Kinetics under Different Phenanthrene and p-Cresol Concentrations

The extent of PHE and p-cresol degradation can be determined by calculating the degradation rate constant. In this study, the PHE and p-cresol degradation rate in Run-1 and Run-9 were evaluated using zero-order and first-order kinetics. The zero-order PHE and p-cresol degradation rate constants were 0.34 mg/L-d (R² of 0.9) and 0.3 mg/L-d (R² of 0.88), and the first-order PHE and p-cresol degradation rates were 0.03/d (R² of 0.94) and 0.045/d (R² of 0.95), respectively, as shown in Fig. 4-11.

The first-order model provided a slightly superior fit to the PHE and p-cresol degradation data than the zero-order model. However, Chang et al. (2002) reported that the degradation rate of PAHs was found to fit well with the first-order kinetics.

Fig.4-11 Kinetics of phenanthrene and p-cresol biodegradation. (a) zero-order and (b) first-order kinetics in Run-1; (c) zero-order and (d) first-order kinetics in Run-9. The presented data are mean values of duplicate incubations.

y = -0.3436x + 16.106

The calculated zero-order (k₀) and first-order (k₁) PHE biodegradation rates in Part-1 and Part-2 are shown in Table 4-3. The results indicate that k₀ and k₁ values are decreasing with the increasing concentration of p-cresol in Part-1. The highest k₀ (0.34 mg/L．d) and k₁ (0.03/d) values were observed in Run-1 (degradation of PHE without p-cresol); whereas, the lowest k₀ (0.23 mg/L．d) and k₁ (0.016/d) values were

observed in Run-5. The reason for the decrease in k₀ and k₁ values can be attributed to the decrease in PHE concentration and the simultaneous increase in p-cresol concentration in Part-2.

Table 4-3 Rates of phenanthrene biodegradation in the inhibition study

Parameter

Part I & II Part I Part II

Run-1 Run-2 Run-3 Run-4 Run-5 Run-6 Run-7 Run-8

k0 0.35 0.34 0.32 0.27 0.23 0.23 0.15 0.1

k1 0.03 0.028 0.026 0.02 0.016 0.026 0.029 0.041

Where k0 : The k value of zero-order kinetic (mg/L．d) k₁ : The k value of first-order kinetic (1/d)

4.7 The effect on p-cresol to phenanthrene degradation

In Part-1, PHE was significantly inhibited due to the increase in addition of p-cresol, and at the same time, more quantity of p-cresol was degraded. In Fig. 4-12, the first-order kinetic k value was decreased from 0.03 to 0.016/d with the increase in p-cresol concentration. Nevertheless, △OD660 was increased with the decrease in PHE degradation; the reason for the increase in △OD660 might be due to the increase in p-cresol degradation with the increase in addition of p-cresol. Similarly, the △p (the

theoretical concentration of p-cresol production minus the actual p-cresol concentration observed in the end of experiment period) has good correlation with △

OD₆₆₀. These results illustrate that p-cresol was more suitable for SRB biodegradation than PHE. This could be due to the lesser toxicity and molecular weight of p-cresol compared to PHE.

Fig.4-12 The correlation between phenanthrene/p-cresol consumption and SRB biomass.

Chapter 5 Conclusions

1. p-Cresol is inevitably the metabolite of PHE, which is clear from Run-1. This confirms the experimental observation of the previous study.

2. The addition of p-cresol has considerable reduction in the degradation of PHE.

However, the degradation of p-cresol was increased with the increase in the addition of p-cresol concentration. This could be due to the lesser toxicity and molecular weight of p-cresol compared to PHE.

3. As a whole, this investigation indicates that the presence of simpler metabolite of PHE, i.e. p-cresol, is also a preferred substrate for the SRB used in this study.

However, the presence of p-cresol has the tendency to be utilized as a much more preferred substrate than PHE.

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