Application of ESEM to environmental colloids

Environmental colloids are toxic or radioactive particles suspended in ground or surface water. These hazardous particles can facilitate and accelerate the transport of toxicants and enhance the threat to humans by exposure to pathogenic substances.
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  MICROSCOPY RESEARCH AND TECHNIQUE zyx 5:439-446 1993) z Application of ESEM to Environmental Colloids H.E. NUTTALL AND RAHUL KALE zyxwvutsr epartment zyxwvutsrqp   ChemicallNuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131 1341 KEY zyxwvutsrq ORDS Microscopy, Groundwater, Pollution, Radioactive waste, Transport, Remedia- tion ABSTRACT Environmental colloids are toxic or radioactive particles suspended in ground or surface water. These hazardous particles can facilitate and accelerate the transport of toxicants and enhance the threat to humans by exposure to pathogenic substances. The chemical and physical properties of hazardous colloids have not been well characterized nor are there standard colloid remediation technologies to prevent their deleterious effects. Colloid characterization requires measurement of their size distribution, zeta potential, chemical composition, adsorption capacity, and morphology. The environmental scanning electron microscope ESEM) by ElectroScan, Inc., analyzes particle sizes, composition, and morphology. It is also used in this study to identify the attachment of colloids onto packing or rock surfaces in our development of a colloid remediation process. The ESEM has confirmed the composition of groundwater colloids in our studies to be generally the same material as the surrounding rock. The morphology studies have generally shown that colloids are simply small pieces of the rock surface that has exfoliated into the surrounding water. However, in general, the source and chemical composition of groundwater colloids is site dependent. We have found that an ESEM works best as a valuable analysis tool within a suite of colloid characterization instruments. o 1993 Wiley-Liss, Inc. INTRODUCTION The introduction of pollutants in the subsurface en- vironment is inevitable due to human activities like waste disposal practices, spills, and land application of pesticides. Though care is taken in choosing waste dis- posal sites, climatic and geochemical changes invari- ably cause percolation of contaminants into the groundwater. Contaminant transport by colloidal-sized particles through porous and fractured rock has been widely recognized McCarthy and Zachara, 1989). Col- loids are ubiquitous in groundwater and have been studied for hundreds of years Hiemenz, 1986). The possibility of radioactive contamination of groundwater from nuclear waste disposal sites has been a cause for concern among researchers all over the world. Sparingly soluble radioactive materials are known to readily adsorb onto immobile aquifer media and their movement through porous media is expected to be retarded. However, groundwater colloids, being of the same material as the rock matrix, also offer sites for adsorption of these radioactive materials. Their ex- tremely small size provides a large surface area per unit mass, leading to a significant adsorption potential for the contaminants. These colloids travel much faster in porous media compared to dissolved species and do not easily sorb onto the immobile rocks Jain, 1992; Nuttall et al., 1991; Nuttall, 1986; Travis and Nuttall, 1985). Thus, measured contaminant migration at waste disposal sites has been found to over-predict the- oretical calculations based on diffusiodadsorption of dissolved species Nuttall, 1986, 1989a,b). Besides be- ing adsorbed onto colloids, the radioactive waste can itself precipitate into colloidal-sized particles and mi- grate as radiocolloids. The groundwater basins near nuclear waste disposal sites such as Mortandad Canyon Penrose et al., 1990; Nuttall and Long, 1992) have radioactive contamina- tion far above background levels. The ideal remedia- tion strategy at such a site should strive to eliminate the subsurface of all radioactivity. Existing technology makes that task extremely difficult if not impossible. However, the importance of limiting further migration of the contaminants needs to be emphasized. The pur- pose of our research is the development of a colloid remediation technique designed to retard or stop con- taminant colloid migration in groundwater near waste sites. Colloid immobilization in contaminated aquifers may be possible by polyelectrolyte capture onto the rock matrix Nuttall et al., 1992; Nuttall and Triay, 1992). In very dilute concentrations polyelectrolytes have the ability to flocculate natural colloidal suspen- sions by surface charge neutralization and polymer bridging. Since these natural colloids have the same surface properties as the subsurface porous rocks, they should readily adsorb onto the rocks in the presence of polyelectrolytes. The suggested technique is the intro- duction of polyelectrolytes into the subsurface media through bore-wells, allowing the flocculation of col- Received January 6, 1993; accepted in revised form March 24, 1993. Address reprint requests to H.E. Nuttall, 209 Farris Engineering Center, De- partment of ChemicalNuclear Engineering, University of New Mexico, Albu- querque, NM 87131-1341. O 1993 WILEY-LISS, INC.  44 zyxwvusr ColloidNaste Immobilization Process H.E. NUTTALL AND R. KALE Polymer Attaches to the Rock2 h Polymer Injection zyxwvu   Injection Stage Waste in Colloids SoiVRock or Bind to in Solution.; Taste and Ro Binding Stage Colloids Second Injection Stage Fig. zyxwvuts . Colloid and waste immobilization strategy loids and their subsequent capture onto the substrate. Polymer flocculates are used to capture and fix colloids onto the rock matrix. In order to immobilize both radiocolloids and dis- solved radionuclides, we are testing the concept of add- ing a better clay type colloid to bind nuclides and then through polymer induced bridging, these radioactive clay colloids will be captured and bound along with natural radiocolloid to the rock matrix (Nuttall et al., 1992; Nuttall and Long, 1992). This two-step process of both nuclide and colloid immobilization on the rock matrix is illustrated schematically in Figure 1 EXPERIMENTS A promising environmental remediation strategy is the treatment of a hazardous waste site with a dilute polymer solution to create polymer-induced colloid cap- ture onto the rock matrix. Polyelectrolyte-induced cap- ture of radiocolloids is being investigated in the labo- ratory as a method to retard colloid migration. Our experimental studies, described here, have shown that polyelectrolytes can adhere onto silica-based surfaces and subsequently capture colloids from contaminated groundwater. Laboratory sandlquartz columns treated with polyelectrolytes significantly enhance the reten- tion of colloids. Actual aquifer rock matrix and ground- water flow rates and concentrations were simulated in columns. Both synthetic silica microsphere colloids and natural colloids from Mortandad Canyon (mixture of feldspar, quartz, and clay) were tested. The conceptual remediation process entails injection of dilute polyelec- trolyte into the subsurface contaminated zone to reduce  APPLICATION OF ESEM TO ENVIRONMENTAL COLLOIDS 44 z   Ceaend A zyxwv   Polycarbonate column B Media Quartz,Glass beads and subsurface media C Syringe pump D - Colloidal dispersion E - Three-way valve F - Flow rate selector G - Syringe size selector H - Outlet Fig. 2. Saturated packed column apparatus. colloid release and transport within the aquifer as il- lustrated in Figure l Laboratory studies are utilized in the process design and development. The experimental equipment used in the saturated colloid remediation studies to simulate colloid immobi- lization consists of a laboratory-scale packed column with an attached syringe pump as illustrated schemat- ically in Figure 2. To simulate the porous media, quartz sand was packed in the columns and the colloi- dal suspensions were passed through small disposable polycarbonate columns (20 cm in length and 0.8 cm in diameter obtained from Rainin Inc.) which were capped off at both ends with 20 micron filters. The quartz was obtained from Unimin (Spruce Pine, NC , and was used in our studies as a control standard. It had a particle size distribution ranging from 150-400 microns and a specific gravity of 2.65. A bulk density ranging from 1,523-1,571 kg/m3 (95-98 lb/ft3) gives an average po- rosity of 40 . Actual subsurface rock cores from the Mortandad site are currently being tested in the col- umn; however, no results are available at this time. Once packed with quartz the column is then fixed ver- tically on a stand with a three-way stopcock at the lower end as illustrated in Figure 2. The stopcock is connected to two syringes fixed on syringe pumps (Sage Instruments) by polyethylene tubes. This configuration allows switching of the fluid flowing through the col- umn between DI water and the sample. Thus, either a step input of colloids or a square pulse input is possible. The syringe pump’s flow rate range is from 0.1-60.0 ml per h. The superficial velocity in the column was cho- sen and operated to correspond to values estimated  442 H.E. NUTTALL AND R. KALE Coll zyxwvuts i zyxwvu s Polymer Attachment Agglomeration Cluster Settling and Capture on the Fig. zyxwvutsr . Polymer-induced flocculation and colloid capture. zyx Polymer Fig. 4 Cationic polyelectrolyte for in situ colloid remediation pro- cess (CATFLOC-poly diallyl dimethyl ammonium chloride). from field data. The colloid breakthrough curves were measured using a N4MD photon auto-correlation spec- trometer by Coulter, Inc. The silica colloids were ob- tained from Nissan Chemical Industries Ltd. (Tokyo, Japan) and were packaged in water suspensions with 40 solids by weight. The silica colloids used were 0.3 microns in diameter. The samples were diluted to con- centrations of approximately 220 mg/l with DI water. This represents about the same concentration com- pared to natural groundwater colloids. Groundwater colloid samples were also studied. These were contam- inated radio-colloids from Mortandad Canyon at Los Alamos National Laboratory, New Mexico. The theory of polymer-induced flocculation is a com- mon waste water clarification technology (Schwoyer, 1981); however, its application to in situ colloid trans- port remediation is new and requires further develop- ment. Polymers in parts-per-million concentrations can adsorb onto the surface of particles and then form polymerbridge type colloid clusters as illustrated in Figure 3. In practice, a polymer injection could coagu- late and remove colloids from the groundwater, thus immobilizing the radioactive nuclides and toxic pesti- cides (Nuttall et al., 1992; Nuttall and Long, 1992). The drink water safe cationic polyelectrolyte used in these experiments is CATFLOC-poly diallyl dimethyl am- monium chloride-and its chemical structure of both the monomer and polymer are drawn in Figure 4. This is a commercially available polymer that is commonly used in municipal waste water treatment systems. Colloid size distribution measurements were per- formed using a multi-angle laser light scattering in- strument, the COULTER Sub-Micron Particle Size An- alyzer (Model N4MD), which is capable of measuring particle sizes from 3-3,000 nm (diameters). The same instrument also gives an indication of particle concen- tration by measuring the relative turbidity of suspen- sions. After sonicating for 30 seconds in a Branson 1200 sonicator, colloid suspensions are diluted to the instrument specifications and a sample is pipetted into 5 ml cuvets for analyses. For characterizing particle charge (zeta potential) a Doppler Electrophoretic Light Scattering Analyzer (COULTER DELSA 440) is used, which measures the electrophoretic mobility and/or zeta potential of sus-  APPLICATION OF ESEM TO ENVIRONMENTAL COLLOIDS zyx 43 z Fig. 5. a: Silica colloids on quartz packing. Secured to surface by a polyelectrolyte. b Silica colloids attached to a clear glass slide by a polyelectrolyte. pended particles. The colloidal silica and the ground- Coulter, Inc., DELSA 44 also gives an accurate mea- water colloids were characterized for particle size and surement of conductivity at any desired temperature. surface charge. To characterize the groundwater clay samples by The pH of the colloidal suspensions was monitored shape, optical and electron microscopy techniques were using an Orion pH meter (model: 250A). used. An Olympus BH-2 UMA optical microscope al- The conductivity of samples was measured using a lowed dark field photomicrographs to be taken of the conductivity meter (Corning, Model PS-17). The submicron particles. The environmental scanning elec-
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