Throughout the world, coal is responsible for generating approximately 38% of power. Coal ash, a waste product, generated from the combustion of coal, consists of fly ash, bottom ash, boiler slag, ...and flue gas desulfurization material. Fly ash, which is the main component of coal ash, is composed of spherical particulate matter with diameters that range from 0.1 μm to >100 μm. Fly ash is predominately composed of silica, aluminum, iron, calcium, and oxygen, but the particles may also contain heavy metals such as arsenic and lead at trace levels. Most nations throughout the world do not consider fly ash a hazardous waste and therefore regulations on its disposal and storage are lacking. Fly ash that is not beneficially reused in products such as concrete is stored in landfills and surface impoundments. Fugitive dust emissions and leaching of metals into groundwater from landfills and surface impoundments may put people at risk for exposure. There are limited epidemiological studies regarding the health effects of fly ash exposure. In this article, the authors provide an overview of fly ash, its chemical composition, the regulations from nations generating the greatest amount of fly ash, and epidemiological evidence regarding the health impacts associated with exposure to fly ash.
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•Integrations of K2O/Fe2O3 with amorphous Al2O3 inhibit its interactions with NH4+.•Water-leaching efficiencies of U and Sr are improved via (NH4)2SO4-based treatment.•Hydrolysis of ...Fe and Ti ions stimulates the formation of coprecipitation at > 60℃.•Coprecipitation can be dissolved under 0.1 M H2SO4 with the release of V and Ga.
Coal ash (CA) becomes the most significant industrial solid waste and attracts much attention due to its potential environmental risk and reuse as the supplementary material. In this study, experiments were conducted to investigate the mode of occurrence and the leaching behavior of valuable trace metals (U, V, and Ga) from CA and (NH4)2SO4-treated CA (NCA), based on the recovery of aluminum. Integrations of Fe- and K-oxide with Si-Al glass increased the ash strength and obstructed the activation of NH4+ on amorphous Al-bearing phases, resulting in a limited improvement in the leaching efficiency of trace metals. On the other side, a higher liquidus temperature, contributing to the dissolutions of Al3+ and Ca2+, could promote the leaching of U from NCA as well, whereas the water-leaching behaviors of V and Ga involved a sophisticated trend with temperature > 40℃. Water-leached V/Ga tended to transfer into Fe-Mn oxide-bound and residual V/Ga owing to the noticeable hydrolysis of Fe and Ti ions that facilitated the formation of coprecipitation. However, 0.1 M H2SO4 could re-dissolve that coprecipitation, and thus leaching efficiencies of U, V, and Ga were 1.9, 1.3, and 5.0 times higher than those by directly leaching CA, respectively.
Owing to the growing volumes of ash and sewage sludge waste, there is a requirement for theoretical and practical research into the use of these wastes as a source of nutrients. However, there are ...relatively few studies on the transfer of macronutrients in soil-plant systems amended with ash-sewage sludge mixtures under field conditions. The aim of the study was to determine the effect of bituminous coal ash (AC), biomass ash (AB), and municipal sewage sludge (MSS) on the quantity and quality of a grass-legume mixture. During a 6 year field experiment on a sandy loam soil treated with the wastes, applied as mixtures or separately, the plant yield; N, P, K, Na, Mg, and Ca uptake by plants; macronutrient content and ratios in the plant biomass; and the recovery rate of macronutrients by plants were evaluated. The AB-MSS treatment increased the yield in comparison to that where the wastes were applied separately. The N, P, and Ca contents in the plant biomass and N and P uptake under ash-sludge treatments were in the range observed for the ash and sewage sludge. The AB-MSS co-application resulted in the highest K uptake. The AC-MSS treatment increased K and Mg uptake in relation to AC treatment. When AC or AB was added to the MSS, the Ca uptake increased relative to the MSS treatment. The plant biomass under the AB treatment was optimal for biofuel purposes in terms of the chemical composition. The co-application of AC or AB with MSS resulted in the optimum Ca:Mg ratio for fodder purposes. The recovery rate of the macroelements decreased in the following order: K, N, P, Mg, Na, and Ca. The results support the co-application of solid wastes such as ash and municipal sewage sludge to improve productivity, support the recycling of macronutrients, improve sustainability through the reduction of ash and sewage sludge disposal, and reduce reliance on mineral fertilizer.
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•We examined ash and sewage sludge as a source of macronutrients.•Macronutrient cycling was assessed during a 6 year field experiment.•We evaluated plant yield, macronutrient content and ratios in plant biomass.•Uptake and recovery rate of macronutrients by plants were also examined.•Results can promote co-application of ash and municipal sewage sludge.
Coal fly ash (CFA) is an ideal source for the preparation of heterogeneous catalysts due to its abundant silicon and aluminum oxides, but its activity needs to be improved. In this study, a green and ...moderate approach for CFA activation was proposed, and a series of CFA-based catalysts were prepared for NO selective catalytic reduction (SCR). The results indicated that CFA could be well activated via mechanochemical activation with 3 h of milling duration in 1 mol/L of acetic acid, and 90% of NO removal was achieved over the CFA-based catalyst in 250 to 375 °C. Two activating mechanisms, i.e., the enhanced CFA fragmentation and the motivated Al dissolution, were revealed during the mechanochemical activation. The former facilitated the formation of mesopores and the exposure of Fe components in CFA fragments, which enhanced the capacity of oxygen storage over the as-activated catalyst. The latter motivated the formation of Si-OH groups, which promoted the migration of electrons and the dispersion of V species, thereby increasing the capacity of NH
adsorption over the as-obtained catalyst. Therefore, the performance of NO reduction was improved. The proposed activating approach could be a promising integration for CFA disposal and NO removal from inside coal-fired power plants.
•Heavy metals in Tea Factory Coal Ash reduced due to vermicomposting.•Compost quality and earthworm reproduction was satisfactory.•Metal exposure increased metallothionein content in earthworm ...intestine.•Gut metal content was higher in Eisenia fetida than Lampito mauritii.
Earthworms can accumulate heavy metals in their intestines to a great extent. Impact of feed materials and duration of metal exposure on natural activity of earthworms are rather unclear; this investigation therefore addresses the impact of metal rich Tea Factory Coal Ash (TFCA) on reproduction, composting and metal accumulation ability of Eisenia fetida and Lampito mauritii. Earthworm count and cocoon production increased significantly during vermicomposting. pH of the vermicomposted mixtures shifted toward neutrality, total organic C decreased substantially and total N enhanced significantly compared to composting. High heavy metal (Mn, Zn, Cu, As) accumulation was recorded in the intestine of both the earthworm species. Moreover, gradual increase in the metal-inducible metallothionein concentration indicated the causal mechanism of metal accumulation in these species. TFCA+cow dung (CD) (1:1) were most favorable feed mixture for E. fetida and TFCA+CD (1:2) were good for L. mauritii in regard to metal accumulation and compost quality.
•Efficient and economic production process can mitigate criticality of germanium.•US germanium resources remain unused due to inefficiency in production.•Production processes including leaching, ...precipitation, and separation are reviewed.•Separation includes precipitation, solvent-extraction, ion-exchange, and adsorption.•Ge chemistry differs from other cations as it remains as neutral species in acidic pH.
Germanium (Ge) is one of the critical elements of modern technologies, with supply risk, inefficient production, and increased demand. It is used in high technology applications such as infrared systems, fiber optics, polymer catalysis, electronics, and solar cells. Its demand is expected to increase due to lack of suitable substitutes, increasing demand for solar cells and 5G networks, and the continuous increasing trend of Ge demand for the past 20 years. Globally, 130 tonnes (t) of Ge are being produced (2020) primarily in China. Germanium is recovered as a byproduct from Zn-refineries and coal fly ash. Yet very low amount (<3%) of Ge contained in Zn-ores and fly ash is recovered worldwide, which suggests that the criticality of Ge is due to lack of economical and efficient extraction and recovery process rather than lack of resources. Worldwide, it is estimated that Zn-ores contain 7–13 kt Ge, and coal and coal fly ash, 25–112 kt. The production of Ge involves leaching followed by recovery of Ge by chlorination-distillation, precipitation, solvent extraction, ion flotation, supported liquid membranes, ion exchange, or solid-phase extraction. Leaching from Zn-refinery residues is primarily done with mineral acids, whereas from coal fly ash is achieved with organic ligands. Ge is different from common metal cations, as it exists as neutral species in acidic pH (1–7). Thus, organic ligands are used to form anionic germanium-ligand complexes in solvent extraction and ion-exchange processes so that the species can be separated through anion-exchange. Alternatively, chelating solvent extractants, chelating ion-exchange resins, and ligand functionalized adsorbents in solid-phase extraction (SPE) are used to recover Ge. The primary challenges in Ge hydrometallurgical production are high energy and chemical requirements in precipitation/chlorination-distillation, high chemical consumption, impurities co-extraction, and waste stream generation in solvent extraction, and lack of selective and high-capacity adsorbents in SPE. An improved, efficient, and economical production method, utilizing unconventional resources, can reduce Ge supply risk. SPE has various advantages over other processes, which makes it an attractive option to recover Ge efficiently and economically.
•We examined characteristics of heavy metals in different particle sizes of fly ash.•Volatile metals were enriched in fine particles mainly as F1 and F2 speciation.•Non-volatile metals existed mainly ...as F3 and F4 speciation.•Cl had the same enrichment pattern as volatile metal and Ca had the opposite trend.•Fine particle size of fly ash had higher carcinogenic risk for onsite workers.
During the pretreatment and recycling processes, the re-suspended dust from municipal solid waste incinerator (MSWI) fly ash might pose a significant health risk to onsite workers due to its toxic heavy metal content. In this work, the morphological and mineralogical characteristics of fly ash in different particle sizes are presented. The concentrations of seven trace elements (Zn, Pb, Cu, Cd, Cr, Fe and Mn) in these samples were determined. The results show that volatile metals, such as Zn, Pb, Cu and Cd, were easily concentrated in the fine particles, especially in Dp2.5-1 and Dp1, with soluble and exchangeable substances as the main chemical species. The health risk assessment illustrated that the cumulative hazard indexes for non-carcinogenic metals in Dp10-5, Dp5-2.5, Dp2.5-1, and Dp1 were 1.69, 1.41, 1.78 and 2.64, respectively, which were higher than the acceptable threshold values (1.0). The cumulative carcinogenic risk was also higher than the threshold value (10−6). For the onsite workers, the relatively apparent non-carcinogenic and carcinogenic effects were from Pb and Cr, respectively. The above findings suggest that fine-grained fly ash contained a considerable amount of heavy metals and exhibited a great health risk.
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•Physico-chemical properties of CFA were summarised in detail.•Leachability of CFA was analysed using six aspects.•Adverse effects of CFA on humans, wild animals, and plants were ...reviewed.•Utilisation of CFA in the USA and EU were summarised.•Directions for the reuse of CFA in the near future were proposed.
Coal fly ash (CFA) is a valuable industrial solid waste, but conventional methods used for its disposal can lead to serious and long-lasting environmental issues. The study of technologies for CFA recycling has been of major concern, while the harm caused by CFA is only partially understood, limiting its reuse. In this review, the basic physico-chemical properties of CFA are introduced, followed by a systematic summary and discussion of the leachability of CFA via different leaching methods and the chemical speciation of some typical metal elements in CFA, which is related to its harmful effects. The specific harm that CFA causes to humans, wild animals, and plants and the study status of magnetic property of CFA are presented. Because of the pervasive concerns of many people, the utilisation of CFA in the USA and Europe and an economic and environmental analysis of its disposal is provided and discussed. Finally, some possible directions for future research involving CFA are proposed.
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•Treated wastewater effectively reduces leachable inorganics in IBA for open reuse.•Multi-step washing and screening align IBA properties with environmental standards.•Treatment ...reduces chloride contamination in IBA fractions by 83–89 %.•Sulfate contamination in IBA fractions decreases by 65–74 % after treatment.•Handling of fines and water generated after treatment processes is crucial.
The current study introduces an innovative methodology by utilizing treated wastewater (TWW) from an effluent treatment plant as a washing agent to enhance the characteristics of incineration bottom ash (IBA). This approach addresses sustainability concerns and promotes the circular economy by reusing wastewater generated in municipal solid waste incineration facilities. Previous research has underscored the challenges of open IBA reuse due to elevated leaching of chlorides, sulfates, and trace metal(loid)s. Thus, the experimental setup explores various combinations of washing, with or without screening, to optimize the properties of soil-like material (SLM < 4.75 mm) and overall material (OM < 31.5 mm) fractions of IBA for unrestricted applications. Batch leaching tests were conducted on treated samples, and leaching characteristics were evaluated in accordance with regulatory standards, primarily the Dutch standard for unrestricted IBA reuse. The findings reveal that washing in isolation proves insufficient to enhance IBA properties; however, washing followed by screening, specifically for removing fines (<0.15 mm), proves effective in reducing contamination. The study identifies that multiple steps of washing and screening (with recirculation) process render OM and SLM fractions suitable for unrestricted reuse with a cumulative liquid-to-solid ratio of 6 L/kg and a total washing time of 15 min. The multi-step treatment was found effective in reducing sulfate contamination by 65–74 % and chloride contamination by 83–89 % in IBA fractions. This approach offers a promising solution for overcoming the limitations associated with IBA leaching, thereby promoting sustainable waste reuse practices.
Current concerns about lack of diversity in supply of critical metals have spurred research into utilizing domestic sources, particularly from waste streams. Sustainability strategies like urban ...mining, industrial symbiosis, and the circular economy suggest avenues to realize new supplies of critical metals. In this work we explore the resource and economic potential for extracting rare earth elements (REEs) from industry byproducts (e.g. coal combustion products, red mud) and secondary sources (e.g. waste electronics and light bulbs). Combining materials flow analysis and characterization data, we find that while REE concentrations in waste and byproduct streams are mostly much lower than current REE ores, some secondary sources are richer than ores in high value REEs such as scandium. The quantities of REEs contained in secondary sources could meet current global demand even with low extraction yield rates. Phosphogypsum, coal ash and red mud from aluminum production stand out as promising candidates for recovery due to high concentrations of valuable REEs and sufficient quantities to potentially meet demand. Processes to extract REEs from secondary sources are under development, it is not yet clear which will be profitable at scale and which can be achieved at least environmental impact. This work provides high level guidance on the potential of secondary sources by characterizing quality (concentrations of different rare earths) and quantity (mass of rare earths in global scale wastes and byproducts). This significant first step helps clarify directions for policy and research and development investments.
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