Environmental bioavailability of hydrophobic organochlorines in sediments?A review

Thin-layer capping with granular activated carbon and calcium-silicate to remediate organic and metal polluted harbor sediment - A mesocosm study

Sediments polluted with hydrophobic organic contaminants (HOCs) and metals can pose environmental risks, yet effective remediation remains a challenge. We investigated a new composite sorbent comprising granular activated carbon (GAC) and a calcium-silicate (Polonite®, PO) for thin-layer capping of polluted sediment, with the aim to sequester both HOCs and metals. Box cores were collected in polluted Oskarshamn harbor, Sweden, and the sediments were treated with GAC and/or Polonite in a 10-week mesocosm study to measure endpoints ranging from contaminant immobilization to ecological side effects on native fauna and biogeochemical processes. The GAC particle size was 300-500 μm to reduce negative effects on benthic fauna (by being non-ingestible) and of biogenic origin (coconut) to have a small carbon footprint compared with traditional fossil ACs. The calcium-silicate was a fine-grained industrial by-product used to target metals and as a carrier for GAC to improve the cap integrity. GAC decreased the uptake of dioxins (PCDD/Fs) in the bivalve Macoma balthica by 47 % and the in vitro bioavailability of PCB by 40 %. The composite cap of GAC + Polonite decreased sediment-to-water release of Pb < Cu < Ni < Zn < Cd by 42-98 % (lowest to highest decrease) and bioaccumulation of Cd < Zn < Cu in the worm Hediste diversicolor by 50-65 %. Additionally, in vitro bioavailability of Pb < Cu < Zn, measured using digestive fluid extraction, decreased by 43-83 %. GAC showed no adverse effects on benthic fauna while Polonite caused short-term adverse effects on fauna diversity and abundance, partly due to its cohesiveness, which, in turn, can improve the cap integrity in situ. Fauna later recovered and bioturbated the cap. Both sorbents influenced biogeochemical processes; GAC sorbed ammonium, Polonite decreased respiration, and both sorbents reduced denitrification. In conclusion, the side effects were relatively mild, and the cap decreased the release and bioavailability of both HOCs and metals effectively, thus offering a promising sustainable and cost-effective solution to remediating polluted sediments.

Effects of polycyclic aromatic hydrocarbons fluoranthene on the soil aggregate stability and the possible underlying mechanism

Soil contamination by polycyclic aromatic hydrocarbons (PAHs) is an increasing problem in many countries, impacting the ecological environment's sustainable development. This study investigated the effects of fluoranthene (Fla) on soil aggregate stability. A possible mechanism for the interaction of Fla with soil aggregates was proposed by characterizing the aggregate structure. The results showed that Fla significantly improved the aggregate stability in the concentration range of 0-30.0 mg/kg. The content of macro-aggregates reached the maximum value at 10 mg/kg of Fla, which increased by 24.25% compared with the control group, while the content of large-aggregates decreased by 12.11%. Meanwhile, the mean weight diameter (MWD) and geometric mean diameter (GMD) increased by 56.63% and 37.66%, respectively. However, the macro-aggregates zeta potential value and specific surface area (SSA) decreased by 12.68% and 13.61%, respectively. The cracks of macro-aggregates were also significantly reduced. In addition, Fla-based free radicals were detected on the macro-aggregates. The absorption peak of the C-O group significantly increased, indicating that Fla may be covalently bound to the aggregates by aromatic ether bonds, which is a possible mechanism for the interaction between Fla and aggregates. This study provides theoretical support for revealing the effects of PAHs on soil.

Increasing the bioaccessibility of polycyclic aromatic hydrocarbons in sediment using ultrasound

In this study, the effect of sonication on the distribution of polycyclic aromatic hydrocarbons (PAHs) in the bioaccessible and less bioaccessible fractions of three contaminated sediments (Little Scioto River, OH-LS; Gary, IN-GI; Eagle Harbor, WA-EH) was examined. After 60min sonication, the fractions of naphthalene, phenanthrene and pyrene remaining in the LS sediment were 0.76±0.18, 0.83±0.04 and 0.76±0.05, respectively, indicating ultrasonic degradation of PAHs in the sediment. In addition, there was a significant decrease in PAH concentration (i.e., up to 91.4%) in the less bioaccessible fractions for all three sediments with sonication. The bioaccessible fraction of phenanthrene and pyrene in LS and pyrene in EH increased by 12.9%, 48.3% and 27.8%, respectively, followed by a slight decrease due to degradation. The initial increase suggests that ultrasonic irradiation of sediment either transfers the PAHs from the less bioaccessible sites to the bioaccessible sites for treatment or transforms less bioaccessible sites into bioaccessible sites. A comparatively smaller reduction (i.e., 20.2%) in the less bioaccessible fraction in GI sediment is attributed to the larger fraction of black carbon in the organic carbon content of the sediment hindering the ability of ultrasound to switch the PAHs from the less to the more bioaccessible sites. Overall ultrasonic irradiation of contaminated sediments is a technique to enhance contaminant remediation by reducing the fraction of contaminants in less bioaccessible sites.

Importance of prey and predator feeding behaviors for trophic transfer and secondary poisoning

Hydrophobic contaminants accumulate within aquatic sediments, hence pelagic predators may have limited direct contact with such compounds, but can be exposed via their benthic prey (i.e., via dietary exposure). Here we examine the importance of feeding behaviors of both prey (sediment ingesters or noningesters) and predators (piercers or engulfers) in determining the extent of dietary exposure and toxic effects. A freshwater macroinvertebrate system was used, consisting of two predator species, a piercer (Notonecta glauca) and an engulfer (Ischnura elegans), and three prey species, a sediment noningester (Cloëon dipterum) and two sediment ingesters (Asellus aquaticus, Chironomus riparius). Predators were fed prey previously exposed to artificial sediment dosed with 30 microg/g of 14C benzophenone. The piercer predator accumulated more benzophenone from sediment ingester compared to sediment noningester prey, whereas the engulfer predator accumulated a similar concentration for all three prey species. Toxic effects, in terms of reduced feeding rate, were only observed with the engulfer feeding on sediment noningesters, probably due to the interaction between the narcotic mode of action of benzophenone and predator hunting strategy. The importance of dietary exposure in risk assessments may therefore depend on exposure pathways of prey, feeding behaviors of predators, and the contaminant's toxic mode of action.

Modelling PCB bioaccumulation in a Baltic food web

A steady state model is developed to describe the bioaccumulation of organic contaminants by 14 species in a Baltic food web including pelagic and benthic aquatic organisms. The model is used to study the bioaccumulation of five PCB congeners of different chlorination levels. The model predictions are evaluated against monitoring data for five of the species in the food web. Predicted concentrations are on average within a factor of two of measured concentrations. The model shows that all PCB congeners were biomagnified in the food web, which is consistent with observations. Sensitivity analysis reveals that the single most sensitive parameter is log K(OW). The most sensitive environmental parameter is the annual average temperature. Although not identified amongst the most sensitive input parameters, the dissolved concentration in water is believed to be important because of the uncertainty in its determination. The most sensitive organism-specific input parameters are the fractional respiration of species from the water column and sediment pore water, which are also difficult to determine. Parameters such as feeding rate, growth rate and lipid content of organism are only important at higher trophic levels.