The generation and mobility of colloids in soils Production et mobilité des colloï des dans les sols

Transcription

1 Scientific registration n o : 1112 Symposium n o : 4 Presentation : poster The generation and mobility of colloids in soils Production et mobilité des colloï des dans les sols WATERS Angela, CHITTLEBOROUGH David, GRANT Cameron, OADES Malcolm Department of Soil Science, The University of Adelaide, Waite Campus, Glen Osmond, South Australia, 564, Australia Introduction Mobile soil colloids affect water quality and are intimately associated with pedogenesis. Insofar as most nutrients are held in clay minerals, organic matter and organo-mineral associations, mobility of colloidal material results in loss of nutrients from the soil. Colloids are also recognized as an important medium in the transport of contaminants in natural aqueous systems (McCarthy and Zachara 1989). Mobile colloids promote the translocation of strongly sorbed contaminants such as radionuclides (Buddemeier and Hunt 1988), heavy metals and hydrophobic organic compounds (Roy and Dzombak 1997). It is widely assumed that the most mobile fraction is fine clay, but recent work shows that particle size distribution is dependent on environmental factors such as storm intensity and pore velocity, and the composition of the mobile clays differs from that of the fine clay in the soil (Chittleborough et al. 1997). The experimental work described here examines the factors which influence the nature, quantity and rate of colloids mobilized in a South Australian Alfisol with particular focus on the physico-chemical and morphological properties of the soil matrix. A characteristic feature of this soil, developed on alluvium of the southern Adelaide Plains, is the marked texture contrast between the A and B horizons. Chittleborough and Oades (198) attributed the high content of clay in the B horizon to illuviation of fine clay. It was proposed that dispersion, the main mechanism whereby colloids are generated in soil, is related to the area of aggregates exposed. Colloids generated by passing solutions of varying electrolyte concentration and sodium adsorption ratio through columns of repacked soil aggregates sieved to two size ranges were characterized, as were colloids generated in subsequent leaching with pure water after drying the columns of soil. 1

2 Materials and methods Soil from the A horizon (-1 cm) of a South Australian Alfisol was used in column transport experiments. The soil was sieved air dry to obtain < 5 mm and 2-5 mm fractions. Acrylic columns of 4.5 cm diameter were packed with 2 g of soil to a depth of 1 cm in 4 x 5 g increments with tamping down between each addition to maximize even packing. A 2 mm layer of acid washed sand (1-2 mm diameter) was placed on the soil surface to minimize the generation of colloids by drop impact. The soil was initially leached with water or salt solutions of sodium adsorption ratio (SAR) 5 and total cation concentration (TCC) 2, 5 and 1 mmol (+) l -1, applied to the surface of the air dry soil at a rate of 5 mm h -1 using a drip head with 1 26G syringe needles. 2 ml samples were collected and analyzed for turbidity and electrical conductivity. After 4 pore volumes of solution had been applied the soils were dried at 5 C. The columns were then leached with pure water and 2 ml samples were analyzed for turbidity and electrical conductivity. The concentration of clay in solution was calculated from turbidity using a calibration curve for the < 2 µm fraction of the soil. Table 1. Description of soil used in column transport experiments Aggregate size organic C % Particle size distribution % Sand Silt Clay < 5 mm mm Results and discussion Leachate was first eluted from the columns filled with < 5 mm soil after 5 cm 3 of solution had been applied and after 4 cm 3 from the 2-5 mm filled columns, corresponding to.63 and.5 pore volumes respectively. Thus the flow was unsaturated, that is, not all the pores were participating in the transport of colloids. Release of colloids during first leaching The concentration of clay in the leachates obtained from leaching with water and salt solutions of SAR 5 and varying TCC decreased with increasing electrolyte concentration (Fig. 1). This is attributed to decreased dispersion of the clays and can be explained by DLVO theory. As ion concentration increases repulsion between clay particles decreases as the double layer on the surface is compressed and the particles remain flocculated. The first leachate collected had a very low concentration of clay, in addition to a very high electrical conductivity as entrained salt was dissolved at the wetting front and leached through the soil (Fig. 2). Thereafter the release of colloids increased rapidly to a maximum before tailing off. The rise in colloid release corresponded to a rapid decrease in electrical conductivity of the leachate at all TCC s as 2

3 Clay concentration (mg l -1 ) water SAR 5 TCC 2 SAR 5 TCC 5 SAR 5 TCC Fig. 1. The influence of total cation concentration on concentration of clay in leachate of Urrbrae A horizon < 5mm (c) 2 Clay concentration (mg l -1 ) EC (µs cm -1 ) (d) Clay concentration (mg l -1 ) EC (µs cm -1 ) Concentration of clay Electrical conductivity Fig. 2. The relationship between concentration of clay and electrical conductivity (EC) in the leachate of Urrbrae A horizon < 5mm after leaching with water, SAR 5 TCC 2, (c) SAR 5 TCC 5 and (d) SAR 5 TCC 1. 3

4 < 5mm 2-5 mm < 5mm 2-5 mm Fig. 3. The influence of aggregate size on release of clay from Urrbrae A horizon when leached with water and SAR 5 TCC 1. shown in Fig. 2. Because there was no decrease in the hydraulic conductivity of clogging of the soil columns the tailing of the colloid release was attributed to a reduction in the supply of dispersed material to the conducting pores rather than reduced transport of the clay through the soil. The effect of aggregate size of the matrix is illustrated for the extremes of TCC in Fig. 3. The concentration of clay released has been presented as a proportion of the total clay content of the soil to take into account the lower clay content of the 2-5 mm aggregate fraction (Table 1). It is clearly shown that for both leaching with water and electrolyte solution less clay is generated and mobilized from the 2-5 mm aggregate fraction, supporting the hypothesis that dispersion of clay is related to the area of the aggregates exposed. Release of colloids in subsequent leaching with water Subsequent leaching with water again generated and mobilized colloids. The concentration of colloids in the initial leachate was greater than that at the end of the first leaching with the exception of the water-treated < 5 mm soil (Fig. 4). The increase in colloid concentration was particularly marked in the soils pretreated with salt solutions, as the electrolyte concentration of the soil solution was reduced rapidly for the first time. Colloid release then decreased but there was a recovery in colloid release from those soils pretreated with salt. This behaviour can be explained by the electrical conductivity of the leachate for the < 5 mm soil (Fig. 5). Decrease in colloid concentration coincides with an increase in electrical conductivity as salts were dissolved at the wetting front and leached from the system; after initial flushing the electrical 4

5 water SAR 5 TCC 1 water SAR 5 TCC 1 Fig. 4. The influence of total cation concentration of the first leaching solution on clay release with subsequent leaching with water after drying < 5 mm and 2-5 mm aggregate fraction of Urrbrae A horizon. conductivity decreased rapidly and there was a steep increase in the release of colloids before tailing of the release curve. This phenomena was less readily explained for the 2-5 mm aggregate fraction. It may have been due to flow bypassing smaller pores to a greater extent in the 2-5 mm aggregate system, as evidenced by the appearance of EC (µs cm -1 ) and Concentration of clay x Electrical conductivity Fig. 5. The relationship between fraction of clay released and electrical conductivity (EC) of subsequent leachate of < 5 mm and 2-5 mm aggregate fraction of Urrbrae A horizon after initial leaching with SAR 5 TCC 1 solution. 5

6 < 5mm 2-5 mm < 5mm 2-5 mm Fig. 6. The influence of aggregate size on release of clay from Urrbrae A horizon when initially leached with water and SAR 5 TCC 1 solution. leachate after only.5 pore volumes in this soil, compared with the.63 pore volumes in the < 5 mm soil. Salt diffusion from the larger aggregates would have required more time and therefore the electrical conductivity profile is much flatter than that for the < 5 mm soil. However, at the surface of the aggregates, the effect of increased electrolyte concentration caused less colloid release. The effect of aggregate size in subsequent leaching with water was counter to that observed in the first leaching run (Fig. 6). This could be due to efficient removal of the readily dispersible colloids from the < 5 mm soil, whereas in the 2-5 mm aggregate fraction a smaller proportion of readily dispersible clay was removed. It is possible that the transport of dispersed colloids from aggregates to pores active in mass flow transport is limited by the rate at which they can diffuse to those pores. Diffusion limited transport would be more significant in the 2-5 mm aggregate fraction, as evidenced by the lower concentration of clay in the initial leachates. Consequently in the next leaching more readily dispersible clay was available in the 2-5 mm aggregate fraction, and these clays were remobilized on rewetting of the soil. Conclusions Greater mobility of clay was found in the < 5 mm fraction of a South Australian Alfisol than in its 2-5 mm aggregates when repacked soils were leached with water and salt solutions of sodium adsorption ratio (SAR) 5 and total cation concentration (TCC) 2, 5 and 1 mmol (+) l -1. This supports the hypothesis that dispersion of clay is related to the area of the aggregates exposed. However, after drying and further leaching of these soils with water, there was greater mobility of clay in the 2-5 mm aggregate fraction. The role 6

7 of diffusion-limited transport in the mobility of colloids through this soil is now the subject of further investigation. By understanding the nature of the mobile colloidal material and mechanisms of the generation and mobility of colloids efforts to attenuate the loss of colloids carrying nutrients or pollutants can be more effective. References Buddemeier, R.W. and Hunt, J.R. (1988) Transport of colloidal contaminants in groundwater: radionuclide migration at the Nevada Test Site. Applied Geochemistry 3, Chittleborough, D.J. and Oades, J.M. (198) The development of a Red-brown earth. III The degree of weathering and translocation of clay. Aust. J. Soil Res. 18, Chittleborough, D.J., Kirkby, C.A., Smettem, K.R.J., Jalalian, A., Ranville, J. and Beckett, R. (1997) Particulate movement through a soil with a strong textural differentiation. Soil Sci. Soc. Am. J. in prepn. McCarthy, J.F. and Zachara, J.M. (1989) Subsurface transport of contaminants. Environ. Sci. Technol. 23, Roy, S.B. and Dzombak, D.A. (1997) Chemical factors influencing colloid-facilitated transport of contaminants in porous media. Environ. Sci. Technol. 31, Keywords : colloids, dispersion, colloid mobility, surface properties, aggregation Mots clés : colloïde, dispersion, mobilité des colloïdes, propriétés de surface, agrégation 7