Cr In Fly Ash Research Articles

Concentrations, Speciation, and Potential Release of Hazardous Heavy Metals from the Solid Combustion Residues of Coal-Fired Power Plants.

Hazardous heavy metal-laden coal combustion byproducts exposed to precipitation or surface/groundwater are of environmental concern. This study analyzed fly ash (FA) and desulfurization gypsum (FGD gypsum) samples obtained from 16 coal-fired power plants in Guizhou Province, China. A combination of field and laboratory studies was used to investigate the binding forms of lead (Pb), cadmium (Cd), and chromium (Cr) and their leaching characteristics under natural storage conditions. The results showed that there were significant proportions of residual states of these elements in FA (84–99% for Pb, 83–91% for Cd, and 73–97% for Cr), indicating a lack of migration to other environmental media. FGD gypsum contained high proportions of metals in migratable states, but the environmental risks were low due to their very low concentrations. The release of Pb, Cd, and Cr from FA and FGD gypsum into extracts varied according to pH. This is related to the form of each element in the solid and the series of reactions that occurs during the leaching process. However, aside from Cr in FA, all heavy metals in FA and FGD gypsum samples were present in concentrations below the relevant standards for landfill leachate, indicating very low release rates. The Cr levels (206–273 μg/L) in some of the FA extracts were higher than the limits for water pollution from domestic landfill, indicating that Cr in FA poses a leaching risk. The results of field leachate sampling and indoor simulated rainfall experiments further validated these findings, indicating that the release of Cr from stockpiled coal FA is a cause for concern.

Speciation analysis of Hg, As, Pb, Cd, and Cr in fly ash at different ESP’s hoppers

In the power plants, the particle size of fly ash was gradually decreased from the front row to the last row of the hopper. The high mercury captured rate was shown with the high unburned carbon content in the front row of the hopper. 80% of mercury captured in the front hopper was in the residual fraction and the rest was in the oxidizable fraction. The other four trace elements tended to be captured more with the smaller particle size of fly ash in the latter hoppers. Above 90% of lead in fly ash at different hoppers was in the speciation of the residual fraction. This indicated that the lead was the less tendency to be leached out during landfill. The speciation of acid-soluble, reducible, and oxidizable fraction for As, Cr and Cd was significantly increased in the fly ash at different hoppers. This indicated that the combustion technology in each power plant had a significantly impact on the speciation of these trace elements in different hoppers. Thus, the usage of the separation technology method to better understand the behavior of trace elements in the fly ash to avoid fly ash’s disposal through increasing the beneficial use of fly ash.

Effect of Source-Classified Collection of Municipal Solid Waste on Heavy Metals and Pozzolanic Properties of Incineration Residues

The adoption of source-classified collection of municipal solid waste (MSW) systems has improved waste management and resource conservation in developing countries, including China. Here, we evaluated the effects of a new MSW source-classified collection system on the distribution and leaching of heavy metals in incineration residues as well as on the pozzolanic properties of residues used as additives in cement production. Cu, Cr, and Mn were found primarily in bottom ash, while the volatile metals Zn, Pb, and Cd became enriched primarily in fly ash during combustion. However, effective presorting of MSW prior to incineration reduced the heavy metal content of the MSW source material and increased the combustion efficiency and heat values of MSW. The system of source-classified collection of MSW resulted in decreased concentrations of heavy metals in bottom ash, as well as decreased leachate toxicity, which improved the viability of bottom ash for use as a non-hazardous raw material in cement production. The consistently high temperatures sustained during incineration of source-classified MSW increased the concentrations and leachate toxicity of Cd, Pb, Cu, Zn, and Cr in fly ash. Pozzolanic activity testing revealed that bottom ash may be used as a pozzolanic material in cement production, but that fly ash is not suitable. In contrast to bottom ash from the incineration of mixed MSW, bottom ash from the incineration of source-classified MSW fully satisfies China’s “Pozzolanic materials used for cement production” standards due to its low loss on ignition, high grindability, and the desired rupture strength, compressive strength, and stability that it adds to mortar mixtures.

Biodiversity variability and metal accumulation strategies in plants spontaneously inhibiting fly ash lagoon, India.

Out of 29 plant species taken into consideration for biodiversity investigations, the present study screened out Cyperus rotundus L., Calotropis procera (Aiton) W.T. Aiton, Croton bonplandianus Baill., Eclipta prostrata (L.) L., and Vernonia cinerea (L.) Less. as the most suitable metal-tolerant plant species (high relative density and frequency) which can grow on metal-laden fly ash (FA) lagoon. Total (aqua-regia), residual (HNO3) and plant available (CaCl2) metal concentrations were assessed for the clean-up of metal-contaminated FA disposal site using naturally colonized plants. The total metal concentration (in mg kg-1) in FA followed an order of Mn (229.8)>Ni (228.4)>Zn (89.4)>Cr (61.2)>Pb (56.6)>Cu (51.5)>Co (41.9)>Cd (9.7). The HNO3- and CaCl2-extracted metals were 0.57-15.68% and 0.03-7.82% of the total metal concentration, respectively. The concentration of Ni and Cr in FA in the present study was highest among the previously studied Indian and average world power plants and Cd, Ni, and Cr were above soil toxicity limit. The variation in total, residual, and plant-available metal (single extraction) concentration indicated the presence of different proportions of metals in FA lagoon which affects the metal uptake potential of the vegetation growing on it. It has been reported that plant-available metal extractant (CaCl2) is the most suitable extractant for assessment of metal transfer from soil to plant. However in the present study, Spearman's correlation showed best significant correlation between total metal concentration in FA and shoot metal concentration (r=0.840; p<0.01) which suggest aqua-regia as the best extractant for understanding the bioavailability and transfer of metal, and in calculation of BCF for moderately contaminated site. It can be stated that plant-available extractant is not always suitable for understanding the availability of metal, but total metal concentration can provide a better insight especially for moderate or low metal-contaminated sites. Principle component analysis revealed that all the plants showed positive correlation with Co and Cd which suggest its subsequent uptake in root and shoot. The biological indices (BCF, BAF, and TF) revealed that E. prostrata (10mg Cd kg-1) and C. procera (3.5mg Cd kg-1) can be utilized efficiently for the phytoextraction of Cd and phytostabilization of other potentially toxic metals (Pb, Cr, and Co) from FA lagoon. All the plants were tolerant to Pb pollution (TF>1, BAF>1, and BCF>1); hence, there was a negligible translocation of Pb to the aerial tissues of these plants which shows their suitability in phytostabilization. In addition, V. cinerea accumulated elevated concentration of potentially toxic Cr (50mg Cr kg-1) and Ni (67mg Ni kg-1) which could also help in the phytoremediation of FA lagoon.

Silicon-aluminum additives assisted hydrothermal process for stabilization of heavy metals in fly ash from MSW incineration

Hydrothermal treatment (HT), as a novel promising technology, still could not make fly ash (FA) from municipal solid waste incineration (MSWI) meet the standard of non-hazardous waste, due to the unsuccessful formation of aluminosilicate minerals caused by the lack of Si and Al in FA. In this research the mechanism of zeolites formation and stabilization/migration of heavy metals during hydrothermal process affected by external addition of coal fly ash (CFA), bentonite, and kaolin as silicon-aluminum source, were studied. Tobermorite could be synthesized, attributing to the proper element proportion of Ca/(Si+Al) adjusted by 30% of additives addition. Heavy metals were detected on the surface of tobermorite crystalline. The leaching toxicity of Pb, Zn, Cu, Cd, Cr in FA were significantly reduced after HT, associated with the transformation into the residue fraction. The decrease of heavy metals in liquid phase and the decrease of leaching toxicity in solid phase were observed, indicating that heavy metals were really stabilized in the treated FA, rather than transferred into liquid. Higher reaction temperature could promote the formation of tobermorite, but increase the leaching toxicity and migration of heavy metals. Addition of 30% of CFA and temperature of 150°C would be the recommended technological parameters.

Heavy metal speciation in various types of fly ash from municipal solid waste incinerator

Fly ash (FA) from municipal solid waste incinerators has been known as hazardous waste, which is mostly because of the high content of heavy metal and dioxins. Besides the content, the form of the heavy metals in fly ash is also very important, because it is tightly related with the leaching behavior of fly ash in diverse circumstances. To evaluate the environment potential risk of fly ash, the Tessier chemical extraction method was adopted. In this study, four kinds of fly ash were examined, one sample from China (CFA) and the other three from Japan (RFA, CaFA and NaFA). Five metal elements were detected and they were Ni, Cr, Cd, Pb, and Cu. The result of total heavy metals’ concentration showed CFA has the lowest content. As to the Tessier chemical extraction experiments, the results show that Cd, Pb, and Cu have higher leaching risk in the environment than other heavy metals. The result of leaching test experiment showed that the more exchangeable speciation of Cd, Cr and Pb in FA, the more it could leach out in natural environment.

Translocation and toxicity assessment of heavy metals from circulated fluidized-bed combustion of oil shale in Huadian, China

Oil shale and fly ash collected from two thermal power plants located in Huadian, the northeast city of China were subjected to fraction distribution, translocation regularity and toxicity assessment to provide preliminary assessment of suitability for land application. By Tessier sequential extraction, the results showed that Ni, Cr, Pb and Zn were mostly bounded with iron-manganese and organic bound in oil shale, but Cu and Cd were mostly associated with iron-manganese bound and residue fraction. Through circulated fluidized-bed combustion, high concentration of heavy metals (Cu, Cd, Ni, Cr, Pb, and Zn) was found in iron-manganese bound and residue fraction in fly ash. There was accumulation of all studied metals except Ni and Cr in fly ash and translocation mass of metals were as follows: Pb>Zn>Cu>Cd during circulated fluidized-bed combustion. Fly ash was contaminated with Cd higher than the pollution concentration limits listed in GB15168-1995, China. This work demonstrated that it was unadvisable way to carry out landfill without any treatment. By means of STI model, toxicity assessment of heavy metals was carried out to show that there was notable increase in toxicity from oil shale to fly ash.

04/00551 Spectrophotometric determination of trace chromium in fly ash: Xia, C. et al. Guangpuxue Yu Guangpu Fenxi, 2003, 23, (1), 205–206. (In Chinese)

In HNO3 medium the fading reaction of arsenazo III oxidated with Cr2O7(2-) was used as to determine trace Cr in fly ash containing Cr. The results show that the fading reaction has high sensitivity in the medium of 3.2 mol.L-1 nitric acid, the molar absorptivity of 3.9 x 10(6) L.mol-1.cm-1 at 520 nm, and Beer's law is obeyed for Cr (VI) in the range of 0.0-40.0 micrograms.L-1. Contents of Cr in fly ash were determined with satisfactory results.