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Biosorption of lanthanum, cerium, europium, and ytterbium by a brown marine alga, Turbinaria Conoides

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Biosorption of lanthanum, cerium, europium, and ytterbium by a brown marine alga, Turbinaria Conoides
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  Biosorption of Lanthanum, Cerium, Europium, and Ytterbium by a BrownMarine Alga,  Turbinaria Conoides K. Vijayaraghavan, † M. Sathishkumar, † and R. Balasubramanian* ,‡ Singapore-Delft Water Alliance, National Uni V ersity of Singapore, 2 Engineering Dri V e 2, Singapore 117577,Singapore, and Di V ision of En V ironmental Science and Engineering, National Uni V ersity 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 biosorbentfor 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 wasfound to proceed mainly by ion-exchange reactions between the lanthanide ions and the carboxyl groupspresent on the algal surface, confirmed by the pH edge, desorption, and scanning electron microscopy/energy-dispersive X-ray results. Biosorption isotherms were modeled using the Langmuir, Freundlich, and Tothisotherms, with the latter-described REE isotherms with very high correlation coefficients and lower errorvalues. 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 berapid, achieving 90% of total biosorption within 50 min. Desorption was successful with 0.05 M HCl, andthe biomass was regenerated and reused for three sorption - desorption cycles without a significant loss in thebiosorption capacity. 1. Introduction In recent years, significant quantities of rare-earth elements(REEs) enter the environment through different pathways becauseof the rapid increase of the exploitation of REE resources and itsapplications to modern industry and daily life. 1,2 In addition,millions of tons of fertilizers containing REEs are used worldwidefor enhancing agricultural productivity. 3 For instance, in China,inorganic compounds of REEs, such as Re(NO 3 ) 3 , which acts as amicroelement fertilizer, have been widely applied to agriculturalcrops. 4 As a result, more REEs have entered the environment andaccumulated in the ecosystem. The scattering of chemical sub-stances, such as REEs, has been known to lead to severe changesin the elemental balance in the environment and biosphere, which,in turn, could endanger public health. 1,5 Considering the accumula-tion property of REEs and its relative toxicity toward livingorganisms, there is a need to find a suitable and economicaltreatment method for REE-bearing solutions.It is known for years that biomaterials can bind differentheavy-metal ions. Biosorbents for the removal of heavy-metalions mainly come under the following categories: bacteria, fungi,algae, industrial waste, agricultural wastes, and other polysac-charide materials. 6 Of the different types of biosorbents,macroscopic and low-cost materials are generally preferred forsuccessful biosorption processes. Microbial biosorbents arebasically small particles with low density, poor mechanicalstrength, and little rigidity. Even though they excel with highbiosorption capacities, they suffer with solid - liquid separation,biomass swelling, the impossibility of regeneration/reuse, andthe development of a high pressure drop when used incontinuous column mode. 6,7 Marine algae, popularly known as seaweed, are biologicalresources and are available in many parts of the world. Algaldivisions include red, green, and brown seaweed, of whichbrown seaweed is s found to be an excellent biosorbent. 8 Thisis due to the presence of alginate, which is present in a gelform in their cell walls. Apart from this, their macroscopicstructure offers a convenient basis for the production of biosorbent particles suitable for sorption process applications. 9 Even though many types of seaweed have commercial applica-tions, in certain areas they are plentiful and fast-growing andthreaten the tourist industry by spoiling the environment andfouling beaches. 10 Compared with heavy-metal ions, the potential of biosor-bents for sequestration of REEs has seldom been studied.Only a very few biosorbents have been tested for theirpotential to bind REEs, and these include  Pseudomonasaeruginosa , 11 Sargassum polycystum , 12 and  Platanus orien-talis  leaf powder 13 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 verycommon brown alga found throughout the Pacific and IndianOceans. It is known for its rigidity but is believed to havelow commercial importance. 2. Materials and Methods2.1. REE Stock Solution and Seaweed Preparation.  REEsas nitrate derivatives La(NO 3 ) 3 · 6H 2 O, Ce(NO 3 ) 3 · 6H 2 O,Eu(NO 3 ) 3 · 5H 2 O, and Yb(NO 3 ) 3 · 5H 2 O were purchased fromSigma-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 thebeaches of the Mandapam region (Tamilnadu, India). Thebiomass was extensively washed with deionized water and sun-dried. The dried biomass was then ground in a blender toproduce particles with an average size of 0.75 mm. 2.2. Biosorption Experiments.  Biosorption experimentswere conducted by bringing 0.1 g of   T. conoides  biomass into * To whom correspondence should be addressed. Tel:  + 65-65165135. Fax:  + 65-67744202. E-mail: eserbala@nus.edu.sg. † Singapore-Delft Water Alliance. ‡ Division of Environmental Science and Engineering.  Ind. Eng. Chem. Res.  2010,  49,  4405–4411  4405 10.1021/ie1000373  ©  2010 American Chemical SocietyPublished on Web 04/09/2010  contact with 50 mL of a REE solution, at the desired pH, in250 mL Erlenmeyer flasks kept on a rotary shaker at 200 rpm.The pH of the solution was initially adjusted using 0.1 M HClor NaOH, with the pH of the reaction mixture being controlledin the same manner during experimental runs. After 6 h of contact, the reaction mixture was filtered through a 0.45  µ mpoly(tetrafluoroethylene) (PTFE) membrane filter and analyzedfor the REE (La, Ce, Eu, and Yb) concentrations using aninductively coupled plasma atomic emission spectrometer(Perkin-Elmer Optima 3000 DV).For pH edge experiments, the initial solution pH was variedbetween 2 and 5. It should be noted that the initial solution pHrange was selected such that no REE precipitations wereexperimentally found in the bulk aqueous solution during thebiosorption process. The initial REE concentration is fixed at1000 mg/L. For isotherm experiments, the experimental pro-cedure is the same except that the initial REE concentrationswere varied at optimum pH conditions. Kinetic experimentswere conducted using the same method as that in isothermexperiments, except that the samples were collected at differenttime intervals to determine the time point at which biosorptionequilibrium was attained.The amount of REE sorbed by the biosorbent was calculatedfrom the differences between the REE quantity added to thebiosorbent and the REE content of the supernatant using thefollowing equation:where  Q  is the REE uptake (mg/g),  C  0  and  C  f   are the initialand equilibrium REE concentrations in the solution (mg/L),respectively,  V   is the solution volume (L), and  M   is the massof the biosorbent (g). 2.3. Isotherm and Kinetic Modeling.  Three equilibriumisotherm models were used to fit the REE isotherm experimentaldata, as follows:where  Q max  is the maximum REE uptake (mg/g),  b  the Langmuirequilibrium constant (L/mg),  K  F  the Freundlich constant (mg/ g) (L/mg) 1/  n ,  n  the Freundlich exponent,  b T  the Toth modelconstant (L/mg), and  n T  the Toth model exponent.The experimental biosorption kinetic data were modeled usingthe pseudo-first- and -second-order kinetics, which can beexpressed in their nonlinear forms, as follows:where  Q e  is the amount of REE sorbed at equilibrium (mg/g), Q t   the amount of REE sorbed at time  t   (mg/g),  k  1  the pseudo-first-order rate constant (1/min), and  k  2  the pseudo-second-orderrate constant (g/mg · min). All of the model parameters wereevaluated by nonlinear regression using  Sigma Plot   (version 4.0,SPSS, Chicago, IL) software. The residual root-mean-squareerror (RMSE) was also used to measure the goodness-of-fit.The RMSE can be defined aswhere  Q i  is the observation from the batch experiment,  q i  isthe estimate from the model for the corresponding  Q i , and  m  isthe number of observations in the experimental isotherm. Thesmaller the RMSE value is, the better the curve fitting wouldbe. The   2 test can be defined asIf data from model are similar to the experimental data,   2 willbe a small number. The average percentage error between theexperimental and predicted values is calculated usingwhere  Q exp  and  Q cal  represents experimental and calculated metaluptake values, respectively, and  N   is the number of measure-ments. All experiments were done in duplicate, and the datapresented 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 REEsolution at pH 4.9  (  0.2, was separated from the solution byfiltration. The biosorbent was then brought into contact with aknown volume of 0.05 M NaOH or 0.05 M HCl for 1 h, on arotary shaker at 200 rpm. After desorption, the reaction mixturewas filtered through a 0.45  µ m PTFE membrane filter andanalyzed for REE concentrations. 2.5. Scanning Electron Microscopy (SEM) andEnergy-Dispersive X-ray (EDX) Analysis.  To determine themajor mechanism responsible for REE removal, dried samplesof virgin and REE-exposed  T. conoides  were dried, coated witha thin layer of platinum, and analyzed by SEM along with EDXanalysis (JEOL, JSM-5600 LV). 3. Results and Discussion3.1. pH Edge and Mechanism of REE Biosorption.  Thesolution pH usually plays a major role in biosorption and affectsthe solution chemistry of the metals and the activity of thefunctional groups of the biomass. Results (Figure 1) revealedthat the REE removal efficiencies of   T. conoides  were found tobe severely affected by the solution equilibrium pH. Theoptimum pH for maximum removal was found to pH 4.9  ( 0.2. A further decrease in the solution pH resulted in a decreasein the performance of   T. conoides  toward all REEs. Brown algaemainly consist of alginic acid, which constitutes 10 - 40% of the dry weight of the algae. 8 Alginic acids are linear carboxy-lated copolymers consisting of different proportions of 1,4-linked   - D -mannuronic acid (M block) and  R  - L -guluronic acid (Gblock). Among the different functional groups, carboxyl groupsare abundant, and several investigators reported that they playan important role in metal biosorption at different pHconditions. 15,16 At lower pH values, these negatively chargedfunctional groups are protonated with H + , or other light metalions, which implies that the majority of binding sites wereoccupied and REEs may not be able to compete with these ions Q  )  V  ( C  0  -  C  f  )/   M   (1)Langmuir model:  Q  ) Q max bC  f  1  +  bC  f  (2)Freundlich model:  Q  )  K  F C  f 1/  n (3)Toth model:  Q  ) Q max b T C  f  [1  +  ( b T C  f  ) 1/  n T ] n T (4)Pseudo-first-order model:  Q t   )  Q e [1  -  exp( - k  1 t  )] (5)Pseudo-second-order model:  Q t   ) Q e2 k  2 t  1  +  Q e k  2 t   (6)RMSE  )    1 m  -  2 ∑ i ) 1 m ( Q i  -  q i ) 2 (7)   2 )  ∑ i ) 1 m ( Q i  -  q i ) 2 q i (8) ε  ( % )  ) ∑ i ) 1 m ( Q i  -  q i  /  Q i ) m  ×  100 (9) 4406  Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010  in occupying the sites; therefore, we observed little or no REEuptake at these strong acidic conditions. As the pH increases,the concentration of H + ions decreases and positively chargedREE ions can interact with negatively charged binding sites of the biomass, and therefore we observed an increase in theremoval efficiency with an increase in the pH. It is also worthnoting that the p K  a  value of the carboxyl groups lies at about4.8 and that maximum attraction of the sorbate to the sorbentis expected around this pH value. 17 The hydrochemical behavior of REEs is strongly influencedby their solution speciation. 18 At acidic pH values (pH  e  5),lanthanides exists as Ln 3 + . 19 Under these pH conditions, theinitial hydroxide concentration was negligible and hence theconcentration of La(OH) 2 + was minimal. At pH values greaterthan 5, precipitation of lanthanides starts and thus experimentswere not conducted beyond pH 5. Considering that lanthanidesare trivalent ions and that neither complexation nor hydrolysisoccurs, it can be clear that ion exchange is the major mechanismresponsible for the biosorption of REEs.From the results, it can be inferred that  T. conoides  biosorbedmore La, compared to other REEs. Even though the margin of difference is low, it is significant to establish that the affinityof   T. conoides  varies with different REEs. The magnitude of the removal efficiencies toward each REE by  T. conoides  canbe generalized as La  >  Ce  >  Eu  >  Yb. The affinity of a biomasstoward a particular ion can be correlated with its atomic mass,electronegativity, and ionic radius. 20 The atomic mass is in theorder of La (138.9)  <  Ce (140.1)  <  Eu (151.9)  <  Yb (173.0). Inthe 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 clearcorrelation can be established between the order of the bio-sorption and the properties of REEs. Because of its high ionicradius, low atomic mass, and low electronegativity,  T. conoides biosorbed more La compared to other REEs.SEM of virgin  T. conoides  revealed important informationon the surface morphology (Figure 2a). Surface protuberanceand microstructures can be observed, which may be due to Caand other salt crystalloid deposition. After REE binding, thesurface of   T. conoides  appears flattened in comparison to theraw sample (figure not shown). Apart from that, no furthersignificant morphological changes were apparent in the SEMimages. In EDX analysis, strong Ca peaks were observed invirgin  T. conoides  (Figure 2b). Peaks for Na, K, and Mg werealso recorded in EDX analysis. Upon observation of the EDXspectra of REE-exposed  T. conoides , it is very clear that Capeaks were reduced and new peaks of biosorbed REEs werepresent (Figure 2). Also, none of the spectra on REE-exposed T. conoides  confirmed the presence of Na, K, and Mg. Thissupports our earlier observation that when virgin  T. conoides is exposed to La 3 + solutions, La 3 + cations may replace someof the alkali and alkaline-earth metals naturally present in thecell 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 andretain in an immobilized form. A biosorption isotherm, the plotof uptake ( Q ) versus the equilibrium sorbate concentration inthe solution ( C  f  ), is often used to evaluate the sorptionperformance of the biosorbent. In this study, isotherm curveswere evaluated by varying the initial REE concentrations(99 - 1005 mg/L), while fixing the equilibrium solution pH at5.0 ( 0.1. Figure 3 illustrates the biosorption isotherms observedduring La, Ce, Eu, and Yb removal by  T. conoides . In general,the REE uptake increases with an increase in the concentrationand reaches saturation at higher concentrations. A close analysisof the shape of the isotherm revealed that the isotherm wasfavorable and can be classified as “L-shaped”. 21 This meansthat the ratio between the REE concentration in the solutionand that sorbed onto  T. conoides  decreases with an increase inthe REE concentration, providing a concave curve with a strictplateau. From the experimental isotherm curves, we can inferthat  T. conoides  exhibited a maximum uptake of La followedby Ce, Eu, and Yb.Isotherms pertaining to the biosorption of La, Ce, Eu, andYb onto  T. conoides  were tested using the Langmuir, Freundlich,and Toth models. The model constants, along with the correla-tion coefficient (  R 2 ), error (%), RMSE, and   2 values arepresented in Table 1. The classical Langmuir model incorporatestwo easily interpretable constants:  Q max , which corresponds tothe maximum achievable uptake by a system, and  b L , which isrelated to the affinity between the sorbate and sorbent. Results 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). Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010  4407  indicated that  T. conoides  recorded the highest  Q max  for La,followed by Ce, Eu, and Yb. A similar trend was also observedin the case of   b L , which implies that  T. conoides  possesses ahigh affinity toward La, followed by Ce, Eu, and Yb. 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 . 4408  Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010  The Freundlich model is an empirical equation based onan exponential distribution of sorption sites and energies. Itis also assumed that the stronger binding sites are occupiedfirst and that the binding strength decreases with an increasein the degree of site occupation. High  K  F  and  n  values indicatethat the binding capacity reached its highest value, and theaffinity between the biosorbent and REE was also high. FromTable 1, it can be inferred that the binding capacity andaffinity decreases in the following order: La  >  Ce  >  Eu  > Yb. 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 Freundlich Tothelement  Q max (mg/g)  b (L/mg)  R 2  %error RMSE   2  K  F (g/L)  n R 2  %error RMSE   2  Q max (mg/g) b T (L/mg)  n T  R 2  %error RMSE   2 La 154.7 0.038 0.99 0.43 2.28 0.39 42.8 4.96 0.99 4.31 14.38 13.64 156.1 0.041 1.05 0.99 0.23 2.22 0.31Ce 152.8 0.035 0.99 0.56 2.15 0.43 40.8 4.95 0.98 4.09 13.66 12.46 155.2 0.039 1.09 0.99 0.24 1.96 0.26Eu 138.2 0.032 0.99 0.53 3.04 0.79 37.3 4.85 0.98 3.58 12.55 10.94 139.5 0.034 1.05 0.99 0.36 3.01 0.68Yb 121.2 0.027 0.99 0.44 1.87 0.32 30.5 4.75 0.98 2.64 9.24 6.78 124.5 0.031 1.14 0.99 0.11 1.58 0.16 Figure 4.  Kinetics of REE biosorption by  T. conoides  at pH 4.9  (  0.2 (curves predicted by the pseudo-first-order model). Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010  4409
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