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Competitive effects on mercury removal by an agricultural waste: application to synthetic and natural spiked waters a

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Luciana S. Rocha , Cláudia B. Lopes , Bruno Henriques , Daniela S. Tavares , J.A. Borges , a

Armando C. Duarte & Eduarda Pereira a

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Department of Chemistry/CESAM, University of Aveiro, Aveiro 3810-193, Portugal

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Álvaro Alves Borges, Lda, Alto Brenha-Brenha, Figueira da Foz 3080-437, Portugal Published online: 01 Oct 2013.

To cite this article: Luciana S. Rocha, Cláudia B. Lopes, Bruno Henriques, Daniela S. Tavares, J.A. Borges, Armando C. Duarte & Eduarda Pereira , Environmental Technology (2013): Competitive effects on mercury removal by an agricultural waste: application to synthetic and natural spiked waters, Environmental Technology, DOI: 10.1080/09593330.2013.841267 To link to this article: http://dx.doi.org/10.1080/09593330.2013.841267

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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.841267

Competitive effects on mercury removal by an agricultural waste: application to synthetic and natural spiked waters Luciana S. Rochaa∗ , Cláudia B. Lopesa , Bruno Henriquesa , Daniela S. Tavaresa , J.A. Borgesb , Armando C. Duartea and Eduarda Pereiraa a Department

of Chemistry/CESAM, University of Aveiro, Aveiro 3810-193, Portugal; b Álvaro Alves Borges, Lda, Alto Brenha-Brenha, Figueira da Foz 3080-437, Portugal

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(Received 4 March 2013; accepted 29 August 2013 ) In this work, the efficiency of a local and highly available agricultural waste, the raw rice husk, was used to remove mercury (Hg) from synthetic and natural waters, spiked with concentrations that reflect the contamination problems found in the environment. Different operating conditions were tested, including initial pH, ionic strength, the presence of co-ions (cadmium) and organic matter. The sorption efficiency of rice husk was slightly affected by the presence H+ ions (pH range between 3 and 9), but in the presence of NaNO3 and NaCl electrolytes and in binary solutions containing Cd2+ and Hg2+ , the sorption efficiency was dependent on the nature and levels of the interfering ion and on the initial concentration of Hg2+ used. Nevertheless, in a situation of equilibrium the effect of those ions was negligible and the removal efficiency ranged between 82% and 94% and between 90% and 96% for an initial Hg2+ concentration of 0.05 mg L−1 and 0.50 mg L−1 , respectively. In more complex matrices, i.e. in the presence of humic substances and in natural river waters, the speciation and dynamics of Hg was changed and a fraction of the metal becomes unavailable in solution. Even then, the values obtained for Hg removal were satisfactory, i.e. between 59% and 76% and 81% and 85% for an initial concentration of Hg2+ of 0.05 and 0.50 mg L−1 , respectively. Keywords: mercury; rice husk; competitive ions; humic substances; remediation; natural waters

1. Introduction Metals are among the toxic substances that reach hazardous levels. The excessive release of these elements into the environment, due to industrialization and urbanization, has posed a great problem worldwide.[1,2] Mercury (Hg) is considered by the Environmental Protection Agency (EPA) as a highly dangerous element because of its accumulative and persistent character in the environment and biota.[3] According to the Agency for Substances and Toxic Diseases Registry (ATSDR), in 2011 mercury occupied the third position on the Priority List of Hazardous Substances.[4] As society realizes the environmental impact of persistent and toxic contaminants and, with strict discharge limits on metals being imposed, the removal of mercury and other metals from industrial waste streams is becoming increasingly important.[2,5] Therefore, the research interest towards the development of highly efficient, safe and economically attractive treatment methods for metal removal has increased.[1,6] The application of biowastes for the removal of toxic contaminants or for the recovery of valuable resources from aqueous wastewaters is one of the recent developments in environmental technology. The major advantages

∗ Corresponding

author. Email: [email protected]

© 2013 Taylor & Francis

of biosorption technology over the conventional methods include not only its low cost but also its high efficiency, the minimization of chemical or biological sludges, the ability of biosorbents’ regeneration, the possibility of metal recovery and the broad range of operational conditions (e.g. pH and temperature).[1,2,5,7–9] The potential of rice husk sorbents to remove Hg from waters has already been investigated by some authors. However, the studies developed in this field used initial concentrations of mercury that largely exceed the values found in contaminated environments (between 8 and 2000 mg L−1 ) [10–15], and often several pre-treatments are performed to the rice husk material (calcination and/or chemical modification), wiping out the concept of revaluation of an agricultural waste.[11–15] Moreover, the large majority of the studies using derived materials are addressed to single-component analysis, and few, are intended for the removal of mercury in real-water samples [10] and to the simultaneous sorption of mercury with one or more metal ions.[11,12] Besides that, little information is given about the efficiency of the sorption process with rice husk materials, regarding the levels of Hg2+ remaining in solution and the legal limits established for Hg in waters and

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wastewaters. Recently, the efficiency of the raw rice husk to remove Hg2+ from single aqueous solutions and for realistic concentrations of this metal was evaluated.[16] The study comprised the assessment of the effect of some physico-chemical parameters such as contact time, initial Hg2+ concentration and mass of rice husk, together with the kinetic and equilibrium behaviour of the Hg2+ –rice husk system. The rice husk proved to be efficient in the removal of low levels of Hg2+ (μg L−1 range), however an enormous gap regarding its behaviour under competitive and real conditions, needs to the filled. In this work, the capacity of the unmodified rice husk to remove Hg2+ using concentrations that reflect the contamination problems found in the environment has been investigated under different competitive conditions, namely initial pH, ionic strength, the presence of co-ions and organic matter. The aim of this study was to evaluate: (i) the competitive effects posed by the presence of other cations in solution; (ii) the feasibility of the simultaneous sorption of two metal ions (Hg and Cd) and (iii) the effect of inorganic and organic complexing agents in the speciation, and consequently on the sorption processes of Hg. Furthermore, in order to understand the interactions between different sorbates and the impact of complexing ligands on the dynamics of Hg, the efficiency of unmodified rice husk to remove Hg2+ from natural spiked waters was also evaluated. Finally, the residual levels of Hg2+ obtained after the treatment with rice husk and under the experimental conditions tested in this work were compared with the guideline values for effluents discharges and drinking water.

2. Materials and methods 2.1. Material and chemicals All chemicals were of analytical reagent grade and solutions were prepared with ultrapure water (18.2 M cm, Milli–Q system). The nitric acid 65% (suprapur) and the standard stock solutions of mercury (1001 ± 2 mg L−1 ) and cadmium (1000 ± 2 mg L−1 ) nitrate were purchased from Merck. Sodium humic salt was purchased from SigmaAldrich. Biohit Proline pipettes equipped with disposable tips were used for appropriate dilutions. All glassware used in the experiments was acid-washed prior to use.

2.2.

Biosorbent material preparation and characterization

The rice husk was kindly supplied by the industries of Álvaro Alves Borges Lda. from Figueira da Foz, Portugal, whose activity is related to the peeling, bleaching and other treatments of rice. The rice husk was washed with distilled water and dried in an oven at 60◦ C, until constant weight (ca. 24 h). Afterwards, the material was triturated using a coffee mill (model Taurus aromatic), sieved to obtain a fraction with particles of size ≤ 500 μm and preserved at room

temperature. The procedure used for the preparation of this sorbent was simple and inexpensive. The specific surface area of the rice husk particles was determined by nitrogen adsorption Brunauer–Emmett– Teller measurements were performed with a Gemini V2.0 Micromeritics instrument. The pore size was calculated from the desorption branch using the Barret–Joyner– Hallenda (BJH) and the pore volume was evaluated from the adsorbed amount. Other information, regarding the morphological and chemical characterization of the biosorbent used in this work, can be found in a previous study [16]. 2.3. Sorption kinetic experiments Batch sorption kinetic experiments were performed at room temperature (21 ± 1◦ C), by contact of a certain amount of the unmodified rice husk samples (particle size < 500 μm) with a known concentration of Hg2+ solution. The experimental trials were carried out in volumetric flasks (2 L) under constant stirring conditions (1400 rpm). Two initial Hg2+ concentrations were selected aiming to represent the maximum value for Hg discharges from industrial sectors (0.05 mg L−1 ) and 10 times high, to simulate an eventual situation of an accidental spill (0.5 mg L−1 ). The dose of rice husk (DoseRH ) used in all experiments, 0.50 g L−1 , was selected according to the data from a previous study.[16] These authors verified that using a DoseRH of 0.50 g L−1 the efficiency of the removal process is assured (> 90%) in consecutive treatments, minimizing the amount of sorbent, and therefore issues concerning secondary waste disposal problems are reduced. All experiments were performed in duplicate and the results were always expressed as the mean value obtained. Experiments without mercury (blank) and without rice husk (control) were run in order to assess, respectively, a possible Hg contribution from the rice husk material and the losses of Hg2+ due adsorption to the glass vessels and in the sample filtration procedure (for more information please consult supplement material). Mercury(II) solutions were prepared by diluting the standard stock solution to the desired concentration in ultrapure water. Experiments started when a known mass of biosorbent was added to Hg2+ solutions and stirring was initiated. Aliquots (10 mL) were collected at increasing times, filtered through a pre-acid-washed 0.45 μm Millipore membrane, and then the filtrate was adjusted to pH < 2 with nitric acid. Each experiment was maintained until the Hg2+ concentration in the solution remained constant. Mercury analysis was performed by cold vapour atomic fluorescence spectroscopy (CV-AFS), on a PSA cold vapour generator, model 10.003, associated with a Merlin PSA detector, model 10.023, using SnCl2 as reducing agent. The Hg2+ concentration was quantified through a calibration curve of five standards prepared in a nitric acid solution (2% v/v; Merck), by dilution from the certified standard solution of mercury(II) nitrate, whose concentrations ranged from 0.0 to 0.5 μg L−1 . In this range, the limit of detection of

Environmental Technology the method is 0.02 μg L−1 , and the precision and accuracy are 9) the precipitation of Hg(OH)2 could occur. However, Zhang et al. [18] proved that for Hg concentrations below 120 mg L−1 , a value considerably higher than the initial concentration used in this study (0.50 mg L−1 ), no significant changes were found

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on the dissolved fraction of Hg in the pH range between 1 and 12, indicating that the Hg(OH)2 remains dissolved in the solution. For the current experimental conditions, that observation is in agreement with the speciation data obtained by Visual MINTEQ, which indicate that the dominant Hg species in aqueous solution at pH 9 is the dissolved form of Hg(OH)2 (100%). Additionally, it was found that during the experimental trial at pH 9 there was a decrease in the pH values with time, from 9.0 to 5.9, particularly after a period of contact between sorbate/sorbent for more than 48 h, indicating the release of protons to solution. This phenomenon suggests the occurrence of an ion-exchange mechanism. Based on these details and considering that the dominant species of mercury available in solution at pH 6 and 9 is Hg(OH)2 (99.94% and 100%, respectively), it is possible to understand why the qe values at pH 9 were close to that at pH 6. Additionally, for the experiments performed at pH 6, the pH remained constant during the entire range of time. At low pH values (pH ca. 3), the excess of H+ ions in solution will compete with the available Hg species (50.8% of Hg2+ , 30.7% of Hg(OH)2 and 18.5% of HgOH+ ) for the active sites in the rice husk surface. Besides, according to some studies with the rice husk material, at pH below 4 the surface of this sorbent material will be surrounded by the hydronium ion (H+ ), producing a positive surface charge and as a result, the sorption of cationic species will be unfavourable [11,19] Additionally, at the end of the experiment the final pH was ca. 3.2, so all of this might explain the slightly lower qe values at the equilibrium for pH 3.

3.2. Effect of ionic strength The ionic strength of the background electrolyte is another variable that may influence the sorption process, due to the competition between several ions for a limited number of binding sites. Also, in cases where electrostatic attractions have an important role in the mechanism of metal removal, metal uptake can be sensitive to changes in the

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Figure 2. Variation of qt (mg g−1 ) with time (t, h) in the presence of NaNO3 and for an ionic strength (I ) of: 0 M (), 0.1 M (♦) and 0.5 M (). Other experimental conditions: CHg,0 of 0.50 (a) and 0.05 mg L−1 (b) and DoseRH of 0.50 g L−1 . For clarity, the error bars were omitted.

concentration of the electrolyte.[17,20] In this study, the effect of ionic strength on the removal of Hg2+ by unmodified rice husk was evaluated using two electrolyte solutions, NaNO3 and NaCl. The pH was monitored while testing the ionic strength effect and the variations observed during the experimental trial ranged between 3% and 18% of the initial pH. 3.2.1. Effect of NaNO3 electrolyte medium The kinetic profile of the amount of Hg2+ sorbed per gram of unmodified rice husk, under different ionic strengths of NaNO3 (INaNO3 ), are shown in Figure 2. The results reveal that the uptake of Hg2+ is affected by the presence of NaNO3 , but different responses can be obtained depending on the ionic strength of electrolyte, the period of contact between sorbate/sorbent and also on the initial Hg2+ concentration (CHg,0 ). For an initial Hg2+ concentration of 0.50 mg L−1 , the kinetic curves obtained in NaNO3 medium and for both ionic strength tested (0.1 and 0.5 M) were identical to the curve obtained in ultrapure water. However, the presence of NaNO3 in solution affects the removal rate of Hg2+ by the unmodified rice husk, in particular in the beginning of the sorption process and for the highest ionic strength. This fact may be ascribed to several factors, namely the competition between Na+ and Hg2+ for the available binding sites in the rice husk surface, changes in the activity of the aqueous Hg2+ ions and the existence of an electrical double layer between the biomass surface and the solute in aqueous solution that restricts the access of Hg2+ to the sorbent surface.[17,20] As mentioned before, the effect of the ionic strength is more pronounced for shorter periods of contact between sorbate/sorbent than at equilibrium. For example, after 10 min, the removal of Hg2+ by unmodified rice husk decreases to 56% for an INaNO3 of 0.1 M and 71% for an INaNO3 of 0.5 M, when compared with the removal in ultrapure water. The influence of the NaNO3 ionic strength decreases with time, and after some point, the presence of competitive ions is negligible for both ionic strengths (t ≥ 3 h and t ≥ 6 h for INaNO3 of 0.1 and 0.5 M,

respectively), achieving Hg2+ removal efficiencies identical to the one obtained in ultrapure water. At equilibrium, the amount of Hg2+ sorbed (qe ) for 0, 0.1 and 0.5 M NaNO3 was, respectively, 0.92 ± 0.001, 0.88 ± 0.006 and 0.87 ± 0.004 mg g−1 (n = 4), and the percentage of removal was higher than 90% for all ionic strengths. The effect of ionic strength of NaNO3 was also evaluated for an initial Hg2+ concentration of 0.05 mg L−1 . The results demonstrate that for a low concentration of Hg2+ the presence of this electrolyte affects the removal efficiency of Hg2+ by the unmodified rice husk, especially when the highest ionic strength is used. The effect of NaNO3 is more notorious for shorter times and tends to minimize, as the contact time increases (e.g. in NaNO3 of 0.5 M after 10 min, occurs a reduction on the removal efficiency, ca. 46%, in comparison with the system in ultrapure water, and after 48 h the reduction is 21%). For longer contact times, the presence of NaNO3 electrolyte ceases to inhibit the removal of Hg2+ , and under some circumstances NaNO3 starts to favours it. For example, in 0.5 M NaNO3 and for a period of contact t ≥ 72 h, the Hg2+ removal increased ca. 10%, when compared with the value obtained in the absence of the electrolyte. The equilibrium was attained for t ≥ 96 h showing an efficiency of removal of ca. 91 and 95% for NaNO3 0.1 and 0.5 M, respectively, and an amount of Hg2+ sorbed at equilibrium of 0.086 ± 0.001 mg g−1 and 0.092 ± 0.0002 mg g−1 (n = 4), in the same order. In NaNO3 medium, the speciation of Hg (data obtained from Visual MINTEQ) does not play an important role on the sorption process, since the dominant species in solution is the same as that in ultrapure water, i.e. Hg(OH)2 (99.9%), with a very small contribution from HgOH+ (0.1%). Nevertheless, the experimental results indicate that the initial concentration of Hg2+ plays an important role on the sorption process to the rice husk surface. For higher concentrations of Hg2+ , the resistance to mass transfer between the aqueous and solid phases can be easily overcome[21] and under these conditions the kinetic profile is less susceptible to the presence of competitive ions and the kinetic curves that were identical to the ones obtained in the absence of NaNO3 , i.e. in ultrapure water.

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Figure 3. Variation of qt (mg g−1 ) with time (t, h) in the presence of NaCl and for an ionic strength (I ) of: 0 M (•) and 0.5 M (•). Other experimental conditions: CHg,0 of 0.50 (a) and 0.05 mg L−1 (b) and DoseRH of 0.50 g L−1 . For clarity, the error bars were omitted.

3.2.2. Effect of NaCl electrolyte medium In the presence of some inorganic ligands such as chloride, the formation of chloro complexes with some metals might affect the sorption process.[20] The formation of these complexes can enhance or decrease the sorption capacity, depending on their affinity to the sorbent surface. Additionally, some anions can also interact with the biomass, changing the state of the active sites in a way that the binding capacity of the sorbent can be either enhanced or reduced.[2] The effect of NaCl electrolyte in the removal of Hg2+ was studied and the kinetic curves were compared with those obtained in ultrapure water (Figure 3). The results indicate that the presence of NaCl in solution has a drastic effect on the removal efficiency of Hg2+ , in particular at the beginning of the sorption process, exerting a much higher influence than the NaNO3 salt. This result indicates that the inhibition of the sorption of Hg2+ ions onto unmodified rice husk, in the presence of NaCl electrolyte, is affected not only by the presence of Na+ competitive ions, but also mostly by the presence of Cl− ions, that form chloro complexes with mercury. The speciation of Hg in solution was modelled, and the data provided by Visual MINTEQ indicate that at pH 6 and for a chloride concentration of 0.5 M, the domi− nant Hg species in solution are HgCl2− 4 (73.7%) and HgCl3 (21.9%), and that only a small fraction of Hg is in the form of HgCl2 (4.4%). These results are in agreement with the dominance diagram of chloro complexes of mercury reported by Allard and Arsenie [22] and Boszke et al.[23] which shows that under these experimental conditions (pH ca. 6 and for a chloride concentration of 0.5 M), the dominant species of − mercury in solution are HgCl2− 4 and HgCl3 .[22,23] This fact suggests that the chlorine complexes, such as HgCl2− 4 and HgCl− 3 , due to their size and charge, have lower mobility and affinity to the sorbent surface than the Hg(OH)2 ions. Recently, El-Shafey [15] used a chemically modified form of rice husk and also found that the formation of stable complexes in chlorine media, such as HgCl2 , HgCl− 3 and HgCl2− 4 prevent Hg(II) from binding onto the sorbent at low pH, resulting in less Hg removal. For the highest initial concentration of Hg2+ (0.50 mg L−1 ), and for the time interval between 5 min and

96 h, the amount of Hg sorbed per weight of rice husk in the presence of NaCl electrolyte, decreases up to 85% in comparison with the results obtained in ultrapure water (Figure 3(a)) and the removal efficiency was less than 10%. Unexpectedly, after a period of contact t ≥ 144 h, the uptake of Hg2+ increased drastically, reaching a situation of equilibrium. Additionally, the control experiments indicate that no Hg2+ losses (less than 10%) occurred under these experimental conditions during the entire trial (see supporting information). The efficiency of the removal process was ca. 96%, and the amount of Hg sorbed at equilibrium was 0.99 ± 0.002 mg g−1 (n = 4), a value slightly higher than the qe of 0.92 ± 0.001 mg g−1 (n = 4) obtained in ultrapure water. For an initial Hg2+ concentration of 0.05 mg L−1 , a similar pattern was observed, i.e. a strong inhibition of the sorption process during the first 48 h, followed by a huge increase in the amount of Hg(II) sorbed (Figure 3(b)), achieving an identical removal efficiency to the one obtained in the absence of 0.5 M NaCl electrolyte. The equilibrium was achieved after 96 h, with a removal efficiency of ca. 83% and an amount of Hg(II) sorbed per weight of sorbent of 0.088 ± 0.003 mg g−1 (n = 4). Like in NaNO3 medium, in electrolyte solutions containing NaCl, the initial Hg2+ concentration plays an important role on the sorption process onto rice husk surface. However, the removal of Hg2+ was clearly more affected for the highest C0,Hg value in NaCl medium. Since the chloride ions in solution are always in excess, this behaviour might be due to the formation of a higher number of chloro complexes, when the concentration of Hg2+ is 10-folds. Since the chloro complexes have a lower affinity to the rice husk surface than the Hg(OH)2 (dominant Hg species in ultrapure water at pH 6), a higher number of these complexes will cause a decrease in the removal efficiency.

3.3. Effect of metal ions Natural waters and wastewaters generally contain several dissolved metals and metal complexes. If the chemistry of the metals is similar, the competition between them for the available binding sites can occur, reducing the sorption

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Figure 4. Variation of qt (mg g−1 ) with time (t, h) for the sorption process of Hg2+ onto rice husk in single and binary metal ion systems. Experimental conditions used: (a) 0.05 mg L−1 of Hg (•) and 0.05 mg L−1 Hg + 0.20 mg L−1 Cd (◦); (b) 0.50 mg L−1 of Hg (), 0.50 mg L−1 Hg + 0.50 mg L−1 Cd () and 0.50 mg L−1 + 2.0 mg L−1 Cd (♦). Other experimental conditions: pH∼6.0 and DoseRH of 0.50 g L−1 . For clarity, the error bars were omitted.

capacity of the metal to be preferentially sorbed. The competitive uptake of Hg2+ from a binary mixture containing Hg2+ and Cd2+ ions was evaluated for different metal concentrations. The results were compared with the ones obtained for single system containing only Hg2+ . Under the experimental conditions, the dominant Hg and Cd species in solution are Hg(OH)2 and Cd2+ , respectively, both of them representing 99.9% of each metal species in solution (data obtained from Visual MINTEQ). For the lowest concentrations of both metals (Figure 4(a)), the sorption of Hg2+ by the unmodified husk rice is reduced by the presence of Cd2+ in solution during the first 48 h. However, when the equilibrium starts to be attained (t ≥ 72 h), the effect of the presence of Cd2+ on the removal of Hg2+ is negligible. The equilibrium was attained after a period of contact between sorbate/sorbent of 96 h, with a percentage of removal of ca. 87%. The amount of Hg2+ sorbed per weight of rice husk (qe ) was 0.081 ± 0.001 mg g−1 (n = 4), while in the absence of Cd2+ ions, the value obtained was 0.082 ± 0.002 mg g−1 . Clearly, under these experimental conditions the presence of Cd2+ ions has a strong impact on the kinetics and on the initial removal rate, but not on the equilibrium behaviour. For equal concentrations of Hg2+ and Cd2+ (i.e. 0.50 mg L−1 of Cd and Hg), the percentage of Hg2+ removal and the amount of Hg2+ sorbed were hardly unaffected by the presence of Cd2+ (Figure 4(b)). At the equilibrium, qe values were basically the same for single (0.92 ± 0.001 mg g−1 , n = 4) and binary (0.91 ± 0.001 mg g−1 , n = 4) systems. When the concentration of Cd2+ is tenfolds the limit of Cd discharges and fourfolds the concentration of Hg2+ (i.e. 2 mg L−1 of Cd and 0.5 mg L−1 of Hg), there is a small decrease in the Hg2+ initial sorption rate when both metals ions coexist in solution. At equilibrium, no noteworthy effect was observed on the sorption process, indeed the amount of Hg2+ sorbed per gram of sorbent (0.95 ± 0.004 mg g−1 , n = 4) and the percentage of removal (ca. 95%) were just slightly higher than the ones obtained in ultrapure water.

Independent of the composition of the binary solution, in a situation of equilibrium the efficiency of Hg2+ removal in the presence and absence of Cd is similar and as a result, the overall process was not affected (no interaction) by the presence of the co-ion. These results are divergent with the ones obtained by El-Said et al.[12] According to those authors, the sorption of Hg2+ onto rice husk ash, from a binary mixture containing Hg2+ and Cd2+ , is inhibited by the presence of the co-ion in solution, presenting antagonist behaviour. It would be expected that the effect of the presence of the co-ion in the removal of Hg2+ by the unmodified rice husk should be less noticeable for the lowest concentration of both metals than for the highest ones, since a lower number of Hg2+ and Cd2+ ions would compete for the available binding sites on the rice husk surface. However, the use of a higher initial concentration of metals offers the driving force to overcome the resistance to mass transfer between the aqueous and solid phases [18], and consequently the effect of the mixture on the sorption of Hg2+ becomes less pronounced. In fact, when both Cd2+ and Hg2+ concentrations are equal to 0.50 mg L−1 , the removal of Hg2+ is barely affected by the presence of the competitive co-ion and when the metal concentrations are 10 times higher than the legal limits, the removal of Hg2+ is favoured. 3.4. Effect of organic matter Dissolved HS are present in aquatic systems (e.g. pore waters, surface waters and shallow ground waters) and since they strongly bind to Hg2+ , they can have an important role in the speciation and mobility of this metal in aqueous environments. Aquatic HS can act as a strong complexing agent, due to the presence in its composition of carboxyl, and in some cases sulphidic groups, which enhances the stability of Hg complexes.[22] In this work, the effect of HS on the Hg2+ removal by unmodified rice husk (Figure 6) was evaluated for two concentrations of humic acids (HS = 5 and 10 mg L−1 ) that mimic the ones found in natural aquatic systems

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t (h) Figure 5. Variation of Ct /C0 with time (t, h) for the sorption process of Hg2+ onto rice husk for an initial Hg2+ concentrations of 0.05 mg L−1 (a and a’) and 0.50 mg L−1 (b and b’). The concentrations of HS used were: 0 mg L−1 (•, ), 5 mg L−1 (•, ) and 10 mg L−1 (◦, ♦). The lines correspond to the values of Ct /C0 along the time for the controls (experiments without rice husk and for a concentration of 5 mg L−1 (– –) and 10 mg L−1 (• • •) of humic salt. Other experimental conditions: pH∼6.0 and DoseRH of 0.50 g L−1 . For clarity, the error bars were omitted.

(1–10 mg L−1 ).[22] The kinetic profile of the removal of Hg2+ by the unmodified rice husk is clearly affected by the presence of dissolved HS (for both HS concentrations). The results show that, independent of HS and Hg2+ ions concentrations, the presence of HS has a strong effect on the initial removal rate, as can be seen from the different slopes of the removal curves for the first hours (Figures 5(a’) and 5(b’)). However, the effect of HS on the Hg2+ removal by the unmodified rice husk tends to decrease with time and at equilibrium no relevant differences are observed between the systems with and without HS. Experiments without rice husk were run as a control, to check the available Hg2+ in solution (the fraction of Hg2+ that does not complexes with HS), and the results show a decrease in the concentration of Hg2+ with time, in particular during the first 48 h of contact, for both concentrations of HS and both initial Hg2+ concentrations. Afterwards, the concentration of Hg2+ in the controls continues to decrease, and reaches its minimum value at the end of the trial. The decrease observed, when compared with the initial values, was ca. 35–37% for C0,Hg of 0.05 mg L−1 and ca. 39–45% for C0,Hg of 0.5 mg L−1 . In order to obtain some information about the solution speciation of Hg in the presence of HS, the NICA-Donnan model in Visual MINTEQ was used. The data obtained for a C0,Hg of 0.05 mg L−1 indicate that 75.6–78.5% of Hg is organically complex to the carboxylic functional groups of HS, 21.3–23.5% of Hg complexed to the phenolic groups of HS, and only a diminutive fraction of Hg(OH)2 is available in

solution (< 0.2%). For C0,Hg of 0.50 mg L−1 , the speciation data are dependent on the concentration of HS. Therefore, for an HS concentration of 5 mg L−1 56.6% and 13.2% of Hg are organically complexed to the carboxylic and phenolic functional groups of HS, respectively, and a considerable fraction of Hg is available in solutions as Hg(OH)2 (30.2%) and HgOH+ (0.02%); for 10 mg L−1 of HS, 76.3% of Hg is organically complexed to the carboxylic and 17.9% to the phenolic functional groups of HS, and a small fraction of Hg (5.8%) is available in solution as Hg(OH)2 . The experimental results are not consistent with the speciation diagram obtained with visual MINTEQ, since the metal fraction, determined experimentally, which is not complexed to HS is higher than the one predicted by the model. As a consequence of Hg complexation with HS, the Ct /C0 values for the sorption experiments with rice husk do not reflect the effective removal of Hg2+ by this material, instead by a combination of both processes, i.e. the uptake by rice husk and the formation of Hg–humic complexes. Also, it is important to mention that a previous study demonstrated that the natural/unmodified rice husk had no ability for the sorption of humic acid,[24] indicating that there is no competition between this species and the Hg2+ for the available binding sites on the rice husk surface. Considering the fraction of Hg not complexed in solution, estimated from the values of Hg in solution of the control experiments, the amount of metal effectively sorbed by the unmodified rice husk (qt ) is highly reduced by the

presence of dissolved HS, for both initial Hg2+ concentrations (data not shown). This happens because the quantity of Hg available in solution to be sorbed by the rice husk is considerably lower than that in the absence of HS. For an initial Hg2+ concentration of 0.05 mg L−1 , and by comparison with the values obtained in ultrapure water, the qe values were suppressed by ca. 50% in the presence of HS, while increasing the initial Hg2+ concentration 10 times (0.50 mg L−1 ), the qe values decreased ca. 40% for 5 mg L−1 of HS and 50% for 10 mg L−1 of HS. The highest amount of Hg2+ effectively sorbed by unmodified rice husk was achieved after a period of contact of 144 h with values of: 0.082 ± 0.002 mg g−1 (0 mg L−1 HS), 0.036 ± 0.001 mg g−1 (5 mg L−1 HS) and 0.041 ± 0.001 mg g−1 (10 mg L−1 HS) for an initial Hg2+ concentration of 0.05 mg L−1 ; and 0.92 ± 0.001 mg g−1 (0 mg L−1 HS), 0.54 ± 0.01 mg g−1 (5 mg L−1 HS) and 0.46 ± 0.01 mg g−1 (10 mg L−1 HS) for 0.50 mg L−1 of Hg2+ . Moreover, considering only the Hg available in solution, i.e. not complexed with HS, the values of removal efficiency obtained were satisfactory and depending on the concentration of HS, ranged between 73% and 76% for CHg ,0 of 0.05 mg L−1 and between 81% and 83% for CHg,0 of 0.50 mg L−1 .

9

the Vouga River, located in central Portugal and that empties into the Atlantic Ocean through a delta called ’Ria de Aveiro’. This river is 148 km long, has a basin with an area of 3700 km2 and an annual run-off at the mouth, on average, of 1900 hm3 . The physical and chemical characterization of the water indicates that the pH value of river sample was 7.0, its conductivity was 58.7 μS cm−1 and the following elements were detected: 1.6 (K), 1.1 (Mg), 2.8 (Ca), 7.1 (Na) and 4.4 (Si) at mg L−1 concentrations and 194 (Al), 7.3 (Ba), 364 (Fe), 3.2 (Li), 49.0 (Mn), 33.5 (P), 17.4 (Sr) and 7.7 (Zn) at μg L−1 concentrations. The total carbon and organic carbon in the dissolved fraction found were 3.5 and 1.2 mg L−1 , respectively, and the amount of suspended particulate matter (SPM) was 7.9 mg L−1 . The amount of Hg2+ in the natural water was found to be very low (3.8 ng L−1 ), typical of non-contaminated waters. In order to obtain a concentration of Hg2+ of 0.05 and 0.50 mg L−1 , the river water was spiked with a standard solution of Hg(NO3 )2 . To evaluate the amount of Hg2+ that became unavailable in the spiked river water, due to the formation of stable complexes with dissolved organic matter and/or due to the association of this metal to the particulate fraction, an experiment without rice husk was run as a control. The kinetic curves obtained in the sorption process of Hg2+ by unmodified rice husk sorbent using spiked natural water from Vouga River were compared with those obtained in ultrapure water (Figure 6). The kinetic curves obtained in river water showed an identical behaviour to the one obtained in Section 3.5, where the effect of the dissolved organic matter was evaluated. According to the curves in Figure 6, the efficiency on the removal of Hg2+ for shorter

3.5.

Application of unmodified rice husk sorbent to remove mercury from spiked natural water The feasibility of unmodified rice husk for the treatment of natural waters contaminated with Hg2+ was evaluated in spiked river water. The natural water was collected from

(a) 1.00 a´

1.00 0.80

Ct /C0

Ct /C0

0.80 0.60

0.60 0.40 0.20

0.40

0.00

0.20

0

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4 t (h)

6

8

0

2

4 t (h)

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50

(b) 1.00

100 t (h)

150

200 b´

0.80

1.00

0.80 Ct /C0

Ct /C0

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Environmental Technology

0.60

0.60 0.40 0.20

0.40

0.00

0.20 0.00 0

50

100

150

200

t (h)

Figure 6. Variation of Ct /C0 with time (t, h) for the sorption process of Hg2+ onto rice husk using spiked river water (◦ and ♦) and spiked ultrapure water (• and ) and for initial Hg2+ concentrations of 0.05 mg L−1 (a and a’) and 0.50 mg L−1 (b and b’). The dash lines correspond to the Ct /C0 values for the controls (without rice husk). Other experimental conditions: pH∼6.0 and DoseRH of 0.50 g L−1 . For clarity, the error bars were omitted.

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periods of contact is reduced in river water, reflecting the higher concentrations of Hg2+ in solution (expressed in normalized values Ct /C0 ) than the ones obtained in spiked ultrapure water. As the time of contact increases (t > 48 h), the Ct /C0 values are closed to the Ct /C0 values achieved in ultrapure water. Moreover, the Ct /C0 values of the controls presented a marked decline until 48 h (ca. 30–33%) and after that the decrease was more moderate, with the highest losses attained at the end of the trial (ca. 50%). The speciation data obtained from the NICA-Donnan model of Visual MINTEQ corroborate these results, which indicate that 31.5% and 16.5% of Hg is organically complexed to the carboxylic and phenolic functional groups of fulvic substances, respectively, and 52.0% is available in solution as Hg(OH)2 . For an initial Hg2+ concentration of 0.05 mg L−1 , the decrease in the Ct /C0 values for the experiment in the presence of rice husk was quite similar to that of control, within 6 h of contact. These values indicate that the decrease achieved in Ct /C0 is probably not due to the removal of Hg by the unmodified rice husk but is due to the formation of complexes with the dissolved organic matter, and eventually to the association of this metal to the suspended particulate matter. The results from the NICADonnan model of Visual MINTEQ indicate that for this Hg2+ concentration, the dominant Hg species in solution are Hg(OH)2 (89.6%) and the remaining fraction of Hg is organically complexed to the carboxylic (7.3%) and phenolic groups (3.1%) of fulvic substances. After a period of contact of 24 h and taking into account the available Hg in solution (estimated from the concentration of Hg2+ the control experiments), 45% of this metal ion is removed by rice husk sorbent and at the equilibrium the removal efficiency was around 59%. The residual concentration of Hg in solution was 0.008 mg L−1 , resulting from the combination of: the sorption by unmodified rice husk and matrix’s complexity (mainly due to the formation of stable complexes between Hg and organic matter in solution and its association to the particulate fraction). Starting with an initial Hg2+ concentration of 0.50 mg L−1 , the Ct /C0 values for Hg2+ in spiked river water decreased with time, and the removal efficiency (assuming the Ct /C0 decline in the control) increased from 29% to 53% in the time interval [0.12-24] hours, while in ultrapure water this increase was from 39% to 76%. For t > 24 h, the normalized concentration of Hg2+ obtained in river and ultrapure water were identical, but the efficiency in the removal process was not the same, considering the Hg losses from the control experiments. At the equilibrium, and considering the available concentration of Hg in solution (estimated from the control experiments), the removal efficiency was ca. 85%, a value somewhat inferior to the 93% obtained in ultrapure water. The residual concentration of Hg obtained after the equilibrium was 0.034 mg L−1 . The effectiveness in the decontamination process is not completely achieved in this type of natural water, but still

the contribution of rice husk sorbent in the removal process is evident and significant. 3.6. Sorption mechanisms The biosorption of metals to biological material is a complex process and involves several mechanisms, such as adsorption on the biomass surface and pores, ion-exchange, surface precipitation and complexation and chelation, may be operative under given operation conditions. To understand the mechanisms involved on biosorption process, exact information regarding cell wall structure (functional groups, type and size of pores) and solution chemistry is required.[25] Depending on these factors, i.e. type of system, single or multiple mechanisms may be involved in metal removal from solution. In this work, the Hg/rice husk system was studied under several degrees of complexity, and consequently different types of mechanisms might have taken place, depending on the physico-chemical parameters of the solution. According to Rocha et al.,[16] the main functional groups of unmodified rice husk responsible for metal removal are the carboxyl, carbonyl and silanol, and besides the high content of carbon, oxygen and silicon, the EDS analysis also identifies the presence of calcium and potassium. Additionally, since the pore size of rice husk is ca. 48 Å, and the ionic radius of Hg2+ is 1.02 Å,[26] there is a strong possibility of the occurrence of an ion-exchange mechanism. This possibility was investigated, for some experimental conditions (at pH 6 and at 0.1 M sodium nitrate ionic strength), by following the concentration of several elements (Hg, Na, Ca, Mg, K and Si) in solution with time. The results show that: (i) Initially the solutions are free of Ca2+ , Mg2+ , K+ and Si4+ ions and contain Hg2+ in the concentration selected (μg/L) and Na+ from pH or ionic strength adjustments (mg/L); (ii) With time, occurs the gradual removal of both Hg2+ and Na+ from solution and the release of Ca2+ , Mg2+ , K+ and Si4+ from the rice husk to the solution (see supplement material); (iii) The total amount of Ca2+ , Mg2+ , K+ and Si4+ released to the solution (in meq) is considerably lower than the total amount of Hg2+ and Na+ removed from solution (also in meq) and (iv) there is a significant correlation (r > .92, P = .05) between the amount of Hg2+ removed from solution at each time and the correspondent amount of total Ca2+ , Mg2+ , K+ and Si4+ released to the solution, and there is no correlation in the case of Na+ . The amount of Ca2+ , Mg2+ , K+ and Si4+ released due to the sorption process was calculated by subtracting the amount of cations released from the blank experiment (in the case of Ca2+ , Mg2+ and K+ , those values were lower than the LOD of the method) from the amount of cations measured in the sorption experiments. The strong correlation (r > .92, P = .05) observed between the amount of Hg2+ removed from solution and the correspondent amount of total elements released to the solution, suggest that

Environmental Technology ion-exchange might be an important mechanism involving in Hg2+ removal by unmodified rice husk. Theoretically, for an ideal ion-exchange mechanism, 1 mol of Hg2+ will replace 2/n mol of Mn+ , according to the following mechanism:

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RH-M2/n (s) + Hg2+ (aq) ⇔ RH-Hg (s) +

2 (aq), nMn+

where RH is the rice husk sorbent, M represents Ca2+ , Mg2+ , K+ or Si4+ and n represents the charge. However, it was also observed that the amount of total Ca2+ , Mg2+ , K+ and Si4+ (in meq) was always much higher than the amount of Hg2+ removed (also in meq), suggesting that some Na+ may also be exchangeable. Additionally, in the case of Na+ removal, other mechanisms in addition to ion-exchange must be involved. This fact is supported by the lack of correlation with the total amount of cations exchangeable and by the high amount of Na+ that is removed from solution relatively to the total amount of cations released. Additionally, and from the results obtained in section 3.1, the experiments performed at pH 9 indicate a substantial decrease in solution pH with time (from 9.2 to 5.7), which suggests the partial hydrolysis of the sorbent, and as a result, the following mechanisms can also occur:

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the increase in the amount of some cations (mainly K+ and Mg2+ ) in solution at the end of the sorption experiments. So, under these experimental conditions complexation and ionexchange mechanisms might be involved in Hg2+ removal from solution. This study shows that increasing the degree of complexity of the Hg/rice husk system may lead to a change, from a single to multiple biosorption mechanisms, involved in Hg removal.

3.7.

Efficiency of the sorption process considering the legal limits of Hg in waters The levels of Hg2+ remaining in solution after the treatment process with unmodified rice husk were compared with the permissible limits established in the Portuguese legislation, for all the experimental conditions tested in this work. According to the Directive 98/83/EC, to obtain the criterion of drinking water quality the levels of Hg2+ must not exceed 0.001 mg L−1 [27] and based on the Directive 84/156/EEC the maximum value for Hg discharges from industrial sectors is 0.05 mg L−1 .[28] Table 1 displays the values for the residual concentration of Hg2+ in solution (CHg,res ) and the percentage of removal, obtained after the

RH-M2/n (s) + 2H2 O (aq) 2 (aq) + 2OH− (aq), nMn+ RH-H2 (s) + Hg2+ (aq) ⇔ RH-Hg (s) + 2H+ (aq). ⇔ RH-H2 (s) +

Since no relevant variation on pH solution was observed during the experiments conducted at pH 3 and 6, the effect of protons release on the ion-exchange mechanism may be despised. In the presence of sodium chloride the exact mechanisms involved cannot be anticipated at this stage, but the results suggest that due to the dominance of the negative chloro 3− complexes (HgCl2− 4 , HgCl ) the ion-exchange mechanism that prevails in the previous operational conditions is strongly inhibited. The time profiles of Hg2+ concentration in the presence of organic matter and in spiked river water are similar (Figures 5 and 6) suggesting the same biosorption mechanisms. When the chemistry of solution is more complex and includes organic matter, additional mechanisms are expected to occur. Contrary to the other experimental conditions, in the presence of organic matter it was observed a decrease in Hg2+ concentration in the controls with time, attributed to the metal complexation with the organic matter. Moreover, according to the modelling results obtained using the NICA-Donnan model in the Visual MINTEQ program, one part of the Hg2+ initially added to the solution is organically complexed to the carboxylic and phenolic functional groups of dissolved humic acids. The Hg2+ ions that are not immediately bound to the organic matter are gradually removed by ion-exchange mechanism, confirmed by

Table 1. Concentration of Hg2+ in solution (CHg,res , mg L−1 ) and the percentage of removal, after the treatment process with unmodified rice husk. C0 Hg2+ (mg L−1 )

Experimental conditions

CHg,res ± SDa Removal (×10−3 mg L−1 ) (%)

0.05

MQ water, pH = 6 NaCl 0.5 M NaNO3 0.1 M NaNO3 0.5 M Hg:Cd (1:4 C0 ratio) Hum 5 ppm Hum 10 ppm River water

8.8 ± 1.0 9.5 ± 0.4 3.8 ± 1.0 2.0 ± 0.6 5.8 ± 0.5 6.4 ± 0.1 6.6 ± 0.4 8.0 ± 0.8

82.4 83.0 91.7 94.5 87.6 76.1b 73.4b 59.1b

0.50

MQ water, pH = 3 MQ water, pH = 6 MQ water, pH = 9 NaNO3 0.1 M NaNO3 0.5 M NaCl 0.5 M Hg:Cd (1:4 C0 ratio) Hg:Cd (1:1 C0 ratio) HS 5 ppm HS 10 ppm River water

28.8 ± 1.9 38.1 ± 2.6 22.5 ± 2.4 39.1 ± 2.8 46.3 ± 1.9 20.0 ± 0.8 25.2 ± 2.2 36.2 ± 0.4 62.1 ± 0.9 47.6 ± 2.0 33.7 ± 1.6

93.6 91.9 95.4 91.9 90.4 96.1 95.0 92.6 81.4b 82.8b 85.1b

All of the experimental conditions used in this work are presented in this table. a Standard deviation (SD) calculated by means of 4 values (n = 4). b The removal efficiency by rice husk sorbent in river water was calculated considering the concentration of available Hg2+ in solution given by the control.

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remediation treatment with unmodified rice husk, under the experimental conditions used in this work. Starting with an initial concentration of Hg2+ of 0.05 mg L−1 , the levels of Hg2+ of the remediated samples, varied between 0.002 and 0.010 mg L−1 and the efficacy in the removal treatment varied between 59% and 95%. The levels of Hg2+ after the treatment with unmodified rice husk are dependent on both the experimental conditions and the complexity of the matrix, and not always the lowest residual concentration corresponds to the highest removal values. The results obtained indicate that the unmodified rice husk is capable to remove Hg2+ from waters with a moderate level of contamination (CHg,0 of 0.05 mg L−1 ). However, the effectiveness in the decontamination process is not completely accomplished applying this treatment, since the criterion of drinking water quality (CHg < 0.001 mg L−1 , Directive 98/83/EC) is not achieved, in any of the experimental conditions applied. Still, in some of the tests performed, for example in NaNO3 0.5 M medium, the Hg2+ levels found at the end of the treatment were close (ca. 2 times higher) to the levels demanded to obtain a water with drinking quality. For an initial concentration of Hg2+ of 0.50 mg L−1 , the removal efficacy with rice husk sorbent in most of the cases showed values higher than 90%. The lowest percentages of removal, i.e. between 81 and 85%, were obtained in an aqueous medium containing organic matter (natural river samples and synthetic samples containing HS), that is responsible for the complexation of some of the Hg2+ in solution, affecting the amount of Hg2+ available. For most of the experimental conditions applied in this work, the levels of Hg2+ in solution after the remediation process were lower than 0.05 mg L−1 (the maximum value for Hg discharges from industrial sectors, Directive 84/156/EEC) and ranged between 0.020 and 0.048 mg L−1 . The only exception was the Hg2+ level of 0.062 mg L−1 found in a synthetic solution containing 5 mg L−1 of HS that slightly exceeds the legal concentration for Hg discharges from industrial sectors. 4. Conclusions In this work, the efficiency of the natural/unmodified form of rice husk to remove Hg using concentrations that reflect the real contamination problems found in the environment was investigated under different operating conditions. The competitive effects posed by the presence of other ions in solution and the feasibility of the process in binary solutions containing metals with identical characteristics were assessed. The results showed that the presence of H+ ions had a little effect on the removal capacity of Hg2+ by rice husk in the pH range between 3 and 9. The effect caused by the presence of NaNO3 and NaCl electrolytes and Cd2+ co-ion proved to be dependent on the initial concentration of Hg2+ and also on the levels of the interfering ions, but in a situation of equilibrium the effect posed by those ions was

negligible. The removal values in NaNO3 and NaCl electrolyte media and in the presence of Cd2+ co-ion, ranged between 82% and 94% and between 90% and 96% for an initial Hg2+ concentration of 0.05 mg L−1 and 0.50 mg L−1 , respectively. In the presence of HS, a fraction of Hg2+ in solution becomes unavailable due to the formation of stable Hg–HS complexes, resulting in a decrease in the uptake of this metal. Still, the removal efficiency of Hg ranged between 73% and 76% (depending on the concentration of HS) for CHg,0 of 0.05 mg L−1 and between 80% and 83% for CHg,0 of 0.50 mg L−1 . In spiked natural river waters, the fraction of available Hg2+ is also affected by the complexity of the matrix, which plays an important role on the speciation and dynamics of this metal. Under these circumstances, the removal efficiency decreased, but the values obtained were satisfactory, i.e. 59% and 85% for an initial concentration of Hg2+ of 0.05 and 0.50 mg L−1 , respectively. Acknowledgements Thanks are due to University of Aveiro/CESAM and Fundação para a Ciência e a Tecnologia (FCT).

Funding The authors thank Fundação para a Ciência e a Tecnologia (FCT) (PTDC/MAR-BIO/3533/2012; PEst-C/MAR/LA0017/2011), FSE and POPH for funding. Luciana S. Rocha and Cláudia B. Lopes thank their Post-doc grants to FCT [grant number SFRH/BPD/ 47166/2008] and [grant number SFRH/BPD/45156/2008]. Bruno Henriques thanks his PhD grant to FCT [grant number SFRH/BD/62435/2009].

Supplemental data Supplemental data for this article can be accessed http://dx.doi.org/ 10.1080/09593330.2013.841267.

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