User:Kelseymflanagan/Silver Nitrate as a Disinfectant

Introduction
Silver nitrate can be used in water treatment to inactivate microorganisms posing a threat to the health of consumers. Much research has been done in evaluating the bactericidal ability of the silver ion against Escherichia coli, a microorganism commonly used as an indicator for fecal contamination and as a surrogate for pathogens in drinking water treatment. Concentrations of silver nitrate evaluated in inactivation experiments range from 10-200 micrograms per liter as Ag+. The germicidal properties of silver nitrate have been observed for thousands of years and scientifically studied for over a century. The biocidal ability of the silver ion is put to wide variety of uses including use as a water disinfectant in hotels and hospitals, as a part of point-of-use water filtration systems, as a preservative for cosmetics, as an alternative to laundry detergent, and as a coating on catheters to prevent infection.

Applications
The antimicrobial ability of silver nitrate has been put to many uses, including:
 * 1) Water disinfection in hotels and hospitals
 * 2) Postharvest cleaning of oysters
 * 3) Inhibition of bacterial growth on chicken farms
 * 4) Water recycling aboard space shuttles
 * 5) Home purification of water in US
 * 6) Point of use disinfectant for water and vegetables in Mexico
 * 7) Alternative to antibiotics (not recommended by FDA)
 * 8) Alternative to laundry detergent
 * 9) Application to eyes of newborn babies to prevent infection
 * 10) Coating on catheters to prevent infection

Kinetics
It is well-documented that the silver ion is effective in the inactivation of Escherichia coli, also referred to as E. coli. ,, , , , , , , , , , . However, there are many inconsistencies in the literature regarding the kinetics of the inactivation of E. coli by the silver ion. These inconsistencies may be due to several factors. First, the kinetics may depend on the source of the silver ion being used. In recent years, research has focused largely on electrolytically generated silver ions or colloidal silver. Most studies in which the inactivation kinetics of E. coli by silver nitrate were explored extensively date back several decades. Even within this smaller group of studies, vast inconsistencies exist, likely due to inaccurate analytical methods for measuring the concentration of silver in solution. Monitoring the decay of the silver ion in solution is imperative as silver tends to both adsorb readily to organic matter in the water and to be light reactive. Furthermore, silver tends to adsorb to glassware, which can lead not only to a decrease in the silver concentration within a given experiment but also to a release of the silver in subsequent experiments unless measures further than general glassware washing are taken for the removal of silver from the glassware surface. Therefore studies must both minimize the external factors effecting the concentration and to measure the changes in concentration that take place throughout the experiment.

Effects of Various Parameters
Despite the inconsistencies in the literature regarding the kinetics of the inactivation of E. coli by silver nitrate, important information can still be taken from the work. A study by Wuhrmann and Zobrist investigated the effect of various parameters upon the kinetics. First, they studied the effect of several ions in the water, including calcium, phosphates and chloride, all of which were found to decrease the bactericidal effect of silver. These effects are important to consider when designing an experiment. Because of the effect of phosphates, it is undesirable to use phosphate buffer to run experiments, as this creates a phosphate concentration much higher than that found in natural waters and will falsely slow the inactivation kinetics. Furthermore, it is important to avoid touching any glassware with bare hands, as chloride from sweat may contaminate the glassware, again slowing inactivation. Chambers, Proctor and Kabler established the importance of using an effective neutralizer solution made of a combination of sodium thioglycolate and sodium thiosulfate, rather than sodium thiosulfate alone, which though it is effective in neutralizing other disinfectants does not sufficiently stop the bactericidal action of silver nitrate. Both tested the effect of pH on the kinetics, finding that a higher pH increased the bactericidal action. , Wuhrmann and Zobrist further established that at a higher temperature, inactivation occurs faster.

Kinetic Models
A further complication of the inactivation kinetics by silver is the question of which model to use. With most disinfectants, the inactivation is effectively modeled using a first-order Chick-Watson model, which states that a certain level of disinfection will occur at a certain CT, or concentration *time value. According to this model, the same amount of inactivation should take place when a concentration of 0.2 mg/L is applied for 10 minutes as when 0.02 mg/L is applied for 100 minutes. Wuhrmann and Zobrist found rate kinetics that followed this model for all conditions, which agrees fairly well with a study by Chambers and Proctor, while another study by Renn and Chesney found curves that did not follow this law. It is therefore unclear whether this law sufficiently models inactivation by the silver ion.

Most recent papers regarding the disinfection of E. coli by silver nitrate have simply plotted the level of disinfection against time. ,, , , While this method of data analysis does not risk making false assumptions about first-order kinetics, it does nothing to account for the applied concentration, which is essential to any kinetics. Therefore, different curves need to be generated for each concentration that might be applied. Furthermore, it does not account for changes in concentration that might take place during the experiment, and which may vary based on many factors.

A third model which has been suggested for the inactivation kinetics by silver nitrate is that of Cs*T, or chemisorbed silver onto the cell body times time. This model suggests that the rate of inactivation depends not on the concentration in the water at a given time, but rather on the silver that has been chemisorbed by the bacteria. It is assumed, according to this model that C0 = C1 + C2 + C3, where C0 is the initial concentration, C1 is the silver still in solution, C2 is the silver lost to adsorption to glassware or other factors in the solution, and C3 is the silver chemisorbed to the bacteria. C0 is measured at the beginning of the experiment, C1 is measured throughout the experiment, and C2 is determine in a control experiment without bacteria. C3, or the Cs value, is then determined to be C0-C1-C2. , According to Hwang, et al., this model was successful in estimating inactivation of E. coli by silver nitrate. Although it is possible that this model does not sufficiently account for all of the possible fates of the initial silver nitrate added to the solution, it is certainly a compelling method of data analysis. Because it is a new model, it has not been extensively studied by various researchers.

Health Risks
Although silver nitrate is non-toxic and is currently not regulated in water sources by the Environmental Protection Agency, when between 1-5 g of silver have accumulated in the body, a condition called argyria can develop. Argyria is a permanent cosmetic condition in which the skin and internal organs turn a blue-gray color. The United States Environmental Protection Agency had a maximum contaminant limit for silver in water until 1990, but upon determination that argyria did not impact the function of organs effected, removed the regulation. Although this condition is a small threat when application of silver is on the order of micrograms per liter, it is certainly a reason to limit the concentration of silver to a low level.