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Removal of Uranium from Contaminated Water by Clay Ceramics in Flow-Through Columns Charles Florez 1 , Young Ho Park 2, * 1 2 3 4

*

ID

, Delia Valles-Rosales 1 , Antonio Lara 3 and Emilio Rivera 4

Industrial Engineering Department, New Mexico State University, Las Cruces, NM 88003, USA; [email protected] (C.F.); [email protected] (D.V.-R.) Mechanical and Aerospace Engineering Department, New Mexico State University, Las Cruces, NM 88003, USA Chemistry and Biochemistry Department, New Mexico State University, Las Cruces, NM 88003, USA; [email protected] Mass Spectrometry Core Laboratory, Vanderbilt University, Nashville, TN 37240, USA; [email protected] Correspondence: [email protected]

Received: 8 September 2017; Accepted: 29 September 2017; Published: 2 October 2017

Abstract: Uranium contamination of groundwater increasingly concerns rural residents depending on home wells for their drinking water in communities where uranium is a source of contamination. Established technologies to clean up contaminated aquifers are ineffective in large contaminated areas or are prohibitively expensive. Permeable reactive barriers (PRBs) are a low-cost alternative to these methods. In this paper, the applicability of clay ceramic pellets was investigated as permeable reactive barriers (PRBs) material for the treatment of uranium-contaminated groundwater. Flow-through columns were fabricated and used to mimic the flow path of a contaminant plume through the reactive media. Experiment results show that clay ceramic pellets effectively remove uranium from uranium-contaminated water and also can be a cost-efficient technique for remediating uranium contaminated groundwater by a clay pellet barrier. Using clay ceramic pellets is also a practical treatment method for uranium removal from drinking water and can supply potable water for households in the affected areas. Keywords: clay ceramics; permeable reactive barrier; uranium; cation exchange capacity; flow-through column

1. Introduction Uranium contamination of groundwater increasingly concerns rural residents depending on home wells for their drinking water in communities with a legacy of mining [1] as well as those living in areas naturally occurring uranium is a source of contamination [2–4]. In the United States, naturally occurring elevated uranium in groundwater is widespread in the west and is scattered in Eastern states [2,5]. Also significant problems stemming from the legacy of uranium development still exist today in the Colorado Plateau area [6]. Uranium in ground water is most commonly found in its hexavalent oxidation state U(VI) as the uranyl ion (UO2 2+ ) [7]. The aqueous solubility of U(VI) makes it difficult to physically remove uranyl from water. Clean-up of contaminated aquifers is difficult due to the inaccessibility to the subsurface and the volume of soil and groundwater requiring treatment [8]. Established technologies such as pump-and-treat and soil excavation are ineffective in large contaminated areas or are prohibitively expensive [9]. Permeable reactive barriers (PRBs) are a low-cost alternative to these methods [10]. The PRB is an in situ permeable treatment zone designed to intercept and remediate a contaminant plume. Several mechanisms have been proposed for the treatment of uranium using elemental iron Water 2017, 9, 761; doi:10.3390/w9100761

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(zero-valent iron) as PRB material, including reductive precipitation [11,12], sorption onto hydrous ferric oxide [13], and co-precipitation with iron oxides [14]. The purpose of the present study is to investigate the applicability of low temperature sintering clay ceramics as PRB material for the treatment of uranium-contaminated groundwater. Natural clays are characterized with sorption ability of different chemical compositions [15–20] and thus are suitable materials for environmental technologies. Clay adsorption capacity is generally increased by granulation [21]. However, such materials have a weak point. Considering the colloidal character of clay minerals, after purification of polluted water such water contains another kind of pollution—colloidal particles of clay containing adsorbate. Low temperature sintered clay ceramics can help to resolve this disadvantage for water purification technologies [22]. The sintering treatment makes clay ceramics insoluble in water. Clay ceramics used in this study have negatively charged sites on their surfaces which adsorb and hold positively charged uranyl ions by electrostatic force. Clay ceramics are water-insoluble and easy to remove from water after treating contaminated water. Flow-through columns were built and used to mimic the flow path of a contaminant plume through the reactive media. A peristaltic pump slowly fed the contaminant medium. Uranyl nitrate was used as a source of Uranium(VI). We also tested the efficiency and practicability of clay ceramic pellets to facilitate abatement of uranium from drinking water. Underprivileged communities do not have access to expensive water purification systems and current methods for purifying water on a small scale are temporary solutions for a small volume of purified water and expensive. The clay ceramic pellets can be used to remove uranium in any environment where individuals are at risk of ingesting uranium contaminated water. 2. Materials and Methods Two different clays from Arizona Cheto (smectite mineral) and New Mexico Gallup (illite mineral) were used for production of ceramic pellets. Both clays are 2:1 layer minerals where an octahedral sheet is bonded to two tetrahedral sheets. These clay minerals have significant permanent negative charges which contribute to the cation exchange capacity (CEC) of clays [23]. The permanent negative charge in the clay results from the substitution of divalent cations for trivalent cations in the octahedral sheet [24]. For example, the permanent negative charge in the smectite group results from substitution of divalent cations Mg2+ for trivalent cations Al3+ in the octahedral sheet. The CEC is a measure of the clay’s ability to hold positively charged ions. Uranium sorbs to clay through this unique characteristic. 2.1. Fabrication of Clay Pellet and Flow-Through Column Dry clays from Arizona Cheto and New Mexico Gallup were mixed with water and spread into a silicon mold in the form of a pellet. These samples were sintered (thermal curing) in a laboratory furnace. A first densification took place below 150 ◦ C from drying the residual water on clay surface [25]. The clay pellets were further heated until chemically bonded water with clay molecules escaped from the clay. The curing process is well described in Reference [22]. Flow-through columns were fabricated using acrylic tubes having an effective height of 25.4 cm and internal diameter of 2.5 cm. The columns have four lateral sampling ports capped with Mininert valves. The columns were equipped with check valves at the inlet and outlet, which allowed for draining liquids out of the columns. These columns were used for flow-through experiments for uranium removal from groundwater as well as water purification experiments for uranium removal from drinking waters as shown in Figure 1.

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Figure 1. Flow-throughcolumn columnapparatus apparatus and and experimental experimental setup flow-through Figure 1. Flow-through setupfor fornonstationary nonstationary flow-through experiments (left) and stationary water purification experiments (right). experiments (left) and stationary water purification experiments (right). Figure 1. Flow-through column apparatus and experimental setup for nonstationary flow-through experiments (left) and stationary water purification experiments (right).

2.2. Experimental Procedure and Sampling

2.2. Experimental Procedure and Sampling

2.2. The Experimental and Sampling columnsProcedure were packed with clay ceramic pellets. Uranium solution was prepared by The columns were packed with clay ceramic pellets. Uranium solution was prepared by dissolving dissolving 0.0015 g uranyl nitrate hexahydrate into 2 L of deionized water (about 400 ppb uranium The columns packed into with2 clay ceramic pellets. solution was prepared byA 0.0015 g uranyl nitrate were hexahydrate L of deionized waterUranium (about 400 ppb uranium solution). solution). A peristaltic pump was used to feed the uranium medium at 5.75 mL/hour to the column. dissolving 0.0015 g uranyl nitrate hexahydrate into 2 L of deionized water (about 400 ppb uranium peristaltic pumpwas wascollected used to in feed uranium platy medium at 5.75 mL/h to the column. Waste Waste liquid a 2the L playtypus bottle. Samples were taken from the side liquid ports towas solution). A peristaltic pump was used to feed the uranium medium at 5.75 mL/hour to the column. collected a 2 L playtypus platy bottle. Samplesaswere taken from the side ports toneedle obtainwas concentration obtain in concentration profile within a column shown in Figure 2. A 20-gauge injected Waste liquid was collected in a 2 L playtypus platy bottle. Samples were taken from the side ports to profile within columnport as shown in Figure 2. A 20-gauge needle was through Mininert through the aMininert valve (Sigma-Aldrich, St. Louis, MO, USA) to injected withdraw 5 mL ofthe fluid with obtain concentration profile within a column as shown in Figure 2. A 20-gauge needle was injected porta disposable valve (Sigma-Aldrich, St. Louis, MO, USA) to withdraw 5 mL of fluid with a disposable syringe. syringe. Samples were taken every 24 h and data were collected for five days. Samples through the Mininert port valve (Sigma-Aldrich, St. Louis, MO, USA) to withdraw 5 mL of fluid with Samples were taken every 24 h 7500 and data collected fiveCA, days. Samples werewith analyzed using were analyzed using Agilent Serieswere ICP-MS (Santafor Clara, USA) equipped a Agilent a disposable syringe. Samples were taken every 24 h and data were collected for five days. Samples Agilent 7500 Series ICP-MS (Santa Clara, CA, USA) equipped Agilent micromist nebulizer micromist nebulizer part No. (G3266-65003) (Agilent, Santa Clara,with CA, a USA) and Cetac (Omaha, NE, were analyzed using Agilent 7500 Series ICP-MS (Santa Clara, CA, USA) equipped with a Agilent 520 Autosampler (Cetac, Omaha, NE, USA). partUSA), No. ASX (G3266-65003) (Agilent, Santa Clara, CA, USA) Cetac USA), ASXNE, 520 micromist nebulizer part No. (G3266-65003) (Agilent, Santaand Clara, CA,(Omaha, USA) andNE, Cetac (Omaha, Autosampler (Cetac, Omaha, NE, USA). USA), ASX 520 Autosampler (Cetac, Omaha, NE, USA).

Mininert Mininert

Port 4 Port34 Port Port 3 Port 2 Port12 Port

Medium

Peristaltic pump Peristaltic

+ Medium U(VI) + U(VI)

pump

Port 1

Waste Waste

Check Check

Figure 2. Schematic of the flow-through experimental setup.

Figure 2. Schematic ofof the Figure 2. Schematic theflow-through flow-throughexperimental experimental setup. setup. 3. Results and Discussion

3. Results and Discussion 3. Results Discussion 3.1. Clayand Characterization 3.1. Ceramic Clay Characterization pellets were made from Arizona Cheto clay and New Mexico Gallup clay as shown in 3.1. Clay Characterization Figure 3. These samples were imaged using the scanning electron microscopy (SEM) (Hitachi High Ceramic pellets were madefrom from Arizona Cheto Cheto clay New Gallup clay as shown in Ceramic pellets were clayand and NewMexico Mexico Gallup shown Technologies, Dallas, TX, made USA). FigureArizona 4 shows SEM images of Cheto (Cheto, AZ, USA)clay and as Gallup Figure 3. These samples were imaged using the scanning electron microscopy (SEM) (Hitachi High in Figure 3. These samples were imaged using the scanning electron microscopy (SEM) (Hitachi Technologies, Dallas, TX, USA). Figure 4 shows SEM images of Cheto (Cheto, AZ, USA) and Gallup

High Technologies, Dallas, TX, USA). Figure 4 shows SEM images of Cheto (Cheto, AZ, USA) and

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Gallup clay (Gallup, NM, USA) pellet samples. Both clays have open structures formed by laminar clay (Gallup, NM, USA) pellet samples. Both clays have open structures formed by laminar particles particles which keep edge-to-edge and edge-to-face contacts. clay formed particles asparticles small and clay (Gallup, NM, USA) pellet Both clays have openCheto structures byappear laminar which keep edge-to-edge andsamples. edge-to-face contacts. Cheto clay particles appear as small and irregularly-shaped flakes, which aisdistinctive morphology ofof smectite clay. Itappear has been which keep edge-to-edge andisedge-to-face contacts. Cheto clay particles as reported small andthat irregularly-shaped flakes, which a distinctive morphology smectite clay. It has been reported clays in Northern New Mexico were formed by smectite and illite [26]. The SEM image of Gallup irregularly-shaped flakes, is were a distinctive morphology of illite smectite It has beenof reported that clays in Northern Newwhich Mexico formed by smectite and [26]. clay. The SEM image Gallupclay shows abundant pseudo-hexagonal illite platelets, which suggests that Gallup clay has a morphology that clays in Northern Mexico were formed smectitewhich and illite [26]. The imageclay of Gallup clay shows abundantNew pseudo-hexagonal illite by platelets, suggests thatSEM Gallup has a closer that ofabundant illite than that of smectite. claytoshows pseudo-hexagonal platelets, which suggests that Gallup clay has a morphology closer to that of illite than that illite of smectite. morphology closer to that of illite than that of smectite.

Figure 3. Arizona Cheto clay pellet (0.7 cm in diameter, white) and New Mexico Gallup clay pellet

Figure 3. Arizona Cheto clay pellet (0.7 cm in diameter, white) and New Mexico Gallup clay pellet Figure Cheto (0.9 cm 3. in Arizona diameter, red). clay pellet (0.7 cm in diameter, white) and New Mexico Gallup clay pellet (0.9 cm in diameter, red). (0.9 cm in diameter, red).

Figure 4. SEM images of the samples of Cheto clay (left) and Gallup clay (right). Figure 4. SEM images of the samples of Cheto clay (left) and Gallup clay (right).

Figure 4. SEM images of the samples of Cheto clay (left) and Gallup clay (right). 3.2. Flow-Through Experiments for Uranium Removal from Groundwater 3.2. Flow-Through Experiments for Uranium Removal from Groundwater Uranium concentration profiles wereRemoval determined along the length of the column after 24 h of 3.2. Flow-Through Experiments for Uranium from Groundwater Uranium concentration profiles weretime determined the5 length theconcentration column after profiles 24 h of operation, when the hydraulic retention was 12 h.along Figure depictsofthe Uranium concentration profiles weretime determined the length of the column after h of operation, when thethe hydraulic retention was 12 h.along Figure 5 depicts the concentration profiles along the length of column for uranium abatement with Gallup and Cheto clay pellets. In 24 this operation, when the hydraulic retention time was 12 h. Figure 5 depicts the concentration profiles along the length of the column uranium abatement with andGallup Cheto clay clay pellets pellets.and In this experiment, uranium levels werefor reduced from 370 ppb to 107Gallup ppb with 370 along the length of the column for uranium abatement with and Cheto clay pellets. In experiment, uranium levels were reduced from 370 ppb to 107Gallup with Gallup clay pellets andclay 370this ppb to 17 ppb with Cheto clay pellets. Cheto clay pellets have appb faster reaction rate than Gallup experiment, uranium levels were reduced 370 ppb to 107 ppbreaction with Gallup clay pellets ppb to 17 ppb with Cheto clay pellets. Chetofrom clay pellets have a Arizona faster rate Gallup pellets because of the high cation exchange capacity (CEC). Cheto claythan belongs to clay theand 370smectite ppb to 17 ppb with Cheto clay pellets. Cheto clay pellets have a faster reaction rate than Gallup pellets because of the exchange capacity Arizona clay belongs to the clay group andhigh has acation high CEC value (about 120(CEC). cmolc/kg) [27,28].Cheto Gallup clays are expected smectite clay group and a high CEC value (about 120CEC cmol c/kg)Arizona [27,28]. expected clayto pellets because thehas high cation exchange capacity (CEC). Cheto belongs cclay /kg) are [26]. Thatto is,the have lower CECof values since illite has generally low values (about Gallup 40 cmolclays /kg) [26]. is, have lower CEC values illite has generally low values (about cmolcion, 1tokg ofclay Cheto and Gallup could take 120 (about centimols and 40 centimols of40 uranyl respectively. smectite group and has clay asince high CEC value 120CEC cmol [27,28]. Gallup clays areThat expected c /kg) 1 kg data of Cheto andvalues Gallup clay could take 120 may centimols and 40 centimols of 40 uranyl respectively. This indicates that slow reaction kinetics require a treatment system with aion, longer hydraulic to have lower CEC since illite has generally low CEC values (about cmol /kg) [26]. That is, c This data indicates that slow reaction kinetics may require a treatment system with a longer hydraulic retention time. 1 kg of Cheto and Gallup clay could take 120 centimols and 40 centimols of uranyl ion, respectively. time. Figure 5 shows rate of uranium removal in each asolution. Hydraulic retention timehydraulic in this Thisretention data indicates thatthe slow reaction kinetics may require treatment system with a longer Figure 5 shows the rate of uranium removal in each solution. Hydraulic retention time in2–3 this experiment was 12 h. The sample medium retrieved from Port 1 was exposed to pellets about h retention time. experiment was 12 h.medium The sample medium retrieved from Port about 1 was 9–10 exposed to pelletsremoval about 2–3 h whereas the sample in Port 4 was exposed to pellets h. Uranium was Figure 5 shows the rate of uranium removal in each solution. Hydraulic retention time in this whereas the sample medium wasfollowed exposedby to apellets 9–10 h. Uranium removal flew was fastest during the initial 2–3 hinofPort the 4test, more about gradual decline as the solution experiment was 12 h. The sample medium retrieved from Port 1 was exposed to pellets about 2–3 h fastestPort during initial of the test, a more gradual that decline the solution flew from 1 tothe Port 4 in2–3 theh column. In followed order to by make a judgment theasobserved uranium whereas the sample medium in Port 4 was exposed to pellets about 9–10 h. Uranium removal was from Port 1between to Port Port 4 in 1the column. order to make a judgment the observed uranium abatement and Port 4 In is statistically meaningful, the that two-sample t-test [29] was fastest during the initial 2–3 h of the test, followed by a more gradual decline as the solution flew from abatement between Port 1 and Port 4 is statistically meaningful, the two-sample t-test [29] was conducted using Minitab Statistical Software [30]. Analysis results in Table 1 showed a statistically Port 1 to Port 4 in the column. In order to make a judgment that the observed uranium abatement conducted using Minitab Statistical Software [30]. Analysis results in Table 1 showed a statistically

between Port 1 and Port 4 is statistically meaningful, the two-sample t-test [29] was conducted using

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Minitab Statistical Water 2017, 9, 761 Software [30]. Analysis results in Table 1 showed a statistically significant difference 5 of 8 (p ≤ 0.05) between Port 1 and Port 4. The results indicate a significant difference between the two significant difference ≤ 0.05) between means at the 95 percent(pconfidence level.Port 1 and Port 4. The results indicate a significant difference between the two means at the 95 percent confidence level.

Port

Port 1

Port 2

Port 2

Port 3

Port 3

Port 4

Port 4

Figure 5. Concentration inthe theflow-through flow-throughcolumn column with Gallup Figure 5. Concentrationprofiles profilesfor forUranium Uranium abatement abatement in with Gallup clayclay pellets (top) and Cheto clay pellets (top) and Cheto claypellets pellets(bottom) (bottom)after after 24 24 h. h. Table Statisticalanalysis analysisof ofuranium uranium concentration concentration data Port 4 4 Table 1. 1. Statistical datafor forPort Port1 1and and Port

Clay Type Cheto Clay Cheto Clay Port location Port 1 Port 4 Port location Port 1190.1 Port 4 Mean value 107.5 Mean value 190.1 107.5 Standard deviation 18.2 14.7 Standard deviation 18.2 14.7 Standard error mean 8.2 6.6 Standard error mean 8.2 6.6 p-value 0.000 p-value 0.000 Clay Type

Gallup Clay Gallup Clay Port 1 Port 4 60.1Port 1 17.54 Port 4 60.1 17.54 17.8 4.61 17.8 4.61 8.9 8.9 2.3 2.3 0.019 0.019

3.3. Water Purification Experiments for Uranium Removal from Drinking Water

3.3. Water Purification Experiments for Uranium Removal from Drinking Water The US Environment Protection Agency (EPA) established the drinking water standard of 30 The US Environment Protection Agency (EPA) established the drinking water standard of 30 ppb ppb for uranium in public drinking water supplies. Feasibility of uranium abatement in contaminated fordrinking uraniumwater in public drinking supplies. Feasibility of uranium abatement contaminated was tested usingwater clay ceramic pellets. The uranium concentration was in recorded until drinking water was tested using clay ceramic pellets. The uranium concentration was recorded the concentration was decreased to 30 ppb. Uranyl nitrate solution was used in the experiments asuntil a thecontaminated concentrationwater was source. decreased 30 ppb. Uranyl nitrate was used in the and experiments Thetoflow-through column wassolution packed with clay pellets uranium as a contaminated water by source. The flow-through column packed pellets and uranium medium and sealed rubber plugs. Samples were takenwas from Port 1with of theclay column and analyzed medium and sealed by rubber plugs. Samples were from Port 1 rates of thewere column using ICP-MS to measure uranium concentration. Thetaken uranium removal fasterand thananalyzed those of flow-through experiments. A total 68% of uranium removed during the initial two hours using ICP-MS to measure uranium concentration. Thewas uranium removal rates were faster than for those Gallup clay pellets and 89% was removed in uranium the first hour Arizona clay pellets. of flow-through experiments. A total 68% of waswith removed during the initial two hours for As shown inand Figure 6, was uranium concentration in hour the uranyl a rapid initial drop Gallup clay pellets 89% removed in the first with solution Arizonashowed clay pellets. inAs theshown first hour of the experiment, followed by a more gradual decline during the of the in Figure 6, uranium concentration in the uranyl solution showed remainder a rapid initial drop test.first At constant ambient conditions, the uranium concentration decreased from the 370 remainder ppb to 30 ppb in the hour of the experiment, followed by a more gradual decline during of the for Cheto pellets in 10 h and to 22.75 ppb for Gallup pallets in 34 h.

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test. At constant ambient conditions, the uranium concentration decreased from 370 ppb to 30 ppb for Cheto Waterpellets 2017, 9, in 76110 h and to 22.75 ppb for Gallup pallets in 34 h. 6 of 8

(a)

EPA safe drinking water limit

(b)

EPA safe drinking water limit

Figure 6.6.Uranium with Gallup Gallupclay claypellet pellet(34 (34h,h, and Cheto clay pellet h, b). Figure Uraniumabatement abatement in in time time with a)a) and Cheto clay pellet (8 h,(8b).

4. Conclusions 4. Conclusions Flow-through experimentssuggest suggestthat that clay clay pellet pellet barriers intercept and remove Flow-through experiments barrierscan caneffectively effectively intercept and remove uranium from contaminated groundwater. The removal efficiency was not decreased considerably uranium from contaminated groundwater. The removal efficiency was not decreased considerably in in and timenoand no noticeable permeability was in found in the column. Clayare pellets are not time noticeable permeability changechange was found the column. Clay pellets not susceptible susceptible to clogging or rapid passivation by reaction products unlike iron corrosion products. In to clogging or rapid passivation by reaction products unlike iron corrosion products. In the drinking the drinking water purification experiments, the uranium concentration was reduced to a water purification experiments, the uranium concentration was reduced to a concentration below the concentration below the EPA’s safe drinking water limit of 30 ppb. The manageable end-product is EPA’s safe drinking water limit of 30 ppb. The manageable end-product is easy to handle and dispose easy to handle and dispose of. Results show that clay ceramic pellets effectively remove uranium of. Results show that clay ceramic pellets effectively remove uranium from uranium-contaminated from uranium-contaminated water and can be a cost-efficient technique for remediating uranium water and can be a cost-efficient technique for remediating uranium contaminated groundwater. contaminated groundwater. This method is also a practical treatment method for uranium removal This method is also a practical treatment method forforuranium removal drinking water and can from drinking water and can supply potable water households in thefrom affected rural areas.

supply potable water for households in the affected rural areas.

Acknowledgments: This work is supported in part by the U.S. Department of Agriculture (USDA) (Grant NO.

Acknowledgments: 2014-38422-22078).This work is supported in part by the U.S. Department of Agriculture (USDA) (Grant NO. 2014-38422-22078). Author Contributions: Y.H.P. conceived the subject of the paper, performed the literature review, analyzed

Author Contributions: Y.H.P. conceived the subject of the paper, performed the literature review, analyzed data, and wrote the paper. D.V.-R. performed the statistical analysis. A.L. and E.R. provided guidance in data, and wrote the paper. D.V.-R. performed the statistical analysis. A.L. and E.R. provided guidance in experimental design fabricated clay ceramic pellets. C.F. fabricatedthe theflow-through flow-throughcolumns columnsand andcarried carriedout experimental design andand fabricated clay ceramic pellets. C.F. fabricated the experiments. the out experiments. Conflicts of Interest: The authorsdeclare declareno noconflict conflictof ofinterest. interest. Conflicts of Interest: The authors

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