A comparative study on metal sorption by brown seaweed

Chemosphere 65 (2006) 51–57 www.elsevier.com/locate/chemosphere

A comparative study on metal sorption by brown seaweed Martin T.K. Tsui a, K.C. Cheung a, Nora F.Y. Tam b, M.H. Wong

a,*

a

b

Croucher Institute for Environmental Sciences and Department of Biology, Hong Kong Baptist University, Kowloon Tong, Kowloon, Hong Kong SAR, PR China Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, PR China Received 4 August 2005; received in revised form 14 February 2006; accepted 1 March 2006 Available online 2 May 2006

Abstract This study compared the sorption of Ag, Cd, Co, Cd, Mn, Ni, Pb and Zn by a Ca-treated Sargassum biomass at pH 5.0, under low and high ionic strength (IS) conditions. The sorption isotherms of As [As(V)] and Cr [Cr(III) and Cr(VI)] were also determined at low IS. The isotherm data for the eight cationic metals and Cr(III) were well fitted by Langmuir equations. Generally, the maximum metal uptake (Umax) followed: Cr(III) > Pb  Cu > Ag  Zn  Cd > Ni  Mn  Co  Cr(VI)  As(V) at low IS and Pb > Cu > Co > Mn  Cd > Zn  Ag > Ni at high IS. As(V) did not bind to the seaweed at pH 5.0. The results indicated that sorption of Pb was not affected by the increasing IS, though the percentage of free Pb ions in the water was greatly reduced as predicted by the speciation model. High IS lowered Umax by 10–36% (except Co and Pb), and lowered the affinity constant of the metal by 33–91% for all cationic metals, as compared to low IS. Moreover, the removal efficiency of the cationic metals and Cr decreased exponentially with initial metal concentrations and was lower at high IS. Ion-exchange was the mechanism responsible for the cationic metal sorption onto the seaweed, and Na ion interfered with the cationic metal binding through electrostatic interaction. In conclusion, this study showed the differential binding capacity of the Sargassm biomass for different metals and oxidation states and the differential effects of IS. According to the present results, Sargassum may be considered a good biosorbent for cationic metals (especially Pb) in both low and high-salt containing wastewater. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Biosorption; Sargassum hemiphyllum; Heavy metals; Ionic strength; Ion exchange

1. Introduction Contamination of toxic metals in the aquatic environment is a widespread phenomenon, especially in the developing countries where high-cost remediation technology is not affordable. Biosorption is a low-cost technology for removing anthropogenic chemicals from water, using readily available biomass from nature (Volesky, 2003). Among many biosorbents, marine seaweed is an excellent biosorbent for metals (Volesky and Holan, 1995). A number of studies (e.g., Davis et al., 2000; Diniz and Volesky, 2005) demonstrated a high metal binding capacity of brown seaweed. In terms of metal sorption capacity, *

Corresponding author. Tel.: +852 3411 7054; fax: +852 3411 7743. E-mail address: [email protected] (M.H. Wong).

0045-6535/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2006.03.002

brown seaweed is superior to other algal species such as red and green seaweeds (Schiewer and Wong, 1999, 2000; Jalali et al., 2002; Hashim and Chu, 2004; Sheng et al., 2004). Among different brown seaweeds, it has been noted that Sargassum spp. possess a relatively high metal binding capacity (Hashim and Chu, 2004). However, most of these studies mainly focused on only a few metals such as copper, cadmium and nickel. It is therefore essential to determine if this brown seaweed can potentially remove other metals which commonly occur in industrial effluents, and also to study the effect of increasing ionic strength on the sorption of metals by this seaweed because many industrial effluents may have highsalt content (Gao et al., 2004; Vijayaraghavan et al., 2005). This information is critical to the design of an efficient remediation system to remove different heavy metals

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in industrial facilities. Comparing sorption of different metals at different ionic strength conditions can give a better conclusion than comparing the results from different studies (due to different methodologies adopted). Furthermore, it is equally important to understand the underlying mechanism of metal binding by the seaweed under both low and high ionic strength conditions. 2. Materials and methods 2.1. Seaweed and pretreatment Brown seaweed, Sargassum hemiphyllum, was collected as life stocks in a relatively uncontaminated coastal site at Port Shelter, Clear Water Bay (Yung et al., 2001). After transport back to the laboratory, the seaweeds were first washed thoroughly with deionized water and then air-dried for 2–3 days. Afterwards, the dried seaweeds were blended in a homogenizer into finer particles. Sizes of seaweed ranging from 0.2 to 1.0 mm were obtained by passing through stainless steel standard sieves. In order to remove the original cations on the seaweeds, the biomasses were pretreated by soaking in 50 mM Ca2+ (as Ca(NO3)2 Æ 4H2O) for 24 h (Diniz and Volesky, 2005) with agitation at 150 rpm, and a solid-to-liquid ratio of 20 g l1. Afterwards, the Ca-saturated seaweeds were washed with deionized water 10 times at 20 g l1 in order to remove the excess Ca2+. The washed seaweeds were dried overnight at 50 °C in an oven and stored in dessicators before use. 2.2. Metals Ten metals or metalloids including arsenic [As(V)], cadmium (Cd), chromium [Cr(III) and Cr(VI)], cobalt (Co), copper (Cu), lead (Pb), manganese (Mn), nickel (Ni), silver (Ag) and zinc (Zn) were selected for this investigation. All chemicals are of analytical grade or better. Metal salts of nitrate were employed (except Co and Mn, in chloride form) because this should give the least interaction with the metal ions. Arsenic as KH2AsO4, and Cr(III) as CrCl3 and Cr(VI) as K2Cr2O7 were tested. Different metal solutions were prepared in ultrapure water prior to the experiments. 2.3. Metal sorption experiments In this study, the metal sorption performance by the seaweed was compared at low (0 mM NaNO3) and high ionic strengths (100 mM NaNO3 or equivalent to 9.74 mS cm1) (Schiewer and Wong, 2000), with the exception of As and Cr (low IS only). The conductivity of the high IS solution used in this study was within the same order of magnitude of (but usually higher than) the conductivity of different kinds of industrial effluents (Gao et al., 2004; Vijayaraghavan et al., 2005). In each set of the sorption experiments, there were 5–8 concentrations (from 0.1 to 10 mM) and there were triplicate flasks for each concentration. For each

replicate, 100 ml of metal solution was prepared in a 250 ml Erlenmeyer borosilicate glass flask. Before adding biosorbents, 10 ml of metal solution was collected from every flask to measure the initial concentration. Afterwards, 200 mg of Ca-loaded seaweeds were placed in contact with 90 ml of metal solution. The pH of the seaweed-metal slurry was monitored using a daily-calibrated pH meter (Orion Model 230A; Boston, MA, USA), and the pH of the mixture was maintained at 5.00 ± 0.30 using 0.1 M HNO3 or 0.1 M Ca(OH)2 during the course of the contact period. The reason for using Ca(OH)2 instead of NaOH was to avoid the introduction of the fourth cationic species in the system of the low IS experiments (i.e., besides H+, Ca2+, and metal ion). The flasks were shaken at 150 rpm for 24 h in the dark in a temperature-controlled incubator (at 20 °C) to reach equilibrium for the sorption system (Hashim and Chu, 2004). After 24 h, 10 ml of the supernatant of the solution was collected for measuring the equilibrium (or final) metal concentration and the amount of Ca2+ released from the biomass (except the treatments of As and Cr). The metal uptake by the seaweeds was calculated using the following mass balance equation (Davis et al., 2000): U ¼ ½ðC i  C f Þ  V =m

ð1Þ

where U = metal uptake (mmol kg1); Ci = initial metal concentration (mM); Cf = equilibrium metal concentration (mM); V = volume of the solution (l); and m = dry mass of seaweed (kg). The Langmuir sorption model was used to fit the sorption data: U ¼ ðU max  C f Þ=ðb1 þ C f Þ

ð2Þ

where Umax = maximum metal uptake (mmol kg1); and b = affinity constant (l mmol1). These two parameters can reflect the nature of the sorbent material and compare the biosorption performance: Umax represents the maximum attainable binding capacity and b indicates the affinity of the metal ion toward the biomass. Moreover, the removal efficiency (RE, %) of the biosorbent on the metal in the solution was determined under low or high IS conditions by the following equation: RE ¼ ðC i  C f Þ  100=C i

ð3Þ

The release of excess Ca from the biomass in the water was also assessed, which was performed by adding Caloaded biomass into the water without any spiked metals and the pH was controlled at 5.0 as mentioned above. After 24 h of incubation, the supernatant was retrieved and analyzed for Ca concentration. Therefore, the Ca2+ released from the biomass in the actual experiment were obtained by subtracting this ‘excess Ca’ concentration. 2.4. Analytical chemistry and regression analyses All metals were analyzed using inductively coupled plasma optical emission spectroscopy (Perkin-Elmer Optima 3000 DV, Wellesley, MA, USA), with calibration

M.T.K. Tsui et al. / Chemosphere 65 (2006) 51–57

standards covering the range from 0.1 to 20 mg l1 using high-purity standards (Charleston, SC, USA). All regression analyses on the data were performed on SigmaPlot 9.0 (Systat Software, Point Richmond, CA, USA).

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system for metals in previous studies (Davis et al., 2000; Sheng et al., 2004). The pH of the solution was frequently maintained at pH 5.0 during the day and became more or less stable at night. After 24 h of mixing, the equilibrium sorption isotherms of different metals by the Caloaded biomass under both low and high ionic strengths are shown in Fig. 1. Generally, high ionic strength (IS) reduced the sorption of metals at high initial metal concentrations, with the exception of Pb. Overall, the experimental data were well fitted by the Langmuir sorption isotherms and the calculated Langmuir parameters are shown in Table 1.

3. Results and discussion 3.1. Metal sorption capacity at different ionic strengths Twenty four h was assumed to be adequate for the sorption system to achieve a state of equilibrium, which was far longer than the time to achieve an equilibrated

1.5

Cadmium (Cd)

Cobalt (Co)

Lead (Pb)

Copper (Cu)

1.2

Metal uptake (mmol g-1)

0.9 0.6

Low IS High IS

0.3 0.0 1.5

Silver (Ag)

Nickel (Ni)

Manganese (Mn)

Zinc (Zn)

1.2 0.9 0.6 0.3 0.0 0

2

4

6

8

10

12 0

2

4

6

8

10

12

0

2

4

6

8

10

12

0

2

4

6

8

10

12

Equilibrium metal concentration (mM) Fig. 1. Sorption isotherms of eight cationic metals by Ca-loaded Sargarssum biomass at pH 5.0 under low (0 mM NaNO3) and high (100 mM NaNO3) ionic strength (IS) conditions. Data are means ± standard deviation (n = 3).

Table 1 Langmuir parameters for metal biosorption on Ca-loaded Sargassum biomass under low (0 mM NaNO3) and high (100 mM NaNO3) ionic strength (IS) conditions at pH 5.0 Metals

Cadmium Cobalt Copper Lead Manganese Nickel Silver Zinc

Low IS

Cd Co Cu Pb Mn Ni Ag Zn

High IS

% change from low to high ionic strengths

Umax (mmol g1)

b (l mmol1)

r2

Umax (mmol g1)

b (l mmol1)

r2

Umax

b

0.693 ± 0.0385 0.616 ± 0.0212 1.11 ± 0.0299 1.15 ± 0.0261 0.628 ± 0.0615 0.603 ± 0.0335 0.935 ± 0.0837 0.706 ± 0.0732

5.08 ± 1.50 2.45 ± 0.475 6.93 ± 1.09 61.9 ± 9.82 1.01 ± 0.421 1.59 ± 0.322 0.317 ± 0.0555 2.32 ± 1.14

0.962 0.990 0.995 0.994 0.959 0.975 0.993 0.950

0.570 ± 0.0409 0.885 ± 0.0869 0.979 ± 0.0438 1.22 ± 0.0433 0.567 ± 0.0226 0.385 ± 0.0644 0.718 ± 0.0943 0.517 ± 0.04

2.06 ± 0.647 0.225 ± 0.0537 3.33 ± 0.669 21.5 ± 7.38 0.283 ± 0.0287 1.06 ± 0.687 0.158 ± 0.038 0.819 ± 0.202

0.973 0.990 0.991 0.992 0.998 0.896 0.996 0.985

18 +44 12 +6 10 36 23 27

60 91 52 65 72 33 50 65

Note: Umax = maximum metal uptake; and b = affinity constant. Data are means ± standard error.

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From the calculated maximum metal uptake (Umax), the sequence was in the following order: Pb  Cu > Ag  Zn  Cd > Ni  Mn  Co at low IS and Pb > Cu > Co > Mn  Cd > Zn  Ag > Ni at high IS. The effect of increasing IS from 0 to 100 mM NaNO3 decreased the Umax of Cd, Cu, Mn, Ni, Ag and Zn by 10–36%, but increased the Umax of Co and Pb by 44 and 6%, respectively. The Umax of Co was greatly increased by one data point having exceptional high Co uptake at high IS and Co equilibrium concentration (Fig. 1). For the affinity constant (b), the order of b values was: Pb  Cu  Cd > Co  Zn > Ni  Mn > Ag at low IS and Pb  Cu  Cd > Ni  Zn > Mn  Co  Ag at high IS. As IS elevated, b values decreased among different cationic metals, from 33% to 91%. For the sorption of As and Cr, there was virtually no binding of As(V) to the seaweed (even negative values in the data) while the seaweed had exceptional binding capacity for Cr(III) but very low for Cr(VI) at pH 5.0 (Fig. 2A). The Umax and b for the sorption isotherms of Cr(III) were 1.39 ± 0.0987 mmol g1 and 10.9 ± 5.21 l mmol1; the Umax of Cr(III) was even higher than Pb and Cu at low IS (Table 1). The Umax and b values could not be calculated for As(V) and Cr(VI), because the sorption data did not fit the Langmuir equation and the sorption was very minimal at pH 5.0. The sorption of Pb by brown seaweed was very strong among different metals at pH 5.0 in this study, even though

the IS was increased from 0 to 100 mM NaNO3. Therefore, it pointed to the fact that brown seaweed is a potentially important biosorbent for removing Pb from industrial effluents, regardless of the salt content (0–100 mM NaNO3). The binding capacity of Cu with the seaweed was also very high but was greatly reduced in the presence of a large amount of monovalent Na ions in the effluents. Therefore, careful consideration should be taken for remediating Cu if the industrial effluents contained a high salt content. In addition, the sorption capacity for Cr(III) was found to be very high in this study. The percentage of total metal as free ionic form under both IS conditions was calculated using MINEQL+ (Version 4.5, Environmental Research Software) at a total metal concentration of 1 mM (Fig. 3). In general, as the IS of the solution increased, the percentage of free ions decreased. The percentage of Pb as free ions at high IS was lowest among the heavy metals studied, however, the sorption isotherm for Pb at high IS was almost identical to that at low IS (Fig. 1), implying that free ions may not be necessarily the only form to bind onto the seaweed. The major form of Pb at high IS solution was Pb(NO3)+. Nevertheless, for other metals, the lower percentage of free metal ions accompanied the decreased uptake of metals at high IS. In this study, pH 5.0 was chosen as the fixed pH for all experiments, while the pH values were usually set at 4.5 or lower for many previous biosorption studies. Therefore, in

A. Sorption isotherm

B. Removal efficiency

2.0

100

As

-1

1.5

80

1.0

60 40

0.5

20

0.0

0 -0.5 2.0

100

Cr

r 2 = 0.967

1.5

Cr 80

r 2 = 0.978

1.0

60

Cr(VI) Cr(III)

Removal efficiency (%)

Metal uptake (mmol g )

As

40

0.5

r 2 = 0.872

0.0

20 0

-0.5 0

2

4

6

8

10

12 0

Equilibrium metal concentration (mM)

2

4

6

8

10

12

14

Initial metal concentration (mM)

Fig. 2. (A) Sorption isotherms of As(V), Cr(III) and Cr(VI) by Ca-loaded Sargarssum biomass at pH 5.0 and low ionic strength condition (0 mM NaNO3). (B) Removal efficiency of As(V), Cr(III) and Cr(VI) by Ca-loaded Sargarssum biomass at different initial metal concentrations under pH 5.0 and low ionic strength condition (i.e., 0 mM NaNO3). Data are means ± standard deviation (n = 3).

M.T.K. Tsui et al. / Chemosphere 65 (2006) 51–57

Low IS High IS

Metal in free ionic form (%)

120 100 80 60 40 20 0 Cd

Co

Cu

Pb

Mn

Ni

Ag

Zn

Fig. 3. The calculated percentage of total metal as free metal ions for different metal species in this study at pH 5.0 under low (0 mM NaNO3) and high (100 mM NaNO3) ionic strength (IS) conditions. The total metal concentrations are 1 mM for all metals.

this study the concentrations of proton were much lower and more metal sorption was expected to occur than the previous studies. Nevertheless, the Umax for several metals at low IS in this study were close to the values reported by other studies using Sargassum sp. (Davis et al., 2000; Hashim and Chu, 2004; Kalyani et al., 2004; Sheng et al., 2004). In addition, the Umax for Ni and Zn in this study was found to be close to that by a species of green seaweed (Ulva lactuca) collected in Hong Kong (Lau et al., 2003).

100

Cadmium (Cd)

55

Therefore, it is possible that the higher Umax for Sargassum than other types of seaweed is metal-specific and it requires further studies to verify for different metals and seaweed combinations (Hashim and Chu, 2004). Cationic Cr(III) was found to be the main form of Cr to bind to the seaweed (Kratochvil et al., 1998). Even anionic Cr(VI) was found to be sorbed by seaweed, because Cr(VI) was reduced to Cr(III) first under acidic pH and then bound to the seaweed (Kratochvil et al., 1998). Therefore, this study indicated that at pH 5.0, there was no ‘direct’ sorption of Cr(VI) but strong sorption of Cr(III). On the contrary, there was no detectable uptake of As(V) by the seaweed (0 mmol g1), which was related to the anionic form of As(V), as similar to the case of Cr(VI) anion. Rather, chitosan and chitin as well as microbial biomasses were successfully used to remove anionic metals in waters (Mcafee et al., 2001; Kartal and Imamura, 2005; Seki et al., 2005). Therefore, biosorbents other than seaweed may be effective to remove anionic metals. 3.2. Removal efficiency and binding mechanism Fig. 4 shows the removal efficiency (RE) of metals as a function of initial metal concentrations (Ci) at two different IS conditions, instead of equilibrium metal concentrations (Cf) as shown in Fig. 1. As Ci increased, the RE generally decreased exponentially. The data were well fitted into an exponential decay function (r2 ranging from 0.637 to

Cobalt (Co)

Copper (Cu)

Lead (Pb)

Nickel (Ni)

Silver (Ag)

Zinc (Zn)

80

Low IS

60

High IS

Removal efficiency (%)

40 20 0 100

Manganese (Mn) 80 60 40 20 0 0

5

10

15

0

5

10

15

0

5

10

15

0

5

10

15

Initial metal concentration (mM) Fig. 4. Removal efficiency of metals by Ca-loaded Sargarssum biomass at different initial metal concentrations under low (0 mM NaNO3) and high (100 mM NaNO3) ionic strength (IS) conditions at pH 5.0 with a biomass concentration of 2.22 g l1. Data are means ± standard deviation (n = 3). The equations and correlation coefficients are provided in Table 2.

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Table 2 Equations and correlation coefficients (r2) to describe the relationships between removal efficiency (RE, %) and initial concentrations (Ci, mM) of different metals in the sorption system, under low (0 mM NaNO3) and high (100 mM NaNO3) ionic strength (IS) conditions at pH 5.0 with a biomass concentration of 2.22 g l1 Metals

Low IS

Cadmium Cobalt Copper Lead Manganese Nickel Silver Zinc

Cd Co Cu Pb Mn Ni Ag Zn

High IS 2

Equation

r

Equation

r2

RE = 97.0exp(0.223Ci) RE = 85.8exp(0.247Ci) RE = 101exp(0.137Ci) RE = 107exp(0.137Ci) RE = 77.2exp(0.271Ci) RE = 81.3exp(0.297Ci) RE = 54.4exp(0.206Ci) RE = 94.3exp(0.259Ci)

0.993 0.976 0.983 0.956 0.932 0.951 0.820 0.956

RE = 74.9exp(0.225Ci) RE = 44.7exp(0.168Ci) RE = 95.7exp(0.167Ci) RE = 106exp(0.128Ci) RE = 28.8exp(0.136Ci) RE = 50.1exp(0.277Ci) RE = 40.4exp(0.376Ci) RE = 54.2exp(0.244Ci)

0.990 0.877 0.996 0.955 0.963 0.903 0.637 0.931

0.993) and the equations for each metal at a specific IS are shown in Table 2. Nevertheless, at high Ci similar RE occurred for each metal even at both IS conditions. The RE of a metal may therefore relate to the solid-to-liquid ratio, pH, and other ambient factors. For example, the RE would be expected to increase as the solid-to-liquid ratio in the sorption system increased (Kalyani et al., 2004). Therefore, this study represented a single situation only (i.e., one solid-to-liquid ratio and pH), but this study compared the RE across different metals and different IS. The RE of As and Cr is shown in Fig. 2B; there was no As removal at all while the RE for both Cr(III) and Cr(VI) decreased exponentially with initial Cr concentrations, the exponential equations for the RE of Cr(III) and

Cr(VI) were RE = 105 exp(0.103Ci) and RE = 18.8exp (0.317Ci), respectively. The (negative) decay constant in the exponential decay equation represents the magnitude of decrease in RE with the initial metal concentration. The decay constants for Cu, Pb and Cr(III) were substantially lower than the other metals, regardless of IS conditions. This implies that Sargassum seaweed could be an efficient sorbent for these metals in the wastewater, especially at low to middle levels of metal concentrations. Strong and linear relationships were found between metal uptake and Ca released from the Ca-loaded Sargassum biomass for the eight cationic metals under both high and low IS conditions (Fig. 5). The y-intercept of the

3.0

Cadmium (Cd)

2.5

Copper (Cu)

Cobalt (Co)

r 2 = 0.901

2.0

Lead (Pb)

r 2 = 0.981

r 2 = 0.840

2

r = 0.974

1.0

r2 = 0.986

-1

Ca release (meq g )

1.5

0.5

r2 = 0.975

r2 = 0.908

r2 = 0.989

0.0 3.0

2.0 1.5

Silver (Ag)

Nickel (Ni)

Manganese (Mn)

2.5

Low IS High IS

r 2 = 0.743

r 2 = 0.987

r 2 = 0.526

r 2 = 0.995

1.0 r2 = 0.988

0.5 0.0 0.0

Zinc (Zn)

0.5

1.0

1.5

r2 = 0.984

r2 = 0.997 2.0

2.5

3.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0 0.0

0.5

1.0

1.5

r2 = 0.882 2.0

2.5

3.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Metal uptake (meq g-1) Fig. 5. Relationship between metal uptake and Ca release from Ca-loaded Sargassum biomass under low (0 mM NaNO3) and high (100 mM NaNO3) ionic strength (IS) conditions. Data are means ± standard deviation (n = 3).

M.T.K. Tsui et al. / Chemosphere 65 (2006) 51–57

graphs for the low IS for the eight metals almost passed through the origin (0.013 ± 0.055 meq g1, where n = 8) while that for the high IS for the eight metals corresponded to 1.05 ± 0.12 meq g1 (n = 8), which indicated that Ca was exchanged from the biomass due to the presence of a large amount of Na ions on the surface of the biomass and since there was no significant amount of Na in the low IS treatment, therefore there was no such effect on Ca release from the biomass. The electrical double layer around the biomass binding sites was full of Na ions at high IS treatment and therefore Na ions competed for electrostatic binding on the negatively charged binding sites with Ca ions and metal ions, even though Na ions did not actually bind to the biomass, but rather through electrostatic interaction (Schiewer, 1999). Sheng et al. (2004) employed infrared spectroscopy to show the chelating character of the metal ion coordination to the carboxyl group in the brown seaweed. The authors demonstrated that the functional groups involved in bivalent metal sorption included carboxyl, ether, alcoholic and amino groups, but concluded that sulfonate groups were not mainly responsible for metal sorption, which was opposite to the findings reported by another study (Fourest and Volesky, 1996). The results of the present study corroborated the findings of previous studies that ion exchange is the main mechanism for the sorption of cationic metal ions onto marine seaweed (Williams and Edyvean, 1997; Kratochvil et al., 1998). 4. Conclusions The present study demonstrated the relatively high binding capacity for a number of metals by Sargassum biomass (Volesky and Holan, 1995) which was pretreated with Ca, and the inability of Sargassum to remove anionic metals close to neutral pH (i.e., pH 5). The effect of ionic strength on the sorption performance may be metal-specific, but generally led to decrease of sorption capacity and removal efficiency. Sargassum was found to be a promising tool to remediate Pb in wastewater, regardless of the salt content in the water tested in this study. The removal efficiency of the sorption system decreased exponentially with increasing metal concentration. Moreover, an ion-exchange process was found to be the mechanism responsible for cationic metal sorption onto the seaweed, and Na ions were shown to lower the number of available binding sites for target metal ions through electrostatic interaction. Acknowledgements The authors thank Mr. Ben Yeung for collecting the macroalga and Mr. Eric Yuen for providing analytical support. Two anonymous reviewers and Dr. Silke Schiewer

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