To understand the proposed mechanism, let us consider the phenol degradation and by-product formation of P5 shown in Figure 7c. According to this figure, the first 360–400 h of phenol degradation was dominated by phytopolymerization and phytooxidation at a rate constant of 3.3x10-3 h-1. This is 11 times faster than the phenol removal rate without vetiver grass (3x10-4 h-1), presumably due to hydrolysis or photodegradation. Microbes might not be effective in degrading phenol at this high concentration (500 mg/L), as phenol is inhibitory to microbes at this concentration (Hugo, 1978; Nweke and Okpokwasili, 2010). Instead, H2O2 and POD produced by vetiver roots assisted in the first phase of phenol transformation. Phenol was detoxified via POD-catalyzed transformation to phenol radicals, followed by polymerization to non-toxic polyphenols or regioselective polymerization with natural organic matters prior to being precipitated as particulate polyphenols (PPP) or particulate organic matter (POM), respectively (Figure 8). This is evident by the rapid removal of dissolved COD (because dissolved phenol contributes to dissolved COD) and the rapid increase of particulate COD (because PPP and POM contribute to particulate COD) (Figure 7c). In addition, because the total COD decreased exponentially in the first phase, phytooxidation assisted by root-produced H2O2 and peroxidase (POD) yielded more oxidizing VOA or CO2 as by-products, which also occurred at the same time as the phytopolymerization of phenol and the precipitation of PPP, as suggested above.