Unwanted uranium released in the aquatic environment from uranium mining and nuclear fuel industry has become a growing threat to human health and environment safety due to its radiological and chemical toxicity. Biosorbents from agro-industrial waste are the most preferred materials for the removal of uranium from the wastewater due to their good cost-to-performance ratio. Brewer’s spent grain (BSG), a widely produced by-product from the beer brewery industry, is an inexpensive and readily available feedstock for the production of uranium biosorbents. In the current work, the use of BSG as a promising starting feedstock for low-cost and efficient adsorbents with high adsorption capacity, fast kinetics, selectivity, and reusability, is investigated. Functionalization methods such as thermal treatment, chemical modification (oxidation), and polymer grafting were explored, and the selectivity was tuned using surface ion-imprinting technology. The adsorption performance of adsorbents prepared from BSG was tested under various conditions for practical application, and structure affinity principles were derived from the characterization, data modeling and experimental results (Fig. 1).
In the first part of this work, BSG is successfully converted into altered BSG (ABSG), an effective biosorbent, by mild hydrothermal treatment approach (150 ℃, 16 h). Compared with the conventional hydrothermal carbonation method (up to 250 ℃), the current method is carried out at a significantly lower temperature without any additional activation process, which minimizes the energy consumption and environmental impact during the treatment. Maillard reaction plays an important role in increasing the adsorption capacity by forming various Maillard reaction products (methylglyoxal-derived hydroimidazolone-1 with the highest content) and melanoidins with a large number of functional groups. In addition, other pathways such as dehydration, decarboxylation, aromatization and oxidation also contribute to the increased adsorption capacity. Therefore, the content of carboxyl groups in ABSG increases up to 1.46 mmol/g with maximum adsorption capacities for La(Ⅲ), Eu(Ⅲ), Yb(Ⅲ) (pH = 5.7), and U(Ⅵ) (pH = 4.7) of 38, 68, 46 and 221 mg/g, respectively (estimated by the Langmuir model). Moreover, FT-IR spectra show that both O- and N-containing functional groups are involved in the adsorption of studied ions.
The second part of this work demonstrates for the first time the successful oxidization of BSG using 85 wt% H3PO4 and NaNO2, increasing the carboxyl groups content from 0.15 mmol/g for BSG to 1.3 mmol/g for oxidized BSG (OBSG). OBSG exhibits fast adsorption kinetics in 1 h and an adsorption capacity for U(Ⅵ) of 297.3 mg/g (c0(U) = 900 mg/L, pH = 4.7), which is superior to other biosorbents reported in the literature. Possible adsorption mechanisms are based on ion-exchange between UO22+ and H+ released from carboxyl groups, and the complexation of UO22+ with the two oxygen atoms of carboxyl groups. For practical application, adsorption/desorption studies show that OBSG retains 60% of original adsorption capacity (167 mg/g) with a desorption ratio of 89% after 5 adsorption/desorption cycles. Evaluation of OBSG performance in simulated seawater (10.8 mg/g, c0(U) = 10 mg/L, 193 mg/L NaHCO3 and 25.6 g/L NaCl, pH0 = 7.7) indicates a potential usage at low concentration, high salinity, and in the presence of carbonate.
In the third part of this work, brewer’s spent grain supported superabsorbent polymers (BSG-SAP) with various cross-linking density are prepared for the first time via one-pot swelling and graft polymerization of acrylic acid (AA) and acrylamide as low-cost and environmentally friendly adsorbents. A 7 wt% NaOH solution was used as a swelling agent for BSG and as a neutralization agent for AA without generating alkaline effluents. The use of BSG and graft polymerization can significantly increase the available hydroxyl, carboxyl and amide groups, resulting in a highly cross-linked and highly hydrophilic three-dimensional polymer network of BSG-SAP. The BSG-SAP (BSG-SAP-H) prepared with high cross-linking density exhibits better properties with exceptional adsorption capacity for U(VI) of 1465 mg/g (estimated by the Toth model) at pH0 = 4.6 within 45 min. It also shows good selectivity for U(VI) in the presence of several metal ions (V(V), K(I), Na(I), Mg(II), Zn(II), Co(II), Ni(II), and Cu(II)) with selectivity coefficients (SU) higher than 72%. In simulated seawater, BSG-SAP-H showed higher adsorption capacity (17.6 mg/g for c0(U) = 8 mg/L, pH0 = 8) compared to the currently reported adsorbents based on natural polymers. In the experiments with the fixed bed column (c0(U) = 30 mg/L), the uranyl ions could be concentrated up to 15 folders in U(VI)-spiked water and up to 13 folds in simulated seawater. Moreover, after four cycles, BSG-SAP-H was able to maintain 80% of adsorption capacity in U(VI)-spiked water (254.4 mg/g) and 90% in simulated seawater (37.4 mg/g). FT-IR and 13C solid-state NMR spectra show the function of amide groups for U(VI) adsorption, the bidentate binding structure between UO22+ and the carboxyl groups, and the cation exchange between Na+ in BSG-SAP and UO22+.
The fourth part of this work describes a new strategy for the preparation of surface ion imprinted brewer’s spent grain (IIP-BSG) using binary functional monomers (2-hydroxyethyl methacrylate and diethyl vinylphosphonate) for selective removal of U(VI). A high monomer/template molar ratio of 500:1 is used to ensure high site accessibility and easy template removal. IIP-BSG exhibits a maximum U(VI) adsorption capacity of 165.7 mg/g (pH0 = 4.6, estimated by the Sips model), a high selectivity (SU > 80%) for U(VI) in the presence of an excess amount of Eu(III) (Eu/U molar ratio = 20), and good tolerance to salinity (47.4 mg/g for U(VI) at ionic strength = 1 mol/L and c0(U) = 0.5 mM = 120 mg/L). After 5 adsorption and desorption cycles, IIP-BSG retains 90% of its adsorption capacity (36.9 mg/g) and high selectivity (SU > 92%) in binary U(VI)/Eu(III) solution (c0 = 0.5 mM = 120 mg/L). In addition, FT-IR spectra show the electrostatic interaction and a coordination of uranyl ions by carboxyl and phosphoryl groups, the site energy distribution theory shows the predominant contribution of high-energy (specific) sites during selective adsorption, and the kinetic model shows that the internal mass transfer is the rate-determining step of U(VI) adsorption.
In the last part of this work, the additional tests were performed for BSG and its derived adsorbents to evaluate their potential for practical application. BSG and most of its derived adsorbents retain 90% of their adsorption capacity after aging in water for 6 days, except for ABSG (60% decrease in adsorption capacity). IIP-BSG shows efficient separation of U(VI)/Ln(Ⅲ) (e.g. La(III), and Nd(III), Sm(III)) in weakly acidic nuclear wastewater (pH0 = 3.5) and U(VI) concentration in carbonate-rich-mine water (e.g. Schlema mine water, pH0 = 7.1) and tailings water (e.g. Helmsdorf tailings water, pH0 = 9.8), demonstrating a high potential for practical use. Selectivity of IIP-BSG is also given for acidic mine water (e.g. Königstein mine water, pH0 = 2.6). In addition, the unmodified BSG and BSG-SAP-H could effectively remove uranyl ions from acidic mine water with high selectivity. In particular, the cost efficiency and the availability of unmodified BSG make it of great interests for the remediation of uranium containing acidic mine water (Table 1).
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:81251 |
| Date | 10 October 2022 |
| Creators | Su, Yi |
| Contributors | Weigand, Jan J., Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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