Separation of critical element Germanium using novel catechol-based adsorbents
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- Author:
- Patel, Madhav
- Graduate Program:
- Energy and Mineral Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 30, 2024
- Committee Members:
- Jeremy Gernand, Program Head/Chair
Sarma Pisupati, Major Field Member
Mohammad Rezaee, Major Field Member
Athanasios Karamalidis, Chair & Dissertation Advisor
Konstantinos Alexopoulos, Outside Unit, Field & Minor Member - Keywords:
- Germanium
Solid-Phase Extraction
Critical element
Catechol
Linear Free-Energy Relationship
Microwave-based synthesis
Solid-Phase Extraction
Catechol
Linear Free-Energy Relationship
Microwave-based synthesis
Fixed-bed column adsorption
Aqueous complexation modeling
Critical Elements - Abstract:
- Germanium (Ge) is one of the critical elements of modern technologies, which faces supply risks, inefficient production, and increasing demand. Its high-tech applications include infrared systems, fiber optics, electronics, and solar cells, with its demand expected to surge further due to the lack of viable substitutes and the increasing adoption of solar cells and 5G networks. Despite significant quantities of Ge being present in resources like zinc ores and coal fly ash—estimated at 7-11 kt and 24.6-112 kt, respectively, less than 3% Ge is extracted worldwide, with the majority of Ge production being concentrated in China. The current methods for Ge production involve leaching followed by separation and recovery techniques such as chlorination-distillation, precipitation, solvent extraction, ion flotation, and ion exchange. These existing methods struggle to compete due to process inefficiencies, associated costs, and environmental concerns, resulting in the absence of Ge production within the USA. To address these challenges, innovative approaches like solid-phase extraction (SPE) offer promising solutions. SPE, utilizing functionalized adsorbents that are selective for target ion, presents advantages over traditional methods, providing opportunities for more efficient and economical Ge recovery. The goal of this doctoral study was to develop novel, selective ligand-functionalized adsorbents for solid-phase extraction of germanium and to understand the behavior of these ligands once tethered to the surface, addressing inefficiencies in traditional Ge extraction methods. The goal was divided into three different research objectives: 1) Developing synthesis processes for Ge-selective adsorbents and examining their physical and chemical properties to assess performance in various adsorption experiments, 2) Evaluating these adsorbents in fixed-bed adsorption columns to determine their capacity, selectivity, and regeneration potential for industrial use, 3) Investigating the adsorption mechanisms of Ge(IV) on these adsorbents and the differences in metal complexation between ligands in solution and those tethered to the surface, which is key to predicting and improving adsorbent performance. To achieve the first research objective, novel catechol-based adsorbents were synthesized and developed; namely, chitosan functionalized with catechol (C-Cat), polystyrene with catechol (A-Cat), polystyrene with nitro-catechol (A-Cat-N), and polystyrene with pyrogallol (A-Py), for the selective separation and recovery of Ge. A novel microwave-based functionalization method was developed to functionalize polystyrene-based adsorbents with catechol, nitro-catechol, and pyrogallol, and grafting yields of 50%, 41.7%, and 62%, respectively, were achieved. The surface functionalizations were confirmed with FTIR, and the surface ligand densities were obtained using XPS and CHNS analysis. The adsorption mechanism was understood using XPS in combination with aqueous complexation modeling. All four adsorbents exhibited exceptional selectivity for Ge in the presence of competing ions. In a comparative study, C-Cat demonstrated higher selectivity for Ge over the commercial resin Purolite S108 (N-methylglucamine resin). Moreover, our adsorbents exhibited higher Ge(IV) adsorption capacity than any other catechol-based adsorbents documented in the literature. The adsorption data conformed well to the Langmuir isotherm, suggesting surface complexation as the predominant adsorption mechanism. At pH 3, the Langmuir adsorption capacities were measured as 22.72 mg/g (C–Cat), 29.76 mg/g (A-Cat), 39.14 mg/g (A-Cat-N), and 37.13 mg/g (A-Py). The kinetic data agreed well with the pseudo-2nd order kinetic model, suggesting surface complexation as the adsorption mechanism. The adsorbents were regenerated using 2 M HCl, and after an initial loss of adsorption capacity in the first two cycles due to incomplete desorption of adsorbed Ge, the adsorbents displayed consistent adsorption (within 90-100% of reduced capacity) and desorption (with >95% desorption) in each cycle. The second research objective was addressed by investigating the fixed-bed column adsorption of Ge(IV) using adsorbents A-Cat, A-Cat-N, and A-Py. All three adsorbents exhibited good Ge adsorption (>25 mg/g) and successful desorption in single-element solutions. However, only A-Cat demonstrated efficient Ge desorption in multi-element adsorption experiments. While all three adsorbents displayed high selectivity for Ge, Ge did not desorb from A-Cat-N and A-Py using 2 M HCl, making them unsuitable for industrial Ge recovery. The flow rate and initial concentration of Ge affected the adsorption in the fixed-bed column containing A-Cat. The lower flow rate resulted in increased adsorption capacity, achieving a capacity of 30 mg/g (at 9 bed-volumes per hour (BVH)), while increased inflow concentration increased capacity to 62.8 mg/g (at 80 mg/L Co). Adsorbent A-Cat maintained over 70% capacity after 5 adsorption-desorption cycles with rapid desorption step using 2 M HCl (~50% desorption in 10 minutes and >90% in 80 minutes). Additionally, A-Cat demonstrated selectivity for Ge in a synthetic Zn refinery residue solution, indicating its suitability for industrial Ge recovery from Zn refinery residue. In contrast, commercial N-methylglucamine resin exhibited low selectivity for Ge, saturating at very low BV levels (<10BV). New Linear Free Energy Relationships (LFERs) were developed and used to estimate the log KML of the Metal/Metalloid-Ligand (ML) moieties, followed by aqueous complexation modeling. The modeling simulated the Ge(IV) selectivity but underestimated the extent of Ge(IV) as well as of other competing ions (e.g., Fe(III), Ga(III), Al(III)) complexation, indicating that the actual stability constants of grafted ligand to be higher than solution log KML. The grafted ligands exhibit higher adsorption affinity for ions that have higher metal-ligand complex stability constant (log KML) in free solution. However, upon grafting the ligand onto the surface, the discrepancy in free-solution affinity among different ligands for the same element, such as catechol, nitro-catechol, and pyrogallol for Ge(IV), diminished, e.g., the nitro-catechol has a >4 order of magnitude larger log KML for Ge compared to pyrogallol, but both A-Cat-N and A-Py showed similar adsorption capacity. This discrepancy between solution and surface underscores the greater dependence of adsorption capacity on surface ligand density rather than the solution log KML. Furthermore, the pH behavior of Ge-ligand complexation underwent a shift after grafting as Ge-L complexation was observed at pH<3. In free-solution, Ge(IV) solely forms ML3 complexes in the presence of catechol, with ML3 not existing at pH < 3. However, Ge(IV) adsorption was observed at pH < 3 across all adsorbents (C-Cat, A-Cat, A-Cat-N, and A-Py). This shift in behavior was attributed to the formation of 1:1 and 1:2 Ge-ligand complexes on the adsorbent surface, enabling the utilization of functionalized adsorbents at significantly lower pH values compared to the use of the same ligands in free-solution. The understanding of the difference in the ligand behavior on the surface compared to the free-solution helped achieve the third objective of the current work. In conclusion, the adsorbent A-Cat has shown capabilities to separate and recover Ge(IV) from complex solutions at the lab scale, while complexation modeling showed the difference in the behavior of tethered ligands, completing the overall goal of the current work. However, further investigation through pilot-scale studies is required to fully integrate solid-phase extraction for Ge recovery in industrial applications. Additionally, the current work attempts to relate the ligand behavior in free-solution and on grafted surfaces, highlighting how a strong affinity in free-solution can translate into high adsorption selectivity and how the ligand behavior changes after functionalization; however, it lacks quantification of the changes between the free-solution ligand and the ligand grafted on the surface. Establishing quantitative relationships to quantify the changes after grafting can help predict the stability constants of the grafted ligand using free-solution thermodynamic properties. These relationships can aid in designing better adsorbents and predicting the exact behavior of the adsorbents.
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