E ﬃ cient Removal of Ni(II) from Aqueous Solution by Date Seeds Powder Biosorbent: Adsorption Kinetics, Isotherm and Thermodynamics

: Adsorption investigations in batch approaches were performed to explore the biosorption of Ni(II) ions from aqueous solutions on date seeds powder. The e ﬀ ects of pH, particle size, initial concentration of Ni(II) ions, adsorbent mass, temperature, and contact on the adsorption efficacy were studied. The maximum removal obtained was 90% for an original Ni(II) ion solution concentration of 50 ppm was attained at pH 7 after 30 min and with 0.30 g of an added adsorbent. The four adsorption models, namely Freundlich, Langmuir, Dubinin–Radushkevich (D–R), and Temkin were examined to ﬁt the experimental ﬁndings. The adsorption system obeys the Freundlich model. The system was found to follow the pseudo-second order kinetic model. Thermodynamic factors; entropy ( ∆ S ◦ ), enthalpy ( ∆ H ◦ ), and Gibbs free energy ( ∆ G ◦ ) changes were also assessed. Results proved that adsorption of Ni(II) ions is exothermic and spontaneous. Sticking probability value was found to be less than unity, concluding that the process is dominated by physical adsorption. sites at elevated temperatures.


Introduction
The presence of heavy metals in water streams is among one of the most dangerous environmental problems arising from the disposal of untreated industrial effluents [1][2][3][4]. Many industries comprise of final treatment processes where discharged metal compounds may lead to pollution in the effluent water [2,5,6]. Most of these heavy metals are non-biodegradable or with long biological half-life leading to potential accumulation and human exposure through food or water [1].
Ni(II) ions exist naturally in water as nitrates, sulfides, and oxides. Nickel intake above the permissible limit causes skin dermatitis, fibrosis, vomiting, pulmonary, nausea, and many other diseases [3,[6][7][8][9]. Most of the methods used for Ni(II) removal from artificial wastewater-like cation exchange and precipitation are costly and produce toxic sludge [2,9]. Recently, some economical, renewable, and effective agricultural and natural materials have been studied as alternative biosorbents [2].

Collection and Treatment of Adsorbent
Dates seeds were collected locally from Abha city, Saudi Arabia. Seeds were rinsed with tap water and then by deionized water, dried at room temperature and were then grounded to powder size using a ball mill before being sieved. The efficiency of the room-temperature dried DSP was compared with small amount of oven-dried DSP and no differences were noticed on the results. Thus, to minimize the cost, drying of DSP by oven was not used in this study.

Reagents
Deionized water (>18 Ω/cm, Milli-Q) was used throughout this work for the solutions and DSP preparations. A stock solution of 1000 ppm Ni(II) ions was prepared using NiNO 3 (LOBA Chemie, Laboratory Reagents and fine Chemicals, Mumbai, India).

Batch Adsorption
Adsorption batch experiments were conducted at different operating conditions (pH, time, adsorbent dosage, adsorbent particles size, and temperature) by adding the desired amount of DSP to 50 mL of Ni(II) ion solution under each particular condition. NaOH (0.25 M) and/or HCl (0.25 M) were used to control the pH value. Mechanical thermostated shaker (WSB, Witeg, Belrose, Germany) was used throughout all the experiments. The solutions were filtered and then analyzed using Atomic Absorption Spectroscopy (AAS) (SpectrAA 220, Varian, Australia) to measure the remaining Ni(II) ions concentrations. The removal efficiencies (R%) were calculated using Equation (1).
where C 0 and C e are the initial and equilibrium Ni(II) ion concentrations, respectively.

Characterization of Biosorbent
The Brunauer-Emmett-Teller (BET) surface area, pore size and pore volume after and before the adsorption process were investigated using Quanta Chrome NOVA 4200E Surface Area Analyzer. The morphology of the DSP was investigated using a scanning electron microscope technique (SEM) JEOL 6360 (Japan). Accelerating voltage of 20 kV was used. The functional groups of DSP before and after the adsorption process were investigated by ATR-FTIR (Cary 630 FTIR from Agilent) in the range of 4000-400 cm −1 at a spectral resolution of 8 cm −1 . DSP samples were analyzed without any pretreatment. Table 1 displays the results of the analysis of DSP by surface area analyzer. Results prove that DSP poses a mesoporous arrangement. Mesopores are detected over the entire sample surface forming a highly uniform and interpenetrating permeable media. Moreover, results also confirm that DSP has a large surface area compared to some other adsorbents used to adsorb Ni(II) ions [13].

ATR-FTIR Spectrum
Investigations of the ATR-FTIR spectra from DSP after and before the adsorption process ( Figure 1) proves the presence of functional groups, which are among the major characteristic to DSP. The presence of developed aliphatic groups was identified by the absorption band ascribed to the stretching vibrations of carbon-hydrogen bonds in the range between (2980-2840 cm −1 ). OH functional groups were recognized by the broad band ascribed to stretching vibrations of oxygen-hydrogen bonds in the range from 3600 to 3100 cm −1 , whereas, ether structures were identified by the band ascribed to stretching vibrations of carbon-oxygen in the range (1100-1000 cm −1 ). Moreover, CO groups were also detected by the band ascribed to stretching vibrations of C=O bonds in the range from 1750 to 1500 cm −1 . On the other hand, the bands of aromatic compounds bond groups overlay with those obtained by the bonds of other structures. Generally, the FTIR spectrum proves the multiplicity of the structure of DSP. The presence of these aforementioned functional groups on the DSP surface indicates its potential ability to act as a promising adsorbent [14].

Scanning Electron Microscope Technique (SEM)
The SEM technique investigations provided an insight into the diverse morphology of the DSP where some larger constituents show asymmetrical form, some other constituents have an extended rod-like construction whereas other smaller constituents display rectangular form. Overall, most of the constituents have a reedy and coarse structure with irregular ends. Particle size of the DSP ranges from 5-15 µm. It can be noted from Figure 2a the availability of numerous available cavities and holes

Scanning Electron Microscope Technique (SEM)
The SEM technique investigations provided an insight into the diverse morphology of the DSP where some larger constituents show asymmetrical form, some other constituents have an extended rod-like construction whereas other smaller constituents display rectangular form. Overall, most of the constituents have a reedy and coarse structure with irregular ends. Particle size of the DSP ranges from 5-15 μm. It can be noted from Figure 2a the availability of numerous available cavities and holes enabling Ni(II) ions to be adsorbed, while Figure 2b shows that these holes are occupied by Ni(II) ions indicating good adsorption capacity for DSP.

Scanning Electron Microscope Technique (SEM)
The SEM technique investigations provided an insight into the diverse morphology of the DSP where some larger constituents show asymmetrical form, some other constituents have an extended rod-like construction whereas other smaller constituents display rectangular form. Overall, most of the constituents have a reedy and coarse structure with irregular ends. Particle size of the DSP ranges from 5-15 μm. It can be noted from Figure 2a the availability of numerous available cavities and holes enabling Ni(II) ions to be adsorbed, while Figure 2b shows that these holes are occupied by Ni(II) ions indicating good adsorption capacity for DSP.

Effect of pH Values
A pH range of 1-11 was assessed to define the optimum value for the removal of Ni(II) ions by adsorption. Results are presented in Figure 3. It has been noted that the adsorption efficiency increases with increasing pH value up to 7, after which no alteration was noted with further increase in the pH value. These results attributed to the competition between Ni(II) ions and H + ions for adsorption spots on the DSP surface at low pH values [15]. As the pH value increases, less H + ions are present; hence, more adsorption sites are available for Ni(II) ions. The optimal pH value was defined as 7; thus, been used throughout this work.
adsorption. Results are presented in Figure 3. It has been noted that the adsorption efficiency increases with increasing pH value up to 7, after which no alteration was noted with further increase in the pH value. These results attributed to the competition between Ni(II) ions and H + ions for adsorption spots on the DSP surface at low pH values [15]. As the pH value increases, less H + ions are present; hence, more adsorption sites are available for Ni(II) ions. The optimal pH value was defined as 7; thus, been used throughout this work.

Effect of Adsorbent Particle Size
Different particle sizes ranging from 100, 150, 250, 400, and 600 μm have been examined and the obtained results ( Figure 4) showed that as the particle size decreases, the removal amount increases from 82% to 90%. This is due to the availability of more surface area obtainable for the removal of Ni(II) ions as the particle size decreases. However, no difference in adsorption was observed with particle sizes of 100 and 150 μm. Particle size of 100 μm was used throughout the study.

Effect of Adsorbent Particle Size
Different particle sizes ranging from 100, 150, 250, 400, and 600 µm have been examined and the obtained results ( Figure 4) showed that as the particle size decreases, the removal amount increases from 82% to 90%. This is due to the availability of more surface area obtainable for the removal of Ni(II) ions as the particle size decreases. However, no difference in adsorption was observed with particle sizes of 100 and 150 µm. Particle size of 100 µm was used throughout the study.

Effect of Adsorbent Mass
The dependence of the adsorption efficiency on DSP mass was studied to determine the optimal mass. Results displayed in Figure 5 show that the removal efficiency of Ni(II) ions increased as the DSP mass increases from 0.05 g to 0.30 g. Increasing the DSP mass to more than 0.30 g has no significant effect on adsorption effectiveness. This is due to the fact that the surface

Effect of Adsorbent Mass
The dependence of the adsorption efficiency on DSP mass was studied to determine the optimal mass. Results displayed in Figure 5 show that the removal efficiency of Ni(II) ions increased as the DSP mass increases from 0.05 g to 0.30 g. Increasing the DSP mass to more than 0.30 g has no significant effect on adsorption effectiveness. This is due to the fact that the surface area of the adsorbent increases with its mass. The optimal DSP mass (0.30 g) was throughout this study.

Effect of Adsorbent Mass
The dependence of the adsorption efficiency on DSP mass was studied to determine the optimal mass. Results displayed in Figure 5 show that the removal efficiency of Ni(II) ions increased as the DSP mass increases from 0.05 g to 0.30 g. Increasing the DSP mass to more than 0.30 g has no significant effect on adsorption effectiveness. This is due to the fact that the surface area of the adsorbent increases with its mass. The optimal DSP mass (0.30 g) was throughout this study.

Effect of Contact Time
In typical contaminant removal experiments, the contact time is considered a significant feature because it directly influences the adsorbent lifetime and the adsorption efficiency. Figure 6 shows the results obtained at different time intervals while all the other conditions (pH = 7.00, particle size = 100 µm, adsorbent mass = 0.30 g and temperature = 25.0 • C, revolutions per minute (rpm) = 150) were kept constant. It was found that the DSP reached the maximum adsorption of 90% for Ni(II) after 30 min.

Effect of Contact Time
In typical contaminant removal experiments, the contact time is considered a significant feature because it directly influences the adsorbent lifetime and the adsorption efficiency. Figure 6 shows the results obtained at different time intervals while all the other conditions (pH = 7.00, particle size = 100 μm, adsorbent mass = 0.30 g and temperature = 25.0 °C, revolutions per minute (rpm) = 150) were kept constant. It was found that the DSP reached the maximum adsorption of 90% for Ni(II) after 30 min.

Adsorption Kinetics
Kinetics of the adsorption process is the key feature for designing efficient adsorption experiments and this requires the use of proper kinetic model. Several kinetic parameters values are shown in Table 2. Adsorption kinetics control the rate, g the efficacy of DSP [16]. Several kinetic models were examined such as intraparticle diffusion, pseudo-second-order model, and

Adsorption Kinetics
Kinetics of the adsorption process is the key feature for designing efficient adsorption experiments and this requires the use of proper kinetic model. Several kinetic parameters values are shown in Table 2. Adsorption kinetics control the rate, g the efficacy of DSP [16]. Several kinetic models were examined such as intraparticle diffusion, pseudo-second-order model, and pseudo-first-order. ln q e − q t = ln q e − k 1 t The pseudo-second order kinetic model is denoted by Equation (3).
where q e and q t are the equilibrium and adsorption capacities at time (t) and equilibrium, and k 1 , k 2 are rate constants for pseudo-first-order and pseudo-second-order, respectively. Moreover, 0.6977 and 0.9937 are values of correlation coefficients (R 2 ) for pseudo-first-order and pseudo-second-order models (Table 2), respectively. Figure 7 proves that the system obeys the pseudo-second-order kinetics model. This is in good agreement with previous studies [12,13,15]. ln q − q = ln q − k t The pseudo-second order kinetic model is denoted by Equation (3).
where qe and qt are the equilibrium and adsorption capacities at time (t) and equilibrium, and k1, k2 are rate constants for pseudo-first-order and pseudo-second-order, respectively. Moreover, 0.6977 and 0.9937 are values of correlation coefficients (R 2 ) for pseudo-first-order and pseudo-second-order models (Table 2), respectively. Figure 7 proves that the system obeys the pseudo-second-order kinetics model. This is in good agreement with previous studies [12,13,15].

Intra-Particle Diffusion Kinetic Model
Intra-particle diffusion kinetic model is displayed by Equation (4) q t = k id t 1/2 + I (4) k id is the intra-particle diffusion rate constant (mg/g. min (1/2) ) and I is a constant that associated to the boundary layer thickness (mg/g). The value of (k id ) was determined from the slope of Equation (4) and presented in Table 2. The relationship between q t and t 1/2 was non-linear, demonstrating that several processes are governing the adsorption process. The initial curved portion of the plot is due to the impact of boundary layer diffusion. The curved portion denotes that the intra-particle diffusion is controlled by the rate of constant k id .

Adsorption Isotherm
Adsorption models are frequently exploited to explain the adsorbate/adsorbent interactions to determine the adsorption capacity of the adsorbent. To evaluate the adsorption isotherms for the DSP, Freundlich, Langmuir, Temkin, and Dubinin-Radushkevich (D-R) adsorption models were examined.

Langmuir Model
Equation (5) (5) where q e is the equilibrium quantity of Ni(II) ions adsorbed on the DSP surface at equilibrium (mg/g), C e is the equilibrium concentration of Ni(II) ions in solution (mg/L), q m is the maximum adsorption of Ni(II) ions (mg/g), and b (L/mg) is the Langmuir constant. According to Equation (4), values of C e /q e were plotted against C e and the results are displayed in Figure 8a. Values of q m and b were obtained from the slope and intercept, respectively. The b value refers to the adsorption binding energy [14], whereby a higher b value means more binding affinity between adsorbent and adsorbate. The parameters (q m , b and R 2 ) are displayed in Table 3.

Freundlich Model
This is an experimental association relating the adsorption of solutes from a liquid onto an adsorbent surface and adopts that various adsorption layers with a number of adsorption energies are involved. This model describes the affinity between the quantities of Ni(II) ions adsorbed per the dosage of the DSP, qe, and the concentration of the Ni(II) ions at equilibrium, Ce. Linear Freundlich model is represented by Equation (6) [12]. ln q = ln K + 1 n ln C where n and Kf represent Freundlich constants describe the process intensity capacity. Values of Kf and n are obtained from the intercept and slope of Figure 8b, respectively. Value of n is an indicator to the adsorption nature according to the following way: if n < 1, adsorption is classified as a physical process, if n = 1, adsorption is linear and if n > 1, adsorption is considered as a chemical process. The range of n values and Kf value are given in Table 3. Results indicate that adsorption of Ni(II) ion on the surface of the DSP is a physical process [13].

Temkin Isotherm
Temkin model proposes that the adsorption heat of all particles in the layer decrease sharply, rather than logarithmic with coverage [18]. The adsorption potential of DSP to Ni(II) ions can be verified by applying Temkin isotherm model. The linear formula of Temkin is shown in Equation (7):

Freundlich Model
This is an experimental association relating the adsorption of solutes from a liquid onto an adsorbent surface and adopts that various adsorption layers with a number of adsorption energies are involved. This model describes the affinity between the quantities of Ni(II) ions adsorbed per the dosage of the DSP, q e , and the concentration of the Ni(II) ions at equilibrium, C e . Linear Freundlich model is represented by Equation (6) [12].
ln q e = ln K f + 1 n ln C e (6) where n and K f represent Freundlich constants describe the process intensity capacity. Values of K f and n are obtained from the intercept and slope of Figure 8b, respectively. Value of n is an indicator to the adsorption nature according to the following way: if n < 1, adsorption is classified as a physical process, if n = 1, adsorption is linear and if n > 1, adsorption is considered as a chemical process. The range of n values and K f value are given in Table 3. Results indicate that adsorption of Ni(II) ion on the surface of the DSP is a physical process [13].

Temkin Isotherm
Temkin model proposes that the adsorption heat of all particles in the layer decrease sharply, rather than logarithmic with coverage [18]. The adsorption potential of DSP to Ni(II) ions can be verified by applying Temkin isotherm model. The linear formula of Temkin is shown in Equation (7): where R is the universal gas constant, T is the absolute temperature, b t is Temkin constant associated with adsorption heat, and A is a constant related to adsorption capacity. A and b t values are found from the slope and intercept of Figure 8c and given along with R 2 in Table 3.

Dubinin-Radushkevich (D-R) Isotherm Model
This model estimates the energy of adsorption. It is commonly used to understand the mechanism of adsorption [19]. This isotherm is not proposed only for constant adsorption potential or homogeneous adsorbent but also for both heterogonous surfaces. The linear equation of this model is presented in Equation (8): ln q e = ln q m − β ε 2 (8) ε is given by Equation (9): β is a constant associated with the adsorption free energy, q m is the theoretical saturation capacity based on D-R isotherm (mg/g). Values of β, q m and R 2 are obtained from Figure 8d and shown in Table 3. The free sorption energy E s , is the change in free energy when one mole of adsorbate is stimulated to the solid surface and is calculated by Equation (10).
The adsorption type can be deduced from the E s value. The adsorption process is considered chemical when E s value is in the range 8.0 to16.0 kJ/mol and physical when E s is less than 8.0 kJ/mol. In this work, the E s value was determined as 2.24 kJ/mol concluding that the adsorption taking action is of physical nature.

Adsorption Thermodynamics
Thermodynamical parameters are vital in any adsorption investigations as the temperature is strongly connected to the kinetic energy of adsorbate. In this study, adsorption tests were performed at different temperatures, viz. 298, 308, 318 and 328 K for the sorption of initial Ni(II) ion

Adsorption Thermodynamics
Thermodynamical parameters are vital in any adsorption investigations as the temperature is strongly connected to the kinetic energy of adsorbate. In this study, adsorption tests were performed at different temperatures, viz. 298, 308, 318 and 328 K for the sorption of initial Ni(II) ion concentration (50 mg/L) on DSP at their particular optimum pH values, DSP mass and contact time. Entropy (∆S • ), enthalpy (∆H • ), and Free energy (∆G • ) change are governed by Equations (11) and (12).
where K D value were calculated by Equation (13) The thermodynamical parameters are listed in Table 4. Negative signs of ∆G • indicate that the process is spontaneous. It is clear that the negative values of ∆G • decrease as the temperature increases. This is attributed to the fact that additional positions on the surface of the DSP are destroyed at elevated temperatures. Values of ∆G • for Ni(II) ions adsorption onto the DSP were found in the range of −3.5 to −5.7 kJ/mol. It is well established that physical adsorption free energy change (∆G • ) is ranging between −20 and 0 kJ/mol and chemical adsorption between −400 to −80 kJ/mol [13]. Thus, adsorption process is mainly a physical sorption process. This finding is in good agreement with the parameters obtained from Freundlich, Dubinin-Radushkevich (D-R) and Temkin models. ∆S • and ∆H • were calculated from the intercept and slope of Figure 10 and presented in Table 3. The negative value of ∆H • (−27.0 kJ/mol) proves that the adsorption is an exothermic process. The positive value of ∆S • (71.0 J/mol) designates the affinity of the DSP towards the Ni(II) ions. Values of sticking probability (S*) and activation energy (Ea) were calculated by applying the modified Arrhenius equation that shown in Equations (14) and (15) [19]: C Figure 10. Relationship between ln K D and 1/T. Values of sticking probability (S*) and activation energy (Ea) were calculated by applying the modified Arrhenius equation that shown in Equations (14) and (15) [19]: where θ is the surface coverage, C i represents the original concentration of Ni(II) ions, E a is the activation energy of the system and S* is the sticking probability. The sticking probability, S*, is a function of the adsorbent/adsorbate adsorption process but should fulfil the circumstance 0 < S* < 1 and depends on the temperature of the process. E a and S* values were obtained from the slope and intercept of Figure 10, and are recorded in Table 4. The Ea value is very low (−2.01 kJ/mol) demonstrating the facile adsorption process and the negative charge of E a value indicates an exothermic process. This is in good agreement with the negative value of ∆H • [15]. Since the value of S* <<< 1, therefore, the probability of Ni(II) ion to stick on the DSP is very high and thus the adsorption process is favorable [18].  Table 5 presents the results of this study compared with other biosorbents. Under the same experimental conditions in terms of optimum pH, temperature, DSP proved high adsorption efficiency in contrast to the chosen biosorbents. It is interesting to note that the DSP was used without any further chemical or thermal modifications. This reduces its cost and hazard to the minimum.

Conclusions
In this study, the powder obtained from date seeds has been tested as a competent (removal effectiveness > 90%), inexpensive, eco-friendly and natural material for adsorption of Ni(II) ions from artificial wastewater. The date seeds powder retained significant amounts of Ni(II) readily. The maximum adsorption capacity for DSP was found to be 41 mg/g. The optimal conditions for efficient removal of 50 ppm of Ni(II) ion were found to be 0.30 g of DSP after 30 min of shaking time at pH 7.00. This adsorbent has positive characteristics making it appropriate to purify wastewater. Such characteristics include (i) cost-effective material: date seeds powder is an expensive material. The only cost of this material would be for the collection, transportation and grinding. It does not need any further chemical or thermal modification (ii) availability: date seeds are abundantly existing in Saudi Arabia (iii) efficiency: date seeds powder can remove more than 90% of Ni(II) ions, which present in artificial wastewater.