current research
Zonal Centrifugation-a
Tool for Environmental Studies
W. P. Bonner, Tsuneo Tamura, C. W. Francis, and J. W. Amburgey, Jr.1 Health Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
Although most separation techniques either destroy a component of interest o r chemically alter the system, recent developments in zonal-centrifugation techniques, especially for biological systems (Anderson, 1966; Lammers, 1967), suggest several ways of separating environmental samples into their various components. The major problem in separating samples containing clay-size inorganic minerals by Z-C is flocculation, preventing individual component banding and other components from reaching their isodensity or isopycnic point. When the Z-C technique is applied to a calcareous pond sediment, five distinct density fractions o r bands are obtained. The technique also offers the possibility of concentrating and identifying minor components such as anatase and dolomite. This paper reports the results of laboratory tests and describes a satisfactory method for suspending and banding environmental samples containing clay minerals, with suggestions of possible applicstions of the described technique.
Z
onal-centrifugation involves preparing a continuous density-gradient solution in a tube, layering the test sample on the gradient solution, and centrifuging the tubes to allow esch component to collect at its isopycnic point (Amburgey, 1966). Earlier techniques for separating multicomponent mineral systems utilized a series of solutions of different densities ; this procedure was especially time consuming since the floating and settled fractions had t o be collected and reintroduced to solutions of higher or lower densities (Baniel, Mitzmager, et ul., 1963 ; Clemmons, Stacy, et ul., 1957). A modification of the “sink-float’’ technique consists of carefd introduction of a series of solutions of Present address. Separation Systems Division, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Tenn. 37830.
differing densities into a tube; however, the interface of the separate solutions imposes a barrier to particle penetration. Though this problem is, in part, overcome by centrifugation, particles accumulating at o r near interfaces are subject to question regarding their densities (Anderson, 1955). With the continuous gradient solution produced by differentially driven pistons, the problem of interface barrier is eliminated and the step-wise preparation of solution is avoided. The density gradient former used in this laboratory was developed at the Oak Ridge Gaseous Diffusion Plant (Fox, 1968). Methods and Results
Density gradient solutions in the range of 1.8 to 2.8 g./cc. necessary for inorganic mineral separation either require heavy organic liquids which are generally not miscible with water or heavy inorganic salts which favor flocculation of clays with high-charge density in the diffuse double layer. To prevent flocculation, several approaches were taken. In Table I selected combinations of the different approaches and the observations regarding flocculation o r dispersion are shown for several materials, including an acidic kaolinitic soil, a calcareous pond sediment, pond suspended solids, and standard clays. Negative results (flocculation) are included to show the need for a suitable suspending agent (numbers 1-7, 9-13, and 16). Note that the greater than 0.2-p fraction of kaolinitic soil and the greater than 2.0-p fraction of pond suspended solids show dispersion without the aid of a dispersing agent (8, 15, and 17). This observation is in keeping with current stability theory of low-charge density materials (van Olphen, 1963). For most samples with a particle size of less than 2.0 p the use of a dispersing agent proved to be most satisfactory for preventing flocculation (14 and 18-21). Although there are many suspending agents compatible with water systems, the search for suspending agents was limited primarily to materials compatible with the organic density gradient system of tetrabromoethane (TBE) and ethyl Volume 4, Number 10, October 1970 821
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Table I. Test Trials to Obtain Dispersion of Environmental Samples Density gradient Sample particle Sample preparation media Suspending media dia. ( w ) Thallium formate Water Suspended pond sediment, Oven dried water, p = 1.80 . 0 8 to 0 . 2 2.2a Thallium formate Thallium formate Suspended pond sediment, Oven dried water, p = 2.4water, p = 2 . 4 0 . 0 8 to 0 . 2 2.7 Diiodomethane Same as gradient Suspended pond sediment, Oven dried dimethylsulfoxide, 0 . 0 8 to 0 . 2 p = 2.2 TBEb = benzene, EtOH Suspended pond sediment, Oven dried p = 1.8 0.08 to 0 . 2 TBE EtOH, EtOH,' Suspended pond sediment, Wet samples washed with TBE p = 1.8-2.8 p = 1.8 EtOH, then acetone 0 . 0 8 to 0 . 2 EtOH TBE TBE EtOH, Suspended pond sediment, Saturate with methylene p = 1.8-2.8 blue, then wash with 0.08 to 0 . 2 EtOH and acetone TBE = EtOH, EtOH, Suspended pond sediment, Saturate with anionic guar TBE p = 1.8-2.8 p = 1.8 polymer, then wash with 0 . 0 8 to 0 . 2 EtOH TBE EtOH, TBE EtOH, Suspended pond sediment, Oven dried followed by p = 1.8-2.8 p = 1.8 EtOH and acetone wash >2 TBE EtOH, EtOH, Oven dried TBE Bottom pond sediment p = 1.8 p = 1.8
Observation Flocculation
+
+
+
+
+ +
+ +
+
+ +
+ +
10
Bottom pond sediment
Oven dried
TBE = EtOH, p = 1.8
11
Bottom pond sediment
Oven dried
TBE
12
Standard clays,
