4. RESULTS AND DISCUSSIONS 103
4.3. Pyrene Biodegradation in TPPB System
4.3.3. Pyrene biodegradation experiments in the TPPB system
4.3.3.1. Single substrate condition
enhancement factor was observed at aeration rates of 1.5 and 2.5 l min-1. These observations clearly indicate a high influence of operating conditions on the oxygen mass transfer in two liquid phase systems. Also, it could be said from these results, that in general, addition of any quantity of organic phase with a high affinity for dissolved oxygen will promote an enhanced total rate of oxygen transfer to the aqueous phases in a two liquid phase partitioning reactor system. It was thus concluded that for pyrene biodegradation experiments employing this system, the following set of conditions were applicable: silicone oil fraction: 20%, agitation rate: 600 rpm, aeration rate: 1.5 l min-1
using the solvent; but when the solvent was changed the authors reported 3-d and 12-d of lag in pyrene degradation with heptamethylnonane and paraffin oil, respectively. During the active degradation phase in the present study, degradation rates were found to be 82, 139, 230 and 270 mg l−1 d−1 for the initial concentrations of 200, 400, 600 and 1000 mg l−1, respectively. MacLeod and Daugulis (2003) in their study reported pyrene degradation rate of 138 mg l−1 d−1 with Mycobacterium PYR-1, which is so far the highest pyrene degradation rate reported till date. In the present study the degradation rates of pyrene at all initial concentrations (except at 200 mg l−1) are much higher, suggesting superior performance of M. frederiksbergense in degrading pyrene employing the TPPB system.
Time (h)
0 50 100 150 200 250 300
Pyrene concentration in silicone oil (mg l-1)
0 200 400 600 800 1000 1200
200 mg l-1 400 mg l-1 600 mg l-1 1000 mg l-1
Figure 4.24: Pyrene degradation profiles obtained in the TPPB system.
It should be noted here that abiotic loss of pyrene in the current TPPB system was found negligible (data not shown), which is in accordance with the observations by other
authors regarding loss of pyrene due to evaporation (Lei et al., 2007) or adsorption to reactor vessel surface (Knightes and Peters, 2003).
Microbial biomass concentrations in TPPB systems are measured, in general, by sampling the aqueous phase of TPPB, thereby ignoring the same in NAPL of such systems. Due to inhomogeneous nature of such TPPB systems, biomass estimation may lead to erroneous interpretation. Hence, in this study, models that consider substrate degradation profile as a function of time rather than biomass were used for modeling the pyrene degradation data.
To model the pyrene degradation profiles followed in this system, five different models belonging to two types, namely: Monod variants (I - simple Monod, II - logistic growth Monod, III - logarithmic growth Monod) and variants of mechanistic three-half- order models (IV - three-half order with linear growth and V - three-half-order with logarithmic growth) were chosen in this study. The mathematical forms of these models are presented in Table 4.10.
Non-linear least square analysis was performed using MATLAB® curve fitting tool to estimate the various kinetic parameters in these models by duly considering constraints for positive integer value or appropriate initial guess of the parameters. Table 4.11 presents the coefficient of determination (R2) values obtained by fitting the various models considered in this study. The table clearly shows that the simple Monod model completely failed to fit the entire data. Though three-half order model (with linear growth) could fit the degradation profile of 400 mg l−1, it however, failed to fit the other data. Monod variants with logarithmic growth could reasonably fit the kinetic data obtained for 1000 mg l−1 (R2 = 0.9152) and 400 mg l−1 (R2 = 0.9231) concentrations, but
not for other concentrations. And the exponential growth form of the three-half-order model provided the best fit for entire data in this study.
The kinetic parameters were thus estimated from this particular model and are given in Table 4.12. The value of k1 (first-order proportionality constant) determined from the best fit three-half-order kinetic model are low if compared with those reported by Brunner and Focht (1984) and Scow et al. (1986) in their studies, but were close to the values reported by Metzger et al. (1999), where it was estimated to be 1.06×10-3 h-1. Estimated k1 in our study for an initial substrate concentration of 400 mg l−1 was maximum compared to those of other concentrations, and this value also seem to be consistent with the observation that the lag period in pyrene degradation is minimum at this concentration.
When similar degradation rates within a given concentration range are encountered in a TPPB system, the uptake mechanism could be said as an interfacial one (MacLeod and Daugulis, 2005). And, since in our study the degradation rates varied with initial concentration of pyrene in silicone oil phase, the interfacial based uptake mechanism was not found to be true. Due to very high partition coefficient of pyrene for silicone oil in TPPB system, no pyrene was found to precipitate during the experiments.
Therefore, different aqueous phase pyrene concentration, depending on its concentration in silicone oil, could be attained for easy uptake by the microorganism growing in aqueous phase. Hence, it could be surmised that either “direct pollutant uptake from the organic phase” or “prior pollutant transfer to the aqueous phase before uptake” or a combination of these two could be a probable mechanism of pyrene uptake in this system.
161 Model Mathematical form a Necessary
condition
Estimable
parameters Reference
I Simple Monod / 2
max 0 max / 0 max /
( )
( )
x s s
x s x s
Y k S
t μ X μ Y S S μ Y S
= +
+ − Nil ks; Yx/s; µmax Bandyopadhyay et al., 1998
II Logistic Monod 0 [(0 0 )( 0 0) ]
0 0
1 ( )
1 ( ) max ks S X t S S X S
X S e μ +
= +
+ S0<<Ks ks; µmax
III Logarithmic Monod S S0 = +1 (X S0 0)(1−eμmaxt) S0>>Ks µmax
Stephen and Martin, 1984
IV Three half order (linear growth) S S0 =e− −k t1 ((k t22) / 2) Nil k1; k2
V Three half order (exponential growth) 1 0( 1) / 0
E t
k t e
S S e
μ
μ
− − − −
= Nil k1; µ
Brunner and Focht, 1984
a E0, Initial level of enzyme concentration (mg l-1); k1, First-order proportionality constant (h-1); k2, Second-order proportionality constant (h-2); ks, Half-saturation constant (mg l-1); S, Substrate concentration at any time (mg l-1); S0, Initial Substrate concentration (mg l-1); t, Time (h); X0,Initial microbial biomass concentration (mg l-1); Yx/s, Yield coefficient (mg cell produced/mg of substrate degraded); µ, Specific growth rate (h-1); µmax, Maximum specific growth rate (h-1)
Table 4.11: Calculated coefficient of determination (R2) values for the various models applied to the pyrene degradation data in the TPPB system.
Initial pyrene concentration (mg l-1) Models
200 400 600 1000
I. Simple Monod 0.3475 0.4564 0.3322 0.4382
II. Logistic Monod 0.4593 0.9137 0.6701 0.7804
III. Logarithmic Monod 0.567 0.9231 0.8693 0.9152 IV. Three half order (linear growth) 0.5325 0.9333 0.6765 0.7254 V. Three half order (exponential growth) 0.9991 0.9929 0.9960 0.9971 Table 4.12: Estimated kinetic parameters from the best fit three-half-order model.
Initial pyrene concentration (mg l-1) Estimated rate constant
200 400 600 1000 k1 (h-1) ×10-3 0.1219 3.258 0.134 0.874