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

**Abubakr Elkhaleefa 1,2 , Ismat H. Ali 3,\* , Eid I. Brima 3,4 , A. B. Elhag 5,6 and Babiker Karama <sup>7</sup>**


Received: 20 July 2020; Accepted: 14 August 2020; Published: 17 August 2020

**Abstract:** Adsorption investigations in batch approaches were performed to explore the biosorption of Ni(II) ions from aqueous solutions on date seeds powder. The effects 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.

**Keywords:** adsorption; date seeds; kinetics; isotherm; thermodynamics

### **1. 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 [1–4]. 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 [3,6–9]. 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].

Date (*Phoenix dactylifera*) seeds, which are composed of lignin, hemicellulose, and cellulose are effective biosorbents; thus, eliminates various contaminants from wastewater [4,10]. The efficiency of this inexpensive biosorbent is due to oxygenated functional groups in the lignocellulosic materials such as cellulose and phenolic compounds [10]. The removal of Ni(II) ions onto bentonite/grapheme oxide was previously studied [11] and the results revealed that it follows Langmuir isotherm with high uptake capacity. Mehrmad et al. [12] have reported that the removal of Ni(II) ions by functionalized henna powder depends on the experimental circumstances, mainly the Ni(II) concentration, the biosorbent mass, and the pH of the medium, the Freundlich and Langmuir isotherm models were used to define the process. The kinetic data were fitted with the pseudo-second-order reaction. The adsorption of Ni(II) ions from wastewater by natural clay had previously been investigated [13] and it was reported that the process is controlled by the pH value of the medium; the sorption process was rapid whereas the maximum adsorption capacity had been attained within 120 min and it was found that the system follows a pseudo-second-order reaction.

The adsorbent used in this study was prepared by a direct, facile, and economic technique. The process does not require any chemicals addition nor high-temperature calcination. It is believed that only few adsorbents were tested without any modification in the removal of heavy metals from aqueous solutions while being powerfully recyclable. *Phoenix dactylifera* is one of the most plentiful plants in Saudi Arabia and the region. Using date seed powder as an adsorbent for Ni(II) ions is considered the novelty of this research.

This work aims to assess the potential of date seed powder to act as an inexpensive and environment-friendly material for the removal of Ni(II) from artificial wastewater. This study was designed to assess, compare, and characterize the adsorption of Ni(II) ions by date seeds powder (DSP) without the addition of any chemicals or thermal treatment. DSP was initially characterized using Brunauer–Emmett–Teller (BET) surface area, scanning electron microscope technique (SEM), and attenuated total reflection-Fourier transform infrared spectrometer (ATR-FTIR). The factors that influence the adsorption efficiency such as mass, concentration of Ni(II) ion, pH, temperature, and contact time, were investigated in this work.

#### **2. Materials and Methods**

#### *2.1. 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.

#### *2.2. 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<sup>3</sup> (LOBA Chemie, Laboratory Reagents and fine Chemicals, Mumbai, India).

#### *2.3. 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).

$$\text{R\%} = \frac{\text{C}\_{\text{o}} - \text{C}\_{\text{e}}}{\text{C}\_{\text{o}}} \times 100 \tag{1}$$

where C<sup>0</sup> and C<sup>e</sup> are the initial and equilibrium Ni(II) ion concentrations, respectively.

#### *2.4. 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−<sup>1</sup> at a spectral resolution of 8 cm−<sup>1</sup> . DSP samples were analyzed without any pretreatment.

#### **3. Results and Discussion**

#### *3.1. Characterization of the DSP*

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].

**Table 1.** Properties of the DSP surface.


#### *3.2. 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−<sup>1</sup> ). 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−<sup>1</sup> , whereas, ether structures were identified by the band ascribed to stretching vibrations of carbon-oxygen in the range (1100–1000 cm−<sup>1</sup> ). 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−<sup>1</sup> . 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].

#### *3.3. 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. *Processes* **2020**, *8*, x FOR PEER REVIEW 4 of 16

*Processes* **2020**, *8*, x FOR PEER REVIEW 4 of 16

**Figure 1.** Attenuated total reflection-Fourier transform infrared spectrometer (ATR-FTIR) spectra for **Figure 1.** Attenuated total reflection-Fourier transform infrared spectrometer (ATR-FTIR) spectra for the date seeds powder (DSP) before and after Ni(II) ions adsorption process. 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.

the date seeds powder (DSP) before and after Ni(II) ions adsorption process.

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