Selenite and Selenate Removal by a Functionalized Alumina Adsorption Technology
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IWC 12‐26 Selenite and Selenate Removal by a Functionalized Alumina Adsorption Technology NANCY SHERWOOD, MELISSA HAYES, REBECCA UNKEFER and GINA SACCO, MAR Systems Inc. Solon, Ohio IWC 12‐26 Keywords: adsorbent media, functionalized alumina, selenite, selenate, selenium speciation, fixed bed, bed volumes, mining, contact time, water quality, temperature, organic acid pretreatment ABSTRACT Selenium speciation presents unique challenges for removal to low ppb trace levels. The selenite oxyanion is reported to be much more readily removed from water by adsorption technology in flow-through column studies than the selenate oxyanion. Selenate removal however, is paramount to wastewater decontamination in the coal mine process and FGD scrubber water markets. The removal of both species is discussed as a function of water quality, pH and temperature. Water conditioning pretreatment options that improve selenate removal by adsorbent technology are discussed and treatment case histories providing selenium removal in coal mine process water ponds to very low levels of selenium are provided. IWC 12‐26 INTRODUCTION pertinent to reducing contaminant levels in Over the past two decades, treatments of these wastewaters. reverse osmosis, ion exchange, reduction by iron, active and passive microbial reduction, Environmental impact studies on the effects in situ microbial reduction and wetlands of coal mining in the Appalachian Plateau have emerged as leading technologies for indicate that concentrations of selenium in selenium removal (Golder, 2009). However sediment ponds and small streams the market continues to look for new frequently exceed the aquatic wildlife innovation in selenium removal to mitigate standard of 5 ppb. (Bevans, 2005). The issues of ease-of-use, capital cost, ancillary selenium in these waters is generally present waste streams and the ability to reach low in a dissolved state and speciation studies selenium levels discharge with these report that the majority of this soluble technologies. selenium is present as the selenate (Se+6, SeO4) species. (Bevans, 2005). The Adsorbent technology has been identified University of West Virginia found that 90% that addresses many of these issues. of the selenium in the Mud River, West Ongoing research utilizing a proprietary Virginia, was present as selenate, and functionalized alumina adsorbent selenate was measured at 70% levels in technology in a single pass through a other West Virginia valley fill streams standard filtration vessel with no ancillary (Bevans, 2005). Selenate is the waste stream, has found interesting predominant species reported by coal variables that affect the removal of selenium companies to be present in their mining oxyanions. This paper describes laboratory tailings ponds and other process waters. and field removal studies that examine Selenate removal has typically been more selenite and selenate removal from synthetic difficult than the other common soluble +4 and coal process waters and suggests ways species, selenite (Se , SeO3) because of its to improve selenium removal. The findings high oxidation state. To mitigate the indicate that removal is not necessarily selenium problem, the Appalachian region species driven and that this adsorbent coal industry is actively seeking effective technology is highly capable of removing selenium removal technologies. both selenite and selenate to achieve a reduction in total selenium applicable to In flue gas desulfurization (FGD) systems, both coal mines and FGD scrubber the newer forced oxidation units drive a wastewaters. majority of the selenium to the highest oxidation state. EPRI reports selenate levels FGD wastewater streams and coal mine up to 100% when high oxidation air rates are process waters increasingly face discharge used for sulfur oxidation (Sandy,2010). regulations regarding selenium sourced from Like the coal mining waters, selenate is the coal. Selenium occurs naturally in coal and predominant species and the industry is tracks with the sulfur content of the coal seeking effective removal technologies. throughout the processes in coal fired power plants and coal mining operations, THE ADSORPTION TECHNOLOGY - A eventually ending up in the wastewater. The specialized adsorbent media can be effective removal of selenium in both of its oxyanion in solving coal mine and FGD selenium forms, selenite and selenate, is highly contamination. The media is produced from IWC 12‐26 an activated alumina substrate upon which capable of attracting selenium oxyanions various proprietary chemistries are reacted and other metals, performance for both throughout the high porosity. These varied species was anticipated. The tests were inorganic chemistries form functionalized conducted using commercially available sites in the media for selected soluble metals selenite and selenate standards diluted in to covalently bond and be removed from Solon, Ohio, tap water to provide a water aqueous streams. At least three of the with ionic strength for speciation chemistry components forming these comparative studies. Selenium removal of functionalized sites are known to complex the two species was studied separately by with soluble selenium in the selenate and flowing 25 ppb selenium containing water selenite oxyanion forms. The removal is through a packed bed of the adsorbent media accomplished by the formation of various at a 10 minute water-to-media contact time selenite and selenate complexes such as for 100 bed volumes of treatment at a aluminum selenite, aluminum selenate and hydraulic loading typical of adsorbent others. treatment vessels (1 to 6 gpm/ft2). The removal of both species by the media was SPECIATION STUDIES high, sustained and nearly identical, Figure 1. The adsorbent technology was evaluated for the removal of both selenite and selenate EXPERIMENTAL PROCEDURE and species in packed bed flow-through columns RESULTS FOR COLUMN TESTS USING of adsorbent media. Traditionally, plain un- WATER PREPARED FROM SELENIUM functionalized alumina is known to have a STANDARDS - The selenite standard was high adsorption capacity for the selenite purchased commercially and contained species only (Kapoor, 1994). However, selenium (+4) oxide in a 2% nitric acid in because this media contains reacted low TOC water (<50 ppb). As prepared, the chemistries throughout the alumina that are standard contained 1000 µg Se (+4) per IWC 12‐26 milliliter of solution. The selenate standard then prepared by diluting 50 ml of the was purchased commercially and contained intermediate standard to 2000 ml with tap anhydrous sodium selenate in ACS reagent water. grade water. The standard contained 1000 µg Se (+6) per one milliliter of solution. The functionalized alumina media was Both standards had valid expiration dates packed into two fixed-bed glass columns. still in effect. Each column had an effective length of 30 cm and an inner diameter of 1 cm. A screen Solon, Ohio, tap water is a moderate distributor was placed into the bottom of the hardness water characteristic of the Great column and a small piece of glass wool was Lakes region. It nominally contains 31 ppm inserted on top of the fitting to act as a water calcium, 8 ppm magnesium, 25 ppm distributor. Media sized to 1680 µm x 420 chloride, 27 ppm sulfate, 0.5 ppm µm was packed into the columns and back- phosphorus, 0.03 ppm iron, pH of 6.9 and 4 flushed to remove fines prior to use. 47.5 ppm suspended solids. It is consistent in bed volumes or 2.0 liters of the selenite quality and other metals removal studies water was pumped up-flow through the using this water have been reproducible. column at 4.2 ml/min in series. These conditions allowed for a 10 minute contact Selenium analysis for all testing was done time and a flow flux of 1.35 gpm/ft2. A by an outside EPA certified laboratory using similar experiment was performed in a ICP method E200.7. When results were down-flow configuration with the same near the method detection limit of 5 ppb, the selenite water. No difference was observed EPA certified lab implemented a check in selenium removal between the water in an standard procedure to report numbers below up-flow or down-flow direction. Since no 5 ppb and/or samples were sent for low level apparent difference was evident between up- selenium analysis by method SM3114C-M flow and down-flow modes, all subsequent gaseous hydride atomic fluorescence. testing was performed in the up-flow configuration. A removal of >90% of the All columns used in these studies were selenite was found. Inlet value of 25 ppb packed with commercial Sorbster™ dry was reduced to less than 2 ppb, Figure 2. media in a standard pellet form, supplied by MAR Systems Inc. The beds were then SELENATE PACKED BED TEST #1: backwashed with Solon tap water to wet the FUNCTIONALIZED ALUMINA ALSO media, rinse off fines and remove entrapped REMOVES SELENATE - The 25 ppb air prior to introduction of the test selenium selenate water was prepared with the same water. procedure as the selenite water however, 1.81 ml of the stock standard was diluted to SELENITE PACKED BED TEST #1: 1000 ml in order to prepare an intermediate FUNCTIONALIZED ALUMINA THAT containing 1000 ppb Se as selenate. The 25 REMOVES SELENITE - Selenite water ppb Se water containing selenate was tested was prepared at 25 ppb by diluting a one in the same column configuration as the milliliter aliquot of the 1000 ppm selenite selenite test in the up-flow direction. A standard to 1000 ml with tap water to removal of >90% of the selenate was found. produce an intermediate water containing An analyzed value of 22 ppb selenate was 1000 ppb Se. The 25 ppb test water was reduced to below 2 ppb, Figure 3. IWC 12‐26 IWC 12‐26 SELENITE AND SELENATE REMOVAL These column tests were run in 36” long by IN LARGER COLUMNS WITH HIGHER 1” internal diameter glass columns. In place VOLUME OF TREATMENT- Confirming of glass wool, a screen supplied with the tests using larger packed bed columns and fitting was used as a distributor and to higher treatment volumes were run that eliminate any plugging of the tubing by the achieved the same results as the packed bed.