# Bioaugmentation Treatment of a PAH-Polluted Soil in a Slurry Bioreactor
## 3 Results and Discussion
### Chemical Analysis  
The three bioslurry (Treatments A, B, and C) treatments were monitored regularly for slurry phase pH, temperature, and dissolved oxygen. The temperature was constant at around 28 \({}^{\circ}\)C throughout all of the treatments, as initially set up. However, the DO levels (Figure 2) started close to 8.0 mg/L, but were then rapidly reduced to a quasi-constant value of 5 mg/L in the three experiments, thereby indicating an active consumption in the context of an aerobic process. Similarly (Figure 2), the pH showed an increase at early time points from around 7.0, up to levels close to 8.0 in all of the treatments. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Texture**} & Sand (2 mm–63 \(\upmu\)m) & 43\% \\ \cline{2-3}  & Silt (63–2 \(\upmu\)m) & 19\% \\ \cline{2-3}  & Clay (\textless{}2 \(\upmu\)m) & 39\% \\ \hline \multirow{6}{*}{**Chemical parameters**} & pH & 8.2 \\ \cline{2-3}  & Conductivity & 0.13 dS/m \\ \cline{2-3}  & Organic matter & 0.20 \% _w/w_ \\ \cline{2-3}  & Nitrogen & 0.03 \% _w/w_ \\ \cline{2-3}  & C/N ratio & 3.9 \\ \cline{2-3}  & Phosphorus & \textless{}2.6 mg/kg \\ \cline{2-3}  & Carbonate content & 0.8 \% _w/w_ \\ \hline \hline
**Microbiology** & Total heterotrophs & \(2.3\times 10^{8}\) CFU/g soil \\ \hline
**PAH content** & mg/kg & 332.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Original soil characteristics; the values shown are an average of three measurements on distinct samples. Figure 2: _Cont._Regarding PAH depletion, after 15 days of treatment, the bioaugmentation treatments (with _Rhodococcus_ and _Pseudomonas_) registered slightly lower PAH concentrations than the biostimulation treatment alone (named Autochthonous in Figure 3). However, the evolution of the three experiments was not same. The depletion of PAHs was noticeably faster when _P. stutzeri_ was added, whereas the effect of _R. erythropolis_ appeared to be slower. However, the extent of degradation finally achieved in both experiments was similar and analogous to that obtained in the biostimulation experiment alone. A more detailed overview of the degradation results is shown in Table 3. Of note, there was an apparent favored degradation of the lighter PAHs (two- and three-ring compounds) with respect to the heavy PAHs (four- to six-ring), regardless of the treatment. This finding is consistent with the differences in the bioavailability of the contaminants [46]. However, the differences between the depletion achieved for lighter or heavier PAHs were not notable in our case (between only 10% and 20%, depending on the experiment). This result could be attributable to the controlled and favorable conditions of the reactor experiments to increase bioavailability, including the addition of  
Figure 3: Evolution of the total polycyclic aromatic hydrocarbon (PAH) concentration during the bioslurry reactor experiments. Figure 2: Evolution of the dissolved oxygen (top) and pH (bottom) of the three bioslurry experiments. the surfactant and even possible biosurfactant production by some of the degrading microorganisms present (see below). Our findings indicate that the microorganisms showed a sufficient degradation capacity to deplete quasi-recalcitrant compounds such as heavy PAHs (more than 70% of degradation); i.e., the controlled optimal conditions of the SB facilitated a greater yield than that achieved in the pilot-scale on-site biopile treatments (as reported in the literature [27], after 60 days less than 40% of heavy PAHs were degraded). In fact, bioavailability constraints are essential factors for hydrocarbon biodegradation [41; 47], especially significant in aged soils such as the one studied here, where lipophilic contaminants had enough time to diffuse into the soil micropores, thereby becoming inaccessible to microorganisms [48]. Subsequently, a residual contaminants fraction, usually associated with the finer grain-sizes (note the remarkable presence of clays in our case), typically remains in the soil [49].