CHEMICAL ENGINEERING TRANSACTIONS VOL. 62, 2017 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Fei Song, Haibo Wang, Fang He Copyright © 2017, AIDIC Servizi S.r.l. ISBN 978-88-95608- 60-0; ISSN 2283-9216 Adsorption and Anoxic Degradation of Typical PAHs in Bohai Coastal Sediments Shuaijie Wang*a, Lu Qina, Ying Liub, Yao Guana, XiaoShan Tana aSchool of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China bChina Electronics Engineering Design Institute, Beijing 100142, China wangshuaijie@ysu.edu.cn This paper aims to disclose the adsorption and anoxic degradation processes of three typical polycyclic aromatic hydrocarbons (PAHs) in the coastal sediments of China’s Bohai Sea region. The PAHs are naphthalene (Nap), phenanthrene (Phe) and pyrene (Pyr). Several sediments samples were collected from Qinhuangdao Port, and subjected to adsorption and biodegradation experiments. Then, pseudo-first-order and pseudo-second-order kinetic models were tested to explain the kinetics of liquid-solid phase adsorption systems. It is discovered that the adsorption of the three PAHs on the coastal sediments matched pseudo- second-order kinetics, revealing that adsorption processes were essentially chemical reactions. In terms of equilibrium adsorption capacity, the three PAHs are ranked as Pyr>Phe>Nap. Next, Freundlich and Langmuir models were employed to fit the experimental data. The fitting results show that the adsorption of Nap, Phe and Pyr on the coastal sediments can be explained as mono-molecular layer adsorption. All the adsorptions of Nap, Phe and Pyr were exothermic reactions. In the anoxic sediments, 93.2% of Nap was removed after 30h, 96.6% of Phe was removed after 7d, and 98.1% of Pyr was removed after 42d. The detected metabolites of Nap were [4,4-dimethyl-2-cyclohexene-1-ol], [1-carboxyl,2-hydroxy, 4-methyl-cyclohexane], the metabolites of Phe were [3-ethoxy ethyl benzoate], [Phthalate (2-methyl propyl) ester] and the metabolites of Pyr were [2- methyl succinic acid, double (2-methyl propyl) ester], [Phthalate (2-methyl propyl) ester], [Phthalic acid butyl ester, 14 alkyl ester]. The findings shed new light on the research of PAHs adsorption and anoxic degradation in coastal sediments. 1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are toxic, potentially carcinogenic and mutagenic. Sixteen PAHs compounds have been included in the European Union and US EPA priority pollutant lists (Chang et al., 2002; Lu et al., 2012; Quan et al., 2009). In recent years, the PAHs pollution has spread to the marine environment due to damaging human activities like oil spill, incomplete combustion of fossil fuel, etc. (Mirandola and Lorenzini, 2016). The highly hydrophobic PAHs could be strongly adsorbed on the organic matters in sediment, resulting in massive accumulation in coastal areas (Chen and Chen, 2011; Wang et al., 2012). The PAHs in the coastal sediments poses a direct risk to organisms and an indirect risk to humans (Haritash and Kaushik, 2009). Therefore, it is highly necessary to remove the PAHs from costal sediments and minimize their adverse effects. Among the various removal techniques, bioremediation has gained prominence in recent years (Chang et al., 2002; Fuchedzhieva et al., 2008; Haritash and Kaushik, 2009). During the bioremediation of PAHs, the catabolic activity of microbes mainly uses oxygen as terminal electron acceptors. However, the polluted sediments are mostly anoxic due to the limited amount of oxygen in subsurface sediments (Dou et al., 2009). Therefore, the predominant metabolic mode of PAHs removal from costal sediments lies in anoxic biodegradation. In China, the Bohai Sea region is one of the regions most heavily polluted by the PAHs. Lin et al. (2005) investigated the spatial distribution of the PAHs in surface sediments of the Bohai Sea region, and discovered that the highest concentration of total PAHs was located in the coastal area at Qinhuangdao with a detection rate of 43 %. The results show that PAHs have presented a high ecological risk to the local area. In light of the above, this paper explores the adsorption and anoxic degradation behaviour of the typical PAHs in Bohai DOI: 10.3303/CET1762204 Please cite this article as: Shuaijie Wang, Lu Qin, Ying Liu, Yao Guan, XiaoShan Tan, 2017, Adsorption and anoxic degradation of typical pahs in bohai coastal sediments, Chemical Engineering Transactions, 62, 1219-1224 DOI:10.3303/CET1762204 1219 costal sediments. The samples were collected from Qinhuangdao Harbour. The typical PAHs include naphthalene (Nap), phenanthrene (Phe) and pyrene (Pyr). The purpose is to lay a scientific basis for artificial restoration of Bohai coastal sediments. 2. Materials and methods 2.1 Chemicals The Nap (purity=99%), Phe (purity=99%) and Pyr (purity=99%) were supplied by Anshan Tianchang Chemical Company. The dichloromethane (analytical grade) was purchased from Qinhuangdao Chemical Reagent Factory. 2.2 Sampling The samples of coastal water and sediments were collected from Qinhuangdao Harbour, China’s Hebei Province. The sampling location for sediments was 10~20cm deeper than the surface sediment. The samples were sieved to remove plants and other debris, and stored at 4°C for further study. The fresh sediment contained 8.73% of water. For the coastal water samples, the salinity was 31‰, and the pH values were 7.4~8.2. The contents of Nap, Phe and Pyr in the sediment samples were 33.4μg/kg, 118.2μg/kg and 165.9μg/kg, respectively. 2.3 Adsorption experiments Adsorption kinetics: 100μg Nap was added to a 50mL conical flask, which contained 10g sterilized sediment and 10mL deionized water. Then, the flask was shaken on an oscillator at 25°C and 175rpm. Finally, the residues of the PAH in water was measured at 0, 10, 20, 30, 60, 90, 180, 300 and 420min by ultraviolet spectrophotometry. The same steps and parameters were applied to the Phe and Pyr. Adsorption isotherm: Different amounts of Nap were added to six 50mL conical flasks, which contained 10g sterilized sediment and 10mL deionized water. The dosage was 50μg, 100μg, 160μg, 200μg, 240μg and 320μg, respectively. Then, the flasks were shaken on an oscillator at 25 °C (35 °C, 45 °C) and 175rpm. Finally, the residue of the PAH in water was measured at the moment of adsorption equilibrium. Except for dosage, the same steps and parameters were applied to the Phe and Pyr. For the Phe and Pyr, the dosage was 4μg, 10μg, 50μg, 100μg, 160μg and 200μg respectively. 2.4 Biodegradation experiments Fresh coastal sediments, each equivalent to 10g dry sediment, were placed in several 50mL conical flasks. An appropriate amount of PAHs were added, respectively, into the sediment to obtain a concentration of 20µg/g (Nap/sediment), 10µg/g(Phe/sediment) and 10µg/g (Pyr/sediment). To maintain a moist environment, 3mL seawater was added to each flask to cover the sediment. The flasks were then placed on an oscillator and shaken for 2h at 200rpm. Then, the flasks were blanketed with nitrogen, sealed up, and left for incubation at 23±1℃. The residues of PAHs in the sediments were determined at different intervals. 2.5 Analytical techniques The PAHs were extracted from the sediments using 10mL dichloromethane. After oscillation, ultrasonic extraction, cleaning and quantitative dilution, the residues dissolved in dichloromethane were taken for ultraviolet spectrophotometry analysis. The optimal wavelength of absorption was 274nm for Nap, 249nm for Phe, and 238nm for Pyr. The recovery rate of Nap residues in the sediment samples at 5, 10 and 20μg/g fell in 100.9%~109.0%, with the relative standard deviation (RSD) ranging from 2.65% to 4.77%; the recovery rate of Phe residues in the sediment samples at 1, 5 and 10μg/g fell in 108.3%~115.0%, with the RSD ranging from 2.85% to 3.05%; the recovery rate of Pyr residues the sediment samples at 1, 5 and 10μg/g fell in 102.5%~118.4%, with the RSD ranging from 4.03% to 4.27%. The metabolic products were analysed by an Agilent 7890 Series Plus gas chromatograph linked to a quadrupolar mass spectrometer (Agilent 5975C). 3. Results and Discussions 3.1 Adsorption of PAHs on Bohai coastal sediment (1) Adsorption kinetics Featuring high octanol-water partition coefficient (Kow) and low water solubility, organic pollutants like PAHs easily adsorbed by sediments in the coastal environment. According to the adsorption behaviour curves (Figure 1) of Nap, Phe and Pyr on coastal sediments, the adsorption capacities of the three PAHs increased 1220 rapidly in the initial 20min, the adsorption rates slowed during 20~60min, and no further adsorption was observed after 60min. Therefore, 60min was regarded as the moment of adsorption equilibrium for the three PAHs. Figure 1: Adsorption curves of Nap, Phe and Pyr on Bohai coastal sediment Two kinetic models, i.e. pseudo-first-order and pseudo-second-order kinetic models, were tested to explain the kinetics of liquid-solid phase adsorption systems. The pseudo-first-order and pseudo-second-order equations can be expressed as: ln(Qe-Qt)=lnQe-k1×t t/Qt=1/(k2•Qe2)+t where Qe and Qt (mg/kg) are the equilibrium adsorption capacity and the adsorption capacity at time t, respectively, k1(min-1) and k2(kg/(mg•min) )are the adsorption rate constants for the pseudo-first order and pseudo-second order kinetics, respectively. The fitting results were shown in Table 1. Table 1: Adsorption kinetic parameters of the pseudo-first-order and pseudo-second-order models PAHs pseudo-first-order model pseudo-second-order model Experimental adsorbance (mg/kg) k1 (min-1) Qe (mg/kg) correlation coefficient k2 (kg/(mg·min)) Qe (mg/kg) correlation coefficient Nap 0.0873 7.17 0.9814 0.023 5.78 0.9989 5.65 Phe 0.1505 22.33 0.9033 0.027 7.73 0.9982 7.58 Pyr 0.1491 12.21 0.9741 0.086 8.28 0.9998 8.24 From Table 1, it can be seen that the pseudo-second-order kinetic model had a higher fitting degree than pseudo-first-order kinetic model; the experimental equilibrium adsorption capacities obtained by the pseudo- second-order kinetic model were closer to the theoretical results than those obtained by the pseudo-first-order kinetic model. The results show that the adsorption of the three PAHs on the coastal sediments is consistent with pseudo-second-order kinetics, and that chemical adsorption played a major role in the adsorption process. In terms of equilibrium adsorption capacity, the three PAHs are ranked as Pyr>Phe>Nap. This is because the Kow of Nap, Phe and Pyr increases with the number of benzene rings. The greater the Kow, the easier the PAHs to be adsorbed by sediments. (2) Adsorption isotherm Figure 2 displays the adsorption isotherms of Nap, Phe and Pyr at different temperatures. As shown in Figure 2, there was a nonlinear relationship between the equilibrium adsorption amount of PAHs on the sediments (Qe) and the equilibrium concentration of PAHs in the solution (Ce). Besides, the adsorption amount wave negatively correlated with the temperature, as long as the latter fell within the scope of the experimental temperature. The results show that the adsorptions of the three PAHs onto costal sediments were exothermic reactions. 0 30 60 90 120 150 180 210 240 270 300 330 360 0 2 4 6 8 10 A d s o rp ti o n C a p a c it y (m g /k g s e d im e n t) time(h) Nap Phe Pyr 1221 Figure 2: Adsorption isotherms of three PAHs on sediment at different temperatures In view of the similar adsorption isotherms at 25 ˚C, 35 ˚C and 45 ℃ for three PAHs, the adsorption isotherms at 25 ℃ were chosen to be fitted. Freundlich and Langmuir models were employed to fit the experimental data. The two models are expressed as: Freundlich model: Qe=KF×Cen Langmuir model: Qe=Q[Ce/(Ce+A)] where Q(mg/kg) is the maximum adsorption capacity, KF and n are empirical coefficients, A is a constant related to adsorption energy. The fitting results were shown in Table 2. Table 2: The fitting results of adsorption isotherm models PAHs Langmuir model Freundlich model A correlation coefficient n KF correlation coefficient Nap 257.3 0.9941 0.9695 0.653 0.9886 Phe 252.7 0.9996 1.012 0.687 0.9971 Pyr 22.11 0.9980 0.9110 0.916 0.9968 As can be seen from Table 2, both Langmuir model and Freundlich model fitted the adsorption of the three PAHs well. In comparison, the better fitting effect belonged to the Langmuir model. This means the adsorption of Nap, Phe and Pyr on the coastal sediments can be explained as mono-molecular layer adsorption. 3.2 Anoxic degradation of PAHs in coastal sediment (1) Anoxic degradation behaviour 5 10 15 20 25 30 35 2 4 6 8 10 12 14 16 18 Q e (m g /k g ) Ce(mg/L) 25℃ 35℃ 45℃ Nap 0 5 10 15 20 0 2 4 6 8 10 12 14 16 Q e (m g /k g ) Ce(mg/L) 25℃ 35℃ 45℃ Phe 0 5 10 15 20 0 2 4 6 8 10 12 14 16 Q e (m g /k g ) Ce(mg/L) 25℃ 35℃ 45℃ Pyr 1222 Figure 3: Anoxic degradation curves of the three typical PAHs The removal ratio (%) was calculated as follows: Removal ratio = (C0-Ct)/C0×100% where C0 is the initial concentrations of the PAHs; Ct is the instantaneous concentrations of the PAHs. As shown in Figure 3, the Nap, Phe and Pyr added into Bohai coastal sediments were well utilized after a certain period of time under the action of native PAH-degrading microbes. The effective use is attributable to the abundance of microorganisms and presence of electron acceptors (e.g. NO2 -, NO3 - and SO4 2 -) in natural coastal sediments. The microbes could use the electron acceptors to degrade PAHs in the sediments to different degrees. Overall, 93.2% Nap was removed after 30h, 96.6% of Phe was removed after 7d, and 98.1% of Pyr was removed after 42d. The pseudo-first-order kinetic model was often introduced to depict the pollutant biodegradation in sediments/soil. lnCt= -k•t+lnC0 where k is the degradation rate constant obtained by linear regression of the data lnCt versus the incubation time (t). The fitting results are given in Table 3. Table 3: The degradation rate constants (k) and regression coefficients ( R2) PAHs k R2 Nap 0.0967h-1 0.9685 Phe 0.5601d-1 0.9284 Pyr 0.1108d-1 0.9576 Table 3 shows that the experimental data were well fitted with the results of the pseudo first-order kinetic model. Hence, the degradation rates were proportional to the concentrations of Nap, Phe and Pyr. (2) Metabolism of PAHs Through gas chromatography-mass spectrometry (GC-MS) detection, two metabolites of Nap, two metabolites of Phe and three metabolites of Pyr were detected during the anoxic degradation process in Bohai coastal sediments (Table 4). 0 3 6 9 12 15 18 21 24 27 30 33 0 10 20 30 40 50 60 70 80 90 100 R e m o v a l ra ti o (% ) time(h) Nap 0 1 2 3 4 5 6 7 8 0 20 40 60 80 100 R e m o v a l ra ti o (% ) time(d) Phe 0 6 12 18 24 30 36 42 48 0 20 40 60 80 100 R e m o v a l ra ti o (% ) time(d) Pyr 1223 Table 4: Metabolites of Nap, Phe and Pyr PAHs Metabolites Nap 4,4-dimethyl-2-cyclohexene-1-ol 1-carboxyl,2-hydroxy, 4-methyl-cyclohexane Phe 3-ethoxy ethyl benzoate Phthalate (2-methyl propyl) ester Pyr 2-methyl succinic acid, double (2-methyl propyl) ester Phthalate (2-methyl propyl) ester Phthalic acid butyl ester,14 alkyl ester 4. Conclusions This paper investigates the adsorption and anoxic degradation behaviours of Nap, Phe and Pyr in Bohai coastal sediments. The main conclusions are listed as follows. First, the adsorption equilibriums of Nap, Phe and Pyr were achieved within 60min. The adsorption kinetics could be described by the pseudo-second-order kinetic model. The adsorption processes of the three PAHs were essentially chemical reactions. Second, the adsorption isotherms of Nap, Phe and Pyr were nonlinear. The Langmuir model simulated the adsorption processes of the three PAHs more accurately than the Freundlich model, indicating that the adsorption of Nap, Phe and Pyr on the coastal sediments can be explained as mono-molecular layer adsorption. All the adsorptions of Nap, Phe and Pyr were exothermic reactions. Third, in the anoxic sediments, 93.2% of Nap was removed after 30h, 96.6% of Phe was removed after 7d, and 98.1% of Pyr was removed after 42d. 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