Fly Ash Leachate Research Articles

Dispersive Solid–Liquid Microextraction Based on the Poly(HDDA)/Graphene Sorbent Followed by ICP-MS for the Determination of Rare Earth Elements in Coal Fly Ash Leachate

A dispersive solid-phase microextraction (DSPME) sorbent consisting of poly(1,6-hexanediol diacrylate)-based polymer microspheres, with embedded graphene microparticles (poly(HDDA)/graphene), was synthesized by microfluidic emulsification/photopolymerization and characterized by optical microscopy and X-ray fluorescence spectrometry. This sorbent was applied for simple, fast, and sensitive vortex-assisted DSPME of rare earth elements (RREs) in coal fly ash (CFA) leachate, prior to their quantification by inductively coupled plasma mass spectrometry (ICP-MS). Among nine DSPME variables, the Plackett–Burman screening design (PBD), followed by the central composite optimization design (CCD) using the Derringer desirability function (D), identified the eluent type as the most influencing DSPME variable. The optimum conditions with maximum D (0.65) for the chelating agent di-(2-ethylhexyl) phosphoric acid (D2EHPA) amount, the sorbent amount, the eluting solvent, the extraction temperature, the centrifuge speed, the vortexing time, the elution time, the centrifugation time, and pH, were set to 60 μL, 30 mg, 2 M HNO3, 25 °C, 6000 rpm, 1 min, 1 min, 5 min, and 4.2, respectively. Analytical validation of the DSPME method for 16 REEs (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) in CFA leachate samples estimated the detection limits at the low ppt level, the recovery range 43–112%, and relative standard deviation within ± 22%. This method was applied to a water extraction procedure (EP) and acetic acid toxicity characteristic leaching procedure (TCLP) for leachate of CFA, from five different coal-fired thermoelectric power plants. The most abundant REEs in leachate (20 ÷ 1 solid-to-liquid ratio) are Ce, Y, and La, which were found in the range of 22–194 ng/L, 35–105 ng/L, 48–95 ng/L, and 9.6–51 μg/L, 7.3–22 μg/L, 2.4–17 μg/L, for EP and TCLP leachate, respectively. The least present REE in TCLP leachate was Lu (42–125 ng/L), which was not detected in EP leachate.

Leaching of coal fly ash with sulphuric acid for synthesis of wastewater treatment composite coagulant

ABSTRACT The results of leaching of coal fly ash in H2SO4 solution are presented. The effects of leaching time, H2SO4 concentration, temperature, and solid/liquid ratio were explored for the dissolution process. Particle size distribution results showed that changes in particle size occurred during the dissolution process; XRD, XRF and FTIR results indicated the same characteristics of the mineral matter in the raw and treated coal fly ash samples; and SEM showed that the raw coal fly ash particles have smooth spherical surfaces while a pricked and corroded surface and a stem-like structure were formed after leaching. The dissolution rate of coal fly ash is controlled by mixed-controlled mechanism (diffusion and chemical reaction). The corresponding activation energies were calculated to be 59.8 kJ/mol for iron, 7.3 kJ/mol for aluminium and 5.8 kJ/mol for silicon. The reaction order was determined to be zero order kinetic model.

Hydrochloric Acid Leaching of Philippine Coal Fly Ash: Investigation and Optimisation of Leaching Parameters by Response Surface Methodology (RSM)

Coal Fly Ash (CFA) is a hazardous waste from coal-fired power plants, but has increasingly become a popular supplementary cementitious material for cement in the construction industry. As a secondary resource of REE, its main advantage lies in its fine particle size that eliminates the need for costly and energy-intensive comminution. In this study, the potential of CFA from the Philippines as a secondary REE resource was investigated by direct leaching of REEs with hydrochloric acid (HCl). The CFA sample came from a coal-fired power plant with a circulating fluidized bed combustion (CFBC) technology. For the leaching tests, the effects of HCl concentration, leaching time and leaching temperature on REE extractions were elucidate optimized via Response Surface Methodology (RSM). The RSM results showed that the optimum leaching occurred at 3M HCl, 65 °C and 270 min with extractions of Nd, Er, Eu, Tb and Dy at 70.8%, 76.34%, 88.02%, 90.01% and 73.38%, respectively. According to these results, the CFA from the Philippines is a promising secondary resource of REE and the extraction method employed was effective in achieving a relatively high REE dissolution. Moreover, the empirical model that was established accurately predicted the dissolution of REE with an accuracy of 98.20%, 96.66%, 97.09%, 98.17% and 97.78% for Nd, Er, Eu, Tb and Dy, respectively.

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 98.3wt% Fe, 0.45wt% Al, and 0.18wt% 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.

Hydraulic performance, consolidation characteristics and shear strength analysis of bentonites in the presence of fly-ash, sewage sludge and paper-mill leachates for landfill application

A rapid upsurge in urban and industrial developments leads to increased generations of solid wastes. The most accepted technique of waste discarding around the world is landfilling. Leaching chemicals from municipal dumping grounds can pollute the groundwater source and the surrounding environment without appropriate precautionary measures. Bentonite is a low-cost constituent used as a liner material in landfills due to its low permeability, high sealing ability, high specific surface area, and the ability to hold up the impurity migration through adsorption. However, leachate interaction with bentonite may alter its properties and reduce its usefulness as a barrier material in the long term. Also, bentonite having different chemical and mineralogical compositions will behave differently due to the leachate interaction. Therefore, it is necessary to compare the performance of various bentonites in the presence of leachates. In the present investigation, two Indian bentonites of different mineralogical compositions were studied for their change in the index properties, swelling, swelling potential, swelling pressure, hydraulic conductivity, consolidation parameters and shear strength properties in the presence of fly ash, sewage sludge and paper mill leachates. The outcomes showed that in the presence of all the leachates, liquid limit, free swell, compression index, swelling potential, swelling pressure, time to complete 90% of consolidation and shear strength dropped; whereas, hydraulic conductivity and coefficient of consolidation increased. Besides, the quality of bentonite prominently influenced the hydraulic, strength and swelling behaviour. The bentonite having a higher cation exchange capacity, liquid limit, specific surface area, and swelling capability undergoes a higher variability in the free swell (80.0, 73.8 and 76.9% decline), liquid limit (73.5, 61.7 and 69.2% decline), swelling potential (61.3, 55.7 and 51.0% decline), swelling pressure (53.3 and 56.4% decrease), and hydraulic conductivity (57.5, 8.6 and 41.1 times increase at a void ratio of 1.2) values when infused with fly ash, sewage sludge and paper mill leachates, respectively. The study also showed that the fly ash leachate interaction causes a higher variation in bentonite behaviour than sewage sludge and paper mill leachates. The study's findings would prove beneficial to design engineers for selecting bentonite types for landfill liners.

Stability evaluation of potentially toxic elements in MSWI fly ash during carbonation in view of two leaching scenarios

Carbonation treatment (CT) by alkaline fly ash (FA) affects the stability of potentially toxic elements (PTEs). This study investigated the leachability and environmental risk of six PTEs contained in FA during natural and accelerated carbonation (NC, AC) using two typical leaching scenarios with distilled water (DW) and acetic acid (AA). The leaching of Pb/Cu/Cr/Ni in solidified/stabilized FA decreased due to CT in DW leaching, but the leaching of Pb/Zn/Cu/Cd increased due to CT in AA leaching. The leaching of the six PTEs (especially Pb/Cd) in AA leaching was significantly higher than that in DW leaching. CT was a promoting factor to increase the environmental risk level of PTEs in FA leachate, especially in AA leaching with H+ input. In the early stage of NC, under DW leaching tests, the environmental risk level of PTEs in FA leachate can be weakened due to the formation of carbonate minerals in the FA matrix. However, excessive NC increases the environmental risk of leached PTEs due to the decalcification of carbonate minerals. Both NC and AC increased the potential environmental risk of PTEs contained in the carbonated FA matrix. The nucleation and dissolution of carbonate minerals were interdependent with the immobilization and leaching of PTEs, which played a dominant role in the CT and leaching tests respectively. They jointly affected the occurrence behavior of PTEs in the FA matrix in CT tests and the leachability of PTEs in leaching tests. This study demonstrates that it is more scientific to evaluate the leachability of PTEs in carbonated FA according to the actual disposal scenarios.

Flow battery electrolyte from carbon black incineration fly ash: A feasibility study of an environment friendly disposal process.

Fly ash represents a kind of finely divided solid waste which is derived from industrial or municipal waste incineration and contains various metal elements. In this work, we focus on the waste-to-resource conversion of one fly ash which is generated from the incineration of petroleum coke gasification waste. The leaching behaviors and waste conversion of this fly ash to value-added products under different treatments were investigated. The majority of the identified elements in leachate demonstrated different leaching patterns as the species of leached element from fly ashes depend on the chemical properties of leaching agents. Moreover, A pathway is developed to harvest toxic vanadium from this fly ash, and the vanadium-containing leachate can be further converted into electrolyte for vanadium redox flow battery (VRFB). The vanadium electrolyte was synthesized by using fly ash leachate as the resource materials in two different ways afterwards: reducing vanadium in the leachate directly (electrolyte from leachate) and synthesizing V2O5 from leachate as the intermediate product (electrolyte from leachate-derived V2O5). The electrochemical behavior and performance of these electrolytes were analyzed to investigate the feasibility of these approaches. The measurement of electrochemical performance proves that the electrolyte from leachate-derived V2O5 is comparable to the standard electrolyte in terms of columbic efficiency (CE), energy efficiency (EE) and stability. Based on experimental results, our research provides a potential solution for the establishment of vanadium flow battery plant. Cost-benefit analysis proves that the payback period of electrolyte synthesized from alkaline leachate and leachate-derived V2O5 is 2.1years and 1.0year, respectively.

Phosphate Polymer Nanogel for Selective and Efficient Rare Earth Element Recovery.

Demand for rare earth elements (REEs) is increasing, and REE production from ores is energy-intensive. Recovering REEs from waste streams can provide a more sustainable approach to help meet REE demand but requires materials with high selectivity and capacity for REEs due to the low concentration of REEs and high competing ion concentrations. Here, we developed a phosphate polymer nanogel (PPN) to selectively recover REEs from low REE content waste streams, including leached fly ash. A high phosphorus content (16.2 wt % P as phosphate groups) in the PPN provides an abundance of coordination sites for REE binding. In model solutions, the distribution coefficient (Kd) for all REEs ranged from 1.3 × 105 to 3.1 × 105 mL g-1 at pH = 7, and the sorption capacity (qm) for Nd, Gd, and Ho were ∼300 mg g-1. The PPN was selective toward REEs, outcompeting cations (Ca, Mg, Fe, Al) at up to 1000-fold excess concentration. The PPN had a Kd of ∼105-106 mL g-1 for lanthanides in coal fly ash leachate (pH = 5), orders of magnitude higher than the Kd of major competing ions (∼103-104 mL g-1). REEs were recovered from the PPN using 3.5% HNO3, and the material remained effective over three sorption-elution cycles. The high REE capacity and selectivity and good durability in a real waste stream matrix suggest its potential to recover REEs from a broad range of secondary REE stocks.

Sustainable Transportation, Leaching, Stabilization, and Disposal of Fly Ash Using a Mixture of Natural Surfactant and Sodium Silicate.

The present study evaluates the transportation, leaching, and stabilization ability of novel saponin extracted from the fruits of Acacia auriculiformis. To enhance the dispersing behavior of the fly ash slurry (FAS) at a lower dosage of sodium silicate, A. auriculiformis was incorporated in FAS. In addition to the rheological study, an attempt has been made to remove heavy metals through leaching for the safe disposal of FAS. Critical factors such as the fly ash (FA) concentration, saponin dosage, surface tension, ζ potential, temperature, and combination of saponin and sodium silicate, affecting the rheology of FAS, were extensively studied. The addition of a nonionic natural surfactant saponin has been proved to enhance the wettability of FA particles by decreasing the surface tension of FAS. The obtained rheology results were compared with the stabilization yield of the previously reported commercial surfactant cetyltrimethylammonium bromide. The incorporation of sodium silicate in the FAS system was found to be phenomenal in the settling and stabilization of FAS, thereby developing reaction products like sodium aluminum silicate (N-A-S). This facilitates the sustainable disposal of FA preventing air pollution after dewatering. The formation of N-A-S was further supported by scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies.

Reuse of power plant pollution sources in the cultivation of thermophilic cyanobacteria PKUAC-SCT E542

The growth of hermophilic cyanobacteria Thermosynechococcus elongatus PKUAC-SCT E542 was not inhibited when the leachate from the flue fly ash of waste incineration plant was added to the culture of cyanobacteria, and the growth of cyanobacteria was not inhibited. Some elements in the BG-11 medium could be used with smoke Dow fly ash leachate was replaced, and the growth rate was increased by 6% to 12% compared with the use of BG-11 medium. In addition, combining the use of fly ash leachate and simulated flue gas to replace Na2CO3 in the microalgae culture system as the sole carbon source, the growth of cyanobacteria was still active, and there was no significant difference compared to BG-11. After 4 days of cultivation, the biomass yield was 0.805 ± 0.05 g/L and 0.751 ± 0.02 g/L, achieving a higher biomass accumulation efficiency, and realizing the feasibility of replacing medium components with leachate and flue gas at the same time. In this system, the cultivation of thermophilic cyanobacteria and the two major pollutants of power plants were mutually beneficial, and at the same time achieve the results of resource utilization of pollution sources, reduction of cultivation costs and production of phycobiliprotein(PBPs).

Effect of Municipal Solid Waste Incineration Fly Ash Leachate on the Hydraulic Performance of a Geosynthetic Clay Liner

Effect of Municipal Solid Waste Incineration Fly Ash Leachate on the Hydraulic Performance of a Geosynthetic Clay Liner

Electrolysis designed for clean production of selective iron products from coal fly ash leachate

With the development of society, solid waste, such as coal fly ash has caused serious environmental problems. Extraction of high value metals from coal fly ash is vital for sustainable development and maintenance of social economy. This work aimed to present a nonhazardous and efficient electrolysis method to extract selective iron products from coal fly ash leachate. Compared with current precipitation, biological, and organic extraction processes, this electrolysis method can selectively prepare different iron products. No chemicals or expensive membranes were used to prevent the introduction of pollutant and membrane limitations. Moreover, no waste was produced enabling realization of zero pollution discharge. Batch experiments indicated that the degree of reduction of iron can be controlled during the electrolysis process, thereby allowing for the production of different types of iron products. The purity of iron electrolytic product was 98.30 wt% Fe2O3, 1.35 wt% SO3, and 0.09 wt% Al2O3, and current efficiency reached 60.3% under the optimal conditions: c (Fe2+) = 0.1 M, t = 4.0 h, and J = 2500 A/m2. Different iron products like Fe3O4, FeOOH and Fe can be prepared sequentially during electrolysis process. Moreover, O2 is generated on anode, and H2 is generated on cathode as clean energy, which increases economic benefits. The wastewater (containing H2SO4) is widely used as a leaching reagent during leaching of coal fly ash, which can return to leaching process enabling zero hazardous substance discharge. This study would provide an alternative clean and efficient method for extracting selective iron products from coal fly ash leachate.

Optimization of Metal Recovery from MSWI Fly Ash by Acid Leaching: Findings from Laboratory- and Industrial-Scale Experiments

A major part of Swiss fly ashes (FA) from municipal solid waste incineration (MSWI) are treated with the acid fly ash leaching process (FLUWA) in order to recover heavy metals prior to deposition. The FLUWA process uses scrub water from wet flue gas cleaning to leach heavy metals from FA. The leaching efficiency is strongly dependent on the leaching conditions (e.g., pH, Eh, L/S-ratio). This case study presents the optimization of the FLUWA process at the MSWI plant Linth, Switzerland, through determination of ideal process parameters for optimal metal recovery. By means of laboratory- and industrial-scale experiments, the process was adjusted towards a more efficient leaching of Zn, Pb, Cu, and Cd. This included the use of an oxidizing agent (hydrogen peroxide). Laboratory experiments proved to be a powerful tool for simulating process optimizations at industrial scale. An ideal leaching pH of 3.8 was determined and it was observed that the process stability is significantly influenced by the L/S-ratio applied to the leaching process. In the course of the study, the recovery could be improved to 67% Zn, 66% Pb, 30% Cu, and 91% Cd. It can be concluded that for optimal metal recovery the process has to be individually adjusted to the composition of the processed FA and scrub water of each specific FLUWA process.

Suitability and characteristics of combustion residues from renewable power plants for subbase aggregate materials, in Thailand

Physico-chemical properties and the environmental impacts were studied relative to the leaching of rubber fly ash and bottom ash. The pozzolanic properties of fly ash and bottom ash were confirmed by the chemical composition, including silicon oxides, calcium oxides, and aluminum oxides. The geo-technical characteristics of rubber wood fly ash and bottom ash, i.e., modified compaction, plasticity, and the soaked California Bearing Ratio, were evaluated to assess the feasibility of fly ash or bottom ash mixed with lateritic soil as aggregate materials for the subbase in road construction in order to optimize the replacement of lateritic soil by fly ash or bottom ash. The leachates from rubber fly ash and bottom ash did not exceed standard thresholds. The measured characteristics of fly ash or bottom ash mixed with lateritic soil were in good alignment with the effective engineering thresholds. Recommendations were developed for safe reuse of byproducts from rubber renewable power plant in subbase road construction.

Coagulation Efficiency of Biomass Fly Ash Leachate in Thermomechanical Pulping (TMP) Pressate

In this study, the coagulation performance of a biomass-based fly ash leachate (FLC) and alum on a thermomechanical pulping (TMP) pressate was compared systematically. Fly ash leachate (FLC) was first produced via mixing fly ash with water and extracting the soluble part of fly ash. Then, the coagulation performance of FLC in TMP pressate was studied and compared with that of alum, as a benchmark. The effects of the dosage of the coagulant and treatment time on the chord length of particles in the TMP pressate as well as the chemical oxygen demand (COD) and lignin content of the TMP pressate were systematically monitored. It was observed that FLC and alum coagulated the dissolved materials of the TMP pressate. The COD and lignin removals were 18.4% and 26.9%, respectively, when FLC was added to a TMP pressate under the conditions of 5060 mg/kg FLC/TMP pressate, 200 rpm, pH 12.5 298 K and 30 min. Calcium ions of FLC were the main contributor to the coagulation performance of FLC for TMP pressate. The chord length analysis of solution revealed that the particles in the 1–10 and 10–50 µm ranges were rapidly coagulated, forming larger aggregates in the 50–150 and 150–300 µm size ranges and reaching the maximum mean chord length within approximately 4 min when the coagulant was an alum. FLC led to the slower conversion of particles in 1–10 µm range and a faster conversion in the 10–50 and 50–150 µm ranges than alum did. As FLC can be readily produced inexpensively onsite, it could be a suitable coagulant for eliminating the dissolved components of the pulp and paper effluents.

Waste Wood Fly Ash Treatment in Switzerland: Effects of Co-Processing with Fly Ash from Municipal Solid Waste on Cr(VI) Reduction and Heavy Metal Recovery

In Switzerland, waste wood fly ash (WWFA) must be treated before deposition on landfills due to its high pollutant load (Cr(VI) and heavy metals). Acid fly ash leaching, the process used for heavy metal recovery from municipal solid waste incineration fly ash (MSWIFA), represents a possible treatment for heavy metal depletion and Cr(VI) reduction in WWFA. The co-processing of WWFA with MSWIFA during acid fly ash leaching was investigated in laboratory- and industrial-scale experiments with different setups. Of interest were the effects on heavy metal recovery efficiency, the successful outcome of Cr(VI) reduction and consumption of neutralizing chemicals (HCl, H2O2). Detailed chemical and mineralogical characterization of two WWFA types and MSWIFA showed that MSWIFA has higher concentrations in potentially harmful elements than WWFA. However, both WWFA types showed high concentrations in Pb and Cr(VI), and therefore need treatment prior to deposition. Depending on the waste wood proportion and quality, WWFA showed chemical and mineralogical differences that affect leaching behavior. In all experimental setups, successful Cr(VI) reduction was achieved. However, WWFA showed higher consumption of HCl and H2O2, the latter resulting in a particularly negative effect on the recovery of Pb and Cu. Thus, co-processing of smaller WWFA portions could be expedient in order to diminish the negative effects of Pb and Cu recovery.

Assessment of heavy metals mobility and correlative recovery and decontamination from MSWI fly ash: Mechanism and hydrometallurgical process evaluation

Fly ash from municipal solid waste incineration (MSWI) enriches many leachable toxic metals which readily migrate into the environment, posing serious risks to the ecosystem and human. In this study, the elements mobility, leaching availability as well as the potential maximum amounts of heavy metals in fly ash were thoroughly evaluated. To decontaminate the toxic elements from resulting fly ash leachates, The aqueous zinc (Zn) was recovered using Cyanex 572, cadmium (Cd) and copper (Cu) were effectively removed through adsorption process by a self-assembled hierarchical hydroxyapatite (HAP) nanostructure. The removal mechanism of Cd, Cu and Zn by leaching, extraction and adsorption was revealed with the results from XRD, ICP-MS and SEM. The results showed that fly ash has a high mobility under maximum availability leaching test (95% of fly ash was dissolved), a recovery rate of 91% for Zn can be obtained using Cyanex 572, and a high adsorption rate (> 95% for both Cu and Cd) was reached using HAP for the pristine fly ash leachate. The outcomes from isothermal and kinetic study revealed that Langmuir isotherm and pseudo-second order model can well describe the Cd and Cu adsorption behavior. Economic assessment suggested that the application of HAP for the removal of Cd and Cu is a technically sound and economically feasible approach. The findings of this study demonstrated that this comprehensive process integrated leaching, solvent extraction and consequential decontamination can be a practical strategy for MSWI fly ash treatment.

Municipal Solid Waste as Secondary Resource: Selectively Separating Cu(II) from Highly Saline Fly Ash Extracts by Polymer-Assisted Ultrafiltration

Urban mining from fly ash resulting from municipal solid waste incineration (MSWI) is becoming more and more important due to the increasing scarcity of supply-critical metals. Metal extraction from acid fly ash leaching has already been established. In this context selective Cu recovery is still a challenge. Therefore, our purpose was the separation of Cu(II) from MSWI fly ash extracts by polymer-assisted ultrafiltration (PAUF). We investigated three polyethyleneimines (PEIs) with regard to metal retention, Cu(II) selectivity, Cu(II) loading capacity, and the viscosity of the PEI containing solutions. A demanding challenge was the highly complex matrix of the fly ash extracts, which contain up to 16 interfering metal ions in high concentrations and a chloride content of 60 g L−1. Overcoming that, Cu(II) was selectively enriched and separated from real fly ash extract at pH 3.0. At pH 1.0, a PEI-free Cu(II) concentrate was obtained and PEIs could be regenerated for reuse in further separation cycles. The PAUF conditions developed at laboratory scale were successfully transferred to pilot scale, and hyperbranched PEI (HB-PEI) was found to be the most suitable reagent for PAUF in a technical scale. Moreover, HB-PEI enables photometric control of the Cu(II) enrichment.

Sorption of rare-earth elements onto a ligand-associated media for pH-dependent extraction and recovery of critical materials

Toward recovering rare-earth elements, we prepared a solid-phase media that binds and elutes metals for extraction and enrichment. We report the synthesis and methodology for a solid-phase media that exhibits pH-dependent binding of aqueous rare-earth elements. The media shows pH-dependent binding of NdIII and retains efficiency over at least six cycles of binding and elution. Mixed solutions of rare-earth elements demonstrated a preference for mid and heavy rare-earth elements based on thermodynamic binding preferences of the ligand, and selectivity for rare-earth elements over iron and aluminum were observed with coal fly ash leachate. We expect this new recovery method to be a significant step towards aqueous-based extraction and enrichment of critically important rare-earth elements.

Evaluation of chemical stabilisation methods of coal-petcoke fly ash to reduce the mobility of Mo and Ni against environmental concerns

Reducing the potential leaching of Mo and Ni from the fly ash (FA) of petroleum coke is an increasingly important issue as Asia and Europe’s demand is expected to drastically intensify as continuing urbanisation and technological innovation demands ever more electricity. In the present study, we investigated coal combustion products (CCP) from a large coal-fired power station fed with a 56:44 coal/petroleum coke blend. Results revealed that leachable concentrations of Mo and Ni from FA were in the upper non-hazardous limit and in the inert limit, respectively (2003/33/EC). Whilst common prevention measures for Mo and Ni based on the adsorption capacity of boiler slag (BS), a mixture of BS: goethite, and jarosite, were considered insufficient to reduce the potential leaching of Mo into FA leachates, a novel chemical stabilisation method based on an aggregate product of portlandite and FA immobilised both Mo and Ni such that the resulting concentrations were below the limits established in the abovementioned 2003 EC Decision. Precipitation may be responsible for the fixation of Mo and Ni in the FA: portlandite aggregates as Ca(MoO4) and NiMoO4, respectively. The findings of this novel study support the use of this aggregate to reduce FA pollutants, which will be of particular interest to nations that remain largely coal/petroleum coke-dependant.