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Coal fly ash (CFA) -an industrial solid waste- has tremendous potential to be used as a starting material for development of valuable porous catalysts and adsorbents because of its silicon and aluminum content. Among various products fabricated from CFA by chemical synthesis process, CFA-based porous catalysts have recently gained remarkable interest among researchers. Each CFA–based catalyst has different properties, the most important of which is the ion exchange capability that depends on the chemical composition and structure of the synthesized product. Studies proved that CFA-based compounds can be used as catalysts/photocatalysts in different environmental processes such as degradation of pollutants. Chemical conversion reactions and synthesis of fine chemicals are among other applications, in which CFA is used as substrate for developing different catalysts. In this review paper, CFA-based catalysts have been classified based on their properties and applications. Methods of characterizations including kinetics and isotherm models are discussed. Furthermore, the effect of several parameters including reaction time, reaction temperature, and the ratio of active compounds to CFA substrate on chemical reactions catalyzed by CFA-based catalysts are discussed. This review paper reveals that CFA-based catalysts are highly efficient compounds not only for application in environmental pollution remediation processes, but also in achieving comparable results in chemical conversion reactions for synthesizing fine chemicals. It can be concluded that CFA as a solid waste should be considered as a low-cost source of alumino-silicate that is a promising candidate for developing inexpensive methods of manufacturing highly efficient and eco-friendly porous catalysts for a wide array of applications. [ABSTRACT FROM AUTHOR]

This paper addresses the detailed characterization of coal fly ashes with respect to their utilization in alkali-activated cement systems, thus maximizing the use of fly ashes in the construction industry. A technique was developed to estimate the reactivity of low calcium fly ashes in alkali-activated systems. The technique is based on a K-value which combines three characteristics of a fly ash. Two characteristics – amorphous phase percentage and specific surface area determined through Blaine measurement – are observed quantities. The third characteristic – degree of polymerization of silica in the amorphous phase of fly ash – is a calculated parameter. To take into account the water demand of fly ashes, which will influence the amount of water needed to get a workable mix, the shape factor was used to adjust the Blaine specific surface area. The relationship between the proposed K-value and compressive strength of alkali-activated fly ash pastes is approximated by a linear function. The correlation coefficient of the relationship varied from 0.961 to 0.833 for 1 and 91 days compressive strength respectively. The proposed K-value can firstly be used to rank fly ashes for their suitability to produce high strength alkali-activated materials and secondly when calibrated for a specific activator and curing conditions, to predict the compressive strength of alkali-activated fly ash binders. [ABSTRACT FROM AUTHOR]

In order to evaluate the ability of granulated coal ash (GCA), a byproduct of coal thermal electric power stations, to remove hydrogen sulfide from organically enriched sediments, a pilot study was carried out at oyster farming sites, where sediments were enriched with oyster feces and dead oysters. Concentration of hydrogen sulfide in the interstitial water of the sediment decreased to nearly zero in both experimental sites, whereas it remained over 0.2 mg/l in the control site. Concentration of acid volatile sulfide (AVS) in the sediment also decreased significantly in both experimental sites, while remained over 0.4 mg/g in the control site. Increases were observed in both the number of benthic microalgae species and the individual number of benthic animals in the surface sediments. This may have been due to the decrease in hydrogen sulfide. [ABSTRACT FROM AUTHOR]

Selenium (Se) is a bioaccumulative constituent of coal fly-ash that can disrupt reproduction of oviparous wildlife. In food webs, the greatest enrichment of Se occurs at the lowest trophic levels, making it readily bioavailable to higher consumers. However, subsequent enrichment at higher trophic levels is less pronounced, leading to mixed tendencies for Se to biomagnify. We used stable isotopes (1 5N and 13C) in claws to infer relative trophic positions and relative carbon sources, respectively, of seven turtle species near the site of a recently-remediated coal fly-ash spill. We then tested whether Se concentrations differed with relative trophic position or relative carbon source. We did not observe a strong relationship between δ15N and Se concentration. Instead, selenium concentrations decreased with increasing δ13C among species. Therefore, in an assemblage of closely-related aquatic vertebrates, relative carbon source was a better predictor of Se bioaccumulation than was relative trophic position. [Copyright &y& Elsevier]

Coal fly ash (CFA) is a significant environmental pollutant that presents a respiratory hazard when airborne. Although previous studies have identified the mineral components of CFA, there is a paucity of information on the structural habits of these minerals. Samples from UK, Polish and Chinese power stations were studied to further our understanding of the factors that affect CFA geochemistry and mineralogy. ICP-MS, FE-SEM/EDX, XRD, and laser diffraction were used to study physicochemical characteristics. Analysis revealed important differences in the elemental compositions and particle size distributions of samples between sites. Microscopy of HF acid-etched CFA revealed the mullite present possesses a fibrous habit; fibres ranged in length between 1 and 10 μm. Respirable particles (<10 μm) were frequently observed to contain fibrous mullite. We propose that the biopersistence of these refractory fibres in the lung environment could be contributing towards chronic lung diseases seen in communities and individuals continually exposed to high levels of CFA. [Copyright &y& Elsevier]

We investigated the abatement of indoor pollution achieved when two types of coal–biomass briquettes (L-BBs and H-BBs) were used in place of honeycombed coal briquettes (H-coal) in household stoves in rural Chongqing, China. Indoor concentrations of sulfur dioxide (SO2), carbon monoxide (CO), and gaseous fluoride were measured. Additionally, we evaluated the factors that affected indoor concentrations of these gases, including the amount of fuel used as well as its sulfur content, the sulfur-emission ratio determined from the amount of sulfur retained in the combustion ash, and the combustion temperature in the stoves. The average 8-h and 24-h SO2 concentrations for L-BBs were nearly equal to or less than the World Health Organization''s 40ppb guideline, whereas those for H-coal and H-BBs exceeded the guideline. The average 8-h SO2 concentrations for L-BBs were from 63 to 89% lower than those for H-coal, even though the 8-h average weight of fuel and its sulfur content for L-BBs were equal to those of H-coal. A chemical analysis of combustion ash indicated that the sulfur-emission ratio was from 26 to 48% for L-BBs, as compared with 86% for H-coal, and this difference resulted in reduction of indoor SO2 concentrations for L-BBs as compared with H-coal. Most of the 8-h average concentrations of CO and gaseous fluoride for all fuels were lower than the WHO guidelines. We concluded that BBs are a useful domestic fuel for the abatement of indoor air pollution. [Copyright &y& Elsevier]

Glass, glass–ceramic and ceramic materials were produced from thermal power plant fly ash without any additives. X-ray diffraction (XRD) analysis revealed the amorphous phase of the glass sample. Augite phase was detected in the glass–ceramic sample, while the enstatite and mullite phases occurred in the ceramic samples. Scanning electron microscopy (SEM) investigations showed that tiny crystallites homogeneously dispersed in the microstructure of the glass–ceramic sample and elongated crystals formed in the ceramic samples. Density values of the obtained samples are comparable to those of the commercially produced glass, glass–ceramic and ceramic samples. Toxicity characteristic leaching procedure (TCLP) results indicated that the produced samples could be taken as non-hazardous materials. Produced samples showed high resistance to alkali solutions in contrast to acidic solutions. Microstructural, physical, chemical and mechanical properties of the produced glass–ceramic samples are better than those of the produced glass and ceramic samples. [Copyright &y& Elsevier]

The solid by-product from power plant fueled with municipal solid waste and coal was used as a raw material to synthesize zeolite by fusion-hydrothermal process in order to effectively use this type of waste material. The effects of treatment conditions, including NaOH/ash ratio, operating temperature and hydrothermal reaction time, were investigated, and the product was applied to simulated wastewater treatment. The optimal conditions for zeolite X synthesis were: NaOH/ash ratio=1.2:1, fusion temperature=550°C, crystallization time=6–10h and crystallization temperature=90°C. In the synthesis process, it was found that zeolite X tended to transform into zeolite HS when NaOH/ash ratio was 1.8 or higher, crystallization time was 14–18h, operating temperature was 130°C or higher. The CEC value, BET surface area and pore volume for the synthesized product at optimal conditions were 250cmolkg−1, 249m2 g−1 and 0.46cm3 g−1 respectively, higher than coal fly ash based zeolite. Furthermore, when applied to Zn2+ contaminated wastewater treatment, the synthesized product presented larger adsorption capacity and bond energy than coal fly ash based zeolite, and the adsorption isotherm data could be well described by Langmuir and Freundlich isotherm models. These results demonstrated that the special type of co-combustion ash from power plant is suitable for synthesizing high quality zeolite, and the products are suitable for heavy metal removal from wastewater. [Copyright &y& Elsevier]

Abstract: Mercury (Hg) has been known to society and used since ancient times. The metal has drawn considerable attention and concern due to its toxicity, persistence, bioaccumulation and long range transport in the atmosphere after emission from coal-fired utilities as well various other high temperature processes. Coal is an important fuel for the production of heat and electricity and in recent years annual hard coal production has approached a level of around 5000 million metric tonnes (Mt, t=1000kg). Global Hg flows via coal and coal utilization by-products (CUBs) are presented in this paper, which are important in light of the regulations to limit the global emissions of Hg and its cycling as well as its circulation via coal and the CUBs. There are no detailed statistics on the global production and consumption of coal fly ash (FA) and in this study, we have estimated the total global FA production for the year 2003 based on ash content in coals and typical flue gas control technology for pulverized coal combustion. The mode of occurrence and concentration of Hg in coal and coal FA for different countries have been evaluated and presented in this study. The total Hg amount in coals processed worldwide was found to be 1534t in 2003 based on a global average concentration of 0.3mg/kg in coal. In addition, “hidden” flows of Hg through export and import of coal assessed during this study, add up to about 149t. In this study, the economic uses of the FA in different sectors such as cement industry, agriculture, land reclamation, filers for asphalt, plastic and many others have been discussed in details. However, there is not much information on uses of coal FA in the developing countries. In the final part of the paper, a short survey has been focused on a few coal producing countries including Australia, China, EU-states, India, Japan, Korea, Russia, South Africa and South American countries, addressing the status of coal and coal FA use and the fate of the Hg that is mobilized. [Copyright &y& Elsevier]

Aiming at the development of a phosphorus removal technology for waste water, phosphate (PO4 3−) retention behavior of bituminous and lignitous coal ashes was investigated using a batch reactor. Ash samples, including fresh and weathered fly and bottom ashes, were studied for their sorption isotherms and reversibility. Fly ashes had a much higher phosphate retention capacity (4000–30,000mgP/kg) than bottom ashes (15–600mgP/kg). Lignitous coal ashes were more capable of retaining phosphate than bituminous coal ashes. The retention process was largely irreversible, and the irreversibility increased with the increase in the retention capacity. Weathering enlarged the retention capacity of the bituminous bottom ash, but substantially lowered that of the fly ash, likely due to the difference in the weather-induced changes between the fly and bottom ashes. Sorption isotherms of fly ashes were found to be adequately represented by the Langmuir model while those of bottom ashes fitted better to the Freundlich model. Concentrations of Ca2+ and PO4 3− in the aqueous phase were measured at the end of sorption and desorption experiments, and were compared with solubilities of three calcium phosphate minerals. The aqueous solutions were saturated or super-saturated with respect to tricalcium phosphate (Ca3(PO4)2) and hydroxyapatite (Ca5(PO4)3OH), and slightly under-saturated with respect to amorphous calcium phosphate. It is concluded that precipitation of calcium phosphate is the predominant mechanism for phosphate retention by coal ash under the conditions studied. There is a strong and positive correlation between alkalinity and phosphate sorption capacity. Consequently, acid neutralization capacity (ANC) can be used as an indicator of phosphate sorption capacity of coal ashes. [Copyright &y& Elsevier]

When recycling bottom ash from municipal solid waste incinerators (MSWIs), salts and heavy metals contents must be considered; in particular, chloride ions must be addressed because they cause serious corrosion in metals. Therefore, only limited amounts of bottom ash can be utilized as a substitution for material or the bottom ash must be treated at high temperatures prior to use. These factors markedly decrease the applications of bottom ash. In addition to the distribution characteristics of chloride ions, this study also investigates the characteristics change before and after de-chlorination using a counter-flow pipe column and three different flow fluxes for different refuse incinerators as the experiment variables. Thus, this study attempts to determine the appropriate conditions for de-chlorination and an appropriate policy for use of bottom ash as concrete aggregate. The experimental results show that a negative correlation exists between the natural logarithm of the chloride ion concentration and particle size in bottom ash. Characteristics of de-chlorinated bottom ash, such as pH value, mud content, loss on ignition, chloride ion concentration, turbidity, and species intensity, all decrease, meaning that de-chlorination decreased chloride ion content and generates a cleaning effect. The per-unit-time efficiency of de-chlorination is highest in the high flux flow. When flow flux is 80mL/min, the de-chlorination efficiency is >0.3%/h. However, the shortest time required for bottom ash de-chlorination does not reduce in proportion to the legally prescribed concentration of chloride ion. [Copyright &y& Elsevier]

The disposal of fly-ash (FA) from coal-fired power stations causes significant economic and environmental problems. Use of such contaminated sites for crop production and use of contaminated water for irrigation not only decreases crop productivity but also poses health hazards to humans due to accumulation of toxic metals in edible grains. In the present investigation, three rice cultivars viz., Saryu-52, Sabha-5204, and Pant-4 were grown in garden soil (GS, control) and various amendments (10%, 25%, 50%, 75% and 100%) of FA for a period of 90 days and effect on growth and productivity of plant was evaluated vis-a-vis metal accumulation in the plants. The toxicity of FA at higher concentration (⩾50%) was reflected by the reduction in photosynthetic pigments, protein and growth parameters viz., plant height, root biomass, number of tillers, grain and straw weight. However, at lower concentrations (10–25%), FA enhanced growth of the plants as evident by the increase of studied growth parameters. The cysteine and non-protein thiol (NP-SH) content showed increase in their levels up to 100% FA as compared to control, however, maximum content was found at 25% FA in Saryu-52 and Pant-4 and at 50% FA in Sabha-5204. Accumulation of Fe, Si, Cu, Zn, Mn, Ni, Cd and As was investigated in roots, leaves and seeds of the plants. Fe accumulation was maximum in all the parts of plant followed by Si and both showed more translocation to leaves while Mn, Zn, Cu, Ni and Cd showed lower accumulation and most of the metal was confined to roots in all the three cultivars. As was accumulated only in leaves and was not found to be in detectable levels in roots and seeds. The metal accumulation order in three rice cultivars was Fe>Si>Mn>Zn>Ni>Cu>Cd>As in all the plant parts. The results showed that rice varieties Saryu-52 and Sabha-5204 were more tolerant and could show improved growth and yield in lower FA application doses as compared to Pant-4. Thus, Sabha-5204 and Saryu-52 are found suitable for cultivation in FA amended agricultural soils for better crop yields. [Copyright &y& Elsevier]