Adsorption of cationic dye on nanostructured biocarbons: kinetic and thermodynamic study

Nanostructured bio-adsorbents were prepared by physical or chemical activation of the residue of supercritical extraction of raspberry seed. Their physicochemical properties were determined by elemental analysis, low-temperature nitrogen adsorption/desorption, Boehm titration and scanning electron microscopy. The biocarbon obtained as a result of physical activation of the precursor showed basic character of the surface and its SBET was 700 m2/g. The chemical activation of the residue of supercritical extraction of raspberry seed with potassium carbonate favored generation of acidic functional groups and SBET of this biocarbon was 1177 m2/g. The nanostructured biocarbons were used for removal of Rhodamine B from its aqueous solutions. The process was best described by the Langmuir isotherm and the maximum capacity of the monolayer was 181.82 mg/g and 277.83 mg/g for the physically and chemically activated samples, respectively. The adsorption energy obtained from the Dubinin–Radushkevich isotherm indicated that the process observed was physisorption, while the kinetics of the process was best described by the pseudo-second-order model. The negative values of Gibbs free energy indicated the spontaneous character of the process. For the chemically activated sample, the highest sorption capacities toward Rhodamine B were obtained in an acidic environment, while for the physically activated sample—in a basic environment. The yield of desorption decreased for the media: distilled water > hydrochloric acid > acetic acid, which means that Rhodamine B molecules were weakly bound to the biocarbon surface.


Introduction
Adsorption studies have been in the area of interest of contemporary science for a long time.The process of adsorption permits reduction of significant amounts of gas and liquid pollutants emitted to the environment (Lemus et al. 2017;Ma et al. 2022), including pesticides, detergents, organic dyes and metal ions from mono-and multi-component solutions (Ojedokun and Bello 2017;Xiang et al.2020;Qi et al. 2023).A large group of effective adsorbents are carbon materials of which the most popular is activated carbon (Wiśniewska et al. 2022).It is obtained as a result of carbonization of organic materials in the neutral gas atmosphere followed by activation, usually with carbon (IV) oxide.Adsorbents of this type can be also obtained by impregnation of an organic precursor with the activating agents, such as sodium or potassium hydroxides and carbonates, followed by thermochemical treatment in a neutral gas atmosphere.Industrial methods of production of carbon adsorbents are based on conventional heating, which, however, requires pyrolysis and activation in high temperatures.An interesting alternative is the use of microwave radiation, which permits running the process at lower temperatures, shortening of the time of heating and energy saving (Bazan-Wozniak et al. 2022).
Although activated carbons have been known for many years, the methods of their syntheses and modifications are continuously developed (Mubarak et al. 2022).The interest in carbon materials increases and many research centers work on synthesis of new carbon adsorbents that would be characterized by high selectivity toward particular pollutants.Yu et al. 2022 have reported the study on synthesis of new biochar/iron oxide composite.The source of carbon was green algae Aegagropila linnaei, which was subjected to pyrolysis in a tube furnace and activated with potassium hydroxide.In the next step, the product was functionalized with ferrous sulfate for effective soldering of Fe 3 O 4 nanoparticles onto the surface of biochar through hydrothermal method.The obtained composite was found highly effective in removal of bisphenol-A.Zhang et al. (2022) studied adsorption of Rhodamine B on the biochar obtained from tobacco midrib.The sorption capacity of this material was 588 mg/g, so much higher than those of the earlier described adsorbents (Adekola et al.2019;Hou et al. 2019).Moreover, this biochar (Zhang et al. 2022) was subjected to regeneration and proved to maintain over 90% of its initial effectiveness.
Rhodamine B belongs to a group of cationic dyes used e.g., in textile, leather or cosmetic industries.It has been estimated that as much as 20% of Rhodamine B is released into the environment because of disposal of untreated wastes of the above-mentioned branches of industry.The presence of N-ethyl groups on both sides of the aromatic rings is responsible for the toxic and cancerogenic properties of this compound.In view of the above, much effort is devoted to minimize the adverse effects of this dye on living organisms (Sharma et al. 2022).
The main aim was to develop low-cost and effective adsorbent from the residue of supercritical extraction of raspberry seed to eliminate Rhodamine B from aqueous solution.The impact of different experimental parameters on the course of adsorption, like contact time, pH, and solution concentration, was studied.Additionally, different isothermal models, such as Freundlich, Langmuir, Temkin and Dubinin-Radushkevich models, were analyzed with the adsorption data to explain the adsorption process.Adsorption kinetics was studied by fitting the experimental data with the inter-particle pseudo-first-order and pseudo-second-order kinetic models.Finally, the thermodynamics of the sorption process was analyzed to establish the practical applicability of the adsorbent for wastewater treatment from liquid pollutants.

Precursors and materials reagents
The biocarbons precursor was the residue of supercritical extraction of raspberry seed obtained from the Łukasiewicz Research Network-New Chemical Syntheses Institute.The precursor in the form of powder with particles size range of 0.15-0.75mm and moisture content in air 5.7 wt% was washed a few times with distilled water and dried at 105 °C for 3 days.Rhodamine B, hydrochloric acid, and sodium hydroxide were of analytical grade, purchased from Merck (Darmstadt, Germany).Deionized water was used in the preparation of the solutions.

Preparation of biocarbons
The precursor (R) was carbonized under a nitrogen atmosphere (170 mL/min) at 400 °C (RP).The process was carried out in a microwave oven.The physical activation of carbonization products was carried at temperature of 700 °C under a stream of carbon (IV) oxide for 30 min (RPA).
Another sample of the biocarbon was obtained by chemical activation (C) of the precursor with potassium carbonate as the activating agent.The weight ratio of precursor to activator was 1:2.The impregnated raw material was activated in a nitrogen atmosphere (330 mL/min) at 700 ºC for a time of 30 min in a microwave oven (RC).
The final biocarbons were subjected to the washing procedure with a hot 5% solution of HCl and later with deionized water.Finally, the samples were dried to constant mass at 105 °C.

Characterization of precursor and biocarbons
The ash content was determined by burning the residue of supercritical extraction of raspberry seed, biochar, and biocarbons in a microwave muffle furnace at 850 °C for 60 min.Thermo Scientific FLASH 2000 Elemental Analyzer was employed to establish the elemental composition of precursor and biocarbons.
The obtained biocarbons were characterized for their porous properties using N 2 adsorption/desorption isotherms measured at 77 K using an analyzer AutosorbiQ instrument, provided by Quantachrome Instruments.Before adsorption measurements, the biocarbons were degassed under vacuum for 12 h, at 300 °C.The specific surface area was calculated from the nitrogen adsorption isotherm data according to the Brunauer-Emmett-Teller method.The total pore volume was estimated from the volume of nitrogen adsorbed at the relative pressure of p/p 0 = 0.99, which is the equilibrium pressure divided by the saturation pressure and converted to the volume of nitrogen in the liquid state at a given temperature.The average pore size was calculated from the Eq. ( 1): (1) where: D -average pore size, V t -total pore volume, S BET -surface area.SEM images were obtained using a SEM microscope (PHILIPS, The Netherlands) in the following conditions: working distance of 14 mm, accelerating voltage of 15 kV and digital image recording by DISS.The pH values of the precursor and biocarbons were measured by the procedure described in detail in our earlier paper (Bazan-Wozniak et al. 2022).The content of oxygen functional groups of all materials studied was determined by the Boehm method (Boehm 1994).

Adsorption study
The biocarbons portions of 0.025 g were placed in flasks and flooded with 0.050 L of a Rhodamine B solution of a given concentration (50-150 mg/L), and the mixtures were shaken for 12 h (300 rpm/min).After shaking, the solid samples were separated from the solution using a laboratory centrifuge.The concentration of the dye in the solution was determined spectrophotometrically at the maximum absorption wavelength of 553 nm, using a Carry 100 Bio spectrometer.The amount of Rhodamine B adsorbed on the biocarbons was calculated from Eq. ( 2): where: C 0 -initial dye concentration (mg/L), C e -equilibrium dye concentration (mg/L), m -the mass of sample (g), V -volume of dye solution (L).
The experimental adsorption studies were carried out three times, and the results are shown with a standard deviation error.
At the beginning, the effect of the biocarbon dose used on their sorption capacities was studied.Flasks were charged with different portions of the biocarbons studied of 0.015 g, 0.025 g and 0.035 g and the biocarbons were flooded with 0.050 L of the Rhodamine B solution of the concentration of 100 mg/L.The mixtures were shaken for 12 h on a shaker.After this time, spectrophotometric measurements were performed.
In the next step, the impact of agitation rate on the sorption capacities of the adsorbents studied was checked for four agitation rate values 100, 200, 300 and 400 rpm/min.The process of agitation at a given rate was carried out for 12 h for the biocarbons (dosage biocarbon-0.025g, concentration of Rhodamine B-100 mg/L).
In our work, we used the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models to explain mechanism of Rhodamine B adsorption on samples obtained.The equation of Langmuir isotherm can be written as (3): (2) where: C e -equilibrium dye concentration (mg/L), q e -experimental amount of the adsorbed Rhodamine B (mg/g), K L -the Langmuir equilibrium constant (L/mg), q e -maximum amount of the adsorbed Rhodamine B (mg/g).We also calculated dimensionless constant (R L ), (4): Its value informs about the shape of the Langmuir isotherm: (R L = 1)-linear, (R L = > 1)-unfavorable, (0 < R L > 1)-favorable and (R L = 0)-irreversible.
The form of Freundlich isotherm is given by formula (5): where: K F -the Freundlich constant describing adsorption capacity (mg/g(mg/L) 1/n ).
Another model, proposed by Temkin is given by the Eq. ( 6): where: B -a constant equal to B = RT/B T , where B T is the Temkin constant (J/mol), R -gas constant (J/mol × K), T -temperature (K), A T -Temkin isotherm equilibrium binding constant (L/mg).
To check the effect of contact time on the effectiveness of the adsorption of Rhodamine B studied, flasks were charged with 0.025 g of biocarbons flooded with 0.050 L of Rhodamine B (100 mg/L).The loaded bottles were shaken on a shaker working for 8 h after which absorbance of the solution was measured.
Experimental data were fitted to pseudo-first-order (8) and pseudo-second-order (9) models: (3) 1 3 where: q t -the amount of Rhodamine B adsorbed in a given time (mg/g), k 1 -the adsorption constant in the pseudo-firstorder equation (1/min), k 2 -the adsorption constant in the pseudo-second-order equation (g/mg × min).
The effect of the pH of Rhodamine B solution on the sorption capacities of the biocarbons was determined for the pH values varied in the range of 2-12.Measurements were performed for 0.025 g of the sample, 0.050 L of the dye solution of 100 mg/L concentration.
The possibility of reuse of biocarbons in adsorption is of great importance.To check the possibility of reuse, at first, 0.5 g of sample was loaded with 100 mg/L Rhodamine B solution under the same conditions used for the previous adsorption tests.Upon equilibrium, the dye-loaded samples were separated and repeatedly washed with distilled water to remove any unsorbed dye; then, they were dried until constant mass.An amount of 0.1 g of the biocarbons loaded with the dye was contacted with 0.050 L of H 2 O, 0.1 M HCl and 0.1 M CH 3 COOH.Desorption efficiency was calculated using the following Eq.( 13): (10)

Physiochemical properties of precursor and biocarbons
The study was begun with determination of physicochemical properties of the precursor and biocarbon adsorbents.
Results are given in Table 1.The content of elemental carbon in the precursor was low and did not exceed 45 wt%.It was lower than that obtained for the precursors which were the residues of supercritical extraction of marigold flowers (Bazan et al. 2016) or common nettle (Bazan-Wozniak et al. 2022).Activation of the precursor leads to an increase in the content of C daf and a decrease in the content of the other elements.The magnitude of these changes depends on the method of activation.The content of C daf in Biocarbon RC is over twice greater than that of the precursor and by about 10% greater than that in sample RP.The content of H daf in samples RP, RPA, and RC lower than that in the precursor is a consequence of gasification of rich in hydrogen fragments of the precursor structure, and for biocarbon RP also gasification of the aromatic structures formed in the process of carbonization.A decrease in the content of nitrogen in the obtained adsorbents probably follows from a low resistance of nitrogen groups to carbon(IV) oxide in the activation with potassium carbonate (Kaźmierczak-Raźna et al. 2019).As implied by the obtained data, the activation with potassium carbonate also contributes to removal of sulfur.The content of oxygen in the obtained biocarbons varies in the range 5.0-13.5 wt%.
As follows from further analysis of the data presented in Table 1, the content of mineral substance in samples RP, RPA and RC, depends on the thermochemical treatment of the biomass.The content of ash in RP carbonizate is 9.8 wt%, while in the RPA adsorbent obtained by activation of the former, it is 5.8 wt%.However, it should be mentioned that adsorbent RP showed a much lower content of mineral substance than the biocarbons obtained by physical activation of the precursor (Bazan et al. 2016).It is probably a consequence of the fact that part of the ash has been removed from the structure of biocarbon RPA as a result of a two-stage washing of the sample with hydrochloric acid and distilled water.The content of ash in sample RC was 2.1 wt%.Relatively low content of mineral substance in the obtained biocarbons may suggest that the process of adsorption of Rhodamine B from its aqueous solution on samples RPA and RC will not be blocked by ash.The next step was determination of acidic-basic properties of the precursor and biocarbons obtained from it.According to the results presented in Table 1, the samples differed in the type and number of oxygen groups.The residue of supercritical extraction of raspberry seed showed a clearly acidic character of the surface as its content of acidic groups was over 4 times greater than that of basic groups.Moreover, the pH of the water extract of the precursor also confirmed the acidic character of the precursor.The processes of carbonization and activation lead to changes in the acidic-basic properties of the samples.Carbonizate RP contains 0.76 mmol/g of acidic groups and 1.45 mmol/g of basic ones.The activation of this carbonizate with carbon(IV) oxide leads to further increase in the basic groups and a decrease in the content of acidic groups, relative to their content in sample RP.The basic character of sample RPA is related to the content of ash in its structure, which usually has alkaline nature because of the type of compounds it contains (mainly metal oxides and carbonates).Moreover, the use of carbon(IV) as an activator favors formation of basic groups (Bazan-Wozniak et al. 2020).Chemical activation of the precursors induces acidic character of the obtained biocarbons.The surface of sample RC is acidic, (pH 5.9) and contains 2.89 mmol/g of acidic groups and 2.05 mmol/g basic ones.
The porous structure of the obtained biocarbons is one of the factors determining their use as adsorbents of liquid pollutants (Pereira et al. 2023).The textural parameters of samples RPA and RC are presented in Table 2 and Fig. 1.
The most developed surface area, of 1177 m 2 /g, was found for the adsorbent activated with potassium carbonate, whereas the surface area of the sample after physical activation was about 500 m 2 /g lower.Thus, potassium carbonate proved to be more reactive.Perhaps the S BET value of sample RPA is to some extent a consequence of ash presence in its structure, which could block the access of the activator to the carbon matrix.Physical and chemical activation of the precursor leads to bio-adsorbents of micro-meso-porous structure as follows from the average pore diameter values collected in Table 2, and the course of low-temperature nitrogen adsorption/desorption isotherms shown in Fig. 1a.For both bio-adsorbents, a hysteresis loop is observed, whose occurrence suggests the presence of meso-pores in these samples.The shape of the loop resembles a H4 (type of hysteresis loops recommended by IUPAC, Thommes et al. 2015) type loop, which is the characteristic of materials containing narrow slit pores.
The morphology of samples RPA and RC is confirmed by SEM images displayed in Fig. 2. As can be inferred from the images, the shapes and distribution of pores in the two samples are different.The SEM images also confirm that the sample obtained by chemical activation of the precursor with potassium carbonate shows greater and better developed surface area than that obtained by physical activation.The SEM of bio-adsorbent RPA reveals bright spots that confirm the content of mineral substance in its structure.

Adsorption study
In the first test, the impact of the mass of adsorbent on the sorption capacity was studied (Fig. 3).The mass of biocarbon varied from 0.015 to 0.035 g.The amount of Rhodamine B adsorbed by a mass unit of a biocarbon sample RPA or RC decreased with increasing mass of the adsorbent.This finding can be attributed to an increase in the biocarbon area and the availability of more adsorption sites with the increasing mass of the RPA/RC.For the samples of 0.025 and 0.035 g, no significant changes were observed because of a limited availability of Rhodamine B in the vicinity of the biocarbon (Rattanapan et al. 2017).With increasing mass of the adsorbent, the removal of Rhodamine B increased, but up to about 0.025 g.For greater masses of the adsorbent, a small increase in the Rhodamine B removal was observed, which suggests that at a certain mass of adsorbent, a maximum capacity was reached.In view of the above, in further studies, the adsorbents were used in the mass of 0.025 g.
In the subsequent tests, the impact of the rate of shaking on the effectiveness of Rhodamine B adsorption was analyzed.The measurements were performed for four shaking speeds: 100, 200, 300 and 400 rpm/min.According to the results, presented in Fig. 4, with increasing shaking speed, the effectiveness of the dye removal increases.The greatest sorption capacity was obtained for the shaking speed of 300 rpm/min, while for the speed of 400 rpm/min, the sorption capacity was a bit lower.This observation can be explained by the fact that at the shaking speed greater than 300 rpm/min, the centripetal force is too strong, which induces the repulsive interaction between the adsorbed molecules of Rhodamine B and if these repulsive forces are stronger than the force of their binding to the adsorbent, the process of desorption begins (Baidya and Kumar 2021).Therefore, in further studies, the shaking speed of 300 rpm/ min was used, at the adsorbent mass of 0.025 g.The adsorption of Rhodamine B on biocarbons RPA and RC was studied in the adsorbate concentration range 50-150 mg/L.The isotherms obtained and presented in Fig. 5 imply that the effectiveness of the dye removal increases with increasing concentration of the solution.
At the beginning of the process, at low concentrations of the dye, its adsorption is a random process.Moreover, a large number of free active sites on the adsorbent surface permit fast capturing of the dye molecules.At higher concentrations of the dye, its molecules are closely packed on the adsorbent surface, which hinders further adsorption so also the reaction between the adsorbent and the dye, leading to the adsorption equilibrium (Ndagijimana et al. 2021).On the basis of Eq. ( 2), given above, the experimental sorption capacities of the studied biocarbons were found to be 178 mg and 270 mg for samples RPA and RC, respectively.These sorption capacities were much different, which indicates that the method of precursor activation determining the textural parameters has a strong effect on adsorption of Rhodamine B. As follows from this difference, close to 100 mg, the bioadsorbent obtained by chemical activation has much better sorption abilities.
The experimental data were also used for determination of the parameters of the four models: Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (Table 3).They were found to be the best described by the Langmuir model, which means that on the surface of samples RPA and RC, a monolayer of Rhodamine B is formed (Laksaci et al. 2023).
Moreover, the maximum sorption capacities calculated from the Langmuir isotherm (q m ) are close to the experimental values.For the biocarbon obtained by physical activation of the precursor, the difference between q max and q e is 3.82 mg/g, while for the sample obtained by chemical activation, it is 7.78 mg/g.The value of coefficient (1/n) for both adsorbents was below 1, which suggests that the process of Rhodamine B adsorption was favorable and easy to carry out (Yurtay and Kılıç 2023).The Temkin isotherm model is based on the assumption that the heat of adsorption of all molecules in a layer decreases linearly because of the adsorbent-adsorbate interaction, and the adsorption is characterized by homogeneous distribution of the binding energy.However, the values of R 2 obtained for the fit with the Temkin isotherm indicate that this model should be rejected for the bio-adsorbents studied and Rhodamine B. The value of parameter E for the Dubinin-Radushkevich model comes from the range 1.414-4.454kJ/mol (E < 8 kJ/ mol), which means that for the adsorbents and the adsorbate studied, we deal with physical adsorption (Hamidon et al. 2022).
The kinetics of Rhodamine B adsorption on the biocarbons studied is illustrated in Fig. 6.The experimental results were also used for the calculation of adsorption parameters characteristic of the pseudo-first-order model and pseudosecond-order model (Table 4.).
Kinetics of the dye adsorption was studied for 8 h.However, as follows from the course of the kinetic curves, the adsorption equilibrium was reached after 5 h of the process.After this time, the sorption capacity of RPA was 175 mg/g, while that of RC was 200 mg/g.According to the correlation coefficient values, R 2 , for the pseudo-first-order model and pseudo-second-order model, the latter seems to be more suitable for description of the process of adsorption in the systems studied.This conclusion is confirmed by the curves presented in Fig. 7.
The review of literature data reveals that the pseudo-firstorder model has been many times used for description of dyes adsorption on porous materials when this process had the character of physical diffusion.The pseudo-second-order model has been mainly applied to describe the process of chemical adsorption (Ezzati 2020).The more accurate fitting of the experimental data with the pseudo-second-order model is corroborated by the q e,cal values (Table 4).
The effect of temperature on the sorption capacities was studied for three temperatures: 298, 318, and 338 K.The results are presented in Fig. 8 and Table 5 and imply that adsorption of Rhodamine B on biocarbons RPA and RC was the most effective at 338 K.
Higher temperatures lead to a decrease in the solution viscosity, which is conducive to the diffusion of the dye molecules from the external layer of the biocarbon to the inside of its pores.The observed increase in the sorption capacities indicates that the reaction was endothermic.The higher the temperature, the more heat is supplied to the system and Fig. 2 SEM images of RPA and RC samples converted into kinetic energy, leading to greater mobility of the dyes toward the adsorbent surface (Mohammadi et al. 2010).The effect of temperature was greater for biocarbon RC.For this sample, the increase in temperature from 298 to 338 K gave the sorption capacity increase by 56 mg/g, while for RPA sample -30 mg/g.The results presented in Fig. 8 are confirmed by those from Table 5.For both samples, the values of ∆H 0 were positive, which means that in the process of this dye adsorption, heat is absorbed and the process has endothermic character.Moreover, it is assumed that in the Fig. 3 Effect of the adsorbent mass on the effectiveness adsorption of Rhodamine B on the samples obtained a b process of chemisorption, the value of ∆H 0 is higher than 80 kJ/mol (Lee and Zaini 2020).The results obtained for samples RPA and RC are 17.85 kJ/mol and 22.97 kJ/mol, respectively, which indicates the process of physisorption.The more negative the value of ∆G 0 , the greater the degree of spontaneity and the more favored the reaction (Rezazadeh et al. 2022).The most negative values of Gibbs free energy were obtained at 338 K, which also indicates that at that temperature, the adsorption of Rhodamine B was the most favorable.
The effect of the initial pH of the dye solution on the dye adsorption on samples RPA and RC is illustrated in Fig. 9.As follows from the data presented in this figure, the dye adsorption on the studied samples is a complex process that depends on the pH of the dye solvent and the method of precursor activation.For biocarbon RPA, the effectiveness of removal of the dye increased with increasing pH of its solution, which may be explained by the electrostatic interactions between the negatively charged biocarbon surface and Rhodamine B molecules (Largura et al. 2010).Moreover, the value of pH pzc for sample RPA, determined by the method of drift, was 8.0.For sample RC, the increase in the dye solution pH from 2 to 4 caused an increase in the sorption capacity, and at pH 6, the sorption capacity was practically unchanged, while at the pH increased from 6 to 12, the sorption capacity decreased.Similar relations were observed for the activated carbon obtained by chemical activation with KOH of rice husks (Ding et al. 2014).It should be noted that the changes in sorption capacity of sample RC throughout the pH range studied are small.The pH pzc value of sample RC is 6.4, which means that the numbers of acidic and basic groups on its surface are similar.Moreover, Rhodamine B is a basic dye whose molecules can assume different forms depending on the dye solution pH.At pH < 3.5, the dye molecules assume a monomeric form, while at pH ≥ 3.5, a zwitterionic form (Wang and Zhu 2007).The H + ions may compete with the dye cation, while OH − groups may compete with the dye anion.
At the last stage of the study, the process of Rhodamine B desorption from the surface of biocarbons RPA and RC to demineralized water, hydrochloric acid and acetic acid 7.00 × 10 -4 6.00 × 10 -4 q e,cal [mg/g] 176.57204.08 was analyzed.The yield of desorption was low, lower than 30% (Table 6) and it was the highest for demineralized water in which the values of 19.36 and 29.44% were obtained for RPA and RC, respectively.The yield of desorption decreased in the order H 2 O > HCl > CH 3 COOH, which may suggest that the dye molecules are weakly bound to the biocarbons surface.Moreover, some of them may penetrate inside the pores of samples RPA and RC.Low percentage desorption of Rhodamine B from biocarbons surface may be attributed to a possibility of large net adsorption energy because of several contact points between large dye molecules and bio-adsorbents (Inyinbor et al. 2016).
The maximum sorption capacities of the biocarbons obtained from the residue of supercritical extraction of raspberry seed were compared to the values reported in literature for other adsorbents (Table 7).Samples RPA and RC are more effective adsorbents of Rhodamine B than magnetic biochar from alkali-activated rice straw (Ren et al. 2020) whose specific surface area was 396.9 m 2 /g, and the maximum sorption capacity was 73.47 mg/g.Biocarbon RPA showed lower sorption capacity to Rhodamine B than activated carbons from bagasse pith (Gad and El-Sayed 2009) or gelatin/activated carbon composite beads (Hayeeye et al. 2017).Gad and El-Sayed (2020) used H 3 PO 4 for production of activated carbon by one-step chemical activation.The q max value obtained for this adsorbent was 263.85 mg/g, so over 80 mg higher than the result obtained for RPA.For gelatin/activated carbon composite beads (Hayeeye et al. 2017), the value of q max was 256 mg/g.It should be emphasized that the adsorbents described in (Hayeeye et al. 2017) and (Gad and El-Sayed 2009) showed lower adsorption efficiency toward Rhodamine B than sample RC obtained from the residue of supercritical extraction of raspberry seed by chemical activation with potassium carbonate.The outstanding result presented in Table 7 was obtained for Raphia hookerie fruit epicarp (Inyinborab et al. 2016) which showed much higher sorption capacity toward Rhodamine B than our RPA and RC samples.The process of adsorption of this dye on Raphia hookerie fruit epicarp had physical character and its q max was 666.67 mg/g.Higher sorption capacity than that of samples RPA and RC was also obtained for the biochar derived from wakame (Undaria pinnatifida), (Yao et al. 2020).This adsorbent was obtained as a result of chemical activation of the initial material with KOH.The authors of the paper have shown that this adsorbent was characterized by microporous structure (S BET 1156.25 m 2 /g), and its maximum sorption capacity toward Rhodamine B calculated assuming the Langmuir model was 574.71 mg/g.

Conclusion
The above presented and discussed results indicate that the residue of supercritical extraction of raspberry seeds may be successfully used for obtaining bio-adsorbents highly effective in removal of Rhodamine B from its aqueous solutions.Physicochemical characterization of samples RPA and RC showed that the method of activation has a significant effect on the specific surface area and chemical nature of the adsorbents.Sample RC obtained as a result of chemical activation of the precursor had S BET of 1177 m 2 /g, which was by almost 500 m 2 /g more than sample RPA obtained  by physical activation, its S BET was 700 m 2 /g.The chemical activation of the precursor with potassium carbonate was found to favor the formation of acidic than basic groups.Sample RPA obtained by physical activation had more basic than acidic groups on the surface.Analysis of the kinetics of Rhodamine B adsorption with the use of Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models proved that the most suitable for this process description was the Langmuir model.For this model, the highest values of the correlation coefficient R 2 were obtained, 0.9949-0.9975,which implies the formation of an adsorption monolayer of Rhodamine B on the biocarbons studied.The maximum sorption capacity of the monolayer on sample RPA was 181.82 mg/g, while for sample RC, it was 277.83 mg/g.The kinetics of adsorption is best described by the pseudosecond-order model, because for this model, higher correlation coefficient values, R 2 , for samples RPA and RC were obtained.Moreover, a close relation was found between the experimental results and the theoretical value calculated for this model.It further supports the suitability of this model for description of Rhodamine B adsorption on the adsorbents studied.The results of thermodynamic study indicate that the Rhodamine B adsorption is a physical process as the value of ∆H 0 for sample RPA was 17.85 kJ/mol, while for sample RC, it was 22.97 kJ/mol.The sorption capacities of both samples increased with increasing temperature of the process, which shows an endothermic character of the reaction.Moreover, for sample RPA, a high sorption capacity was achieved in a basic medium, while for sample RC-at pH of 4, which means that the process of adsorption is complex and needs further study.

Fig. 1
Fig. 1 Low-temperature N 2 adsorption-desorption isotherms (a) and pore size distribution (b) of RP and RC samples

Fig. 4
Fig. 4 Effect of agitation rate on the adsorption of Rhodamine B on samples obtained

Fig. 7
Fig. 7 Pseudo-first-order (a) and pseudo-second-order (b) kinetic plots for adsorption of Rhodamine B onto samples obtained

Fig. 8
Fig. 8 Effect of temperature on the adsorption of Rhodamine B onto samples obtained

Fig. 9
Fig. 9 Effect of pH on the adsorption of Rhodamine B onto samples obtained

Table 1
Characterization of the precursor and biocarbons 1 Dry ash-free basic 2 By difference

Table 2
Textural parameters of samples RPA and RC

Table 3
The isotherm adsorption parameters of Rhodamine B

Table 4
Kinetic parameters for adsorption of Rhodamine B

Table 5
Thermodynamic parameters of the adsorption of Rhodamine B