Magnetic activated carbon (MAC) mitigates contaminant bioavailability in farm pond sediment and dietary risks in aquaculture products

Highlights

• A predictive model was established to inform the optimal MAC dosage.

• 3% dosage of MAC had most cost-effective stabilization for heavy metals and PAHs.

• Reduced contaminant bioavailability was linked to dietary risk reduction in clams.

• 70% of MAC was recovered from sediment via magnetic separation.

Abstract

Heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs) are among the contaminants of concern in aquaculture ponds due to their frequent detection and high bioaccumulation in aquatic products and hence high dietary risks to human beings. In this study, magnetic activated carbon (MAC) was added as a stabilization and removal adsorbent to native pond sediment with known contamination of HMs and PAHs to reduce the tissue residues and dietary risks of HMs and PAHs in a model aquaculture species (Venerupis philippinarum) in the course of a 28-day bioaccumulation experiment. Meanwhile, passive sampling techniques based on diffusive gradient in thin films (DGT) and polydimethylsiloxane (PDMS) were applied to sense the bioavailable fraction of HMs and PAHs in sediment during the stabilization process. The results showed that 3% dosage of MAC to sediment achieved the most cost-effective stabilization for HMs and PAHs. A remarkable decrease was observed with the tissue residues of HMs and PAHs in V. philippinarum (28–47% for HMs and ~76% for ∑PAHs), which was quantitatively linked to the decline in their bioavailable concentrations in sediment pore water (31–46% for HMs and ~76% for ∑PAHs). Consequently, the target hazard quotients (THQs) posed by HMs and incremental lifetime cancer risks (ILCRs) by PAHs in V. philippinarum were reduced by 38% and 46%, respectively. Along with the magnetic recovery of ~70% MAC from the sediment, HMs (4.8–13%) and PAHs (2–60%) can be effectively removed. We further established a multi-domain equilibrium sorption model that was able to predict the optimal amendment of MAC for quantitative mitigation of bioavailable PAHs in sediment pore water within a certain range of MAC dosage. Future studies are warranted to explore the applicability domain of MAC for in situ remediation in aquaculture ponds to ensure the quality of farming organisms or to serve other purposes in aquatic systems.

Introduction

Aquatic products are an important source of high protein (Tacon and Metian, 2013), however, the accumulation of contaminants in aquatic products is becoming a big concern (Gadelha et al., 2019). Sediments in aquaculture ponds, a reservoir of multiple pollutants, due to diffusion, advection and bio-stirring during the culturing process, are currently considered to be reliable media reflecting the potential of aquaculture pollution (Zang et al., 2017; Maletić et al., 2019) and determining the quality of aquatic products. Among the pollutants, heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs) are the two commonly concerned groups due to their high risk and persistence for both human beings and the environment via food-chain delivery and biomagnification (Analuddin et al., 2017; Abdel-Shafy and Mansour, 2016). Our previous research showed that PAHs and HMs were widely detected in aquaculture animals in eastern China (Li et al., 2019). Therefore, controlling the release (i.e. stabilizing or removal) of HMs and PAHs from the sediments of aquaculture ponds is highly required in terms of food safety and environmental health.

Among the existing methods for stabilization or removal of HMs and PAHs in sediments, adsorption was found to be the most cost-effective means (Taneez et al., 2018). Especially, adsorption is least likely to produce harmful chemical/biological transformation products throughout the process and is possible to be regenerated and reused (Janssen and Beckingham, 2013). With excellent adsorption capacity and affinity for hydrophobic organic pollutants (HOCs) and HMs, activated carbon (AC) has been widely used as a stabilization agent for HOCs (Menya et al., 2017; Hilber, 2010; Ghosh et al., 2011) and HMs (Ghosh et al., 2011; Gilmour et al., 2013; Que et al., 2018) in sediments.

However, normal AC can hardly be separated from the sediment after adsorption, which may cause potential ecological risks (Jonker et al., 2009; Nybom et al., 2012). There has been another new type of activated carbon formed by loading magnetic material on its surface, named magnetic activated carbon (MAC). The greatest advantage of MAC is that the recovery can be independent of the shape and size of activated carbon (Oliveira et al., 2002) and can be reused for multiple times (Lompe et al., 2017; Karunanayake et al., 2018). As the adsorption of PAHs (from water and sediments) and HMs (from water) were hardly affected by the magnetic impregnation (Han et al., 2015a; Han et al., 2015b), MAC has been successfully applied in water treatment (Castro et al., 2009), the mining industry (Gómez-Pastora et al., 2014), and in agriculture as controlling the release of N and P from sediments (Choi et al., 2016; Lin et al., 2019). However, the investigation regarding the adsorption of HMs and PAHs from aquaculture ponds sediments with MAC is lacking. It is worth noting that MAC remediation may have toxic effects on sediment-dwelling organisms, which can then be partially alleviated by the magnetic sorbent recovery (Han et al., 2017). In aquaculture, the toxic effects of the MAC, if any, would obviously be of great concern and should be assessed in related studies.

Furthermore, there has been a lack of effective procedures to evaluate the stabilization efficacy of the bioavailable pollutants after carbon-sorption remediation processes. Bulk burden of sediment-associated contaminants has usually been defined as the accumulation of pollutants in sediments, which, however, cannot reliably predict their bioavailability and toxicity (Hu et al., 2018). Han et al. (2015a) and Choi et al. (2016) measured the reduction of labile PAHs in sediment pore water via passive sampling so as to evaluate stabilization efficacy of the bioavailable fraction. As a further refinement, recently, the bioavailability of sediment-associated PAHs and HMs has been successfully evaluated involving both bioaccumulation experiments and passive sampling technology. Specifically, polydimethylsiloxane (PDMS) was utilized for PAHs and diffusive gradient in thin film (DGT) for HMs (Li et al., 2018a; Chen et al., 2019). In most cases, the stabilization effect of ACs for HMs and PAHs were analyzed alone in respective studies. The simultaneous stabilization effect of ACs for HMs and PAHs existing together in sediments is worth investigation for application but has seldom been found.

Therefore, in this study, the MAC prepared by the co-precipitation, which has been successfully used for the control of nitrogen and phosphorus in sediments (Lin et al., 2019), was added as the stabilization and removal materials to the sediments of aquaculture ponds, and V. philippinarum was used as the test organism for bioaccumulation experiments to investigate the stabilization efficacies of HMs and PAHs simultaneously. The bioaccumulation experiments were combined with passive sampling techniques based on DGT and PDMS to evaluate the changes of bioavailability of HMs and PAHs during the stabilization process. Besides, pollutants removal from sediment via recovery of MAC was evaluated. The current study was designed to understand (1) the efficacy of MAC amendment to sediment in reducing the tissue residues of HMs (Cu, Pb, Cd, Cr) and PAHs in V. philippinarum; (2) the efficacy of MAC amendment in mitigating the bioavailability of sediment-associated HMs and PAHs in sediments; (3) the quantitative dose-effect relationship during the stabilization of PAHs via an equilibrium sorption model to inform the optimal addition of MAC; and (4) the removal of bioavailable HMs and PAHs by magnetic separation of MAC from sediment. These quantitative tasks paved the way for future exploration of the potential of MAC for in situ sediment remediation in aquaculture ponds or in other aquatic settings.