Adsorption of Cadmium and Methylene Blue Using Highly Porous Carbon from Hemp Seeds

: Toxins in water, such as metal ions and dyes, have become a global challenge to humanity by causing several serious illnesses. Removal of these toxins from water is needed for human health and environmental concerns. This work investigated the use of hemp seeds as an environmentally friendly adsorbent for applications in water treatment. Pristine hemp seeds (PHSs) were carbonized at 500 and 700 ◦ C to obtain carbon-based hemp seeds (CHS-500 and CHS-700 for the removal of cadmium and methylene blue. The morphological and functional groups of the carbonized adsorbents and pristine hemp seeds were determined by SEM and FTIR. Isotherm studies showed that the Langmuir model best described the adsorption process based on homogenous surfaces. The maximum adsorption capacities were obtained with CHS-700 with a 36.88 and 52.61 mg/g uptake of cadmium and methylene blue, respectively. The effect of contact time showed that the sorption process rate was rapid initially, followed by a slower increment due to the saturation of active sites. ∆ S ◦ values were positive, demonstrating the increased randomness and degree of freedom of cadmium and methylene blue in water. The adsorption trends for cadmium and methylene blue by CHS-500 and CHS-700 increased as the temperature rose; a decrease was observed for adsorption by PHSs. This result suggests that the adsorption was endothermic for CHS-500 and CHS-700 and exothermic for PHSs. The pseudo-second-order model better described the uptake of both pollutants by the adsorbents. This research illustrates the great potential of carbon-based hemp seeds in removing cadmium and methylene blue dye from water.


Introduction
Water systems globally are constantly compromised when wastewater contaminated with toxic metals and dyes enters the environment, threatening the ecosystem and human health [1]. It is vital to seek suitable methods to analyze water contaminants [2]. Several toxic metal ions, such as cadmium (Cd), and dyes, including methylene blue (MB), are discharged into surface water [3]. Cadmium is a corrosion-protective coating for steel and iron, batteries, solar cells, alloys, plastic stabilizers and pigments [4,5]. Cadmium and its compounds are dangerous, causing cardiovascular, neurological, renal, reproductive, gastrointestinal and respiratory cancer [1]. It is released from the body very slowly and has coactive toxicity with other metal ions, making it very dangerous [1]. Baskaran and Abraham (2022) [1] used carbonized coconut shells to absorb cadmium. Their results indicated a maximum capacity of 44.67 mg/g. Zhu et al. (2022) [6] used phosphate-modified clay to remove cadmium, showing a maximum adsorption capacity of 198.34 mg/g. Yan et al. (2022) [7] removed cadmium, lead and copper from water by bleaching ethylene tetramethylene phosphonic acid into UiO-66. Their adsorbent had a large adsorption capacity with good stability and was considered new. All the adsorbents mentioned above still had some flaws in their adsorption processes, such as poor thermal and chemical stability, low adsorption capacity and sorption rate, poor recyclability and high process cost.
Methylene blue is one of the organic dyes used for dyeing wool, silk and cotton fabrics [8]. When untreated pigments are released into the environment, they cloud the water, decrease aquatic plants' photosynthetic activity and interrupt the ecosystem [9]. Pollutants from methylene blue are highly toxic and cause mutagenic, allergenic and carcinogenic effects [10]. Therefore, it is essential to eliminate methylene blue from wastewater. Li et al. (2023) [11] used fly ash as a biomass adsorbent for removing methylene blue. Their results indicated that raw fly ash had a low methylene blue adsorption capacity of 71.22 mg/g compared to that of biochar fly ash, which was 139.93 mg/g. Biochar fly ash is easily dispersed in the adsorption system and has low reusability [12]. Zhang et al. (2022) [13] prepared porous biochar from wood and bamboo-derived heavy bio-oil to absorb methylene blue. Their results showed that the methylene blue adsorption capacity increased and then decreased when the carbonization temperature was raised to 800 • C for 2 h. Although a highly porous adsorbent was obtained in that study, a high temperature of 800 • C was not favorable for the sorption of methylene blue.
Activated carbon has been used as an adsorbent in treating many toxic ions and dyes and can be produced from different materials. However, using activated carbon involves high costs and difficulty in regeneration [14,15]. The use of low-cost raw materials may alleviate these problems. This study highlights hemp seeds as an alternative for water treatment. Although many studies have reported on cadmium and methylene blue adsorption, there is little information regarding removing these pollutants using hemp seeds. This work attempts to contribute to solving environmental pollution challenges by utilizing hemp seeds. Most studies to date emphasize the adsorption of a single pollutant at a time. Few studies have been published on the simultaneous adsorption of different contaminants [16]. However, wastewater effluents are composed of mixtures of pollutants, and the challenge of simultaneously removing them remains [17]. Several studies have used different parts of hemp as an adsorbent for various pollutants. Pejic et al. (2009) [18] wrote on the adsorption of Pb(II), Cd(II) and Zn(II) using hemp fibers. Vukcevic et al. (2014) [19] described the uptake of Zn(II), and Kyzas et al. (2015) [20] studied the adsorption of Ni(II), using hemp fibers. Few studies have reported on hemp seeds as potential adsorbents. Shooto and Thabede (2022) [21] used carbonized hemp seeds for the simultaneous adsorption of Cr(VI) and Cd(II). Mphuthi et al. (2023) [22] conducted experiments on the adsorption of Pb(II), MB and ibuprofen using hemp seeds deposited with MnO/CuO and MnO/ZnO. Methylene blue (MB) has been extensively used in important applications, including in the textile and dyeing industries; wastewater from these industries contains high concentrations of MB dye. Similarly, metal ions (Cd(II) and Pb(II)) are used as mordant agents in the dyeing process [23]. Several studies have reported on the simultaneous removal of Cd(II) and MB; Giraldo et al. (2022) [23] used orange peel waste to remove Cd(II) and MB from water, while Yunusa et al. (2021) [24] used porous carbon from Albizia lebbeck pods for the adsorption of Cd(II) and MB. The adsorption of Cd(II) and MB with membranes has been reported by Zeng et al. (2011) [25]. Song et al. (2019) [26] prepared xanthate-modified baker's yeast to remove Cd(II) and MB from an aqueous solution.
Hemp seeds are consumed as food and are also used as a medicine for controlling cholesterol, cardiovascular and gastrointestinal problems, as well as other diseases [27]. A few studies have been conducted using hemp as an adsorbent. However, carbonized hemp seeds have not been used as cadmium and methylene blue adsorbents. The objective of this study was to use pristine hemp seeds (PHSs) and carbonized hemp seeds at 500 • C (CHS-500) and 700 • C (CHS-700) to adsorb methylene blue and cadmium from water. We hypothesize that cadmium and methylene blue adsorption will be higher using carbon-based hemp seeds. Furthermore, the materials' batch adsorption, equilibrium, thermodynamics, kinetics and isotherms were assessed.

Pristine Hemp Seeds
The hemp seeds were ground to a fine powder with a coffee grinder. A sieve was used to obtain particle sizes of 0.8 to 1.0 mm. Exactly 10 mg of the powdered hemp seeds was soaked in a 100 mL hexane solution for 30 min and then dried at 35 • C for 24 h. This material was labelled pristine hemp seeds (PHSs).

Carbon-Based Hemp Seed Adsorbents
For one hour, PHSs were carbonized at 500 and 700 • C at 10 • C/min using nitrogen gas at 110 mL/min. The samples were heated for 2 h and removed from the furnace to cool. The adsorbents heated at 500 and 700 • C were named CHS-500 and CHS-700, respectively.

Batch Adsorption Experiments
The effect of concentration on the adsorption rates was investigated using different concentrations (20,40,60,80, 100 mg/L) for 720 min. Contact time was studied between 5 and 720 min using a working solution of 100 mg/L. The effect of pH was carried out at pH 2, 4, 6, 8 and 10 using 100 mg/L for 720 min. The temperature effect was measured between 288 and 308 K using 100 mg/L. All the parameters were investigated using 20 mg of the adsorbent in 50 mL of the working solution. The batch experiments were conducted in triplicate.
Equations (1) and (2) were used to calculate (q e ) cadmium and methylene adsorbed (mg/g); (C o ) and (C e ) denote the initial and final amounts of cadmium and methylene left in the solution (mg/L), respectively. The volume (V) of the working standard was measured in (mL) and the (m) mass of the adsorbent was measured in (g).

Kinetic Calculations
Kinetic rates were estimated using the pseudo-first-order (PFO) and pseudo-secondorder (PSO) models with the following equations.
In the above equations, (q e ) and (q t ) indicate the amounts of adsorbed cadmium and methylene blue at the equilibrium time (t), respectively. K 1 denotes the pseudo-first-order rate constant (min −1 ) and K 2 denotes the pseudo-second-order rate constant (g.mg/min). The experimental data were used in Equation (5) to estimate the intra-particle diffusion (IPD) model. (q t ) and (k i ) were determined using KyPlot software to calculate the amount adsorbed and the rate constant. (k i ) is the rate constant in (g/g min 1/2 ), and C is the cadmium and methylene blue concentration on PHS, CHS-500 and CHS-700 surfaces.

Isotherms
The Freundlich and Langmuir models were used to calculate isotherms using the nonlinear equations shown in Equations (6) and (7), respectively, using KyPlot version 6.0. (Q o ) represents the maximum uptake of the adsorbent (mg/g), (b) the solute surface interaction energy, (k f ) the Freundlich capacity factor and (1/n) the isotherm linearity parameter.

Analytical Methods
The morphology of the adsorbents was characterized using SEM, FEI Nova Nano 200, with an EXD spectrometer (Thermo Fisher, Waltham, MA, USA). The functional groups were identified with an FTIR spectrum 400 (Perkin Elmer, Waltham, MA, USA). The methylene blue concentration was obtained with a UV-visible Evolution 220 (Thermo Fisher Scientific) spectrophotometer. The pH was determined using a Metrohm pH meter. The Cd(II) ion concentrations were analyzed with an ICP, iCAP 7000 plus series ICP-OES spectrometer (Thermo Fisher Scientific). The operational details of some of the equipment used is described further in the literature [28][29][30][31].

FTIR Analysis
FTIR provides important information about superficial groups on the adsorbent surfaces [32]. Figure 1 shows the FTIR spectra of PHSs, CHS-500 and CHS-700. The PHS spectrum displays a broad peak at 3292 cm −1 , which is characteristic of O-H stretching vibrations in structures consisting of cellulose, lignin, hemicellulose and adsorbed water [32]. The band at 2912 and 2845 cm −1 was ascribed to C-H vibrations of methyl and methoxy groups (Thabede et al. 2020a) [33]. The peak at 1735 cm −1 was due to C=O vibrations of the ketonic group of the hemicellulose structure [34,35]. The peaks at 1621 and 1511 cm −1 were ascribed to asymmetric and symmetric stretching vibrations of (C=O) and (NH) of the amide group, respectively [36]. The peaks between 1300 and 1000 cm −1 were due to the C-O vibration of the alcohols and carboxylic acids [37]. Comparing the spectra of CHS-500 and CHS-700 with PHSs showed that many bands had decreased intensity, while others disappeared in CHS-500 and CHS-700. The disappearance of bands is due to the decomposition of organic compounds associated with the cellulose and lignin structures [38].
The band at 2912 and 2845 cm −1 was ascribed to C-H vibrations of methyl and methoxy groups (Thabede et al. 2020a) [33]. The peak at 1735 cm −1 was due to C=O vibrations of the ketonic group of the hemicellulose structure [34,35]. The peaks at 1621 and 1511 cm −1 were ascribed to asymmetric and symmetric stretching vibrations of (C=O) and (NH) of the amide group, respectively [36]. The peaks between 1300 and 1000 cm −1 were due to the C-O vibration of the alcohols and carboxylic acids [37]. Comparing the spectra of CHS-500 and CHS-700 with PHSs showed that many bands had decreased intensity, while others disappeared in CHS-500 and CHS-700. The disappearance of bands is due to the decomposition of organic compounds associated with the cellulose and lignin structures [38].

SEM Analysis
SEM is used for imaging the surface of adsorbents on a sub-microscopic scale, showing the sorbent materials' topographies, structures and shapes [39]. The PHS, CHS-500 and CHS-700 morphologies were assessed using the SEM images in Figure 2a-f. Figure  2a,b indicate that the surface of the sorbent material was rough and composed of irregular pores. However, after carbonization, the porosity of CHS-500, seen in Figure 2c,d, and CHS-700, seen in Figure 2e,f, increased significantly with small amounts of carbon debris and micropores of different sizes. These pores could allow for greater cadmium and methylene blue adsorption and more ion exchange sites [40]. The porosity also presents the possibility of metal ions and dyes being trapped inside them [40].

SEM Analysis
SEM is used for imaging the surface of adsorbents on a sub-microscopic scale, showing the sorbent materials' topographies, structures and shapes [39]. The PHS, CHS-500 and CHS-700 morphologies were assessed using the SEM images in Figure 2a-f. Figure 2a,b indicate that the surface of the sorbent material was rough and composed of irregular pores. However, after carbonization, the porosity of CHS-500, seen in Figure 2c,d, and CHS-700, seen in Figure 2e,f, increased significantly with small amounts of carbon debris and micropores of different sizes. These pores could allow for greater cadmium and methylene blue adsorption and more ion exchange sites [40]. The porosity also presents the possibility of metal ions and dyes being trapped inside them [40].

Initial Concentration Effect
The effect of concentration on cadmium and methylene blue adsorption was determined at 298 K and is indicated in Figure 3a,b. Both figures show that cadmium and methylene blue sorption increased as the concentration increased. This means that cadmium and methylene blue adsorption on PHSs, CHS-500 and CHS-700 depended on the concentration. A similar observation was made by Mabungela et al. (2022) [41]. This type of behaviour by adsorbents was explained in a study by Ocampo-Perez et al. (2011) [42], which states that the increased sorption capacity is due to high mass transfer resistance. At a high concentration of 100 mg/L, there were more collisions between the cadmium and methylene blue and adsorbents' surfaces; hence, higher sorption capacities were observed.
It was also shown that CHS-700 had a higher cadmium and methylene blue adsorption capacity than PHSs and CHS-500. Appl

Initial Concentration Effect
The effect of concentration on cadmium and methylene blue adsorption was determined at 298 K and is indicated in Figure 3a,b. Both figures show that cadmium and methylene blue sorption increased as the concentration increased. This means that cadmium  [41]. This type of behaviour by adsorbents was explained in a study by Ocampo-Perez et al. (2011) [42], which states that the increased sorption capacity is due to high mass transfer resistance. At a high concentration of 100 mg/L, there were more collisions between the cadmium and methylene blue and adsorbents' surfaces; hence, higher sorption capacities were observed. It was also shown that CHS-700 had a higher cadmium and methylene blue adsorption capacity than PHSs and CHS-500.

Contact Time Effect
The effectiveness of PHSs, CHS-500 and CHS-700 in adsorbing cadmium and methylene blue was assessed at time intervals between 5 and 720 min (Figure 4a,b). Figure 4a,b display a similar trend showing that the sorption capacities of all the adsorbents increased as contact time increased. The adsorption occurred in two stages. Stage 1 is where a fast adsorption rate occurs at the beginning of the adsorption process. This was attributed to the readily available adsorption sites, pores and functional groups on the surfaces of the adsorbents, which caused rapid adsorption; a similar result was observed by Mabungela et al. (2022) [41]. Stage 2 involves a slower increment due to the saturation of the active sites as contact time advances. PHSs, CHS-500 and CHS-700 reached equilibrium after 100 min, and CHS-500 and CHS-700 showed higher sorption capacities than PHSs.

Contact Time Effect
The effectiveness of PHSs, CHS-500 and CHS-700 in adsorbing cadmium and methylene blue was assessed at time intervals between 5 and 720 min (Figure 4a,b). Figure 4a,b display a similar trend showing that the sorption capacities of all the adsorbents increased as contact time increased. The adsorption occurred in two stages. Stage 1 is where a fast adsorption rate occurs at the beginning of the adsorption process. This was attributed to the readily available adsorption sites, pores and functional groups on the surfaces of the adsorbents, which caused rapid adsorption; a similar result was observed by Mabungela et al. (2022) [41]. Stage 2 involves a slower increment due to the saturation of the active sites as contact time advances. PHSs, CHS-500 and CHS-700 reached equilibrium after 100 min, and CHS-500 and CHS-700 showed higher sorption capacities than PHSs. tration. A similar observation was made by Mabungela et al. (2022) [41]. This type of be-haviour by adsorbents was explained in a study by Ocampo-Perez et al. (2011) [42], which states that the increased sorption capacity is due to high mass transfer resistance. At a high concentration of 100 mg/L, there were more collisions between the cadmium and methylene blue and adsorbents' surfaces; hence, higher sorption capacities were observed. It was also shown that CHS-700 had a higher cadmium and methylene blue adsorption capacity than PHSs and CHS-500.

Contact Time Effect
The effectiveness of PHSs, CHS-500 and CHS-700 in adsorbing cadmium and methylene blue was assessed at time intervals between 5 and 720 min (Figure 4a,b). Figure 4a,b display a similar trend showing that the sorption capacities of all the adsorbents increased as contact time increased. The adsorption occurred in two stages. Stage 1 is where a fast adsorption rate occurs at the beginning of the adsorption process. This was attributed to the readily available adsorption sites, pores and functional groups on the surfaces of the adsorbents, which caused rapid adsorption; a similar result was observed by Mabungela et al. (2022) [41]. Stage 2 involves a slower increment due to the saturation of the active sites as contact time advances. PHSs, CHS-500 and CHS-700 reached equilibrium after 100 min, and CHS-500 and CHS-700 showed higher sorption capacities than PHSs.

pH Effect
The cadmium and methylene blue uptake by PHSs, CHS-500 and CHS-700 was evaluated at pH 2, 4, 6, 8 and 10 (Figure 5a,b). PHS, CHS-500 and CHS-700 sorption capacities were greater in a basic solution at pH 8 than in an acidic medium between pH 2 and 6. All the adsorbents recorded a maximum adsorption capacity at pH 8. These results were ascribed to the adsorbents' surface deprotonation, which resulted in enhanced electrostatic interaction and higher adsorption. At lower pH values, cadmium and methylene blue uptake was minimal due to weak electrostatic interactions at acidic pH conditions because the functional groups on the surface of the adsorbents were protonated and acquired a positive charge [43].
The cadmium and methylene blue uptake by PHSs, CHS-500 and CHS-700 was evaluated at pH 2, 4, 6, 8 and 10 (Figure 5a,b). PHS, CHS-500 and CHS-700 sorption capacities were greater in a basic solution at pH 8 than in an acidic medium between pH 2 and 6. All the adsorbents recorded a maximum adsorption capacity at pH 8. These results were ascribed to the adsorbents' surface deprotonation, which resulted in enhanced electrostatic interaction and higher adsorption. At lower pH values, cadmium and methylene blue uptake was minimal due to weak electrostatic interactions at acidic pH conditions because the functional groups on the surface of the adsorbents were protonated and acquired a positive charge [43].

Temperature Effect
Experiments were carried out at different temperatures (288-308 K) to assess the effect of temperature on the adsorption of cadmium and methylene blue by PHSs, CHS-500 and CHS-700. The results are shown in Figure 6a,b. The plots show increased cadmium and methylene blue sorption capacities at 288 to 308 K for all the adsorbents. The graphs show that the adsorption capacity increases with increasing temperature, indicating that adsorption is an endothermic process. This may be related to the enhanced mobility of the dye and cadmium ions from the bulk solution towards the adsorbent surfaces of PHSs, CHS-500 and CHS-700 with increasing temperature. At higher temperatures, many molecules may also obtain adequate energy to interact with active sites at the surface of adsorbents. A similar adsorption trend for both pollutants means that they were adsorbed on similar sites with identical hydrogen bonds, electrostatic interactions or π-π interactions between the aromatic rings and, therefore, different adsorption capacities [44].

Temperature Effect
Experiments were carried out at different temperatures (288-308 K) to assess the effect of temperature on the adsorption of cadmium and methylene blue by PHSs, CHS-500 and CHS-700. The results are shown in Figure 6a,b. The plots show increased cadmium and methylene blue sorption capacities at 288 to 308 K for all the adsorbents. The graphs show that the adsorption capacity increases with increasing temperature, indicating that adsorption is an endothermic process. This may be related to the enhanced mobility of the dye and cadmium ions from the bulk solution towards the adsorbent surfaces of PHSs, CHS-500 and CHS-700 with increasing temperature. At higher temperatures, many molecules may also obtain adequate energy to interact with active sites at the surface of adsorbents. A similar adsorption trend for both pollutants means that they were adsorbed on similar sites with identical hydrogen bonds, electrostatic interactions or π-π interactions between the aromatic rings and, therefore, different adsorption capacities [44].

Isotherm Models
The Langmuir and Freundlich isotherms (Table 1) were assessed using equilibrium data obtained at 288 K to calculate the maximum sorption and interaction mechanism for cadmium and methylene blue by PHSs, CHS-500 and CHS-700. Table 1 displays that the Langmuir isotherm model was a better fit for the results with good correlation coefficients.

Isotherm Models
The Langmuir and Freundlich isotherms (Table 1) were assessed using equilibrium data obtained at 288 K to calculate the maximum sorption and interaction mechanism for cadmium and methylene blue by PHSs, CHS-500 and CHS-700. Table 1 displays that the Langmuir isotherm model was a better fit for the results with good correlation coefficients. This result suggested that the adsorption of cadmium and methylene blue by PHSs, CHS-500 and CHS-700 took place on active sites on homogenous surfaces in a monolayer coverage [45].

Kinetic Models
The rate at which methylene blue and cadmium are adsorbed from water is important. The rate was estimated to determine the efficiency of PHSs, CHS-500 and CHS-700 for water treatment. The sorption of cadmium and methylene blue by PHSs, CHS-500 and CHS-700 was fitted into the pseudo-first-order (PFO) and pseudo-second-order (PSO) models and intraparticle diffusion (IPD). The parameters in Table 2 show that the PSO model is a better fit for the cadmium and methylene blue uptake by PHSs, CHS-500 and CHS-700 than the PFO model. The R 2 indicates that values are closer to 1, suggesting that the intra-particle diffusion (IPD) model did not fit.

Thermodynamic Parameters
Thermodynamic parameters were used to determine the spontaneity and feasibility of cadmium and methylene blue adsorption by PHSs, CHS-500 and CHS-700 (Table 3). The parameters evaluated at different temperatures show positive ∆H • values for all adsorbents, which implied an endothermic reaction. Generally, an endothermic reaction increases the sorption process for pollutants as temperature increases [35]. ∆S • values were also positive; this indicated increased randomness and degree of freedom for cadmium and methylene blue in the solution. The negative values for ∆G • implied that the uptake of cadmium and methylene blue by PHSs, CHS-500 and CHS-700 was feasible and spontaneous.

Adsorption Mechanism for Methylene Blue and Cadmium
The proposed uptake mechanisms for methylene blue and cadmium are shown in Figure 7. The FTIR spectra (Figure 1) show the presence of -OH, -CH, -NH 2 and -C=O groups on the adsorbent's surface for cadmium and methylene blue uptake. Similar observations were reported by   [46]. The mechanism showed different interaction types between cadmium, methylene blue and the surface structures' functional groups. Methylene blue is a cationic dye and positively charged ion in an aqueous solution.
Protons are available at lower pHs and on the surfaces of the adsorbents. Therefore, the resulting ionic repulsion with methylene blue's positively charged surface leads to a lower uptake [47]. Cationic dye sorption is ideal at pH > pHpzc because of the occurrence of -OH on the adsorbent surface [48]. This also applies to the adsorption of cadmium. The adsorbents' surface changed from positively to negatively charged with increasing pH-hence the higher sorption ability since there was no repulsion. At above pH 8, the C-OH bond on the surface becomes C-O − . Therefore, electrostatic attractions between positively charged dye ions, cadmium and negatively charged adsorption sites cause an increase in adsorption [47]. Based on the information above, it can be concluded that different interactions enhanced cadmium and methylene blue uptake by PHSs, CHS-500 and CHS-700. Table 4 compares the cadmium and methylene blue adsorption capacity of CHS-500 and CHS-700 with other adsorbents converted to carbonaceous materials. The adsorption capacities of CHS-500 and CHS-700 are amongst the highest reported values. The results of CHS-500 and CHS-700 suggest that carbonized hemp is a promising material for water purification, with some of the advantages of hemp seeds including that there are several functional groups that enhance adsorption capacity and that they are inexpensive. From Table 4, it can be seen that hemp seeds performed better than other sorption materials.

Comparison Study
hence the higher sorption ability since there was no repulsion. At above pH 8, the C-OH bond on the surface becomes C-O − . Therefore, electrostatic attractions between positively charged dye ions, cadmium and negatively charged adsorption sites cause an increase in adsorption [47]. Based on the information above, it can be concluded that different interactions enhanced cadmium and methylene blue uptake by PHSs, CHS-500 and CHS-700.  Table 4 compares the cadmium and methylene blue adsorption capacity of CHS-500 and CHS-700 with other adsorbents converted to carbonaceous materials. The adsorption capacities of CHS-500 and CHS-700 are amongst the highest reported values. The results of CHS-500 and CHS-700 suggest that carbonized hemp is a promising material for water purification, with some of the advantages of hemp seeds including that there are several functional groups that enhance adsorption capacity and that they are inexpensive. From Table 4, it can be seen that hemp seeds performed better than other sorption materials.

Conclusions
In this study, pristine hemp seeds (PHSs) were successfully carbonized at 500 and 700 • C to the adsorbents CHS-500 and CHS-700, respectively. Batch adsorption studies were carried out using different parameters, including pH value (2, 4, 6, 8 and 10), contact time (5-720 min), initial concentration (20,40,50,80 and 100 mg/L) and temperature (288-308 K). FTIR and SEM analyses showed that PHSs, CHS-500 and CHS-700 were porous with -CH, -OH, -NH 2 and -C=O groups on their surfaces. All adsorbents demonstrated a gradual increase in concentration between 20 and 100 mg/L. The time effect showed a fast adsorption rate at the beginning of the adsorption process, with a slower increment at a later stage after 100 min, where CHS-500 and CHS-700 showed higher sorption capacities than PHSs. The temperature effect showed that the sorption capacities increased from 288 to 308 K for all the sorbents. All adsorbents displayed their highest cadmium and methylene blue sorption capacities at pH 8. The Langmuir isotherm model was the best fit for the experimental data, with correlation coefficients close to 1. PSO better described the adsorption process of cadmium and methylene blue by PHSs, CHS-500 and CHS-700 than the PFO and IPD models. ∆G • indicated that cadmium and methylene blue sorption by PHSs, CHS-500 and CHS-700 was feasible. The proposed uptake mechanism was due to electrostatic attraction as well as π and hydrogen bonding. The maximum adsorption capacities of CHS-500 were 31.12 and 49.89 mg/g for cadmium and methylene blue, respectively. On the other hand, CHS-700 had capacity values of 35.08 and 53.98 mg/g for cadmium and methylene blue, respectively. Both carbon adsorbents adsorbed more methylene blue than cadmium, with the highest being obtained for CHS-700.