COMPARATIVE STUDY OF THE LOCAL VEGETABLE ACTIVATED CARBON WITH COMMERCIAL ONES FOR ADSORPTION OF METHYLENE BLUE

. Activated carbons have great applicability in the conditioning of wines: discoloration, removal of foreign taste and smell, correction of organoleptic parameters, etc. The purpose of this work was to compare the structural and sorption characteristics of local vegetal activated carbon obtained from apricot stones (AC-C, Republic of Moldova) with that of commercial activated carbons (Granucol ® BI/GE/FA, Germany). The physico-chemical characteristics of studied activated carbons have been evaluated by standard methods (nitrogen sorption isotherms, IR spectroscopy, pH value of activated carbons suspension etc.) and the adsorption capacity by using methylene blue dye as a reference substance. Experimental data were analysed by theoretical models: Langmuir and Equilibrium isotherm models, and pseudo-first-order, and pseudo-second-order kinetic models. The adsorption capacity of the local activated carbon (AC-C, 690 mg/g) is higher by 30% than that of activated carbons from Granucol ® series (approx. 535 mg/g).


Introduction
Activated carbons due to their distinct qualities like high specific surface area, high porosity, and desired surface functionalization have considerable applications in adsorption/separation, pollution removal, water treatment, food and beverages processing, etc. [1,2].
During the production of wines, among the important processes are the technological treatments with adjuvant materials, these being administered with the aim of facilitating the clarification and stabilization of the wine.Today there is a complex of adjuvant substances that are used in the wine industry, differing according to their nature and specific properties.Most of them adsorb macromolecular compounds on their surface or facilitate the formation of conglomerates, which leads to sedimentation and clarification of the wine [3].Among the adjuvant substances used in winemaking, there are activated carbons, in a wide variety; these act by surface tension (adsorption) and serve not only for clarification, but more often for decolorization and removal of unpleasant odours and tastes from wine.In general, the activated carbons used in winemaking should meet certain quality criteria, among them are: the brightening power of methylene blue should be 70-75%, ash content 6%, iron content no more than 2% [3].
In winemaking, commercially available activated carbons of Granucol ® type (FA, BI, GE) (ERBSLÖH, Germany), which are of vegetable origin and contain up to 20% of bentonite in the mixture, have gained a wide spread [4][5][6][7][8].The activated carbons of Granucol ® type are especially recommended to decolorize wines and to remove mold, yeast and barrel odours, as well as the extremely offensive mouse odour.Depending on treatment, the activated carbons of Granucol ® type are applied as follow [4]: Granucol ® FAfor the elimination of reddish off-colours due to browning reactions; Granucol ® BIfor the reduction of tannins and polyphenols and for the elimination of brownish high-colour; Granucol ® GEfor the off-taste and off-odours adsorption.
The practice of using different organic substances as reference materials for testing adsorbents and evaluating their adsorption capacity is quite widespread.Among others methods, the methylene blue test has become popular because it is easily applicable, needs no special equipment and yield accurate results.Methylene blue is a basic dye belonging to the group of quinone imine dyes, containing a phenothiazine ring.In aqueous solutions, methylene blue is in the form of a cation with aligned bonds and has an affinity for materials of amphoteric nature [9].Methylene blue dye has been used for determining surface specific area of various materials for several decades [9].
The present work aimed to compare the adsorption properties of a local vegetal activated carbon obtained from apricot stones (AC-C, Republic of Moldova) with that of commercial activated carbons (Granucol ® BI/GE/FA, Germany) used in winemaking by using methylene blue as model compound.Here are presented the preliminary results.
Methylene blue (MB) dye was purchased from Fluka.MB is a cationic dye with a molecular mass of M= 319.85 g/mol and molecule size of 1.69x0.74x0.38 nm, it is stable and often used as a standard dye/sorbate for determining the sorption characteristics of adsorbents towards organic molecules or dyes (Figure 1) [9].

Methods
Some physico-chemical characteristics of the activated carbons (humidity, ash content and pH value of the activated carbon suspension) were determined by standard methods [11].In brief, the activated carbon humidity (U, %) was determined by drying the samples in an oven at a temperature of 110°C until the constant mass; the ash (A, %) content was determined by calcination of the activated carbon samples in an oven at 850°C until the constant mass; the pH value of the activated carbon suspension was determined by equilibrating 0.1 g of activated carbon with 100 mL double-distilled water (solid:liquid ratio 1:1000) and recording the pH value of the suspension after 24 h of contacting-stirring.
Textural parameters of the activated carbons were determined from the adsorption isotherms of nitrogen at 77 K by means of the AUTOSORB-1MP equipment (Quantachrome) [12].The specific surface area (SBET) was determined using the Brunauer-Emmett-Teller (BET) equation.The total pore volume (Vtotal) was determined from the amount of nitrogen gas adsorbed at the relative pressure of 0.99.In order to determine the volume of the micropores (Vmicro) the t-method was used, while the volume of mesopores (Vmeso) was calculated from the difference between the total volume and the volume of micropores.The NLDFT (Non-Linear Density Function Theory) equilibrium model was employed for evaluating the effective (predominant) radius of the pores and the distribution of the pores volumes as a function of the pores radii.
Infrared (FTIR) spectra of activated carbons were recorded on the FT-IR Spectrometer Spectrum 100 (PerkinElmer, USA) in ATR mode.
Adsorption of methylene blue.MB stock solution was prepared by dissolving a weighed quantity of C16H18ClN3S in distilled water and MB solutions in range of 50-1000 mg/L were obtained by diluting the stock solution.
The adsorption process of methylene blue was studied under static conditions; the adsorption isotherms were determined from aqueous solutions at room temperature by contacting the dry activated carbon samples with a solution of the methylene blue of different concentrations (in ascending order) at the same solid: liquid ratio (1:1000), under stirring for a reasonable period of time in order to reach the adsorption equilibrium.Temperature was controlled by a thermostatic bath and monitored throughout each experiment (temperature meter WTW 538).The equilibrium concentrations of methylene blue were determined with the JENWAY 6505 UV-Vis spectrophotometer at λ= 665 nm (after separation of phases by filtration), and the adsorption value, expressed in mg/g, was calculated from Eq.( 1).

𝑎
where, C0 is the initial concentration of MB (mg/L); Ce the equilibrium concentration of the MB (mg/L); V is the volume of contact solution (L); m the mass of the dry activated carbon (g).
The evaluation of specific surface area (SMB, m 2 /g) of adsorbents can be performed by determining the maximal adsorption of methylene blue on the adsorbent surface, using the Eq.(7).
where, amax= Q0 which is the methylene blue adsorption capacity in the monolayer (mg/g) at 25°C, determined from Langmuir isotherm; AMB is the area occupied by one methylene blue molecule (m 2 per molecule); NA is the Avogadro number (6.02·10 23 molecules per mol); MMB is the methylene blue molar mass (319.85 g/mol).

Characterization of activated carbon samples
The activated carbon samples from the Granucol ® series (FA, BI, GE) have a low moisture content, 2-3%, ash content of 16-20%, and the surface of the activated carbons is slightly acidic, being in accordance with the values presented in the technical passport of these carbons (Table 2) [4].Unlike the activated carbon samples from Granucol ® series, the local activated carbon (AC-C) has a low ash content (approx.6%), and the surface is weakly basic (Table 2).The characteristics of the porous structure (SBET, Vmicro, Vmeso, Vtotal) determined from the nitrogen sorption-desorption isotherms reveal that the activated carbon sample obtained from apricot stones, under laboratory conditions, are very close to those of the activated carbons from the Granucol ® series, the specific surface is between the values 1220-1385 m 2 /g, the total volume of the pores 0.95-1.2cm 3 /g, and the share of mesopores is approx.65-70% of the total pore volume (Table 3, Figures S1-S4).
= k 1 (q e −q t ) qt and qe are the amount of adsorbate sorbed per mass of sorbent (mg/g) at any time and equilibrium, respectively; k1 is the rate constant (min −1 ).
(2) [13] Pseudo-secondorder kinetic model (Ho and McKay) k2 is the rate constant, qt is the adsorbate uptake capacity at any time t. ( Separation factor: qe is the adsorption capacity (mg/g); Ce is the adsorbate concentration at equilibrium (mg/L); KL is the Langmuir constant of the sorption equilibrium (L/mg); Q0 is the monolayer formation capacity (mg/g).C0 is the initial concentration (mg/L); KL is Langmuir constant (L/mg).
(4) (5) [15] Freundlich   =     1/ KF is the empirical constant that provides information about the adsorption capacity of the sorbent; n is the empirical constant indicating the intensity of the sorption process.

Adsorption of methylene blue
Kinetics and adsorption isotherms of methylene blue at different temperatures on activated carbons of Granucol ® type (FA, BI, GE) are presented in Figures 2-7 and for local activated carbon AC-C in Figures 8 and 9.The concentration of methylene blue decreases rapidly during the first 5-10 min of phase contact, after which the speed of the methylene blue adsorption decreases until a relatively constant value is reached.After 10 minutes of phase contact (at the initial concentration of methylene blue of 500 mg/L) activated carbon Granucol ® FA adsorbs approximately 80% of MB from the solution, Granucol ® Bi approximately 50%, Granucol ® Ge approximately 65%, and activated carbon AC-C adsorbs the most, 90% of methylene blue molecules.As seen from the kinetic curves, the adsorption equilibrium of methylene blue on activated carbons Granucol ® FA, Granucol ® GE and AC-C is established after approx.100 min (Figures 2, 6 and 8), but for Granucol ® BI the equilibrium is established after approx.250 min (Figure 4).The kinetic adsorption curves represent dependencies that are linear at small contact times between phases, and then become curved.According to the literature, this indicates that diffusion within the sorbent grain controls the overall process rate [23].The experimental data of methylene blue adsorption on studied activated carbons (Granucol ® type and AC-C) are very well described by the pseudo-second-order kinetic model, since the calculated adsorption values are close to the experimental ones, and the value of the correlation coefficient (R 2 ) is close to 1 compared to the adsorption values and R 2 obtained for the pseudo-first-order kinetic model (<0.894) (Tables 4 and 5).According to some authors, the adsorption process of MB on Granucol® type activated carbons proceeds in two stages, the first of which is mentioned physical adsorption [24].Results regarding the adsorption of MB on activated carbons from hazelnut shells and walnut shells indicate that the adsorption process occurs through both physical and chemical adsorption [25,26].
The adsorption isotherms of methylene blue on activated carbons of Granucol ® series (FA, GE, BI) at temperatures of 25, 35 and 45°C are presented in Figures 3, 5 and 7.The experimental adsorption isotherms for local activated carbon AC-C are presented in Figure 9.According to the results, the adsorption capacity of the studied activated carbons for methylene blue increases with the temperature, but insignificantly.
Furthermore, the adsorption capacity of AC-C activated carbon at 25°C is approximately 30% higher compared to Granucol ® activated carbons.
The adsorption process of methylene blue dye on activated carbons has been described by the Langmuir and Freundlich adsorption isotherm models, which are mathematical models that describe how the adsorbed substance interacts with the adsorbent, providing information about the nature of interactions between the adsorbate and the adsorbent.In the Langmuir adsorption isotherm model, it is assumed that the maximum adsorption corresponds to a monolayer of molecules saturated on the adsorbent surface.According to this theory, once a molecule has occupied an active site, no further sorption can take place on that site, and possible molecular interactions between active sites of the adsorbent are also excluded [15].The Freundlich adsorption isotherm model describes non-ideal and reversible adsorption, unrestricted by the formation of a monolayer of adsorbed molecules on the adsorbent surface.The model is used to describe adsorption in heterogeneous systems, taking into account the molecular interactions between active sites as well as those between adsorbate molecules [16].The adsorption equilibrium data for methylene blue on studied activated carbons from Granucol ® series are best described by the Langmuir isotherm model and correlate well with literature data (Table 6) [23,25,26].According to adsorption isotherms, at temperature of 25°C, the Granucol ® type samples adsorb approx.535 mg/g of methylene blue (Figures 3, 5 and 7).The maximum monolayer adsorption capacity of Granucol ® type activated carbons (calculated according to the Langmuir model) correlates well with values of adsorption capacity obtained experimentally (Table 6).For all activated carbons from Granucol ® series the RL value fell within the limits of 0.001÷0.04,which means that the process of adsorption of methylene blue on the studied samples is favourable [23].
The adsorption isotherms of methylene blue dye on AC-C activated carbon were obtained under the same conditions as for the samples from in the Granucol ® series.The adsorption capacity of the AC-C sample for methylene blue dye at all three temperatures is very close in value, being approximately 690 mg/g.Similarly, close values were obtained from theoretical calculations using the Langmuir adsorption model (Figure 9, Table 7).The separation factor RL has values smaller than 1, indicating that the adsorption process of methylene blue dye on AC-C activated carbon is favourable (Table 7).The adsorption capacity for methylene blue of the AC-C activated carbon (690 mg/g) is higher than that of activated carbons from Granucol ® series (approx.535 mg/g) by 30%.
It was interesting to evaluate how the amount of dye adsorbed on the activated carbon samples correlates with their surface area.For methylene blue adsorption in monolayer, we can deduce the surface area of activated carbon covered by the dye molecules, which is equal to the total surface area of the adsorbed methylene blue molecules, expressed by Eq.( 7).The activated carbon area covered by one molecule of methylene blue may change because attachment may be done with different orientations [27].
The calculated values of the total surface area of the methylene blue molecules adsorbed compared to the SBET value of the studied activated carbons, determined from the sorption-desorption isotherms of nitrogen, are presented in Table 8.The total surface area of the methylene blue molecules adsorbed on activated carbons was calculated using the Q0 value obtained at 25°C (Tables 6 and 7).According to the obtained results, the (calculated) surfaces covered with methylene blue molecules differs from that the SBET values.For Granucol ® FA and Granucol ® GE the value of SMB is smaller than that of SBET and can be explained as follow, due to the dimensions of the methylene blue molecule, pores with dimensions smaller than approximately 1.3 nm 2 will not be accessible for adsorption.Regarding the activated carbons samples for which were obtained values of SMB higher than that of SBET it can be assumed that methylene blue molecules were adsorbed on the activated carbons surface in more than one orientation (Table 8).The results obtained regarding the adsorption of methylene blue on the studied activated carbons (Granucol ® type and AC-C) highlight that further research is needed to elucidate the mechanism of methylene blue adsorption.However, comparing the results obtained regarding the physico-chemical characteristics and adsorption capacity of methylene blue of the AC-C activated carbon from apricot stones, obtained under laboratory conditions (Laboratory of Ecological Chemistry, Institute of Chemistry), with those of commercial activated carbons of Granucol ® type (FA, BI, GE), it can be concluded that AC-C activated carbon can be recommended for applications in winemaking.

Conclusions
The physico-chemical characteristics of new activated carbon from apricot stones AC-C, evaluated by standard methods (nitrogen sorptiondesorption isotherms, IR spectroscopy, pH value of activated carbons suspension), are comparative with that of the commercially activated carbons of Granucol ® type.The surface area (SBET) of AC-C sample is of 1385 m 2 /g and for activated carbons of Granucol ® type ranges between 1220-1385 m 2 /g.Adsorption studies using methylene blue dye as a reference substance revealed a higher adsorption capacity of AC-C activated carbon (690 mg/g) by 30% than that of activated carbons from Granucol ® series (approx.535 mg/g).Kinetic studies showed that the kinetic data were well described by the pseudo-second-order kinetic model.The adsorption also closely fit the Langmuir isotherm rather than the Freundlich model, suggesting monolayer adsorption rather than multilayer adsorption.
According to the obtained results, the local vegetal activated carbon (AC-C) has proven to be effective compared to commercial ones (Granucol ® series) in removing methylene blue dye from solutions and can be recommended for further research in order to be used in winemaking.