Particles, deposition resuspension

Thatcher, T.L. and D.W. Layton (1995). Deposition, resuspension, and penetration of particles within a residence, Atmos. Environ., 29, 1487-1497. 

The concentrations of the corrosion products in the reactor water are controlled by a number of parameters, including feedwater input, particle deposition and resuspension, precipitation and dissolution, and quality of performance of the reactor water cleanup system. As a consequence, these concentrations vary considerably from plant to plant and also within a plant, frequently without showing a discernible trend. The concentrations of total iron may vary by more than 3 orders of magnitude, primarily as a result of variations in the concentration of insoluble iron species, while that of dissolved iron is relatively constant and typically below 10 ppb. The concentration range for total cobalt in different plants is considerably smaller, and is typically in the range 10 to 200 ppt. In general, even when extreme fluctuations are ignored, there is no consistent trend in the concentrations of the corrosion products in the reactor water. 

Water column model. A comprehensive chemical mass balance in the water column should account for mass change with time, advection and dispersion, particle deposition, soluble release, particle resuspension from the bed, evaporation to air and degradation. Over a differential distance x in the direction of flow (L) these processes are. 

The dispersion term is absent since dividing the reach into Ax completely mixed segments accomplishes dispersion numerically. In equation 1 t is time (t), Ct is soluble, particulate, and colloidal, concentration (M/L ), U is average water velocity (M/t), Ds is particle deposition flux (M/L t), h is water column depth (L), m v is suspended solids concentration (M/L ), fp and fd are fractions chemical on particles and in solution, kf is the soluble fraction bed release mass-transfer coefficient (L/t), Cs is the total, soluble and colloidal, concentration at the sediment-water interface (M/L ), Rs is particle resuspension flux (M/L t), ms is the particulate chemical concentration in the surface sediment (M/L ), fps Cts is the fraction on particles and total chemical concentration in the surface sediment (M/L ), Kl is the evaporation mass-transfer coefficient (L/t), Ca is chemical vapor concentration in air (M/L ), H is Henry s constant (L / L ) and Sx is the chemical lost by reaction (M/L t). It is conventional to use the local or instantaneous equilibrium theory to quantify the dissolved fraction, fd, particulate fraction, fp, and colloidal fraction, fooM in both the water column and bed. The equations needed to quantify these fractions appear elsewhere (4, 5, 6) and are omitted here for brevity. 

Together equations 1 and 2 are the CFaT portion of the riverine model. It is dependent open a solids mass balance module for determining the suspended solids concentration plus the particle deposition and resuspension fluxes. The hydrodynamic, particle balance as well as the biota up-take modules are beyond the scope of this study. In addition to the noted coupling to other models, equations 1 and 2 are coupled at the sediment-water interface through the flux expressions. 

This section contains a discussion of how the processes of deposition and resuspension are applied in practice to estimate fluxes of solids and associated contaminants across the sediment-water interface as a result of particle deposition and bed erosion. 

The models described in Section 10.3 are whole system models that include all processes involved in the transport and fate of contaminants in aquatic systems. Application of these models therefore demonstrates how particle deposition and resuspension affect the transport and fate of contaminants associated with those particles. The case study presented in Section 10.4 for the Lower Fox River is an example of how research knowledge and appropriate field data collection on sediment deposition and resuspension processes (reviewed in Section 10.2) have been incorporated into conceptual and numeric models (reviewed in Section 10.3) that support assessment and remediation of contaminated sediment sites. 

The main disadvantage of the perfect sink model is that it can only be applied for irreversible deposition of particles the reversible adsorption of colloidal particles is outside the scope of this approach. Dahneke [95] has studied the resuspension of particles that are attached to surfaces. The escape of particles is a consequence of their random thermal (Brownian) motion. To this avail he used the one-dimensional Fokker-Planck equation... 

Besides the resuspension of particles, the perfect sink model also neglects the effect of deposited particles on incoming particles. To overcome these limitations, recent models [72, 97-99] assume that particles accumulate within a thin adsorption layer adjacent to the collector surface, and replace the perfect sink conditions with the boundary condition that particles cannot penetrate the collector. General continuity equations are formulated both for the mobile phase and for the immobilized particles in which the immobilization reaction term is decomposed in an accumulation and a removal term, respectively. Through such equations, one can keep track of the particles which arrive at the primary minimum distance and account for their normal and tangential motion. These equations were solved both approximately, and by numerical integration of the governing non-stationary transport equations. 

Where the waves and currents weaken, resuspended sediment settles back down to the seafloor. Given the small particle sizes of the suspended material (mostly 3 to 10 pm), redeposition can take many years. The resulting redistribution of sediments creates patches of clay, mud, and exposed rock on the continental margins. In other words, resuspension from waves and currents can cause some sediments to become reUct deposits. Hard bottoms can serve as good habitats for some members of the benthos as they promote the formation of coral reefs. For paleoceanographers, relict deposits are problematic because they represent gaps, or imconformities, in the sedimentary record. 

Also special care should be taken to reduce uncertainties on emission data and measurements. The validation of an aerosol model requires the analysis of the aerosol chemical composition for the main particulate species (ammonium, sulphate, nitrate and secondary organic aerosol). To find data to perform this kind of more complete evaluation is not always easy. The same applies to emissions data. The lack of detailed information regarding the chemical composition of aerosols obliges modellers to use previously defined aerosols components distributions, which are found in the literature. Present knowledge in emission processes is yet lacunal, especially concerning suspension and resuspension of deposited particles [37]. 

The conditions in Nevada are favourable for resuspension of Pu from the ground. Because the area round the N.T.S. is arid, Pu has not been moved down the soil profile by leaching or by cultivation, and more than 50% of the Pu was found to be in the top 20 mm of soil about 15 a after deposition (Anspaugh et al., 1975). The mechanisms of resuspension of particles from the ground are considered in a later chapter. The resuspension factor Kr is defined ... 

Ten years after the spread of activity, about two-thirds of the deposited Pu was in the top 50 mm of soil and reduced availability for resuspension probably accounts for the fall in air concentrations shown in Fig. 5.6. The region is moderately arid, with an annual rainfall of 400 mm, and the area near the source of Pu is untilled grassland. Because the Pu in air was mostly attached to soil particles, the particle size was large, and only about 25% was in the respirable fraction (Volchok etal., 1972). 

Most radioactive particles and vapours, once deposited, are held rather firmly on surfaces, but resuspension does occur. A radioactive particle may be blown off the surface, or, more probably, the fragment of soil or vegetation to which it is attached may become airborne. This occurs most readily where soils and vegetation are dry and friable. Most nuclear bomb tests and experimental dispersions of fissile material have taken place in arid regions, but there is also the possibility of resuspension from agricultural and urban land, as an aftermath of accidental dispersion. This is particularly relevant to plutonium and other actinide elements, which are very toxic, and are absorbed slowly from the lung, but are poorly absorbed from the digestive tract. Inhalation of resuspended activity may be the most important route of human uptake for actinide elements, whereas entry into food chains is critical for fission products such as strontium and caesium. 

Garland (1979, 1982, 1983) used the wind tunnel shown in Fig. 6.13 to measure resuspension of radioactive particles from grassland at Harwell. The fan and motor were mounted on a turntable, and the working section could be positioned as required. Radioactive particles were deposited on a strip of grass about 10 m long, and air was then drawn over it in the tunnel. Samplers measured the amount of resuspended activity in the air downwind of the strip. The horizontal flux of activity was deduced and expressed as the rate constant A of resuspension. 

A more detailed study of the role of rain in resuspension onto shoots was carried out by Dreicer et al. (1984). Tomato plants were grown outdoors and the quantity of soil particles <53 pm and 53-105 pm measured between 0-40 cm and >40 cm above ground level after four successive storm events. Table 7-12 shows that most deposition onto the foliage occurred within 40 cm of the ground and that there was generally an increase in this deposition with a successive number of storms. There are few studies of this type and there is a requirement for controlled experiments on the influence of different types of rain events (duration, intensity, droplet size) on both resuspension onto leaves and also removal of such contamination. 

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