GAC Adsorption Processes for Chloroform Removal from Drinking Water

Transcription

1 Tanzania Journal of Natural and ISSN Applied Sciences (TaJONAS) Faculty of Natural and Applied Science May-July 2011: Volume 2, Issue 1, GAC Adsorption Processes for Chloroform Removal from Drinking Water Zinabu Tebeje Haramaya University, P. O. Box 1069 Code 1110, Addis Ababa, Ethiopia ABSTRACT A study was made to check suitability of Freundlich and Langmuir adsorption isotherm models and determine the capacity of bituminous Granular Activated Carbon (GAC) Calgon F200 for removal of Chloroform (CHCl 3 ) in water. Chloroform was considered as Trihalomethanes (THMs) basic indicator compound. Bituminous based Calgon F200 GAC was selected for the study as it was recommended by the supplier for removal of THMs (CHCl 3 ). Adsorption isotherm model was carried out for spiked deionized water with CHCl 3 (Co=3.85mg/l, ph of 7.0 ± 0.2, T=20 O C). After sample bottles were agitated at the speed of 25 rpm in a mechanical shaker for 15 days, measurements were taken using Gas Chromatography with electron capture detector (ECD). The results obtained were checked with Freundlich adsorption isotherm model and the model expresses well adsorption of CHCl 3. The adsorption capacity coefficient (K) and strength (1/n) were found to be and 0.74 respectively with R 2 =0.97. Similarly, the results obtained were checked with Langmuir adsorption isotherm model and the model expresses well adsorption of chloroform with R 2 =0.93. Key words: Adsorption; Bromoform; Chloroform; GAC; THM. * To whom correspondence may be addressed: zinabut@yahoo.com

2 INTRODUCTION In many developing and developed countries, drinking water quality is a crucial factor to safeguard human health. Many water sources contains contaminates (micro-organisms) which can easily be managed by disinfection process. Chlorine for example is a cheap but strong disinfectant (Roy, 1995). In 1974, researchers in the Netherlands and the United States demonstrated that disinfectant by-products (DBPs) like trihalomethanes (THMs) were being formed from the interaction of chlorine/ bromide with various organic substances in water (Al-Naseri, S.K. and Abbas, T. R. 2009; USEPA, 1999). These chlorinated organic compounds and other disinfection byproducts (DBPs) potentially cause cancer, miscarriages and they are mutagenic. Studies also linked THMs to heart, lung, kidney, liver, and central nervous system damage (Fearing et al., 2004; Edward, 2005). For this reason, different countries set specific limits for THMs concentrations in potable water. The permissible value for chloroform recommended by WHO is 30 µg/l (Gallard, 2002). The current regulations of United States Environmental Protection Agency (USEPA) demand stage 1 D/DBPR (Disinfectants / Disinfection By Products Rule) target of 80 µg/l for total THMs (TTHMs), and a more stringent level of 40 µg/l is proposed to be in effect as the stage 2 target (Alicia and Alvarez, 2000; Lin et.al., 1999). The European commission has proposed a new council directive with parametric values of 40 µg/l for chloroform (Capar and Yetis, 2002). Hence, removal of THMs is necessary and it is achieved mostly by applying GAC filtration ( Fearing et al., 2004; Edward, 2005; Qasim et al., 2004). In general, THMs are hydropobic and low molecular weight, characteristics which make them difficult to remove by most physiochemical processes. For example, the removal of THMs in disinfected water requires GAC adsorption. THM species removal by GAC filtration is in the order as follows CHCl 3 <CHBrCl 2 <CHBr 2 Cl<CHBr 3 (Letterman, 1999; Potwora, 2006). The more number of bromine atoms attached to THM species leads to high absorbability by GAC (Potwora, 2006; Kruithof, 2007). For this reason the adsorption isotherm is extremely valuable in getting an evaluation of the effectiveness of GAC for treating contaminated water. For the study bituminous based Calgon F200 were selected since it was recommended by its manufacturer for best adsorption of THM species. Beside, Chloroform was considered as trihalomethanes (THMs) basic indicator compound since it is the most common and less adsorbed species of THMs (Potwora, 2006). The design of full scale GAC adsorption process involve time consuming and expensive pilot plant studies. Hence, batch adsorption isotherm tests could be conducted to predict the capacity of GAC for THM adsorption in a short time (Crittenden et al, 1986; McGuire, 1991). Thus, this study was initiated to check suitability of Freundlich and Langmuir batch adsorption isotherms models for chloroform adsorption by GAC (Calgon F200). There by, to investigate the capacity of Calgon F200 GAC for chloroform adsorption. MATERIAL AND METHODS Experimental Setup and Operational Procedure M odel Wat er Pr eparation For batch adsorption isotherm, model water was prepared batchwise in L brown glass bottle from de-ionized water. 1mmole of NaHCO 3 and the target contaminant Chloroform (3.85mg/l) was added to the bottle. Then, the glass bottle was covered by parafilm to avoid air formation at the top of the bottle and mechanically stirred for one day to achieve complete mixing. P G AC Pr eparat ion The grain size of GAC used in batch experiments was 200x400. This size was obtained by crushing 12x40 mesh GAC supplied by Calgon (Table 2-1). After GAC grains were crushed, the appropriate size for batch was achieved by sieving. The GAC grains that did not pass the upper sieve were returned and crushed again until it passed the sieve. GAC grains of appropriate size were stored in a beaker covered with aluminum foil. Fines produced during crushing and sieving were removed by washing. GAC of appropriate size was stored in a clean beaker filled with de-ionized water. The TaJONAS, May-July 2011, Vol. 2, Issue

3 GAC was stirred with a glass rod and allowed to settle. After the GAC particles settled in one to three minutes, the supernatant was poured off and new de-ionized water was added. The stirring and settling was continued using fresh de-ionized water until clear supernatant was achieved. The wet GAC was placed in an oven at 105 o C for at least two days. The dried GAC was stored in a dark amber bottle with teflon lined caps in a decicator. B at ch Experimental S etup Batch experiment was conducted to investigate GAC removal capacity of THM, chloroform. Model water was prepared using chloroform. Powdered granular activated carbon (PGAC 200x400 mesh size) was used in batch adsorption experiments to reduce the time necessary to reach equilibrium and to ensure a representative carbon samples. Glass bottles of ml volume were stored in 0.5 M hydrochloric acid (HCl) for two days and cleaned with de-ionized water. Different PGAC dosages (0.02-8g) were introduced to eight batch reactor bottles (seven for Calgon F200 GAC and one blank in duplicate). These bottles ( ml) were subsequently filled with model water followed by ph adjustment of 7.0 ± 0.2 by adding 0.1 and 1.0 M HCl using METROHM-691 ph meter. Once the ph was adjusted, the bottles were covered with double layer parafilm to make air free surface before closing the cap. Then, the bottles were placed on the mechanical shaker and shaken at speed of 25 rpm for 15 days. This contact time was ample to achieve equilibrium in the system for all the THM components studied. After 15 days of adsorption, samples were taken and measured. Analytical Methods G a s Chrom at ograph y (GC ) GC method use headspace for the determination of volatile halogenated hydrocarbons in the samples. Chloroform was detected by an electron capture detector (ECD). Identification was based on retention time while quantification is based on the intensity of the ECD signal using a five-point calibration. During sample preparation 5 ml of sample using pipette was added into a 20 ml headspace vial and the vial was closed with a crimp cap with a silicon septum. The sample was heated in a closed headspace vial in order to obtain equilibrium between the concentration of the volatile halogenated hydrocarbons in the headspace above the sample and the concentration in the sample. By purging the headspace with Helium, the volatile hydrocarbons in the headspace were transferred to the GC where they were separated. The components were detected by an electron capture detector (ECD) and calibration curve and control standards were observed using software (Turbochrom). Identification was based on retention time (5.26 min and min for chloroform and bromoform respectively) while quantification was based on the intensity of the ECD-signal using a five-point calibration. These retention times slightly vary due to aging of the column and were corrected. The quantification of the components was done automatically by the software using linear regression of the second order. Finally results for chloroform < 0.5 μg/l were reported as < 0.5 μg/l. When the total THM result was < 2 μg/l, the sum of the THMs was reported as < 2 μg/l. But, when the result of one of the components was > 50 μg/l, the samples were diluted and re-analyzed. Data Analysis The results were interpreted using Freundlich and Langmuir adsorption isotherm equation. The Freundlich adsorption isotherm equation is commonly used for adsorption capacity calculations. This equation is accepted as a standard and is strictly an empirical approach (Qasim et al., 2004; Roy, 1995). The Freundlich Adsorption Isotherm is expressed as:... Equation 1 TaJONAS, May-July 2011, Vol. 2, Issue

4 Where q e (mg/g) represents the amount of THM adsorbed (mg) per unit mass of GAC (g), Ce (mg/l) is the concentration of residual in contaminated water after the GAC and the contaminated water reach adsorptive equilibrium. K [(mg/g)(l/mg) 1/n ] is Freundlich adsorption capacity parameter and 1/n (unit less) is Freundlich adsorption intensity parameter for each sample tested (Qasim et al., 2000; Crittenden et al., 2005). For fixed values of Ce and 1/n, the larger the K value the higher is the adsorptive capacity (qe). For fixed values of K and Ce, on the other hand, the smaller the value of 1/n the lesser would be the concentration dependence of adsorption (Ce). Conversely, if the value of 1/n is large, the adsorption bond is weak and the value of qe changes distinctly with small changes in Ce. The Langmuir adsorption isotherm is based on the theoretical principle of ideal localized monolayer model that only a single adsorption layer exists The Langmuir adsorption isotherm is expressed as:... Equation 2... Equation 3 Where: qe represents the amount solute adsorbed per unit mass of adsorbent (wt/wt). Q0 and K are Langmuir constants related to capacity and energy of adsorption, respectively and Ce is equilibrium concentration of adsorbate in solution after adsorption. RESULTS AND DISCUSSION Batch Isotherm Study Batch isotherms experiments with powdered granular activated carbon (PGAC) media were carried out with model water containing chloroform to exmine the effectiveness and adsorption capacity of Calgon F200 GAC for chloroform removal. Data for isotherm obtained by treating fixed volume of the contaminated water with known weights of GAC. Two models, Freundlich and Langmuir adsorption isotherm, were checked for the expression of experimental results. B at ch adsorption isotherm for chloroform Batch adsorption isotherm experiments were conducted with Calgon F200 for model water containing 3.85mg /L chloroform at ph of 7.0 ± 0.2. Data obtained for Calgon F200 PGAC fitted well the Freundlich isotherm model and adsorption is favorable (1/n<1) (Fig. 1). Where, adsorption capacity coefficient (K) and strength (1/n) were found to be and 0.74 respectively with R 2 =0.97. Similarly, data obtained for Calgon F200 fitted well Langmuir adsorption model. Where, adsorption capacity (Q 0 ) and energy (K) of adsorption were found to be and respectively with R 2 =0.93 (Fig. 3). Conversely, the value of 1/n is large by 6 percent for Calgon F200 in comparison to literature value. This implies that Calgon F200 has weak adsorption bond toward chloroform and the adsorption capacity (qe) changes distinctly with small changes in equilibrium concentration (Ce) in comparison to literature value. Comparison of Freundlich isotherm results for Calgon F200 with literature results for THM adsorption on bituminous GAC (Potwora, 2006) show very similar adsorption isotherm as shown in Fig. 2. CONCLUSION Results from batch adsorption experiment conducted with Calgon F200 GAC and model water containing chloroform (3.85mg /L) well fitted the Freundlich (R 2 =0.97) adsorption isotherm model and adsorption is favorable (1/n<1). Where, the adsorption capacity coefficient (K) and strength (1/n) were found to be and 0.74 respectively. Similarly, Langmuir adsorption isotherm model express the result well (R 2 =0.93). TaJONAS, May-July 2011, Vol. 2, Issue

5 ACKNOWLEDGMENT My first intellectual debt is to Professor Gary Amy, Dr. B. Petruseviski, Dr. K. Ghebremichael for helping me to consider different, more interesting analytical and methodological insights. I would like to thank the editors also for their wise questions and constructive critiques which rendered this study readable and acceptable piece of work. It is difficult to single out all peoples who assisted me with my study however; I am particularly indebted to my wife Melat G. for the important role she plays and her keen cooperation when it was needed. My respected thanks goes to all the peoples and institutions who contributed for fruition of this study: UNESCO-IHE, Coca-Cola Company for its generous financial supports and Haramaya University. Finally, I would like to praise God for giving me courage and being my strength. Fig. 1 Freundlich isotherm for chloroform adsorption, Calgon F200, 200x400 mesh PGAC; model water: deionized water, Co=3.85mg/l, ph of 7.0 ± 0.2, T=20 O C Fig. 2 Freundlich isotherms for chloroform (i) Calgon F200, 200x400 mesh PGAC; model water: Co=3.85mg/l, ph of , T=20 O C 2; (ii) adsorption isotherm for bituminous GAC (Potwora, 2006) TaJONAS, May-July 2011, Vol. 2, Issue

6 1/Loading, 1/chloroform/PGAC, 1/mg/g Tebeje, Z y = 0.013x R 2 = Calgon /Equilibrium concentration( 1/Ce), 1/mg/l Fig. 3 Langmuir isotherm for chloroform adsorption, Calgon F200, 200x400 mesh PGAC; model water: deionized water, Co=3.85mg/l, ph of 7.0 ± 0.2, T=20 O C Table 1 Used GACs Specifications and General Characteristics Specific and General Characteristics Supplier Calgon GAC name Calgon F200 Iodine number Total surface area (B.E.T.) m2/g 850 Apparent density kg/m3 - Ball-pan hardness - 95 Effective Size D10 mm Uniformity coefficient REFERENCE Al-Naseri, S.K. and Abbas, T. R. (2009). Predicting NOM Removal by Fixed-Bed GAC Adsorbers. Jordan Journal of Civil Engineering, 3(2): Crittenden, J. C., Trussell, R. R., Hand, D. W., Howe, K. J. and Tchobanoglous, G. (2005). Water Treatment: Principles and Design, 2 nd edition. John Wiley and Sons, Inc., USA. Crittenden, J. C., Berrigan, J. K., and Hand, D. W. (1986). Design of Rapid Small Scale Adsorption Test for a Constant diffusivity. Water Pollution Control Federation, 58 (4): Capar G. and Yetis U. (2002). Removal of THM precursors by GAC. Wat Res J, 36(10): Edward, E. B. (2005). Water Treatment Plant Design, 4 th edition. Mc-Graw Hill Publishing Company, New York. Fearing, D.A., Banks, J., Wilson, D., Hillis, P.H., Campbell, A.T. and Parsons, S.A. (2004). NOM Control Options: The Next Generation. Water Science and Technology: Water Supply 4 (4): Gallard H. (2002). Chlorination of Natural Organic Matter: kinetics of THM formation. Water Resource Journal, 36(1): Lin C. H., Huang Y., Hao O. (1999). Ultrafiltration processes for removing humic substances. Wat Res J, 33(5): McGuire, M. (1991). Evaluating GAC for Trihalomethanes Control. Journal of American Water Works 83: TaJONAS, May-July 2011, Vol. 2, Issue

7 Potwora, J. R Trihalomethanes Removal with Activated Carbon, Water conditioning and purification (date accessed: June, Qasim, S. R., Motley, E. M. and Zhu, G. (2000). Water Works Engineering: Planning, Design, and Operation. Prentice-Hall PTR, Upper Saddle River, NJ. Roy, G. M. (1995). Activated Carbon Applications in the Food Pharmaceutical Industries. Technomic Publishing Company, Inc., Lancaster, Pennsylvanai USA. USEPA (United States Environmental Protection Agency) ICR Manual for Bench and Pilot Scale Treatment Studies. Report No. EPA 814/B USA. TaJONAS, May-July 2011, Vol. 2, Issue