A geochemistry study of arsenic speciation in overburden from Mae Moh Lignite Mine, Lampang, Thailand

Size: px

Start display at page:

Download "A geochemistry study of arsenic speciation in overburden from Mae Moh Lignite Mine, Lampang, Thailand"

Bertha Cross
6 years ago
Views:

1 DOI /s SPECIAL ISSUE A geochemistry study of arsenic speciation in overburden from Mae Moh Lignite Mine, Lampang, Thailand Kanitta Wongyai Savitri Garivait Oliver Donald Received: 24 February 2009 / Accepted: 22 December 2009 Ó Springer-Verlag 2010 Abstract The aim of this study was to investigate the geochemical characteristics of arsenic in the solid material samples of the Mae Moh Mine and also the Mae Moh power plants fly ash samples were systematically studied. Arsenic concentration in overburden, coal lignite and fly ash are variable (depending on source of solid samples). The results show that the strata of overburden, J seam of coal and fly ash are rich in arsenic and also relatively soluble from fly ash; it occurs as a surface precipitate on the ash particle. The experimental study on speciation in the strata also indicates that the arsenic speciation of Mae Moh solid samples are mainly arsenate, As (V), which are approaching exceed 80%. Arsenic content in the main of overburden is in the range of mg/kg, which is larger than the arsenic background soil values. Solid materials polluted wastewater; the arsenic speciation was present predominantly as arsenate in the surface water of a series of Mae Moh solid materials basins. Keywords Trace elements Lignite Overburden Arsenic Mine drainage Mae Moh Mine K. Wongyai (&) Laboratory Section, Geology Department, Mae Moh Mine, EGAT, Lampang, Thailand kanitta.w@egat.co.th K. Wongyai S. Garivait The Joint Graduate School of Energy and Environment, King Mongkut University of Technology Thonburi, 91 Pracha-uthit Road, Bangmod, Tungkru, Bangkok 10140, Thailand O. Donald Laboratoire de Chimie Analytique Bioinorganique et Environnement, UMR CNRS 5034, Hélioparc Pau Pyrénées, Pau, France Introduction Mae Moh Mine is located in Lampang province, in the northern part of Thailand (Fig. 1). The mine is one of the largest mines in Thailand and in South East Asia with an annual production of million tons of coal. The coal is supplied to ten steam power plants within the mining area, with exploration data indicating that identified lignite reserves, more than 1,460 million ton, sufficient for a generating plant capacity of 2,400 MW. It started operation in 1955, supplying about 15% of total electrical power requirements and the only lignite-based electrical power producer in the country. About million tons of overburden has to be removed annually. Overburden is the major solid waste materials requiring disposal during the mining operation. In the dumping process, the overburden is dumped by 15-m-high spreaders to form 10-m-high dumps. The dumps are located within the previously excavated areas and in new areas. The largest fraction of dumping areas occurs off-site. These areas are located near the pit and are usually located at a high elevation. The overburden as dumped is essentially granular; however, most of the material breaks down rapidly to clay. This feature will help to reduce the permeability of dumps and limit the volume of leachate draining from dumps in the long period. Currently 40,000 tons of lignite per day are crushed and transported to the stockpile for power station by belt conveyor. Generally, the Mae Moh Power plants are the conventional lignite-fired type and are designed to burn low quality of lignite. The heating values and sulfur are of the primary importance for bending lignite quality, parameters property are shown in Table 1 for carefully monitored to ensure that the acceptable feed is maintained in coal-fire operation condition. The ash content is approximately 25%

$Fig. 1 Location of Mae Moh Basin Table 1 Mae Moh lignite quality data Parameter (AR-basic) Preferred limited Acceptable limited Comment Sulfur (%) \2.00 3.$

2 Fig. 1 Location of Mae Moh Basin Table 1 Mae Moh lignite quality data Parameter (AR-basic) Preferred limited Acceptable limited Comment Sulfur (%) \ Critical for control of SO 2 emission Ash (%) \ Increases disposal requirement Heating values (kcal/kg) [2,600 2,000 Energy available Moisture (%) \ Handing/grinding problems CaO (% sulfate free) \23 23 Slagging of the total tonnage of coal feed to the power stations. It is important to know how to dispose of the bulk of this ash without associated environment problem. Approximately 70% of Mae Moh fly ash is currently recycled, mainly in the building and construction industries. All 30% of bottom ashes without of specification for recycle transported to the ash dumping area by conveyor belts. Hart et al. (1995) studied the arsenic contamination in fly ash from three boiler types in the Mae Moh power plant and reported that arsenic can be accumulated in the fly ash samples. Arsenic was enriched in samples of bottom ash (BA) relative to electrostatic precipitator ashes (ESPA). The arsenic generally increases in concentration going from BA samples through the sequence ESPA samples and reaches the maximum of 352 ppm in ESPA, showed a distinct concentration increase with decreasing particle size. The highest concentration of arsenic occurred in the samples with the finest average particle size. The arsenic analysis of the lignite samples described in the study includes three main seams (J, K, and Q) of Mae Moh basin. The arsenic distribution of J seam shows a wide range from 3.07 to 515 mg/kg, which includes both the highest content of arsenic in the Mae Moh coal and sulfur occurrence, as indicated in the arsenic content varies between and mg/kg of K seam and also showed the arsenic content varies between 3.07 and 350 mg/kg of Q seam. The arsenic analysis of the blending feed coal lignite and fly ash samples described in the study

3 includes all of the Mae Moh power plants, the arsenic distribution of feed coal lignite and fly ash varies between and mg/kg in dry weight (Bashkin and Wongyai 2002). Arsenic speciation is very important in assessing the geochemistry, transport and fate of arsenic in soil environments, since the geochemical behavior depends on its species (Pongratz 1998; Carbonell-Barrachina et al. 1999). The main factors that control the distribution of arsenic species include redox conditions, ph, and microbial activity. Redox conditions not only affect the distribution of arsenic species, but also the chemistry of iron and sulfur closely related to the chemistry of arsenic (Lumsdon et al. 2001). Arsenate, As (V) is frequently found under oxic conditions, whereas arsenite, As (III) and arsenic sulfides occur under anoxic conditions. The specific chemical arsenic species that dominates in an aquatic environment depends on the physicochemical and biological characteristics of the water and sediment. In general, arsenite is considered more toxic and soluble than arsenate in natural environments (Korte and Fernando 1991). Toxicity and the mobility of trace metals depend strongly on their specific chemical forms and on their binding state such as precipitated with primary or secondary minerals, complexed by organic ligands. It uses a succession of chemical reagents that sequentially extract various targeted phases in sample. The results are useful for obtaining information about origin, mode of occurrence, bioavailability, potential mobility and transport of elements in natural environments. Sequential extraction can provide information about the identification of the main binding sites, the strength of metal binding to the particulates and the phase associations of trace elements in samples. This could help us to understand the geochemical processes governing heavy metal mobilization and potential risks induced. Arsenic is of particular interest and concern because it occurs commonly in coal-bearing rock and waste products such as fly ash associated with the burning of coal and disposal of fly ash. It is however widely accepted that, arsenic mostly occurs in the mineral fraction and likely with pyrite in coal. The amount of arsenic in world coals is in the range of mg/kg (Swaine 1990). However, there are extremely exceptional cases that have lead to arsenosis which have been noted in Czechoslovakia and China. In Czechoslovakia, coals with 900 1,500 mg/kg (Swaine 1995) have been burned, whereas in China, coals with as much as 35,000 mg/kg of arsenic have been burned. Finkelman et al. documented the devastating effects on human health of using high arsenic. Chinese burn coal indoors for cooking and drying vegetables; indeed, high-arsenic coals (1,400 and 2,000 mg/kg) are also found in Canada and the United States. However, because high-arsenic coals in Canada and the United States represent unusual geologic occurrences of coal, they are neither used domestically nor by industry. In the case of the Canadian coals, such high arsenic contents are related to thermal alteration processes (Van der Flier 1991). The arsenic contents of some of the individual layers of in situ coal seams, such as the high ash partings ( mg/kg), are greater than that of the feed coal from the same seam (2.4 mg/kg) (Gentzis et al. 1996). Coal is largely composed of organic matter, but it is the inorganic matter in coal minerals and trace elements that have been cited as possible causes of health, environmental, and technological problems associated with its use. Some trace elements in coal are chemically toxic such as arsenic; questions have been raised concerning possible risk from arsenic. In order to accurately address these questions and to predict the mobility of arsenic element during the coal fuel-cycle, it is important to determine the concentration, distribution, and form of arsenic elements in coal and fly ash. Assessment of the arsenic exposure from coal burning is critically dependent on the concentration of arsenic elements in coal and in the fly ash that remains after combustion. Then rainwater seeps into the solid wastes and begins to dissolve the metals and carry them into the general environment. In order to manage environment risk due to arsenic in water, arsenic speciation of overburden, feed coal and fly ash have been developed. Materials and methods The feed lignite samples were automatically collected daily at the conveyor belt before feeding to the Mae Moh power plants and fly ash samples were also collected from fly ash silo every day and one weekly averaged sample composed of bottom ash were taken during January 2002 December These samples were prepared and decomposed by digestion method. The collected samples will be kept in separate labeled bottles. These samples will be preparation and decomposed by digestion method, follow as ASTM 3201, The concentrations of arsenic in the overburden and feed coal samples and power plant ashes were determined using hydride atomic absorption spectroscopy. Speciation analysis was conducted by ICP-MS-HPLC with separation on separations of four available arsenic species were investigated. The optimization of HPLC and ICP-MS coupling provided chromatograms of the four arsenic species such as As (III), As (V), MMA and DMA for a mixed arsenic standard of 0.5 lg/l of each of the compounds. The chromatographic conditions are the following: an anionexchange column [Hamilton PRP-X100 ( mm; 10 lm)] with phosphate buffers 2 mm PBS/0.2 mm EDTA/

4 5.0 mm NaNO 3 as mobile phase, flow rate (1 ml/min), injection volume (50 ll). Repeatability and accuracy on the lowest sensitivity scale were calculated from ten consecutive measurements of an artificial standard containing 0.4 lg As/ L of each of the four compounds of interest. Field studies included monthly 16 water sampling stations for 5-year period. Sump water samples which leaching of waste of overburden. Seepage of ash pond from the lignite-fired generating plants and reservoir near the surface mining area and power stations. Field analysis water-quality parameters such as ph and specific conductance will be kept in separate labeled plastic bottle. Results and discussion Concentration of arsenic in feed low rank coals used in 10 units of Mae Moh Power plants that were burning range in lignite coals were examined in this study range of mg/kg in dry weight (Fig. 2). Concentrations (mg/kg) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month of samples Fig. 2 Arsenic content of Mae Moh power plants feed lignite samples (mg/kg in dry weight) The arsenic analysis of the fly ash samples described in this section includes the three main units of Mae Moh power plants (units 4 8, units 9 10 and units 11 13). The total production of fly ash collected in the ESP was about 5 million tons per study year. The arsenic analyses were conducted for 52 composite samples and the relevant results were in the wide range between of mg/kg in dry weight (Fig. 3). It is important to know as how to dispose of the bulk of this ash without associated environment problem. Fly ash of the Mae Moh has leached through ash ponds and contaminated seeps water almost all over the sump area of mine. Natural waters also have been heavily contaminated because of the presence of power stations near the natural waters. Fly ash also has contaminated the soil around the fly ash ponds with presence of arsenic in the soil Table 2. Arsenic content of the overburden composition samples used for leaching experiments is given in Fig. 4, Table 3. The solutions resulting from the leaching experiments with distil water and simulated at ph 5 7. The chromatograms shown in Fig. 4 are for two main different concentrations of arsenic species indicating good chromatographic separation of the arsenite and arsenate species. Arsenite and arsenate were found in the Mae Moh overburden samples. The leachate extraction of overburden samples for arsenic speciation shows chromatogram of arsenate as the predominant species (by comparison with standard retention times) with a small amount of arsenite [As (III)]. Chappell et al. (1995) investigated the solid phase speciation of arsenic at former cattle dip sites where As (III)-based pesticides were used to control cattle ticks. They reported more than 95% of total arsenic in the soils was present as As (V) (Fig. 5). The speciation of arsenic in soils determines its behavior and mobilization. Arsenate has generally been reported to dominate the soil solid phase (Chappell et al. 1995). Arsenate and arsenite As (III) have been reported to undergo similar sorption reactions in soils although the Fig. 3 Arsenic content of Mae Moh fly ash lignite-burning samples (mg/kg in dry weight) Concentrations (mg/kg) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month of Samples

5 Table 2 Summary of TDS, hardness and sulfate (mg/l) and arsenic (ppb) in Mae Moh area water samples Location Concentration range (mg/l) ph ECD TDS Hardness Sulfate Arsenic (ppb) Ban Tha Si ± ± ± ± ± 3.2 Mae Kram Reservoir ± ± ± ± ± 2.4 Mae Chang Reservoir ± ± ± ± ± 3.6 Sump NE 6.7 4,030 ± 4.3 3,850 ± 2.1 1,590 ± 4.6 2,600.5 ± ± 14.3 Seepage 6.7 5,250 ± 5.2 5,198 ± 3.4 2,580 ± ,630.0 ± ± 9.8 Settling pond 7.0 3,090 ± 2.3 2,567 ± 2.7 1,789 ± 5.3 2,226.5 ± ± 5.7 NE wetland 7.2 2,090 ± 1.4 1,950 ± ± 4.7 1,226.5 ± ± 4.5 Outlet NE wetland 7.3 1,985 ± 2.3 1,756 ± ± ± ± 3.4 SW wetland 7.1 1,245 ± 1.5 1,081 ± ± ± ± 10.6 Outlet SW wetland 7.2 1,445 ± ± ± ± ± 13.2 Coal stockpile drain 6.5 2,600 ± 2.3 1,500 ± 1.6 1,234 ± 6.7 1, ± ±13.1 Main drain 7.8 4,900 ± 4.1 1,800 ± 1.9 1,800 ± 7.5 2,145.0 ± ± 3.7 Ash water lake 8.3 4,825 ± 5.8 1,523 ± 1.5 1,554 ± 0.3 3,825.0 ± ± 4.8 Power plant wetland 7.3 2,200 ± 2.3 2,000 ± 2.6 1,425 ± 2.5 1,955.3 ± ± 6.2 South wetland 7.0 1,095 ± ± ± ± ± 15.1 Mae Moh Reservoir 7.1 1,225 ± 2.4 1,080 ± ± ± ± 15.1 Table 3 Range of arsenic speciation concentrations in the overburden Mae Moh area Overburden As range (ppb) As average ph 5 (ppb) As average ph 6 (ppb) As average ph 7 (ppb) JP KP QP Fig. 4 Arsenic content of Mae Moh Mine overburden samples (mg/ kg in dry weight) sorption of As (III) is generally of a lower magnitude that that of As (V) (Smith et al and Manning and Goldberg 1997). In this study, we investigate the overburden and leachate water distribution of arsenic speciation in watershed collected from contaminated Mae Moh mining and power plants area. Arsenic and other metals were applied to these samples (Table 3). As expected, the solutions become acidic (ph 5 and rich in sulfate in both leachates). The same ph values and sulfate concentrations were observed in leaching experiments composition of the leachates obtained in the laboratory experiments is quite similar to that found in some water samples collected at the mining areas. The chromatograms shown in Fig. 6 are two main different concentrations of arsenic species indicating good chromatographic separation of the arsenite, As (III) and arsenate, As (V) species. Arsenite and arsenate were found in the Mae Moh overburden, lignite and fly ash samples in similar ratio by HPLC ICP-MS. As shown in the Tables 3, 4, the amount of arsenic leached out from solid sample was found to be of higher level and other trace metals found to be lower level than the regulatory limit in Thailand. However, acid washing has a drawback in that a great deal of wastewater containing the metal is inevitably leached out. Another attempt has also been made for solidification of trace metals by the production of lightweight aggregate with solid samples. Arsenic is typically found in uncontaminated soils at concentrations up to 50 mg/kg (Alloway 1990). The concentration of arsenic in the lignite, fly ash and soil sample from the Mae Moh area were over this background soil values. The samples from the higher concentrations of arsenic in the lignite, fly ash from the Mae Moh may result from the use of ionizing wet scrubbers at this facility,

6 which can increase the removal of these elements from gaseous emissions into solid waste streams. Conclusions Fig. 5 TDS, hardness, sulfate (mg/l) and arsenic concentration (ppb) in Mae Moh water samples Fig. 6 Arsenic speciation chromatograph of leachate solid samples (ppb) Water quality from mine drainage depends on the lignite composition and the geological structure of mines. 16 surface water sampling stations, water samples which will be taken from sump and open pit mine which would be drained from mining activity, external mine and power plants activity. The Mae Moh wetlands applications, wetlands are used particularly for the treatment of relatively high salinity and some trace metals which flow from the mine sumps, mine waste dumps and power plants activities. Water drainage inflows to the mine are directed toward sumps, flowing from or caused by surface mining, lignite refuse piles and contaminants in water include high total dissolved solids, hardness, conductivity, sulfate with elevated levels of dissolved metals in highly acidic. Surface mining of coal lignite often exposes pyretic spoil material and leachate of coal lignite, to remove sulfur, also creates refuse materials with enhanced concentrations of pyrite. These are generally constructed for treatment of runoff before it enters natural receiving waters. Determination of arsenic speciation found that all of geological origins of Mae Moh material samples were Table 4 Composition of leachate solution water samples collected from a pond on site Mae Moh mine Power plants Flyash seep Mae Moh wetland ph Conduct 4,000 4,500 2,800 2,400 Trace elements (mg/l) Fe Al Mn Ca Mg Na Cr Ni 1,200 1,467 1, Cu 900 1, Zn 2,900 3,377 1,600 1,700 Cd N/A N/A Pb Anions (mg/l) SO 4 2-2,228 2,612 1, Cl NO 3 3- PO 4 \ \0.1 3 \ \0.1 9

7 mainly inorganic arsenic compound. The results of arsenic speciation indicate that 30% of arsenic in most of the feed coals is in the form of arsenite As (III) and 70% is in form of arsenate As (V). However, for the same rank of coal, but with higher sulfur content, the results of this study showed that the quality of reference sample was a water of low salinity such as EC approximately us/cm, hardness mg/l as CaCO 3, sulfate 2 49 mg/l with a slightly alkaline ph Wastewater from Mae Moh Mine and Mae Moh power plant drainage were characterized by high salinity. The contaminants of wastewater showed high concentration in many parameters such as conductivity, TSD, hardness and contain moderate to high concentration of sulfate and arsenic. The highest concentrations were monitored in water drainage of mine sumps. The high contamination of the wastewater in the study area was in the wide range of variables depending on the period and sampling station. The total quantities of toxic and potentially toxic arsenic contained in the solid samples produced by power plants can be estimated from the total quantities of solid samples produced and the elemental composition of the fly ashes. Acknowledgments I would like to express my sincere and appreciation to Assoc. Prof. Dr. Benjavun Ratanasathien for his suggestions and advice in continuing our work. Special thanks are also to the EGAT staff at Mae Moh Mine, with special for Geology Department. This study was undertaken at Laboratory section Mae Moh Mine, Perkin Elmer s Korea Laboratory and Laboratoire De Chimie Analytique Bio-Inorganique Et Environnement (LCLBIE). I wound like to specially thank Dr. Fabienne Seby, for her direct supervision in developing the analytical work procedure, assistance and just plain attitude with ICP-MS HPLC analyses, Pau University, France. My special thanks to the supported staffs of laboratory section for their assistance during my study. References Alloway BJ (1990) Heavy metals in soils. Wiley and Sons, New York Bashkin VN, Wongyai K (2002) Environmental fluxes of arsenic from lignite mining and power generation in northern Thailand. Environ Geol 41: Carbonell-Barrachina A, Jugsujinda A, Burlo F, Delaune RD, Patrick WH Jr (1999) Arsenic chemistry in municipal sewage sludge as affected by redox potential and ph. Water Resour 34: Chappell J, Chiswell B, Olszowy H (1995) Speciation of arsenic in a contaminated soil by solvent extraction. Talanta 42: Gentzis T, Goodarzi F, Koukouzas CN, Foscolos AE (1996) Petrology, mineralogy and geochemistry of lignites from Crete, Greece. Int J Coal Geol 30: Hart BR, Powell MA, Fyfe WS (1995) Geochemistry and mineralogy of fly ash from the Mae Moh lignite deposit, Thailand. Energy Source 17:23 40 Korte NE, Fernando Q (1991) A review of arsenic (III) in groundwater. Crit Rev Environ Control 21:1 39 Lumsdon DG, Meeussen JCL, Paterson E, Garden LM, Anderson P (2001) Use of solid phase characterization and chemical modeling for assessing the behavior of arsenic in contaminated soils. Appl Geochem 16: Manning BA, Goldberg S (1997) Adsorption and stability of arsenic (III) at the clay mineral water interface. Environ Sci Technol 31: Pongratz R (1998) Arsenic speciation in environmental samples of contaminated soil. Sci Total Environ 224: Smith AH, Biggs MU, Moore L, Haque R, Steinmaus C, Chung J, Hemandez A, Lopipero P ( 1999) Cancer risks from arsenic in drinking water: implication for drinking water standard. Arsenic exposure and health effects III, pp Swaine DJ (1990) Trace elements in coal. Butterworths, London, p 278 Swaine DJ (1995) Environmental aspects of trace elements in coal. Springer, Berlin, p 324 Van der Flier-Keller E (1991) Platinum-group elements in Tulameen coal, British Columbia, Canada. Econ Geol 86(2):

A FIELD DEMONSTRATION OF AN ALTERNATIVE COAL WASTE DISPOSAL TECHNOLOGY GEOCHEMICAL FINDINGS. Paul T. Behum. Liliana Lefticariu. Y.

A FIELD DEMONSTRATION OF AN ALTERNATIVE COAL WASTE DISPOSAL TECHNOLOGY GEOCHEMICAL FINDINGS Paul T. Behum Office of Surface Mining, Mid-Continent Region, Alton, IL Liliana Lefticariu Department of Geology,