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[Display omitted] • A Ti-Al-C MAX phase (Ti 3 AlC 2) is used to recover gold from acidic solutions. • High gold extraction capacity (>6000 mg/g) is achieved at pH < 1. • High selectivity for gold is observed in the presence of interfering metal ions. • Gold recovery is likely to proceed via a reduction reaction similar to cementation. • High purity gold (92 mass%) is recovered without any posttreatment. Given the scarcity of natural gold sources and associated mining challenges, the recovery of gold from secondary sources, such as electronic equipment waste (e-waste), becomes imperative. E-waste is typically treated with aqua regia to obtain highly acidic gold-containing solutions, which are subsequently processed by various methods to recover gold. Gold recovery materials offer numerous benefits but often exhibit insufficient selectivity, efficiency, and cost-effectiveness. To address this problem, we herein used a nonmodified pristine Ti-Al-C MAX phase (Ti 3 AlC 2) to realize the one-pot room-temperature recovery of gold from highly acidic solutions (pH < 1) in the form of easily collectable bulky ball-shaped Au0 aggregates. High gold selectivity (K d = 2.03 × 106 mL/g) and extraction efficiency (∼99 %) were observed even in the presence of interfering metal ions (Pd2+, Pt4+, Co2+, Ni2+, Cu2+, Zn2+), and the maximum gold extraction capacity (>6000 mg/g) surpassed that of previously reported gold recovery materials. The recovered gold was highly pure (>90 mass%) and contained minimal amounts of the original MAX phase, which obviated the need for intensive postpurification. The results of instrumental analyses suggested that gold recovery relied on electron transfer, similar to cementation, and primarily involved the reduction of solution-phase Au3+ to Au0 via electron transfer from the Ti and Al of Ti 3 AlC 2. Thus, our work demonstrates the significant potential of pristine Ti 3 AlC 2 for the energy-efficient recovery of gold from highly acidic solutions and paves the way for the efficient valorization of e-waste. [ABSTRACT FROM AUTHOR]

• A porous hydrogel was synthesized by incorporating a polymer network into a β -CD-based MOF structure. • The β -CD-MOF hydrogel demonstrated increased structural stability. • The β -CD-MOF hydrogel exhibited excellent adsorption properties for Au3+, Ag+, and Pb2+ ions. • The β -CD-MOF hydrogel showed preferential adsorption of Au3+ over Ag+, potentially useful for gold recovery. Wastewater containing heavy metal ions produced by electroplating, smelting and other industries poses a threat to human safety. Recently, metal–organic frameworks (MOFs) have emerged as porous crystalline adsorbents for the removal of heavy metal ions. Starch-derived cyclodextrins (CDs), a series of cyclic oligosaccharides, have been explored for fabrication of cost-efficient MOF materials. However, the coordination bonds in CD-based MOFs can be destroyed by water, which limits their application as absorbent materials for the removal of heavy metal ions from wastewater. In this work, a β -CD-MOF-based porous hydrogel was developed by incorporating a crosslinked hydrogel network based on acrylic acid (AA) and N,N'-methylenebis(acrylamide) (MBA) monomers into a β -CD-based MOF structure. The synthesized porous hydrogel, denoted A/M−CDMOF−gel, not only showed enhanced structural stability but also exhibited excellent properties for simultaneous adsorption of Au3+, Ag+, and Pb2+ ions. A kinetic study revealed that the A/M−CDMOF−gel had a satisfactory binding rate within 60 min of contact, and the higher fit coefficient from the pseudo-second-order model indicated its chemical adsorption property. Moreover, according to the Langmuir isotherm model fitting results, the maximum capacity of the A/M−CDMOF−gel reached 316.4, 60.9, and 414.2 mg/g for Au3+, Ag+, and Pb2+, respectively. Therefore, the regenerable A/M−CDMOF−gel adsorbent is expected to provide ideas for the development of renewable absorbent materials in the field of wastewater treatment and remediation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The preparation of metal–organic frameworks with super large pore structures through the method of "MOF in MOF" has been proposed. • The adsorbent solely based on its pore advantage achieved a gold adsorption capacity of 1184 mg/g. • This work obtained gold with a purity of 23.8 k from a mineral leaching solution. The design of an efficient adsorbent for Au(III) has remained a challenging research focus, as the adsorbent needs to overcome competition from other ions while simultaneously addressing pore blockage caused by the reduction of Au(III) to elemental gold. In this study, we report the synthesis of a hierarchically porous metal–organic frameworks (MOFs) with ultra-large pores (300 nm) using the "MOF in MOF" method. This material facilitates enhanced adsorption kinetics by increasing the rate of gold ion adsorption and effectively prevents pore blockage resulting from Au(III) reduction, enabling access to the majority of adsorption sites. Batch experiments were conducted, and various complex water mixtures, including freshwater, simulated electronic waste solution, and gold ore leachate, were tested to demonstrate the rapid extraction of trace amounts of gold by the developed adsorbent. Remarkably, without additional functionalization, the adsorbent solely based on its pore advantage achieved a gold adsorption capacity of 1184 mg/g and reached 80 % adsorption within just 5 min (C0 = 100 mg/L, V/m = 2 L/g). Furthermore, the gold recovered from the leachate of gold ore exhibited a purity of 23.8 K (99.2 % purity). [ABSTRACT FROM AUTHOR]

• Electrodeposition and sodium dithionite reduction for gold recovery was compared. • Gold began to deposit at the potential of lower than −100 mV Hg/HgO. • Cl−, S 2 O 3 2− and SO 3 2− exhibited different effects on gold deposition/reduction. • Sodium dithionite reduction displayed better gold recovery performance. • Sodium dithionite reduction had better solution stability and lower recovery cost. A comparative study was performed on electrodeposition and sodium dithionite reduction for recovering gold in the gold-rich solution from the adsorption of thiosulfate solution by ion exchange resin. Using electrodeposition method, gold started to deposit when the potential was lower than −100 mV Hg/HgO. The rise of Cl− concentration improved gold deposition while the increases of SO 3 2− and S 2 O 3 2− concentrations were unbeneficial to gold deposition. When the solution was deposited at the optimum potential of −600 mV Hg/HgO for 5 h, the gold deposition percentage reached 99.33 %. With sodium dithionite reduction method, the gold reduction percentage achieved 100 % after only 5 min as S 2 O 4 2− dosage was eight times of the theoretical amount. The increases of SO 3 2− and S 2 O 3 2− concentrations hindered gold reduction while the augment of Cl− concentration had no obvious effect. Overall, sodium dithionite reduction method has the advantages of shorter reaction time, higher gold recovery percentage, better solution stability and lower recovery cost, and thus it has a broader industrial application prospect. [ABSTRACT FROM AUTHOR]

[Display omitted] • The low-cost and acid-stable PILs were designed to recover gold from e-waste. • The PILs exhibited excellent adsorption capacity and rapid equilibrium time. • The PILs achieved remarkable selectivity to Au(III) in actual CPUs leachate. • A satisfactory reusability of more than 8 cycles was realized. • An economical and feasible flow chart of gold recovery from e-waste was provided. Recycling gold from e-waste is not only beneficial to environmental protection, but also to resource sustainability. However, it remains a great challenge due to the limitations of high cost, lengthy balance process and the interference of coexisting metals. Herein, a family of low-cost, nitrogen-rich content and acid-stable imidazolium-based poly(ionic liquid)s, designated as P(nDVB-mVBIMCl)s, were synthesized for recovery of gold from e-waste. The obtained P(2DVB-8VBIMCl)s achieves outstanding adsorption capacity (820.5 mg·g−1) and rapid equilibrium time (15 min) and satisfactory reusability (more than 8 cycles). XPS, EDS and ion chromatography revealed that the excellent adsorption performance was attributed to synergistic effect of ion exchange and chemical reduction. In addition, the P(2DVB-8VBIMCl)s showed excellent selectivity for gold in actual e-waste leaching solution, exhibited satisfactory selectivity coefficient for Fe3+ (1.46 × 104) and Ni2+ (1.12 × 104), obtaining 23.4 karat high-purity gold after calcination. In addition, the P(2DVB-8VBIMCl)s shows its good economic feasibility. Overall, this work provides a feasible adsorbent, which becomes a promising candidate for industrial gold recovery from e-waste. [ABSTRACT FROM AUTHOR]

• Gold was recovered from pregnant thiosulfate solution by ion exchange resin. • Na 2 SO 3 and NaCl displayed evident "synergistic role" on gold desorption. • The behavior of gold desorption with the mixed desorbent was revealed. • A two-step desorption mechanism was proposed based on thermodynamic analysis. • The mechanism was demonstrated by microscopic characterization and DFT calculation. The behaviors and mechanisms of gold desorption from its loading resin with the mixed solution of Na 2 SO 3 and NaCl were studied. The desorbent displayed evident "synergistic role" on the gold desorption and had good desorption performance over wide ranges of reagent concentration, flow rate and gold loading amount. Based on the thermodynamic analysis, a two-step desorption mechanism was proposed: (1) SO 3 2– reacted with [Au(S 2 O 3) 2 ]3– to generate [Au(S 2 O 3)(SO 3)]3–; (2) The resin had weak affinity for the two-ligand gold complex which was thus replaced by Cl– readily. SEM-EDS, characteristic color of gold complex, Raman spectra and binding energy calculation results indicated SO 3 2– could react with [Au(S 2 O 3) 2 ]3– to form [Au(S 2 O 3)(SO 3)]3–, and the affinity of the resin for the newly-generated complex was weaker than that for Cl–, which demonstrated the above mechanism. The mechanism model of gold recovery from pregnant thiosulfate solution by ion exchange resin was finally constructed. [ABSTRACT FROM AUTHOR]

• A new RBS-functionalized thermo-responsive polymer was synthesized. • This polymer was used as a polymeric surfactant for flotation of Au(III). • The surfactant exhibited selective capture and excellent adsorption property. • The surfactant was easily regenerated by reduced release and thermal precipitation. The selective flotation recovery of gold from secondary resources is a sustainable strategy to fill the ever-increasing demand for gold resources, and reduce the environmental pollution. In the present work, a novel method to realize the selective and fast recovery of Au(III) in the flotation process involving the use of thermo-responsive amphiphilic polymer poly(ethylene glycol)- b -poly(N-isopropylacrylamide- co -glycidyl methacrylate) (PEG 113 - b -P(NIPAM 108 - co -GMA 62)) integrated with rhodamine B thiohydrazide (RBS) affinity ligand was studied. The polymer possessed the selective capture property toward Au(III) salt and excellent adsorption performance on bubble surfaces, driven by specific coordination interactions between Au(III) salt and RBS ligand, and hydrophobic interactions between GMA moieties and bubbles. Flotation using the thermo-responsive polymeric surfactant displayed a high separation efficiency and fast flotation kinetics, achieving a separation efficiency of 85% for Au(III) salt at 28 min, which was comparable with sulfur-functionalized mesoporous materials. This flotation process followed pseudo-second-order kinetics and Freundlich isotherm adsorption model. Furthermore, in flotation recovery of Au(III) salt form actual e-waste leaching solution, separation efficiency of 81% was achieved, while Au grade was improved from 1.55 % to 34.76%. This polymeric surfactant was easily regenerated by the reduction-induced release the bound Au(III) salt and thermal precipitation, leading to retain 81% of separation efficiency after five cycles. This work provides an effective way to fabricate flotation surfactant, which can meet the urgent demand for performance improvement and chemical consumption reduction in practical applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • [C 2 MIM][N(CN) 2 ] ILs replace water-based electrolyte for Au electrodeposition. • ILs improve the current efficiency and avoid the electric energy consumption of HER. • Regenerated iodine was reused in cyclic leaching-electrodeposition to recover Au. • Applied potential, temperature and stirring speed were studied to improve diffusion. The iodination method is a cyanide-free and efficient gold-leaching process, but a large amount of electric energy is consumed for hydrogen evolution during the process of gold recovery by electrodeposition in the iodide/H 2 O system. Herein, [C 2 MIM][N(CN) 2 ] ionic liquids were developed as electrolytes to overcome the limitations of water-based solvents for higher current efficiency by extending the electrochemical window. And the three-dimensional carbon felt with a high surface area and abundant active sites was selected as the cathode material to enhance the deposition rate. Cyclic voltammetry test demonstrated that both Au+ reduction and I- oxidation are irreversible processes controlled by diffusion with the diffusion coefficients of 1.48 × 10-7 cm2/s and 2.91 × 10-7 cm2/s at 20 °C, respectively. The optimal current efficiency (88.03%), specific energy consumption (216.37 kWh/t) and recovery of gold (95.77%), and iodine (85.75%) were obtained by regulating the electrodeposition parameters, including applied potential, temperature, and stirring speed. Moreover, the gold leaching and deposition efficiency were stable at approximately 93% and 95% with 0.2% iodine supplement for four cyclic leaching-electrodeposition. SEM-EDS, XRD, and XPS analysis showed the deposits on the carbon fibers had good uniformity, and the deposit composition contained Au0, Cu0, and CuO. [C 2 MIM][N(CN) 2 ] ionic liquid is an attractive alternative to non-concentrated aqueous electrolytes for high-efficiency and low-power electrodeposition applications. [ABSTRACT FROM AUTHOR]

Phloroglucinol/Resorcinol coupled Triazine-based porous organic polymer for the removal of gold and cationic dyes and paraquat pollutant. [Display omitted] • A facile catalyst-free solvothermal reaction was explored to tune the pore size, and surface area of Phloroglucinol/Resorcinol coupled Triazine-based hyper cross-linked porous organic polymer (HCPs). • The HCPs display coral/fibre-like morphology and fast and complete removal towards large classes of various organic pollutants (cationic dyes, anionic dye and herbicide) over multiple cycles via batch and column methods. • HCPs exhibit good removal performance toward gold ions under acidic conditions over multiple cycles. Due to the various anthropogenic activities, the continuous release of organic pollutants is a serious global threat. Thus, developing effective removal and remediation technologies for hazardous pollutants is essential to minimize the adverse effects on human health and the environment. Herein, we developed hyper-crosslinked polymers (HCPs) via catalyst-free solvothermal Friedel-Crafts alkylation reaction using tris-phloroglucinol/tris-resorcinol substituted triazine and dimethoxymethane. The obtained HCPs (HCP-CPF, HCP-CRF) exhibit variation in the surface area, pore size and morphology. The cross-linking of Phloroglucinol-triazine-based planar monomers via methylene spacer leads to porous coral-like morphology with a high surface area (S area = 480 m2/g) in HCP-CPF. In contrast, the cross-linking of Resorcinol-substituted triazine-based monomers displayed a fibre network with comparatively low surface area (S area = 35.7 m2/g) in HCP-CRF. The presence of densely functionalized multiple hydroxyl groups, good surface area and aqueous dispersibility in HCPs facilitate the fast and efficient removal capacity of various cationic dyes and paraquat herbicide in water. The HCP-CPF displayed fast removal capacity towards the large class of different organic dyes, and it removed 100% of most of the dyes within 10 min from the aqueous solution. Furthermore, the recovery of gold from an aqueous solution is one of the essential and crucial tasks. HCP-CPF and HCP-CRF exhibited a good removal capacity towards Au3+ via in-situ reduction (Q max = 497.5 mg/g for HCP-CPF and Q max = 463.33 mg/g for HCP-CRF). [ABSTRACT FROM AUTHOR]

[Display omitted] • A new adsorbent consisting of a 3D layered aerogel skeleton embedded with TRF resin was creatively synthesized. • This work provided a general method for the difficult recovery of small particle size adsorbents. • Both experimental and theoretical studies reveal that the thiourea group plays a key role in adsorption. Current adsorbents for Au(III) recovery suffer from high costs, low adsorption capacity, recycling difficulty, as well as small-scale applications, making most of them have no commercialization potential. We report a complex adsorbent SA-TRF in this work. The aerogel made of sodium alginate (SA) serves as the carrier, and TRF resin (a resin composed of thiourea, resorcinol and formaldehyde) is used as the functional element. The thin-layered SA aerogel is evenly embedded with functionalized micron adsorbent (TRF) particles. Owing to the multiple functional groups of TRF, SA-TRF exhibited a high adsorption capacity. TRF was capable of making complete contact with Au (III) in the solution owing to the thin-layered SA aerogel. The SA aerogel's macrostructure makes it simple to collect SA-TRF in solution. The repeating unit of TRF's C S and NH functional groups were identified as being responsible for Au(III) binding, with its sulfhydryl functional group performing as the strongest binding site. The conveniently synthesized SA-TRF outperformed most other adsorbents and demonstrated significant adsorption capacity for Au (III) up to 600.2 mg/g in 70 mg/L Au-spiked water (corresponding removal efficiency is 85.7 %), which makes it particularly promising for industrial applications. [ABSTRACT FROM AUTHOR]

In this present work, an electrogenerative flow-through reactor with an activated reticulated vitreous carbon (RVC) cathode was developed and used in recovering gold from an E-waste. The system resulted in more than 99% gold being recovered within 2 h of operation for 100 mg L −1 of initial gold(III) chloride concentration. Besides, the flow cell system in series shows around 98% gold being recovered within 4 h. From three different cathodes studied, activated RVC serves as the best cathode material having the highest recovery rate with more than 99% of gold from E-waste being recovered in 2 h of operation. It has an exceptional high void volume, highly porous surface structure, rigid structure and low resistance to fluid flow. It can be seen that the developed electrogenerative method is environmentally friendly and could recover the gold fully (>99%), even at a low concentration of gold. Finally, the morphology of gold deposits on an activated RVC cathode was also investigated. [ABSTRACT FROM AUTHOR]

[Display omitted] • A magnetic, efficient, selective, and low-cost material was developed for gold recovery. • Reduction-recovery mechanism was thoroughly analyzed. • Comprehensive evaluation of magnetic iron sulfide materials for gold recovery. • Over 2000 mg/g gold recovery capacity. Gold is a highly valuable resource in our society, an efficient and selective recovery technology is necessary for gold recycling from wastes to alleviate resource shortage and environmental pollution caused by traditional mining activities. In this paper, Fe 3 S 4 /Fe 7 S 8 materials were applied for Au(III) recovery for the first time, one of the materials showed ultra-high capacity (2020.7 mg/g). Various characterizations and adsorption tests proved its excellent properties, including high capacity and selectivity, good magnetism, low cost, and reusability. Characterization results revealed Au(III) is recovered on the adsorbents through in-situ reduction, and the reduction mechanism was studied through intensive XRD and XPS analyses. This work introduced magnetic iron sulfide materials in Au(III) recovery, and demonstrated its promising features. [ABSTRACT FROM AUTHOR]

• A novel sulfydryl-modified MOF was prepared by externally optimization methods. • The theoretical adsorption capacity of Au(III) was increased by almost 24.57%, and the reaction kinetics was also accelerated by more than 180%. • In the existence of competing ions, Au(III) recovery was enhanced by 26.53%. • The adsorption mechanisms were electrostatic interaction, reduction and chelation. • DFT calculation revealed that sulfhydryl functional group was the strongest binding site. A novel sulfhydryl-modified MOF (UiO-66-MSA) was prepared by externally optimizing UiO-66-NH 2 with mercaptomalic acid (MSA) and its capacity in selective recovering of Au(III) from solution was examined. Tests indicated that external modification of UiO-66-NH 2 by MSA increased its theoretical maximal adsorption capacity at pH 5 by 24.57% from 578.0 to 720.0 mg/g. Simulation of the kinetics data revealed that Au(III) binding on both UiO-66-MSA and UiO-66-NH 2 followed the pseudo-second-order model, with the former material having significantly higher kinetics. In the existence of competing ions, Au(III) recovery by sulfydryl-modified UiO-66-NH 2 (UiO-66-MSA) was increased from 72.06% to 98.59% at an initial Au(III) concentration of 56.9 mg/L. This material also had a fairly good reusability. At 5 times of regeneration, it was still able to capture 99.42% Au(III) from solution. Analysis of the changes in XPS patterns and the electrokinetic properties of UiO-66-MSA suggested that Au(III) binding on this MSA-modified MOF was complex. The electrostatic interaction, reduction and chelation may have all actively participated in the adsorption process. DFT calculation revealed that Au(III) binding was attributed to the -SH, -NH and C=O functional groups residing on the repeating unit of UiO-66-MSA, with its sulfhydryl functional group being the strongest binding site. [ABSTRACT FROM AUTHOR]

A novel polymeric membrane adsorber with a high density of adsorption sites that can selectively capture Au(III) ions, is proposed as an efficient alternative to recover gold from dilute solutions. Poly-thiosemicarbazide (PTSC), a polymer that contains one chelate site per monomeric unit, was used to fabricate the membranes. This polymer can be easily processed into membranes by a phase inversion technique, resulting in an open and interconnected porous structure suitable for high flux liquid phase applications. This method overcomes the usual low capacities of membrane adsorbents by selecting a starting material that contains the adsorption sites within it, therefore avoiding the necessity to add an external agent into the membrane matrix. The resulting mechanically stable PTSC membranes can operate in a pressure driven permeation process, which eliminates the diffusion limitations commonly present in packed column adsorption processes. This process can selectively recover 97% of the gold present in a solution containing a 9-fold higher copper concentration, while operating at a flux as high as 1868 L/m 2 h. The maximum gold uptake measured without sacrificing the mechanical stability of the membrane was 5.4 mmol Au/g. Furthermore the gold can be easily eluted from the membrane with a 0.1 M thiourea solution and the membrane can be reused for at least three cycles without any decrease in its performance. Finally, the ability of this membrane for recovering metals from real-life samples, like seawater and tap water, was tested with promising results. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel MOF containing abundant N, S atoms was prepared by one-step method. • The structure of the MOF was well characterized by SCXRD, FT-IR, SEM and TGA. • The adsorption uptake of Au(III) onto the MOF can be up to 2878 mg/g. • The MOF has rapid adsorption rate, excellent selectivity and good reusability. • Redox is the main adsorption mechanism, and electrostatic interaction is significant. Due to the special status of precious metals in the preparation of high-tech materials, the recycling of precious metals has important practical significance. A novel metal–organic framework called as Zn-AMT containing large number of functional atoms (N, S) was prepared as a highly efficient adsorbent to recover Au(III) from acid solution. Compared with the reported adsorbents, Zn-AMT shows remarkable adsorption efficiency towards Au(III) with the maximum adsorption capacity up to 2040 mg·g−1 at 25 °C. Increasing temperature to 42 °C could make the adsorption capacity increased to 2878 mg·g−1. The adsorption ratio of 100% could be achieved for the low Au(III) concentration within 100 mg·L-1. The Zn-AMT shows both excellent selectivity and good reusability. The adsorption kinetics data follow the pseudo-second-order kinetics and the adsorption isotherms data are well fitted by the Langmuir model. The adsorption mechanism was deduced that the Au(III) was firstly adsorbed onto the surface of Zn-AMT through electronic attraction and the coordination interaction of -SH, and then in situ reduced to Au(I). With the prominent adsorption performance, excellent selectivity, and admirable reusability, the Zn-AMT adsorbent could be a promising candidate for gold recovery in practical application. [ABSTRACT FROM AUTHOR]

Highlights: [•] Novel method of foam separation was applied to gold recovery. [•] The effect of an average foam diameter was investigated on the separation performance. [•] A parameter chart of the operational conditions on the separation performance was presented. [•] The effect of a bubble column diameter was also investigated on the separation performance. [Copyright &y& Elsevier]

Abstract: The selective transport of Au(III) over Cu(II) and Hg(II) using ω-thiocaprolactam as extractant and carrier in solvent extraction and polymer inclusion membrane transport experiments is discussed. Solvent extraction experiments were carried out using the extractant dissolved in chloroform to establish the chemical equilibria involved in the Au(III)-ω-thiocaprolactam separation system. Au(III) is extracted into the organic phase quantitatively (E (%)>99%) at low extractant concentrations from aqueous HCl solutions. Cu(II) extraction kinetics is slow compared to Au(III) extraction, allowing for the efficient separation of both metals. Polymer inclusion membranes (PIMs) are designed as alternatives to the use of high amounts of hazardous and expensive reagents. Several stripping phases were tested. Up to 70% of Au(III) can be recovered from 1M HCl feed aqueous phase solutions. Interference from Cu(II) or Hg(II) is negligible at short times (160min) in competitive ([M(II, III)]0,feed =0.1mM) membrane transport experiments using a cellulose triacetate/NPOE/ω-thiocaprolactam PIM and a potassium iodide–hydrochloric acid aqueous stripping solution. Apparent diffusion coefficients in the membrane for Au(III) and Hg(II) in the system were estimated to be 2.2×10−11 and 3.4×10−15 m2 s−1, respectively. [Copyright &y& Elsevier]

Abstract: Hydrogel beads were synthesized from radical polymerization of poly(ethylene glycol) monoacrylate (PEGA) and poly(ethylene glycol) diacrylate (PEGDA), and applied for the recovery of Au(III) from hydrochloric acid solutions. The acrylate hydrogels had poly(ethylene glycol) units as side chain, and retained water as much as 70% of their dry-weight. Due to a strong interaction between Au(III) and poly(ethylene glycol) units in the hydrogel structure, the metal was adsorbed into the hydrogel beads with the percent adsorption of more than 90% irrespective of the acid concentration. The adsorption was well correlated with the Freundlich isotherm equation, and the adsorbed amount of Au(III) reached as high as 121mg-Au/g-dry gel at maximum. Moreover, selective separation of Au(III) was attained since other heavy metals such as Cu(II), Zn(II), Fe(III) and Al(III) were excluded from the hydrogel beads. The present hydrogel beads were preferable in treating dilute acid solutions to avoid the hydrolysis of ester group in the gel structure at high acidities. [Copyright &y& Elsevier]

Abstract: We introduce a nanostructured Mn2O3-based adsorbent and a low-cost method that is capable of extracting gold, in the form of metallic nano-to-micrometer-size particles, from down to sub-ppm-level aqueous solutions including seawaters with high yield, good selectivity and recyclability, and environmental benignity. The Mn2O3 adsorbent, prepared by low-temperature synthesis followed by acid treatments, exhibits a mass-fractal-like morphology of agglomerated nanometer-size crystallite grains covered with active protonated sites and a surface area of about 120m2/g. For aqueous solutions containing ∼100ppm gold, a yield of 70mg of gold/g of adsorbent was achieved. A Langmuir-type adsorption isotherm was observed, showing a rapid uptake of gold. The method is well suited to gold recovery from very dilute solutions. For example, over 95% of the gold was recovered from seawater samples containing 0.1 and 1ppm of added gold. A pilot study of seawater with 1ppt added gold showed similar results. The protonation–suspension–filtration–washing process can be recycled without noticeable degradation in yield. [Copyright &y& Elsevier]

• Enhanced AuIII adsorption capacity and selectivity of GCC beads with GA crosslinking. • Electrostatic attraction, chelation, and reduction of AuIII as major mechanisms. • Reduction of AuIII to AuI and then to Au0 particles formed within the bead structure. • Successful separation of gold with high purity from acidic e-waste leaching solution. • Good reusability of GCC beads in repeated AuIII adsorption–desorption cycles. Recovery of gold from electronic waste (e-waste) has received significant attention due to its high economic value. This study reports glutaraldehyde-crosslinked chitosan (GCC) beads as highly efficient bioadsorbents for the separation of auric ions (Au(III)) from acidic e-waste leaching solutions. With increasing extent of glutaraldehyde-crosslinking, the GCC beads showed enhanced adsorption selectivity and capacity toward Au(III). The Au(III) adsorption capacity, however, was saturated with a further increase in the crosslinking time due to increasing self-oligomerization of glutaraldehyde. High adsorption selectivity was also observed for other precious metals such as Pd(II), Pt(IV), Ir(III), Os(IV), and Ag(I), but negligible adsorption was found for most other metals. The high Au(III) adsorption capacity and selectivity of GCC beads was attributed to multiple mechanisms such as electrostatic attraction, chelation, and reduction of Au(III), based on a range of spectroscopic evidence (FTIR, XRD, XPS, SEM, and TEM) and Au(III) adsorption behavior. The adsorbed Au(III) was reduced to Au(I) and then to Au(0), forming Au-nanoparticles as the main Au species with increasing adsorption time. The GCC beads showed a good reusability in repeated Au(III) adsorption–desorption cycle tests and were used to successfully separate high purity gold (96.7%) from an acidic e-waste leaching solution. [ABSTRACT FROM AUTHOR]