Effects of humic acid on solubility and biodegradation of polycyclic aromatic hydrocarbons in liquid media and mangrove sediment slurries

The effects of humic acid (HA) on the solubility and biodegradability of mixed polycyclic aromatic hydrocarbons (PAHs) (phenanthrene (PHE), pyrene (PYR), and benzo[a]pyrene (BAP)) in liquid media and mangrove sediment slurries were investigated. The addition of HA to the liquid media (0-1.6%, w/v) significantly enhanced the solubility of all mixed three PAHs and the biodegradation of PHE and PYR (but not BAP) by MP-PYR1, a PYR-degrading bacterium isolated from mangrove sediment. Amendment with 0.2% HA to the sediment slurries exhibited little enhancement in either PAH solubility or degradation. Although amendment with 1.6% HA increased the aqueous phase PAHs in the sediment slurries, it did not have any significant enhancement effect on biodegradation. Natural attenuation of PHE in sediment was evident, with 91% degraded after 7 d. The highest biodegradation of PHE and PYR was found in the sediment slurries inoculated with MP-PYR1, and the degradation efficiency was even higher than that in the liquid media (99% vs. 85% for PHE and 97% vs. 63% for PYR). The degradation capacity of MP-PYR1 for both PHE and PYR was comparable when it was inoculated to the sterile and non-sterile sediment slurries, implying that the inoculum was able to compete with the indigenous microorganisms. The reason why BAP was not degraded in either liquid media or mangrove sediment slurries was more likely due to the lack of degraders than its low solubility. These results suggested that the success of PAH degradation did not rely solely on the amounts of soluble PAHs which could be enhanced by the HA amendment; the presence of a suitable degrader was also important.