Presence of carbonate species - Bicarbonate and carbonate ions present in the background water matrix will scavenge hydroxyl radicals and reduce the reaction rate with organics.
Presence of natural organic matter - Natural organic matter (NOM) present in the background water matrix will scavenge hydroxyl radicals and reduce the reaction rate with organics.
pH - The pH dictates the level at which certain ions important to AOPs will be present, including carbonate ion, bicarbonate ion, and the anion of hydrogen peroxide. The pH affects the charge on target organics if they are weak acids or bases and in some cases the ionic form has a rate constant one or two orders of magnitude higher than the molecular form.
Presence of reduced metal ions - Reduced metal ions, such as Fe(II) and Mn(II), present in the background water matrix will scavenge hydroxyl radicals and reduce the reaction rate with organics.
Reactivity of the target compound with hydroxyl radicals - The second order hydroxyl radical rate constant is an indication of how the AOP reactions will proceed. AOP reactions tend to be quite rapid with second order hydroxyl radical rate constants on the order of 108 to 1010 L/mole•s. The higher the second order hydroxyl radical rate constant, the more amenable the compound is to reduction by an AOP. Although the second order rate constants of bicarbonate ion, carbonate ion, and NOM are typically lower than the target compounds, their concentrations are often several orders of magnitude higher than the target compounds, increasing the importance of the presence of these species in the background water matrix.
Assimilable Organic Carbon - Reactions between hydroxyl radicals and natural organic matter present in the background water matrix will produce assimilable organic carbon (AOC). By definition, AOC is biodegradable and EPA encourages biological filtration to remove AOC when ozonation is applied. The UV/H2O2 process has typically been applied to waters with low TOC so the biological filtration step has not been consistently applied. AOC should be considered in the application of an AOP.