Stabilization of hydrophobic organic contaminants in sediments by natural zeolites: bioavailability-based assessment of efficacy using equilibrium passive sampling

Purpose: As natural zeolites have been widely used as cost-effective adsorbents for plant nutrients and heavy metals in sediments, it is worthwhile to verify the potential of natural zeolites for mixed-contaminant stabilization including hydrophobic organic contaminants (HOCs). The effectiveness of natural zeolite amendment in sediment on reducing the bioaccumulation of sediment-associated HOCs in V. philippinaram (C_b) was assessed. Then, the role of sediment pore water freely dissolved HOCs (C_free) in C_b prediction was further identified and modeled. Materials and methods: In this study, a bioavailability-based assessment of the HOC-stabilization efficacy of natural zeolites in maricultural sediments was performed using equilibrium passive sampling. V. philippinaram was adopted as a biological indicator for HOC bioaccumulation. Polydimethylsiloxane (PDMS) was used as a passive sampler for monitoring the concentration of the C_free. The assumption that C_free is a central mediator driving the bioaccumulation process of HOCs from the bulk sediment to the exposed organisms was validated by correlating the reductions in C_b and C_free of the sediments amended with natural zeolites. Subsequently, a model based on this assumption was built and verified for the bioavailability-based assessment of the stabilization efficacy of natural zeolite amendment on sediment-associated HOCs. Results and discussion: The results showed the bioaccumulations of four kinds of HOCs (including polycyclic aromatic hydrocarbons, polybrominated diphenyl ethers, pyrethroids, and organochlorine pesticides) in V. philippinaram were reduced by 7.3–38% after the natural zeolite amendment (10% dry weight in sediments), and the C_free values measured with PDMS were reduced proportionally. It supported the assumption that C_free is a central mediator driving the bioaccumulation process of HOCs. K_ow of HOCs and two properties of sediment were found to be significantly correlated with the reductions of the C_free values. Based on these findings, a model to predict the C_b values of PAHs and PBDEs in V. philippinaram was built. The model was then verified by a significant correlation between the predicted and measured values of C_b. Conclusions: The potential of natural zeolites for the stabilization of mixed-contaminants in sediments, including HOCs, was proved as notable reductions of C_b and C_free of HOCs in the sediments amended by natural zeolite were observed. The results also suggested PDMS is a promising tool for predicting the bioaccumulation of sediment-associated HOCs in V. philippinaram and further for assessing the stabilization efficacy of natural zeolites in maricultural sediments. Modifying natural zeolites to further improve the effectiveness of HOC stabilization is warranted.