Charles F. Croft

Existing analysis methods for sulfite and sulfate are mostly focused on the determination of one of the two analytes, and even when both analytes are targeted, the corresponding methods are not fully automatic. Herein, the development, optimisation, and validation of an automatic sequential injection analysis (SIA) method for the simultaneous determination of sulfite spectrophotometrically and sulfate turbidimetrically in water samples is reported for the first time. This SIA method is based on the rapid reaction of sulfite with 5,5’-dithiobis(2-nitrobenzoic acid) (DTNB), a non-hazardous and inexpensive reagent, and the precipitation of sulfate with BaCl2. Calibration curves for sulfite and sulfate were characterised by linear ranges of 0 - 40 mg L-1 and 0 - 300 mg L-1, LODs of 0.1 mg L-1 and 4 mg L-1, and RSDs (n = 10) of < 2.51% (10 mg L-1 SO32-) and < 2.05% (100 mg L-1 SO42-), respectively, with a sampling rate of 16 h-1. The SIA method has been successfully applied to synthetic acid rainwater, tap water, and bottled waters spiked with sulfite and sulfate species and good recovery in the range 94 – 115% was obtained.

The development of deep eutectic solvents (DESs) is a key issue for the realization of green and efficient metal extraction processes. The present study aims to experimentally construct the phase diagram of the binary system consisting of tri-n-octylphosphine oxide (TOPO) and 4,4,4-trifluoro-1-phenyl-1,3-butanedione (HBTA) and, thus, determine its eutectic composition for the solvent extraction of Li⁺. Differential scanning calorimetry was used to characterize the phase transitions (melting temperatures and enthalpies) over the entire composition range of the binary mixture. Its eutectic composition was established at HBTA:TOPO mass ratio of 60:40. For further validation of the eutectic composition from the experimentally measured thermal effects for melting of different HBTA:TOPO mass ratios, a Tammann diagram was also constructed. Only mixtures with HBTA:TOPO mass ratios of 70:30, 60:40 (eutectic composition), and 50:50 were liquids at 30 °C, while at room temperature of 25 °C, the 70:30 mixture formed crystals. All three mixtures, which were liquids at 30 °C, were found to extract Li⁺ effectively. However, at a room temperature of 25 °C, only the eutectic mixture (60:40 mass ratio) extracted Li⁺ effectively, while the mixture with HBTA:TOPO mass ratio of 50:50 formed crystals when mechanically agitated and, therefore, was deemed as unsuitable for Li⁺ extraction.

Rare earth elements (REEs) are becoming increasingly important in the development of modern and green energy technologies with the demand for REEs predicted to grow in the foreseeable future. The importance of REEs lies in their unique physiochemical properties, which cannot be reproduced using other elements. REEs are sourced through mining, with global exploration of additional commercially viable mining sites still ongoing. However, there is a growing need for recycling of REEs due to the current supply of REEs not matching the growing demand, the environmental impact of REE mining and processing (the so-called “balance problem”), and the generation of large volumes of harmful electronic waste (e-waste). Industrial REE processing is mainly carried out by hydrometallurgy processes, particularly solvent extraction (SX) and ion exchange (IX) technologies. However, these methods have a significant environmental impact due to their intensive use of harmful and nonsustainable reagents. This Review highlights the development of approaches involving polymer-based extracting materials for REE manufacturing as more sustainable alternatives to current industrial REE processing methods. These materials include supported liquid membranes (SLMs), solvent impregnated resins (SIRs), macro and micro capsules, polymer inclusion membranes (PIMs), and micro polymer inclusion beads (μPIBs). Polymer-based extracting materials have the advantage of more economical regent usage while applying the same extractants used in commercial SX, enabling applications analogous to the current industrial process. These materials can be fabricated by a variety of methods in a diverse range of physical formats, with the advantages and disadvantages of each material type described and discussed in this Review along with their applications to REE processing, including e-waste recycling and mineral processing.

A composite material, referred to as micropolymer inclusion beads (μPIBs), was fabricated for the first time using a microfluidic technique and applied successfully for the recovery of Au(III) from simulated digests of electronic scrap. Best results for the extraction of Au(III) were achieved with μPIBs consisting of 55% (m/m) poly(vinyl chloride) as the base polymer, 35% (m/m) Aliquat 336 as the extractant, and 10% (m/m) 1-tetradecanol as a modifier. The size and surface morphology of the μPIBs were examined using optical microscopy and scanning electron microscopy, respectively. A batch of 200 mg μPIBs allowed the complete and selective extraction of Au(III) (2.85 mg) from 50 mL of a simulated digest of electronic scrap containing other metal ions, including 1365 mg Cu(II). The extracted Au(III) was quantitatively stripped from the μPIBs into 50 mL of 0.1 mol L^–1 solution of thiourea. No Cu(II) and only sub-microgram levels of Cd(II) and Zn(II) were detected in this solution, thus confirming the suitability of the μPIBs for the efficient recovery of Au(III) from digests of electronic scrap. The microfluidic method used in this study is expected to be applicable for the fabrication of μPIBs for the selective separation of other chemical species by varying the composition of the beads.