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Zeolite Performance as an Anion Exchanger for Arsenic Sequestration in Water Siddhesh Shevade* and Robert G. Ford National Risk Management Research Laboratory, U.S. Environmental Protection Agency, Ada, O K 74820 *Corresponding author:
[email protected]
Zeolites are well known for their use in ion exchange and acid catalysis reactions. The use of zeolites in anion or ligand exchange reactions is less studied. The NH form of zeolite Y (NY6, Faujasite) has been tested in this work to evaluate its performance for arsenic removal from water in continuous— flow reactions. Zeolite NY6 performed well for arsenate removal under simple and complex inlet chemistries. The results from column studies indicate that contact time and zeolite particle size can be varied to optimize both the physical and chemical performance of the continuous-flow process. Evaluation of a physical mixture of NY6 and the NH form of zeolite ZSM-5 indicate the potential for treatment of complex contaminant streams containing As, Cd, Pb and M T B E . Overall, these experimental results indicate that synthetic zeolites offer significant flexibility in designing continuous— flow treatment processes for the removal of inorganic and organic contaminants in aqueous waste streams. +
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© 2005 American Chemical Society
In Advances in Arsenic Research; O'Day, Peggy A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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Introduction Arsenic is of environmental concern due to its toxicity and carcinogenicity (/,2). Its presence in water is due to the dissolution of minerals from subterranean strata or from an anthropogenic origin such as the leaching of manmade arsenic compounds from smelting of metal ores, agricultural pesticides, desiccants and wood preservatives. It causes arsenical dermatitis, skin cancer (3) , neurological effects, enlargement of liver, heart disease and internal cancers (4) . Techniques such as adsorption (5,6), anion exchange (7), reverse osmosis (8) and coagulation processes (9,10) are commonly used for arsenic removal from water. Alumina (5) and iron oxide/hydroxides (70) are commonly employed for arsenic removal in adsorption and coagulation methods. Iron oxide coated polymer (//), and silica containing iron oxides (12) have also been tested for arsenic removal from drinking water. Zeolites are well known for their ion exchange capacity (13,14,15). Due to the negative structural charge in zeolites, they are most commonly employed as cation exchangers. The role of zeolites in the conversion of solid and liquid hazardous wastes into environmentally acceptable products has also been successfully evaluated (16,17). Several zeolites, namely clinoptilolite (18), chabazite (18% SZP1 (19,20), 13X (21) and 5A (21) have been identified for arsenic removal from water. Synthetic zeolites Faujasite Y , ZSM-5 and Beta also showed very good arsenic removal properties in our earlier studies (22). In published reports on arsenic removal using activated alumina, adsorption via ligand exchange has been identified as the removal mechanism (5,23). A similar reaction has been postulated for arsenic sorption onto Al-rich zeolites (22,24). The focus of this study was to examine arsenic removal in continuous-flow column applications. Factors that were examined included pH, empty bed contact time, and the influence of competing ions on arsenic uptake. The main aim was to screen synthetic zeolite Faujasite Y for arsenic removal with short contact time over a wide pH range, which may be a potential water treatment process for drinking water purification and industrial effluent treatment. Zeolite performance was assessed relative to the current and proposed arsenic M C L of 50 and 10 ppb, respectively (25).
Experimental Zeolites Faujasite Y [NY6] (Si:Al = 6) and ZSM-5 [NZ] (Si:Al = 40) were procured in N H form (ZEOLYST International, U S A ) . Arsenate solutions were prepared by dissolving sodium arsenate [ N a H A s 0 7 H 0 (J.T. Baker)] into de-ionized water (Millipore 18 ΜΩ). Zeolite N Y 6 was chosen for study, since our previous research demonstrated this material performed well for arsenate removal in batch systems. Zeolite ZSM-5 was employed in this study to +
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In Advances in Arsenic Research; O'Day, Peggy A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
308 evaluate the use of zeolite mixtures for treatment of complex waste streams containing both inorganic and organic contaminants. Companion studies in our laboratory have indicated that ZSM-5 is capable of degrading methyl tert-butyl ether (MTBE). Due to the geochemical conditions that develop in ground water plumes resulting from gasoline spills, M T B E and redox sensitive metals such as arsenic are often encountered as co-contaminants.
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Reactions The continuous-flow reactions were carried out in up-flow, fixed-bed glass column reactors (Kontes Flex-Column, 1.0 cm ID). Zeolite samples of different particle size (0.125-2 mm [120-10 mesh sizes]) were loaded into the column to achieve an equivalent bed volume of 1.8-2.2 ml. Different size fractions of zeolite N Y 6 were generated by pelletizing the initial fine-grained material and crushing the resultant pellet. Size fractions were subsequently isolated from the crushed material by sieving. Solutions containing 1000 ppb A s were passed through the column using a peristaltic pump. The pH of inlet and outlet solutions were measured throughout each experiment. Reaction parameters that were tested included empty bed contact time (EBCT), inlet pH, zeolite particle size and the affect of competing ions including nitrate (Ca(N0 ) ) and phosphate (Na HP0 ). A column study was also carried out to assess the capability to treat complex waste streams using a zeolite mixture. For this test, an inlet solution containing 1000 ppb Pb, 1000 ppb Cd and 1000 ppb M T B E along with 1000 ppb As was passed through a mixture of N Y 6 and N Z (1.0 g of each). v
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Analysis and Characterization Chemical analysis of dissolved As, C d and Pb in aqueous samples was carried out using a Perkin Elmer Optima 3300DV inductively coupled plasma optical emission spectrometer (ICP-OES) or a Perkin Elmer 5100ZL graphite furnace atomic absorption spectrophotometer (GFAAS). M T B E analysis was carried out by gas chromatography using a Tekmar 2000 purge and trap system coupled to an Hewlett-Packard 5890 G C with a flame ionization detector. The limits of arsenic quantitation for ICP-OES and G F A A S were 33 ppb and 2 ppb, respectively.
Results and Discussion In recent work (22) we have studied arsenic removal from pollutant water by using synthetic zeolites. Arsenic removal was examined in batch reaction
In Advances in Arsenic Research; O'Day, Peggy A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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experiments as a function of pH to assess zeolite performance and stability. The inferences from our previous work are: zeolites achieve arsenic removal via a ligand exchange mechanism; high aluminum containing zeolites showed better arsenic removal capacity; structural stability and pH buffering capacity of zeolites enabled arsenic removal from waste streams with initial pH varying from 2 to 12. Arsenic removal to desired levels was achieved on zeolites within 15 min of reaction time in batch studies (22,24), which leads us to the continuousflow reaction studies presented in this paper.
Effect of Empty Bed Contact Time (EBCT) In continuous-flow reactions, the time for interaction between the sorbate and sorbent is a critical factor in determining performance for contaminant removal. We investigated the influence of interaction time by varying E B C T , a design parameter required for process scale-up. The results of arsenate removal on zeolite N Y 6 for three EBCTs (0.36, 1.5 and 4.5 min) are shown in Figure 1. It was observed that the effectiveness of arsenate removal increased with increasing E B C T . For an E B C T of 0.36 min, removal of arsenate below the current E P A M C L (50 ppb) was only achieved through 50 bed volumes (BV). Increase of the E B C T to 1.5 min and 4.5 min resulted in an increase of the number of bed volumes effectively treated to approximately 250 and greater than 300 B V , respectively. Arsenate was removed to a concentration of
