Oxidation-assisted alkaline precipitation : the effect of H2O2 on the size of CuO and FeOOH nanoparticles

H2O2 was demonstrated to narrow the size distribution and decrease the size of CuO and hydrous FeOOH (2-line ferrihydrite) nanoparticles under conditions of high supersaturation. We introduce oxidation-assisted alkaline precipitation (Ox-AP) and compare it to traditional alkaline precipitation (AP). While for AP, a metal salt solution (e.g., CuCl2) is mixed with an alkali (e.g., NaOH), for Ox-AP, the more reduced form of that metal salt solution (e.g., CuCl) is simultaneously mixed with that alkali and an oxidant (e.g., H2O2). The resulting precipitates were characterized with SEM, XRD, DLS and single particle ICP-MS and shown to be nanoparticles (NPs). Ox-AP CuO NPs were up to 3 times smaller than AP NPs. Ox-AP FeOOH NPs were up to 22.5% smaller than AP NPs. We discuss and propose a possible mechanism of Ox-AP through careful consideration of the known reaction chemistry of iron and copper. We propose that an increased monomer formation rate enhances the nucleation rate, which ultimately results in smaller particles with a more narrow distribution. The more distinct effect of Ox-AP on copper, was attributed to the fast formation of the stable CuO monomer, compared to AP, where the Cu(OH)(2) and/or Cu-2(OH)(3)Cl monomers are more likely formed. Although, the exact mechanism of Ox-AP needs experimental confirmation, our results nicely demonstrate the potential of using Ox-AP to produce smaller NPs with a more narrow distribution in comparison to using AP

Microbial bioanodes with high salinity tolerance for microbial fuel cells and microbial electrolysis cells

Increasing the conductivity of the electrolytes used in microbial electrochemical systems is an essential prerequisite to the large-scale success of these technologies. Microbial bioanodes formed from a salt marsh inoculum under constant acetate feeding generated up to 85 A·m-² in media containing 776 mM NaCl (45 g·L-¹, 1.5 times the salinity of seawater). These values were the highest salinities accepted by a microbial anode so far and the highest current densities reported with felt graphite electrodes

Membrane electrolysis for separation of cobalt from terephthalic acid in industrial wastewater

Recovery of valuable metals from wastewaters containing both metals and organics is challenging with current technologies, in part due to their interactions. Typical approaches are chemical intensive. Here, we developed a membrane electrolysis system coupled to an acidic and alkaline crystallizer to enable separate precipitation of the organics and metals without additional chemicals. The target industrial wastewater contained mainly purified terephthalic acid (PTA), benzoic acid (BA), p-Toluic acid (PA), cobalt (Co), and manganese (Mn). We examined the removal and recovery efficiency of PTA and cobalt from two types of synthetic stream and the real process stream using several configurations. The acidic crystallizer reached a removal efficiency of PTA of 98.7 +/- 0.2% (Coulombic efficiency 99.71 +/- 0.2%, pH 3.03 +/- 0.18) in batch tests of the simple synthetic stream. The alkaline crystallizer achieved a cobalt recovery efficiency of 94.51 +/- 0.21% (Coulombic efficiency 87.67 +/- 0.31%, pH 11.37 +/- 0.21) in batch tests of the simple synthetic stream (TPA and Co). Then, the system was operated continuously with complex synthetic stream (TPA, BA, PA, Co and Mn). The alkaline crystallizer achieved a cobalt recovery efficiency of 97.78 +/- 0.02% (Coulombic efficiency 90.45 +/- 0.17%)at pH 11.68 +/- 0.02. The acidic crystallizer obtained a PTA removal efficiency of 61.2 +/- 0.1% (Coulombic efficiency 62.3 +/- 0.2%) over 144 h (pH 3.71 +/- 0.03). A real stream was tested over 5 h runs in batch showing 31.1 +/- 1.0% PTA (Coulombic efficiency 26.5 +/- 0.2%) and 82.92 +/- 0.22% cobalt removal (Coulombic efficiency 75.27 +/- 0.31%) at pH 2.71 +/- 0.12 and 8.07 +/- 0.02, respectively. However, micron-scale precipitates were generated from real stream tests. To conclude, the membrane electrolysis cell coupled with acidic and alkaline crystallizers enabled simultaneous separation of PTA and cobalt as solid precipitates from a complex stream with no chemical addition. The efficiencies were lower with the real stream than the synthetic streams, showing the impact of matrix effects and the need to optimize the performance of the crystallizers

Gas diffusion electrodes on the electrosynthesis of controllable iron oxide nanoparticles

The electrosynthesis of iron oxide nanoparticles offers a green route, with significant energy and environmental advantages. Yet, this is mostly restricted by the oxygen solubility in the electrolyte. Gas-diffusion electrodes (GDEs) can be used to overcome that limitation, but so far they not been explored for nanoparticle synthesis. Here, we develop a fast, environmentally-friendly, room temperature electrosynthesis route for iron oxide nanocrystals, which we term gas-diffusion electrocrystallization (GDEx). A GDE is used to generate oxidants and hydroxide in-situ, enabling the oxidative synthesis of a single iron salt (e.g., FeCl_2) into nanoparticles. Oxygen is reduced to reactive oxygen species, triggering the controlled oxidation of Fe^(2+) to Fe^(3+), forming Fe_(3-x)O_(4-x) (0 <= x <= 1). The stoichiometry and lattice parameter of the resulting oxides can be controlled and predictively modelled, resulting in highly-defective, strain-heavy nanoparticles. The size of the nanocrystals can be tuned from 5 nm to 20 nm, with a large saturation magnetization range (23 to 73 A m^2 kg^(-1)), as well as minimal coercivity (similar to 1 kA m^(-1)). Using only air, NaCl, and FeCl_2, a biocompatible approach is achieved, besides a remarkable level of control over key parameters, with a view on minimizing the addition of chemicals for enhanced production and applications

Spin transition nanoparticles made electrochemically

Materials displaying novel magnetic ground states signify the most exciting prospects for nanoscopic devices for nanoelectronics and spintronics. Spin transition materials, e.g., spin liquids and spin glasses, are at the forefront of this pursuit; but the few synthesis routes available do not produce them at the nanoscale. Thus, it remains an open question if and how their spin transition nature persists at such small dimensions. Here we demonstrate a new route to synthesize nanoparticles of spin transition materials, gas-diffusion electrocrystallization (GDEx), wherein the reactive precipitation of soluble metal ions with the products of the oxygen reduction reaction (ORR), i.e., in situ produced H_2O_2, OH^-, drives their formation at the electrochemical interface. Using mixtures of Cu^(2+) and Zn^(2+) as the metal precursors, we form spin transition materials of the herbertsmithite family-heralded as the first experimental material known to exhibit the properties of a quantum spin liquid (QSL). Single-crystal nanoparticles of similar to 10-16 nm were produced by GDEx, with variable Cu/Zn stoichiometry at the interlayer sites of Zn_xCu_(4-x)(OH)_6Cl_2. For x = 1 (herbertsmithite) the GDEx nanoparticles demonstrated a quasi-QSL behavior, whereas for x = 0.3 (0.3 < x < 1 for paratacamite) and x = 0 (clinoatacamite) a spin-glass behavior was evidenced. Finally, our discovery not only confirms redox reactions as the driving force to produce spin transition nanoparticles, but also proves a simple way to switch between these magnetic ground states within an electrochemical system, paving the way to further explore its reversibility and overarching implications

Exploring natural vs. synthetic minimal media to boost current generation with electrochemically-active marine bioanodes

One of the greatest challenges in the development of Microbial Electrochemical Technologies (METs) is the achievement of efficient bioanodes, which not only can operate for long term but which can also be effectively and promptly regenerated for the sustained and successive production of electricity out of waste organics contained in aqueous streams. Simple strategies that facilitate the engineering of these systems are then pursued. Sustainable electricity generation was here achieved using electrochemically-active marine biofilms, which reached up to 6.8 A m−2 in the best case. These biofilms showed deteriorated current generation after successive transfers in fresh natural media. The electricity-generation functionality of these marine biofilms was recuperated after their relocation into synthetic minimal media (i.e. up to about 3.8 A m−2 after a decay down to about 1–2 A m−2). Upon this relocation, the overall electrochemical mechanisms were preserved. Fluctuating nutrient stress intensified the effect of minimal media on current generation. The change from natural to minimal media showed an important impact on the selection and adaptation of microbial communities, characterized by CE-SSCP profiles; yet, robust bioanodes in which some microbial species were preserved were obtained in synthetic minimal media, these being sufficient for a reproducible electrochemical functionality. The systematic cycling between natural media and minimal media, regarded as periodic stress conditioning, is therefore proposed as a convenient strategy to boost current generation in robust electrochemically-active marine bioanodes

Metal recovery by microbial electro-metallurgy

Raw metals are fundamental to the global economy as they are essential to maintain the quality of our life as well as industrial performance. A number of metal-bearing aqueous matrices are appealing as alternative supplies to conventional mining, like solid industrial and urban waste leachates, wastewaters and even some natural extreme environments (e.g. deep marine sediments, geothermal brines). Some of these sources are already managed for recovery, while others are not suitable either because they are too low in content of recoverable metals or they contain too many impurities that would interfere with classical recovery processes or would be cost-prohibitive. Microbial electro-metallurgy, which results from the interactions between microorganisms, metals and electrodes, in which the electron transfer chain associated with microbial respiration plays a key role, can contribute to overcome these challenges. This review provides the state of the art on this subject, and summarizes the general routes through which microbes can catalyse or support metal recovery, leading to nano- and macro-scale materials. Competing sorption and electrochemical technologies are briefly revisited. The relevant sources of metals are highlighted as well as the challenges and opportunities to turn microbial electro-metallurgy into a sustainable industrial technology in the near future. Finally, an outlook to pursue functional materials through microbial electrometallurgy is provided

Nanocomposite SPEEK-based membranes for Direct Methanol Fuel Cells at intermediate temperatures

Novel nanocomposite membranes were prepared by infiltration of a blend of sulfonated PEEK (SPEEK) with polyvinyl alcohol (PVA), using water as solvent, into electrospun nanolibers of SPEEK blended with polyvinyl butyral (PVB). The membranes were characterized for their application on Direct Methanol Fuel Cells (DMFCs) operating at moderate temperatures (>80 degrees C). An important role of the solvent on the crosslinking temperature for the SPEEK-PVA system was observed. A mat of hydrated SPEEK-30%PVB nanofibers revealed higher proton conductivity in comparison with a dense membrane of similar composition. Incorporation of the nanoliber mats to the SPEEK-35%PVA matrix provided mechanical stability, methanol barrier properties and certain proton conductivity up to a crosslinking temperature of 120 degrees C. Not remarkable effect of the nanofibers was found above that crosslinking temperature. The combined effect of the nanofibers and crosslinking temperature on the properties of the membranes is discussed. DIV1FC performance experiments concluded promising results for this new low-cost type of membranes, although further optimization steps are still required.This research has been funded by the R&D Support Programmes of the Polytechnic University of Valencia (project 24761) and the Spanish Ministry of Science and Innovation (project SP-ENE-20120718).Mollá Romano, S.; Compañ Moreno, V. (2015). Nanocomposite SPEEK-based membranes for Direct Methanol Fuel Cells at intermediate temperatures. Journal of Membrane Science. 492:123-136. https://doi.org/10.1016/j.memsci.2015.05.055S12313649

Towards Increased Recovery of Critical Raw Materials from WEEE– evaluation of CRMs at a component level and pre-processing methods for interface optimisation with recovery processes

Increasing recovery of critical raw materials (CRMs) from waste electrical and electronic equipment (WEEE) is a strategic priority to mitigate supply risks. Today, CRM recovery rates are generally low, with increases requiring new recovery processes and interface optimisation with pre-processing to ensure appropriate material flows for efficient recovery are generated. Here, results from an industrial trial to increase CRM recovery from WEEE are presented to inform development of pre-processing strategies which generate such material flows. Au, Ag, Co, Ga, Mg, Nb, Ru, Pd, Ir, Y, Nd, Sb, Ta and W are identified with XRF in components of a range of WEEE samples including within individual printed circuit board (PCB) components. CRM distribution in PCBs is mapped by visual inspection with reference to this data. Cost-effective methods to disassemble WEEE; isolate CRM bearing components, and upgrade/concentrate CRMs are evaluated for industrial adoption. A guillotine is found most suitable for LCD disassembly and separation of Au edge-contacts from PCBs, while cryocracking is best for isolation of internal components of digital media devices. Thermal PCB disassembly with a solder bath for simultaneous SMD removal and subsequent sieving to sort SMDs thereby concentrating CRMs for recovery is a promising approach. Microwave ashing of PCBs to concentrate CRMs is promising although off-gas treatment would be required. Recovery potential of identified CRMs from material streams generated is found to be poor due to lack of suitable recovery infrastructure except for precious and platinum group metals in PCBs, but available pyrometallurgical recovery permanently dissipates other CRMs present