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Seawater Desalination Using Modified Ceramic Membranes V. Perez-Moreno,* C. B. Bonilla-Suarez, M. Fortanell-Trejo, and G. Pedraza-Aboytes Centro de Estudios Academicos sobre Contaminaci on Ambiental, Facultad de Química, Universidad Autonoma de Queretaro, Centro Universitario, Cerro de las Campanas, Santiago de Queretaro, Qro, C. P. 76010, Mexico ABSTRACT: Nanofiltration has many applications such as pretreatment and partial demineralization in seawater desalination processes. This paper studies the performance of commercial ceramic membranes, modified by depositing either platinum or silver, to reject salts of seawater. The membrane used is multilayer structured with a titania layer (5 kD MWCO) manufactured by TAMI Industries (Nyons, France) and anionic impregnation was used to depose platinum or silver onto the membrane. The effect of salts rejection of membranes was investigated using seawater from the Mexican Pacific coast. The tests were carried out at pressure of 6 bar and the experimental results showed that modified membranes could achieve a salt rejection close to 30% in total dissolved solids (TDS) whereas the rejection in nonmodified membranes was less than 5%. Anion and cation rejection was also determined, with higher values obtained for cation removal.
1. INTRODUCTION Desalination of seawater has become increasingly important as a source of water supply to contend with the worldwide scarcity of this resource. Fresh water for most human activities requires a content of total dissolved solids (TDS) lower than 1000 mg/L, and to achieve this value there are different processes. Membranes processes have dominated the desalination market in recent times, mainly reverse osmosis using polymeric membranes. Thermal processes, which were the first industrial processes to remove salts from seawater, are now less used because of the larger environmental and economical costs.1 In Mexico, there were about 435 desalination plants at the beginning of of the 21st century, but around 30% of them were not operating in a regular way, due to maintenance or design problems. The total gross capacity up to 2007 was 193 771 m3/d but only around 135 000 m3/d was real operating capacity.2 Reverse osmosis (RO) as a membrane process is a pressure driven filtration, and the seawater components (TDS, organic matter, turbidity, or hardness) may produce problems such as scaling, fouling, increase in energy requirements, and aging of membranes. Nanofiltration (NF) membranes may be employed as pretreatment to decrease seawater desalination problems and to reduce their effects on productivity and costs.3 Additionally, NF is able to reduce part of the content of seawater salts and be considered as a desalination process itself.4 Membrane processes are classified according to the size of particles that they can remove. NF has properties between ultrafiltration (UF) and RO, in both size of removed matter and in the operating pressure. NF is able to remove particles in the range of 0.010.001 μm. However, there is an increasing development in polymeric materials for membranes; porous ceramic membranes have some advantages such as higher chemical and thermal resistance, longer lifetime, and capacity to be modified.5,6 Ceramic membranes surfaces can be treated to selectively reject some chemical ions. The modification can be done by anionic impregnation using a metallic salt onto the pores of the membrane. In this way, the dissolved ions concentration can be reduced due to rejection/attraction of them with the r 2011 American Chemical Society
electric charges of the deposed metal even though the membrane pore is larger than the rejected particle and without losing the membrane porosity during the operation.7 Many researchers have studied the rejection of different particles with NF membranes, even ionic compounds, but they have used mainly polymeric membranes and low salt concentration in their studies. Afonso et al.8 had good results of ion removal using low concentrations with some polymeric membranes, reaching values of 95% with divalent ions. Hilal et al.9 had similar values with low concentrations but they dropped to 41% when using higher concentrations (up to 25 000 ppm) and using polymeric membranes. Al-Zoubi and Omar10 used high concentration of individual chemical ionic compounds with polymeric membranes and obtained rejection from 15 to 60% for different ions, and variating pressure up to 9 bar. Mazzoni et al.11 measured the permeability of ceramic membranes using low concentrated solutions of different separated electrolytes reaching values of 50% for CaCl2 and 25% for NaCl. In this work, ceramic modified membranes and seawater (high concentration solution) were used. This may establish the viability of using this type of membrane as a precursor step in the desalination process. We present the rejection of some anions and cations using modified membranes and they are compared with that of nonmodified membranes.
2. EXPERIMENTAL SECTION 2.1. Seawater Characterization. For this work, we employed seawater from the Mexican Pacific Coast, specifically from Playa Blanca, Guerrero (17° 340 45.0800 N, 101° 270 21.9700 W). The water samples were first obtained from this point and analyzed. Special Issue: AMIDIQ 2011 Received: April 29, 2011 Accepted: September 2, 2011 Revised: August 9, 2011 Published: September 02, 2011 5900
dx.doi.org/10.1021/ie2009313 | Ind. Eng. Chem. Res. 2012, 51, 5900–5904
Industrial & Engineering Chemistry Research
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Figure 1. Diagram of the equipment used in this study.
Then, samples along 80 km were also spatially taken and analyzed. A yearly characterization was also done and we could evaluate the parameters of the seawater to take into account the geographic and time variation. Physicochemical and microbiological parameters were determined according to Mexican norms. 2.2. Materials. Commercial membranes provided by TAMI were used. They are made of four layers: the largest layer is a mixture of α-Al2O3, TiO2, and ZrO2, the two next layers are made of TiO2 and TiO2/ZrO2, and the last layer is made of pure TiO2. The external diameter of the membranes is 10 mm and membranes with one and seven channels with a total length of 25 cm were used. The contact surface is 0.0047 m2 for the membrane of one channel and 0.0132 m2 for the seven-channel membrane. The pore size employed was 5 kD (molecular weight cutoff, MWCO). For the modification, the platinum precursor was H2PtCl6, provided by Merck, in aqueous solution. The silver precursor was silver acetate reactive grade provided by Meyer in a solution of pyridine and acetic anhydride, both ACS reagent grade also provided by Meyer. 2.3. Membrane Modification. 2.3.1. Impregnation with Platinum. The technique used was based on the previous work of Perez et al.12 The support membrane was immersed into a stirred solution of hexachloroplatinic acid (pH = 2.5, [Pt] = 2.5 104 mol L1) for 4 h after being soaked in distilled water. Then the membrane was washed three times during 20 min in 0.1 N nitric acid. Later, the membrane was dried in a furnace under nitrogen atmosphere at 100 °C for 1 h and then temperature was increased at a rate of 2 °C min1 to reach 300 °C. Finally, hydrogen was fed into the system for 8 h in order to reduce the Pt (II) to metallic particles. 2.3.2. Impregnation with Silver. The technique followed was based on the previous work of Triwahyono et al.13 In this case the metal was deposed in the pores, forming a silver ketenide by the reaction of two solutions: a mixed solution of silver acetate and pyridine and a mixed solution of acetic anhydride and pyridine. The concentration of the mixed solutions was 5 wt % of Ag and impregnation was done for 12 h in a dark room. The silver ketenide supported membrane was then dried at room temperature for 24 h followed by calcinations in furnace at 300 °C for 8 h at a rate of 2 °C min1. 2.4. Ion Removal in Seawater. The experimental equipment was a metallic bench-scale membrane reactor which operates horizontally with tubular membranes. A volumetric pump supplies a cross-flow rate with pressure from 0 to 6 bar. Experiments were carried out at room temperature and with a transmembrane pressure measured by two gauges, located upstream and downstream of the membrane and regulated by a manual valve, as shown in Figure 1. The maximum operating pressure of the equipment is 7 bar and experiments were done at 6 bar. Transmembrane
Figure 2. Location of sampling and seawater characterization zone of the study along the Mexican Pacific coast.
pressure difference, ΔP (i.e., P2 P1), was calculated between the inlet and outlet sections of the membrane module. In all the experiments only minor differences were observed (ΔP = 0), therefore, the composition values were taken as constant along the module and equal to the respective inlet values. In this system, nonmodified membranes, membranes impregnated with platinum, and membranes impregnated with silver were tested and salt rejection was determined. TDS in feedwater, permeate, and retenate (brine discharge) were analyzed with a gravimetric method. Concentration of cations (Na+, K+, Mg2+, and Ca2+) was determined with atomic absorption in a Perkin-Elmer spectrophotometer model A Analyst 200. Anion concentration (Cl, SO42-, and HCO3) was determined by titration. Similarly, a laboratory pilot RO plant was also tested, for comparison purposes, using commercial polymeric membrane Desal TFM-50, a polyamide membrane provided by Osmonics. The RO membrane requires higher pressure, and in this study 10 bar was used. Rejection, R, was calculated using the following equation:14 R ¼ 1
Cp Cf
ð1Þ
where Cp and Cf are permeate and feed concentration (ppm), respectively. 2.5. Comparative Energy Cost. Evaluation of energy was done as specific energy consumption (SEC), which is defined as the electrical energy necessary to produce a cubic meter of permeate. To simplify the presentation of this value, the required electrical energy is assumed to be the same as the pump work and considering pump efficiency as 100% as was done by Zhu et al.15 The SEC is given by SEC ¼
W pump Qp
ð2Þ
where Qp is the permeate flow rate and Wpump is the rate of work done by the pump measured as electrical consumption.
3. RESULTS AND DISCUSSION 3.1. Seawater Characterization. The variation of physicochemical and microbiological parameters of the seawater was investigated in a geographical zone along 80 km in the Mexican Pacific coast between the points Troncones (17° 47.0000 N, 101° 43.7300 W) and El Calvario (17° 23.0800 N, 101° 09.6440 W) and considering as central point Playa Blanca, as indicated in Figure 2. 5901
dx.doi.org/10.1021/ie2009313 |Ind. Eng. Chem. Res. 2012, 51, 5900–5904
Industrial & Engineering Chemistry Research Seawater quality had no significant difference in the different points sampled. In the same way the seasonal variation was not relevant during the period of this study, which was from July 2009 until June 2010. The values obtained are within the normal values of seawater and there were no signs of anthropogenic perturbation or visible suspended matter. Selected water quality parameters are presented in Table 1. 3.2. TDS Rejection. The main objective of membrane impregnation with different metals such as platinum and silver was determining its effectiveness in salt content removal. The achievement obtained is remarkable as shown in Figure 3. The value of TDS rejection of nonmodified membrane is 4.87%, using the optimal pressure of 6 bar. The impregnated membranes have a rejection value of 2230%, at same pressure. This value is lower than the obtained by other researchers,1618 but they used lowconcentration solutions. The rejection of the RO membrane is almost 85% (at 10 bar). Mazzonni et al.,11 using TAMI nonimpregnated membranes, obtained values close to 50% for CaCl2 using feed solutions of 5 mol m3, but rejection dropped to 5% when the concentration was 10 mol m3. During this research, there was no drop in flux, nor was any chemical or biological fouling detected (turbidity or variation in feed pH), during the experiments. So, the observed behavior can be related to the electric charges of the impregnated metal with the dissolved ions in seawater. Table 1. Seawater Quality during the 1-Year Experiment parameter
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3.2.1. Effect of Platinum/Silver Impregnation in TDS Removal. In the case of platinum there was a larger rejection in the case of seven-channel membranes. But in the case of silver a larger rejection was obtained with the one-channel membrane. This suggests that platinum was deposed inside the narrower channels or inside the porous material. So, when the surface is increased in the seven-channel membrane, the rejection of ions is higher as there is a larger surface, due to a similar platinum concentration in the surface of both types of membranes. Perez et al.12 indicated that platinum has very even anionic impregnation on
Figure 4. Anions rejection by different impregnated membranes, platinum (Pt) or silver (Ag), and comparison with reverse osmosis membrane (RO).
mean value or range
temperature, °C
19.024.4
pH
8.1
SDI, % min1
4.55.6
conductivity, mS cm1 @20 °C
48.9
color, mg L1 Pt/Co
