Adsorption of emerging environmental pollutants by marine sediment in relation to sediment organic diagensis

Abstract

Ever-growing discharges of various emerging chemical contaminants are imposing a great threat of pollution to the coastal environment. Adsorption by sediment plays an essential role in the transport and fate of pollutants in the aquatic system. The sorption of emerging contaminants onto sediment is believed to be largely dependent on the sediment organic matter (SOM). In the present study, laboratory experiments were carried out on the changes of the adsorption behavior of sediment during the sediment aging and diagenesis process. A few EDCs and antibiotics were selected as the model emerging compounds for the adsorption tests. The results demonstrated that both the quantity and the quality of the SOM affected the adsorption of the model pollutants, such as 17α-ethinyl estradiol (EE2) and bisphenol A (BPA), onto the marine sediment collected from Victoria Harbour, Hong Kong. The adsorption isotherms can be well described by the linear partition model.

Natural and artificial sediment with a high SOM content was incubated for 4-6 months to simulate the natural diagenesis process. The most rapid degradation of labile SOM occurred in the first 1 month or so and afterward, SOM reduction became slower. Microbial activity played an important role in SOM degradation and transformation. A rapid initial bacterial growth was observed in the sediment, followed by a slow endogenous decay. The dynamics of biomass growth and decay first transformed the labile SOM into biomass and microbial byproducts. After the exhaust of readily biodegradable SOM, the biomass decay produced humic-like substances, resulting in more refractory and condensed SOM residues in the sediment.

More importantly, the degradation and transformation of SOM displayed a profound impact on the adsorption behavior of the sediment. For the selected EDCs and antibiotics, including BPA, EE2, nonylphenol (NP), phenanthrene (PHE) and tetracyclines (TCs), the adsorption capacity indicated by the partition coefficient, Kd, decreased at the beginning of SOM diagenesis. The Kd values for different chemicals recovered lately to different extents as the result of the SOM condensation and humification. All of the organic matter normalized partition coefficients, KOM, of the concerned pollutants increased considerably in the late phase of SOM diagenesis. Based on the experimental results, a general conceptual model was established to describe SOM diagenesis and its impact on chemical adsorption by the sediment. According to the model prediction, the SOM profile would become more dominated by the condensed and refractory fractions during sediment diagenesis with an increasing affinity and partition capacity for organic contaminants.

Moreover, the release of adsorbed contaminants from marine sediment in the simulated digestive fluids was investigated. In general, the presence of gastric pepsin and bile salts helped the desorption of hydrophobic pollutants from the sediment into the digestive solutions. The influence of the SOM diagenetic status on chemical desorption from the sediment varied between EDCs of different chemical properties. It is apparent that aged sediment could bring more emerging pollutants into the digestive system of receiving organisms, imposing a potential risk to human health through the food chain.