Geoscience Reference
In-Depth Information
interception, where the particle size and trajectory are such that it encounters the
collector grain while flowing past, and (3) Brownian diffusion, where the particle is
brought into contact with a collector due to its Brownian motion (Yao et al. 1971 ;
Elimelech et al. 1995 ). Geometric models (Sakthivadivel 1966 , 1969 ) suggest that
straining could have a significant influence when the ratio of the particle diameter
to the median grain diameter of the porous medium is greater than 0.05. Similarly,
a limiting ratio of 0.154 for predicting straining of particles in constrictions has
been proposed (Herzig et al. 1970 ). However, recent experimental evidence sug-
gests that straining could be important at much smaller particle to grain size ratios
(Bradford et al. 2003 ). Mobilization (detachment) of deposited particles also is a
key process governing colloid transport and fate. Mobilization can take place
following drastic changes in pore water chemistry and when the hydrodynamic
forces overcome the adhesive forces between particles and the medium grains
(Amirtharajah and Raveendran 1993 ).
Deposition and trapping of contaminants on colloidal materials and other sus-
pended particles may occur during their transport through the vadose zone and thus
create an additional route for pollutant distribution in the subsurface. Below
hazardous waste sites, for example, an unexpected transport process of cationic
radionuclides (e.g., Pu, Am) or various heavy metals has been observed, which can
be explained only by colloid-facilitated transport (McCarthy and Zachara 1989 ;
Penrose et al. 1990 ; Ryan and Elimelech 1996 ). Laboratory experiments testing
colloid-facilitated redistribution in the partially saturated zone confirmed that
colloids can accelerate the transport of cationic and anionic metals (e.g., Vilks
et al. 1993 ) or toxic organic chemicals (e.g., Vinten et al. 1983 ). Colloidal
materials involved in the process of enhanced redistribution of contaminants in the
subsurface include inorganic matter like clay minerals, oxides and carbonate
particles, with sizes in the range of 10 nm to a few micrometers, as well as organic
colloids like humic substances and microbial exudates.
Vinten et al. ( 1983 ) demonstrated that the vertical retention of contaminated
suspended particles in soils is controlled by the soil porosity and the pore size
distribution. Figure 5.8 illustrates the fate of a colloidal suspension in contami-
nated water during transport through soil. Three distinct steps in which contami-
nant mass transfer may occur can be defined: (1) contaminant adsorption on the
porous matrix as the contaminant suspension passes through subsurface zones, (2)
contaminant desorption from suspended solid phases, and (3) deposition of con-
taminated particles as the suspension passes through the soil.
The suspended solid particle size and the volume of effluent also must be
considered in examining deposition in the subsurface. For example, under leaching
of a waste disposal site or following irrigation with sewage effluent, the coarse
fraction of suspended solids is retained in the upper layer, while the finer colloidal
fraction is more mobile, and its transport is controlled by the porosity of the
subsurface solid phase.
Particle deposition from aqueous suspensions onto stationary surfaces is a
dynamic phenomenon characterized by a transient or time-dependent rate of
deposition. The deposition of contaminated suspended particles is affected by the
Search WWH ::




Custom Search