Y. D. & Huang, Q. “Jack”. Electrochemical oxidation of PFOA and PFOS in concentrated waste streams. Remediation 28, 127–134 (2018).3. Conte, L., Falletti, L., Zaggia, A. & Milan, M. Polyfluorinated Organic Micropollutants Removal from Water by Ion Exchange and Adsorption. in CHEMICAL ENGINEERING TRANSACTIONS 43, (2015).4. Zaggia, A., Conte, L., Falletti, L., Fant, M. & Chiorboli, A. Use of strong anion exchange resins for the removal of perfluoroalkylated substances from contaminated drinking water in batch and continuous pilot plants. Water Res. 91, 137–146 (2016).5. Chularueangaksorn, P., Tanaka, S., Fujii, S. & Kunacheva, C. Regeneration and reusability of anion exchange resin used in perfluorooctane sulfonate removal by batch experiments. J. Appl. Polym. Sci. 130, 884–890 (2013).6. Deng, S., Yu, Q., Huang, J. & Yu, G. Removal of perfluorooctane sulfonate from wastewater by anion exchange resins: Effects of resin properties and solution chemistry. Water Res. 44, 5188–5195 (2010).

Nitrate Reduction Using Iron and Copper Bimetallic Nanoparticles Supported by Chelating Resin: Effect of Solution Chemistry, Mechanism, and Regeneration

AbstractNanoscale zero-valent iron/copper supported on chelating resin (D-Fe/Cu) was prepared for removing nitrate from water. Batch experiments were carried out to investigate the effect of soluti...

Release and stability of water dispersible biochar colloids in aquatic environments: Effects of pyrolysis temperature, particle size, and solution chemistry

Pathways for the physical disintegration of biochar (BC) and the release of water dispersible BC colloids (WDBC) have received much attention due to their unique impacts on carbon loss and contaminant. However, the current understanding of the mechanisms involved in WDBC formation and associated influencing factors is rather limited. This study systematically explored the effects of pyrolysis temperature, initial particle size, and solution chemistry on WDBC formation in aqueous solutions and examined the formation and colloidal stability of WDBC in natural solutions. Results showed that pyrolysis temperature determined the abrasion resistance of pyrolyzed BC, and the submicron fragment rate decreased in the order 400 °C (BC400) > 700 °C (BC700) > 200 °C (BC200). The WDBC yield decreased in the order BC400 (77.5–331 mg g−1) > BC700 (33.5–173 mg g−1) > BC200 (16.8–125 mg g−1) depending on BC size at a solution ionic strength (IS) ≤ 1 mM, which was positively correlated with the submicron fragment rate of bulk BC. With the exception of BC200, increasing IS (0.1–20 mM) and decreasing pH (3.0–10.0) significantly inhibited WDBC yield. Release and sedimentation dominated the WDBC formation processes with the former being more susceptible to solution chemistry. Derjaguin-Landau-Verwey-Overbeek interactions properly explained the effect of IS on WDBC from BC400 and BC700, while the steric resistance of abundant dissolved organic carbon on BC200 was mainly responsible for the high formation of WDBC at high IS (20–50 mM). WDBC had high colloidal stability and could form and stabilize well in natural surface waters and soil solutions, suggesting the relevant risk of long-distance migration of WDBC in environments. These findings represent new knowledge regarding the physical decomposition and the fate of BC in the environment.

The transport of graphitic carbon nitride in saturated porous media: Effect of hydrodynamic and solution chemistry

Understanding transport behavior of graphitic carbon nitride (g-C3N4) in porous media plays an important role in preventing its possible causing the underground environmental problems. The transport behavior of g-C3N4 in porous media were investigated by packed column experiments at different flow rates, ionic strengths (ISs), pHs and multivalent cations. The experimental results showed that the transport ability of g-C3N4 decreased with the IS increasing, and most of the g-C3N4 was retained in the sand column for the IS greater than 0.0001 M. The flow rate had little effect on the transport behavior of g-C3N4, and the recovery of g-C3N4 increased slightly with increasing flow rate. In addition, the migration ability of g-C3N4 under acidic conditions was drastically reduced compared with neutral alkaline conditions. Moreover, it was found that 1.51%, 30.33%, 34.91%, and 60.54% of g-C3N4 was retained in the column when g-C3N4 was leached through the quartz sand column at Al3+, Ca2+, Mg2+, and K+, which was consistent with the Schulze-Hardy rule. Finally, FTIR spectrum showed that the infrared absorption peak of the g-C3N4 mixed quartz sand were shifted to certain degrees under different conditions, which confirmed that hydrogen bond was formed in the transport of carbon nitride with the quartz sand surface. This study provides a new perspective on the role of hydrogen bond in the transport and fate of nanomaterials.

Sorption of Polyfluoroalkyl Surfactants on Surface Soils: Effect of Molecular Structures, Soil Properties, and Solution Chemistry.

Zwitterionic, cationic, and anionic per- and polyfluoroalkyl substances (PFASs) are identified in aqueous film-forming foam (AFFF) concentrates and AFFF-impacted sites. However, the mobility potential of zwitterionic and cationic PFASs is poorly understood, preventing reliable site assessment. The study aimed to elucidate the mobility behaviors of PFASs of various charge states in saturated soil-water systems and assess critical influencing factors. Five anionic, three zwitterionic, and one cationic PFASs were investigated in five soils through batch sorption experiments. Pairwise comparison revealed that the quaternary ammonium group imparted a strong affinity of cationic perfluorooctaneamide ammonium compound (PFOAAmS) for soils. The influence of the quaternary ammonium group is mitigated in polyfluoroalkyl betaines, yet perfluorooctane sulfonamidoalkyl betaine (PFOSB) showed strong sorption in selected soils. Two soil bulk properties showed some correlations with the soil-water distribution coefficient (Kd). A positive correlation with the fraction of soil organic carbon was found only for anionic PFASs, whereas cation exchange capacity had an approximate positive correlation with Kd only for PFOAAmS. Water chemistry (Ca2+ and pH) influences the sorption of nonanionic PFASs in very distinct fashions or even in opposite trends to what was known for anionic PFASs. Sorption was insensitive to pH changes except for PFOSB; PFOSB underwent profound sorption reduction because its speciation occurs around neutral pH, while the two other betaines and PFOAAmS have pKa values that are outside of the environmentally relevant range. The lack of correlations suggests that the transport potential of PFASs is probably highly site-specific. It remains a challenge in deciphering PFAS sorption mechanisms and predicting how AFFF plumes migrate.

Removal of aqueous arsenite by simultaneous ultraviolet photocatalytic oxidation–coagulation of titanium salts: effect of precipitate particles size and solution chemistry

Removal of aqueous arsenite by simultaneous ultraviolet photocatalytic oxidation–coagulation of titanium salts: effect of precipitate particles size and solution chemistry

Theoretical constraints on the effects of added cations on clumped, oxygen, and carbon isotope signatures of dissolved inorganic carbon species and minerals

Carbonate clumped isotope thermometry has emerged as a useful tool for the reconstruction of precipitation temperatures for minerals from surface and subsurface environments. Few studies have examined the impact of variable solution chemistry and mineral chemistry on clumping. Therefore, we used theoretical calculations to predict the impact of the presence of cations in a solution on the equilibrium clumped, oxygen (18O/16O and 17O/16O, θ and κ), and carbon isotopic (13C/12C) compositions of dissolved inorganic carbon species (DIC), and the impact of cation type and stoichiometry on carbonate mineral composition within the calcite-dolomite-magnesite solid solution. We examined the impact of the presence of Mg2+, K+, Na+, and Ca2+ on equilibrium Δ63 values of carbonate ion groups in DIC species and the composite DIC pool using the B3LYP/6-311++G(2d, 2p) model. We developed new cluster models for dolomite, magnesite, and high-magnesium calcite (Ca0.83Mg0.17)CO3 and (Ca0.67Mg0.33)CO3) and examined the effect of extraneous cations substituted into the crystal lattices of the carbonate minerals. We find the presence of added cations in the DIC pool increases equilibrium Δ63 values slightly, depending upon their solution concentrations. For the cations examined, the identity of the added cation does not seem to be significant. Cumulative effects may account for small-scale differences (<0.02‰) but are unlikely to be substantial unless cation/anion association and ion pairing are important in the solution that carbonates are growing from. We also find that at a given temperature, stoichiometric dolomite and magnesite Δ63 values are similar to calcite and less than aragonite. Surprisingly, substitution of an extraneous cation into a carbonate mineral (e.g., as observed in high-magnesium calcite) generally lowered Δ63 by a slight amount, which suggests that high and low-magnesian calcites may have resolvable differences in equilibrium clumped isotope signatures, potentially providing an explanation for observations of different Δ47 values in slow-growing minerals. These results imply the cationic composition of the solution may be a minor factor that should be considered for crystals that rapidly grow from solution, and that cationic substitution into carbonate mineral lattices will influence equilibrium compositions of carbonate minerals. Solution chemistry effects on mineral composition are likely to be observed at high growth rates. In contrast, equilibrium isotope compositions arising from cationic substitution in minerals, such as differences between high-Mg calcite and low-Mg calcite, or high-Mg calcite and dolomite, would likely be observed at slow growth rates.

Soil Aggregates as a Source of Dissolved Organic Carbon to Streams: An Experimental Study on the Effect of Solution Chemistry on Water Extractable Carbon

Over the past two decades, headwater streams of the northern hemisphere have shown increased amounts of dissolved organic carbon (DOC), coinciding with decreased acid deposition. The exact nature of the mechanistic link between precipitation composition and stream water DOC is still widely debated in the literature. We hypothesize that soil aggregates are the main source of stream water DOC and that DOC release is greater in organic rich, riparian soils versus hillslope soils. To test these hypotheses, we collected soils from two main landscape positions (hillslope and riparian zones) from the acid-impacted Sleepers River Research Watershed in northeastern Vermont. We performed aqueous soil extracts with solutions of different ionic strength (IS) and composition to simulate changes in soil solution. We monitored dynamic changes in soil particle size, aggregate architecture and composition, leachate DOC concentrations, dissolved organic matter (DOM) characteristics with fluorescence spectroscopy and trends in bioavailability. In low IS solutions, extractable DOC concentrations were significantly higher, particle size (by laser diffraction) was significantly smaller and organic material was separated from mineral particles in scanning electron microscope observations. Furthermore, higher DOC concentrations were found in Na+ compared to Ca2+ solutions of the same IS. These effects are attributed to aggregate dispersion due to expanding diffuse double layers in decreased IS solutions and to decreased bridging by divalent cations. Landscape position impacted quality but not quantity of released DOC. Overall, these results indicate that soil aggregates might be one important link between Critical Zone inputs (i.e. precipitation) and exports in streams.

Endogenous release of metals with dissolved organic carbon from biochar: Effects of pyrolysis temperature, particle size, and solution chemistry

Metals are released from biochar (BC) in either the free or dissolved organic carbon (DOC)-combined form. The complexation of metals with DOC influences their toxicity and bioavailability in the environment. The endogenous release of metal species with heterogeneous DOC from BC is very complex; this process has been neglected and remains unaddressed in the literature to date. In this study, the yield and chemical properties of labile DOC from BC were characterized, and the release of endogenous metal/metalloid elements (K, Mg, Mn, Fe, Al, Cu, and Si) and their species from BC with various pyrolysis temperatures and particle sizes were systematically investigated under various solution chemistries. The results showed that pyrolysis temperature of BC significantly influenced the yield and composition of DOC and DOC-metal/metalloid complexes, while particle size had lower impact. The yield of BC-derived DOC significantly decreased and the components gradually changed from low-molecular weight and low-aromaticity hydrophilic humic acid-like substances to complex high-molecular weight and high-aromaticity hydrophobic substances as pyrolysis temperature increased from 200 to 700 °C. The release of total dissolved metals decreased with increasing pyrolysis temperature, while the highest total dissolved Si was released from BC with the moderate pyrolysis temperature (500 °C). The metal elements were mainly released in the DOC-combined form, while the released Si was mainly in the free form in the neutral water environment. The release of DOC increased while that of dissolved metals decreased with increasing solution pH. The release of total dissolved metals/metalloids increased but the ratio of the DOC-combined metals/metalloids decreased with increasing solution ionic strength. These results provide new insight into the understanding of endogenous metal/metalloid release from BC in the natural environment.

(Corrosion Division Morris Cohen Graduate Student Award Address) Active Metallic Corrosion in Weak Acid Solutions: A Unified Mechanistic View to Cathodic Reactions

Metallic corrosion in an aqueous acidic solution is one of the most common corrosion scenarios in industrial applications and energy sector, in particular the oil and gas extraction and transmission facilities. In such systems corrosion is usually caused by the presence of the co-produced aqueous phase, acid gases, and other dissolved acidic compounds. The three major compounds that are conventionally believed to be causing the high corrosivity in such environments are carboxylic acids, carbon dioxide (CO2), and hydrogen sulfide (H2S). The presence of these species in the aqueous phase form an acidic, highly corrosive solution. Nevertheless, mild steel remains the inevitable choice of material due to practical and economical limitations of such large scale infrastructures. The use of mild steel, with its low resistance to corrosion, makes the understanding, prediction, detection, and mitigation of corrosion an essential aspect of this industry. The exceptionally higher corrosivity of the aqueous solutions containing CO2, H2S and carboxylic acids (e.g. acetic acid or HAc in short) as compared to solutions of strong acids (e.g. HCl) with the same pH, has been known for decades. Species like carbonic acid (H2CO3), bicarbonate ion (HCO3 -), H2S, and HAc, as weak acids, are only partially dissociated in the solution. Hence, they are present both in their dissociated and undissociated forms. In the conventional mechanistic view, the higher corrosivity of these solutions have been associated with the additional reduction reactions of the undissociated weak acids (to hydrogen and their corresponding anion) including H2CO3, and HCO3 - in CO2 corrosion, as well as HAc, and H2S. With the focus on the cathodic reactions, the mechanism of corrosion in aqueous solutions containing HAc, CO2, and H2S are revisited in a series of investigations. In all three cases the experimental data and the quantitative evaluations showed that the direct electrochemical reduction of H2CO3, HCO3 -, HAc, and H2S are not occurring to any appreciable extend, in contrast to the conventional mechanistic understanding. Furthermore, the deficiencies in previous studies both in the experimental approaches and quantification of the results leading to misinterpretation of the data, were discussed. In the mechanistic view developed in these studies, the corrosion of mild steel in all of the abovementioned scenarios is an extension of simple acidic corrosion of steel with only two electrochemical reactions: hydrogen ion (H+) reduction as the sole cathodic reaction, and iron dissolution as the sole anodic reaction. However, the combined effect of the coupled chemical reactions along with solution pH, and other environmental conditions such as temperature and fluid flow on the surface concentration of H+, leads to a great diversity in the observed behavior of the cathodic polarization curves and corrosion rate trends, as have been observed in the previous studies. In its simplest form, the chemical dissociation of the weak acid at the vicinity of the metal surface acts as an additional source of H+, hence, results in increased limiting currents. Nevertheless, the results of this study showed that depending on the chemical properties of the involved weak acid and other environmental conditions, the same mechanism may lead to more complex polarization behavior. The reduction of HCO3 - and H2S are examples of such scenario, where the cathodic polarization curves show two “waves”, as demonstrated in Figure 1. Even though the existence of this “double wave” was considered as solid proof of the direct reduction of these weak acid until very recently, the mathematical simulations based on the newly proposed mechanism showed that this behavior is also merely another example of the effect of solution chemistry on the surface concentration of H+. In light of this cumulative knowledge, a unifying and generic mechanistic view for Active corrosion in weak acid solutions can now be put forward. Based on this mechanistic view a generic categorization of weak acids, simply based on their pKa values, is developed to represent their extent of influence on the cathodic polarization and corrosion rates. Such a framework can be used to assess the expected behavior of any given species and provides the guidelines for any future research in this field of study. Figure 1

Effect of solution chemistry on the iodine release from iodoapatite in aqueous environments

To ensure the safe disposal of nuclear waste, understanding the release process of radionuclides retained in the nuclear waste forms is of vital importance. Iodoapatite Pb9.85(VO4)6I1.7, a potential waste form for iodine-129, was selected as a model system for ceramic waste forms in this study to understand the effect of aqueous species on iodine release. Semi-dynamic leaching tests were conducted on bulk samples in cap-sealed Teflon vessels with 0.1 mol/L NaCl, Na2CO3, Na3PO4, and Na2SO4 solutions under 90 °C, fixed sample surface area to solution volume ratio of 5/m, and periodic replacement of leaching solutions. The reacted solutions were then analyzed by Inductively Coupled Plasma-Mass Spectrometry and Inductively Coupled Plasma-Optical Emission Spectrometry; the leached surfaces were characterized by X-ray diffraction, scanning electron microscopy, and infrared spectroscopy. The result shows that, compared to deionized water, the ion-rich solutions enhanced the iodine release as a result of the increased ionic strength, reduced activity coefficient of dissolved species, and increased solution pH. Surface reactions can lead to the formations of secondary phases by ion-exchange and precipitation. These findings suggest that an ion-rich environment in the geological repository can be detrimental to the disposal safety of the nuclear waste form.

A critical review on remediation of bisphenol S (BPS) contaminated water: Efficacy and mechanisms

Bisphenols have drawn increasing attention from regulatory agencies and scientific communities due to their widespread occurrences, distribution and endocrine disrupting effects, and severe toxicity. As one of the bisphenol A (BPA) substitutes, bisphenol S (BPS) is most frequently detected in the environment. Although numerous studies have shown its occurrence, distribution, and toxicity to certain aquatic species, investigation on BPS removal from aqueous environments is lacking. Thus, in this review, we summarize the state-of-art about BPS removal approaches including biodegradation, sorption, and advanced oxidation processes. Particular attention has been paid to the BPS sorption mechanisms, active species for BPS biodegradation, and the corresponding degradation pathways. The primary degradation intermediates formed during BPS oxidation (e.g. p-hydroxybenzenesulfonic acid) are discussed. The effects of solution chemistry (pH, ionic strength, anion, and surfactants) on BPS decontamination are also emphasized. In addition, knowledge gaps, current challenges, and future research needs of BPS decontamination in real environments have been discussed briefly. Through this review we demonstrate the overarching scientific opportunities for a comprehensive understanding of the efficiency and mechanisms of the bio-and chemical remediation approaches of BPS contaminated water.HighlightsMicroplastics may become a new source of bisphenol S (BPS) in aquatic environment.BPS is biodegradable but it may take a long time.Major mechanisms for BPS sorption are summarized and discussed.BPS degradation mechanisms and pathways are summarized.

Deposition and release of carboxylated graphene in saturated porous media: Effect of transient solution chemistry

Chemical perturbation of pore-water in porous media may remobilize and release deposited colloids/nanomaterials into bulk flow. This re-entrainment process is important to accurately assessing the fate and transport of colloids/nanomaterials in the subsurface. This study investigated deposition and subsequent release of carboxylated graphene nanomaterials (CG) in water-saturated sand columns by first depositing CG at 100 mM NaCl or 2 mM CaCl2 (Phase 1), followed by Phase 2 (elution with sequences of 50, 10, and 1 mM NaCl, or sequences of 0.5 and 0.1 mM CaCl2), and then Phase 3 elution using deionized water. Approximate 89.2%–98.7% of injected CG was retained in sand through Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions, Ca2+ bridging, and straining in Phase 1. Sequential reduction of ionic strength in Phases 2 and 3 resulted in increased release of deposited CG mainly due to the expansion of the electrical double layer thickness and thus decreased depth of the attractive secondary minimum. With increasing pulses of flushing solution, unrecoverable CG increased because weakly associated CG via the secondary minimum was likely translated to immobile regions. Significant tailing of CG released in Phase 3 suggests that CG retained in CaCl2 was more resistant upon detachment than in NaCl. In cation exchange experiment, only 0.7% of applied CG was released, possibly ascribed to the CG remobilized by cation exchange was immediately re-entrained by the secondary minimum in 50 mM NaCl. Our findings indicate that retained nanomaterials (e.g., CG) can be remobilized and transported downward in transient solution chemistries, raising concerns about their potential migration risk to groundwater.