water

Article Salts Removal from Synthetic Solution-Potash by Non-Planted Constructed Wetlands

Warawut Chairawiwut *, Dena W. McMartin † and Shahid Azam †

Environmental Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2, Canada; [email protected] (D.W.M.); [email protected] (S.A.) * Correspondence: [email protected] or [email protected]; Tel.: +1-306-585-5551 † These authors contributed equally to this work.

Academic Editor: Alan Howard Received: 8 February 2016; Accepted: 21 March 2016; Published: 24 March 2016

Abstract: Four pilot-scale non-planted constructed wetlands (CWs) were employed to study the fate and transport of the two dominant chloride salts contained in brine resulting from solution mining activities in the potash industry. The simulated brine contained a 10:1 ratio of NaCl:KCl based on authentic brine characteristics. The multi-layer functioned as a main salt filtering component comprising of Regina , sand and gravels. The CW systems were operated in three batches of 16 days (experiments 1–3). K+ were removed by 92% (4.6 mg/L) from the effluent, while Cl´ and Na+ were removed in lower proportions of 51% (85.3 mg/L) and 45% (53.2 mg/L), respectively. Over time, the retained quantities of the three target salt ions decreased, indicating that clay capacity may have been reached. This study demonstrated that Regina Clay has substantial sorbent capacity for salt ions contained in simulated potash brine solution.

Keywords: chloride salts; ; brine treatment; Regina Clay

1. Introduction Solution mining techniques are employed for extraction of from potash (potassium bearing) deposits at several locations in Saskatchewan, Canada. Water is used to dissolve and extract the relatively deep potash salts for subsequent surface-level refining. Such methods produce large volumes of salt-affected waters () that must be stored in surface impoundments prior to their release to the environment. Given the climate (winter extremes in ´30 ˝C range and summer extremes in 30+ ˝C) prevalent in the area (that precludes plant life for approximately 1/2 of the year), the containment facilities are mostly non-vegetated constructed wetland (CW) systems that utilize local materials. For the purposes of this research, experiments were conducted at ambient room ranging between 19 and 21 ˝C, which does not accurately reflect the winter in constructed wetlands, but does recognize the above ´0 ˝C (4–10 ˝C in-field range) that is experienced during winter containment. The ready availability of an active clay in Regina, Saskatchewan, make CW systems an attractive approach for salt removal (otherwise causing and groundwater contamination). The clay has been used as a compacted liner for various applications: treatment plant [1], livestock manure storage [2], and municipal landfill [3]. The high specific surface area (50 m2/g) along with a high cation exchange capacity (40 cmol(+)/kg) of Regina clay [4] are useful in retaining several ionic species present in salt-rich water. A combination of physical and chemical processes are operative in active clay media such as exchange, filtration, settling, precipitation, volatilization, and [5].

Water 2016, 8, 113; doi:10.3390/w8040113 www.mdpi.com/journal/water Water 2016, 8, 113 2 of 9

Recent research reports that the application of non-planted CWs receiving landfill results in high heavy removal efficiency [6]. Unlike planted CWs, the sensitivities of biological systems to such factors as temperature changes and toxic chemicals are not as significant for the function of soil and other media in non-planted CWs. The treatment mechanics and efficiency are less affected by these factors, as are their planted counterparts. However, the alterations of soil particles such as desiccation and creaking are reported to cause failures to the clay liner in landfills [7]. Therefore, the application of a compacted clay layer as the primary pollutant filter bed in non-planted CWs might experience the same issue. Because of the desiccation cracks of soil, the hydraulic conductivity of clays may be changed and increase [8], meaning that the contact time between the clay bed and contaminated inflow is dramatically decreased. Seepage also may occur, resulting in the rapid migration of pollutants and short-circuiting of the soil layers resulting in high outflow contaminant concentrations. CWs technology with no application of plants is likely to contribute minimally to the removal of soluble organics, phosphorous, and nitrogen as well as pathogens due to lack of microbial activities and plant uptake mechanisms present in the system [5]. However, such non-planted systems are well-suited to assessments of soil-contaminant interactions for the removal of potash extraction´related salts and constituent ions. Wetland plants typically contain sufficient concentrations of Na+,K+ and Cl´ in their cells, and thus the requirement for biological uptake of these three ions is sufficiently low so that non-planted systems represent the majority of ion-exchange and sorption removal from the aqueous phase. The main objective of this paper was to understand the fate and transport of two dominant chloride salts (NaCl and KCl) present in solution-potash brines in constructed wetland. Four pilot-scale CW setups were operated in three batches over 16 days using simulated brine.

2. Materials and Methods

2.1. Synthetic Brine Preparation The synthetic brine was prepared to produce a concentration of chloride salts at a ratio of 10:1 for NaCl:KCl. The concentration ratio was determined from field data obtained from a typical solution potash mine [9]. The brine concentrations used in experiments within the 10:1 ratio are presented in Table1.

Table 1. 10:1 brine concentrations used in batch experiments.

Dissociation Syn. Brine NaCl:KCl Na+, mg/L K+, mg/L Cl´, mg/L Concentration mg/L:mg/L Eq ISE sd Eq ISE sd Eq ISE Sd C1 DI water 0 1.9 1.3 0 0.4 0.3 0 1 0.7 C2 75:7.5 37.5 28.1 6.7 3.8 4.9 0.8 41.3 38.6 1.9 C3 150:15 75 56.9 12.8 7.5 8.2 0.5 82.5 81.2 0.9 C4 300:30 105 117.8 22.8 15 17.3 1.6 165 171.6 4.6

The experimental design takes into consideration that the two dominant salt compounds are not completely dissociated, especially the Na+ ion. The standard deviation (sd) reflects the gap between theoretical expectation (Eq) and actual laboratory data using ion selective electrodes (ISE) instrumentation and analyses.

2.2. Instrumentation and Analysis Both influent and effluent samples collected during batch experiments 1´3 were analyzed for salt ions concentration using ISEs. To obtain precise and accurate results, each ISE was conditioned and stored in recommended solutions (Table2) before performing a daily calibration test. The two point slopes check as recommended by the manufacturer was used to inspect the ISE functioning. Water 2016, 8, 113 3 of 9

Therefore each salt parameter was provided in three different proportions which are 1000, 100 and 10 mg/L. The calibration test succeeded when two slope values as shown on a WTW /ION 3400i meter were recorded between 56 and 58 mV. All calibration solutions were prepared biweekly from a WTW sodium standard solution 10 g/L Na+ (NaCl), WTW potassium standard solution 10 g/L K+ (KCl) and WTW chloride standard solution 10 g/L Cl´(NaCl).

Table 2. Conditioning and storing solutions for ISE Calibration.

Water 2016, 8, 113 3 of 9 Standard Calibration Reference pH/Ion ISEs Conditioning Storing Solution Electrode 3400i + Slope+ a WTW sodium standard solution 10 g/L Na (NaCl), WTW potassium(STD) standardSeries solution (mg/L) 10 g/L K (KCl) and WTW chloride standard solution 10 g/L Cl−(NaCl). Check (mV) 3 Type 10 mg/L of 1 mg/L of 10 g/L 2 3 R-503/D Required + + + 10, 10 , 10 56–58 10-205-3064Na Table 2. ConditioningSTD Naand storingSTD solutions Na forNa ISE (NaCl)Calibration. 3 Not 10 mg/L of 1 mg/L of Standard10 g/L Calibration2 3 K-800 ReferenceRequired pH/Ion + + + 10, 10 , 10 56–58 ISEsrequired STDConditioning K STDStoring K SolutionK (KCl) Slope Electrode 3400i Series (mg/L) 3 (STD) Check (mV) Not 10 mg/L of 1 mg/L of 10 g/L 2 3 Cl-800 Type Required 103 mg/L´ of 1 mg/L of´ 10´ g/L 10, 10 , 10 56–58 required R-503/D Required STD Cl STD Cl Cl (NaCl) 10, 102, 103 56–58 10-205-3064Na STD Na+ STD Na+ Na+(NaCl) Not 103 mg/L of 1 mg/L of 10 g/L K+ K-800 Required 10, 102, 103 56–58 To perform Na+ ionrequired measurements, bothSTD Type K+ 10-205-3064STD K+ (KCl) Na electrode and R-503/D reference Not 103 mg/L of 1 mg/L of 10 g/L Cl− Cl-800 Required 10, 102, 103 56–58 electrode must be connectedrequired to the pH/IONSTD 3400i Cl− meterSTD Cl at− the(NaCl) same time. The reference probe was not required in order to operate the K-800 and Cl-800 electrodes. All ISEs, the pH/ION 3400i meter and the standardTo perform solutions Na+ ion were measurements, purchased both from Type Hoskin 10-205-3064 Scientific, Na electrode Vancouver, and R-503/D BC, Canada. reference electrode must be connected to the pH/ION 3400i meter at the same time. The reference probe was 2.3. Experimentalnot required Cell in Design order to and operate Multi-Layer the K-800 Soils and Cl-800 electrodes. All ISEs, the pH/ION 3400i meter and the standard solutions were purchased from Hoskin Scientific, Vancouver, BC, Canada. All experimental cells were identically designed as non-plug flow systems (simulating constructed wetlands)2.3. using Experimental clay, sand Cell Desi andgn gravel. and Multi-Layer Based onSoils vertical flow theory, the synthetic brine was fed on top of the CWAll cells. experimental Then, it graduallycells were flowedidentically down designed through as thenon-plug multi-layer flow systems soil media, (simulating and effluents were collectedconstructed at the wetlands) bottom using of CW clay, cells. sand The and studygravel. alsoBased intended on vertical to flow simulate theory, thethe synthetic experiment brine cells as tailing ponds,was fed so on the top application of the CW cells. of plants Then, wasit gradually not required flowed withindown through this research. the multi-layer The clay soil wasmedia, collected and effluents were collected at the bottom of CW cells. The study also intended to simulate the from a construction site in Regina (Saskatchewan, Canada) whereas sand and gravel were purchased experiment cells as tailing ponds, so the application of plants was not required within this research. from a retailThe clay location was collected that sources from a materialsconstruction locally. site in Regina (Saskatchewan, Canada) whereas sand and Materialgravel were gradation purchased was from determined a retail location using that sieve sources analysis materials [ 10locally.] and hydrometer analysis [11] (Figure1). TheMaterial clay was gradation found was to contain determined 95% grainsusing sieve finer analysis than 0.075 [10] mmand andhydrometer 70% materials analysis finer[11] than 0.002 mm.(Figure The 1). D10 The (grain clay was size found pertaining to contain to 10%95% material)grains finer of than sand 0.075 was mm found and 70% to be materials 0.27 mm finer and this than 0.002 mm. The D10 (grain size pertaining to 10% material) of sand was found to be 0.27 mm and number fit well in the design criteria (0.1 mm to 0.4 mm) for primary in subsurface flow [12] this number fit well in the design criteria (0.1 mm to 0.4 mm) for primary substrate in subsurface as the gravelflow [12] was as sized the gravel between was sized 6.3mm between to 9.1 6.3 mm. mm to 9.1 mm.

100

90

80

70

60

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40 Regina clay Percent Passing (%) Passing Percent 30 Sand 20 Gravel 10

0 0.001 0.01 0.1 1 10 Particle Size (mm)

FigureFigure 1. Grain 1. Grain size size distribution distribution of selected selected materials. materials. The CW system was installed in clear rectangular plastic containers comprising about 50 mm Thethick CW layers system of wasclay, installedsand and gravel in clear for rectangular the upper, middle, plastic and containers bottom laye comprisingrs, respectively. about The 50 top mm thick layers oflayer clay, functioned sand and as gravel the salt for filtering the upper, layer while middle, the sand and and bottom gravel layers, provided respectively. water pathways The for top layer functioneddrainage. as the The salt porosity filtering ( ) layer was whilemeasured the sandas 0.3 andand gravel0.4 for sand provided and gravel water layers, pathways respectively. for drainage. Compaction loads were applied to the clay in order to form a compacted layer with hydraulic conductivity closer to that expected in a full-scale CW. About 10 kg of dried clay was moistened with deionized water and filled into the rectangular wooden frame which was placed on top of a geo-synthetic fabric (35 cm × 55 cm). A round rubber hammer was used to produce a point load and

Water 2016, 8, 113 4 of 9

The porosity (ï) was measured as 0.3 and 0.4 for sand and gravel layers, respectively. Compaction loads were applied to the clay in order to form a compacted layer with hydraulic conductivity closer to that expected in a full-scale CW. About 10 kg of dried clay was moistened with deionized water and filled into the rectangular wooden frame which was placed on top of a geo-synthetic fabric (35 cm ˆ 55 cm). A round rubber hammer was used to produce a point load and compact the clay Water 2016, 8, 113 4 of 9 layer as desired. A well-graded and fine leveling surface was achieved using a marble roller and a bubblecompact level. the The clay pilot-scale layer as CWsdesired. consisted A well-graded of a cell and tube, fine multi-layers leveling surface soils, was an achieved effluent storageusing a and drainagemarble pipes roller (Figure and a 2bubble). level. The pilot-scale CWs consisted of a cell tube, multi-layers soils, an Theeffluent geotechnical storage and properties drainage pipes of the (Figure clay are 2). shown in Table3. The high liquid limit and plastic limit indicate aThe high geotechnical water adsorption properties of and the retention clay are shown capacity in Table of the 3. The clay. high Likewise, liquid limit the and bulk plastic density was measuredlimit indicate as a 1.4high g/cm water3 adsorptionindicating and that retentio the samplen capacity did of the not clay. achieve Likewise, fully the compacted bulk density status was measured as 1.4 g/cm3 indicating that the sample did not achieve fully compacted status (i.e., (i.e., Db ě 1.6 g/cm3). Thus, the clay layer permitted higher transmission of water through the upper Db ≥ 1.6 g/cm3). Thus, the clay layer permitted higher transmission of water through the upper layer layerthan than ideally ideally present present under under field field conditions. conditions.

Figure 2. Schematic of pilot-scale CWs. Figure 2. Schematic of pilot-scale CWs.

Table 3. Geotechnical properties of the clay media. Table 3. Geotechnical properties of the clay media. Parameter Unit Value Test Method ParameterField dry density Unit g/cm3 1.37 Value [13] Test Method 3 Field drySpecific density gravity, Gs g/cm - 2.711.37 [13] [13] Specific gravity, G - 2.71 [13] Dry density,s d g/cm3 1.02 d = /(1 + w) 7 3 7 7 Dry density, d g/cm 3 1.02 d = /(1 + w) Bulk density, , Db 3g/cm 1.4 Core Method Bulk density, 7,Db g/cm 1.4 Core Method ρ 3 3 Particle density,Particle D density,ρ D g/cm g/cm 2.212.21 Core Method Core Method Porosity,Porosity,ï - - 0.4 0.4 = 1 − (Dï=b 1/D´ρ)(D b/Dρ) 3 Bed volumeBed volume cm cm3 27402740 = VV/Vï=T, VVVV/V = BVT,V V = BV Water content,Water wcontent, w % % 37 37 [14] [14] Liquid limit, LL % 74.5 [15] Plastic limit,Liquid PL limit, LL % % 74.5 28.3 [15] [15] Dimension ofPlastic CW cells limit, PL % 28.3 [15] cm2 62 ˆ 38 (lengthDimensionˆ width) of CW cells cm2 62 × 38 (length × width) 2.4. Experiments 2.4. Experiments Three batch experiments were completed in which each of the four experimental cells were fed Three batch experiments were completed in which each of the four experimental cells were fed 5 L/day5 L/day influents influents (synthetic (synthetic brine brine and and deionized deionized water). water). Synthetic Synthetic brine brine influent influent waswas introducedintroduced over a periodover ofa 13period days of and 13 days then and cleansed then cleansed with deionized with deionized water water for three for daysthree todays facilitate to facilitate salt removalsalt fromremoval the clay from media. the clay media. EffluentsEffluents from from each each experimental experimental cell cell in batchin batch experiments experiments 1´ 13− were3 were collected collected once once every every 24 24 hours. The concentrationshours. The concentrations of salt ions, of Nasalt+ ,Kions,+ and Na+, Cl K´+ ,and were Cl− measured, were measured for each for 24 each h sample 24 h sample in all in three all sets of experiments.three sets of A experiments. summary of A the summary experimental of the ex designperimental and operationdesign and is operation described is indescribed Table4. in Table 4.

Table 4. Batch experiments.

Water 2016, 8, 113 5 of 9

Table 4. Batch experiments.

Batch Operational Feeding Cleaning HRT Effluent Sampling WaterExperiments 2016, 8, 113 Formats Cells Stage (Day) Stage (Day) (h) Frequency Total5 of 9 B 1, 2, 3 16-day 4 13 3 24 at 24 h 192 Batch Operational Feeding Cleaning HRT Effluent Sampling Experiments FormatsNote: Cells HRT = Hydraulic Stage (Day) Retention Stage Time (Day) of CW (h) cells. Frequency Total B 1, 2, 3 16-day 4 13 3 24 at 24 h 192 All laboratory data wereNote: compiled HRT = inHydraulic a Microsoft Retention Excel Time spreadsheet of CW cells. for statistical and mathematic calculations.All laboratory Trend data data were were plotted compiled on time-sequencein a Microsoft Excel figures spreadsheet to illustrate for changesstatistical inand all key parametersmathematic over calculations. time. Trend data were plotted on time-sequence figures to illustrate changes in all key parameters over time. 3. Results and Discussion 3. Results and Discussion 3.1. CW System Performance and Removal Efficiencies 3.1. CW System Performance and Removal Efficiencies The CW systems demonstrated promising results for cation removal, especially the K+ ions. Each experimentalThe CW systems cell system demonstrated was able promising to maintain results a for consistent cation removal, removal especially effectiveness the K+ ofions. K+ ions throughoutEach experimental the 13 day cell exposure system was to the able synthetic to maintain brine a consistent feed, while removal the ability effectiveness to remove of K+Na+ ions ions decreasedthroughout from the Day 131 day forward, exposure resulting to the synthetic in minimal brine treatment feed, while potential the ability by to Day remove 13. Na+ The ions majority decreased from Day 1 forward, resulting in minimal treatment potential by Day 13. The majority of of Cl´ ion removal was achieved during Day 1 to Day 4. The daily variations of average quantities Cl− ion removal was achieved during Day 1 to Day 4. The daily variations of average quantities removed in Cells 2, 3 and 4 are presented in Figure3. removed in Cells 2, 3 and 4 are presented in Figure 3.

200 Cell2 Cell3 Cell4 (a) 150

100

50

0 0 2 4 6 8 10 12 14 16 Removed Na+, mg/l Na+, Removed -50

-100 30 (b)

20

10

0

Removed K+, mg/l 0246810121416 -10 300 (c) 200

100

0 0246810121416

Removed Cl-, mg/l Cl-, Removed -100 -200 Time, day Figure 3. The variation of average quantities of (a) Na+; (b) K+; and (c) Cl− removed in the three Figure 3. The variation of average quantities of (a) Na+;(b)K+; and (c) Cl´ removed in the three experimental CW cells with 95% confidence intervals (n = 3) provided. All data are corrected per n experimentalcontrol CWs. CW cells with 95% confidence intervals ( = 3) provided. All data are corrected per control CWs. Figure 3 also draws attention to the fact that cations are more highly attracted to the clay versus Figurethe anions3 also due draws to the high attention CEC of to the the natural fact that clay. cations In other are words, more clay highly media attracted had ability to the to remove clay versus the anionsthe anion due in to a theshort high period, CEC but of also the becomes natural clay.quickly In exhausted. other words, Despite clay both media cations had abilityhaving toequal remove + + the anionpositive in acharge, short the period, K ion but was also more becomes highly favored quickly in exhausted. uptake kinetics Despite vs. the both Na cations ion, most having likely equal due to their atomic size in the lyotropic series. Therefore, the CW system would potentially positive charge, the K+ ion was more highly favored in uptake kinetics vs. the Na+ ion, most likely contribute maximum and consistent removal efficiency of K+ > Na+ > Cl− ions. due to theirThe atomic results size of maximum in the lyotropic removal series. efficiency Therefore, calculations the CW for system each of would the three potentially key salt contribute ions + + ´ maximuminvestigated and consistent (Table 5) during removal the efficiencythree brine-fed of K batch> Na operations> Cl ions. of the CW systems clearly support Thethe above results statements of maximum regarding removal K+ efficiencyion removal calculations efficacy, with foreach lower of and the threeless consistent key salt ions removal investigated of (TableNa5)+ duringand Cl−. the three brine-fed batch operations of the CW systems clearly support the above statementsThe regarding highest removal K+ ion removalefficiency efficacy, was recorded with lower at nearly and 92% less for consistent clay uptake removal of the of K+ Na ion+ andwhile Cl ´. Thethe maximum highest removal removal efficiencypercentages was for Na recorded+ and Cl at− ions nearly were 92% substantially for clay uptake lower at of 45% the and K+ ion50%, while + the maximumrespectively. removal It is apparent percentages that, although for Na small+ and quantities Cl´ ions of were K ions substantially were present lowerin the effluents, at 45% and the 50%, removal percentage was still nearly twice as high as compared to the other two salt ions examined.

Water 2016, 8, 113 6 of 9 respectively. It is apparent that, although small quantities of K+ ions were present in the effluents, the removal percentage was still nearly twice as high as compared to the other two salt ions examined. This resultWater 2016 indicates, 8, 113 that the clay medium demonstrated a higher affinity to attract and remove6 of 9 the largerThis atomic result cations indicates present that the in clay the influentmedium demonstr brine. ated a higher affinity to attract and remove the larger atomic cations present in the influent brine. Table 5. Mean concentration values and removal efficiencies for three key salt ions exposed to synthetic brineTable (n = 3).5. AllMean data concentration are corrected values per controland removal CWs. efficiencies for three key salt ions exposed to synthetic brine (n = 3). All data are corrected per control CWs. Influent Concentration Outflow Concentration Removal Efficiency Ion DayInfluent Concentration Outflow Concentration Removal Efficiency Ion Day (mg/L) (mg/L) (%) (mg/L) (mg/L) (%) + ˘ ˘ Na+ 1 1 117.8 ± 117.8 7.6 7.6 64.5 64.5 ± 87.187.1 45 45 K+ 8 4.9 ˘ 3.2 0.3 ˘ 3.1 92 + ClK´ 8 1 4.9 ± 171.63.2 ˘ 15.4 0.3 86.2 ± 3.1˘ 195 92 50 Cl− 1 171.6 ± 15.4 86.2 ± 195 50

3.2. Cation3.2. Cation Removal Removal 3+ 2+ 2+ + + + AccordingAccording to theto the lyotropic lyotropic series series (Al (Al3+> > CaCa2+ > Mg Mg2+ > >K K+ > Na> Na+ > H>+), H cations), cations that thatcarry carrya a significantsignificant positive positive charge charge along along with with being being ofof largelarge atomic atomic size size are are more more likely likely to be to attracted be attracted to and to and absorbedabsorbed by theby the negatively negatively charged charged surfaces surfaces ofof clay [16]. [16]. If Ifthe the degree degree of ofthe thevalence valence is equal, is equal, the the largerlarger ion size ion willsize will contribute contribute to producingto producing a stronger a stronger substitution substitution power power [[17].17]. For the purposes of of this study,this it isstudy, important it is important to note thatto note the that ionic the radii ionic of radii K+, reportedof K+, reported as 1.38 as Å 1.38 and Å 1.51 and Å 1.51 in theÅ in structures the + of aluminastructures (octahedral) of alumina and (octahedral) silica (tetrahedral), and silica (tet respectively,rahedral), respectively, are bigger are than bigger those than of those Na+ atof Na 1.02 and at 1.02 and 1.18 Å, respectively [18]. Therefore, at the point at which the availability of negatively 1.18 Å, respectively [18]. Therefore, at the point at which the availability of negatively charged surface charged surface area on clays becomes limited, the K+ ion will bond preferentially over Na+ to an area on clays becomes limited, the K+ ion will bond preferentially over Na+ to an ion-exchange site. ion-exchange site. FigureFigure4 presents 4 presents the the daily daily deviations deviations of of cation cation removalremoval including including the the three-day three-day flushing flushing stage, stage, duringduring which which deionized deionized water water was appliedwas applied to extricate to extricate the sorbedthe sorbed ions fromions from the Regina the Regina Clay Clay exchange sites.exchange Each experimental sites. Each cell experimental system was cell able system to maintain was able a consistent to maintain removal a consistent effectiveness removal of the K+ ion throughouteffectiveness the of 13-daythe K+ ion exposure throughout to the the synthetic 13-day exposure brine feed, to the while synthetic the ability brine feed, to remove while the Na+ ion decreasedability to fromremove Day the 1 forward,Na+ ion decreased resulting infrom minimal Day 1 treatment forward, potentialresulting byin minimal Day 13. Therefore,treatment this resultpotential confirms by that Day the13. Therefore, daily K+ ion this elimination result confirms percentage that the daily in all K+ experimental ion elimination cells percentage is approximately in all + doubleexperimental that for Na cells+. is approximately double that for Na .

Na K

200 (a)

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Cell 2 Removal 2 Removal Cell 0 2 4 6 8 10 12 14 16 -100

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Efficiency, % 0 2 4 6 8 10 12 14 16 Cell 3 Removal 3 Removal Cell -100 200 (c) 100

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Cell 4 Removal 4 Removal Cell 0 2 4 6 8 10 12 14 16 -100 Day

+ + FigureFigure 4. Na 4. Naand+ and K Kremoval+ removal efficiencies efficiencies in ( aa)) Cell Cell 2; 2; (b (b) )Cell Cell 3; 3;and and (c) (Cellc) Cell 4 throughout 4 throughout the three the three experimentalexperimental CW CW cells cells with with 95% 95% confidence confidence intervalsintervals ( (nn == 3) 3) provided. provided. All Alldata data are arecorrected corrected per per controlcontrol CWs. CWs. Highlighted Highlighted areas areas represented represented datadata obtained from from three-day three-day flushing flushing stage. stage.

3.3. Chloride Ion Removal

Water 2016, 8, 113 7 of 9

3.3. Chloride Ion Removal Water 2016, 8, 113 7 of 9 The amount of Cl´ ions was dramatically reduced in the early stages of experimental cell operation − due to formationsThe amount of insolubleof Cl ions salt was compounds dramatically with reduced available in cationsthe early and stages precipitation of experimental on clay cell surfaces operation due to formations of insoluble salt compounds with available cations and precipitation on via anion exchange processes [16]. Clays naturally exhibit very low anion exchange capacity (AEC). clay surfaces via anion exchange processes [16]. Clays naturally exhibit very low anion exchange By Daycapacity 4 of the(AEC). experiments, By Day the4 of process the experiments, of insoluble the salt process formation of insoluble was significantly salt formation decreased was from ´ thatsignificantly originally observed, decreased asfrom recorded that originally by reduced observed, Cl asremoval recorded efficiency.by reduced AtCl− thisremoval point efficiency. in time, it is likelyAt that this accessibilitypoint in time, to it both is likely the cationsthat accessibility and AEC-related to both the processes, cations and which AEC-related were limited processes, from the start,which became were fully limited exhausted from the or start, depleted. became In fully Days exhausted 5 through or 13 depleted. of the experiments, In Days 5 through the amount 13 of the of free Cl´ ionsexperiments, that had the accumulated amount of free in the Cl− clay ions solutionthat had wasaccumulated leached outin the on clay a daily solution basis was by leached the introduction out of freshon a synthetic daily basis brine by the that introduction forced any of accumulatedfresh synthetic Clbrine´ through that forced the any system. accumulated Cl− through the system. 3.4. Flushing Stage 3.4. Flushing Stage A flushing or cleaning process was completed using deionized water feed for all experimental A flushing or cleaning process was completed using deionized water feed for all experimental cells to allow the time and opportunity to leach bonded ions from the clays. The effluents were cells to allow the time and opportunity to leach bonded ions from the clays. The effluents were collected and the concentration of all three key salt ions monitored through the three-day flushing collected and the concentration of all three key salt ions monitored through the three-day flushing process.process. Through Through this this process, process, the the quantityquantity of of all all three three ions ions was was dramatically dramatically reduced reduced (Figure (Figure 5). 5). Additionally,Additionally, all experimentalall experimental cells cells contained contained minimal quantities quantities of ofretained retained salt saltions ionsthat thathad been had been accumulatedaccumulated into into the Reginathe Regina Clay Clay media media and, thus,and, werethus, carriedwere carried forward forward to the subsequentto the subsequent experiment as comparedexperiment to as the compared initial concentrations to the initial concentrations by Day 16. by Day 16.

Day14 Day15 Day16 Syn.Brine 120 (a)

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Sodium, mg/l Sodium, 0 Cell 2 Cell 3 Cell 4 30 (b) 20

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0 Potassium, mg/l Cell 2 Cell 3 Cell 4 -10

300 (c) 200

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0 Chloride, mg/l Chloride, Cell 2 Cell 3 Cell 4 -100 Experimental Cells

FigureFigure 5. Mean 5. Mean daily daily effluent effluent concentrations concentrations of (a) Na ++; ;((bb) )KK+; +and; and (c) (cCl)− Cl in´ thein flushing the flushing stage stage includingincluding 95% 95% confidence confidence interval interval error error bars bars forfor n = 3. 3. All All data data are are corrected corrected per percontrol control CWs. CWs.

Due to the weakness of the Van der Waals’ forces of the silica sheet, hydrogen ions (H+) from Due to the weakness of the Van der Waals’ forces of the silica sheet, hydrogen ions (H+) from water can enter across the edge of the silica layers located on top of the alumina layer in the 2:1 waterstructure can enter of smectite across the to edgeaffect ofhydrogen the silica bond layerss with located groups on of top oxygen of the or alumina hydroxyl layer [17]. inThe the 2:1 structureseparation of smectite and defection to affect of hydrogen the silicabonds layer may with then groups occur. of oxygenThis action, or hydroxyl known as [17 broken]. The separationedge, and defectionreleases a large of the quantity silica layerof negative may thencharges occur. to the This clay action, surface known because as ofbroken the incomplete edge, releases structural a large quantityformation of negative of the silica charges sheet.to In theaddition, clay surfacethe clay mineral because itself ofthe may incomplete recharge the structural negatively formationcharged of the silicasurfaces sheet. by substitution In addition, of thecations clay in mineral the alumina itself layer. may This recharge process the increases negatively the negative charged charges surfaces by substitutionavailable ofduring cations replacement in the alumina of a lower layer. valence This cation process with a increases higher valence the negative one [14]. charges available during replacement of a lower valence cation with a higher valence one [14]. Water 2016, 8, 113 8 of 9

4. Conclusions The experimental CW systems were found to serve as a satisfactory treatment method for overall salt ion removal from the synthetic solution-potash brine containing a 10:1 ratio of NaCl:KCl. The data obtained from three batch experiments indicate systems removal efficiencies that demonstrate promising applications for non-vegetated CWs under the conditions tested. The removal percentage for each ion investigated in the experimental cells increased with increasing initial concentrations. The experimental results clearly identified that K+ ions were most readily removed in significant quantities (in the range of 92% removal) in the Regina Clays media. Both Na+ and Cl´ ions were removed at no more than 45% and 50%, respectively, throughout the experiments. Across the experiments, the retained amounts of K+, Na+ and Cl´ ions decreased in the outflow with increasing operational time. Therefore, this study has proven that Regina Clay has significant application potential to serve as adsorbent media for the removal of salt ions from solution-potash brine.

Acknowledgments: The authors gratefully acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada to Dena W. McMartin (2015-04296). Further funding was received from the University of Regina. Author Contributions: Warawut Chairawiwut designed and carried out the experiments, conducted the data analysis and led the authorship of the resulting paper; Dena W. McMartin and Shahid Azam conceived of the research and experiments, and provided academic reviews and technical comments during manuscript preparation. Conflicts of Interest: The authors declare no conflict of interest.

References

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