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Biosorption of Lanthanum, Cerium, Europium, and Ytterbium by a Brown Marine Alga, Turbinaria Conoides K. Vijayaraghavan,† M. Sathishkumar,† and R. Balasubramanian*,‡ Singapore-Delft Water Alliance, National UniVersity of Singapore, 2 Engineering DriVe 2, Singapore 117577, Singapore, and DiVision of EnVironmental Science and Engineering, National UniVersity of Singapore, Singapore 117576, Singapore
The ability of a brown marine alga, Turbinaria conoides, to remove four rare-earth elements (REEs; lanthanum, cerium, europium, and ytterbium) was evaluated. Results showed that T. conoides was an excellent biosorbent for all four REEs. The equilibrium pH was found to severely affect the biosorption performance; pH 4.9 ( 0.2 was found to be an optimum pH for favorable biosorption of REEs. The biosorption mechanism was found to proceed mainly by ion-exchange reactions between the lanthanide ions and the carboxyl groups present on the algal surface, confirmed by the pH edge, desorption, and scanning electron microscopy/energydispersive X-ray results. Biosorption isotherms were modeled using the Langmuir, Freundlich, and Toth isotherms, with the latter-described REE isotherms with very high correlation coefficients and lower error values. Maximum biosorption uptakes, according to the Langmuir model, were recorded as 154.7, 152.8, 138.2, and 121.2 mg/g for La, Ce, Eu, and Yb, respectively. Biosorption kinetics of REEs was found to be rapid, achieving 90% of total biosorption within 50 min. Desorption was successful with 0.05 M HCl, and the biomass was regenerated and reused for three sorption-desorption cycles without a significant loss in the biosorption capacity. 1. Introduction In recent years, significant quantities of rare-earth elements (REEs) enter the environment through different pathways because of the rapid increase of the exploitation of REE resources and its applications to modern industry and daily life.1,2 In addition, millions of tons of fertilizers containing REEs are used worldwide for enhancing agricultural productivity.3 For instance, in China, inorganic compounds of REEs, such as Re(NO3)3, which acts as a microelement fertilizer, have been widely applied to agricultural crops.4 As a result, more REEs have entered the environment and accumulated in the ecosystem. The scattering of chemical substances, such as REEs, has been known to lead to severe changes in the elemental balance in the environment and biosphere, which, in turn, could endanger public health.1,5 Considering the accumulation property of REEs and its relative toxicity toward living organisms, there is a need to find a suitable and economical treatment method for REE-bearing solutions. It is known for years that biomaterials can bind different heavy-metal ions. Biosorbents for the removal of heavy-metal ions mainly come under the following categories: bacteria, fungi, algae, industrial waste, agricultural wastes, and other polysaccharide materials.6 Of the different types of biosorbents, macroscopic and low-cost materials are generally preferred for successful biosorption processes. Microbial biosorbents are basically small particles with low density, poor mechanical strength, and little rigidity. Even though they excel with high biosorption capacities, they suffer with solid-liquid separation, biomass swelling, the impossibility of regeneration/reuse, and the development of a high pressure drop when used in continuous column mode.6,7 Marine algae, popularly known as seaweed, are biological resources and are available in many parts of the world. Algal * To whom correspondence should be addressed. Tel: +6565165135. Fax: +65-67744202. E-mail:
[email protected]. † Singapore-Delft Water Alliance. ‡ Division of Environmental Science and Engineering.
divisions include red, green, and brown seaweed, of which brown seaweed is s found to be an excellent biosorbent.8 This is due to the presence of alginate, which is present in a gel form in their cell walls. Apart from this, their macroscopic structure offers a convenient basis for the production of biosorbent particles suitable for sorption process applications.9 Even though many types of seaweed have commercial applications, in certain areas they are plentiful and fast-growing and threaten the tourist industry by spoiling the environment and fouling beaches.10 Compared with heavy-metal ions, the potential of biosorbents for sequestration of REEs has seldom been studied. Only a very few biosorbents have been tested for their potential to bind REEs, and these include Pseudomonas aeruginosa,11 Sargassum polycystum,12 and Platanus orientalis leaf powder13 and crab shell.14 Thus, the objective of the present study is to explore the biosorption potential of Turbinaria conoides toward lanthanum, cerium, europium, and ytterbium from aqueous solutions. T. conoides is a very common brown alga found throughout the Pacific and Indian Oceans. It is known for its rigidity but is believed to have low commercial importance. 2. Materials and Methods 2.1. REE Stock Solution and Seaweed Preparation. REEs as nitrate derivatives La(NO3)3 · 6H2O, Ce(NO3)3 · 6H2O, Eu(NO3)3 · 5H2O, and Yb(NO3)3 · 5H2O were purchased from Sigma-Aldrich. Stock solutions were prepared by addition of the required amount of REE salts in deionized water. A fresh biomass of T. conoides was collected from the beaches of the Mandapam region (Tamilnadu, India). The biomass was extensively washed with deionized water and sundried. The dried biomass was then ground in a blender to produce particles with an average size of 0.75 mm. 2.2. Biosorption Experiments. Biosorption experiments were conducted by bringing 0.1 g of T. conoides biomass into
10.1021/ie1000373 2010 American Chemical Society Published on Web 04/09/2010
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contact with 50 mL of a REE solution, at the desired pH, in 250 mL Erlenmeyer flasks kept on a rotary shaker at 200 rpm. The pH of the solution was initially adjusted using 0.1 M HCl or NaOH, with the pH of the reaction mixture being controlled in the same manner during experimental runs. After 6 h of contact, the reaction mixture was filtered through a 0.45 µm poly(tetrafluoroethylene) (PTFE) membrane filter and analyzed for the REE (La, Ce, Eu, and Yb) concentrations using an inductively coupled plasma atomic emission spectrometer (Perkin-Elmer Optima 3000 DV). For pH edge experiments, the initial solution pH was varied between 2 and 5. It should be noted that the initial solution pH range was selected such that no REE precipitations were experimentally found in the bulk aqueous solution during the biosorption process. The initial REE concentration is fixed at 1000 mg/L. For isotherm experiments, the experimental procedure is the same except that the initial REE concentrations were varied at optimum pH conditions. Kinetic experiments were conducted using the same method as that in isotherm experiments, except that the samples were collected at different time intervals to determine the time point at which biosorption equilibrium was attained. The amount of REE sorbed by the biosorbent was calculated from the differences between the REE quantity added to the biosorbent and the REE content of the supernatant using the following equation: Q ) V(C0 - Cf)/M
(1)
where Q is the REE uptake (mg/g), C0 and Cf are the initial and equilibrium REE concentrations in the solution (mg/L), respectively, V is the solution volume (L), and M is the mass of the biosorbent (g). 2.3. Isotherm and Kinetic Modeling. Three equilibrium isotherm models were used to fit the REE isotherm experimental data, as follows: QmaxbCf 1 + bCf
(2)
Freundlich model: Q ) KFCf1/n
(3)
Langmuir model: Q )
Toth model: Q )
QmaxbTCf [1 + (bTCf)1/nT]nT
(4)
where Qmax is the maximum REE uptake (mg/g), b the Langmuir equilibrium constant (L/mg), KF the Freundlich constant (mg/ g) (L/mg)1/n, n the Freundlich exponent, bT the Toth model constant (L/mg), and nT the Toth model exponent. The experimental biosorption kinetic data were modeled using the pseudo-first- and -second-order kinetics, which can be expressed in their nonlinear forms, as follows: Pseudo-first-order model: Qt ) Qe[1 - exp(-k1t)] (5) Pseudo-second-order model: Qt )
Qe2k2t 1 + Qek2t
(6)
where Qe is the amount of REE sorbed at equilibrium (mg/g), Qt the amount of REE sorbed at time t (mg/g), k1 the pseudofirst-order rate constant (1/min), and k2 the pseudo-second-order rate constant (g/mg · min). All of the model parameters were evaluated by nonlinear regression using Sigma Plot (version 4.0, SPSS, Chicago, IL) software. The residual root-mean-square
error (RMSE) was also used to measure the goodness-of-fit. The RMSE can be defined as RMSE )
1 m-2
m
∑ (Q
i
- qi)2
(7)
i)1
where Qi is the observation from the batch experiment, qi is the estimate from the model for the corresponding Qi, and m is the number of observations in the experimental isotherm. The smaller the RMSE value is, the better the curve fitting would be. The χ2 test can be defined as m
χ2 )
∑ i)1
(Qi - qi)2 qi
(8)
If data from model are similar to the experimental data, χ2 will be a small number. The average percentage error between the experimental and predicted values is calculated using m
ε (%) )
∑ (Q
i
- qi /Qi)
i)1
m
× 100
(9)
where Qexp and Qcal represents experimental and calculated metal uptake values, respectively, and N is the number of measurements. All experiments were done in duplicate, and the data presented are the average values of two experiments. 2.4. Desorption Experiments. The REE-loaded T. conoides, which was previously exposed to 100 mg/L of each of REE solution at pH 4.9 ( 0.2, was separated from the solution by filtration. The biosorbent was then brought into contact with a known volume of 0.05 M NaOH or 0.05 M HCl for 1 h, on a rotary shaker at 200 rpm. After desorption, the reaction mixture was filtered through a 0.45 µm PTFE membrane filter and analyzed for REE concentrations. 2.5. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDX) Analysis. To determine the major mechanism responsible for REE removal, dried samples of virgin and REE-exposed T. conoides were dried, coated with a thin layer of platinum, and analyzed by SEM along with EDX analysis (JEOL, JSM-5600 LV). 3. Results and Discussion 3.1. pH Edge and Mechanism of REE Biosorption. The solution pH usually plays a major role in biosorption and affects the solution chemistry of the metals and the activity of the functional groups of the biomass. Results (Figure 1) revealed that the REE removal efficiencies of T. conoides were found to be severely affected by the solution equilibrium pH. The optimum pH for maximum removal was found to pH 4.9 ( 0.2. A further decrease in the solution pH resulted in a decrease in the performance of T. conoides toward all REEs. Brown algae mainly consist of alginic acid, which constitutes 10-40% of the dry weight of the algae.8 Alginic acids are linear carboxylated copolymers consisting of different proportions of 1,4-linked β-D-mannuronic acid (M block) and R-L-guluronic acid (G block). Among the different functional groups, carboxyl groups are abundant, and several investigators reported that they play an important role in metal biosorption at different pH conditions.15,16 At lower pH values, these negatively charged functional groups are protonated with H+, or other light metal ions, which implies that the majority of binding sites were occupied and REEs may not be able to compete with these ions
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Figure 1. Effect of the equilibrium pH on the biosorption of La, Ce, Eu, and Yb onto T. conoides (initial REE concentration ) 98.9 ( 1.2 mg/L).
in occupying the sites; therefore, we observed little or no REE uptake at these strong acidic conditions. As the pH increases, the concentration of H+ ions decreases and positively charged REE ions can interact with negatively charged binding sites of the biomass, and therefore we observed an increase in the removal efficiency with an increase in the pH. It is also worth noting that the pKa value of the carboxyl groups lies at about 4.8 and that maximum attraction of the sorbate to the sorbent is expected around this pH value.17 The hydrochemical behavior of REEs is strongly influenced by their solution speciation.18 At acidic pH values (pH e 5), lanthanides exists as Ln3+.19 Under these pH conditions, the initial hydroxide concentration was negligible and hence the concentration of La(OH)2+ was minimal. At pH values greater than 5, precipitation of lanthanides starts and thus experiments were not conducted beyond pH 5. Considering that lanthanides are trivalent ions and that neither complexation nor hydrolysis occurs, it can be clear that ion exchange is the major mechanism responsible for the biosorption of REEs. From the results, it can be inferred that T. conoides biosorbed more La, compared to other REEs. Even though the margin of difference is low, it is significant to establish that the affinity of T. conoides varies with different REEs. The magnitude of the removal efficiencies toward each REE by T. conoides can be generalized as La > Ce > Eu > Yb. The affinity of a biomass toward a particular ion can be correlated with its atomic mass, electronegativity, and ionic radius.20 The atomic mass is in the order of La (138.9) < Ce (140.1) < Eu (151.9) < Yb (173.0). In the case of electronegativity, La (1.1) < Ce (1.12) ) Eu (1.12) < Yb (1.21). On the contrary, the ionic radius is in the order of La (117.2) > Ce (115) > Eu (108.7) > Yb (100.8). Thus, a clear correlation can be established between the order of the biosorption and the properties of REEs. Because of its high ionic radius, low atomic mass, and low electronegativity, T. conoides biosorbed more La compared to other REEs. SEM of virgin T. conoides revealed important information on the surface morphology (Figure 2a). Surface protuberance and microstructures can be observed, which may be due to Ca and other salt crystalloid deposition. After REE binding, the surface of T. conoides appears flattened in comparison to the raw sample (figure not shown). Apart from that, no further
significant morphological changes were apparent in the SEM images. In EDX analysis, strong Ca peaks were observed in virgin T. conoides (Figure 2b). Peaks for Na, K, and Mg were also recorded in EDX analysis. Upon observation of the EDX spectra of REE-exposed T. conoides, it is very clear that Ca peaks were reduced and new peaks of biosorbed REEs were present (Figure 2). Also, none of the spectra on REE-exposed T. conoides confirmed the presence of Na, K, and Mg. This supports our earlier observation that when virgin T. conoides is exposed to La3+ solutions, La3+ cations may replace some of the alkali and alkaline-earth metals naturally present in the cell wall through an ion-exchange mechanism. 3.2. Biosorption Isotherms and Modeling. The quality of a biosorbent is judged by how much sorbate it can attract and retain in an immobilized form. A biosorption isotherm, the plot of uptake (Q) versus the equilibrium sorbate concentration in the solution (Cf), is often used to evaluate the sorption performance of the biosorbent. In this study, isotherm curves were evaluated by varying the initial REE concentrations (99-1005 mg/L), while fixing the equilibrium solution pH at 5.0 ( 0.1. Figure 3 illustrates the biosorption isotherms observed during La, Ce, Eu, and Yb removal by T. conoides. In general, the REE uptake increases with an increase in the concentration and reaches saturation at higher concentrations. A close analysis of the shape of the isotherm revealed that the isotherm was favorable and can be classified as “L-shaped”.21 This means that the ratio between the REE concentration in the solution and that sorbed onto T. conoides decreases with an increase in the REE concentration, providing a concave curve with a strict plateau. From the experimental isotherm curves, we can infer that T. conoides exhibited a maximum uptake of La followed by Ce, Eu, and Yb. Isotherms pertaining to the biosorption of La, Ce, Eu, and Yb onto T. conoides were tested using the Langmuir, Freundlich, and Toth models. The model constants, along with the correlation coefficient (R2), error (%), RMSE, and χ2 values are presented in Table 1. The classical Langmuir model incorporates two easily interpretable constants: Qmax, which corresponds to the maximum achievable uptake by a system, and bL, which is related to the affinity between the sorbate and sorbent. Results
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Figure 2. SEM picture of raw T. conoides (a) and EDX spectra of raw (b), La-exposed (c), Ce-exposed (d), Eu-exposed (e), and Yb-exposed (f) T. conoides.
indicated that T. conoides recorded the highest Qmax for La, followed by Ce, Eu, and Yb. A similar trend was also observed
in the case of bL, which implies that T. conoides possesses a high affinity toward La, followed by Ce, Eu, and Yb.
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Figure 3. Isotherms during biosorption of different REEs onto T. conoides at pH 4.9 ( 0.2 (curves predicted by the Toth model). Table 1. Biosorption Isotherm Model Constants at pH 4.9 ( 0.2 Langmuir Qmax element b(L/mg) (mg/g) La Ce Eu Yb
154.7 152.8 138.2 121.2
0.038 0.035 0.032 0.027
R2 0.99 0.99 0.99 0.99
Freundlich
% RMSE error 0.43 0.56 0.53 0.44
2.28 2.15 3.04 1.87
χ2
KF (g/L)
n
R2
0.39 0.43 0.79 0.32
42.8 40.8 37.3 30.5
4.96 4.95 4.85 4.75
0.99 0.98 0.98 0.98
The Freundlich model is an empirical equation based on an exponential distribution of sorption sites and energies. It is also assumed that the stronger binding sites are occupied first and that the binding strength decreases with an increase in the degree of site occupation. High KF and n values indicate
Toth
% RMSE error 4.31 4.09 3.58 2.64
14.38 13.66 12.55 9.24
χ2 13.64 12.46 10.94 6.78
Qmax bT (mg/g) (L/mg) 156.1 155.2 139.5 124.5
0.041 0.039 0.034 0.031
nT
R2
1.05 1.09 1.05 1.14
0.99 0.99 0.99 0.99
% RMSE error 0.23 0.24 0.36 0.11
2.22 1.96 3.01 1.58
χ2 0.31 0.26 0.68 0.16
that the binding capacity reached its highest value, and the affinity between the biosorbent and REE was also high. From Table 1, it can be inferred that the binding capacity and affinity decreases in the following order: La > Ce > Eu > Yb.
Figure 4. Kinetics of REE biosorption by T. conoides at pH 4.9 ( 0.2 (curves predicted by the pseudo-first-order model).
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Table 2. Biosorption Kinetic Model Constants at pH 4.9 ( 0.2 pseudo first order element
experimental Qe(mg/g)
Qe (mg/g)
k1 (L/min)
La Ce Eu Yb
150.5 147.5 131.0 115.5
148.7 145.8 128.0 112.1
0.051 0.050 0.051 0.049
pseudo second order
R2
% error
RMSE
0.99 1.00 1.00 1.00
1.13 0.72 0.41 0.06
3.41 2.89 2.06 2.44
The Toth isotherm is derived from the potential theory and is applicable for heterogeneous adsorption. It assumes a quasiGaussian energy distribution. Most sites have adsorption energy lower than the maximum adsorption energy. The results of the model constants revealed that T. conoides favored La biosorption, followed by Ce, Eu, and Yb. On the basis of error analysis, it was determined that both Langmuir and Toth models were able to describe REE isotherms with high correlation coefficients and lower percent error, RMSE, and χ2 values (Table 1). 3.3. Kinetics and Modeling. Figure 4 presents a typical set of kinetic experimental results for the biosorption of REEs onto T. conoides biomass. The sorption kinetics in a wastewater treatment is significant because it provides valuable insight into the reaction pathways and the mechanism of a sorption reaction. Because biosorption is a metabolism-independent process, it would be expected to be a very fast reaction.6 Experimental kinetic data coincided with this aspect because more than 90% of total removal occurred in the first 50 min. This initial quick phase was followed by the slow attainment of equilibrium. These two different phases are common in most sorption systems because the sorbent contains a high concentration of exchangeable binding sites for metal binding in the beginning. As time progresses, these binding sites get saturated and subsequently uptake decreases. The experimental biosorption kinetic data were modeled using the pseudo-first- and -second-order kinetic models. The model rate constants and predicted equilibrium REE uptakes, along with correlation coefficients, percent error, RMSE, and χ2 values, are presented in Table 2. The biosorption process may proceed
χ2
Qe (mg/g)
k1 × 104 (g/mg min)
R2
% error
RMSE
χ2
3.31 2.12 0.53 0.82
164.5 161.7 142.2 125.0
4.32 4.27 4.87 5.28
0.98 0.98 0.98 0.99
0.99 1.44 1.83 2.37
6.29 6.59 4.20 3.78
4.82 5.60 3.24 3.55
by diffusion of REE ions through the boundary layer at the biosorbent layer at the biosorbent surface, and this may be the rate-determining step of the overall process. In that case, biosorption may likely follow the pseudo-first-order equation of the Lagergren. Results revealed that the pseudo-first-order model was able to describe the REE kinetic data with high correlationcoefficient(0.99-1.00),lowpercenterror(0.06-1.13%), low RMSE (2.06-3.41), and low χ2 (0.53-3.31) values. The values for the equilibrium biosorption capacity as predicted by the pseudo-first-order model coincided well with the experimental uptake values. The pseudo-first-order rate constant, k1, followed the same trend as that of uptake values, with the values in the order of La > Ce > Eu > Yb. Analysis of the experimental data with the pseudo-second-order kinetic model shows a good agreement of the sets of data, which is reflected in the high correlation coefficient values (Table 2). However, close analysis revealed that the pseudo-second-order model overpredicted the equilibrium uptake values. While considering percent error, RMSE, and χ2 values, it is clear that the pseudo-second-order data slightly deviates from the experimental data. The curves as predicted by the pseudo-first-order model for biosorption kinetic data of different REEs are shown in Figure 4. 3.4. Desorption and Regeneration. In desorption experiments, REE-loaded T. conoides were exposed, separately, to 0.05 M HCl or 0.05 M NaOH. Only about 11.9-12.5% of the initially sorbed REE could be desorbed by 0.05 M NaOH. Because the major mechanism for REE biosorption is ion exchange, a positively charged ion is required to replace La3+ in carboxyl binding sites. Even though Na+ can bind with
Figure 5. Reuse of regenerated T. conoides for REE biosorption (elutant ) 0.05 M HCl).
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carboxyl sites, it is a soft ion and affinity toward the carboxyl group is the least compared to other metal ions.22 In contrast, 0.05 M HCl performed very well and exhibited elution efficiencies greater than 99% for all REEs. This was due to the presence of excess H+ ions in the system, which successfully eliminates La3+ ions from the carboxyl sites. This fact is well supported by the sorption experimental results (Figure 1), which pointed out that less or no biosorption occurred at pH values of less than 2. Also, it is worth noting that this highly acidic desorbent medium had no influence on the physical structure of the biomass because no significant biosorbent weight loss was observed. The regeneration experiments explore the repeated biosorption potential of T. conoides for REEs over three sorption-desorption cycles (Figure 5). The biomass was able to retain its first cycle REE uptake capacity throughout the three cycles examined, aided by consistently high elution efficiencies by 0.05 M HCl. The percent decrease in the uptake in the third cycle compared to the first cycle was very minimal because only 0.9, 1.6, 3.2, and 4.9% decreased uptakes were observed for La, Ce, Eu, and Yb, respectively. 4. Conclusions This work demonstrates the feasibility of using a brown marine alga, T. conoides, for the biosorption of La, Ce, Eu, and Yb. The algal biomass showed great potential for the removal of all REEs in the order of La > Ce > Eu > Yb. The predominant ion-exchange mechanism involving carboxyl groups of alginate was confirmed. Typically, the biosorption capacity increases with an increase in the pH in the ranges of 2.0-5.0. Maximum biosorption capacities of 154.7, 152.8, 138.2, and 121.2 were obtained for La, Ce, Eu, and Yb, respectively, at pH 4.9 ( 0.2, according to the Langmuir model. Desorption and subsequent reuse of T. conoides biomass was possible with 0.05 M HCl, which performed well with elution efficiencies greater than 99% for all REEs. Thus, the high efficiency of the biomass, low biomass cost, and lower dependence on the biomass (due to reuse) makes this process an effective and alternative technique for the treatment of REE-laden solutions. Acknowledgment The authors gratefully acknowledge the support and contributions of this project to the Singapore-Delft Water Alliance (SDWA). The research presented in this work was carried out as part of the SDWA’s research programme (Grant R-264-001002-272). Literature Cited (1) Volokh, A. A.; Gorbunov, A. V.; Gundorina, S. F.; Revich, B. A.; Frontasyeva, M. V.; Chen, S. P. Phosphorus Fertilizer Production as a Source of Rare Earth Elements Pollution of the Environment. Sci. Total EnViron. 1990, 95, 141.
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ReceiVed for reView January 07, 2010 ReVised manuscript receiVed March 23, 2010 Accepted March 26, 2010 IE1000373
