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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. [Display omitted] • Defatted microalgal adsorbents were prepared with or without treating with concentrated H 2 SO 4. • Adsorbents treated using concentrated H 2 SO 4 exhibited better Au(III) adsorption capacity and selectivity. • Au(III) was reduced to metallic form during the adsorption. • Loaded and reduced Au was successfully desorbed using HCl and thiourea mixed solution. • Component of microalgae would affect the adsorption behavior. [ABSTRACT FROM AUTHOR]

The growing demand for gold, particularly due to its use in electronics, underscores the need for efficient recovery methods from secondary sources. This is not only economically beneficial but also crucial for environmental protection. Traditional gold recovery methods, relying on toxic substances like mercury and cyanide, pose significant environmental and health hazards. This review concentrates on the challenges of gold ion adsorption and the development of innovative adsorbents enriched with nitrogen (N) and sulfur (S) functional groups. Various materials such as ligands, polymers, and metal-organic frameworks (MOFs) have been recently developed to improve the efficiency and selectivity of gold ion recovery. The review explains how N and S functional groups interact with Au ions, delving into the chemical bonding, coordination, ionic, and redox reactions that facilitate high adsorption capacities. It also assesses the synthesis processes and elucidates the mechanisms behind Au ions adsorption, showcasing the strengths and weaknesses of current technologies. Despite advancements, challenges remain, particularly in the long-term stability, scalability, and environmental impact of these novel materials in practical applications. The review calls for comprehensive lifecycle analysis studies, scalable synthesis methods, and a deeper understanding of molecular adsorption mechanisms. It highlights the potential of hybrid materials combining different functional groups for improved performance. This review not only summarizes current progress but also guides future research towards more sustainable, efficient, and environmentally friendly gold recovery methods. [Display omitted] • Materials contains N and S groups exhibit improved gold selective recovery. • Polymers, ligands, and MOF offer promising solutions for sustainable gold recovery. • Efficient materials in recovering gold from leachate. [ABSTRACT FROM AUTHOR]

Development of selective and cost-effective sorbents used for gold separation from electronic waste (e-waste) leachates is crucible to simplify post-processing steps and reduce the capital costs associated with gold recovery technologies. Despite numerous reported sorbents showing promising results, the practical applications are hindered by the use of expensive chemicals and complex synthesis procedures. Herein, we proposed a facile and low-cost composite of coconut fibers biochar (CFB) modified with zero-valent iron (ZVI), named CFB@Fe, and examined the gold sorption properties from simulated acid-leached e-waste solutions, which was compared to those of CFB and its acid-modified biochar (HCFB). All materials exhibited rapid adsorption kinetics of Au(III), reaching equilibrium sorption in 5 min. The ZVI modification of CFB compensated for the pH dependence, maintaining high Au(III) adsorption efficiency (∼100%) across the pH range of 1 and 7. In addition, CFB@Fe demonstrated a significant enhancement in Au(III) sorption capacity, with Freundlich constants (K F) related to adsorption capacity in the order of CFB@Fe (155.4) > CFB (122.2) > HCFB (101.4). The selective sorption of Au(III) on CFB@Fe was enhanced in the presence of high concentrations of co-leached ions. Examining the Au(III) sorption mechanisms on CFB@Fe revealed an adsorption-reduction process, leading to the generation and deposition of metallic gold on the sorbent. The mechanisms were discussed concerning the roles played by the surface–OH functional groups of CFB and ZVI particles. These findings suggest a great potential for CFB@Fe as a low-cost and efficient material for large-scale separation and enrichment of gold from acidic e-waste leachates. • Low-cost ZVI-modified biochar was proposed for selective and efficient uptake of Au(III). • The modification of ZVI showed a significant enhancement of Au(III) adsorption. • The Au(III) adsorption was fast and reached equilibrium within 5 min. • Successful separation of gold (99.6%) was obtained from a simulated aqua regia leachate. • Adsorption-reduction mechanisms were proposed for the Au(III) adsorption. [ABSTRACT FROM AUTHOR]

The application of non-cyanide baths in electroplating for gold recovery has drawn significant attention due to their clear benefits on the environment and human health. Among these alternatives, thiourea has emerged as a promising non-cyanide bath, and yet high energy and reagent consumption has impeded its widespread adoption. Herein, an energy-efficient gold recovery process via electroplating was developed to reduce bath depletion. The redox potentials of gold deposition using thiourea electrolyte were examined through linear sweep voltammetry, and the consumed energy during the electrochemical plating of gold was quantified. The gold deposition was achieved in the two-electrode system enabling a 97% recovery under 0.3 V of the cell voltage. The proposed system exhibited a significant reduction in energy consumption to 0.129 kWh∙kg−1, implying that the oxidation of free thiourea on the anode mainly contributed to the decrease in the applied voltage and energy. Furthermore, the suppression of the bath consumption was evaluated compared with electroless plating, another reliable deposition process, throughout monitoring gold desorption and deposition efficiencies during the cyclic use of thiourea. The retardation of the sorbent consumption was observed in the electroplating during the consecutive cycles, which inferred that the reduction of its oxidized form, formamidine disulfide, led to the regeneration of the agent on the cathode. Thus, the suggested electroplating process with a gold-thiourea bath shows great potential as a workable means of gold recovery from an economic standpoint with its low energy and sorbent consumption to realize the commercialization of urban mining in the electronic industry. [Display omitted] • Energy-efficient gold recovery process via electrochemical plating was performed. • Gold deposition was achieved in 0.129 kWh∙kg−1 of energy consumption. • Thiourea consumption was retarded during recycling cycles in gold electroplating. • Participation of free thiourea enabled to reduce applied energy and bath depletion. • Electroplating using thiourea eased to recover gold from an economic point of view. [ABSTRACT FROM AUTHOR]

The industrial application potential of current adsorbents for Au(III) recovery is limited due to their high costs, poor adsorption capacity, and complicated preparation processes. This study introduces a novel approach to address these challenges by synthesizing metal sulfides through a one-step hydrothermal method. The synthesized metal sulfides exhibit exceptional adsorption capacities, exceeding 1500 mg/g. Particularly, petal-shaped CuS demonstrates outstanding performance in extracting gold from complex solutions, including fresh water and simulated leaching liquid of electronic circuit boards. With an impressive adsorption capacity of 2536 mg/g in a 700 mg/L Au (III) spiked solution, CuS achieves a remarkable 90.6% removal efficiency. The adsorption process of CuS on gold can be well described by a pseudo-second-order kinetic model and the Langmuir model. Furthermore, CuS is successfully loaded onto sodium alginate (SA) aerogel for large-scale applications. The layered structure of SA overcomes the challenge of solution retrieval, making it a promising material with outstanding removal performance for industrial use. In cases where a high specific surface area does not confer advantages, CuS demonstrates superior performance compared to numerous gold ion adsorbents. This observation motivates us to emphasize the content of active functional groups during the design of gold ion adsorbents. This research significantly advances the field of Au(III) recovery by providing cost-effective and efficient adsorbents with simplified synthesis processes and strong adsorption capabilities. [Display omitted] • CuS demonstrates excellent adsorption capacity (2536 mg/g), achieving a 90.6% removal efficiency. • DFT theory was utilized for the first time to study the impact of hydrochloric acid on gold ion adsorption. • CuS was further loaded on sodium alginate (SA) aerogel for large-scale applications. [ABSTRACT FROM AUTHOR]

Considering the high costs associated with bio-based adsorbents, chemically modified adsorbents, metal organics, and N- or S-containing organic polymers, this study proposed the recovery of gold from printed circuit boards (PCBs) using solid waste coal gasification slag (CGS) as adsorbents. This study found that CGS can quickly adsorb AuCl 4 - in solution, and the adsorption process was less affected by pH value, stirring speed and temperature. The results of adsorption kinetics and adsorption isotherms indicated that gold adsorption process better fitted pseudo-second-order kinetics and Langmuir model. The gold adsorption process was controlled by the chemical process. After adsorption, the gold ions aggregated on CGS surface to form micron-sized zero-valent gold, as demonstrated by several characterization methods. The formation environment of CGS and possible presence of elemental aluminum explained its high reducibility. Because of their reducing properties, gold ions are more likely to obtain electrons on the surface of CGS than activated carbon. The values were detected in the electrochemical circuits comprising CGS and AuCl 4 - in different reaction areas, which further proved the promotion effect of the chemical reduction reaction on the adsorption of gold ions. The AuCl 4 - in the leaching solution of the PCBs were recovered by the CGS, and the saturated adsorption amount reached 40.3 mg/g. Using CGS to recover gold not only recycles solid waste, but also realizes the recycling of useful resources in waste PCBs at a low cost. • Coal gasification slag (CGS) was used to recovery gold from printed circuit boards. • The reduction characteristic of CGS promotes the adsorption of AuCl 4 -. • Gold maximum adsorption capacity is 109.28 mg/g. • CGS shows advantages in energy saving because of its inherent characteristics. [ABSTRACT FROM AUTHOR]

• Untreated cut human hair waste was investigated on recovery of precious metals. • Human hair waste exhibited high selectivity for Au with reduction and high capacity. • Cysteine and cystine in human hair played a key role of Au adsorption and reduction. • Stepwise recovery of pure metallic Au from other precious metals was achieved. • Stable and abundant hair waste can be an excellent bio-adsorbent for Au recovery. 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. [ABSTRACT FROM AUTHOR]

• A facile process combining a single-step gold leaching and adsorption was proposed. • Novel chlorate reagent showed ∼100 % leaching efficiency of gold from e-waste. • Chitosan-based beads showed selective separation of gold from the chlorate leachate. • Gold with 97.8 % purity was obtained from pyrolysis of gold-adsorbed chitosan beads. • The entire process is simple, safe, and inexpensive for gold recovery from e-waste. Recovery of gold from electronic wastes, particularly waste printed circuit boards (WPCBs), has received significant attention due to its high gold content and economic value. This study reports a novel method for efficient and selective recovery of gold from WPCBs using an integrated chlorate leaching and chitosan-based bioadsorption. The experimental conditions of the leaching step were optimized, and ∼100 % leaching efficiency of gold could be achieved when 0.5 g of WPCB sample was treated with 12.5 mL (liquid to solid ratio of 25) of a mixture of chlorate (0.2 M) and chitosan (1% w/v) in 2 M HCl at 55 °C for 5 h. The WPCB leachate containing gold and other metal ions (e.g. Cu and Al) was diluted three-fold and then treated with glutaraldehyde-crosslinked chitosan (GCC) beads, achieving selective and complete adsorption of gold at GCC dose of ≥1 g/L. The adsorbed gold on GCC could be fully desorbed by using a thiourea/HCl mixture (each 0.2 M). The high efficiency of gold adsorption-desorption could be maintained up to four successive cycles, signifying long-term reusability of the GCC. As an alternative approach, thermal decomposition of the gold-adsorbed GCC at 650 °C was shown to generate sponge-like gold residue with 97.8 % purity. These results indicated high efficiency of the proposed method for recovering gold from e-waste along with its simple process steps, low chemical usage and low cost. [ABSTRACT FROM AUTHOR]