Environ. Sci. Technol. 2000, 34, 2490-2497
Colloid Movement through Stable Soils of Low Cation-Exchange Capacity ANGELA G. NOACK, CAMERON D. GRANT,* AND DAVID J. CHITTLEBOROUGH Department of Soil & Water, The University of Adelaide, Waite Campus PMB No. 1, Glen Osmond, SA 5064, Australia
Factors controlling the transport of mobile colloids through soils are poorly understood yet have major environmental impacts. This study attempted to identify the relative significance of two physical factors controlling movement of different illitic clays through columns of various stable, nonreactive soils: (i) pore size distribution of the soil matrix and (ii) average size of mobile colloids. The columns were leached with various clay suspensions, and colloid concentrations were measured in relation to effluent solution chemistry. No single factor dominated colloid mobility through the soils tested: pore size distribution exerted some control, but so too did size of the mobile colloids (partly affected by cation status). The most mobile colloids in low-charge soils may simply be fine enough to escape physical filtration by the soil matrix. This applies mainly to near-saturated soils, because as soils dry, greater salt concentrations and particle interactions increase mobile-particle sizes and thus increase the degree of possible physical filtration.
Introduction Colloid transport through soils impacts significantly on reservoir water quality and ecology (1) and to a variable degree is directly related to manageable soil factors (2). Mobile colloids are potential vectors in the environment for materials that become associated with them (3), and while this is widely accepted, the quantitative prediction of colloid-facilitated contaminant transport is extremely complex and difficult to predict in natural ecosystems (4). The extent to which these problems can be understood and managed is limited primarily by our understanding of why and how colloids move in the first place (5). Although DLVO theory predicts strong dependence of attachment efficiency on particle size, experimental work (4) shows that retention of colloids can be virtually independent of this. Charge characteristics, which are often invoked to explain the mobility of soil colloids, certainly depend strongly on their size (1). Thus, it is paradoxical that surface area and charge characteristics of many colloids depend strongly on their size while natural mobile particles interact with and attach to matrixes in ways that seem to be size-independent. This apparent inconsistency probably arises from numerous interactions that occur between the colloids themselves and the soil matrix through which they move, plus the chemistry of the percolating solution. The complexity of the * Corresponding author phone: 61-8-8303-7404; fax: 61-8-83036511; e-mail:
[email protected]. 2490
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000
interactions is immense, and few studies have attempted to separate the various factors involved (4, 6, 7). The aim of the experiments reported herein was to determine, under controlled hydrological conditions, the relative importance of influent colloid size and matrix pore size on the vertical mobility of suspended colloids. This study used columns of stable, nondispersive soils of low cation-exchange capacity (CEC) to minimize the impact of physicochemical factors.
Materials and Methods Soils. In this column-leaching study, it was important to ensure that no clay particles originated from the soils themselves. We therefore selected subsoil samples (free of organic matter) from two nominally stable nondispersive soils of low CEC and checked them for clay dispersion. When no colloids appeared in the effluent after leaching with 10 pore volumes of distilled water, it was considered reasonable to assume that colloidal material in any future effluent from these two soils would derive strictly from the influent suspension. The Oxisol (clay texture; 27-60 cm depth from Lismore, New South Wales) had a CEC of only 50.2 mmol+ kg-1. The Spodosol (sandy clay texture; 100-120 cm depth from Willow Creek, South Australia) also had an even smaller CEC of 24.2 mmol+ kg-1; both soils were known to be highly resistant to dispersion (8). All colloids in the leachates were thus reasonably expected to represent those that had not been adsorbed or filtered by the soil through which they passed. Clay Suspensions. Clay suspensions were prepared from lacustrine mineral deposits of Muloorina illite (9) and Fithian illite (10) plus the 0.08-0.2-µm fraction isolated from the C horizon of an Alfisol known locally as the Urrbrae fine sandy loam (11). The mineralogical composition of the clay fraction in the Alfisol was determined by X-ray diffraction analysis (XRD; Philips PW1729 X-ray generator, Co tube, 40 kV, 30 mA, scanned 3-35° 2θ); it comprised illite, kaolinite, and interstratified minerals. Crushed samples were dispersed by Na saturation with 1 M NaCl and washing with distilled water until the clay began to disperse. The Muloorina illite was known to have a uniform diameter of approximately 0.07 µm (12), thus easily collected by centrifugation. A coarse fraction (1-2 µm) and a fine fraction (0.08-0.2 µm) from the Fithian illite and the fine Alfisol, were also collected by centrifugation. Half the suspension for each clay fraction was flocculated with 1 M NaCl and half with 0.5 M CaCl2, followed by further washing with 1 M NaCl and 0.5 M CaCl2 to prepare Na- and Casaturated clays, which were dialyzed against distilled water until free of salt. Suspensions were diluted to 100 mg of clay dm-3 with distilled water for use in the column-leaching experiments; the EC of the clay suspensions was
