Solution mining techniques are employed for extraction of minerals 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 (brines) 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 temperature ranging between 19 and 21 °C, which does not accurately reflect the winter temperatures 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 soil salinity and groundwater contamination). The clay has been used as a compacted liner for various applications: wastewater 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 ion exchange, filtration, settling, precipitation, volatilization, and adsorption [5
Recent research reports that the application of non-planted CWs receiving landfill leachate results in high heavy metal 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 mineral 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+
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 Table 1
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 (Table 2
) before performing a daily calibration test. The two point slopes check as recommended by the manufacturer was used to inspect the ISE functioning. 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 ph/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−
To perform Na+ ion measurements, both Type 10-205-3064 Na electrode and R-503/D reference electrode must be connected to the pH/ION 3400i meter at the 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 standard solutions were purchased from Hoskin Scientific, Vancouver, BC, Canada.
2.3. Experimental Cell Design and Multi-Layer Soils
All experimental cells were identically designed as non-plug flow systems (simulating constructed wetlands) using clay, sand and gravel. Based on vertical flow theory, the synthetic brine was fed on top of the CW cells. Then, it gradually flowed down through the multi-layer soil media, and effluents were collected at the bottom of CW cells. The study also intended to simulate the experiment cells as tailing ponds, so the application of plants was not required within this research. The clay was collected from a construction site in Regina (Saskatchewan, Canada) whereas sand and gravel were purchased from a retail location that sources materials locally.
Material gradation was determined using sieve analysis [10
] and hydrometer analysis [11
] (Figure 1
). The clay was found to contain 95% grains finer than 0.075 mm and 70% materials finer than 0.002 mm. The D10 (grain size pertaining to 10% material) of sand was found to be 0.27 mm and this number fit well in the design criteria (0.1 mm to 0.4 mm) for primary substrate in subsurface flow [12
] as the gravel was sized between 6.3 mm to 9.1 mm.
The CW system was installed in clear rectangular plastic containers comprising about 50 mm thick layers of clay, sand and gravel for the upper, middle, and bottom layers, respectively. The top layer functioned as the salt filtering layer while the sand and gravel provided water pathways for drainage. The porosity (ɳ) 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 × 55 cm). A round rubber hammer was used to produce a point load and compact the clay layer as desired. A well-graded and fine leveling surface was achieved using a marble roller and a bubble level. The pilot-scale CWs consisted of a cell tube, multi-layers soils, an effluent storage and drainage pipes (Figure 2
The geotechnical properties of the clay are shown in Table 3
. The high liquid limit and plastic limit indicate a high water adsorption and retention capacity of the clay. Likewise, the bulk density was measured as 1.4 g/cm3
indicating that the sample did not achieve fully compacted status (i.e.,
Db ≥ 1.6 g/cm3
). Thus, the clay layer permitted higher transmission of water through the upper layer than ideally present under field conditions.
Three batch experiments were completed in which each of the four experimental cells were fed 5 L/day influents (synthetic brine and deionized water). Synthetic brine influent was introduced over a period of 13 days and then cleansed with deionized water for three days to facilitate salt removal from the clay media.
Effluents from each experimental cell in batch experiments 1−3 were collected once every 24 hours. The concentrations of salt ions, Na+
, were measured for each 24 h sample in all three sets of experiments. A summary of the experimental design and operation is described in Table 4
All laboratory data were compiled in a Microsoft Excel spreadsheet for statistical and mathematic calculations. Trend data were plotted on time-sequence figures to illustrate changes in all key parameters over time.
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.