Drivers and Pathways for the Recovery of Critical Metals from Waste-Printed Circuit Boards.

The ever-increasing importance of critical metals (CMs) in modern society underscores their resource security and circularity. Waste-printed circuit boards (WPCBs) are particularly attractive reservoirs of CMs due to their gamut CM embedding and ubiquitous presence. However, the recovery of most CMs is out of reach from current metal-centric recycling industries, resulting in a flood loss of refined CMs. Here, 41 types of such spent CMs are identified. To deliver a higher level of CM sustainability, this work provides an insightful overview of paradigm-shifting pathways for CM recovery from WPCBs that have been developed in recent years. As a crucial starting entropy-decreasing step, various strategies of metal enrichment are compared, and the deployment of artificial intelligence (AI) and hyperspectral sensing is highlighted. Then, tailored metal recycling schemes are presented for the platinum group, rare earth, and refractory metals, with emphasis on greener metallurgical methods contributing to transforming CMs into marketable products. In addition, due to the vital nexus of CMs between the environment and energy sectors, the upcycling of CMs into electro-/photo-chemical catalysts for green fuel synthesis is proposed to extend the recycling chain. Finally, the challenges and outlook on this all-round upgrading of WPCB recycling are outlined.

Crystalline restacking of 2D-materials from their nanosheets suspensions.

We report here the highly ordered restacking of the layered phosphatoantimonic dielectric materials H3(1-x)M3xSb3P2O14, (where M = Li, Na, K, Rb, Cs and 0 ≤ x ≤ 1), from their nanosheets dispersed in colloidal suspension, induced by a simple pH change using alkaline bases. H3Sb3P2O14 aqueous suspensions are some of the rare examples of colloidal suspensions based on 2D materials exhibiting a lamellar liquid crystalline phase. Because the lamellar period can reach several hundred nanometers, the suspensions show vivid structural colors and because these colors are sensitive to various chemicals, the suspensions can be used as sensors. The structures of the lamellar liquid crystalline phase and the restacked phase have been studied by X-ray scattering (small and wide angle), which has followed the dependence of the lamellar/restacked phase equilibrium on the cation exchange rate, x. The X-ray diffraction pattern of the restacked phase is almost identical to that of the M3Sb3P2O14 crystalline phase, showing that the restacking is highly accurate and avoids the turbostratic disorder of the nanosheets classically observed in nanosheet stacking of other 2D materials. Strikingly, the restacking process exhibits features highly reminiscent of a first-order phase transition, with the existence of a phase coexistence region where both ∼1 nm (interlayer spacing of the restacked phase) and ∼120 nm lamellar periods can be observed simultaneously. Furthermore, this first-order phase transition is well described theoretically by incorporating a Lennard-Jones-type lamellar interaction potential into an entropy-based statistical physics model of the lamellar phase of nanosheets. Our work shows that the precise cation exchange produced at room temperature by a classical neutralization reaction using alkaline bases leads to a crystal-like restacking of the exfoliated free Sb3P2O143- nanosheets from suspension, avoiding the turbostratic disorder typical of van der Waals 2D materials, which is detrimental to the controlled deposition of nanosheets into complex integrated electronic, spintronic, photonic or quantum structures.

Intense pH Sensitivity Modulation in Carbon Nanotube-Based Field-Effect Transistor by Non-Covalent Polyfluorene Functionalization.

We compare the pH sensing performance of non-functionalized carbon nanotubes (CNT) field-effect transistors (p-CNTFET) and CNTFET functionalized with a conjugated polyfluorene polymer (labeled FF-UR) bearing urea-based moieties (f-CNTFET). The devices are electrolyte-gated, PMMA-passivated, 5 µm-channel FETs with unsorted, inkjet-printed single-walled CNT. In phosphate (PBS) and borate (BBS) buffer solutions, the p-CNTFETs exhibit a p-type operation while f-CNTFETs exhibit p-type behavior in BBS and ambipolarity in PBS. The sensitivity to pH is evaluated by measuring the drain current at a gate and drain voltage of -0.8 V. In PBS, p-CNTFETs show a linear, reversible pH response between pH 3 and pH 9 with a sensitivity of 26 ± 2.2%/pH unit; while f-CNTFETs have a much stronger, reversible pH response (373%/pH unit), but only over the range of pH 7 to pH 9. In BBS, both p-CNTFET and f-CNTFET show a linear pH response between pH 5 and 9, with sensitivities of 56%/pH and 96%/pH, respectively. Analysis of the I-V curves as a function of pH suggests that the increased pH sensitivity of f-CNTFET is consistent with interactions of FF-UR with phosphate ions in PBS and boric acid in BBS, with the ratio and charge of the complexed species depending on pH. The complexation affects the efficiency of electrolyte gating and the surface charge around the CNT, both of which modify the I-V response of the CNTFET, leading to the observed current sensitivity as a function of pH. The performances of p-CNTFET in PBS are comparable to the best results in the literature, while the performances of the f-CNTFET far exceed the current state-of-the-art by a factor of four in BBS and more than 10 over a limited range of pH in BBS. This is the first time that a functionalization other than carboxylate moieties has significantly improved the state-of-the-art of pH sensing with CNTFET or CNT chemistors. On the other hand, this study also highlights the challenge of transferring this performance to a real water matrix, where many different species may compete for interactions with FF-UR.

Do Aqueous Suspensions of Smectite Clays Form a Smectic Liquid-Crystalline Phase?

Bottom-up strategies for the production of well-defined nanostructures often rely on the self-assembly of anisotropic colloidal particles (nanowires and nanosheets). These building blocks can be obtained by delamination in a solvent of low-dimensionality crystallites. To optimize particle availability, determination of the delamination mechanism and the different organization stages of anisotropic particles in dispersion is essential. We address this fundamental issue by exploiting a recently developed system of fluorohectorite smectite clay mineral that delaminates in water, leading to colloidal dispersions of single-layer, very large (≈20 µm) clay sheets at high dilution. We show that when the clay crystallites are dispersed in water, they swell to form periodic one-dimensional stacks of fluorohectorite sheets with very low volume fraction (<1%) and therefore huge (≈100 nm) periods. Using optical microscopy and synchrotron X-ray scattering, we establish that these colloidal stacks bear strong similarities, yet subtle differences, with a smectic liquid-crystalline phase. Despite the high dilution, the colloidal stacks of sheets, called colloidal accordions, are extremely robust mechanically and can persist for years. Moreover, when subjected to AC electric fields, they rotate as solid bodies, which demonstrates their outstanding internal cohesion. Furthermore, our theoretical model captures the dependence of the stacking period on the dispersion concentration and ionic strength and explains, invoking the Donnan effect, why the colloidal accordions are kinetically stable over years and impervious to shear and Brownian motion. Because our model is not system specific, we expect that similar colloidal accordions frequently appear as an intermediate state during the delamination process of two-dimensional crystals in polar solvents.

Efficient Electrocatalyst Nanoparticles from Upcycled Class II Capacitors.

To move away from fossil fuels, the electrochemical reaction plays a critical role in renewable energy sources and devices. The anodic oxygen evolution reaction (OER) is always coupled with these reactions in devices but suffers from large energy barriers. Thus, it is important for developing efficient OER catalysts with low overpotential. On the other hand, there are large amounts of metals in electronic waste (E-waste), especially various transition metals that are promising alternatives for catalyzing OER. Hence, this work, which focuses on upcycling Class II BaTiO3 Multilayer Ceramic Capacitors, of which two trillion were produced in 2011 alone. We achieved this by first using a green solvent extraction method that combined the ionic liquid Aliquat® 336 and hydrochloride acid to recover a mixed solution of Ni, Fe and Cu cations, and then using such a solution to synthesize high potential catalysts NiFe hydroxide and NiCu hydroxide for OER. NiFe-hydroxide has been demonstrated to have faster OER kinetics than the NiCu-hydroxide and commercial c-RuO2. In addition, it showed promising results after the chronopotentiometry tests that outperform c-RuO2.

Activated recovery of PVC from contaminated waste extension cord-cable using a weak acid.

Waste electronic and electrical equipment are complex mixtures of valuable and/or toxic materials, which pose serious challenges in their recycling or disposal, for example, electrical transmission wires insulated in polyvinyl chloride materials. These materials are frequently found contaminated with toxic chemical elements, such as Pb, Hg, Cr, or Cd, and are discarded without decontamination. To resolve this problem, we developed a microwave-assisted extraction process to remove toxic metals from plastic e-waste. We processed diluted (30 wt%) citric acid at 210 °C for 1 h inside a pressurized vessel heated by microwave, and found it was suitable not only for the extraction of the toxic metals (∼100%) but also for a significant plastic recovery (>50 wt%). To predict an optimized process window, the support vector regression machine learning algorithm was applied, which reduced the amount of experimentation required while still giving accurate results. Conditions optimized for the reference sample also led to maximum extraction of toxic metals from real-life extension cord waste. We also report that the recovered plastic's properties remained intact after the extraction.

Direct reuse of electronic plastic scraps from computer monitor and keyboard to direct stem cell growth and differentiation.

Reuse of electronic wastes is a critical aspect for a more sustainable circular economy as it provides the simplest and most direct route to extend the lifespan of non-renewable resources. Herein, the distinctive surface and micro topographical features of computer electronic-plastic (E-plastic) scraps were unconventionally repurposed as a substrate material to guide the growth and differentiation of human adipose-derived mesenchymal stem cells (ADSCs). Specifically, the E-plastics were scavenged from discarded computer components such as light diffuser plate (polyacrylates), prismatic sheet (polyethylene terephthalate), and keyboards (acrylonitrile butadiene styrene) were cleaned, sterilized, and systematically characterized to determine the identity of the plastics, chemical constituents, surface features, and leaching characteristics. Multiparametric analysis revealed that all the E-plastics could preserve stem-cell phenotype and maintain cell growth over 2 weeks, rivalling the performance of commercial tissue-culture treated plates as cell culture plastics. Interestingly, compared to commercial tissue-culture treated plastics and in a competitive adipogenic and osteogenic differentiation environment, ADSCs cultured on the keyboard and light diffuser plastics favoured bone cells formation while the grating-like microstructures of the prismatic sheet promoted fat cells differentiation via the process of contact guidance. Our findings point to the real possibility of utilizing discarded computer plastics as a "waste-to-resource" material to programme stem cell fate without further processing nor biochemical modification, thus providing an innovative second-life option for E-plastics from personal computers.

Fine tuning the structural colours of photonic nanosheet suspensions by polymer doping.

Aqueous suspensions of nanosheets are readily obtained by exfoliating low-dimensional mineral compounds like H3Sb3P2O14. The nanosheets self-organize, at low concentration, into a periodic stack of membranes, i.e. a lamellar liquid-crystalline phase. Due to the dilution, this stack has a large period of a few hundred nanometres, it behaves as a 1-dimensional photonic material and displays structural colours. We experimentally investigated the dependence of the period on the nanosheet concentration. We theoretically showed that it cannot be explained by the usual DLVO interaction between uniform lamellae but that the particulate nature of nanosheet-laden membranes must be considered. Moreover, we observed that adding small amounts of 100 kDa poly(ethylene oxide) (PEO) decreases the period and allows tuning the colour throughout the visible range. PEO adsorbs on the nanosheets, inducing a strong reduction of the nanosheet charge. This is probably due to the Lewis-base character of the EO units of PEO that become protonated at the low pH of the system, an interpretation supported by theoretical modeling. Oddly enough, adding small amounts of 1 MDa PEO has the opposite effect of increasing the period, suggesting the presence of an additional intermembrane repulsion not yet identified. From an applied perspective, our work shows how the colours of these 1-dimensional photonic materials can easily be tuned not only by varying the nanosheet concentration (which might entail a phase transition) but also by adding PEO. From a theoretical perspective, our approach represents a necessary step towards establishing the phase diagram of aqueous suspensions of charged nanosheets.

Value-added products from thermochemical treatments of contaminated e-waste plastics.

The rise of electronic waste (e-waste) generation around the globe has become a major concern in recent times and its recycling is mostly focused on the recovery of valuable metals, such as gold, silver, and copper, etc. However, e-waste consists of a significant weight fraction of plastics (25-30%) which are either discarded or incinerated. There is a growing need for recycling of these e-waste plastics. The majority of them are made from high-quality polymers (composites), such as acrylonitrile butadiene styrene (ABS), high impact polystyrene (HIPS), polycarbonate (PC), polyamide (PA), polypropylene (PP) and epoxies. These plastics are often contaminated with hazardous materials, such as brominated flame retardants (BFRs) and heavy metals (such as Pb and Hg). Under any thermal stress (thermal degradation), the Br present in the e-waste plastics produces environmentally hazardous pollutants, such as hydrogen bromide or polybrominated diphenyl ethers/furans (PBDE/Fs). The discarded plastics can lead to the leaching of toxins into the environment. It is important to remove the toxins from the e-waste plastics before recycling. This review article gives a detailed account of e-waste plastics recycling and recovery using thermochemical processes, such as extraction (at elevated temperature), incineration (combustion), hydrolysis, and pyrolysis (catalytic/non catalytic). A basic framework of the existing processes has been established by reviewing the most interesting findings in recent times and the prospects that they open in the field recycling of e-waste plastics.

On-line spectroscopic study of brominated flame retardant extraction in supercritical CO2.

Removal of brominated flame retardants (BFRs) from polymers before disposal or recycling will alleviate negative environmental effects and ensure safe usage of recycled products. Extraction of BFRs in supercritical CO2 is appealing but also presents challenges to industries due to limited solubility and lack of kinetic studies. For a more comprehensive evaluation of supercritical extraction potentialities, we (i) developed an on-line pressure apparatus that is compatible with both the FTIR and UV-vis spectrometers to enable kinetic and thermodynamic studies; (ii) studied kinetic extraction involving three conventional and two novel BFRs as well as three typical polymeric matrix. Solubilities were determined using the gravimetric method or X-ray fluorescence. FTIR exhibited a superior applicability compared to UV-vis in the following BFR extraction's time-dependency binary and ternary systems. We observed that faster stirring speed, higher temperature, and finer particle size can accelerate the overall extraction kinetics. In binary systems, it took less than 2 h to achieve equilibrium for each BFR at 60 °C, 25 MPa and 1000 rpm. In the presence of polymeric matrix, slower extraction kinetics were observed due to the occurrence of competitive dissolution and molecular diffusion within the matrix. Mathematical models derived from irreversible desorption and Fick's diffusion laws fitted well with the observed extraction kinetics of BFRs, thus enabling us to identify the rate-determining step. The high solubilization rate coefficients that we measured for BFRs revealed that the dynamic extraction process in up-scaling design could compensate for the low solubility with flowing supercritical CO2.

Destabilization of the Nematic Phase of Clay Nanosheet Suspensions by Polymer Adsorption.

Complex aqueous mixtures comprised of swelling clays and hydrosoluble polymers naturally occur in soils and play a major role in pedogenesis. They are also very often used for formulating oil-well drilling fluids, paints, and personal-care products. The suspensions of some natural clays, thanks to their large nanoparticle aspect ratio, spontaneously form nematic liquid-crystalline phases where the particles align parallel to each other, which affects their flow properties. We observed that adding small amounts of hydrosoluble polymers to these clay suspensions destabilizes the nematic phase with respect to the isotropic (disordered) phase. The polymers that we used (poly(ethylene oxide) and dextran) were too small to adopt particle-bridging conformations and small-angle X-ray scattering experiments showed that the structure of the nematic phase is not altered by polymer doping. However, the adsorption isotherm shows that the macromolecules adsorb onto the clay nanosheets, effectively coating them with a polymer layer. Our extension of Onsager's theory for polymer-coated platelets properly captures the experimental phase diagram and shows how the nematic phase destabilization can be due to the polymer adsorbing more on the platelet faces than at the rim. Because the flow properties of the nematic phase are very different from those of the isotropic phase, the presence or absence of the former phase is an important factor to be determined and considered to explain the rheological behavior of these complex systems.

A microfluidic study of synergic liquid-liquid extraction of rare earth elements.

A microfluidic technique is coupled with X-ray fluorescence in order to investigate the origin of the so-called synergy effect observed in liquid-liquid extraction of rare earth elements (REEs) when special combinations of two extractants - one solvating and one ionic - are used. The setup enables kinetic studies by varying the two phases' contact time. The results obtained are compared with those obtained using a standard batch extraction method at identical contact time. We then determine variations of free energies of transfer for five rare earth elements present in a solution together with a non-target ion (Fe3+) at different pH. Analysis of the effect of temperature and of surface charge density of the coexisting cations allows separating electrostatic effects from complexation effects. We finally show that all non-linear (synergic) effects are quadratic in mole fraction. This demonstrates that in-plane mixing entropy of the bent extractant film, in the first nanometer around rare earth ions, is the determining term in the synergy effect. Surprisingly, even when the third phase is present, free energies of transfer could still be measured in the dilute phase, which is reported for the first time, to our knowledge. We hence show that the extractive power of the dense third phase is stronger than that of conventional reverse aggregates in equilibrium with excess water.

Isotropic, nematic, and lamellar phases in colloidal suspensions of nanosheets.

The phase diagram of colloidal suspensions of electrically charged nanosheets, such as clays, despite their many industrial uses, is not yet understood either experimentally or theoretically. When the nanosheet diameter is very large (∼100 nm to 1 µm), it is quite challenging to distinguish the lamellar liquid-crystalline phase from a nematic phase with strong stacking local order, often called "columnar" nematic. We show here that newly upgraded small-angle X-ray scattering beamlines at synchrotron radiation facilities provide high-resolution measurements which allow us to identify both phases unambiguously, provided that single domains can be obtained. We investigated dilute aqueous suspensions of synthetic Sb3P2O143- nanosheets that self-organize into two distinct liquid-crystalline phases, sometimes coexisting in the same sample. Close examination of their X-ray reflection profiles in the directions perpendicular to the director demonstrates that these two mesophases are a columnar nematic and a lamellar phase. In the latter, the domain size reaches up to ∼20 µm, which means that each layer is made of >600 nanosheets. Because the lamellar phase was only rarely predicted in suspensions of charged disks, our results show that these systems should be revisited by theory or simulations. The unexpected stability of the lamellar phase also suggests that the rims and faces of Sb3P2O143- nanosheets may have different properties, giving them a patchy particle character.

Determining the Partial Pressure of Volatile Components via Substrate-Integrated Hollow Waveguide Infrared Spectroscopy with Integrated Microfluidics.

A microfluidic system combined with substrate-integrated hollow waveguide (iHWG) vapor phase infrared spectroscopy has been developed for evaluating the chemical activity of volatile compounds dissolved in complex fluids. Chemical activity is an important yet rarely exploited parameter in process analysis and control. Access to chemical activity parameters enables systematic studies on phase diagrams of complex fluids, the detection of aggregation processes, etc. The instrumental approach developed herein uniquely enables controlled evaporation/permeation from a sample solution into a hollow waveguide structure and the analysis of the partial pressures of volatile constituents. For the example of a binary system, it was shown that the chemical activity may be deduced from partial pressure measurements at thermodynamic equilibrium conditions. The combined microfluidic-iHWG midinfrared sensor system (µFLUID-IR) allows the realization of such studies in the absence of any perturbations provoked by sampling operations, which is unavoidable using state-of-the-art analytical techniques such as headspace gas chromatography. For demonstration purposes, a water/ethanol mixture was investigated, and the derived data was cross-validated with established literature values at different mixture ratios. Next to perturbation-free measurements, a response time of the sensor <150 s ( t90) at a recovery time <300 s ( trecovery) has been achieved, which substantiates the utility of µFLUID-IR for future process analysis-and-control applications.

Gas sensor array based on metal-decorated carbon nanotubes.

Here we demonstrate design, fabrication, and testing of electronic sensor array based on single-walled carbon nanotubes (SWNTs). Multiple sensor elements consisting of isolated networks of SWNTs were integrated into Si chips by chemical vapor deposition (CVD) and photolithography processes. For chemical selectivity, SWNTs were decorated with metal nanoparticles. The differences in catalytic activity of 18 catalytic metals for detection of H(2), CH(4), CO, and H(2)S gases were observed. Furthermore, a sensor array was fabricated by site-selective electroplating of Pd, Pt, Rh, and Au metals on isolated SWNT networks located on a single chip. The resulting electronic sensor array, which was comprised of several functional SWNT network sensors, was exposed to a randomized series of toxic/combustible gases. Electronic responses of all sensor elements were recorded and the sensor array data was analyzed using pattern-recognition analysis tools. Applications of these small-size, low-power, electronic sensor arrays are in the detection and identification of toxic/combustible gases for personal safety and air pollution monitoring.

Label-free detection of DNA hybridization using carbon nanotube network field-effect transistors.

We report carbon nanotube network field-effect transistors (NTNFETs) that function as selective detectors of DNA immobilization and hybridization. NTNFETs with immobilized synthetic oligonucleotides have been shown to specifically recognize target DNA sequences, including H63D single-nucleotide polymorphism (SNP) discrimination in the HFE gene, responsible for hereditary hemochromatosis. The electronic responses of NTNFETs upon single-stranded DNA immobilization and subsequent DNA hybridization events were confirmed by using fluorescence-labeled oligonucleotides and then were further explored for label-free DNA detection at picomolar to micromolar concentrations. We have also observed a strong effect of DNA counterions on the electronic response, thus suggesting a charge-based mechanism of DNA detection using NTNFET devices. Implementation of label-free electronic detection assays using NTNFETs constitutes an important step toward low-cost, low-complexity, highly sensitive and accurate molecular diagnostics.

Integration of cell membranes and nanotube transistors.

We report the integration of a complex biological system and a nanoelectronic device, demonstrating that both components retain their functionality while interacting with each other. As the biological system, we use the cell membrane of Halobacterium salinarum. As the nanoelectronic device, we use a nanotube network transistor, which incorporates many individual nanotubes in such a way that entire patches of cell membrane are contacted by nanotubes. We demonstrate that the biophysical properties of the membrane are preserved, that the nanoelectronic devices still function as transistors, and that the two systems interact. Further, we use the interaction to study the charge distribution in the biological system, finding that the electric dipole of the membrane protein bacteriorhodopsin is located 2/3 of the way from the extracellular to the cytoplasmic side.

Charge transfer from ammonia physisorbed on nanotubes.

We report the use of nanotube field-effect transistor devices for chemical sensing in a conducting liquid environment. Detection of ammonia occurs through the shift of the gate voltage dependence of the source-drain current. We attribute this shift to charge transfer from adsorbed ammonia molecules, with the amount of charge estimated to be as small as 40 electrons for the smallest shift detected. Using the concentration dependence of the response as an adsorption isotherm, we are able to measure the amount of charge transfer to be 0.04 electron per ammonia molecule.

Dilute liquid crystals used to enhance residual dipolar couplings may alter conformational equilibrium in oligosaccharides.

The solution structures of a trisaccharide and a pentasaccharide containing the Lewis(x) motif were determined by two independent approaches using either dipolar cross-relaxation (NOE) or residual dipolar coupling (RDC) data. For the latter, one-bond 13C[bond](1)H RDC enhanced by two different mineral liquid crystals were used alone. Home-written programs were employed firstly for measuring accurately the coupling constants in the direct dimension of non-decoupled HSQC experiments, secondly for transforming each RDC data set into geometrical restraints. In this second program, the complete molecular structure was expressed in a unique frame where the alignment tensor is diagonal. Assuming that the pyranose rings are rigid, their relative orientation is defined by optimizing the glycosidic torsion angles. For the trisaccharide, a good agreement was observed between the results of both approaches (NOE and RDC). In contrast, for the pentasaccharide, strong discrepancies appeared, which seem to result from interactions between the pentasaccharide and the mesogens, affecting conformational equilibrium. This observation is of importance, as it reveals that using simultaneously NOE and RDC can be hazardous as the former represent 99% of the molecules free in solution, whereas the latter correspond to less than 1% of the structure bound to the mesogen.