Gold(III) recovery from aqueous solutions by raw and modified chitosan: A review

Adsorption isotherms, kinetics, and thermodynamics of Au(III) on chitosan/palm kernel fatty acid distillate/magnetite nanocomposites

This study examined the adsorption isotherms, kinetics, and thermodynamics of Au(III) onto chitosan/palm kernel fatty acid distillate/magnetite nanocomposites (CPMNs) to enhance the understanding of adsorption behavior and mechanisms. Adsorption experiments were conducted across various initial Au(III) concentrations, contact times, and temperatures. The experimental data were analyzed using nonlinear isotherm and kinetic models, and thermodynamic parameters were evaluated. The results revealed that the Langmuir model best fits the adsorption equilibrium data, showing a maximum monolayer adsorption capacity of 1.102-1.163 mmol/g (217-229 mg/g). The pseudo-first-order model best describes the kinetic data, suggesting first-order kinetics and a physisorption-dominated process. Thermodynamic analysis indicated that the adsorption is spontaneous, endothermic, entropy-driven, and highly favorable, primarily governed by physisorption. This study provides significant insights into the adsorption mechanisms of CPMNs for Au(III), contributing to advancing cost-effective and eco-friendly adsorbents for industrial use, such as wastewater treatment and metal recovery in mining, metallurgy, and electronic waste recycling industries.

Enhanced Removal of Ultratrace Levels of Gold from Wastewater Using Sulfur-Rich Covalent Organic Frameworks

In view of the increasing global demand and consumption of gold, there is a growing need and effort to extract gold from alternative sources besides conventional mining, e.g., from water. This drive is mainly due to the potential benefits for the economy and the environment as these sources contain large quantities of the precious metal that can be utilized. Wastewater is one of these valuable sources in which the gold concentration can be in the ppb range. However, the effective selective recovery and recycling of ultratrace amounts of this metal remain a challenge. In this article, we describe the development of a covalent imine-based organic framework with pores containing thioanisole functional groups (TTASDFPs) formed by the condensation of a triazine-based triamine and an aromatic dialdehyde. The sulfur-functionalized pores served as effective chelating agents to bind Au3+ ions, as evidenced by the uptake of more than 99% of the 9 ppm Au3+ solution within 2 min. This is relatively fast kinetics compared with other adsorbents reported for gold adsorption. TTASDFP also showed a high removal capacity of 245 mg·g-1 and a clear selectivity toward gold ions. More importantly, the material can capture gold at concentrations as low as 1 ppb.

Advanced membrane-based high-value metal recovery from wastewater

Considering the circular economy and environmental protection, sustainable recovery of high-value metals from wastewater has become a prominent concern. Unlike conventional methods featuring extensive chemicals or energy consumption, membrane separation technology plays a crucial role in facilitating the sustainable and efficient recovery of valuable metals from wastewater due to its attractive features. In this review, we first briefly summarize the sustainable supply chain and significance of sustainable recovery of aqueous high-value metals. Then, we review the most recent advances and application potential in promising state-of-the-art membrane-based technologies for recovery of high-value metals (silver, gold, rhenium, platinum, ruthenium, palladium, iridium, osmium, and rhodium) from wastewater effluents. In particular, pressure-based membranes, liquid membranes, membrane distillation, forward osmosis, electrodialysis and membrane-based hybrid technologies and their mechanism of high-value metal recovery is thoroughly discussed. Then, engineering application and economic sustainability are also discussed for membrane-based high-value metal recovery. The review finally concludes with a critical and insightful overview of the techno-economic viability and future research direction of membrane technologies for efficient high-value metal recovery from wastewater.

Sulfhydryl functionalized chitosan-covalent organic framework composites for highly efficient and selective recovery of gold from complex liquids

Developing candidate adsorbents with high capacity and selectivity for adsorption is a critical approach to recover gold from secondary sources such as electronic waste (e-waste). This measure not only contributes to the recycling of valuable resources but also benefits environment protection and mitigates the detriment to the ecosystem. Traditional covalent organic frameworks (COFs) have garnered great potential in various fields owing to their unique structures and innate functional sites. However, the construction of specific functionalized COFs and their application in efficient gold extraction are still in their nascent stages. Herein, inspired by the photo-induced thiol-ene click reaction, a novel chitosan-modified COFs material (DhaTab-V@chitosan-SH) was first demonstrated by choosing a precursor COF (DhaTab-V) abundant with vinyl groups to crosslink with a thiol-functionalized chitosan (chitosan-SH). Significantly, the coexistence of imine, amide, hydroxyl and thioether groups in the skeleton could serve as functional adsorption sites to combine with the Au (III) via electrostatic interaction and coordination, imparting DhaTab-V@chitosan-SH with prominent adsorption potential for swift, selective and efficient gold capture (970.15 mg·g-1). Additionally, DhaTab-V@chitosan-SH exhibited superior reusability, and realized real gold capture from the leaching solution of e-waste.

Efficient recovery of gold from solution with a thiocyanate-modified Zr-MOF: adsorption properties and DFT calculations

The design and development of new large-capacity and selective materials for extracting rare precious metals via electronic waste is practically essential. In this paper, a new efficient UiO-66-NCS has been obtained as a consequence of the modification of the classical Zr-MOF (UiO-66-NH2), and its ability to recover gold has been investigated. These batch results adequately illustrated that UiO-66-NCS exhibited good adsorption capacity (675.5 mg g-1) and exceptional selectivity. In addition, UiO-66-NCS achieved faster adsorption equilibrium times of about 120 min. Adsorption kinetics and isotherms demonstrated that the pseudo-second-order adsorption scheme and a Langmuir-type procedure were shown by the adsorption of Au(III) on UiO-66-NCS. Characterized by pH effect experiments, TEM, XRD, and XPS, the adsorption of UiO-66-NCS with Au(III) relies on coordination, which further results in reduction, and the generated Au(0) is uniformly dispersed in the MOF. The adsorbent has considerable advantages for cyclic regeneration. Finally, DFT fitting results showed that the adsorption binding energy of UiO-66-NCS with [AuCl4]- was -8.63 kcal mol-1 for the adsorption process. UiO-66-NCS is likely to be an ideal substance for gold recovery.

An O/N/S-rich porous Fe-based metal-organic framework (MOF) for gold recovery from the aqueous phase with excellent performance

Recovering gold from wastewater has both economic and environmental benefits. However, how to effectively recover it is challenging. In this work, a novel Fe-based metal-organic framework (MOF) was synthesized and decorated with 2,5-thiophenedicarboxylic acid to have a well-developed porous architecture to effectively recover Au(III) from water. The maximum Au(III) sorption capacity by the finally-synthesized porous material MIL-101(Fe)-TDCA reached 2350 mg/g at pH = 6.00 ± 0.15, which is one of the highest among all literature-reported relevant materials including MOFs, and high sorption strength can be maintained within a wide pH range from 2.0 to 10.0. Besides, Au(III) sorption efficiency at low concentrations (i.e., 3.5 × 104 mg/mL) reached over 99%. Mechanically, outstanding Au(III) sorption by MIL-101(Fe)-TDCA resulted from the O/N/S-containing moieties on its surface, large surface area and porosity. The N- and S-containing functionalities (CS, CONH) served as electron donors to chelate Au(III). The O-containing (FeOFe, COFe, COOH, and coordinated H2O) and N-containing (CONH) moieties on MIL-101(Fe)-TDCA interacted with OH groups on the hydrolyzed species of Au(III) (AuCl3(OH)-, AuCl2(OH)2-, and AuCl(OH)3-) by hydrogen bond, which further increased Au(III) sorption. Furthermore, about 45.71% of Au(III) was reduced to gold nanoparticles by CS groups on the decorated 2,5-dithiophene dicarboxylic acid during sorption on MIL-101(Fe)-TDCA. Over 98.35% of Au(III) was selectively sorbed on MIL-101(Fe)-TDCA at pH 4.0, much higher than that of the coexisting heavy metal ions including Cu(II), Zn(II), Pb(II), and Ni(II) (< 5%), despite their same concentration at 0.01 mg/mL. Although sorption selectivity of a noble metal Pt(IV) by MIL-101(Fe)-TDCA is relatively poor (68.23%), it could be acceptable. Moreover, reusability of MIL-101(Fe)-TDCA is also excellent, since above 90.5% Au(III) still can be sorbed after two sorption-desorption cycles. Overall, excellent sorption performance and the roughly-calculated gold recycling benefits (26.30%) highlight that MIL-101(Fe)-TDCA is a promising porous material for gold recovery from the aqueous phase.

Advances in natural polysaccharides for gold recovery from e-waste: Recent developments in preparation with structural features

The increasing demand for gold because of its high market price and its wide use in the electronic industry has attracted interest in gold recovery from electronic waste (e-waste). Gold is being dumped as solid e-waste which contains gold concentrations ten times higher than gold ores. Adsorption is a widely used approach for extracting gold from e-waste due to its simplicity, low cost, high efficiency, and reusability of adsorbent material. Natural polysaccharides received increased attention due to their natural abundance, multi-functionality, biodegradability, and nontoxicity. In this review, a brief history, and advancements in this technology were evaluated with recent developments in the preparation and mechanism advancements of natural polysaccharides for efficient gold recovery. Moreover, we have discussed some bifunctional modified polysaccharides with detailed gold adsorption mechanisms. The modified adsorbent materials developed from polysaccharides coupled with inorganic/organic functional groups would demonstrate an efficient technology for the development of new bio-based materials for efficient gold recovery from e-waste. Also, future views are recommended for highlighting the direction to achieve fast and effective gold recovery from e-waste in a friendly and sustainable manner.

Importance of a nano-sized molybdenum composite synthesized using a microwave oven in the sorption enhancement of Au(iii) from the aqueous phase

Sorption and reduction of Au(iii) by CS-MoCOM-Th on different scales.

Enhanced adsorption selectivity of Au(III) over Cu(II) from acidic chloride solutions by chitosan/palm kernel fatty acid distillate/magnetite nanocomposites

This work aimed to develop a modified chitosan adsorbent with enhanced adsorption selectivity for Au(III) over Cu(II) from acidic chloride solutions using low-cost and green raw materials. Various adsorbents, i.e., chitosan powder, chitosan microbeads, chitosan/palm kernel fatty acid distillate (PKFAD) microcomposites, magnetite nanoparticles, and chitosan/PKFAD/magnetite nanocomposites (CPMNs), were first evaluated for their ability to adsorb Au(III) and Cu(II) from single- and binary-metal solutions across different pH levels, followed by parametric analysis of Au(III) and Cu(II) adsorption from binary- and multi-metal solutions onto CPMNs, Au(III) desorption from Au(III)-loaded CPMNs, and reusability of CPMNs. Finally, Au(III)-loaded CPMNs were characterized with SEM-EDX, XRD, FTIR, and XPS to confirm the proposed adsorption mechanisms. Among all the adsorbents studied, CPMNs exhibited outstanding performance in adsorbing Au(III) from an equimolar binary Au(III)-Cu(II) solution, achieving the highest equilibrium adsorption capacity of 0.479 mmol/g (94.4 mg/g) without reaching saturation. Under optimal adsorption conditions of pH 3, 1 g/L CPMN dosage, and 90 min contact time, CPMNs adsorbed 96 % of Au(III) with a selectivity over Cu(II) exceeding 99 %. CPMNs demonstrated excellent reusability, maintaining over 80 % adsorption and desorption efficiencies for 5 cycles. The proposed adsorption mechanisms of CPMNs for Au(III) encompass electrostatic attraction, hydrogen bonding, solvation, and reduction.

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

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

In vitro digestion and fecal fermentation of Siraitia grosvenorii polysaccharide and its impact on human gut microbiota

In this study, the structure of Siraitia grosvenorii polysaccharides (SGPs) changed significantly after digestion. After 48 h of in vitro fecal fermentation, M_w decreased and the content of C_R showed a trend of increasing and then decreasing. The monosaccharide composition (glucose) of SGPs showed a trend of decreasing and then stabilizing during fecal fermentation, indicating that SGPs were partially degraded during in vitro fermentation and significantly degraded and utilized by the human intestinal microbiota. In addition, SGPs fermentation for 48 h increased the production of SCFAs especially acetic acid, propionic acid, and butyric acid. Moreover, after in vitro digestion and enzymatic digestion, the in vitro hypoglycemic activity of SGPs remained relatively high afterward, albeit reduced. This study contributes to a better understanding of the potential digestion and enzymatic mechanisms of SGP, which is important for the future development of SGP as a functional food and drug.

Construction of MoS2@Activated Alumina Beads as Catalysts for Rapid Gold Recovery from Au(S2O3)23- Solution

Gold recovery from thiosulfate leaching solution Au(S₂O₃)₂^3- is regarded as a tough task because of the low efficiency and complex procedure in current technology, which hindered the industrial application of this eco-friendly technique. In this work, a MoS₂@activated alumina bead composite (MoS₂@AA) was constructed through a simple hydrothermal anchoring method and served as a catalyst to recover gold from Au(S₂O₃)₂^3- solution for the first time. The microstructure and chemical component of MoS₂@AA were systematically analyzed. In addition, batch experiments were carried out to explore the recovery behavior of Au(S₂O₃)₂^3- (concentration: 10 to 200 ppm). Ascribing to the extraordinary optical property of MoS₂@AA, Au(S₂O₃)₂^3- could be directly reduced to Au⁰ by photogenerated electrons and then form a two-phase interface of gold/MoS₂@AA. As a result, the recovery of Au(S₂O₃)₂^3- can reach up to 98% on MoS₂@AA, which was much higher than traditional methods. More importantly, the reduced Au⁰ could be desorbed from MoS₂@AA through a supersonic method, achieving one-step Au⁰ recovery from Au(S₂O₃)₂^3-. This novel strategy used in this research has great significance to the development of Au(S₂O₃)₂^3- recovery in the future.

Adsorptive removal of fluoride using ionic liquid-functionalized chitosan - Equilibrium and mechanism studies

In this study, novel biosorbents, based on chitosan and imidazolium ionic liquid, were prepared for the removal of fluoride from aqueous solutions. The adsorbents were characterized by FTIR, SEM-EDS and low-temperature nitrogen adsorption-desorption. To investigate the adsorption mechanism and behavior of chitosan adsorbents, batch experiments were conducted under different adsorbent dosages (2, 4, 10 g/L), pH (4, 7, 9) and initial concentration (0.5-25.0 mg/L). The influence of the method of synthesis of ionic liquid on the adsorption performance were also studied. Experimental data were evaluated by Freundlich, Langmuir and Sips models. The introduction of ionic liquid significantly improved the uptake of fluoride compared to pure chitosan. The adsorption was influenced by the experimental conditions, as well as the method of ionic liquid synthesis. The highest fluoride removal was observed at pH 4 and found to decrease with increasing pH. The removal efficiency and adsorption capacity values indicated that the dose of 4 g/L was the optimum adsorbent dosage. The equilibrium data fitted best with the Sips isotherm and the maximum adsorption capacity reached 8.068 mg/g for modified chitosan beads. The mechanism of fluoride adsorption onto ionic liquid-modified chitosan involves electrostatic attraction, ion exchange and ion pair interaction.

Chitosan: A Sustainable Material for Multifarious Applications

Due to the versatility of its features and capabilities, chitosan generated from marine crustacean waste is gaining importance and appeal in a wide variety of applications. It was initially used in pharmaceutical and medical applications due to its antibacterial, biocompatible, and biodegradable properties. However, as the demand for innovative materials with environmentally benign properties has increased, the application range of chitosan has expanded, and it is now used in a variety of everyday applications. The most exciting aspect of the chitosan is its bactericidal properties against pathogens, which are prevalent in contaminated water and cause a variety of human ailments. Apart from antimicrobial and water filtration applications, chitosan is used in dentistry, in water filtration membranes to remove metal ions and some heavy metals from industrial effluents, in microbial fuel cell membranes, and in agriculture to maintain moisture in fruits and leaves. It is also used in skin care products and cosmetics as a moisturizer, in conjunction with fertilizer to boost plant immunity, and as a bi-adhesive for bonding woods and metals. As it has the capacity to increase the life span of food items and raw meat, it is an unavoidable component in food packing and preservation. The numerous applications of chitosan are reviewed in this brief study, as well as the approaches used to incorporate chitosan alongside traditional materials and its effect on the outputs.

An Overview of Modified Chitosan Adsorbents for the Removal of Precious Metals Species from Aqueous Media

This mini-review provides coverage of chitosan-based adsorbents and their modified forms as sustainable solid-phase extraction (SPE) materials for precious metal ions, such as gold species, and their complexes in aqueous media. Modified forms of chitosan-based adsorbents range from surface-functionalized systems to biomaterial composites that contain inorganic or other nanomaterial components. An overview of the SPE conditions such as pH, temperature, contact time, and adsorbent dosage was carried out to outline how these factors affect the efficiency of the sorption process, with an emphasis on gold species. This review provides insight into the structure-property relationships for chitinaceous adsorbents and their metal-ion removal mechanism in aqueous media. Cross-linked chitosan sorbents showed a maximum for Au(III) uptake capacity (600 mg/g), while S-containing cross-linked chitosan display favourable selectivity and uptake capacity with Au(III) species. Compared to industrial adsorbents such as activated carbon, modified chitosan sorbents display favourable uptake of Au(III) species, especially in aqueous media at low pH. In turn, this contribution is intended to catalyze further research directed at the rational design of tailored SPE materials that employ biopolymer scaffolds to yield improved uptake properties of precious metal species in aqueous systems. The controlled removal of gold and precious metal species from aqueous media is highly relevant to sustainable industrial processes and environmental remediation.