Dechlorination of TCE and PCBs using bimetallic nanoparticles has received increasing interest in recent years. However, due to the extremely high reactivity, nanoparticles prepared using current methods tend to either react with surrounding media or agglomerate, resulting in the formation of much larger flocs and significant loss in reactivity. To overcome these drawbacks, we developed a simple and green approach for synthesizing palladized iron (Fe-Pd) nanoparticles. We modified the conventional methods by applying a water-soluble starch as a stabilizer. The starched nanoparticles displayed much less agglomeration but greater dechlorination power than those prepared without a stabilizer. TEM analyses indicated that the starched nanoparticles were present as discrete particles as opposed to dendritic flocs for nonstarched particles. The mean particle size was estimated to be 14.1 nm with a standard deviation of 11.7 nm, which translated to a surface area of approximately 55 m2 g(-1). While starched nanoparticles remained suspended in water for days, nonstarched particles agglomerated and precipitated within minutes. The starched nanoparticles exhibited markedly greater reactivity when used for dechlorination of TCE or PCBs in water. At a dose of 0.1 g L(-1), the starched particles were able to destroy 98% of TCE (C0 = 25 mg L(-1)) within 1 h. While trace amounts (<25 microg L(-1)) of 1,1-DCE were detected in the initial stage (<20 min) of degradation, no other intermediate byproducts such as vinyl chloride, cis-, or trans-dichloroethene were detected. The starched nanoparticles at approximately 1 g L(-1) were able to transform over 80% of PCBs (C0 = 2.5 mg L(-1)) in less than 100 h, as compared to only 24% with nonstarched Fe-Pd nanoparticles. The application of an innocuous stabilizer may substantially enhance the performances of palladized iron nanoparticles for environmental applications.
This study reports a new strategy for stabilizing palladized iron (Fe-Pd) nanoparticles with sodium carboxymethyl cellulose (CMC) as a stabilizer. Compared to nonstabilized Fe-Pd particles, the CMC-stabilized nanoparticles displayed markedly improved stability against aggregation, chemical reactivity, and soil transport. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) analyses indicated that the CMC-stabilized nanoparticles with a diameter <17.2 nm are highly dispersed in water. Fourier transform infrared (FTIR) spectroscopy results suggested that CMC molecules were adsorbed to iron nanoparticles primarily through the carboxylate groups through monodentate complexation. In addition, -OH groups in CMC were also involved in interactions with iron particles. Batch dechlorination tests demonstrated that the CMC-stabilized nanoparticles degraded trichloroethene (TCE) 17 times faster than their nonstabilized counterparts based on the initial pseudo-first-order rate constant. Last, column tests showed that the stabilized nanoparticles can be readily transported in a loamy-sand soil and then eluted completely with three bed volumes of deionized (DI) water.
Zerovalent iron (ZVI) nanoparticles of various sizes were synthesized by applying various types of carboxymethyl cellulose (CMC) as a stabilizer. At an initial Fe2+ concentration of 0.1 g/L and with 0.2% (w/w) of CMC (Mr = 90 000), nanoparticles with a hydrodynamic diameter of 18.6 nm were obtained. Smaller nanoparticles were obtained as the CMC/Fe2+ molar ratio was increased. When the initial Fe2+ concentration was increased to 1 g/L, only 1/4 of the CMC was needed to obtain similar nanoparticles. On an equal weight basis, CMC with a greater Mr or higher D.S. (degree of substitution) gave smaller nanoparticles, and lower the synthesizing temperature favored the formation of smaller nanoparticles. It is proposed that CMC stabilizes the nanoparticles through the accelerating nucleation of Fe atoms during the formation of ZVI nanoparticles and, subsequently, forms a bulky and negatively charged layer via sorption of CMC molecules on the ZVI nanoparticles, thereby preventing the nanoparticles from agglomeration through electrosteric stabilization. In agreement with the classical coagulation theory, the presence of high concentrations of cations (Na+ and Ca2+) promoted agglomeration of the nanoparticles. The strategy for manipulating the size of the ZVI nanoparticles may facilitate more effective applications of ZVI nanoparticles for in situ dechlorination in soils and groundwater.
In water treatment processes that involve contaminant reduction by zerovalent iron (ZVI), reduction of water to dihydrogen is a competing reaction that must be minimized to maximize the efficiency of electron utilization from the ZVI. Sulfidation has recently been shown to decrease H formation significantly, such that the overall electron efficiency of (or selectivity for) contaminant reduction can be greatly increased. To date, this work has focused on nanoscale ZVI (nZVI) and solution-phase sulfidation agents (e.g., bisulfide, dithionite or thiosulfate), both of which pose challenges for up-scaling the production of sulfidated ZVI for field applications. To overcome these challenges, we developed a process for sulfidation of microscale ZVI by ball milling ZVI with elemental sulfur. The resulting material (S-mZVI) exhibits reduced aggregation, relatively homogeneous distribution of Fe and S throughout the particle (not core-shell structure), enhanced reactivity with trichloroethylene (TCE), less H formation, and therefore greatly improved electron efficiency of TCE dechlorination (ε). Under ZVI-limited conditions (initial Fe/TCE = 1.6 mol/mol), S-mZVI gave surface-area normalized reduction rate constants (k') and ε that were ∼2- and 10-fold greater than the unsulfidated ball-milled control (mZVI). Under TCE-limited conditions (initial Fe/TCE = 2000 mol/mol), sulfidation increased k and ε ≈ 5- and 50-fold, respectively. The major products from TCE degradation by S-mZVI were acetylene, ethene, and ethane, which is consistent with dechlorination by β-elimination, as is typical of ZVI, iron oxides, and/or sulfides. However, electrochemical characterization shows that the sulfidated material has redox properties intermediate between ZVI and FeO, mostly likely significant coverage of the surface with FeS.
Electrocatalytic reduction of carbon dioxide (CO2ER) in rechargeable Zn–CO2 battery still remains a great challenge. Herein, a highly efficient CO2ER electrocatalyst composed of coordinatively unsaturated single‐atom copper coordinated with nitrogen sites anchored into graphene matrix (Cu–N2/GN) is reported. Benefitting from the unsaturated coordination environment and atomic dispersion, the ultrathin Cu–N2/GN nanosheets exhibit a high CO2ER activity and selectivity for CO production with an onset potential of −0.33 V and the maximum Faradaic efficiency of 81% at a low potential of −0.50 V, superior to the previously reported atomically dispersed Cu–N anchored on carbon materials. Experimental results manifest the highly exposed and atomically dispersed Cu–N2 active sites in graphene framework where the Cu species are coordinated by two N atoms. Theoretical calculations demonstrate that the optimized reaction free energy for Cu–N2 sites to capture CO2 promote the adsorption of CO2 molecules on Cu–N2 sites; meanwhile, the short bond lengths of Cu–N2 sites accelerate the electron transfer from Cu–N2 sites to *CO2, thus efficiently boosting the *COOH generation and CO2ER performance. A designed rechargeable Zn–CO2 battery with Cu–N2/GN nanosheets deliver a peak power density of 0.6 mW cm−2, and the charge process of battery can be driven by natural solar energy.
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