Arsenic is a metalloid that occurs naturally in the earth and in the seas. It is odorless and tasteless. Arsenic is not an abundant element in the earth's crust, the average crustal concentration of 1.8 mg/kg ranks it fifty-two between tin and molybdenum (Demayo, 1985). However, through geogenic processing of crustal materials, arsenic can be concentrated in soils to a typical range of 2 to 20 mg/kg (Yan-Chu, 1994). In nature, it is widely distributed in a number of minerals, mainly as the arsenides of copper, nickel and iron, or as arsenic sulfide or oxide. Arsenic occurs naturally as an impurity in metal ores (e.g. arseno-pyrite, copper, gold), in sulfide compounds (e.g. realgar = AsS, orpiment = As2S3), coal, shales, and in rock-phosphate, among others (Wachope,1983; Tamaki and Frankenberger, 1992). Oxidized forms of arsenic are usually found in sedimentary deposits. Inorganic arsenic in high amounts has been known for centuries as a fast acting human poison. Human activity generates anthropogenic arsenic, making it the third most common regulated inorganic contaminant found at CERCLA sites. Modern usage of arsenic includes formulation of pesticides and herbicides, decolorization of glass, treatment/ preservation of wood, paint manufacturing, and the production of semiconductors. The last few decades have witnessed a substantial increase in public concern over arsenic pollution in both soils and water. Arsenic is listed by the Environmental Protection Agency as one of 129 priority pollutants. Immense use of arsenic containing fungicides, pesticides, wood preservatives, and through mining and burning of coal has contributed to environmental pollution. Thus, anthropogenic use makes arsenic a common inorganic toxicant found at contaminated sites worldwide. Arsenic is troublesome concerning mobility and treatment. The toxic effects of arsenic are related to its oxidation state, determination of individual species of arsenic and the examination of factors affecting the speciation of arsenic is important. Increased arsenic concentrations in soils and soil solution have been responsible for yield decreases and contamination of edible products (Woolson, 1973; Carbonell-Barrachina et al., 1995; Smith et al., 1998) thus causing diseases such as bone marrow depression, liver diseases, and various forms of cancer in humans (Hall, 2002). Consequences of long-term exposure to inorganic forms of arsenic are serious because these compounds have been recognized as skin and lung carcinogens in humans. In addition, arsenic has been reported to affect the vascular system in humans and has been associated with the development of diabetes. Arsenic in the solid phase is safer than in the aqueous and bio-available form, i.e. it is not readily available to biological receptors. Enormous interest exists in the processes and factors that govern the transition from soluble to solid As phases. Especially it's of great importance to many communities that suffer from elevated arsenics levels in potable and irrigation waters.
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Arsenic is a steel-gray brittle metalloid and a member of the nitrogen group in the fourth period of the periodic table. It has an atomic weight of 74.92 and an electronic configuration of [Ar]18 4s2 3d10 4p3. Arsenic appears in three allotropic forms: yellow, black and grey; the stable form is a silver-gray, brittle crystalline solid. It tarnishes rapidly in air, and at high temperatures burns forming a white cloud of arsenic trioxide. In oxygenated environments, arsenic oxidizes rapidly, and the positively charged metalloid center associates readily with three to four oxygen atoms forming either AsO33- (arsenite = As(III)) or AsO43- (arsenate = As(V)) depending on the pH and redox potential (Eh) of its surroundings. Arsenate and arsenite are the two most prevalent forms of arsenic in the environment, however, elemental arsenic (As0) and arsenide (As-3) are known to exist as well (Tamaki and Frankenberger, 1992). The main organic arsenic species, methylarsonic acid (MA(V)) and dimethylarsinic acid (DMA(V)) are generally present in smaller amounts than the inorganic forms, arsenite (AsO33-) and arsenate (AsO43-). Ligand substitution of oxygen by methyl groups results in various methylated arsenic species (Smith et al., 1998). Different methylated species, monomethylarsonic acid (MMAA), dimethylarsinic acid (DMAA) and trimethyl arsine oxide (TMAO) are shown in Figure 1-1. In the study of arsenic species in the environment, as it moves through the environment various processes lead to changes in the chemical form of the arsenic. The processes can be chemical, biological or geological. Various arsenic forms in the environment are shown in Figure 1-2.
Figure1-1. Environmental Arsenic Compounds.
Figure 1-2. Transformation of Arsenic in the Environment.
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Arsenate is a weak triprotic acid with acid dissociation constants (pK) ranging from 2.20, 6.97, to 11.53 (Wagman et al., 1982). Reported pKa values for arsenous and arsenic acids are given in Table 1-1.
Table 1-1. pKa Values for Arsenous and Arsenic acids.
*( Wagman et al., 1982)
With respect to the pH ranges of most natural environments (pH 5-9), the negatively charged H2AsO4- and HAsO4 2- species are dominant in aqueous solution. Within the pH range from 2.3 to 11.6, As (V) acts as an amphiprotic substance, i.e., it accepts as well as donates protons (H+). Arsenate may thus act as both a Bronsted acid and a Bronsted base. Arsenate also acts as an electron pair acceptor (Lewis Acid = Bronsted Acid) and an electron pair donor (Lewis Base = Bronsted Base), because a proton is considered an electron pair acceptor and its conjugate base is the electron pair donor (H+ vs. AsO43-). Arsenate, given its chemical similarity to phosphate (hard base), is considered to be a borderline hard base. Its chemistry is complex and there are many different compounds of both inorganic and organic arsenic. In general, As(III) is more mobile than As(V).
In water, arsenic is usually found in the form of arsenate or arsenite. Arsenic is mainly transported in the environment by water. In oxygenated water, arsenic usually occurs as arsenate, but arsenite predominates under reducing conditions. Leaching of arsenate is slow, due to binding to hydrous oxides of iron and aluminium. The interrelationship among abiotic and biotic factors controls As(V)-As(III) cycling, and in turn, the solubility, mobility, and subsequent bioavailability of arsenic. The As (III) is more mobile and more toxic than the As (V). The burning of coal and smelting of metals are major sources of arsenic in air. The common valence states of arsenic in nature include -3, 0, +3 and +5. In soils and groundwater the most often encountered arsenic forms are inorganic As (III) (arsenite) and As (V) (arsenate). They are readily interconverted and hence are usually found together, with arsenate being the thermodynamically favored form under normal environmental oxygen levels (Smith et al., 1998).
The mode of occurrence of arsenic in coal is an important factor that influences the behavior of the element. Coal produces up to 30% of its weight as fly ash after combustion. Arsenic is enriched on fly ash particles as a consequence of the condensation of volatile arsenic species onto the surfaces of the particles. There is concern that leaching of this arsenic will result in contamination of aquatic disposal sites. While studies have shown that adsorption, dissolution, and precipitation are important in controlling desorbed arsenic concentrations, there is considerable variation among fly ash samples. Arsenic in coal is often contained in iron sulfide minerals such as pyrite, arsenopyrite, and chalcopyrite (Boyle and Jonasson, 1973). The behavior of arsenic during disposal of coal-combustion waste products is a significant cause for concern because of its toxicity, environmental persistence, and tendency to bio-accumulate. The main concern is that pollution from the ash leaches out and contaminates nearby groundwater that is used for drinking. Inorganic arsenic species in contaminated sites are arsenite, arsenate, arsenic sulfide (HAsS2), elemental arsenic and arsine gas (AsH3) forms. Arsenate species include H3AsO4, H2AsO4-, HAsO42- and AsO43-. Arsenite species are the reduced inorganic arsenic species and include H3AsO3, H2AsO3-, HAsO32-, and AsO33-. The inorganic trivalent species of arsenic are considerably more toxic than inorganic pentavalent and organic trivalent species (National Academy of Sciences, 1977; U.S. Atomic Energy Commission, 1973). The main As (V) species are H2AsO4- and HAsO42- (Bowell, 1994).
In closed or batch systems leaching of arsenic is related to the solubility of arsenic species in water. Solubility of some arsenic compounds is shown in Table 1-2. Solution properties such as Eh and pH affect the solubility and stability of arsenic species to a great extent. At high Eh values, arsenic acid species (H3AsO4, H2AsO4-, HAsO42-, and AsO43-) are stable, whereas low Eh values induce reducing conditions, forming stable arsenous acid species (H3AsO3, H2AsO3-, HAsO32-) (Ferguson and Gavis, 1972) with low aqueous solubility. Alkaline pH causes As (III) oxidation and increases As (III) uptake by soils.
Table 1-2. Solubility product constants of some arsenic salts at 25oC.
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The leaching process is a specific extraction process, where the temperature, solid-liquid extraction ratio, and solid particle size must be taken into account in order to assess the behavior of the leaching process.
The arsenic that has leached into groundwater may be adsorbed by the surrounding subsurface soils. Adsorption of inorganic As (III) and As (V) on soil mineral surfaces is an extremely important process that affects the fate and mobility of arsenic in groundwater. There is potential for arsenic release when there is fluctuation in Eh, pH, soluble arsenic concentration and sediment organic content. At higher soil redox levels (200-500 mV), arsenic solubility is low and the major part (65-98%) of the arsenic in solution is present as As(V).
Amorphous hydroxides of iron and aluminum (Pierce and Moore, 1982; Sakata, 1987), clay content (Elkhatib et al., 1984), and pH (Pierce and Moore, 1982; Sakata, 1987; Elkhatib et al., 1984; Pierce and Moore, 1980) are the soil properties reported to affect the sorption of arsenic. The main process controlling the mobility of As in the soil-water systems is the adsorption of arsenic by soil constituents rather than by precipitation (Livesey and Huang 1981).
Speciation of arsenic is significantly affected by the Eh and pH. Arsenic acid has been found to be predominant at strong acidic pH below 2.3 and Eh ranging from 0.55-1.2 V. At pH between range 2.3-6.8 and at Eh range of (0.1 - 0.9 V), H3AsO4- has been found to be stable. With shift in pH from neutral to basic (pH 6.8-11.6) and Eh ranging between (0.1-(-0.3) V), HAsO42- is predominant and for pH (11.6-14), Eh (0.3-0.5V), AsO42- was the stable species. It has been found that for low Eh values (0.6-(-0.3) V) and pH of 9, arsenous acid is the most stable species but for pH in the range of 9-11 and Eh of (-0.3-(-0.4) V), most stable form was H2AsO3-. In a relative alkaline pH (11-13.5) and Eh of (-0.4-(-0.6) V), it forms HAsO32-. For strongly alkaline, pH greater than 13.5 and Eh between (-0.6-(-0.7) V), AsO33- species was predominant (Figure 1-3). The pH and Eh for our samples ranged between 6.89-8.19 and 207.84-246.14 respectively. From stability diagram (Figure 1-3) we could conclude that most of the arsenic existed as H2AsO4-and HAsO4-2 species.
Figure 1-3. Standard Eh-pH stability diagram for arsenic-water system @ 25oC and 1 bar.
Ash was collected from Ash Basin sites by installing piezometers. The three locations for basin #8 are shown in Figure 1-4. Sampling depths have been near surface, mid-depth and bottom of the ash deposit. Ash samples for basin #8 are PZ8-3(5-9'), PZ8-3(15-19'), PZ8-3(30.5-34.5'), PZ8-4(10-14'), PZ8-3(5-9'), PZ8-4(20-24'), PZ8-4(30-34'), PZ8-5(10-14'), PZ8-5(20-24'), PZ8-5(20-25'). Locations have been defined by PZ8-3, PZ8-4, PZ8-5 and numbers in brackets are the depths from where ashes were collected. The soils that are used in the study are obtained from near the ash basins. The soils used in the study were MW8-8A, MW8-8B(60-65'), MW8-8C(80-88'), MW8-9A(40-50'), MW8-9B(58-68'), MW8-9C(78-88'), MW8-10A(25-35'), MW8-10B(45-55'), MW8-10C(65-75'), MW8-11A, MW8-11B(45-55'), MW8-11C(65-75'), MW8-11C(G), MW8-12A, MW8-12B(45-55').
Solid phase analysis using acid digestion has been performed. The various elements that were analyzed are aluminum, antimony, arsenic, barium, beryllium, boron, cadmium, calcium, chromium, copper, iron, lead, lithium, magnesium, manganese, molybdenum, nickel, selenium, silver, sodium, strontium, zinc, chloride, fluoride, and sulfate. Particle size analysis was performed on ash samples. Eh, pH, moisture, specific gravity, organic matter, inorganic carbon analysis was performed.
The ash samples from basin # 8 were found to have pH from neutral to slightly basic. The redox potential was between 200-250 mV. The Eh of the sample was affected by its pH and viceversa. From Figure 1-5 it can be concluded that increase in Eh leads to decrease in pH (R2= 0.85, P=0.001) for these samples.
Figure 1-4. Basin #8 Site.
Figure 1-5: Eh vs pH for ash samples @ 25oC and 1 atm N2.
Total moisture content varied from 14-26 (%), having highest for PZ8-4(30-34'). Specific gravity and porosity for all the ash samples was nearly the same, in range 2.03-2.33 and 0.31-0.36 respectively. Organic matter content for ash samples was low (0-0.86). Clay content was found to be low in ash samples except in ash samples PZ8-5(20-24') and PZ8-5(20-25'), which was determined to be 32.3 and 33.4 respectively. Highest silt content was seen in ash PZ8-(30.5-34.5') (70.4%), the deepest of location PZ8-3 and sand content in PZ8-5(10-14') (54.7%). Physical properties of ash samples are listed in Table 1-3.
-Table 1-3. Physical Properties of Ash from Basin #8.
When acid digestion on the ash samples was performed, iron was found to be in highest amount (18500-33400 mg/kg), the second highest element was aluminum (3750-28400 mg/kg). Arsenic content of ash ranged from 8 to 261 mg/kg. Calcium was found in the range of 1220-5100 mg/kg, whereas boron was below detection limit for some ashes and highest value is 24 mg/kg. Manganese was between 20.8-78.6 mg/kg. Sulfate range varied from 69 to 439 mg/kg. Inorganic Carbon (IC) ranged from 1.16 and 3.10 percent. Range of nickel, chromium, copper, lead and lithium was between 0 and 100 mg/kg. Antimony, beryllium, selenium, molybdenum, and silver were present at low levels. In general, the elements had the highest concentration in deepest ash sections for all the locations, indicating that the shallower sections of ash were more weathered (leached) as compared with the deeper sections.List of all the elements with concentration has been shown in Table 1-4.
Table 1-4. Chemical Content of Ash (mg/kg) from Basin #8.
1.2 RESEARCH OBJECTIVES
This research focuses on the leaching characteristics of arsenic from ash derived from a thermal power plant along with attenuation studies of arsenic in nearby subsurface soils. Batch and continuous leaching experiments were carried out to determine the leaching characteristics of arsenic from alkaline coal fly ash.
1.2.1 Primary Research Objectives
The primary research objectives were to:
Determine the leaching of arsenic from alkaline coal fly ash with DI water using multi-stage batch sequential leaching experiments using a single solid-liquid extraction ratio.
Determine the leaching of arsenic from coal fly ash with DI water in continuous column leaching experiments using a constant flow rate of leaching solution.
This study evaluated the effect of ash chemical content, physical properties and leachate properties on leaching of arsenic from ash. Primary focus in this research was to determine the source of arsenic in ash and leachate and the factors controlling the leaching of arsenic. Many researchers observed that more than 90% of arsenic is in the form of arsenate (van der Sloot et al., 1985; Turner, 1981; Silberman et al., 1984). Yager (1998) suggested calcium arsenate [Ca3(AsO4)3] as a probable host for arsenic in alkaline coal fly ash. Various factors affecting leaching of arsenic are pH, Eh, alkalinity, particle size and the chemical content of ash. Therefore, the Eh, pH, TDS, alkalinity, particle size analysis, and the chemical content of ash and leachate were determined. Literature has shown that leaching of arsenic may be directly affected by the pH of ash. Generally arsenic in fly ash is present in the silicate matrix, alumina, oxides of iron and manganese. Calcium phase controls the leaching process of arsenic (van der Hoek et al.,1994). Tessier sequential extraction was helpful in determining the chemical association of arsenic in fly ash in various fractions and the fraction from where arsenic is leaching. Kinetics experiments were performed to determine the leaching rate of arsenic and other constituents in leachate. These results helped with determining the ash fraction from which arsenic and other leachate constituents were leaching.
Column leaching was performed on fly ash because they generates results that reflect true natural environment systems. A basic assumption in column leaching is that the distribution of the leaching solutin is uniform and that all particles are exposed equally to the leachant solution. Batch sequential leaching was also performed and compared with column leaching because column methods are more expensive and operationally more complex than the batch sequential leaching. Kinetics study, Tessier sequential extraction and data analysis were helpful in obtaining various factors affecting both short term and long term leaching of arsenic. Particle size affects the leaching of arsenic. Generally arsenic is present in silt and clay fraction of ash.
1.2.2 Secondary Research Objectives
The secondary research objectives were to:
1. Determine the speciation of arsenic in ash-water system during batch sequential leaching using equilibrium speciation software.
2. Determine the leaching of arsenic from coal fly ash with DI water in single-stage batch leaching experiments using several solid-liquid extraction ratios.
Determine the adsorption of arsenic from ash leachates and synthetic arsenic solutions onto subsurface soil/ geological materials.
This study evaluated the effect of leachate and soil properties on adsorption of arsenic. Various subsurface soils were used to examine the physical and chemical properties that influence arsenic adsorption processes in soils using ash leachates and synthetic arsenic solutions. The main factors affecting adsorption of arsenic on soils are pH, Eh, reaction time, and oxidation states of arsenic. The soil properties include particle size, cation exchange capacity, aluminum, iron, phosphorous, calcium, organic matter, and moisture. Arsenic adsorbs on iron oxides, hydroxide and oxyshydroxide in soils. Iron was present in high concentration in all soil samples. The nature of phosphorous is similar to that of arsenic and it competes with arsenic for adsorption on the soil surface. The pH of soil and leachate also affects the adsorption of arsenic. The higher solution pH is causing higher desorption relative to adsorption of arsenic on soil. Binding strength of arsenic is affected by the particle size distribution in soils.