*3.4. E*ff*ect of pH Values*

(**a**) (**b**) 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<sup>+</sup> ions for adsorption spots on the DSP surface at low pH values [15]. As the pH value increases, less H<sup>+</sup> 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.

**Figure 2.** SEM technique images of the DSP surface (**a**) before and (**b**) after adsorption.

*3.4. 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+

**Figure 3.** Effect of changing pH values on the efficiency of removal of Ni(II) ions.

#### **Figure 3.** Effect of changing pH values on the efficiency of removal of Ni(II) ions. *3.5. E*ff*ect of Adsorbent Particle Size*

*3.6. Effect of Adsorbent Mass* 

study.

study.

*3.5. 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 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.

observed with particle sizes of 100 and 150 μm. Particle size of 100 μm was used throughout the

*Processes* **2020**, *8*, x FOR PEER REVIEW 6 of 16

**Figure 5.** Effect of the DSP mass on the removal efficiency of Ni(II) ions. **Figure 4.** Effect of particle size of the DSP on the efficiency of removal of Ni(II) ions.

**Figure 4.** Effect of particle size of the DSP on the efficiency of removal of Ni(II) ions.

The dependence of the adsorption efficiency on DSP mass was studied to determine the

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

#### *3.6. E*ff*ect of Adsorbent Mass* **Figure 4.** Effect of particle size of the DSP on the efficiency of removal of Ni(II) ions.

study.

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. *3.6. 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

*Processes* **2020**, *8*, x FOR PEER REVIEW 6 of 16

**Figure 5.** Effect of the DSP mass on the removal efficiency of Ni(II) ions. **Figure 5.** Effect of the DSP mass on the removal efficiency of Ni(II) ions.

#### *3.7. E*ff*ect of Contact Time Processes* **2020**, *8*, x FOR PEER REVIEW 7 of 16

*3.8. Adsorption Kinetics* 

pseudo-first-order.

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. *3.7. 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.

**Figure 6.** Effect of contact time on the efficiency of removal of Ni(II) ions. **Figure 6.** Effect of contact time on the efficiency of removal of Ni(II) ions.

models were examined such as intraparticle diffusion, pseudo-second-order model, and

**Table 2.** Kinetic parameters for the removal of Ni(II) ion by DSP. **Kinetic Models Parameters** 

> qe (mg/g) 12.2 k1 (min−1) 4.0 × 10−<sup>4</sup> R2 0.6988

qe (mg/g) 3.1 k2 (g/mg min) 0.3 R2 0.9937

kid (mg/g. min(1/2)) 0.056 I 2.6 R2 0.7978

Pseudo-first-order model

Pseudo-second-order model

Intraparticle diffusion model

### *3.8. 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.


**Table 2.** Kinetic parameters for the removal of Ni(II) ion by DSP.

#### 3.8.1. Pseudo-First-Order Model *Processes* **2020**, *8*, x FOR PEER REVIEW 8 of 16

This model is denoted by Equation (2) [17]. 3.8.1. Pseudo-First-Order Model

3.8.2. Intra-Particle Diffusion Kinetic Model

diffusion is controlled by the rate of constant kid.

$$
\ln\left(\mathbf{q}\_{\rm e} - \mathbf{q}\_{\rm t}\right) = \ln\mathbf{q}\_{\rm e} - \mathbf{k}\_{\rm l}\mathbf{t} \tag{2}
$$

The pseudo-second order kinetic model is denoted by Equation (3). lnሺqୣ − q୲ሻ = ln qୣ − kଵt (2) The pseudo-second order kinetic model is denoted by Equation (3).

$$\frac{\mathbf{t}}{\mathbf{q}\_{\rm t}} = \frac{1}{\mathbf{k}\_2 \mathbf{q}\_{\rm e}^2} + \frac{\mathbf{t}}{\mathbf{q}\_{\rm e}} \tag{3}$$

where q<sup>e</sup> and q<sup>t</sup> are the equilibrium and adsorption capacities at time (t) and equilibrium, and k1, k<sup>2</sup> are rate constants for pseudo-first-order and pseudo-second-order, respectively. Moreover, 0.6977 and 0.9937 are values of correlation coefficients (R<sup>2</sup> ) 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]. 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 (R2) 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].

**Figure 7.** Pseudo-second-order kinetics. **Figure 7.** Pseudo-second-order kinetics.

kid 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 (kid) was determined from the slope of Equation (4) and presented in Table 2. The relationship between qt and t1/2 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

q୲ = k୧ୢ tଵ/ଶ + I (4)

#### 3.8.2. Intra-Particle Diffusion Kinetic Model

Intra-particle diffusion kinetic model is displayed by Equation (4)

$$\mathbf{q}\_{\rm t} = \mathbf{k}\_{\rm id} \,\mathbf{t}^{1/2} + \mathbf{I} \tag{4}$$

kid 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 (kid) was determined from the slope of Equation (4) and presented in Table 2. The relationship between q<sup>t</sup> and t1/<sup>2</sup> 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 kid.

#### *3.9. 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.

#### 3.9.1. Langmuir Model

Equation (5) presents the Langmuir linear equation

$$\frac{\mathbf{C\_e}}{\mathbf{q\_e}} = \frac{\mathbf{C\_e}}{\mathbf{q\_m}} + \frac{1}{\mathbf{b}\mathbf{q\_m}}\tag{5}$$

where q<sup>e</sup> is the equilibrium quantity of Ni(II) ions adsorbed on the DSP surface at equilibrium (mg/g), C<sup>e</sup> is the equilibrium concentration of Ni(II) ions in solution (mg/L), q<sup>m</sup> is the maximum adsorption of Ni(II) ions (mg/g), and b (L/mg) is the Langmuir constant. According to Equation (4), values of Ce/q<sup>e</sup> were plotted against C<sup>e</sup> and the results are displayed in Figure 8a. Values of q<sup>m</sup> 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 (qm, b and R<sup>2</sup> ) are displayed in Table 3.


**Table 3.** Parameters obtained from various isotherm models.

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**Figure 8.** Isotherm models of adsorption of Ni(II) ions by the DSP (**a**), Langmuir, (**b**) Freundlich, (**c**) Temkin and (**d**) D-R. **Figure 8.** Isotherm models of adsorption of Ni(II) ions by the DSP (**a**), Langmuir, (**b**) Freundlich, (**c**) Temkin and (**d**) D-R.

#### 3.9.2. Freundlich Model 3.9.2. 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]. 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].

$$
\ln \mathbf{q}\_{\text{e}} = \ln \mathbf{K}\_{\text{f}} + \frac{1}{\mathbf{n}} \ln \mathbf{C}\_{\text{e}} \tag{6}
$$

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]. where n and K<sup>f</sup> represent Freundlich constants describe the process intensity capacity. Values of K<sup>f</sup> 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<sup>f</sup> 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].

#### 3.9.3. Temkin Isotherm 3.9.3. Temkin Isotherm

(7):

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 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):

$$\mathbf{q}\_{\mathbf{e}} = \frac{\mathbf{RT}}{\mathbf{b}\_{\mathbf{t}}} \ln \mathbf{A} + \frac{\mathbf{RT}}{\mathbf{b}\_{\mathbf{t}}} \ln \mathbf{C}\_{\mathbf{e}} \tag{7}$$

ln Cୣ (7)

ln A +

b୲

b୲

where R is the universal gas constant, T is the absolute temperature, b<sup>t</sup> is Temkin constant associated with adsorption heat, and A is a constant related to adsorption capacity. A and b<sup>t</sup> values are found from the slope and intercept of Figure 8c and given along with R<sup>2</sup> in Table 3.

#### 3.9.4. 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):

$$
\ln \mathbf{q}\_{\mathbf{e}} = \ln \mathbf{q}\_{\mathbf{m}} - \boldsymbol{\mathfrak{E}} \,\,\varepsilon^2 \tag{8}
$$

ε is given by Equation (9):

$$
\varepsilon = \text{RT} \ln \left( 1 + \frac{1}{\mathcal{C}\_{\text{e}}} \right) \tag{9}
$$

β is a constant associated with the adsorption free energy, q<sup>m</sup> is the theoretical saturation capacity based on D–R isotherm (mg/g). Values of β, q<sup>m</sup> and R<sup>2</sup> are obtained from Figure 8d and shown in Table 3. The free sorption energy Es, is the change in free energy when one mole of adsorbate is stimulated to the solid surface and is calculated by Equation (10).

$$\mathbf{E}\_{\mathbf{s}} = \frac{1}{\sqrt{2\beta}}\tag{10}$$

The adsorption type can be deduced from the E<sup>s</sup> value. The adsorption process is considered chemical when E<sup>s</sup> value is in the range 8.0 to16.0 kJ/mol and physical when E<sup>s</sup> is less than 8.0 kJ/mol. In this work, the E<sup>s</sup> value was determined as 2.24 kJ/mol concluding that the adsorption taking action is of physical nature.

#### *3.10. E*ff*ect of Temperature*

*3.11. Adsorption Thermodynamics* 

towards the Ni(II) ions.

where KD value were calculated by Equation (13)

(12).

Figure 9 displays the influence of temperature on the adsorption of Ni(II) ions onto the DSP surface. The adsorption efficiency decreases as the temperature increases. This is justified due to the damage of some adsorption sites at elevated temperatures. *Processes* **2020**, *8*, x FOR PEER REVIEW 12 of 16

**Figure 9.** Effect of temperature on the removal efficiency of Ni(II) ions by the DSP. **Figure 9.** Effect of temperature on the removal efficiency of Ni(II) ions by the DSP.

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 (ΔS°), enthalpy (ΔH°), and Free energy (ΔG°) change are governed by Equations (11) and

ln Kୈ <sup>=</sup>−∆H<sup>୭</sup>

RT <sup>+</sup>

Kୈ <sup>=</sup>qୣ Cୣ

The thermodynamical parameters are listed in Table 4. Negative signs of ΔG° indicate that the process is spontaneous. It is clear that the negative values of ΔG° 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 ΔG° 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 (ΔG°) 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. ΔS° and ∆H° were calculated from the intercept and slope of Figure 10 and presented in Table 3. The negative value of ΔH° (−27.0 kJ/mol) proves that the adsorption is an exothermic process. The positive value of ΔS° (71.0 J/mol) designates the affinity of the DSP

∆S<sup>୭</sup>

∆ G<sup>୭</sup> = RT ln Kୈ (11)

<sup>R</sup> (12)

(13)

#### *3.11. 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).

$$
\Delta \mathbf{G}^{\mathbf{0}} = \mathbf{R} \mathbf{T} \ln \mathbf{K}\_{\mathbf{D}} \tag{11}
$$

$$
\ln \mathbf{K}\_{\rm D} = \frac{-\Delta \mathbf{H}^{\rm o}}{\mathbf{R} \mathbf{T}} + \frac{\Delta \mathbf{S}^{\rm o}}{\mathbf{R}} \tag{12}
$$

where K<sup>D</sup> value were calculated by Equation (13)

$$\mathbf{K}\_{\rm D} = \frac{\mathbf{q}\_{\rm e}}{\mathbf{C}\_{\rm e}} \tag{13}$$

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. *Processes* **2020**, *8*, x FOR PEER REVIEW 13 of 16

**Figure 10.** Relationship between ln KD and 1/T. **Figure 10.** Relationship between ln K<sup>D</sup> and 1/T.

<sup>θ</sup> = ൬1 − Cୣ

where Ɵ is the surface coverage, Ci represents the original concentration of Ni(II) ions, Ea is the

**Table 4.** Values of thermodynamic parameters.

71.7 <sup>−</sup>27.0 <sup>−</sup>2.01 0.002 308 6.7 −4.9

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

**∆H° (kJ/mol)** 

**Ea (kJ/mol)** 

**∆S° (J/mol K)** 

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. Ea 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 Ea 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

modified Arrhenius equation that shown in Equations (14) and (15) [19]:

activation energy of the system and S\* is the sticking probability.

DSP is very high and thus the adsorption process is favorable [18].

**∆G° (kJ/mol)** 

**T, K KD**

*3.12. Comparisons with Other Adsorbents* 

298 10.0 −5.7

318 5.0 −4.2 328 3.6 −3.5

Values of sticking probability (S\*) and activation energy (Ea) were calculated by applying the

ି

C୧

ୖ (14)

൰ (15)

**S\* (J K/mol)** 

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]:

$$\mathbf{S}^\* = (1 - \theta)\mathbf{e}^{\frac{-\mathbf{E}\_\mathbf{a}}{\mathbf{RT}}} \tag{14}$$

$$\Theta = \left(1 - \frac{\mathbf{C\_e}}{\mathbf{C\_i}}\right) \tag{15}$$

where θ is the surface coverage, C<sup>i</sup> represents the original concentration of Ni(II) ions, E<sup>a</sup> 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<sup>a</sup> 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<sup>a</sup> 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 4.** Values of thermodynamic parameters.

#### *3.12. Comparisons with Other Adsorbents*

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.


**Table 5.** Comparison of the DSP with other biosorbents used for the removal of Ni(II) ions.

\*\* not reported.

Orange peel 162.6 Langmuir 2nd order 5.50 [26] Tea factory waste 18.4 Langmuir \*\* 4.00 [27]

#### **4. 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.

**Author Contributions:** Conceptualization I.H.A. and A.E.; methodology, A.E.; validation, E.I.B.; formal analysis, E.I.B.; investigation A.E.; resources, A.B.E.; writing—original draft preparation, I.H.A. and A.E.; writing—review and editing, E.I.B.; supervision, B.K.; project administration, I.H.A.; funding acquisition, A.B.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by DEANSHIP OF SCIENTIFIC RESEARCH, KING KHALID UNIVERSITY, grant number R.G.P.1/139/40.

**Conflicts of Interest:** The authors declare no conflict of interest.
