Chapter 21
Arsenic Removal from Drinking Water Using Clay Membranes 1
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Jun Fang , Baolin Deng*, and T. M. Whitworth
Department of Civil and Environmental Engineering, University of Missouri at Columbia, Coumbia, MO 65211 Department of Geological and Petroleum Engineering, University of Missouri at Rolla, MO 65409 Corresponding author:
[email protected]
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While many treatment technologies are available for arsenic removal from drinking water including coagulation/filtration, lime softening, activated alumina adsorption, ion exchange, and membrane processes, most of these approaches are expensive and more suitable for large water systems. In this study, membranes made of low-cost clay minerals were explored for arsenate removal. Montmorillonite, kaolinite, and illite were selected for membrane preparation. Feed water spiked with arsenate was pumped through the compacted clay membranes and the effluent was collected at the lower pressure side for arsenic analysis. The ability of clay membranes to retain arsenic was investigated at different initial arsenic concentrations and ionic strengths controlled by sodium chloride. The influence of applied pressure and the permeate flux on arsenic removal efficiency was also examined. The results indicated that a greater than 90% of arsenic rejection could be achieved for water with 50-100 μg/l of arsenate using the clay membranes. The required pressure for clay membrane filtration was, however, significantly higher than that of synthetic organic membranes.
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© 2005 American Chemical Society
In Advances in Arsenic Research; O'Day, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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Introduction Arsenic is of increasing environmental concern due to increased awareness of its adverse effects to human health. Excessive amounts of arsenic can cause acute gastrointestinal and cardiac damage. Long term exposure to arsenic via drinking water leads to health problems such as vascular disorders and skin cancer (1-3). To minimize the risk, the US Environmental Protection Agency has revised the Maximum Contamination Level (MCL) of arsenic in drinking water from 0.05 mg/1 to 0.01 mg/1. Viable water treatment technologies are being reassessed for their efficacy in removing arsenic from drinking water and for their potential applicability to meet the new standard (4). The technologies investigated include coagulation/filtration (5, 6), lime softening (7), ion exchange, activated alumina adsorption (8), and membrane filtration processes (9-12). A key characteristic of a membrane is its ability to restrict or prevent the passage of some solutes in solution while permitting transport of others. The solute rejection by membranes is based on size and/or electrical restrictions. The membrane would be ideal if it could absolutely retain a solute. Typical membranes used in arsenic removal studies are synthetic polymeric membranes. It has been shown that membrane filtration processes, such as reverse osmosis (RO) and nanofiltration (NF), are capable of arsenic removal, particularly at low influent arsenic concentrations (13). Brandhuber and Amy (10) reported As(V) could be effectively treated by R O and N F with rejection rate higher than 95%, and membrane surface charge might be a more important factor in determining the rejection of As(V) than the molecular weight cut-off of the membranes. However, the applications of polymeric membranes are limited by lack of heat, chemicals, and corrosion resistance, as well as their high cost. Development of other low-cost inorganic membranes should be explored for future uses. Clays are known to have membrane properties and can reject ionic solutes in solution (14-17). Clay mineral surfaces are negatively charged due to the substitution of the lower valence for higher valence cations within the mineral structure, as well as the broken bonds on the mineral surface. The ideality of clay membranes is a function of the membrane's surface charge density and porosity as well as the concentration of ions in solution (18). Clay membrane efficiency increases greatly when the double layer of adjacent clay platelets overlaps under compaction. A possible mechanism for charged compacted clay membranes rejecting ion solutes is that the anions attempting to pass through the clay membranes are repelled by the negative charge on the clay platelets, cations tend to remain with their co-ions in order to maintain the electrical neutrality in the solution, thus their movement across the clay is also restricted (19). For uncharged membranes, like kaolinite, the rejection arises predominantly from their size exclusion properties (20).
In Advances in Arsenic Research; O'Day, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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296 Some studies have been previously conducted on the potential uses of clays as practical membranes. Ishiguro et al. (21) applied an uncompacted 0.5 mm thick montmorillonite layer as a reverse osmosis membrane to separate salt solutions and non-ionized organics. The clay membrane exhibited characteristics typical of a charged membrane, rejecting sodium chloride solute less effectively with increasing solute concentrations. The rejection rate of non-ionized organics was very low. They also found that the separation of amino acids depended greatly on the net charge carried by the amino acid molecule (21). In another study, L i et al. (22) used a compacted bentonite membrane to purify oil-field produced water that contained total dissolved solids (TDS) of 196,250 mg/L. To maximize the flow through the membrane, ultrathin (0.04mm to 0.06 mm thick) bentonite clay membranes were used and results demonstrated that the membrane efficiency for inorganic solutes decreased with increasing solute concentration and with increasing total dissolved solids (22). Arsenic is considered a metalloid that exists in various forms and oxidation states in the environment. Two species commonly found in drinking water are arsenate and arsenite. Arsenate is the thermodynamically stable form of inorganic arsenic in oxic water, and usually predominates in surface water. Under typical drinking water pH (5.5 - 8.5) conditions, arsenate exists as anionic H As0 "and H A s 0 ' . Clay membranes, which are negatively charged, should be capable of rejecting arsenate from drinking water. To our knowledge, however, this has not been experimentally demonstrated. The objective of this study is to examine the potential arsenic removal by thin compacted clay membranes at different initial arsenate and background electrolyte concentrations. 2
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Experimental Methods Clay minerals used in this study were obtained from the Clay Minerals Society Source Clay Repository (Department of Geological Sciences, University of Missouri-Columbia). Samples of sodium montmorillonite (SWy-2) from Crook County, W Y , poor crystallized kaolinite (KGa-2), and illite (IMt-2) from Silver Hill, M T , were used for membrane preparation. The illite fragments were ground with a mortar and pestle to pass a 75μπι sieve before the preparation. To purify the clay minerals, each of the three clay/water slurries was poured into a plastic jar with marble mill media and milled on a mechanic roller for two days. Standard sedimentation technique was then used to obtain the smaller size fraction. Following separation, the clay slurries were dialyzed, freeze dried in a benchtop freeze drier (Labconco, Model 4.5), and finally the freeze dried clays were stored in sealed bags prior to use (17). Figure 1 is the schematic of the experimental setup, followed a design similar to the one used by Fritz and Whitworth (23). The membrane filtration
In Advances in Arsenic Research; O'Day, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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system consisted of a feedwater reservoir, a high performance liquid chromatographic (HPLC) pump capable of delivering fluid at constant flow rates ranging from 0 to 10 ml/min (Gilson Model 305), pressure gauge, an acrylic cell with two caps, and sample collection bottles. The membrane area in this cell was 15.52±0.01 cm . 2
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H P L C Pump
Pressure gauge
Feed water
0.1 μηι membrane
Stainless steel frit
Sample collection Figure I. Schematic of the experimental setup.
The clay membrane was prepared in the experimental setup. A 316 stainless steel porous frit was first set into the lower cap, then two pieces of membrane filter papers (Millipore, 0.1 μιη) were placed on top of the frit. The acrylic cell was carefully inserted into the lower cap without wrinkling the filter papers, with an O-ring installed for leak prevention. A clay suspension, formed by mixing 0.30 g of freeze-dried sodium montmorillonite with 120 ml deionized water, was transferred to the cell. The cell was then sealed with the upper cap and O-ring, followed by bolting the cell and caps tightly with threaded steel rods. Using this typical down- flow filtration system, deionized water was pumped through the
In Advances in Arsenic Research; O'Day, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
298 port in the center of upper cap into the cell to compact the clay suspension, forming a thin layer of membrane on the top of the filter paper. A n Ashcroft pressure gauge ranging from 0 to 2000 psi was installed in front of the cell to measure the applied pressure to the cell. The time period used for membrane formation usually lasted for two to four days. The internal pressure of the cell depended on the flow rate of the system. Since the acrylic cell used in the system could only endure a maximum continuous pressure around 1500 psi, the flow rate of the system was set no more than 0.2 ml/min to maintain the pressure at approximately 1100 psi. Permeability (L ) of the clay membrane to deionized water was measured after the clay membrane formation, when the flow rate and applied pressure reached a steady state. Under this condition, L could be calculated from the equation (1): p
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p
L
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