Stabilization of hydrophobic organic contaminants in sediments by natural zeolites: bioavailability-based assessment of efficacy using equilibrium passive sampling
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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 (Cb) was assessed. Then, the role of sediment pore water freely dissolved HOCs (Cfree) in Cb 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 Cfree. The assumption that Cfree 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 Cb and Cfree 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 Cfree values measured with PDMS were reduced proportionally. It supported the assumption that Cfree is a central mediator driving the bioaccumulation process of HOCs. Kow of HOCs and two properties of sediment were found to be significantly correlated with the reductions of the Cfree values. Based on these findings, a model to predict the Cb 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 Cb.
The potential of natural zeolites for the stabilization of mixed-contaminants in sediments, including HOCs, was proved as notable reductions of Cb and Cfree 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.
KeywordsEquilibrium passive sampling Freely dissolved concentration Hydrophobic organic contaminants Maricultural sediment Natural zeolite Stabilization
Aquaculture in the world has surpassed wild catch as a major supply source of the world’s fish consumption (Taylor 2009). As of 2016, being the country with the largest fishery industry of the world, China had an annual production volume of 69 million tons, where mariculture contributed approximately half (Wang et al. 2016). Ensuring the quality and safety of both cultured species and surrounding environment is the central dogma for the sustainability of mariculture. One concern is the high frequency of plant-nutrient pollution (eutrophication) which could lead to serious ecological events such as algal blooms where mariculture products are exposed to algal toxins and further pose adverse risks to the human consumers (Gibble et al. 2018). Plant nutrients (such as nitrogenous and phosphorus compounds) derived from industrial and agricultural wastewater discharges and also the feed residues and excreta of mariculture itself are believed to be the reason of the eutrophicaiton. These plant nutrients tend to enter sediment (Glibert and Burkholder 2011; Schumacher et al. 2014), and can be released from the sediment to the overlying seawater which would further augment the eutrophication (Guan et al. 2009; Zhang et al. 2010).
Another rising concern is the health-related risks caused by toxic-chemical contamination (Guo et al. 2008; Liang et al. 2016; Gu et al. 2017). Toxic chemicals such as heavy metals and hydrophobic organic pollutants (HOCs) are of potential toxicity and of ability to be bioaccumulated in sediment and organisms (Kibria et al. 2012; Wang et al. 2014; Lu et al. 2015; Trellu et al. 2016). HOCs such as polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenyl ethers (PBDEs), pyrethroids (PEs), and organochlorine pesticides (OCPs) in mariculture sediments were found to be significantly correlated with the chemicals remaining in bivalve species (Yap et al. 2004; Zhang et al. 2009; Bao et al. 2012; Mwakalapa et al. 2017; Amato et al. 2018; Tang et al. 2018). Meanwhile, sediments are one main source of heavy metals and HOCs in aquaculture ponds via the process of diffusion, advection, and biological agitation (Reible et al. 1996; Giusti et al. 1999; Smalling et al. 2010; Frognerkockum et al. 2016).
Therefore, for a high-density mariculture, it is quite necessary to control plant-nutrient pollutants, heavy metals, and HOCs in the sediments simultaneously. Currently, sediment amendment with adsorbents has been demonstrated as an effective technique for controlling pollutants in aquatic systems (Ghosh et al. 2011). Biochar is a solid material obtained from the thermo-chemical conversion of biomass in an oxygen-limited environment, and it has been found effective for HOC stabilization (Khan et al. 2015). In comparison with other carbonaceous materials, biochar is also of lower cost and higher sustainability, thus becoming a widely used adsorbent (Kah et al. 2017). However, biochar would form a large amount of hydroxyl radicals during the process of stabilization and damage the cell membranes, thereby inhibiting the growth of marine organisms, the adverse effect of which could be more pronounced than that of contaminants (heavy metals and HOCs) themselves (Liao et al. 2014). Moreover, a considerable content of PAHs (638–12,347 μg kg−1) was found to be produced during biochar synthesis, the potential negative effects of which during biochar application have not yet been properly considered (Wang et al. 2017; Wang et al. 2018). Besides, the efficacy of biochar on immobilizing ionizable pollutants (such as nitrogen and heavy metals) was found to be much poorer compared with nature zeolites (Misaelides 2011; Mukherjee and Zimmerman 2013).
Natural zeolites are a type of rack-shaped aluminosilicate mineral containing alkali metal and alkaline-earth metals. Natural zeolites are composed of an elementary structure of an aluminosilicate framework which comprises of a tetrahedral arrangement of silicon cations (Si4+) and aluminum cations (Al3+) that are surrounded by four oxygen anions (O2−). Each oxygen ion within the Si-O and Al-O bonds connects two cations and is shared between two tetrahedrons, thus yielding a macromolecular three-dimensional framework of SiO2-and-AlO2-tetrahedral-building blocks. Being of significant worldwide occurrence, natural zeolite has been widely applied to control the release of ammonium from sediment due to its high capacity of cation exchange (Wang and Peng 2010; Li et al. 2018a). Previous studies of our group showed that natural zeolite has a significant stabilization efficacy on phosphorus compounds (Lin et al. 2007) and heavy metals (Li et al. 2018b) in the sediment.
The Si-O-Si bonds on the surface of natural zeolites are also found to be hydrophobic (Chen 1976; Gilchrist et al. 1993; Wirawan et al. 2015). Thus, natural zeolite is of great potential of adsorbing HOCs along with plant nutrients and heavy metals, by different mechanisms though. The proposed underlying mechanism of bioaccumulation reduction of PAHs is mass transfer from porewater to biochar sorptive sites (Khan et al. 2015). However, few studies were reported on stabilizing organic pollutants by amending sediments with natural zeolites. Moreover, natural zeolites contain essential elements that can be absorbed and utilized by animals via selective ion exchange and therefore considered healthy for organisms (Baek et al. 2018). Therefore, it is worthwhile to evaluate natural zeolites as a eco-friendly, cost-effective material for mixed-contaminant stabilization including HOCs.
It has been shown that the freely dissolved concentration in sediment pore water (Cfree) drives the bioaccumulation process of HOCs from the bulk sediment to the exposed aquatic organisms (USEPA 2012; Ghosh et al. 2013; Lang et al. 2015; Li et al. 2018b). Thus, Cfree as an indicator for the stabilization effectiveness of HOCs by natural zeolite amendment in sediment is worth of further investigation, thus bypassing chemial analysis in organisms. The equilibrium passive sampling technique using polymers (e.g., polydimethylsiloxane or PDMS) has been well established to measure Cfree for a broad spectrum of HOCs (such as PAHs, PCBs, PBDEs, PEs, and dioxins) based on their PDMS-water partition coefficients (KPDMS-water) (Hunter et al. 2010; Mayer et al. 2014; Li et al. 2016, 2018b).
In the present study, natural zeolites were used as the HOC-stabilization material in mariculture sediments and exposed with Venerupis (V.) philippinaram for bioaccumulation testing. PDMS was applied for measuring Cfree and was also used in bioaccumulation data interpretation. The specific tasks of this study included (1) assessing the effectiveness of natural zeolite amendment in sediment on reducing the bioaccumulation of sediment-associated HOCs in V. philippinaram; (2) identifying the correlation between the accumulation of HOCs in V. philippinaram and Cfree using PDMS; and (3) validating the feasibility and reliability of using PDMS for predicting HOC bioaccumulation in benthic organisms and further the stabilization effect of natural zeolite in maricultural sediment.
2 Materials and methods
2.1 Chemical analysis
The target compounds included eight OCPs (α-HCH, β-HCH, γ-HCH, δ-HCH, p,p′-DDD, p,p′-DDE, hexachlorobenzene, heptachlor epoxide) (Dr. Ehrenstorfer Inc., GER), 16 PAHs (NAP, ANY, FLU, ACE, PHE, ANT, FLA, PYR, CHR, BaA, BbF, BkF, BaP, DBA, IPY, BghiP) (Sigma-Aldrich Inc., USA), four PEs (λ-cyhalothrin, bifenthrin, permethrin, fenpropathrin), and eight PBDEs (BDE-17, BDE-28, BDE-66, BDE-71, BDE-85, BDE-100, BDE-153, BDE-154) (Accustandard Inc., USA). For PAH analysis, pre-deuterated standards were used as internal standards, including acenaphthene-d10, chrysene-d12, perylene-d12, and phenanthrene-d10 which were purchased from Dr. Co. For PE analysis, 13C6-λ-cyhalothrin, 13C6-cis-permethrin, and 13C6-trans-permethrin (Cambridge Isotope Laboratories, Andover, MA, USA) were used as internal standards. For OCP analysis, 13C-α-HCH, 13C--β-HCH, 13C-γ-HCH, 13C-δ-HCH, 13C-p,p′-DDD, 13C-p,p′-DDE, 13C-hexachlorobenzene, and 13C-heptachlor epoxide were used (Cambridge Isotope Laboratories Inc.). For PBDE analysis, 13C-BDE47, 13C-BDE99, 13C-BDE100, 13C-BDE153, and 13C-BDE154 were used (Cambridge Isotope Laboratories Inc.) These HOCs were measured following the methodologies published previously by our group (Chen et al. 2013; Li et al. 2015). Details of the instrumental methods were also provided in Section S1 (Electronic Supplementary Material - ESM).
Quality control procedures for sample analysis included triplicate analysis, method blanks, and spiked samples. For each sample, the relative standard deviation (RSD) of the analytical results was less than 15%. For every ten samples analyzed, a blank sample was included to check laboratory contamination. Pre-extracted samples were used as the blank samples. For recovery analysis, the pre-extracted sediment samples were spiked with OCPs, PAHs, PEs, and PBDEs of 0.1, 1, and 10 ng g−1 dw; 5, 50, and 500 ng g−1 dw; 1, 10, and 100 ng g−1 dw; and 1, 10, and 100 ng g−1 dw, respectively; the pre-extracted Venerupis philippinarum samples were spiked with OCPs, PAHs, PEs, and PBDEs of 10, 100, and 1000 ng g−1 lipid; 100, 1000, and 10,000 ng g−1 lipid; 100, 500, and 1000 ng g−1 lipid; and 100, 500, and 1000 ng g−1 lipid, respectively. Additional details of limit of quantification (LOQ), average recoveries of each compound, and internal standards are provided in Table S1 (ESM). GraphPad Prism 6 (GraphPad Software, Inc., USA) and Spass 8.0 were used for statistical analysis.
2.2 Sediment and natural zeolite
Surface sediment samples (0–10 cm) from a Portunus trituberculatus (P. trituberculatus) pond (S1) in Shanghai (N30° 53′,E121° 58′), a Venerupis philippinarum (V. philippinarum) pond (S2), and a Perca fluviatilis (P. fluviatilis) pond (S3) in Huzhou of Zhejiang Province (N30° 55′, E120° 5′) in China were collected by the Peterson sampler (PSC-1/16) in August 2018. These ponds were selected based on our previous investigation on HOC concentrations in the cultured animals from aquaculture ponds around Yangtze River Delta (Table S2, Fig. S1 - ESM). Specifically, the concentrations of PAHs, PEs, PBDEs, and OCPs in these cultured animals ranged from 291–493 ng g−1 dw, 62–72 ng g−1 dw, 89–244 ng g−1 dw, and 39–64 ng g−1 dw, respecively, at the higher end of the comparison. Moreover, these three seletected species, i.e., two kinds of bentho (P. trituberculatus and V. philippinarum) and one kind of nekton (P. fluviatilis) were typically cultured and consumed in Eastern China. Sediment samples were put in solvent-rinsed foil bags right after collection and placed on ice, and then delivered to the lab and stored at − 20 °C until analysis.
Natural zeolite was purchased from Jinyun, Zhejiang Province, China. The cation exchange capacity of this natural zeolite is 1300~1800 mmol kg−1, the molar ratio of Si/Al is 4.25~5.25, and the chemical composition is SiO2 (70%), Al2O3 (12%), Fe2O3 (0.87%), K2O (1.1%), CaO (2.6%), MgO (0.13%), and Na2O (2.6%). X-ray diffraction analysis revealed that the natural zeolite contained 66% clinoptilolite, 19% mordenite, and 15% silica (Li et al. 2018a). The natural zeolite was ground and sieved to obtain particles of 0.15–0.18 mm diameter, and then washed five times with deionized water prior to the experiments.
2.3 Animal husbandry
V. philippinaram were purchased from Shanghai Luchao Aquatic Product Market and were acclimated under laboratory conditions for 21 days before experiments. The V. philippinaram used in the study were healthy adult ones with a shell length of 32 ± 2 mm and a shell height of 12 ± 2 mm. During V. philippinaram acclimation, the salinity and temperature of the seawater were kept at 25 ± 2 and 20 ± 1 °C, respectively, and the water was aerated continuously. The photoperiod was 16 h light followed by 8 h dark, and V. philippinaram was fed daily with a regular amount of algae (Phaeodactylum) (2 g/tank) (Ngo et al. 2016). The lipid content was determined using the method in the previous study (Chen et al. 2017). The lipid content of V. philippinarum was 0.51% ± 0.15%.
2.4 Experiment design
For the bioaccumulation experiment, the sediments from each sampling site were divided into two groups: a natural zeolite-amended group and a natural zeolite-free group. Each group had three replicates. A preliminary experiment was conducted to determine the maximum amount of zeolite to be added to the sediment for ensuring a < 10% mortaility of V. philippinaram (Li et al. 2018a). The addition amount of zeolite was then set 10% based on sediment dry weight (dw), since it would yield the best adsorption rate on phosprous and heavy meatals with the relatively highest survival rate of organisms (Lin et al. 2007; Li et al. 2018a).
For the natural zeolite-amended group, approximately 2 kg (wet weight with 30% water content) of each sediment sample was placed in a 5-L glass tank, and the natural zeolite was then added and homogenized. The homogenized mixture of sediments and natural zeolite were kept away from direct sunlight at the temperature of 4 °C for 28 days before use in the following-up experiments. Same procedure was followed for the zeolite-free group.
2.4.1 Bioaccumulation experiment
The sediments of two groups in the tanks were covered with 4 L artificial seawater (ISO-10253). Twenty-five pieces of V. philippinarams were added into each tank. The bioaccumulation experiment lasted for 28 days in line with the EPA recommended method (600/R-99/064). During the accumulation period, the mortality rate of V. philippinarams in the natural zeolite-free group was less than 10%. Upon finishing the accumulation, V. philippinarams and sediments were sampled and freeze-dried for chemical analysis using the methods provided in Table S1 and Section S1 (ESM).
2.4.2 Determination of Cfree using PDMS-based equilibrium sampling
Polydimethylsiloxane (PDMS) disk (density of 0.97 g mL−1, Specialty Silicone Products Inc., Ballston Spa, NY) was used as passive samplers to assess the bioavailable fraction in the sediment experiments. The volume and the weight of the PDMS disks were 15.07 μL and 17.6 mg, respectively. The minimum sediment mass was the most important factor in the sediment matrix equilibrium passive sampling to satisfy the criteria of < 5% depletion in relation to the entire sediment. The required sediment weight (as dry weight (dw)) is listed in the Table S3 and Table S4 (ESM). More sediment (500 g wet weight) than the minimum amount needed according to these theoretical considerations in order to ensure the non-depletive conditions for the system of sediment, DOC, and pore water. Polydimethylsiloxane (PDMS) was cut into pieces of 16 mm in diameter. The PDMS sheets were ultrasonically cleaned three times with 15 mL of n-hexane/acetone (V/V = 1:1) for 15 min each time. The cleaned PDMS film was keeped in a cleaned 100 mL bottle filled with methonal/water (V/V = 1:1) before use. One piece of pre-cleaned PDMS and approximately 500 g (wet weight, ww) of aged sediment was added to 1 L glass jar (water content adjusted to 60%).
Where CPDMS and KPDMS-water represent the concentration of a target analyte in PDMS after equilibrium partitioning and the partition coefficient of the compound between PDMS and water, respectively. Literature data of KPDMS-water were sought and listed in Table S5 (ESM).
2.5 Statistical analysis
Spearman correlation analysis was carried out for the correlation between the decline rates in Cb and in Cfree, the decline rates of the chemicals in sediments and their original levels in the sediments, and for the correlation between logKow values and decline rates in Cfree. t test analysis was carried out for the differentiation between the decline rates in Cb and in Cfree. Pearson correlation analysis was carried out for the comparison of the measured 38 individual HOC concentrations that were detected in V. philippinaram to their predicted values based on the model. Origin 8.0 (OriginLab, USA), GraphPad Prism 5.0 (USA), and Spass 20.0 (IBM, USA) were used for these statistical analyses.
3 Results and discussions
3.1 Concentrations and compositions of HOCs in the sediments of maricultural ponds
Concentrations and compositions of OCPs, PAHs, PEs, and PBDEs in the surface sediments from three ponds were shown in Table S6 and Fig. S2 (ESM). Thirteen out of 16 targeted PAH congeners were detected in the sediment samples (1234–3526 ng g−1 dw for two-ring congeners, 33–51 ng g−1 dw for three-ring congeners, 29–71 ng g−1 dw for four-ring congeners, and 120–864 ng g−1 dw for five-ring congeners). Two-ring and five-ring congeners were the dominating PAHs, followed by three- and four-ring congeners. Of note, the concentration of FLU and DBA ranged from 1234–2939 ng g−1 dw and 35–528 ng g−1 dw, respectively, which exceeded the mid-value of reaction interval in Sediment Quality Guidelines (SQG) of USA for these two chemicals (Long et al. 1995). The mid-value SQG of FLU and DBA is set as 540 ng g−1 dw and 260 ng g−1 dw, respectively, which is an indicator of potential ecological risk in the sediments proposed by Long et al.
Four out of the 10 target PBDEs were detected in the sediment samples, the concentrations of ∑4PBDEs ranged from 35 to 76 ng g−1 dw, which was significantly higher than those found in Holland Scheldt River (De et al. 2003) and New Brunswick aquaculture farm (Sather et al. 2006). Wherein, BDE85 and BDE154 account for 69–91% of ∑4PBDEs (Fig. S2 - ESM), being the dominant congeners. Gu et al. (2017) found that compared with a non-aquaculture area, PBDE concentrations were significantly higher in all matrices in the aquaculture area.
β-HCH was the only kind of OCPs detected. PEs in the maricultural ponds was mainly consisted of lambda-cyhalothrin, accounting for 60–89% of ∑2PEs. The concentrations of ∑OCPs and ∑2PEs ranged from 8–15 ng g−1 dw and 8.8–69 ng g−1 dw, respectively. Compared with the pesticide residue levels in the 42 surface sediment samples collected along the southern coast of the East China Sea (2.4 to 7.2 ng g−1 dw) (Lin et al. 2012), the concentrations of this study were at a relatively higher level, which might be attributed to an excessive application of pesticides in Yangtze River Delta, a region with a high density of agriculture activities. No DDT was detected in all samples, which indicates no source of DDT sources in Yangtze River Delta (Wang et al. 2014).
3.2 Effectiveness of natural zeolite amendment in reducing bioaccumulated HOCs in V. philippinaram
Hazard index (HI) of OCPs, PEs, and PBDEs in V. philippinaram was calculated for risk assessment. The HI is a non-carcinogenic risk index, has been used as an indication of risk from oral exposure (Teuschler and Hertzberg 1995; Evans et al. 2015; Domenech and Martorell 2017). After adding zeolite to the sediments, the non-carcinogenic risk index (HI) of ∑OCPs, ∑PEs, and ∑PBDEs decreased by 9–20%, 36–39%, and 36–55% (Table S8 - ESM). The potential incremental lifetime cancer risk (ILCR) index of PAH congeners (Li et al. 2018c) was also calculated and a decrease of 11–54% was observed (Table S9 - ESM). Our results suggest that natural zeolite amendment can reduce the health risk through bivalve consumption and improve the quality of surrounding media to a certain degree.
Undeniably, the effect of natural zeolite on reducing the bioaccumulation of HOCs is not as remarkably good as biochar (Fang and Gan 2014). However, in comparison with biochar, natural zeolites are more available and provide higher nitrogen-immobilization efficiency. For a comprehensive mixed-contaminant stabilization, the value of natural zeolites on HOC stabilization should not be neglected for the noticeable effectiveness. Therefore, the role of natural zeolite in HOC stabilization should be further investigated for improving the efficacy.
3.3 Effectiveness of natural zeolite amendment in stabilizing sediment-associated HOCs
Cfree of HOCs in sediment pore water was determined by the concentrations in PDMS (CPDMS) and the PDMS-water partition coefficient (KPDMS-water) according to Eq. (1) (Mayer et al. 2003). Compared with the zeolite-free group, the total freely dissolved concentration (Cfree) of HOCs in sediment zeolite-amended group showed decreases (Table S5 - ESM). Specifically, Cfree of ∑OCPs, ∑PAHs, ∑PEs, and ∑PBDEs reduced by 6.8–11%, 10–17%, 10–16%, and 15–20%, respectively (Fig. 1). This result was consistent with the theory that natural zeolites reduce Cfree and the bioavailability of sediment-associated HOCs simultaneously.
Both Cb and Cfree showed a certain degree of decrease in the zeolite-amended group, showing significantly different decline rates (t test, P < 0.05) with higher rate for Cb of ∑PAHs, ∑PEs, and ∑PBDEs (Fig. 1). As V. philippinaram would inevitably ingest particle-borne contaminants, in the natural zeolite amendment group, a reduction in the assimilation of particle-associated HOCs in V. philippinaram might have occurred. It might be the reason for the higher decline rate for Cb than Cfree (Klaassen et al. 1994).
3.4 Factors related to efficacy of natural zeolite amendment in stabilizing sediment-associated HOCs
As mentioned above, natural zeolite is a type of clay mineral with siloxane-type surface where HOCs could be absorbed by weak polarity or non-polarity instead of entering cavities of zeolite. The apolar component and polar component of surface tension for the natural zeolite were measured at 20 °C to be 16 mJ m−2 and 14 mJ m−2, respectively, following the method described by Lee et al. (2000). Therefore, HOC adsorption in mineral zeolite occurs primarily on the surface of hydrophobic siloxanes (Gilchrist et al. 1993; Jaynes 2015). The stronger the hydrophobicity of the compound, more easily it will be adsorbed on the surface of the siloxane. Higher Kow value leads to better adsorption of the hydrophobic HOCs by the zeolite, which in turn affects the values of Cfree. It is consistent with the results of Xie et al. (2012) on the effect of the hydrophobicity of phenolic compounds during the stabilization by modified zeolite in wastewater. Jovanović et al. (2006) also came to a similar conclusion that the higher the hydrophobicity of the pesticides, the stronger adsorption capacity on zeolite.
In addition to the Kow values of HOCs, the property of sediments was found to be another factor affecting the stabilization of chemicals in natural zeolites. For the same chemical, the decline rates in the zeolite-amended groups were different for different sediment characteristics. The organic matter content and external surface properties of the sediments were found to be the key factors determining the adsorption capacity of organic pollutant (Toul et al. 2003). It can be seen that the physical and chemical properties of the sediments from three different ponds were different (Table S3 - ESM). A positive correlation was found between the particle size of sediments and decline rates of Cfree for PEs, and a negative correlation was found between the specific surface area of the sediments and decline rates of Cfree for PBDEs (Table S10 - ESM). It is understandable that smaller particle size would also provide a larger surface area, resulting in a stronger retention of HOCs onto the sediments (Meng et al. 2014). Due to the limited number of samples, this statistical result of correlation needs further experimental verification is needed in the future.
3.5 Bioavailability-based assessment of the efficacy of natural zeolite amendment
The potential of natural zeolites for HOC stabilization in sediments was proved. The bioaccumulations of four kinds of HOCs (PAHs, PBDEs, PEs, and OCPs) in V. philippinaram were reduced by 10–38% after adding natural zeolite (10% dry weight) in sediments. The freely dissolved concentration of HOCs in sediment pore water (Cfree) measured by using PDMS passive sampler was reduced proportionally. PDMS passive sampling method was then assessed for competently predicting the concentration of HOCs in V. philippinaram and the stabilization efficacy of natural zeolite. Kow of HOCs and the properties of sediments were found to be influencing factors of the stabilization efficacy. Modifying natural zeolite to further improve the HOC-stabilization efficacy is warranted. Further studies can be focused on the influence of sediment characteristics/organism taxa on the bioavailability of sediment-associated HOCs and on the in situ application of passive samplers.
The present research was supported by Shanghai Science and Technology Commission Key Support Fund (18050502100), Shanghai Municipal Science and Technology Commission Fund (D-8003-18-0043), Shanghai Collaborative Innovation Center Fund (A1-2037-16-0001-12), Shanghai Ocean University Technology Development Fund (A2-0203-00-100223), and Shanghai Ocean University Doctoral Foundation (A2-0203-00-100352).
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