Surface modifications of Sargassum muticum algal biomass for mercury removal: A physicochemical study in batch and continuous flow conditions

Chemical Engineering Journal 229 (2013) 378–387

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Surface modifications of Sargassum muticum algal biomass for mercury removal: A physicochemical study in batch and continuous flow conditions Leticia Carro, José L. Barriada, Roberto Herrero ⇑, Manuel E. Sastre de Vicente Departamento de Química Física e Ingeniería Química I, Universidad de A Coruña, c/Rúa da Fraga 10, 15008 A Coruña, Spain

h i g h l i g h t s  Treated macroalga S. muticum constitutes a good material for mercury elimination.  Lipid removal from the native material improves mercury removal from solution.  Competition studies with organic dyes show no influence in mercury removal.  Column experiments confirm the combined reduction–sorption process.  SEM/EDS allow identifying mercury deposits as mercury (I) and metallic mercury.

a r t i c l e

i n f o

Article history: Received 19 April 2013 Received in revised form 5 June 2013 Accepted 9 June 2013 Available online 17 June 2013 Keywords: Chemical treatment Algae Mercury Sorption Reduction

a b s t r a c t Mercury elimination onto brown alga Sargassum muticum has been studied in order to enhance process performance and to go further in the analyses of the combined sorption–reduction mechanism that is taking place. Several chemical treatments of the natural biomass have improved material stability and diminished weight losses, as it was confirmed through TOC analyses. Fast kinetics and high mercury removal capacities (0.9 mmol g1) were found with native and treated material. Competition with dyes has shown that mercury uptake is not affected for these organic compounds in solution; in addition, high metal and dye elimination percentages were obtained simultaneously in mercury/Methylene Blue mixtures. Column experiments have confirmed that mercury interaction with biomass is a complex mechanism where nearly 70% of the metal is reduced and 30% is adsorbed onto material surface. Ó 2013 Published by Elsevier B.V.

1. Introduction Some of the most widespread metals studied are key environmental pollutants of major toxicity such as lead, copper, cadmium, chromium or mercury; the present work is focused on mercury elimination from solution. Mercury, together with lead and cadmium, are considered to be the ‘‘Big Three’’ and the most toxic heavy metals with the greatest potential hazard to humans and the environment. Their poisoning effects are not immediately obvious and, in the case of mercury, the two major responses to its toxicity involve neurological and renal disturbances [1]. This study is based on the analysis of the natural materials capacity to remove mercury from solution owing to the risk for human health and environment. Mercury can exist in solution as univalent or divalent ions as well as elemental mercury. The distribution of this metal between the three oxidation states is determined by the redox potential, pH ⇑ Corresponding author. Tel.: +34 981167000x2126; fax: +34 981167065. E-mail address: [email protected] (R. Herrero). 1385-8947/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.cej.2013.06.014

and other species present in solution [1]. Two different states are the most common in the aquatic environment: as divalent cation, being Hg(OH)2 and HgCl2 the predominant species in most surface waters or as organic ion, mainly as monomethyl mercury [2,3]. Mercury and its compounds are used in dental preparations, thermometers, fluorescent and UV lamps, pharmaceuticals, fungicides in paints and industrial process waters [1]. All this human activities render many sources and discharges of mercury in the environment; therefore, it is necessary to find a good technique which allows obtaining high metal elimination, but at the same time a technique economically and environmental friendly. In last decades, sorption technology has been widely studied in order to optimize experimental conditions and to go further in the knowledge of all the parameters that are directly implied in the elimination processes. In addition, high efficiency of sorption to remove different compounds from solution even at high initial concentrations was clearly proved [4,5]. Conventional methods to remove pollutants from wastewaters usually present low efficiency performance, especially at low concentrations, expensive

379

L. Carro et al. / Chemical Engineering Journal 229 (2013) 378–387

chemicals are required and large amounts of wastes or sludge are produced [6]. On the other hand, one of the most important advantages of sorption techniques includes low operation costs if ‘‘low cost’’ sorbents are used and moreover low quantity of sludge is produced [7]. Previous studies based on Hg(II) elimination onto natural materials have demonstrated that mercury uptake is a complex mechanism which involve adsorption–reduction combined process [8– 10]. Studied materials present in their structure different functional groups which can be implied in the sorption process such as carboxyl and sulfate groups. These groups are located in cell wall polysaccharides [11,12], as well as other organic functional groups which can also play an important role in metal reduction [13]. Among different kinds of materials tested in pollutant elimination studies, high efficiency for metal removal using algae as cost effective sorbent has been specially proved [14–17]. Sargassum muticum algal biomass was selected as a potentially good material to provide high mercury eliminations through a combined adsorption–reduction process, as it has been already proved [9]. The main objective of the present work is to carry out chemical modifications to obtain the best mercury elimination capacities using S. muticum as sorbent. These treatments can modify the main functional groups in the material surface and improve native material performance as well as provide additional information about the combined mechanism implied in mercury elimination. Moreover, alga modifications improve the mechanical stability of the material and reduce the weight losses in uptake process; these aspects are particularly important for industrial applications. Different alga treatments and their efficiency for mercury removal are reported in this work. In addition, a complete elimination study has been carried out to compare the capacity of the treated materials for mercury removal compared to the native one. A screening with all the treatments, equilibrium and kinetic studies and continuous flow experiments are presented. The influence of Methylene Blue (MB) and Acid Bue 25 (AB 25) in mercury elimination has been analyzed using binary mixtures (metal:dye). S. muticum capacity to remove both compounds simultaneously was also studied in these experiments. All the previous works developed with the same brown alga [9,18] and the results present in this study allow comparing different uptake capacities obtained with S. muticum collected in different points or seasons, as additional information for sorbent description and its influence in mercury removal.

2. Materials and methods 2.1. Biomass S. muticum biomass was collected in Galician coast (NW Spain). The brown alga was washed with deionised water and was ovendried at 60 °C during 24 h in a forced air circulation oven. The dried material was ground with an analytical mill, sieved and stored in polyethylene bottles. Particles with size ranging from 0.5 to 1 mm diameter were used in all the experiments. Several portions of this biomass were chemically modified following different treatments: 1. Extraction of the lipidic fraction: – Three 1 g portions of alga were put in contact with 100 mL of aqueous solutions 50% (v/v) of acetone, methanol and ethanol separately. The mixtures were shaken during 24 h and then the biomass was filtered, rinsed with deionised water and oven-dried at 60 °C overnight [19,20].

– 1 g of the brown alga was put in contact with a mixture of 2:1 chloroform/methanol. Then, it was shaken during 24 h at room temperature. After this period the biomass was rinsed with deionized water, filtered off and dried in an oven at 60 °C overnight [20]. 2. Modification of the carboxyl groups: – Esterification of the carboxylic acids [20,21]: 3 g of biomass were mixed with 200 mL of methanol and 1.8 mL of concentrated HCl to obtain a solution 0.1 M of HCl. This mixture was also shaken during 24 h and rinsed, filtered and dried following the same procedure described above. The general reaction scheme is: Hþ

RCOOH þ CH3 OH ! RCOOCH3 þ H2 O – Treatment with citric acid, a cross-linking process previously described in literature [22] was followed: 2.5 g of citric acid were dissolved in a small volume of water and it was added over 5 g of S. muticum. The mixture was dried in an oven for 24 h at 60 °C. Afterwards the dry mixture was left in the oven 24 h at 120 °C to carry out the reaction between the biomass groups and the citric acid. Then, the obtained product was suspended in 1 L of deionized water and pH was adjusted to 2. Suspension was magnetically stirred during 30 min. The mixture was filtered and the material rinsed with at least 2 L of deionized water. The biomass was dried in the oven at 60 °C. 3. Treatment with Methylene Blue – 1 g of S. muticum was put in contact with 200 mL of 1.5 mmol/L MB solution during 24 h. MB remaining in solution was measured and it was estimated that the dye concentration on the biomass was 0.25 mmol/g. This material was filtered off and oven dried overnight at 60 °C. 2.2. Reagents Reagents used for biomass treatment were ethanol, methanol, chloroform and acetone from Panreac. (Panreac Quimica S.A.Barcelona, Spain). HCl and Citric acids also from Panreac were added in the acid treatments of the biomass functional groups. NaOH p.a. (pro analysi) and HNO3 (Suprapur) both from Merck (Merck KGaA, Darmstadt, Germany) were used to adjust the pH of solutions. 4,40 Bis(dimethylamino)-thiobenzophenone from Merck was used as a complexing agent in the spectrometric determination of mercury. 1-propanol from Panreac was used to prepare mercury complexing agent solutions. Mercury salt, namely HgCl2, was obtained from Merck. Methylene blue and Acid Blue 25 from Panreac were used as organic compounds to study the competition with metal elimination of the treated biomass. All solutions were prepared with deionised water. 2.3. Batch removal experiments A screening with the treated biomass was carried out by bringing in contact 0.1 g of the selected material with 40 mL of mercury solution (2.5 mmol L1). The mixture was shaken during 24 h at 175 rpm adjusting the pH to 5 by addition of the necessary volumes of concentrated NaOH or HNO3. The metal concentration remaining in solution was measured using colorimetric or cold vapor techniques. Mercury competition with organic compounds was also carried out following the same procedure as in the screening experiments. A concentration of 1 mmol L1 of both mercury and dye in solution was chosen because higher concentrations are difficult to measure. The pH of mercury-organic dye solution was adjusted to the optimum value for mercury removal, which was determined to be

380

L. Carro et al. / Chemical Engineering Journal 229 (2013) 378–387

between 5 and 5.5 for metal removal onto S. muticum in previous studies [9]. Mercury isotherms were determined at 25 °C for native and treated alga. 0.1 g portions of biomass were stirred during 24 h with 40 mL of several mercury solutions in a concentration range from 0.25 mmol L1 to 3.5 mmol L1. An isotherm to study the competition of MB with mercury was also determined. For this experiment mercury concentration range was varied from 0.25 to 3.5 mmol L1 while maintaining dye concentration fixed to 1 mmol L1 in all the metal/dye mixtures. On the other hand, kinetics studies were performed by putting in contact 100 mL of the metal solution (2.5 mmol L1) in a thermostated cell at 25 °C solution with 0.25 g of biomass. pH was controlled periodically and adjusted to 5 with small amounts of acid or basic solutions as required. All the kinetic studies were followed during 24 h to ensure that equilibrium was reached. 2.4. Column experiments Mercury uptake with the native and treated materials was also studied under continuous flow conditions. The columns used in these experiments filled with 21 g of biomass were 3 cm of internal diameter. The first column was performed with native S. muticum, a solution of 0.5 mmol L1 of mercury (II) and a bed depth of 30 cm. The second continuous flow experiments was carried out filling the column with S. muticum treated with ethanol and a solution of Hg(II) 0.5 mmol L1 was passed through the column; in this case, the bed depth was 33 cm. Additionally, another column was filled with native S. muticum but the solution studied was a binary mixture of mercury and MB of equimolar concentration (0.5 mmol L1); in this third column the bed depth attained was 28 cm. The differences found between the bed depths despite of having the same amount of biomass depend on the material used and the packed arrangement obtained. A porous sheet was placed at the bottom of the column to support the material and to obtain a good liquid phase distribution as well as uniform inlet flow into the column. The top of the bed was filled with glass beads to avoid biomass losses. All the solutions studied were introduced in the column in upflow mode with a peristaltic pump (Watson–Marlow). The flow rate was selected around 9 mL min1, obtaining experimental residence times between 23 and 27 min, the variations found in these values were related to the different bed depths described above. These experimental residence times are within the range of the theoretical values time for a compound in the column which is established between 15 and 35 min [23]. Samples were collected at the top of the column and the metal and dye concentration were measured. 2.5. Analytical techniques Mercury concentration in solution was measured through colorimetric determination following the Michler´s thioketone method described in previous works [18]. Some of the measurements were carried out by cold vapour atomic absorption spectroscopy to check the colorimetric technique efficiency. Results obtained with both methods were in close agreement. Methylene Blue and Acid Blue 25 were determined using spectrophotometric measurements. The maximum absorption peaks for these compounds are 665 nm and 602 nm, respectively. 2.6. SEM and EDS The biomass used in continuous flow experiments was collected after elimination process, dried and analyzed by SEM (Scanning Electron Microscopy) (JEOL JSM 400) equipped with an Oxford Inca

Energy 200 equipped for EDS (Energy Dispersive X-ray Spectroscopy). This characterization was performed to observe material surface after the contact with metal and dye solutions and to determine metal deposits presence in alga structure. EDS analyses allow confirming metal deposits composition. 3. Results and discussion 3.1. Batch removal experiments Batch removal experiments were carried out to compare the native alga capacity with those obtained with seven different treatments for mercury removal. In Table 1 it can be observed that all the removal percentages are above 70% except with the citric acid treatment. The best results were obtained with methanol, ethanol and acetone treatments, percentages near 100% for mercury elimination were reached. The extraction of the biomass with these solvents implies the elimination of the lipidic and protein fractions of the material surface. High capacities found with these treatments can be associated to the higher accessibility of the binding sites for metal removal after the extraction process [19]. This process provides additional sorption sites that could be occupied by heavy metals or organic compounds, increasing the surface area and, therefore, improving the uptake capacity of the material. The esterification of the carboxylic groups of the biomass was carried out to study the contribution of these functional groups in mercury removal. Carboxylic group is generally the main binding group especially in brown alga, such as S. muticum and is related with the elimination from solution of different metallic or organic cations [24,25]. Based on the results showed in Table 1, the esterification reaction does not improve the mercury elimination compared to native material, although metal removal above 70% is reached. This result indicates that carboxylic group could contribute to mercury removal but less availability of this group in the biomass surface does not prevent achieving high metal eliminations. This behaviour found with mercury is completely different of other pollutants such as dyes [21] or metallic cations [25], where carboxylic group plays a significant role in sorption process, resulting in a decrease of the percentages of pollutants elimination after esterification treatment. In addition, it is important to take into account the role of the solution pH in these experiments; the esterification process or the biomass treatment with citric acid make solution pH lower than that with native material and more difficult to maintain at the optimum value for mercury removal. As it was previously proved [26], there is a clear pH dependence in metal elimination, so the difficulty to keep the pH around 5 through all the elimination process is one of the reasons why in both acid treatments, mercury removal capacity is lower than that with other modifications.

Table 1 Removal percentages obtained from the screening with native S. muticum, six different alga treatments and biomass loaded with Methylene Blue (MB). Initial mercury concentration 0.5 mmol L1. Biomass dose 2.5 g L1 and pH 5. Materials

Removal (%)

Native Methanol/CHCl3 Methanol Ethanol Acetone Esterification Citric Acid MB

86 ± 2 78 ± 3 94 ± 1 93 ± 1 94 ± 1 80 ± 1 65 ± 2 76 ± 2

L. Carro et al. / Chemical Engineering Journal 229 (2013) 378–387

The purpose of treatment with citric acid is to obtain a product highly cross-linked to increase the ion-exchange potential of the alga. The experimental conditions were optimized based on previous works [22] to maximize the thermochemical reaction of the biomass with the citric acid. However, the results obtained with this treatment are far from the maximum capacity for mercury elimination reached with other alga modifications. This fact can be associated with the pH of the solution, as it was explained before, but also with the availability of the functional groups that bind the metal after the citric acid treatment. This modification not only is not specific for mercury elimination, but it is also based on the increment of the cationic exchange properties of the biomass. As it was previously tested [8,9], the mercury elimination is not associated to metal free form Hg2+, but it is supposed to occur through metal neutral species. Therefore, the citric acid treatment can diminish the number of functional groups related to mercury caption, such as hydroxyl group, which are directly implied in the reaction with citric acid [22]. Furthermore, it is remarkable the mercury elimination obtained using the biomass treated with MB. This result shows high biomass capacity to eliminate both compounds, even at high concentration, which can be associated to different uptake mechanisms. The capacity of the brown alga to remove both compounds from solution makes the material very adequate for industrial application. This high capacity found with S. muticum has been tested in previous studies with mercury [9] and MB individually [27], as well as the main mechanism that is taking place for each compound removal. One of the most important reasons to carry out some chemical modifications into the native biomass is based on improving the stability and mechanic properties of the material for industrial applications. TOC and weight loss studies were performed with all the treated materials in order to analyze some possible changes in the biomass structure. Results are shown in Table 2. Two different TOC analysis were made, one studying the solution TOC content after mercury elimination; the other putting in contact the biomass with deionised water in the same experimental conditions than with mercury solution but without metal. These results demonstrate that the highest biomass leaching and weight loss occurs with the native material. Similar weight losses were registered with all the biomass modifications. The amount of carbon released in solution is not related to the mercury removal process because no clear trend was found between the TOC with or without mercury in solution. Good results obtained with the treated biomass are remarkable comparing to native alga, which confirm that chemical modifications can improve biomass stability and avoid high weigh losses.

3.2. Sargassum muticum: comparison of the uptake capacity The high capacity of S. muticum for pollutant removal has been proved previously not only with mercury but also with other heavy metals and organic compounds [27–30]. Brown alga affinity for

Table 2 Percentages of weight losses of algal biomass and total organic carbon measurements (TOC) after washing with deionized water. Total organic carbon after mercury sorption (TOC(Hg)).

Native Methanol/CHCl3 Methanol Ethanol Acetone Esterification

% Weight lose

TOC (mg/L)

TOC (Hg) (mg/L)

38.6 16.9 15.8 15.7 18.8 16.4

104 41.9 35.3 32.7 36.9 56.5

110 42.6 41.4 39.2 40.6 30.6

381

mercury has been tested in the present work and in two previous studies [9,18]; so the maximum removal capacities for this alga can be compared. In order to compare the results obtained, S. muticum was described as S. muticum 1 for the first work in 2009 [18], S. muticum 2 for the second one [9] and S. muticum 3 for the alga used in the present study. Maximum sorption capacities found were 1.03 mmol g1 with S. muticum 1, 0.69 mmol g1 with S. muticum 2 and 0.94 mmol g1 with S. muticum 3. As it can be observed, high mercury removal capacities were obtained with this alga species, being one of the best sorbent tested for mercury removal. However, differences were found among the three algal batches studied. These differences can be explained taking into account the alga composition; all the algae were collected in Galician coast (Spain) but in different seasons and years (from 2005 to 2010). The alga used in this study, S. muticum 3, was collected in 2010 during the spring season in the coast of A Coruña (Galicia). High metal removal capacity of the brown algae is mainly associated to the polysaccharide matrix. The uronic acids that constitute the alginates in brown algae occur in different ratios and quantities depending on the strains but also in the age, season and origin of the alga [5]. These results highlight the importance of material composition on mercury elimination and the necessity of characterization techniques to determine the functional groups on the biomass surface. Moreover, it is necessary to find a good sorbent, not only easily obtained but also that keeps constant structural properties to obtain similar eliminations capacities; it is here where chemical modifications could play an important role in order to obtain a material more stable and homogeneous.

3.3. Competition with organic compounds Generally, industrial wastewaters are composed of more than one pollutant so it is necessary to analyze the sorbent capacity to remove all the compounds present in solution. Moreover, the study of the influence of other species in mercury elimination can provide additional information to describe the interaction between the metal and the material. In the present work, mercury elimination onto native alga or modified biomass was tested in presence of two different dyes, as a model of organic pollutants, Methylene Blue and Acid Blue 25. Methylene Blue (319.85 g mol1) is a cationic dye with high affinity for anionic pollutants while Acid Blue 25 (416.38 g mol1) is an anionic dye which presents good capacity to retain cationic compounds. Both dyes are widely used in textile industry as well as models in adsorption studies. The structures of these two dyes are shown in Fig. 1. The elimination was studied using binary mixtures metal/dye of equimolar concentrations. Results for MB/mercury mixtures are shown in Fig. 2. It can be observed that mercury elimination is not affected for dye presence in solution; almost 100% of relative mercury removal is reached with all the materials tested. Similar behaviour was found in the mixtures AB25/mercury for mercury removal. These results are in agreement with previous studies where mercury elimination onto S. muticum was tested in the presence of other organic pollutants or divalent cations [9] For the binary mixtures with MB, the high uptake capacity of S. muticum with both compounds was demonstrated; not only almost 100% of initial mercury was removed, but also 100% of the dye. Mercury and MB are retained onto the alga surface following different mechanisms which do not interfere in the pollutant interactions with the material surface, at least at current experimental conditions. High elimination of both compounds can be also explained taking into account that the optimum pH for mercury elimination is also the optimum value for the cationic dye (MB) removal onto the brown alga [27]. In addition, in these binary

382

L. Carro et al. / Chemical Engineering Journal 229 (2013) 378–387

Fig. 1. Methylene Blue (a) and Acid Blue 25 (b) structures.

Fig. 2. Relative removal percentages of mercury and MB onto native and treated S. muticum in binary mixtures. Initial concentration of both compounds 1 mmol L1. pH 5. The reference value 100% represents the compound removal in the absence of the other component of the mixture.

mixtures, pH is easily kept around 5 during all the elimination process which helps mercury removal. On the other hand, it was observed that AB 25 elimination is not equal with all the tested materials and is lower than MB removal. Some of the relative mercury uptakes were over 90%, while several of them were around 70%. This fact can be explained considering the different form of both dyes in solution; MB is a cationic dye while AB 25 is anionic, therefore sorption interactions with the functional groups over the surface biomass are different. In addition, pH dependence for AB 25 elimination is the opposite of mercury or MB sorption [27]; AB 25 removal is optimum at low pH values as it was observed with other anionic dyes [31]. Based on these results, it can be concluded that mercury uptake is not affected for the presence of dyes in solution. Moreover, mercury removal conditions are optimum also for MB elimination; unfortunately, experimental parameters to eliminate high concentrations of the anionic dye are quite different than the ideal conditions for mercury uptake so it is difficult to reach high capacities for removal of both compounds simultaneously, at least using the brown algae biomass.

ethanol or acetone. These treated materials reach mercury elimination percentages over 90%, which improve the values obtained with the native alga. It is also remarkable that 50% of the initial mercury concentration was eliminated in less than 30 min not only with ethanol, methanol and acetone treatments but also with native biomass, considering the high initial metal concentration, 2.5 mmol L1. These results are in agreement with batch studies, where, as it was explained in the previous section, these three treatments are the most adequate to obtain high mercury eliminations, improving native material results. However, longer times to reach the equilibrium were observed with the citric acid modification. The fast mercury uptake observed for the different treatments represent a clear advantage for practical uses, as it will facilitate the preparation of shorter elimination columns, ensuring efficiency and economy. Usually, experimental data obtained from kinetic studies are fitted to different kinetics models in order to analyze the potential rate controlling step that can include particle adsorbate diffusion or chemical reaction processes. Weber–Morris model can describe the process if the intraparticle diffusion of the adsorbate into the pores of the adsorbent is considered the rate-limiting step. Kinetics of mercury eliminations obtained in this study were fitted to the intraparticle diffusion model. The rate equation of this model was proposed by Weber and Morris in 1963 [32] and is based on a linear relationship between the uptake capacity (qt) of the material and the square root of time (t1/2), according to the expression:

qt ¼ ki t 1=2 where qt (mmol g1) is the metal uptake at time t (min) and ki (mmol g1 min0.5) is the intraparticle diffusion constant. The con-

3.4. Kinetic studies Kinetic studies were carried out in order to determine different equilibrium times for each treated material. In Fig. 3 it can be observed that the results are similar with all the tested materials, the maximum elimination percentage is reached in 5 h approximately. There are three treatments that showed higher capacity for mercury elimination: extraction of the lipid fraction with methanol,

Fig. 3. Mercury removal percentages versus time by native S. muticum, loaded with MB and modified alga with six different treatments. Initial metal concentration 2.5 mmol L1 and 2.5 g L1 of sorbent dose, pH  5.

L. Carro et al. / Chemical Engineering Journal 229 (2013) 378–387

stant (ki) for each material tested was determined from the slope of the straight-line plots of qt versus square root of time. Intraparticle diffusion constants are given in Table 3. As it can be seen, the model fits well the experimental data and all the fittings show good regression coefficients. Moreover, the intercept of the plots passes very close to the origin; therefore intraparticle diffusion is considered to be the rate controlling-step. Small deviations from zero can be associated to experimental error if they are small. Higher deviation from the origin could indicate that other processes also contribute to metal uptake onto the biomass, these processes are taking place at same time that diffusion [32]. It was observed that the intraparticle diffusion constant value is almost the same with the ethanol, methanol and acetone treatments and similar to native material. This constant is smaller with the citric acid modification and presents the same values for esterification process and with the biomass loaded with MB. This result could indicate that the lipidic fraction extraction of the treated biomass with ethanol, methanol and acetone improve material porosity, rendering the binding sites more available for metal sequestration and making easier the pollutant diffusion into the material structure. Intraparticle diffusion constants obtained in this study can be compared with previous works using the same native alga at same initial metal concentration. The values given in the present work are higher than the previous study [9], which can be associated to small differences in S. muticum species collected in a different season and different places in the Galician coast, as it was explained before. However, diffusion constants found for mercury elimination onto the brown macroalga S. muticum are much higher than the constant values given for the divalent cations sorption onto another green macroalga [33]. Kinetics results confirm that mercury elimination onto the native or modified biomass is a fast process and S. muticum as a good sorbent to go further in removal studies and applications.

383

Based on the results obtained in batch experiments and kinetic studies, the biomass treated with ethanol was selected among the different modifications done to determine the maximum metal elimination capacity of this material and to compare these results with those of the native alga. Moreover, the maximum of mercury elimination with the brown alga was evaluated in competition with MB in solution. Therefore, three isotherms at 25 °C were carried out; results are showed in Fig. 4. Fitting parameters from Langmuir and Langmuir–Freundlich models can be seen in Table 4. Similar results were obtained with the native alga and the alga after the extraction with ethanol. Maximum of mercury elimination capacity (Qmax) was 0.9 mmol g1 with all materials studied and no differences were found for the values obtained between the two models applied to fit metal uptake data. These outcomes confirm that ethanol treatment on S. muticum is an adequate process to improve biomass characteristics, which allow obtaining mercury elimination capacities similar or even better than with other natural materials, resins or activated carbons, as it can be observed in Table 5. On the other hand, it can be observed that in competition with MB, mercury elimination is even higher than the value found without dye presence in solution. This result supports the idea of different mechanisms taking part for the elimination of both chemical compounds and no competitive processes taking place. Moreover, in binary mixtures with MB is easier to keep the pH at the optimum value for mercury elimination; very small additions of base or acid are necessary to adjust this parameter. This fact renders removal onto S. muticum as an operational process easier to carry out, reaching high elimination values for two different pollutants. Mercury elimination by native and modified alga can also be evaluated by using a dimensionless constant separation factor or equilibrium parameter, RL, defined as:

RL ¼

1 1 þ bC o

ð1Þ

3.5. Equilibrium studies The analysis of equilibrium data is performed with the adsorption isotherms models. The sorption equilibrium provides information about the affinity of the sorbent for the pollutant, and its distribution in the liquid and solid phase. Metal removal is a complex process which implies different mechanisms simultaneously and, in most of the cases, is far from ideal conditions and assumptions described in the isotherm model. However, the main reason for the extended used of these isotherms is that they incorporate constants easily interpretable which can be used to compare elimination performance [29,34]. In the present work, Langmuir and Langmuir–Freundlich models were selected to analyze experimental data. The parameters obtained from both isotherms allow comparing maximum uptake capacities (Qmax), the affinity of the biosorbent for the sorbate (b) and evaluate the degree of heterogeneity of the material surface (n).

where b is the Langmuir constant and C0 is the initial metal concentration in solution (mg L1). This parameter allows seeing whether elimination process is favorable; if 0 < RL < 1 the process is considered to be favorable, if RL > 1 biosoption equilibrium is unfavorable. RL = 0 for the irreversible case and RL = 1 for the linear case [35]. It was checked that all the RL values are in the range 0–1 in the

Table 3 Results for diffusion model fitting kinetic data of mercury sorption onto native and treated S. muticum at pH 5. Numbers after ± correspond to the standard error of each parameter obtained from the fitting. Treatments

ki (102 mmol g1 min0.5)

r2

Native Methanol/CHCl3 Methanol Ethanol Acetone Esterification MB Citric Acid

9.7 ± 0.8 6.6 ± 0.3 10.2 ± 0.6 10.2 ± 0.3 10.7 ± 0.5 7.7 ± 0.7 7.7 ± 0.3 2.9 ± 0.1

0.96 0.98 0.98 0.99 0.99 0.96 0.99 0.98

Fig. 4. Isotherms data for native S. muticum (squares), treated alga with ethanol (triangles) and competition with MB (circles) at 25 °C and pH 5. Mercury concentration from 0.25 to 3.5 mmol L1 and MB concentration of 1 mmol L1. Lines represent Langmuir model fitting curves.

384

L. Carro et al. / Chemical Engineering Journal 229 (2013) 378–387

Table 4 Results obtained from the fitting of the equilibrium data at pH 5 with Langmuir and Langmuir–Freundlich model. Numbers after ± correspond to the standard error of each parameter obtained from the fitting. b½c Langmuir Q e ¼ Q max 1þb½c

S. muticum native S. muticum EtOH S. muticum MB

Langmuir–Freundlich Q e ¼ Q max

Qmax (mmol g1)

b (L mmol1)

r2

Qmax (mmol g1)

b (L mmol1)

n

r2

0.94 ± 0.04 0.87 ± 0.03 0.97 ± 0.02

30.9 ± 4.5 31.7 ± 5.7 17.6 ± 2.1

0.96 0.97 0.98

0.88 ± 0.02 0.91 ± 0.06 1.03 ± 0.06

36.3 ± 2.4 25.9 ± 7.6 15.1 ± 3.2

0.6 ± 0.1 1.2 ± 0.2 1.1 ± 0.2

0.98 0.97 0.98

Table 5 Hg(II) uptake capacities obtained with different types of materials. Materials

Uptake capacity (mmol g1)

References

Activated carbon from walnut shell Activated carbon from furfural Duolite resin GT-73 Cystoseira baccata (Brown alga) Thiol-functionalized spent grain Modified chitosan Coarse quitin Treated S. muticum with ethanol S. muticum native

0.76

[37]

0.87 1.81 1.0 1.1 0.95 0.35 0.91 0.88

[38] [39] [40] [41] [42] [43] This study This study

experimental conditions, which indicates a favorable process for mercury removal onto native S. muticum. Similar results were found for ethanol treated material and for the isotherm in competition with MB. In addition to Langmuir and Langmuir–Freundlich models, Dubinin–Radushkevich (DR) isotherm was also applied to the experimental data obtained in the present work. This model does not assume a homogeneous surface or a constant sorption potential. DR equation can be described as follows [24]:

Q e ¼ Q max ebe

2

ð2Þ 2 2

where b is the coefficient related to the sorption energy (mol J ), Qmax is the maximum removal capacity, which is also described as Dubinin–Radushkevich constant, and e is the Polanyi potential (J mol1), that is expressed as:



e ¼ RT ln 1 þ

1 Ce

 ð3Þ

b is related to the mean sorption energy (E) though the equation:

1 E ¼ pffiffiffiffiffiffi 2b

ðb½cÞ1=n 1þðb½cÞ1=n

ð4Þ

The plot of ln Qe versus e2 should lead to a straight line, and the slope gives the b coefficient, so mean sorption energy can be calculated. In general, DR model has been used to calculate E values. Obtaining the metal sorption energy may provide information to know if elimination is related to a chemical or a physical process; some authors have defined the mean energy (E) in a range between 1 and 16 kJ mol1, which indicates a physical electrostatic force related to the sorption process [6]. Mean sorption energies were calculated from the slope of the plots using Eq. (4) and it was obtained 8.45 kJ mol1 for mercury elimination onto native material, 11.8 kJ mol1 for mercury elimination with biomass treated with ethanol and 10.0 kJ mol1 in the isotherm for mercury in competition with MB. As it can be observed, sorption energies values are similar in the three isotherms studied and they correspond to a physical process, according to theoretical ranges described above. These data are similar to a

previous study evaluating S. mutiucm capacity to eliminate mercury which have shown process enthalpy values corresponding to physical interactions [18]. Mercury elimination with natural materials is a complex process that involved different interactions and mechanism occurring simultaneously, which can include chemical and physical processes. DR can provide some information about mean sorption energy of the global process but mercury elimination cannot be described only in terms of adsorption onto the material surface. Batch studies allow optimizing all the experimental conditions in a elimination process and provide useful information to go further in the knowledge of the interaction between the chemical compound and the biomass. However, once all these experiments were carried out, the most effective configuration of the elimination system is based on a flow-through fixed bed type reactor or column. 3.6. Continuous flow experiments One of the most important advantages to carry out chemical modifications in the native biomass is related to improve the mechanical stability in their application in continuous flow experiments. Raw biomass usually is very soft and presents high weight losses. In addition, native materials used in column fillings can promote clogging due to the biomass swelling. Apart from improving uptake capacity and extending binding sites, material modifications provide appropriate rigidity and swelling characteristics of biosorbent particles. In order to compare the behaviour of native and treated materials in mercury elimination, two different columns were performed at the same experimental conditions using as sorbent native S. muticum and the alga treated with ethanol. Moreover, the competition between mercury and MB in continuous flow studies was also determined using a column filled with raw material and using as feed solution a binary mixture of both compounds in equal molar concentrations. Mercury breakthrough curves in the three different columns are presented in Fig. 5. As it can be observed, similar results were obtained in the three experiments, however small differences were registered in breakthrough times. Taking into account that the same flow rate was used in all the columns, the breakthrough time for mercury removal onto biomass treated with ethanol was set at 32 h, so 16.7 L of metal free solution were obtained; using the native material, the breakthrough time was 26 h, 13.5 L of effluent solution without mercury were obtained and with the column in competition with MB, the mercury breakthrough time was establish at 22 h and 11.5 L of clean solution were collected. Based on these results, the biomass treated with ethanol is the best sorbent for mercury elimination in continuous flow experiments. In addition, the differences found between the three columns can be explained based on the bed depth for each column despite of using the same quantity of biomass. As it was observed, an increased in bed height correspond to a rise in the breakthrough time, this may be associated to higher surfaces areas of the biosorbent available, improving the contact with the metal. However, the

L. Carro et al. / Chemical Engineering Journal 229 (2013) 378–387

Fig. 5. Breakthrough curves for mercury sorption onto native S. muticum (squares), treated alga with ethanol (triangles) and competition with MB (circles). Mercury initial concentration 0.5 mmol L1. Bed depth 30 cm for native biomass, 33 cm with ethanol treatment and 28 cm in the column with MB. Flow rate 8.7 mL min1.

higher bed height, the more broad mass transfer zone, so the breakthrough curves obtained are less steep. This shape of the curves increasing bed depth implies more difficulties to exhaust the material; same behaviours in continuous flow experiments were previously explained in literature [36]. On the other hand, the breakthrough curves obtained with MB and mercury are shown in Fig. 6. As it was explained before, in this column with the binary mixture, less volume of mercury free effluent was obtained comparing to the column without MB. However no clear influence of the dye in mercury elimination was registered. The differences found can be associated to operational problems fixing the bed height and continuous flow conditions. MB breakthrough time was set at 139 h, so in this colum 11.5 L of mercury free solution were obtained and 72 L without the dye. These results confirm that S. muticum is a very good sorbent to eliminate both compounds from solution at high concentrations and in batch and continuous experiments.

385

Fig. 6 allows comparing the different curve shapes found with the metal and the dye. MB presents the typical curve found in most of the continuous flow studies with metals or organic compounds [36] where the final concentration in the effluent is the same as the feed solution. However, the mercury curve attained a plateau approximately at 40% of initial metal concentration. This behaviour was previously found for mercury in column experiments [8,9] and it is associated to the complex elimination process which includes not only mercury sorption onto material surface, but also metal reduction. It is remarkable that the plateau was reached at the same concentration value in the three columns; this fact indicates that functional groups implied in reduction process do not change due to the presence of the other compound in solution or due to alga modification with ethanol. Natural materials present in their structures a great amount of easily oxidized functional groups that can be implied in metal reduction processes. Some of these functional groups together with their oxidation reactions were described in previous studies [13]. Based on the breakthrough curve shape, it can be assumed that the adsorption capacity of the material is saturated when the plateau in the curves is reached, however maximum reduction potential is much higher and is not attained in these experiments. Taking into account the selected areas in Fig. 6a–c, the total amount of mercury removed from solution, once the plateau was attained, can be assumed to be the areas b and c, while a, the area under the curve, corresponds to the mercury eluting the column. Following the behaviour of a pure adsorption process, the sorption capacity of the material can be estimated as the area above the curve (area b in Fig. 6) while the quantity of metal reduced can be interpreted as area c. The area under the curve (area a in Fig. 6) was determined through the numerical integration of data obtained. Thus, sorption and reduction percentages for mercury elimination onto S. muticum were calculated using areas b for adsorption and c for reduction. Results are showed in Table 6. High reduction percentages were found in the three processes which indicate that alga biomass presents higher content in oxidizable groups than binding sites for mercury adsorption. However, the lack of data of the amount of mercury that appears as metallic mercury or mercury (I) and the complexity of material composition make difficult to predict the exact mechanism taking place. Based on these results it is just assumed a combined sorption–reduction process with neutral mercury species involved in it. 3.7. SEM and EDS analysis

Fig. 6. Breakthrough curves for a solution containing equimolar concentrations (0.5 mmol L1) of mercury (circles) and MB (squares) onto native S. muticum. Bed depth 28 cm and flow rate 8.7 mL min1. Selected areas represent the amount of mercury that passes through the column at established time. (a) describes the mercury that elutes the column and (b and c) are the mercury adsorbed and reduced, respectively.

Based on the results obtained in the column experiments it was assumed that mercury was reduced by the functional groups on the material structure. Therefore, S. muticum biomass used in continuous flow studies was analyzed through SEM in order to find mercury deposits over material surface that confirm reduction process. Fig. 7 shows a representative area of all the biomass analyzed. It can be clearly observed two different metal deposits, mercury drops and crystalline precipitates. Both types of mercury deposits were found in the biomass collected from the three different columns, without significant differences among them. EDS characterization allows confirming the composition of these precipitates; drops are mainly composed by mercury and crystalline deposits present a mercury:chloride composition 1:1. This characterization confirms mercury reduction into metallic mercury and mercury (I) which is mainly associated to chloride; similar results were found previously using natural materials for mercury removal [8,9].

386

L. Carro et al. / Chemical Engineering Journal 229 (2013) 378–387

Table 6 Mercury adsorption and mercury removal quantities obtained from the breakthrough curves in column experiments. Adsorption and reduction percentages observed for the three different columns calculated when the plateau is reached, in 39 h for native alga, 49 h for ethanol treated material and 38 h for the competition with MB: Metal initial concentration 0.5 mmol L1 and 21 g of material.

Native Ethanol Competition MB

Mercury adsorption (mmol g1)

Mercury elimination (mmol g1)

Adsorption (%)

Reduction (%)

0.15 0.18 0.13

0.45 0.58 0.40

33.7 31.8 33.0

66.3 68.2 67.0

Fig. 7. Scanning electron micrograph of ethanol treated alga after column experiments with mercury (II). Two different mercury deposits are shown: mercury drops and crystalline precipitates. Insets correspond to EDS graphs of both types of deposits.

4. Conclusions

Acknowledgements

Chemical modifications carried out with the alga S. muticum have improved mechanical stability of the native material and reduced the weight losses during the mercury elimination process. High metal uptake values were attained specially with the lipid extraction treatments with ethanol, methanol and acetone. Simultaneously, the same chemical treatments provided materials with fast elimination kinetics. The good fitting of the kinetics data to the intraparticle diffusion model proposed by Weber and Morris supports the idea of an improvement in the material porosity and binding site availability achieved through the extraction of the lipid fraction of the biomass used in this study. Additionally, it has been checked that competition with dyes does not affect metal elimination; moreover, high uptake values were reached for mercury and MB elimination in binary mixtures onto the brown alga, not only in batch studies but also in continuous flow experiments. Finally, columns studies have shown the high capacity of the native and ethanol treated S. muticum to clean large volumes of mercury polluted water. Continuous experiments data have also confirmed the combined sorption–reduction for mercury elimination. Biomass characterization through SEM and EDS showed metal deposits over material surface which clearly confirm mercury reduction in column experiments.

L. Carro thanks to Ministerio de Educación for the fellowship granted through FPU program. Authors thank Ministerio de Economía y Competitividad for the financial support through the research Project CTM2012-37272. References [1] L.K. Wang, J.P. Chen, Y. Hung, N.K. Shammas, Heavy Metals in the Environment, CRS Press Taylor & Francis, Boca Raton, 2009. [2] Q. Wang, D. Kim, D.D. Dionysiou, G.A. Sorial, D. Timberlake, Source and remediation for mercury contamination in aquatic systems – a literature review, Environ. Pollut. 131 (2004) 323–336. [3] J. Wase, C.F. Forster, Biosorbents for Metal Ions, Taylor & Francis, London, 1997. [4] H.K. Alluri, S.R. Ronda, V.S. Settalluri, J. Singh, S.V. Bondili, P. Venkateshwar, Biosorption: An eco-friendly alternative for heavy metal removal, Afr. J. Biotechnol. 6 (2007) 2924–2931. [5] B. Volesky, Z.R. Holan, Biosorption of heavy metals, Biotechnol. Prog. 11 (1995) 235–250. [6] J. Febrianto, A.N. Kosasih, J. Sunarso, Y.H. Ju, N. Indraswati, S. Ismadji, Equilibrium and kinetic studies in adsorption of heavy metals using biosorbent: a summary of recent studies, J. Hazard. Mater. 162 (2009) 616– 645. [7] K. Chojnacka, Biosorption and bioaccumulation – the prospects for practical applications, Environ. Int. 36 (2010) 299–307. [8] L. Carro, V. Anagnostopoulos, P. Lodeiro, J.L. Barriada, R. Herrero, M.E. Sastre de Vicente, A dynamic proof of mercury elimination from solution through a combined sorption–reduction process, Bioresour. Technol. 101 (2010) 8969– 8974.

L. Carro et al. / Chemical Engineering Journal 229 (2013) 378–387 [9] L. Carro, J.L. Barriada, R. Herrero, M.E. Sastre de Vicente, Adsorptive behaviour of mercury on algal biomass: competition with divalent cations and organic compounds, J. Hazard. Mater. 192 (2011) 284–291. [10] M. Cox, E.I. El-Shafey, A.A. Pichugin, Q. Appleton, Removal of mercury(II) from aqueous solution on a carbonaceous sorbent prepared from flax shive, J. Chem. Technol. Biotechnol. 75 (2000) 427–435. [11] R.H. Crist, K. Oberholser, D. Schwartz, J. Marzoff, D. Ryder, D.R. Crist, Interactions of metals and protons with algae, Environ. Sci. Technol. 22 (1988) 755–760. [12] T.A. Davis, B. Volesky, A. Mucci, A review of the biochemistry of heavy metal biosorption by brown algae, Water Res. 37 (2003) 4311–4330. [13] B. Deng, A.T. Stone, Surface-catalyzed chromium(VI) reduction: reactivity comparisons of different organic reductants and different oxide surfaces, Environ. Sci. Technol. 30 (1996) 2484–2494. [14] E. Romera, F. González, A. Ballester, M.L. Blázquez, J.A. Muñoz, Comparative study of biosorption of heavy metals using different types of algae, Bioresour. Technol. 98 (2007) 3344–3353. [15] E. Romera, F. González, A. Ballester, M.L. Blázquez, J.A. Muñoz, Biosorption of heavy metals by Fucus spiralis, Bioresour. Technol. 99 (2008) 4684–4693. [16] P. Lodeiro, R. Herrero, M.E. Sastre de Vicente, Thermodynamic and kinetic aspects on the biosorption of cadmium by low cost materials: a review, Environ. Chem. 3 (2006) 400–418. [17] V.J.P. Vilar, C.M.S. Botelho, R.A.R. Boaventura, Methylene blue adsorption by algal biomass based materials: biosorbents characterization and process behaviour, J. Hazard. Mater. 147 (2007) 120–132. [18] L. Carro, R. Herrero, J.L. Barriada, M.E. Sastre de Vicente, Mercury removal: a physicochemical study of metal interaction with natural materials, J. Chem. Technol. Biotechnol. 84 (2009) 1688–1696. [19] R.S. Bai, T.E. Abraham, Studies on enhancement of Cr(VI) biosorption by chemically modified biomass of Rhizopus nigricans, Water Res. 36 (2002) 1224–1236. [20] E. Rubín, P. Rodríguez, R. Herrero, M.E.S. Vicente, Adsorption of methylene blue on chemically modified algal biomass: equilibrium, dynamic, and surface data, J. Chem. Eng. Data 55 (2010) 5707–5714. [21] Y.Z. Fu, T. Viraraghavan, Dye biosorption sites in Aspergillus niger, Bioresour. Technol. 82 (2002) 139–145. [22] R.E. Wing, Corn fiber citrate: preparation and ion-exchange properties, Ind. Crop. Prod. 5 (1996) 301–305. [23] D.O. Cooney, Adsorption Design for Wastewater Treatment, Lewis Publishers, Boca Raton, Fl., 1999. [24] B. Volesky, Sorption and Biosorption, BV Sorbex, St. Lambert, Quebec, 2003. [25] P. Lodeiro, A. Fuentes, R. Herrero, M.E. Sastre de Vicente, CrIII binding by surface polymers in natural biomass: the role of carboxylic groups, Environ. Chem. 5 (2008) 355–365. [26] L. Carro, J.L. Barriada, R. Herrero, M.E.S. de Vicente, Adsorptive behaviour of mercury on algal biomass: competition with divalent cations and organic compounds, J. Hazard. Mater. 192 (2011) 284–291. [27] E. Rubín, P. Rodríguez, R. Herrero, J. Cremades, I. Bárbara, M.E. Sastre de Vicente, Removal of methylene blue from aqueous solutions using as

[28]

[29]

[30]

[31]

[32] [33]

[34]

[35]

[36]

[37]

[38]

[39] [40]

[41]

[42] [43]

387

biosorbent Sargassum muticum: an invasive macroalga in Europe, J. Chem. Technol. Biotechnol. 80 (2005) 291–298. P. Lodeiro, C. Rey-Castro, J.L. Barriada, M.E. Sastre de Vicente, R. Herrero, Biosorption of cadmium by the protonated macroalga Sargassum muticum: Binding analysis with a nonideal, competitive, and thermodynamically consistent adsorption (NICCA) model, J. Colloid Interface Sci. 289 (2005) 352–358. P. Lodeiro, B. Cordero, Z. Grille, R. Herrero, M.E. Sastre de Vicente, Physicochemical studies of Cadmium (II) biosorption by the invasive alga in Europe, Sargassum muticum, Biotechnol. Bioeng. 88 (2004) 237–247. E. Rubín, P. Rodríguez, R. Herrero, M.E. Sastre de Vicente, Biosorption of phenolic compounds by the brown alga Sargassum muticum, J. Chem. Technol. Biotechnol. 81 (2006) 1093–1099. A. Ozer, G. Akkaya, M. Turabik, The biosorption of Acid Red 337 and Acid Blue 324 on Enteromorpha prolifera: the application of nonlinear regression analysis to dye biosorption, Chem. Eng. J. 112 (2005) 181–190. W.J. Weber Jr., A.M. Asce, J.C. Morris, Kinetics of adsorption on carbon from solution, J. Sanit. Eng. Div. Proc. Am. Soc. Civil Eng. 89 (1963) 31–59. P. Pavasant, R. Apiratikul, V. Sungkhum, P. Suthiparinyanont, S. Wattanachira, T.F. Marhaba, Biosorption of Cu2+, Cd2+, Pb2+ and Zn2+ using marien green macroalga Caulerpa lentillifera, Bioresour. Technol. 97 (2006) 2321–2329. P. Lodeiro, J.L. Barriada, R. Herrero, M.E. Sastre de Vicente, The marine macroalga Cystoseira baccata as biosorbent for cadmium (II) and lead (II) removal: kinetic and equilibrium studies, Environ. Pollut. 142 (2006) 264–273. K.R. Hall, L.C. Eagleton, A. Acrivos, T. Vermeule, Pore and solid diffusion kinetics in fixed bed adsorption under constant-pattern conditions, Ind. Eng. Chem. Fundam. 5 (1966) 212–223. P. Lodeiro, R. Herrero, M.E. Sastre de Vicente, The use of protonated Sargassum muticum as biosorbent for cadmium removal in a fixed-bed column, J. Hazard. Mater. B137 (2006) 244–253. M. Zabihi, A.H. Asl, A. Ahmadpour, Studies on adsorption of mercury from aqueous solution on activated carbons prepared from walnut shell, J. Hazard. Mater. 174 (2010) 251–256. F. Yardim, T. Budinova, E. Ekinci, N. Petrov, M. Razvigorova, V. Minkova, Removal of mercury (II) from aqueous solution by activated carbon obtained from furfural, Chemosphere 52 (2003) 835–841. S. Chiarle, M. Ratto, M. Rovatti, Mercury removal from water by ion exchange resins adsorption, Water Res. 34 (2000) 2971–2978. R. Herrero, P. Lodeiro, C. Rey-Castro, T. Vilariño, M.E. Sastre de Vicente, Removal of inorganic mercury from aqueous solutions by biomass of the marine macroalga Cystoseira baccata, Water Res. 39 (2005) 3199–3210. L.Y. Chai, Q.W. Wang, Q.Z. Li, Z.H. Yang, Y.Y. Wang, Enhanced removal of Hg(II) from acidic aqueous solution using thiol-functionalized biomass, Water Sci. Technol. 62 (2010) 2157–2166. C. Jeon, W.H. Höll, Chemical modification of chitosan and equilibrium study for mercury ion removal, Water Res. 37 (2003) 4770–4780. J.L. Barriada, R. Herrero, D. Prada-Rodríguez, M.E. Sastre de Vicente, Interaction of mercury with chitin: a physicochemical study of metal binding by a natural biopolymer, React. Funct. Polym. 68 (2008) 1609–1618.