Uranium speciation in coal bottom ash investigated via X-ray absorption fine structure and X-ray photoelectron spectra.

Similar to chromium contamination, the environmental contamination caused by uranium in radioactive coal bottom ash (CBA) is primarily dependent on the chemical speciation of uranium. However, the relationship between uranium speciation and environmental contamination has not been adequately studied. To determine the relationship between uranium speciation and environmental contamination, X-ray absorption fine structure (XAFS) and X-ray photoelectron spectra (XPS) analyses were performed to determine the uranium speciation in CBA exposed to different chemical environments and simulated natural environments. The leachability of the different forms of uranium in the CBA was studied via a simulated acid rain leaching experiment, and the results showed that 57.0% of the total uranium was leached out as U(VI). The results of a linear combination fit (LCF) of the X-ray absorption near edge structure (XANES) spectrum revealed that in the raw CBA, the uranium mainly occurred as U3O8 (71.8%). However, in the iron-rich particles, the uranium mainly occurred as UO2 (91.9%) after magnetic separation. Magnetite is a ubiquitous ferrous-bearing oxide, and it was effective for the sorption of U(IV). The result of FeSO4 leaching experiment indicated that 96.57% of total uranium was reduced from U(VI) to U(IV) when infiltrated with the FeSO4 solution for 6months. This result clearly demonstrated the changes in chemical valence of uranium in the coal ash and provided a conceptual principle for preventing uranium migration from ash to the surrounding soil and plants.

Distribution and mode of occurrence of uranium in bottom ash derived from high-germanium coals.

The radioactivity of uranium in radioactive coal bottom ash (CBA) may be a potential danger to the ambient environment and human health. Concerning the limited research on the distribution and mode of occurrence of uranium in CBA, we herein report our investigations into this topic using a number of techniques including a five-step Tessier sequential extraction, hydrogen fluoride (HF) leaching, Siroquant (Rietveld) quantification, magnetic separation, and electron probe microanalysis (EPMA). The Tessier sequential extraction showed that the uranium in the residual and Fe-Mn oxide fractions was dominant (59.1% and 34.9%, respectively). The former was mainly incorporated into aluminosilicates, retained with glass and cristobalite, whereas the latter was especially enriched in the magnetic fraction, of which about 50% was present with magnetite (Fe3O4) and the rest in other iron oxides. In addition, the uranium in the magnetic fraction was 2.6 times that in the non-magnetic fraction. The experimental findings in this work may be important for establishing an effective strategy to reduce radioactivity from CBA for the protection of our local environment.

Novel synthesis of cyano-functionalized mesoporous silica nanospheres (MSN) from coal fly ash for removal of toxic metals from wastewater.

Trace amounts of toxic metals are usually difficult to be purified by conventional chemical precipitation or physical adsorption in wastewater. In this study, in order to realize high-value utilization of coal fly ash for wastewater purification, a novel method was applied to prepare high-performance mesoporous silica materials from coal fly ash. In comparison with a commonly used method, characterizations revealed that the new method obtained mesoporous silica nanospheres with uniformly distributed cyano groups (denoted by MSN), while the common method only obtained irregular sponge-like microstructure (denoted by ISM). Besides, MSN showed better hydrothermal stability, higher specific surface area (693m2/g) and more ordered mesopores from the comparison. Moreover, the sorption experiments of simulated wastewater suggested that MSN was better in removing toxic metals (Ni2+ and Cd2+) than ISM. For the practical wastewater from a battery plant, 2g/L dosage of MSN showed excellent performance for purification of trace amounts of various toxic metals (Ni, Cd, Mn, Zn, Hg and Pb), the concentration of which reduced to ppb level after MSN treated. The results suggested that MSN can be an effective and low-cost sorbent for removing various toxic metals from wastewater.

Removal of uranium and gross radioactivity from coal bottom ash by CaCl2 roasting followed by HNO3 leaching.

A roast-leach method using CaCl2 and HNO3 to remove uranium and gross radioactivity in coal bottom ash was investigated. Heat treatment of the ash with 100% CaCl2 (900°C, 2h) significantly enhanced uranium leachability (>95%) compared with direct acid-leaching (22.6-25.5%). The removal efficiency of uranium and gross radioactivity increased steeply with increasing CaCl2 content, from 10% to 50%, and a HNO3 leaching time from 5 min to 1h, but remained nearly constant or decreased slightly with increasing CaCl2 dosage >50% or acid-leaching time >1h. The majority of the uranium (87.3%), gross α (92.9%) and gross ß (84.9%) were removed under the optimized roast-leach conditions (50% CaCl2, 1M HNO3 leaching for 1h). The mineralogical characteristics of roasted clinker indicated that molten CaCl2 promoted the incorporation of Ca into silica and silicates and resulted in its progressive susceptibility to acid attack. Uranium and other radionuclides, most likely present in the form of silicates or in association with miscellaneous silicates in the highest density fraction (>2.5g mL(-1)), were probably leached out as the result of the acid decomposition of newly formed "gelatinizing silicates".