Black Tea Waste as Green Adsorbent for Nitrate Removal from Aqueous Solutions

The aim of the study was to prepare effective low-cost green adsorbents based on spent black tea leaves for the removal of nitrate ions from aqueous solutions. These adsorbents were obtained either by thermally treating spent tea to produce biochar (UBT-TT), or by employing the untreated tea waste (UBT) to obtain convenient bio-sorbents. The adsorbents were characterized before and after adsorption by Scanning Electron Microscopy (SEM), Energy Dispersed X-ray analysis (EDX), Infrared Spectroscopy (FTIR), and Thermal Gravimetric Analysis (TGA). The experimental conditions, such as pH, temperature, and nitrate ions concentration were studied to evaluate the interaction of nitrates with adsorbents and the potential of the adsorbents for the nitrate removal from synthetic solutions. The Langmuir, Freundlich and Temkin isotherms were applied to derive the adsorption parameters based on the obtained data. The maximum adsorption intakes for UBT and UBT-TT were 59.44 mg/g and 61.425 mg/g, respectively. The data obtained from this study were best fitted to the Freundlich adsorption isotherm applied to equilibrium (the values R2 = 0.9431 for UBT and R2 = 0.9414 for UBT-TT), this assuming the multi-layer adsorption onto a surface with a finite number of sites. The Freundlich isotherm model could explain the adsorption mechanism. These results indicated that UBT and UBT-TT could serve as novel biowaste and low-cost materials for the removal of nitrate ions from aqueous solutions.


Introduction
Innovative environmentally friendly methods to recycle industrial and food waste are needed at present because of the recent acceleration of climate change. The world is increasingly heading toward a closed-circuit economy, an alternative economic model centered on the idea of ending the life cycle of a commodity [1][2][3][4].
Recent findings from laboratory research indicate that it may be possible to develop effective and reasonably priced adsorbents using various by-products from the agro-food industry, such as tea waste, coffee waste, or seed and fruit waste, to remove organic compounds and heavy metals from contaminated aquatic environments. An abundant natural substance or one that is a waste product or byproduct of industry or agriculture that requires little to no processing is considered a low-cost adsorbent [5][6][7][8][9][10].
Following the "3 Rs", the circular economy seeks to address pollution issues by returning of materials to another life cycle. In such a situation, waste is seen as a valueadded substance [7,10,11].
Due to their high solubility and nutritional importance for aquatic plants, nitrates are important components of agricultural fertilizers. In order to meet their nutrient needs, plants are gradually replacing the commercially available adsorbents because they have several key advantages, including low cost, availability, the ability to be recycled into products with added value for waste removal, a rich chemical composition with high surface area, and the potential for modification. The literature reveals a growing demand for biosorbents that are environmentally friendly and which have low costs [29,30].
One of the inexpensive green adsorbent plant materials for the elimination of various contaminants is tea. Tea leaf cellulose and hemicelluloses, lignin, structural proteins, and condensed tannins make up most of the cell wall [18,31].
Due to the variety of functional groups contained in tea leaves, they have excellent potential for the removal of organic or inorganic contaminants from wastewater. Large amounts of solid tea waste could be used to improve soil fertility [9,13]. The second most popular beverage after water is tea. According to a study by the United Nations Food and Agricultural Organization (FAO), global exports are rising and are projected to reach 750,981 tons by 2023 [3,31].
In the current study, black tea waste was evaluated as a potential biosorbent for the removal of nitrate ions from synthetic wastewater samples. The adsorbents were obtained either by thermally treating spent tea to produce biochar (UBT-TT), or by employing the untreated tea waste (UBT) to obtain convenient bio-sorbents.
The adsorbents were characterized before and after nitrate adsorption by Scanning Electron Microscopy (SEM), Energy Dispersed X-ray analysis (EDX), Infrared Spectroscopy (FTIR) and Thermal Gravimetric Analysis (TGA). The experimental conditions, such as pH, temperature and nitrate ions concentration, were studied to evaluate the interaction of the nitrate with adsorbents and the potential of the adsorbents for the nitrate removal from synthetic solutions. The Langmuir, Freundlich and Temkin isotherms were applied to derive the adsorption parameters based on the obtained data.
The reusability potential of the adsorbents was assessed to elucidate their efficacy.

Materials and Reagents
All chemicals and reagents used in this study were of analytical grade and were utilized as supplied without any additional treatment.
Potassium nitrate (KNO 3, analytical reagent, ≥98.0%, Merck, Darmstadt, Germany) was used to prepare the model (synthetic) solutions. A stock solution of a concentration of 1000 mg L −1 (as N-NO 3 − ) was prepared by dissolving 1.6290 g KNO 3 in 1000 mL demineralized water. The experimental solutions were prepared by diluting the stock solution to the desired concentration (50-500 mg L −1 ). Used black tea adsorbents were prepared from commercial fresh black tea leaves.

Preparation of Adsorbent
Used black tea adsorbent (UBT) was prepared from commercial fresh black tea leaves by boiling with distilled water for 20 min and then filtering. UBT was boiled repeatedly until the remaining solution became colorless. This treatment was repeated eight times. Washing should remove all substances contained in spent tea that could cause contamination (such as polysaccharides, tannins, and colored components). After the extraction of black tea liquor, the leaves were dried in an oven (Binder, Germany) at 105 • C for 48 h. Dried tea residues were ground and sieved through the sieves of mesh sizes ≤ 0.5 mm, and they were stored in air-tight bottles for adsorption experiments. These adsorbents were labeled 'used black tea' (UBT).
Portions of these samples were further thermally treated at 400 • C for 2 h, and the resultants were labeled 'thermally treated black tea waste' (UBT-TT). The carbonaceous material was carbonized at 400 • C in a muffle furnace (Nabertherm, Germany). The final biochar obtained was washed thoroughly after cooling with distilled water until the pH of the rinsing water was neutral. The adsorbent thus obtained was dried in an oven at 105 • C for 24 h, and then sieved with an 80-200 mesh sieve.
Finally, the resulting adsorbents were used for the experimental adsorption studies.

Preparation of the Samples Artificially Contaminated with NO 3 − Ions
Water samples artificially contaminated with NO 3 − ions were prepared by dissolving the required amounts of KNO 3 in deionized water to a final concentration of 500 mg/L stock solution. Further dilutions were prepared in the same solvent.
The pH of the solutions was adjusted to the desired value using 0.1 mol/L HCl or NaOH solutions. The calibration curve for the determination of NO 3 − at the selected pH value was prepared by measuring different concentrations of NO 3 − (50-500 mg/L) at 410 nm.
The concentration ranges existing in the industrial raw effluents was selected for this research, and after the variation of the initial pollutant concentration, the value 300 mg/L was chosen for the working concentration. The desired concentrations were obtained by a dilution technique. All other chemicals used in the present study were of laboratory grade, and standard procedures were followed for all experiments.

Adsorbent Characterization
UBT and UBT-TT morphology and surface characteristics were studied using a field emission scanning electron microscope coupled with energy-dispersive spectra EDS (FEI Scios DualBeam FIB SEM, Thermo-Fisher, Brno, Czech Republic). The Fourier transform infrared spectrometer (FT-IR) (Shimadzu IR TRACER-100, Kyoto, Japan) was used for the identification of functional groups on the UBT and UBT-TT surface involved in nitrate adsorption.
The thermal decomposition characteristics of UBT and UBT-TT adsorbents were monitored using TGA/DTG (thermogravimetric/derivative thermogravimetric).

Adsorption Equilibrium Studies
To evaluate the nitrate adsorption performance of used black tea (UBT) adsorbents, batch experiments were carried out in a batch-type procedure using a shaker (benchtop shaker laboratory incubator 300 × 400 MM, 70DEG C, Essex, UK). Samples in Erlenmeyer flasks were removed from the shaker at predetermined time intervals. After filtration, the residual nitrate concentration was determined at 410 nm using a UV/Vis spectrophotometer (CECIL 1021, 1000 Series, Bristol, UK) according to salicylic acid method [32,33].
The influence of the nitrate concentration (50-300 mg/L), pH (2.0-10.0), and temperature (15-40 • C) on the adsorption process were studied. The initial solution pH value was adjusted by adding 0.1 mol/L HCl or NaOH solutions. For all experiments, the adsorbent weight was 0.2 g and the contact time was 90 min.
The kinetics were examined at various nitrate concentrations, and the isotherms were carried out at different temperatures. The amount of nitrate retained by the adsorbents, q e (mg g −1 ) for all samples was calculated by Equation (1) [12][13][14][15]29]: where q e is the nitrate uptake (mg g −1 ); C 0 (mg/L) is the initial concentration of the nitrate; C e (mg/L) is the equilibrium nitrate concentration, at equilibrium; V is the volume of the solution (L), and W is the weight of the adsorbent (g).

SEM Analysis
To determine the structure and morphology of the adsorbent materials, Scanning Electron Microscopy (SEM-FEI Scios DualBeam FIB SEM) coupled with Energy-Dispersive Spectra (EDS), equipped with a field emission gun and a 1.2 nm resolution X-ray energy dispersive spectrometer with a resolution of 133 eV were used (Thermo Fischer). The FEI Scios™ is an ultra-high resolution DualBeam™ analytical system that achieves outstanding 2D imaging for a wide range of samples.
The original adsorbent, which was waste black tea (UBT), had a diversified, coarse and highly porous surface composed of numerous fibrous bonds and spongy formations, providing a wide field for the adsorption of the nitrate (Figure 1a,b). The main components of tea are cellulose and hemicellulose, and it was observed that UBT presented a stem structure ( Figure 1a). solution (L), and W is the weight of the adsorbent (g).

SEM Analysis
To determine the structure and morphology of the adsorbent materials, Scanning Electron Microscopy (SEM-FEI Scios DualBeam FIB SEM) coupled with Energy-Dispersive Spectra (EDS), equipped with a field emission gun and a 1.2 nm resolution X-ray energy dispersive spectrometer with a resolution of 133 eV were used (Thermo Fischer). The FEI Scios™ is an ultra-high resolution DualBeam™ analytical system that achieves outstanding 2D imaging for a wide range of samples.
The original adsorbent, which was waste black tea (UBT), had a diversified, coarse and highly porous surface composed of numerous fibrous bonds and spongy formations, providing a wide field for the adsorption of the nitrate (Figure 1a,b). The main components of tea are cellulose and hemicellulose, and it was observed that UBT presented a stem structure (Figure 1a).
A rougher surface area and the broadly distributed pores of UBT-TT can offer an effective surface area and more opportunities for the binding of nitrate ions. The UBT-TTloaded adsorbent showed uniform coverage of UBT-TT by nitrate, leading to a reduction in surface porosity. This surface characteristic would substantiate the higher adsorption capacity of UBT-TT (Figure 1c  Energy Dispersed X-Ray (EDX) Measurements The EDX spectra of adsorbents (UBT and UBT-TT) are shown in Figure 2. The EDX spectra of adsorbent sample UBT showed that on the surface of all the samples, carbon and oxygen were mostly present. The EDX spectra also showed the presence of other elements, such as Mg, K, Mn, Si, P, S and Mn (Figure 2a). After the thermic treatment, the content of carbon increased from 54.13 to 63.06%, while the oxygen decreased from 45.05 to 36.59% due to the loss of moisture and some phosphate and sulfate groups. This affirmation is supported by the decrease in the values recorded for P and S from 0.20 to 0.13%, and 0.18 to 0.1% (Figure 2b), respectively. This behavior was also reported by P. Wijeyawardana et al. [34] and S. Suman et al. [35]. A rougher surface area and the broadly distributed pores of UBT-TT can offer an effective surface area and more opportunities for the binding of nitrate ions. The UBT-TTloaded adsorbent showed uniform coverage of UBT-TT by nitrate, leading to a reduction in surface porosity. This surface characteristic would substantiate the higher adsorption capacity of UBT-TT (Figure 1c,d).

Energy Dispersed X-ray (EDX) Measurements
The EDX spectra of adsorbents (UBT and UBT-TT) are shown in Figure 2. The EDX spectra of adsorbent sample UBT showed that on the surface of all the samples, carbon and oxygen were mostly present. The EDX spectra also showed the presence of other elements, such as Mg, K, Mn, Si, P, S and Mn (Figure 2a). After the thermic treatment, the content of carbon increased from 54.13 to 63.06%, while the oxygen decreased from 45.05 to 36.59% due to the loss of moisture and some phosphate and sulfate groups. This affirmation is supported by the decrease in the values recorded for P and S from 0.20 to 0.13%, and 0.18 to 0.1% (Figure 2b), respectively. This behavior was also reported by P. Wijeyawardana et al. [34] and S. Suman et al. [35]. spectra of adsorbent sample UBT showed that on the surface of all the samples, carbon and oxygen were mostly present. The EDX spectra also showed the presence of other elements, such as Mg, K, Mn, Si, P, S and Mn (Figure 2a). After the thermic treatment, the content of carbon increased from 54.13 to 63.06%, while the oxygen decreased from 45.05 to 36.59% due to the loss of moisture and some phosphate and sulfate groups. This affirmation is supported by the decrease in the values recorded for P and S from 0.20 to 0.13%, and 0.18 to 0.1% (Figure 2b), respectively. This behavior was also reported by P. Wijeyawardana et al. [34] and S. Suman et al. [35].

ATR-FTIR Analysis
Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) was used to characterize the samples before and after the addition of NO 3 − . The surface chemistry of UBT and UBT-TT was studied using a FTIR Spectrophotometer (Shimadzu IR TRACER-100, Kyoto, Japan) in the region of 4000-400 cm −1 . The spectra of UBT and UBT-TT samples before and after nitrate adsorption were examined and compared to identify the functional groups on the adsorbent (Figures 2-4). Like other biomasses, black tea leaves primarily consist of cell walls (comprised of cellulose, hemicellulose, lignin, proteins, condensed tannins, etc.). These components have a variety of functions (hydroxyl, carboxylate, etc.) that can effectively help remove various contaminants [8].
the UBT sample exhibits a broad band around 3310 cm and a peak at 1624 cm , which i attributed to the stretching vibration of the banded -OH groups from alcohols [15]. Th peaks at 2358, 2918 and 2848 cm −1 are attributed to C≡N stretch [36] and the C-H and −CH2-stretching vibration modes [1]. The absorption peaks in the region of 1458-837 cm − indicates the presence of esters and ethers groups [37].
After thermal treatment, the corresponding absorption peaks to esters, ether groups and C-H and −CH2stretching vibration modes decrease significantly in terms of inten sity. The UBT-TT exhibit a broad peak at 3313 cm −1 attributed groups from alcohols, [15 and two peaks at 2358 and 1595 cm −1 assigned to the C≡N stretch [36] and the amine group [38]. The FT-IR spectra of tea biomass before and after nitrate loading were very simila ( Figure 4). The only significant difference was the decrease in peak absorbance at 3310 1624 and 1031 cm −1 that indicated that the -OH and C-O groups were involved in nitrat adsorption [28].  The same behavior was also observed for thermal treated tea biomass (UBT-TT) before and after nitrate adsorption ( Figure 5). However, a slight difference was observed consisting of the splitting of the band at 1595 cm −1 into two small sharp bands at 1562 and 1543 cm −1 , probably due to the N-H bending vibration of the secondary amide and the primary amine [39]. To study the effect of thermal treatment on biochar structures, the FT-IR analysis was used to identify the functional groups from UBT and UBT-TT. As can be seen in Figure 3, the UBT sample exhibits a broad band around 3310 cm −1 and a peak at 1624 cm −1 , which is attributed to the stretching vibration of the banded -OH groups from alcohols [15]. The peaks at 2358, 2918 and 2848 cm −1 are attributed to C≡N stretch [36] and the C-H and −CH 2 -stretching vibration modes [1]. The absorption peaks in the region of 1458-837 cm −1 indicates the presence of esters and ethers groups [37].
After thermal treatment, the corresponding absorption peaks to esters, ether groups, and C-H and −CH 2 -stretching vibration modes decrease significantly in terms of intensity. The UBT-TT exhibit a broad peak at 3313 cm −1 attributed groups from alcohols, [15] and two peaks at 2358 and 1595 cm −1 assigned to the C≡N stretch [36] and the amine group [38].
The FT-IR spectra of tea biomass before and after nitrate loading were very similar ( Figure 4). The only significant difference was the decrease in peak absorbance at 3310, 1624 and 1031 cm −1 that indicated that the -OH and C-O groups were involved in nitrate adsorption [28].
The same behavior was also observed for thermal treated tea biomass (UBT-TT) before and after nitrate adsorption ( Figure 5). However, a slight difference was observed consisting of the splitting of the band at 1595 cm −1 into two small sharp bands at 1562 and 1543 cm −1 , probably due to the N-H bending vibration of the secondary amide and the primary amine [39]. The same behavior was also observed for thermal treated tea biomass (UBT-TT) before and after nitrate adsorption ( Figure 5). However, a slight difference was observed consisting of the splitting of the band at 1595 cm −1 into two small sharp bands at 1562 and 1543 cm −1 , probably due to the N-H bending vibration of the secondary amide and the primary amine [39].

Thermogravimetric Analysis: Heat Treatment of Bio-Sorbents and the Adsorption Performance
The extent of heating and carbonization was evidenced from the thermo-gravimetric analysis (Figures 6-8). Since the thermal treated adsorbents have already undergone a thermal decomposition process in the production step, the weight loss of the bio-sorbent in TGA testing was not particularly large.
The thermal decomposition curves of UTB and UTB-TT are presented in Figures 6-8. As can be seen for both samples, the first degradation step with a weight loss of 6.45% for UTB and 8.84%, for UTB-TT, respectively, occurred between 40-130 • C due to the elimination of moisture and volatile compounds [3]. The UTB biomass exhibits a maximum weight loss of 58.53% around 352 • C that is caused by the pyrolysis of the cellulose and hemicellulose content [37]. The slow weight-loss process of lignin pyrolysis took place for UTB in the region from 520 to 700 • C ( Figure 6). Compared with UTB, the UTB-TT exhibits only the degradation step of the lignin residue [40] at 472 • C with a weight loss of 66.78% because of the thermal treatment at 400 • C, which caused the degradation of cellulose and hemicellulose. This observation is in agreement with FT-IR data.
The TGA curves of UTB and UTB-TT after nitrate adsorption (Figures 7 and 8) had similar degradation behavior, with the mention of the presence of an additional decomposition step of 9.25% around the temperature of 200 • C for UTB and 9.53% at 240 • C for UTB-TT, which can be caused by the degradation of the absorbed nitrate anion.
hemicellulose content [37]. The slow weight-loss process of lignin pyrolysis took p UTB in the region from 520 to 700 °C ( Figure 6). Compared with UTB, the UTB-TT e only the degradation step of the lignin residue [40] at 472 °C with a weight loss of because of the thermal treatment at 400 °C, which caused the degradation of cellulo hemicellulose. This observation is in agreement with FT-IR data.

Adsorption Performance
Numerous factors, such as initial adsorbate concentration, adsorbent dose, pH and temperature can affect the adsorption process differently. The adsorbent amount was 0.2 g and the contact time was 90 min.

Effect of the Initial Concentration
The adsorption capacity of UBT and UBT-TT samples, as a function of the initial concentration of a nitrate in aqueous solution is presented in Figure 9a,b The adsorbent sample was added to 50 mL of the nitrate solution. The influence of the initial nitrate concentration (50-300 mg/L) was studied.

Adsorption Performance
Numerous factors, such as initial adsorbate concentration, adsorbent dose, p temperature can affect the adsorption process differently. The adsorbent amount w g and the contact time was 90 min.

Effect of the Initial Concentration
The adsorption capacity of UBT and UBT-TT samples, as a function of the init centration of a nitrate in aqueous solution is presented in Figure 9a,b The adsorben ple was added to 50 mL of the nitrate solution. The influence of the initial nitrate c tration (50-300 mg/L) was studied.
When the nitrate concentration was low, there were more adsorption sites surface of the adsorbent, and the adsorption efficiency of the nitrate was higher. Ho the amount of nitrate exceeded the saturated adsorption sites of the adsorbent wh initial concentration of nitrate was higher, and the removal efficiency decreased ally. As a result, increasing the initial nitrate concentration reduced the removal eff of the adsorbent due to the limited number of adsorption sites. By increasing the When the nitrate concentration was low, there were more adsorption sites on the surface of the adsorbent, and the adsorption efficiency of the nitrate was higher. However, the amount of nitrate exceeded the saturated adsorption sites of the adsorbent when the initial concentration of nitrate was higher, and the removal efficiency decreased gradually. As a result, increasing the initial nitrate concentration reduced the removal efficiency of the adsorbent due to the limited number of adsorption sites. By increasing the initial concentrations of nitrite, the specific sites of the adsorbents become saturated, and the exchange sites fill up. Although the kinetics of the process are slowed by higher initial nitrate concentrations, the uptake percentage increases [6,19].

Effect of Initial pH
In this study, 0.2 g of adsorbent was added to 50 mL of nitrate solution. The influence of the initial pH was studied in the domain of 2.0-10.0. The initial solution pH value was regulated by 0.1 mol/L HCl or NaOH solutions. Solution pH has an important effect on nitrate adsorption. Most adsorbents lose their adsorption capacity under strongly acidic or basic conditions. The change in adsorption with pH ( Figure 9) can be explained by electrostatic interactions between adsorbents and adsorbates. The lowest values of the adsorption capacity were obtained at pH 2 and 4 for UBT and UBT-TT. Adjusting the pH with HCl affects the adsorption capacity. At lower pH values, the H+ ions compete with nitrate ions for the electrostatic surface charges in the system, decreasing the percentage of sorption [41]. The best values of the adsorption capacity were obtained at pH 6.5 for UBT and pH 8 for UBT-TT (Figure 10a,b). The highest adsorption capacity was achieved at neutral pH, mainly due to strong electrostatic interactions between the adsorption sites and the nitrate anions. A lower adsorption of nitrate at alkaline pH may be due to abundant HO-ions competing with contaminants for the same sorption sites.
concentrations of nitrite, the specific sites of the adsorbents become saturated, exchange sites fill up. Although the kinetics of the process are slowed by highe nitrate concentrations, the uptake percentage increases [6,19].

Effect of Initial pH
In this study, 0.2 g of adsorbent was added to 50 mL of nitrate solution. The in of the initial pH was studied in the domain of 2.0-10.0. The initial solution pH va regulated by 0.1 mol/L HCl or NaOH solutions.
Solution pH has an important effect on nitrate adsorption. Most adsorbents lo adsorption capacity under strongly acidic or basic conditions. The change in ads with pH ( Figure 9) can be explained by electrostatic interactions between adsorbe adsorbates. The lowest values of the adsorption capacity were obtained at pH 2 a UBT and UBT-TT. Adjusting the pH with HCl affects the adsorption capacity. A pH values, the H+ ions compete with nitrate ions for the electrostatic surface ch

Effect of Temperature
The adsorption capacity of nitrate anions reached the highest values at 15 • C for UBT-TT and at 25 • C for UBT (Figure 11a,b). Temperature is a factor that influences the mobility of nitrate ions in aqueous solutions as well as the surface properties of the adsorbent. The increased adsorption at higher temperatures is caused by the adsorption sites being more readily available [15,29]. the system, decreasing the percentage of sorption [41]. The best values of the adsorption capacity were obtained at pH 6.5 for UBT and pH 8 for UBT-TT (Figure 10a,b). The highest adsorption capacity was achieved at neutral pH, mainly due to strong electrostatic interactions between the adsorption sites and the nitrate anions. A lower adsorption of nitrate at alkaline pH may be due to abundant HO-ions competing with contaminants for the same sorption sites.  TT and at 25 °C for UBT (Figure 11a,b). Temperature is a factor that influences the of nitrate ions in aqueous solutions as well as the surface properties of the adsor increased adsorption at higher temperatures is caused by the adsorption sites be readily available [15,29].

Regeneration Experiments
The effect of the type of the desorbing agent should be evaluated while ta account the sorbentʹs potential for reuse. Each adsorbent must have good reusab high adsorption capacity to significantly increase the economic value of the pro to the long drying time, physical regeneration methods such as water regenera heat regeneration are less effective, less stable, and therefore more costly. The a is frequently regenerated using straightforward chemical reagents such as acid, a

Regeneration Experiments
The effect of the type of the desorbing agent should be evaluated while taking into account the sorbent's potential for reuse. Each adsorbent must have good reusability and high adsorption capacity to significantly increase the economic value of the process. Due to the long drying time, physical regeneration methods such as water regeneration and heat regeneration are less effective, less stable, and therefore more costly. The adsorbent is frequently regenerated using straightforward chemical reagents such as acid, alkali, and salts, because the chemical regeneration method offers the advantages of high regeneration efficiency and low cost [29,42].
For the regeneration of the adsorbents, several attempts were performed to regenerate nitrate ions from the adsorbed support by using HCl, NaOH and Na 2 CO 3 solutions at the concentration of 0.1 mol/L. Among them, Na 2 CO 3 and HCl solutions had the best regeneration effect due to the enhancement of electrostatic repulsion. After the sorption process, the dried and weighed sorbents (with an initial solution concentration of 300 mg/L) were stirred for 120 min with the desorbing agents HCl, NaOH or Na 2 CO 3 at a concentration of 0.1 mol/L. After completion of the stirring, the adsorbent was separated by filtration and was then dried at room temperature for 24 h.
The desorption percentage was calculated using the expression [18,19,42]: where: C o (mg/L) is the initial concentration of NO 3 − ions in the feed solution, C e (mg/L) is the NO 3 − concentration at equilibrium, V (L) is the volume of feed solution, C r (mg/L) is the NO 3 − concentration in solution after regeneration, and V t (L) is the volume of the regeneration solution. Figure 12a-c showed the repeatability of nitrate adsorption by each adsorbent after 0.1 mol/L HCl, NaOH, and Na 2 CO 3 treatment. The results showed that the third cycle was almost 100% for all regeneration reagents, which demonstrated that each adsorbent prepared from spent black tea had good reusability. tion efficiency and low cost [29,42].
For the regeneration of the adsorbents, several attempts were performed to regener ate nitrate ions from the adsorbed support by using HCl, NaOH and Na2CO3 solutions a the concentration of 0.1 mol/L. Among them, Na2CO3 and HCl solutions had the best re generation effect due to the enhancement of electrostatic repulsion. After the sorption pro cess, the dried and weighed sorbents (with an initial solution concentration of 300 mg/L were stirred for 120 min with the desorbing agents HCl, NaOH or Na2CO3 at a concentra tion of 0.1 mol/L. After completion of the stirring, the adsorbent was separated by filtra tion and was then dried at room temperature for 24 h.
The desorption percentage was calculated using the expression [18,19,42]: where: Co (mg/L) is the initial concentration of NO3 − ions in the feed solution, Ce (mg/L) is the NO3 − concentration at equilibrium, V (L) is the volume of feed solution, Cr (mg/L) is the NO3 − concentration in solution after regeneration, and Vt (L) is the volume of the re generation solution. Figure 12a-c showed the repeatability of nitrate adsorption by each adsorbent after 0.1 mol/L HCl, NaOH, and Na2CO3 treatment. The results showed that the third cycle was almost 100% for all regeneration reagents, which demonstrated that each adsorbent pre pared from spent black tea had good reusability.

Isotherm Analysis
In this study, the relationship between adsorption capacity and equilibrium concen tration was analyzed by using the isotherm models of Langmuir, Freundlich and Temkin to fit the experimental data ( Table 1).
The design and optimization of an adsorption process for the removal of inorgani and organic contaminants from aqueous solutions critically depends on the analysis of th equilibrium isotherms (Figures 13-15).  [3,10,18,28].

Isotherm Analysis
In this study, the relationship between adsorption capacity and equilibrium concentration was analyzed by using the isotherm models of Langmuir, Freundlich and Temkin to fit the experimental data (Table 1). Table 1. The equations of the Langmuir, Freundlich and Temkin isotherm models [3,10,18,28].
Langmuir q e = q m · K L · C e 1 + K L · C e Langmuir linearized form: C e q e = C e q m + 1 q m · K L q e = adsorption capacity determined at equilibrium (mg·g −1 ); q m = maximum adsorption capacity (mg·g −1 ); K L = Langmuir constant (L·mg −1 ) Freundlich q e = K F · C e 1/n Freundlich linearized form: ln q e = ln K F + 1 n ln C e K F = adsorption capacity; n = intensity of adsorption; q e (mg/g) = the equilibrium sorption concentration of nitrate per gram of adsorbent; C e (mg/L) = the concentration of the solute in solution at equilibrium The design and optimization of an adsorption process for the removal of inorganic and organic contaminants from aqueous solutions critically depends on the analysis of the equilibrium isotherms (Figures 13-15).   The Langmuir parameters can also be used to predict the affinity between the nitrate and each sorbent using a dimensionless separation factor (R L ) [3,10,31]: where K L is the Langmuir constant, and C i is the initial concentration (mg/L).   According to the criteria listed in Table 2, the value of the separation factor R L can be used to determine whether the adsorption process was favorable or unfavorable.
The value of R L for the adsorption of nitrate onto UBT and UBT-TT adsorbents is shown in Figure 16, which indicates that the sorption of nitrate ions was favorable (see Table 2). The R L value was between 0 and 1, which signifies a strong relation for adsorption ( Figure 16).  The Langmuir parameters can also be used to predict the affinity between the and each sorbent using a dimensionless separation factor (RL) [3,10,31]: where KL is the Langmuir constant, and Ci is the initial concentration (mg/L).
According to the criteria listed in Table 2, the value of the separation factor RL used to determine whether the adsorption process was favorable or unfavorable.    [3,10,31].

Separation Factor (R L )
Type of Isotherms The value of RL for the adsorption of nitrate onto UBT and UBT-TT adsorbent shown in Figure 16, which indicates that the sorption of nitrate ions was favorable ( Table 2). The RL value was between 0 and 1, which signifies a strong relation for adso tion (Figure 16).  The intensity of the adsorption process is indicated by the values of the Freundlich parameter n F . A favorable adsorption process is represented by n F values between 1 and 10. The results showed that the value of 1/n F is less than the unity, indicating that the nitrate pollutant is favorably adsorbed by the adsorbents prepared from black tea waste. This is in strong agreement with the findings regarding the R L values [2,15,31].
The data obtained from this study were best fitted to the Freundlich adsorption isotherm applied to the equilibrium (the values were R 2 = 0.9431 for UBT and R 2 = 0.9414 for UBT-TT), assuming the multi-layer adsorption onto a surface with a finite number of sites [10,31].
Since the Langmuir and Freundlich isotherm models did not sufficiently explain the adsorption mechanism, the Temkin isotherms were also fitted to the experimental data. The adsorption parameters calculated using the Temkin model indicated that the electrostatic interactions participated in the nitrate sorption process.
The data obtained from this study (Table 3) were best fitted to the Freundlich adsorption isotherm applied to the equilibrium (the values R 2 = 0.9136 for UBT and R 2 = 0.9506 for UBT-TT), assuming the multi-layer adsorption onto a surface with a finite number of sites. Table 3. The parameters of the Freundlich, Langmuir and Temkin adsorption isotherm models for nitrate removal.

Thermodynamic Analysis
The values of the thermodynamic parameters of Gibbs free energy (∆G), change in enthalpy (∆H), and change in entropy (∆S) were calculated by using the Langmuir isotherm constant K L at different temperatures ( Table 4). The following expression can be used to associate the Langmuir adsorption constant with the free energy of adsorption ∆G [31,42,43]: where T is temperature (Kelvin), R is the gas constant (8.314 · 10 −3 kJ/mol·K), and K L is the equilibrium constant obtained from the Langmuir isotherm model. Enthalpy and entropy changes are also related to the Langmuir equilibrium constant by the following equation [3,10,42]: The values of enthalpy change (∆H) and entropy change (∆S) were calculated from the slope and intercept of the plot lnK L versus 1000/T (Tables 4 and 5).
The values of Gibbs free energy (∆G > 0) indicate that the adsorption process is nonspontaneous and requires a small amount of energy; the adsorption process occurs naturally. The negative value of the standard enthalpy change ∆H shows that the adsorption process is exothermic. Negative values of ∆S represent a stable arrangement of nitrate ions on the adsorbent surface [16,42]. The value of enthalpy is also an indicator of the adsorption mechanism; it suggests an adsorption mechanism by physio-sorption through van der Waals forces if the value of enthalpy is less than 20 kJ mol −1 , or an electrostatic type of forces between a pollutant and the adsorbent if it is between 20 and 80 kJ mol −1 [43].
A nitrate is a monovalent anion, and this makes the positively charged surface be a possible adsorption site that can adsorb nitrate ions through electrostatic attraction. The adsorption of nitrate ions onto adsorbents containing surface functional groups, such as the hydroxyl, carboxyl and amine groups, may relate to the electrostatic attraction, and these adsorbents become protonated at strongly acidic conditions [44].

Environmental Significance
Model solutions are generally used to investigate the applicability of an adsorbent, particularly where inexpensive (non-conventional) adsorbents are involved. The experimental results of this study demonstrated that spent black tea (without a chemical or thermal treatment) and thermally treated spent black tea represent abundant sources to prepare efficient and stable adsorbents, which can remove nitrate ions from aqueous solutions, and these sorbents can be easily regenerated.

Comparison of the Maximum Adsorption Capacity of Various Bio-Adsorbents for Nitrates
In the present study, the adsorption capacity of spent black tea and thermally treated black tea was compared with other natural or synthetic adsorbents used in the nitrate adsorption process, and the results are reported in Table 6.

Conclusions
In recent years, green chemistry strategies have been proposed for the utilization of plant waste, such as energy recovery, value-added products, or use as adsorbents. Tea waste can be carefully processed and converted into efficient biosorbent materials, and its disposal is not without environmental side effects.
The results of this study suggest that spent black tea (UBT) and thermally treated black tea (UBT-TT) could be effectively used as bio-adsorbents for the removal of nitrate ions from synthetic wastewater. These materials were characterized by different analytical and spectroscopic methods.
The adsorption isotherms were simulated by the Langmuir, Freundlich, and Temkin models. The maximum adsorption capacities (q e ; mg/g) for UBT and UBT-TT were 59.44 mg/g and 61.425 mg/g, respectively. The data obtained from this study were best fitted to the Freundlich adsorption isotherm applied to the equilibrium (the values were R 2 = 0.9431 for UBT and R 2 = 0.9414 for UBT-TT), assuming the multi-layer adsorption onto a surface with a finite number of sites. The Freundlich isotherm model could explain the adsorption mechanism. However, it was observed from the thermodynamics and adsorption isotherm results that a physical process also occurred in the UBT and UBT-TT adsorption process.
The current study indicates that spent black tea could be considered as an efficient biowaste and cost-effective material for the removal of nitrates from aqueous solutions.
In further research we propose to study the influence of competitive ions in wastewater and to determine a cost-benefit analysis to assess the implementation of these adsorbents on a large scale. For example, tea waste could also be utilized to make a bio-organic fertilizer that can be used in farming, because plant growth is aided by organic fertilizers which improve both the soil structure and the crop yields. Funding: The authors thankfully acknowledge the Petroleum-Gas University of Ploiesti, Romania for the financial support and for providing the platform to perform this research.