Leaching Mechanism Research Articles

A thermodynamic analysis on thiosulfate leaching of gold under the catalysis of Fe3+/Fe2+ complexes

• Thermodynamic analysis of Fe-catalyzed thiosulfate leaching of gold is performed. • Catalysis of EDTA 4– , ox 2– , CN – and cit 3– complexes of Fe 3+ /Fe 2+ are involved. • Possible mechanisms for Fe-catalyzed thiosulfate leaching of gold are elucidated. • Reasons for low reactivity of Fe complexes towards thiosulfate are proposed. Thiosulfate is an eco-friendly gold lixiviant that is promising to replace the highly toxic cyanide. Copper is a conventional catalyst in accelerating the rate of thiosulfate leaching of gold, but it causes high thiosulfate consumption and complex gold recovery process. Attempts on the replacement of copper by other transitional metals such as nickel, cobalt and iron have been made to address the two issues. Among the catalysts, iron is cheap with great potential of development and industrial application. Few research has focused on the thermodynamics for iron-based thiosulfate systems. In this paper, a systematic thermodynamic analysis on the thiosulfate leaching of gold catalyzed by Fe 3+ /Fe 2+ and the ligand of EDTA 4– , oxalate (ox 2– ), cyanide (CN – ) or citrate (cit 3– ) is conducted by constructing a series of predominance area and speciation diagrams. Information on the stability, speciation, and redox behavior of iron species under various solution conditions are provided to better understand the complex solution chemistry and possible mechanisms for the iron-catalyzed gold thiosulfate leaching. The likely reasons for the high stability and low consumption of thiosulfate under the catalysis of Fe 3+ /Fe 2+ complexes are also proposed.

A cost-effective method for enhancing manganese leaching from rhodochrosite caused by surfactant-regulated crystal growth of CaSO4·2H2O

The cost-effective leaching of manganese (Mn2+) from rhodochrosite is critical for achieving energy saving, curtailing pollutants, and cleaner production of electrolytic metallic manganese (EMM). In this work, different surfactants were used to enhance Mn2+ leaching from rhodochrosite. The effects of the surfactants: trisodium citrate (TC), sodium oleate (SOL), ethylenediaminetetraacetic acid (EDTA), sodium dodecyl sulfate (SDS), and tetradecyltrimethyl ammonium bromide (TTAB) on the Mn2+ leaching efficiency were evaluated. The enhanced mechanism of Mn2+ leaching is discussed, and an economic evaluation is presented. The leaching efficiency of Mn2+ reached 91.8% in the presence of 5 mg/L TC, which is 13.8% higher than that in the absence of TC using a temperature of 50 °C, a solid-liquid ratio of 1:5, an acid-to-ore ratio of 0.58:1, and reaction time of 3.5 h. In the leaching mechanism, TC inhibited the growth of the 020, 040, and 041 crystal planes of CaSO4·2H2O; thus, CaSO4·2H2O crystals with a shortened columnar morphology and smaller sheet shapes were obtained. The grain refinement of CaSO4·2H2O did not readily wrap on the surface of rhodochrosite, consequently increasing the probability for rhodochrosite to react with H+, and improving the leaching efficiency of Mn2+. Moreover, TC did not remain in the leachate to impact the subsequent electrolytic deposition. This study provides a cost-effective method for leaching Mn2+ from rhodochrosite.

Innovative and sustainable separation and recovery of valuable metals in spent CIGS materials

In recent years, the rapid development of copper indium gallium selenide (GIGS) solar cells makes it necessary to recycle spent CIGS in order to realize circular economy. In this work, a novel and efficient method for comprehensive recovery valuable metals in spent CIGS materials was investigated, mainly including three steps: (1) leaching of spent CIGS with nitric acid to separate indium; (2) precipitating to obtain copper gallium selenite mixture with magnesium oxide as precipitant; (3) calcination of the residue and the precipitation to recover selenium. X-ray photoelectron spectroscopy (XPS) analysis indicates that the valence state of selenium was converted from negative divalent to positive tetravalent after oxidative leaching. In addition, the results of X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR) and scanning electron microscope/energy dispersive spectrometer (SEM/EDS) shows that the dissolution of metals in CIGS during leaching were significantly different. Copper and gallium were almost completely leached, while indium entered the slag phase as indium selenite, which achieved the separation of indium. Moreover, the reaction mechanism of leaching was analyzed from the perspective of thermodynamics. After leaching, copper, gallium and selenium can be completely recovered from the leach liquor using magnesium oxide as precipitant. Subsequently, the separation of Se in the residue and precipitates can be achieved at 800 °C for 90 min. Finally, the recovery yields of Cu, In, Ga and Se were 99.23%, 96.82, 98.08% and 96.38% respectively. This research represents an important innovation from the perspective of circular economy and provides a new idea for comprehensive utilization of secondary solar cells.

Dissolution of molybdenite roasting flue dust in sulfuric acid: kinetics and mechanism for molybdenum and rhenium leaching

Dissolution of molybdenite roasting flue dust in sulfuric acid: kinetics and mechanism for molybdenum and rhenium leaching

Thermodynamic analysis of gold leaching by copper-glycine-thiosulfate solutions using Eh-pH and species distribution diagrams

Glycine and thiosulfate, as alternatives to cyanide, are all considered to be one of the most promising lixiviants for gold. In this paper, a novel copper-catalytic glycine thiosulfate collaborative system of gold leaching was proposed. Based on the available thermodynamic data, a series of Eh-pH and species distribution diagrams for gold/copper-glycine-thiosulfate system under various conditions were constructed using thermodynamic calculations. The analysis results for thermodynamic diagrams indicated that the copper-glycine-thiosulfate system had many potential advantages over the copper-ammonia/citrate-thiosulfate system, such as wider pH range and lower thiosulfate consumption. The electrochemical models for the copper-catalytic mechanism of gold leaching with glycine-thiosulfate were also proposed in this study. Moreover, the relevant results obtained in this study provide a theoretical basis for the optimization of gold leaching with copper-catalytic glycine thiosulfate system.

Modification of carbonate-activated binder for lead-zinc mine tailings based cemented paste backfill

CPB promotes a green development of the mining industry. At the same time, the high cost of OPC limits the large-scale application of CPB. This study aimed to use carbonate-activated binders as cementitious materials in LZT based CPB. The binder was modified by CLDH and/or HM when considering the carbonate-activated binder's low reaction rate and bad performance. The effect of 3–5 wt% CLDH and 0.5–1 wt% HM on the reaction kinetics and phase assembles of Na2CO3-activated slag was investigated via isothermal calorimetry, XRD, and TG analysis. Flowability, setting time, and compressive strength of CPB samples were used to assess the feasibility of the binder in CPB. The immobilization effect and leaching mechanism of heavy metals from the CPB samples were then evaluated via TCLP, sequential extraction test, and semi-dynamic leaching. The incorporation of CLDH and HM accelerated binder reaction through consuming CO32−. The coupling addition of 3 wt% CLDH and 0.5 HM presented the best performance, reaching about 2.29 MPa at 28 days. This value was higher than OPC-based CPB samples by around 10%.

Selective extraction and recovery of tin from hazardous zinc-leaching residue by oxalic acid/sulfuric acid mixture leaching and hydrolytic precipitation

Zinc-leaching residue (ZLR) with 0.46% Sn and 54% Pb brings potential threats to environment but is worthy to recover valuable metals. Reduction of Sn(IV) to Sn(II) is critical for tin extraction. In this work, selective extraction and recovery of tin from ZLR via oxalic acid/sulfuric acid leaching and hydrolytic precipitation was proposed. Various reductants were attempted, and the oxalic acid shows superior tin leaching efficiency. The effects of reduction leaching and hydrolysis parameters on the separation of Sn and Pb were systematically investigated. After leaching by 24 g/L oxalic acid in 0.50 mol/L H2SO4 at 60 °C for 30 min with L/S ratio of 10 mL/g and stirring speed of 400 r/min, 90.5% Sn is leached into the solution, and 99.8% Pb is simultaneously concentrated in the leaching residue. In the hydrolysis process, tin sediment with recovery of 93.8% and content of 46.4% was obtained. The as-prepared tin and lead products can directly act as concentrates for the tin and lead smelting. The oxalic acid reduction leaching mechanism of tin was also discussed. Based on the reduction leaching behaviors of tin from ZLR, the possible Sn leaching enhancement via oxalic acid addition in the existing industrial zinc-leaching process was proposed in this work.

Mild sulphuric acid pre-treatment for metals removal from biosolids and the fate of metals in the treated biosolids derived biochar

Biosolids are contaminated with heavy metals (HMs) and alkali and alkaline earth metals (AAEMs). These metals limit the suitability of biosolids for land application as well as their pyrolytic conversion to high-quality products. In this work, a mild sulphuric acid pre-treatment of biosolids was carried out at different stirring speeds (300–900 rpm), temperatures (25–100 °C), extraction time (0–180 min), and acid concentration (1–5% v/v) to reduce the metals load in biosolids and their biochar derived from pyrolysis. The metal leaching process was very rapid and reached equilibrium in less than 30 min. The optimum conditions removed about 75–95% HMs and 80–95% AAEMs except Ca due to the formation of CaSO4 hydrates. Temperature was the driving parameter for Cd and Ni extraction, whereas temperature and acid concentration played the leading roles in Cu extraction. The shrinking core product layer diffusion and surface chemical reaction models described the extraction kinetics of Ni, Cu and Cd. A leaching activation energy of 10.02 kJ/mol and 7.37 kJ/mol was estimated for Ni and Cd, respectively. FTIR, SEM and XRD characterisation of the treated biosolids jointly indicated that the leaching mechanism was dominated by acid dissolution of metal-containing components followed by ion exchange of metal species with protons from H2SO4. Treatment at 25 °C and 3% H2SO4 lowered the biosolids ash content by 50% and preserved the physicochemical attributes, which enhanced the pyrolysis upcycling of the treated biosolids. Pre-treatment influenced the migration characteristics of the metals during pyrolysis and the produced biochar had several folds lower HMs and AAEMs contents than the raw biosolids-derived biochar.

The driving mechanism of cement hydrated products leaching on as-built cement concrete bridge deck pavements

Recently, a new distress type in as-built dense-graded asphalt concrete (DGAC) paved on concrete bridge decks, namely alkali leaching, has been detected in China. The water carries dissolved alkali ions from the cement concrete deck to the pavement surface and reacts with CO2 in the air to form whitish carbonate deposits. The “bottom-up” water transportation process in the presence of the natural environment without vehicle loading effect is a complicated problem. Thereby, scrutiny of the driving force of the “bottom-up” water transportation process in cement concrete bridge deck pavements (CCBDPs) plays a vital role in realizing the mechanism of alkali leaching. In the present study, three potential driving mechanisms that may cause this process are proposed and simulated in the laboratory, including water vapor diffusion under action of multiple temperatures and relative humidity (RH), capillary rise, and leaching water caused by variation of atmospheric pressure. The microstructure of alkali leaching samples cored from distress pavement is also carefully examined by X-ray CT. The obtained results show that the diffusion mass of water vapor demonstrates a rapid lessening with the reduction of the relative humidity differences. A highly linear correlation between the RH difference and the diffusion flux of the water vapor for different temperatures is also identified, indicating that the DGAC with various air voids follows Fick’s first law of diffusion. Further, capillary rise indeed occurs in the DGAC but does not contribute to the alkali leaching process. The pressure difference could continuously drive the leaching water to the surface of DGAC samples, and the critical pressure difference (0.3–0.7 kPa) would be generally less than the daily variation of the atmospheric pressure (approximately 0.8 kPa in weather alternate days). The quantitative analysis of the internal structure for alkali leaching cores also reveals that the distressed cores present relatively higher void ratios, especially for small volume and narrow pores. The entrapped air within unsaturated pores provides the precondition for water transport subjected to the atmospheric pressure differences. These findings contribute to the understanding of static moisture transportation in the dense-graded asphalt mixtures and clarify the main driving mechanism of moisture-induced alkali leaching on CCBDPs.

Kinetics and mechanisms of leaching of rare earth elements from secondary resources

The study of the kinetics of REEs from secondary resources and the understanding of the mechanisms governing the leaching reactions are fundamental aspects for the design and the optimization of industrial processes. Despite their interest, the kinetic aspect of these valuable elements from secondary resources has not been reviewed yet. Therefore, a deep understanding of the major phenomena involved in the elementary stages of REEs leaching is extremely crucial and would allow setting up a comprehensive methodology for kinetics investigation. This review provides a state of the art on various kinetics approaches describing the leaching kinetics and mechanisms of REEs from secondary resources notably low-grade minerals, industrial residues as well as end-of-life products. Different existing kinetics models applied for the leaching were reviewed and deeply discussed. Meanwhile, comparative study of different leaching mechanisms was also realized for a better understanding of the REEs leaching reactions. More importantly, the effect of different leaching parameters on the kinetics was discussed for further optimization and development of REEs extraction processes from secondary resources. In addition, the comparison of different REEs secondary resources was provided in terms of content, availability and economic opportunities to target the most potentially and economically viable resources.

Long-term leaching mechanism of chromium-containing slag after vitrification and heat treatment

Vitrification and heat treatment are economical methods for utilizing Cr-containing slag; however, the prepared products have different Cr solidification abilities. Herein, the effects of the two routes on the long-term leaching behavior of Cr-containing slags were studied. The Mg, Al, and Cr contents and heat treatment were found to influence the type and relative content of the mineral phases, which further promoted the Cr leaching behavior of the samples. Among them, the 5Al-3Cr glass and heat-treated samples exhibited relatively higher Cr leaching accumulation. Compared with the heat-treated samples, the glass samples had a better solidification ability, particularly for the high-Cr-containing group. The Cr leaching accumulation decreased by 70%–90%, indicating that the vitrification of the melt can effectively inhibit the formation of the Cr-leachable phase. During the leaching process, the dissolved Ca/Mg/Al ions influence the pH and redox potential of the liquid environment, resulting in the oxidation of Cr3+ to Cr6+. This research showed that vitrification is a safer and more effective route for the utilization of Cr-containing slag.

Studying the leaching mechanism of spent SCR catalyst with different leaching agents (NaOH, H2SO4, HCl and HNO3) using DFT calculations

The leaching mechanism of the spent selective catalytic reduction (SCR) catalyst was investigated by density functional theory (DFT) calculations and experiments. Three models, namely V2O5/TiO2, WO3-V2O5/TiO2, and WO3/TiO2, were created to simulate the spent SCR catalyst. Interaction between the surface clusters and the leaching agents of NaOH, H2SO4, HCl and HNO3 was investigated by DFT calculations. The adsorption site was determined through electrostatic potential and adsorption energy. The calculations showed that NaOH tended to be adsorbed on V and W atoms. H2SO4, HCl and HNO3 tended to be adsorbed on specific O atoms. H2SO4 dissociated on all three models, HCl dissociated on V2O5/TiO2 and WO3-V2O5/TiO2, while HNO3 dissociated only on V2O5/TiO2. The order of leaching ability among leaching agents was NaOH > H2SO4 > HCl > HNO3. Experiments showed that the leaching ratios of V and W in NaOH solution were 82.4% and 54.3%. In acid solution, the leaching ratio of V was 69.0% for H2SO4, 52.5% for HCl, and 42.2% for HNO3, while W was hardly leached out. After acid leaching, the change in the TiO2 lattice parameter was greater than that after alkaline leaching. The experimental results were found to be consistent with the calculations.

Low-carbon treatment of zinc contaminated iron tailings using high-calcium geopolymer: Influence of wet-dry cycle coupled with acid attack

Zinc contaminated iron tailings can be stabilized by an environmentally friendly high-calcium geopolymer as a new filler material for clean reutilization. The durability and toxic leaching of stabilized materials are vital for sustainable engineering applications, especially when exposed to complex environments. This study comprehensively studied the strength and leaching performance of four types of high-calcium geopolymer stabilized tailings (M series and C series; M1, M2, C1, and C2) subjected to the erosion of the Wet-Dry cycles coupled with Acid rain (WDA). Tests including unconfined compression test, apparent integrity, and mercury intrusion test were carried out to evaluate the strength and durability of stabilized tailings. These results showed that the erosion resistance of the C series was worse than that of the M series. Meanwhile, the leachate properties, e.g., pH, electrical conductivity, and typical ion (calcium, magnesium, and zinc) leaching concentrations, were examined to evaluate the leaching durability of the stabilized tailings. The relationships between chemical properties and ion leaching concentration were analyzed, which helped understand the erosion and leaching mechanisms. These results reflected that the occurrence of erosion is due to the coupling effect between the structural damage and hydration reaction. Furthermore, the stabilized materials after the WDA erosion were investigated the mechanisms of immobilization, erosion, and leaching via the X-ray diffraction and scanning electron microscope-energy dispersive spectroscopy tests. The mechanisms of zinc immobilization were precipitation, adsorption and solid-solution structure, and physical encapsulation. After the WDA erosion, the number of hydration products would decrease, but the zinc-sulfate precipitates would increase to reduce zinc leaching. Therefore, the increase of zinc leaching concentration was related to zinc desorption and damage of physically encapsulated zinc. The paper deepens the understanding of the durability of these stabilized materials under wet-dry and acid erosion, improving the reuse reliability of clean and green materials.

Release of heavy metals from conventional and reflective cool cement pavements

Cool cement blocks are used for pavement construction to mitigate the heat island effect. Cement block pavements in contact with water will produce leachates, which may affect soil, surface water and groundwater quality. Therefore, the objective of this work was to determine and compare the leaching behavior of heavy metals (Cd, Co, Cr, Fe, Mn, Ni, Pb, Zn) from conventional and reflective cool cement blocks using the mass transfer tank leaching test, according to USEPA method 1315 and the batch leaching test as a function of pH, according to method CEN/TS 14429. Comparison of the results of the two tests indicates that equilibrium was not reached during the mass transfer test. In all cases, the Leachability Index values are higher than 8, indicating limited mobility. Observed diffusivity values differ by several orders of magnitude (10−11 – 10−20 m2/s) and are very small, indicating very slow diffusion. Leaching curves as a function of pH were very consistent and close to each other, indicating that the leaching mechanisms were the same for all cement blocks. Α two-domain one-dimensional mathematical model describing heavy metal diffusion from the pavements to the underlying soil was developed and tested. Simulations with this model showed that emissions from reflective cool cement pavements to the underlying soil are not different from the conventional ones in the long run.

A Novel Deep-Eutectic Solvent with Strong Coordination Ability and Low Viscosity for Efficient Extraction of Valuable Metals from Spent Lithium-Ion Batteries

A new type of deep-eutectic solvents (DESs), consisting of ethylene glycol (EG) and sulfosalicylic acid dihydrate (SAD), were designed for efficient leaching valuable metals from cathode active materials (LiCoO2 and Li14.8Ni1.7Co8.5MnO30.5) of spent lithium-ion batteries (LIBs) for the first time. The as-prepared DESs possessed strong coordination ability and low viscosity, which were suitable for metal extraction. The influences of experiment parameters on extraction of Co and Li were systematically investigated, and the leaching mechanism and kinetics were elucidated in detail. Under the optimal conditions, the leaching efficiencies of Co and Li from pure LiCoO2 reached 93.5% and 98.3%, while those of Co, Ni, Mn, and Li achieved 94.8%, 99.1%, 100%, and 100% from the spent Li14.8Ni1.7Co8.5MnO30.5, respectively. Results revealed that the Li+ ions located in the interlayer of layered LiCoO2 were replaced by H+ ions and entered into solution, while Co3+ ions in the skeleton structure were reduced and coordinated by EG and sulfosalicylic acid (SA) to form four soluble Co2+–SA complexes, realizing synergistic and efficient leaching of Li and Co from the LIB cathode active material. Moreover, Co and Li leaching kinetics conformed to the shrinking core model, in which the interfacial chemical reaction was the limiting step, and the apparent activation energies were separately about 77.38 and 79.54 kJ/mol.

Comparison of Nature and Synthetic Zeolite for Waste Battery Electrolyte Treatment in Fixed-Bed Adsorption Column

To support a sustainable energy development, CO2 reduction for carbon neutralization and water-splitting for hydrogen economy are two feasible technical routes, both of which require a significant input of renewable energies. To efficiently store renewable energies, secondary batteries will be applied in great quantity, so that a considerable amount of energy needs to be invested to eliminate the waste battery electrolyte pollution caused by heavy metals including Cu2+, Zn2+ and Pb2+. To reduce this energy consumption, the removal behaviors of these ions by using clinoptilolite and zeolite A under 5, 7 and 10 BV h−1 in a fixed-bed reactor were investigated. The used zeolites were then regenerated by a novel NH4Cl solution soaking, coupled with the ultrasonication method. Further characterizations were carried out using scanning electron microscopy, N2 adsorption and desorption test, and wide-angle X-ray diffraction. The adsorption breakthrough curves revealed that the leaching preference of clinoptilolite was Pb2+ > Cu2+ > Zn2+, while the removal sequence for zeolite A was Zn2+ > Cu2+ > Pb2+. The maximum removal percentage of Zn2+ ions for clinoptilolite under 5 BV h−1 was 21.55%, while it was 83.45% for zeolite A. The leaching ability difference was also discussed combining with the characterization results. The fact that unit cell stayed the same before and after the regeneration treatment approved the efficacy of the regeneration method, which detached most of the ions while doing little change to both morphology and crystallinity of the zeolites. By evaluating the pH and conductivity changes, the leaching mechanisms by adsorption and ion exchange were further studied.

A review of gold extraction using alternatives to cyanide: Focus on current status and future prospects of the novel eco-friendly synthetic gold lixiviants

Cyanide is currently the predominant lixiviant for gold extraction. However, cyanide is highly toxic, causing potential environmental hazards and high detoxification costs. This has initiated extensive research aimed at seeking and developing eco-friendly substitutes for cyanide. Although significant research has been undertaken, most, if not all, of the traditional non-cyanide lixiviants such as thiosulfate, thiourea, halide and thiocyanate have proven to be difficult to achieve widespread adoption at gold mines because of their inherent limitations. In recent years, a range of novel eco-friendly synthetic gold lixiviants (NESGLs) such as “Jinchan”, one of the most representative products, have been developed successfully. Compared with cyanide, these NESGLs are much less or non-toxic, and have achieved comparable gold recoveries from ores without changing the original cyanidation process and equipment. The successful development of NESGLs appears to be making significant inroads into the dominance of cyanide, as evidenced by a growing number of their industrial applications at gold mines worldwide. This paper reviews the current status of gold extraction from its ores using the NESGLs with details focused on their leaching efficacy, synthesis, and composition as well as possible leaching mechanisms. Their future perspectives and developing trends of considerable concern are also discussed.

Geochemical tests to study the effects of cement ratio on potassium and TBT leaching and the pH of the marine sediments from the Kattegat Strait, Port of Gothenburg, Sweden

Cement is a key construction material in civil and geotechnical engineering. However, its application for stabilization of marine sediments needs further investigation. This paper tests the effects of varied amounts of Portland cement added to soil samples on the leaching of tributyltin (TBT) from the contaminated marine sediments and evaluates the pH level. Identifying the best combinations of Portland cement / slag / Cement Kiln Dust (CKD) for effective treatment of soil samples is a central challenge in marine geotechnical investigations with major implications for stabilization of dragged coastal sediments. The paper aims to test the TBT leaching as well as pH level and potassium (K) content in soil samples. Materials included marine sediments collected from the seabed of the Port of Gothenburg, Kattegat Strait, southwest Sweden. The methodology applied the existing specifications of the Swedish Institute for Standards (SIS) for geochemical tests of soil stabilized by Portland cement/slag/CKD. Leaching in soil samples was examined for 2.25, 9 and 36 days. Variations in the speed of decline of TBT leaching were noted depending on the ratio of Portland cement. The methodology follows the SIS instructions regarding the procedure of leaching tests: SS-EN 15863. Mixtures with pure Portland cement and cement / CKD gave pH values between 11.5–12 in the surface leaching experiments. Mixing of slag / CKD or slag lowered the pH range to 11–11.5 and 10–10.5, respectively. The leaching of TBT was affected by the changed amounts of seawater in the surface leaching experiments. The study shows that leaching reduces over time when the mobile fraction is being washed away and replaced by other leaching mechanisms and processes. Furthermore, in models in which leaching of TBT and potassium were assessed, there were, on average, changes in behaviour on the 9th day during the experiment time treatment and stabilization afterwards.

Characteristic comparison of leaching valuable metals from spent power Li-ion batteries for vehicles using the inorganic and organic acid system

This work presents a new insight: the leaching rules of various batteries and leaching agents were explored. In this study, LiNi0.5Co0.2Mn0.3O2 (NCM) and composite material batteries (CMB) were treated by environmentally friendly acid (phosphoric acid, citric acid, and acetic acid). The properties and composition of NCM and CMB were characterized and analyzed, and the optimal leaching conditions were investigated. For NCM, the leaching efficiencies of Li, Ni, Co, and Mn by phosphoric acid were 95.4%, 99.8%, 99.5%, and 98.0%, respectively, while those by citric acid were 91.0%, 85.7%, 99.9%, and 93.6%, respectively. For CMB, the optimal leaching efficiencies of phosphoric acid were Li 92.9%, Ni 93.3%, Co 94.8%, Mn 94.3%, while citric acid were Li 88.2%, Ni 72.5%, Co 94.0%, Mn 90.5%. Comparing the leaching behavior revealed that NCM was more easily leached than CMB, that may be correlated with the structure and composition of the active components. Three acids were evaluated for their efficacy and availability: phosphoric acid > citric acid > acetic acid. And kinetics and leaching mechanisms suggested that the kinetic behaviors of NCM and CMB in the three leaching systems were all following the Avrami model.

Equilibrium leaching of selected ultraviolet stabilizers from plastic products

Despite the importance of (micro)plastics in the release of plastic additives, the leaching mechanism of organic plastic additives from various plastic materials is poorly understood. In this study, the equilibrium leaching of five highly hydrophobic ultraviolet (UV) stabilizers (UV326, UV327, UV328, UV329, and UV531) from three plastics (low-density polyethylene (LDPE), polyethylene terephthalate (PET), and polystyrene (PS)), was investigated employing acetonitrile-water cosolvent systems. Their extrapolated water solubilities were in the 0.15–0.54 μg L−1 range, limiting their transport as “dissolved” in water and (micro)plastics are likely those particulate carriers. The equilibrium leaching of UV stabilizers from plastics was better explained by the Flory-Huggins model incorporating the nonideal behavior caused by the size disparity between UV stabilizers and polymer materials and their compatibility. Specifically, leaching of UV stabilizers from LDPE showed a positive deviation from Raoult’s law, whereas slight negative deviations were observed in PET and PS. In addition, the equilibrium concentration of the benzotriazoles in LDPE increased linearly with the volume fraction up to only 0.4%. These observations could be explained by the unfavorable interactions of UV stabilizers with polyethylene, indicating that polymer type should be also important when evaluating the fate of hydrophobic additives. Because equilibrium distribution of additives between (micro)plastics and water is crucial for evaluating the fate and transport of hydrophobic plastic additives, further studies on the leaching equilibrium of various additives from different plastic materials are necessary.