Removal of aqueous phenanthrene by brown seaweed Sargassum hemiphyllum: Sorption-kinetic and equilibrium studies

Separation and Purification Technology 54 (2007) 355–362

Removal of aqueous phenanthrene by brown seaweed Sargassum hemiphyllum: Sorption-kinetic and equilibrium studies M.K. Chung a , Martin T.K. Tsui a,1 , K.C. Cheung a , Nora F.Y. Tam b , M.H. Wong a,∗ a

Croucher Institute for Environmental Sciences and Department of Biology, Hong Kong Baptist University, Kowloon Tong, Kowloon, Hong Kong, SAR, China b Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, SAR, China Received 30 June 2006; received in revised form 9 October 2006; accepted 11 October 2006

Abstract The batch sorption-kinetics and equilibrium uptake of phenanthrene (PHE), a hydrophobic organic compound (HOC), in aqueous compartment were investigated using dead tissue of brown seaweed Sargassum hemiphyllum under various conditions for 24 h. It was found that the higher the shaking rates (50–250 rpm) and temperatures (15–35 ◦ C), the higher the sorption rates of PHE, but no significant changes were observed for the maximum sorption capacities. Study with different initial PHE concentrations (50–1000 ␮g L−1 ) showed that a higher ambient level of PHE resulted in a faster initial uptake rate and greater sorption capacity. The presence of ionic species (0.01–1 M NaCl) changed the sorption-kinetics of PHE markedly by altering the maximum removal of PHE, but not the initial sorption rates. No significant removal variations were noted under various initial pH (pH 2–11), while constant alkalinity resulted in an alleviated PHE sorption by Sargassum. The sorption-kinetics of PHE typically followed more closely to the pseudo-second-order model (r2 > 0.85) than pseudo-first-order equation (r2 > 0.72). Removal capacities were in the range of 430–460 ␮g g−1 for tests spiked with 1000 ␮g L−1 PHE. Typical percentage removals of aqueous PHE by Sargassum for all the investigated factors (e.g. initial pH and ionic strength) were in the range of 91.7–98.4%. Log Sargassum-water and the organic carbon normalized partition coefficient were calculated as 3.83 and 3.96 mL g−1 , respectively. The present study provided valuable information for achieving optimal sorption of aqueous PHE using Sargassum as an effective sorbent for removing HOCs in wastewaters and urban runoffs. © 2006 Elsevier B.V. All rights reserved. Keywords: Biosorption; HOCs; PAHs; Organic matter; Seaweed; Macroalgae

1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are aromatic hydrocarbons with two or more fused carbon rings. They are classified as persistent toxic substances (PTS) by United Nations Environmental Programme (UNEP) since they are characterized not only by their ubiquity and persistency in the environment, but also their toxicities to various organisms. Because of their low water solubility [1], PAHs are also commonly known as hydrophobic organic compounds (HOCs). Among these compounds, 16 of them are especially notorious for their mutagenic and carcinogenic properties and named as priority pollutants by the U.S. EPA [2]. Contamination of PAHs in urban runoffs has

∗

Corresponding author. Tel.: +852 3411 7746; fax: +852 3411 7743. E-mail address: [email protected] (M.H. Wong). 1 Present address: Water Resources Science Program, University of Minnesota, St. Paul, MN 55108, USA. 1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.10.008

been found [3–5] and subsequently to receiving water bodies [5] and edible fish [6] were also reported. Biosorption is a branch of biotechnology that uses dead or inactive biomass from various origins to reduce chemical concentrations in the aqueous compartment [7]. The cost is lower than ion exchange technology while still retaining comparable removal efficiency on various kinds of pollutants, from heavy metal ions, to soluble toxic chemicals such as phenol in pulp mill or dyes in industrial wastewaters [7]. The use of bacterial biomass as a sorbent to remove aqueous phenanthrene (PHE) was demonstrated by Stringfellow and Alvarez-Cohen [8], however, the removal percentages of PHE varied (42–91%). The effectiveness for removing metallic ions and polar organics in wastewaters by the marine brown seaweed, Sargassum, has long been recognized [9,10]. In contrast to bacterial biomass, little has been done to examine its potential for PHE sorption. Furthermore, Sargassum possesses other attractive features, such as relative ease of collection, abundant in worldwide coastal areas, stable quality than fermentation originated bacteria and

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its cellulose structure enabling a minimum pretreatment including immobilization and granulation of microbes [7]. Therefore, it has become one of the popular choices of biosorbents for removing toxicants, especially heavy metals in wastewaters. Most of the available studies are focused on ionic species such as heavy metals in contaminated wastewaters, and to certain extent, the polar organic pollutants such as organic dyes and phenolics. The characteristics of Stockholm POPs have caught our attention. In this study, we explored the effectiveness of using dried Sargassum hemiphyllum to remove aqueous PHE. PHE was chosen as a model of medium HOCs because of its abundance in the aquatic environment [11]. Various factors such as temperature, shaking rate and initial PHE concentration that may affect the uptake of PHE were also investigated. The sorption-kinetic and equilibrium experiments can provide important information for understanding the dynamic interactions between soption and environmental factors in batch systems and thus optimal conditions for remediation of HOCs by seaweed can be tailored. 2. Materials and methods 2.1. Algal sampling and preparation S. hemiphyllum was harvested as life stocks from Clear Water Bay, Hong Kong. It was rinsed thoroughly with deionized water and dried in oven at 50 ◦ C overnight. Subsequently it was sieved to retain 0.18–1 mm fraction by standard meshes (Endecotts Ltd., London) and stored in desiccator before use. 2.2. Standard setup In order to compare kinetic and equilibrium sorptions caused by different factors, a standard sorption condition was given and only one factor was varied each time while others were kept constant. Briefly, 0.4 g of prepared biomass was added to 200 mL deionized water in a 250 mL glass Erlenmeyer flask. Addition of PHE was by means of spiking 1000 ␮g L−1 methanol-dissolved PHE stock (>96%, Aldrich Chemical) by glass syringe at a final concentration <0.1% to avoid cosolvent effect. Sodium azide (>99%, Riedel-de Ha¨en) was added to a working concentration of 1 mg kg−1 to minimize biological degradation of PHE and the flasks were wrapped firmly by aluminium foil. The flasks were shaken in dark by an orbital shaking incubator at 25 ± 1 ◦ C for 24 h to achieve equilibrium sorption between the biomass and the PHE. 2.3. Sorption experiments Five factors each with five levels of progressive variations were investigated. These included two external factors such as shaking speed (50–250 rpm) and temperature (15–35 ◦ C) during sorption plus three other factors consisting of modification in pH (pH 2–11), ionic strength (0.01–1 M NaCl) and initial concentrations of PHE (50–1000 ␮g L−1 ) of the test solution. Only glasswares were in contact with the spiked solution throughout the experiments since significant loss of aqueous

PHE was found when using plastics (even Teflon). Totally there were 13 subsamplings obtained within 24 h. Flasks were shaken thoroughly before subsampling, and 1.5 mL of subsample was filtered by a glass fiber filter (GF-75, 0.7 ␮m, Advantec) and drained directly into 2 mL amber vials, in which the filters were pre-rinsed with another 1 mL aqueous sample to minimize loss of PHE by equilibrating the filters. The vials were measured immediately to diminish loss of PHE. Triplicates were prepared for each level. In addition, three Sargassum-free controls with identical setups were assembled to quantify the loss of aqueous PHE to other factors than Sargassum, under predefined conditions. 2.4. Quantification of phenanthrene Quantification of aqueous PHE was performed by high performance liquid chromatography (HPLC) equipped with one micro vacuum degasser, a phootodiode array detector (DAD) and a fluorescence detector (FLD) (1100 Series, Agilent). Aliquots of 10 ␮L of sample was drawn and injected into the HPLC by an autosampler. The separation was done by the analytical reverse-phase Zorbax C18 column, with 4.6 i.d. × 150 mm ˚ pore size (Eclipse XDBdimension, 5 ␮m particle size and 80 A C18, Agilent), thermostated at 25 ◦ C. Eluting reagent comprised of 90% acetonitrile (HPLC grade, >99.9%, Tedia) and 10% milli-Q water (Millipore Corp.) at a flow rate of 1 mL min−1 with isocratic gradient for 5 min. Quantitation of peaks were calculated by signals from FLD using external standards. Absorbance wavelength was set to 254 nm for DAD, while the excitation and emission wavelengths for FLD detector were 244 and 360 nm, respectively. Detection limit was calculated as three times the signal to noise ratio as 2 ␮g L−1 for FLD. 2.5. Calculation of sorbed phenanthrene by Sargassum Mass of PHE sorbed by Sargassum in equilibrium was calculated by mass balance. Loss of PHE on GF filters was reproducible (CV < 5%, n = 3) ranging from 20 to 4% along sampling intervals. Thus, concentrations of PHE were corrected in advance. The amount of PHE taken up by sorbent was calculated as follows: qt =

Cctt Vt − Ct Vt × 1000 m

(1)

where qt (␮g g−1 ) is the amount of sorbed PHE by Sargassum at time t; Cctt and Ct (␮g L−1 ) are the aqueous concentrations of PHE in control and in experimental setup at time t, respectively; Vt (L) is the volume of solution at time t; and m (g) is the mass of Sargassum used. The mean concentration of PHE in control flasks under standard condition at 24 h was 927 ± 36 ␮g L−1 (93% of the spiked amount). 2.6. Kinetic modelings Two kinetic models, pseudo-first-order equation [12] and pseudo-second-order equation [13] were used to fit the experimental data. The non-linear lines in the kinetic graph (Figs.

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Fig. 1. Sorption-kinetics of Phe on Sargassum powder (␮g g−1 ) under various environmental effects for 24 h (mean and S.D. of three replications): (a) shaking speeds (rpm) and (b) temperatures (◦ C).

1, 2 and 4) were calculated from pseudo-second-order equation. All the graphs were plotted by Sigmaplot® Version 9.0.

2.7. Removal efficiency of phenanthrene by Sargassum The removal capacity (RC) of PHE is calculated as: RC =

V0 C0 − Vf Cf m

(2)

While the percentage removal of aqueous PHE (%R) is obtained as: %R =

(V0 C0 − Vf Cf ) × 100% V 0 C0

(3)

2.8. Partition coefficient The Sagassum-water partition coefficient (log Ksw ) of PHE at 25 ◦ C was calculated as follow: qeq Ksw = (4) Ceq Ksw values were obtained from regression analysis of qeq versus Ceq for five different initial PHE levels. Since the sorption of HOCs typically depended on the organic carbon content of the sorbent, and consequently the organic carbon (OC) normalized partition coefficient Koc can be calculated as [14]: Koc =

Ksw . OC%

(5)

3. Results and discussion 3.1. Kinetic studies

where V0 and C0 are the initial volume and concentration of solution; Vf and Cf are the final volume and concentration of aqueous PHE (t = 24 h).

Table 1 compares the kinetic data among treatments. The data generated was fitted into the linearilzed pseudo-first-order

Fig. 2. Sorption-kinetics of Phe on Sargassum powder (␮g g−1 ) under various pH for 24 h (mean and S.D. of three replicates): (a) initial acidic and alkaline conditions and (b) maintained with constant alkalinity.

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Table 1 Rate constants and correlation coefficients for pseudo-second order kinetic model for batch sorption studies of PHE under the influence of various factors at 25 ◦ C for 24 h k2

r2

(A) Rotation speed (rpm) 50 100 150 200 250

0.0004 0.0014 0.0033 0.0072 0.0103

0.84 0.98 0.99 0.99 0.99

(B) Sorption temperature (◦ C) 15 20 25 30 35

0.0024 0.0027 0.0033 0.0047 0.0045

0.99 0.99 0.99 0.99 0.99

(C) Initial sorption (pH) 2 5 7 9 11

0.0031 0.0023 0.0024 0.0022 0.0026

0.99 0.99 0.99 0.99 0.99

(D) Constant sorption (pH) 2 5 7 9 11

0.0031 0.0023 0.0024 0.0027 0.0033

0.99 0.99 0.99 0.99 0.99

(E) Ionic strength (M of NaCl) 0.01 0.05 0.1 0.5 1

0.0032 0.0036 0.0038 0.0047 0.0051

0.99 0.99 0.99 0.99 0.99

(F) Initial concentration (␮g L−1 ) 50 0.0506 250 0.0093 500 0.0054 750 0.0040 1000 0.0033

0.99 0.99 0.99 0.99 0.99

and pseudo-second-order kinetic equations. For most of the conditions investigated, the pseudo-second-order model was able to describe the kinetic behavior of PHE more accurately (r2 = 0.85–1.00) when compared with the pseudo-first-order model (r2 = 0.72–0.99). Thus, we will focus to examine the corresponding rate constant (k2 ) of pseudo-second-order model further (Table 1). 3.2. Environmental effects—agitation rate and sorption temperature Fig. 1a shows the performance of batch kinetic sorption of aqueous PHE by Sargassum under various agitating speed (50–250 rpm) for 24 h. Table 1a lists the corresponding rate constants k2 and demonstrated they were proportional to agitating speed while Table 2a reveals that the maximum sorption capacities of sorbate remained largely constant at equilibrium. This finding is in accordance with the basic theory of sorption that

Table 2 Batch removal capacity (RC) and percentage removal (%R) of PHE by Sargassum under influence from different factors at 25 ◦ C for 24 h RC (␮g g−1 ) (A) Rotation speed (rpm) 50 100 150 200 250

%R

453.0 454.9 450.6 455.7 445.9

± ± ± ± ±

5.9 a 3.2 a 4.4 a 2.3 a 3.2 a

93.2 93.6 92.7 93.8 91.7

± ± ± ± ±

1.2 a 0.7 a 0.9 a 0.5 a 0.7 a

(B) Sorption temperature (◦ C) 15 455.4 20 458.4 25 450.6 30 452.6 35 454.6

± ± ± ± ±

3.6 a 3.2 a 4.4 a 8.7 a 1.4 a

93.7 94.3 92.7 93.1 93.5

± ± ± ± ±

0.7 a 0.7 a 0.9 a 1.8 a 0.3 a

(C) Initial sorption pH 2 5 7 9 11

437.6 456.0 447.9 452.0 430.8

± ± ± ± ±

5.7 a 1.4 b 2.4 ab 1.6 ab 4.6 a

92.0 95.8 94.1 94.8 91.4

± ± ± ± ±

1.2 a 0.3 b 0.5 ab 0.3 ab 1.0 a

(D) Constant sorption pH 2 5 7 9 11

437.6 456.0 447.9 414.2 374.2

± ± ± ± ±

5.7 a 1.4 b 2.4 ab 2.0 c 1.7 cd

92.0 95.8 94.1 86.9 79.4

± ± ± ± ±

1.2 a 0.3 b 0.5 ab 0.4 c 0.4 cd

(E) Ionic strength (M of N NaCl) 0.01 447.4 0.05 442.4 0.1 427.8 0.5 417.4 1 399.7

± ± ± ± ±

2.1 a 2.3 b 0.4 c 4.7 d 1.7 e

92.0 92.4 92.9 95.3 98.4

± ± ± ± ±

0.4 a 0.5 a 0.1 b 1.1 b 0.4 b

(F) Initial concentration (␮g L−1 ) 50 22.7 250 108.1 500 210.0 750 330.9 1000 450.6

± ± ± ± ±

0.1 a 0.8 b 3.4 c 0.5 d 4.4 e

96.2 94.6 91.7 92.2 92.7

± ± ± ± ±

0.4 a 0.7 a 1.5 ab 0.1 ab 0.9 a

Groups sharing the same letter within each treatments (vertical section) are not significantly different as classified by one-way ANOVA test (Tukey HSD, p < 0.05).

increasing the contact rate between PHE and sorbent favours the mass transfer of PHE between aqueous and solid phase. Various temperatures were selected (15–35 ◦ C) and maintained during the experiment. However, the effects were significantly weaker (Fig. 1b) in contrast to those caused by aforementioned factor (shaking speed). The k constants increased in coherent with the increasing temperature treatments from 15–30 ◦ C (Table 1b), but the situation did not hold true at 35 ◦ C as a slight drop in k value was observed when comparing with the k value at 30 ◦ C. In contrast, the equilibrium removal capacities of the PHE were similar among treatments (Table 2b). Although not strong, this general relationship between k constant and temperature suggested the sorption of hydrophobic PHE to Sargassum was chiefly an endothermic reaction. The low enthalpy change involved in the organic-water partitioning was well known [15] and this is in line with the results presented here.

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Fig. 3. Comparison of initial and final pH values for various pH treatments under experimental conditions for 24 h (mean and S.D. of three replicates): (a) without maintaining pH and (b) designated pH were maintained after each samplings.

3.3. Chemicals interaction in the solution—effect of pH and ionic strength Fig. 2a plots the sorption-kinetic graph for PHE by Sargassum under different initial pH. The pH series span from acidic (pH 2) through neutral (pH 7) to alkaline (pH 11) conditions. Interestingly, the equilibrium removal capacities were generally lower in very acidic (pH 2) and alkaline (pH 11) solution, but higher for treatments around neutral (pH 5, 7, 11. Table 2c). This bi-polar phenomenon was also observed in their corresponding k values (Table 1c). Fig. 3a presents further investigation by comparing the initial and final solution pH, and the Sargassum biomass was very resisting to the alkaline environment. To address the effects of prolong alkaline period on sorption behavior, another batch experiments were carried out by maintaining the alkalinity after each sampling (Figs. 2b and 3b). There were significant decline in the equilibrium removal capacities and higher k values at pH 9 and 11 (Tables 1d and 2d). A very strong clear brown color was developed during the pH-maintaining experiments, and it is believed that a reduction in PHE immobilization was caused by

an increase of dissolved organic matter (DOM) content in the solution. The release of colloidal compounds from dead biomass and their influence on the solubility of HOCs were also noted by other studies [16,17]. The DOM behaves as a cosolvent-like colloid, thus boosting the solubility of PHE and enhanced their solubility in aqueous compartment. Fig. 4a indicates ionic content in the aqueous solution induced profound effects on the sorption performance of the aqueous PHE. The initial sorption rates within the first 3 h were apparently very similar, a weak trend of increasing k values (Table 1e) implied mass transfer of PHE from aqueous phase was possibly faster along with elevated ionic concentration in solution. Equilibrium removal capacity of sorbate was the lowest (400 ␮g g−1 ) for the most salty solution (1M NaCl) (Table 2e), but in contrast, the k values were inversely proportional to the ionic strength. The types of sorption interactions included but not limited to sorbate–solvent, sorbate–sorbent, sorbate–container surface; salt–solvent, salt–sorbent, salt–sorbate. It is expected that the elevated electrolyte in the solution interacts more strongly with water molecules than PHE-water interaction, thus reducing

Fig. 4. Sorption-kinetics of Phe on Sargassum powder (␮g g−1 ) under different conditions for 24 h (mean and S.D. of three replicates): (a) ionic strength of the solution (M of NaCl) and (b) initial concentrations of PHE.

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the solubility of the aqueous PHE or even DOM-PHE interaction (salting out) and forcing the PHE to adhere on other surfaces such as Sargassum, glass flask or escape to vapour phase. Our results ruled out the increased sorption of PHE by Sargassum. Ionic species can interact strongly with Sargassum as it is known to be a strong sorbent for heavy metals such as Pb and Cd [18], the biomass may sorb and accumulate the ionic species such that the ionic charge is higher than the ambient environment. However, whether these interactions are significant in controlling the sorption behaviours of PHE are still unknown. Hoffman et al. [5] demonstrated that urban runoff is a major source of PAHs discharging to coastal areas. Typical salinity of seawater is roughly equivalent to 0.6 M NaCl and the present finding of a weaker sorption capacity of PHE by Sargassum under a higher (above 0.1 M NaCl) salinity suggested that a decrease in the sorption of medium HOCs (log Kow around 4–5) by the organism may happen in coastal areas receiving urban discharges. This further implied that the toxicities caused by PAHs in the discharges to coastal organisms should be lower when compared with freshwater habitats. 3.4. Effect of initial concentration According to Fig. 4b, which shows the sorption-kinetics of a series of initial PHE concentrations (50–1000 ng mL−1 ), the sorption of aqueous PHE was a concentration-dependent process. Higher initial concentrations of sorbate resulted in a higher probability of collision, thus drove smaller k values (Table 1f) and in turn a greater uptake by the biomass (Table 2f). This relationship was in consistent with other studies using biomass of banana and microalgae for sorption of pollutants [19,20]. 3.5. Sorption isotherms Data in Fig. 4b can be transformed and plotted as an equilibrium isotherm (Fig. 5). The isotherm reveals the potential

Fig. 5. Isotherm plot of Phe sorption by Sargassum powder under standard condition at 25 ◦ C (mean and S.D. of three replicates).

maximum uptake of sorbate by sorbent in a defined system at fixed temperature. The strong linear relationship observed (r2 = 0.985) implied that the Sargassum is an effective sorbent for the removal of PHE at low concentration (maximum initial concentration as 1000 ␮g L−1 ). A linear relationship was also obtained using bacterial biomass as sorbent [8], since isotherm frequently exhibited linearity in narrow concentration range. 3.6. Removal efficiency of phenanthrene Understanding how the investigated factors influencing biosorption are crucial to determine the potential design for a cost and time effective strategy for the treatment of runoff and wastewater. The RCs and %Rs of PHE by Sargassum under different conditions are tabulated in Table 2. RCs for PHE were generally in the range of 430–460 ␮g g−1 , except for the test with different initial concentrations. The percentage removals of aqueous PHE at constant alkalinity were below 85% while greater than 90% when considered all cases. It Table 3 Partition coefficients of aqueous PHE for Sargassum hemiphyllum at 25 ◦ C as calculated from this study and coefficients for various sorbents adapted from others log Kd

log Koc

(A) Biotic Cellulosec Sargassum hemiphyllum Ligninc Algaec Green River kerogenc Cuticlec Oxidized humic acidc Degraded algaec Humic acidc Collagenc Pula kerogenc

2.98 3.83 4.03 4.13 4.19 4.22 4.28 4.43 4.45 4.47 4.71

3.33 3.96e 4.18 4.39 4.64 4.50 4.56 4.66 4.67 4.72 4.88

(B) Abiotic Sedimentb DOM from soild Coala Traffic soota Activated carbona Coal soota Wood soota Black carbon in New York Harbor Sedimentb Oil Soota Black carbon in Boston Harbor Sedimentb Diesel soot (SRM 1650)a Charcoala Fly asha Graphitea

3.90 4.00 4.34 4.60 4.75 4.91 5.00 5.60 5.93 6.10 6.16 6.25 7.45 8.76

N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A.

Log selected compound, water (Kd ) (mL g−1 ) and log organic carbon, water partition coefficients (Koc ) (mL g−1 ) are listed. N.A.: not available. a Jonker and Koelmans [22] at 20 ◦ C. b Lohmann et al. [23]. c Salloum et al. [24]. d Poerschmann and Kopinke [25]. e Based on the organic carbon content as equal to crude protein, crude lipid and total dietary fiber obtained by freeze-dried method [26].

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was also revealed that the relationship between RC and %R was inversely proportional, and especially prominent under the effects exerted by different ionic strength and initial PHE concentrations. However, it should be noted that a small portion of aqueous PHE is removed not by the biomass but by the adsorption on the internal surface of glassware. Thus, actually RCs and %Rs by Sargassum may be lower than the reported values in Table 2. 3.7. Comparison of partition coefficient Partition coefficient (Kd ) is an effective parameter to estimate the distribution of target chemicals between two phases and hence their fate and mobility in the environment. By comparing the sorbent-water Kd values for PHE of different compounds, the ranking of sorption efficiency of these sorbent to aqueous PHE and other HOCs can be estimated. Table 3 lists various partition coefficients of PHE to different common phases and the log Kd and log Koc values calculated from this study. Biotic-origin sorbents are having log Kd values from 2.98 to 4.71, in contrast with 3.90–8.76 for the abiotic soot or soot-like materials. The calculated log Kd for Sargassum was 3.83, implied its sorption affinity for the PHE and other HOC was low when compared with the soot-like materials, which has been found to be exceptionally strong for immobilization of HOCs in aquatic ecosystems [21]. Although the log Kd value was low (3.83), but given the large Sargassum population that is typically found in coastal sea floor, it could be a significant contributor to control the moderately HOCs contained in urban discharge from moving deeper to the ocean. 4. Conclusions It was found that factors affecting sorption rate of PHE by Sargassum were shaking rate of the sorption system, and to less extent, the environmental temperature during sorption. Initial concentration of aqueous PHE, ionic strength and the alkalinity of the solution were factors influencing the sorption capacity of PHE. However, more study is needed to understand how exactly the ionic strength and alkalinity of the solution affect the sorption capacity by Sargassum. Sargassum typically can remove more than 90% of aqueous PHE under investigated conditions and thus it can be an effective candidate for tackling HOC pollutions. Acknowledgments The authors thank B. Yeung from Department of Biology (CUHK) for collecting macroalgae; Y.K. Chung, from Department of Statistics and Actuarial Science (HKU) and K. So, from School of Computer Science and Engineering (UNSW, Australia) for their advice on using mathematical models and statistical analyses. This study was supported by the Area of Excellence (AoE) Scheme under the University Grants Committee of the Hong Kong Special Administrative Region (CITYU/AoE/0304/02).

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