Can e-waste recycling provide a solution to the scarcity of rare earth metals? An overview of e-waste recycling methods

Recycling e-waste is seen as a sustainable alternative to compensate for the limited natural rare earth elements (REEs) resources and the difficulty of accessing these resources. Recycling facilitates the recovery of valuable products and minimizes emissions during their transportation. Numerous studies have been reported on e-waste recycling using various techniques, including thermo-, hydro- and biometallurgical approaches. However, each approach still has technical, economic, social, or environmental limitations. This review highlights the potential of recycling e-waste, including outlining the current unutilized potential of REE recycling from different e-waste components. An in-depth analysis of e-waste generation on a global scale and Australian scenario, along with various hazardous impacts on ecosystem and human health, is reported. In addition, a comprehensive summary of various metal recovery processes and their merits and demerits is also presented. Lifecycle analysis for recovering REEs from e-waste indicate a positive environmental impact when compared to REEs produced from virgin sources. In addition, recovering REEs form secondary sources eliminated ca. 1.5 times radioactive waste, as seen in production from primary sources scenario. The review outcome demonstrates the increasing potential of REE recycling to overcome critical challenges, including issues over supply security and localized dependency.

Low-temperature method for desiliconization of polymetallic slags by ammonium bifluoride solution

Throughout the period of operation of non-ferrous metal deposits, a significant amount of waste has been accumulated. The accumulated waste contains valuable metals in concentrations that allow considering them as valuable raw materials. However, it is worth noticing the presence of problems that previously did not allow for more complete extraction of the target components. Such problems include the presence of significant amounts of silicon dioxide in the form of a silicate matrix, the removal of which will allow the extraction of valuable components with the elimination of industrial waste areas. The paper considers a method for removing silicon from the polymetallic slags. According to the results of the work, it was found that silicon passes into solution in the form of ammonium hexafluorosilicate. Iron, aluminum, and a number of other components react with ammonium hydrofluoride, but do not leach into the solution due to their low solubility in the resulting system. After removing silicon, the solid residue was subjected to pyrohydrolysis to obtain a product that can be subjected to magnetic separation to obtain a magnetic iron concentrate and a non-ferrous metal concentrate. The formed concentrate can later be used to extract zinc, lead, silver, etc. The productive solution was directed to the deposition of silicon with the subsequent production of silicon dioxide. The resulting solution can be directed to evaporation in order to regenerate and reuse ammonium hydrodifluoride.

Microbial assemblage for solid waste bioremediation and valorization with an essence of bioengineering

Environmental solid waste bioremediation is a method of treating contaminated solid waste that involves changing ecological conditions to foster the growth of a broad spectrum of microorganisms and the destruction of the target contaminants. A wide range of microorganisms creates metabolites that may break down and change solid waste-based pollution to various value-added molecules. Diverse bioremediation technologies, their limitations, and the procedure involve recycling solid waste materials from the environment. The existing environmental solid waste disposal services are insufficient and must be upgraded with more lucrative recovery, recycling, and reuse technologies to decrease the enormous expenditures in treatment procedures. Bioremediation of solid waste eliminates the toxic components. It restores the site with the advent of potential microbial communities towards solid waste valorization utilizing agriculture solid waste, organic food waste, plastic solid waste, and multiple industrial solid wastes.Bioengineering on diverse ranges of microbial regimes has accelerated to provide extra momentum toward solid waste recycling and valorization. This approach increases the activity of bioremediating microbes in the commercial development of waste treatment techniques and increases the cost-effective valuable product generation. This framework facilitates collaboration between solid waste and utilities. It can aid in establishing a long-term management strategy for recycling development with the advent of a broad spectrum of potential microbial assemblages, increasing solid waste contamination tolerance efficiency and solid waste degradability. The current literature survey extensively summarises solid waste remediation valorization using a broad spectrum of microbial assemblages with special emphasis on bioengineering-based acceleration. This approach is to attain sustainable environmental management and value-added biomolecule generation.

A novel process for preparing Fe-Cr-Ni-C alloy: synergetic reduction of stainless steel dust and laterite nickel ore

In order to improve the recovery ratio of valuable metals in stainless steel dust, reduce environmental pollution, and promote solid waste resource recovery and sustainable development of industry, the synergistic reduction process for preparing Fe-Cr-Ni-C alloy was studied in detail by changing the addition of laterite nickel ore and reduction process conditions. The results show that with the addition of laterite nickel ore, the basicity of raw materials is reduced, the precipitation and aggregation of metal particles are promoted, the separation effect of metals and slags from reduction products is improved, and the metal recovery ratio also improved in the synergistic reduction process. When the ratio of stainless steel dust to laterite nickel ore is 94%:6%, reduction temperature is 1400 °C, reduction time is 20 min, and FC/O is 0.8, the metals and slags of the reduction product can be separated naturally after cooling; the recoveries of Fe, Cr, and Ni are 90.6%, 90.1%, and 91.2%, respectively. The grades of Fe, Cr, and Ni in the Fe-Cr-Ni-C alloy are 62.7%, 18.9%, and 4.1%, respectively. The content of harmful elements S and P in the alloy is low, so it can be directly used as raw material for stainless steel smelting.

From hazardous waste to fertilizer: Recovery of high-value metals from smelter slags

This work proposes a method to valorize lead slag for fertilizer purposes. The research concept was to selectively recover valuable microelements (Cu(II), Fe(II), Zn(II) in an amount of at least 0.2% m/v of each) by chemical leaching while retaining toxic elements in the slag (i.e. As and Pb). Among acids, hydroxides, salts and their mixtures tested for slag treatment, it was potassium hydrogen sulfate and ammonia liquor under strongly oxidizing conditions (in the presence of hydrogen peroxide) that proved to be the most effective leaching agents. Response Surface Methodology applied to optimize the slag leaching conditions set the most favorable process parameters (concentration of leaching agents, slag to reagent weight ratio, and temperature). As a result, the concentration of Cu(II) in the extract was 3751 mg/L (for ammonia liquor) and Fe(II) and Zn(II) concentrations in potassium hydrogen sulfate were 4738 mg/L and 6102 mg/L, respectively. To close the life cycle of the waste, immobilization in polyethylene and binding to cement were indicated as methods to manage the solid waste material after leaching. The mixed extracts rich in Cu(II), Fe(II) and Zn(II) ions were tested in germination tests on cucumber. No phytotoxic effect was observed, which raises the possibility of utilizing the solutions after chemical leaching of slag as an alternative source of micronutrients for the production of multicomponent fertilizers. The results are promising and fit in the assumptions of circular economy.

Metallurgical Wastes as Resources for Sustainability of the Steel Industry

The industrial pollution caused by metallurgical waste accumulation has a negative impact on the three environmental factors: soil, air and water. Therefore, the correct management of these wastes would lead to: protection of the environmental factors, the saving of natural resources and sustainability of the steel industry. The purpose of this paper is to assess the chemical and mineralogical compositions of metallurgical wastes landfilled in the Păgida slag dump (Alba County, Romania), for sustainability of the steel industry and metal conservation. The chemical compositions of the two waste samples were analyzed by the XRF (X-ray fluorescence) technique. According to the chemical characterization, magnesium oxide (MgO) has potential to be used as an additional and raw material in the cement industry. The presence of oxides such as CaO, SiO2 FeO and Al2O3 in the compositions of the metallurgical waste samples indicate that they have the potential for use as clinker materials in cement production. The iron and manganese contents from metallurgical wastes can be reused in the iron and steel industry. The presence of V2O5 and TiO2 is connected with the making of stainless steel, and for this reason they have the potential to be reused in the stainless steel industry. The predominant chemical compounds are SiO2, Fetotal, Cao and MgO. The mineralogical compositions were analyzed by the XRD (X-ray diffraction) technique. The mineralogical compounds presenting reuse potential in different domains are Fayalite, Magnetite, Magnesioferrite and Periclase. The mineralogical compounds from metallurgical wastes can be reused as: raw and/or additional materials in the process from which they originate (steelmaking); raw and/or additional materials in road construction and concrete production; pigments in paints; micronutrients in fertilizers; ore of iron, etc. Then, the theoretical assessments of the recovery potentials of the metals were estimated for slag dumps. Copper (Cu), vanadium (V), molybdenum (Mo) and nickel (Ni) have high recovery potential. The total economic value of the recovery potential of metals from slag dumps was assessed to be USD 1175.7440 million.

Using Sawdust Derived Biochar as a Novel 3D Particle Electrode for Micropollutants Degradation

This work examined the use of a 3D combined electrochemical process based on particle electrodes from sawdust-derived biochar pyrolized at T = 550–850 °C to remove persistent pollutants. The as-prepared biochar was characterized by scanning electron microscopy with an X-ray energy dispersive spectrometer (SEM/EDS), nitrogen adsorption (BET method) and X-ray diffraction (XRD) techniques. The use of sawdust biochar pyrolized at 650 °C led to a significant increase in efficiency against the sum of conventional 2D electrochemical systems and adsorption, and the synergy index estimated equal to 74.5% at optimum conditions. Sulfamethoxazole (SMX) removal was favored by increasing particle electrode loading. Despite that, the reaction was slightly favored in near-neutral conditions; the system retained most of its activity in the pH range 3–10. The proposed 3D system could degrade different micropollutants, namely SMX, Bisphenol A (BPA), Propylparaben (PP), and Piroxicam (PR). Of particular interest was that no significant reduction in degradation was observed in the case of complex or real water matrices. In addition, the system retained its efficiency regarding SMX removal after five sequential experiments in the 3D combined electrochemical process. However, further investigation is needed to estimate the contribution of the different mechanisms of micropollutant removal in the proposed system.

Biochar caged zirconium ferrite nanocomposites for the adsorptive removal of Reactive Blue 19 dye in a batch and column reactors and conditions optimizaton

Biochar caged zirconium ferrite (BC-ZrFe2O5) nanocomposites were fabricated and their adsorption capacity for Reactive Blue 19 (RB19) dye was evaluated in a fixed-bed column and batch sorption mode. The adsorption of dye onto BC-ZrFe2O5 NCs followed pseudo-second-order kinetics (R 2 = 0.998) and among isotherms, the experimental data was best fitted to Sips model as compared to Freundlich and Langmuir isotherms models. The influence of flow-rate (3–5 mL min−1), inlet RB19 dye concentration (20–100 mg L−1) and quantity of BC-ZrFe2O5 NCs (0.5–1.5 g) on fixed-bed sorption was elucidated by Box-Behnken experimental design. The saturation times (C t /C o  = 0.95) and breakthrough (C t /C o  = 0.05) were higher at lower flow-rates and higher dose of BC-ZrFe2O5 NCs. The saturation times decreased, but breakthrough was increased with the initial RB19 dye concentration. The treated volume was higher at low sorbent dose and influent concentration. Fractional bed utilization (FBU) increased with RB19 dye concentration and flow rates at low dose of BC-ZrFe2O5 NCs. Yan model was fitted best to breakthrough curves data as compared to Bohart-Adams and Thomas models. Results revealed that BC-ZrFe2O5 nanocomposite has promising adsorption efficiency and could be used for the adsorption of dyes from textile effluents.

Progress in research and development of particle electrodes for three-dimensional electrochemical treatment of wastewater: a review

A three-dimensional (3D) electrochemical technology is regarded as a very effective industrial wastewater treatment method as it has high treatment efficiency, high current efficiency, and low energy consumption, and especially can completely mineralize nonbiodegradable organic pollutants. The core of the 3D electrochemical technology is a particle electrode, and the particle electrode plays several important roles for removing pollutants during the electrochemical reaction process. Many types of particle electrodes have been developed and used for different types of wastewater treatment. In this paper, a comprehensive review on the research and development of particle electrodes of the 3D electrochemical reactors for wastewater treatment is conducted. Specifically, the role that the particle electrode plays during the 3D electrochemical treatment of wastewater is thoroughly investigated and systematized. In addition, the different types of particle electrodes used in the 3D electrochemical wastewater treatment are classified into several types according to the presence or absence of a catalyst and the main components of the particle electrode or carrier. Also, focusing on the recent research results, the structural characteristics, performance, advantages and defects, and the role of catalyst components of each particle electrodes are evaluated. Finally, the direction and prospect of future research on the particle electrode is presented.

Hydrothermal synthesis, characterization and photocatalytic activity of Mg doped MoS2

In this research work nanoparticles of Mg (0, 1, 2 and 3%) doped MoS2 are prepared by Hydrothermal method at 200 °C for 9 h. Scanning Electron Microscope (SEM) for surface morphology, Fourier Transform Infrared Spectroscopy (FTIR) for structural and chemical bonding and UV-visible spectroscopy for optical properties are used. SEM showed that sheet-like structure has changed into stone-like shaped when Mg has doped into MoS2. From FTIR, Mo–O, Mo=S, and H–O bond peaks are becoming dim and new chemical bonds S=O, Mo=O, Mg–O, CH and OH are forming with the increase of Mg doping. UV-visible spectroscopy showed that MoS2 has an indirect bandgap 2.21 eV. Band gap decreased from 1.84 to 1.82 eV when the Mg doping was increased from 1 to 2%, respectively. As Mg concentration was increased i.e. 3% then band gap increased to 1.88 eV. Photocatalytic activity (PCA) of undoped and Mg doped MoS2 is appraised by degrading rhodamine blue (RhB) and methylene blue (MB) dyes. The results showed that PCA (in presence of visible light) Mg doped MoS2 is greater than pure MoS2 which significantly increased the photocatalytic properties.

Green and efficient utilization of stainless steel dust by direct reduction and self-pulverization

Stainless steel dust is a solid waste and contains a large number of valuable Fe, Cr, and Ni metal oxides, which should be recovered efficiently. Through direct reduction and self-pulverization separation, the goals of high metal recovery ratios and grades in the new process of comprehensive stainless steel dust utilization were achieved. Combined with theoretical analysis and experimental research, the effects of different conditions (FC/O ratio, reduction temperature, reduction time) on the reduction process and self-pulverization of reduction products were studied. The results showed that the optimal FC/O ratio was 0.8, reduction temperature was 1450 °C and reduction time was 20 min for the metal oxides in stainless steel dust to be completely reduced by carbon-thermal reduction; the self-pulverization holding temperature was 1100 °C, the holding time was 15 min, the conversion ratio of Ca₃SiO₅ to Ca₂SiO₄ reached the maximum, the content of γ-Ca₂SiO₄ in the reduced slag after cooling was increased, and a higher degree of self-pulverization was achieved. Fe-Cr-Ni-C alloy with an iron grade of 66.82%, chromium grade of 20.02% and nickel grade of 4.12% was manufactured successfully from stainless steel dust. The recoveries of iron, chromium and nickel in the stainless steel dust were 92.50%, 92.02% and 93.74%, respectively.

Synthesis and characterization of magnetically separable La1−x Bi x Cr1−y Fe y O3 and photocatalytic activity evaluation under visible light

A series of Bi and Fe doped La1−x Bi x Cr1−y Fe y O3 (x = 0.00–0.10 and y = 0.02–0.12) perovskites were fabricated through a facile microemulsion method and were characterized by XRD, DC electrical-resistivity, dielectric, VSM, and UV–Visible measurements. Orthorhombic phase of synthesized substituted chromite nanocrystallite was confirmed by powdered XRD analysis with crystallite size in 47.8–32.9 nm range. DC electrical resistivity was observed to increase from 1.70–39.99 × 108 Ω-cm. Dielectric parameters analyzed in frequency range of 20 kHz–20 MHz were decreased, while magnetic parameters were observed to increase with the increase in dopant (Bi+3 and Fe+3) concentration. Whereas coercivity values was low (narrow hysteresis loop), which indicate the soft ferromagnetic of the prepared material materials which are quite useful to employ in storage devices and electronics. Moreover, La1−x Bi x Cr1−y Fe y O3 degraded 90.80% Rhodamine B dye under visible light irradiation within 55 min. The increase in electrical resistivity, while decrease in dielectric parameters was also observed with increase in dopant concentration, ferromagnetic nature and excellent photocatalytic properties make this material suitable for high frequency energy devices, microwave appliances as well as an excellent magnetically separable photocatalyst for the purification of contaminated wastewater.

Kinetics, equilibrium and thermodynamics of dyes adsorption onto modified chitosan: a review

In view of promising sorption capacity, stability, biodegradability, cost-effectiveness, environmental friendly nature, regeneration and recycling ability, the chitosan (CS) based adsorbents are highly efficient for the sequestration of dyes. Since CS offers variable chemical structures and CS have been modified by incorporating different moieties. The CS composites with unique properties have been employed successfully for dye adsorption with reasonably high adsorption capacity versus other similar adsorbents. Modifications of CS were promising for the preparation of composites that are extensively studied for their adsorption capacities for various dyes. This review highlights the CS and its modification and their applications for the adsorption of dyes. The removal capacities of CS-based adsorbents, equilibrium modeling, kinetics studies and the thermodynamic characteristics are reported. Moreover, the FTIR, BET, SEM, TGA and XRD were employed for the characterization of CS modified adsorbents are also discussed. Results revealed that the modified CS is highly efficient and can be employed for the sequestration of dyes from effluents.

Kinetics, equilibrium and thermodynamics of dyes adsorption onto modified chitosan: a review

In view of promising sorption capacity, stability, biodegradability, cost-effectiveness, environmental friendly nature, regeneration and recycling ability, the chitosan (CS) based adsorbents are highly efficient for the sequestration of dyes. Since CS offers variable chemical structures and CS have been modified by incorporating different moieties. The CS composites with unique properties have been employed successfully for dye adsorption with reasonably high adsorption capacity versus other similar adsorbents. Modifications of CS were promising for the preparation of composites that are extensively studied for their adsorption capacities for various dyes. This review highlights the CS and its modification and their applications for the adsorption of dyes. The removal capacities of CS-based adsorbents, equilibrium modeling, kinetics studies and the thermodynamic characteristics are reported. Moreover, the FTIR, BET, SEM, TGA and XRD were employed for the characterization of CS modified adsorbents are also discussed. Results revealed that the modified CS is highly efficient and can be employed for the sequestration of dyes from effluents.

ZnO, CuO and Fe2O3 green synthesis for the adsorptive removal of direct golden yellow dye adsorption: kinetics, equilibrium and thermodynamics studies

In the present investigation, ZnO, CuO and Fe2O3 were prepared via green route and utilized for the sequestration of DGY (Direct Golden Yellow) dye. Affecting variables i.e., temperature, contact time, adsorbent dose and pH were optimized for maximum sequestration of dye from aqueous medium. The pH 2, adsorbent dose 0.1 g/50 mL dye solution, temperature 30 °C and 50 mg/L dye initial concentration were best levels for efficient dye adsorption and equilibrium was attained in 30 min reaction time. The dye sequestration on to ZnO, CuO and Fe2O3 was an exothermic process. Freundlich and Temkin adsorption isotherms explained well the dye adsorption onto nanoadsorbents and dye adsorption followed pseudo first order kinetic model. Effect of electrolytes and heavy metal ions was also investigated and both affected the adsorption process significantly. In the presences of surfactant/detergent, the removal of dye was reduced and 0.5 N NaOH efficiently desorbed the dye from nanoadsorbents. Findings depicted that the nanoadsorbents are effectual for the sequestration of DGY dye, which can be employed for the remediation of textile effluents.

Removal of Ammonium from Aqueous Solutions Using Zeolite Synthesized from Electrolytic Manganese Residue

This paper carried out the study on removal of ammonium from aqueous solutions by zeolite derived from electrolytic manganese residue (EMR) via a fusion method. The variables of pH, contact time, EMRZ (EMR-based zeolite) dosage, initial ammonium concentration, and competitive cations and anions on the ammonium uptake capacity were systematically investigated in an attempt to illustrate adsorption performance of EMRZ. The results show that these influence factors had a remarkable impact on the ammonium uptake capacity of EMRZ. Maximum ammonium uptake capacity was achieved at pH value 8.0, EMRZ dosage 0.2 g/100 mL, contact time 100 min, initial ammonium concentration 200 mg/L, and temperature 35°C. Under optimized conditions, ammonium uptake capacity onto EMRZ was up to 27.89 mg/g. The competitive degree of cations in ammonium adsorption process follows the sequence of Na+>K+>Ca2+>Mg2+, and the sequence of anion effect on ammonium removal onto EMRZ is CO32− > Cl− > SO42− > PO43−. The adsorption kinetic was explored and best represented by pseudo-second-order kinetic model. And the adsorption isotherm experimental data had best fitness with the Freundlich and Koble–Corrigan model, suggesting that heterogeneous uptake was the principal mechanism adopted in the process of ammonium adsorption. Moreover, calculation of thermodynamic parameters such as change in free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) was carried out and it was determined to be −15.77∼−14.03 kJ·mol−1, +37.66 kJ·mol−1, and +173.38 J·mol−1·K−1, respectively. These parameters confirmed that ammonium uptake onto EMRZ was an endothermic and spontaneous process. Moreover, no obvious deterioration tendency was observed for the regenerated EMRZ compared with fresh EMRZ. These results indicate that EMRZ has wide application prospects in removing ammonium from wastewater.