Removal of ammonia by natural clay minerals using fixed and fluidised bed column reactors

Transcription

1 Removal of ammonia by natural clay minerals using fixed and fluidised bed column reactors M.S. Çelik*, B. Özdemir*, M. Turan**, I. Koyuncu**, G. Atesok* and H.Z. Sarikaya** * Istanbul Technical University, Mining Engineering Department, Ayazaga Istanbul-Turkey ** Istanbul Technical University, Environmental Engineering Dept., Ayazaga Istanbul-Turkey Abstract A series of fixed and fluidised bed ion exchange column runs were conducted to identify the ability of natural clay minerals, sepiolite and clinoptilolite, to remove ammonia from a contaminated drinking water reservoir. Ion exchange column tests using both fixed and fluidised bed were initially carried out with synthetic water composed of NH 4 Cl. Breakthrough curves as a function of flow rate, particle size, and initial ammonia concentration reveal that sepiolite does not have as high ion exchange capacity as clinoptilolite but maintains a steady adsorption up to higher bed volumes. The adsorption capacity was modified by using regeneration cycles at both acidic and alkaline ph. Furthermore, fluidised bed runs with clinoptilolite utilising water and air as fluidiser resulted in inferior results compared to those of fixed bed runs. This was respectively ascribed to the presence of ammonia in the circulating water and competition of exchangeable ions released in water and the ability of air to adsorb nitrogen. Tests conducted with natural raw water contaminated with sewage indicated that clinoptilolite adsorbs ammonia the same as the synthetic water. Regenerated clinoptilolite is capable of removing ammonia from both synthetic and actual raw water at a much higher rate than the untreated clinoptilolite. Keywords Adsorbent, ammonia, clinoptilolite, sepiolite, wastewater treatment, zeolite. Introduction Wastewater produced from municipal, agricultural and industrial sites creates ammonia nitrogen which subsequently mixes into lakes, rivers and particularly drinking water reservoirs. Ammonium nitrogen decreases the dissolved oxygen required for the aquatic life and also accelerates the corrosion of metals and construction materials. McLaren and Farquhar (1973) and Train (1979) explained that the maximum concentration of ammonia and ammonia compounds allowed for the fish at a temperature 18 C and ph of 5 7 was approximately 2 mg l 1. The maximum limit set by the European Association for drinking water is approximately 0.5 mg l 1 and also a guide level is given as 0.05 mg l 1 (AWWA, 1990; Gaspard et al., 1983). Therefore, raw water with high ammonia concentration must be treated before it reaches the consumer and also wastewater before arriving at the receiving water. Biological nitrification and denitrification, air stripping and ion exchange are the most preferred methods both in terms of performance and cost (Mercer et al., 1970; Reeves, 1972; Culp et al., 1978). Biological methods are limited to a minimum 5 mg l 1 of concentration due to the formation of undesirable chemical compounds. Contrary to this, it is possible to decrease the ammonia level to 1 mg l 1 using air stripping methods. But the requirement of using water over 15 C and carbonate deposition are the disadvantages of this method. The ion exchange method is preferred over the other methods since it is stable, suits automation and quality control, and is easy to maintain (McLaren and Farquhar, 1973; Breck, 1973). Several investigators have explained the mechanism of ammonia removal from wastewater by natural clinoptilolite (Gaspard et al., 1983; Mercer et al., 1970; Jorgensen,1975; Mumpton, 1988; Sirkecioǧlu and Şenatalar, 1995; Baykal et al., 1996; Kurama and Kaya, 1998). Mercer et al. (1970) showed that ammonium ions are able to exchange Na, Ca and Water Science and Technology: Water Supply Vol 1 No1 pp IWA Publishing

2 M.S. Çelik et al. Mg ions from clinoptilolite more selectively than other ions. The first mobile ion exchange unit built near Lake Tahoe (Nevada, US) with a capacity of 22,700 td 1 is reported to be the first industrial unit known where clinoptilolite was used (Ciambelli et al., 1985). In addition, the Rosemount plant in Minnesota with a capacity of 2300 td 1, a plant in Virginia with a capacity of td 1 and a plant in Colorado with a capacity of td 1 are the main examples in the USA. Torii (1978) reports the details of the first plant using clinoptilolite in Japan. The high selectivity of clinoptilolite against ammonium ions can be explained with the following three mechanisms: molecular size properties, hydration of cations, and anionic corner separation (Si/Al ratio). The ion exchange capacity of zeolites varies in the range of 1.2 and 2.2 meqg 1. Clinoptilolites composed of rings of 8 and 10 elements or 5.6 Å unit window show an ideal molecular size for ammonium ion. In addition to these structural properties, ion exchange capacity is also dependent on the conditions of the mineral bed (McLaren and Farquhar, 1973). A drinking water reservoir in Istanbul was highly contaminated through the penetration of sewage waters in the vicinity of a populated urban area. Analyses indicated the presence of ammonia levels of as high as 15 mg/l. In the study, the ammonia adsorption capacity of Sivrihisar sepiolite and Gördes clinoptilolite was investigated under different conditions using fixed and fluidised bed columns. The performance of clinoptilolite is tested on contaminated reservoir water. Experimental Clinoptilolite and sepiolite samples used in the experiments were received from Incal zeolite and Mayas sepiolite companies. The samples were classified into different size groups: +4mm (+ 4 mesh), mm (4 6 mesh), mm (6 8 mesh), 2+1 mm (8 16 mesh). The chemical analysis of the clinoptilolite sample is given in Table 1. Since the sample is rich in Ca ++ and K + it was identified as Ca-clinoptilolite. Gördes clinoptilolite has the following properties: cation exchange capacity meq g 1, pore diameter 4 Å, purity 92 %, bed porosity 40 %, density 2.15 gcm 3, apparent density 1.30 gcm 3, and suspension ph The x-ray diffraction and chemical analysis of sepiolite indicates that calcite and dolomite are the major impurities accompanying sepiolite. Table 1 Chemical analyses of Gördes clinoptilolite and Sivrihisar sepiolite. Constituent Clinoptilolite Sepiolite % by wt. % by wt. SiO CaO K 2 O SO AI 2 O MgO TiO P 2 O Fe 2 O Na 2 O LOI The laboratory scale experimental set-up consists of fiberglass ion exchange columns, raw water and regenerant solution tanks, feed pumps, valves and treated water tank. The cylindrical column has dimensions of cm. The bed height was kept as 50 cm. The particle size of clinoptilolite and sepiolite was selected as 2+1 mm with a bed height of 50 cm. The

3 amount of 2+1 mm clinoptilolite and sepiolite samples filling the 50 cm height weighed 370 and 180 g, respectively. A peristaltic pump, Masterflex 100, was used to feed ammonia containing solutions to the column at a rate of 6 mh 1. The empty bed contact time (EBCT) can be obtained as, EBCT = V R / Q ( 1 ) where V R is the bed volume, Q F denotes the flow rate in the bed. Since bed volume and flow rate were measured as 350 cm 3 and 70 cm 3 min 1, respectively, EBCT was found to be 5 min. During the adsorption process, the samples were taken in 2 hour-periods and analysed using the Nessler method. The breakthrough curves were obtained plotting C/Co versus bed volume (BV). When C/Co value approached 1, the process was terminated and regeneration started. The bed volume (BV) is defined as, M.S. Çelik et al. BV = V F / V R ( 2 ) where V F is the total water volume passed in the column during the adsorption process. The regeneration solution under the optimum conditions consisted of 30 gl 1 of NaCl and 1.5 gl 1 of NaOH. The ph was kept approximately at 11.5 which was close to that used elsewhere ( Sirkecioǧlu and Şenatalar, 1995). Regeneration solution was pumped to the bottom of the bed using a pump that maintains both mixing and expansion of the bed. Thus, the upflow in the bed accelerated desorption of ammonia from zeolite particles. Results and discussion Ion exchange column tests were conducted against particle size, bed height, and initial ammonia concentration using sepiolite and clinoptilolite. These are discussed below. Fixed bed column tests with sepiolite The breakthrough curves were constructed against bed volume for different bed heights of 27, 33 and 50 cm, which correspond to the material loads of 100, 125 and 180 g, respectively (Figure 1). It is seen that the ratio of concentration of ammonia in treated water to the initial ammonia concentration (C/Co value) exhibits a marginal change with bed height. However, when the results are plotted as C/Co versus running time, the differences become more pronounced. The higher the bed height, the more the curve shifts to the left. But generally due to the overall low sorption capacity of ammonia by sepiolite, the differences are apparently masked. Fluidised bed column tests with sepiolite Figure 2 presents the comparison of breakthrough curves obtained with fixed and the fluidised bed columns using 180 g (50 cm bed height) sepiolite. Evidently, while the adsorption in the fixed bed ends at 150 BV, that in the fluidised bed levels off at 230 BV. Similarly, the breakthrough point in the fixed bed occurs at 23 BV corresponding to the C/Co value of 0.84, whereas the fluidised bed exhibits 41 BV and C/Co =0.80; this indicates that the plateau value in fixed bed is reached much faster than that in the fluidised bed. Regeneration A good adsorbent, in addition to its high adsorption capacity, must also exhibit a good regeneration for multiple use. Therefore, the regenerability of sepiolite has been tested by a series of systematic experiments. These are shown in Table 2. 83

4 Table 2 Methods used in regeneration experiments and their features. Method Fixed bed Fluidised bed Circulation pump Flow rate 0.25 l/min 0.17 l/min 1.37 l/min M.S. Çelik et al. Figure 1 Breakthrough curves of original sepiolite against bed volume for different bed heights (A:100g, B: 125g, C:180g). Figure 2 Comparison of breakthrough curves obtained with fixed and fluidised bed columns using 180 g sepiolite (50 cm bed height) 84 Regenerability of ammonia loaded into a fixed bed column against time is presented in Figure 3. The regeneration curve reveals that the amount of ammonia in sepiolite increases in the first 10 min up to an ammonia concentration of 20 mg/t followed by a decrease at the same rate and then practically becomes nil at the end of 20 min. The regenerability of the fluidised bed column system is illustrated in Figure 3. Ammonia released from the sepiolite matrix starts at a level of 4 mg/l and gradually decreases down to about 40 min and then levels off at 0.35 mg/l at 60 min. A slightly higher ammonia concentration was recorded at the outlet water compared to that in the circulating water tank. This indicates that some of the released ammonia is circulating back into the

5 circulating tank. This also explains the inability of ammonia level to fall down to 0 mg/l with the fixed bed column. Regeneration of ammonia from the fixed bed using the circulation pump alone given in Figure 3 shows that the regeneration starts at an ammonia concentration perhaps higher than 6 mg/l but remains at high ammonia levels of 2.1 mg/l. The areas under the regeneration curves in Figure 3 together with the initial effluent and final concentrations were used to calculate the regeneration efficiencies for each system. Accordingly, the fixed and fluidised bed columns have adsorbed and mg/l and the corresponding released ammonia concentrations are and mg/l, respectively. The calculated regeneration efficiencies for the fixed and fluidised systems are respectively found to be 59.9 % and 20.0 %. The regeneration made by the circulation pump alone yielded an efficiency of 57.8 %. M.S. Çelik et al. Figure 3 Regeneration of ammonia from sepiolite in different beds. Optimum regenerant composition was determined by saturating each type of bed by a desired regenerant as shown in Table 3. Samples of 1.5 g were taken at the end of each run and mixed with 10 mg/l of ammonia (30 ml) and the residual concentration (CR) was determined for each case. This identified the optimum type of condition for regeneration. Table 3 Identification of optimum regeneration conditions for sepiolite. Regeneration condition ph Bed type C R 1 30 g/l NaCl 10 Fixed bed g/l NaCl 11.5 Fixed bed g/l MgCl Fixed bed N HCl 0.5 Fixed bed N HCl 30 g/l NaCl 0.5 Fixed bed N HCl 5 g/l MgCl Fixed bed g/l NaCl 11.5 Circulation pump g/l NaCl 11.5 Fixed bed g/l NaCl 11.5 Fluidised bed Untreated sepiolite Table 3 shows that regeneration conducted in a fixed bed with untreated sepiolite using 30 g/l NaCl at ph 11.5 yielded the maximum ammonia removal, i.e. the minimum residual ammonia concentration. 85

6 M.S. Çelik et al. Column tests with clinoptilolite Ammonia adsorption tests with clinoptilolite were conducted by means of three different alternatives: fixed bed, water-driven fluidised bed and air-driven fluidised bed. The fixed bed column with 50 cm of bed height loaded with 310g untreated clinoptilolite is presented in the form of breakthrough curves in Figure 4. The ammonia removal remains at relatively high levels of % up to 90 BV, above which C/Co value increases almost linearly; the breakthrough point corresponds to about 8 h of running period. The breakthrough curve of water-driven fluidised bed created by a circulation pump is again illustrated in Figure 4. The height of the column allowed loading of 260 g clinoptilolite only. Although no definite breakthrough point is observed, the C/Co starts at about 0.25 and continues up to 90 BV and then follows a slightly higher rate. It is clear that the waterdriven fluidised bed produces inferior ammonia removal compared to that obtained with the fixed bed. The breakthrough curve for the air-driven fluidised bed is also presented in Figure 4. The C/Co value remains below 0.1 up to 35 BV and then stays at up to 110 BV. Figure 4 Breakthrough curve for the removal of ammonia by clinoptilolite in different bed types (260g untreated clinoptilolite). 86 When all the curves are examined together they all exhibit some type of breakthrough point at BV. However, while the fixed bed is capable of removing ammonia up to %, water- and air-driven fluidised bed systems can only capture about % of ammonia. This is attributed to the sorption of nitrogen in air by clinoptilolite itself and the consequent decline in the adsorption of ammonia. Similarly, in the case of the water-driven system, the exchangable ions such as Na +, Ca +, Mg + release and subsequently mixes into the circulation tank and compete with ammonia for adsorption. Also the higher flow rates applied to keep the bed suspended result in shorter contact time and in turn lead to lower ammonia removal efficiency. Elmali reservoir water with 15 mgl 1 ammonia concentration was used in the adsorption experiments. The ammonia concentration was found to remain under 0.1 mgl 1 up to 100 BV. This corresponds to the C/Co ratio of 0.09 and 0.28 for the reservoir water and synthetic water, respectively. After 23 hours of running the C/Co value was obtained as 0.32 and 0.23 for regenerated clinoptilolite using reservoir and synthetic waters, respectively. The breakthrough curves showed that reservoir and synthetic water have similar characteristics (Figure 5). It is shown that the regenerated clinoptilolite adsorbs ammonium ion better than the original one; this is a clear advantage for industrial applications. Since clinoptilolite used in the experiments is a relatively cheap natural zeolite with high ion exchange capacity, it has a potential advantage over biological and chemical treatment

7 methods. However, sepiolite appears to be particularly effective on odorous organic impurities that are not easily removed by other means. It is plausible to use sepiolite in combination with zeolite for the removal of ammonia and other difficult-to-remove organic components. M.S. Çelik et al. Figure 5 Breakthrough curves for ammonia removal from synthetic and reservoir raw water using untreated and regenerated clinoptilolite of 2 +1 mm in size (B6: untreated clinoptilolite/synthetic water; B7: once regenerated clinoptilolite/synthetic water; A5: once regenerated clinoptilolite/reservoir raw water. Conclusions In this study, Sivrihisar sepiolite and Gördes clinoptilolite samples both from Turkey were tested in different types of material beds to evaluate their ability to adsorb ammonia from aqueous solutions. The salient features are presented below. 1. Untreated sepiolite is generally not very receptive to the adsorption of ammonia. The fluidised bed column is more efficient than the fixed bed due to better exposure of ammonia to sepiolite. 2. Optimum conditions for the regeneration of sepiolite are found to occur with the addition of 30 g/l NaCl at ph Under the optimum regeneration conditions the fixed bed was found to produce a higher ammonia adsorption compared to the water-driven fluidised bed and circulating-pump driven fluidised bed. All the regenerants, i.e. MgCl 2, HCl, and NaCl performed well compared to the untreated sepiolite. However, NaCl at ph 11.5 yielded the best results. 3. Clinoptilolite is a superior adsorbent for ammonia from aqueous solution. Testing of three different methods, viz. fixed bed, water-driven and air-driven fluidised beds led to the conclusion that the fixed bed gives the best results because nitrogen in air and the presence of exchangable ions in the recirculating tank reduce the ability of ammonia to be effectively captured by clinoptilolite. While the fixed bed removes % of ammonia the fluidised beds remove %. 4. Regenerated clinoptilolite is capable of removing ammonia from both synthetic and actual raw water at a much higher rate than the untreated clinoptilolite. 87

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