Table 3 shows the results of bioremediation on day 270. The
bioremediation was most successful in sandy soil with more than
70% degradation of TPH. The removal percentagewas relatively low
in clay. In Table 4 the results of the analysis of variance have been
presented. The results shows that soil type significantly affects
bioremediation (Pvalue < 0.05), and the variation in moisture content
does not significantly affect the TPH removal in the tested range. The
removal percentages in other soils were comparable to each other.
The results confirm that the presence of sand in the soil is advantageous
in bioremediation. The low bioremediation in clay could be
due to inefficient oxygen transfer in the soil. Fine grained clay with
high surface area formed a sticky texture in the presence of water,
blocking efficient oxygen transfer through the soil. Sandy soils on the
other hand aremore porous than clays. Higher porosity allowsbetter
oxygen transfer in the soil which is essential to biodegradation of
hydrocarbons. Larger pores provide also enough space for microbial
growth. Akbari and Ghoshal (2015) demonstrated that pores smaller
than 3 micrometers are not accessible to bacteria. Another reason for
low degradation of hydrocarbons in clay could be strong adsorption
of the pollutants on the surface of soil particles.
Figs. 1e4 show the results of bioremediation for the soils as a
function of time and moisture content. For the sandy soil (Type I)
the gasoil residual was 69.63 g/kg at the beginning and decreased
to 20.6 g/kg on day 270. For the clay soil (Type II) the gasoil residual
was 69.7 g/kg at the beginning and decreased to 52.9 g/kg on day
270. For the coarse soil (Type III) the gasoil residual was 71.61 g/kg
at the beginning and decreased to 26.35 g/kg on day 270, and finally
for the coarse soil with high clay content (Type IV) the gasoil residual
was 69.52 g/kg at the beginning and decreased to22.55 g/kg
on day 270. Overall the results of this part showed that sandy soils
are the best candidate for bioremediation while clay soils resist
bioremediation.
The results of the complementary experiment on the clay soil
have been presented in Table 5. The results indicate that by
frequent mixing the TPH removal increased to 57% in one month.
The removal in unmixed soil samples was lower than the samples
with mixing. The best result here was for the moisture content of
10%. Higher moisture content negatively affected the TPH removal
in the samples. Table 6 shows the results of the analysis of variance
for this experiment. The results confirm that the moisture content
and blending both significantly affect the TPH removal in clay soils.
Comparing the results of this experiment with the previous one, it
can be concluded that the main reason for low degradation of TPH
in the previous experiment was low availability of oxygen to the
microbes. The large size of the sample (30 kg) made it difficult to
mix it efficiently and this prevented proper exposure of the soil
aggregates with air. In the latter experiment small size of the
samples (500 g) and frequent mixing allowed efficient exposure to
the air and improved the rate of biodegradation.