H₂O₂-Mediated Oxidation of Zero-Valent Silver and Resultant Interactions among Silver Nanoparticles, Silver Ions, and Reactive Oxygen Species

The H₂O₂-mediated oxidation of silver nanoparticles (AgNPs) over a range of pH (3.0–14.0) is investigated here, and an electron charging–discharging model capable of describing the experimental results obtained is developed. AgNPs initially react with H₂O₂ to form Ag⁺ and superoxide, with these products subsequently reacting to reform AgNPs (in-situ-formed AgNPs) via an electron charging–discharging mechanism. Our experimental results show that the AgNP reactivity toward H₂O₂ varies significantly with pH, with the variation at high pH (>10) due particularly to the differences in the reactivity of H₂O₂ and its conjugate base HO₂^– with AgNPs whereas at lower pH (3–10) the pH dependence of H₂O₂ decay is accounted for, at least in part, by the pH dependence of the rate of superoxide disproportionation. Our results further demonstrate that the in-situ-formed AgNPs resulting from the superoxide-mediated reduction of Ag⁺ have a different size and reactivity compared to those of the citrate-stabilized particles initially present. The turnover frequency for AgNPs varies significantly with pH and is as high as 1776.0 min^–1 at pH 11.0, reducing to 144.2 min^–1 at pH 10.0 and 3.2 min^–1 at pH 3.0

Silver NanoparticleAlgae Interactions: Oxidative Dissolution, Reactive Oxygen Species Generation and Synergistic Toxic Effects

The short-term toxicity of citrate-stabilized silver nanoparticles (AgNPs) and ionic silver Ag­(I) to the ichthyotoxic marine raphidophyte <i>Chattonella marina</i> has been examined using the fluorometric indicator alamarBlue. Aggregation and dissolution of AgNPs occurred after addition to GSe medium while uptake of dissolved Ag­(I) occurred in the presence of <i>C. marina</i>. Based on total silver mass, toxicity was much higher for Ag­(I) than for AgNPs. Cysteine, a strong Ag­(I) ligand, completely removed the inhibitory effects of Ag­(I) and AgNPs on the metabolic activity of <i>C. marina</i>, suggesting that the toxicity of AgNPs was due to the release of Ag­(I). Synergistic toxic effects of AgNPs/Ag­(I) and <i>C. marina</i> to fish gill cells were observed with these effects possibly attributable to enhancement in the generation of reactive oxygen species by <i>C. marina</i> on exposure of the organism to silver

Effects of Aggregate Structure on the Dissolution Kinetics of Citrate-Stabilized Silver Nanoparticles

Aggregation and dissolution kinetics are important environmental properties of silver nanoparticles (AgNPs), and characterization of the interplay between these two processes is critical to understanding the environmental fate, transport, and biological impacts of AgNPs. Time-resolved dynamic light scattering was employed to measure the aggregation kinetics of AgNPs over a range of monovalent electrolyte (NaCl) concentrations. The fractal dimensions (<i>D</i>_f) obtained from aggregation kinetics and static light scattering were found to be dependent on the aggregation mechanism and, in accord with expectation, varied from 1.7 for diffusion-limited cluster aggregation to 2.3 for reaction-limited cluster aggregation. An aggregation–dissolution model, in which the proportion of accessible reactive sites on primary particles as well as the aggregate size and <i>D</i>_f are assumed to be key determinants of reactivity, is found to provide an excellent description of the decline of normalized rate of dissolution of AgNPs during aggregation for different NaCl concentrations. This model also provides fundamental insights into the factors accounting for the observed change in rate of dissolution of AgNPs on injection into seawater thereby facilitating improved prediction of the likely toxicity of AgNPs in the marine environment

Capacitive Membrane Stripping for Ammonia Recovery (CapAmm) from Dilute Wastewaters

A novel cost-effective flow-electrode capacitive deionization unit combined with a hydrophobic gas-permeable hollow fiber membrane contactor (designated “CapAmm”) is described here and used for efficient recovery of ammonia from dilute synthetic wastewaters. During operation, ammonia migrates across a cation exchange membrane and selectively accumulates in the cathode chamber of a flow electrode followed by transformation to dissolved NH₃ with subsequent stripping via a membrane contactor and recovery as ammonium sulfate. Our results demonstrate that the CapAmm process can achieve an ammonia removal efficiency of ∼90% and a recovery efficiency of ∼60%. At current densities of 5.8 and 11.5 A m^–2 (normalized by the effective cation exchange membrane area) and a hydraulic retention time of 1.48 min, the energies required for ammonia recovery were 9.9 and 21.1 kWh (kg of N)⁻¹, respectively, with these values being comparable with those of other similar electrochemical ammonia recovery systems. These findings suggest that the CapAmm technology described here has the potential for the dual purposes of cost-effective salt removal and the recovery of ammonia from wastewaters, with greater stability, better flexibility, and greater energy efficiency compared to those of other methods

Optimizing the Design and Synthesis of Supported Silver Nanoparticles for Low Cost Water Disinfection

Silver nanoparticles (AgNPs) were successfully synthesized and impregnated on silica using chemical reduction methods. XPS and Ag <i>K</i>-edge XANES analysis revealed that the impregnation of AgNPs onto silica using a chitosan + sodium borohydride (NaBH₄) method results in higher silver loading and Ag(0)/Ag­(I) ratio compared to that obtained using NH₃ + NaBH₄/glucose methods. The effects of the dosage of chitosan on silver loading, Ag­(I) release, and bactericidal activities of AgNP-impregnated silica were investigated, with results showing that, at high dosages of chitosan, Ag­(I) released from AgNP-impregnated silica plays an important role in disinfection, while AgNP-mediated bactericidal action dominates at low dosages of chitosan. To further decrease the manufacturing cost, partially oxidized “black rice husk ash” containing substantial residual carbon was applied as AgNP support and found to lead to a greater degree of silver impregnation and to exhibit a longer disinfection lifetime than that of lower carbon content silica supports. On the basis of these findings, it is clear that considerable scope exists for careful optimization in the design and production of AgNP-based bactericidal materials for water treatment purposes

Experimental Investigation on the Behavior of Supercritical CO₂ during Reservoir Depressurization

CO₂ sequestration in saline aquifers is a promising way to address climate change. However, the pressure of the sequestration reservoir may decrease in practice, which induces CO₂ exsolution and expansion in the reservoir. In this study, we conducted a core-scale experimental investigation on the depressurization of CO₂-containing sandstone using NMR equipment. Three different series of experiments were designed to investigate the influence of the depressurization rate and the initial CO2 states on the dynamics of different trapping mechanisms. The pressure range of the depressurization was from 10.5 to 4.0 MPa, which covered the supercritical and gaseous states of the CO₂ (named as CO₂(sc) and CO₂(<i>g</i>), respectively). It was found that when the aqueous phase saturated initially, the exsolution behavior strongly depended on the depressurization rate. When the CO₂ and aqueous phase coexisting initially, the expansion of the CO₂(sc/g) contributed to the incremental CO₂ saturation in the core only when the CO₂ occurred as residually trapped. It indicates that the reservoir depressurization has the possibility to convert the solubility trapping to the residual trapping phase, and/or convert the residual trapping to mobile CO₂

Effect of Structural Transformation of Nanoparticulate Zero-Valent Iron on Generation of Reactive Oxygen Species

While it has been recognized for some time that addition of nanoparticlate zerovalent iron (nZVI) to oxygen-containing water results in both corrosion of Fe⁰ and oxidation of contaminants, there is limited understanding of either the relationship between transformation of nZVI and oxidant formation or the factors controlling the lifetime and extent of oxidant production. Using Fe <i>K</i>-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, we show that while nZVI particles are transformed to ferrihydrite then lepidocrocite in less than 2 h, oxidant generation continues for up to 10 h. The major products (Fe­(II) and H₂O₂) of the reaction of nZVI with oxygenated water are associated, for the most part, with the surface of particles present with these surface-associated Fenton reagents inducing oxidation of a target compound (in this study, ¹⁴C-labeled formate). Effective oxidation of formate only occurred after formation of iron oxides on the nZVI surface with the initial formation of high surface area ferrihydrite facilitating rapid and extensive adsorption of formate with colocation of this target compound and surface-associated Fe­(II) and H₂O₂ apparently critical to formate oxidation. Ongoing formate oxidation long after nZVI is consumed combined with the relatively slow consumption of Fe­(II) and H₂O₂ suggest that these reactants are regenerated during the nZVI-initiated heterogeneous Fenton process

Modeling the Kinetics of Contaminants Oxidation and the Generation of Manganese(III) in the Permanganate/Bisulfite Process

Permanganate can be activated by bisulfite to generate soluble Mn­(III) (noncomplexed with ligands other than H₂O and OH^–) which oxidizes organic contaminants at extraordinarily high rates. However, the generation of Mn­(III) in the permanganate/bisulfite (PM/BS) process and the reactivity of Mn­(III) toward emerging contaminants have never been quantified. In this work, Mn­(III) generated in the PM/BS process was shown to absorb at 230–290 nm for the first time and disproportionated more easily at higher pH, and thus, the utilization rate of Mn­(III) for decomposing organic contaminant was low under alkaline conditions. A Mn­(III) generation and utilization model was developed to get the second-order reaction rate parameters of benzene oxidation by soluble Mn­(III), and then, benzene was chosen as the reference probe to build a competition kinetics method, which was employed to obtain the second-order rate constants of organic contaminants oxidation by soluble Mn­(III). The results revealed that the second-order rate constants of aniline and bisphenol A oxidation by soluble Mn­(III) were in the range of 10⁵–10⁶ M^–1 s^–1. With the presence of soluble Mn­(III) at micromolar concentration, contaminants could be oxidized with the observed rates several orders of magnitude higher than those by common oxidation processes, implying the great potential application of the PM/BS process in water and wastewater treatment

Fluoride Removal from Brackish Groundwaters by Constant Current Capacitive Deionization (CDI)

Charging capacitive deionization (CDI) at constant voltage (CV) produces an effluent stream in which ion concentrations vary with time. Compared to CV, charging CDI at constant current (CC) has several advantages, particularly a stable and adjustable effluent ion concentration. In this work, the feasibility of removing fluoride from brackish groundwaters by single-pass constant-current (SPCC) CDI in both zero-volt and reverse-current desorption modes was investigated and a model developed to describe the selective electrosorption of fluoride and chloride. It was found that chloride is preferentially removed from the bulk solution during charging. Both experimental and theoretical results are presented showing effects of operating parameters, including adsorption/desorption current, pump flow rate and fluoride/chloride feed concentrations, on the effluent fluoride concentration, average fluoride adsorption rate and water recovery. Effects of design parameters are also discussed using the validated model. Finally, we describe a possible CDI assembly in which, under appropriate conditions, fluoride water quality targets can be met. The model developed here adequately describes the experimental results obtained and shows how change in the selected system design and operating conditions may impact treated water quality

Effects of Good’s Buffers and pH on the Structural Transformation of Zero Valent Iron and the Oxidative Degradation of Contaminants

The presence of Good’s buffers caused rapid ZVI corrosion and a dramatic release of Fe­(II) leading to the Fe­(II)-catalyzed transformation of ferrihydrite to lepidocrocite and/or the direct formation of lepidocrocite from the oxidation of Fe­(II) in the pH range 4.0–6.2. In comparison, in the absence of Good’s buffers, elution of Fe­(II) was insignificant with ferrihydrite being the only Fe­(III) oxyhydroxide detected following the oxidative transformation of ZVI. The rapid ZVI corrosion in the presence of Good’s buffer is possibly due to either (i) disruption of the Fe oxide surface layer as a result of attack by Good’s buffers and/or (ii) interaction of Good’s buffer with the outer Fe oxide surface and surface-associated Fe­(II)/Fe­(III) causing the Fe oxide surface layers to be more porous with both these processes facilitating continuous O₂ access to the Fe(0) core and allowing the diffusion of Fe atoms outward. Our results further show that the deprotonated forms of Good’s buffers and the surface charge of the Fe oxides formed at the ZVI surface strongly affect the sorption of the target compound (i.e., formate) and hence the oxidation of these compounds via surface-associated Fe­(II)-mediated heterogeneous Fenton processes