Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.mgumus.chem.msu.ru/publication/2016/2016-volikov-etal.pdf Äàòà èçìåíåíèÿ: Fri Dec 11 11:44:36 2015 Äàòà èíäåêñèðîâàíèÿ: Sat Apr 9 23:24:19 2016 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 20

Chemosphere 145 (2016) 83e88

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Chemosphere
journal h omepage: ww w.elsevier.com/l ocate/ch em o s phere

Nature-like solution for removal of direct brown 1 azo dye from aqueous phase using humics-modified silica gel
Alexander B. Volikov a, Sergey A. Ponomarenko Kirk Hatfield c, Irina V. Perminova a, *
a b c

a, b

, Andrey I. Konstantinov a,

Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1-3, Moscow 119991, Russia Enikolopov Institute of Synthetic Polymeric Materials, RAS, Profsoyuznaya Str. 70, Moscow 117393, Russia Engineering School of Sustainable Infrastructure and Environment, University of Florida, Gainesville 32611, FL, USA

highlights
Silanol humic derivatives are obtained by reaction with aminoorganosilane in water. Silanol humic derivatives are immobilized onto silica gels using aqueous sorption. Humics-modified silica gels efficiently remove trisazo dye from aqueous phase. Similar approach can be used for removal of azo dyes released to aquatic environments.

graphical abstract

article info
Article history: Received 16 March 2015 Received in revised form 4 September 2015 Accepted 19 November 2015 Available online xxx Handling Editor: Min Jang Keywords: Azo dye Direct brown 1 Removal Silica gel Humic substances Organosilanes Immobilization

abstract
The objective was to estimate suitability of humics-modified silica gels for adsorptive removal of the Direct Brown 1 trisazo dye from aqueous phase. The major advantage of the proposed adsorbents is that of an ecologically sound procedure of immobilizing silanized humic derivatives onto silica gel in aquatic solutions. The silanized humic derivatives, in turn, are obtained without a use of organic solvent by reacting natural humic materials from peat and coal with 3-aminopropyltriethoxyorganosilane in water. These silanized derivatives are surface active and are capable of self assembling into humic adlayers at the water solid interface. A use of this approach allows for immobilization of up to 220 mg of humic materials per 1 g of SiO2. The adsorption capacity of humics-modified silica gels with respect to the Direct Brown 1 trisazo dye varied from 3.5 up to 8.8 mg per 1 g of SiO2. The maximum sorption obtained for the silanized derivative with 50% modification degree was comparable to adsorption capacity of activated coal to this dye (7.5 mg gþ1). The results of this adsorption study, warrant further studies of azo dye removal from aqueous environments. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Azo dyes account for more than 50% of all the produced dyes (Chequer et al., 2011). As a result, they are the most common group of synthetic colorants released into the environment (Chung and

* Corresponding author. E-mail addresses: ponomarenko@ispm.ru (S.A. Ponomarenko), kirk.hatfield@ essie.ufl.edu (K. Hatfield), iperm@org.chem.msu.ru (I.V. Perminova). http://dx.doi.org/10.1016/j.chemosphere.2015.11.070 0 045-6535/© 2015 Elsevier Ltd. All rights reserved.


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Stevens, 1993; Maguire and Tkacz, 1991; Saratale et al., 2011; Solis et al., 2012). Many azo dyes and products of their degradation (e.g., aromatic amines) are toxic and expose mutagenic and carcinogenic effects on living organisms that raises serious environmental concerns (Mathieu-Denoncourt et al., 2014; Pereira and Alvis, 2012). Adsorption is one of the most effective processes of dye removal from contaminated water widely employed by industries (Allen and Koumanova, 20 05; Babel and Kurniawan, 20 03; McKay et al., 1980). Recent research has focused on developing cost-effective alternatives to activated carbon (Mohan et al., 20 02; Yagub et al., 2014). Many have reported the feasibility of using various low cost adsorbents such as peat, lignite, clay, zeolites, coal fines, fly ash, different agricultural by-products and biosorbents (Crini, 20 06; Gupta and Suhas, 20 09; Mittal et al., 2013; Ong et al., 20 07; Srinivasan and Viraraghavan, 2010). In this context, the adsorption of pollutants by humic substances (HS) deserves specific attention (Perminova and Hatfield, 2005). HS are natural hyperbranched polyelectrolytes, which possess aromatic backbone highly substituted with functional groups. In nature, humic coatings attached to the surface of mineral particles engender higher sorption capacities for soils and sediments with respect to organic chemicals including azo dyes (MathieuDenoncourt et al., 2014; Olcay et al., 20 09). This suggests that the immobilization of humic coatings onto surface of solid sorbents might increase substantially their sorption affinity for azo dyes. Indeed, in the prior work of Prado et al. (20 03), it was reported that modification of silica gel with humic acid yielded sorbent with enhanced sorption capacity for removal of indigo carmene dye from aqueous phase. However, the process of sorbent preparation included treatment of silica gel with aminoorganosilane in toluene at 140 C during 72 h, which makes the proposed procedure prohibitively expensive and ecologically not safe, while the prepared sorbent might contain traces of organic solvent. The objective of this study was to propose a protocol of preparation of humics-modified silica gel suitable for azo dye removal from aqueous phase, which would be free of the above disadvantages. For this purpose, humic material is first reacted with aminoorganosilane in the aqueous phase; after which, the resultant silanized derivative is immobilized onto silica gel surface. Both reactions are carried out in aqueous phase that exclude residual contamination of the sorbent with organic solvent. Schematically the proposed approach is presented in Fig. 1. Sorption properties of the prepared humics modified silica gel were demonstrated on azo dye Direct Brown 1. This dye was chosen as a representative of the sulfonated azo dyes which are highly water soluble and most refractory to biodegradation (Olcay et al.,

2009). 2. Material and methods 2.1. Materials Direct Brown 1 (C.I. 30045) azo dye belonging to direct application class and trisazo chemical class was employed as the test dye. Its molecular formula is C31H22N8Na2O6S, and molecular weight is 680.615. Structural formula is shown in Supplementary Materials. Direct Brown 1 was purchased from SigmaeAldrich. Stock solution of the dye was prepared in MilliQ water at concentration of 200 mg/L. All other reagents used were of analytical reagent grade. Two types of humic materials were used in this study. The first one was commercially available potassium humate (Sakhalin Humate) isolated from leonardite and designated as CHS. Peat sodium humate (PHS) was obtained from low moor peat by single extraction with 1 M NaOH. 3-aminopropyltriethoxysilane (APTES) was purchased from Penta Ltd. (Russia, Moscow). It was of technicalgrade and used without further purification. Silica gel 60 was purchased from Merck and had specific surface area of 540 m2 gþ1. 2.2. Functionalization of humic materials using APTES A wide range of reagent ratios were used for modification of humic materials with APTES. The reaction stoichiometries were calculated on the basis of carboxyl group available in CHS and PHS. The content of carboxyl groups was determined using Ca-acetate method (Swift, 1996) and accounted for 2.7 and 2.9 mmol gþ1 for CHS and PHS, respectively. Given that 1 g APTES contains 4.5 mmol of amine-groups, a 1:1 M ratio of COOH and NH2 groups was reached by treating 1 g of CHS with 1.67 g (1.75 mL) of APTES. The reaction was run at three different APTES-to-humics molar ratios, nominally: 0.2, 0.5, and 1.0. These ratios corresponded to 20, 50, and 10 0% modification degree of carboxyl groups available within the humic backbone. Prior to synthesis, a weight of potassium or sodium humate (CHS or PHS, respectively) was dissolved in distilled water in a beaker while stirring with a magnetic stirrer. A required aliqoute of APTES was added dropwise to the obtained humate solution under continued stirring, and pH of the reaction mixture was adjusted to 4 with 5 M HCl. Then, the solution was transferred to a round bottom flask, and rotor-evaporated to dryness. The resulting products were stored in sealed vials. They looked like dark brown amorphous powders. The names of the samples obtained were constructed as CHS-APTES-X or PHS-APTES-X, where X is modification degree. In

Fig. 1. Schematic reaction pathways for synthesis of silanized humic materials using 3-aminopropyltriethoxysilane (APTES) in aquatic medium and of their immobilization onto the silica gel surface in aqueous phase.


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85

total, 3 samples were obtained for CHS and 3 samples e for PHS. 2.3. Characterization of the functionalized humic materials Elemental analyses (C, H, N) were performed on Vario EL analyzer. The Si content was determined spectrophotometrically using Specord M40. Ash content was determined manually. Oxygen content was calculated as a difference. Size Exclusion Chromatography (SEC) analyses were conducted according to Perminova et al. (1998). The SEC column was 25 á 20 0 mm packed with Toyopearl HW-55S. Phosphate buffer (0.028 M, pH 6.8) was used as a mobile phase at a flow rate of 1 mL minþ1. The SEC column was calibrated using sodium polystyrenesulfonates (PSS), kDa: 14.0; 20.7; 45.1; and 80.84. Blue dextran (20 0 0 kDa) served as a void volume probe, and acetone e as a permeation volume probe. 2.4. Sorption of silanol humic derivatives onto silica gel Sorption experiments were conducted at room temperature (T ¼ 22 ± 2 OC) in the presence of air on the day light using silica gel as sorbent. Solutions of humic materials at concentrations of 0.01e4 g Lþ1 were prepared by dissolving corresponding amounts of solid samples in several drops of concentrated NaOH and adjusting volumes to 10 mL with phosphate buffer (0.028 M, pH 6.0). An aliquot (10 mL) of each solution was added with 0.1 g of silica gel. Resulting suspensions were placed into an overheadshaker for 1 day. To measure dissolved concentration of humic derivatives, the optical density of the solutions was monitored at 254 nm using UVevis spectrometer (Cary 50, Varian Inc., U.S.A.). The calibration curves were plotted for each humic derivative used in this study to account for a substantial decrease in light absorbing capacities of the derivative along with an increase in its modification degree. This was an expected change due to an increase in aliphatic character of the highly modified humic materials. The amount of HS sorbed was calculated as shown below: Qsorb. ¼ (0 þ w) V/m, (1)

2.6. Sorption of Direct Brown 1 onto silanol-humics treated silica gels Sorption of Direct Brown azo dye on the modified silica gel was studied using equilibrium isotherm technique. The dye solutions were prepared in distilled water in the concentration range from 5 to 20 0 mg Lþ1. The aliqoutic parts of these dye solutions (10 mL) were placed into 15 mL centrifuge tubes containing 50 mg of silica gel (or modified silica gel) and agitated on the overhead shaker during 24 h. All experiments were run at 22 ± 2 C. Then, the suspensions were centrifuged and the supernatant was analyzed for dye content by measusring its optical density at 380 nm. The sorbed amount of the dye (Qsorb) was calculated using same equation as in case of HS (1). 3. Results and discussion Modifications of coal and peat humates (CHS and PHS, respectively) with organosilane were carried out in water. This is a principal difference to our prior works (Karpiouk et al., 2012; Perminova et al., 2012) where we demonstrated feasibility of efficient silanization of humic materials in dimethyl formamide (DMF), but searched instantly for alternative to this solvent to avoid contamination of the final product. Here we propose a solution to this problem by using water as a solvent and amine-functionalized organosilane (e.g. 3-aminopropyltriethoxysilane (APTES)) for the purpose of modification. Rationale behind was that amine group of organosilane does not react with water, but it is capable of binding to abundant carboxyl-groups of humic materials dissolved in water yielding surface active humic derivatives functionalized with silanol groups. The corresponding modifications were carried out at three different molar ratios of APTES to HS varying from 0.2 up to 1.0 on the basis of carboxylic acidity of the initial humic material. Surface activity of the silanized derivatives was characterized using equilibrium sorption onto silica gel. The corresponding sorption isotherms are shown in Fig. 2. The obtained derivatives were also characterized by data on their elemental compositions, carboxyl group content, and molecular weight characteristics. Together with sorptive characteristics, they are given in Table 1. The obtained isotherms show clearly that modification of the both humic materials used in this study with APTES enhances drastically their sorption capacity for silica gel: it starts from zero sorption in case of initial humic materials (CHS-K and PHS-Na) and ends up with 20 0 mg gþ1 for the same humic material silanized with APTES at 10 0% modification degree. These data are comparable with sorption affinity of the alkoxysilyl-derivatives of humic materials obtained in organic solvent (Perminova et al., 2012) and exceed sustantially the reported data on sorption capacity of the aminized silica gels with respect to humic acids (80 mg HS$gþ1 SiO2) (Prado et al., 20 03). Given that the isotherms from Fig. 2 are S-type and do not plateau even at maximum concentrations used in this study, they were characterized with point values of maximum sorption (Qmax) and distribution coefficient Kd. The former was calculated as a sorption of HS in mg per 1 g of SiO2 at the maximum HS concentration of 4 g Lþ1, and the latter was determined at the same concentration of HS. The corresponding data are presented in Table 1. It can be deduced from Table 1 that the amount of Si gradually increased with increasing rates of APTES in the reaction mixtures indicating progressive modification of HS. This trend was followed by an increase in C/N ratio indicative of the contribution of nitrogeneous organosilane into humic structure, and by a decrease in the content of free carboxyl groups in the modified humic materials. Molecular weight characteristics of the silanized derivatives indicate an increase in weight average molecular weight (Mw) and

where: qsorb. e amount of HS (mg) sorbed onto 1 g of silica gel (SiO2), 0 and w e initial and equilibrium concentration of HS in the solution (mg Lþ1), resepctively, V e solution volume (mL), m e mass of silica gel (g). 2.5. Preparation of humics-modified silica gel sorbents for azo dye experiments To prepare humics-modified silica gel sorbents, we used three silanol humic derivatives which showed the highest sorption affinity for silica gel (CHS-APTES-50, CHS-APTES-10 0, and PHAAPTES-10 0). The two former derivatives were obtained from coal humic materials, the latter one e from peat humic acid. An aliquot (10 0 mL) of the respective derivative at a concentration of 4 g Lþ1 was added to 1 g of silica gel in a 0.028 M phosphate buffer at pH 6.0 and then shaken for 48 h until sorption equilibrium was reached. Decreases in humic derivative concentration were monitored by measuring its UV-absorbance at 254 nm. The silica-gel was then centrifuged and washed repeatedly with distilled water. The product obtained was then dried in a vacuum desiccator over P2O5 for 48 h and labeled CHS-APTES-50-SiO2, CHS-APTES-10 0-SiO2, and PHA-APTES-50-SiO2. The content of humic material in the resultant products was calculated from the measured loss of the derivative in solution. The content or organic carbon in the same products were determined from the data on elemental composition of the corresponding derivative.


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Fig. 2. Sorption isotherms of the silanized derivatives of coal humic substances (CHS) (A) and peat humic substances (PHS) (B) onto silica-gel. Sorption conditions: 0.028 M phosphate buffer, pH 6.0, 0.1 g SiO2 per 10 mL of HS solution, equilibrium time 24 h, T ¼ 22 ± 2 C.

Table 1 Structural, molecular weight, and sorption characteristics of the humic materials. Name CHS CHS-APTES-20 CHS-APTES-50 CHS-APTES-100 PHS PHS-APTES-20 PHS-APTES-50 PHS-APTES-100
a b

C, % 31.7 32.4 31.6 31 40.9 40.1 38.8 31.4

Si % 6.3 6.4 7.5 8.8 0.6 2.1 4.4 5.5

H/C 1.08 0.9 1.34 1.21 1.2 1.3 1.33 1.46

C/N 0.04 0.05 0.08 0.11 0.08 0.09 0.11 0.14

COOH, mmol gþ 2.7 2.4 2.0 1.8 2.9 2.2 2.4 1.7 ± ± ± ± ± ± ± ± 0.1 0.4 0.3 0.4 0.2 0.4 0.4 0.4

1

Mw, kDa 12.7 16.9 28.8 26.3 28.7 26.3 27.8 62.8

Mw/Mn 3.6 5.4 3.2 10.2 4.5 4.5 4.5 7.6

Q

max

,a mg HS gþ1 SiO

2

Kd,b L g 0.59 27 69 170 0.48 36 107 205

þ1

1 30 194 221 1 144 175 223

Qmax is a sorption value of HS in mg per 1 g of SiO2 at HS concentration of 4 g Lþ1. Kd is a ratio of the HS equilibrium concentration in solution to the sorbed HS in mg per 1 g of SiO2 at an initial concentration of humic derivatives 4 g Lþ1.

Fig. 3. SEC chromatograms of CHS (A), PHS (B) and their silanized derivatives. The SEC column was 25 á 200 mm packed with Toyopearl HW-55S. Phosphate buffer (0.028 M, pH 6.8) was used as a mobile phase at a flow rate of 1 mL minþ1.

polydispersity (Mw/Mn) values of the derivatives along with an increase in modification degree. Inspection of the corresponding SEC chromatograms (Fig. 3) shows that the more substantial shift towards higher molecular weight was characteristic for the CHS derivatives as compared to the PHS derivatives. This could be connected to the lower molecular weight of the parental coal HS (12.7 kDa) versus much heavier PHS (28.7 kDa). The intense peak at the void volume of the column might be connected to formation of colloidal matter in the solutions of silanized derivatives with higher modification degree which is excluded at the void volume of the column. Supposedly, this colloidal matter is silica sol formed as a result of partial hydrolysis of the silane residues within humic derivatives. Structural alterations in APTES-modified HS were characterized

using FTIR spectroscopy on the example of CHS-APTES-10 0 derivative. FTIR spectra of the parental coal HS (potassium humate, CHSK) and APTES are given for comparison (Fig. 4). The typical FTIR spectrum of the silanized derivative is characterized by appearance of two strong absorption bands of SieOeC bonding at 110 0 and 1050 cmþ1, which can be also seen in the spectrum of APTES. The sharp peak of carboxylate positioned at 1560 cmþ1 in the spectrum of parental humate (CHS-K), has broadened up to 1640 cmþ1 in the spectrum of CHS-APTES-10 0, which is indicative of large contribution of amide >C¼O (amide I) usually located at 1630e1695 cmþ1. The presence of amide bond is also confirmed by high peak intensity at 1560 cmþ1 (amide II and carboxylate) and a weak band at 1265 cmþ1 (amide III). The similar changes were observed for all APTES-HS derivatives. They can


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Fig. 4. FTIR spectra of the parental coal HS (potassium humate, CHS), APTES, and the CHS-APTES-10 0 derivative.

indicate that attachment of APTES to HS in aquatic solutions occurs both via covalent bonding and ionic interaction of protonated aminoorganosilane with negatively charged polyanions of HS. This also explains the presence of residual carboxyl groups in the silanized derivatives with the highest modification degree (Table 1). Despite of that, the obtained derivatives are characterized with the same high maximum values of sorption capacity (220 mg HS$gþ1 SiO2) as those reported for the silanized derivatives synthesized in organic solvent (Karpiouk et al., 2012). These values are comparable with the content of HS in the organic rich geosorbents e mollisols, sediments, and others. To estimate efficacy of the humics-modified silica gels obtained with a use of the proposed approach for azo dye removal from aqueous phase, we used batch experimental set up. Direct Brown 1 was used as a representative of sulfonated azo dyes which are the most problematic for water treatment and remediation due to their high solubility in water, low retardation, and stability towards biodegradation (Ali, 2010; Yagub et al., 2014). Silica gels used for this experiment were obtained by modification with the silanized humic derivatives which showed maximum sorption capacity onto SiO2 (Table 1): CHS-APTES-50, CHS-APTES-10 0, and PHS-APTES10 0. Modification of silica gel was conducted in the concentrated solution of the corresponding humic derivative (4 g Lþ1), which enabled immobilization of the largest amount of humic material. Desorption experiments run on thus modified silica gels have shown irreversible sorption of the silanized humic materials onto silica gel: there was no bleaching observed of the sorbed humic material upon multiple washing of the modified silica gels with

Fig. 5. Sorption of Direct Brown 1 trisazo dye onto silica gel modified with coal humic derivatives (CHS-APTES-50 and CHS-APTES-10 0) and peat humic derivative (PHAAPTES-10 0). T ¼ 22 ± 2 C, pH ¼ 6.0, distilled water. Solid to liquid ratio: 10 0 mg SiO2 per 10 mL of dye solution.

both 0.028 M phosphate buffer and distilled water (the data are not shown). Equilibrium sorption isotherms of the Direct Brown 1 azo dye onto the humics-modified silica gels are shown in Fig. 5. The amount of the sorbed dye was determined from a decrease in optical density of the solution at 390 nm which was used for monitoring adsorptive removal of the dye. The obtained isotherms are of L-type and fit well to Langmuir model. From the obtained data it can be concluded that much higher adsorptive removal of the dye was observed onto humics-modified silica gels as compared to non-modified silica gel. At the same time, among the humics-treated silica gels the highest efficacy was characteristic to the silica gel treated with humic derivative of the lowest modification degree - CHS-APTES-50. The value of maximum sorption reached onto this sorbent (8.8 mg gþ1) exceeded those (in mg gþ1) onto coal based sorbents such as lignite coal (4.1), bituminous coal (2.04), charfines (6.4), and activated carbon (7.69 mg gþ1) as reported by Mohan et al. (20 02) under comparable experimental conditions. This makes the developed sorbent of substantial interest for a use in removal of azo dyes from aqueous phase. The lower sorption capacity of Direct Brown azo dye onto the both silica gels treated with the silanized humic derivatives of 10 0% modification degree (3.2 and 3.6 mg per g of SiO2 for CHS-APTES10 0 and PHS-APTES-10 0, respectively) can be explained by a substantial decrease in the pore volume of the treated silica gel due to deposition of the colloidal silica released as a result of hydrolysis of the silanol derivatives which intensifies along with an increase in HS:APTES ratio. This is consistent with the SEC-results (Fig. 3) and with the data of BET measurements which showed a substantial decrease in pore volume of the silica gel at the highest loadings with the humic material. Hence, derivatives of smaller modification degree are preferrable for modification of silica gel. This also complies well with optimization of the expenses for preparing modified silica gel because of a reduced amount of organosilane needed to react with natural humic material. The proposed approach can be realized in two different ways: the first one implies preparation of the humics-coated silica gel as described in this study, without a use of organic solvent, followed by its deployment as a sorbent in columns or in slurries for removing dye from waste water effluents similar to silica gels, coalbased and other low cost adsorbents (McKay et al., 1980; Mohan et al., 20 02; Gupta and Suhas, 20 09); the second one includes in situ deployment of the solutions of silanolized humics for cleaning up dye-contaminated ground water by pumping them into the aquifer. In the second case, the removal of dye from groundwater occurs via its sorption onto humic adlayer which is self-assembled at the wateresolid interface and attached to granular aquifer support via SieOeSi linkages (Karpiouk et al., 2012; Perminova et al., 2012). The first deployment strategy is most suitable for off-site wastewater cleanup, whereas the second strategy might be deployed on site through in situ installation of the humic permeable reactive barriers. In case of the first strategy, regeneration of the humics-modified silica-sorbents might be needed. It can be achieved via treatment with diluted solutions of KOH or NaOH (0.01 M) similar to the protocols reported for chitosan-based sorbents loaded with the anionic diazo-dyes (Shen et al., 2011; Szygula et al., 20 08). This treatment will recycle silica gel by removing humic adlayer with the sorbed azodye. The dye bearing humic waste can be easily compacted by acidification and separation of the humic-dye precipitate, which can be incinerated or biodegraded (Yagub et al., 2014). The regenerated silica gel can be re-modified with silanolized humics and reused for the next cycle. The advantage of using the proposed environmental solutions stems from ecological


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safety of humics-based materials and richness of their low-cost reserves such as lignites, lignins, composts, etc. 4. Conclusion The nature-inspired appoach is proposed to adsorptive removal of the Direct Brown azo dye from aqueous phase based on a use of humics-modified silica gels. The major novelty of this approach is that modification of silica gel was conducted in water by using surface-active silanized humic materials. The latter are natural hyperbrached polyelectrolytes capable of both ionic, hydrophobic, and donoreacceptor interactions due to the presence of numerous functional groups and amphiphilic properties of their structures. As a result, humic adlayers can efficiently adsorb not only azo dyes present in the effluent but concomittant contaminants, such as heavy metals and other organic chemicals, as well. The humic adlayers can also serve as a solid support and redox shuttles for azo dye degrading microorganisms (Cervantes et al., 20 01; Hong et al., 20 07) which increases an added value of their use in dye removal and remediation processes (MartÌnez et al., 2013). Another advantage of the proposed approach is that the silanized humic derivatives can be also employed in situ (e.g., injected into contaminated aquifer) for creating humic adlayers onto granular support. Recently, we have demonstrated feasibility of this approach for removal of plutonium (Shcherbina et al., 2014). To show its applicability for waters contaminated with azo dyes is our nearest goal. Acknowledgments We would like to acknowledge assistance of Dr. Alexander Vyacheslavov from the Department of Materials Science of Lomonosov MSU in conducting BET-measurements. This research was supported by Russian Foundation for Basic Research (grant #13-04-018531-a and #11-03-12177-OFI-M-2011). Supplementary materials Structural formula of Direct Brown 1 azo dye, BETmeasurements and linearized isotherms for the Direct Brown 1 trisazo dye adorption onto modified silica gels are available for this paper. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2015.11.070. References
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