Utilizing sustainable hemp biomass as an eco-friendly for potentially toxic elements removal from water

Potentially toxic elements in water is one of the important environmental problems. In this study, it was aimed to produce an environmentally friendly and cost-effective biosorbent using sustainable hemp biomass. The chemical composition of hemp biosorbents was characterized by FTIR, SEM, and XRD, and the results showed that the biosorbent could be a good alternative. A batch system was used to investigate the effects of initial concentration, pH, contact time, and temperature on the removal performance of Cu (II) and Zn (II) ions. The statistical analysis was performed, and the amount of adsorbed substance, kinetic values, and experiment results were evaluated for suitability. Kinetic data determined the best fit to pseudo-second-order kinetics for Cu (II) and Zn (II) ions. Adsorption determined the Langmuir model for Cu (II) ions and the Freundlich model for Zn (II) ions. The maximum adsorption capacity in the batch system was determined as 25.59 mg/g for Cu (II) and 12.97 mg/g for Zn (II) ions. The obtained thermodynamic data confirmed the endothermic nature of the adsorption. In desorption studies, after three cycles, the adsorption efficiency decreased from 83.3% to 52.8% for Cu (II) and from 82.1% to 49.7% for Zn (II). Study results showed that hemp biosorbent may be an alternative adsorbent that can be used to remove PTEs from wastewater.


Introduction
In recent years, potentially toxic elements (PTEs) in the aquatic environment have attracted attention worldwide due to their properties, such as persistence, toxicity, bioaccumulation, and non-biodegradability [1][2][3].In fact, PTEs are among the most widely released environmental pollutants, affecting the environment and human health.Key PTEs include arsenic, chromium, manganese, mercury, lead, iron, cadmium, cobalt, nickel, copper, platinum, zinc, silver, tin, gold, vanadium, molybdenum, and titanium [4].Apart from natural activities such as volcanic eruptions and weathering of rocks, PTEs have been released into the environment due to anthropogenic activities such as mining, pesticide use, metal smelting, galvanic coating, and the discharge of domestic and industrial wastewater [5,6].The extraction of PTEs globally has increased by over 75% over 35 years [7].It is known that PTEs such as Copper, Zinc, Lead, and Nickel are produced more than 1000 m 3 tons per year, and accordingly, their consumption values increase in this context [7,8].As a result of increased exposure to PTEs, they affect human health negatively by penetrating into water resources [9].Although PTEs such as Copper (Cu (II)) and Zinc (Zn(II)) are necessary for the metabolic activities of living organisms, the accumulation of PTEs in soil, organisms and humans at concentrations above their toxicity levels is a cause of great concern [10,11].For this reason, the World Health Organization has set some limit values for PTEs, for example the Cu (II) limits of 1 mg/l for natural mineral waters; Zn (II) limit value is 3 mg L −1 in drinking water [12].The Environmental Protection Agency (EPA) has set a regulation of 1.3 mg/l for the action level of Cu in tap water samples [13].For Zn, the available data are summarized, and it is concluded that the concentration is generally well below 5 mg L −1 [14].In addition, in our previous field study, high Cu and Zn concentrations were

Chemical treatment of Hemp
The hemp hurds were first purified by washing several times with distilled water.Then it was dried in an oven at 105 m 3 °C for 12 h.The drying process reduced the water content of hemp hurds and stabilized their physical properties.The modification of hemp hurds was conducted according to Kyzas et al [37].Kyzas et al [37] used hemp fibers and shives in their study.In this study presented; A more sustainable goal was aimed by choosing their waste (hurds) instead of hemp fibers.Additionally, Kyzas et al [37] used Nickel (Ni (II)) metal, while in the presented study, Zn (II) and Cu (II) were tested separately.Briefly, in the present study, hemp hurds were mercerized in NaOH for 1 h with 500 rpm shaking using a solid-liquid ratio of 50 g/l.At this stage, the mechanical strength and hydroxyl groups of hemp were increased, and the general mechanical properties were improved.Subsequently, the filtrate was rinsed with deionized water until its pH reached 7. The washed hemp hurds were dried at 60 °C for 12 h.In analyses; 40 °C, 50 °C, 60 °C, 70 °C, and 80 °C treatments have been tried; However, 60 °C was chosen as optimum since the material stability is the same at temperatures of 60 °C and above and there is a risk of burning of this material treated with acid and base solutions at extremely high temperatures.In terms of duration; The minimum duration was chosen according to the 1, 3, 6, 12, 24 h treatment applied based on reasons such as reducing the amount of moisture contained in this material, increasing its stability, and increasing its durability.As a result, with these washing and drying processes, excess NaOH was removed from the hemp hurd, and the desired properties were obtained in the material.
Afterward,the hemp hurds were shaken in 100 g L −1 citric acids at room temperature at 500 rpm for 30 min.Wet hemp hurds were incubated at 60 °C for 24 h and cross-linked at 120 °C for 90 min.At this stage, the modification made with citric acid changed the surface chemistry and increased the surface capacity and the adsorption capacity of hemp.In addition, it is aimed to adsorb metal ions more easily by creating microporosity by treatment with citric acid.Cross-linking was performed to make the material more resistant to various chemical environments.Finally, Hemp hurds were washed with deionized water until the citric acid was removed, dried in an oven at 60 °C for 24 h, and stored in a desiccator.In this way, excess modification material was cleared, and the final form was determined.Briefly, the hemp hurds were treated with NaOH followed by esterification using citric acid to produce bioadsorbent for the removal of Cu (II) and Zn (II) from aqueous solutions.The aim is to improve the sorption capacity of hemp hurds towards Cu (II) and Zn (II) in aqueous solution [37].The schematic diagram including the production and metal adsorption processes of hemp biosorbent is shown in figure 1.

Adsorption experiment
Batch experiments were performed in an incubator shaker (Edmund Bühler-TH30) operating at a constant speed of 125 rpm with a working volume of 100 ml prepared at 30 °C under desired conditions.The effects of adsorption of metal ions (Cu (II) and Zn (II)) on hemp biosorbents, contact time (0-720 min), pH(3-4-5 and natural), initial metal concentration(10-40 mg L −1 ), and temperature(30 °C-50 °C), were investigated.For the desired initial metal concentration (10 mg L −1 ), a certain amount of hemp biosorbent (2.5 g L −1 ) was added to 100 ml of the total working volume, and the solutions were mixed at a certain speed (125 rpm) at the natural pH of the solution (pH Cu =6.18 -pH Zn =6.23), and 30 °C.An adsorption study was carried out with a contact time of 720 min and the natural pH of Cu (II) and Zn (II) metal solutions.Hemp biosorbents reached saturation in 120 min, and the adsorption equilibrium time was selected as 120 min.For the pH study, the pH of the metal solutions was adjusted by adding 0.1 N H 2 SO 4 or NaOH to the desired value (between 3-4-5 and natural).Next, the biosorbents were separated using a 0.45 μm membrane filter through which they were filtered.Using an atomic adsorption spectrophotometer, the filtrate was then analyzed for the concentration of Cu(II) and Zn(II) ions.
The equations used to calculate the percentage removal and the amount of adsorbed metal ions (Cu (II) and Zn (II)) during analysis are shown below [44,45]: Removal efficiency, R (%) Retained amount of metal ion, q (mg/g) where C 0 = initial concentration of metal ion (mg/L), C= metal ion concentration after sorption (mg/L), V= volume of solution (L), and G = weight of hemp hurds (g).

Analysis
Cu (II) and Zn (II) concentration in solution was analyzed by Unicam 420 AAS atomic absorption spectrometer.
To determine the effectiveness of the prepared biosorbent, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscope (SEM) -energy-dispersive spectroscopy (EDS) analyses were conducted.XRD measurements were carried out on a SmartLab instrument, FTIR measurements were performed on a Tensor27 instrument, and SEM images were obtained using a JEOL JSM-7001 device.XRD patterns were recorded on an XRD diffractometer using CuKα radiation (0.15405 nm) at 40 kV and 40 mA in the 5-50°2θ range for crystal phase identification.SEM images clearly showed morphological changes on the surface of hemp fibers after treatment with a solution containing Cu (II) and Zn (II) ions.Additionally, EDS analysis was conducted to highlight the presence of Cu (II) and Zn (II) ions.FTIR spectra of the samples were obtained using potassium bromide disks (thickness ∼500 μm).The spectra were recorded in the range of 4000 to 400 cm −1 with a resolution of 4 cm −1 , presented after baseline correction, and converted to absorbance mode.

Desorption and regeneration studies
Adsorption and desorption are two processes that usually follow each other.Desorption studies help explain adsorbent recovery and regeneration, process efficiency, and adsorption mechanism.Desorption studies have been carried out because the desorption process provides economic advantages such as effective use of materials, reduced production costs, increased product quality, and environmental sustainability.0.25 g of hemp biosorbent was taken from 100 ml of Cu (II) and Zn (II) solutions at a concentration of 10 mg/l and mixed for 2 h.The adsorbent was then removed from the solution.The obtained metal-loaded adsorbents were mixed separately with HCl, NaOH, NaCl, ethanol, and citric acid desorption agents in a volume of 100 ml, prepared at a concentration of 0.1 M, in a shaker at a speed of 125 rpm for 24 h.After this process, hemp biosorbents were removed from the solution, and the concentrations of Cu (II) and Zn (II) transferred into the solution were measured in the AAS instrument.The percentage of metals desorbed from the adsorbents was calculated using the following formula [28]: To test the reuse of renewed adsorbents, a regeneration study of the material was carried out by performing adsorption-desorption studies in three cycles.

Statistical analysis
Statistical analysis was performed by IBM ® SPSS ® Statistics version 22.0 (IBM, Armonk, NY, USA).One-way analysis of variance (ANOVA) followed by Duncan's multiple range tests were used to compare the mean values.Also, the student's t-test was performed to analyze significant differences between Cu and Zn in the same treatments.P values less than 0.05 were considered significant.Statistical differences were marked with different letters.Capital letters indicate significant differences among the treatments for the same metal, and lower-case letters indicate significant differences between Cu and Zn in the same treatments.

Results and discussion
3.1.Characterization of the hemp hurds SEM analysis presented in figure 2 illustrates the morphological changes in the unloaded and loaded biosorbent.After the biosorption of heavy metal ions, the surface of the biosorbent became smoother and less porous, suggesting possible physical retention and adsorption of the metal ions onto the biosorbent [37,46].The surface morphology of the biosorbent is distinctive and rough, providing a favorable environment for adsorption.The voids present on the surface of the biosorbent serve as channels for the diffusion of metal ions into the material, allowing them to access micro-and mesopores where functional groups can interact with the surface.This interaction facilitates the adsorption process.Adsorption of pollutants via hemp-based adsorbents depends on adsorption mechanisms such as hydrogen bonds, electrostatic and π-π interactions, and ion exchange [40].In this context, surface groups (e.g., hydroxyl, carboxyl groups) found in hemp biosorbent can attract metal ions.These groups are thought to adsorb Cu (II) and Zn (II) metal ions by interacting with the negative charges on the surface of the biosorbent.It is predicted that the attraction between opposite charges causes Cu (II) and Zn (II) metal ions to adsorb to the surface of the biosorbent.It is thought that adsorption occurs as a result of the active groups (such as hydroxyl groups) on the biosorbent forming complexes with Cu (II) and Zn (II) metal ions.Finally, hydrogen bonds are predicted to cause adsorption by interacting between metal ions and functional groups on the surface of the biosorbent [47].Figures 2(E) and (F) of the EDS analysis revealed strong peaks attributed to Cu and Zn after the adsorption of metal ions.The EDS analysis indicated that the raw biosorbent comprises 50.3% carbon, 46.7% oxygen, 2% calcium, 0.8% palladium, and 0.2% magnesium.In the biosorbent characterization after Cu(II) adsorption, a composition of 47.5% carbon, 45.3% oxygen, and 4.5% copper, which was not present before, was observed.Similarly, in the characterization of the biosorbent after Zn(II) adsorption, a composition of 46.9% carbon, 44.5% oxygen, and 4.7% zinc, which was not present previously, was observed.The SEM/EDS analysis confirmed the uptake of Cu(II) and Zn(II) by the biosorbent.
Figure 3 presents the results of FTIR analysis, which reveals the functional groups present on the surface of the biosorbent and their interactions with metal ions.The identified functional groups include hydroxyl (-OH), carbonyl (-C=O), carboxylic (-COOH), and ester (RCOOR′), as indicated by the corresponding wave numbers of the peaks [37].The rapidly increasing adsorption of metal ions by the hemp biosorbent in the first 30 min indicates an intensive utilization of active sites, particularly those supported by specific functional groups and surface areas.It is thought that the hydroxyl groups (-OH) on the surface of the hemp biosorbent are attracted to the metal ions, and the carboxyl groups (-COOH) form a complex with the metal ions and contribute to rapid adsorption.Additionally, the high surface area of hemp biosorbents provides more space for metal ions to adsorb.In the presented study, it was determined that adsorption increased in the first 30 min, adsorption slowed down as time progressed, and the adsorbent reached saturation after 120 min, which indicates the use of these areas [40].Accordingly, the change in the vibration frequency of these functional groups after metal ion adsorption in figures 3(B) and (C) shows that metal ions are involved in the retention process.Specifically, a band observed in the 3500-3200 cm −1 region corresponds to -OH groups present in the hemp biosorbent.Possible sources of -OH groups may be cellulose, hemicellulose and lignin [48].Furthermore, vibrations originating from carbonyl groups are identified in the 1700-1600 cm −1 region, indicating the presence of carbonyl in the carboxyl group of citric acid.Possible sources of C=O groups may be hemicellulose and lignin.The bands in this region confirm the presence of carboxylic acids contained in the hemp biosorbent.Additionally, it is postulated that the bands observed in the 1000 cm −1 region, which correspond to both pectin and cellulose, may result from bending of carbon and oxygen bonds.These findings provide insight into the chemical structure of the biosorbent and its ability to adsorb metal ions [49,50].
The XRD analysis presented in figure 4 depicts the changes in the diffraction peaks of the biosorbent subsequent to metal ion adsorption.The results of the XRD analysis suggest that hemp biosorbent has a crystalline structure, as evidenced by the sharp peaks observed in figure 4.These peaks correspond to different crystallographic directions within the sample.Notably, the highest peak intensity of the hemp biosorbent was observed at 2θ = 22.5 degrees.However, the presence of the 22.5°peak, which is lower in intensity compared to pristine cellulose, confirms the crystalline structure of cellulose, this peak remains intact after adsorption and it can be concluded that modifications occur only on the surface and amorphous areas [50].The XRD spectra exhibited significant variations at 2θ orders of 15, 17, and 22, which corresponded to Cu (II) and Zn (II) peaks.According to this, the XRD analysis results demonstrate that the crystalline structure of the hemp biosorbent remains unchanged after interaction with metal ions [37].This is an important finding, as it supports the use of hemp biosorbent as a stable adsorbent for metal ions.

Adsorption of Cu (II) and Zn (II) metals with hemp biosorbents 3.2.1. Effect of contact time
The time dependence of the adsorption process at a metal concentration of 10 mg/l is presented in figure 5.The effect of contact time on Cu (II) and Zn (II) was measured using hemp biosorbent, with contact times ranging from 1 to 720 min.The adsorption of Cu (II) and Zn (II) on hemp biosorbents increased rapidly within the first 30 min.It was observed that the removal rates in the system increased up to 120 min and then reached saturation.At 120 min, Zn (II) removal efficiency is statistically higher than Cu (II) (p<0.05).The high removal rates in the initial minutes were attributed to the more significant active regions of the biosorbent.However, over time, the metal removal gradually decreased due to the reduction and filling of these active regions, leading to a rapid equilibrium [51].The saturation of active sites on the biosorbent can reduce the adsorption capacity of metal ions.Diffusion processes that allow metal ions to reach the biosorbent may be affected over time, which  may cause the removal rate to decrease.In addition, desorption events, which are the release of adsorbed metal ions back under certain conditions, may also be effective in decreasing metal removal rates over time [52].

Kinetics of adsorption
Adsorption reaction kinetics are calculated to understand the rate and mechanism of these adsorption reactions.This kinetic information is crucial for optimizing adsorption processes, enhancing process efficiency, and contributing to the development of new materials for various applications [53].In this study, four kinetic models, namely pseudo-first-order, pseudo-second-order, intraparticle diffusion, and Elovich models, were used to investigate the adsorption of Cu (II) and Zn (II) (Figure S1a-d).Kinetic constants and correlation coefficients were determined by plotting their graphs, and the values are shown in table 1 and table S1.
As presented in table 1, higher R 2 values were obtained for the pseudo-second-order model compared the other adsorption rates for Cu (II) and Zn (II).The Δq (the standard deviation between the experimental and computed adsorption capacities) values in table 1 also proved that the experimental results are an excellent fit for the pseudo-second-order model.This shows that a chemical bond is formed between the adsorbate and the adsorbent, where the adsorption mechanism is dependent on chemisorption.However, the pseudo second order kinetic model alone cannot explain the diffusion mechanism [54].The transfer of solute in adsorption is characterized by the intra-particle diffusion model.Therefore, the intra-particle diffusion model has also been applied to explain the diffusion mechanism.The intra-particle diffusion model showed that the adsorption process did not depend on the intra-particle diffusion model alone since the R 2 value was low [53].Additionally, the fact that the mentioned lines do not pass through the origin indicates that fine diffusion or intra-particle diffusion co-occurs.Therefore, the results of this research indicate that the pseudo-second-order kinetic model mechanism plays an important role in metal adsorption.

Effect of pH
The pH values of industrial wastewater are an important factor to consider in the initial adsorption studies of metal ions [16].pH can change the solubility of metal ions by altering the surface charge of the hemp biosorbent.
The effect of initial pH on the adsorption of Cu (II) and Zn (II) onto hemp biosorbents was investigated at pH values of 3, 4, 5, and natural.At higher pH values, metal hydroxide precipitates may form, making adsorption studies impossible [55,56].In addition, working within this pH range is appropriate as it aligns with the typical pH values observed in wastewater generated by the metal surface treatment industry [47].As seen in figure 6, the sorption capacities for Cu (II) and Zn (II) were found to be lowest at pH 3 (p < 0.05).This statistical significance underscores the impact of pH on the adsorption behavior of the studied metal ions.This statistical significance underscores the impact of pH on the adsorption behavior of the studied metal ions.This decrease in sorption can be explained by the positive charge of the hemp biosorbents, which creates a repulsion between the positively charged metal ions and the hemp surface.Additionally, the presence of hydrogen ions at low pH levels can compete with metal ions for active sites on the hemp surface, further reducing the sorption capacity for Cu (II) and Zn (II).
The maximum sorption levels were observed at pH 5 for Cu (II) (3.4 mg g −1 ) and at pH 4 for Zn (II) (3.77 mg g −1 ), indicating that these pH values are favorable for the adsorption of these metal ions.This suggests a strong affinity of the hemp biosorbents for Cu (II) and Zn (II) in this pH range.Increasing the initial pH of the solution above 6.18 for Cu and 6.23 for Zn resulted in the precipitation of hydroxyl ions due to their higher concentration.As a result, no further adsorption experiments were conducted at pH levels higher than the natural solution pH values.

Effect of initial Cu (II) and Zn (II) concentrations
The results depicted in figure 7 demonstrate the impact of the initial concentrations of Cu (II) and Zn (II) on the adsorption process.It was observed that the equilibrium adsorption amount increased by an increment in the initial concentrations of Cu (II) and Zn (II).The statistical analysis demonstrates a significant correlation between the equilibrium adsorption amount and increasing initial concentrations (p < 0.05).This can be attributed to the fact that at lower initial concentrations, the metal ions can easily bind to the abundant adsorption sites present on the surface of the biosorbent.Additionally, an increase in the driving force of the concentration gradient, resulting from the rise in Cu (II) and Zn (II) concentrations, can also be attributed to the observed behavior.However, when the initial metal concentrations were higher, the number of available adsorption sites remained constant, leading to limited access of metal ions to the adsorption sites and a subsequent decrease in adsorption efficiency [19].Therefore, it can be concluded that the hemp biosorbents exhibit high separation efficiency at low metal ion concentrations, but their effectiveness diminishes with higher initial metal concentrations.

Adsorption isotherms
Data on Langmuir, Freundlich, Dublin-Radushkevich (D-R) and Temkin isotherm models for Cu (II) and Zn (II) adsorption onto hemp biosorbents are shown in table 2 (figures S2(a)-(d)).For Cu (II), the lowest R 2 (0.93) was observed in the D-R isotherm model, and the highest R 2 (0.9924) was observed in the Langmuir isotherm model, while for Zn (II), the lowest R 2 (0.6898) was observed in the D-R model and the highest R 2 (0.8926) was observed in the Freundlich model.For Cu (II) adsorption, the Langmuir isotherm model had the lowest SSE value (0.0003), and for Zn (II), the Freundlich isotherm model had the lowest SSE value (0.063).Therefore, the Langmuir isotherm model for Cu (II) and the Freundlich model for Zn (II) were considered to represent best the experimental data on the adsorption of hemp biosorbents (table 2 and table S2).
The suitability of adsorption of Cu (II) and Zn (II) metals on hemp biosorbent was evaluated by R L , which indicates the nature of adsorption.R L > 1 represents negative, R L = 1 linear, 0 < R L < 1 positive, and R L = 0 represents the irreversible nature of adsorption.In table 2, R L (separation factor for Ci=10 mg/l) is 0 < R L < 1, indicating that adsorption occurs normally [57].It reveals that the adsorption is not very strong but is visible.In the Freundlich isotherm model, 'n' values are used to indicate whether the process is physical or chemical.The type of process is determined by the parameter n: the process is linear when n = 1, chemical when n > 1, and physical when n < 1 [58,59].In this study, it is thought that n < 1 and the adsorption of Cu(II) and Zn (II) metals to hemp biosorbent is a physical process.Since n < 1, hemp biosorbent proved to be accurate as an adsorbent for Cu (II) and Zn (II).In D-R isotherm models, E values also express the nature of the process, with E values < 8 kJ mol −1 indicating the physical process and E values between 8 and 16 kJ mol −1 indicating the chemical process [53].Since the current E value of the D-R model is 0.791 for Cu (II) and 2.236 kJ/mol for Zn (II), it represents a physical adsorption process.
The selectivity of the material becomes a very important factor when evaluating the suitability of materials for a particular application.Selectivity refers to the ability of the material to interact preferentially with certain substances over others.In heavy metal adsorption, properties such as the electronegativity of ions, ionic diameter, and hydrated ion radius determine the adsorbent's selectivity order and adsorption capacity.The important ionic properties of Cu (II) and Zn (II) are given in table 3. It is pointed out that the electronegativity of Cu (II) is higher than that of Zn (II), so the adsorption capacity of Cu (II) is greater than that of Zn (II).It is thought that Cu (II) ions, which have the lowest hydrated radius, adhere more easily to the surface of the hemp biosorbent [60,61].
At the same time, the coexistence of humic acid affects the adsorption of metal ions by hemp biosorbent.Since the humic acid found in the hemp plant can chelate metals, it contributes positively to adsorption.In the presence of humic acid, metals in the solution are chelated and removed from the solution more quickly [62][63][64].

Effect of temperature
Temperature is an important factor affecting the effectiveness and capacity of adsorbents.The diffusion rate of metal ions from the solution to the biosorbent surface increased with temperature, resulting in a significant increase in the adsorption rate.The effect of temperature on the adsorptive removal of Cu (II) and Zn (II) metal ions by hemp biosorbent was examined in the temperature range of 30, 40, and 50 °C.Figure 8 shows the removal of Cu (II) and Zn (II) metals using hemp biosorbent and the effect of temperature on the adsorption capacity.The highest removal efficiencies for 10 mg/l Cu (II) and Zn (II) solutions were recorded as 85% and q max the maximum saturated adsorption capacity (mg/g), K L Langmuir adsorption constant related to adsorption energy (L/mg).R L the separation parameter K F the Freundlich constant (mg/g), n the degree of heterogeneity (L/mg, k D-R , D-R isotherm constant (mol 2 /kJ 2 ), E average free energy (KJ/mol).
A the equilibrium constant of Temkin adsorption (mg/L), B the adsorption heat of Temkin adsorption (J/ mol).95% at 50 °C, respectively (figures 8(A) and (B)).As the temperature increased from 30 °C to 50 °C, the equilibrium adsorption capacity increased from 12.46 to 13.47 mg g −1 and from 11.32 to 11.66 mg/g for 40 mg/ l Cu (II) and 40 mg/l Zn (II), respectively (figures 8(C) and (D)).The observed increase in both adsorption capacity and removal efficiency with an elevation in temperature has been determined to be statistically significant (p < 0.05).This finding aligns with the principles of thermodynamics, where elevated temperatures often facilitate more favorable adsorption kinetics and stronger interactions between the adsorbent and adsorbate molecules.The increase in adsorption with temperature can be explained by the fact that metal ions gain more energy at higher temperatures and, therefore, interact more with the adsorbent surface.The diffusion of metal ions into the pores may have increased due to the increased mobility of the ions at high concentrations and temperatures.In addition, it is thought that the increasing adsorption capacity of hemp biosorbent with temperature increase may have contributed to the adsorbent surface activation [65].Therefore, it turns out that the process is an endothermic reaction between adsorbate and adsorbent and is spontaneous.

Thermodynamic parameters
Thermodynamic parameters such as the Gibbs standard free energy (ΔG°), changes in enthalpy (ΔH°) and entropy (ΔS°) of adsorption can be determined by the following equations [66]: where K c represents the distribution coefficient of the adsorbate, q e and C e represent the equilibrium metal concentration on hemp biosorbent (mg/g) and in the solution (mg/L).R stands for the universal gas constant (8.314J/mol.K), and T stands for temperature (K).ΔH°and ΔS°parameters are calculated from the slope and intersection point of the ln Kc and 1/T graphs, respectively.Thermodynamic parameters are shown in table 4 (figures S3(a)-(b)).A decrease in ΔG°values was observed with increasing temperature.Adsorption processes with ΔG°values between −20 and 0 kJ mol −1 are spontaneous physical processes [67].Negative values of ΔG°indicate a spontaneous and thermodynamically favorable adsorption process, while positive values of ΔH°indicate an endothermic reaction.Positive ΔS°values may be related to the affinity of the adsorbent for metal ions and the randomness of the entire process during the sorption process [65]

Desorption and reusability
Desorption and regeneration of adsorbent substances are critically important in industrial processes and applications.These processes enable adsorbent materials to be effectively reused, save energy and costs, increase environmental awareness, and increase process efficiency.Thanks to the renewal of adsorbents used in environmental aspects and the usability of renewed adsorbents, the problems that may occur during the disposal of used adsorbents can be partially reduced.Therefore, it is necessary to investigate whether the adsorbent can be reused after adsorption.In this study, to investigate the reusability of hemp biosorbents were loaded with Cu (II) and Zn (II) solutions (10 mg/l) until they reached equilibrium.Then, the solution was filtered, and metal concentrations were measured using an AAS.It was tested for the desorption of Cu (II) and Zn (II) metal solutions using different eluents such as 0.1 M NaOH, HCl, NaCl, citric acid, and ethanol.For desorption, hemp biosorbents loaded with Cu (II) and Zn (II) metals were dried in an oven at 60 °C and contacted with 100 ml of different eluents The mixture was shaken at 30 °C for 24 h.Then, the solutions were filtered, residual metal concentrations were measured in the ASS, and the percentage of metals desorbed from the hemp biosorbent was calculated.The results are shown in figure 9(A), and according to these findings, the optimum elution (>%80) was determined to be 0.1 M HCl (p<0.05).
Adsorption-desorption studies were performed in triplicate in a batch setup, and average values were obtained.Figure 9(B) shows the simultaneous adsorption-desorption of metals onto hemp biosorbent.The observed percent desorption of hemp biosorbents for 0.1 M HCl in the first cycle was 83.3% for Cu(II) and 82.1% for Zn(II).In the statistical comparison of Cu (II) and Zn (II) in terms of desorption, the 1st cycle is higher than the other cycles (p < 0.05).After three cycles of desorption, the adsorption efficiency of Cu (II) and Zn (II) metals decreased from 83.3% to 52.8% for Cu (II) and from 82.1% to 49.7% for Zn (II).As the cycle number increases, the desorption effect decreases statistically (p < 0.05).It was observed that efficiency decreased after three cycles of frequent use.There are several potential reasons why metal retention in the adsorption-  desorption process decreases over time.One of these is that the adsorbent loses its activity over time.Clogging of the active sites on the surface may reduce the effectiveness of the adsorbent due to chemical reactions or other pollutants.Another is that thermal deactivation of the adsorbent due to high temperatures can reduce the desorption ability of the metal.Additionally, physical losses, particle size, aggregation, or other physical changes may prevent the effective retention of metal particles.Nevertheless, these results are promising regarding the potential recovery and reuse of the adsorbent [39].Although the reduced adsorption capacity after recycling creates a negative effect, the hemp biosorbents produced have shown that they can be used as a practical and edible material in industrial wastewater treatment due to their economical and environmentally friendly structure.

3.3.
Comparison of hemp biosorbent with other adsorbates in terms of adsorption of Cu (II) and Zn (II) Adsorption capacity is a critical parameter used to evaluate the adsorption capacity of adsorbents.In table 5, it was determined that the adsorption capacity of hemp biosorbent was generally higher compared to other adsorbents.Compared with other adsorbents in Cu (II) removal (table 5), the adsorption capacity of hemp biosorbents is more significant than other natural materials.Only Palm oil fruit shells [68] adsorption capacity has a higher adsorption capacity than hemp biosorbent in Cu (II) removal.Table 5 shows that hemp biosorbents are generally very effective among natural biosorbents, especially in Zn (II) removal.For Zn (II), only the adsorption capacity of the Groundnut shell [69] adsorbent is higher than the hemp biosorbent.When the adsorption capacity of commercial adsorbents is examined in table 5, it is seen that Amberlite R-120 resin [70] and graphene oxide [71] have considerably higher adsorption capacities than hemp biosorbent.It has been observed that activated carbon [72,73] and zeolites [74,75] have higher adsorption capacities than hemp biosorbents, depending on their type.These commercial adsorbents can perform well to remove heavy metals, but are often expensive and environmentally unsustainable [41].It appears that the hemp biosorbents used in this study exhibit excellent Cu (II) and Zn (II) removal performance with high sorption capacity, especially compared to other renewable materials.Although the adsorption capacities of the developed biosorbents are not slightly superior to those of ion exchange resins and certain zeolites and activated carbons, this is compensated by a significantly lower production cost.For this reason, hemp biosorbents stand out with their low costs compared to commercial adsorbents, natural and renewable resources, biodegradability and biocompatibility advantages, effectiveness in various application areas, and easy availability [76].These properties make hemp biosorbents valuable as an environmentally friendly, sustainable, and economical pollutant removal alternative.However, the specific application conditions and requirements regarding their use must be considered.

Conclusion
Hemp biosorbents offer an effective solution for the removal of potentially toxic elements from the aquatic environment due to their adsorption capacity.Therefore, in the study, effects such as contact time,pH, initial metal concentration effect, and temperature on the removal of Cu (II) and Zn (II) PTEs from the aquatic environment by adsorption using hemp biosorbents were evaluated.In addition, the characterization tests of hemp biosorbents revealed their high surface area, molecular structure, and biodegradability ability to remove PTE.The pseudo-second-order kinetic model was the best to describe the biosorption of Cu (II) and Zn (II) on hemp biosorbents.According to the Langmuir isotherm, the maximum adsorption capacity of hemp hurds for Cu (II) and Zn (II) obtained in this study were 26.59 and 12.97 mg g −1 , respectively.According to thermodynamic data, the endothermic nature of the adsorption was confirmed.The desorption results of hemp biosorbents are promising regarding possible recovery and reuse of the biosorbent.For these reasons, hemp biosorbents offer a promising solution in environmental applications to remove potentially toxic elements from aquatic environments, as they have renewable and biodegradable properties and can be removed after use without harming the environment.Besides, this natural material can be used to treat industrial application, potentially reducing water pollution and contributing to environmental sustainability goals.However, there may be economic difficulties in the transition of this study to industrial application, such as increased costs, the necessity of large facilities and processes, and the use of more raw materials and energy.Careful engineering studies, pilot-scale trials, and economic analysis are important for successful industrial applications.However, based on the success of similar projects, this type of scaling is considered planable and manageable.

Table 1 .
Kinetic parameters for removal of Cu(II) and Zn(II) by hemp biosorbent.
e is equilibrium adsorption capacity (mg/g), k 2 is second-order rate constant (g/mg min), K id is a rate constant (mg/g min 1/2 ), C is a film layer thickness, α is the adsorption rate constant (mg/g min), β (g/mg) is the desorption rate constant.

Table 2 .
Equilibrium isotherm modelling for the adsorption of Cu (II) and Zn (II) by hemp biosorbent.