Protective colloidal contaminants

Whatever the technology used, and regardless of any treatment used preparatory to bitartrate stabilization, wine treated with artificial cold must be clean, i.e. not excessively contaminated with yeast or bacteria, as is often the case with wines stored in large vats. These wines should, therefore, be filtered on a simple continuous earth filter. Another advantage of filtration is the elimination of part of the protective colloids. Fine filtration is not useful at this stage, and is certainly not recommended, as there is a risk of eliminating microcrystals likely to act as crystallization nuclei. 

A. Irreversible Colloids. — Some irreversible colloids are sensitive to electrolytes, while others remain unaffected. Some degree of r u-larity of behavior is manifested only by colloidal metals (free from contamination with other colloids), salts, and sulfides. Here the law obtains with a fair amount of certainty, that electrolytes cause precipitation. In general very small concentrations of salts, bases, and acids suffice to coagulate the colloid in question. The precipitate is not soluble in water. Nonelectrolytes, on the contrary, usually do not cause coagulation. The sensitiveness may be completely destroyed by the addition of often very small quantities of protective colloids. Even traces of these last-named substances may influence the reaction to a considerable degree. 

High levels of rot in the harvest increase juice turbidity and make clarification difficult, due to the protective colloidal effect of glucan produced by Botrytis. A low concentration of rot (less than 5%) tends to facilitate juice clarification, due to a pectinase activity in contaminated grapes that is nearly 100 times higher than in healthy grapes. 

PVP, a water soluble amine-based pol5mer, was found to be an optimum protective agent because the reduction of noble metal salts by polyols in the presence of other surfactants often resulted in non-homogenous colloidal dispersions. PVP was the first material to be used for generating silver and silver-palladium stabilized particles by the polyol method [231-233]. By reducing the precur-sor/PVP ratio, it is even possible to reduce the size of the metal particles to few nanometers. These colloidal particles are isolable but surface contaminations are easily recognized because samples washed with the solvent and dried in the air are subsquently not any more pyrophoric [231,234 236]. 

Principally purification and characterization methods of monometallic nanoparticles are directly applied to those of bimetallic nanoparticles. Purification of metal nanoparticles dispersed in solution is not so easy. So, in classical colloid chemistry, contamination is carefully avoided. For example, people used pure water, distilled three times, and glass vessels, cleaned by steam, for preparation of colloidal dispersions. In addition, the reagents which could not byproduce contaminates were used for the preparation. Recently, however, various kinds of reagents were used for the reaction and protection. Thus, the special purification is often required especially when the nanoparticles are prepared by chemical methods. 

In addition, bacterias need to be removed from well, river or lake water before water can be rendered potable. A properly selected microfiltration ceramic membrane is effective for bacteria decontamination. When used preceding a reverse osmosis or an ion-exchange unit, the microflltiaiion membrane protects the downstream separation process from bacteria contamination and possible fouling due to colloids. 

The metals have the tendency to form compounds of low solubility with the major divalent cations (Pb, Cd being found in natural water. Hydroxide, carbonate, sulfide, and, more rarely, sulfate may act as solubility controls in precipitating metal ions from water. A significant fraction of lead and, to a greater extent, cadmium carried by river water is expected to be in an undissolved form. This can consist of colloidal particles or larger undissolved particles of lead carbonate, lead oxide, lead hydroxide, or other lead compounds incorporated in other components of surface particulate matter from runoff. The ratio of lead in suspended solids to lead in dissolved form has been found to vary from 4 1 in rural streams to 27 1 in urban streams. The US Environmental Protection Agency (USEPA) has reported Maximum Contaminant Levels in water that are permissible to be 0.005 m L for cadmium and 0.015 mg/L of lead. ... 

Tn the scientific and engineering community, concern for protection of aquatic environments from chemical pollution is shared by individuals from diverse disciplines. An area of overlapping interests is the study of particulates in water, their characterization, their interaction with the solution phase, and their control. Because many chemical and microbiological water contaminants are associated with colloidal or suspended particulate matter, a symposium on this subject seemed timely to provide a forum for various specialists to exchange ideas, methods, and models used to investigate the fate and effects of particulates and their associated materials in various aqueous environments. 

Puls, R.W., J.H. Eychaner, and R.M. Powell. 1990. Colloid-facilitated transport of inorganic contaminants in groundwater Part I. Sampling considerations. Environmental Research Brief, EPA/600/M-90/023. U.S. Environmental Protection Agency, Washington, DC. 

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