Utilizing of bio-adsorbent in zero waste concept: adsorption study of crystal violet onto the centaurea solstitialis and verbascum thapsus plants Sıfır atık konseptinde biyo-adsorban kullanımı: kristal violet'in centaurea solstitialis ve verbascum thapsus bitkileri üzerine adsorpsiyon çalışması

Öz Adsorption is one of the most widely used methods for dye removal from water. At the end of the adsorption process, the dyed adsorbents emerging as a second-order waste which is the main disadvantage of this process. In this study, the removal of a synthetic dye Crystal Violet (CV) from the synthetic wastewater by using Centaurea solstitialis (CS) and Verbascum Thapsus (VT) plants was studied by adsorption. At the end of the adsorption process, the reusing potential of the dyed plant was explored by measuring the dyed plant calorific values. Experimental design and modeling were performed using the surface response method (RSM). The regression coefficients for developed models were 0.86 for the CS plant and 0.95 for the VT plant. Adsorption process for CS and VT plants were fitted by Dubinin-Radushkevich isotherm, and Temkin isotherm, respectively. The two plants were pseudo-second-order, endothermic, and found to be chemically. CS plant had a capacity of 84.03 mg.g -1 with a contact time of 85 min. The capacity of the VT plant reached 109.89 mg.g -1 at a contact time of 55 min. The calorific values results show increases in the calorific values for the two plants after the adsorption process. The CS plant increased from 4003.774 to 4458.059 Kcal.Kg -1 . Whereas the VT plant increased from 3206.028 to 4120.330 Kcal.Kg -1 . These values indicate the using possibility of the two plants as solid fuel by burning the dyed plants in emission controlled industrial facilities by applying the zero-waste concept.


Introduction
With the increase in industrial activities in recent years, the amount of dyes used to color products has also increased [1]. Significant amounts of dyes are used, especially in the textile, paper, leather, paint, printing, food and plastic industries. In industries where dyes are used, the discharged wastewater resulting from the process without color removal sufficiently causes significant environmental problems. The discharged dyes affect the chemical structure of the water. Also, it disrupts the aesthetic appearance of the water and prevents the passage of light into the water body. All these have negative effects on the aquatic ecosystem [1], [2].
Following the Regulation on Water Pollution Control (RWPC) in Turkey published in the Official Gazette dated 31.2.2004, and * Corresponding author/Yazışılan Yazar numbered 25687 within the scope of the Environmental Legislation, the color parameter was added to the discharge criteria for treated wastewater with the amendment dated 24/4/2011 and numbered 27914. According to this regulation, the color parameter for the composite samples after 2 h should be 280 Pt-co and 260 Pt-co after 24h [3].
Crystal Violet (CV), also methyl violet or basic violet, is a cationic triphenylmethane dye [4]. Triphenylmethane dyes are well soluble in water since they contain sulfonic acid [5]. CV dye can be used as a pH indicator, it is used to classify gram bacteria, textiles, food additives, cosmetics, plastics, and paper prints [6], [7]. Despite its wide range of applications, CV dye is known to have toxic effects [8], [9]. For these reasons, to protect water resources and human health, the CV dye should be treated before being delivered to the receiving environment [9].
There are different methods used in dye removal from water. Some of them; coagulation-flocculation [10], photocatalytic oxidation [11], reverse osmosis [12], ozonation [13], chemical oxidation [14], membrane filtration [15] and adsorption. Adsorption is a preferred method for dye removal among other advanced treatment techniques, due to its economic and environmental friendliness [7]. The limitation of this technique is the generation of the second-order contaminant.
Adsorption is the process by which molecules in one substance adhere or attach to the surface of another substance [16]. In the adsorption process, the properties of the adsorbents and the adsorbed substances are affected by factors such as pH, temperature, presence of ions in the environment, adsorption kinetics and time [17].
Adsorbent materials are divided into two as natural and synthetic adsorbent materials. Natural adsorbents are classified as inorganic (zeolite, clay, perlite, etc.) and organic according to their structure. The most common synthetic and inorganic adsorbent is activated carbon. It has the highest adsorption capacity. Because of its costs, the researchers are investigating alternative adsorbent materials. The use of natural, organic, and inanimate adsorbents such as agricultural by-products and plant wastes as adsorbents is cheaper and environmentally friendly [18], [19].
These adsorbents are preferred because they do not dissolve in water, have a large surface area, and contain functional groups such as lignin, hydroxyl (OH) and carboxyl (COOH), which are connect to the dyes in wastewater and remove it from the water [20]. It is also possible to increase the adsorption capacity by replacing these functional groups with simple activation processes [21]. The only disadvantage of natural herbal adsorbent materials is that they are not suitable for every dye [22].
As mentioned above, the limitation of the adsorption process is the generation of the second-order contaminant. The dyed adsorbents remaining after the adsorption process are evaluated as hazardous wastes. These wastes cannot be disposed of in landfills, but they can be disposed of in hazardous waste disposal facilities. Hazardous waste incineration license plant is one of the possible solutions as they can be used within the scope of Waste Derived Fuel (ATY). According to Turkish Communique (2014), the wastes with the energy larger than 2500 kcal/kg will be used together as additional fuel to obtain energy in incineration plants. The communique stated that absorbents and filter materials contaminated with dangerous substances can be accepted to the facilities as ATY [23].
Response Surface Method (RSM) is a method using statistical and mathematical techniques for experimental design, model development and investigation of the effect of independent variables [24], [25]. It is important to use RSM in experimental design and modeling to reduce the use of chemicals and shorten the test times.
Centaurea solstitialis (CS) is a thorny plant observed in many parts of the world. It also found in agricultural areas in Turkey in the vacant land and arid slopes [26]. It obstacle the development and production yield of the plant grown on agricultural land. The CS plant, which is removed from agricultural lands by various means, emerges as waste with no economic value [27].
Verbascum Thapsus (VT) is a leaf hairy plant that grows in many parts of Turkey. The flower part of this plant contains essential oil and glycoside. Also, because the seeds are poisonous to fish is used in fishing. The flower part and the leaf part are generally considered to be good for some diseases such as colds and are considered to be medicinal plants by local people [28].
This study aims to investigate the CV removal possibility by CS and VT plants as low-cost bio-adsorbents from the aqueous solution. It also aims to study the potential reusing of the dyed plants in emission controlled industrial facilities to eliminate the second-order contaminants by applying the zero-waste concept.

Adsorbent preparation
The CS plant was obtained from Çevttepesi of Erdemli District of Mersin Province. The VT plant was collected from the Harfilli Neighborhood of Erdemli District of Mersin Province. CS and VT plants were washed with pure water to remove dust, soil, etc. It was then dried in a HAREUS oven at 105 °C for 24 hours. The dried adsorbents were milled with the ISOLAB blender. Based on the preliminary experiment results, the adsorbents were passed through a 35 mesh sieve (Figures 1 and 2).

Crystal violet preparation
The chemical formula of the cationic CV dye is C25H30N3Cl (Figure 3), with a molecular weight of 407.98 g.mol -1 . 1000 ppm CV stock solution prepared from MERCK brand dye was used in the experiments. In the experiments, 0.1 M NaOH and 0.1 M HCl were used to adjust the pH.

Adsorbent Characterization
In this study, the weights were measured by ISOLAB precision balance. MICRO TEST-ELECTROFOREST shaking incubator was used to mix the solutions at definite temperatures and 150 rpm. ISOLAB pH meter was used for acid-base determination. UV spectrophotometer (HACK DR-3900) was used to determine the color change in the experiments at 585 nm wavelength. Fourier Transform Infrared Spectroscopy (FTIR) was used to measure the infrared spectra of adsorbents (CS and VT). The surface area determinations were carried by Brunauer, Emmett, and Teller (BET). Scanning Electron Microscopy (SEM) (Gemini Zeiss Supra 55) devices were used to determine surface morphology.

Adsorption experiments
In the adsorption experiments; pH, adsorbent mass, initial concentration of crystal violet, temperature and the contact time were optimized. 100mL solution volume was used. The solutions were placed in 250 mL flasks in case of overflow during agitation. After the experiment, efficiency (%) and adsorbent capacity (qe) (mg.g -1 ) were calculated using Equation 1 and Equation 2, respectively [29].
Where, Ci is the initial concentration (mg.L -1 ) of the CV dye; Cf is the CV final concentration (mg.L -1 ); V solution volume (L); m indicates the adsorbent mass (g).
Desorption process was carried out as shown in the previous studies [27], [29] with some modification. The dyed adsorbent obtained at the end of the adsorption process was shaken with 0.1 M HCl and 0.1 M NaOH solutions at 150 rpm for 24 hours to calculate the percentage of recovery (desorption). The recovery percentage was calculated using Equation 3:

Adsorption isotherms
To explain the relationship between adsorbent and adsorbed substance; Langmuir [30], Freundlich [31], Temkin [32] and Dubinin-Radushkevich (D-R) isotherms were used [27], [32]. Langmuir isotherm is based on the assumption of single-layer adsorption on the homogeneous surface of the adsorbent (Equation 4) [30]. Freundlich isotherm refers to an empirical equation in the adsorption process (Equation 5) [31]. Temkin isotherm explains the adsorption mechanism by energy distribution (Equation 6) [32]. Dubinin-Radushkevich (D-R) isotherm explains the distribution of Gaussian energy on a heterogeneous surface [27]. Furthermore, D-R isotherm is related to the porous structure of the adsorbent and provides information about the physical or chemical adsorption process (Equation 7). If the term [RTLn (1 + 1 C A )] 2 has a value higher than 8 kJ.mol -1 , the adsorption occurs chemically [33].

Adsorption kinetics
Adsorption kinetic models have been used to express the behavior of the adsorption reaction. Also, the models describe the mechanism by which the adsorbent substance interacts with the adsorbed substance [17], [32]. The linear forms of the pseudo-first-order kinetic model [34] and pseudo-secondorder kinetic model [35] are shown in Equation 8 and Equation  9 respectively.
In the equations; qe is the amount of adsorbed material in equilibrium; qt, is the amount of adsorbed material at the end of time t; K1 represents the first-order rate constant (min -1 ) of adsorption and K2 shows the false second-order rate constant (g.mg -1 .h-1 ).

Response surface method (RSM)
RSM is a modeling technique derived from a group of mathematical and statistical data based on experimental data [37]. In these experiments, the ranges of pH, CS and VT masses, CV concentration, contact time, and temperature were determined. A model was created by Design Expert v11 program. The ranges of the parameters are shown in Table 1.

Adsorbent characterization
In this study, the surface areas for CS and VT plants were determined by BET analysis. The surface area decreased for both plants after the adsorption process. BET and Langmuir surface areas of both plants decreased after adsorption ( Table 2). Surface morphologies for two plants were obtained by SEM analysis. Figure 4 shows the adsorbent morphology before and after the adsorption process.
The FTIR analysis shows a shıftıng ın the peaks. It also shows change in the intensity of those peaks. This can support the occurrences of the adsorptıon process. By comparison between the graphs before and after the adoption process, many peaks disappeared at the end of the adsorption process ( Figures 5 and 6). In CS plant, peaks at bands of 490.79, 536.11, 1317.14, and 1603.52 cm -1 were not recorded after the adsorption process.
In contrast, many new peaks were noticed. The peak at the band of 2328 cm -1 shows C≡N starch bond. C-H bend bond was recorded at the peak of 1455cm -1 . N-O asymmetric stretch bond at 1506 cm -1 was detected. In addition to that, a new peak at 1584 cm -1 shows a possible N-H bend bond formation. C-CL bond was also noticed at the band of 828.28 cm -1 . The formation of new peaks gives a hint about the type of adsorption.
In the VT plant, new peaks were not be recorded. By contrast, many peaks have been shown in the raw VT and have not been recorded after the adsorption process.

RSM results
The developed models for the CV adsorption onto the adsorbents (CS and VT) were established by RSM. The two models were tested statistically and founded to be significant.

Effects of parameters
Upon adsorption of CV dye onto CS plant; the effects of pH and mass parameters were determined. The pH has the minimum effect for the adsorption capacity. The mass has an inverse relationship with capacity. The maximum capacity was reached at 0.5 g of plant and pH 8, Figure 7(a).
For the VT plant; the capacity has a proportional relationship with pH, although it has an inverse relationship with mass. The maximum capacity has been reached at pH 8, and 0.5 g of plant mass is shown in Figure 7(b).  The effect of temperature on the adsorption process was examined. In CS plant the capacity has increased sharply with temperature. VT plant capacity had also increased with temperature but with more flat trend. The temperature effects are shown in Figure 9.

Adsorption isotherm
In this study, Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherms were examined. Isotherm coefficients and regression coefficient results are given in Table 3. For the CS plant, the D-R isotherm had a higher regression value (R=0.9906) than the other isotherms. The adsorption expected to occur chemically, because the value of [RT Ln (1+ (1/CA))] 2 (23.57 kJ.mol -1 ) is higher than 8 kJ.mol -1 . For the VT plant, Temkin isotherm had the best fit with a regression value of 0.9221.

Adsorption kinetics
In this study, pseudo-first-order and pseudo-second-order kinetics were examined. The kinetics coefficient results are given in Table 4. The regression coefficients of CS and VT plants were 0.9942 and 0.9943; pseudo-second-order kinetics are 0.9998 and 0.9999, respectively. According to these results, the adsorption kinetics of both plants were found to be suitable for pseudo-secondorder kinetics ( Table 4). The Kinetic results explain the absence of chemical and physical equilibrium. It also states that there are two reactions. The first is faster and reaches the equilibrium state faster, the second one is slow reaction and reaches the equilibrium state in a long time.

Adsorption thermodynamics
In this study, enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) were studied for the CS and VT plants. The results are given in Table 5. In both plants; • The adsorption event occurs spontaneously since the Gibbs free energy (∆G) was found to be negative for all temperatures, • The positive values of the Enthalpy (∆H) indicate that the adsorption is the endothermic reaction, • Entropy values (∆S) were positive. This indicates that there may be structural changes between CS and VT plants and CV dye.

Desorption
Desorption experiments were carried out as a result of adsorption experiments. The plants with dye were shaken in 0.1 M HCl and 0.1 M NaOH solutions at 150 rpm in the shaker for 24 hours. The maximum desorption yield was obtained by the interaction of both plants with acid for 120 minutes. Desorption percentages for the CS plant and VT plant were 0.76% and 1.88%, respectively. According to the results, the adsorption process was considered to be chemical because there was no dye recovery.

Zero waste concept utilization
The zero waste concept was successfully applied in this study. The second order contaminants from the adsorption process were planned to be reused as solid fuel in emission controlled industrial facilities. To achieve that, the calorific values were measured and recorded as shown in Table 6.   [41]. Kulkarni et al (2017) founded the adsorption capacity of CV onto water hyacinth to be 322.58 mg.g -1 [42]. Table 7 shows some of these studies.
Even though the adsorption capacity stated in this study is relatively smaller than the literature studies, both plants show a high removal efficiency of CV from aqueous solution. In addition to that the capacity is larger than the powdered activated carbon (PAC) and Afsin-Elbistan fly ash obtained by Eren and Nacar (2006) [43].

Conclusion
In this study, the differences between adsorption processes of Centaurea solstitialis (CS) and  [47], [48]. According to these values, the dyed adsorbents can be used within the scope of waste derived fuel (ATY). Cheremisinoff (2003) has reported that the waste can be converted to energy by incineration method, which is the second most frequent method [49]. Concerning that, the adsorption by-products could be used to generate energy by using it as a possible partial replacement of coal in the emission controlled industrial factories. Thus, many benefits will be achieved; CV will be removed from aqueous solution by lowcost materials, the adsorption by-products will be used as fuel, the uses of coal expected to be lower. In addition to that, the air pollution resulting from the raw plant burning process as a disposal method by the farmers will be decreased.
As a result from this study, the two plants (CS and VT) had been used successfully in CV removal from aqueous solution. Also the utilization of these plant in zero waste concept was achieved successfully.

Acknowledgements
The authors acknowledge the funding provided by the department of scientific research projects of Mersin University (Project Number: 2020-1-TP3-4010).

Author contribution statements
In the scope of this study, Mutlu YALVAÇ in the formation of the idea, the writing and editting, and the literature review; Hüdaverdi ARSLAN in the supplying the materials used; Mohammed SALEH in the formation of the idea, the writing and editing, the design and the literature review, Melis GÜN in the assessment of obtained results, and examining the results; Muhammed Şahin HEKİM in the experiment step in terms of content were contributed.

Ethics committee approval and conflict of interest statement
There is no need to obtain permission from the ethics committee for the article prepared.
There is no conflict of interest with any person / institution in the article prepared.