Precipitation, surface

This case study involves an unnamed petrochemical plant located about 15 miles inland from the Texas Gulf Coast, described by Donaldson and Johansen.184 It illustrates two approaches to injecting incompatible wastestreams to prevent well plugging by precipitation surface treatment and multiple injection wells. 

An autocatalytic reaction is one promoted by its own reaction products. A good example in geochemistry is the oxidation and precipitation of dissolved Mn11 by C>2(aq). The reaction is slow in solution, but is catalyzed by the precipitated surface and so proceeds increasingly rapidly as the oxidation product accumulates. Morgan (1967) studied in the laboratory the kinetics of this reaction at 25 °C and pH > 9. 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

Astringents such as tannic acid (home remedy black tea) or metal salts precipitate surface proteins and are thought to help seal the mucosal epithelium. Protein denaturation must not include cellular proteins, for this would mean cell death. Although astringents induce constipation (cf AP salts, p. 166), a therapeutic effect in diarrhea is doubtful. 

It precipitates surface proteins at ulcer base and act as a physical barrier, preventing acid, pepsin and bile from coming in contact with ulcer base. It also augments gastric mucosal PG synthesis thereby enhancing protective action. It has no acid neutralizing action. 

Solidification with cement generally is accomplished with a Portland cement and other additives. The quantity of cement can be varied according to the amount of moisture in the waste. Heavy metal cations in the waste form insoluble carbonates and hydroxides at the high pH of the mixture. The surface of the hardened mass can be coated with asphalt or other material to reduce leaching of hazardous components. If the waste is mixed with anhydrous cement and water there is the possibility of ions incorporation in the cement structure during the hydrolysis process. Heavy metal ions could bind with the cement by the process of chemisorption, precipitation, surface adsorption,... 

We use here the term sorption as the retention of a compound on the surface of a solid particle that removes it from the aqueous medium. This phenomenon affects the composition of water by transferring the compound or ion from the aqueous medium to a solid (mainly a sediment in suspension or a colloid). Then, it may no longer be present in water, especially if the sediment settles. Sorption may be identified and associated with adsorption, surface precipitation, surface complexation, and/or ion exchange (or even absorption). 

Accurate comparison of results requires knowledge of reaction site density per unit surface area. Calcite materials used for kinetic study have included natural marbles, limestones, hydro-thermal crystals of Iceland spar, tests of calcareous organisms and laboratory and commercial precipitates. Surface areas, estimated by BET methods and graphical methods (based on particle size distribution) range from about 0.005 to 2 m g . There are apparent discrepancies between graphical and BET surface areas and the question is raised as to which type of surface area estimate is most representative of the reacting surface area. 

A chief goal of this book is to help the reader understand controls on the chemical quality of surface-and subsurface-waters, both pristine and polluted. The focus is on inorganic processes and on the chemistry of soil and groundwaters, with less said about the chemistry of precipitation, surface-waters, or the ocean. The book leans heavily on the principles of chemical thermodynamics and the concept of chemical equilibrium. Chemical equilibrium, whether attainable or not, represents the reference state for purposes of explaining the concentrations of aqueous species in the hydrosphere. Concepts of chemical kinetics are introduced when they are known and seem applicable. 

Surfactants used to characterize the main properties of the precipitate surface i.e. the specific surface area and heterogeneity are 1 sodium dodecane sulfonate-SDS (ref. Merck 12146) with a Krafft point of 31°C and solubility of 6.5T0 M [8] in water at 25°C and 1 sodium tetradecane sulfonate -STS (ref. Merck 12362) with a Krafft point of 46°C and solubility of 1.36T0 M [8] in water at 25°C. 

Precipitate flotation is applied in the analysis of natural waters. Trace ions in an aqueous solution are co-precipitated with colloidal metal hydroxide collectors and floated with the aid of a gas stream [94-96]. Tiny gas bubbles are trapped in the interstitial spaces and on the surfaces of the precipitates to give sufficient buoyancy. Surfactant ions having the charge opposite to the precipitate surfaces are used to make the surface hydrophobic. Another important role of the surfactants e.g., sodium oleate, sodium dodecylsulphate) is to form a stable froth layer to support the precipitate on the solution surface, which is important for complete separation of the precipitate. 

Thus in contact with aluminium and ferric phosphates, the aluminium and ferric ion concentrations are reduced and so the phosphate ion concentration is increased to maintain the solubility product at their constant levels. AIPO4 represents various hydrated and hydroxyl phosphates of aluminium, including any adsorbed or precipitated surface layers on oxides and alumino-silicates. FePO similarly, represents various hydrated and hydroxyl phosphates of iron including adsorbed or precipitated surface layers on iron oxide. 

From the viewpoint of water management the use of pesticides is a serious danger particularly when these substances are introduced into precipitation, surface, ground-, drainage or wastewaters. The cases of water contamination are often due to the neglect of essential safety measures during their application, purification and decontamination of machines used for their application and transport, and aerial application, particularly in unsuitable periods. 

The main source of drinking water comes from atmospheric precipitations, surface and ground inflow and capillary elevation of the groundwater. The amount of water taken up, its binding and motion depend on the relationship between the soil and water, which involve the statics and dynamics of the soil water. The statics of the soil water characterizes the content of water in the soil, expressed by the moisture and water capacity of the soil. 

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