Approved Abstracts

Assisted biodegradation of PAHs polluted soils using cyclodextrins and rhamnolipids



Author(s): Madrid F; Rubio-Bellido M; Florido MC; Villaverde J; Morillo E;
Presenter: Esmeralda Morillo

Three agricultural soils (CR, CS, TM) have been artificially contaminated with a mixture of polycyclic aromatic hydrocarbons (PAHs), from two to six condensed rings (acenaphthylene (ACL), acenaphthene (ACE), anthracene (ANT), fluorene (FLU), phenanthrene (PHE), fluoranthene (FLT), pyrene (PYR), benzo(a)anthracene (BaANT), chrysene(CHR), benzo(a)pyrene (BaPYR), benzo(b)fluoranthene (BbFLT), dibenzo(a,h)anthracene (DBahANT), benzo(g,h,i)perylene (BPER), indeno(1,2,3,c,d)pyrene (IPYR), considered as priority pollutants by the USEPA. In a second assay the same soils were also contaminated with a mixture of only 7 PAHs with three (ACL, ACE, ANT, FLU and PHE) or four (PYR and FLT) condensed rings to study if the presence of the heavier PAHs (molecular weight > PYR) influenced the degradation of these 7 PAHs. The natural biodegradation of PAHs was studied with the endogenous microflora of each soil and compared with the biodegradation assisted with compounds that increase the solubility of PAHs and their bioavailability, in order to observe if the extent of PAHs degradation could be enhanced. The compounds used were hydroxypropyl-β-cyclodextrin (HP), methyl-β-cyclodextrin (RAMEB) and a biosurfactant (rhamnolipid, RL).
All 3 soils showed the ability to naturally dissipate the 7 lighter PAHs, especially ACE, ACL and FLU, although the remaining concentrations of ANT, PHE, FLT and PYR used to be lower than 5-6%. Also BaANT and BaPYR were partially degraded, with remaining percentages in the range 30 – 75 %, depending on the soil. The rest of PAHs were not degraded.
The extractants used as bioavailability enhancers shown quite variable results in PAHs biodegradation depending on the type of soil. HP and RL increased the degradation rate of the 7 lighter PAHs in two of the soils (CR and CS), with decreasing half-life values (DT50) especially in the case of HP. RL decreased the initial degradation rate, but after 30 days the effects of this amendment was similar to HP. On the contrary, for TM soil (with the highest clay fraction and organic matter content) the addition of HP and RL did not improve their degradation in relation to the control.
The addition of RAMEB affected negatively to the PAHs degradation, with a remaining percentage of the ∑7 PAHs studied in the range 45-70% of their initial content. It can be due to the higher stability of the complexes formed with the seven PAHs under study and, as a consequence, their lower degradability. In addition, it can be also a consequence of the low degradability of RAMEB, what would have prevented its use as biostimulant for the soil microflora (the contrary that occurred with HP and RL).
Due to the high number of experiments and data obtained, a multiway Analysis of Variance (ANOVA), principal component analysis and the modelization of data obtained were carried out. Principal component analysis groups PAHs in 3 main components what explain 80% of the total variance of the system. The first one explains 38.0% of the total variance and is highly correlated with the same seven PAHs selected for the second assay, showing their higher degradability. On contrary, the second component explains 26.9% of the total variance and is correlated with the heaviest PAHs (5 and 6 rings, BaPYR, BbFLT, DBahANT, BPER and IPYR), whereas the third component (14.4% of the total variance) correlate with the other 4 ringed PAHs (BaANT and CHR). The Analysis of Variance indicated that 3 of the 7 PAHs (ACE, ACL and FLU) were influenced by the presence of the high molecular weight PAHs, showing an increased degradation, but not the rest of PAHs studied (ANT, PHE, FLT, PYR). For ACE and ACL such increased degradation was influenced by the type of soil, but not in the case of FLU.


Keywords: PAHs; Soil bioremediation; Cyclodextrin

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