Effects of hydroxypropyl-β-cyclodextrin and β-cyclodextrin on the distribution and biodegradation of phenanthrene in NAPL-water system

• HPCD and β-CD could increase partitioning of phenanthrene in NAPL-water system.
• HPCD and β-CD could enhance biodegradation of phenanthrene dissolved in two NAPLs.
• Effect of HPCD was higher as compared to β-CD, as confirmed by molecular docking.
• Biodegradation rates depend on apparent equilibrium concentrations of phenanthrene.
• Increasing concentration of HPCD may cause competition between HPCD and degraders.

Biodegradation of polycyclic aromatic hydrocarbons (PAHs) in contaminated sites is often limited by the presence of nonaqueous-phase liquids (NAPLs). To increase the bioavailability of PAHs in NAPLs, we investigated the effect of cyclodextrins (CDs) [including hydroxypropyl-β-cyclodextrin (HPCD) and β-cyclodextrin (β-CD)] on the equilibrium partitioning and biodegradation of phenanthrene (Phe) dissolved in two NAPLs, hexadecane and di-2-ethylhexyl phthalate (DEHP). The result showed that the extent of β-CD- or HPCD-mediated phenanthrene partitioning from NAPL into the aqueous phase increased with increasing concentrations of the CD in the aqueous phase and the relative abundance of phenanthrene in NAPL. Overall, the capacity of HPCD to partition phenanthrene was higher compared to β-CD, as confirmed by molecular docking. The effect of CD on the apparent concentration of phenanthrene in the hexadecane-water system was about twice that in the DEHP-water system. As a result of the increased partitioning of phenanthrene into the aqueous phase by CD, the biodegradation of phenanthrene was enhanced, and was influenced by the type and concentration of CD as well as the and type of NAPL used. When the concentration of β-CD was increased from 0.72 mM to 7.2 mM, it was found that there was a linear correlation between apparent equilibrium concentrations and biodegradation rates of phenanthrene (R2 > 0.97). At the same time, increases in the concentration of CD led to increases in competition between CD and the phenanthrene-degraders for phenanthrene. When the concentration of HPCD was increased beyond 3.6 mM, the enhancing effect of HPCD on the biodegradation rate of phenanthrene was not obvious. Thus, although CD appeared to be a promising agent for promoting bioremediation, the concentration and type of CD should be carefully chosen for different areas to reflect its ability to enhance the biodegradation.