STUDY ON THE EFFECTS OF SOIL: RESULTS AND DISCUSSIONS

DISCUSSIONS

Many of the hydrocarbons are resistant to degradation in the natural environment. The overall degradation rate of hydrocarbons biodegradation in soils is strictly limited by a variety of parameters Rockne et.al 2002. Two of the most important soil factors that affect hydrocarbons degradation are soil pH and available nutrients. The results of the present study reveal considerable effects of soil pH and addition of nutrients (in the form of N-P-K fertilizer) on the hydrocarbons degradation of crude oil contaminated soil.

The results obtained from the detailed remediation studies carried out with soil spiked with different amounts of crude oil under different pH values are presented in Table 3, Table 4 and Table 5. The results estimate the loss of TPH from the crude oil contaminated soil samples for each treatment options employed to study the effect of pH on remediation.

As already mentioned above, remediation experiment was also carried out using petroleum hydrocarbons contaminated soils (only with initial TPH 3.0%) under pH 7.5, original pH and at different N-P-K environments. The results of the study are presented in Table 6, Table 7 and Table 8. The results determine the loss of petroleum hydrocarbons from the crude oil contaminated soil samples for each treatment options employed to study the effect of additional N-P-K fertilizer at optimum pH on remediation.

Soil pH is an important parameter that predominantly affects the biodegradation process. This is because each type of microorganisms has a preferred pH range for optimal growth and activity.[3] Following important observations can be made from the detailed remediation (enhanced degradation measured with loss of TPH) studies carried out with soil spiked with different amounts of crude oil under different pH values.

a) The common order of degradation (DpH, measured as total percentage loss in TPH after six months) for each initial TPH (except for initial TPH 0.3 %) according to pH variation is as follows-
D7.5>D4.5>DORIGINAL > D5.5>D6.5>D8.5>D3.5
For initial TPH 0.3% the order is slightly different-
D7.5>D4.5> D5.5>D6.5>D8.5> DORIGINAL >D3.5

In the second case the less degradation at original pH may be due to the non-treatment condition i.e., absence of pH controlling reagents which requires further investigations.

b) The degradation was very good at pH 4.5 and showed identical TPH degradation (enhanced degradation) pattern similar to the ‘Control soil’ which had a pH of 5.0(original pH). As the ‘Control soil’ is acidic in nature these observations may be attributed to the members of the indigenous microbial community. This may be due to the fact that biological activity in the soil is less affected by small pH variation.
c) The soil samples whose pH was adjusted to 5.5, 6.5 showed enhanced degradation for lowest initial TPH (0.3%) concentration and decreased degradation for other higher initial TPH concentrations. This may perhaps be due to the fact that low TPH concentration is lost favorably due to evaporation and remaining part is used in physical processes. At other higher TPH concentrations these processes are not predominant due to high initial TPH. The soil sample whose pH was adjusted to 8.5 showed identical TPH degradation as above. The soil pH between pH 5.5 and 8.5 encourage microbial activity.
d) The soil sample whose pH was adjusted to 3.5 showed decreased TPH degradation for all initial TPH concentrations. This may be due to the decreased microbial activities at very low pH (3.5) as compared to original pH (5.0).
e) The study conducted at different pH’s showed that the highest degradation of petroleum hydrocarbons occurred at pH 7.5. This may be due to the fact that microbial activity is greater at or near neutral pH, which enhances degradation processes, mineralization, and nitrogen transformations (e.g., nitrogen fixation and nitrification).

The degradation continued to improve with time and it was observed that TPH continued to degrade more and more up to 180 days. Above observations are graphically represented in the Figure 1-Figure 4.
Thus, the pH factor affected total petroleum hydrocarbons (TPH) degradation and remediation of hydrocarbon-contaminated soil appears to be feasible.

Another most important soil factor that affects degradation is nutrient availability. The nutrient status of soil has direct impact on microbial activity and hence biodegradation of hydrocarbons in soil. The positive effects as well as the negative effects of different N-P-K levels on the biodegradation of hydrocarbons have been reported by different authors. [25,26] In the present study, role of N-P-K fertilizer added at optimum pH and original pH on the degradation of petroleum hydrocarbons was evaluated. There was significant degradation of petroleum hydrocarbons with the addition of N-P-K fertilizer. The common order of degradation according to N-P-K contents (DpH/NPK%, only for initial TPH 3.0%) after six months of experimentation is shown below. The term “NPK%” means the amount (in the unit of gram) of N-P-K fertilizer added to 100g soil system.

D 7.5/90%> D7.5/60%> D7.5/30%>D7.5/0%> D0RIGINAL/0%

The degradation continued to improve with increase in concentration of additional N-P-K fertilizer. Thus, soil responded most positively to 90% additional N-P-K fertilizer. This is clearly visible from the Figure 5. It is important to mention that the results of the present study may vary from the results obtained by other studies due to the difference in crude oil compositions, climatic conditions, soil characteristics, soil microbial community and many other important factors. Present findings will, ultimately, help to carry out further investigations to prepare a suitable in situ method for the degradation of hydrocarbons in oil field soil.