Sinks a Vital Element of Modern Waste Management 2nd International Conference on Final Sinks 16 – 18 May 2013 Espoo, Finland
Trace element extractability from bottom and fly ash of an industrial-scale (120 MW) power plant using artificial sweat and gastric fluids M. Mäkelä (1), R. Pöykiö (2), O. Dahl (1), K. Manskinen (3), G. Watkins (1) and H. Nurmesniemi (3) (1) Aalto University, School of Chemical Technology, Finland (
[email protected],
[email protected],
[email protected]), (2) City of Kemi, Finland (
[email protected]), (3) Stora Enso Oyj, Finland (
[email protected],
[email protected])
Abstract In-vitro methods, involving artificial sweat and gastric fluids, can be used as tools for the determination of extractable trace elements in a range of industrial wastes. Although in-vitro tests using artificial body fluids have been reported to have limitations, they provide a rapid and inexpensive means to determine the bioavailability of trace elements in wastes and are therefore relatively widely used. Hence the artificial sweat and artificial gastric fluid extractable concentrations of a wide range of elements in industrial bottom and fly ash were determined to assess potential occupational risks associated with ash handling. Except for Mo and Se, the extractable element concentrations in the artificial gastric fluid were clearly higher than those in the artificial sweat fluid. These results are reasonable in the light of the fact that the pH of the gastric fluid was extremely acidic both before (pH 1.54) and after (bottom ash; pH 1.70; fly ash, pH 2.0) extraction. However, the higher extractable concentrations of Mo and Se in the artificial sweat are reasonable, because Mo and Se are able to form oxyanions, which means that their extractability clearly increases from acidic pH value to neutral and alkaline conditions. Due to the relatively high extractability of certain elements in fly ash using an artificial gastric fluid, e.g., Al (19,900 mg kg-1, d.w.), Ba (568 mg kg-1, d.w.), Pb (29.8 mg kg-1, d.w.), Ni (12.7 mg kg-1, d.w.), V (65.4 mg kg-1, d.w.) and Zn (372 mg kg-1, d.w.), careful handling of this ash residue is recommended in order to prevent the ingestion and penetration of ash particles across the human gastrointestinal tract. Keywords: artificial body fluid, bottom ash, extraction, fly ash, trace element
Sinks a Vital Element of Modern Waste Management 2nd International Conference on Final Sinks 16 – 18 May 2013 Espoo, Finland
1
Introduction
Traditionally disposal in landfill sites has been the most widely used method in the waste management of ash materials. The dry matter content of the fly ash is generally very high (ca. 90 – 99 %), which is a disadvantage as it may cause considerable dust problems during handling (Jalovaara et al. 2003). Many researchers have investigated the risks to workers arising from prolonged exposure to ash (Chen et al. 2005, Cprek et. al. 2007). In terms of human health risk assessment, there are three main exposure pathways for trace element metals/metalloids generally present in ash materials. The main area of concern is the oral/ingestion pathway followed by the dermal and respiratory exposure routes (Wragg & Cave 2002). Extraction tests are widely used as tools to estimate the potential release of trace elements from ash over a range of possible waste management activities, including recycling or reuse, for assessing the efficiency of waste treatment processes, and after disposal. As extraction does not necessarily denote total decomposition, the extractable recoveries of analyses are generally lower than the respective total concentrations. Recoveries can only reach total values if the element is completely soluble in the extraction solvent. The importance of extraction studies is based on the fact that the potential toxicity of trace elements in ash is not related to its total concentration, but to its extractability (Filgueiras et al. 2002). In-vitro methods, involving artificial sweat and gastric fluids, have been used as tools in many environmental studies for the determination of extractable trace elements in a range of industrial wastes such as coal fly ash (Lu et al. 2009), chromite ore processing residue (Horowitz & Finley 1993), chromate copper arsenate-treated wood (Nico et al. 2006), as well as in contaminated urban roadside (Wang et al. 2007) and industrial soils (Kim et al. 2002). Although in-vitro tests using artificial body fluids have been reported to have limitations, for instance they cannot contain all the constituents of human fluids (e.g., proteins and enzymes), they provide a rapid and inexpensive means to determine the bioavailability of trace elements in wastes and are therefore relatively widely used. The aim of this investigation was to determine artificial sweat and artificial gastric fluid extractable concentrations of a wide range of elements for assessing potential occupational risks associated with bottom and fly ash handling.
2
Material and Methods 2.1
Ash sampling
The bottom and fly ash investigated in this study originated from a large-sized (120 MW) bubbling fluidized bed (BFB) boiler at the power plant of a fluting board mill located in Finland (Manskinen et al. 2010). The sampled bottom ash was a mixture of the bottom ash from the outlet of the BFB boiler and the bed sand material used in the BFB boiler. The electrostatic precipitator (ESP) at the power plant has three fields (i.e., electrodes), and in the current configuration of the plant´s ash collecting system, fly ash fractions from all three fields are collected and combined into one ash silo. Thus, the fly ash investigated in this study was a mixture from these three ESP fields. Sampling was carried out over a period of fifteen days, and five individual samples (1 kg per sampling day) were combined to give one composite sample with a weight of 5 kg for both ash fractions. The sampling period represented normal process operating conditions for the combustion plant, e.g., in terms of O2 content and temperature. During the sampling period approximately 50% of the energy produced by the BFB boiler originated from the incineration of commercial peat fuel, and 50% from the incineration of clean forest residues (i.e., bark, wood chips and sawdust). Approximately 74% of the forest residue was clean bark from the mill´s wood handling process of the mill. Approximately 98% of the barked wood was birch (Betula verrucosa and B. pubescens), and 2% was alder (Alnusi insana and A. glutinosa). The peat fuel originated near the fluting mill and was thus of domestic origin. 2.2.
Determination of mineralogical composition and the physical and chemical properties of the ashes For the determination of the mineralogical composition of the ashes, an X-ray diffractogram of a powdered sample was obtained with a Siemens D 5000 diffractometer (Siemens AG, Karlsruhe, Germany) using CuK radiation. The scan was run from 2 to 80° (2-theta-scale), with increments of 0.02° and a counting time of 1.5
Sinks a Vital Element of Modern Waste Management 2nd International Conference on Final Sinks 16 – 18 May 2013 Espoo, Finland
seconds per step. Operating conditions were 40 kV and 40 mA. Peak identification was done with the DIFFRACplus BASIC Evaluation Package PDFMaint 12 (Bruker axs, Germany) and ICDD PDF-2 Release 2006 software package (Pennsylvania, USA). The pH and electrical conductivity (EC) of the ashes were determined by a combination pH/EC analyser equipped with a Thermo Orion Sure Flow pH electrode (Turnhout, Belgium) and a Phoenix conductivity electrode (Phoenix Electrode Co., Texas, USA) with a cell constant of 1.0. The determination of pH and EC was according to European standard SFS-EN 13037 at a solid to liquid (S/L) ratio of 1:5. Determination of the dry matter content of the ashes was carried out according to European standard SFSEN 12880, in which a sample is dried overnight to a constant mass in an oven at 105 °C. A comprehensive review of the standards, analytical methods and instrumentation is given in our previous study (Manskinen et al. 2010). 2.3. Determination of pseudo-total element concentrations in the ashes For the determination of pseudo-total trace element concentrations in the the ashes, the dried samples were digested with a mixture of HCl (3 mL) and HNO3 (9 mL) in a CEM Mars 5 microprocessor controlled microwave oven with CEM HP 500 Teflon vessels (CEM corp., Matthews, USA) using USEPA method 3051A (Yafa & Farmer 2006). The cooled solutions were transferred to 100 mL volumetric flask and the solutions were diluted to volume with ultrapure water. The ultrapure water was generated by an Elgastat Prima reverse osmosis and Elgastat Maxima ion exchange water purification system (Elga, Ltd; Bucks, England). All reagents and acids were suprapure or pro analysis quality. Analyte quantification was performed with a Thermo Elemental IRIS Intrepid II XDL Duo (Franklin, USA) inductively coupled plasma optical emission spectrometer (ICP-OES). 2.4. Element extraction procedure by artificial sweat and gastric fluids The artificial sweat was prepared by dissolving 5 g NaCl, 1 g lactic acid and 1 g urea in 1 L of deionized water and adjusting the pH to 6.53 with ammonia (Song et al. 2007). The artificial gastric fluid was prepared by dissolving 60.06 g glycine in 2 L of deionized water and adjusting the pH to 1.54 with HCl (Wang et al. 2007). The extraction was carried out in polypropylene bottles by shaking 1 g of ash on a dry weight (d.w.) basis with 100 mL of the extract (i.e., artificial sweat or artificial gastric fluid) for 1 hour by end-over-end mixing at 37 °C. After extraction, the extract was separated from the solid residue (i.e., the undissolved ash) by filtration through a 0.45 µm membrane filter. The pH of the extract was then measured and the metal concentrations in the extract determined with an ICP-OES.
3
Results and Discussion
3.1 Mineralogical composition and the physical and chemical properties of the ashes According to the acquired XRD data (see Manskinen et al. 2010), the bottom ash contained only silicate minerals (i.e., anorthite, magnesiohornblende, microcline and quartz). The fly ash contained anorthite, microcline and quartz, but also biotite, which is a silicate mineral, as well as hematite, which is an iron oxide. The existence of Ca-based minerals in the form of anorthite, magnesiohornblende and anorthite in the ashes is reasonable due to the natural calcium content of wood residues (Vanvuka and Kakaras 2011), which were a part of the boiler fuel mix during this investigation. The existence of silicate minerals in the bottom ash is reasonable when considering that the bed material of fluidized bed boiler furnaces usually consists of silica sand. Furthermore, the existence of silicate minerals both in the bottom and fly ash fractions may also partly due to the sand and soil particle contamination of forest residues during harvesting transportation and handling (Steenari & Lindqvist 1999). In addition, it may partly derive from the decomposition of plant tissue-derived Si-based minerals during incineration, such as phytolith (SiO2 × nH2O), which is often a structural component of plant tissue, deposited between and within plant cells (Humbhreys 2004). According to Vamvuka and Kakaras (2011), hematite could be produced from oxidation of organic iron or siderite during the combustion process. Both ashes were strongly alkaline (i.e., bottom ash pH 11.3; fly ash pH 12.0; see Maskinen et al. 2010). According to van Herck and Vandecasteele (2001), an alkaline pH indicates that part of the dissolved metals occur as basic metal salts, oxides, hydroxides and/or carbonates. Therefore, the proportion of soluble basic
Sinks a Vital Element of Modern Waste Management 2nd International Conference on Final Sinks 16 – 18 May 2013 Espoo, Finland
metal salts, oxides, hydroxides and carbonates in the ashes outweighs the proportion of soluble acid components, and the ashes subsequently generate an alkaline pH. The very low total organic carbon (TOC) value of the bottom ash (i.e.,
