Sands natural soils, studies

Pentachlorophenol, a widely used wood preservative, is considered to be moderately biorefractory with a biodegradation rate constant of 3 x 1012 L/ cell/hr, a log of 5.01, and a vapor pressure of 1.1 x 10-4 mmHg at 20°C. Watts et al. (1990) carried out completely mixed batch tests by treating penta-chlorophenol-contaminated soils with Fenton s reagent. Mineralization of pentachlorophenol (PCP) was studied in commercially available silica sand and two natural soils by removal of parent compound and total organic carbon with corresponding stoichiometric recovery of chloride. The soluble iron concentration decreased over the first 3 hr of treatment, and the concentration remained relatively constant thereafter. A possible mechanism for iron precipitation was proposed as follows ... 

Miscellaneous Identified Inhibitors. 3-Acetyl-6-methoxy-benzaldehyde is present in the leaves of the desert shrub Encelia farinosa. It is apparently leached from the leaves and washed into the soil by rain. Concentrations of approximately 0.5 mg. per gram of dried leaf material have been measured. In sand culture studies, growth of tomato seedlings was inhibited by 50 p.p.m. while 115 p.p.m. reduced growth by 50% (53). A concentration of 250 p.p.m. killed the test plants within one day. The structure was confirmed by synthesis, and the synthetic material was shown to be as active as the natural product (54). Derivatives were also prepared in which a cyano, nitro, or amino group was substituted for the aldehyde moiety. The amino derivative was reported to be the most highly toxic. 

A field study was conducted by Larson et al. [31] to characterise the impact of effluent discharges on a sandy soil about 0.5 m below the surface. A 2.5 m thick unsaturated zone and a 3—4 m thick unconfined sand/gravel aquifer underlaid the tile field. LAS concentrations in the effluent plume decreased over a distance of 10 m from 10 000 to 30 xg L-1. A further object of study was a laundromat pond exposed to LAS-containing sewage for more than 25 years. A clay layer separated the natural pond from the vadose zone made up of porous sand. Measurements of LAS levels as a function of soil depth beneath the pond showed a rapid decrease from about 220 mg kg-1 at 30 cm to... 

The modern history of ion exchange began in about 1850 when two English chemists, Thompson(4) and Way(5), studied the exchange between ammonium ions in fertilisers and calcium ions in soil. The materials responsible for the exchange were shown later to be naturally occurring alumino-silicates 6. History records very much earlier observations of the phenomenon and, for example, Aristotle(7), in 330 BC, noted that sea-water loses some of its salt when allowed to percolate through some sands. Those who claim priority for Moses(8) should note however that the process described may have been adsorption ... 

Natural waters always contain colloid particles. They originate from the rocks and soils and contain mineral particles (clay, sand, silt, etc.) and organic matter (humic substances). Since the size of the particles is small, they have great specific surface area and high adsorption capacity. The concentration of colloids is usually studied by the measurement of turbidity. 

Soils vary greatly in composition and reactivity. Many complex and dynamic processes occur continuously in most soils composed primarily of mineral and organic matter, water, and air. The soil atmosphere is composed of oxygen, carbon dioxide, nitrogen, and several minor gases whereas the mineral fraction varies in amounts of sand, silt, and clay and in types and amounts of clay minerals. Moreover, hydration and base saturation of the clay minerals also vary considerably. The organic matter and mineral colloids present in the soil contribute directly and indirectly to the extremely active nature of pesticide-soil systems. Since soil water contains many soluble compounds, it serves as an essential medium for many chemical and physical processes. The extreme complexity of these soil systems has been the primary reason that so few fundamental studies have been undertaken involving the ultimate fate of pesticides in soils. 

Values of k for natural porous materials vary widely. Typical permeability values for soils are clean gravel 10 9 10 7 m2, clean sand 10-11 10 10 m2, peat 10 13 10-11 m2, stratified clay 10-16 10-13 m2, and unweathered clay 10-2° 10-16 m2. Table I shows typical values of permeability. In some literature, especially in the early studies, the unit of Darcy (cm2cP/atms = 0.987 X 10 12 m2) is also used. 

The occurrence of thermal diffusion in nature was first observed by Severinghaus et al. (1996) for soil gas in sand dunes. The fractionation of soil air by water vapor diffusion, gravitational settling, and thermal diffusion studied by these authors may also have a small effect on dissolved noble gases in ground water, and hence the calculation of noble gas paleotemperatures. However, these comparatively minor effects have not yet been further studied in the soil-air-ground-water system. 

This goes against molecular expectations because one would predict both glass and rubber spheres to adhere similarly with a force around 100 nN. Evidently, it is important to study the detailed nature of the contact between particles in order to understand these differences. The purpose of this chapter is to define the contact between spheres, to understand how the adhesion is developed, and to use this knowledge to explain phenomena such as powder friction and sintering, which are vitally important to the smokes, dusts, soils, sands and particulate masses which occupy much of the planet. 

There is a great interest in the nature of the interface between water and silicate minerals (see for example Davis and Hayes (1986)). Much of the chemical activity in soils, sediments and porous rocks occurs at such an interface. Experimentally, it is very difficult to examine this interface because it is such a small part of the liquid-solid system. Hydrated smectites and vermiculites have water between all of the silicate layers and therefore the percentage of the sample which is interface is enormously larger than the interface between, for example, a grsin of quartz in contact with liquid water. Another way to look at this is that the surface are of a quartz sand is probably much less than 1 m gram while a typical smectite has a surface area of as much as 800 m /gram. For these, and other reasons, intercalated clays have been extensively studied. 

---