Project Title: Effect of Bicarbonate Alkalinity on Performance of Advanced Oxidation Processes [Project 533]

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Harry Bond
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1 Project Title: Effect of Bicarbonate Alkalinity on Performance of Advanced Oxidation Processes [Project 533] Principal Investigator: Gary R. Peyton, Illinois State Water Survey Objectives: Preliminary calculations indicated that carbonate radical may participate in organic compound removal during AOP application, and that in some cases, this participation might be further enhanced by relatively minor adjustments to the water chemistry, during a pretreatment step. The objectives of this project were to investigate those effects, develop a model for prediction of the effects on organic compound removal efficiency under drinking water treatment conditions, determine changes in by-product formation that could be expected under conditions of high alkalinity, and to determine the significance of these effects in drinking water treatment. Background: The advanced oxidation processes (AOPs), which utilize hydroxyl radical for the destruction of organic compounds, represent attractive drinking water treatment alternatives, because they are generally effective for the removal of most organic compounds found in water and do not leave chlorinated by-products. Hydroxyl radical is most frequently generated by the composition of ozone during the ozonation of water. Because ozone and hydroxyl radical are so reactive, they also react with other nontarget material in the water, reducing the efficiency of attack on the target organic compound and thereby increasing the amount of ozone required for treatment. It is widely recognized that the presence of high carbonate-bicarbonate alkalinity in the water to be treated may result in lowered organic compound removal efficiency, due to scavenging of the hydroxyl radical (OH) by carbonate and bicarbonate ions. However, another free radical, carbonate radical, is formed during the scavenging reaction, and its effects on organic compound removal and by-product formation were not known prior to this project.

2 Highlights: In the general case, enhancement will not typically occur during AOP water treatment. However, exceptions to the typical cases can occur and the criteria in this report should be applied specifically for cases of interest. Step-by-step procedures for application of the criteria to specific water treatment scenarios are given in Chapter 7, Recommendations to Utilities and Procedures for Calculating Alkalinity Effect for Specific Waters

3 Approach: The effect of carbonate-bicarbonate alkalinity on the efficiency of, and byproducts from, treatment using advanced oxidation processes (AOPs) was investigated theoretically and in the laboratory. A mathematical model for AOPs was developed. This kinetic model was used to predict conditions under which organic compound removal would be aided by carbonate radical reactions. A step-by-step procedure was developed wherein criteria can be successively applied to a water treatment case of interest to determine whether enhancement of treatment (beyond that afforded by hydroxyl radical alone) is already occurring in a given instance or can be promoted by pretreatment such as carbonation, decarbonation, or removal of dissolved organic carbon (DOC). By-product studies were conducted for one aliphatic compound (ethanol) and three aromatic compounds (phenol, aniline, and benzoic acid) in order to determine differences that may be expected between hydroxyl radical and carbonate radical products formed during AOP treatment. Results/Findings: It is well known that alkalinity and Natural Organic Material (NOM) can have a detrimental effect on AOP treatment due to scavenging of the hydroxyl radicals by carbonate or bicarbonate ions and NOM. For most AOP water treatment operations, this will be the most significant effect of carbonate-bicarbonate alkalinity on capital and operating costs. The report shows this effect as a function of bicarbonate alkalinity and dissolved organic carbon (DOC) content of the water. When AOPs are applied to natural water to destroy an organic compound that is present at relatively low concentration, the process is usually quite inefficient. This report contains information that can be used to estimate the economic impact of decarbonation or ph change on AOP water treatment, by calculating the decrease in the capital and operating costs that result from a decreased ozone generation capacity requirement.

4 Several criteria were developed for use in determining whether successful enhancement could occur as a result of carbonate radical reactions. Enhancement simply means the contribution of carbonate radical to organic compound removal, whether already occurring in an existing AOP treatment system or predicted to occur after some proposed pretreatment step. Determination of whether the criteria for enhancement are satisfied consists of substituting water quality information (i.e., solute concentrations), treatment goals (e.g., initial and final concentration of organic compound, maximum practical oxidant dose), and reaction rate constants for the solutes into the mathematical expressions derived for the criteria. The following general observations were made in the project: Does Enhancement Occur? Substitution into the Enhancement Factor equation indicated that enhancement can typically still occur even if the rate constant kcr for reaction of carbonate radical with organic compound is about three orders of magnitude smaller than its hydroxyl radical rate constant, kor. Can the desired treatment be performed with a reasonable ozone dose? Substitution into the Limit of Reasonable Efficiency (LRE) equation indicated that for typical waters, a value of kcr(106 is required. Thus, this is a more stringent requirement in some cases, and 1,1,1-TCA enhancement would not occur even at a rather high ozone dose of 10 mg/l. Is removal by carbonate radical significant compared to removal by ozone? For typical water quality, it was found that the value of kcr must be 2 to 3 orders of magnitude larger than the ozonation rate constant, koz,r for reaction with carbonate radical to be significant compared to destruction by ozone. The results of the by-product experiments indicated that carbonate radical reactions could result in lower by-product concentrations by preferentially destroying byproducts at the expense of parent organic compound destruction. Coupling of byproducts leads to polymeric material which, because of its size, may not be easily

5 assimilated by organisms and may escape analytical detection. Coupling can also remove by-products from bioavailability by binding them to the aromatic groups of the natural organic material that is present in raw water. Impact: Probably the most significant use to which this study s findings will be applied is the improvement of AOP treatment efficiency by the reduction of the alkalinity of the water to be treated. Dramatic savings in oxidant costs may be realized by removal of bicarbonate alkalinity and/or conversion of carbonate alkalinity to bicarbonate alkalinity. The sensitivity of the treatment efficiencies to DOC content of the water indicates that AOP treatment to remove organics should follow the flocculation and filtration steps, and that small improvements in DOC removal by flocculation and filtration may also result in significant improvement in the economics of AOP treatment following filtration. Although enhancement of organic compound removal by reaction with carbonate radical is possible in principle, several factors combine to prevent the effect from being of much significance in the range of conditions that are most typically encountered during drinking water treatment using ozonation-base AOPs. There may, however, be exceptions to this general case, and the step-by-step criteria provided in this report should be applied to specific cases of interest, when considering pretreatment to promote enhanced organic compound removal.

Kinetic studies using UV-VIS spectroscopy Fenton reaction

Kinetic studies using UV-VIS spectroscopy Fenton reaction Abstract The goal of this exercise is to demonstrate the possibility of using a modern in situ spectroscopic method (UV-VIS spectroscopy) to investigate