Chemical Modification of Sargassum sp. for Prevention

Ind. Eng. Chem. Res. 2005, 44, 9931-9942

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Chemical Modification of Sargassum sp. for Prevention of Organic Leaching and Enhancement of Uptake during Metal Biosorption J. Paul Chen* and Lei Yang Division of Environmental Science and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

A significantly high amount of organic leaching has been observed in the treatment and recovery of heavy metals by raw biosorbents. In this study, acid, base, calcium, formaldehyde, and glutaraldehyde are used for the modification of locally derived raw seaweed (RSW), Sargassum sp., so that the modified seaweeds (MSW) have less organic leaching while the metal biosorption capacity is maintained. It is determined that 0.2% of formaldehyde is the best, in regard to chemical modifications. The organic content of the filtrated water samples is only 3.84 mg/L total organic carbon (TOC), 80% less than that when the RSW is used. The metal biosorption capacity is greatly improved, while the uptake kinetics is similar to that of the RSW. The metal biosorption follows a descending sequence: lead > copper > zinc ≈ cadmium > nickel. Higher pH causes higher metal biosorption. Ion exchange has an important role in the metal uptake. A surface diffusion model well describes the biosorption kinetics. It is determined that 0.2 M hydrochloric acid (HCl) is the best, in regard to metal desorption. Approximately 90% of metal ions can be eluted from the metal-loaded MSW, which requires ∼20 min to complete. A fivecycle operation of metal sorption and desorption confirms that the MSW is much better than the RSW. The Fourier transform infrared (FT-IR) analysis demonstrates that the hydroxyl, amino, and carboxyl functional groups in the MSW provide the major biosorption sites for the metal binding. Scanning electron microscopy (SEM) analysis shows a strong coordination crosslinkage between the copper ions and the organic functional groups of the biomass. 1. Introduction Heavy metals in a water environment are an extreme concern, because of their toxicity. They come from various industrial sources, such as metal mining, electroplating, metal finishing, metal molding, and coil coating. Conventional heavy-metal treatment methods, although used for many decades, have disadvantages in terms of efficiency and operational cost. A series of studies has been intensively conducted to investigate the possibility of using biomaterials (termed as biosorbents) for heavy-metal removal and recovery from aqueous solutions through biosorption.1 Marine algae have been identified as good biosorbents, because of their low cost, renewable nature, and high metal biosorption capacity.1-3 They can effectively remove heavy-metal ions with concentrations ranging from few ppm to several hundred ppm. Maximum metal biosorption capacity ranges from 0.1 to 1.5 mmol/g biosorbent.1-3 The biosorption of cations is more effective than that of anions. The biosorption capacity is normally much higher than commercial adsorbents and ion-exchange resins. Marine algae have rich contents of polysaccharides in the cell wall, which are mainly responsible for higher metal biosorption. Several important functional groups, such as carboxyl, sulfate, and amino, are identified in marine algae.4,5 Few key chemical interactions, including ion exchange, surface complex formation, microprecipitation, chelation, and coordination, are used to explain the biosorption mechanisms. * To whom correspondence should be addressed. Fax: +1-831-303-8636, +65-6872-5483. E-mail: [email protected], [email protected].

Marine algae contain a variety of light-metal ions, which can release into the water.3,6 After the algae are used, the total dissolved solids and hardness of the water can increase. This would not cause significantly negative environmental impacts, because their toxicity is less important. Marine algae contain a high amount of organic substances, such as carbohydrates, protein, lips, and pigments; as a result, some of them can inevitably become dissolved in the aqueous solutions during the biosorption operation.2,7,8 It is common to observe that the water after biosorption changes to a yellowish or green color. Kratochvil and Volesky reported that the total organic carbon (TOC) of the effluent from a Sargassum-packed column was ∼24 mg/L at the early phase of operation. The TOC during the desorption can be as high as 55 mg/L.2 The organic leaching from the biosorbents can lead to a secondary pollution and retard the biosorption technology in water and wastewater treatment. Therefore, it is important to modify the raw marine algae before they are used. There are two major options: encapsulation (entrapment) and surface modification. In the encapsulation, various supporting materials such as polymers are used to encapsulate the biosorbents.9 Poly(vinyl alcohol), collagen fiber, and alginate as immobilization matrixes are often used. Through the encapsulation, the leaching can be avoided; however, the mass transfer is normally reduced. In the surface modification, acid, base, calcium, and aldehyde can be used.7,8,10 This approach is more cost-effective, because the modification agents are normally less expensive than entrapment materials, the sorptive capacity is enhanced, and the mass transfer is not affected.

10.1021/ie050678t CCC: $30.25 © 2005 American Chemical Society Published on Web 11/02/2005

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Ind. Eng. Chem. Res., Vol. 44, No. 26, 2005

Because of the abundant resource marine algal biomass, we have studied the biosorption properties of several algae.3,6 Sargassum sp. was determined to have better biosorption capacity and kinetics, among the algae. The aim of this study was to screen different surface modification methods on Sargassum sp. for the prevention of organic leaching and the enhancement of heavy-metal removal. The biosorption properties, as functions of the operational conditions (e.g., pH), were determined. A series of desorption experiments was conducted. A multicycle biosorption experiment was performed. Finally, the biosorption mechanisms were studied by scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. 2. Materials and Methods Sargassum sp. was collected from Singapore West Coast. This brown algae lives on seashore rocks. At low tide, Sargassum sp. emerges and covers rocks. Scissors was used to cut the lower stem and the root was left on the rocks, to protect the seashore vegetation environment. The averaged length of algae is ∼50 cm. After being washed with deionized (DI) water and dried at 60 °C overnight in an oven, the biomass was milled with a blender and sieved to particles with sizes ranging from 0.5 mm to 0.85 mm. A 50% glutaraldehyde (OdCHCH2-CH2-CH2-CHdO) solution was supplied by Fluka. A 37% formaldehyde solution and all heavy-metal salts of analytical grades were purchased from Merck (Germany). An orbital shaker with a speed of 150 rpm (Daiki ADK-OS010) was used in the equilibrium experiments. The concentrations of metal ions were measured by inductively coupled plasma-emission spectroscopy (ICPES) (Perkin-Elmer Optima 3000, USA). The TOC was used to evaluate the degree of organic leaching during biosorption, which was measured using an TOC analyzer (Shimadzu TOC Analyzer Model 5000A, Japan). The pH was measured by an ORION 525A pH meter. A statistic standard error analysis of metal concentration measurement was conducted by 20 identical biosorption experiments. With a credibility of 95%, an average standard deviation of 1.89% in the measurements was obtained. 2.1. Screening of Chemical Modification Methods. Thirteen modification methods listed in Table 1 were used. One gram of raw seaweed (RSW), Sargassum sp., was reacted with 100-mL chemical solutions (Table 1) for 24 h. The resulted modified seaweeds (MSWs) were then filtered from the mixture, washed with 300mL DI water several times, and dried in an oven overnight at 60 °C. The weight-loss percentage (Wcm %) that is due to the chemical modifications can be determined by

Wcm% (%) )

Wi - Wf × 100 Wi

(1)

where Wi is the dry weight of RSW and Wf is the dry weight of MSW. It is observed that the organic leaching from RSW can be decreased if it is pre-washed by the DI water. To compare this approach with the chemical modification approaches, the RSW with a weight of 1 g was washed with DI water for several times, filtrated, and dried. The weight loss was then determined.

Table 1. List of Modification Methodsa description glutaraldehyde

fomaldehyde

methods index a b c d e f g h i j k l m n

0.1 M NaOH 0.1 M HCl 0.1 M HCl + 0.1 M CaCl2 0.1 HCl + 10% formaldehyde solution 0.1 M CaCl2 0.02% formaldehyde solution 0.2% formaldehyde solution 2% formaldehyde solution 10% formaldehyde solution 0.02% glutaraldehyde solution 0.2% glutaraldehyde solution 2% glutaraldehyde solution 10% glutaraldehyde solution no modification (raw seaweed)

a Note: the pretreatment contact time is 24 h; the solid-to-liquid ratio (S/L) is 10 g/L.

The MSW with a dosage of 1 g/L was used to remove copper that had an initial concentration of 6 mM and pH 5.0. Higher initial concentration of copper ions was used so that the MSW became saturated with the metal ions. The contact time was controlled at 24 h. Weight loss and organic leaching that were due to the biosorption, and the metal removal efficiency of different MSW, were determined and used to screen chemical modification methods. The biosorption capacity (i.e., the amount of metal adsorbed per gram of sorbent applied) at equilibrium (qe), in units of mg/g or mmol/g, can be calculated as follows:

qe )

(C0 - Ce) × V m

(2)

where C0 is the initial concentration of metal ions in solution (expressed in units of mg/L or M), Ce the equilibrium concentration of metal ions in solution (given in units of mg/L or M), V the volume of solution (given in liters), and m the mass of sorbent applied (given in grams). Considering a weight loss due to the modification, the biosorption capacity of RSW and MSW can also be compared based on the mass of seaweed before the modifications. Therefore, the metal biosorption capacity with such as consideration can be expressed as

qe-OSW ) qe(1 - Wcm%)

(3)

where qe-OSW is the biosorption capacity based on the weight of seaweed before modifications (expressed in units of mmol metal/g OSW) and qe is the biosorption capacity based on the weight of MSW or RSW (expressed in units of mmol metal/g MSW or RSW) and can be determined using eq 2. Wcm% can be obtained using eq 1. Because the heavy metals are strongly adsorbed onto the RSW and the MSW, the amount of heavy ions sorbed on the solids must be taken into consideration in the calculation of weight loss due to the biosorption. Some

Ind. Eng. Chem. Res., Vol. 44, No. 26, 2005 9933

of light metals (e.g., Ca ions) are released into the solution due to the ion exchange discussed later. The weight loss due to the biosorption (Wbiosorption%) with such a consideration can be determined as follows:

Wbiosorption% (%) )

(wti - wtf) + wtm - wtmo × 100 wti (4)

where wti is the weight of biosorbents before biosorption, wtf the weight of biosorbents after biosorption, wtm the weight of heavy metal ions (e.g., Cu2+) adsorbed onto the biosorbents, and wtmo the weight of metal ions (e.g., Ca2+) released from the biosorbents. It must be recognized that there is a weight loss during the chemical modifications, as stated in eq 1. Thus, a weight loss based on the mass of the seaweeds before modification can be calculated:

Wbiosorption-OSW% (%) )

Wbiosorption% × 100 1 - Wcm%

(5)

2.2. Determination of Light-Metal Content in Biosorbents. To determine the original metal content in the biosorbents, 0.3 g of RSW and 0.2% formaldehyde MSW were washed for 2 h with 100 mL of 0.1 mM HNO3. The supernatant was removed for analysis of the calcium, magnesium, potassium, sodium, aluminum, iron, and zinc concentrations. The acid-washing procedure was repeated three times, so that the light-metal contents decrease to a neglectable level. The total contents of each metal ion in the RSW and MSW can be determined by summing the respective metal content from each washing cycle. 2.3. Biosorption Study. A series of experiments for the determination of biosorption kinetics, isotherms, and pH effects was performed. In the biosorption kinetic experiments, 1 g of 0.2%formaldehyde-treated Sargassum sp. was added to a 1000-mL metal solution, the pH of which was maintained at 5.0. The solution was then stirred at a constant rate. The samples were taken at different time intervals and analyzed by ICP-ES. The biosorption capacity (q), as a function of time (t), can be determined by

q)

(C0 - C)V m

(6)

where C is the concentration at time t (given in units of mg/L or M). In the isotherm experiment, 0.1 g of 0.2%-formaldehyde-treated biosorbents was added into a 100-mL metal solution with different initial concentrations. The solution pH was controlled at 5.0. The solution was shaken, with the temperature being controlled at 25 °C for 6 h to obtain equilibrium. The concentrations of metal ions were measured by ICP-ES. The metal concentrations in the solution and solid phases (Ce and qe) can be related by several empirical equations and theoretical models. The Langmuir equation shown below was used to determine the copper isothermal adsorption capacity:

qe )

qmax bCe 1 + bCe

(7)

where qmax is the maximum adsorption capacity and b is the Langmuir constant. In the pH effect experiment, 0.1 g of biosorbents was added into a 100-mL metal solution. The solution pH was controlled at different values by hydrochloric acid (HCl) or sodium hydroxide (NaOH). The contact time was controlled at 24 h. Other procedures were the same as those in the isotherm experiments. 2.4. Desorption Study. To determine the feasibility of reusing biosorbents, the desorption experiments were performed. The copper-loaded biosorbents were first prepared: 2 g of 0.2% formaldehyde MSW was contacted with 2 L of a 2.5 mM copper solution at pH 5.0 overnight. The copper-loaded biosorbent was then filtered, washed by DI water, and dried at 60 °C in an oven for 6 h. Several desorption reagents were tested for their suitability for the recovery of biosorption capacity. HCl, HNO3, H2SO4, EDTA (sodium salt), Na2CO3, and NaHCO3 were used in the screening. The small quantity (0.1 g) of biosorbents were collected in 25-mL beakers, each containing 10 mL of a 0.2 mM solution of the desorption agents. The solid-to-liquid ratio (S/L) was 10 g/L. The metal elution and the organic leaching were determined. The elution efficiency by desorption agents, as a function of time (t), can be defined as

elution efficiency (%) )

CstVs (C0 - Ce)V

× 100

(8)

where Vs is the volume of solution in the desorption (L) and Cst is the concentration of metal ions in the solution at time t (mg/L or M). C0, Ce, and V are defined in eq 2. The desorption time was controlled at 24 h. The ultimate elution efficiency can be calculated by

ultimate elution efficiency (%) )

CsuVs (C0 - Ce)V

× 100 (9)

where Csu is the ultimate concentration of metal ions in the solution (expressed in units of mg/L or M). Among the above desorption agents, it was found that HCl was the most efficient. The effect of HCl concentration was thus studied; its concentration, ranging from 0.01 M to 1.0 M, was used to “strip” the metal ions from the metal-loaded MSW. In the experiment, the S/L value of 10 g/L was controlled. Similarly, the metal elution and organic leaching were determined. 2.5. Multicycle Biosorption. To determine the reusability of 0.2% formaldehyde MSW for metal removal and recovery, a five-cycle biosorption-desorption batch experiment was performed. The biosorption and desorption experiments were conducted for 6 and 2 h, respectively. The details of biosorption experiments were the same as those previously described. HCl (0.2 M) was used in the desorption experiments. When each cycle was accomplished, the biosorbent was washed by the DI water and placed in the acid solution, and then it was transferred to the metal solution for the next biosorption cycle. Both the biosorption and elution of metal ions were determined. 2.6. Scanning Electron Microscopy. The surface morphology of the biosorbents was visualized by an SEM device (JEOL, model JSM-5600V, Japan). The SEM analysis enables the direct observation of the changes in the surface microstructures of the biosor-

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bents that are due to the chemical surface modifications, biosorption, and desorption. The chemical compositions of raw, formaldehyde-treated, copper-loaded, and copper-desorbed Sargassum sp. were determined. 2.7. Fourier Transform Infrared Spectroscopy. FT-IR spectroscopy was used to determine the vibration frequency changes in the functional groups in the biosorbents. The spectra were collected using a model FTS-135 spectrometer (Bio-Rad, USA) within the wavenumber range of 400-4000 cm-1. Specimens of various biosorbents were first mixed with KBr and then ground in an agate mortar (Merck, for spectroscopy) at an approximate ratio of 1/100 for the preparation of pellets (weight of 100 mg). The resulting mixture was pressed at 10 tons for 5 min. In regard to recording the spectra, 16 scans and 8-cm-1 resolutions were applied. The background obtained from a scan of pure KBr was automatically subtracted from the sample spectra. All spectra were plotted using the same scale on the absorbance axis. 3. Results and Discussion 3.1. Pretreatment of Sargassum sp. Weight loss due to chemical modification is an important parameter for selection of modification approaches. High weight loss means a waste of natural biomass. An approach leading to a higher weight loss should not be recommended, even though the resulting modified sorbent could have lower organic leaching. The simplest approach to modify seaweeds is to wash them using DI water. Our measurement shows that the weight loss due to the washing is 29.5%. To reduce the weight loss and enhance the biosorption capacity, a total of thirteen different modification methods, including acid, base, aldehydes, and their combinations, were used for modification of raw Sargassum sp. The weight loss percentage of ∼24.5% for CaCl2-, glutaraldehyde-, and formaldehyde-modified Sargassum sp. is demonstrated in Figure 1. A more serious weight-loss percentage can be observed when an acid or base is used. The use of 0.1 M HCl and 0.1 M NaOH leads to higher weightloss percentages (33.8% and 43.3%, respectively). Weightloss percentages of 30.0% and 28.1% are observed when 0.1 M HCl, followed by CaCl2 or 10% formaldehyde, is used. As shown in Figure 2, all of the MSW show higher copper capacity (qe) than the RSW. When the weight loss (Wcm%) is considered, the metal biosorption capacities (qe-OSW) for MSW and RSW are virtually similar to each other, except that the acid is used for the modification. This indicates that, the portion of biomass removed during the chemical modifications is not greatly involved in the metal biosorption. It is also observed that the lowest biosorption capacity (qe-OSW) occurs when 0.1 M HCl is used for the modification. The biosorption capacity of the HCl MSW is 73% of the RSW, which can be due to strong hydrolysis reactions of organic substances from the seaweeds by the acid. The weight loss in the MSW and the RSW during the biosorption is illustrated in Figure 3. The weight loss of RSW (Wbiosorption%) during the copper biosorption is the highest (24.1%) among all the biosorbents. The second-highest weight loss (Wbiosorption% ) 19.1%) occurs when the 0.1 M HCl MSW is used. The average weightloss percentage of the aldehydes MSW is 11.3%, far below the highest (which belongs to RSW). When the

Figure 1. Comparison of weight loss percentages of different modification approaches. S/L ) 10 g/L, contact time ) 24 h.

Figure 2. Comparison of copper biosorption capacities of raw and modified Sargassum sp.: biosorption, modified seaweeds (MSW) or raw seaweed (RSW) ) 1 g/L; pH ) 5; [Cu]0 ) 6 mM, contact time ) 24 h.

weight loss during the chemical modifications is considered, most of the MSW have lower weight loss (Wbiosorption-OSW%) than the RSW (24.1%). The weight loss of HCl MSW (Wbiosorption-RSW% of 28.8%) is higher than that of the aldehydes MSWs (15.1%) and the RSW.


Figure 3. Comparison of the weight loss of different modification approaches during copper biosorption: MSW or RSW ) 1 g/L; for copper: pH 5, [Cu]0 ) 6 mM, contact time ) 24 h.

The TOC of solution after biosorption is a direct measurement of the organic pollution that is caused by using biosorbents, the main compositions of which are organic. The organic pollution can directly affect the industrial applications of the biosorption technology, because high organic content is always undesirable after toxic metal ions are removed. It was observed during the experiment that, when the RSW was used, the solutions became brown or dark brown in color, which resulted from the pigments being leached from the raw seaweeds. There are many types of pigments in brown seaweeds, including chlorophyll and carotene. Most of these pigments are soluble in organic solvents, but are less soluble in water.11 A preliminary experiment was conducted: 1 g RSW/L was placed into contact with DI water for 24 h. The water that was filtrated with laboratory filter paper had TOC values of 110.9 and 186.3 mg/L at controlled pHs of 5.0 and 2.0, respectively. Figure 4 shows the organic leaching during copper biosorption by both MSW and RSW. Note that the TOC reported here is just a measure of organic substances that can pass through a 0.45-µm membrane filter. In some cases, such as the RSW, the actual TOC values are far above those in the figure. Nevertheless, the values can provide a good indication of organic leaching during the biosorption. As shown, the leaching from MSW pretreated by aldehydes (0.02%-2%) and CaCl2 is much lower than that from the RSW. The averaged organic content in the biosorption is 4.62 mg/L TOC; the 0.2% formaldehyde MSW has a value of 3.84 mg/L TOC, which is much lower than the leaching from the RSW (19 mg/L TOC) (i.e., a decrease of 80%). Higher organic leaching can be observed when acid or 10% aldehydes is used, which are due to organic dissolution by acid and the high dosage of aldehydes applied in modifications, respectively.

Figure 4. Total organic carbon (TOC) leaching from raw and modified Sargassum sp. during copper biosorption. Biosorption: MSW or RSW ) 1 g/L; pH 5, [Cu]0 ) 6 mM, contact time ) 24 h. Water samples were filtrated using 0.45-µm membrane filters.

Based on weight loss during the modification and the metal biosorption, organic leaching, and heavy-metal removal capacity, the chemical modification by 0.2% formaldehyde is the best for copper biosorption. Thus, the 0.2% formaldehyde MSW was used in the subsequent biosorption study. It has been reported that alginate is the major cell wall component of brown algae, accounting for up to 40% of the dry weight. Guluronate is one of the basic chemical components of alginate and its derivates. In the presence of multivalent metal ions (e.g., Ca ions in seawater), a cross-linkage among the metal ions and the O atoms within the chains of guluronates can be established.12 The formation of cross-linkage essentially solidifies the biomass or biopolymers, which prevents organic leaching during metal biosorption. The evidences can be found in the weight loss and organic leaching of modification method e in Figures 3 and 4. When an acid is involved in the modification, the H ions alter the cross-linkage, which cause dissolution of organic solids. Thus, the weight loss and the organic leaching increase, of whose evidence can be found in modification methods b, c, and d of Figures 1 and 4. In the presence of a base such as NaOH, hydrolysis reactions can occur, which causes high dissolution of organic substances from the seaweeds, as shown in Figure 1. The hydrolysis reactions can lead to the formation of more carboxylic (-COOH), carboxylate (-COO-), and alcohol (-OH) groups in the MSW, which enhances the cationic biosorption, as demonstrated in Figure 2. Aldehydes such as formaldehyde used in this study are commonly used for preservation of plant and animal tissues.13,14 Formaldehyde and glutaraldehyde can cause

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Figure 5. Leaching of light-metal ions from RSW and MSW. Biomass content is 2 g/L.

polymerization. Polymerized aldehydes can react with primary amine groups in seaweeds, which are illustrated as follows:

R1-CHO + R2-NH2 f R1-CHdN-R2 + H2O (10) where R1 represents the aldehydes carbon chain and R2 represents the biopolymer chain in algae. Because of the aldehydes fixation, the cross-linkage in the seaweeds can be established. This leads to solidification of the biosorbents: the weight loss of 24.5% in aldehydes modifications (Figure 1) versus that of 29.5% in water washing. The effect of cross-linkage can be further confirmed by the lower weight loss and organic leaching given in Figures 3 and 4. In addition, the effect of cross-linkage by heavy metal ions can also contribute the solidification of biomass. The heavy metals can exchange with Ca ions within the MSW and form a more stable structure. It is consistent with other findings in the literature.15 3.2. Acid-Washable Light-Metals Content. Light metals can attach to seaweeds through covalent or electrostatic attraction. Calcium, magnesium, sodium, and potassium could be involved in the heavy-metal biosorption process through ion exchange, metal complex formation, coordination reactions, and other mechanisms.1,3,6 Figure 5 gives the acid-washable sodium, potassium, magnesium, and calciumm contents in the RSW and the 0.2% formaldehyde MSW in the three cycles. After three acid-washing cycles, the corresponding contents decrease to a neglectable level. In addition, the presence of aluminum, iron, and zinc were not detectable. The major ions in the biosorbents are Na, K, Mg, and Ca. The total cation contents of RSW and 0.2% formaldehyde MSW are 4.46 mequiv/g RSW and 3.09 mequiv/g MSW, respectively. Because Mg and Ca are considered as major cations for heavy-metal-ion uptake, their contents are more important. The contents of RSW and MSW are 2.22 mequiv/g RSW and 2.36 mequiv/g MSW, respectively. Both biosorbents have very similar Ca and Mg contents; however, they have copper uptake capacities of 1.23 mmol/g RSW (or 2.46 mequiv/g RSW) and 1.61 mmol/g MSW (or 3.22 mequiv/g MSW), respectively, as shown in Figure 2. It clearly indicates that

Figure 6. Plot of pH effects on heavy-metal biosorption by treated Sargassum sp. [MSW] ) 1 g/L; contact time ) 6 h; [Cu]0 ) 1.17 mM, [Pb]0 ) 1.06 mM, [Cd]0 ) 0.81 mM, [Zn]0 ) 1.08 mM, and [Ni]0 )0.93 mM.

the ion exchange is not the sole mechanism. Other mechanisms also have important roles in the copper uptake. 3.3. Effect of pH. A chemical equilibrium program (MINEQL+, Version 4.5) was used to calculate the distribution of metal species in aqueous solution, as a function of pH.16 In the simulation, five heavy-metal saltssCd(NO3)2, Ni(NO3)2, Pb(NO3)2, Zn(NO3)2, and Cu(NO3)2swere assumed. All calculations are based on open-atmosphere systems with a carbon dioxide (CO2) pressure of 10-3.5 atm. The metals are all in their free ionic forms at pH zinc ≈ cadmium > nickel. At pH 5.0, the metal ions are present in ionic forms and their removal reaches approximately the maximum; thus, pH 5.0 was chosen in subsequent kinetics and isotherm experiments. The pH dependence of metal biosorption can be explained by the competitive effect by H ions with heavy-metal ions through a combination of mechanisms: ion exchange and the formation of surface metal complexes.

R1-M1m1+ + M2m2+ f R1-M2m2+ + M1m1+ R1-M1m1+ + H+ f R1-H++ M1m1+ m2+

R 2 + M2

m 2+

(11a) (11b)

f R2-M2

(12a)

R2 + H+ f R2-H+

(12b)

where R1 and R2 represent organic functional groups in the MSW. Both of them can be carboxyl, sulfonic

Ind. Eng. Chem. Res., Vol. 44, No. 26, 2005 9937 Table 2. Langmuir Constants of Metal Biosorption by Raw Sargassum sp. and Modified Sargassum sp. qmax b metal biosorbenta (mmol/g) (mM-1)

De (m2/s)

kf (m/s)

Pb(II) Pb(II)

MSW RSWb

1.46 1.16

132.84 14.23

3.5 × 10-12 4.7 × 10-5

Cu(II) Cu(II)

MSW RSWb

1.37 0.99

10.42 8.78

3.7 × 10-12 1.3 × 10-4

Ni(II) Ni(II)

MSW RSWb

1.22 0.61

5.69 4.6

4.5 × 10-12 1.3 × 10-4

a MSW ) modified seaweed; RSW ) raw seaweed. b Note: The biosorption data of raw Sargassum sp. were obtained from Sheng et al.3

Figure 8. Kinetics of heavy-metal biosorption by treated Sargassum sp. [MSW] ) 1 g/L; pH controlled at 5.0; [[Ni]0 ) 1.20 mM.

Figure 7. Effect of light-metal ions on copper biosorption: (a) initial light-metal concentration and (b) mechanism study. [MSW] ) 0.1 g, volume ) 100 mL, [Cu]0 ) 0.9 mM, contact time ) overnight, and pH 4.5.

groups, hydroxyl, and amino groups. M1m1+ represents the cations initially in the MSW, which are exchanged with the heavy-metal ions (M2m2+) in the solutions. At low pH, the H ions strongly compete with heavymetal ions for the adsorptive sites, as shown in the aforementioned equations. Thus, the metal uptake is lower. When the pH is increased, the competitive effect becomes less important and, hence, more heavy-metal ions are removed. 3.4. Effect of the Presence of Light Metals. Lightmetal ions commonly exist in water. Calcium and magnesium are particularly important in the area where the hardness is high. The presence of the alkaline-earth metals can affect metal biosorption capacity. Our previous study has indicated that calcium has an important role in the metal biosorption onto the RSW through an ion-exchange mechanism.3 Four light-metal ions of calcium, magnesium, sodium, and potassium were studied, with the results being illustrated in Figure 7a. With an increase in the initial concentration of calcium, the copper biosorption significantly decreases. The effect by magnesium is less important. However, sodium and potassium have virtually no effect on the biosorption. To test the role of ion exchange that is caused by the light-metal ions, the change in the concentrations of the

light metals versus that of the copper is plotted. As shown in Figure 7b, the Ca ions have an important role in the copper biosorption. The linear relationship clearly demonstrates that 1 mol of calcium in the MSW is exchanged with 1 mol of Cu ions in the solution. The points for magnesium, sodium, and potassium are far away from the straight line (1/1), indicating that they do not greatly participate in the metal biosorption. 3.5. Sorption Isotherm. Metal sorption isotherm experiments were conducted; the data are nicely fit by the Langmuir model (the figure is not shown here). The values of qmax and b for the metal biosorption onto the 0.2% formaldehyde MSW are listed in Table 2. The biosorption isotherm parameters for the RSW reported by Sheng et al.3 are also listed in the table. Comparison of the qmax values of MSW and RSW shows that the metal biosorption is enhanced by 26%-200% after the chemical modification is applied. This clearly demonstrates that the chemical modification significantly improves the biosorption capacity. The qmax value for copper is slightly lower than the biosorption capacity given in Figure 2. The initial pH in the experiment of “screening” was 5, and it increased during the metal uptake. Therefore, more metal ions were adsorbed, which caused greater metal biosorption, as shown in Figure 2. In addition, the metal removal in the experiment can be enhanced by precipitation reactions inside of the biosorbent, because of the higher initial copper concentration (6 mM). 3.6. Biosorption Kinetics and Its Modeling Simulation. Figure 8 shows the kinetics of metal biosorption by the 0.2% formaldehyde MSW. More than 90% of the maximum metal uptake can be achieved within 40 min.

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No further heavy-metal removal is observed after 3 h for all the cations. Comparison of this finding with those reported in the literature shows that the modification does not alter the biosorption kinetics.2,3 The biosorption kinetic involves a rapid process, followed by a slightly slow process. The specific surface area of biosorbents is normally very low. Mameri et al. reported that nonliving Streptomyces rimosus biomass (a bacterial antibiotic waste that is obtained from fermentation) had a specific surface area of 156 m-1;17 Leusch and Volesky found that the specific surface area of Sargassum fluitans biomass was 58.8 m-1 when the particle size was 0.500.84 mm (the same size as that used in this study).18 Therefore, it is reasonable to assume that the surface diffusion controls the biosorption kinetics.19 Constant physical properties are assumed. A mathematical model can be formulated as follows:

(

De

)

∂2q 2 ∂q ∂q + ) ∂t ∂r2 r ∂r

(for 0 e r e ap, t > 0) (13)

Figure 9. Effect of HCl concentration on metal desorption. In biosorption, MSW was saturated by Cu ions at pH controlled at 5.0 with biosorption capacity of 1.58 mmol/g; in desorption, S/L ) 10 g/L.

The initial and boundary conditions may be specified as

∂q )0 ∂r De

r)0

∂q F ) kf(C - C*) ∂r p q)0

t)0

(14) r ) ap

(15) (16)

where C and q are the concentrations of the metal ions in the bulk and solid phases, respectively; C* is the aqueous phase concentration at the particle surface, in equilibrium with the corresponding concentration in the solid phase q*; De is the effective diffusivity within the particles; Fp is the particle density, r is the radius of the particle, ap is the radius for the particle (from the center to the surface of particle), kf is the external masstransfer coefficient, and t is the time. Equation 13, with the initial and the boundary conditions, can be solved.19 As shown in Figure 8, the surface diffusion model well describes the biosorption kinetics. The diffusivity and the external mass-transfer coefficient in ranges of 10-12 m2/s and 10-4 m/s, respectively, that are given in Table 2 were used in the modeling. The former is slightly lower than the diffusivity,20 while the latter is higher than as that reported in the literature.18 3.7. Desorption. Acid, base, and complex agents can be used to release the metal ions that are adsorbed onto sorbents. Sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), NaOH, HCl, nitrate acid (HNO3), sulfuric acid (H2SO4), and ethylenediamine tetraacetic acid (EDTA) (sodium salt) were used to recover the copperloaded MSW. The copper elution efficiency is determined by eq 9. The desorption efficiency by Na2CO3, NaHCO3, and NaOH was observed to be in the range of 8%-16%. Conversely, the three mineral acids and EDTA led to the desorption of copper above 90%. Among these, HCl caused the least organic leaching after a reaction time of 24 h. Thus, HCl was selected as a desorbent in all subsequent studies. Figure 9 demonstrates the effect of HCl concentration on the copper desorption. With an increase in the concentration, the elution of copper dramatically increases. When the concentration reaches 0.2 M, the

Figure 10. Elution efficiency of copper as a function of time. Cu ions were adsorbed onto the MSW with a biosorption capacity of 1.23 mmol/g; 0.1 M HCl was used for the metal elution.

elution achieves its maximum of 90%. A further increase in the concentration does not help in the elution. Meanwhile, organic leaching (TOC) increases as the acid concentration is increased. Consideration of both elution efficiency and organic leaching leads to the selection of 0.2 M HCl for the recovery of heavy metals from the used MSW. The copper desorption kinetics is illustrated in Figure 10. The desorption sharply increases during the first several minutes; almost 70% of maximum desorption occurs in 2 min, and it is completed within 20 min. The kinetics of desorption is much faster than that of biosorption (with an equilibrium time of 4 h), as shown in Figure 8. 3.8. Multicycle Biosorption/Desorption Experiments. The reusability of treated Sargassum sp. for the recovery of copper was investigated during five repeated cycles of biosorption/desorption in batch mode. Figure 11 shows the process performance in a five-cycle operation when the RSW and the 0.2% formaldehyde MSW are used. As illustrated in Figure 11a, when 0.5 g MSW or RSW is used, the metal uptake ranges from 0.77 mmol Cu to 0.64 mmol Cu by MSW, whereas that by the RSW is 0.58-0.55 mmol Cu. The elution of copperloaded MSW is 85%-90%, whereas that of RSW is


Figure 11. Process performance as a function of operation cycle: (a) metal treatment and recovery, and (b) reusability of biosorbents. Each cycle is started with biosorption and followed by desorption of metal-loaded MSW. The amount of RSW and MSW initially added was 0.5 g. In biosorption, the RSW or the MSW was added into 5.58 mM copper with a volume of 500 mL; pH was controlled at 5; in desorption, the copper-loaded sorbents were washed by 300 mL of deionized (DI) water, and 0.2 M HCl with a volume of 50 mL was used for the metal elution.

Figure 12. Scanning electron microscopy (SEM) micrographs (1000× magnification) of (a) raw Sargassum sp. (b) modified Sargassum sp. (c) modified Sargassum sp. after biosorption of 2.5 mM of copper, and (d) 0.2-mM-HCl-eluted Sargassum sp. that was modified and adsorbed copper.

78%-94%. The eluted copper concentrations were 552705 mg/L and 709-836 mg/L for the RSW and the MSW, respectively. The Cu ions, which are present in such a higher concentration, can be easily recovered, using chemical reduction approaches such as the use of hydrazine and electrochemical deposition.21 Figure 11b shows that, with an increasing number of operational cycles, the metal uptake capacity decreases, which follows downward at an angle of 45°. At the end of operation, the weight-loss percentages of

MSW and RSW are 27.3% and 45.5%, respectively. The reduction in the metal uptake is due to the weight loss during the operations. In addition, some of the heavymetal ions can form cross-linkage with the organic functional groups; as a result, they would not return to the solution. From the aforementioned results, it is obvious that the chemical modifications for the raw seaweed significantly improve the metal uptake capacity and reduce the loss in the organic functional groups.

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Table 3. Elemental Composition of Copper-Loaded Modified Sargassum sp.a Composition element

wt %

at. %

C O Mg Al Si S K Ca Fe Cu total

25.68 53.33 0.83 4.74 6.62 1.22 0.29 0.12 0.25 6.91 100

35.18 54.83 0.56 2.89 3.88 0.63 0.12 0.05 0.07 1.79 100

a Note: the data were obtained from spectrum processing of the SEM image in Figure 12c.

3.9. Scanning Electron Microscopy Analysis. Figure 12 shows SEM micrographs of raw and modified

Sargassum sp. (before/after biosorption and after desorption). Surface protuberance can be observed in Figure 12a, which may be due to calcium or some other salt crystalloid deposition. Table 3 shows that silicon, which is a major element in the diatom outer shell, has a weight content of 6.6% on the biomass surface. The silicon oxides also can cause the microstructures on the surfaces. The protuberance disappears after the modification, as shown in Figure 12b. Instead, bold frames and elliptically shaped units appear in the “wrinkled” biomass surface. The surfaces of MSW contain less microstructures and become smoother. Egg-shaped microstructures appear on the surfaces of MSW. The -OH group of RSW can react with the formaldehyde to form acetal (shown below), which causes the transformation of the surface morphology. In addition, more than half of the Na and K ions are removed during the modification, as shown in Figure 5, which can contribute the changes in the morphology.

Figure 13. Fourier transform infrared (FT-IR) spectra of biosorbents: (a) MSW versus copper-loaded MSW, and (b) MSW versus MSW after copper desorption.


4. Conclusion

Figure 12c shows modified Sargassum sp. after it has undergone complete copper biosorption. The threedimensional microstructures become more obvious. The edge of each microstructure looks sharper than that before the copper biosorption. The energy-dispersive X-ray (EDX) analysis results in Table 3 show a copper content of 6.9% on the surface of MSW. This clearly demonstrates a strong coordination cross-linkage between the Cu ions and the organic functional groups. However, the surfaces become smoother and the sharper edge disappears after the copper-loaded MSW is contacted with the HCl solution, as shown in Figure 12d. The egg-shaped microstructures appear again on the surfaces, similar to those observed in Figure 12b. The acid elution strips not only the Cu ions but also the alkaline-earth metals, which causes the relaxation of structures in the biosorbent. 3.10. Fourier Transform-Infrared Analysis. FTIR spectroscopic analysis of treated, adsorbed, and desorbed Sargassum sp. was conducted. Figure 13a shows the IR spectra of virgin and Cu-ion-bound MSW. The band at 3433.1 cm-1 represents pendent -OH and -NH groups in the virgin MSM. The shift in the band to 3436.9 cm-1 indicates changes in the amino group during the copper sorption. This also suggests that Cu2+ -OH interaction competes with the hydrogen bonding between OH groups. Carboxylate exhibits dual bands at 1631.7 and 1423.4 cm-1 for the virgin MSW. Both bands observed shifts to different extents after the copper biosorption to 1635.5 and 1419.5 cm-1. This shift can be explained by the associations of the carbonyl group with metal ions.3 Figure 13b gives FT-IR spectra of the virgin MSW and the MSW that have undergone copper biosorption and elution (0.2 M HCl and S/L ) 10 g/L). After the desorption process, bands assigned to the -OH and -NH2 functional groups shift back from 3436.1 to 3433.1 cm-1, which indicates the restoration of binding sites. Similarly, the carboxylate functional group is restored at 1423.4 cm-1 after desorption, whereas a further shift of carbonyl group to a higher wavelength (from 1635.5 to 1639.4 cm-1) can be observed. No chemical bonds are destroyed or created, because of the presence of Cu ions in the biomass, as shown in Figure 13a. However, Figure 13b demonstrates one type of CdO absorption band at 1735.8 cm-1, with no prior peaks observed for reference on spectra associated with the virgin MSW and the copper-loaded MSW. The appearance of the peak could indicate that carboxylic acid groups generated from the acidification of copper/ calcium carboxylate react with alcoholic groups (i.e., esterification) under the catalysis of HCl. The new peaks at 879.5 and 663.5 cm-1 are less obvious and can be neglected. This FT-IR analysis shows the coordination of metals with functional groups present in the MSW. The amino and carboxyl functional groups provide the major biosorption sites for the metal binding (e.g., calcium and copper). Other functional groups, such as ether and alcoholic functional groups, experience less-obvious changes, and, thus, they do not have important roles in the metal uptake.

Among sodium hydroxide (NaOH), hydrochloric acid (HCl), calcium chloride (CaCl2), formaldehyde, and glutaraldehyde, 0.2% formaldehyde is observed to be the best, in regard to the chemical modifications of Sargassum sp. The modified seaweeds (MSW) have a weight loss of 24.5% during the modification, which is lower than that observed in acid and base modifications. The metal biosorption capacity of the MSW is higher than that of the RSW, whereas its weight loss is 53% less than the raw seaweed (RSW). The organic content of the filtrated finished samples is only 3.84 mg/L TOC, compared with that of 19.0 mg/L TOC when the RSW is used. Higher pH would cause higher metal biosorption. The metal biosorption follows a descending sequence: lead > copper > zinc ≈ cadmium > nickel. The metal uptake by the MSW is finished with 4 h, which is similar to that of the RSW. Ion exchange between Ca ions in the MSW and the heavy-metal ions in the solution has an important role in the metal uptake. A surface diffusion model well describes the biosorption kinetics. Among HCl, NaOH, sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), nitric acid (HNO3), sulfuric acid (H2SO4), and ethylenediamine tetraacetic acid (EDTA), 0.2 M HCl is the best, in regard to the metal desorption. Approximately 90% of the metal ions can be eluted from the biosorbent; the desorption is completed within 20 min. A five-cycle operation of metal sorption and desorption confirms that the MSW is much better than the RSW. The concentration of eluted copper solution is >700 mg/L, which can be further recovered by chemical reduction technologies. The Fourier transform infrared (FT-IR) analysis demonstrates that the hydroxyl, amino, and carboxyl functional groups in the MSW provide the major biosorption sites for the metal binding. The scanning electron microscopy (SEM) study shows the strong cross-linkage between metal ions and organic functional groups. Acknowledgment The financial support provided to J.P.C. by the National University of Singapore (NUS) is appreciated. The authors thank Dr. Liang Hong (Department of Chemical and Biomolecular Engineering, National University of Singapore) for his valuable comments on the SEM and IR analyses. Literature Cited (1) Volesky, B. Biosorption of Heavy Metals; CRC Press: Boca Raton, FL, 1990. (2) Kratochvil, D.; Volesky, B. Biosorption of Cu from ferruginous wastewater by algal biomass. Water Res. 1998, 32, 2760. (3) Sheng, P. X.; Ting, Y. P.; Chen, J. P.; Hong, L. Sorption of lead, copper, cadmium, zinc, and nickel by marine algal biomass: characterization of biosorptive capacity and investigation of mechanisms. J Colloid Interface Sci. 2004, 275, 131. (4) Raize, O.; Argaman, Y.; Yannai, S. Mechanisms of biosorption of different heavy metals by brown marine macroalgae. Biotechnol. Bioeng. 2004, 87, 451. (5) Lodeiro, P.; Cordero, B.; Grille, Z.; Herrero, R.; Sastre de Vicente, M. E. Physicochemical studies of cadmium(II) biosorption by the invasive Alga in Europe, Sargassum muticum. Biotechnol. Bioeng. 2004, 88, 237.

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(6) Sheng, P. X.; Tan, L. H.; Chen, J. P.; Ting, Y. P. Biosorption Performance of Two Brown Marine Algae for Removal of Chromium. J. Dispersion Sci. Technol. 2004, 25, 681. (7) Matheickal, J. T.; Yu, Q. M. Biosorption of lead(II) and copper(II) from aqueous solutions by pretreated biomass of Australian marine algae. Bioresour. Technol. 1999, 69, 223. (8) Figueira, M. M.; Volesky, B.; Ciminelli, V. S. T.; Roddick, F. A. Biosorption of metals in brown seaweed biomass. Water Res. 2000, 34, 196. (9) Chen, J. P.; Lie, D.; Wang, L.; Wu, S. N.; Zhang, B. P. Dried Waste Activated Sludge as Biosorbents for Metal Removal: Adsorptive Characterization and Prevention of Organic Leaching. J. Chem. Technol. Biotechnol. 2002, 77, 657. (10) Holan, Z. R.; Volesky, B.; Prasetyo, I. Biosorption of Cadmium by biomass of marine algae. Biotechnol. Bioeng. 1993, 8, 819. (11) Henry, G. M. The chemistry of brown algae. In Economic Botany: A Textbook of Useful Plants and Plant Products, 2nd Edition; Hill, A. F., Ed.; McGraw-Hill: New York, 1952; p 174. (12) Braccini, I.; Pe´rez, S. Molecular basis of Ca2+-induced gelation in alginates and pectins: the Egg-Box model revisited. Biomacromolecules 2001, 2, 1089. (13) Jayakrishnan, A.; Jameela, S. R. Review: Glutaraldehyde as a fixative in bioprostheses and drug delivery matrixes. Biomaterials 1996, 17, 471. (14) Gerrarda, J. A.; Brown, P. K.; Fayle, S. E. Maillard crosslinking of food proteins I: the reaction of glutaraldehyde, formaldehyde and glyceraldehyde with ribonuclease. Food Chem. 2002, 79, 343.

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Received for review June 10, 2005 Revised manuscript received August 18, 2005 Accepted September 17, 2005 IE050678T

Chemical Modification of Sargassum sp. for Prevention

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