Acidic Thiourea Solution Research Articles

Palladium(II) extraction from hydrochloric acid solutions with 2,6-bis[(methylsulfanyl)methyl]cyclohexan-1-one

Palladium(II) extraction from hydrochloric acid solutions with 2,6-bis[(methylsulfanyl)methyl]cyclohexan-1-one, obtained from natural methanethiol, was studied using chloroform as a diluent. Palladium(II) is recovered rapidly and with high efficiency by the extractant from solutions of 0.1–10 mol/L HCl and can be separated with high selectivity from Pt(IV), Cu(II), Ni(II), Zn(II) and Fe(III). A coordination mechanism of the palladium(II) extraction from 1 M HCl solutions was established. The extraction agent is coordinated to the Pd(II) ion through donor sulfur atoms in an extracted compound of the [PdCl2L] type. Stripping of Pd(II) is effectively carried out with a 1 M NH4OH solution or an acidic (0.1 mol/L HCl) thiourea solution.

Defatted microalgae Haematococcus pluvialis: A sustainable source for gold recovery

The defatted microalgae Haematococcus pluvialis (HP), obtained from various production lots, were subjected to two different treatments: drying without chemical treatment (DH1, DH2, and DH3) and treatment with concentrated sulfuric acid (TH1, TH2, and TH3). These treated materials were employed as efficient and eco-friendly adsorbents to investigate the adsorption behavior of Au(III) in acidic chloride media. The treated adsorbents demonstrated exceptional performance, achieving 100 % adsorption of Au(III) across a broad range of hydrochloric acid (HCl) concentrations from 0.01 to 5.00 M, whereas the dry adsorbents exhibited up to 90 % adsorption only at low HCl concentrations. Both dry and treated adsorbents exhibited superior selectivity for Au(III) in the presence of a mixture of precious metal ions. The adsorption isotherms of Au(III) followed the Langmuir type of adsorption mechanism, and the maximum adsorption capacities for Au(III) were estimated as 1.57, 0.88, and 1.52 mol kg–1 for DH1, DH2, and DH3, and 6.17, 4.80, and 5.37 mol kg–1 for TH1, TH2, and TH3, respectively. The adsorbed Au(III) ions were reduced to the elemental state, as confirmed by X-ray diffraction patterns, scanning electron microscopy/energy-dispersive X-ray analyses, and digital micrographs of the adsorbents after Au(III) adsorption. Successful desorption of the adsorbed Au(III) was achieved using acidic thiourea solution.

Thiourea leaching of gold: Elucidating the mechanism of arsenopyrite catalyzed thiourea oxidation by Fe3+ and the beneficial role of oxalate through experimental and density functional theory (DFT) investigations

The combination of bio-oxidation pretreatment of refractory sulfide gold ore and thiourea leaching is a suitable method for gold extraction due to its environmental friendliness and compatibility in the two leaching systems. Arsenopyrite is an important auriferous sulfide mineral where the encapsulated gold can be effectively exposed by bioleaching pretreatment. The issue of high thiourea (TU) consumption, however, severely hinders the commercial application of this technology. In this paper, the effect of arsenopyrite on gold leaching in an acidic TU solution was investigated. Leaching results showed that the presence of arsenopyrite seriously impeded TU leaching of gold represented as the much decrease in gold dissolution and TU stability. The dominant cause for the detrimental effect of arsenopyrite was that arsenopyrite can catalyze the oxidation of TU by Fe3+ and thus bring about high TU consumption, further resulting in the marked deterioration of TU dissolution of gold. The catalytic mechanism is believed to be that TU adsorbs on arsenopyrite surface and then electrons transfer from TU to the oxidant Fe3+ through the conduction bands of arsenopyrite. Oxalate was used to improve TU stability and gold dissolution, and its mechanism was proposed as follows: oxalate not only weakened the interaction between Fe3+ and TU through complexing with Fe3+ but also competed with TU to absorb on the arsenopyrite surface, which decreased the TU adsorption. Besides, the formation of Fe-oxalate complexes, as the terminal electron acceptor, distinctly reduced the oxidation capacity of Fe3+, thus alleviating the catalytic effect of arsenopyrite on TU oxidation by Fe3+.

Thiourea leaching of gold in presence of jarosite and role of oxalate

The effect of jarosite generated from bio-oxidation on gold leaching in acidic thiourea solution was studied, and the role of the oxalate additive was also investigated. The biogenetic jarosite was found to markedly raise the thiourea consumption and hinder the gold dissolution. The dominant cause for the detrimental effect of jarosite is likely that iron ions were released from jarosite dissolution in the acidic solution and thus increased the solution potential, resulting in the accelerated decomposition of thiourea and the aggravated passivation of gold surface by the passivating species from thiourea decomposition. However, the addition of oxalate to solution was shown to be effective in weakening the adverse effect of jarosite manifested as much reduced thiourea consumption and enhanced gold dissolution. The possible acting mechanisms of oxalate were revealed by the analyses of solution potential, Zeta potential and XPS. Oxalate could not only weaken the interaction between Fe3+ and thiourea via complexing with Fe3+ but also prevent the passivation species from coating on gold surface through indiscriminate adsorption and electrostatic repulsion.

Effect of spacer length between N atoms of linear alkyl triamines on adsorption of anionic platinum(IV) ions

In this study, the effect of carbon chain length between N atoms in linear alkyl triamines on the adsorption of anionic platinum ions was studied. For this purpose, bis(3-aminopropyl)amine (BAPA-SG) and diethylenetriamine (DETA-SG) groups were covalently bonded to the silica gel surface. The modified silica gels were characterized through C, H, N elemental analyses and FTIR spectroscopy before being utilized for adsorption and preconcentration Pt(IV) ions from aqueous solutions. Various factors influencing the batch adsorption of Pt(IV) ions with BAPA-SG and DETA-SG, such as nitric acid, chloride, initial Pt(IV) concentration, and contact time, were investigated. The adsorption of Pt(IV) ions was most efficient in the solutions containing 0.1 M hydronium and 0.1 M chloride ions. The adsorbed amount of Pt(IV) increased with an increase in the initial concentration of Pt(IV). The adsorption of Pt(IV) ions with BAPA-SG and DETA-SG reached equilibrium in 2 and 4 h, respectively. The kinetics of Pt(IV) adsorption on both modified silica gels were found to be compatible with the Pseudo-second-order kinetic model. Langmuir isotherm accurately described the adsorption Pt(IV) ions with BAPA-SG and DETA-SG, with maximum monolayer adsorption capacities of 158.7 and 227.3 mg/g, respectively. In the column solid phase extraction studies, Pt(IV) ions could be quantitatively recovered from 500 mL sample solutions using both modified silica gels by eluting with acidic thiourea solutions. The recovery of Pt(IV) by DETA-SG from chloride-containing solutions was more effective than with BAPA-SG. As a result, it was concluded that diethylenetriamine-bonded silica gel could adsorb Pt(IV) ions more effectively than bis(3-aminopropyl)amine bonded silica gel.

Effect of metal ions on gold adsorption onto activated carbon in the acidic thiourea solution: Experiment study and DFT calculation

Thiourea leaching has been regarded as a promising method to substitute cyanidation for gold recovery from ores or concentrates. Activated carbon used in traditional cyanide leaching solution can also be used for gold recovery from thiourea leaching solution. In this paper, the effect of metal ions including Cu2+, Ag+, Pb2+ and Zn2+ on gold adsorption onto activated carbon from acidic thiourea solution was studied for the first time. Results showed that the adsorption process of activated carbon for gold can be well described by the Freundlich isotherm model, and followed the Pseudo-second kinetic model. The adsorption of gold onto activated carbon in thiourea solution may be a ligand exchange reaction. The S of the activated carbon is the electron-accepting atoms, which can form the bond of Au-S with Au in Au[CS(NH2)2]2+. The presence of Pb2+ and Zn2+ did not interfere gold adsorption at gold concentration of 5 mg/L, but hindered gold adsorption at gold concentration of 100 mg/L. When Cu2+ and Ag+ were added in the whole studied concentration range, the gold adsorption could be significantly declined. The adsorption capacity of gold onto activated carbon in the presence of metal ions followed the sequence of Cu2+ < Ag+ < Pb2+ < Zn2+ < no addition of metal ions. Meanwhile, metal ions also adsorbed onto activated carbon, and their adsorption capacity on activated carbon obeyed the descending order of Cu2+ > Ag+ > Pb2+ > Zn2+, consistent with the complexing ability of metal ions with thiourea. The decrease of active site of activated carbon occupied by metal ions was responsible for the decrease of gold adsorption. Based on the DFT analysis, it can be inferred that the ability of metal ions to lose electrons caused the different influence of metal ions on gold adsorption.

Role of foreign ions in the thiourea leaching of gold

Thiourea, as an environmentally friendly gold lixiviant, has the potential to replace highly toxic cyanide. The effect of foreign ions including Cu2+, Ag+, Pb2+, Zn2+, AsO33− and AsO43− on gold leaching in an acidic thiourea solution was investigated in this paper. Leaching results showed that the foreign ions had an adverse effect on thiourea leaching of gold. The gold dissolution (except low Pb2+ concentrations, i.e., < 20 mg/L) and thiourea stability both were in order of no foreign ions ≈ Zn2+ > Pb2+ > AsO33− > AsO43− > Ag+ > Cu2+. The thiourea dissolution of gold could be improved at low concentrations of Pb2+ (<20 mg/L), although the thiourea consumption was also increased. The thermodynamic analysis and the scanning electron microscope (SEM) and X-ray photon spectroscopy (XPS) analyses suggested that the foreign ions resulted in insignificant changes in the corrosion characteristics of leached gold surface and the quantity and chemical state of surface species. The possible mechanism for the effect of foreign ions on thiourea leaching of gold was proposed that the thiourea consumption in the presence of foreign ions was significantly increased, leading to the marked deterioration of thiourea dissolution of gold.

New compound N-methyl-N-isopropyl octanthioamide for palladium selective extraction and separation from HCl media

To seek more efficient extractant for Pd recovery from leaching solution of spent automotive catalysts, a new compound N-methyl-N-isopropyl octanthioamide (TA-813) was synthesized and characterized through FT-IR, 1H NMR, ESI-MS and elemental analysis. Its extraction and stripping behaviors for Pd were studied. To evaluate its reusability performance, extraction−stripping cycles were carried out. Finally, TA-813 was applied to separating Pd from simulated leaching solution of spent automotive catalysts. TA-813 shows fast extraction kinetics, high extraction efficiency and specific selectivity for Pd. During extraction two TA-813 molecules coordinate with one Pd. The loaded Pd in the organic phase is efficiently stripped by neutral and acidic thiourea solutions. TA-813 shows no loss in activity after eight extraction−stripping cycles. Even from simulated leaching solution of spent automotive catalysts, in which the concentrations of some purities (La, Ce, Mg, Al) are much higher than that of Pd, Pd is still selectively extracted.

Preconcentration and speciation analysis of mercury: 3D printed metal scavenger-based solid-phase extraction followed by analysis with inductively coupled plasma mass spectrometry.

A selective method for preconcentration and determination of methylmercury (MeHg) and inorganic mercury (iHg) in natural water samples at the ng L-1 level has been developed. The method involves adsorption of Hg species into a 3D printed metal scavenger and sequential elution with acidic thiourea solutions before ICP-MS determination. Experimental parameters affecting the preconcentration of MeHg and iHg such as the sample matrix, effect of the flow rate on adsorption, eluent composition, and elution mode have been studied in detail. The obtained method detection limits, considering the preconcentration factors of 42 and 93, were found to be 0.05ngL-1 and 0.08ngL-1 for MeHg and iHg, respectively. The accuracy of the method was assessed with a certified groundwater reference material ERM-CA615 (certified total iHg concentration 37±4ngL-1). The determined MeHg concentration was below MDL while iHg concentration was determined to be 41.2±0.5ngL-1. Both MeHg and iHg were also spiked to natural water samples at 5ngL-1 concentration and favorable spiking recoveries of 88-97% were obtained. The speciation procedure was successfully applied to two lake water samples where MeHg and iHg concentrations ranged from 0.18 to 0.24ngL-1 and 0.50-0.62ngL-1, respectively. The results obtained demonstrate that the developed 3D printed metal scavenger-based method for preconcentration and speciation of Hg is simple and sensitive for the determination of Hg species at an ultra-trace level in water samples.

Hydroxypicolinic acid tethered on magnetite core-silica shell (HPCA@SiO2@Fe3O4) as an effective and reusable adsorbent for practical Co(II) recovery

The soaring demand and future supply risk for cobalt (Co) necessitate more efficient adsorbents for its recycling from electronic wastes, as a cheaper and less hazardous option for its production. Herein, a magnetic adsorbent covalently tethered with 5-hydroxypicolinic acid (HPCA) as Co(II) ligand was developed. The magnetic component (Fe3O4) was protected with silica (SiO2), then silanized with chloroalkyl linker and subsequently functionalized with HPCA via SN2 nucleophilic substitution (HPCA@SiO2@Fe3O4). Results from FTIR, TGA, EA, and XPS confirm the successful adsorbent preparation with high HPCA loading of 2.62 mmol g−1. TEM-EDS reveal its imperfect spherical morphology with ligands well-distributed on its surface. HPCA@SiO2@Fe3O4 is hydrophilic, water-dispersible and magnetically retrievable, which is highly convenient for its recovery. The Co(II) capture on HPCA@SiO2@Fe3O4 involves monodentate coordination with carboxylate (COO−) and lone pair acceptance from pyridine (aromatic –N = ) moiety of HPCA, with minor interaction from acidic silanols (Si–O-). The binding occurs at 2 HPCA: 1 Co(II) ratio, that follows the Sips isotherm model with competitive Qmax = 92.35 mg g−1 and pseudo-second order kinetics (k2 = 0.0042 g mg−1 min−1). In a simulated LIB liquid waste, HPCA@SiO2@Fe3O4 preferentially captures Co(II) over Li(I) with αLi(I)Co(II)=166 and Mn(II) with αMn(II)Co(II)=55, which highlights the importance of HPCA for Co(II) recovery. Silica protection of Fe3O4 rendered the adsorbent chemically stable in acidic thiourea solution for its regeneration by preventing the deterioration of the magnetic component. Covalent functionalization averted ligand loss, which allowed HPCA@SiO2@Fe3O4 to deliver consistent and reversible adsorption/desorption performance. Overall results demonstrate the potential of HPCA@SiO2@Fe3O4 as a competitive and practical adsorbent for Co(II) recovery in liquid waste sources.

Separation of palladium and silver from E-waste leachate: effect of nitric acid concentration on adsorption to Thiol scavenger

The development of recovery techniques for metals present in low concentrations in E-waste, such as silver and palladium, is important from the aspect of the circular economy. Adsorption of palladium and silver was studied in detail in a batch process with silica-based Thiol scavenger from nitric acid leachate of waste printed circuit boards (PCBs). High adsorption efficiencies of Pd(II) and Ag(I), >97%, were reached in nitric acid concentrations below 3 mol L−1. At higher acid concentrations, adsorption efficiency of Ag(I) decreased drastically which enables the separation of Ag(I) and Pd(II) based on nitric acid concentration in sample solution. Pd(II) and Ag(I) followed pseudo 2nd order kinetical model with fast adsorption with contact times of 60 and 30 min, respectively, in all studied acid concentrations. Adsorption followed Langmuir isotherm reaching highest loading capacities of 199 mg g−1 for Ag(I) and 105 mg g−1 for Pd(II) in 1 mol L−1 nitric acid media, after which loading capacities decreased while nitric acid concentration increased. Thiourea-based desorption agents were most effective for desorption with >80% desorption efficiencies for Ag(I) and 55 - 88% efficiencies for Pd(II) in a one-step desorption. However, separation of Pd(II) and Ag(I) is not entirely possible in desorption step, hence the highest separation between Pd(II) and Ag(I) was found with two-step adsorption in which Pd(II) and Ag(I) were separated initially at adsorption step based on nitric acid concentration, followed by desorption step with acidic thiourea solution.

Batch sorption and fixed-bed elution for Pd recovery using stable amine-functionalized melamine sponge

Sorption-based techniques have been widely studied for metal recovery. Conventionally, sorption and desorption steps are operated in fixed-bed column system under one-pass mode (FO); however, such sorption requires a large volume of solution passing the bed when the feed concentration is low. In this study, highly stable amine-coated alginate melamine (ML/APG) sponge was prepared and applied in a combined process involving sorption under fixed-bed recirculation mode (FR, simulating batch system) and desorption under FO for Pd(II) concentration. The efficiency was evaluated in terms of both recovery of Pd(II) and utilization of the sponge. Results show that ML/APG sponge is stable in 3 M of nitric acid, sodium hydroxide and sodium chloride after shaking at 300 rpm for 72 h with mass loss less than 0.9%. The maximum sorption capacity obtained by Langmuir model for Pd(II) increases from 9.7 mg g−1 to 134.3 mg g−1 after amine functionalization. The sorption equilibrium could be achieved by a less amount of solution and less time when using FR compared to FO. For example, it only needs 0.7 L of Pd(II) solution with a metal concentration of 20 mg L−1 to achieve the maximum sorption capacity under FR, while the required volume is 1.8 L at flow rate of 1 mL min−1 and more than 5 L at 10 mL min−1 under FO. This means that FR improved the rational use of the sorbent and thus increased sorption efficiency. For elution step, FO only required 40 mL to completely desorb Pd(II) from exhausted sponge, while that needed more than 200 mL under FR. The combined process allows both effective sorption and elution. The reusability of ML/APG sponge and Pd(II) recovery from low-concentration solution were studied by combining FR for sorption step and FO for elution step. Results show that the sorption efficiency maintains more than 96% after 100 min of reaction during 10 cycles. The elution performance is stable with a 100% desorption efficiency by passing 50 mL of acidic thiourea solution. The concentration factor (CF, ratio of metal concentration in the eluate over its concentration in the sorption used for sorption), is above or close to 70 for the first 10 mL-sample and much more than 40 for the second 10 mL-sample.

Hydrometallurgical recovery of platinum-group metals from spent auto-catalysts – Focus on leaching and solvent extraction

To ensure the supply of raw materials for products of extreme importance in strategic sectors, the recovery of critical metals from secondary sources becomes increasingly urgent. Platinum group metals (PGMs), being rare and very valuable, fall into this demand, and the catalytic converters that contain them are recognized as one of the main sources. Hydrometallurgical processes have been proposed as an alternative to pyrometallurgical ones, with leaching and separation by solvent extraction being core operations in this type of processing. This article investigates these two operations, seeking to optimize conditions and propose new arrangements to improve them. Two different catalyst samples were considered in the experimental work. Leaching was carried out involving concentrated HCl solutions (with H2O2 as oxidant) and low liquid/solid ratios, seeking to maximize PGMs recovery, guaranteeing their high concentration in leachates and minimizing aluminum co-dissolution. Cerium leaching was also followed since this is a rare-earth metal with potential interest. Temperature, HCl concentration, liquid/solid ratio (L/S), time and particle size factors were evaluated, and the optimized conditions found were 11.6 mol L−1 HCl, 1%vol H2O2, 60 °C, L/S = 2 L kg−1 and 3 h, leading to PGM yields of 90–98% Pt, 99% Pd and 70–96% Rh, and leachate compositions of 0.41–0.78 g L−1 Pt, 1.6 g L−1 Pd, 0.062–0.066 g L−1 Rh, depending on the catalyst sample.For solvent extraction (SX), several commercial extractants dissolved/diluted in toluene were checked, firstly with a model solution, and then applied to the real spent auto-catalyst (SAC) leachates produced in-situ. The overall results showed that the most promising SX systems among those tested were Cyanex® 471X and Cyphos® IL 101. Cyanex® 471X allowed the quantitative extraction of Pd(II) and Fe(III), but the latter was conveniently scrubbed by water prior to Pd(II) stripping by an acidic thiourea solution. Pt(IV) and Pd(II) extraction values by Cyphos® IL 101 were very encouraging, as only Fe(III) and Zn(II) were appreciably co-extracted, however, scrubbing of the contaminating metals, and Pd(II) and Pt(IV) stripping, did not work. Hence, investigation to find proper scrubbing/stripping agents for Cyphos® IL 101 SX system, to recover PGMs from recycled SACs, is further needed.

Intensification reactive recovery of tetravalent platinum from spent catalyst via synergism of TBP/Cyanex 302 system

Synergistic reactive extraction of platinum (IV) from aqueous solution was conducted using an organic phase consisting of tributylphosphate (TBP) and bis (2,4,4-trimethylpentyl monothiophosphinic acid (Cyanex 302) in kerosene. Influence of different type of single extractants, mixtures and their composition towards synergistic effect were investigated. Recovery of platinum (IV) from loaded platinum-organic phase was conducted using an acidic thiourea solution. Results show that the extraction efficiency of platinum using 0.10 M Cyanex 302 and 0.10 M TBP were 10 and 0.5 % in 1 hour, respectively. About 80% of platinum (IV) was extracted using 0.05 M Cyanex 302 + 0.05 M TBP with highest distribution ratio (D = 4.08) and synergist factor (Rf =2.22). Both TBP and Cyanex 302 acted as a base and synergist extractant, respectively. Through synergism system, the concentration of the both extractants (0.10 M) was reduced by 50% (0.05 M). Such findings contribute towards intensification of this reactive extraction since comparing to the single system, the synergistic and productive extraction with minimal use of extractants was obtained via abovementioned mixture system. Meanwhile, almost 98% of platinum (IV) was successfully recovered using 3.0 M thiourea in 3.0 M hydrochloric acid (HCl) in two stages of stripping process.

Recovery of Pd(II) in chloride solutions by solvent extraction with new vinyl sulfide ploymer extractants

Unlike sulfur-containing extractants, extraction efficiency of extractants containing N atoms is often affected by the concentration of H+, but strong affinity of sulfur to Pd(II) may cause stripping process not smoothly. Since sulfur atom conjugated with CC decreases electron cloud density on S atom, a novel polymer extractant containing vinyl sulfide motifs (Poly-EDDYPF) was designed and prepared in this paper, the investigation of this acid resistant extractant shows that the polymer has high extraction efficiency of Pd(II) over 99% with nicely acid resistant. The loading experiment shows that the capacity of Pd(II) of Poly-EDDYPF is up to 268 mg/g. Nearly 95.6% Pd(II) in loaded organic phase could be easily stripped using acid thiourea solution (1 mol/L HCl + 0.1 mol/L thiourea) and the organic phrase could be reused at least 5 cycles without significant decrease of the extraction efficiency. The optimized conditions for the extraction process are given: [HCl] =1 mol/L, contact time = 20 min, amount of extractant =0.7 g/L, O/A ratio = 1/1. H+ concentration and Cl – concentration exhibit practically no effect on extraction efficiency.

In Situ Electrochemical AFM: Travelling Waves and Pit Growth during Au Dissolution

Kinetics, mechanism and self-organization evolution during electrochemical dissolution of gold (gold leaching) in acidic thiourea or alkaline thiosulfate solutions can be clarified through in Situ electrochemical (EC) AFM at the mesoscopic level. Pit-induced layer dissolution and travelling waves as well as the transition from pit-induced layer dissolution to wave-type dissolution could be observed by using in situ EC-AFM at open-circuit potential (OCP) of Au(111) bead electrode in acidic thiourea solutions. A proposed scheme gives an interpretation for the pit-induced layer and wave dissolution of Au(111) through the bonding interaction of sulfur atoms with gold atoms located at different positions on Au(111). Pits and pit-induced layer dissolution of Au(111) plane could be observed through in situ EC-AFM as the potential applied potentiostat above OCP in cupric ammoniacal thiosulfate solutions, where the pits dissolution present autocatalytic growth in depths, radius and numbers. The role of copper(II) anions and ammonia during the electrochemical dissolution of Au(111) plane is to dissolve the passivated sulfur comes from the decomposition of thiosulfate or chemical dissociation of polythionates produced by thiosulfate on the surface of Au(111) bead electrode, rather than to oxidize Au0 to Au+ for copper(II) ammonia complex according to the our study with in Situ EC-AFM.

Adsorption of Gold from Acid Thiourea and Chloride Solutions by Ion Exchange Resins.(Dept.M)

Adsorption of Gold from Acid Thiourea and Chloride Solutions by Ion Exchange Resins.(Dept.M)

Uptake and Recovery of Gold from Simulated Hydrometallurgical Liquors by Adsorption on Pine Bark Tannin Resin

The recovery of critical and precious metals from waste electrical and electronic equipment (WEEE) is an environmental and economic imperative. Biosorption has been considered a key technology for the selective extraction of gold from hydrometallurgical liquors obtained in the chemical leaching of e-waste. In this work, the potential of tannin resins prepared from Pinus pinaster bark to sequester and recover gold(III) from hydrochloric acid and aqua regia solutions was assessed. Equilibrium isotherms were experimentally determined and maximum adsorption capacities of 343 ± 38 and 270 ± 19 mg g−1 were found for Au uptake from HCl and HCl/HNO3 (3:1 v/v) solutions containing 1.0 mol L−1 H+. Higher levels of acidity (and chloride ligands) significantly impaired the adsorption of gold from both kinds of leaching solutions, especially in the aqua regia system, in which the adsorbent underperformed. Pseudo-first and pseudo-second order models successfully described the kinetic data. The adsorbent presented high selectivity towards gold. Actually, in simulated aqua regia WEEE liquors, Au(III) was extensively adsorbed, compared to Cu(II), Fe(III), Ni(II), Pd(II), and Zn(II). In three adsorption–desorption cycles, the adsorption capacity of the regenerated adsorbent moderately decreased (19%), although the gold elution in acidic thiourea solution had been quite limited. Future research is needed to examine more closely the elution of gold from the exhausted adsorbents. The results obtained in this work show good perspectives as regards the application of pine bark tannin resins for the selective extraction of Au from electronic waste leach liquors.

Gold recovery from precious metals in acidic media by using human hair waste as a new pretreatment-free green material

We herein propose an advanced gold recovery process employing biomass waste of human origin. Au(III) adsorption on human hair (HH) was found to be efficient and to concurrently reduce Au(III) to metallic gold. The maximum loading capacities of gold on HH and white HH were determined using Langmuir equation fitting to be 3.04 and 3.24 mmol g−1, respectively. This result indicates that melanin pigment in HH is not involved in Au(III) adsorption. The concurrent reduction of Au(III) was confirmed using X-ray diffraction analysis and scanning electron microscopy before and after Au(III) adsorption. To gain more detailed understanding of Au(III) adsorption at a molecular level, complexations of Au(III) with each amino acid component found in the peptides of HH was investigated. l-Cystine and l-cysteine were found to play a role in both the adsorption and reduction of Au(III). The mechanism of Au(III) adsorption and reduction on HH was investigated using proton nuclear magnetic resonance spectroscopy. The stepwise purification of three precious metals was also demonstrated: Pt(IV) and Pd(II) were successfully removed from HH by washing with acidic thiourea solution, while pure gold could be recovered by incineration. Thus, HH waste has shown potential as beneficial and efficient utilization on the separation and recovery of precious metals.

Gold Leaching Using Thiourea from Uranium Tailing Material, Gabal El-Missikat, Central Eastern Desert, Egypt

The objective of this study was to investigate thiourea leaching of gold present in the uranium tailings. Sulfuric acid treatment for raw material is important in order to dissolve uranium as well as to remove easily leachable metals. The acidic thiourea solution was proposed as a non-toxic possible alternative process route to cyanide in gold dissolution. The acidic system of thiourea-sodium sulfite was studied in the gold dissolution process. The presence of sodium sulfite can significantly reduce thiourea decomposition, accelerate the gold dissolution process, decrease the leaching time, and decrease the activation energy of gold dissolution. The activation energy for gold dissolution was evaluated. The activation energy solution with sodium sulfite is 7.62 kJ mol−1 which is much lower than 9.32 kJ mol−1 in the absence of sodium sulfite. Low activation energy confirms the surface reaction dissolution mechanism. This method demonstrates the possibility of extracting gold directly from low-grade ores without any physical or chemical pre-concentration method with a more environmentally friendly process.