|Anduang Ofuo Odiongenyi*|
|Corresponding Author: Anduang Ofuo Odiongenyi, Department of Chemistry, Akwa Ibom State University Ikot Akpaden, Mkpat Enin Local Government Area, Akwa Ibom State, Nigeria|
|Received: April 10, 2020; Accepted: May 14, 2020;|
|Citation: Odiongenyi AO. (2020) Experimental and Computational Chemistry Studies on Decolouration of Water Polluted by Neutral Red Dye by Adsorption unto Wood Saw Dust. J Chem Sci Eng, 3(2): 143-151.|
|Copyrights: ©2020 Odiongenyi AO. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.|
In attempt to investigate the possibility of decolorizing water polluted by neutral red dye, wood saw dust was selected as a cheap raw material for the adsorption of the dye from aqueous solution. Batch adsorption method was used to investigate the effect of concentration of the dye, adsorbent dosage, period of contact and temperature. Concentration of the dye was determined using an ultraviolet-visible spectrophotometer. Results obtained indicated that wood saw dust has the potential removal efficiency in the range of 85 to 91 % (which corresponded to equilibrium adsorption concentration of 6.42 to 13.25 mg/g respectively). Adsorption of the dye increased with increase in concentration. Also, within some respective certain critical points, the adsorption of the dye was also observed to increase with increase in the period of contact, temperature and adsorbent dosage. The adsorption of the dye agreed with Langmuir, Temkin and Dubinin-Raduskevich isotherms. Calculated thermodynamic and other data indicated that the adsorption of neutral red dye is also spontaneous and supports the mechanism of chemisorptions. A pseudo second order kinetic and intra particle diffusion models were also obeyed by the adsorption data. Computational chemistry Fukui function and molecular orbital analysis indicated that the site for the adsorption of the dye is through the pyridine nitrogen bond of the dye. This proposed electron donation site was supported by Huckel charge analysis, MM2 simulation and HOMO and LUMO profiles. Regeneration experiment indicated that 68 % of the adsorbent was restored.
Keywords: Water pollution, Dye, Remediation, Adsorption, Wood sawdust
Clean water is highly needed for various industrial, natural and domestic purposes. One of the major requirements is that the water must be colorless and free from toxic content . Dyes of various forms have been identified as the commonest source of color pollution to our water system. Sources of dyes pollution may include textile industries, ternary industries, some manufacturing industries, paint industries and chemical laboratories .
Some dyes have been confirmed to be toxic, indicating that they can be poisonous when present in significant concentration. In view of this, several studies have been reported on the removal of different dyes using adsorption and other techniques. Adsorption technique has proven to be economical, eco-friendly and one of the best methods . The use of wood or timber waste for the adsorption of dyes has been reported for a number of dyes . The use of some materials to remove dyes from aqueous solution has been reported. For example,
· Use of bentonite to remove methylene dye from aqueous solution 
· Use of clay and modified clay to adsorb some dyes from aqueous solution 
· Use of some adsorbents to remove some dyes from aqueous solution and found that some plant materials are excellent materials for the removal of some dyes from aqueous solution 
· Use of clay-based materials for the adsorption of some dye and reported excellent adsorption efficiency 
· Natural clay is found to be a good adsorbent for some cationic and anionic dyes 
· Use of some plant materials for the removal of some cationic and anionic dyes 
· Studies on the removal of neutral red dye by adsorption process have been reported for cationic neutral red dye using rice husk 
Batch adsorption process is used in this study. The study shall be supported by computational chemistry, which is aimed at finding the possible adsorption site through Fukui function calculations. Knowledge of mechanism of adsorption can provide a clue to future study of possible remediation measures. The chemical structure of NR dye is shown in Figure 1.
The chemical name for the compound is N7, N7, 7-trimethylphenazine-2, 8-diamine. The compound contains hetero atoms; it is aromatic and has functional groups suitable for adsorption.
MATERIALS AND METHODS
Batch adsorption process as reported elsewhere was used to study the effect of concentration, contact time, adsorbent dosage and temperature . Equilibrium concentration of the dye was calculated using equation 1
Where C0 is the initial concentration of the dye, Ce is the equilibrium concentration of the dye, V is the volume of solution and m is the mass of the adsorbent. Computational chemistry calculations were performed using DFT method in Hyper Chem package. Full geometry optimization was achieved using AMBER method of MM method in the same Hyper Chem package. Molecular mechanics (MM) simulation was done using MM2 package in the ChemBio 2018. ChemBio 2018 was also used to developed graphic for input into hyperChem and MM calculations.
RESULTS AND DISCUSSIONS
Batch adsorption study
Beer-Lambert law which proposes a linear relationship between absorbance and concentration according to the equation 2,
Where A is the absorbance, eis the absorptivity, l is the path length and C is the concentration of the dye. The equation suggests a linear plot (with zero intercept) when values of absorbance are plotted against concentrations. The above model was used to develop a calibration curve through which concentrations of the dye were obtained by extrapolation. In this study, effect of concentration, period of contact, pH and particle size on adsorption of NR dye were investigated. Figure 2 shows variation of equilibrium concentration of NR dye adsorbed with concentration. It is observed in the plot that the amount of NR adsorbed increases with increase in concentration which implies that increase in the amount of NR molecules diffusing onto the adsorbent also favors increase in the rate of adsorption.
The amount of dye adsorbed was also found to vary with adsorbent dosage as shown in Figure 3. The equilibrium concentration of NR adsorbed, first increased with increase in adsorbent dosage up to a critical point, beyond which, the amount adsorbed started decreasing with increase in adsorbent dosage. The observed trend suggests that the number of adsorption sites might have increase with increase in the amount of adsorbent, thus giving ways for adsorbate to stick to the surface until maximum amount of adsorbate had been adsorbed. It is also possible that diffusion rate of adsorbate unto the adsorbate decreases after the critical point since much of the adsorbate’s molecules had already been adsorbed. Also, at this point, desorption can set in or there may be no significant increase in the number of adsorbate molecule.
Figure 4 reveals the pattern obtained for the variation of the amount of NR dye adsorbed with time. From the plot, it is evident that the equilibrium amount of NR dye adsorbed by wood saw dust increases with increase in the period of contact until it approaches a maximum, after which further increase in time did not increase the amount of dye adsorbed. Given a fixed amount of adsorption site and concentration of adsorbent, the more the adsorbate solution remains in contact with the adsorbent, probability of adsorption will increase but at a maximum, most of the adsorption sites have been occupied. Consequently, provided there is no desorption, then the amount NR dye adsorbed will increase with time and then remain constant.
Temperature also affected the extent of adsorption of NR dye unto wood saw dust. Temperature can affect adsorption in two ways. Firstly, the extent of adsorption may increase with increase in temperature leading to chemisorptions mechanism and secondly, the extent of adsorption can decrease with increase in temperature, which points toward physisorption mechanism.
The adsorption of NR dye was found to support chemisorption mechanism, indicating that the equilibrium amount of dye adsorbed increased with increase in temperature and then remains constant after a certain temperature. The graph is shown in Figure 5.
Adsorption isotherm gives the relationship between the amount of dye adsorbed with concentration at a given temperature. It is a unique factor that can be used to study the adsorption behavior of dyes and other adsorbates. Attempts to obtain best suited adsorption isotherm through fitting of data led to the conclusion that Langmuir, Temkin and Dubinin-Raduskevich isotherms best described the adsorption behavior of NR dye.
The Langmuir adsorption isotherm can be written according to equation 3 
= 1 / (qm b) + Ce/qm
Where Ce is the equilibrium
concentration of adsorbate (mg/l), qe is the amount of adsorbate
adsorbed per unit mass of the adsorbent (mg/l), b is the Langmuir adsorption constant
which is related to affinity between the adsorbate and the adsorbent while qm
is the theoretical monolayer saturation capacity. From the Langmuir equation, a
form of the Temkin isotherm can be written according to equation 4 
Where: A is the Temkin isotherm constant (L/g), B = RT/b, b is the Temkin constant related to heat of sorption (J/mol), R is the gas constant (8.314 J/mol K), and T is the absolute temperature (K). Temkin adsorprion plot for the adsorption of NR dye unto wood saw dust is shown in Figure 7.
From the plot, Temkin
adsorption constants (B = 3.034, ln A = 2.2162, b = 830.3039 and R2
= 0.9540) were favorable and indicate excellent fitness of the adsorption
process to the Temkin model.
The Dubinin-Radushkevich (DRK) adsorption model takes the form shown in equation 5 below
where qe is the equilibrium amount
of the dye that is adsorbed (mg/g), qm
is the theoretical amount of dye adsorbed,
ε = RTln (1+ 1/Ce
The value of the adsorption energy can be obtained from the following relation,
Application of the DRK equation requires that
a plot of
From the slope of the plots, the adsorption energy is 13.55 J/mol for NR dye. Generally, adsorption energy in the range of 1 to 8 kJ/mol point toward physiosorption while adsorption energy in the range of 8 to16 kJ/mol is an indication of chemisorptions. Therefore, NR dye is adsorbed unto wood saw dust through the mechanism of chemisorptions, as stated before.
In kinetic study of dye removal, two significant pseudo order models are pseudo first and pseudo second order kinetic models. The expressions for the models are provided in equations 4.6 and 4.7 respectively .
qt ) = lnqe- k1 t
t /qt = 1 /
(k2 qe2) + t/qe
Curve fittings to kinetic models indicated
that the adsorption of NR dye best agree with the pseudo second order kinetic
(i.e equation 9). Therefore plots of
Pseudo second order adsorption parameters obtained from the plot (Fig.8) were qe (mg/g) = 166.67, k2 = 0.0040 mol/s, h = 0.009, t0.5 = 1.5000 and R2 = 0.896.
INTRA PARTICLE DIFFUSION MODEL
The existent of intra particle diffusion can be established if equation 10 is obeyed,
qe= kp t0.5 (10)
Thermodynamic adsorption parameter can be derived from the Gibb Helmholtz equation, which relates equilibrium constant with temperature ,
∆G0 = -RT(CAe/Ce
Where R is the gas constant, T is the temperature, CAe is the solid phase concentration of the dye and Ce is the concentration of the dye in solution. Consequently, since Keq = CAe/Ce , then,
From equation 12, a plot of
FTIR spectrum of the dye
COMPUTATIONAL CHEMISTRY STUDY
Semi empirical indices are central for
evaluating molecular reactivity, stability, electron donor-acceptor potentials
and other electronic parameters associated with adsorption. Calculated semi empeirical parameters for NR
dye were Total energy (-2802.55 eV),
Electronic energy (-19389.344 eV), Core core repulsion (16586.79 eV), Cosmo
area (283.88 Å2), Cosmo volume (303.13 Å3), ionization
potential (8.19 eV), Energy of the HOMO (-8.19 eV), Energy of LUMO (-1.12 eV),
Number of filled levels (48) and molecular weight (252.31 g/mol). These
parameters compare favourable with data obtained for two dyes that were
adsorbed effectively on ground nut shell surface .
Computational chemistry can be adequately used to predict or interpret the mechanism of adsorption or predict adsorption behavior of a given adsorbate or adsorbent . Of great interest is the use of Fukui function, which enables the reactivity of a molecular species to be viewed in terms of the individual atoms that constitute the molecule. The Fukui functions for nucleophilic, electrophilic and radial attacks can be technically defined as follows ,
function analysis is based on the magnitude and sign of the calculated Fukui
function (Table 1). The site that
favors electrophilic attack is the site with the highest value of
On the other hand, the sites for nucleophilic attack are in the alkene carbons (C1 and C2). This assertion is partly supported by values of calculated Huckel charged (which are also presented in Table 1) which uniquely identified the two pyridine nitrogen atoms (N7 and N10) as the two atoms that have the highest negative values of Huckel charge. Similarly, C9. C13 and C1 seems to have the highest positive values of Huckel charges, which suggest that the nucleophilic sites for attack resides along the benzene bonds surrounding this zone. The frontier molecular orbital theory of reactivity and selection suggests that electrophilic attack takes place in the HOMO and transfer electron from the HOMO to LUMO, which then facilitates adsorption . The HOMO itself can be likened to electrophilic Fukui function while the LUMO should correspond to the nucleophic Fukui function. This approach is demonstrated in the HOMO and LUMO diagrams shown in Fig.11. In the diagram, green stands for negative while red stands for positive. It is clearly indicated in the diagram that the HOMO orbital is concentrated in the amine nitrogen (N7 and N10) while the LUMO orbital is concentrated in those atoms that has positive values of Df. Adsorption involves either electron transfer (chemisorptions) or charge transfer (physiosorption) or both. Adsorption of NR dye unto wood saw dust proceded predominantly through the mechanism of chemisorptions, therefore, electron transfer from charge NR molecule to charged saw dust surface must have facilitated the adsorption process .
Molecular mechanics analysis indicated that the amine nitrogen (N7 and N10) has one lone pair of electrons each, hence it can donate the electron to a vacant orbital in the adsorbent and this enhances adsorption (Figure 12 and 13). Nitrogen is a hetero atom, known for its strong tendency to facilitate adsorption through electron donation [22-25].
The experimental results obtained from this study qualify Musanga cecropioides saw dust is a good adsorbent for the removal of dye from aqueous solution. The adsorbent functions through mechanism of adsorption and displayed mono layer adsorption model, best described by Langmuir, Temkin and Dubinin-Radushkevich isotherms. The adsorption performance of wood saw dust vary with the period of contact, temperature, adsorbent dosage and concentration. A close examination of these parameters as well as established kinetic and thermodynamic models can provide relevant information in improving the adsorption efficiency of the wood saw dust.
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