3. APPLICABLE CONCEPTS AND TECHNOLOGIES
3.6 Factors Decreasing Decomposition Effectiveness
Various factors may decrease decomposition effectiveness of AOP and non-radical pathway decomposition processes.
3.6.1
Oxidant Scavengers
Oxidant scavengers discussed in this section include scavengers of hydroxyl radicals, ozone, and hydrogen peroxide. They are called scavengers because the will consume oxidants, resulting in lower decompositional effectiveness.
3.6.1.1 Carbonate and Bicarbonate
The detrimental impact of alkalinity (defined as the ability of water bodies to neutralise acids) on the effectiveness of AOPs has been comprehensively studied (Kommineni et al., 2001). Both alkaline carbonate and bicarbonate will scavenge hydroxyl radicals to create carbonate radicals which, in turn, react with other organic or inorganic compounds present, albeit at a much slower rate (Hoigné and Bader, 1983a and b; (Kommineni et al., 2001). The reaction for the scavenging of hydroxyl radicals by bicarbonate ions is shown below in Reaction 3-61 (Morel and Hering, 1993; Kommineni et al., 2001):
kOH·
The rate constants for the reactions of hydroxyl radicals with carbonate and bicarbonate at neutral conditions are 3.8×108 and 8.5×106 M-1sec-1, respectively (Buxton et al., 1988). Of relevance to the oxalate-based systems under study in this thesis is the observation that the rate constants in near neutral conditions (i.e. pH of around 7) are slower than the reaction rate constant of hydroxyl radicals with oxalate, 109 M-1sec-1 (Munter, 2001) at the same pH. At near neutral conditions, the hydroxyl radical reactions with carbonate and bicarbonate are second-order reactions. That is the reaction rate, r, is a function of the relevant rate parameter, kOH·, the hydroxyl radical concentration, [OH·], and the concentration of carbonate or
bicarbonate, [CO3 or HCO3]. Equation 3-1 shows the reaction of hydroxyl radicals with
carbonate or bicarbonate.
r = kOH·[OH·][CO3 or HCO3 ] (eq. 3-1)
Water can contain carbonate and bicarbonate ions at concentrations several orders of magnitude higher than oxalate and, thus, the reaction of hydroxyl radicals with carbonate and bicarbonate can proceed as fast as their reaction with oxalate (Munter, 2001; Kommineni et al., 2001). For oxalate decomposition, the scavenging effect of hydroxyl radicals by carbonate and bicarbonate can be significant. The potential significance is because carbon dioxide and water are the primary products associated with the oxidative mineralisation of oxalate (Davis
et al., 2009; Zuo and Deng, 1997; Lagunova et al., 2012; Minakata et at., 2011). Per Munter, 2001, CO2 readily dissolves in water, a process that becomes more favourable as pH increases with bicarbonate formation dominating at pH > 6.4 and carbonate formation predominating at pH > 10.3. Therefore, the significance of bicarbonate and carbonate as scavengers becomes even more substantial as the oxalate destruction proceeds and both the amount of CO2 generated and the pH of the slurry increases.
With the solubility of carbonate and bicarbonate exceeding that of ozone (Minakata et al.,
2011; Battino et al., 1983; Glaze and Kang, 1989), its effect could quickly dominate over other scavengers. In fact, when investigated by Battino et al., 1983; Glaze and Kang, 1989; Olson and Barbier, 1994; Chiang et al., 2006, it was concluded that carbonate actively lowers the TOC removal rates, while bicarbonate could completely inhibit TOC removal. To mitigate against this impedance, waters high in carbonate and bicarbonate ions can be:
1) Treated with carbon dioxide stripping (e.g. by sparging with oxygen) before AOP treatment (Peyton et al., 1998),
2) Administered higher concentrations of ozone (Kommineni et al., 2001; Munter, 2000), or
3) Exposed for longer ozonation times (Kommineni et al., 2001; Munter, 2000).
3.6.1.2 Nitrates, Nitrites, Phosphates, and Sulfates
Nitrite is another commonly identified scavenger for both hydroxyl radicals and ozone. Published rate constants for the reaction of nitrite with ozone and hydroxyl radicals under neutral conditions are 4×108 M-1sec-1 and 1×1010 M-1sec-1 respectively (Neta et al., 1988; Farhataziz and Ross, 1977) – highly competitive with the rate constant for the reaction of hydroxyl radicals with oxalate.
Phosphates, PO43-, and sulfates, SO42-, also have the potential to scavenge ozone and hydroxyl radicals. However, they are extremely slow in reacting with OH· (Gottschalk et al., 2000; Kommineni et al., 2001), and their scavenging effect can usually be neglected for ozone/peroxide/UV systems (Hoigné, 1998).
Since nitrate, nitrite, phosphates and sulfates all are present in HLW sludge, their concentration and potential scavenger impact to the ECC Process are evaluated in Chapter 4.
3.6.2
Impacts to UV Light Effectiveness
Since the conceptual design process for ECC discussed in Section 2.3 includes UV, impacts to UV light in both AOP and non-AOP water-treatment processes are investigated as part of the literature review.
Although different terms and quantifications are used by the various authors, the impacts could be group into two general categories.
The first impact category, entitled solution properties, includes factors directly associated with the characteristics of the solution being oxidised. The factors can be summarised as transmissivity, turbidity, and suspended solids, and metals concentrations (Karim and Gehr, 2001).
It is important to note that any UV light absorbing constituent present in solution undergoing ozonation will decrease the rate of formation of radicals, this includes the effects of common anions such as nitrite/nitrate.
The second category includes UV lamp sheath fouling, breakage, and ageing.
3.6.2.1 Solution Properties
Anion Concentrations
Nitrates and nitrites adsorb UV light in the range of 230 to 240 nm and 300 to 310 nm; consequently, high nitrate and nitrite concentrations (either >1 mg/litre) have been shown to limit the effectiveness of UV transmissivity (Calgon, 1996; Kommineni et al., 2001).
Water Quality Measured Properties
UV transmissivity (UVT), refers to the percentage of light that passes through a solution sample at a specific wavelength, normally 254 nm. It is usually reported for a path length of 1 cm. Related properties include turbidity, hardness, and pH.
Turbidity is a measure of the degree to which the water loses its transparency due to the presence of suspended particulates (Gottschalk et al., 2000). Similar to the effect from suspended solids, systems relying on UV irradiation for the dissociation of H2O2 or O3 exhibit a decrease in efficiency as turbidity increases. Turbidity lowers the transmittance of the light into solution and, thus, increased turbidity drops the penetration of the UV radiation into the water (Kommineni et al., 2001; Gottschalk et al., 2000; Avramescu et al., 2008).
pH, which affects the solubility of metals and thus compromises water clarity. The ideal value for suspended solids is < 10 ppm, for UVT is > 85% at a 254 nm wavelength (Kommineni et al., 2001).
3.6.2.2 Fouling, Breakage, and Ageing
Concerns associated UV system degradation include lamp fouling, subsequent lamp sleeve cleaning. Lamp breakage and ageing are other potential problems since UV intensity output decreases with time. Water quality parameters used as possible predictors of fouling were COD, suspended solids, temperature, pH, UV transmission, and metal concentrations, mainly Fe and Ca (Karim and Gehr, 2001).
According to Peng et al., 2005, a significant problem with UV disinfection of wastewater is the accumulation of fouling materials at sleeve-water interfaces. Even though techniques cleaning has been perfected in the last 20 years, some permanent fouling will occur. Although often automated, chemical and mechanical cleaning can remove most fouling materials
satisfactorily, permanent foulants, which cannot be wholly eradicated using conventional cleaning operations, remain on the quartz sleeves and reduce effectiveness (Peng et al., 2005).