Article pubs.acs.org/EF
Determination of Boron in Coal Using Closed-Vessel Microwave Digestion and Inductively Coupled Plasma Mass Spectrometry (ICPMS) Shifeng Dai,† Weijiao Song,† Lei Zhao,† Xiao Li,† James C. Hower,*,‡ Colin R. Ward,§ Peipei Wang,† Tian Li,† Xin Zheng,∇ Vladimir V. Seredin,⊥,∥ Panpan Xie,† and Qingqian Li† †
State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (Beijing), Beijing 100083, China ‡ Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, Kentucky 40511, United States § School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia ∇ ThermoFisher Scientific, Beijing 10083, China ⊥ Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetny per. 35, Moscow 119017, Russia ABSTRACT: Boron in coal is of great interest, mainly because it can be used as paleosalinity indicator; it is also of environmental concern because of its toxic effects, especially on land plants. This paper describes a new method for determination of the boron concentration in coal by inductively coupled plasma mass spectrometry (ICP-MS) after closed-vessel microwave digestion. Samples of raw coal rather than coal ash were used for the determination to avoid the loss of volatile organically bound boron. Addition of H3PO4 to the HNO3 and HF used as reagents in the process could also significantly diminish boron volatilization during acid-drying after sample digestion. A 2% ammonia solution, which was injected into an ICPMS spray chamber to eliminate the memory effect of boron, may reduce boron signals to blank levels (within 120 s) during ICPMS analysis. The boron concentrations of National Institute of Standards and Technology (NIST) standard reference coal samples obtained using the ICP-MS in high-resolution mode and with 103Rh or 115In used as online addition internal standards are in good agreement with the certified values.
1. INTRODUCTION Boron in coal is of interest, both because it can be used as paleosalinity indicator for the coal’s sedimentary environment1−4 and because it may have adverse environmental effects.5−7 For example, Goodarzi and Swaine8 successfully used the total boron concentration to develop fresh/brackish and brackish/marine indices of coal sedimentary environments. Their work places the fresh/brackish and brackish/marine boundaries at 50 and 110 μg/g B and, although this paleosalinity indicator remains controversial,2,9,10 these values have been widely used by others.2,4,11−13 However, an elevated concentration of boron in coal may also be derived from hydrothermal fluids,10 volcanic activity,2,9 and climatic variations.9 High boron concentrations may have toxic effects on land plants,5,6 and this may preclude the use of fly ash as a soil ameliorant and limit disposal options.14,15 Relatively small excesses may be detrimental, but the impact depends in part on the mobility of the boron in the ash material. Some studies have shown that a large fraction of the boron in fly ash is in soluble form and it is often the most mobile trace element in coal ash.16−20 The susceptibility to harm is also plant speciesspecific,7,21 and this provides a further complication in evaluating the environmental effects of boron in coal and its combustion products.14 On the other hand, Yunusa et al.15 reported that boron phytotoxicity from coal ash may be minimized in the field situation, where the soil volume is not © 2014 American Chemical Society
limited and where leaching is possible. Indeed, once the roots extend beyond the zone of ash incorporation, the plants may be able to regain much of the early loss in growth and actually benefit from the ash-derived nutrients.15 A very few studies have also shown that high boron concentrations may affect male reproductive capabilities in animals,22 although probably not in humans where no such negative findings have been reported.23 Three modes of occurrence of boron in coal are commonly recognized, namely, bound to the organic matter, associated with clay minerals (mainly illite), and bound within the crystal lattice of tourmaline.2,7,14,24−28 Although illite and tourmaline may be locally important modes of occurrence, the organically bound mode is generally considered to be the most common.7,12,14,24 The mode of occurrence exerts dominant control on the partitioning behavior of boron, both during coal ashing and during coal utilization. Boron present in coal as tourmaline is retained in ash at a temperature of 1000 °C, while organically bound boron is easily volatilized.14 Ashing experiments have shown a significant loss of boron, in some cases at 370 °C and, in one case, at 150 °C during low-temperature radio frequency Received: April 22, 2014 Revised: June 4, 2014 Published: June 23, 2014 4517
dx.doi.org/10.1021/ef500912a | Energy Fuels 2014, 28, 4517−4522
Energy & Fuels
Article
(1000 μg/mL, single-element Rh standard solution GSB 04-17462004, National Center of Analysis and Testing for Nonferrous Metals and Electronic Materials), 115In (100 μg/mL, CCS4, Inorganic Ventures), and 9Be (100 μg/mL, CCS4, Inorganic Ventures) were used as the internal standards for online addition during ICP-MS analysis. The standard solution (10 μg/mL, THM-TS-1, Inorganic Ventures), containing multielements, Li, Co, In, and U, was used to prepare the tuning solution. All standards were used without further purification and prepared in 2% (v/v) HNO3. Metal-oxide-semiconductor (MOS) reagent HF (40%, v/v), the guaranteed reagents HNO3 (65%, v/v) and H3PO4 (85%, v/v) were used for sample digestion. Ultrapure 99.999% Ar was used without further purification as the cooling, auxiliary, and nebulizer gas. National Institute of Standards and Technology (NIST) standard reference coal samples, including SRM1632c, SRM2682b, and SRM2685b, were used to evaluate the suitability of the ICP-MS technique for boron determination, based on the unashed coal material. 2.3. Sample Digestion. To allow for the volatile behavior of organically bound boron,14,29 50 mg of each NIST standard coal sample, crushed to
