Colloid Remediation in Groundwater by ... - ACS Publications

Chapter 6

Colloid Remediation in Groundwater by Polyelectrolyte Capture

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 21, 2016 | http://pubs.acs.org Publication Date: May 13, 1992 | doi: 10.1021/bk-1992-0491.ch006

H. E. Nuttall, Jr.1, S. Rao1, R. Jain1, R. Long2, and Ines R. Triay3 1Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, N M 87131 2Department of Chemical Engineering, New Mexico State University, Las Cruces, N M 88003 Los Alamos National Laboratory, Los Alamos, N M 87545 3

The presence of radioactive colloids (radiocolloids) in groundwater has been documented in several studies. There is significant evidence to indicate that these colloids may accelerate the transport of radioactive species in groundwater. Because field experiments are often fraught with uncertainties, colloid migration in groundwater is an area of active research and the role and existence of radiocolloids is being investigated. This paper describes an ongoing study to characterize groundwater colloids, to understand the geochemical factors affecting colloid transport in groundwater, and to develop an in-situ colloid remediation process. The colloids and suspended particulate matter used in this study were collected from a perched aquifer site (located at Los Alamos National Laboratory's Mortandad Canyon in northern New Mexico, USA) that has radiation levels several hundred times the natural background and where previous researchers have measured and reported the presence of radiocolloids containing plutonium and americium. At this site, radionuclides have spread over several kilometers. Inorganic colloids collected from water samples are characterized with respect to concentration, mineralogy, size distribution, electrophoretic mobility (zeta potential), and radioactivity levels. Presented are the methods used to investigate the physiochemical factors affecting colloid transport and the preliminary analytical results. Included below are a description of a colloid transport model and the corresponding computational code, water analyses, characterization of the inorganic colloids, and a conceptual description of a process for in-situ colloid remediation using the phenomenon of polyelectrolyte capture. Several studies indicate that radioactive colloids are present in groundwater and can contribute to accelerated migration of radioactive species through the subsurface (1,2,

0097-6156/92/0491-O071$06.00/0 © 1992 American Chemical Society

Sabatini and Knox; Transport and Remediation of Subsurface Contaminants ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


72

TRANSPORT AND REMEDIATION OF SUBSURFACE CONTAMINANTS

3, 4,5). Others (7,8) have indicated the likely potential for facilitated transport of contaminants by groundwater colloids. Longworth and Ivanovich (7) have measured both the concentration of radiocolloids and the amount of radioactive species adsorbed on colloids which were collected from a variety of aquifers. The partitioning of natural series actinides has been measured and both uranium and thorium were found on the colloids. Adsorption of radionuclides onto organic colloids was greater than for inorganic colloids but both colloid types would be considered radiocolloids since they contain radionuclides. It has been suggested in the literature that subsurface transport of contaminants and transuranic elements can be due in part to colloid migration (5,9). At Los Alamos National Laboratory (LANL), two waste sites, T A 21 DP West and Mortandad Canyon, have been monitored over many years for the presence and migration of radiation. Predicted actinide migration distances were on the order of a few millimeters; however, monitoring results showed that at the T A 21 DP West site (6) plutonium and americium have migrated vertically downward nearly 33 m and at Mortandad Canyon, plutonium and americium were detected over two miles from the source (5). It should be noted that the presence of colloids at Mortandad Canyon does not prove that radiocolloids were the cause of the contaminant migration. Further studies are needed to verify the role of colloids at both the T A 21 DP West and the Mortandad Canyon sites. Several other radioactive sites have been monitored for radionuclide migration in groundwater. Monitoring data has shown actinide and/or radionuclide migration in excess of 100 meters in groundwater. Sites other than Mortandad Canyon and T A 21 DP West where radionuclides have migrated considerable distances include the radioactive waste burial sites at Maxey Flats in Kentucky (70), the Idaho National Energy Laboratory—INEL (9) and at the Nevada Test Site—NTS (4). Field data from these sites underline the need to better understand and predict the role of radiocolloids in contaminant migration. To meet this objective, a joint project was initiated. The objective is to understand and predict colloid-contaminant migration in groundwater through colloid transport modeling, water sampling with colloid characterization, and laboratory experiments. Using this combined information, the plan is to develop an effective and scientifically based colloid immobilization strategy. At DOE's Mortandad Canyon site, we are investigating the potential role of radiocolloids in the migration of plutonium and americium. Our progress in understanding groundwater colloids and their transport is summarized in this paper. In the first section, the site modeling and colloid transport code is described. Next are studies characterizing colloids and groundwater from Mortandad Canyon. To date we have investigated the groundwater composition, colloid size distribution, composition, zeta potential, and radiation levels. This characterization information is used in both the modeling and in the design of laboratory experiments. Lastly, we describe in a conceptual overview our proposed in-situ colloid remediation technique using polymer induced capture. Colloid Transport Modeling The purpose of modeling is to better resolve the role of colloids in facilitated radionuclide migration. As a first step, we are modeling the Mortandad Canyon hydrology and solute transport. Next, for the same hydrology conditions, we will model colloid transport.



6. NUTTALL ET AL.

Colloid Remediation in Groundwater

73

Colloid transport modeling is also coupled to the design and interpretation of laboratory column experiments. The transport modeling can be divided into two activities : 1 ) study of Mortandad Canyon hydrology with solute transport and 2) colloid transport. The TRACR3D (77) code, which is well established and recognized for this application, is being used to provide groundwater velocity profiles and baseline solute transport predictions. Results from the TRACR3D simulations of the hydrology and solute transport at Mortandad Canyon were presented in a recent M.S. thesis (72). The water velocity profiles calculated from the TRACR3D simulations will be used as input to the Colloid Transport Code, (CTC) (13,14) for colloid transport simulation studies. Using the velocity profiles calculated in TRACR3D and appropriate submodels describing colloid capture, CTC will be used to model and predict the extent of colloid migration at Mortandad Canyon. To describe colloidal transport CTC, which is a new code, solves the coupled solute transport and population balance equations. The population balance (75) is a continuity equation for the number of particles and the dependent variable is number density of particles. Particle properties such as colloid size are treated as additional orthogonal axes. The CTC code has been tested on a number of standard problems and successfully solved the problem of particle-wall interactions in two-dimensional col loidal transport through saturated/unsaturated fractures (14). The population balance model and code are very generic in nature and can be used to model colloid transport in a wide range of applications. Submodels specific to applications can be added to the code by the user. In view of the general nature of the model formulation, CTC was designed to solve unsteady, nonlinear, coupled, second-order differential systems in up to four spatial axes. It is written in standard Fortran 77 and can operate on most UNIX/ V M S workstations as well as Cray computers under the UNICOS operating system. The code automatically discretizes all the spatial derivatives by user-selected finite differencing schemes thus converting all partial differential equations into sets of ordinary differen tial equations. Β oundary conditions of the most general form can be specified by the user and are added to the system of ordinary differential equations. The resulting system of equations is solved using the robust and efficient LSODPK (16) solver package. Graphical output is best viewed using NCSA Image graphics software (77). To date, preliminary modeling results for the site hydrology and solute transport for Mortandad Canyon (12) have been consistent with the literature, i.e., results showed that dissolved species transport acting under normal ionic equilibrium behavior would not account for the extent of radionuclide migration present at Mortandad Canyon (5). Non-site specific, colloid transport modeling studies have shown that colloids can migrate very rapidly through small fractures under certain idealized conditions. This fact may help explain observations of rapid colloid migration both in the laboratory and in field experiments. A l l of these results are preliminary and the issues of contaminant migration at Mortandad Canyon are continuing to be investigated. The next section describes the colloid sampling and characterization activities. Actual groundwater colloid properties (size, composition, mobility, and mineralogy) are essential for both colloid modeling and for the design and interpretation of laboratory experiments.


74

TRANSPORT AND REMEDIATION OF SUBSURFACE CONTAMINANTS


Colloid Sampling / Characterization Water chemistry and colloid properties affect colloid migration and must be measured in order to predict colloid mobility. The majority of this work is being performed by the colloid characterization laboratory located at the University of New Mexico. Groundwater colloid samples used in this study were collected from Mortandad Canyon. Water samples were drawn from three properly cased monitoring wells (MCO-4B, -6B and -7 A) in the lower portions of Mortandad Canyon from boreholes spaced approximately a quarter of a mile apart. Samples were collected at a depth of approximately 40 to 60 feet in the perched aquifer. More information on the Mortandad Canyon site is available in a recent L A N L report (18). Water analyses for the three well samples are reported in Table I. Note that for the samples in columns Β and C the solids were acid digested while for column A similar water chemistry analyses were performed but without acid digestion by the Soil Water Air Testing laboratory at New Mexico State University (NMSU). The size distributions of the larger colloids contained in the three unfiltered water samples were analyzed using the Coulter Electronics Ltd. Multisizer (range = 1.07 to 30 μπι) and the size distribution information is presented in Table II. It can be seen that the mean size of the colloids is fairly consistent over the three samples. The size distribution of the smaller colloids was analyzed using the Coulter Electron ics Ltd N 4 M D Submicron Particle Analyzer (range = 3 to 3000 nm) and the results are given in Table ΙΠ. The electrophoretic mobility of the groundwater colloids was measured using a Coulter Electronics Ltd DELS A 440. The results showing that the colloids are negatively charged are presented in Table IV. Additional analyses performed on the water sample from well MCO-6B include measurement of: (a) Total Suspended Solids (TSS) : Method 2540 D of Standard Methods (79). (b) Total Dissolved Solids (TDS) : Method 2540 C of Standard Methods. (c) Electrical Conductivity : Method 2510 Β of Standard Methods, and the results are presented in Table V . Characterization of the water samples from the three Mortandad Canyon wells indicates a number of factors. First ,the total suspended solids, TSS, values reported in Table V are sensitive to gravity settling indicating that the original samples contained large particulate matter in addition to colloids. The existence of large particles was also confirmed by S E M analyses. Particles as large as 43 μπι were observed. Hence, the unagitated TSS analyses are likely to be more representative of actual colloid concen trations. Better water sampling techniques are called for in future sampling. The measured colloid sizes varied considerably due to the gravity settling effects and the presence of large particulates. The water is of low ionic strength with a pH of about 8.7. The Mortandad colloids are negatively charged.


6. NUTTALL ET AL.

75

Colloid Remediation in Groundwater


Table I. Water Analysis of Samples from Mortandad Canyon Monitoring Wells MCO-7A Sample MCO-6B MCO-4B C Β A Element Units Β C A A Β C Ag Al (mg/L) As ^g/L) Β Ug/L) Ba Ug/L) Be ( Rg/L) Ca (mg/L) Cd Ug/L) CI (mg/L) CN (mg/L) Co Ug/L) Cr ^g/L) Cu ^g/L) Fe (mg/L) Hg Ug/L) Κ (mg/L) La (mg/L) Mg (mg/L) Mn (mg/L) Na (mg/L) Ni Ug/L) N03-N (mg/L) Ρ (mg/L) Pb (μκΛ-) Rb ^g/L) Sb Ug/L) Se Ug/L) Si02 (mg/L) Sn ^g/L) S04 (mg/L) Sr (mg/L) Sulfides (mg/L ) Tl Ug/L) V Ug/L) Zn TDS (mg/L) pH Cond. μιτύιο/οιη A=NM State Univ;

_

Colloid Remediation in Groundwater by ... - ACS Publications

Recommend Documents