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Arsenic removal from drinking water using thin film composite nanofiltration membrane
Desalination 252 (2010) 75–80
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Arsenic removal from drinking water using thin film composite nanofiltration membrane R.S. Harisha a, K.M. Hosamani a,⁎, R.S. Keri a, S.K. Nataraj b, T.M. Aminabhavi b a b
P. G. Department of Studies in Chemistry, Karnatak University, Dharwad 580 003, India Membrane Separation Division, Center of Excellence in Polymer Science, Karnatak University, Dharwad 580 003, India
a r t i c l e
i n f o
Article history: Received 21 August 2009 Received in revised form 25 October 2009 Accepted 30 October 2009 Available online 25 November 2009 Keywords: Arsenic Drinking water Membrane Nanofiltration Water treatment
a b s t r a c t Arsenic removal from drinking water is a major problem in many parts of the world. Acute and chronic arsenic exposure through drinking water has been reported in many countries, especially Argentina, Bangladesh, India, Mexico, Mongolia, Thailand and Taiwan, where a large proportion of ground water is contaminated with arsenic at levels from 100 to over 2000 μm/L. In this work, we have investigated the arsenic removal by the membrane-based nanofiltration (NF) technique. The results of this study demonstrate that arsenate ions removal by NF with a high rejection of 99.80% along with total dissolved solids (TDS) and other contaminates were achieved. Significant flux was retained at the end of each NF experiment with operation time of 180 min, which suggests that membranes were not affected by the fouling phenomenon during the process run. The feasibility of membranes for treating drinking water by varying feed pressure, pH and feed concentration was tested on the separation performance of the thin film composite NF membranes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Arsenic is widely distributed in air, water and soil as a metalloid and in the form of chemical compounds [1]. Average concentrations of arsenic in earth's crust normally range from 1.5 to 5 mg/kg. Higher concentrations were found in some igneous and sedimentary rocks, particularly in iron and manganese ores. Arsenic may be released from these ores to soil, surface water and ground water. Recently, the presence of dissolved arsenic in drinking water has emerged as a problem of global magnitude. Promulgation of new maximum contaminant levels (MCL) by the United States Environmental Protection Agency (USEPA) and the World Health Organization (WHO) has resulted in a demand for more efficient arsenate ions removal technologies. Discovery of arsenic in ground water of alluvial deposits of West Bengal (India) and Bangladesh referred to as the Bengal delta contaminating up to 97% of domestic water supply in rural areas based on tube wells, threatens millions of people who are exposed to high concentrations of arsenic. Recent studies indicate the additional contamination of the food chain due to irrigation with arsenic contaminated water [2–4]. In 1993, WHO lowered the guideline value for arsenic in drinking water from 50 μg/L down to 10 μg/L [5]. In contrast to the Southwest region of Taiwan, West Bengal and Bangladesh face an ongoing problem with widespread exposure to very high concentration levels of arsenic in drinking
⁎ Corresponding author. Fax: +91 836 747884, +91 836 2771275. E-mail address: [email protected] (K.M. Hosamani). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.10.022
water ranging between 35 and 77 million of Bangladesh and West Bengal residents are now thought to be affected by exposure to arsenic in drinking water that exceeds 2000 μg/L [4,6]. In West Bengal, an estimated one million residents are exposed to ground water containing up to 3900 μg/L [7]. Large populations of these regions, coupled with the concentrations of arsenic in drinking water in hundreds or thousands of μg/L, would pose a major threat to public health and hygiene. Furthermore, because some cancers have a latency period of 20 to 30 years, and widespread exposure to these very high concentrations of arsenic began only in 1970s due to the drilling of millions of tube wells; it could be also possible that the full effect of arsenic exposure in West Bengal and Bangladesh is yet to be seen. Arsenic can exist in natural water sources in both organic and inorganic forms; however, only inorganic forms viz., arsenate [As (V)] and arsenite [As (III)] are considered in this study because available data indicate that the amount of organic arsenic in drinking water sources is almost insignificant. Studies have linked the long-term exposure to arsenic in drinking water to bladder cancer, lungs, skin, kidney, nasal passages, liver and prostate [8]. Besides its tumorigenic potential, arsenic has been shown genotoxic effects [9,10]. Arsenic removal from drinking water using alum (90% removal), iron salt (90 % removal), iron fillings (90% removal) and bucket or tea-bag method (80–90% removal have been described by many researchers [11,12]. There is a report on arsenic removal from soil using pteris vittata (brake fern) [13]. Common treatment methods such as water softening, carbon filters and sediment filters cannot adequately remove arsenic from drinking water. In these days, distillation,
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R.S. Harisha et al. / Desalination 252 (2010) 75–80
Fig. 1. Schematics of nanofiltration pilot plant in its cross-flow filtration configuration containing thin film composite polyamide NF membrane in spiral wound module.
nanofiltration (NF) and reverse osmosis (RO) with pretreatment techniques are the only effective methods approved for arsenic reduction. Thus, there is a growing need to develop the cost effective methods for arsenic removal from drinking water. In the literature, there are many methods that have been used for removing arsenic from drinking water [14–17]. However, membranebased separation techniques have been widely employed in different countries for treating a wide variety of industrial effluents [18–24]. The effectiveness of NF membrane processes in water and wastewater treatment has now become one of the most reliable standard techniques to obtain good quality drinking water. Earlier, arsenic removal by the membrane-based separation was attempted by several researchers [25–27]. In continuation of our earlier work [28– 30] on the application of membrane-based techniques, we now investigate the feasibility of arsenic removal from simulated arsenic contaminated water by an indigenously built nanofiltration pilot plant with commercial thin film composite (TFC) membrane of polyamide was used. Experimental data were collected at ambient (30 °C) temperature and results are discussed in terms of variables that have greater practical importance. 2. Experimental 2.1. Materials and feed preparation The commercial thin film composite (TFC) NF-300 was purchased from Permionics, Vadodara, India. The membrane was specified for seawater desalination with the catalogue designation Perma PPT-9908 with an effective area of 2.5 m2, module length of 40 in. and diameter of 2.4 in.. Feed concentration for the pH range of 2–11 was employed during the process operation. Sodium arsenate (NaH3AsO4), sodium arsenate heptahydrate (Na2HAsO4), sodium sulfate (Na2SO4) and all the chemicals were purchased from S. D. Fine Chemicals, Mumbai, India. Deionized water for cleaning and analytical standard preparations was generated from the same reverse osmosis (RO) system. Simulated feed mixtures were prepared using tap ground water, adding 3 µM, 0.5 µM and 0.01 µM concentrations of each arsenic salt solution and for the comparison experiments another set of simulated contaminated waste water were prepared in deionized (DI) water. The pH of these solutions was adjusted by using hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions. 2.2. Analytical methods Concentrations of As, K and Na ions were determined by the atomic absorption spectrometer (AAS) (GBC Avanta 932, Dangenong, Victoria, Australia). Samples of these solutions were collected at every 15 min interval of time for the analysis. Conductivity and pH of the
solutions were also measured using the conductivity meter coupled with pH sensor (Jenway Model 4330, Essex, UK). Chloride ion concentrations were measured by volumetric titration method. Total dissolved solid (TDS) content in mg/L level was estimated directly using the TDS meter (TDScan 1, Singapore). Turbidity was measured using Nephelometric Turbidimeter (Singapore). 2.3. Pilot plant description A pilot-scale skid-mounted system as shown schematically in Fig. 1 was built by incorporating the commercial thin film composite nanofiltration (TFC NF) polyamide membrane with an inlet pressure of 70 bar inside an fiber reinforced plastic (FRP) cylindrical pressure vessel; 2.5 in. diameter × 21 in. length was the size of each module. A feed tank of 30 L capacity made of SS-316 was provided for storage and supply of effluent to the system as well as collection of the recycled concentrate. A cooling coil was installed inside the feed tank for circulating cold water to maintain the constant feed temperature within the range of 27–29 °C. A high-pressure pump capable of maintaining a pressure of 100 bar was installed for transporting feed liquid throughout the system. A 2 HP single-phase motor ran the pump. A restricting needle valve was provided on the concentrate outlet of the membrane pressure vessel to pressurize the feed liquid to the desired value, as indicated by the pressure gauge installed at the upstream side of the valve. Permeate and concentrate flow rates were measured by two glass rotameters containing metal floats. An adjustable safety valve was fixed on the pump discharge line to prevent the system pressure from exceeding the maximum desired Table 1a Analytical data of the arsenic contaminated ground water collected from one of the local site of Manvi, India and these data were used as reference data for the preparation of simulated water for this study. Analyte
Concentration, mg/L
pH Alkalinity total (as CaCO3) Calcium Chloride Silica Chromium Manganese Nitrate (as N) Magnesium Fluoride Sulfate Iron Potassium Sodium Aluminum Arsenic
8.0 95.5 6.9 0.55 130.1 0.005 0.001 0.2 0.28 1.15 9.9 0.035 5.3 62.3 0.05 0.051
R.S. Harisha et al. / Desalination 252 (2010) 75–80 Table 1b Specifications of the simulated mixture of arsenic contaminated water along with other common contaminants found in the ground water sources.
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Table 2 Conductivity of the dissolved ionic species and total dissolved solids removal pattern as a function of pressure for NF processes with initial feed concentration of 100 mg/L.
Contaminate
Concentration (mg/L)
Pressure (bar)
Flux (L/min)
Conductivity (mS/cm)
TDS (mg/L)
Arsenic Chloride Calcium Fluoride Iron Sulfate Total dissolved solids Magnesium and potassium
0.015–0.020 305 550 0.27 0.108 60 368 b5
Initial conc. 15 20 25 30
100 330 1360 1760 2480
0.809 0.456 0.308 0.202 0.200
485 274 183.6 121.2 120.8
value (70 bar). All the accessories were connected by 0.5 in. outerdiameter SS-316 piping in high-pressure region, whereas lowpressure outlets from concentrate and permeate sections were made of poly vinyl chloride (PVC) braided tubing.
removal of monovalent ions. It does not require the same energy to perform the separation. In RO membrane process, the separation performance of the membrane is judged by the % rejection (% R) of total dissolved solid (TDS), chemical oxygen demand (COD) and other feed components, which are calculated as: % R = ðCp = Cf Þ
ð1Þ
3. Experimental procedure and sampling Nearly 30 L of synthetic feeds (different concentrations) were poured in the feed tank after thoroughly cleaning the membrane systems and wetting with the deionized water. The high-pressure pump was employed to transport the feed to the spiral wound membrane module and the system pressure was adjusted at a value greater than the osmotic pressure, by means of restricting needle valve. Using the control valve, retentate flow rate was maintained constant (10 L/min) throughout the experiments, to ensure steadystate hydrodynamic conditions inside the membrane module. Feed pressure was varied from 0 to 30 bar by keeping the feed TDS concentrations constant (for NF, a series of approximate TDS concentration having 100 mg/L, 1000 mg/L, 1875 mg/L, 2575 mg/L, 5000 mg/L and a series of known arsenate standard solutions in the feed having concentrations; 0.1 mg/L, 0.5 mg/L, 1 mg/L, 2 mg/L, 5 mg/ L and 10 mg/L), Permeate samples were collected after 15 min with every 5 bar increase in feed pressure. Flux was noted by collecting the permeate in a measuring jar for up to 1–2 min around 300–3000 mL of permeate was collected in each step of pressure increase for a thorough analysis. The experiment was repeated thrice with every feed samples for reproducibility of the permeate characteristics, which generally falls within 3% standard error. 3.1. Flux and % rejection Nanofiltration, in concept and in operation, is much the same as reverse osmosis (RO). However, the key difference is the degree of
Where Cp represents the concentration of a particular component in permeate and Cf is its feed concentration. Flux (J) is the volume of permeate (V) collected per unit membrane area (A) per unit time (t), which is calculated as: J = V = At
ð2Þ
3.2. Membrane fouling and its prevention Fouling of the membrane surface is the bane of membrane operations. The concentration of arsenic in the form of arsenate (V) in addition to calcium and magnesium contaminants in drinking water was reasonably high along with naturally according Fe (II) in very low concentrations, which would cause the scale (cake) formation on membrane surface, thereby causing subsequent fouling. There are two types, colloidal flocculation or inorganic scaling above the membrane surface, in addition to adsorption of organics to the membrane. However, the membrane permeability always decreased whenever amounts of oxidized iron and/or manganese were present in feed water. Also, the manganese oxide particles offers specific resistance of the cake compared to the iron oxide particles and their mixture. Fouling layers were removed by using appropriate cleaning procedures. The most common procedure was using caustic detergent for the removal of dirt, oils and colloidal material, acids for metal hydroxides, and oxidizing agents for adsorbed organics. The calcium carbonate (CaCO3) scaling was controlled by treating the membrane at the end of day's experiments with an acid solution (HCl, pH = 2), which converts carbonate to CO2 and also removes other metal precipitates as well as mineral scales. Washing with 1% aqueous solution of tetrasodium ethylene diamine tetra acetic acid (EDTA), a chelating agent useful in removing organics and silt, was done on alternate days. To prevent the biological fouling, membrane was washed thoroughly with deionized water and stored in 0.5% solution of sodium bisulfate (NaHSO3) at the end of the experimental runs. The above methods were efficient enough in restoring the water flux.
Table 3 Conductivity of the dissolved ionic species and total dissolved solids removal pattern as a function of pressure for NF processes with initial feed concentration of 1000 mg/L.
Fig. 2. Effect of raw/ground/tap water turbidity containing arsenic as one of contaminate as a function of arsenic rejection at optimized pressure of 50 bar.
Pressure (bar)
Flux (L/min)
Conductivity (mS/cm)
TDS (mg/L)
Initial conc. 15 20 25 30
1000 522 1220 2120 2560
0.795 0.357 0.302 0.221 0.221
477 215 181.4 133.2 131.9
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Fig. 3. Rejection of total dissolved solids (TDS) as a function of different applied pressure for NF processes.
4. Results and discussion Specifications of arsenic contaminated water were collected from one of the local points in India and these averages were to simulate the wastewater for this study as given in Table 1a. Flow rates of the permeates at different pressures were determined with their respective electrical conductivity, pH and for all other contaminants. 4.1. NF experiments NF experiments were carried out to remove the arsenic. Our NF module effectively removed the particles in colloidal size range, which were found in raw/tap/ground water used for preparing simulated mixtures. The prepared simulated waste water samples free from suspended particles were used as feed for NF experiment with the characteristics as shown in Table 1b. Fig. 2 shows the variation of arsenic concentration with residual water turbidity. Turbidity is a measure of the degree to which the water loses its transparency due to the presence of suspended particulates. Turbidity is a principal physical characteristic of water and is an expression of the optical property that causes light to be scattered and absorbed by particles and molecules rather than transmitted in straight lines through a water sample. The more total suspended solids in the water, the murkier it seems and the higher the turbidity. It is caused by suspended matter or impurities that interfere with the clarity of the water. These impurities may include clay, silt, finely divided inorganic and organic matter, soluble colored organic compounds, and plankton and other microscopic organisms. The suspended particles absorb heat from the sunlight, making turbid waters become warmer, and so reducing the concentration of oxygen in the water (Table 2). For water samples tested, an exponential relationship was noticed as depicted in Fig. 2. Feed with a TDS concentration of 331 mg/L was run to remove other common contaminates along with arsenic. At the optimal pressure range of 30–50 bar, permeate was having all contaminates below the (MCLs) norms. Thus, our analysis showed that permeate concentration was reduced to 102 mg/L of TDS and conductivity from 0.65 to
Table 4 Conductivity of the dissolved ionic species and total dissolved solids removal pattern as a function of pressure for NF processes with initial feed concentration of 1875 mg/L.
Fig. 4. Effect of different initial concentrations of the feed mixture on % rejection of arsenic as a function of applied pressure.
0.062 mS/cm. At the optimal pressure range of 40–50 bar, the rejection of 99% arsenic along with TDS was achieved. This indicates the high efficiency offered by the NF membrane module in rejecting possible bivalent and trivalent ions that include calcium, magnesium, and ironlike trace metals. As shown in Table 3, a high efficiency of NF membrane module in rejecting the possible bivalent and trivalent ions that include calcium, magnesium, and iron-like trace metals, is observed. Thus, it was effective in removing other common contaminates available in ground water sources as shown in Fig. 3. In any case, NF demonstrated the successful removal of TDS along with arsenic. 4.2. Effect of initial concentration To check the effectiveness of our new system developed for different potential applications, we have carried out NF experiments with different initial concentrations. Natural concentrations of arsenic in ground water sources typically range from 0.1 to 40 mg/L, with an average concentration of 5 to 6 mg/L. Through erosion, dissolution and weathering, arsenic gets released to ground water. Water from these sources would thus cause an elevated arsenic level in rivers downstream. Effect of initial concentration on the removal efficiency of NF membrane is displayed in Table 4. With the operating pressure of 10 to 60 bar, 0.0005 M concentration, a highest arsenic removal of 98.98% was achieved at 50 bar pressure. For 0.0003 M initial concentrations, rejection rate were increased to 99.82% at 50 bar pressure and 99.99% rejection was achieved for concentration of 0.0001 M solution at 50 bar applied pressure (Fig. 4). Even though at higher applied pressures, the same trend was observed for 40 to 50 bar pressures, which was indeed the optimal level. For a higher initial concentration, membrane was found to be highly effective in achieving 99.99% of rejection and these values are much below the MCL. Conductivity as a function of applied pressure at three different initial concentrations is shown in Table 5. Electrical conductivity of
Table 5 Conductivity of the dissolved ionic species and total dissolved solids removal pattern as a function of pressure for NF processes with initial feed concentration of 2575 mg/L.
Pressure (bar)
Flux (L/min)
Conductivity (mS/cm)
TDS (mg/L)
Pressure (bar)
Flux (L/min)
Conductivity (mS/cm)
TDS (mg/L)
Initial conc. 15 20 25 30
1875 440 1340 2920 3280
0.786 0.455 0.354 0.207 0.355
472 273 213 123.8 214
Initial conc. 15 20 25 30
2575 500 1460 2440 2640
0.938 0.527 0.403 0.211 0.368
561 317 241 126.8 220
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Table 6 Conductivity of the dissolved ionic species and total dissolved solids removal pattern as a function of pressure for NF processes with initial feed concentration of 5000 mg/L.
Fig. 5. Conductivity of dissolved ionic species present in the contaminated feed mixture as a function of applied pressure for NF processes.
the permeate solution decreased exponentially with time. In cases where 40 and 50 bar pressures were applied, continuously a decrease in electrical conductivity was observed as shown in Fig. 5. A significant difference was observed in electrical conductivity of the permeate at the end of the operation for 50 bar for three different initial concentrations with higher rejection rates of the ion, which has resulted in achieving a very low conductivity of 0.084 μS. 4.3. Effect of pH As mentioned before, arsenic can exist in two stable forms viz., As (III) and As (V). Its predominant oxidation state depends on the pH and redox potential. Predominant forms of arsenic in ground and surface waters are: As (V) and As (III). Inorganic compounds found in the environment include oxides (As2O3) and sulfides (As2O5). Also inorganic arsenic species that are stable in oxygenated waters includes 2− 3− arsenic acid species viz., H3AsO4, H2AsO− 4 , HAsO4 and AsO4 . The effect of pH on transport of arsenic is shown in Fig. 6. Transfer of ion is greater at lower pH, which decreases with increasing pH. However, the relative abundance (see Table 6) of a particular complex depends upon the concentration of arsenic ion and pH of the solution. As can be seen, the
Pressure (bar)
Flux (L/min)
Conductivity (mS/cm)
TDS (mg/L)
Initial conc. 15 20 25 30
5000 480 1480 2240 2880
1.046 0.565 0.384 0.365 0.336
631 338 231 218 202
pH of water has a strong effect on % arsenic removal at the given pressures. Arsenic in the commonly high oxidation state of (V) can be thus effectively removed by the NF technique. At lower pH of 5, rejection rate was extended up to 99.99%. In general, in the pH range of 5–7, an increase in percentage arsenic rejection was noticed. These results highlight the fact that the effects of sulfate as well as iron ions depend not only on arsenic rejection, but also the concentration of other electrolytes present. With further attention on the removal of weakly acidic arsenic species in water by NF at sufficiently high pH values was made possible by the use of antiscalants. This was taken care to prevent any damage to the membrane. However, the practical processes can then be developed with NF to remove all the major species of other contaminates as well. The membrane performance was reasonably good, along with a decreased solute rejection; a decrease in flux was also observed, indicating the possible concentration polarization of the solute, which increases the osmotic pressure at the membrane surface, thereby causing a loss in the effective transmembrane pressure. 5. Conclusions The present study demonstrates the effective usage of an NF process for the removal of arsenic from drinking water. The commercial TFC NF membranes of polyamide origin used in this study could decrease arsenic ion concentration to the level set by World Health Organization (WHO) for arsenic contaminated drinking water. Due to high fluxes obtained, significant rejection rates of TDS, conductivity etc., were achieved. However, the efficiency of this process depends upon the pH of water and the presence of other ions. NF results of this study showed the turbidity caused by suspended matter or organic impurities that interfere with the clarity of the water was reduced substantially to produce potable quality water along with arsenate ion removal from the synthetic water. The system is very simple to operate, with a minimum of pumps and controls, in addition to low requirements for operator attention. Typical, daily maintenance includes monitoring of the performance parameters. Acknowledgements The authors thank Department of Science and Technology, New Delhi – 110 016 (Ref. No SR/S1/OC-08/2005 dated 05-09-2005) for financial assistance and Center of Excellence in Polymer Science, Karnatak University, Dharwad, India, for providing the facilities. References
Fig. 6. Effect of the simulated feed mixture pH on the % rejection of arsenic as a function at different applied pressure.
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Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Arsenic removal from drinking water using thin film composite nanofiltration membrane R.S. Harisha a, K.M. Hosamani a,⁎, R.S. Keri a, S.K. Nataraj b, T.M. Aminabhavi b a b
P. G. Department of Studies in Chemistry, Karnatak University, Dharwad 580 003, India Membrane Separation Division, Center of Excellence in Polymer Science, Karnatak University, Dharwad 580 003, India
a r t i c l e
i n f o
Article history: Received 21 August 2009 Received in revised form 25 October 2009 Accepted 30 October 2009 Available online 25 November 2009 Keywords: Arsenic Drinking water Membrane Nanofiltration Water treatment
a b s t r a c t Arsenic removal from drinking water is a major problem in many parts of the world. Acute and chronic arsenic exposure through drinking water has been reported in many countries, especially Argentina, Bangladesh, India, Mexico, Mongolia, Thailand and Taiwan, where a large proportion of ground water is contaminated with arsenic at levels from 100 to over 2000 μm/L. In this work, we have investigated the arsenic removal by the membrane-based nanofiltration (NF) technique. The results of this study demonstrate that arsenate ions removal by NF with a high rejection of 99.80% along with total dissolved solids (TDS) and other contaminates were achieved. Significant flux was retained at the end of each NF experiment with operation time of 180 min, which suggests that membranes were not affected by the fouling phenomenon during the process run. The feasibility of membranes for treating drinking water by varying feed pressure, pH and feed concentration was tested on the separation performance of the thin film composite NF membranes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Arsenic is widely distributed in air, water and soil as a metalloid and in the form of chemical compounds [1]. Average concentrations of arsenic in earth's crust normally range from 1.5 to 5 mg/kg. Higher concentrations were found in some igneous and sedimentary rocks, particularly in iron and manganese ores. Arsenic may be released from these ores to soil, surface water and ground water. Recently, the presence of dissolved arsenic in drinking water has emerged as a problem of global magnitude. Promulgation of new maximum contaminant levels (MCL) by the United States Environmental Protection Agency (USEPA) and the World Health Organization (WHO) has resulted in a demand for more efficient arsenate ions removal technologies. Discovery of arsenic in ground water of alluvial deposits of West Bengal (India) and Bangladesh referred to as the Bengal delta contaminating up to 97% of domestic water supply in rural areas based on tube wells, threatens millions of people who are exposed to high concentrations of arsenic. Recent studies indicate the additional contamination of the food chain due to irrigation with arsenic contaminated water [2–4]. In 1993, WHO lowered the guideline value for arsenic in drinking water from 50 μg/L down to 10 μg/L [5]. In contrast to the Southwest region of Taiwan, West Bengal and Bangladesh face an ongoing problem with widespread exposure to very high concentration levels of arsenic in drinking
⁎ Corresponding author. Fax: +91 836 747884, +91 836 2771275. E-mail address: [email protected] (K.M. Hosamani). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.10.022
water ranging between 35 and 77 million of Bangladesh and West Bengal residents are now thought to be affected by exposure to arsenic in drinking water that exceeds 2000 μg/L [4,6]. In West Bengal, an estimated one million residents are exposed to ground water containing up to 3900 μg/L [7]. Large populations of these regions, coupled with the concentrations of arsenic in drinking water in hundreds or thousands of μg/L, would pose a major threat to public health and hygiene. Furthermore, because some cancers have a latency period of 20 to 30 years, and widespread exposure to these very high concentrations of arsenic began only in 1970s due to the drilling of millions of tube wells; it could be also possible that the full effect of arsenic exposure in West Bengal and Bangladesh is yet to be seen. Arsenic can exist in natural water sources in both organic and inorganic forms; however, only inorganic forms viz., arsenate [As (V)] and arsenite [As (III)] are considered in this study because available data indicate that the amount of organic arsenic in drinking water sources is almost insignificant. Studies have linked the long-term exposure to arsenic in drinking water to bladder cancer, lungs, skin, kidney, nasal passages, liver and prostate [8]. Besides its tumorigenic potential, arsenic has been shown genotoxic effects [9,10]. Arsenic removal from drinking water using alum (90% removal), iron salt (90 % removal), iron fillings (90% removal) and bucket or tea-bag method (80–90% removal have been described by many researchers [11,12]. There is a report on arsenic removal from soil using pteris vittata (brake fern) [13]. Common treatment methods such as water softening, carbon filters and sediment filters cannot adequately remove arsenic from drinking water. In these days, distillation,
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Fig. 1. Schematics of nanofiltration pilot plant in its cross-flow filtration configuration containing thin film composite polyamide NF membrane in spiral wound module.
nanofiltration (NF) and reverse osmosis (RO) with pretreatment techniques are the only effective methods approved for arsenic reduction. Thus, there is a growing need to develop the cost effective methods for arsenic removal from drinking water. In the literature, there are many methods that have been used for removing arsenic from drinking water [14–17]. However, membranebased separation techniques have been widely employed in different countries for treating a wide variety of industrial effluents [18–24]. The effectiveness of NF membrane processes in water and wastewater treatment has now become one of the most reliable standard techniques to obtain good quality drinking water. Earlier, arsenic removal by the membrane-based separation was attempted by several researchers [25–27]. In continuation of our earlier work [28– 30] on the application of membrane-based techniques, we now investigate the feasibility of arsenic removal from simulated arsenic contaminated water by an indigenously built nanofiltration pilot plant with commercial thin film composite (TFC) membrane of polyamide was used. Experimental data were collected at ambient (30 °C) temperature and results are discussed in terms of variables that have greater practical importance. 2. Experimental 2.1. Materials and feed preparation The commercial thin film composite (TFC) NF-300 was purchased from Permionics, Vadodara, India. The membrane was specified for seawater desalination with the catalogue designation Perma PPT-9908 with an effective area of 2.5 m2, module length of 40 in. and diameter of 2.4 in.. Feed concentration for the pH range of 2–11 was employed during the process operation. Sodium arsenate (NaH3AsO4), sodium arsenate heptahydrate (Na2HAsO4), sodium sulfate (Na2SO4) and all the chemicals were purchased from S. D. Fine Chemicals, Mumbai, India. Deionized water for cleaning and analytical standard preparations was generated from the same reverse osmosis (RO) system. Simulated feed mixtures were prepared using tap ground water, adding 3 µM, 0.5 µM and 0.01 µM concentrations of each arsenic salt solution and for the comparison experiments another set of simulated contaminated waste water were prepared in deionized (DI) water. The pH of these solutions was adjusted by using hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions. 2.2. Analytical methods Concentrations of As, K and Na ions were determined by the atomic absorption spectrometer (AAS) (GBC Avanta 932, Dangenong, Victoria, Australia). Samples of these solutions were collected at every 15 min interval of time for the analysis. Conductivity and pH of the
solutions were also measured using the conductivity meter coupled with pH sensor (Jenway Model 4330, Essex, UK). Chloride ion concentrations were measured by volumetric titration method. Total dissolved solid (TDS) content in mg/L level was estimated directly using the TDS meter (TDScan 1, Singapore). Turbidity was measured using Nephelometric Turbidimeter (Singapore). 2.3. Pilot plant description A pilot-scale skid-mounted system as shown schematically in Fig. 1 was built by incorporating the commercial thin film composite nanofiltration (TFC NF) polyamide membrane with an inlet pressure of 70 bar inside an fiber reinforced plastic (FRP) cylindrical pressure vessel; 2.5 in. diameter × 21 in. length was the size of each module. A feed tank of 30 L capacity made of SS-316 was provided for storage and supply of effluent to the system as well as collection of the recycled concentrate. A cooling coil was installed inside the feed tank for circulating cold water to maintain the constant feed temperature within the range of 27–29 °C. A high-pressure pump capable of maintaining a pressure of 100 bar was installed for transporting feed liquid throughout the system. A 2 HP single-phase motor ran the pump. A restricting needle valve was provided on the concentrate outlet of the membrane pressure vessel to pressurize the feed liquid to the desired value, as indicated by the pressure gauge installed at the upstream side of the valve. Permeate and concentrate flow rates were measured by two glass rotameters containing metal floats. An adjustable safety valve was fixed on the pump discharge line to prevent the system pressure from exceeding the maximum desired Table 1a Analytical data of the arsenic contaminated ground water collected from one of the local site of Manvi, India and these data were used as reference data for the preparation of simulated water for this study. Analyte
Concentration, mg/L
pH Alkalinity total (as CaCO3) Calcium Chloride Silica Chromium Manganese Nitrate (as N) Magnesium Fluoride Sulfate Iron Potassium Sodium Aluminum Arsenic
8.0 95.5 6.9 0.55 130.1 0.005 0.001 0.2 0.28 1.15 9.9 0.035 5.3 62.3 0.05 0.051
R.S. Harisha et al. / Desalination 252 (2010) 75–80 Table 1b Specifications of the simulated mixture of arsenic contaminated water along with other common contaminants found in the ground water sources.
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Table 2 Conductivity of the dissolved ionic species and total dissolved solids removal pattern as a function of pressure for NF processes with initial feed concentration of 100 mg/L.
Contaminate
Concentration (mg/L)
Pressure (bar)
Flux (L/min)
Conductivity (mS/cm)
TDS (mg/L)
Arsenic Chloride Calcium Fluoride Iron Sulfate Total dissolved solids Magnesium and potassium
0.015–0.020 305 550 0.27 0.108 60 368 b5
Initial conc. 15 20 25 30
100 330 1360 1760 2480
0.809 0.456 0.308 0.202 0.200
485 274 183.6 121.2 120.8
value (70 bar). All the accessories were connected by 0.5 in. outerdiameter SS-316 piping in high-pressure region, whereas lowpressure outlets from concentrate and permeate sections were made of poly vinyl chloride (PVC) braided tubing.
removal of monovalent ions. It does not require the same energy to perform the separation. In RO membrane process, the separation performance of the membrane is judged by the % rejection (% R) of total dissolved solid (TDS), chemical oxygen demand (COD) and other feed components, which are calculated as: % R = ðCp = Cf Þ
ð1Þ
3. Experimental procedure and sampling Nearly 30 L of synthetic feeds (different concentrations) were poured in the feed tank after thoroughly cleaning the membrane systems and wetting with the deionized water. The high-pressure pump was employed to transport the feed to the spiral wound membrane module and the system pressure was adjusted at a value greater than the osmotic pressure, by means of restricting needle valve. Using the control valve, retentate flow rate was maintained constant (10 L/min) throughout the experiments, to ensure steadystate hydrodynamic conditions inside the membrane module. Feed pressure was varied from 0 to 30 bar by keeping the feed TDS concentrations constant (for NF, a series of approximate TDS concentration having 100 mg/L, 1000 mg/L, 1875 mg/L, 2575 mg/L, 5000 mg/L and a series of known arsenate standard solutions in the feed having concentrations; 0.1 mg/L, 0.5 mg/L, 1 mg/L, 2 mg/L, 5 mg/ L and 10 mg/L), Permeate samples were collected after 15 min with every 5 bar increase in feed pressure. Flux was noted by collecting the permeate in a measuring jar for up to 1–2 min around 300–3000 mL of permeate was collected in each step of pressure increase for a thorough analysis. The experiment was repeated thrice with every feed samples for reproducibility of the permeate characteristics, which generally falls within 3% standard error. 3.1. Flux and % rejection Nanofiltration, in concept and in operation, is much the same as reverse osmosis (RO). However, the key difference is the degree of
Where Cp represents the concentration of a particular component in permeate and Cf is its feed concentration. Flux (J) is the volume of permeate (V) collected per unit membrane area (A) per unit time (t), which is calculated as: J = V = At
ð2Þ
3.2. Membrane fouling and its prevention Fouling of the membrane surface is the bane of membrane operations. The concentration of arsenic in the form of arsenate (V) in addition to calcium and magnesium contaminants in drinking water was reasonably high along with naturally according Fe (II) in very low concentrations, which would cause the scale (cake) formation on membrane surface, thereby causing subsequent fouling. There are two types, colloidal flocculation or inorganic scaling above the membrane surface, in addition to adsorption of organics to the membrane. However, the membrane permeability always decreased whenever amounts of oxidized iron and/or manganese were present in feed water. Also, the manganese oxide particles offers specific resistance of the cake compared to the iron oxide particles and their mixture. Fouling layers were removed by using appropriate cleaning procedures. The most common procedure was using caustic detergent for the removal of dirt, oils and colloidal material, acids for metal hydroxides, and oxidizing agents for adsorbed organics. The calcium carbonate (CaCO3) scaling was controlled by treating the membrane at the end of day's experiments with an acid solution (HCl, pH = 2), which converts carbonate to CO2 and also removes other metal precipitates as well as mineral scales. Washing with 1% aqueous solution of tetrasodium ethylene diamine tetra acetic acid (EDTA), a chelating agent useful in removing organics and silt, was done on alternate days. To prevent the biological fouling, membrane was washed thoroughly with deionized water and stored in 0.5% solution of sodium bisulfate (NaHSO3) at the end of the experimental runs. The above methods were efficient enough in restoring the water flux.
Table 3 Conductivity of the dissolved ionic species and total dissolved solids removal pattern as a function of pressure for NF processes with initial feed concentration of 1000 mg/L.
Fig. 2. Effect of raw/ground/tap water turbidity containing arsenic as one of contaminate as a function of arsenic rejection at optimized pressure of 50 bar.
Pressure (bar)
Flux (L/min)
Conductivity (mS/cm)
TDS (mg/L)
Initial conc. 15 20 25 30
1000 522 1220 2120 2560
0.795 0.357 0.302 0.221 0.221
477 215 181.4 133.2 131.9
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Fig. 3. Rejection of total dissolved solids (TDS) as a function of different applied pressure for NF processes.
4. Results and discussion Specifications of arsenic contaminated water were collected from one of the local points in India and these averages were to simulate the wastewater for this study as given in Table 1a. Flow rates of the permeates at different pressures were determined with their respective electrical conductivity, pH and for all other contaminants. 4.1. NF experiments NF experiments were carried out to remove the arsenic. Our NF module effectively removed the particles in colloidal size range, which were found in raw/tap/ground water used for preparing simulated mixtures. The prepared simulated waste water samples free from suspended particles were used as feed for NF experiment with the characteristics as shown in Table 1b. Fig. 2 shows the variation of arsenic concentration with residual water turbidity. Turbidity is a measure of the degree to which the water loses its transparency due to the presence of suspended particulates. Turbidity is a principal physical characteristic of water and is an expression of the optical property that causes light to be scattered and absorbed by particles and molecules rather than transmitted in straight lines through a water sample. The more total suspended solids in the water, the murkier it seems and the higher the turbidity. It is caused by suspended matter or impurities that interfere with the clarity of the water. These impurities may include clay, silt, finely divided inorganic and organic matter, soluble colored organic compounds, and plankton and other microscopic organisms. The suspended particles absorb heat from the sunlight, making turbid waters become warmer, and so reducing the concentration of oxygen in the water (Table 2). For water samples tested, an exponential relationship was noticed as depicted in Fig. 2. Feed with a TDS concentration of 331 mg/L was run to remove other common contaminates along with arsenic. At the optimal pressure range of 30–50 bar, permeate was having all contaminates below the (MCLs) norms. Thus, our analysis showed that permeate concentration was reduced to 102 mg/L of TDS and conductivity from 0.65 to
Table 4 Conductivity of the dissolved ionic species and total dissolved solids removal pattern as a function of pressure for NF processes with initial feed concentration of 1875 mg/L.
Fig. 4. Effect of different initial concentrations of the feed mixture on % rejection of arsenic as a function of applied pressure.
0.062 mS/cm. At the optimal pressure range of 40–50 bar, the rejection of 99% arsenic along with TDS was achieved. This indicates the high efficiency offered by the NF membrane module in rejecting possible bivalent and trivalent ions that include calcium, magnesium, and ironlike trace metals. As shown in Table 3, a high efficiency of NF membrane module in rejecting the possible bivalent and trivalent ions that include calcium, magnesium, and iron-like trace metals, is observed. Thus, it was effective in removing other common contaminates available in ground water sources as shown in Fig. 3. In any case, NF demonstrated the successful removal of TDS along with arsenic. 4.2. Effect of initial concentration To check the effectiveness of our new system developed for different potential applications, we have carried out NF experiments with different initial concentrations. Natural concentrations of arsenic in ground water sources typically range from 0.1 to 40 mg/L, with an average concentration of 5 to 6 mg/L. Through erosion, dissolution and weathering, arsenic gets released to ground water. Water from these sources would thus cause an elevated arsenic level in rivers downstream. Effect of initial concentration on the removal efficiency of NF membrane is displayed in Table 4. With the operating pressure of 10 to 60 bar, 0.0005 M concentration, a highest arsenic removal of 98.98% was achieved at 50 bar pressure. For 0.0003 M initial concentrations, rejection rate were increased to 99.82% at 50 bar pressure and 99.99% rejection was achieved for concentration of 0.0001 M solution at 50 bar applied pressure (Fig. 4). Even though at higher applied pressures, the same trend was observed for 40 to 50 bar pressures, which was indeed the optimal level. For a higher initial concentration, membrane was found to be highly effective in achieving 99.99% of rejection and these values are much below the MCL. Conductivity as a function of applied pressure at three different initial concentrations is shown in Table 5. Electrical conductivity of
Table 5 Conductivity of the dissolved ionic species and total dissolved solids removal pattern as a function of pressure for NF processes with initial feed concentration of 2575 mg/L.
Pressure (bar)
Flux (L/min)
Conductivity (mS/cm)
TDS (mg/L)
Pressure (bar)
Flux (L/min)
Conductivity (mS/cm)
TDS (mg/L)
Initial conc. 15 20 25 30
1875 440 1340 2920 3280
0.786 0.455 0.354 0.207 0.355
472 273 213 123.8 214
Initial conc. 15 20 25 30
2575 500 1460 2440 2640
0.938 0.527 0.403 0.211 0.368
561 317 241 126.8 220
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Table 6 Conductivity of the dissolved ionic species and total dissolved solids removal pattern as a function of pressure for NF processes with initial feed concentration of 5000 mg/L.
Fig. 5. Conductivity of dissolved ionic species present in the contaminated feed mixture as a function of applied pressure for NF processes.
the permeate solution decreased exponentially with time. In cases where 40 and 50 bar pressures were applied, continuously a decrease in electrical conductivity was observed as shown in Fig. 5. A significant difference was observed in electrical conductivity of the permeate at the end of the operation for 50 bar for three different initial concentrations with higher rejection rates of the ion, which has resulted in achieving a very low conductivity of 0.084 μS. 4.3. Effect of pH As mentioned before, arsenic can exist in two stable forms viz., As (III) and As (V). Its predominant oxidation state depends on the pH and redox potential. Predominant forms of arsenic in ground and surface waters are: As (V) and As (III). Inorganic compounds found in the environment include oxides (As2O3) and sulfides (As2O5). Also inorganic arsenic species that are stable in oxygenated waters includes 2− 3− arsenic acid species viz., H3AsO4, H2AsO− 4 , HAsO4 and AsO4 . The effect of pH on transport of arsenic is shown in Fig. 6. Transfer of ion is greater at lower pH, which decreases with increasing pH. However, the relative abundance (see Table 6) of a particular complex depends upon the concentration of arsenic ion and pH of the solution. As can be seen, the
Pressure (bar)
Flux (L/min)
Conductivity (mS/cm)
TDS (mg/L)
Initial conc. 15 20 25 30
5000 480 1480 2240 2880
1.046 0.565 0.384 0.365 0.336
631 338 231 218 202
pH of water has a strong effect on % arsenic removal at the given pressures. Arsenic in the commonly high oxidation state of (V) can be thus effectively removed by the NF technique. At lower pH of 5, rejection rate was extended up to 99.99%. In general, in the pH range of 5–7, an increase in percentage arsenic rejection was noticed. These results highlight the fact that the effects of sulfate as well as iron ions depend not only on arsenic rejection, but also the concentration of other electrolytes present. With further attention on the removal of weakly acidic arsenic species in water by NF at sufficiently high pH values was made possible by the use of antiscalants. This was taken care to prevent any damage to the membrane. However, the practical processes can then be developed with NF to remove all the major species of other contaminates as well. The membrane performance was reasonably good, along with a decreased solute rejection; a decrease in flux was also observed, indicating the possible concentration polarization of the solute, which increases the osmotic pressure at the membrane surface, thereby causing a loss in the effective transmembrane pressure. 5. Conclusions The present study demonstrates the effective usage of an NF process for the removal of arsenic from drinking water. The commercial TFC NF membranes of polyamide origin used in this study could decrease arsenic ion concentration to the level set by World Health Organization (WHO) for arsenic contaminated drinking water. Due to high fluxes obtained, significant rejection rates of TDS, conductivity etc., were achieved. However, the efficiency of this process depends upon the pH of water and the presence of other ions. NF results of this study showed the turbidity caused by suspended matter or organic impurities that interfere with the clarity of the water was reduced substantially to produce potable quality water along with arsenate ion removal from the synthetic water. The system is very simple to operate, with a minimum of pumps and controls, in addition to low requirements for operator attention. Typical, daily maintenance includes monitoring of the performance parameters. Acknowledgements The authors thank Department of Science and Technology, New Delhi – 110 016 (Ref. No SR/S1/OC-08/2005 dated 05-09-2005) for financial assistance and Center of Excellence in Polymer Science, Karnatak University, Dharwad, India, for providing the facilities. References
Fig. 6. Effect of the simulated feed mixture pH on the % rejection of arsenic as a function at different applied pressure.
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