alexa Biosorption of Neodymium by Selected Photoautotrophic and Heterotrophic Species | OMICS International
ISSN: 2157-7048
Journal of Chemical Engineering & Process Technology

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Biosorption of Neodymium by Selected Photoautotrophic and Heterotrophic Species

Marcus Heilmann1, Wojciech Jurkowski2, Rainer Buchholz1, Thomas Brueck2 and Anna Maria Becker1*
1 Institute of Process Engineering, Faculty of Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Paul-Gordan-Straße 3, Erlangen, Germany
2 Industrial Biocatalysis, Center of White Biotechnology, Department of Chemistry, Technische Universität München, Lichtenbergstraße 4, Garching, Munich, Germany
Corresponding Author : Anna Maria Becker
Institute of Bioprocess Engineering
Faculty of Engineering
Friedrich-Alexander-Universität Erlangen-Nürnberg
Paul-Gordan-Straße 3, 91052 Erlangen, Germany
Tel: 499131-85-23054
E-mail: [email protected]
Received August 17, 2015; Accepted September 03, 2015; Published September 07, 2015
Citation: Heilmann M, Jurkowski W, Buchholz R, Brueck T, Becker AM (2015) Biosorption of Neodymium by Selected Photoautotrophic and Heterotrophic Species. J Chem Eng Process Technol 6: 241. doi:10.4172/2157-7048.1000241
Copyright: © 2015 Heilmann M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

In a search for alternative methods for isolation of rare earth elements from diluted solutions, a possibility of their preconcentration on biomass of mainly phototrophic species, that can be easily cultivated in open or closed systems, was investigated in this study. A screening of 19 biological species, consisting of 17 microalgae, one yeast and one moss, in regard to the biosorption of neodymium (Nd) on their lyophilized biomass was performed. Maximum sorption capacity (qM) was determined after incubation for 3 h at room temperature and centrifugation through measuring the Nd content in the supernatant via a spectrophotometric assay. The obtained qM values ranged from 0.26 mmol*g-1 for Chlamydomonas reinhardtii to 0.48 mmol*g-1 for Calothrix brevissima and up to 0.74 mmol*g-1 for Physcomitrella patens. Kinetic measurements show the reaching of qM after 30 min for C. reinhardtii and 180 min for P. patens. Based on titration studies the proton exchange capacity was found to be 0.39 mmol*g-1 for C. reinhardtii and 0.44 mmol*g-1 for P. patens leading to a conclusion that proton exchange was mainly responsible for the biosorption mechanism. To our knowledge it is the first report of screening regarding such a wide scope of microalgae for a sorption of Nd.

Keywords
Biosorption; Microalgae; Lyophilized biomass; Recovery of Nd from highly diluted solutions; Mechanism
Introduction
Rare earth elements (REEs) consist of the lanthanides plus yttrium and scandium. Global demand for REEs in 2015 was estimated to 210,000 tons [1]. These elements are chemically very similar and randomly distributed in nature. Hence, their production is difficult and challenging due to many byproducts such as thorium, uranium, heavy metals and acidic waste waters [2]. Moreover, they can be found together in bulky rocks, so their separation is sophisticated. The classical exploitation of REE sources currently pollutes nature heavily. Due to that and the fact that around 97% of the worldwide need of REEs are provided by China [3], making REE-consuming industries excessively dependent on rationing and the rising world market prizes, there is a great interest in alternative and environmentally friendly methods for delivering these elements. One important representative of the REEs is neodymium (Nd), with its applications ranging from auto catalysts, petroleum refining, hard drives in laptops, headphones, hybrid engines up to permanent magnets in wind turbines [1,4].
The idea of binding metal ions from aqueous solutions via biosorption is not new. There are several methods reported in literature and applications for biosorption of heavy metals by various microorganisms such as bacteria, yeasts, fungi and algae [5-8]. During the last decade, REE biosorption and bioaccumulation by plant biomaterial became of increased interest [9-11], where so called “hyperaccumulators” such as Pronephrium simplex, Sargassum sp. and Dicropteris dichotoma were employed. Also microalgae were involved into this research field [12-14] with special emphasis on biosorption of REEs from acidic aqueous solutions. Microalgae are an interesting group of organisms for such research since they have numerous members and most of them can be produced, due to their high productivity in comparison to land plants, in high amounts under relatively simple conditions.
Our work aims to develop a potent method for recovery of Nd from highly diluted sources. In order to reach this aim, primarily, species have to be identified, that can be produced easily and bind Nd with high efficiency and preferably selectively with respect to heavy metals. In this paper we present a broad algal screening concerning Nd uptake from defined solutions and the first studies aiming at the elucidation of the binding mechanism of Nd.
Materials and Methods
Instrumentation and chemicals
Ultraviolet/Visible (UV/Vis) absorption spectra were recorded using a spectrophotometer (SPECORD 205, Analytik Jena, Germany) and a multilabel reader (EnSpire 2300, Perkin Elmer, USA). For measuring of pH-values, a pH-meter (pH 510, EUTECH Instruments, Germany) was used in combination with a micro electrode (InLab Micro, Mettler Toledo, USA). Biomass was centrifuged from culture medium using a Contifuge (Stratos, Heraeus, Germany). Centrifugation steps during the experiments were performed utilizing a lab centrifuge (5810R, Eppendorf, Germany). Lyophilized biomass was prepared by using a freeze dryer (Alpha 1-2, Christ, Germany). Additional determination of neodymium contents was performed via inductively coupled plasma atomic emission spectroscopy (ICP-AES, CIROS CCD, Spectro, Germany) at the Institute of Chemical Reaction Engineering, Friedrich-Alexander University of Erlangen-Nürnberg, Germany.
Neodymium (III) was investigated as neodymium (III) nitrate hexahydrate (99.9%, Aldrich, USA) and xylenol orange disodium salt (XO; FluKa, Germany) was used for UV/Vis-spectroscopic assay. ICP AES machine was calibrated using an Nd3+-ICP standard solution with 1000 mg*l-1 in diluted nitric acid (VWR, USA).
Biomass material and cultivation
Nineteen species included in the screening were either available in the in-house algae library or ordered from the Culture Collection of Algae at Goettingen University (SAG), Germany. In-house algae library contains mostly algae strains originating from SAG, but also from Chlamydomonas Research Center at the University of Minnesota (CC), Microbial Culture Collection at National Institute of Environmental Studies (NIES), ATCC and International Moss Stock Center at Freiburg University (IMSC).
In order to obtain enough biomass for following experiments, different microalgae (Arthronema africanum, Arthrospira platensis, Calothrix brevissima, Chlamydomonas reinhardtii, Chlorella sorokiniana, Euglena gracilis, Euglena mutabilis, Euglena stellata, Euglena viridis, Galdieria sulphuraria, Lyngbya taylorii, Nostoc ellipsosporum, Nostoc punctiform, Porphyridium purpureum, Prymnesium saltans, Schizochytrium limacinum, Tetraselmis chuii), yeast (Saccharomyces cerevisiae) and moss (Physcomitrella patens) were cultivated either in a well-established photobioreactor screening module (PSM) [15], a 25 l loop photobioreactor, developed at the Institute of Biotechnology, Department of Bioprocess Engineering, TU Berlin [16] or in 2 l Erlenmeyer flasks. For investigation of the sorption mechanism, the green alga Chlamydomonas reinhardtii (CC- 125) and the moss Physcomitrella patens (IMSC 40001) were cultivated according to Heining et al. [15]. All other species were obtained from other works performed at the Institute of Bioprocess Engineering at the University Erlangen-Nürnberg (for conditions see supporting information, SI).
Preparation of stock solutions
A Nd3+ stock solution was prepared by dissolving Nd (NO3)3 x 6 H2O (4.82 g) in 1 l ultrapure water resulting in a final concentration of 10 mM of Nd3+. The concentration of Nd3+ was monitored using ICPAES. Working solutions used for sorption experiments and calibration were prepared from the stock solution by appropriate dilutions with ultrapure water. XO stock solution was prepared by dissolving XO (71.65 mg) in 50 ml ultrapure water reaching a final concentration of 2 mM of XO. The XO concentration for calibration and unknown solutions was held constant at 200 μM by dilution of 100 μl 2 mM XO stock solution in 1 ml.
Quantification of Nd3+
The UV/Vis-spectroscopic assay employing XO as a complexing agent, as described previously, was used with slight modifications for determination of unknown Nd3+ concentrations in aqueous solutions obtained from all experiments described below [17]. Absorption spectra from 300 nm to 700 nm were recorded. Calibration solutions ranging from 10 μM to 100 μM Nd3+ with 200 μM XO were prepared in a 96-well microtiter plate made from quartz (Hellma, Germany) and the absorbance of the Nd-XO-complex was measured at 572 nm. Absorbance values were constant for at least 3 h when stored in closed reaction vessels. The influence of sodium chloride was investigated through repetition of the calibration in presence of NaCl (1.71 mM and 17.1 mM).
Biosorption screening
To harvest the biomass the cultivation medium was centrifuged, resuspended in little deionized water, freezed at -80°C and lyophilized (vacuum, 24 hr). The dried biomass was homogenized using mortar and pestle prior further use and each experiment was performed in triplicates. Next, 2 ml of ultrapure water were added to the dry biomass (10-40 mg) in polycarbonate tubes (12 ml, Thermo Scientific, USA) and stirred at room temperature (rt) for 1 hr. After centrifugation (10 min, 10000×g, rt), the supernatants were removed and the residual water into the biomass was determined by weighing. Sorption experiments were performed through incubation with 2 ml of the Nd3+ stock solution (10 mM) under stirring at rt for 3 hr. After the incubation the biomass samples again were centrifuged (10 min, 10000×g), biomass removed and the neodymium content in the supernatant measured using spectroscopic method described above. The supernatants were prediluted 1:49 (100 μl of the supernatant plus 4,900 μl ultrapure water) in order to fit the working range of the assay and 200 μl of the prediluted solution were added to a solution containing 100 μl XO (2 mM) and 700 μl ultrapure water to reach a final XO concentration of 200 μM. The maximum sorption capacity (qM) was determined based on the difference in the amounts of substance of Nd3+ in solution before (ni) and after (nf) incubation with the biomass:
With ci=initial Nd3+-concentration, cf=final Nd3+-concentration, V=volume and m=biomass dry weight. In order to check if the maximum sorption capacity was reached, each species was investigated twice with the same concentration of Nd3+ but two different amounts of biomass. Additionally, pH-values of the biomass supernatant were determined after washing with ultrapure water and after sorption.
Temporal and pH-behavior
Kinetic investigations utilizing lyophilized biomass without further treatment were conducted by measuring the Nd3+ content in the supernatant after centrifugation over 4 hr in duplicates. For generation of one data point, the biomass was centrifuged (5 min, 10,000×g) and the concentration of Nd3+ in the supernatants were measured as describe above. Predilution was carried out through addition of 10 μl of the supernatants to 490 μl of ultrapure water. Additionally, pH-value of the supernatant was determined for every data point.
Titrations of biomass
In order to determine the pKa values and maximum proton exchange capacity of the biomass species titration experiments with the biomass (50 mg) of moss (P. patens) and green alga (C. reinhardtii) were conducted in duplicates. The native lyophilized biomass was fully protonated before titration through stirring in 10 ml of 0.1 M HClsolution at rt for 2 hr. After centrifugation (10 min, 10,000×g) the biomass was washed with ultrapure water until the conductivity was lower than 20 μS*cm-1 to guarantee a defined starting point. Finally, the biomass was suspended in 10 ml of 1 mM NaCl-solution and titrated with 0.1 M NaOH or HCl by sequential addition of 500 μl of respective acid/base solution.
The pKa values were determined using ProtoFit 2.1. Fit parameters were adjusted as 4 discrete binding sites, all amphoteric in a nonelectrostatic adsorption behavior, calculated with the extended Debye- Huckel equation.
Swelling behavior
For assessing the swelling behavior of the lyophilized algal biomass, 50 mg of C. reinhardtii and P. patens in duplicates were incubated in ultrapure water (10 ml) for 1 hr, centrifuged (10,000×g, 10 min), the supernatant was removed and the residual wet biomass was weighed.
Results and Discussion
Quantification of Nd3+
For an easy quantification of unknown Nd concentrations, a spectrophotometric assay employing XO was established. Optimization of the XO-assay is shown here, where absorption spectra (300 nm to 700 nm) of different XO-concentrations (100 μM, 200 μM, 1000 μM; Figures 1-3 SI) in presence of Nd3+ (from 10 μm to 100 μM) were recorded. The free dye shows an absorption maximum at 430 nm, whereas the maximum of the Nd3+-XO-complex lays at 580 nm, hence they are separated adequately for quantifying Nd3+ in solution. When measuring with 100 μM XO, the upper limit of the Nd3+ concentration is limited to approximately 70 μM due to the fact, that the Nd3+-XOcomplex exists at a 1 : 1 ratio of both molecules [18] and XO should be in excess. Therefore, XO was employed in concentrations of 200 μM and 1000 μM for Nd3+-concentrations between 10 μM and 100 μM. The use of 1000 μM XO (Figure 2) shows decreased absorbances for the XO-Nd3+-complex at 580 nm when compared to XO of 100 μM due to broadening of the absorption peak of XO. For this reason a XOconcentration of 200 μM (Figure 3) was selected for the assay.
The calibration curve (Figure 1) prepared with Nd3+ concentrations ranging from 10 μM up to 100 μM in presence of 200 μM XO results in two different linear slopes indicating possible formation of two different complexes. This interpretation is supported by findings of Budesinsky et al. [18] who reported various extinction coefficient for different metal-to-XO-ratio resulting in higher signals for a 1 : 1 ratio of metal ions to XO that could explain a higher slope for a concentration between 60 μM and 100 μM Nd3+ and 200 μM XO. For quantifications of Nd3+-concentrations in sorption experiments the calibration between 10 μM and 50 μM Nd3+ was used and samples to be analyzed were diluted adequately to fit the working range.
Biosorption screening
The maximum Nd3+-uptake of 19 species including: 9 green, 2 red and one heterotrophic microalgae, 5 cyanobacteria, one moss and one yeast, were investigated in this study. The results are shown in Figure 2. The species showed very different maximum sorption capacities of Nd3+ ranging from 0.08 mmol*g-1 dry biomass (N. ellipsosporum) to 0.74 mmol*g-1 dry biomass (P. patens). Within the microalgae the highest metal-uptake was registered for C. brevissima (0.48 mmol*g-1 dry biomass), T. chuii (0.36 mmol*g-1 dry biomass), E. mutabilis (0.35 mmol*g-1 dry biomass) and L. taylorii (0.32 mmol*g-1 dry biomass). These results correspond well with previously published biosorption experiments with the brown macroalga Sargassum sp., where the found sorption capacities ranged between 0.60 mmol*g-1 and 0.70 mmol*g-1 dry biomass for La, Nd, Eu and Gd [10].
Examination of sorption mechanisms
In order to elucidate the mechanism of the REE-uptake, kinetic, pH- and titration measurements with the according biomass were performed. Results of the temporal sorption and pH-behavior can be seen in Figure 3. Starting from the pH-value of the Nd3+ stock solution (pH 5.2) a pH drop to 4.3 for C. reinhardtii and to 3.9 for P. patens were observed after 4 h. When repeated without Nd3+ in the solution (incubation of 30 mg dry biomass in 2 ml ultrapure water) no pH change for C. reinhardtii could be registered and an increase from pH 6.0 to 7.0 was observed for P. patens. Hence, it could be followed that pH drops for C. reinhardtii and P. patens were induced exclusively by the metal-uptake resulting in a proton release into the medium suggesting the proton exchange mechanism.
During the same experiments the maximum Nd3+-uptake was reached already after 30 min for C. reinhardtii whereas for P. patens it could be reached only after around 90 min. Simultaneously, the pH drop correspond well with the time of reaching the maximum sorption capacity of Nd3+ also supporting the thesis that ion exchange was a predominant mechanism of the metal-uptake. In comparison, reaching a steady state of the REE sorption was previously reported to range from 0.5 hr up to 24 hr for various species [8,19,20]. REE sorption at moss biomass has not been reported previously in peer reviewed literature, however Sari et al. [21] investigated biosorption behavior of Cd (II) and Cr (III) at other moss species - Hylocomium splendens - where they found that the maximum of biosorption was reached after 90 min. This fits well with results reported here.
Titration studies were performed with C. reinhardtii and P. patens (Figure 4). Proton exchange capacities (PEC), maximum Nd3+-uptake capacities (MUC) and pKa values for C. reinhardtii and P. patens, calculated from titration curves, are shown in Table 1. Taking the last titration data point PEC could be calculated as 0.39 mmol*g-1 for C. reinhardtii and 0.44 mmol*g-1 for P. patens. These PKa values can be compared with those of functional groups of glycoproteins. For C. reinhardtii pKa=4.4 can be aligned to carboxyl groups in asparagine or glutamine and pKa=8.9 corresponds to α-amino groups [22]
The cell wall of C. reinhardtii contains several layers of hydroxyproline-rich glycoproteins [23] bearing many free hydroxyl groups that are potential candidates for metal ion binding. Nieboer and Richardson [24] classified metal ions due to their covalent or ionic binding character in metal-ion/ligand complexes. Following their definition, REEs are added to Class A metals, which prefer O-bearing donor molecules. Hence, it can be concluded that a main binding mechanism is proton-to-Nd3+-exchange at hydroxyl groups. Taking into account that Nd3+ possesses 3 positive charges, it can be proposed, that one Nd3+ occupies more than one binding site. The proton-to-Nd3+ ratio for C. reinhardtii is 1.5, meaning 2 Nd3+ ions releases 6 protons in the hypothesis of an exclusive proton exchange mechanism. For P. patens proton-to-Nd3+ ratio is 0.6 indicating a combination of more sorption mechanisms such as electrostatic interactions, precipitation and complexation. Martins et al. [25] postulated a competing process between protons and metal ions while sorption of Cd (II) and Zn (II) among the aquatic moss Fontinalis antipyretica.
Outlook
Experiments shown here will be adapted to Europium, Cerium and Dysprosium in the near future. Furthermore, we will investigate selective binding of REEs with respect to heavy metals. With the best applicable microalgal species, we want to build up a minor cartridge for REE production from high diluted aqueous solutions.
Acknowledgements
We would like to acknowledge the Institute of Chemical Reaction Engineering at the Friedrich-Alexander-Universitat Erlangen-Nürnberg for ICP measurements and the Bavarian State Ministry of the Environment and Consumer Protection for financial support of the project.
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