A friend or foe to the groundwater industry?
By Roger Miller
In our world the reaction of “oxidation” can be found everywhere and the results can be beneficial in some cases and detrimental in others. Our knowledge and understanding of oxidation in groundwater can guide us in decisions such as design parameters, material selection, and operational controls.
The original definition of oxidation provided by the scientific community was: “the union of a substance with oxygen.” Over time with more study of various aspects of chemistry, it was determined the reaction of oxidation did not always involve the element oxygen and the definition was altered to its current form: “a chemical reaction in which an element or ion is increased in positive valence, losing electrons to an oxidizing agent.”
To consider the action of transferring electrons, scientists established the “oxidation state” of known elements. Therefore, as elements reacted with each other and electrons were transferred, the oxidation state of the elements would change. As electrons are negative in value, the loss of electrons would
increase the oxidation state; the gain of electrons, termed as reduction, would decrease the oxidation state. Understandably, there cannot be oxidation without reduction and this aspect of chemical reactions is referred to as “redox.”
One of the most common oxidation reactions involving the element oxygen is found in combustion or fire, which generates heat and light from the rupture of the electron bond. The smoke is the oxidized product of the reaction.
Another common one is respiration, the reaction where animals inhale oxygen and exhale carbon dioxide and use the oxygen to oxidize organic nutrients (food), yielding energy.
Along with these known reactions are a multitude of other oxidation uses such as hydrogen burning in the presence of chlorine, liberating heat and light just like fire, and producing hydrogen chloride (hydrochloric acid)—which we use in the groundwater industry every day.
However, the most notable of the oxidation reactions in groundwater activities are corrosion and disinfection.
Corrosion is the deterioration of a material due to the interaction with its environment. Many forms of corrosion that are observed in the groundwater industry are attributed to the electron transfer process and the associated movement of metal ions from one location to another, producing metal deterioration.
General or uniform corrosion proceeds uniformly over the entire surface of the metal exposed. The mechanism of attack is the electrochemical process at the surface of the metal. Small differences in metal composition can create anodes and cathodes that facilitate the corrosion process.
The basis of many forms of corrosion is the electrochemical reaction involving an anode and a cathode, and an electrolyte producing the components for ion transfer. In a groundwater well system, the anode and cathode are provided by the various metals of construction and the electrolyte is the water.
Metals are arranged into a series with the less noble metals at the top and the more noble ones at the bottom. This type of arrangement is known as the galvanic series (Figure 1). In the electrochemical reaction, the less noble metals will become the anodic site and sacrifice their metal ions to the more noble metals at the cathodic site.
As noted in the series, low carbon steel is less noble than stainless steel and will sacrifice ions to the stainless steel cathodic site if the electrolyte—water—provides an adequate electrical connection. This activity is referred to as dissimilar metal corrosion and can be addressed in the material selection
process of the well design.
Concentration cell corrosion, also referred to as under deposit or oxygen cell corrosion, actually occurs in the absence of oxygen. This electrochemical reaction develops underneath deposit buildup on the metal surface and produces a localized pitting damage. This type of corrosion is the most rapid and
damaging to the well structure and equipment and is greatly influenced by the ion concentration of the aquifer water. Understandably, periodic removal of deposits from the metal surfaces within the well will reduce the occurrence of this type of corrosion.
Microbial influenced/induced corrosion is material degradation caused by bacterial activity. This includes their production of acids and enzymes, pitting, and the most common degradation by iron oxidizing bacteria. In relationship to the prior-discussed concentration cell or under deposit corrosion,
the predominant cause of this type is bacterial growth. As a residual problem within the water well environment, it is not only the material degradation of iron oxidation but also the accumulation of this oxidized iron into the bacterial slime and fouling the flow paths within the well structure.
Within the groundwater industry we have standardized our initial evaluation of corrosion potential on the saturation index of the aquifer water. The Langelier Saturation Index is the most common calculation used. However, the Ryznar Index is similar and uses the same set of parameters to make
the evaluation, but Dr. John Ryznar modified the Langelier calculation based on actual field results he studied over time.
The end result of these calculations is a determination if the water is saturated and will form mineral scale, or if it is undersaturated and will be aggressive or corrosive. With the understanding these saturation indexes require laboratory analysis to generate some of the parameters required by the
index calculation, a fast and simple field test can be performed to assess general corrosion of the aquifer water, the Nail Test.
This test uses a low carbon nail placed in a glass container of aquifer water and observed for 24 hours. If a pink color develops within the water or around the nail in the first three to five hours, the water is determined to be fairly corrosive.
Disinfection is the application of energy or chemistry to kill pathogenic organisms. The need for this process was initiated around 1900 when the science community established the cause of worldwide epidemics of typhoid fever, cholera, and dysentery was waterborne bacteria. The response to this
need was the application of oxidation chemicals.
Although there are a variety of biocidal chemicals available, not all are applicable to the potable water industry and therefore oxidation chemicals have evolved as the standard form of water disinfection. The oxidation chemicals seen in the groundwater industry to varying degrees are chlorine dioxide, hydrogen peroxide, potassium permanganate, bromine, and chlorine.
Chlorine dioxide has actually been used in water treatment in the United States since the mid-1940s and provides a much stronger oxidation than the more common chlorine. However, chlorine dioxide must be generated at the point of demand and most conventionally performed in a vessel or generator requiring
a controlled chemical reaction. Although chlorine dioxide does not produce disinfection byproducts and shows superior biofilm penetration, its use throughout the industry has been limited potentially from the site generation requirement and the chemical’s short half-life.
Hydrogen peroxide is the strongest oxidizer used in water disinfection when in its acid form of peracetic acid. However, it is not a strong biocide and is not approved as a stand-alone disinfectant in the U.S. and therefore is commonly found in mixed oxidation technologies. Additionally, the release of nascent
oxygen into the well environment is a detriment to its use in well disinfection.
Potassium permanganate is considered a strong oxidizer and is most effective on iron and manganese, creating an insoluble particulate. Although there are no disinfection byproduct issues with potassium permanganate, it is not considered an effective biocide, and with the strong staining potential of the resulting solution when mixed with water, it is not normally used in well disinfection.
Bromine is a halogen chemistry like chlorine but is found to be more effective as a disinfectant at higher temperatures where chlorine compounds are less stable. Bromine is not used for disinfection of water wells, but is used in the drilling process to create high density drilling fluids.
Chlorine, the most available and cost effective oxidation chemistry used for water disinfection, is available in many forms. The initial use back in the early 1900s was chloride of lime or calcium hypochlorite, with liquid sodium hypochlorite being used more in groundwater today. Of course, gas chlorine is still used in many water treatment plants with the addition of ammonia forming chloramines to reduce the creation of unwanted disinfection byproducts.
Regardless of what type of chlorine used when added to water, they all form hypochlorous acid and hypochlorite ions. The hypochlorous acid is the most biocidal form and is highly dependent on pH values near neutral to maintain its existence. In our industry today, there are a variety of chlorine enhancing chemistries designed to buffer the pH to the desired values and providing the most effective disinfection activities.
An additional oxidation chemical used in groundwater, but one that is not a disinfectant, is persulfate. This oxidant is most effective in degrading chlorinated solvents and petroleum products, even the difficult carbon tetrachloride. Persulfates have been used for remediation of contaminated soils through the process of in situ chemical oxidation.
Although the measurement and evaluation of disinfection potential within our industry has been accomplished by the value of “free total chlorine,” the value can be compromised with changes in pH value and reaction with amines, contributing to chlorine values but with little oxidizing capacity.
To this point the water industry has accepted the value of oxidation reduction potential (ORP) for assessment of biocidal capabilities. The ORP value is an equilibrium between the various forms of chlorine and is regardless of the pH value. The World Health Organization recognized in 1972 an ORP
value of 650 mV provided instantaneous disinfection of viral bacteria. The ORP value in current water technologies is used for monitoring and controlling many aspects of water treatment and corrosion control processes, providing a valuable tool to our industry.
The oxidation reaction is a friend to the groundwater industry—most prominently with the process of disinfection and residual benefits of general corrosion providing some metal passivation, effervescences of the reaction providing agitation during disinfection, and the contaminant removal process using persulfates.
On the other hand, the oxidation reaction is a foe to the industry in the degrading of well systems through corrosion, the accumulation of iron oxides in well fouling, the potential encrusting of biofilm from excess oxidation, and the potential formation of disinfection byproducts.
Roger Miller serves as a senior consultant for Water Systems Engineering Inc. His work over the past 30 years has involved research and development, analytical procedures, site assessment, and project oversight in the groundwater industry.