Soil removal mechanisms

Another soil-removal mechanism is also working, simultaneously, to remove soil albeit a much slower process (i.e., up to several hours). When the bath/soil interface becomes curved due to a change in (see Figure 4d), its chemical potential is... 

Since the detergent power of many surfactants cannot be directly related to their efficacy as emulsifiers, there exists some question as to the importance of emulsification as a primary soil removal mechanism and redeposition control. Certainly, for efficient solubilization to occur, the area of surfactant solution—soil interface must be maximized, which implies a reduction in the solid substrate-oil interface. 

In addition to lowering the interfacial tension between a soil and water, a surfactant can play an equally important role by partitioning into the oily phase carrying water with it [232]. This reverse solubilization process aids hydrody-namically controlled removal mechanisms. The partitioning of surface-active agents between oil and water has been the subject of fundamental studies by Grieser and co-workers [197, 233]. 

Adsorption of bath components is a necessary and possibly the most important and fundamental detergency effect. Adsorption (qv) is the mechanism whereby the interfacial free energy values between the bath and the soHd components (sofld soil and substrate) of the system are lowered, thereby increasing the tendency of the bath to separate the soHd components from one another. Furthermore, the soHd components acquire electrical charges that tend to keep them separated, or acquire a layer of strongly solvated radicals that have the same effect. If it were possible to foUow the adsorption effects in a detersive system, in all their complex ramifications and interactions, the molecular picture of soil removal would be greatly clarified. 

Four means of soil removal have been proposed mechanical action ... 

Hexachloroethane released to water or soil may volatilize into air or adsorb onto soil and sediments. Volatilization appears to be the major removal mechanism for hexachloroethane in surface waters (Howard 1989). The volatilization rate from aquatic systems depends on specific conditions, including adsorption to sediments, temperature, agitation, and air flow rate. Volatilization is expected to be rapid from turbulent shallow water, with a half-life of about 70 hours in a 2 m deep water body (Spanggord et al. 1985). A volatilization half-life of 15 hours for hexachloroethane in a model river 1 m deep, flowing 1 m/sec with a wind speed of 3 m/sec was calculated (Howard 1989). Measured half-lives of 40.7 and 45 minutes for hexachloroethane volatilization from dilute solutions at 25 C in a beaker 6.5 cm deep, stirred at 200 rpm, were reported (Dilling 1977 Dilling et al. 1975). Removal of 90% of the hexachloroethane required more than 120 minutes (Dilling et al. 1975). The relationship of these laboratory data to volatilization rates from natural waters is not clear (Callahan et al. 1979). 

The system must be operated properly so the vadose zone does not become completely saturated with water, thus reducing the effective permeability to the point that gases and vapors cannot be recovered. When the soil-heating process has progressed to the extent that gaseous steam reaches the recovery wells, all the water within the soil zone has been vaporized. At this juncture, S VE becomes the primary removal mechanism. A blower or vacuum pump can be used to induce airflow in the subsurface, which will facilitate the removal of any remaining residual liquids. 

Mirex is a very persistent compound in the environment and is highly resistant to both chemical and biological degradation. The primary process for the degradation of mirex is photolysis in water or on soil surfaces photomirex is the major transformation product of photolysis. In soil or sediments, anaerobic biodegradation is also a major removal mechanism whereby mirex is slowly dechlorinated to the 10-monohydro derivative. Aerobic biodegradation on soil is a very slow and minor degradation process. Twelve years after the application of mirex to soil, 50% of the mirex and mirex-related compounds remained on the soil. Between 65--73% of the residues recovered were mirex and 3-6% were chlordecone, a transformation product (Carlson et al. 1976). 

Under anaerobic conditions, mirex was slowly dechlorinated to the 10-monohydro derivative by incubation with sewage sludge bacteria for two months (Andrade and Wheeler 1974 Andrade et al. 1975 Williams 1977). The primary removal mechanism for mirex was anaerobic degradation as demonstrated by the 6-month stability of the compound in nine aerobic soils and lake sediments (Jones and Hodges 1974). 

From this description, it is obvious how agitation and buoyant effects of the soil could speed up this mechanism (in fact, roll up cannot occur at all unless buoyant or agitation forces act on the soil), but soil-removal rate is a kinetic question and will not be pursued further here. 

All cresol isomers can be rapidly removed from environmental media. The dominant removal mechanism in air appears to be oxidation by hydroxyl radical during the day and nitrate radical at night, with half-lives on the order of a day. In water under aerobic conditions, biodegradation will be the dominant removal mechanism half-lives will be on the order of a day to a week. Under anaerobic conditions, biodegradation should still be important, but half-lives should be on the order of weeks to months. In soil under aerobic conditions, biodegradation is also important, but half-lives are less certain, although probably on the order of a week or less. 

The results obtained with C19 and as model soils suggested that the crystal form of the hydrocarbon can affect surfactant penetration and hence removal. Hexacosane (C26), with a melting point of 57 °C, allows investigation of the effect of temperature over a wider range than the systems described above. Additional details about the relationship of surfactant phase behavior to solid soil removal can be obtained, and the efficiency of the displacement mechanism can be explored further, using C2g as a model soil. 

Solid hydrocarbon soils can be rapidly removed from the surface of a ZnSe IRE by alkyl polyethylene oxide) surfactants. The removal mechanism involves penetration of a small amount of the surfactant into the hydrocarbon layer, which causes an increase in methylene chain defects in the soil, and displacement of solid soil from the substrate. Solubilization of a large fraction of the solid soil is not required. 

In ambient air, the primary removal mechanism for acrolein is predicted to be reaction with photochemically generated hydroxyl radicals (half-life 15-20 hours). Products of this reaction include carbon monoxide, formaldehyde, and glycolaldehyde. In the presence of nitrogen oxides, peroxynitrate and nitric acid are also formed. Small amounts of acrolein may also be removed from the atmosphere in precipitation. Insufficient data are available to predict the fate of acrolein in indoor air. In water, small amounts of acrolein may be removed by volatilization (half-life 23 hours from a model river 1 m deep), aerobic biodegradation, or reversible hydration to 0-hydroxypropionaldehyde, which subsequently biodegrades. Half-lives less than 1-3 days for small amounts of acrolein in surface water have been observed. When highly concentrated amounts of acrolein are released or spilled into water, this compound may polymerize by oxidation or hydration processes. In soil, acrolein is expected to be subject to the same removal processes as in water. 

I will briefly review some of the mechanisms of soil removal involved in aqueous and nonaqueous textile cleaning before proceeding to the experiments and results of this study. This discussion provides background information on the two methods of cleaning. 

During the last few decades extensive attention has been paid to the hazards arising from contamination of the environment by arsenic. Decontamination of heavy metals in the soil and water around industrial plants has been a challenge for a long time. The use of microorganisms for the recovery of metals from waste streams (Joshi et al, 2008 Patel et al, 2006,2007 Maeaskie and Dean, 1990), as well as the employment of plants for landfill application (Tripathi et al., 2007), has received increasing attention. Recent developments and improvements have resulted in the construction of bioreactors (Oehmen et al, 2006) that have a smaller footprint, and treat the metals more effectively. Many studies have demonstrated primary removal mechanisms for the metals by arsenate-reducing bacteria, which transform arsenate to arsenite (Cohen, 2006 Afkar et al, 2003 Mukhopadhyay et al, 2002). Plants have been... 

The non- and mono-ortho CBs have been quantitated accurately in the principal source, namely, commercial PCB mixtures [15, 16] additional environmental sources such as incineration have been identified [17,18] their presence in every ecosystem including the pristine polar regions has been shown [19,20] estimates of their flux in air, water, soil, and the removal mechanisms such as OH reactions in atmosphere and sediment burial in rivers and oceans have been proposed [21] their microbial degradation and biotransformation in organisms have been studied [22,23] a battery of in-vitro and in-vivo bioassays using mammalian, avian, and piscian models for the benefit of risk-assessment of these CBs have been developed [24]. Studies like these in the last decade have resulted in a new awareness of these important class of industrial contaminants. 

---