Emission of hazardous trace elements (HTEs) from energy production is receiving much attention due to concerns about the toxicity to the ecosystem and human health. This study presented new field measurement data on the HTEs partitioning behavior and size-segregated elemental compositions of gaseous particular matter (PM) generated from a commercial circulating fluidized bed (CFB) power plant. Mineralogical and morphological characteristics of combustion ash and PM2.5 (particle diameter less than 2.5 μm) were determined by X-ray diffractometer (XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS). Functional groups alteration during CFB combustion was characterized by Fourier transform infrared spectroscopy (FTIR). The presence of aliphatic hydrogen at 2910 cm−1 and 2847 cm−1 in the PM2.5 suggested that the aliphatic carbon-rich volatiles were absorbed on the fine particles with large surface area. Fine fly ash (PM2.5) occurred as irregular glass particles or/and as unburned carbon. The typical irregular particles were mainly composed of Al-Si-Ca or Al-Si-Fe phases. The enrichment behavior of HTEs was determined for the airborne size-segregated particular matter. Elemental occurrences, combustion temperature, unburnt carbon, and limestone additives during CFB combustion were critical in the transformation behavior of HTEs. The total potentially mobile pollutants that exit the CFB power plant every year were estimated as follows: 0.22 tons of Cr, 0.12 tons of Co, 0.73 tons of Ni, 0.04 tons of As, 0.07 tons of Se, 3.95 kg of Cd, and 3.34 kg of Sb.

This paper investigated the application of graphite particle electrodes to the removal of Zn, Pb, Cu, and Cd from municipal solid waste incineration (MSWI) fly ashes in a three-dimensional (3D) electrokinetic reactor. The influences of the voltage gradient, mass ratio of graphite powers to fly ashes, nitric acid concentrations, proposing times, and liquid-solid (L-M) ratios on the remedial efficiencies of MSWI fly ashes were comprehensively studied in an orthogonal deign and a sequential double-factor setup. Significant analysis showed that changes in the mass ratios and nitric acid concentrations both had a statistically significant effect on the removals of Zn and Pb. Proposing times and L-M ratios both remarkably affected the removals of heavy metals (HMs) in a 3D electrochemical system. The graphite powers had a narrower distribution interval and slightly larger surface areas compared with MSWI fly ashes, which relented pH gradients over the time in the electrochemical experiments and minimized the bubble barricade caused by the hydrolysis. The particle electrode had increased the residue factions of Zn, Pb, Cu, and Cd in S1 region by approximately 216%, 136%, 309%, and 950%, respectively, compared with the raw MSWI fly ashes. The addition of graphite powders to a two-dimensional (2D) electrochemical process strengthened hydrolysis reactions, shortened time for the redistribution of pH balance, decreased the tortuosity of migration path, and increased the desorption concentrations of HMs in the sample area.

Nano-structured zinc sulfide (Nano-ZnS) has been demonstrated to be a promising alternative to activated carbon (AC) for controlling mercury emission from coal combustion flue gas. The ultimate fate of the mercury-laden Nano-ZnS after mercury capture is mostly disposed in landfill with fly ashes. Thus, the stability of mercury adsorbed on the Nano-ZnS is of considerable significance in the secured disposal of fly ash after mercury removal and in the commercial application of the Nano-ZnS sorbent for removal of mercury from coal combustion flue gas. In this work, a modified toxicity characteristic leaching procedure (TCLP) was conducted to evaluate the leachability of mercury on the Nano-ZnS. The effects of leachate pH value, leaching time, liquid-to-solid ratio, and acid rain types on mercury leaching from the mercury-laden Nano-ZnS were systematically investigated. The TCLP results show that the concentration of mercury in leachate was far below the safe limit (200 μg/L) as imposed by the US Environmental Protection Agency (EPA) for classifying a material as a hazardous waste. All the key parameters that generally affected metal leaching rate exhibited slight effect on mercury leaching from the mercury-laden Nano-ZnS. Leaching tests at various highly severe conditions resulted in less than 0.01% mercury leaching from the mercury-laden Nano-ZnS. Sequential selective extraction tests demonstrated that mercury sulfide (HgS) was the dominant adsorption product on the Nano-ZnS, which guaranteed the excellent stability of mercury adsorbed on the Nano-ZnS. Graphic abstract ᅟ

In this study, ash samples were collected from five locations situated in the boiler of a circulating fluidised bed municipal solid waste incinerator (high- and low-temperature superheater, evaporator tubes and upper and lower economiser). These samples represent a huge range of flue gas temperatures and were characterised for their particle size distribution, surface characteristics, elemental composition, chemical forms of carbon and chlorine and distribution of polychlorinated dibenzo-p-dioxins (PCDD), dibenzofurans (PCDF) and biphenyls (PCB). Enrichment of chlorine, one of the main elements of organochlorinated pollutants, and copper, zinc and lead, major catalytic metals for dioxin-like compounds, was observed in lower-temperature ash deposits. The speciation of carbon and chlorine on ash surfaces was established, showing a positive correlation between organic chlorine and oxygen-containing carbon functional groups. The load of PCDD/F and PCB (especially dioxin-like PCB) tends to rise rapidly with falling temperature of flue gas, reaching their highest value in economiser ashes. The formation of PCDD/F congeners through the chlorophenol precursor route apparently was enhanced downstream the boiler. Principal component analysis (PCA) was applied to study the links between the ash characteristics and distribution of chloro-aromatics. The primary purpose of this study is improving the understanding of any links between the characteristics of ash from waste heat systems and its potential to form PCDD/F and PCB. The question is raised whether further characterisation of fly ash may assist to establish a diagnosis of poor plant operation, inclusive the generation, destruction and eventual emission of persistent organic pollutants (POPs).

In this study, the adsorption behavior of waste boiler fly ash has been explored for purification of nickel heavy metal ion bearing water. The raw boiler fly ash (RBFA) was alkali modified to improve the adsorption characteristics. The modified boiler fly ash (MBFA) was characterized using the SEM, XRF, XRD, BET, and TGA analyzers to confirm the improved textural, mineralogical, porosity and thermal characteristics of the adsorbent. The adsorption studies were conducted in a batch mode by varying different operational parameters like pH, contact time, heavy metal ion concentration, and time. The MBFA showed higher adsorption capacity (~ 86 mg/g) as compared to RBFA (~ 64.8 mg/g) at optimized conditions. The equilibrium data for Ni(II) sorption were analyzed using Langmuir, Sips, and Freundlich isotherm models. Sips model proves to be superior with R² = 0.99. Thermodynamics of Ni(II) removal showed that the process of adsorption is endothermic and spontaneous in nature. Enthalpy calculated was 2.95 and 18.65 kJ/mol for RBFA and MBFA, respectively. The adsorption kinetics of Ni(II) by both RBFA and MBFA were modeled using pseudo-first-order, fractional order, and intra-particle diffusion equations. The results indicate that the fractional order kinetic equation and intra-particle diffusion model were suitable to describe the nickel adsorption.

Heavy metals can be serious pollutants of natural water bodies causing health risks to humans and aquatic organisms. The purpose of this study was to investigate the removal of five heavy metals from water by adsorption onto an iron industry blast furnace slag waste (point of zero charge (PZC) pH 6.0; main constituents, Ca and Fe) and a coal industry fly ash waste (PZC 3.0; main constituents, Si and Al). Batch study revealed that rising pH increased the adsorption of all metals with an abrupt increase at pH 4.0–7.0. The Langmuir adsorption maximum for fly ash at pH 6.5 was 3.4–5.1 mg/g with the adsorption capacity for the metals being in the order Pb > Cu > Cd, Zn, Cr. The corresponding values for furnace slag were 4.3 to 5.2 mg/g, and the order of adsorption capacities was Pb, Cu, Cd > Cr > Zn. Fixed-bed column study on furnace slag/sand mixture (1:1 w/w) revealed that the adsorption capacities were generally less in the mixed metal system (1.1–2.1 mg/g) than in the single metal system (3.4–3.5 mg/g). The data for both systems fitted well to the Thomas model, with the adsorption capacity being the highest for Pb and Cu in the single metal system and Pb and Cd in the mixed metal system. Our study showed that fly ash and blast furnace slag are effective low-cost adsorbents for the simultaneous removal of Pb, Cu, Cd, Cr and Zn from water.

Cement solidification is an important pre-treatment technology for municipal solid waste incineration (MSWI) fly ash into landfill. The physicochemical properties and leaching characteristics are the foundation for assessing the long-term stability of the fly ash solidified with benchmark cement in landfills. The leaching performances of bulk components (Na, K, Ca, Cl, CO₃²⁻, and SO₄²⁻) and heavy metals (Cu, Zn, Cr, Pb, and Zn) were analyzed based on the percolation column test and pH dependent test respectively. The research showed that in the cement-solidified fly ash, Na and K were mainly in the form of soluble chloride salts and would be washed out severely at the initial leaching stage due to the weak fixation effect of cement. Moreover, a considerable amount of Ca was washed out simultaneously with Na and K, causing a temporary increase in pH value, and then Ca leaching was controlled by the solubility of minerals, mainly calcium carbonate, ettringite formed with CO₃²⁻ and SO₄²⁻. Cement solidification reduced the cumulative release of mobile Cu, Zn, Cr, Pb, and Cd contained in MSWI fly ash. In the cement-solidified fly ash, the leaching of Cu and Zn was controlled by mineral solubility under alkaline conditions, Cr was dependent on the redox conditions, and Pb was related to the complex structures formed with Si–O bonds of silicates. A further research on the long-term stability of the cement-solidified fly ash in landfills was needed.

Based on this study, the Al₂O₃ content of Jungar coal ash is over 45%, and the resistivity of high-Al₂O₃ ash in Jungar reaches up to 10¹² Ω​·cm. These results seriously influenced the electric characteristics of fly ash, and the collection efficiency of electrostatic precipitators (ESPs) evidently decreased. To facilitate the effective collection of fine particle in the flue gas generated before and after coal blending via ESP, the fly ash obtained from a power plant electrostatic precipitation was analyzed in terms of resistivity, size distribution, and cohesive force through a portable dust electrical resistivity test instrument, Bahco centrifuge, and a cohesive force test apparatus invented by the researchers. The mixed ratio of else coal is higher than 50%, the resistivity of the fly ash in the flue gas was lowered to approximately two orders of magnitude, and the size distribution showed an evident decrease in the PM2.5 and PM10 content in fly ash. In addition, the adhesive force and efficiency increase from 95.9 to 99.5% in the electrostatic precipitation. Therefore, the combustion of blending coal is an effective approach to improve the efficiency of ESP used to collect high-Al₂O₃ fly ash.

In the vulnerable Arctic environment, the impact of especially hazardous wastes can have severe consequences and the reduction and safe handling of these waste types are therefore an important issue. In this study, two groups of heavy metal containing particulate waste materials, municipal solid waste incineration (MSWI) fly and bottom ashes and mine tailings (i.e., residues from the mineral resource industry) from Greenland were screened in order to determine their suitability as secondary resources in clay-based brick production. Small clay discs, containing 20 or 40% of the different particulate waste materials, were fired and material properties and heavy metal leaching tests were conducted before and after firing. Remediation techniques (washing in distilled water and electrodialytical treatment) applied to the fly ash reduced leaching before firing. The mine tailings and bottom ash brick discs obtained satisfactory densities (1669–2007 kg/m³) and open porosities (27.9–39.9%). In contrast, the fly ash brick discs had low densities (1313–1578 kg/m³) and high open porosities (42.1–51. %). However, leaching tests on crushed brick discs revealed that heavy metals generally became more available after firing for all the investigated materials and that further optimisation is therefore necessary prior to incorporation in bricks.

The alkalinity (AKₐₛₕ), BCR sequential extraction method, and principle component analysis (PCA) were adopted to investigate the heavy-metal partitioning and their speciation redistribution in solid phase in ash-melting process. The results indicated that the conversion of Zn-OXI (oxidisable fraction) into Zn-RES (residual fraction) and the decomposition of Cu-OXI fraction were prevailing in solid-phase reaction. Moreover, the alkalinity reduction from AKₐₛₕ = 2.0 to AKₐₛₕ = 1.2 had positive implications for the concentration reduction of As-RED (reducible fraction), Zn-RED, and Pb-RES fractions in slags. The modified synthesis toxicity index (STIM) calculation model was introduced to investigate the potential ecological risk (PEI) of heavy metals in the recycling and utilization of molten slag. Based on STIM calculation model, PEI of heavy metal in hazardous materials was classified into five ranks, such as serious pollution (STIM > 462), heavy pollution (330 < STIM < 462), moderate pollution (132 < STIM < 330), mild pollution (0 < STIM < 132), and no pollution (STIM = 0). The molten slags produced from fly ash can be directly reused as building materials like freestone and ceramics due to the mild PEI.