Membrane Filtration of Colloidal Activated Carbon: Considerations for Optimization of Head Loss Reduction and Small Molecule Adsorption

Transcription

1 Membrane Filtration of Colloidal Activated Carbon: Considerations for Optimization of Head Loss Reduction and Small Molecule Adsorption Erin Partlan a, Pauline Amaral b, Mengfei Li a, Patrick Ayerle c, David A. Ladner a * Affiliation: a Clemson University, Dept. of Environmental Engineering and Earth Sciences b Federal University of Santa Catarina, Dept. of Environmental and Sanitary Engineering, c University of Maryland, College Park, Dept. of Material Science and Engineering *Corresponding author: ladner@clemson.edu Abstract Colloidal activated carbon produced by pulverization in a wet bead mill has faster adsorption of small molecules than conventionally sized powdered activated carbon (PAC). To separate these superfine particles, termed S-PAC, microfiltration membranes are used. However, the formation of a tight S-PAC cake layer on MF membranes results in high head loss. Effects of source water quality and particle size were evaluated with regards to filtration flux and small molecule adsorption efficiency. The presence of natural organic matter (NOM) proved to be a confounding factor: NOM can foul the membrane but can also be adsorbed by S-PAC. Two methods were tested for improving filtration flux: chemical coagulation and membrane modification using surfactants. Chemical coagulation resulted in floc formation but not flux improvement. Calcium proved to be effective at floc formation and fouling mitigation. Several surfactants were tested as membrane coatings and evaluated for coating resiliency, flux improvement, and modification of surface charge. Polyethylenimine (PEI) was the most resilient but also caused additional fouling. Didecyl dimethyl ammonium chloride (DDAC) was also fairly resilient and resulted in the highest flux recovery through backwashing. Introduction Activated carbon with particle diameters of 2 7 nm are referred to as superfine PAC, or S- PAC, and are beneficial in cases where adsorption rates are important, such as with a short contact time or in competitive adsorption with natural organic matter (NOM) 1,2. Faster adsorption is made possible by reducing the particle size, and thus reducing the distance that molecules travel by diffusion into adsorbent pores 3 6. S-PAC is produced through wet milling, a common process used for the pulverization of solid materials into fine powders and generally entails high velocity contact between grinding media and the product material while suspended in a carrier fluid. Wet milling results in a substantially decreased particle size with smaller particles resulting from longer milling durations. Additionally, changes to physical and chemical characteristics can occur that modify S- PAC in relation to the initial PAC material 7. To employ S-PAC was a water treatment alternative, it is also necessary to incorporate a method of removal. Microfiltration (MF) membranes have been shown to be effective in complete removal of S-PAC from water.

2 Additionally, the application of S-PAC onto MF membranes as a pre-coat has been explored 6,8. The intentional deposition of carbon onto a membrane before use creates a version of an adsorption column or packed bed filter with very short contact times. Ellerie et al. (213) showed that adsorption through a membrane coating occurred faster than adsorption in a continuously stirred tank reactor. However, the issue with S-PAC filtration and coating comes in the form of membrane fouling due to formation of a cake layer, which results in a loss of filtration rate and efficiency. There exists a tradeoff between contaminant adsorption and flux decline. While smaller particles have faster adsorption kinetics compared to larger molecules, they also cause higher degrees of flux decline since the particles can pack more tightly into a cake layer. Particles processed for 15 3 minutes, that is, passing through the mill 3 45 times, was found to be optimal among carbons processed for shorter and longer times, up to 6 hours 9. This optimum may shift if flux decline can be mitigated. Here, chemical coagulation is used to produce S-PAC flocs that act like larger particles when collected on the membrane surface. Also, surfactants are used as membrane coatings to prevent irreversible attachment of foulants to the membrane. Previous studies have shown that surfactant coating can improve filtration flux over time and reduce membrane susceptibility to fouling 1. Surfactant modification has the added benefit that it can improve particle rejection by reducing the effective pore size either by size exclusion or electrostatic repulsion. In both cases, the issue of hydraulic backwashing must be addressed, but the problem may be more unique with surfactant coatings in which the modifier compounds must stick in one situation but remove in another. Materials and Methods A commercial carbon, Watercarb-8 (bituminous coal, Standard Purification) was milled as received using a flow-through bead mill (MiniCer, Netzsch Premier Technologies, Exton, PA, USA). S-PAC was stored as a slurry in distilled and deionized (DDI) water until used. Aggregation tests using ferric chloride and a varied water matrix were performed in 2 L gator jars; 1 L is taken from the sampling port for filtration analysis. The carbon concentration was set at 5 mg/l for all tests following analysis of atrazine breakthrough though carbon filter cakes of varying masses. After settling, contents of the jar test beaker are transferred to a pressure vessel for dead end filtration. S-PAC, both aggregated and unaggregated, was filtered through flat sheet polymeric membranes housed in a plastic membrane cell (Amicon, Millipore). Pressure for filtration, fixed at 1 psi, was supplied by a nitrogen tank connected to an 8 ml pressure vessel. Permeate is collected in a flask on a balance; mass information is transferred to a computer for determination of flux. Atrazine was used as a model micropollutant and radiolabeled molecules were used as an adsorbate tracer. Permeate samples were measured by liquid scintillation counting to determine atrazine concentrations. The membranes used to evaluate source water effects were polymeric membranes made of hydrophilic PVDF. Other polymeric membranes made of mixed cellulose esters with three different pore sizes.8 µm,.45 µm, and.22 µm were tested for modification with six surfactants: didecyl dimethyl ammonium chloride (DDAC), sodium dodecyl sulfate (SDS), sodium docecyl benzene sulphonate (SDBS), Tween-8, and 1, 7, and 75 kda

3 polyethylenimine (PEI). The surface charge of the membranes along with resiliency of the surfactant coating was evaluated by titration using the SurPASS Electrokinetic Analyzer to determine the zeta potential. The uncoated membrane flux was determined via distilled and deionized (DDI) water filtration for 2 minutes. Carbon coatings were made by adding carbon directly to the membrane cell after sonication for particle disaggregation (S- 4, Qsonica, LLC). Backwashing trials were performed by reversing the membrane and filtering at the same pressure through the nonactive side of the membrane. Backwashing efficiency was evaluated by returning the membrane to the original direction and measuring the clean water flux. Results and Discussion Membrane Coating Flux Decline In clean water trials, S-PAC with a mean diameter of 23 nm resulted in 6% flux decline. Larger S- PAC resulted in less flux decline: particles from 3-7 nm resulted in 1-2% flux decline. Scanning electron microscopy reveals the difference in packing densities (Figure 1). PAC particles are large relative to the membrane material and pack loosely. The smallest S-PAC, milled for 6 hours, was too small to be individually discerned at the magnification used and the cake layer had very tight packing. Packing density is also affected by the source water quality. The cake structure is stabilized by the presence of NOM, which is seen as rough matter on the surface of the carbon particles. FIGURE 1: SCANNING ELECTRON MICROSCOPE IMAGES OF CARBON PARTICLES COATED ON A MEMBRANE AND USED TO FILTER WATER CONTAINING NATURAL ORGANIC MATTER. ABOVE: PAC, BELOW: 6 H MILLED S-PAC The stabilized S-PAC layer causes very high flux decline while the PAC layer caused little to no flux decline. This flux trend, little decline for larger particles and higher decline for smaller particles, was true for all carbons (Figure 2). Additionally, the flux decline associated with S- PAC is not only a function of its particle size; there is a clear difference with respect to carbon type, which may be due to any number of physical and chemical characteristic differences. Thus, the optimal milling time may vary between carbon types, though generalizations can be made. It is likely that short milling durations are sufficient to produce useful S-PACs that adsorb well and do not result in excessive flux decline.

4 FIGURE 2: FLUX DECLINE ASSOCIATED WITH FILTRATION OF S-PAC ONTO MICROFILTRATION MEMBRANES IS A RESULT OF PARTICLE SIZE ONLY WITHIN EACH CARBON TYPE. 1 Flux Decline Mitigation Ferric chloride coagulation resulted in varied flux declines (Figure 3). In DDI water, fouling led to approximately 5% flux decline. Coagulation of the S-PAC using 1 mg/l of ferric chloride changed the flux decline as a function of the ph of coagulation. At ph 7, there was no change in flux decline, at ph 6, there was additional flux decline, and at ph 8, there was less flux decline. In tests with water containing ionic strength, the addition of ferric chloride reduced the fouling slightly at ph 7. However, the addition of calcium resulted in more flux decline mitigation. There are two likely reasons for the varied ferric chloride results: first, extra coagulant that did not complex with the carbon is available to complex with the membrane, and second, precipitated coagulant can result in pore blocking. In other tests, aluminum hydroxide coagulation resulted in severe fouling, likely for similar reasons. Alum has a strong affinity for polymeric membrane materials. Calcium is able to create flocs without these issues and thus performs as a coagulant without the ph dependency. Normalized Flux Normalized Flux Time (min) Baseline Flux FeCl3 ph 6 FeCl3 ph 7 FeCl3 ph Time (min) Baseline Flux 1mM NaCl +.1mM CaCl2 FeCl3+1mM NaCl FIGURE 3: FLUX DECLINE AS A RESULT OF AGGREGATED S-PAC TOP: FERRIC CHLORIDE PERFORMANCE DEPENDS ON PH BOTTOM: IONIC STRENGTH PRESENCE REDUCES FLUX DECLINE.

5 Surfactant modified membranes were evaluated by zeta potential measurement and filtration. Readings of modified membranes reflected the charge of the surfactant coatings and breaks in this data revealed loss of surfactants from the membranes, particularly for SDS and SDBS (Figure 4). PEI coatings proved more resilient. PEI greatly inhibited filtration flux, decreasing clean water flux to as low as 15% of its uncoated value, but improved foulant removal over all nonpolymeric surfactants besides DDAC. DDAC resulted in the best foulant removal and highest recovered clean water flux value at roughly 8%. However, the method of coating removal still remained to be addressed. Since PEI resisted removal through DDI rinsing, removal of PEI from the membranes was explored using a 5% hydrogen peroxide solution to induce hydrolysis, and resulted in increased recovery flux values. Zeta Potential (mv) Bare 1.25 g/l SDS 1.25 g/l SDBS ph.45 µm MCE membranes coated in 5g/L 75 kda PEI were tested with six H 2O 2 enhanced backwashing methods and compared with the recovered flux from unenhanced backwashing (Figure 5). H 2O 2 was applied both in the forward flow direction as well as the reverse flow direction. Reverse flow was more effective. Additionally, while increasing the solution concentration resulted in increased removal, adding factors of temperature and time also resulted in increased removals. Heating the solution to 35 C improved the removal, but soaking for 1 minutes resulted in even better removals. From an energy standpoint, soaking is the most economical method of removing the surfactant layer. Then the layer can be reapplied before the next filtration. Post Wash CWF (lmh)) FIGURE 4: SURFACE ZETA POTENTIAL OF AN UNCOATED MEMBRANE AND MEMBRANES COATED WITH 1.25 G/L SDS AND 1.25 G/L SDBS. THE BREAK IN DATA REVEALS AN UNRESILIENT COATING THAT WASHES AWAY BETWEEN TITRATIONS. FIGURE 5: FLUX AFTER BACKWASHING AND CHEMICALLY ENHANCED BACKWASHING USING HUDROGEN PEROXIDE. HEAT INCREASED RECOVERY SLIGHTLY, BUT SOAKING WAS MORE EFFECTIVE.

6 Conclusions Colloidal carbon is useful as an adsorbent coating on microfiltration membranes, but results in severe flux decline for the smallest particles. Methods of flux decline mitigation are possible through coagulation of S-PAC and surface modification of membranes. Surfactant modified membranes resulted in additional rejection and less flux decline. Methods of polymer removal were also explored. Acknowledgements This project is supported by the National Science Foundation under grant number CBET 12367, REU grant number , CU COMSET and the School of Materials Science and Engineering. The views expressed do not necessarily reflect those of the funding agency. SEM images were created at the Clemson University Electron Microscopy Laboratory. References (1) Matsui, Y.; Yoshida, T.; Nakao, S.; Knappe, D. R. U.; Matsushita, T. Characteristics of competitive adsorption between 2-methylisoborneol and natural organic matter on superfine and conventionally sized powdered activated carbons. Water Res. 212, 46 (15), (2) Matsui, Y.; Nakao, S.; Taniguchi, T.; Matsushita, T. Geosmin and 2-methylisoborneol removal using superfine powdered activated carbon: shell adsorption and branched-pore kinetic model analysis and optimal particle size. Water Res. 213, 47 (8), (3) Pelekani, C.; Snoeyink, V. L. Competitive adsorption between atrazine and methylene blue on activated carbon: the importance of pore size distribution. Carbon N. Y. 2, 38 (1), (4) Matsui, Y.; Murai, K.; Sasaki, H.; Ohno, K.; Matsushita, T. Submicron-sized activated carbon particles for the rapid removal of chlorinous and earthy-musty compounds. J. Water Supply Res. Technol. 28, 57 (8), 577. (5) Ando, N.; Matsui, Y.; Kurotobi, R.; Nakano, Y.; Matsushita, T.; Ohno, K. Comparison of natural organic matter adsorption capacities of super-powdered activated carbon and powdered activated carbon. Water Res. 21, 44 (14), (6) Ellerie, J. R.; Apul, O. G.; Karanfil, T.; Ladner, D. A. Comparing graphene, carbon nanotubes, and superfine powdered activated carbon as adsorptive coating materials for microfiltration membranes. J. Hazard. Mater. 213, 261C, (7) Partlan, E.; Davis, K.; Ren, Y.; Apul, O. G.; Mefford, O. T.; Karanfil, T.; Ladner, D. A. Effect of bead milling on chemical and physical characteristics of activated carbons pulverized to superfine sizes. Water Res (8) Hamad, J..; Kennedy, M..; Hofs, B.; Heijman, S. G..; Amy, G.; Schippers, J. Super ground PAC in combination with Ceramic Micro- filtration II; 28. (9) Amaral, P.; Partlan, E.; Li, M.; Lapolli, F.; Mefford, O. T.; Karanfil, T.; Ladner, D. A. Superfine powdered activated carbon (S-PAC) coatings on microfiltration membranes: Effects of milling time on contaminant removal and flux; 215. (1) Chen, V.; Fane, A. G.; Fell, C. J. D. The use of anionic surfactants for reducing fouling of ultrafiltration membranes: Their effects and optimization. J. Memb. Sci. 1992, 67 (2-3),