Biocides, oxidizing chlorine

Liang et al. (2006) ° developed a novel hydantoinylsiloxane that could be used to functionalize silica gel for use as a biocide upon exposure to dilute household bleach. The hydantoirtylsiloxane could be used in either monomeric or polymeric form. The authors covalently bonded hydantoinylsiloxane to the silica gel, and proved that the coated silica gel particles, upon exposure to household bleach, were able to inactivate Staphylococcus aureus and Escherichia coli. The authors also claim that the coatings are stable to flowing water and can be rechlorinated if the oxidative chlorine is lost during activity. Such coatings could be conveniently employed in protective fabrics. ... 

The first point to emphasize is that crevice corrosion of passive alloys does not occur in environments deprived of oxidizing species other than water. Generally, it does not occtrr in deaerated solutions. On the contrary, oxidizing agents such as many biocides (hypochlorite, chlorine, chlorine dioxide, etc.) may catrse crevice corrosion. 

Chlorine Tolerance Most of the best RO membranes are attacked by oxidants, and they are particularly susceptible to chlorine. A particularly sensitive locus for attack is the amidic hydrogen. Cellu-losic membranes are generally less sensitive, and pass the chlorine into the permeate giving downstream biocidal activity, veiy useful for under-the-sink RO. These factors are largely responsible for CA s survival in RO membranes. Chlorine, whatever its vices, has the virtue of being a known, effective, residual bactericide and a good inhibitor of... 

Chlorine is desirable as a bulk pretreatment biocide for inlet water, but its subsequent removal upstream of the membrane is absolutely necessary ana difficult. NaHSO,3 is a common additive to dechlorinate before membranes. It is customarily added at 3-5 mg/1, an excess over the stoichiometric requirement. NH3 is sometimes added to convert the chlorine to chloramine, a much less damaging biocide. Heavy metals present in seawater seem to amplify the damaging effects of chlorine and other oxidants. 

One will loose biocidal activity due to the presence of iron and manganese in the pool water because chlorine and chlorine related products will oxidize these cations ... 

PHMB is very toxic to fish and aquatic life. It is moreover irritating to skin and may cause sensitization by skin contact. It can cause irritation to the eyes, nose and respiratory tract. The PHMB is not compatible with most common swimming pool chemicals. Not compatible with chlorine and chlorinated chemicals and bromine donors. Not compatible with ionic sterilizers, copper based QAC-algicides, anionic detergents, water softening chemicals, persulfate oxidants etc. The defence of the inventors of PHMB is that one should not combine it with other biocides because it should be a bactericide/algicide. But the algicidal properties of PHMB are very weak in brochures and manuals the dose is 200 ppm. 

Resistance to antimicrobial agents is of concern as it is well known that bacterial resistance to antibiotics can develop. Many bacteria already derive some nonspecific resistance to biocides through morphological features such as their cell wall. Bacterial populations present as part of a biofilm have achieved additional resistance owing to the more complex and thicker nature of the biofilm. A system contaminated with a biofilm population can require several orders of magnitude more chlorine to achieve control than unassociated bacteria of the same species. A second type of resistance is attributed to chemical deactivation of the biocide. This deactivation resistance to the strong oxidizing biocides probably will not occur (27). 

The method of addition is not conducive to good control, and therefore this oxidizer is suitable only for the smallest cooling systems. Due to the high chlorine content and rapid solubility, care should be exercised to ensure that the materials of construction can tolerate this biocide. (Often some galvanized packaged towers and plastic bottle towers are unsuitable.) Peak free chlorine reserves can easily exceed 2 to 3 ppm CI2 unless good control is exercised. 

Where chlorine dioxide is used in large process cooling systems, it is not uncommon for significant levels of sulfides to be present in the cooling water system. A benefit of chlorine dioxide biocide is its additional ability to readily oxidize this sulfide however, good monitoring and control is necessary, as high initial sulfide levels can result in mineral acid formation and the potential for corrosion to occur ... 

Nevertheless, chlorine dioxide does have a valuable role to play. It is a useful chemical tool for solving particular, tough, combined biocidal and oxidation problems. 

In practice, the required ratio can be different from theoretical, as quite often additional bleach is required to provide HOC1 as an oxidant for algal slimes and other forms of chlorine demand. Also, it is necessary to have a permanent source of oxidant available to effect the promotion of HOBr. However, not all the available bromine generated is lost by biocidal reaction or by (limited) volatility. There is, in fact, some degree of recycling of the bromide ion (Br ) back into HOBr, so monitoring of bromine plus the combined free and total chlorine is necessary to strike the correct halogen balance. 

Biocide/biostat programs based on sodium bromide oxidation provide a much safer option to cooling system operators than gaseous chlorine and a lower cost option compared with the addition of bleach (also the total volume of liquid consumed is decreased compared with bleach). 

As discussed in the previous section, the powerful oxidizing properties of chlorine and the other biocides are widely employed to kill and dislodge... 

Chlorine (as a biocide/biostat and to oxidize process contaminants) Note pH over 8.0, which limits the efficiency of chlorine. 

Basic problem of poor biological control. The pH level of the recirculating water is too high for efficient use of chlorine. Much of the chlorine is used as a process contaminant oxidizer and not as a biocide. No biodispersant is used. Use of DBNPA was a poor choice. Most of the biocidal duty falls on isothiazoline, but even this cannot effectively deal with algae, given that the pH is over 8.0 and the weather is extremely sunny. 

During an on-line clean, the quality of the clean will normally be enhanced by the use of an initial chlorine dosing procedure at a pH of 7.2 to 7.6. This practice is to be recommended, irrespective of whether the current cooling system maintenance biocide program employs chlorine. With severely fouled cooling systems, it may be necessary to provide many hundreds of ppm of HOC1 in order to satisfy the oxidation demand. This in turn may require the use of a temporary, supplementary corrosion inhibitor, and almost certainly the use of some antifoam. 

Typically limit to 0.5 ppm total Fe maximum in the cooling water. Above this level, phosphonate can be used to control problems initiated by iron. However, if the total Fe rises to 3.0 ppm, adsorption of the phosphonate onto the iron takes place and a loss of inhibitor function can occur. Iron salts present a serious fouling risk in cooling systems. Dissolved iron quickly oxidizes (especially where chlorine, bromine, or other oxidizing biocides are used) and forms ferric oxide/hydroxide, which acts as a flocculant,... 

Uncooked starch suspensions can easily spoil. If suspensions are to be held for more than a few hours, a preservative (biocide) needs to be added. Some commonly used reagents, such as chlorine or sodium hypochlorite, will attack (oxidize) starch. This reaction has to be considered when choosing a preservative. 

Microbial fouling is best dealt with before biofilm becomes mature. Biofilm protects the microorganisms from the action of shear forces and biocidal chemicals used to attack them. Microbes can be destroyed using chlorine, ozone, ultraviolet radiation, or some non-oxidizing biocides (see Chapters 8.2.1,8.2.2, 8.1.8, and 8.2.5, respectively). An effective method to control bacteria and biofilm growth usually involves a combination of these measures. Specifically, chlorination or ozonation of the pretreatment system, followed by dechlorination to protect the membranes, or UV distraction followed by periodic sanitation with a non-oxidizing biocide used directly on the membranes. 

Biocides are used widely in industry. There are at least three main classes of industrial chemical biocides. The first class includes the oxidizing and bleaching agents, such as chlorine dioxide, hydrogen peroxide, and sodium hypochlorite. The oxidizing action may directly kill bacteria or fungi or weaken the cell walls so that they are more susceptible to other classes of biocides (see below). Sodium... 

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