Chemical state of boron in coal fly ash investigated by focused-ion-beam time-of-flight secondary ion mass spectrometry (FIB-TOF-SIMS) and satellite-transition magic angle spinning nuclear magnetic resonance (STMAS NMR)

Abstract

The chemical states of boron in coal fly ash, which may control its leaching into the environment, were investigated by focused-ion-beam time-of-flight secondary ion mass spectrometry (FIB-TOF-SIMS) and satellite-transition magic angle spinning nuclear magnetic resonance (STMAS NMR) spectroscopy. The distribution of boron on the surface and in the interior of micron-sized fly ash particles was directly observed by FIB-TOF-SIMS. Coordination numbers of boron and its bonding with different atoms from particles of bulk samples were investigated by STMAS NMR. Boron in coal fly ash with relatively poor leaching characteristics appears as trigonal BO(3) and coexists with Ca and Fe at the outer layer of every particle and inside CaO-MgO particles. In contrast, boron in coal fly ash with better leaching characteristics appears as CaO- or MgO-trigonal BO(3) and tetragonal BO(4), and it is distributed only on the outer surface of each ash particle without showing any correlation with a particular element.

Extensive FE-SEM/EDS, HR-TEM/EDS and ToF-SIMS studies of micron- to nano-particles in anthracite fly ash

“…However, chemical maps are good indicators of relative surface concentration and its change as a function of sample treatment or time. In previous researches this technique provided information on the elemental distributions not only at the surface but also inside the particles (Hayashi et al, 2010). In the present work, generally the composition of this kind of CFAs samples, which are often very complex, included the detection of light elements, heavy metals, and hydrocarbons.…”

Preliminary Study on Additives for Controlling As, Se, B, and F Leaching from Coal Fly Ash

“…Iwashita mentioned that under highly alkaline conditions, the leaching of B and Se tended to decrease [3]. Shun-ichi stated that coal fly ash with better leaching characteristics contained boron in the CaO-and MgO-phases on the surface of coal fly ash particles [8]. Jose and Wang reported that alkaline pH, calcium addition, temperature and the leaching time affected the leaching of As and Se [4,9].…”

“…Trace elements that are also classified as heavy metals are transformed into less soluble compounds, which makes these trace elements non-hazardous in the environment. As, Se, B, and F can react with calcium and form relatively insoluble compounds such as Ca 3 (AsO 4 ) 2 , Ca 4 (OH) 2 (AsO 4 ) 2 •4H 2 O [9,13], CaSeO 3 [9], Ca 2 B 2 O 5 , Ca 3 B 2 O 6 [8] and CaF 2 [24,25]. Therefore, calcium addition through the use of high-calcium-content materials in the leaching process could be applied to control the leaching of trace elements.…”

“…In addition, the pH of the leachate is believed to be correlated with the calcium content in the coal. Hayashi [8] studied the chemical state of boron in coal fly ash and concluded that boron associated with calcium and magnesium is difficult to leach. Wang [9] stated that As and Se have a high mobility in alkaline pH, and Sri et al [10,11] stated that leaching of arsenic is related to calcium oxide content in coal fly ash.…”

Characteristic Sorption of H₃BO₃/B(OH)₄^− on Magnesium Oxide

“…31) These phenomena were not observed in sorption of fluoride on MgO. 26,27) Other phases such as Mg 7 Fig. 4, although the reliability of identification is relatively low with these index cards.…”