Activated carbon adsorption of gold from cyanide-starved glycine solutions containing copper. Part 1: Isotherms

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.

Ecofriendly recovery of copper from spent telecommunication printed circuit boards using an indigenous cyanogenic bacterium

In recent years, electronic waste (e-waste) production has increased due to the population's growth and high consumption. As a result of the high concentration of heavy elements in these wastes, their disposal has posed many environmental problems. On the other hand, due to the non-renewability of mineral resources and the presence of valuable elements such as Cu and Au in electronic waste, these wastes are considered secondary minerals for recovering valuable elements. Among electronic waste, recovery of metals from spent telecommunication printed circuit boards (STPCBs) is significant, which has not been addressed despite their high production worldwide. This study isolated an indigenous cyanogenic bacterium from alfalfa field soil. The 16S rRNA gene sequencing results showed that the best strain has 99.8% phylogenetic affinity with Pseudomonas atacamenisis M7DI(T) with the accession number SSBS01000008 with 1459 nucleotides. The effect of the culture medium, initial pH, glycine concentration, and methionine on the cyanide production of the best strain was investigated. The results showed that the best strain could produce 12.3 ppm cyanide in NB medium with an initial pH of 7 and a concentration of glycine and methionine of 7.5 g/L and 7.5 g/L, respectively. The one-step bioleaching method was performed, which led to the recovery of 98.2% of Cu from STPCBs powder after 5 days. Finally, XRD, FTIR, and FE-SEM analyses were performed to investigate the structure of the STPCBs powder before and after the bioleaching process, confirming the high Cu recovery.

MIL-161 Metal-Organic Framework for Efficient Au(III) Recovery from Secondary Resources: Performance, Mechanism, and DFT Calculations

The recovery of precious metals from secondary resources is significant economically and environmentally. However, their separation is still challenging because they often occur in complex metal ion mixtures. The poor selectivity of adsorbents for gold in complicated solutions prevents further application of adsorption technology. In this study, a Zr-based MOF adsorbent, MIL-161, was synthesized using s-tetrazine dicarboxylic acid (H₂STz) as an organic ligand. MIL-161 demonstrated a high adsorption capacity of up to 446.49 mg/g and outstanding selectivity for gold(III) in a simulated electronic waste solution as a result of the presence of sulfur- and nitrogen-containing groups. In addition, the MIL-161 adsorbents were characterized using Fourier transform infrared (FT-IR), field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TG), Brunner-Emment-Teller (BET), and X-ray photoelectron spectroscopy (XPS). Additionally, the adsorption kinetics, isotherms, and thermodynamics of the MOF adsorbents were also thoroughly examined. More importantly, the experimental results and DFT calculations indicate that chelation and electrostatic interactions are the main adsorption mechanisms.

Exploring bioleaching potential of indigenous Bacillus sporothermodurans ISO1 for metals recovery from PCBs through sequential leaching process

The low efficiency and selectivity limitations of biohydrometallurgy technique compel the researchers to explore novel microbial strains acclimated to metal existence site with higher toxicity tolerance and bioleaching capability in order to improve the role of bioleaching process for e-waste management. The current study aimed to explore bioleaching potential of indigenous Bacillus sporothermodurans ISO1; isolated from metal habituated site. The statistical approach was utilized to optimize a variety of culture variables including temperature, pH, glycine concentration and pulp density that impact bio-cyanide production and leaching efficiency. The highest dissolution of Cu and Ag, 78% and 37% respectively, was obtained at 40 °C, pH 8, glycine concentration 5 g L^-1, and pulp density 10 g L^-1 through One Factor at a Time (OFAT), which was further increased up to 95% Cu and 44% Ag recovery through the interactive effect of key factors in the Response Surface Methodology (RSM) approach. Furthermore, Chemo-biohydrometallurgy approach was utilized to overwhelm the specificity limitation; as higher concentration of Cu in computer printed circuit boards (CPCBs) causes interference to recover other metals. The sequential leaching through ferric chloride (FeCl₃), recovered Cu prior to bio-cyanidation by B. sporothermodurans ISO1 and resulted in the improved leaching of Ag (57%), Au (67%), Pt (60%), etc. The current work reports on B. sporothermodurans ISO1, a new Bacillus strain that exhibits highest toxicity tolerance (EC50 = 425 g L^-1) than earlier reported stains and has higher leaching potential that can be implemented to large-scale biometallurgical process for e-waste treatment to achieve the agenda of sustainable development goal (SDG) under the strategies of urban mining.

Extraction of Gold and Copper from Flotation Tailings Using Glycine-Ammonia Solutions in the Presence of Permanganate

This study presents the novel idea of a cyanide-free leaching method, i.e., glycine-ammonia leaching in the presence of permanganate, to treat a low-grade and copper-bearing gold tailing. Ammonia played a key role as a pH modifier, lixiviant and potential catalyst (as cupric ammine) in this study. Replacing ammonia with other pH modifiers (i.e., sodium hydroxide or lime) made the extractions infeasibly low (<30%). The increased additions of glycine (23–93 kg/t), ammonia (30–157 kg/t) and permanganate (5–20 kg/t) enhanced gold and copper extractions considerably. Increasing the solids content from 20 to 40% did not make any obvious changes to copper extraction. However, gold leaching kinetics was slightly better at lower solids content. It was indicated that the staged addition of permanganate was unnecessary under the leaching conditions. Recovery of gold by CIL was shown to be feasible, and it improved gold extraction by 15%, but no effect was observed for copper extraction. Percentages of 76.5% gold and 64.5% copper were extracted in 48 h at 20 g/L glycine, 10 kg/t permanganate, 20 g/L carbon, pH 10.5 and 30% solids. Higher extractions could be potentially achieved by further optimization, such as by increasing permanganate addition, extending leaching time and ultra-fine grinding.

Pore Evolution in Refractory Gold Ore Formed by Oxidation Roasting and the Effect on the Cyanide Leaching Process

Refractory gold ore is usually affected by the associated carbonaceous matter through the preg-robbing effect, which is eliminated by oxidation roasting, followed by leaching, to achieve a satisfactory gold leaching efficiency. Roasting-leaching experiments, pore structure measurements, scanning electron microscopy (SEM), and X-ray diffraction are used to explore the structural evolution of pores on the surface and its effect on the leaching performance. Pores with optimal sizes were obtained by roasting at 650 °C for 2.0 h with a ventilation of 0.6 m³/h; approximately 92.55% gold could be recovered under these conditions. A porous structure observed by SEM became more compact as the temperature further increased to 850 °C. The formation of CaSiO₃ and CaSO₄ in pores led to pore shrinkage. The mechanism of oxidation roasting, followed by cyanide leaching, was schematically analyzed and revealed the effects of pore structural evolution and phase transformation on the leaching efficiency.

Synthesis and characterization of tetraethylene pentamine functionalized MIL-101(Cr) for removal of metals from water

PURPOSE

Metal contamination in water is a worldwide persistent problem. We developed a nano-adsorbent, TEPA-MIL-101(Cr) that exhibits effective removal of heavy metals from real water samples.

METHODS

MIL-101(Cr) was synthesized under solvo-thermal condition. Then MIL-101(Cr) was dehydrated and degassed at high temperature under vacuum to generate the coordinately unsaturated sites which are used for tetraethylene pentamine (TEPA) grafting. The structures, morphologies, and compositions of the sorbents have been characterized. Langmuir and Freundlich isotherm models were applied for describing the adsorption process onto TEPA-MIL-101(Cr).

RESULTS

The successful grafting of TEPA on MIL-101(Cr) was verified by Fourier transform infrared. The results of X-ray diffraction, scanning electron microscopy, and CHN analysis show that the structure of TEPA-MIL-101(Cr) retains the original structure of MIL-101(Cr). Thermogravimetric analysis indicates thermo-stability of the adsorbent up to 300 °C. Optimal conditions for adsorption were determined as pH = 6.5 and contact time = 1 h. The adsorption capacities of TEPA-MIL-101(Cr) for Pb(II), Cu(II), Cd(II), and Co(II) from aqueous samples were 227.5, 217.7, 221.4, and 215.6 mg/g respectively, which is on average more than 8 times that of MIL-101(Cr). Analysis of Langmuir and Freundlich models for describing the adsorption isotherms of TEPA-MIL-101(Cr) reveals that the metal ions were absorbed onto TEPA-MIL-101(Cr) by a favorable physical absorption process.

CONCLUSIONS

TEPA-MIL-101(Cr) was synthesized successfully by a simple, and cost-effective method. The removal efficiency of TEPA-MIL-101(Cr) for the metal ions achieved more than 95 % in real water samples, which in addition to its thermal stability character make it a promising candidate for water treatment purposes.

Cyanide consumption minimisation and concomitant toxic effluent minimisation during precious metals extraction from waste printed circuit boards

Waste printed circuit boards (WPCBs) constitute a hazardous material with up to 40 different metals, including numerous many heavy metals and environmentally harmful metals. Most hydrometallurgical processing approaches use high concentrations of toxic reagents and generate significant amounts of harmful effluents. This research investigates the use of cyanide-starved glycine solution containing no free cyanide in the leachate to extract precious metals from WPCBs, with most of copper and base metals pre-removed by upstream glycine-only leaching. Under the optimised conditions, 90.1% Au, 89.4% Ag and 70.1% Pd were extracted together with 81.0% Cu and 15.0% Zn. The extraction of other base metals remained low at 8.3% for Al and <5% for Pb, Ni, Co, Fe and Sn, indicating a fairly good selectivity of the leaching system. By comparing with stoichiometric and intensive cyanidation, the cyanide-starved glycine system showed comparable or better performance on precious metals extraction, but cyanide use was reduced by >70% whereas the glycine can be reused. Analysis indicates that with an initial cyanide dose of 250 ppm, the leaching solution was starved of cyanide after 4 h with no free cyanide, which minimises safety and health risks significantly compared with traditional intensive cyanidation ([CN] > 3500 ppm). Glycine and cyanide dose, pH, and particle size dominated leaching kinetics, while staged addition of cyanide did not enhance the extractions. A significant (70-90%) reduction in required cyanide use and cyanide-bearing effluents can be achieved while performing polymetallic metal removal and allowing reagent recycling.

Selective Adsorption, Reduction, and Separation of Au(III) from Aqueous Solution with Amine-Type Non-Woven Fabric Adsorbents

Herein, adsorption, separation, and reduction of Au(III) from its aqueous solution were studied with different amine-type, non-woven fabric (NF) adsorbents fabricated with radiation-induced graft polymerization. The adsorbents exhibited different adsorption capacities of Au(III) over a concentration range of hydrochloric acid (HCl) from 5 mM to 5 M, and the diethylamine (DEA)-type adsorbent performed best under all test conditions. The DEA-type adsorbent was inert toward other metal ions, including Cu(II), Pb(II), Ni(II), Zn(II) and Li(I), within the fixed concentration range of HCl. Flow-through adsorption tests indicated DEA-type adsorbent exhibited a rapid recovery and high adsorption capacity of 3.23 mmol/g. Meanwhile, DEA-type adsorbent also exhibited high selectivity and rapid extraction for Au(III) from its mixed solution with Pt(IV) and Pd(II). After adsorption, the reduction of Au(III) was confirmed by XRD spectra, TEM, and digital micrograph images. The results indicated that nano-sized Au particles were mainly concentrated on the adsorbent in 5 mM HCl solution. In 1 M HCl solution, not only nano-sized Au particles were found, but also micro-size Au plates precipitation occurred. This study provides a novel material for selective and efficient gold uptake from aqueous solution.

Optimized bioleaching of copper by indigenous cyanogenic bacteria isolated from the landfill of e-waste

In this study, indigenous cyanogenic bacterial strains were isolated on nutrient, minimal salt, and soil extract media at various culture conditions from two distinct landfills of e-waste, Iran. Based on their cyanide formation profiles, five most potent isolates were selected for optimization and to this end, the influence of the most effective factors on cyanide production including pH, glycine concentration and temperature were assessed using one-factor at a time method (OFAT). Initial pH of 7, glycine concentration of 2 g/L and temperature of 30°C were obtained as optimal conditions for most of the isolates. Additionally, two bioleaching processes were applied for each bacteria to detect the effect of optimal conditions on bioleaching and to assay their potential in the mobilization of copper. Under optimal conditions and pulp density of 1 g/L, copper recoveries were recorded as 96.73%, 82.49%, 81.17%, 41.72%, and 31.52% by S22, N13, N37, N23, and N41 respectively during 10 days which is approximately 1.5-5 times higher than the recovery obtained without optimization. During the optimization and the bioleaching process, the pH fluctuation of the flasks was monitored which validated the activity of the microorganisms.

Activated Carbon from Yam Peels Modified with Fe3O4 for Removal of 2,4-Dichlorophenoxyacetic Acid in Aqueous Solution

The removal of organic pollutants from water sources can be enhanced using suitable adsorbents. The aim of this research was to study the adsorption capacity and potential reuse of a magnetic adsorbent prepared from agricultural wastes of yam peels (Dioscorea rotundata) for 2,4-dichlorophenoxyacetic (2,4-D) acid removal. The procedure was performed through carbonization and activation at 400 and 500 °C, respectively. Then, the as-prepared activated carbon (AC) was chemically modified using magnetite (Fe3O4) nanoparticles. The AC and magnetic activated carbon (MAC) were characterized and then used for batch adsorption and regeneration tests at different pH, initial concentrations of 2,4-D, and temperature. AC and MAC were showed to have microporous structures with surface areas of 715 and 325 m2/g, respectively. Superparamagnetic behavior was observed for MAC with a saturation magnetization of 6 emu/g. The results from the batch experiments showed higher adsorption capacity at high initial concentration of 2,4-D, low pH, and room temperature. The thermodynamic parameters indicated that the experiments proceeded as exothermic and spontaneous adsorption. Our findings also showed that MAC can be separated from the water medium through a facile magnetic procedure, and from regeneration experiments, MAC showed better results with 60% of its initial adsorption capacity after five cycles. Hence, MAC was found to be a promising alternative adsorbent of pesticides in water.

Tailored high mesoporous activated carbons derived from Lotus seed shell using one-step ZnCl2-activated method with its high Pb(II) capturing capacity

Lotus seed shell was employed using one-step method combining carbonization with ZnCl₂ activation to synthesize activated carbons because of its inexpensiveness and local accessibility. The lotus seed shell-activated carbons (LSSACs) with the highest surface area (2450.8 m²/g) and mesoporosity (98.6%) and the largest pore volume (1.514 cm³/g) were tailored under optimum conditions as follows: impregnation ratio = 2:1, carbonization temperature = 600 °C, and time = 1.0 h. The surface Zn(II), abundant hydroxyl, and carboxyl functional groups from the activation process could result in rapid Pb(II) adsorption onto the LSSAC surface through surface complexation, ion exchange, or precipitation. The maximum monolayer adsorption capacity (q_m) for Pb(II) of 247.7 mg/g at 25 °C could be fitted from the Langmuir isotherm. The Gibbs free energy (△G) and positive enthalpy (△H) indicated that the adsorption process was spontaneous and endothermic, and to some extent, it was explained by the intra-particle diffusion mechanism. Our results may provide a promising way to produce activated carbons with high adsorption capacity using solid waste, which will eventually promote the environmental sustainability.