The Use of Pre-heated Black Cumin Seeds (Nigella sativa) for Sorption Basic Dyes from Aqueous Solutions

The discharge of industrial effluents containing dyes such as methylene blue (MB) and safranine T (ST) into water resources has caused serious problems for humans and organisms which live in water. On the other hand, the seeds of nigella sativa have been known to be effective and economical materials that present an excellent performance of dye removal from various types of wastewater. In this study, a different method from the existing literature was used, where the black cumin seeds (HBCS) were heated for a twenty-four hour period at 85 °C and ground to 300 μm particle size to obtain new adsorbents for (MB) and (ST) dye adsorption from aqueous solutions. Four operating variables, which show a strong impact on adsorption, were tested by conducting a batch mode adsorption experiment and these were: the dye solution’s initial pH (pH), the contact time (t), the initial concentration of the dye (Co), and the mass of the adsorbent (ma). The sorption capacity of the adsorbent (q) and the dye removal efficiency (% Rem) were utilized as indicators of the impacts of selected operating variables on adsorption processes. It was found that the adsorption of MB and ST dyes increased when the initial concentration of the solution is higher. The adsorption of MB and ST dyes onto HBCS with initial concentration of 50 mg/L was 3.39 mg/g and 4.36 mg/g, respectively. The equilibrium values for the MB and ST dyes corresponded with the Freundlich isotherm model onto HBCS highlighting the multi-layer adsorption process occurring which describes the interactions between the modified HBCS and the two dyes. The findings demonstrate that the optimum adsorption processes of the MB and ST dyes accrued in the 120 minutes and obtained at pH 10.7 and 11 for MB and ST, respectively. These conditions provide a negative charge for the surface of HBCS, causing its attraction to the positive charge of the MB and ST dyes. Kinetic studies elucidated that the MB and ST dyes adsorption onto HBCS most corresponded with pseudo-second order. The findings of this study could be implemented functionally in the industrial wastewater treatment sector.


INTRODUCTION
The contamination of freshwater sources is presently regarded a worrying environmental issue to a modern-day world. Due to rapid industrial and unplanned urbanization activities and unskilled utilization of natural water resources, nearly 80% of the world population is facing the problems related to water security and fresh water supply degradation due to contamination [De Gisi et al., 2016].The manufacturing industry is one of the major contributors to aquatic pollution to all over the world. Discharging the contaminated effluent to water resources causes a severe threat to the natural water system, human wellbeing and the whole ecosystem [Lellis et al., 2019]. The dyes in the effluent water from the textile, paper, pulp and dyeing industries should be removed before discarding the effluent into water systems [Tara et al., 2020]. It was reported that about "5,000-10,000" tons of dye are discharged into the water system every year [Benjelloun et al., 2021]. The dyes in water are detectably visible and can affect the color, taste and odor of the water. Many of these dyes have high levels of toxic and recalcitrant compounds, unmanageable organic pollutants, and even carcinogenic materials. This causes disastrous effects to aquatic and human life [Katheresan et al., 2018;Hynes et al., 2020].
Basic dyes are synthetic dyes that have a wide variety of industrial applications due to their concentrated color and easy application procedure [Bayomie et al., 2020;Benjelloun et al., 2021]. On the other hand, these dyes are characterized by their high solubility and poor biodegradation in water. It is estimated that about (20-50)% of them last in the outflow wastewater discharged from manufacturing industries into water bodies [Lellis et al., 2019]. Recently, as legislation has imposed strict requirements on the textile industrial effluent production, much consideration has been paid to treat the dye in the outflow wastewater. Hence, there is a high demand to remove, mineralize or transform the textile industry dyes from wastewater [Tara et al., 2020].
A scope of traditional remedying techniques for dye desegregation have been achieved broadly, including bio-electrochemical degradation, membrane filtration process, oxidation method, microbial technology, coagulation, photocatalytic degradation, and reverse osmosis [Katheresan et al., 2018;Shindhal et al., 2021]. In order to reduce the harmful consequences involved, adsorption has been considered an efficient technique for the removal of various types of dye from the aqueous solutions due to its easy application, low operational expenses, high tolerance to pollutant types, insusceptibility to toxins, capacity to treat different dyes from contaminated wastewater and the ability to regenerate the used adsorbent [ [Srinivasan, 2018].
For the purpose of the present study, MB and ST, being described as concerning pollutants by various health organizations, were chosen as model materials to assess the efficiency of nigella sativa for eliminating the dyes from aqueous solutions. Generally, MB and ST are characterized by their high color intensity, even when in small amounts, and their complex structure makes them highly toxic [Siddiqui et al., 2018]. MB, usually recognized as basic blue 9, is a stain or positivelycharged soluble dye that presents in the structure of a crystalline solid or a blue/green colored powder that separates in aqueous solutions as cations and chloride ions. General names of MB are Aizen methylene blue, methylthionine chloride, and Chromosmon [Al-Ghouti and Al-Absi, 2020].
ST is a basic red dye substance (basic red 2(BR-2) with an IUPAC designation as 3, 7-Diamino -2, 8-dimethyl-5-phenyl phenazinium chlorate and is a cationic dye [Xia et al., 2018]. It is used in different areas such as chemistry, biology, and medicine. ST, brown reddish powder, is aphenazine dye which is soluble in water and is primarily used as edible dye in coloring and flavoring cookies and candy. It is also utilized to color tannin, cotton, silk, bast fibers, leather, wool, and paper [Abukhadra and Mohamed, 2019]. In spite of the beneficial applications of MB and ST in science, these dyes have detrimental impacts on the environment and human life. They are highly water soluble, toxic and mutagenic agents, which have been shown to be carcinogenic [Thabede et al., 2020].
Although many studies have dealt with using the black cumin seeds as an effective adsorptive material in treating wastewater, in this study, the efficiency of modified nigella sativa (pre-heated black cumin seeds (HBCS)) adsorbents to eliminate basic dyes (MB and ST) from aqueous solutions was presented. In the literature, the adsorption studies using HBCS to treat MB and ST from aqueous solutions have not been reported to date. Different experimental factors including solution pH, contact period, concentration of dye, and plant dosage were selected to perform the adsorption experiment.

Preparation of adsorbents
In this study, nigella sativa seeds (black cumin, also known as black caraway, nigella, kalojeera, kalonji or kalanji) were used as an adsorbent. The selection of black cumin seeds as absorbent in this study was due to their: low cost, availability in nature, low environmental impact, accessibility, lack of selection towards contaminants, easy reproduction, reusability, and the potential of their surface to have numerous efficient groups (carboxyl, hydroxyl and amide) [Srinivasan, 2018]. Such features set the black cumin plant in a beneficial place for adsorption technology.
The seeds of black cumin (BCS) were obtained from a wellness store in Babel Center, Babel, Iraq. The (BCS) were cleaned using distilled water before being used as an adsorbent to eliminate dirt. Later, the seeds were modified by heating to evaluate the impact of such condition on the removal of basic dyes. They were dried and heated in the oven for twenty-four hours at 85 °C. Then, the heated black cumin seeds (HBCS) were ground to 300 µm particle size, labeled and stored at a temperature of 25 °C in covered glass containers that were cleaned with distilled water and purified to remove any contaminants, in preparation for the sorption process.

Preparation of dye solutions
Basic dyes (MB and ST) were utilized in this study as adsorbates. MBis a standard powder and has the molecular format C 16 H 18 N 3 SCl (molecular weight: 319.86 g/mol) with CI Categorization Numeral 52015. MB is a basic blue tempera of spectroscopic rung and has a watery dissolvability of 50 g L -1 at 20 °C. In turn, ST (basic red 2) is a standard powder and has the molecular format C 20 H 19 N 4 Cl (molecular weight: 350.84 g/mol) with CI classification numeral 50240. The ST dye is soluble in water and alcohol, it is composed of 3,7-Diamino-2,8-dimethyl-5-phenyl phenazinium chlorate [Wu et al., 2019]. Both dyes were obtained from the marketplace: Babylon Lab Chemical Co., Ltd, Babel, Iraq.
Two stock solutions of 50 mg/L were made by mixing appropriate amounts (0.05 g) of dry powdered dye of MB and ST in 1 L of water which was cleaned by double-distillation. In order to make the working solutions, the stock solution and distillate water were mixed to reach the accurate working solution concentration. The experiment solution pH was set using dilute hydrochloric acid. The UV/VIS Spectrophotometer (PerkinElmer Lamda 25 UV/VIS) was utilized to find the concentrations of MB and ST in solution. The maximum wavelength of λ max = 665 nm for MB and λ max = 520 nm for ST were used to measure the dye absorbance concentration. The absorbance-concentration profile was found by plotting a linear calibration curve of absorbance vs. the dye concentration.

Batch adsorption experiment
Batch experiments were used to determine the adsorption measurement by using a known amount of the modified adsorbent dosage (0.5-2) g mixed with 100 ml of a known concentration of each aqueous dye solutions (MB and ST) in a sequence of conical flasks with 250 ml capacity. The adjustment of pH was conducted by diluting the solution with HCl or NaOH (0.1 mol/L) and then calculating the pH with aSCT-BEN-PH-1 pH meter (JJS, Dallas, TX, USA). The test adsorption experiments were performed in a temperature-monitored (±1 °C) (Oxylab, Babel) water vibrator. The mix solution was agitated at a fixed temperature at 250 rpm and 120 min. For pre-established period of time, the bottles were centrifuged and the concentration of the residual dye in the mixture was analyzed, after equilibrium was reached, by measuring the supernatant absorbance at the wavelength that corresponds to the optimum sample absorbance using an UV/ VIS Spectrophotometer at 650 nm and 520 nm for MB and ST, respectively.
For each dye, the impact of four variables on the adsorption capacity of HBCS was examined and these variables were: the initial pH of the dye solution (pH), contact time (t), the initial dye concentration (C o ) and mass of adsorbent (m a ). The effect of contact time was investigated for each dye at different time durations, which were: 15, 30, 45, 60, 75, 90, 105 and 120 min and with standard concentration of 50 mg/L. Then, 100 mL of each dye was placed into lidded bottles consisting of 2.0 g of the adsorbent. For a time period of 120 min, the impact of MB and ST was investigated at different concentration of: 10, 20, 40, 50, 60, 80, 90 and 100 mg/L. In regards to the MB solution, 100 mL of standard solution (50 mg/L) was placed into lidded bottles consisting of 2.0 g of the adsorbent and was assessed at varying pH of: 3.8, 5.9, 7, 8.3 and 10, to study the impact of pH on the solution. The same process was applied for the ST solution at varying pH of: 4, 5, 7.5, 9.5 and 11. A vibrator was applied for each variable (pH, contact time, dye concentration, and absorbent dose) to reach equilibrium between the adsorbent and working standard solution at 250 rpm.

Equilibrium adsorption quantity and its removal efficiency
Under the adsorption kinetics and adsorption isotherm model, adsorption experiments were set up using various adsorbent doses. The impact of the four chosen variables on the adsorbed quantity (q) of HBCS and the percentages of dye removal (% Rem) with HBCS were investigated.
The amount of adsorbate adsorbed by HBCS expressed per unit mass, q, and dye removal efficiency, % Rem, of adsorption are found by Eqs.
(1) and (2), respectively: where: q -quantity of dye adsorbed by the adsorbent (mg/g); V -the volume of the dye solution (L); C o -initial dye concentration in contact with the adsorbent (mg/L); C f -the concentration of the dye after the batch adsorption work (mg/L); and m a -the adsorbent mass (g).

Kinetics models
Finding the rate at which sorbate is removed from the solution in kinetic order is required to build up a suitable sorption treatment processes [Senthil Kumar et al., 2014]. The adsorption kinetics, therefore, represent the main principle in the sorption process analysis [Benjelloun et al., 2021]. In order to assess the adsorption kinetics, the pseudo first-order (Equation (3)) [Lagergren, 1898] and second-order kinetics (Equation (4)) [Ho and McKay, 1999] demonstrated below are commonly used: where: q e, q t -the adsorbent's adsorption capacities (mg/g) at equilibrium and at a specific time (t)(min), respectively, k 1 is the first-order kinetic rate constant (1/ min) and k 2 is the second-order kinetic rate constant [(g/mg) (1/min)].

Adsorption isotherm model
The obtained data from the isotherm experiments was used to assess the various adsorption isotherm models. Due to their precise descriptions, adsorption isotherms are vital when determining adsorption mechanisms. The Langmuir and Freundlich models were employed to determine the types of adsorption process and evaluate the adsorption isotherm of dyes onto the adsorbent [Benjelloun et al., 2021].
The Langmuir model is suitable for single molecule adsorption, demonstrating the adsorption mechanism on homogeneous surfaces, and monolayer adsorption processes. Equation (5) shows the linear Langmuir model [Langmuir, 1918]: The Freundlich model describes a practical explanation of the single element adsorption equilibrium, and can be employed in a wide variety of fields. The model assumes the process is heterogeneous, multilayer adsorption [Kocaman, 2020]. Equation (6) shows the Freundlich model [Freundlich, 1906]: where: C e -the equilibrium concentration of the dye solution (mg/L); q max , q e -the maximum monolayer and equilibrium adsorption quantity of dye per HBCS mass, (mg/g) respectively; b -the Langmuir equilibrium constant associated with the affinity of a dye molecule to the adsorbent (L/mg); K f -the Freundlich constant [(mg/g) (L/mg)1/n] relating to the adsorptive bond energy, and 1/nthe strength factor of the adsorption or the heterogeneity of the surface.

Impact of pH on dye adsorption
The pH of the solution is an essential controlling parameter during the adsorption experiment. This experiment investigated the impact of pH on the removal capacity of MB and ST dyes using HBCS with the various pH values and Figure 1 displays these results. The removal capacity of MB and ST dyes was higher when pH was increased and the maximum removed capacity of the dyes was obtained at the pH of 10.07 and 11 for the MB dye and ST dye, respectively ( Figure 1).
Furthermore, the removal percentage results also show an increase in the removal efficiency of each dye when the pH was increased and the maximum percentage was 90.45% at pH of 10.07 for the MB dye and 96.28% at pH of 11 for the ST dye. High pH values caused the surface of the adsorbent (HCBS) to become increasingly negatively charged and increases the dye attraction to the active sites of the surface of the adsorbent, explaining the increased removal efficiency of both dyes when pH is increased. The removal efficiency of both dyes with the increase in the solution pH showed a trend that is elucidated by the fact that decreasing pH caused the adsorbent (HBCS) surface to become more positively charged, therefore reducing the attraction of the dye to the active sites of the adsorbent surface [Xia et al., 2018]. The removal of the MB and ST dyes can be impacted by the zero point charge pH (pH zero ) of the adsorbent. Below pH zero , the surface of the adsorbent becomes more positively charged. Following the results presented by this experiment, the values of pH 10.07 and 11, for the MB dye and ST respectively, were selected as best suitable pH values for additional studies. The removal capacities of both dyes showed a similar trend, where the phase of rapid initial adsorption had proceeded to a slower phase of adsorption, when a plateau was attained. The highest adsorption removal of MB (93%) and ST (96%) dyes by HBCS were observed at 120 min. The changes in dye removal efficiencies were consistent with both dyes' change in capacity of adsorption. Thabede et al., 2020, also noted similarly on MB adsorption onto activated carbon powder. Due to the high presence of adsorption sites on HBCS that can be available to interact with the dye molecules of the solution, there is a rapid adsorption phase initially. The rate of adsorption showed a slower increase with time due to the number of the active sites decreasing. When the adsorption and desorption of the dyes reach an equilibrium state, the plateau is established. However, a distinguished difference could be noted on the adsorption capacities of MB (1.83 mg/g) and ST (2.2 mg/g) dyes. The difference in the ionic strength of dye molecules may be the cause of the higher removal capacity of the ST dye in comparison to the MB dye onto HBCS [Xia et al., 2018].  Figure 3 shows the effects of initial dye concentration on the adsorption behavior of the MB and ST dyes by HBCS. The adsorption capacities of HCBS from 0.423 mg/g (at 10 mg/L) to 3.39 mg/g (at 100 mg/L) for the MB dye and from 0.466 mg/g (at 10 mg/L) to 4.36 mg/g (at 100 mg/L) for the ST dye can be noted. The change in the initial dye concentration is the same as the change in the total amount of dye molecules in the determined adsorbent mass and solution volume [Wong et al., 2020].

Impact of initial MB and ST dye concentrations
Therefore, higher adsorption capacity could be caused by more adsorbates binding to the active sites of the adsorbent due to increased concentration of dye molecules. This shows the high adsorption removal of MB (94.7%) and ST (97.3%) dyes at low dye concentrations (10 mg/L). At higher initial dye concentrations, it is expected that a higher number of adsorbates are in competition for the same amount of active sites which results in high adsorption sites saturation and the dye molecules become more distributed throughout the solution without absorption taking place. Consequently, the dye removal percentage of 67.95% (MB) and 87.3% (ST) is observed to decrease at the concentration of 100 mg/L.

Impact of the dose of adsorbent
The impacts of the dose of adsorbent (m a ) on MB and ST adsorption are shown in Figure 4. A notable reduction of adsorption capacity of HBCS can be observed from 6.17 mg/g (at 0.5 g a dose) to 1.99 mg/g (at a 2.0 g dose) for the MB dye, and from 7.26 mg/g (at a 0.5 g dose) to 2.0 mg/g (at a 2.0 g dose) for the ST dye. These results are comparable to the MB adsorption onto cellulosic olive stones biomass [Al-Ghouti and Al-Absi, 2020] and ST adsorption onto zeolite minerals [Abukhadra and Mohamed, 2019]. This trend is caused by the higher number of available active sites on HBCS at an increased dose of adsorbent and the number of molecules of adsorbate stays the same. Moreover, the inverse relationship between the dose of adsorbent and capacity of adsorption could also be the reason for the noticed trend as presented in Eq. (1).

Sorption Kinetics assessment
The kinetic estimation was conducted by applying the experiment results to the pseudo-firstorder (PFO) and pseudo-second-order (PSO) kinetic models, to assess the adsorption of MB onto HBCS. The linear coefficient of determination (R 2 ), kinetic rate constants (k 1 and k 2 ) and the capacity of adsorption at equilibrium (q e ) are presented in Table 1. The fitting plots at various initial MB dye concentrations are presented in Figures 5 (pseudo-first-order model) and 6 (pseudo-second-order model). It can be observed that the R 2 values of the pseudo-firstorder kinetic model ( Figure 5) were higher than those obtained by the pseudo-second-order (Figures 6) and the calculated (q e ) results obtained from the second-order model were comparable with the experimental adsorption findings. The comparison of the R 2 values pointed out that the adsorption values are more suitably fitted to the    Figure 5. Pseudo-first order graphs for the impact of contact time and initial dye concentration on removal of MB using HBCS Figure 6. Pseudo-second order graphs for the impact of contact time and initial dye concentration on removal of MB using HBCS surface adsorption and finally, the internal diffusion process stage. This model demonstrates the existing adsorption process well [Kocaman, 2020], highlighting the complex process of the absorption of dyes by HBCS. Similar adsorption features were noted for the substrate of Pleurotus eryngii [Wu et al., 2019] and the levulinic acidmodified natural shells [Kocaman, 2020].

Adsorption isotherm analysis
The Langmuir and Freundlich isotherms were utilized to work out the maximum capacity of adsorption, demonstrating how the adsorbate and biosorbent interact and to find the solid-liquid interface adsorption. According to equations 5 and 6, the adsorption values were applied to these two models in order to analyze the adsorption isotherm. The model adsorption isotherm variables and data of the coefficient of determination values (R 2 ) are presented in Table 2 as well as illustrated in Figures 7 (Langmuir isotherm) and 8 (Freundlich isotherms). The values of R 2 coefficient of the Freundlich model were higher than those of the Langmuir model, which indicated that the Freundlich model is a better demonstration for the adsorption data of both dyes ( Table 2). The sorption of MB and ST on the HBCS adsorbent followed the Freundlich isotherm model with R 2 being 0.956 and 0.981 for the MB and ST dyes, respectively. Such results indicate the occurrence of multilayer adsorption with heterogeneous nature making an interaction between the adsorbate and the adsorbent, which was also observed in the MB adsorption onto activated carbon particles [Thabede et al., 2020]Fourier transformed infrared (FIR, as well as the ST adsorption onto the spent substrate of Pleurotus eryngii [Wu et al., 2019]. It was shown that the adsorption on the surface of the adsorbent was linear and uniform due to the fact that the Freundlich parameter (n) was established to be in the range of 0-1 ( Table 2). The Freundlich isotherm is one of the most fitted isotherms for demonstrating the process of adsorption occurring on heterogeneous surfaces which can be expressed by the heterogeneous nature of the preheated black cumin seeds.

CONCLUSIONS
The effectiveness of modified pre-heated black cumin seeds (HBCS) was evaluated for the removal of methylene blue (MB) and safranine T (ST) dyes from aqueous solution. A batch mode of adsorption experiment was conducted with variables of pH, contact time, initial dye concentration, and adsorbent dosage. The results demonstrated that the pH of the solution influences the adsorption of MB and ST. It was noted that higher initial concentration of the solutions increased the adsorption capacity for the MB and ST dyes. The findings present that the maximum adsorption processes were noticed at