Surface Recognition Directed Selective Removal of Dyes from

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Materials and Interfaces

Surface recognition directed selective removal of dyes from aqueous solution on hydrophilic functionalized petroleum coke sorbents. A supramolecular perspective Pablo Trujillo, Teresa Gonzalez, Joaquin Brito, and Alexander Briceño Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02020 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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Industrial & Engineering Chemistry Research

Selective removal of cationic dyes from aqueous solution directed by surface recognition on non-porous hydrophilic adsorbents derivate from functionalized petcoke is shown 338x190mm (96 x 96 DPI)

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Surface recognition directed selective removal of dyes from aqueous solution on hydrophilic functionalized petroleum coke sorbents. A supramolecular perspective. Pablo Trujillo, a Teresa González, a Joaquín L. Britoa,b and Alexander Briceño. a* aCentro

de Química, Instituto Venezolano de Investigaciones Científicas (IVIC),

Apartado 21817, Caracas 1020-A, Venezuela. bYachay

Tech University. Urcuqui 100119, Ecuador.

Abstract

Novel controlled hydrophilic non-porous carbon-based sorbents from petroleum coke were obtained by using a hydrothermal oxidation treatment of shot coke with different H2O2 concentrations. The ability of resulting carbon materials for the selective adsorption of cationic dyes from aqueous solution directed by surface recognition is highlighted for the first time. This approach provides an eco-friendly, low-cost alternative for the preparation of sorbents with unprecedented properties to achieve remarkable selective adsorption independently of the presence of intrinsic porosity. A further advantage of these sorbents is their efficient regeneration capability and reusability without appreciable changes in their adsorption capacity. Results reveal that the dye adsorption capacity is significantly enhanced on multivalent hydrophilic surfaces in comparison to the pristine surface. In addition, a deep insight is offered into the explanation of selective adsorption processes on hydrophilic coke in function of surface recognition processes based on cooperative multivalent supramolecular interactions such as classical synthons: pyridine-carboxylic acid, OH···N, cation,  and electrostatic interactions. Likewise, this adsorption capacity is highly dependent on the fine-tuning of donor/acceptor binding sites between adsorbate-sorbent. A further type of important supramolecular interaction was observed on the adsorption of dyes onto hydrophobic coke-raw surface, which is controlled mainly by cation and  interactions. Corresponding author: Email: [email protected]; [email protected] (Alexander Briceño) Keywords: Surface recognition, functionalized petroleum coke, selective adsorption, supramolecular chemistry, hydrophobic/hydrophilic surfaces.


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1. Introduction Venezuela produces huge amounts of petroleum coke (petcoke), around 20.000 Ton/day, as a by-product from oil refinery processes.1 This residual product has drawn little attention as an accessible carbon-containing raw material, or for uses as an energy source due to its high calorific power or as an abundant and cheap source of carbon (~90%). Besides, petcoke contains metals as vanadium and nickel, which have many commercial uses in different industries (cement, metal and steel). Carbonaceous matrices derived from biomass and other waste residues have been widely used as an important material family for environment applications as sorbents and/or as active catalytic supports.2-5 However, these carbon sources have little economic value and often present disposal problems. Therefore, there exists a need to value such by-products as an important inexpensive alternative to existing commercial activated carbons. To date, few reports are known on general and satisfactory strategies for the preparation of novel valuable materials from petcoke.6-8 Conventionally, it is accepted that the performance of an adsorbent is highly dependent on its specific area, pore structure and surface chemical functionality.9 Thus, the design of a highly efficient adsorbent is focused on the selectivity to different adsorbate targets and its tuneable adsorption capacity.9-11 In the case of carbon materials, these targets can be reached by control of the pore structure employing different activation routes, precursors and templates.12-15 However, selectivity is limited due to the complexity of the resulting disordered pore structures. On the other hand, the role of surface chemical functional groups in directing the selective removal of chemical substances is well known.15-21 Therefore, the surface modification of carbon materials provides a fascinating way to fine-tune either physical or chemical adsorption processes.16-19 At this point, molecular recognition on hierarchical and complex interfaces and/or surface solid materials should be taken into account.22-23 Since these design requirements could be satisfied by supramolecular chemistry principles,24-25 the transference of supramolecular self-assembly processes from solution to solid platforms has become an exciting topic of research. Thus, we anticipated that the analysis of solid state molecular self-assemblies derived from crystal structure data may reveal the expected changes occurring when a specific substrate approach at the surface, which are closely related to molecular recognition



events. Therefore, we have considered adsorption phenomena as an extended border effect of how the outer molecules interact with complementary groups on well-defined crystals planes at the bulk crystals. In particular, molecular recognition directed by hydrogen bonds synthons26-29 between a specific receptor and a substrate represents one of the most important and exploited supramolecular tools to induce the self-assembly of a wide variety of well-defined functional structures. Interactions such as pyridine-carboxylic acid, pyridinium-carboxilate and OH···N might provide strength and directionality (Figure 1), allowing a better control on the stability and selectivity of the adsorption process.30 In the literature the description of the adsorption process is ambiguous or very poor from a supramolecular viewpoint. Conventionally, the authors describe the adsorption capacity almost exclusively based on texture properties (superficial area and pore structure), discarding in many cases the contribution of surface chemical functionality and/or possible interactions between the surface and the adsorbate. Generally, the use of robust hierarchical interactions (H-bonding supramolecular synthons and cation-) are omitted with great lightness, while the processes are described based on weaker interactions from the energy viewpoint, which leads to incorrect and/or incomplete interpretations. On the other hand, the existence of simultaneous binding capacity via multiple recognition sites either in the substrate or on the surface (multivalency) as well as, the complementary matching of such interactions may result in a synergistic effect, which might dictate a greater affinity than individual interaction or the sum of the corresponding individual interactions.29 Thus, modified surfaces can be modulated to promote selective adsorption processes in function of surface recognition processes based on cooperative multivalent interactions. These cooperative effects can display energetic values higher than the energy associated to classical physisorption processes inclusive can be found in the order of values assigned to chemical adsorption process.31 Such discrepancies about adsorption classification open an interesting discussion about describing adsorption process on functionalized surfaces in an adequate fashion. Therefore, a deep understanding of such substrate-surface interactions is essential in order to design novel selective molecular adsorbents. Likewise, the deliberate control of the number and/or kind of donor/acceptor hydrogen binding ACS Paragon Plus Environment

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sites on the supramolecular periphery may be a key factor to predict the adsorption capacity of any molecule on a specific supramolecular interface. In this context, taking advantage of our ongoing studies on the use of multivalent

interactions

in

the

crystal

engineering

of

supramolecular

assemblies30 in combination with our recent research on the preparation of new composites bearing carbonaceous materials,19, 31 we have envisaged that the use of multivalent hydrophilic surfaces could provide new possibilities for the design of selective binding receptors or sorbents. Herein, we have outlined an opposite alternative using a non-porous solid platform bearing multiple hydrogen bonding sites for the selective removal of organic dyes directed by surface recognition processes via complementary hierarchical interactions based on well-recognized supramolecular synthons. 26-28, 32-35 The potential of such hypothesis is demonstrated by the controlled surface functionalization of a nonporous carbon matrix (delayed-coke, termed here coke-raw), utilizing a sustainable and green low-cost alternative for the production of improved petcoke by means of different hydrothermal oxidation treatments assisted by H2O2. The selection of a non-porous structure allows us to discriminate and/or minimize the contribution of porosity to the adsorption process. In recent years, the contamination of the environment by synthetic dyes has become a worldwide issue,36-39 such concern being due to their toxic and persistent nature and their harmful effects on human health. In this context, the removal of dyes from wastewater by selective adsorbents with high adsorption capacity and tolerant to different dye groups remains a major challenge in environment applications.36-39 In this work, a comparative adsorption study of several dyes onto functionalized coke and coke-raw is reported for the first time, with results offering important insights into the adsorption behaviour on the different surfaces. Also, a deep comprehensive interpretation of the selective removal of chosen dyes from a supramolecular perspective is provided. Thus, it is shown that hierarchical supramolecular interactions can dominate the adsorption processes, depending on the hydrophobic/hydrophilic nature of the sorbent, pH, charge and potential multivalent acceptor/donor capacity of the dye onto the surface.



2. MATERIALS AND METHODS 2.1. Chemicals. All reagents and solvents were purchased from commercial sources and were used without further purification. 2.2. Hydrothermal functionalization of coke-raw. The petcoke used as raw material in this study came from Puerto la Cruz refining plant in Anzoátegui, Venezuela, provided by INTEVEP-PDVSA and coded as CQ-30. The optimization of the hydrothermal oxidation conditions was carried out for coke-raw with H2O2 at different concentrations, temperatures (120 and 180 ºC) and heating times (12 and 24 h). Five different types of functionalized cokes were prepared from coke-raw, which are referred to as coke-blank (no H2O2), coke-0.5, coke-1, coke-2 and coke-3, where the number in the last four refers to the amount (mL) of concentrated H2O2 used for the oxidation of 500 mg of coke-raw. Thus, a 0.5, 1, 2 and 3 mL volume of 30% (V/V) H2O2 solution was used, respectively. The total volume was always adjusted at a constant value of 10 mL with H2O. 2.3. Characterization. Fourier transform infrared (FT-IR) spectroscopy studies were carried out with a Thermo Scientific Nicolet IS10, using KBr pellets in a ratio coke:KBr::1:99 (w/w). X-ray photoelectron spectra were obtained with a Specs GmbH instrument employing a 150 Phoibos kinetic energy analyzer and a non-monochromatic Al K radiation source (1486.6 eV) operated at a power of 350 W. Vacuum in the analysis chamber was better than 5 x 10-10 Pa. High resolution spectra in the C 1s region were taken with a minimum of 20 scans. Curve-fitting of the C 1s high resolution spectra was carried out using the CASA XPS software after subtraction of the Shirley type base line. The software employs a nonlinear least squares algorithm and mixed Gaussian/Lorentzian peak shape of variable proportion. The surface morphology of coke-raw and functionalized cokes were characterised by FEI QUANTA 250 FEG Scanning Electron Microscope. Surface area determinations by nitrogen adsorption using BET method were carried out on a Micromeritics ASAP 2010 Instrument. The wettability characterization of the coke films was performed by employing a OCA 20 contact angle measurement system. The automated micro-syringe system released a liquid droplet having a volume of 5 mL on the substrate surface. The shape of the droplet on the films was recorded by using a coupled digital camera. All the measurements were performed under ambient conditions (i.e. temperature  25 °C) and were completed within a few seconds to minimize the effect of evaporation on the


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contact angle measurements. The standard deviation in the contact angle measurement is  2° 2.4. Organic dye adsorption. The adsorption experiments were carried out at room temperature (30 ºC) in a stirred batch system. Prior to each adsorption assay, the sorbents were kept in the oven at 80 °C for 48 h for complete removal of moisture. Batches of powdered adsorbent (5 mg each) were added to 20 mL of aqueous solutions of MB, TB, MG, MV, MaG, F, Har, MO, MR (Co 2x10-5 mol L-1). These solutions were mechanically agitated for 48 h. Then, the solutions were separated from the adsorbent by centrifugation and the supernatants were taken by means of a syringe. Finally, the amount of dye concentrations at equilibrium were analysed based on the standard calibration curves on a Genesys UV-VIS 10S Spectrophotometer. The following expressions were used to calculate the amount of adsorbed dye at equilibrium (qe, mg g-1) or the removal efficiency (R). qe = (Co –Ce) x V / m R(%) = (Co –Ce) x 100% / Co where Co and Ce (mg L-1) are the initial and equilibrium concentration of the solutions, respectively, V (L) is the volume used of the solution and m (g) is the adsorbent mass. The experiments were performed without adjusting the external pH. However, the final pH after adsorption assays was measured, showing that the pH values of solution during the adsorption process were practically unchanged (~6.5). The experiments were performed in triplicate, which showed that total uncertainly was less than 5% of the initial concentrations. To get the adsorption isotherm, 10 mg of adsorbent coke-raw or coke-2 were added into different concentrations solutions of MB in the 2x10-6 - 4x10-5 mol L-1 (20 mL) concentration range. The maximum adsorption capacity of both adsorbents was calculated using the Langmuir, Freundlich and Temkin adsorption isotherm models after adsorption equilibrium. To determine the adsorption capacity of MB on coke-raw and coke-2 at various pHs, the pH of MB solutions was adjusted with 0.1 mol L-1 HCl or 0.1 mol L-1 NaOH aqueous solution. Selective organic dye adsorption experiments from binary mixtures: Typically, a fresh coke-2 sample (5 mg) was put in the mixed dyes solutions (20 mL). The mixed dye solutions were prepared from equal volumes of the two corresponding solution with



the same concentrations (2x10-5 mol L-1). UV/Vis spectra were measured to determine the selective adsorption ability of coke-2 at given time intervals. Dye release and competitive assays: Firstly, coke-2 was saturated with 20 mL of MB or MV solutions (Co  2x10-5 mol L-1), the adsorbent was separated by centrifugation and the liquid was discarded. The solid obtained was added to 20 mL of ethanol and saturated aqueous NaCl solution. The dye-releasing was monitored by UV/Vis spectroscopy.

Figure 1. Chemical molecular structures of recognized supramolecular synthons based on robust hydrogen bonding synthons: pyridine-carboxylic acid, OH···N, carboxylatepyridinium (I-III) and cation (IV). 3. RESULTS AND DISCUSSION 3.1. Functionalization of shot coke (coke raw) under hydrothermal conditions and characterization of raw and functionalized cokes. A simple route for functionalization of carbon surfaces consists of the generation via chemical oxidation of hydrophilic groups based on oxygenated moieties (e.g. carboxylic, epoxide, hydroxyl, quinines, among others),12,14-18 which are widely exploited as anchoring groups of inorganic species as a previous step for the preparation of composites, catalysts and/or adsorbents. Many usual synthetic methods require the use of strong and hazardous oxidants (e.g. inorganic acids o bases, chlorates, etc) and very harsh conditions to induce the desired chemical changes on the surface. Therefore, alternative and eco-friendly methods for the oxidation and functionalization of carbon matrixes are


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of great significance.16-18 In this direction, in our previous studies about the synthesis and characterization of new materials based on metal oxide decorated carbon, the functionalization

ability

of

carbon

surfaces

via

hydrothermal

method

was

demonstrated.27 In this work, we show an attractive green approach to functionalize different carbon surface materials via a hydrothermal method assisted by H2O2. Different synthetic parameters (reaction time, H2O2 concentration and temperature) and their effect on the surface of shot petcoke were evaluated and the best conditions to obtain the highest oxidation of the surface were found to be 24 h at 180 °C using 2 mL of 30% (V/V) H2O2. Such conditions were chosen as optimal due to the highest acidity found for the solids evaluated at the different experimental conditions. Figure 2 shows SEM images of raw and functionalized cokes. Coke-raw particles depicted a rudimentary and irregular morphology with a wide size distribution in the micrometer (m) range. All the particles have rough textures with heterogeneous surfaces. Some particles display the characteristic globular array usually found for coke residues. Comparative analysis from SEM images of functionalized cokes revealed that there were no obvious morphological and texture changes from coke-raw after applying different hydrothermal functionalization conditions. Figure S1 (see ESI) displays the results of FT-IR spectroscopy which was used to explore the chemical functionalization of the materials obtained from the hydrothermal-treatment under different oxidation conditions, with variable H2O2 concentration and temperature. FT-IR spectra analysis revealed that the highest H2O2 concentrations produced a gradual increment of some absorption bands related to the presence of oxygenated groups on the surface. Specifically, an increment was observed of the relative intensity of the bands due to O– H (3000–3500 cm−1), C=O and C–O groups at 1605 and 1300-1200 cm−1, respectively. Additionally, the appearance of a new band related to the C=O stretching of carboxyl groups at 1694 cm−1 was also observed as a shoulder. Interestingly, a comparative analysis between the spectra obtained from the functionalization with the highest H2O2 concentration used and that one of the coke-blank submitted to the same experimental conditions (24 h and 180 ºC, without H2O2) revealed an increase of the relative intensity of the absorption bands related to oxygenated moieties. Likewise, this surface oxidation effect is higher with the rise of the hydrothermal treatment temperatures from 120 to 180 ºC (Figure S1). The incorporation of such oxygenated groups on the carbon matrix leads to an improvement of the hydrophilicity as a function of the increased amount of H2O2 concentration, and it can be easily



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dispersed in water in contrast to the intrinsic hydrophobicity of coke-raw (Figure 2). The surface wettability of the raw and functionalized cokes was examined by placing a pure water drop of 5 mL. As shown in Fig. 2, the coke-raw film bears a water contact angle of   102.9°, which is attributable to the lack of oxygenated groups on the surface, similar water contact value was found for coke-H2O (  96.9°). In contrast, the spontaneous spreading of a 5 mL water drop after 7-10 s is illustrated on functionalized cokes (0.5, 1, 2 and 3). In such samples, the static contact angle cannot be determined;   0° (Fig 2). This fact indicates that functionalized surfaces exhibit a total wetting behaviour, which is associated a strong attractive forces between adsorbate-sorbent due to a high degree of oxidation on the surface in all the materials. In consequence, such treated cokes show a fine-tune hydrofilicity related to the increase of amount of diverse oxygenated chemical groups on the surfaces.

(a)

(b)

  102.9°

Coke-raw

(d)

(c)

  0° Total Wetting

Coke-2

Figure 2. SEM micrographs of coke-raw (a-b) and functionalized coke-2 (c-d). Photographs of suspensions in water of coke-raw (top) and coke-2 (bottom), showing the hydrophobic and hydrophilic nature of coke-raw (  102.9°) and modified coke (  0°), respectively


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3.2. Chemical surface characterization. In order to evaluate quantitatively the functionalization degree and type of functional groups on the coke-raw surface and the hydrothermally treated cokes, the Boehm titration method was employed,21,40 the corresponding results being shown in Table 1. These results are in agreement with the evidence on the increase in oxygen functionalities provided by FT-IR spectra. Thus, there is a strong increment of both carboxylic and phenolic groups with H2O2 concentration between 0.5 and 2 mL of H2O2, while the lactone moieties content does not vary greatly in the same range. For the sample treated with the highest H2O2 dose of 3 mL, there is a sharp increase of the amount of lactones while phenolic groups diminish strongly. Generally, the total acidity of the surface increases with the H2O2 amount except for the 3.0 mL dose. Note that even the hydrothermal treatment of the blank coke (without H2O2) leads to an increase of carboxylic groups and a moderate rise of total acidity. These results indicate that a controllable functionalization process can be attained under mild conditions by using a green oxidant. This possibility provides an interesting eco-friendly alternative to induce chemical incorporation of oxygenated functionalities on a given carbonaceous matrix in comparison to conventional oxidation methods. Further evidence for the efficiency of coke-raw H2O2 –induced hydrothermal oxidation was provided by point of zero charge (pzc) measurements by the pH-drift method.41 Table 1 compares the pzc values of the raw and hydrothermally functionalized petcokes at different oxidant concentrations. Increase of the H2O2 concentration resulted in significantly lower pzc of the resulting cokes than that of the coke-raw. These values gradually become more acidic from neutral pzc 7.04 to 3.07, due to the increasing concentration of covalent functionalized surface with acidic groups (Figure S2). This corroborates the presence of hydrophilic groups as potential anchoring sites able to interact with self-complementary chemical groups directed by hydrogen bonding interactions.



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Table 1. Content of oxygenated moieties determined by Boehm titrations. Values of point of zero charge (pzc), BET surface area (SBET) and percent MB removal capacity by raw, blank and hydrothermally functionalized petcokes at different oxidant concentrations; MB concentration (Co  2x10-5 mol L-1) with sorbent loading of cokes 0.25 mg mL-1 in all the cases. Samples Carboxylic Lactones mmol/g mmol/g

Phenolic Total acidity mmol/g mmol/g

pzc

Ads (%) 36  1

aS

BET (m2 g-1)

bAPV

mic (cm3g-1)

cAPD

(Å)

Coke-raw

0.042

0.208

0.160

0.410

7.04

Coke-blank

0.082

0.209

0.179

0.469

4.97 14.2  0.3 2.40.4 0.0072 224.9

Coke-0.5

0.115

0.211

0.239

0.565

4.11

47  1

3.70.1 0.0166 147.1

Coke-1

0.136

0.247

0.359

0.741

4.01

61  2

5.70.1 0.0212

93.9

Coke-2

0.209

0.167

0.567

0.942

3.07

91  3

8.90.1 0.0281

77.2

Coke-3

0.192

0.343

0.123

0.659

4.03

75  2

5.40.4 0.0118 150.3

a:

SBET, Surface area determined by BET method.

b:

APV, Average pore volume.

c:

APD, Average pore diameter by DBJH method.

4.90.1 0.0141 143.6

3.3. XPS Measurements. X-ray photoelectron spectroscopy (XPS) is a technique well suited for the study of the surface composition of solid materials.42 In particular, for the case of carbonaceous materials, the types of oxygen functionalities can be explored by examining either the C 1s or O 1s regions.15,

42

All the analyzed

samples showed the presence, in addition to C and O, of the following elements in the indicated range of contents (as atomic %): S (2.0-2.2); N (1.5-1.8); Si (0.6-0.8); V (0.11-0.12%); Na (< 0.1%). As expected, oxygen content varied as follows: coke-raw (4.1 at.%); coke blank (6.8%); coke-2 (9.1%); coke-3 (9.0%). Thus, increasingly stronger oxidizing treatments resulted in higher surface O contents. Figure S3 shows typical results for curve-fitting (“deconvolution”) of the C 1s signals of coke samples. The main peak in the C 1s region with a binding energy (B.E.) of 284.5 ± 0.1 eV corresponds to graphite-like carbon. Peaks at 286.3 ± 0.2 eV are assigned to C-O (ether, hydroxyl) species, while those at 287.9 ± 0.2 eV are due to C=O (carbonyl, carboxyl) moieties. Table 2 collects the results for selected samples. In agreement with the results from Boehm titration, carboxyl type groups (C=O) exist in lower concentrations than phenolic plus lactones (mostly C-O) moieties. However, the


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concentration of C=O augments from coke-raw to coke-blank and are highest and similar for coke-2 and coke-3, results that parallel those found for the carboxylic functionality from Boehm titration. Table 2. Quantitative results of curve-fitting of C1s signal of samples of coke raw and modified cokes. Signal Binding Energy Assignation Sample Coke-raw Coke-blank Coke-2 Coke-3

Peak A 284.5 ± 0.1 eV Graphitic

Peak B 286.3 ± 0.2 eV C-O

Peak C 287.9 ± 0.2 eV C=O

77.7 % 74.2 % 71.1 % 70.7 %

17.6 % 20.4 % 22.1 % 22.1 %

4.7 % 5.4 % 6.8 % 7.2 %

3.4. Surface Area Measurements. The nitrogen adsorption–desorption isotherms of all the sorbents can be categorized as type II (b) characteristic of non-porous or macroporous aggregate powders (Figure S4). These solids present low apparent BET surface area, which varies in the range of 2.4-8.9 m2 g-1 with average pore volume values in the range of 0.0072-0.0281 cm3 g-1. Table 1 shows the values corresponding to BET areas of the raw and hydrothermally functionalized cokes submitted to different oxidant concentrations. 3.5. Molecular recognition and supramolecular complementarity at surfaces. To illustrate our approach based on the selective removal of organic dyes directed by surface recognition processes via complementary supramolecular synthons, we chose nine dyes with distinctive structural features, including charged dyes, different sizes and shapes and the presence of functional groups with potential affinity with the functionalities available on the sorbents surface (Figure 3). All the functionalized cokes were tested as sorbents for dye removal, including the blank and raw cokes. In a preliminary test probe, methylene blue (MB) was used as a target molecule due to its unique structural features from the supramolecular viewpoint. This molecule is a cationic heterocyclic dye, which displays two hydrogen acceptor sites able to interact with surface groups via archetypal self-complementary heterosynthons such as pyridine···carboxylic acid, OH···N and ionic cation interactions (Figure 1).



CH3

CH3 S

N

H3C

N

CH3

H2N

S

N

CH3

H3C

NO2 S

CH3

CH3

CH3

CH3

N

N

N

N

H3C

CH3

Cl

Methylene Green

Toluidine Blue

CH3

CH3 N

N

N

H3C

Methylene Blue

H3C

CH3

N Cl

Cl

N

CH3

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HO

O

OH

CH3

Cl

Cl

O O

H3C

N

CH3 Fluoresceine

Malachite Green

Methylene Violet

CH3

HOOC N

H3CO N H Harmaline

CH3

N

H3C

N N

+

Na-O3S

N

N

CH3

N

H3C Methyl red

Methyl orange

Figure 3. Chemical molecular structures of the organic dyes evaluated for the adsorption processes, showing chemical groups able to interact with the complementary functional groups on the surface of the different sorbents. Table 1 lists the MB adsorptive capacity of all the prepared sorbents. In general, these values showed a gradual increase of adsorption as a function of the extent of functionalization degree of the solids. The adsorption capacities of the sorbents toward MB were determined by UV-vis spectroscopy (Figure 4(a)). However, while MB adsorption increases continuously from coke-raw/coke-blank to coke-0.5 up to coke-2, there is a significant diminution of adsorption when passing from coke-2 to coke-3. On the other hand, the untreated coke-raw shows a larger MB adsorption value than cokeblank. The adsorption values were a linear function of total acidity - if that of cokeblank is dismissed -, and good correlations with the concentration of both carboxylic and phenolic groups were observed (Figure 4(b)). As anticipated, this tuneable adsorptive capacity is associated to the cooperative effect of COOH and OH (phenolic) groups onto the surface. This showed that hydrophilic solids are suitable for the removal of MB from solution even in lack of apparent porosity in the solids.


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(a) 1

Chemical groups (mmol g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Total acidity

0.8

Carboxylic

0.6

Phenolic 0.4

Lactones 0.2 0 20

40

60

80

100

% Removal MB (b)

Figure 4. (a) Comparative adsorption capacity of MB on functionalized cokes (coke0.5, coke-1, coke-2 and coke-3), and coke-raw. Visible spectra of MB adsorption at equilibrium using a solution of MB (Co  2x10-5 mol L-1) show that the intensity of MB decreases in function of the greater acidity of the sorbents). The sorbent loading was maintained at 0.25 mg mL-1 for all cases, contact time: 48 h. (b) Correlation between surface concentration of acid functionalities derived from Boehm titration results and MB adsorption (%). 3.6. Adsorption isotherms for MB on Coke-raw and coke-2.The adsorption isotherm is a well-known effective method to study the adsorption ability of an adsorbent and understand the interactions between the adsorbate/adsorbent systems. Figure 5 shows the variation of the amount of MB adsorbed at equilibrium on coke-raw and coke-2, with the data fitted by the Langmuir, Freundlich and Temkin isotherm



models43-45 (Table 3). The regression coefficient R2 obtained from the different adsorption models revealed different adsorption mechanisms depending on the kind of coke used (Figs. 5(a-d)). For coke-raw, the Langmuir model coefficient (R2 =0.9709) was much higher than that obtained with the Freundlich and Temkin models, suggesting that the Langmuir isotherm correlates well with the experimental data for this adsorbent, which indicates that the adsorption is due to monolayer coverage. In contrast, the adsorption mechanism for coke-2 is better described by the Temkin model (R2 =0.9743). This result suggests that the adsorption is dominated by ionic interactions between adsorbent and adsorbate. This mechanism is consistent with the presence of possible charge-assisted hydrogen bonds interactions directed by carboxylatepyridinium or carboxylate-ammonium interactions. These results revealed that the dye adsorption capacity was significantly enhanced on hydrophilic surface in comparison to pristine surface, being favourable and spontaneous both adsorption processes over cokeraw and coke-2 (Table 3). In particular, the value for Coke-2 was 4.7 times higher than coke-raw. 20

Langmuir 1.2 1

Coke-2 Coke-raw

12

0.8 Ce/qe

qe(mg/g)

16

8

coke-2

0.6

coke-raw

0.4 4

0.2

(a)

0 0

1

2

3

4

Ce(mg/L)

5

6

(b)

0 0

7

1

2

3

4

5

6

7

Ce (mg/L)

Temkin

Freundlich

20

3

16

2.5 2

12

coke-raw coke-2

8

Ln qe

qe(mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

1.5

coke-raw

1 4 0 -1

-0.5

0

0.5

1

1.5

coke-2

0.5

(c) 2

(d)

0 -0.6

-0.1

0.4

0.9

1.4

1.9

Ln Ce

Ln Ce

Figure 5. Comparative adsorption isotherms of MB with coke-raw and coke-2 as adsorbents at 30 °C (a), using a solution of MB (Co  2x10-5 mol L-1) , the sorbent loading was maintained at 0.5 mg mL-1 for both cases, contact time: 48 h. Adsorption isotherms for MB over coke-raw and coke-2 (a) and the ﬁtting curves with Langmuir (b) Temkin (c) and Freundlich (d) models for the adsorption of MB.


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Table 3. Langmuir, Freundlich and Temkin model parameters for adsorption of MB over coke-raw and coke-2 Isotherm models Langmuir Model =

+

Freundlich Model lnqe = ln KF +

ln Ce

Temkin Model qe =

ln KT +

ln Ce

Parameters and units qm (mg/g) KL R2

Coke-raw

Coke-2

5.865 3.223 0.9743

26.882 0.374 0.9271

n

3.926

1.456

KF R2 b (J/mol) KT R2

4.131 0.4882 2.507 75.415 0.4937

6.912 0.8961 424.584 3.78 0.9709

3.7. pH dependence of dye adsorption. Given the acid-base nature of the adsorbate/sorbent system, specifically in the case of the adsorption of the MB molecule, the pH should be an important parameter affecting the adsorption process. The removal percentage of MB at 30 ºC for different pH values on functionalized coke-2 and cokeraw was measured at equilibrium (Figure 6). The adsorbed amount (%) shows a gradual increase with increasing pH between 2 and 8 for the case of coke-2. Such behaviour could be associated with the following factors: a) in the case of MB adsorption directed by hydrogen bonds interaction in the range of pH 2-8, MB can protonate, which restrains the hydrogen bond interactions. In particular, the protonation of nitrogen of the pyridyl ring prevents the pyridyl-carboxylic acid interaction, which is considered as a strong and robust one in supramolecular chemistry (Synthon type I, Figure 1). In consequence, the stabilisation of MB on the surface should decrease considerably at the lower pHs. b) Likewise, at pH < pzc the surface became positive, which leads to the electrostatic repulsion between MB and the surface of coke-2. The opposite effect should be expected at pH > 7, where the surface is negative and the adsorption capacity should increase via attractive electrostatic interactions. However, the adsorption changes in the basic pH range are very low, which suggests that electrostatic interactions are minimized by directional hydrogen bond interactions. To provide additional evidence for the real contribution of pyridyl-carboxylic acid heterosynthon and/or the cation interaction on the adsorption process, an additional experiment was carried out. The sample of Coke-2 used for the adsorption of MB was saturated with NaCl in order to trigger the ionisation of carboxylic acid groups. This ionic-exchange was expected to provide a more negative surface and thus favour the adsorption leading



to a switch from a neutral heterosynthon pyridyl-carboxylic acid to ionic heterosynthon pyridinium-carboxylate (Synthons type I and III, Figure 1). However, the MB removal percentage at equilibrium decreased from 91  3 to 56  1%. A plausible explanation is that the excess of Na+ ions generated interference due to possible repulsive electrostatic interactions between Na+ and pyridinium-ammonium di-cations and/or it was a strong competitor for hydrophobic adsorption sites, leading to ionic shielding effect of the surface, thus avoiding the stabilisation by cation contribution.34,46 In contrast, when coke-raw is used, pH exerts a lower effect on MB removal (%). This could be related to the low capacity to produce either high positive or negative charges due to the low density of ionisable groups on the surface in function of pH. Therefore, the adsorption varies slightly on the coke-raw from 25.8  0.5 to 32.6  0.3% in the same range of pH 2-8, being pH 5 the optimal value for maximum adsorption of MB (40.0  0.6%). At this pH, MB is likely to be protonated, leading to the formation of a di-cationic species. Therefore, the adsorption could be controlled almost exclusively by the increasing strength of the cation interactions on the surface. Besides, the addition of NaCl caused MB release to the solution, which gradually turned darker blue. A similar desorption behaviour after NaCl addition was observed for all the cationic dyes adsorbed on coke-raw. These results provide further evidence for the hypothesis of the stabilisation by cation adsorbate-surface interactions, which might be one of the key interactions for the relatively high selectivity either of functionalized or coke-raw toward cationic dyes adsorption. Coke-2

100

% Removal MB

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

Coke-raw

80 60 40 20 0 0

1

2

3

4

pH

5

6

7

8

9

Figure 6. Effect of solution pH on the percentage of adsorbed Methylene blue over coke-2 and coke-raw (Co  2x10-5 mol L-1). The sorbent loading was maintained at 0.25 mg mL-1 for both cases, contact time: 48 h.


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3.8. Adsorption dyes properties on functionalized coke-2. Encouraged by such results, and in order to provide a proof-of-concept demonstration, we evaluated the adsorption of similar cationic dyes, selecting coke-2 as adsorbent, which contains the highest surface acid groups density. The adsorption of the following compounds was evaluated: Toluidine blue (TB), Methylene green (MeG), Methylene violet (MV) Malachite green (MaG) and Harmaline (Har). These Methylene blue-derivatives were chosen to assess various effects, such as the change of a dimethylamine N(CH3)2 group by an amine NH2 group in order to incorporate either acceptor or donor hydrogen capacity for the case of TB. This change was expected to affect the adsorption in comparison to MB due to the possible competition either of NH2 or pyridyl groups with the carboxylic groups. Likewise, for MeG, it was expected/anticipated that the presence of a NO2 group on the ring should have some effect on cation and/or  interactions due to its electrodrawing effect and/or the possibility of breaking the planar structure of the by steric effect with N(CH3)2. On the other hand, MV and MaG display similar structures, although MV contains three N(CH3)2 groups, this is, one more than MaG. Therefore, the acceptor hydrogen bonding capacity is similar to that of MB but higher than that of MaG; in consequence, the presence of a higher or lower number of acceptor hydrogen bonding groups should significantly affect the stability of its adsorption onto the surface. Taking into account such structural features and assuming that the adsorption is effectively directed by surface recognition, we expected that the trend in the adsorptive capacity would be larger in the molecules that have a higher acceptor/donor capacity. Figure 7 shows the coke-2 adsorptive capacity of TB, MV, MeG, MaG and Har. These values showed similar trends to that observed for MB under equivalent experimental conditions. Such results showed that this solid is suitable for the removal, in varying degrees, of these cationic dyes from aqueous solution. As anticipated, the adsorptive capacity is closely related to the existence of the number of donor/acceptor sites on the molecule, being greater the adsorption for those molecules with more possibilities to interact via multiple supramolecular interactions with the surface. Thus, the observed trend revealed the following order of adsorptive capacity in function of the removal percentage determined at equilibrium: MB  TB > MeG  MV > MaG > Har. Another interesting result is the appreciable dye adsorptive capacity of coke-raw, despite showing the lowest acidity among the solids. This solid showed lower



adsorption than the functionalized cokes; however, adsorption trends on coke-raw were similar to those observed for coke-2 (Figure 7). These results strongly suggest that the adsorption processes might be controlled by an additional mechanism different to the adsorption directed by hydrogen bond interactions. Given the hydrophobic nature of coke-raw, it would seem that such process could be directed almost exclusively by cation and/or  interactions. Conventionally, authors do not discriminate between these distinct interactions, being cation stronger (5-80 KJ mol-1) than simple  interactions (0-50 KJ mol-1). Recently, cation has been shown to be a robust supramolecular interaction either in solution or in the solid state32-33 (Synthon IV, Figure 1). Although the cation

interaction has not been usually considered or

described as an important hierarchical interaction that contributes to the stabilisation of the molecules on the surface, we contend here that it should be taken into account for the correct and/or full description and interpretation of these adsorption processes. To evaluate the strength of the cation interaction in the adsorption process a new set of assays were carried out with anionic Methyl Orange (MO), Fluorescein (F) and Methyl Red (MR) dyes using either raw or the most hydrophilic functionalized coke-2 under the same experimental conditions. To our surprise, neutral and anionic dyes showed lower affinity and/or adsorption than cationic dyes, the adsorption for neutral dyes being greater than for anionic ones, independently of whether the surface is hydrophobic or hydrophilic, even when the molecules display potential groups available to interact with the surface via hydrogen bonds and  interactions. The complete adsorption series is disclosed in Figure 7. The observed trend revealed the following order in function of the removal percentage determined at equilibrium: MB  TB > MeG  MV > MaG > Har > MR > F > MO. These processes can be visually monitored by the decolouration of the different solutions and quantified by UV-vis spectroscopy (Figs. 8(a)-(h)).


Page 20 of 33

Page 21 of 33

100 90 80 70 60 50 40 30 20 10 0

Coke-2 Coke-raw

% Removal Dyes

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Comparative adsorption capacity of dyes on coke-2 and coke-raw at equilibrium for the entire series of dyes, using solutions with similar concentrations for each dye (Co  2x10-5 mol L-1). The sorbent loading was maintained at 0.25 mg mL-1 for both cases, contact time: 48 h.

In particular, Harmaline is a heterocycle similar to MB and displays either donor or acceptor hydrogen bonding capacity (pyridyl, pyrrole and methoxy groups). However, its adsorptive capacity is lower than that of cationic MB-derivatives, even in comparison to MaG dye, which displays only two binding sites. A similar behaviour was observed for MR, which contains two functional groups able to interact by selfcomplementary supramolecular interaction via supramolecular synthons between carboxylic and N(CH3)2 groups of the surface. We believe that such difference in the adsorption capacity is associated to the lack of positive charge on the molecular structures. These results reveal a highly selective removal of cationic dyes versus anionic dyes. The combination of such results suggest that the selective removal of cationic dyes is closely related to the existence of a synergistic effect due to simultaneous, multiple supramolecular interactions between absorbate/sorbent, where the cation interaction might also play a significant role on the adsorption process.


0.8

TB Initial

0.6

MB at equilibrium

0.6

TB at equilibrium

0.4

(a)

0.2

Absrobance

MB Initial

550

600

650

Wavelength (nm)

700

0.4

(b)

0.2 0

750

400

450

500

550

600

650

Wavelength (nm)

700

750

0.8

MV Initial

0.8

MG Initial

0.6

MV at equilibrium

0.6

MG at equilibrium

0.4 0.2

(c)

Absorbance

Absorbance

500

0.4 0.2

(d)

0

0 400

450

500

550

600

650

700

400

750

450

500

550

600

650

700

750

Wavelength (nm)

Wavelength (nm) MaG Initial

0.8

MR Initial

0.6

MaG at equilibrium

0.6

MR at equilibrium

Absorbance

0.8

0.4 0.2

(e)

0 400

450

500

550

600

650

Wavelength (nm)

700

0.4 0.2

(f)

0

750

300 350 400 450 500 550 600 650 700 750

Wavelength (nm)

0.8

Har Initial

0.8

MO Initial

0.6

Har at equilibrium

0.6

MO at equilibrium

0.4 0.2

(g)

0 300

350

400

450

500

550

600

650

700

750

Absorbance

Absorbance

Page 22 of 33

0.8

0

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Absorbance


0.4 0.2

(h)

0 350

400

Wavelength (nm)

450

500

550

600

Wavelength (nm)

650

700

750

Fig 8. UV-vis spectra of dye adsorption on coke-2 of MB (a), TB (b), MV (c), MeG (d), MaG (e), MR (f), Har (g) and MO (h), using solutions with similar concentrations for each dye (Co  2x10-5 mol L-1). The sorbent loading was maintained at 0.25 mg mL-1 for all cases, contac time: 48 h. Insets: photographic images showing the characteristic colour of the solutions before (left) and after (right) dyes adsorption at equilibrium.

3.9. Desorption and reusability of functionalized coke-2. The regenerability and reusability of coke-2 was evaluated. Given the hydrophilic nature of the sorbent, ethanol was used to remove MB and MV from saturated solids (MB/coke-2 (a) and MV/coke-2 (b)). Simple washing for the removal of cationic dyes was feasible and


Page 23 of 33

effective, leading to dye-releasing up to initial adsorbed values according to UV-vis spectra, which shows that the intensity of MB and MV increase with time (Figure 9(a)(b)). The above results indicated that the dyes are released from the surface due to the competition exchange adsorption of the dye by the polar solvent and the ability to form stable hydrogen bonds with the hydrophilic sites on the surface. There were no obvious changes in adsorption efficiency of the regenerated coke-2, at least after three times. This ability is highly desirable in adsorption processes, and is closely related to the lack of internal porosity. 0.8

0.8

0h

0.6

3h

0.4 0.2

(a)

0

Absorbance

1h

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0h 1h

0.6

3h

0.4 0.2

(b)

0 350

400

450

500

550

600

650

700

750

400

450

Wavelength (nm)

500

550

600

650

700

750

Wavelength (nm)

Figure 9. Representative UV-vis absorption spectra of MB and MV release from MB/coke-2 (a) and MV/coke-2 (b) in ethanol solution (V = 20 mL). Insets: Photographic images show the colour of the solution at 0 h (left) and after 3 h (right) time of addition of ethanol.

3.10. Selective adsorption studies of binary mixtures on functionalized coke2. Studies on the selective removal of dyes from binary mixtures in solution help to understand the parameters involved in the adsorption processes. Thus, we performed two types of competitive sorption experiments employing coke-2 as adsorbent. In the first place, selective adsorption assays of a mixture of dyes chosen on the basis of charge affinity were evaluated. Secondly, mixtures of cationic dyes were also studied, in order to evaluate their ability and efficiency and their distinctive donor/acceptor capacities, for example, molecules with a multivalent available acceptor/donor sites are expected to be stronger competitors than molecules with lower acceptor/donor capacity. The selective adsorptions of the following binary mixtures were tested: MB/MO, MB/F, MB/MV, MB/MaG and MB/MR. Observations were made by the naked eye and quantification performed by UV-vis spectroscopy (Figure 10). For the assays bearing cationic-anionic dyes mixtures the colour resulting from the combination of the dyes and the adsorbent revealed the preferential and selective adsorption of MB over MO, MR and F, respectively. In such cases the colour of the initial solution gradually



changed and became the colour characteristically associated to the pure anionic or neutral dye after addition of coke-2, such changes indicating that cationic MB was selectively removed from solution. A similar effect was also observed for the mixtures of cationic MB/MV and MB/MaG dyes (Figure 10(a)-(f)). Nevertheless, concomitant adsorption of both components was observed for such mixtures. Whereas neutral or anionic dyes are not adsorbed significantly by the hydrophilic coke, competitive assays from mixtures of cationic dyes revealed interesting results, confirming that the molecules with a higher number of donor/acceptor sites show a remarkably larger affinity for the hydrophilic surface than the molecules with lower donor/acceptor capacity. In both cases MB is adsorbed in a higher percentage in comparison to MV and MG, with relative percentage values at equilibrium of MB: 45  1% and MV: 39.2  0.7%; MB: 49  1% and MG: 19.9  0.4 (%), respectively. This effect is even more evident for MV, which contains an additional N(CH3)2 group compared to MaG molecule with only two N(CH3)2 groups. MV is thus preferentially adsorbed over MaG (MV: 37.2  0.5% and MaG: 12.3  0.2%), and after reaching equilibrium, the solution displays the characteristic colour of MaG due to its lower extent of adsorption on the surface. Additional evidence was provided in order to validate the anticipated selective affinity of cationic dyes based on multivalent donor/acceptor interactions with coke-2. After single component adsorption experiments, samples saturated with MV on coke-2 (MV/coke-2), were put in contact with a solution of MB (10 mL at 2x10-5 mol L-1). Interesting competitive cationic-exchange adsorption processes were thus found. The cation in solution with the highest multivalent supramolecular capacity has the ability to partially exchange the adsorbed dye from the surface and the concomitant adsorption of MB, where the sum of the simultaneous adsorption of both dyes is higher than that of the separate adsorption of either component (Figure 11). This result suggests the existence of different heterogeneous adsorption sites, which could be fitted to specific targets depending on size, shape, electronic and charge complementarity. In consequence, the adsorption capacity of the sorbent might be higher in presence of the mixture of different molecular structures. UV-vis spectra show that the intensity of MB in solution decreases with time. Simultaneously, the solution gradually exhibits an increase of the characteristic colour of MV.


Page 24 of 33

0h

0.6

0.5

1h

0.5

0.4

3h

0.3

MB

MO

0.2

Absorbance

0.6

0.1 350

400

450

500

550

600

650

700

750

0.3

3h

0.3

F

0.2

(c)

0.1

(b) 400

450

500

550

600

Wavelength (nm)

0.6

650

700

750

0h

MaG

0.5

Absorbance

Absorbance

0.4

MR

0.2

350

1h

MB

3h

MB

0

0h

0.5

1h

0.4

Wavelength (nm)

0.6

0h

0.1

(a)

0

MB

1h 3h

0.4 0.3 0.2 0.1

(d)

0

0 350

400

450

500

550

600

650

Wavelength (nm)

700

350

750

400

450

500

550

600

650

Wavelength (nm)

700

750

0.6

0h

1

0.5

1h

0.8

1h

0.6

3h

0.4

MV

0.3

3h

MB

0.2 0.1

Absorbance

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Absorbance

Page 25 of 33

MV

MaG

0.4 0.2

(e)

0

0h

(f)

0 350

400

450

500

550

600

Wavelength (nm)

650

700

750

350

400

450

500

550

600

Wavelength (nm)

650

700

750

Figure 10. Representative UV-vis absorption spectra for evaluating the selective adsorption capability of coke-2 toward MB from mixed dyes: MB/MO (a), MB/F(b), MB/MV(c), MB/MaG (d), MB/MR (e) and MV/MaG (f), using solutions with similar concentrations (Co  2x10-5 mol L-1) and similar solution volume (10 mL) for each dye. The sorbent loading was maintained at 0.25 mg mL-1 for both cases, contac time: 48 h. Insets: photographic images show the characteristic colour of the solutions before (left) and after (right) dyes adsorption at equilibrium.

At the present stage, based on all the aforementioned experimental findings, some insights can be drawn to explain the results of the different adsorption assays: a) The results indicate that hydrophilic sorbent coke-2 exhibits a remarkable selectivity, depending on charge and supramolecular periphery of the dyes, which can be highly selective and tuneable for cationic dyes, allowing the discriminated affinity on the surface by simply modifying supramolecular domain and its ability to interact via complementary interactions. b) Such selectivity strongly suggests the existence of a real



positive synergistic effect as a direct consequence of the concomitant participation of multiple interactions between absorbate/sorbent, where the cation interaction might also play a significant role in the adsorption process. In the case of coke-2, it displays an interesting amphiphilic feature, with the molecular adsorption occurring either by hydrogen bonds or hydrophobic interactions. c) On the other hand, the adsorption process on hydrophobic surfaces is dominated mainly by cation, ··· and electrostatic interactions. Thus, several possibilities of supramolecular interactions on the different surfaces are outlined. For example, for the adsorption of MB on coke-2 at least three kinds of interactions can be described based on the combination of supramolecular synthons type I, II and IV, whereas the adsorption on coke-raw can be described by the synthon IV (Figure 1). These interactions are disclosed in Figure 12(a)(b).

Figure 11. Representative UV-vis absorption spectra of competition cationic-exchange adsorption processes from saturated MV/coke-2 in presence of 10 mL of MB (Co  2x10-5 mol L-1) at 30 °C. Inset: photographic images show the characteristic colour of the solutions before (left) and after (right) dyes adsorption at equilibrium.

The comparative adsorption capacity of MB on some activated carbon and other non-porous carbon sorbents is shown in Table 4. These solids have been obtained under different activation conditions, showing distinctive textural and surface properties. In particular, the sorbents obtained in this study show some advantages in comparison to the preparation of activated carbons, for instance, they are obtained by a one-pot process, using green and mild conditions. This approach provides a useful route to


Page 26 of 33

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controlled surfaces with the ability to direct selectivity toward cationic dyes, together with the possibility of obtaining easily regenerable and reusable sorbents from simple ethanol washing. Such structural and functional features are highly attractive from an economic perspective. Table 4. Adsorptive capacity toward Methylene blue of different carbon-based sorbents Adsorptive capacity (mg g-1)

Ref

453.09

[47]

Carbon Black

35.09

[47]

Charcoal

62.7

[48]

Coconut husk based AC

66

[49]

Activated carbon (AC)

9.81

[50]

Apricot stones-AC 750 °C

4.11

[51]

Walmut shell-AC 750 °C

3.53

[51]

Corncob based AC

0.84

[52]

Fruli, Hydrothermal method

83

[53]

Coke-2, Hydrothermal method

26.8

This work

Coke-raw (Coke 30)

5.9

This work

Sorbents NCS* Sucrose and H2SO4

*Nonporous carbon -based sorbent

(a)

(b)

Figure 12. Schematic representation of possible supramolecular interaction modes between hydrophilic coke and MB adsorbate. Combining distinctive hydrogen bonding interactions: (a) via heterosynthon carboxylic acid-pyridine cation and ···. (b) OHpyridine cation, and ···. The presence of heteroatoms such as S and N were omitted in both proposed surface models for simplicity.



4. Conclusions In summary, in this contribution we have described the successful development of novel hydrophilic sorbent materials from the controlled surface modification of pristine petcoke by using hydrothermal oxidation treatment with different H2O2 concentrations. We have demonstrated the potential of the resulting non-porous hydrophilic solids as inexpensive and easily regenerable materials for the selective removal of cationic dyes from aqueous solution directed by surface recognition. This approach provides an interesting low-cost, green alternative as a potential substitute for activated carbon as sorbent. Based on our comparative adsorption studies of different dyes using cationic or anionic molecules on functionalized and coke-raw together with competitive preferential adsorption test of dye molecules, we have attempted to give a picture on how different types of hierarchical supramolecular interactions work on distinctive surfaces in a unifying manner from a supramolecular viewpoint. Thus, we have contributed with a deep insight into a better understanding and/or full interpretation of the adsorption processes on modified surfaces beyond classical interpretation based on the texture properties of sorbent materials (porosity, superficial area, pore volume and average pore diameter).31, 54 It is worth noting that such adsorption processes can occur efficiently on adsorbents, independently of the presence of intrinsic porosity.30 In the last decades, a wide variety of supramolecular synthons have become wellrecognized reliable tools to assemble diverse structures of multicomponent arrays. We expect that the present findings open up endless opportunities for the design of tailormade sorbents based on classic molecular recognition processes to direct the selective adsorption and/or separation of complex mixtures of molecules, including multistep adsorption processes due to tuneable surface in each adsorption step, as well as enantiomeric mixtures separation. Further work on the potential use of other functionalized surfaces is currently being explored on diverse environmental and chemistry applications based on specific supramolecular synthons. Conflicts of interest There are not conflicts to declare Acknowledgements The authors would like to thank to FONACIT for financial support through project PEI2013002269. Also, we thank C. Avendaño, M. Morgado, L. Cubillan, J. Arevalo-Fester,


Page 28 of 33

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Yraida Diaz, Oscar Corona and Brianny Zambrano (Intevep-PDVSA) for technical assistance. We appreciate language revision of the manuscript by staff from Instituto de Lenguas, Universidad Nacional de San Luis, Argentina

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Supplementary Material Information available: FT-IR spectra point charge zero determination (pzc), XPS spectra analysis and N2 adsorption and desorption and pore size distribution of the samples for coke-raw (a), coke-blank (b), Coke-0.5 (c), coke-1 (d), coke-2 (e) and coke-3 (f) of all the sorbents.


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TOC Graphic

Selective removal of cationic dyes from aqueous solution directed by surface recognition on non-porous hydrophilic adsorbents derivate from functionalized petcoke is shown.


Surface Recognition Directed Selective Removal of Dyes from

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