Exfoliate apricot kernels, natural low-cost bio-sorbent for rapid and efficient adsorption of CN ions from aqueous solutions. Isotherm, kinetic and thermodynamic models

Exfoliate apricot kernels were collected and prepared for adsorbing the cyanide ions from the aqueous solution. Fourier transforms infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) and scanning electron microscopic (SEM) were utilized to describe the exfoliate apricot kernels. The adsorption experiments carried out were in a batch experiment, prepared 100 mL of cyanide solution for various temperature, pH, contact time, speed of mixer and dose of the adsorbent. The isotherms models were checked Langmuir, Freundlich, Temkin, and Harkins-Henderson, isotherm models. The isotherm coefficient of Langmuir, Freundlich, Temkin, and Harkins-Henderson models were 0.99, 0.8, 0.95 and 0.68 respectively. The Langmuir isotherm model best fitted for adsorption more than other models. The kinetic were studies pseudo-first-order, pseudo-second-order, intra particle diffusion, and Elovich kinetic models. The kinetic (R 2 ) constant for pseudo-first-order, pseudo-second-order, intra particle diffusion, and Elovich were 0.989, 0.947, 0.969 and 0.905 respectively. Pseudo-first-order giving a better fit for the process.


INTRODUCTION
Industrial activities can generate significant loads of pollutants that are released into water sources. Gold mining uses highly toxic compounds such as cyanide which is considered potentially lethal to any ecosystem (Dash et al., 2009). In contact with water, it produces hydrocyanic acid, a compound that causes serious illnesses in humans and animals, and degradation of soil fertility. The use of cyanide in gold mining is an important cause of biodiversity depletion in developing countries. The term cyanide refers to a singularly charged anion consisting of one carbon atom and one nitrogen atom joined with a triple bond, CN -. The most toxic form of cyanide is free cyanide, which includes the cyanide anion itself and hydrogen cyanide, HCN, either in a gaseous or aqueous state. At a pH of 9.3 -9.5, CNand HCN are in equilibrium, with equal amounts of each present. At a pH of 11, over 99% of the cyanide remains in solution as CN -, while at pH 7, over 99% of the cyanide will exist as HCN. Although HCN is highly soluble in water, its solubility decreases with increased temperature and under highly saline conditions. Both HCN gas and liquid are colorless and have the odor of bitter almonds, although not all individuals can detect the odor (Akcil, 2010;Donato et al., 2017).
Cyanide is one of the anthropogenic contaminants in natural water through several industrial effluents and considered highly toxic for aquatic life and human beings (Paschka et al., 1999). It enters the human body through inhalation, ingestion, absorption through the skin, eyes, and mucous membranes. Industrial exposure even in short term can cause tremors and neurological disorders, while long-term exposure causes weight loss, thyroid dysfunction and nerve damage, and hypoxia (Kulig and Ballantyne, 1991).

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This is an open access article distributed under the terms of the Exposure to cyanide from inhalation through showering and absorption through the skin in bathing is not normally expected to be significant (Brüger et al., 2018). Cyanide is certainly a chemical that is toxic and lethal to humans if ingested (Paschka et al., 1999;Kulig and Ballantyne, 1991). This is likely why its use by the mining industry often instills fear, anger, and opposition by both the communities near the mine and the environmental community (Brüger et al., 2018;Dunbar and Heintz, 1997).
Human industrial activities which lead to the generation of large quantities of wastewater has been on the increase, especially wastewater generated from local food processing industries (cassava processing mills) which have become an environmental challenge in developing countries where the processing of staple foods such as cassava is encouraged to provide both food and employment opportunities for the increasing populace (Behnamfard and Salarirad, 2009;Eletta et al., 2016). The processes involved in starch production from cassava and other allied products such as gari and fufu leads to the release of large quantities of naturally occurring cyanoglycosides compounds present in cassava (Kaewkannetra et al., 2009).
As pressure increases for control of toxic waters, interest in new methods of treatment also increases. The detoxification of cyanide-bearing wastes is one area that has received particular attention. Both physical and chemical methods of removing cyanide from water are feasible and include precipitation (Xie and Wang, 2017), foam separation (Dai et al., 2012), ion exchange (Meenakshi and Viswanathan, 2007;Kurama and Çatalsarik, 2000) air purging (Parga et al., 2003), electrolytic . Lots of methods have been used to deal with cyanide and precious metals . These methods are mainly divided into two kinds, the destruction of cyanide in its effluents and the recovery of cyanide (Lu et al., 2015). Electrochemistry method , chlorine and H2O2 oxidation process (Sun et al., 2014), SO2-air oxidation process (Parga et al., 2003), destruction of cyanide by biological treatment methods (Sheremata and Hawari, 2000;Sarma et al., 2019).
There are two different types of exfoliants: chemical and mechanical, both can be a result of natural ingredients. Certain ingredients may work as a chemical exfoliant due to their acidity or the presence of certain enzymes that will aid in removing dead skin cells without a physical component. A more commonly understood exfoliator is mechanical exfoliants. These utilize microbeads, granules, or other natural materials, such as apricot seed powder, to aid in physically removing dead and damaged skin cells (Zhang, et al., 2019). Exfoliate apricot kernels used as an adsorbent by removal methylene blue from aqueous solutions (Namal and Kalipci, 2020), removal of Cr (VI) (Kalipci and Namal, 2018), removal of malachite green (Abbas, 2020), production carbonaceous adsorbents from apricot kernels (Krasilnikova et al., 2005).
In the present work, exfoliate apricot kernels a novel and natural plant adsorbent, examined for the removal of CNions from aqueous solutions, as adsorbent under several conditions like temperature, adsorbent dose, contact time, and pH, finding a general equation relating these conditions to give its optimum value and studying the adsorption isotherm, kinetics and thermodynamic.

Exfoliate Apricot Kernels
Apricot stone shells mostly consist of lignocellulose material, i.e. of insoluble fibers. Hence, they are a woodlike, inflammable raw material with a characteristic light brown color and a high calorific value. In a dry state, they are extremely durable and their mechanical strength/hardness is similar to olive stone granules.

Surface Characterization of Exfoliate Apricot Kernels
Exfoliate apricot kernels were collected as a low-cost biosorption waste (were collected from local markets), exfoliate apricot kernels were eliminated from the fruit and were milled to a fine powder into 100-200 µm mesh size to use in experiments, by a hammer mill (Fitzmill Mill Hammer, L1A, R24330, China), Fig.1 represents the preparation steps of exfoliate apricot kernels to experimental work. Then, weighed the powder of exfoliator apricot kernels with a sensitive balance of 0.00001 accuracies to prepared for the experiments. The product from milling was dried in an oven at 105°C for 24 h. To avoid further moisture absorption, samples were preserved in desiccators. Dried samples were then taken in a porcelain crucible and covered with a lid to be placed in a control muffle furnace at 150°C for an hour, to get dried powder. The samples were washed with 1M hydrochloric acid solutions firstly, and then with distilled water until the pH value reached 7.0. After washing, the samples were dried in an oven at 105°C for 24 h. Then the dried samples were preserved in desiccators to avoid further moisture absorption. The adsorbent showed a fluffy, highly porous, and rough microstructure, containing some voids and cracks which were suitable for the CNions adsorption. The properties of exfoliate apricot kernels samples were tested by several techniques:  The functional groups were specified by FT-IR (Shimadzu 8400S FTIR spectrometer, equipped with KBr beam splitter) using approximately 5 mg of each sample along with 5 mg KBr. FT-IR spectrophotometer was operated at a spectral range of 4000-400 cm -1 with a maximum resolution of 0.5 ~ 0.85 cm -1 .  Energy dispersive X-ray (EDX) spectroscopy allows elemental identification by measuring the number and energy of X-rays emitted from a specimen after excitation with an electron beam, (Shimadzu device, EDX-7000) it was used for the detection of the elements in the samples (qualitative analysis).  The morphology of the sample was evaluated using an Em-Craft (Korea): Table-top scanning electron microscope (SEM Cube-1000). Samples were dehydrated by putting them into critical point drying equipment or freeze-dried. The exfoliate apricot kernels powder was fixed in an aluminum plate (specimen holder), using an electrically conductive tap and a coating of gold at 10 mbar for 90 s was applied. Each sample was transferred to the microscope for observation. The procedure was applied to gain information about the arrangements of particles that correlated with the structure of samples.

Chemical Reagents
Cyanide solution was prepared by dissolving sodium cyanide in double distilled water to yield a stock solution containing 1 mg (CN -)/mL. For prepared the required concentration samples, diluting the stock solution used in experiments. Utilizing 1M NaOH or 1 M H2SO4 for adjusted the pH of the experimental solutions. The chemicals utilized were of analytical grade.

Procedure
Firstly, 100 mL of prepared solution contain CNions was used to study the ability of exfoliate apricot kernels to adsorb CNions from solution. The factors investigated were, the dose of adsorbent (1, 5, 10 and 50 g) and shaker with a perforated platform, Tablar 2000, Heidolph for mixing the exfoliate apricot kernels, temperature (25, 35, 45 and 55°C) (With heating plate, magnetic stirrer and thermostat to control the heating and mixing solution, also used (water bath) model WNB from Memmert company for heated and controlling the process temperature), the contact time (10, 40, 60 and 90 min.) and pH of the solution (2, 4, 6 and 7). The rotational speed of the mixer was (200 rpm). The adsorption process of CNions needs statistical analysis for the experimental gotten data to develop an optimal adsorption model.
The adsorption capacity q was calculated from the difference between the initial concentration and equilibrium adsorbate compound concentration, which is as follows the equation: Where q is the adsorption capacity (mg/g), C0 and Ce are the initial and equilibrium concentration (mg/L), respectively, M is the adsorbent dosage (g) and V is the solution volume (L). The CNions removal percentage can be calculated as follows:

RESULTS AND DISCUSSION
The properties of the exfoliate apricot kernels were examined using FT-IR, EDX, and SEM.

Fourier Transforms Infrared Spectroscopy (FT-IR) and EDX Spectra Investigation
FT-IR spectroscopy has been utilized to test the structural dynamics, structural composition, conformational changes (effect of binding, temperature, and pH), structural stability, and aggregation of proteins. It has been used to identify functional group of compounds, such as carbohydrates and esters, as well as inter atom chemical bonds in a variety of samples. The FT-IR spectra of exfoliate apricot kernels were indistinguishable in the wavenumber range of 4000-400 cm −1 , only with subtle differences in the intensity of bands/peaks. Infrared spectra of exfoliate apricot kernels indicated by the presence of narrow peak bands at 1743.71 cm -1 attributes to aldehyde and ketone C = O stretching. The position of the C = O stretching indicated the hydrogen bonding and conjugation within the molecules. High intense peaks followed by a peak at 2922.25 cm -1 and 2852.81 cm -1 , attributed to O-H stretching (carboxylic acid) vibrations and aldehyde C-H stretching. These O-H stretching vibrations may be due to carboxylic compounds in the polymer protein matrix. The carboxylic acids (RCOOH) exist as a dimer, except in dilution solution, due to strong intermolecular hydrogen bonding ( Fig. 2A).
The EDX spectra gave an interesting finding. EDX is a very useful tool for identifying elements on the adsorbent surface. The presence of C, O, and Si, ions on the exfoliate apricot kernels surface were confirmed by the peaks at 0.32, 0.6, and 2.2 keV, respectively (Fig. 2B). Fig. 3. Scanning electron microscopic (SEM) of exfoliate apricot kernels

Scanning Electron Microscopic (SEM) Investigations
Cyanide is very reactive, forming simple salts with alkali earth cations and ionic complexes of varying strengths with numerous metal cations; the stability of these salts is dependent on the cation and pH. The salts of sodium, potassium and calcium cyanide are quite toxic, as they are highly soluble in water, and thus readily dissolve to form free cyanide. Operations typically receive cyanide as solid or dissolved NaCN or Ca(CN)2. Weak or moderately stable complexes such as cadmium, copper, and zinc are classified as weak-acid dissociable (WAD). Although metal-cyanide complexes by themselves are much less toxic than free cyanide, their dissociation releases free cyanide as well as the metal cation which can also be toxic. Even in the neutral pH range of most surface water, WAD metal-cyanide complexes can dissociate sufficiently to be environmentally harmful if in high enough concentrations.
Generally, changes of pH were effect by the metal ion's adsorption, due to the activity of protons and ions. Fig. 4 represented the pH changes effect on the CNions adsorption by exfoliate apricot kernels as adsorbent material, test results were showing that the solution pH changing affected slightly on the CNions adsorption. When pH = 4 the CNions uptake 98.8% increased slightly, then pH < 4 decreases slightly. When the solution pH between 6.5 to 7, the adsorption relatively percentage was constant between 98.8% to 99%, but at pH = 7 the adsorption efficiency decreasing to less than 95% because the CNions started to precipitate.
Cyanide forms complexes with gold, mercury, cobalt, and iron that are very stable even under mildly acidic conditions. However, both ferro-and ferri-cyanides decompose to release free cyanide when exposed to direct ultraviolet light in aqueous solutions. This decomposition process is reversed in the dark. The stability of cyanide salts and complexes is pH-dependent, and therefore, their potential environmental impacts and interactions (i.e. their acute or chronic effects, attenuation, and re-release) can be varied. The scanning electron microscope (SEM) is one of the most versatile methods for the examination and analysis of the microstructure, chemical composition, and physical (size and shape) characterizations. It shows the arrangement of starch granules and the protein network in the matrix. The SEM was conducted at a magnification of 1000 x and 1200x. The images (A, B and C) in Fig. 3 for the exfoliate apricot kernels press cake reveals that the press cake had some polygonal and irregular shapes representing the presence of starch molecules in the sample. Whereas, some cracked surfaces represented the presence of protein. Starch granules seem to be surrounded by little pieces of other protein material, giving the appearance of rousing dust in exfoliate apricot kernels press cake. The differences in the picture were the variations in the number of individual granules and compound granules or may be due to different processing conditions and extraction methods to form different structural aggregate. The results of SEM tests are at the bottom of each image.

Effect of Temperature
The effect of temperature on the CNions removal was shown in Fig. 5. When the system temperature went up, the solubility of the CNions in wastewater increased correspondingly, removal of CNions would increase at pH 7, over 99% of the cyanide will exist as HCN. Although HCN is highly soluble in water, its solubility decreases with increased temperature and under highly saline conditions.

Effect of Adsorbent Dose
The effect of adsorbent dosage on the percentage removal of CNions is depicted in Fig. 6 at specific conditions. It can be seen that the removal of CNions increased with increasing adsorbent dosage and attained a maximum value (100%) at an adsorbent dosage of 0.95 gm/100 mL. The phenomenon is associated with an increase in available binding sites for adsorption in higher sorbent dosage, but sorption capacity decreases with an increase in sorbent dosage.

Effect of Contact Time
The relationship between contact time and CNions removal by exfoliate apricot kernels was conducted through batch experiments to achieve the equilibrium as shown in Fig. 7. The CNions removal increased with contact time. The rapid adsorption in the first 5 to 30 minutes can be attributed to the increased availability of vacant surface sites at the initial stages. The optimal contact time was 60 minutes. The adsorption rapidly occurs and is normally controlled by the diffusion process from the bulk to the surface.

Adsorption Isotherms Models
Adsorption isotherms are mathematical models that describe the distribution of the adsorbate species among liquid and solid phases, based on a set of an assumption that is related to the heterogeneity/ homogeneity of the solid surface, the type of coverage, and the possibility of interaction between the adsorbate species (Hattab et al., 2016;Jaafar and Alatabe, 2019).

The Langmuir Models
The Langmuir isotherm is based on the assumption that all the adsorption sites are energetically identical (monolayer adsorption) and adsorption occurs on a structurally homogeneous adsorbent (Langmuir, 1918). The Langmuir isotherm is given: where KL (dm 3 /g ) and aL (dm 3 /mg ) represent Langmuir constants (Langmuir, 1916;Langmuir, 1917).

The Freundlich Model
This isotherm is derived from the assumption that the adsorption sites are distributed exponentially to the heat of adsorption (Freundlich, 1906), and it is given by: qe = Kf Ce l/n (5) Where qe is the adsorbed metal ions (mg/g), Ce is CNions concentration in the solution at equilibrium (mg/L), Kf (mg/g) indicates the multilayer adsorption capacity and n an empirical parameter related to the intensity of adsorption.

Temkin Isotherm Model
This model was obtained with consideration of adsorption interaction and adsorption substances (Hadi et al., 2020;Vadi et al., 2011;Vadi et al., 2010), which consider the effects of the heat of adsorption of all molecules in the layer, would decrease linearly with coverage due to the adsorbate and adsorbent interactions and is given by: qe = lnACe (6) Here ( ) = B (J/mol), which is Temkin constant, A (l/g) is the equilibrium binding constant, R is the universal gas constant and T(K) is absolute solution temperature.

Harkins-Henderson Model
This model explains multilayer adsorption and the existence of heterogeneous pore distribution in the adsorbent (Vadi et al., 2011;Vadi et al., 2010). The Harkins-Henderson isotherm has been applied in the following form: n and K H-H are isotherm constants. From the above isotherm models the results were obtained, the linearized form of Langmuir, Freundlich, Temkin, and Harkins-Henderson isotherm models, using Equations (4) to (7) respectively, were analyzed using Microsoft Excel Software to find the isotherm constants. These constants presented in Table 1, it can be seen that the regression correlation coefficient (R 2 ) of the Langmuir equation (R 2 = 0.998) is more linear when compared with that of other equations, implying that the adsorption isotherm data are well fitted by the Langmuir isotherm. Fig.  8 shows the experimental curve and isotherm model curves.
The fact that the Langmuir isotherm fits the experimental data very well, the adsorption is single-layer and the maximum adsorption corresponds to a saturated monolayer of CNions molecules on the exfoliate apricot kernels surface, the energy of adsorption is constant, and there is no transmigration of CNions in the plane of the surface (Alatabe and Kariem, 2019). The Langmuir isotherm model is based on the assumption that there are a finite number of active sites that are homogeneously distributed over the surface of the adsorbent. These active sites have the same affinity for adsorption of a monomolecular layer and there is no interaction between adsorbed molecules (Vadi et al., 2010;Alatabe and Kariem, 2019).

Adsorption Kinetics
The kinetics of adsorption describes the rate at which adsorbate is adsorbed on the adsorbent. The adsorption kinetics is required for selecting optimum operating conditions for the full-scale batch process. It is also helpful for the prediction of the adsorption rate, giving important information for designing and modeling the process. Several kinetic models are used to analyze adsorption kinetics data (Alatabe and Hussein, 2018) .

Pseudo-First-Order Kinetic Model
This model proposed by Lagergren. The linearized form is shown in Equation (8). log (qe-qt) = log qe-k1t/2.303 (8) qe (mg/g) and qt (mg/g) are adsorption capacity at equilibrium and at time t respectively. k1 is the rate constant of pseudo first-order adsorption (min -1 ) (Pan and Xing, 2010).

Pseudo-Second-Order Kinetic Model
The pseudo-second-order kinetic model can be represented as follows: dq t dt = ks(qe -qt) 2 (9) Where ks is the rate constant of adsorption, g/(mg. min) (Ho and McKay, 1999;Ho, 2006).

Intra Particle Diffusion Study
In the batch mode adsorption process, initial adsorption occurs on the surface of the absorbent. Also, the sorbate can diffuse into the interior pores of the adsorbent. Weber and Morris suggested the following kinetic model to investigate if the adsorption is intraparticle diffusion or not (Saleh et al., 2016;Simonin and Bouté, 2016). The relationship may be given as: qt = kid t 1/2 + C (10) Where kid and C are the intra-particle diffusion rate constant.

Elovich Model
The Elovich equation was developed for describing the kinetics of heterogeneous chemisorption, the equation assumes a heterogeneous distribution of adsorption or activation energies that vary continuously with surface coverage and it's widely used in liquid-solid adsorption (Wu et al., 2009). The Elovich equation is generally expressed as: qt = 1/ β ln (α β) + 1/ β ln t (11) Where, α is the initial bio-sorption rate (mg/g min) and β is related to the extent of surface coverage and the activation energy for chemisorption (g/mg).
From the above kinetic models, the results were obtained, the instantaneous adsorption of the batch process was investigated using four different models. These kinetic models included the pseudo-first-order, pseudo-secondorder, intra-particle diffusion, and Elovich models (Cheung et al., 2000). The experimental results were employed to derive the kinetic parameters using these models.  Table 2 shows the results of these analyses and Fig. 9 to 12 represent the adsorption capacity with the fitted model. Fig. 9 represents the relation of log (qe-qt) and time for pseudo-first-order model, Fig. 10 represents the relation of (time/qt) and time for pseudo-second-order model, Fig. 11 represents the relation of qt and (time) 0.5 for the intraparticle diffusion model, and Fig. 12 represents the relation of qt and ln (time) for Elovich model.
By comparing the correlation coefficient (R 2 ) values of each curve for all four models listed in Table 2, it seems that the kinetics of CNions adsorption onto exfoliate apricot kernels was found to be fitted with a pseudo-first-order model more than other models.

Adsorption Thermodynamic Results
The effect of CNadsorption on the temperature was studied at a temperature ranging from 20 to 60°C. The Gibbs energy change (∆G°) indicates the degree of the spontaneity of an adsorption process, and a higher negative value reflects more energetically favorable adsorption (Hussein and Alatabe, 2019;Alatabe, 2018;Alatabe, 2018). Thermodynamic parameters such as standard free energy change (∆G°) (Liu, 2009), standard enthalpy change (∆H°) and standard entropy change (∆S°) were calculated using the following relations : where R is the universal gas constant (8.314 J/mol. K), T is the temperature (K) and Ke is the thermodynamic equilibrium constant without units (Alatabe, 2018;Ho and Ofomaja, 2006). The enthalpy change (ΔH°) and entropy change (ΔS°) of adsorption are obtained from the following equation lnKe= ∆S°R − ∆H°R T (14) According to Equation (14), ΔH° and ΔS° parameters can be calculated from the slope and intercept of a plot of lnKe versus 1/T, respectively (Fig. 13).
These thermodynamic parameters can offer insight into the type and mechanism of an adsorption process. Values of free energy change ∆G° are negative confirming that CNions adsorption onto exfoliating apricot kernels is spontaneous and thermodynamically favorable since ∆G° became more negative (-0.27, -0.39, -0.55 and -0.63 kJ/mol) with an increase in temperature at 25, 35, 45 and 55C o respectively, indicating a high driving force and hence resulting in higher adsorption capacity at a higher temperature. The positive value of ∆H° indicated the endothermic adsorption process (0.756 kJ/mol). A little but positive value of ∆S° (0.0225 kJ/mol.K) in the temperature range 20-60°C suggested increased randomness at the solid-solution interface because some water molecules were dislodged during adsorption of CNions.

CONCLUSIONS
The study showed that the exfoliate apricot kernels was effective in adsorption CNions from wastewater due to the availability of effective functional groups as shown in FT-IR. 99% CNremoval at temperature 45°C, pH 7, adsorbent dose 0.95 g/100 mL and 60min contact time. The Langmuir equation fits the experimental data for equilibrium isotherm of CNions removal more than other equations. The pseudofirst-order adsorption is predominant for kinetics and thermodynamics studies. Finally, as the exfoliate apricot kernels were efficient in absorbing CNions from the wastewater, we recommend using it in the treatment of wastewater contaminated with CNions.