Recovery and reduction of Au(III) from mixed metal solution by thiourea-resorcinol-formaldehyde microspheres

Efficient capture of iodine in steam and water media by hydrogen bond-driven charge transfer complexes

Untreated radioactive iodine (129I and 131I) released from nuclear power plants poses a significant threat to humans and the environment, so the development of materials to capture iodine from water media and steam is critical. Here, we report a charge transfer complex (TCNQ-MA CTC) with abundant nitrogen atoms and π-conjugated system for adsorption of I2 vapor and I3- from aqueous solutions. Due to the synergistic binding mechanism of benzene/triazine rings and N-containing groups with iodine, special I-π and charge transfer interaction can be formed between the guest and the host, and thus efficient removal of I2 and I3- can be realized by TCNQ-MA CTC with the adsorption capacity up to 2.42 g/g and 800 mg/g, respectively. TCNQ-MA CTC can capture 92% of I3- within 2.5 min, showing extremely fast kinetics, excellent selectivity and high affinity (Kd = 5.68 × 106 mL/g). Finally, the TCNQ-MA CTC was successfully applied in the removal of iodine from seawater with the efficiency of 93.71%. This work provides new insights in the construction of charge transfer complexes and lays the foundation for its environmental applications.

Tailoring hierarchical nanostructures of tannin acid/alginate beads for straightforward selective recovery of high-purity Au(0) from aqueous solution

The utilization of biomass materials with functional properties and rational porous structures holds significant potential for the recovery of precious metals from secondary resources, while facing challenges in achieving rapid reduction and high recovery rates of metallic Au(0). Herein, a novel concept of achieving high-purity Au(0) efficiently by tailoring tannin acid (TA) architecture and porous structure of TA-functionalized alginate beads (P-TOSA). Optimized by structural engineering, the hierarchically nanostructured P-TOSA beads demonstrate exceptional selectivity and recovery capacity (756.1 ± 2.7 mg/g at pH 5), while maintaining a recovery efficiency of over 99 % across a broad range of pH values (1.0-8.0) through the synergistic combination of chelation-based chemisorption and phenolic groups-based redox reaction. Notably, the TA-based nanostructure-boosted reduced Au(0) served as nucleation sites, facilitating elongation and migration of gold crystals across the vein network, thus forming a shell composed with 90.4 ± 0.4 % of element gold. UV radiation exposure could further generate a dynamic redox system and expedite Au (III) reduction to ultra-high purity Au(0) (93.3 ± 1.1 %) via abnormal grain growth mode. Therefore, this study presents a practical and straightforward approach utilizing biomass microbeads for recycling precious metals in metallic form without the use of toxic eluents or additional reductants.

Efficient and selective capture of Au(III) from PCBs by pentaethylenehexamine-modified chloromethylated polystyrene beads

Recycling of gold promotes solving the problems of resource waste and environmental pollution. In this work, pentaethylenehexamine (PEHA)-modified chloromethylated polystyrene beads (PEHA-CMPS) was synthesized for the recovery of Au(III) from actual printed circuits boards (PCBs) leaching solution. PEHA-CMPS exhibited excellent adsorption efficiency at a wide pH range. It was discovered that the pseudo-second-order and Langmuir model provided a superior match for the Au(III) adsorption process. The maximum adsorption capacity for Au(III) was 1186 mg/g. Furthermore, PEHA-CMPS was able to selectively capture trace Au(III) with recovery efficiencies of above 80% from the actual PCBs leaching solution. In addition, the column separation approach was utilized to better assess the practical applications for PEHA-CMPS, proving that the prepared adsorbent exhibited great prospects in industrial applications. The adsorption efficiency still maintained 95% after five adsorption-desorption cycles. The FTIR, XRD, and XPS analyses demonstrated that Au(III) uptake on PEHA-CMPS was a collaborative process involving electrostatic interaction, chelation, and oxidation-reduction. The PEHA-CMPS provided a promising strategy in Au(III) recovery and environmental remediation.

Sulfonated polyhedral oligomeric silsesquioxane-cyclodextrin hybrid polymers for efficient removal of micropollutants from water

Herein, β-cyclodextrin-containing hybrid polymers (P1, P2 and P3) were prepared through crosslinking partially benzylated β-cyclodextrin (PBCD) by octavinylsilsesquioxane (OVS). P1 stood out in screening studies and the residual hydroxyl groups of PBCD was sulfonate-functionalized. The obtained P1-SO₃Na showed greatly enhanced adsorption towards cationic MPs and maintained the excellent adsorption performance towards neutral MPs. The rate constants (k₂) of cationic MPs upon P1-SO₃Na were 9.8-34.8 times larger than those upon P1. The equilibrium uptakes of the neutral and cationic MPs upon P1-SO₃Na were above 94.5 %. Meanwhile, P1-SO₃Na demonstrated appreciable adsorption capacities, excellent selectivity, effective adsorption of mixed MPs at environmental levels and good reusability. These results confirmed the great potential of P1-SO₃Na as effective adsorbent to remove MPs from water.

Immobilization of tannin onto dialdehyde chitosan as a strategy for highly efficient and selective Au(III) adsorption

Recycling of Au(III) from wastewater can not only increase resource utilization but also reduce environmental pollution. Herein, a chitosan-based bio-adsorbent (DCTS-TA) was successfully synthesized via crosslinking reaction between tannin (TA) and dialdehyde chitosan (DCTS) for the recovery of Au(III) from the solution. The maximum adsorption capacity for Au(III) was 1146.59 mg/g at pH 3.0, which fitted well with the Langmuir model. The XRD, XPS, and SEM-EDS analyses demonstrated that Au(III) adsorption on DCTS-TA was a collaborative process involving electrostatic interaction, chelation, and redox reaction. Existence of multiple coexisting metal ions did not significantly affect the Au(III) adsorption efficiency, with >90 % recovery of DCTS-TA obtained after five cycles. DCTS-TA is a promising candidate for Au(III) recovery from aqueous solutions due to its easy preparation, environmental-friendliness, and high efficiency.

Porous Covalent Organic Framework Based Hydrogen-Bond Nanotrap for the Precise Recognition and Separation of Gold

Utilizing weak interactions to effectively recover and separate precious metals in solution is of great importance but the practice remains a challenge. Herein, we report a novel strategy to achieve precise recognition and separation of gold by regulating the hydrogen-bond (H-bond) nanotrap within the pore of covalent organic frameworks (COFs). It is found that both COF-HNU25 and COF-HNU26 can efficiently capture Au^III with fast kinetics, high selectivity, and uptake capacity. In particular, the COF-HNU25 with the high density of H-bond nanotraps exhibits an excellent gold uptake capacity of 1725 mg g^-1 , which is significantly higher than that (219 mg g^-1 ) of its isostructural COF (COF-42) without H-bond nanostrap in the pores. Importantly, the uptake capacity is strongly correlated to the number of H-bonds between phenolic OH in the COF and [AuCl₄ ]^- in water, and multiple H-bond interactions are the key driving force for the excellent gold recovery and reusability of the adsorbent.

Sustainable and Selective Modern Methods of Noble Metal Recycling

Noble metals exhibit broad arrange of applications in industry and several aspects of human life which are becoming more and more prevalent in modern times. Due to their limited sources and constantly and consistently expanding demand, recycling of secondary and waste materials must accompany the traditional mineral extractions. This Minireview covers the most recent solvometallurgical developments in regeneration of Pd, Pt, Rh, Ru, Ir, Os, Ag and Au with emphasis on sustainability and selectivity. Processing-by selective oxidative dissolution, reductive precipitation, solvent extraction, co-precipitation, membrane transfer and trapping to solid media-of eligible multi-metal substrates for recycling from waste printed circuit boards to end-of-life automotive catalysts are discussed. Outlook for possible future direction for noble metal recycling is proposed with emphasis on sustainable approaches.

Highly Efficient and Selective Gold Recovery Based on Hypercross-Linking and Polyamine-Functionalized Porous Organic Polymers

With the continuous increase of electronic products, there is an urgent need to effectively recover gold from e-waste and other secondary resources other than the original mine. Here, hypercross-linking and polyamine-functionalized porous organic polymers (Pc-POSS-POP) were designed and facially synthesized based on multiple azo-coupling polymerizations between 2,9,16,23-tetraaminophthalocyanine (H₂Pc(NH₂)₄) and octa(aminophenyl)-t8-silsesquioxane (OAPS) for the first time. The reaction requires no metal as a catalyst, thereby benefiting the purification of the product and the industrial scalability. Pc-POSS-POPs possess a hypercross-linking structure, highly conjugated frameworks, nitrogen-rich active sites, and extensively visible and near-infrared light absorption, which was utilized as an absorbent to retrieve Au (III). The results demonstrated that Pc-POSS-POPs have a high adsorption capacity (862.07 mg g^-1) and a rapid adsorption rate toward gold recycling. The maximum adsorption capacity could reach up to 1026.87 mg g^-1 as in the case of light irradiation. Due to the strong N coordination sites and the electronic interaction between the -NH₄⁺ groups of Pc-POSS-POPs and AuCl₄^-, Pc-POSS-POPs also exhibited excellent selectivity toward gold over several coordinated metals [Cr (VI), Co (II), Cd (II), Ni (II), and Hg (II)]. These properties together with the good regenerative ability and superior recyclability demonstrated that Pc-POSS-POPs possess promising potential as hypercross-linking polymers for capturing and recycling of Au (III).

Selective recovery of platinum from spent autocatalyst solution by thiourea modified magnetic biocarbons

The precious platinum group metals distributed in urban industrial products should be recycled because of their rapid decline in the contents through excessive mining. In this work, thiourea modified magnetic biocarbons are prepared via an energy-efficient microwave-assisted activation and assessed as potential adsorbents to recover platinum ions (i.e., Pt(IV)) from dilute waste solution. The physicochemical properties of prepared biocarbons are characterized by a series of spectroscopic and analytic instruments. The adsorption performance of biocarbons is carried out by using batch tests. Consequently, the maximum adsorption capacity of Pt(IV) observed for adsorbents is ca. 42.8 mg g^-1 at pH = 2 and 328 K. Both adsorption kinetics and isotherm data of Pt(IV) on the adsorbents are fitted better with non-linear pseudo second-order model and Freundlich isotherm, respectively. Moreover, the thermodynamic parameters suggest that the Pt(IV) adsorption is endothermic and spontaneous. Most importantly, the adsorbents exhibit high selectivity toward Pt(IV) adsorption and preserve ca. 96.9% of adsorption capacity after six cyclic runs. After adsorption, the regeneration of the prepared adsorbents can be effectively attained by using 1 M thiourea/2% HCl mixed solution as an eluent. Combined the data from Fourier transform infrared and X-ray photoelectron spectroscopies, the mechanisms for Pt(IV) adsorption are governed by Pt-S bond between Pt(IV) and thiourea as well as the electrostatic attraction between anionic PtCl₆^2- and cationic functional groups of adsorbents. The superior Pt(IV) recovery and sustainable features allow the thiourea modified magnetic biocarbon as a potential adsorbent to recycle noble metals from spent autocatalyst solution.

Mechanism of selective gold adsorption on ion-imprinted chitosan resin modified by thiourea

Thiourea-modified chitosan-imprinted resin (IM-TUCS) and a corresponding nonimprinted resin (NIM-TUCS) were synthesized and characterized using adsorption experiments. The adsorption results showed that adsorption reached equilibrium within 4 h. The adsorption data were better fitted using the Langmuir model (R²>0.99), and the gold adsorption capacities of IM-TUCS and NIM-TUCS were 933.2 and 373.7 mg·g^-1, respectively. The IM-TUCS adsorbent was more suitable for gold than other coexisting anions and cations. The possible mechanism underlying Au(Ⅲ) adsorption on IM-TUCS was further investigated using X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction analyses. The protonation of the amino group on the resin under low pH conditions promoted Au(Ⅲ) adsorption; O, N and S in the C‒OH, C˭S and C-NH₂ groups contained in the IM-TUCS coordinated with Au(III) ions. The cross-linking of the imprinted resin provided holes that could hold Au(III), thus the imprinted resin supported more Au(III). The adsorption capacity of the IM-TUCS for Au(III) was significantly higher than that of the NIM-TUCS, which is attributed to the cross-linking of the imprinted resin. Moreover, the IM-TUCS showed specific recognition capabilities for Au(III). After elution with the eluent, IM-TUCS was reused for four cycles with a gold recovery rate of approximately 93%, revealing its high potential economic value.

An Imidazole Thione-Modified Polyhedral Oligomeric Silsesquioxane for Selective Detection and Adsorptive Recovery of Au(III) from Aqueous Solutions

Developing a material toward simultaneous detection and recovery of gold ions (Au(III)) is highly desirable for the economy and the environment. Herein, we report a highly efficient dual-function material for simultaneous Au(III) detection and recovery by simply introducing abundant imidazole thione and thioether groups in one system. This material, that is, an imidazole thione-modified polyhedral oligomeric silsesquioxane (POSS-2), was prepared by a mild reaction of an imidazolium-containing POSS and sulfur at ambient temperature. The POSS-2 suspension in water can rapidly and selectively detect Au(III) with a very low limit of detection of 1.2 ppb by fluorescence quenching or a visualized color change from white to dark orange. POSS-2 can also selectively and efficiently capture Au(III) with a maximum adsorption uptake of 1486.5 mg/g. The adsorption process well fits with the pseudo-second-order kinetic and Langmuir models. The intriguing dual-function performance is better than most of the previous Au(III) probes or adsorbents. The mechanism study reveals that the detection and adsorption behavior are mainly caused by the redox reaction and coordination between imidazole thione and thioether groups and Au(III). Furthermore, POSS-2 was successfully utilized to extract gold without interference from a discard CPU. These results indicate the potential application of the present dual-function material for Au(III) detection and recovery from aqueous solutions. More dual-functional materials could be designed and prepared by this simple strategy.

Highly selective enrichment of Au using enaminone covalent organic polymers (COP)

The second recovery of gold has fundamental environmental and economic significance. In this study, an enaminone covalent organic polymer (COP) was synthesized and used to recover gold from secondary resources. The ketene covalent organic polymer (COP) possessed good gold adsorption performance, and the best adsorption capacity could reach up to 1945 mg g⁻¹ at 298 K, which was superior to traditional adsorbents. Meanwhile, the synthesized COP exhibited excellent adsorption selectivity and renewability of Au³⁺. The pseudo-second-order kinetic equation and Langmuir equation were suggested for describing the adsorption process. Moreover, this gold adsorption behavior was intended to be a spontaneous, endothermic, and entropy increasing process.

The second recovery of gold has fundamental environmental and economic significance.

Core-shell Fe3O4@zeolite NaA as an Adsorbent for Cu2

Here, using Fe3O4@SiO2 as a precursor, a novel core-shell structure magnetic Cu2+ adsorbent (Fe3O4@zeolite NaA) was successfully prepared. Several methods, namely X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), Transmission electron microscope (TEM), Brunauer Emmett Teller (BET) and vibrating sample magnetometry (VSM) were used to characterize the adsorbent. A batch experiment was conducted to study the Cu2+ adsorption capacity of Fe3O4@zeolite NaA at different pH values, contact time, initial Cu2+ concentration and adsorbent does. It is found that the saturated adsorption capacity of Fe3O4@zeolite NaA on Cu2+ is 86.54 mg/g. The adsorption isotherm analysis shows that the adsorption process of Fe3O4@zeolite NaA to Cu2+ is more consistent with the Langmuir model, suggesting that it is a monolayer adsorption. Adsorption kinetics study found that the adsorption process of Fe3O4@zeolite NaA to Cu2+ follows the pseudo-second kinetics model, which means that the combination of Fe3O4@zeolite NaA and Cu2+ is the chemical chelating reaction. Thermodynamic analysis shows that the adsorption process of Fe3O4@zeolite NaA to Cu2+ is endothermic, with increasing entropy and spontaneous in nature. The above results show that Fe3O4@zeolite NaA is a promising Cu2+ adsorbent.