Environ. Sci. Technol. 1997, 31, 3226-3231
Mechanism of Arsenic Sorption by Hydrated Lime S. MAHULI, R. AGNIHOTRI, S. CHAUK, A. GHOSH-DASTIDAR, AND L.-S. FAN* Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210
Arsenic, along with mercury and selenium, represents one of the trace metallic emissions from fossil fuel-fired power plants that exhibit a tendency to remain in the gas phase. In this study, the effectiveness of some commonly used mineral sorbents is tested for the removal of arsenic from flue gas. Investigations are conducted with hydrated lime sorbent to identify the mechanism of As/Ca interaction and the chemical state and characteristics of captured species. Arsenic oxide (As2O3) is used as the representative arsenic species, and investigations are conducted in a differential fixed bed reactor at medium (400-600 °C) and high (1000-800 °C) temperature conditions. Comparison of Ca(OH)2 with three other mineral sorbents (kaolinite, alumina, and silica) reveals that calcium hydroxide is the most effective in capturing arsenic. The capture mechanism of Ca(OH)2 does not involve a simple physical adsorption but proceeds by means of an irreversible chemical reaction leading to a solid product. X-ray diffraction and ion chromatography analyses of the post-sorption sample confirm that calcium arsenate is the dominant reaction product. Sorption of arsenic by fly ash is found to be reversible and physical in nature.
Introduction Coal combustion, waste incineration, and metallurgical operations such as smelting of ores generate hot flue gases containing arsenic. Coal-fired power plants have been identified as major sources responsible for trace metal emissions including arsenic and will most likely be required to reduce these emissions by impending EPA regulations. The average arsenic concentration in U.S. coal is reported as 15 µg/g (1). Arsenic and its compounds have been linked to harmful toxicological impacts despite the fact that arsenic is an essential trace element for humans and other animals. The concentration of arsenic species, its phase transformations, and its chemical state have been a subject of debate. Germani and Zoeller (2) reported between 0.7-52% of the in-stack As to be in the form of gas and its concentration in the gas phase to be about 7 µg/m3. Thurnau and Fournier (3) observed that between 5 and 20% of the arsenic remains in the flue gas during incineration of arsenic-laden hazardous soil matrices. Owens et al. (4) found that, in the coal-fired cement kiln process, the vaporization of arsenic is suppressed. They cited the formation of calcium arsenate as the probable cause for the suppression. The elemental state (As) and the oxide form (As2O3) have been postulated to be the two most probable species in the oxidizing flue gas environment. However, arsenic (along with selenium) is much more volatile as an oxide than as an * To whom correspondence should be addressed. Telephone: (614)-292-7907; fax: (614)-292-1929; e-mail address: Fan@ kcgl1.eng.ohio-state.edu.
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element. The saturation concentration of arsenic occurring as As2O3 in flue gas is about 300 mg/m3 at typical stack temperature of 150 °C; while that of the elemental form is only about 0.060 mg/m3. The corresponding flue gas concentration upon combustion is around 1.3 mg/m3 for arsenic at 150 °C and at 3% O2 by volume (1). Thus, flue gas could contain all of the arsenic in the form of an oxide and not as an element. Srinivasachar et al. (5) studied the speciation of As, Cd, Cr, and Pb under simulated incineration conditions. They reported As2O3 under oxidizing conditions doped with one or both of HCl and SO2. Winter et al. (6) also observed As2O3 as the only species following injection of aqueous As2O3 into simulated combustion environment. The high volatility and its existence in the form of gas make arsenic emission control a very difficult task. In the past few years, dry sorbent injection for in-situ capture of metals from hot flue gas has been studied with the aim of developing a potential control technique. In dry sorbent injection, metal sorption by the solid could take place by means of physical adsorption, chemisorption, chemical reaction, or a combination of these processes. Prior investigations have involved studies on the effectiveness of different mineral sorbents in removal of cadmium, lead, and alkali metal compounds from hot flue gas. Uberoi and Shadman (7, 8) have studied silica, alumina, kaolinite, bauxite, emathlite, and lime for sorption of cadmium and lead in a thermogravimetric type reactor system for a reaction temperature of 700-800 °C. Their results showed that the overall sorption process is not physical, but a complex combination of physical and chemical processes that are dependent on the temperature and the type of sorbent. Gullet and Raghunathan (9) used several mineral sorbents, namely, hydrated lime, limestone, kaolinite, and bauxite for upper furnace injection and measured reduction in the amounts of various metallic air toxics. They reported significantly higher capture of arsenic with hydrated lime and limestone as compared to the other sorbents. Ho et al. (10, 11) studied the capture of lead and cadmium compounds by limestone, sand, and alumina during fluidized bed combustion/incineration. Wu et al. (12) have studied multi-functional sorbents for the simultaneous removal of sulfur oxides as well as metallic contaminants from high-temperature gases. Gullett et al. (13) and Jozewicz and Gullett (14) investigated the kinetics and the reaction mechanism of dry Ca-based sorbents with gaseous HCl in a short-time differential reactor. Wouterlood and Bowling (15) studied the removal of As4O6 from flue gases at 200 °C using surface-active agents as well as Ca-based sorbents. They concluded that there was some chemical reaction or chemisorption mechanism with the sorbents. It is known that fly ash acts as a good sink for most of the volatile trace toxics (with the possible exception of highly volatile mercury and halides). Germani and Zoeller (2) reported that the gas-phase As concentration decreases linearly with increase in the in-stack total particulate mass loading over the entire range of their study (11-254 mg/m3). An inverse relationship has been confirmed between the concentration of trace elements and the size of fly ash by a number of researchers (16-20). Davison et al. (16) reported that the concentration of captured As on 5-10 µm fly ash particles was 8-fold more than that on >40 µm particles. Although it is well established that fly ash particles do capture arsenic species, the nature of interaction between fly ash particles and arsenic is not well understood. In this study, the effectiveness of some commonly used mineral sorbents is tested for removal of arsenic. Investigations are conducted with hydrated lime sorbent to identify the mechanism of As/Ca interaction and the chemical state
S0013-936X(97)00212-5 CCC: $14.00
1997 American Chemical Society
TABLE 1. Composition of Investigated Mineral Sorbents and Fly Ash (wt %) composition Ca(OH)2 SiO2 Al2O3 MgO CaCO3 CaO TiO2 Fe2O3 K2O Na2O CaSO4
calcium hydroxidea 94.0 0.9 0.6 1.0 1.0 1.0 0.5
kaoliniteb
aluminac
45.5 38.5 0.06
99.99
0.03 1.5 0.41 0.34 0.05
silicad 99.99
NIST fly ashe 49.3 29.4 0.8 3.1 1.3 12.5 2.3 0.28 1.1
a Linwood Mining & Minerals Co. b Albion Kaolin Co. c Johnson Matthey Co. d Fisher Scientific. e NIST Certified fly ash (SRM 1633b).
FIGURE 1. Schematic of differential bed reactor (DBR) system. and characteristics of captured species. In order to compare the characteristics of capture by sorbents with those of fly ash, a representative fly ash is also tested for its arsenic removal potential.
Experimental Section A schematic of the reactor setup is shown in Figure 1. The main components of this system are the microbalanceequipped toxic gas generation assembly and the differential bed reactor assembly housed in a furnace. The details of the reactor system are described in detail elsewhere (21). Reactor Assembly. The arsenic source is held in a Pt sample pan suspended by a Pt hangdown wire from the ATI Cahn D-200 digital microbalance. As2O3 is used as the arsenic source in all the experiments. The vaporization tube that houses the source pan is a 25.4 mm o.d. quartz tube with provision for carrier gas entry and thermocouple insertion for monitoring the source temperature. Dry nitrogen, which is used as the carrier gas in all the experiments, enters at the top of the tube and flows down over the arsenic source. The vaporization tube is wrapped with heating tape and closely maintained at a specific temperature. Solid As2O3 has a significant vapor pressure above 150 °C (22). The desired concentration of arsenic in the gas phase is achieved by adjusting the temperature of the vaporization tube. A vaporization temperature of about 200 °C gives the desired uniform vaporization rate to achieve the objective of a small yet uniform concentration of arsenic. The flow rate of the carrier gas is typically maintained at an optimum value of 200 mL/ min, since experimental observations suggest that any higher flow rates lead to loss of accuracy of the weight reading. The reactor assembly consists of a ceramic tube, a 1200 °C Lindberg single-zone furnace, and a sorbent holder assembly. The reactor is a 2.54 cm o.d. mullite tube cemented to custom-fabricated stainless steel end-connections at the top and bottom. The top connection has provision for mixing the toxic gas-laden reactant stream with the diluent flow. A 6.4 mm o.d. flexible stainless steel tubing transports the As2O3carrying gas from the vaporization tube to the top of the reactant tube. In order to avoid condensation of the As species in this transport line, its length is kept to a minimum and is maintained at a higher temperature than the vaporization
FIGURE 2. Amount of arsenic captured by various sorbents. As2O3 gas-phase concentration, 13 ppm; sorption temperature, 600 °C; sorption time, 1 h. tube. Before the preheated carrier gas enters the reactor, it is mixed with diluent gas. Dry air is used as the diluent stream. The diluent gas is passed through a Alltech Hydropurge II molecular sieve adsorbent. This trap serves to simultaneously remove moisture and CO2 from the air. The diluent gas is preheated by passing the diluent transport line (3.2 mm o.d. stainless steel) through the reactor furnace. The sorbent is dispersed on a small amount of quartz wool that is supported on the quartz sorbent holder. The gas coming out from the reactor passes through a train of two aqueous impinger solutions (7% HNO3) designed to capture the remaining As2O3 before being vented to exhaust. Experimental Analyses. The chemical composition of the different sorbents and the fly ash (NIST certified) is given in Table 1. During experiments, a small amount of preweighed sorbent (15-20 mg) is dispersed on small quantity (3-5 mg) of quartz wool in the sample holder. The arsenic concentration is calculated from a knowledge of the vaporization rate and the gas flow rates. The experiments are performed with the toxic gas-carrier gas and diluent gas flow rates at 200 and 1000 mL/min (STP), respectively. Ca(OH)2 calcination is realized in-situ by keeping the furnace at the predetermined calcination temperature for a fixed period of time under a continuous flow of N2. After calcination, the furnace temperature is adjusted to the desired sorption temperature, thus avoiding atmospheric exposure of the calcined sorbent.
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dispersant quartz wool itself exhibits no significant capture (
