Particulate materials, movement

In the velocity theory above developed it is apparent that while fundamentally sound, the chief difficulty in practice concerns the measurement of bed velocities. This has been overcome in part by Rubey s analysis of the subject, and By the general theory of Kennedy and Lacey. However, in recent years studies of silt movement have utilized DuBoys (1879) expression of tractive force. This expression is simple and convenient and involves the basic elements of channel hydrology depth and slope. Tractive force means the force activity on the bed causing movement of the particulate material. The force required to impart initial motion to the bed material is called the critical tractive force. General movement is defined as the condition where particles up to and including the largest composing the bed are in motion. 

The extensive amount of research on this compound has indicated that 2,3,7,8-TCDD is both remarkably stable and is almost exclusively associated with particulate material. Because of the low water solubility and high sorption on soils, it moves only very slowly in natural soils. Human exposure to 2,3,7,8-TCDD in soils is thus a function of movement and degradation (primarily photochemical) on soil particles. Enhanced movement of 2,3,7,8-TCDD in soils also contaminated with oils and/or solvents potentially can increase concentrations near the surface, and this process, along with photolysis on particles, requires further investigation. 

Since, from the first law of adhesion, particles always adhere, the natural state of particulate materials is aggregated as in Fig. 10.1 (a), with each particle stuck fast to its neighbors. However, when the particles become contaminated with liquid, the second law of adhesion applies and the adhesion is reduced. In certain circumstances, this reduction in adhesion proceeds so far that the particles remain separate or dispersed in the contaminating medium, as in Fig. 10.1(b). Such bizarre behavior is a challenge to the theorist because it seems to go against the basic laws of adhesion. The purpose of this chapter is to show that several complex mechanisms cause the particles to behave strangely. The most important mechanism is Brownian movement, followed by electric charging, followed by adsorption of polymer molecules at the surface. 

The removal of heavy metal ions from both natural water supplies and industrial wastewater streams is becoming increasingly important as awareness of the environmental impact of such pollutants is fiilly realized. In particular, the likelihood of such metal ions precipitating out of solution and/or coating other materials can have a profound effect on both aqueous and nonaqueous environments. There is considerable evidence in the literature that the primary mechanism for transportation of metal contaminants in aquatic systems is the movement of suspended particulate material containing the adsorbed pollutant metals [1,2]. It is also known that a strong correlation exists between the concentration of trace metals in the (aquatic) environment and the extent to which those metal ions adsorb onto colloidal substrates present in the environment [2,3], A similar correlation between the concentration of trace metals in the (aquatic) environment and their precipitation behavior is not so clear. There is, then, a well-founded need to study adsorption-related phenomena in order to understand and predict the behavior of toxic metals in the environment. 

Refractory ceramic fiber is finding some use in insulating plenum skirts because of its ability to flex and accommodate movement in the plenum skirt. Care must be taken in designing some erosion protection for the lining due to the inability of ceramic fiber to withstand any erosive environment containing particulate material. 

Much less is known about the movement of sands and clays in pipes than of silt in regular channels. In spite of the industrial importance of the subject, the amount of research done is extremely limited. While it may seem that fundamental information could easily be obtained from various installations, this is not generally the case. Wherever sands, clays, or other particulate matter are moved in pipes, little attempt is made to control the concentration of the mixture moved. When such material is moved from a vat or thickener, the most concentrated ma-trial is removed first. In other words, since the concentration of the material moved seems a most important variable at first, and since it usually cannot be controlled, there has been little incentive to experimentation. 

The presence of organic matter (especially particulate organic matter, POM, common in seawater) promotes the coagulation of colloids by serving as binding material that favors their aggregation. Nevertheless, tidal movements re-suspend these settled... 

---