Usability of spent Salvia officinalis as a low-cost adsorbent in the removal of toxic dyes: waste assessment and circular economy

ABSTRACT The removal of methylene blue (MB) and maxilon yellow (MY) dyes from aqueous solutions was investigated by an adsorption process using spent Salvia officinalis (SSO) for the first time. The operating conditions and mechanisms for the MB-SSO and MY-SSO adsorption systems were determined. FTIR-ATR and SEM-EDAX analyses were also performed to characterise the adsorbent. Optimum conditions in each system were similarly obtained, excluding pH (5.7 for MB and 6.5 for MY). The dye removal efficiency was 78% for MB and 56% for MY. It was observed that experimental data fits Freundlich isotherm (R2 ≈ 0.97) and pseudo-second-order kinetic (R2 ≈ 0.99) model in both systems. The maximum adsorption capacities of SSO for MB and MY dyes were determined as 16.53 and 11.79 mg g−1, respectively. The pseudo-second-order kinetic model revealed that adsorption efficiency was affected by a number of active sites on the adsorbent and the concentration of the dye. The thermodynamic parameters confirmed that adsorption was spontaneous, exothermic and physisorption in both adsorption systems. The results indicate that SSO can be used as a promising adsorbent material for removal of MB and MY dye from aqueous solutions. However, in order to remove high dye concentrations, SSO must be activated.


Introduction
Dyes or pigments are used to colour final products in most industries such as textile, paper, plastic, leather, food, cosmetics [1,2].The dyes are cationic, anionic, and non-ionic, and the textile industry uses the majority of the dyes in various colours.Cationic dyes are the most common and stable among them [3].Every year, approximately 7 × 10 5 tons of dyestuffs are produced for use in the textile industry.It is estimated that 1.6 million m 3 of water is consumed in the textile industry with a production of 8 tons and 15-20% of the dye used is discarded into effluent [4,5].Textile industrial wastewater contains a variety of organic and inorganic pollutants, as well as has a wide pH range (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12), different colours, and high temperature (50-70 ± °C).
In this study, the colour removal, which is an important contaminant in textile industry wastewater, with Spent Salvia officinalis was investigated.There are some studies carried out using only its seeds to benefit from the mucilage feature of Salvia officinalis [29,30].In the study, waste recovery was also aimed by using all of the spent Salvia officinalis.The use of Salvia officinalis, which most of us consume at home, instead of being disposed of as garbage after its consumption, will ensure that the adsorption system is low-cost and environmentally friendly.The use of a natural product as an adsorbent material also facilitates its availability.
Dyes of many different colours are used in the textile industry.For this reason, MB and MY dyes, which are blue and yellow, were used in this study.It is in the cationic group in two dyes.Dyes from the same group were selected for easy comparison.MB is used as a drug in addition to being used as a dye.The reason why MY dye was chosen as yellow is that it is more difficult to remove the colour in yellow-coloured wastewater and is to observe the removal of a colour that is difficult to remove with SSO.Dyes are toxic and have a negative effect on body systems such as skin and eyes.They cause carcinogenesis to humans and animals [29,30], mutagenesis, allergic (respiratory problems), dermatitis, and skin irritation [30,31].Vomiting/nausea, anaemia, and hypertension are observed with prolonged exposure to MB [32].This study was considered due to the overconsumption of dyes, their harmful effects and the need to determine the effective (efficient and low cost) adsorbent.

Materials
Sage waste was used as an adsorbent in this study.Sage waste was washed with tap water until the filtrate was colourless.Then sage waste was washed with distilled water and dried in an oven at 105°C for 24 h.To obtain adsorbents in different particle sizes, the dried wastes were ground in a grinder and were sieved in sieves of 75, 150, 180 and 500 µm.
The molecular weight of methylene blue (Sigma Aldrich) used as adsorbate is 319.85 g mol −1 while the molecular weight of maxilon yellow 4GL (Fluka) is 337.4 g mol −1 .The maximum absorbance wavelengths of MB (C 16 H 18 CIN 3 S) and MY (C 15 H 19 N 3 O 4 S) are 410 nm and 665 nm, respectively.Colour analyzes were also performed using the UV-Vis spectrophotometer at these wavelengths.

Methods and batch scale system
100 mg L −1 stock dye solutions were prepared.All experimental studies were conducted with 100 mL of dye solutions in 250 mL flasks.Heidolph brand magnetic stirrer (Germany) was used in the experiments.The pH values of the samples were measured electrometrically with the WTW Multi 3620 IDS SET C brand pH metre (Germany) in accordance with the TS EN ISO 10,523 standard.At the end of the experiments, the samples were centrifuged at 2000 rpm and the absorbance values of the samples were determined with Agilent Cary 80 UV-Vis brand spectrophotometer (USA).For pH adjustment, 0.1 N HCl and 0.1 N NaOH (Sigma Aldrich) were used.All chemicals used in the study are of laboratory quality (> 99% purity) and distilled water was used in the preparation of the necessary solutions.The adsorption system used in the study has given in Figure 1.
FTIR-ATR analyses were performed to determine the change in functional groups in solid and liquid products before and after adsorption in both MB and MY dye.FTIR-ATR data recorded with 32 scans at a 16-cm −1 resolution was conducted with the Perkin Elmer Spectrum 100 device (USA) in the range of 4000-650 cm −1 and the Scanning Electron Microscope-Energy Dispersive X-ray analysis was conducted with (SEM-EDAX) EDS detector (EDAX Element EDS detector) device connected to the JEOL JSM-6390 L device (USA).

Removal of methylene blue with SSO
Spent Salvia officinalis was used as an adsorbent for the zero waste approach in the removal of MB used in both the textile industry and the pharmaceutical industry.The effect of all parameters affecting the adsorption system has been investigated.The optimum operating conditions of the MB-SSO adsorption system and results obtained are given in Figure 2.
As the initial dye concentration increases, the removal efficiency decreased.Approximately 98% colour was removed in 60 minutes at a concentration of 5 ppm.This is an expected result, as the scarcity of the dye molecule reduces inter-molecular competition.A removal efficiency of 79% at 10 ppm and 54% at 50 ppm was achieved.
The effect of stirring speed was studied in the range of 100-400 rpm.The colour removal efficiency has decreased at speeds of 100 rpm and higher more than 200 rpm.The highest removal was achieved at 200 rpm.This result clearly shows the effect of slow mixing and fast mixing.Low-speed mixing indicates that there is not enough contact during the working time, in other words, there is less adhesion, while high-speed mixing causes the adhering pollutant to mix with water again.At the end of 60 minutes, approximately 51% at 100 rpm and approximately 60% removal efficiency at 400 rpm was obtained.
Adsorbent size is a parameter related to the adsorbent surface area.Studies in four different sizes between 75 -> 180 µm were conducted.While approximately 70% removal efficiency was obtained at 75 µm, this value was found as approximately 61% at >180 µm.79% colour removal was achieved at optimum value (180 µm).
Temperature, an important parameter affecting the adsorption process, is a necessary parameter in kinetic calculations.The effect of temperature on the adsorption process was examined at 25-60 C. Different temperature values have been studied for both kinetic calculations and due to the high temperature of the wastewater generated in the textile industry.The temperature did not have a positive effect on the removal of MB by adsorption.As the temperature increased, the removal efficiency decreased because the mobility of the pollutant molecules in the water increased.The highest removal efficiency was determined to be 79% at 25°C although the temperature usually acts as a catalyst for reactions.
pH is a parameter that affects adsorbent activity and decomposition of pollutant molecules.As a result of the work carried out in a wide pH range, the highest removal efficiency (about 79%) was achieved in the natural pH of the dye solution.While a removal efficiency of approximately 33% at pH 1 and 75% at pH 11 was obtained, higher colour removal was achieved at pH conditions close to neutral.
Contact time is the time required for adsorbent and pollutant interaction.The length of the contact time is an indication that the adsorbent is not very active.Commercial powdered activated carbon is very active and provides higher dye removal in much shorter times.Since the activation process is not applied to the SSO, its activity is less.As the contact time increased, the removal efficiency increased.The highest removal efficiency (79%) was obtained in 60 minutes.However, considering the cost, 45 minutes was determined as the optimum value.The removal efficiencies at a contact time of 45 and 60 minutes are very close to each other and the removal efficiency at 45 minutes is 78%.

Removal of Maxilon Yellow 4GL with SSO
In the study, one of the reasons to investigate the removal of Maxilon Yellow 4GL used in the textile industry is to observe the removal of different dye with SSO.In addition, yellow colour dye was chosen due to the difficulty of its removal.The optimum operating conditions of the MY-SSO adsorption system and results obtained are given in Figure 3.
In MY-SSO adsorption system, 10 ppm dye concentration, 200 rpm stirring speed, 0.05 g 100 mL −1 adsorbent dosage, 180 µm adsorbent size, 25°C temperature, natural pH (5.7) and 45 min contact time were determined as optimum values.In these conditions, approximately 56% colour removal efficiency was obtained.The highest removal efficiency was obtained in the same operating conditions with MB except pH.However, although both are cationic dyes, MB was better removed compared to MY.This result shows that two different dyes in the cationic group have different adsorption capacity.It also indicates that the blue colour is easier to remove than the yellow colour.
The removal efficiency decreased while the initial dye concentration increases.Approximately 98% colour was removed in 60 minutes at 5 ppm dye content.This is an expected result, as the scarcity of the dye molecule reduces intermolecular competition.As the initial concentration of the dye was raised from 10 to 50 mg L −1 , the removal rate of the colour was found to decrease from approximately 56% to 30%.
At 100 rpm and higher stirring speeds than 200 rpm, colour removal efficiency was reduced.The highest removal efficiency was achieved at 200 rpm.Low-speed mixing indicates that there is not enough contact during the working time, in other words, there is less adhesion.High-speed mixing causes the adhering pollutant to mix with water again.Approximately 38% removal efficiency was obtained at 100 rpm at the end of 60 minutes, while approximately 43% removal efficiency was obtained at 400 rpm.
As the temperature increased, the removal efficiency decreased.The highest removal rate (about 56%) was achieved at 25°C.
As a result of the study conducted in a wide pH range, the highest removal efficiency (about 56%) was achieved in the natural pH of the dye solution.While a removal efficiency of approximately 30% was obtained at pH 1, this value is 43% at pH 11.Higher colour removal efficiencies were achieved at pH values close to neutral.As the contact time increased, the removal efficiency increased.The highest removal efficiency (about 56%) was obtained for 60 minutes.However, considering the cost, 45 minutes was chosen as the optimum value.The colour removal at 45 minutes was 52%, and this removal rate is too close to the maximum removal efficiency.

Changes of FTIR in SSO and dye wastewater
As a result of FTIR-ATR analysis, mainly 3 peaks were observed in SSO before and after adsorption of MB dye, while 4 peaks were observed in SSO after adsorption of MY dye (Figure 4).The peaks at wavelengths of 3100-2800 cm −1 , 1800-1500 cm −1 , and 1100-900 cm −1 were occurred in SSO before adsorption.In SSO, which adsorbs MB dye, peaks were observed in the band 3100-2800 cm −1 , 1800-1500 cm −1 , and 1100-900 cm −1 , while in SSO, which adsorbs MY dye, peaks were detected in the band 3700-3000 cm −1 , 3000-2800 cm −1 , 1800-1500 cm −1 , and 1100-900 cm −1 .These peaks can be attributed to the N-H elongation vibration in the amines and amides, or the O-H elongation vibration in phenols, respectively, and the C = C bond in the alkene group or the C = O bond in the carbonyl/amide group [9,18,33].The peak at 1100-900 cm −1 may also be attributed to the Si-O stretch in the alkyl halide group [33].The result of the EDAX analysis also confirms the existence of this group (Table S1).Gao et al. [16] have stated that significant changes were not observed in FTIR groups in the adsorbent before and after adsorption in the study of dye removal by adsorption.
FTIR-ATR analysis was performed in wastewater before and after treatment by adsorption in optimum operating conditions (Figure 5).Basically, two peaks occurred in the functional group region (wavelengths above 1500 cm −1 ) in all analyses.The occurrence of the same peaks indicates that these groups are stable.The peaks were observed at wavelengths of 3700-2900 cm −1 and 1700-1500 cm −1 .
These peaks can be attributed to N-H stretching vibration in the amines and amides or O-H stretching vibration in phenols and C = C stretch in the alkene group or C = O stretch in the carbonyl/amide group, respectively.The peak at 3395-3475 cm −1 has been attributed to the stretching vibrations of water molecules.The peak at 1679-1652 cm −1 has been attributed to the H-O-H bending vibration [34].In short, the existence of H-O-H (water of hydration), N-H, O-H, C = C and C = O bonds has been proven.Although there are some shifts in FTIR spectra, in the study of removal of anthraquinone dye by struvite, it was stated that the peak changes in FTIR before and after treatment were similar [34].The reason for this is that the chemical structure of the adsorbent does not deteriorate since the adsorption process takes place on the adsorbent surface [34].
SEM-EDAX analyses were performed to characterise the SSO before and after treatment for both systems.The SEM-EDAX results are given in the appendix (Figure S1-S3).When the results are examined, it is seen that SSO has a cylindrical structure before adsorption.This indicates a good possibility for dyes to be adsorbed into pores [35].It can be said that after the adsorption of MB and MY dyes, adhesion occurs on the surface and homogeneity decreases.As a result of EDAX analyses, B, C, O, Si, Au, Pd were detected in all SSOs before and after adsorption, while Mg and Al were detected only in the MY+SSO adsorption system (Table 1).The B content was measured as 42.08% in SSO before adsorption and 38.62% and 39.62% in SSO after MB and MY adsorption, respectively.SSO contains boron and its decrease after the adsorption process indicates that some of it is mixed with water.

Isoterhms, kinetics and thermodynamics
Different isotherms (Freundlich, Langmuir, Elovich, Temkin, Redlich Peterson, Dubinin-Radushkevich, and etc.), kinetics and thermodynamic calculations are performed to determine the adsorption mechanism [13].The Langmuir and Freundlich isotherm models are the most commonly used isotherms in the studies, and in this study, experimental  The degree of suitability of isotherms for both dyes is Freundlich> Temkin> Langmuir> BET, respectively.The Freundlich isotherm developed by the German physiologist describes the physical adsorption on the surface of the adsorbent in the heterogeneous structure.In physical adsorption, interaction occurs with Van der Waals attraction forces.The attraction of the pollutant to the adsorbent surface does not depend on the specific region, in other words, it may move on the surface and is reversible.Adsorption takes place in a single layer.Moreover, desorption becomes easier and faster [6].The properties of the adsorbent used, not the contaminant, determine which isotherm is more suitable.In some studies conducted with MB in the literature, it has been observed that the Langmuir isotherm is more appropriate [23,24].BET isotherms are a special type of Langmuir isotherm [36] and in the study, R 2 values of BET and Langmuir models were obtained lower.
Kinetic models are used to determine the rate of adsorption.At four different temperatures, pseudo-first order, pseudo-second order and interparticle diffusion kinetic models were used to calculate rate constants and the data obtained are given in Table 2. Kinetic graphics are given in the appendix Figure S6-S7.
In the MB-SSO adsorption system, R 2 values for pseudo-first-order and interparticle diffusion increased with increasing temperature, while R 2 values for pseudo-second-order decreased with temperature increase.It was found that the pseudo-second-order kinetic model was more appropriate to describe both adsorption processes.In studies performed with MB [2,11,23,24] and MY [1] dyes, it has also been stated that adsorption systems fit with pseudo-second-order kinetics.It could be concluded that the adsorption of MB and MY dyes with the SSO are affected both by a number of active sites on the adsorbent and the dye concentration [34].
The thermodynamic coefficients calculated at four different temperatures for both adsorption systems are given in Table 3.Heat is generated during the interaction between the pollutant and the adsorbent.The adsorption process depends on thermodynamic parameters related to temperature and these thermodynamic parameters give information about whether the adsorption process is spontaneous or not.Activation energy graphs are given in the appendix Figure S8-S9.
Enthalpy (ΔH) and entropy (ΔS) changes were negative for both systems.For the MY-SSO adsorption system, Gibbs free energy (ΔG) change was negative at 25°C, while positive values were obtained at other temperatures.Since the heat generated in adsorption occurs spontaneously at constant temperature and pressure, the enthalpy change is always minus (-) signed.As the more irregular molecules in the gas or liquid medium become more stable by being held on the solid surface, the entropy changes, therefore entropy values are always negative [13].The negative ΔH value indicates that the adsorption process is exothermic.In addition, the fact that this change value is <84 kJ/ mol shows that the adsorption mechanism is physisorption.When the results obtained from this study are examined, it is seen that physical adsorption occurs in MB-SSO and MY-SSO adsorption systems.In addition, this result confirms that the efficiency decreases as the temperature increases.ΔG values were obtained between −20 and 0 kJ mol −1 in both adsorption systems.These ΔG values state that the physisorption is based on the electrostatic interaction between the adsorbed units and the active sites in the adsorbent [37].A negative ΔG value indicates that the adsorption process is instantaneous [38].For adsorption to occur spontaneously, it must be exothermic [37].In the present study, the adsorption at 25°C, which is the optimum value, is exothermic for both systems.Almost the same optimum conditions for both adsorption systems (natural pH: 5.7 for MB and 6.5 for MY), adsorbent dosage 0.05 g 100 mL −1 , initial dye concentrations of 10 mg L −1 , stirring speed of 200 rpm, adsorbent size of 180 µm, 45-minute contact time and a temperature of 25°C) were obtained.In other words, the effect of operating parameters has been the same.
It has been stated that OH released in an acidic environment is neutralised by proton, recycling is prevented and removal efficiency is high [39].This situation, which is not valid for every dye, depends on the pH of the dye and the adsorbent.The pH of a medium controls the magnitude of electrostatic charges which are imparted by the ionised dye molecule [33].It has been stated that generally close to neutral and alkaline conditions are more effective in dye removal [18].In this study, highest removal efficiency was obtained at pH values close to neutral.
As the contact time increased, the adsorption efficiency increased.Fast dye adsorption occurred up to the first 30 minutes and then the dye uptake rates slowed down.This change has also been seen in studies conducted by Bayrak-Tezcan et al. [33] and Dincer et al. [40].The slowing down of the adsorption rate indicates that the active sites decrease over time [18].Short contact time is necessary for the economy of the system.
As the dye concentration increased, the adsorption efficiency decreased.This was also observed in the study conducted by Ertugay [18].An increase in dye concentration may cause a driving force for adsorption and increase the possibility of collision between dye molecules and adsorbent particles [18,40].However, at higher dye concentrations, adsorption may be reduced by decreasing the active sites [9].
Temperature is a parameter that changes the adsorption equilibrium by affecting the adsorption capacity and diffusion rate of dye molecules [18].As the temperature increases, the decrease in efficiency indicates that there is an exothermic mechanism.As the temperature increases, the solubility of the dye increases and as a result, the interaction forces between the dye and the adsorbent may be reduced [18,40].In other words, the Brownian movement of the dye molecules may increase and the adsorption efficiency may be decreased [40].
As the amount of adsorbent increases, the efficiency is expected to increase as active regions increase [18].However, in the present study, 0.05 g 100 mL −1 was determined as optimum for both adsorption systems.This situation may be attributed to overlap or aggregations of adsorption active sites.As a result of aggregation, it may cause a decrease in active sites for dye molecules and an increase in the length of the diffusion path [9].Since no activation process was performed for the SSO adsorbent used in the study, the adsorbent is likely to be aggregated.Adsorption capacity is controlled by two mechanisms.These are surface area and pore diameter.The pore diameter is a more effective parameter compared to the surface area.The pore diameter and the surface area should be large.Small pore diameter causes clogging in the pores [41].The relationship between adsorption capacity and particle size depends on the pollutant (chemical structure/ionic charge of the pollutant molecule) and the basic properties of the adsorbent (crystallinity, porosity and rigidity of polymeric chains) [42].Adsorption occurs at the surface.Smaller adsorbent sizes are clarified by larger surface areas and higher removal performance, according to Weber.The highest removal efficiency was obtained in this study at a large particle size (180 μm).This is not an expected result.However, the small size of the adsorbent is not preferred for large applications as it may cause head loss and blockages in the columns [42].Considering this information, some modification processes can be applied to SSO in order to improve the adsorption efficiency without reducing the particle size.
Mixing is a parameter that can affect the distribution of the dye in solution and the formation of the outer boundary film.There was a more frequent collision between the dye and the adsorbent surface at 200 rpm compared to 100 rpm.This situation was also observed in the study conducted by Hanafiah et al. [43].With proper stirring speed, the boundary layer thickness around the adsorbent particles decreases and thus the interaction of adsorbent and dye molecules increases [43].At higher stirring speeds, possibly adsorbed dye molecules pass back into the water.Of course, this possibility may vary depending on the operating conditions.For example, in the adsorption of Acid Blue 25 to sawdust, it has been stated that the dye adsorption remains constant at 500 rpm [43].

Estimated cost
Adsorption, one of the advanced oxidation methods, can be preferred as pre-treatment or final treatment.This method, which is used to reduce toxicity, can also provide water recovery [3,44].Because it is a process that does not use chemicals, it does not cause different colour formations [9].The factors that prevent the adsorption process from being preferred frequently in treatment; the adsorbent is disposable (requires regeneration), and the adsorbent sometimes causes turbidity.The disposable adsorbent increases the solid waste load.Wastes generated instead of commercial activated carbon can be evaluated both to reduce the operating cost and not to increase the solid waste load.
Due to the simplicity of the adsorption system, the investment cost is often economical.When the cost, lifetime, annual interest rate, annual flow and contact time of the adsorption unit are taken as 10,000 $ m −3 , 10 years, 4%, 30,000 m 3 (8 hours a day, 300 days a year) and 45 minutes respectively, the investment cost was calculated as approximately 39$ m −3 .The operating conditions (10 ppm dye concentration, 200 rpm stirring speed, 0.05 g 100 mL −1 adsorbent dosage, 180 µm adsorbent size, 25°C temperature, natural pH (5.7 and 6.5) and 45-min contact time) that the highest removal efficiency is achieved, are similar for both adsorption systems.The operating cost consists of adsorbent, regeneration of used adsorbent, chemicals used for pH adjustment (NaOH, HCI etc.) and energy consumption (system control, temperature, etc.).The adsorbent used (SSO) is an adsorbent produced for the purpose of evaluating waste.For this reason, the adsorbent and regeneration costs can be neglected.In this case, the operating cost consists of energy consumption (temperature, system control, etc.) and maintenance cost.Maintenance cost was calculated as 0.008 $ m −3 with the acceptance of 2% of the investment cost.The electricity consumption cost of the adsorption process, which is an annual flow and contact time of 10,000 $ m −3 and 40 minutes, respectively, was calculated as approximately 0.14 $ m −3 [45].In this study, the reaction time is 45 minutes and the total operating cost was calculated as roughly 0.15 $ m −3 .Total cost was determined as 0.54 $ m −3 .

Conclusion
This study emphasised that spent Salvia officinalis (SSO) can be used as an alternative and effective adsorbent material for the removal of methylene blue (MB) and maxilon yellow (MY) dyes from aqueous solutions.The optimum experimental conditions were determined as natural pH (5.7 for MB and 6.5 for MY), particle size of 180 µm, stirring speed of 200 rpm, adsorbent dosage of 0.05 g 100 mL −1 and 25 C.In these conditions, the highest removal efficiencies of MB and MY were obtained as 78% and 56%, respectively.Adsorption of MB and MY dyes on SSO fit the Freundlich isotherm model (R 2 : 0.965 for MB and 0.973 for MY) better than the others.The maximum adsorption capacities obtained from the isotherm model were 16.53 and 11.79 mg g −1 for MB and MY.The pseudo-second-order kinetic model was found to be the most suitable model for MB-SSO and MY-SSO systems (R 2 :0.998 for MB and 0.985 for MY).The pseudo-second-order kinetic model revealed that adsorption is affected by a number of active sites on the adsorbent as well as the concentration of the dye.Changes in free energy, enthalpy, and entropy showed that the adsorption of dyes onto SSO was thermodynamically favourable, spontaneous, exothermic, physisorption, and increased randomness at the solid/liquid.
More studies need to be done using different dyes or different types of contaminants to better evaluate the capacity of the SSO adsorbent.The estimated cost of the proposed process has been taken into consideration.However, a detailed techno-economic feasibility analysis should be carried out before broader applications.In addition, the enhancability of adsorption capacity and desorption capacity of the adsorbent should be investigated.Thus, information about the reusability and recovery of the adsorbent is provided.
As a result, instead of being evaluated as garbage after the consumption of Salvia officinalis, which we use for different purposes (tea, meditation, spice in meals, health, etc.) in our homes, its usability as an adsorbent will make the adsorption system costeffective.The findings of this study will also give an idea for efficient adsorbent production by using different activation techniques of spent Salvia officinalis.

Figure 2 .
Figure 2. Optimising adsorbent dosage, particle size, temperature, stirring speed, dye concentration and pH for Methylene Blue dye.

Figure 4 .
Figure 4. FTIR group changes of solid SSO before and after treatment (a-clean SSO, b-SSO after MB removal, c-SSO after MY removal).

Figure 5 .
Figure 5. FTIR group changes of dye solutions before and after treatment (a-MY before treatment, b-effluent after MY removal, c-MB before treatment, d-effluent after MB removal).

Table 1 .
R 2 values of the isotherms.Freundlich and Temkin isotherm models.R 2 values of isotherms calculated for MB and MY dyes are given in Table1.Isotherm graphics are given in the appendix FigureS4-S5.

Table 2 .
Kinetic coefficients for both adsorption systems.

Table 3 .
Gibbs free, enthalpy and entropy energies for both adsorption systems.