Hedwig, Sebastian
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Nanofiltration-enhanced solvent extraction of scandium from TiO acid waste
Scandium is a critical raw material with a technological potential to reduce transportation costs and CO2 emissions. However, global supply and market adoption are crucially impaired by the lack of high-grade Sc ores and recovery strategies. A tandem nanofiltration solvent extraction route is demonstrated to enable effective Sc recovery from real-world acid waste from the chloride TiO2 production route. The process involving several filtration stages, solvent extraction, and precipitation was optimized, ultimately producing >97% pure (NH4)3ScF6.
Bioleaching and toxicity of metallurgical wastes
Metallurgical wastes contain metals that are unrecovered during industrial processing. The disposal of these wastes is technically difficult due to the potential release of metals through weathering. Therefore, alternative management methods are currently sought. The high leaching susceptibility of these wastes combined with the need for alternative sources of rare and critical metals creates a need for residual element recovery. This study evaluated the leaching potential of lead matte and copper slag through chemical mineral acid leaching as well as indirect bioleaching with organic acids and direct bioleaching using Acidithiobacillus thiooxidans. The leaching efficiency of these acids was compared based on different normality equivalents. Additionally, the effects of pulp density (1–10%) and extraction time (24–48 h) were assessed. Slag toxicity was assessed with a germination test in concentrated and diluted leachates using Brassica juncea. The results demonstrated that copper slag is particularly suitable for chemical treatment because as much as 91 wt.% Cu and 85 wt.% Zn or 70 wt.% Cu and 81 wt.% Zn were extracted using HNO3 or bacterial leaching, respectively. The residual slag was characterized by significant metal depletion and the presence of gypsum, rendering it more suitable for further use or disposal. Lead matte released 65 wt.% Cu and 8 wt.% Zn using mineral acid leaching while 70 wt.% Cu and 12 wt.% Zn were released using bacterial leaching. Further process optimization is needed for lead matte to generate residue depletion in toxic metals. Toxicity assessment showed toxic characteristics in metal-loaded leachates originating from waste treatment that inhibited germination rates and root development.
Organic solvent free PbI2 recycling from perovskite solar cells using hot water
Perovskite solar cells represent an emerging and highly promising renewable energy technology. However, the most efficient perovskite solar cells critically depend on the use of lead. This represents a possible environmental concern potentially limiting the technologies’ commercialization. Here, we demonstrate a facile recycling process for PbI2, the most common lead-based precursor in perovskite absorber material. The process uses only hot water to effectively extract lead from synthetic precursor mixes, plastic- and glass-based perovskites (92.6 – 100% efficiency after two extractions). When the hot extractant is cooled, crystalline PbI2 in high purity (> 95.9%) precipitated with a high yield: from glass-based perovskites, the first cycle of extraction / precipitation was sufficient to recover 94.4 ± 5.6% of Pb, whereas a second cycle yielded another 10.0 ± 5.2% Pb, making the recovery quantitative. The solid extraction residue remaining is consequently deprived of metals and may thus be disposed as non-hazardous waste. Therefore, exploiting the highly temperature-dependent solubility of PbI2 in water provides a straightforward, easy to implement way to efficiently extract lead from PSC at the end-of-life and deposit the extraction residues in a cost-effective manner, mitigating the potential risk of lead leaching at the perovskites’ end-of-life.
Recovery of scandium from acidic waste solutions by means of polymer inclusion membranes
Scandium is a raw material with properties that promise considerable potential for application in alloys to enable aviation fuel savings and as dopants for use in sustainable energy production using solid oxide fuel cells. Despite these attractive properties, scandium is rarely used due to its scarcity and unreliable supply. Therefore, new strategies for scandium recovery are of economic priority. In this study, polymer inclusion membranes (PIMs) consisting of PVDF-HFP, 2-NPOE and DEHPA, were optimised for selective scandium separation from real TiO2 production waste. With the optimised system, >60% of the scandium was recovered with high selectivity, resulting in scandium mole fraction at more than two orders of magnitude higher in the receiving phase than in the original waste. This suggests PIMs may be an effective way to recover scandium from bulk waste, thus easing the scarcity and insecurity that currently limit its bulk application.
Rapid metal mobilisation through litter, water and bioweathering as the legacy of historical copper smelting
Though activities have long ceased, historical mining sites may continue to represent a risk to the environment and human health through long-time leaching processes. This study was undertaken to assess the immediate environmental risk posed by historic metallurgical slags upon contact with litter (Fagus sylvatica leaves, Picea abies needles) and stream water. Further, the contribution of direct slag phase bioweathering was investigated using a soil solution favouring microbial growth (biostimulation) versus aqueous sterile soil extracts. The slags' exposure to Acidithiobacillus thiooxidans mimicked the extremely acidic conditions that will eventually develop under long-term weathering of the sulfidic phases present in the slags (e.g. bornite, chalcopyrite). The risk of metal mobilisation was assessed by means of both bio-chemical leaching experiments (quantification by triple quadrupole inductively coupled plasma mass spectrometry QQQ-ICP-MS) as well as phytotoxicity (Zea mays germination; direct contact and soil pot experiments). Potential metal donor slag phases were identified by scanning electron microscopy (SEM-EDS). It was shown that slags would be categorised as hazardous waste when remaining in contact with the studied weathering solutions. Lead was the most mobile element leaching from slags (up to 86%) and exceeded the legal limits for classification as a non-hazardous waste for all executed treatments. Biostimulation had little effect on Cu leaching (up to 2.6% versus 2.5% for the sterile soil extract, respectively). Litter derived solutions, in contrast, enhanced glass dissolution instead of heavy metal bearing phases. Metal leaching was rapid, raising concerns for peak loads on slag exposure to changing biogeochemical conditions. Extremely acidic conditions and bioleaching by A. thiooxidans were shown to result in metal-enriched leachates (up to 92% of Zn) as well as the lowest germination rate in Zea mays, implying a long term risk by sulphide bioweathering. Five week pot experiments with a soil/slag mixture and Zea mays revealed poor growth in all studied conditions. However, a bacterially derived citric acid was found to improve root and shoot development, possibly due to alleviating the toxic effect of some elements. Due to the observed phytotoxicity, we conclude that the phytoremediation/rehabilitation of slag impacted soils may be limited. The search for a metal tolerant plant species that would be efficient in terms of biomass production and metals uptake is a perspective of this work.
Ecotoxicity attenuation by acid-resistant nanofiltration in scandium recovery from TiO2 production waste
Sulfur amino acid status controls selenium methylation in pseudomonas tolaasii. Identification of a novel metabolite from promiscuous enzyme reactions
Selenium (Se) deficiency affects many millions of people worldwide, and the volatilization of methylated Se species to the atmosphere may prevent Se from entering the food chain. Despite the extent of Se deficiency, little is known about fluxes in volatile Se species and their temporal and spatial variation in the environment, giving rise to uncertainty in atmospheric transport models. To systematically determine fluxes, one can rely on laboratory microcosm experiments to quantify Se volatilization in different conditions. Here, it is demonstrated that the sulfur (S) status of bacteria crucially determines the amount of Se volatilized. Solid-phase microextraction gas chromatography mass spectrometry showed that Pseudomonas tolaasii efficiently and rapidly (92% in 18 h) volatilized Se to dimethyl diselenide and dimethyl selenyl sulfide through promiscuous enzymatic reactions with the S metabolism. However, when the cells were supplemented with cystine (but not methionine), a major proportion of the Se (∼48%) was channeled to thus-far-unknown, nonvolatile Se compounds at the expense of the previously formed dimethyl diselenide and dimethyl selenyl sulfide (accounting for <4% of total Se). Ion chromatography and solid-phase extraction were used to isolate unknowns, and electrospray ionization ion trap mass spectrometry, electrospray ionization quadrupole time-of-flight mass spectrometry, and microprobe nuclear magnetic resonance spectrometry were used to identify the major unknown as a novel Se metabolite, 2-hydroxy-3-(methylselanyl)propanoic acid. Environmental S concentrations often exceed Se concentrations by orders of magnitude. This suggests that in fact S status may be a major control of selenium fluxes to the atmosphere. IMPORTANCE Volatilization from soil to the atmosphere is a major driver for Se deficiency. “Bottom-up” models for atmospheric Se transport are based on laboratory experiments quantifying volatile Se compounds. The high Se and low S concentrations in such studies poorly represent the environment. Here, we show that S amino acid status has in fact a decisive effect on the production of volatile Se species in Pseudomonas tolaasii. When the strain was supplemented with S amino acids, a major proportion of the Se was channeled to thus-far-unknown, nonvolatile Se compounds at the expense of volatile compounds. This hierarchical control of the microbial S amino acid status on Se cycling has been thus far neglected. Understanding these interactions—if they occur in the environment—will help to improve atmospheric Se models and thus predict drivers of Se deficiency.
Layer-by-layer membrane modification allows scandium recovery by nanofiltration
Aluminium scandium (Sc) alloys are stronger, more corrosion resistant and more heat tolerant than classical aluminium alloys and allow for 3D printing. In particular, the aerospace industry benefits from better fuel efficiency due to lighter materials as well as the advantages of additive manufacturing. However, Sc is currently not available in sufficient quantities and has recently been identified as a raw material critical to the economy. Due to the recentness of the demand, technologies for recovery of Sc from secondary sources are in their infancy. In this study, Sc recovery from titanium dioxide pigment production waste by nanofiltration was investigated. Custom-made layer-by-layer (LbL) modified membranes were optimized with regards to their elemental retention (i.e., selectivity towards Sc) as well as their acid resistance. In model solutions, the optimized membrane retained up to 64% ± 4% Sc, removing the major impurity, iron (Fe), efficiently (12% ± 7% retention) while achieving high flux [32 L m−2 h−1] at a low transmembrane pressure of 5 bar. Acid resistance was shown down to a pH of 0.1, which could be even further increased (up to ≤3 M HCl) by adding more bi-layers and changing the coating conditions. In real wastes, the optimized LbL membrane showed higher Sc retention (60% vs. 50%) compared to a commercial acid resistant membrane, while achieving considerably higher fluxes [27 L m−2 h−1 versus 1 L m−2 h−1, respectively at 5 bar]. It was possible to operate filtration at low transmembrane pressure with up to 70% permeate recovery and flux that was still high [∼10 L m−2 h−1]. In a nutshell, titanium dioxide pigment wastes contained sufficient amounts to satisfy the growing demand for Sc and can be exploited to their full extent by LbL nanofiltration due to the proven advantages of acid stability, Sc retention and selectivity and high achievable fluxes at low pressures.