Evaluation of a sequential extraction process used for determining mercury binding mechanisms to coal combustion byproducts.

ABSTRACT

Leaching of mercury from coal combustion byproducts is a concern because of the toxicity of mercury. Leachability of mercury can be assessed by using sequential extraction procedures. Sequential extraction procedures are commonly used to determine the speciation and mobility of trace metals in solid samples and are designed to differentiate among metals bound by different mechanisms and to different solid phases. This study evaluated the selectivity and effectiveness of a sequential extraction process used to determine mercury binding mechanisms to various materials. A six-step sequential extraction process was applied to laboratory-synthesized materials with known mercury concentrations and binding mechanisms. These materials were calcite, hematite, goethite, and titanium dioxide. Fly ash from a full-scale power plant was also investigated. The concentrations of mercury were measured using inductively coupled plasma (ICP) mass spectrometry, whereas the major elements were measured by ICP atomic emission spectrometry. The materials were characterized by X-ray powder diffraction and scanning electron microscopy with energy dispersive spectroscopy. The sequential extraction procedure provided information about the solid phases with which mercury was associated in the solid sample. The procedure effectively extracted mercury from the target phases. The procedure was generally selective in extracting mercury. However, some steps in the procedure extracted mercury from nontarget phases, and others resulted in mercury redistribution. Iron from hematite and goethite was only leached in the reducible and residual extraction steps. Some mercury associated with goethite was extracted in the ion exchangeable step, whereas mercury associated with hematite was extracted almost entirely in the residual step. Calcium in calcite and mercury associated with calcite were primarily removed in the acid-soluble extraction step. Titanium in titanium dioxide and mercury adsorbed onto titanium dioxide were extracted almost entirely in the residual step.

INTRODUCTION

Mercury, once emitted, can remain airborne for 0.5-2.0 yr before depositing onto soil or surface waters. (1) Once deposited on soils or water bodies, mercury is persistent and may bioaccumulate in ecosystems. (2) In the United States, approximately 30% of the total mercury emitted by anthropogenic sources comes from coal-burning utilities. (3) Although mercury is a trace metal found in coal varying with rank in the range of 0.01-3.3 parts per million by weight, mercury emission from coal combustion is substantial because of the large quantity of coal used every year. (3)

The U.S. Environmental Protection Agency issued the Clean Air Mercury Rule (CAMR) on March 15, 2005, which will reduce total mercury emissions from coal burning utilities in the United States through a market-based cap and trade system. The CAMR will reduce mercury emissions from 48 t/yr to 38 t/yr by 2010 during the first phase of implementation and to 15 t/yr by 2018 in a second phase of reductions. (4) Sorbent addition to coal combustion processes has the potential to remove mercury and has been shown to effectively capture gaseous mercury, suppress the fraction in the solid phase, and transform it to an environmentally benign form. (5,6) Potential sorbents for mercury capture include activated carbon-based sorbents and inorganic sorbents. Inorganic sorbents include silica, titania, alumina, aluminosilicates, and calcium-based compounds. (7-9) Titanium dioxide is an inorganic sorbent that can capture mercury after photo-catalytic oxidation of [Hg.sup.0] under ultraviolet (UV) irradiation. (10) When sorbents are used, the mercury is transferred from the gas phase to solid phases that are collected with the fly ash or as separate byproducts. Elemental mercury does not partition significantly to the particulate phase, and oxidation either in the gas phase or at the particle surface is necessary for mercury capture (11,12); consequently, this study focuses on the sequential extraction of mercury (II). The solid-phase speciation of mercury in combustion byproducts dictates its leachability to the environment and can be investigated using sequential extraction techniques. (13)

Sequential extractions furnish information about the origin, mode of occurrence, bioavailability, and mobility of trace metals in solid samples. (14,15) The mobility of heavy metals depends on the characteristics of the solid, the strength of bond between the heavy metal and the solid matrix, and the composition of the leaching solution. (16) Sequential extraction is a technique in which a series of chemical solutions is applied to one solid in succession. Elements associated with a solid material can be partitioned into specific fractions by selective extraction with appropriate reagents. (14) Each sequential extraction procedure is designed to differentiate among metals bound to different solid phases and mineral fractions. (17) The materials investigated in this study were goethite ([alpha]-FeOOH), hematite ([alpha]-[Fe.sub.2][O.sub.3]), calcite (CaC[O.sub.3]), mercury oxide (HgO), and a titanium dioxide mixture of anatase and rutile ([alpha]-Ti[O.sub.2] and [beta]-Ti[O.sub.2]). These phases, with the exception of [alpha]-FeOOH, are all found in coal combustion fly ashes. On a mass basis, iron oxides may comprise as much as 16% of the fly ash, carbonates such as CaC[O.sub.3] as much as 22%, and titanium comprises approximately 1% and may be present as Ti[O.sub.2]. (18-20) [alpha]-FeOOH was investigated to compare mercury binding mechanisms between different iron minerals. Investigation of mercury binding mechanisms to mercury-laden materials and titanium dioxide (a potential inorganic sorbent) yields information about the future mobility of mercury in the environment and indicates the capability of each material to sequester mercury.

Each phase can retain heavy metals by mechanisms including ion exchange, outer- and inner-sphere complexation, precipitation, and coprecipitation. The potential mechanisms are illustrated in Figure 1. Precipitation (mechanism 1) involves the formation of discrete mercury-containing solid phases on the sorbent surface. Chemical adsorption (mechanism 2) to the surface involves the formation of chemical bonds between mercury atoms and reactive functional groups on the sorbent surface. Electrostatic adsorption (mechanism 3) is a form of physical adsorption in which mercury ions are bound to the surface through electrostatic adsorption and not through coordination to any specific sites on the sorbent surface. Encapsulation (mechanism 4) involves the incorporation of mercury within the solid matrix through either substitution of mercury for another cation or the formation of domains of discrete mercury-containing solid phases within the matrix of another solid. Encapsulation is different from the partitioning of mercury to surfaces that may be present within the interior of porous sorbents; such partitioning would occur through a process of surface precipitation or chemical or physical adsorption. The mercury extracted in the steps of a sequential extraction procedure can be related to the targeted mercury fraction and binding mechanisms.

The goal of sequential extraction is to convert the heavy metals bound to certain phases into soluble forms within the extractant solution. (16) Careful selection of the extractant solutions is required to differentiate among phases and binding mechanisms. In each sequential extraction, extractants are applied in order of increasing aggressiveness, such that the successive fractions obtained correspond with metal associations of lesser mobility. Common extractants used in sequential extraction schemes generally fall into the following groups: unbuffered salts, weak acids, reducing agents, oxidizing agents, and strong acids. (16) Two commonly used procedures are the Tessier and the European Community Bureau of Reference (BCR) (proposed in 1993 by the European Community's Bureau of References, now called the Standards, Measurements and Testing Programme) sequential extraction processes. (14,21) Researchers have made modifications to these sequential extraction schemes based on the solid material studied, the phases of interest, and the trace metals in question. (15-17,22-28)

[FIGURE 1 OMITTED]

Both the sequential extraction process and the toxicity characteristic leaching procedure (TCLP) provide information regarding the leachability of trace metals. The TCLP is a standard method used to simulate the leaching of metals from solids within the moderately acidic environment of a municipal landfill. The TCLP uses a solution of 0.5 M sodium acetate and 0.5 M glacial acetic acid (pH = 4.9 [+ or -] 0.1), (22) which is similar to the acid soluble step used in this study. However, the sequential extraction process provides leachability information under additional conditions. In particular, the reducing extraction step simulates the reducing conditions of a municipal landfill better than the TCLP alone. Sequential extraction processes warrant further investigation and could supplement the TCLP in determining the leachability of toxic metals.