Anthracene Adsorption to Particles and Water-Stable Aggregates of Mangrove Sediment in Jiulong River Estuary, China

Polycyclic aromatic hydrocarbons (PAHs) pollution in mangroves has drawn much attention, but knowledge of the sorption of PAHs in mangrove sediment is limited. This study investigated the particles and water-stable aggregates (WSA) of mangrove sediment in Jiulong River Estuary, China, and the characteristics of anthracene adsorption to them. The adsorption of anthracene was strongly influenced by the physicochemical and structural properties of sediment particles and WSA. The main sorbents of mangrove sediment were carbonized particles and clays. The porous structure of carbonized particles made it easy to sequestrate sequester the anthracene, and the aging al-lowed anthracene to move into deeper sites of the carbonized particles. Clays had high anthracene-fixing capacities, and they included organic matters and formed aggregates. The sorption contents coefficient K f of anthracene with WSA of different sizes increased in the order 0.063 - 0.25 mm > 0.063 mm > 0.25 - 1.0 mm > 1.0 mm. The order was correlated with which due to the contents and characteristics of organic matters in the aggregates.

soil particles could affect the bioremediation of these pollutants. Especially after long years of aging, these compounds can move into the inaccessible compartments of soil particles and lead to sequestration, which may reduce extractability and result in low bioavailability [21] [22] [23] [24] [25]. Therefore, it is important to study the sorption of PAHs in mangrove sediment particles and aggregates. Many studies have focused on the distribution of PAHs in different particles of surface soils or sediments, but the results are various because of different sampling sites [19] [26]- [31].
The mangrove sediment has unique properties such as organic-rich matters and fine grains [3]. It was also under a flooded condition which can damage its physical structures through physical dispersion. So that, the particles and water-stable aggregates of mangrove sediment are important for the sorption of PAHs and then influence their degradation. However, the physical composition and structural properties of the mangrove sediments were not reported and its relationship with the sorption of PAHs in sediments was unknown. This study aims at investigating the particles and water-stable aggregates of mangrove sediments and the sorption of a typical PAH, anthracene in these particles and water-stable aggregates.
The stock solution of anthracene was prepared by dissolving an appropriate amount of regent in anhydrous ethanol, with a final concentration of 100 mg/L.
The stock solution was kept in a brown bottle at 4˚C and wrapped with aluminium foil to avoid any light exposure prior to use. Working solutions of anthracene were prepared by transferring small aliquots of each stock solution into several glass tubes. After allowing evaporation of the solvent by a gentle flow of high-purity nitrogen gas (≥99.99%), Mill-Q water was added to the mark of all the colorimetric tubes to obtain a series of the PAHs working solutions for sorp-Journal of Environmental Protection tion experiment. The trichloromethane and methanol used for PAH analysis were HPLC-grade solvents purchased from Tedia (USA). The calcium chloride (CaCl 2 ) used for adsorption experiment was analysis grade purchased Sigma Aldrich (USA).

Sediment
The surface (0 -10 cm) sediment sample was collected from the mangrove wetland (24˚29'N, 117˚55'E) in Jiulong River Estuary, which is one of the largest river/estuary systems in south China and is the major drinking water source for Xiamen and the main freshwater source for its sea area [11]. The wet sediment was brought back to the laboratory and used for separating particles and water-stable aggregates immediately. The other portion of sampled sediment was air-dried and passed through a 2 mm sieve and the dry sieved sediment was used for the anthracene sorption experiment.
Selected properties of the mangrove sediment were determined [32] and as follows: pH (in H 2 O) 6.66, organic matter 28.4 g/kg, total N 1.13 g/kg, total P 0.85 g/kg, and total K 19.2 g/kg. The background concentrations of the studied PAHs in the sediment proved to be <2% of the lab-added concentrations.

Sediment Particle Separation
The sediment was physically separated by a size and density separation method as described by Ghosh (2000) [26]. Wet sieving was first performed to separate the sediment into four size fractions (>1.0, 1.0 -0.25, 0.25 -0.063, and <0.063 mm). The larger size fractions (>0.063 mm) comprised primarily of sand grains, coal-derived particles, and woody material. It was possible to wash off the lighter fractions (coal and wood) from the heavier sand by swirling with water in a beaker and draining off the entrained lighter particles. Materials in the fine fraction (<0.063 mm) were density separated using a cesium chloride solution having a specific gravity of 1.8. Five grams of wet sediment and 40 mL of cesium chloride solution were centrifuged at 200 rpm for 10 min in 50 mL glass centrifuge tubes. The fine coal-derived and wood particles floated, were decanted, and were collected on filter paper and rinsed with water several times. The heavy clay and silt fractions were similarly washed several times to remove cesium chloride.
Each of these sizes and density separated fractions was then weighted and investigated for particle structures by a scanning electron microscope (SEM, HITACHI S-4800 FE-SEM).

Sediment Water-Stable Aggregates (WSA) Separation
The WSA of the sediment was analyzed by using a slight modification of the wet-sieving technique described by Elliott (1986) [33]. Briefly, the moist sediment samples were wet sieved manually through a series of three sieves to obtain four size fractions: (>1.0, 1.0 -0.25, 0.25 -0.063, and <0.063 mm). The WSA in each size fraction was collected, dried and weighed.

PAH Sorption in Sediment Particles
The PAH contaminated sediments were prepared according to the method described by Hatzinger and Alexander (1995) [34]. The dry sieved sediments were sterilized by an autoclave at 103.4 kPa and 121.3˚C for 2 h to prevent any microbial activity during the sorption experiment. 500 ng/mL anthracene was prepared by taking 250 μL of anthracene stock solutions to a 50 mL brown volumetric flask; methanol was added to the mark after allowing evaporation of the solvent by a gentle flow of high-purity nitrogen gas (≥99.99%). Then 20.00 g portions of sterilized sediments were added to sterilized 50 mL screw-cap test tubes.
To each tube was added 2 mL anthracene solution. The liquid was added dropwise to bring the anthracene concentration to 50 ng/g dry weight (dw) sediment.
Several tubes were placed in a hood for 1 h, and the samples were shaken to allow the methanol to evaporate and to ensure thorough mixing of PAH with the sediments. Deionized water was then added to bring the moisture level of sediments to 50% (W/W); and the other tubes with PAH contaminated sediments incubated in the dark at 20˚C ± 1˚C for aging 100 d [35] [36].
The fresh and aged PAH contaminated sediments were size and density separated by the method described above, each of these sizes and density separated fractions were then analyzed for PAH concentration by GC-MS.

PAHs Sorption in WSA
The

Statistical Analyses
All of these experiments were performed in triplicates and the results presented were average values of the three replicates. Data were analyzed statistically using analysis of variance (ANOVA) and Duncan's multiple range tests were employed to determine the significance of the differences between the parameters.
The statistical package used was the SPSS statistical software package (Version 11.0) and the confidence limit was 95%. Particle structures in each size and density separated fraction were then investigated using SEM, and the results are shown in Figure 1 and Figure 2. Figure 1 shows that the particles in the light fractions of the mangrove sediment are mainly structural organic materials, including plant residues and fragments and carbonaceous particles (such as coal dust, coke, and so on). Plant residues and fragments is the plant residue suffering natural wear and decay which retained some plant tissue structure (Figure 1(a) and Figure 1 (Figure 2(a)); the clay particles are composite grain and have laminated structure; the particle size is small and is more distributed in the fraction of <0.063 mm (Figure 2(b)).

Sediment Particle Characterization
In addition, sediment aggregates particles were observed in >1.

The Distribution of PAH in Sediment Particles
The concentrations of unaged and aged anthracene in each size and density separated fraction were analyzed and the results are shown in Table 2  contents of the PAH in this fraction increased after aging.
The main particles in 0.063 -0.25 mm and 0.25 -1.0 mm heavy fractions, sand and silt, had low sorption capacity of PAHs. That is why the contents of PAHs in these fractions were smaller than others. Clays that existed in <0.063 mm fraction have stratified structures and large surface areas, thus easy to absorb PAHs.
Moreover, the clays made up a high proportion of the mangrove sediment and thus they absorbed most of PAHs. After aging, the contents of the PAHs decreased in <0.063 mm fraction, but increased in >1.0 mm, heavy fraction. It was probably because aging caused the PAHs to penetrate deeper into sediment aggregates, which were observed in >1.0 mm, heavy fraction.

The Physical Structure and Chemical Properties of Sediment Aggregates
The above experiment results showed that sediment aggregates affected the PAHs sorption obviously. So we did further study focused on the characters of sediment aggregates and PAHs sorption in which. The microstructure of WSA with different sizes of the sediment was investigated by using SEM, as shown in Figure 3. Figure 3 shows that the inorganic mineral particles in the sediment bond together or adhere to the skeleton particles and shape microorganisms The chemical properties of sediment aggregate are shown in Table 3. Table 3 shows that TOC content of aggregate with different size were decreased in the order 0.063 -0.25 mm > 0.25 -1.0 mm > 1.0 mm > 0.063 mm. This phenomenon can be explained by the particle components of the sediment as discussed above.
The plant debris and carbonaceous particles mainly distribute in the fraction of 0.25 -1.0 mm and 0.063 -0.25 mm (see Table 1), which made the higher TOC content of the aggregates with these two sizes.

PAH Sorption in Sediment Aggregates
The sorption isotherm data for anthracene in different size aggregates are shown in Figure 4. The fitting parameters from the Linear model and Freundlich model are presented in Table 4. From Table 4 Table 4, it can be seen that, the higher sorption capacity of 0.063 -0.25 mm aggregate can be explained by its higher TOC and C/N.
From Table 4, it can also be seen that the n values of Freundlich model for most types of aggregates were less than 1, which indicated that the sorption of PAHs on these aggregates was nonlinear. The n value of >1.0 mm aggregates was smaller than that of others, which indicated that the non-Linearity of the sorption of PAH in >1.0 mm aggregates was higher than that of others. That may be The different superscript lower-case letters in each column indicate significant differences.

Conclusion
The results showed that the sorption of anthracene was strongly influenced by the physicochemical and structural properties of sediment particles and WSA.
The main sorbents of mangrove sediment were carbonized particles and clays.
The porous structure of carbonized particles made it easy to sequestrate the PAH, and the aging made the PAH come into deeper sites of the carbonized particles; clays had good PAH-fixing capacities and they always combined by or-