Environ. Sci. Technol. 1993, 27, 1139-1 145
New Purification Technique for the Removal of Organics from Aqueous Solutions Using Silicone Polymers Anthony R. J. Andrews, Albert Zlatkls,' Mlchael T. Tang? Wensheng Zhang, and Henry Shanfleld
Department of Chemlstry, Unlverslty of Houston, Houston, Texas 77204-5641
A new technique for water purification has been developed to remove organic contaminants in an aqueous solution using silicone polymers in the form of hollow fiber membranes. The method is simple to use and has a low setup cost, high capacity, and efficiency. A number of parameters such as solute concentration, wall-thickness of tubing, tube length, diameter, effect of flow rate, capacity of the tubing, tubing type, and effect of temperature and pressure have all been investigated. A simple mathematical treatment of the data is presented with good agreement between experimental and theoretical data for the derived equation. Introduction
The Safe Drinking Water Act was amended in 1986 to require the EPA to regulate 83 drinking water contaminants by 1989 (1). Among those contaminants, 14 are volatile organic compounds, and 35 are other organics. The EPA regulates by law (2, 3) the concentration of specified chemicals in both drinking water and effluent water from factories. Recent publications by the EPA have proposed maximum concentration levels (MCLs)for 25 chemicals under the National Primary Drinking Water Regulations (4) and have added new chemicals to the list of compounds that may be regulated in the future (5).The EPA has determined that a number of technologies are applicable to the treatment of organic contaminants in water including biological degradation, steam stripping, carbon adsorption, distillation, incineration, and fuel substitution (7-11). Among these methods, activated carbon is the most popular because of its high adsorption capacity, great affinity for many organic substances, and relatively low cost. However, there are certain drawbacks associated with its use, such as difficulties during regeneration or low mechanical strength. The concentration of most organics ranges from 0.01 to 0.5 ppm after water treatment. One of the important applications of polymeric membranes is the separation of mixtures including both gases and liquids in water (12-16). The separation of compounds from water using membrane permeation is not in itself a new idea, the separation of organic contaminants from water having been described by several authors (17-20). However, none of these methods were specifically directed at the purification of water; they were generally more concerned with using membrane permeation as a way of separating, enriching, and quantifying organics in water. The only paper that these authors could find that specifically dealt with the use of hollow fiber membranes for the express purpose of water purification looked at the removal of ammonia from water back in 1970 (21). The authors of this paper did mention the possibility of ~~
Current address: Scott Speciality Gases, 3714 Lapas Dr., Houston, TX 77023. 0013-936X/93/0927-1139$04.0010
0 1993 American Chemical Society
removing volatile organics by the same method, but until now noone has followed this up in detail. LaPack et al. (22) recently studied the effects of temperature, flow rate, and hollow fiber dimensions on the enrichment of organics from water and air for mass spectrometry, and their results are of interest here as our system requires the maximization of removal of organics from water. Of the three processes occurring, (1)absorption onto the hollow fiber inner surface, (2) diffusion through the hollow fiber, and (3) desorption from the external surface, it is assumed that steps one and three are instantaneous and that step two is the rate-limiting step. This whole process is known as pervaporation. The objective of this research was to seek a new methodology for water purification utilizing the unusual permeability and absorption characteristics of silicone tubing to organic compounds in polluted water via pervaporation. Our primary goal in this study was to achieve 95% or above removal for 1 ppm organic contaminants. Experimental Section
Materials. Stock solutions (5000ppm in methanol) of the following chemicals were used throughout this research: (i) 2-butanone and cyclohexanone; (ii) l,l,l-trichloroethane and 1,1,2-trichloroethane; (iii) trichloroethylene; (iv) tetrachloroethylene; (v) chloroform and carbon tetrachloride; (vi) chlorobenzene, and (vii) bromodichloromethane. All organic compounds tested were purchased from Aldrich Chemical Co. (Milwaukee, WI) except for 2-butanone, tetrahydrofuran, nitrobenzene, dichloromethane, and benzene, which were from Mallinekrodt Speciality Chemicals Co. (Paris, KY),and vinyl chloride, which was from Fluka (Ronkonkoma, NY). Since deionized water might interfere with organic compounds at the 1ppm level, water purified in a Milli-Q filtration system (Millipore; Bedford, MA) was used in the preparation of 1 ppm solutions. Deionized water was utilized directly for the preparation of other standard solutions. The silicone tubing used in this research was manufactured by Brim Electronic, Inc. (Fair Lawn, NJ) and Dow Corning Corp. (Midland, MI) with different dimensions and/or compositions as follows: ( a )i.d., 0.059 in. = 1.5 mm; o.d., 0,091 in. = 2.3 mm; length, 500 f t = 150 m (Brim Catalog No. BSR-15). (PI id., 0.186 in. = 4.7 mm; o.d., 0.226 in. = 5.7 mm; length, 250 f t = 75 m (Brim Catalog No. BSR-5). ( x ) id., 0.25 in. = 6.4 mm; o.d., 0.375 in. = 9.5 mm; length, 100 ft = 30 m (Silastic medical-grade tubing, Dow Catalog No. 601-445). Pretreatment of Glass Container. Several reported procedures for deactivation of glass chromatographic columns were modified to deactivate the surfaces of glass containers in our laboratory (23-26). Each glass container was washed with deionized water, acetone, and methylene chloride, and was dried in an oven. Thereafter, an appropriate amount of hexamethyldisilazane (ca. 1.5 g. of Envlron. Sci. Technol., Vol. 27, No. 6, 1993
1139
+---
Restrictor
Prassure
regulator
Puririee
Silylated g l a s s bottle
i:s:er
Fburo 1. Apparatus for the purificationof standardsolutionscontaining organic contaminants.
HMDS/1 L vol of container) was transferred into this container, and it was closed. The container was heated at 200 "C for 1 h, cooled to room temperature, washed with deionized water, acetone, and methylene chloride, and after drying was ready for use. The silylated glass container was tested by storage of a standard solution containing 1 ppm of chloroform, l,l,l-trichloroethane, carbon tetrachloride, 1,1,2-trichloroethane, trichloroethylene, bromodichloromethane, and chlorobenzene for 3 weeks. No decrease in concentration was seen. Apparatus. A Hewlett-Packard Model 5890A gas chromatograph with a Model 3393A integrator was employed for the quantitative analyses of organic compounds in the standard solutions. This GC system contained a flame ionization detector and a split/splitless capillary inlet system. A 30 m X 0.53 mm X 1.5 hm DB-5 fused silica capillary column (J & W Scientific, Rancho Cordova, CA) was used as the analytical column. Sample injection was done in the split mode, with a ratio of 6:l unless stated otherwise. The injector and detector temperatures were kept at 250 and 300 "C, respectively. The chromatographic column was operated isothermally at 40 "C for the first 2 min, then followed by a temperature program of 10 "C min-l, until reaching a final temperature of 90 "C. Other conditions used are given separately. The apparatus to remove the trace organic compounds in the standard solution is shown in Figure 1. The flow rate of the effluent solution from the silicone trapping tubing could be adjusted either by a pressure regulator or a restrictor at the end of the tubing. Procedure. (A) Investigation of Performance of Silicone Tubing a on Efficiency of Purification. ( i ) Performance on 1,10, and 100ppm Standard Solutions. Standard solutions (100 mL in 2% methanol/water) containing 1ppm of cyclohexanone, l,l,l-trichloroethane, tetrachloroethylene, 1,1,5-trichloroethane,and 2-butanone in 2% methanol/water; 10 ppm of cyclohexanone, l,l,ltrichloroethane, 1,1,2-trichloroethane,trichloroethylene, chloroform,tetrachloroethylene, carbon tetrachloride, and 2-butanone; and 100 ppm of 2-butanone, tetrachloroethylene, 1,1,1-trichloroethane, 1,1,2-trichloroethane, and cyclohexanonewere prepared. A standard solution (1pL) was injected into the GC to acquire area count data for each organic compound before the purification process. The standard solution was collected after passage through the silicone tubing and analyzed immediately. The difference in area counts for each organic compound before and after the water treatment process was calculated, and the percent removal of each organic was determined. (ii) Performance on 10 p p m Standard Solution after Treatment of 100 ppm Standard Solution. A standard 1140
Environ. Sci. Technol., Vol. 27, No. 6, 1993
solution (1L) containing 100ppm of l,l,l-trichloroethane, 1,1,2-trichloroethane, tetrachloroethylene, 2-butanone, and cyclohexanone in 2 % methanol/water was prepared. This standard solution was transferred into a 4-L glass container. The container was inverted, and an inlet nitrogen pressure of 20 psi was applied. The percent removal of each compound was determined as before. (B) Investigation of Performance of Silicone Tubing B on Efficiency of Purification. (i) Consideration of Laminar Flow us Turbulent Flow. Normally, the interaction between an aqueous solution and the surface of the silicone tubing is greater at lower flow rate. Hence, the absorption process should be more complete a t a lower flow rate. However, two types of flow patterns, laminar flow and turbulent flow, might have different effects on absorption on the surface of silicone tubing. Reynolds number, Re, was used to classify these two flows. Reynolds number is defined by the following equation (27): Re = DUP/V where Re is the Reynolds number, D is the diameter of tubing (cm), U is the average linear velocity of solution (cm/s), P is the density (g/cm3), and Y is the viscosity (g/cm-s>. The literature generally states that when Re < 2100, the flow is laminar, and when Re > 3000, the flow is turbulent (28). Assuming that the viscosity of the dilute standard solutions are equal to that of water (0.0089 g/cms at 25 "C) (29),a flow rate of 590 mL min-l is necessary to achieve turbulent flow for tubing with 4.7-mm i.d. A standard solution (2 L) containing 10 ppm of chloroform, l,l,1-trichloroethane, carbon tetrachloride, trichloroethylene, chlorobenzene, bromodichloromethane, and 1,1,2-trichloroethanein 2 % methanoVwater was prepared. The same procedure for water purification under various flow rates, including both turbulent and laminar flows, was performed. The concentration difference was determined as described before. Another standard solution (2 L) containing l ppm of the same organics was prepared. The same experiment was repeated under laminar flow. The efficiency difference between 1 ppm and 10 ppm, standard solutions was compared. (ii)Capacity. A standard solution (2 L) containing 100 ppm of chloroform was prepared and passed through 30 m of this tubing at a flow rate of 500 mL min-1 (in between laminar and turbulent flows). The above procedure was repeated several times until the tubing was saturated with chloroform, at which point the silicone tubing could not remove chloroform from the standard solution any further. If it was still not saturated after the passage of about 30 L of this solution, this procedure was stopped. Instead a standard solution (2L) containing 5000 ppm of chloroform was prepared. This solution was sent through 30 m of this silicone tubing in this same manner repeatedly. The capacity of tubing /3 was then determined. (C) Investigation of Performance of Asilicone Tubing x on Efficiency of Purification. (i) Laminar us Turbulent Flow. 1and 10 ppm standard solutions (2 L) of chloroform, l,l,l-trichloroethane, carbon tetrachloride, bromcdichloromethane, 1,1,2-trichloroethane,trichloroethylene, and chlorobenzene in 2 % methanol/water were prepared. The standard solution passed through this silicone tubing under different inlet nitrogen pressures for both laminar and turbulent flows. The efficiency of
-~ ~
removal for each compound by silicone tubing under various flow rates was compared. (ii) Capacity. A standard solution (2 L) containing 5000 ppm of chloroform was prepared, and the same procedure was employed as in section B (ii) to determine the capacity of this silicone tubing. The flow rate was controlled at 1000 mL min-l. The capacity differences between tubing p and x were compared. (D) Reproducibility and Applicability. To improve reproducibility, an internal standard, which should help eliminate the problems of irreproducible GC injections, was used for further studies. Contaminated water (100 mL) was taken prior to passage through the tubing and spiked with the internal standard. Peak area ratios were compared with those obtained from 100 mL of water similarly spiked after passage through the tubing, and thus, by a comparison of the ratios, the percentage removal of each organic was calculated. After being spiked, the water samples were placed in an ultrasonic bath for 2 min to ensure that an homogeneous mixture was being sampled. The internal standard and its concentration are detailed with each batch of chemicals. Water contaminated with various organics was run in the following batches: Batch one: 10 ppm solutions of chloroform, carbon tetrachloride, trichloroethylene, l,l,ltrichloroethane, 1,1,2-trichloroethane,and chlorobenzene in water were prepared by dilution of standard solutions (10 000 ppm in methanol) with deionized water. Fresh working solutions were made daily. The internal standard was dibutyl ether at a concentration of 15 ppm (from a 10 000 ppm standard solution in methanol). GC analysis conditions were an initial temperature of 60 "C for 2 min and then ramping to 90 "C at 10 "C min-l and hold at 90 "C for 2 min. Batch two: 10 ppm solutions of 1,l-dichloroethylene, 1,2-dichloroethane, cis-1,2-dichloroethylene,trans-l,2dichloroethylene, and dichloromethane in water were prepared by direct dissolution daily. Direct dissolution was used because it was found that if methanolic solutions were used, the methanol peak interfered in the GC analysis. The internal standard was trichloroethylene at 10 ppm (from a 10% solution in methanol). GC analysis was carried out using a 40 "C isothermal run. Batch three: 10 ppm solutions of benzene, 1,4-dichlorobenzene, and 1,2,4-trichlorobenzene in water were prepared by direct dissolution daily. 1,4-Dichlorobenzene was dissolved in benzene prior to addition to the water. The internal standard was chlorobenzene a t 10ppm (from a 10% solution in methanol). GC analysis was carried out using a 150 "C isothermal run. Batch four: Approximately 10 ppm solutions of vinyl chloride were prepared by bubbling vinyl chloride through water while measuring the flow. The internal standard was 1,Qdichloroethane at 20 ppm (from a 20% solution inmethanol). GC analysis was carried using an isothermal run at 30 "C. Batch five: 10 ppm solutions of tetrahydrofuran, 1,3dichlorobenzene, nitrobenzene, and naphthalene were prepared by direct dissolution in water daily. Naphthalene was first dissolved in the other chemicals prior to addition to the water. The internal standard was 10 ppm chlorobenzene (from a 10% methanolic solution). GC analysis was carried out using a 130 "C isothermal run. Batch six: A 10 ppm solution of 2-butanone and 50 ppm solutions of hexachloro-l,3-butadieneand hexachlo-
Table I. Efficiency of Removal of Organics from Aqueous Solution Flowing through a 1.5-mm i.d. X 2.3-mm 0.d. X 150-m Silicone Tubee
tested compound
10 ppmb
2-butanone 1,1,1-trichloroet hane chloroform carbon tetrachloride trichloroethylene 1,1,2-trichloroethane tetrachloroethylene cyclohexanone c
1ppmb
1ppmc
86
100 ppmb 84
86
60
NDd ND ND
ND
ND
-e
ND
-
-
99
-
-
-
ND ND
ND ND
ND ND
ND ND
94
95
94
78
-
a Operational temperature = 25 "C.b Flow rate of 5 mL min-'. Flow rate of 7 mL min-I. ND = not detected. e - = not run.
roethane in water were prepared by dissolution in toluene and then dilution with water every 3 days. The internal standard was 10 ppm chlorobenzene (from a 10% methanolic solution). GC analysis was carried out using a 130 "C isothermal run. Organic-containing water solutions were stored in a polyethylene carboy (Fisher Scientific, Pittsburgh, PA, Catalog No. 02-963BB)for this and future sections of work. Solutions were prepared daily in most cases to avoid problems of absorption and dissolution of the organics into this vessel. (E-G) Effect of Temperature, Vacuum, and Static Mixers. Contaminated water (2 L)was passed through 3 m (10 ft) of tubing 0 or x which was heated or placed under vacuum by the use of a Thelco Model 10 vacuum oven (GCA/Precision Scientific, Chicago, IL). Static mixers (Cole-Parmer, Catalog No. L-04667-14),if present, were placed inside the entire length of the tubing manually.
Results and Discussion
(A) Performance Achieved by Using Silicone Tubing a. (i) For I, 10,and 100p p m Solutions. Table I lists the efficiency of removal of 1, 10, and 100 ppm organics flowing through this silicone tubing at different flow rates. It was noted that 2-butanone and cyclohexanone are much more difficult to remove by silicone tubing under the same conditions. The solubilities of 2-butanone and cyclohexanone in water at 20 "C are 24% and 2.3% respectively (34). However, the solubilities of l,l,l-trichloroethane, 1,1,2-trichloroethane, and tetrachloroethylene are much smaller than those of 2-butanone and cyclohexanone.This result suggests that the more water soluble organics are very difficult to remove by silicone tubing, probably due to their relatively high polarity. At a higher flow rate (7 mL min-l), 2-butanone and cyclohexanone are removed to a lesser extent than at a lower flow rate (5 mL min-l). This is to be expected because of the shorter residence time. However, the percent removal of tetrachloroethylene, l,l,l-trichloroethane, and 1,1,2-trichloroethane are complete (>95%) even at the higher flow rate. This result shows that the use of larger diameter silicone tubing at higher flow rates for water purification is possible for such compounds. In fact, it is not, of course, possible to achieve 100% removal of organics by any method. Instead, the representation not detected (ND) is used, which means that the concentration of a specific organic in water after treatment cannot be detected by the same GC system used to analyze the water standard before treatment. Environ. Scl. Technol., Vol. 27, No. 6, 1993
1141
Table 11. Efficiency of Removal of 10 ppm Organics from 1 L of Solution Flowing through a 1.5-mm i.d. X 2.3-mm 0.d. X 150-m Silicone Tube after Treatment of 100 ppm Organics in 100 mL of solution
Table 111. Efficiency of Removal of 10 ppm Organics from 2 L of Aqueous Solution Flowing at Various Flow Rates through a 4.7-mm i.d. X 5.7-m Silicone Tube
3’6 removal for sample
testedcompound 2-butanone l,l,l-trichloroethane 1,1,2-trichloroethane tetrachloroethylene cyclohexanone
coming out at different time (rninpb 40 53 70 85 100 120 180 -6 ND ND ND 11
17
ND ND ND 14
20 ND ND ND 13
ND ND ND
16 ND ND ND
-6
-1
7
26 ND ND ND 7
11
ND ND ND -4
Operational temperature = 25 O C . Average flow rate = 7 mL min-1.
The percent removal of each compound seems to be independent of its concentration. For example,the percent removal of tetrachloroethylene at 5 mL min-1 flow rate is ND for all three concentrations if the flow rate is kept constant. If the concentration of each component in the water standard does not affect its percent removal by silicone tubing over a wide concentration range, then it would greatly simplify the quantitative description of the process in water purification. With activated carbon, a higher concentration of the contaminant in an aqueous solution increases the degree of adsorption and, hence, removal by activated carbon. This differs from the removal characteristics of silicone tubing. Therefore, a single experiment is capable of providing qualitative and quantitative information on the percent removal for each organic contaminant in water over a wide sample concentration range. (ii)For 10 ppm Solution after Treatment of 100 ppm Solution. Table I1 lists the percent removal of 10 ppm organics using the silicone tubing which has already been utilized for the treatment of a 100 ppm solution. For compounds with low solubilities such as l,l,l-trichloroethane, the percent removal is still virtually complete. However, an intriguing result is that the percent removal of 2-butanone and cyclohexanone is very low. For some sample solutions of this standard collected a t different time intervals, even a *negative” removal for these two particular compounds is seen. It could be that 2-butanone and cyclohexanone do not migrate through the silicone tubing. This is what could have resulted in finding a “negative” percent removal, Le., return of wall-absorbed compounds back to the water. This experimental result shows that silicone tubing is not a good material to remove compounds such as these, especially at low concentrations of these organics right after a high concentration treatment. The same situation also occurs when activated carbon is used for water treatment. (B) Performance Achieved by Using Silicone Tubing& (i)Effect of LaminarFEow us Turbulent Flow. The percent removal of the various organics achieved by using silicone tubing @ at different flow rates is shown in Table 111. All the organics in the standard solution have been essentially removed at a flow rate of 25 mL min-l. However, the performance of silicone tubing decreases greatly at higher flow rates of 205 and 434 mL min-l, which are still in the laminar flow region. Near or well-above the transition flow rate, the performance of silicone tubing is improved slightly. Despite this improvement, the percent removal of each organic at 1142 Envlron. Sci. Technol., Vol. 27, No. 6, 1993
testedcompound chloroform l,l,l-trichloroethane carbontetrachloride trichloroethylene bromodichloromethane 1,1,2-trichloroethane chlorobenzene
% removal at different flow rates” mL m i d 25b 67c 205d 434e 6401 74W
ND ND ND ND ND ND ND
ND ND ND ND 85 ND 99
ND ND ND ND 78 96 99
87 94 89 92 65 71 93
74 92 ND 92 64 79 88
91 95 ND 95 82
85 95
Operational temperature = 25 “C. Reynolds number = 130. Reynolds number = 340. Reynolds number = 1040. e Reynolds number = 22OO.f Reynolds number = 3250. g Reynolds number = 3760.
c 0,
30
’
10 0.0
I
I
I
I
I
0.4
0.8
1.2
1.6
2.0
Amount of Chloroform Passed lg Flgure 2. Capacity information of silicone tubing /3 tested by repeated passage of 2 L of 100 ppm chloroform solution.
turbulent flow is not better than at a slow flow rate below 25 mL min-l. This is because of two opposing factors, residence time and flow pattern; the first being detrimental, while the second contributes toward performance of silicone tubing by forced diffusion. For a limited length of silicone tubing, such as 75 m in this case, the residence time becomes more important than forced diffusion due to turbulence. The significance of turbulent flow on this water treatment process increases with increasing tubing length. (ii)CapacityInformation. For the study of the capacity of a 4.7 mm i.d. X 5.7 mm 0.d. X 30 m silicone tube, the amount of chloroform passed through the silicone tubing was plotted against the percent removal of chloroform for each run. The percent removal of chloroform (Figure 2) did not decrease. Instead, it remained more or less constant at 62 % f 4 % . This indicates that 100 ppm of chloroform solution cannot “saturate”this silicone tubing; i.e., there is continuous removal of chloroform through the tubing wall to the external (air) surroundings. The percent removal of chloroform did not change significantly with the 5000 ppm chloroform solution, indicating a steady-state removal through the tubing wall to the external atmosphere. Average percent removal of the 5000 ppm solution was 62 % f 3 % ,which is almost the same value as that of the 100ppm solution. A strong odor of chloroform was evident during the elution of standard solutions, indicating that this high capacity is due to its high permeability through the silicone tubing wall. Ob-
Table IV. Efficiency of Removal of 1 ppm and 10 ppm Organics from 2 L of Solution Flowing at Various Flow Rates through a 6.4-mm i.d. X 9.5-mm 0.d. X 30-mSilicone Tube 1PPmn 10 ppm4 27 mL/ 1310 mL/ 886 mL/ 1810 mL/ tested compound minb minC mind mine 81 48 60 53 chloroform ND 50 65 61 l,l,l-trichloroethane ND 46 64 60 carbon tetrachloride ND 55 66 62 trichloroethylene 95 19 55 59 bromodichloromethane 89 51 56 51 1,1,2-trichloroethane 96 55 61 56 chlorobenzene a Operational temperature = 25 OC. Reynolds number = 100. Reynolds number = 4860. d Reynolds number = 3290. e Reynolds number = 6730.
viously some method of containment for the removed organics is required. (C) Performance Achieved by Using Silicone Tubing x. (i) Effect of Laminar Flow us Turbulent Flow. Table IV lists the percent removal of each organic for 1 and 10 ppm standard solutions achieved using silicone tubing x at various flow rates. It can be seen that the percent removal of each organic at the 1 ppmlevel is around 95% at the lower flow rate and is reduced to about half this amount at the higher flow rate. There are no significant differences between these two flow rates for the removal of each organic in water with the 10 ppm solution. (ii) Capacity Information. The results of a capacity study of silicone tubing x showed that the performance of this siliconetubing dropped abruptly from 56%to about 25% after a few runs. A possible explanation for this is that the tubing wall is so thick that permeation through the wall is not fast enough, further absorption is prevented causing the drop in percent removal. The wall thickness of our current silicone tubing is 1.6 mm, which is three times as large as the tubing used in section B. As a matter of interest, for this material, the odor of chloroform was not obtained during the elution of the 5000 ppm standard solution as before. This suggests that chloroform could not easily penetrate the wall of this silicone tubing. (D) Reproducibility and Applicability. The reproducibility of the percent removal of each organic for different experiments in this research was excellent. The deviation of the average values was in the 1 % range at the flow rates below 25 mL min-I, generally no more than l o % ,and within 5 % in most of the cases that we studied for higher flow rates. The percentage removal of 16 other chemicals tested with both types of tubing and at two different flow rates are shown in Table V. These results contradict the tentative trend that we suggested earlier in that there seems to be no direct correlation between solubility and percent removal. An example of this is shown by the data obtained for cis-1,2-dichloroethyleneand trans-1,2-dichloroethylene; while the trans isomer has higher water solubility (0.63%by weight at 25 "C) than the cis form [0.35% by weight at 25 "C(30)1,it also has a higher percentage removal, 58 compared to 47 for Brim tubing at 235 mL min-' and 20 "C. Similar trends are seen for other chemicals such as benzene and its derivatives. Further
Table V. Percentage Removal of 16 Other Chemicals Not Previously Studied Using Brim and Silastic Tubing at 20 OC
chemical
BrimD Brimb SilasticC silasticd
67 66 77 32 10 38 58 54 70 47 36 58 39 23 43 58 46 39 38 50 52 43 61 54 48 67 66 20 8 0 68 51 44 37 20 15 14 59 44 38 35 15 10 24 23 64 80 56 43 50 ND ND 60 Flow rate of 1263mL min-l. Flow rate of 235 mL min-l. Flow rate of 667 mL min-1 with static mixers. Flow rate of 235 mL min-' with static mixers. 1,l-dichloroethylene 1,2-dichloroethane trans-1,2-dichloroethylene eis-1,2-dichloroethylene dichloromethane benzene p-dihlorobenzene 1,2,4-trichlorobenzene vinyl chloride tetrahydrofuran rn-dichlorobenzene nitrobenzene naphthalene 2-butanone hexachloroethane hexachloro-1,3-butadiene
50 10 41 29 24 44 33 43 45 20 49
Table VI. Effect of Using Static Mixers within Silastic Medical-Grade Tubing on Percent Removal of Batch One Organics percent removal4 no static with static no static with static mixersb mixersb mixersC mixersc
chemical chloroform l,l,l-trichloroethane carbon tetrachloride trichloroethylene 1,1,24richloroethane chlorobenzene
45 53 47 48 38 41
40 57 59 55 34 48
15 14 40 19 5 15
46 64 ND 38 10 35
Temperature = 20 OC. Flow rtate of 1200 mL min-l (Reynolds number = 4470). Flow rtate of 333 mL min-l (Reynolds number = 1240).
study, such as the use of a homologous series, is required in this area. (E) Effect of StaticMixers. TableVIshows the effect of using static mixers on the removal of batch one organics with the Silastic tubing. These results clearly show that using the static mixers when the flow rate is too low for turbulent flow (Reynolds number 3000);however, there is still an increase for some chemicals. The increase in removal seen with static mixers present in presumably due to the fact that the staticmixers cause 'fresh' solution to be constantly brought into contact with the tubing wall. Without static mixers, a layer of water depleted of organics forms at the wall of the tubing and further organic removal is then governed by diffusion of organics into this layer, and the rate of diffusion of the organics may be less through the water than through the membrane. This explains the decrease in removal seen for some chemicals (e.g., CzCld when the flow rate is reduced with Brim tubing while there is an increase in removal upon a similar decrease with the Silastic tubing. In the first case, the flow changes from turbulent to laminar, and a depleted layer forms at the tubing wall causing diffusion through this layer to be the rate-determining step. While in the second case both flows are turbulent, and hence residence time is more important. Envlron. Sci. Technol., Vol. 27, No. 6, 1993 1143
Table VII. Effect of Increasing Temperature (in "C) on Percent Removal of Batch One Organics with both Brim and Silastic Tubing"
percent removal with Brimb chemical
20 (20)
50 (24)
100 (27)
150 (31)
20 (20)
percent removal with Silasticc 50 (26) 100 (31) 150 (34)
chloroform 29 29 45 47 50 58 61 50 I,l,l-trichloroethane 48 52 61 67 58 65 70 60 carbon tetrachloride 55 64 82 82 70 70 70 60 trichloroethylene 45 46 59 63 58 66 68 62 l,l,Z-trichloroethane 18 21 33 40 40 40 40 38 chlorobenzene 38 38 51 56 54 63 62 62 a The numbers in parentheses are the temperature of the water after passage through the tubing (in O C ) . b Flow rate of 1263mL min-1. c Flow rate of 667 mL min-l (with static mixers). Table VIII. Effect of Varying Pressure from 1 atm to 1 Torr on Percent Removal of Batch One Organics with both Brim and Silastic Tubing at 20 O C
chemical chloroform l,l,l-trichloroethane carbon tetrachloride trichloroethylene 1,1,2-trichloroethane chlorobenzene
% removal with Brima 1atm 1Torr
45 58 58 50 33 43
27 50 58 50 17 39
% removal with Silasticb 1atm 1 Torr
38 56 ND 57 30 61
46 56 50 57 39 58
Flow rate of 235 mL min-I. Flow rate of 235 mL min-l (with static mixers).
(F) Effect of Temperature. The effect of increasing the temperature on the percent removal seen with both Brim and Silastic tubing is shown in Table VII. The temperatures of both the oven and the water after passage through the tubing are shown to indicate that warming of the water is minimal. These results show the expected increase in percentage removal with increasing temperature. This is because the increase in temperature leads to increases in diffusivity and permeability for the organic compounds and water and a consequent increase in percent removed. This is evidence that step two is the rate-limiting step. Although increasing the temperature does increase the overall percent removal of organics, there is a trade-off here to be considered in that in order to increase the temperature it is necessary to supply energy to the system, and this raises the cost. The use of an elevated temperature may also serve to shorten the lifetime of the tubing. (G) Effect of Vacuum. The results obtained by placing both types of tubing in a vacuum oven and measuring the percentage removal at both atmospheric pressure and a pressure of 1 Torr showed no significant increase in percentage removal for any of the chemicals in batch one as shown in Table VIII. Raising the temperature while applying vacuum did not increase the percent removal beyond that seen for merely raising the temperature. The lack of any significant increase in percentage removal with the application of a vacuum indicates that the evaporation of organics from the outside of the tubing is not the rate-limiting step in the process occurring here, which is in agreement with the three-step process described in the Introduction. The unusual result seen with carbon tetrachloride is because the concentration used is very close to the limit of detection for carbon tetrachloride. So small experimental variations are greatly magnified and appear as a change from ND to 50% removal. 1144
Envlron. Scl. Technol., Vol. 27, No. 6, 1993
(H) Performance Index. From Ficks second law (31), d2C/dx2,we can derive an equation which quantitatively describes the process occurring in our system: ln(C/C,) = -KL/F In @/a) where C is the concentration of contaminants in effluent water, C, is the initial concentration of contaminants, F is the flow rate, b and a are the outer and inner radii of the tubing, respectively, and K is a constant. In deriving this equation we assume the following: (1) that a steady state is reached; (2) that the concentration at the tubing wall is equal to the bulk concentration, i.e., that a depleted water layer does not form; (3) that the concentration at the external wall is zero; and (4) that the diffusion coefficient and absorption equilibrium constant are constant at constant temperature and pressure. The derived equation shows that the percent removal is independent of the concentration of a standard solution. This result can explain why we always obtained the same percent removal of each organic at different concentration levels under otherwise constant conditions. The constant K can be defined as an index of performance of a particular tubing. The greater K is, the higher the percent removal. Hence, the value of K offers a simple comparison between two different types of tubing about the ability of removing organic pollutants from water. Using the results from 6.1 m of tubing and applying the derived K to 5 m of tubing, the predicted percent removal of 1,l-dichloroethylene, cis-l,2-dichloroethyleneand benzene are 84,69, and 78%, respectively, which compare to experimental results of 83,62, and 74%. Using the data obtained at 101mL min-1 to predict results at 54 mL min-l with the same length tubing, we get 95, 81, and 89% theoretically and 88, 73, and 81% experimentally. The difference between experimental and theoretical results is presumably due to the fact that assumptions 2 and 3 are difficult to satisfy and are not strictly valid in this system. Further modifications to the equation to take these effects into account are required. (I) Cost. Cost is an important factor relating to the possibility of employing this technology on a large scale. The silicone tubing p is a relatively inexpensive material (less than $100). If the silicone tubing is operated under 90 "C, the estimated service life on this tubing is about 40 years (32). This assumes that the organics will have no detrimental effect on the tubing lifetime, and longevity studies to investigate this are under way. It has been found that the absorption property of silicone tubing does not change noticeably after a 1-year period of continuous use. Due to its permeability characteristic, there should be no regeneration procedure needed for the tubing. This
reduces the total expense of this technique. Some additional cost will be necessary with a larger scale apparatus as trapping of the organics will be required to avoid the problem of merely being shifted from one of water pollution to that of air pollution.
Conclusions The experimentsconductedthus far show the feasibility of using hollow fiber membranes to remove organics from water and thus purify it. The effect of going from laminar flow to turbulent flow by increasing the flow rate (and the use of static mixers) and the competing effects that are occurring during the transition have been demonstrated. The tubing has been found to have no capacity limits as such, which is as expected as the organics are not trapped within the matrix of the hollow fiber membranes but permeate through and evaporatefrom the external surface. Difference types of tubing are found to give different results, and this is due to their differing permeability and selectivity. The ultimate goal of this research has been the development of an efficient technique to remove organic contaminants from water on a large scale, such as the purification of wastewater from industrial companies, decontamination of priority organic pollutants from drinking water, and so forth. Experiments done to date show that our system gives satisfactory results for up to 10 gal/day with a large number of organic chemicals. We intend to build a scaled-up version of our apparatus in the near future capable of running at least 400 gal/day. This device will have the facility to trap the organics removed from the water. Application to real samples obtained from contaminated water sites within the Gulf Coast region is also planned. The use of different types of tubing to remove compounds such as 2-butanone is also under study.
Acknowledgments We thank the Gulf Coast Hazardous SubstanceResearch Center for supporting this project.
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Received for review August 28, 1992. Accepted February 12, 1993.
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