Industrial-scale extraction of high value-added kaolin from excavation waste: Demonstration from Xiamen, China

Excavation waste from the construction of subways and other underground infrastructures is mainly composed of gravel, sand and clay of minimal economic value, which commonly ends up in landfills. Although the coarse sand and gravel of the excavation waste are typically recycled on site, a large amount of the fine-grained residue must be disposed of due to the prohibition of marine land reclamation in Xiamen, China, leading to an increasingly severe shortage of landfills. In this contribution, a new strategy was successfully developed for industrial-scale extraction of high value-added kaolin from the excavation waste of Xiamen. This strategy can overcome the challenges of complex and variable chemical compositions, high iron contents, low industrial grade, and organic contaminants in the raw materials. Characterization using chemical analysis, powder X-ray diffraction, scanning electron microscopy, and infrared spectroscopy showed that the Xiamen excavation waste originated from granite weathering is mainly composed of kaolinite and quartz, along with high Fe contents and other impurities. The excavation waste was subjected to an intensive process of blunging, grinding, sieving, and classifying, as well as successive iron removal by magnetic separation. Subsequently, the extracted products meet commercial requirements, including those for high-quality kaolin with whiteness and plasticity larger than 90° and 17%, respectively. Moreover, an industrial-scale green production line with an annual treatment capacity of one million tonnes of excavation waste at the utilization rate of 100% was implemented. Hence, this work presents an effective approach for exploiting similar excavation waste around the world to promote sustainable development.

Development of a novel sizing approach for passive mine water treatment systems based on ferric iron sedimentation kinetics

The removal of dissolved and particulate iron (Fe) from contaminated mine drainage is an omnipresent challenge in, and legacy of, the mining industry worldwide. The sizing of settling ponds and surface-flow wetlands for passive Fe removal from circumneutral, ferruginous mine water is based either on a linear (concentration-independent) area-adjusted removal rate or flat assignment of an experience-based retention time, neither of which reflects the underlying Fe removal kinetics. In this study, we evaluated the Fe removal performance of a pilot-scale passive system operating in three identical, parallel lines for treatment of mining-influenced, ferruginous seepage water to determine and parameterise a robust, application-orientated model approach for sizing of settling ponds and surface-flow wetlands, each. By systematically varying flow rates (and thus residence time), we were able to demonstrate that the sedimentation-driven removal of particulate hydrous ferric oxides in settling ponds may be approximated by a simplified first-order approach at low to moderate Fe levels. The first-order coefficient was found in the order of 2.1(±0.7) × 10^-2 h^-1, which corresponds well with previous laboratory studies. The sedimentation kinetics may be combined with the preceding Fe(II) oxidation kinetics to estimate the required residence time for pre-treatment of ferruginous mine water in settling ponds. In contrast, Fe removal in surface-flow wetlands is more complex due to the phytologic component, which is why we advanced the established area-adjusted Fe removal approach by parameterising the underlying concentration-dependency for polishing of pre-treated mine water. The quantitative results of this study provide a novel, conservative approach for customised sizing of settling ponds and wetlands in integrated passive mine water treatment systems.

Facile encapsulation of nano zero-valent iron with calcium carbonate: synthesis, characterization and application for iron remediation

In this study, CaCO3 was used as a modifier for nano zero-valent iron (nZVI) surface to prevent rapid aggregation and effectively utilized for iron remediation from aqueous solution. Surface chemistry and morphology of CaCO3 encapsulated nZVI (CaCO3-nZVI) before and after treatment of contaminant iron solution were characterized by scanning electron microscopy-energy dispersive X-ray (SEM-EDX), X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The mechanisms of surface modification as well as iron remediation were well depicted with the help of these characterisation tools. Iron removal efficacy of 96.4% was achieved with 0.25 g/L adsorbent dose for an influent iron of 0.5 mg/L at pH 10 after a 3 h treatment process. When the influent concentration was increased to 10 mg/L, the removal capacity decreased to 92.1%. The study demonstrates that CaCO3 and nZVI in the encapsulated nanoparticle have a significant synergistic effect. The pseudo-second- order reaction kinetics and Freundlich isotherm model correctly portrayed the experimental data for iron removal by CaCO3-nZVI. The CaCO3-nZVI is a viable option for iron removal from various aqueous media due to its facile preparation, high iron removal capability, and reusability.

Simultaneous separation of Fe & Al and extraction of Fe from waste coal fly ash: Altering the charge sequence of ions by electrolysis

A significant amount of coal fly ash is generated and this type of waste material causes severe environmental hazards. Metal (Al and Fe) extraction from coal fly ash is beneficial to the resource utilization of waste coal fly ash. However, the coexistence of Al and Fe in coal fly ash means that the separation of Al and Fe is required, which is a key and difficult step to prepare high value-added products from coal fly ash. This work presents a novel electrolysis method to alter the charge sequence of Al³⁺, Fe³⁺, and H₂O, leading to a process different from their natural tendency for simultaneous separation of Fe³⁺ and Al³⁺, and extraction of Fe. The single iron removal efficiency was 43.48%, and the aluminum extraction efficiency was <0.30% under optimal conditions. The iron product had a purity of 98.3 wt% Fe, 0.45 wt% Al, and 0.18 wt% S. This process occurs without chemical additions and expensive membranes, avoiding impurity introduction, slag generation, and membrane limitations. Fe(s), H₂(g), Al₂(SO₄)₃(aq), and O₂(g) are the main products during the electrolysis. Flake Fe is selectively produced instead of colloidal Fe(OH)₃. Fe is a magnetic substance and is easier to remove from the solution by magnets than colloidal Fe(OH)₃. H₂ is a green fuel. Wastewater (Al₂(SO₄)₃(aq)) can be directly used to further extract alumina. Therefore, this study provides an alternative method of zero pollution discharge for simultaneous separation of Fe³⁺ and Al³⁺, and extraction of Fe from coal fly ash leachate.

Efficient removal of iron from red gypsum via synergistic regulation of gypsum phase transformation and iron speciation

Red gypsum is a type of iron-rich gypsum residue originated from industrial titanium dioxide process using Ilmenite. Currently, it has a low rate of comprehensive utilization about 20%, and deep removal of iron impurity is the crucial factor that directly limits its multipurpose utilization. In this study, the iron was efficiently removed from red gypsum residue by synergistic controlling the phase transformation of gypsum and the iron speciation under hydrothermal conditions. The iron removal efficiency was more than 99% under the optimized treatment condition (i.e. liquid-solid ratio of 10, with 1.5 M HCl as mineralizer, heating at 140 °C for 6 h). The X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) analyses showed that the phase transformation and crystal growth of gypsum accelerated by H⁺ was the essential reasons to fully remove iron. Moreover, H⁺ also provided acidic conditions (pH < 1) to change the iron speciation from amorphous oxide or hydroxide fine particles into soluble Fe³⁺ which release into the solution and easy to be removed by solid-liquid separation. In this work, based on the synergistic regulation of gypsum phase transformation and iron speciation, a feasible method for deep removal of iron from red gypsum was proposed, which is conducive to broadening the comprehensive utilization range of red gypsum. This work would inspire the treatment and resource utilization of industrial gypsum residues containing other contaminants or impurities, including heavy metals and organic matters.

Highly chelating stellate mesoporous silica nanoparticles for specific iron removal from biological media

In this work, the design of a new generation of functionalized large pore silica nanoparticles is addressed for the specific removal of iron from biological environments. Herein, mesoporous silica with a large pore stellate morphology, denoted STMS, were grafted with the highly specific iron chelating agent desferrioxamine B, DFoB. The challenge of this work was the step by step elaboration of the nanoplatform and the evaluation of its chelating efficiency and selectivity. Hence, the controlled covalent grafting of DFoB specific iron chelator, was successfully achieved ensuring a high grafting rate of chelating ligand of 730 nmol·mg^-1 (i.e., 0.85 ligand·nm^-2). Furthermore, these highly chelating STMS silica were able to capture iron(III) stabilized with nitrilotriacetic acid (NTA) in solution at physiological pH with a fast kinetics (less than 30 min). For a stoichiometry 0.85:1 (FeNTA : DFoB), the STMS-DFoB nanoparticles allowed reaching capture capacity and efficiency of 480 nmolFe³⁺/mg SiO₂ and 78%, respectively. Regarding the selectivity features of the removal process, studies were performed with two different media composed of various metal ions: (i) an equimolar solution of various metal cations and (ii) a Barth's buffer mimicking the brain solution composition. In both cases, the chelating STMS-DFoB showed a high selectivity for iron versus other ions at the same (Al³⁺) or different valency (Na⁺, K⁺…). Finally, this work paves the way for new nanosystems for metal overload treatments as well as for future highly chelating nanoplatforms that can be used at the interface between depollution and nanomedecine.

Evaluation of pyrolyzed areca husk as a potential adsorbent for the removal of Fe2+ ions from aqueous solutions

The hurdle of valorisation of Arecanut husk on one side and the pollution of aquatic bodies by heavy metals like Iron on the other end are contemplated together in this study. The areca husk is pyrolyzed at 450°C for two hours to obtain Biochar. Batch adsorption studies were employed to investigate the effect of adsorbent dosage (2-10 g/l), initial concentration of adsorbate (1-5 mg/l) and contact time (30 -360 min) at temperature of 28±2 °C & pH 4.0±0.2 on the removal of Iron from pyrolyzed areca husk. The adsorption capacity was found to increase with increase in initial Iron concentartion and contact time, but decreases with the adsorbent dosage. Langmuir, Freundlich, Temkin and Dubinin-Radushkevich Isotherms was used to analyse the equilibrium data. Langmuir and Dubinin-Radushkevich model best describe the uptake of Iron ions implying a monolayer adsorption with physisorption. Pseudo second order, exhibited the best fit for the effectiveness of Iron adsorbtion indicating the maximum limit of chemisorption. Thermodynamic studies indicated that the adsorption was spontaneous and exothermic in nature. The mechanisms responsible for adsorption of Iron on pyrolysed areca husk was conducted by SEM-EDAX, XRD and FTIR indicating oxidation and precipitaion of Iron into complex compounds of jarosite and ferrous hydroxy sulphates. In conclusion, pyrolyzed areca husk can be technically & economically feasible alternative adsorbent material.

Development of copper recovery process from flotation tailings by a combined method of high‒pressure leaching‒solvent extraction

Sulfide copper mineral, typically Chalcopyrite (CuFeS₂), is one of the most common minerals for producing metallic copper via the pyrometallurgical process. Generally, flotation tailings are produced as a byproduct of flotation and still consist of un‒recovered copper. In addition, it is expected that more tailings will be produced in the coming years due to the increased exploration of low‒grade copper ores. Therefore, this research aims to develop a copper recovery process from flotation tailings using high‒pressure leaching (HPL) followed by solvent extraction. Over 94.4% copper was dissolved from the sample (CuFeS₂ as main copper mineral) by HPL in a H₂O media in the presence of pyrite, whereas the iron was co‒dissolved with copper according to an equation given as C_Cu = 38.40 × C_Fe. To avoid co‒dissolved iron giving a negative effect on the subsequent process of electrowinning, solvent extraction was conducted on the pregnant leach solution for improving copper concentration. The result showed that 91.3% copper was recovered in a stripped solution and 98.6% iron was removed under the optimal extraction conditions. As a result, 86.2% of copper was recovered from the concentrate of flotation tailings by a proposed HPL‒solvent extraction process.

Siderophore-mediated iron removal from chrysotile: Implications for asbestos toxicity reduction and bioremediation

Asbestos fibers are highly toxic (Group 1 carcinogen) due to their high aspect ratio, durability, and the presence of iron. In nature, plants, fungi, and microorganisms release exudates, which can alter the physical and chemical properties of soil minerals including asbestos minerals. We examined whether exudates from bacteria and fungi at environmentally relevant concentrations can alter chrysotile, the most widely used asbestos mineral, and lower its toxicity. We monitored the release of iron from chrysotile in the presence of organic acid ligands and iron-specific siderophores derived from bacteria and fungi and measured any change in fiber toxicity toward peritoneal macrophages harvested from mice. Both fungal and bacterial siderophores increased the removal of iron from asbestos fibers. In contrast, organic acid ligands at environmentally relevant concentrations neither released iron from fibers nor helped in siderophore-mediated iron removal. Removal of plant-available or exchangeable iron did not diminish iron dissolution by both types of siderophores, which indicates that siderophores can effectively remove structural iron from chrysotile fibers. Removal of iron by siderophore lowered the fiber toxicity; fungal siderophore appears to be more effective than bacterial siderophore in lowering the toxicity. These results indicate that prolonged exposure to siderophores, not organic acids, in the soil environment decreases asbestos fiber toxicity and possibly lowers the health risks. Thus, bioremediation should be explored as a viable strategy to manage asbestos-contaminated sites such as Brownfield sites, which are currently left untreated despite dangers to surrounding communities.

Down-flow fixed-structured bed reactor: An innovative reactor configuration applied to acid mine drainage treatment and metal recovery

A down-flow fixed-structured bed reactor (DFSBR) was operated for 277 days treating a mixture of synthetic substrates simulating an iron-rich acid mine drainage (AMD) and the soluble fraction of a sugarcane vinasse. The synthetic sugarcane vinasse was used as electron donor for biological sulfate-reduction, resulting in influent chemical oxygen demand (COD) close to 4000 mg L^-1 and volumetric organic loading rate of 4.8 g L^-1d^-1. The influent sulfate concentration was kept close to 2000 mg L^-1 (volumetric sulfate loading rate of 2.5 g L^-1d^-1) while a gradual increase of iron concentration (2-400 mg L^-1) was applied. COD removal efficiencies were higher than 93% and the sulfate removal efficiencies were close to 100%. With the highest iron concentration (400 mg L^-1) applied, the DFSBR achieved 95% of iron removal efficiency. The precipitate collected at the reactor bottom showed increasing concentrations of fixed suspended solids (FSS), as well as an increasing proportion of iron, indicating the possibility of metal recovery from the system. The association between sulfidogenic and methanogenic processes also enables energy recovery from the methane-rich biogas produced.

Removal of metal from acid mine drainage using a hybrid system including a pipes inserted microalgae reactor

In this study, the microalgae culture system to combined active treatment system and pipe inserted microalgae reactor (PIMR) was investigated. After pretreated AMD in active treatment system, the effluent load to PIMR in order to Nephroselmis sp. KGE 8 culture. In experiment, effect of iron on growth and lipid accumulation in microalgae were inspected. The 2nd pretreatment effluent was economic feasibility of microalgae culture and lipid accumulation. The growth kinetics of the microalgae are modeled using logistic growth model and the model is primarily parameterized from data obtained through an experimental study where PIMR were dosed with BBM, BBM added 10 mg L(-1) iron and 2nd pretreatment effluent. Moreover, the continuous of microalgae culture in PIMR can be available. Overall, this study indicated that the use of pretreated AMD is a viable method for culture microalgae and lipid accumulation.