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Non-linear modelling of the adsorption of Indigo Carmine dye from wastewater onto characterized activated carbon/volcanic ash composite
⁎Corresponding author.
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Over the past couple of years, the resurgence of placing an effective and sustainable amendment to combat against the auxiliary industrial entities like Indigo Carmine (IC), remains a highly contested agenda from a global point. The birth of non-linear modelling for these auxiliary entities is also of significant interest in order to avoid loss of some useful information. With the renaissance of activated carbon (AC), the AC prepared from palm kernel shells (PKSAC) and composite prepared by combining the PKSAC and porous volcanic ash (BVA) from the foot of active volcanic mountain of Cameroon will be of significant contribution for ever increasing pollution problems. Non-linear modelling method was used to model the uptake capacity by adsorption process of IC onto PKSAC and PKSAC/BVA composite. Effects of contact time (0–60 min), adsorbent dose, pH of solution and initial dye concentration (10–20 ppm) were studied on the quantity removal of the hazardous IC dye from aqueous solution in a batch experiment. The prepared PKSAC and PKSAC/BVA composite were characterized using Nitrogen adsorption at 77 K (BET), Fourier transform Infrared spectroscopy (FTIR), Sacanning electron microscopy with energy dispersive X Ray (SEM-EDX), and particle size. The optimum IC uptake was 11.025 and 12.642 mg/g for PKSAC and PKSAC/BVA composites adsorbent respectively. Four Isotherms and kinetic non-linear regression models each were used to model the adsorption data. It results that for the isotherm models, the Langmuir and Freundlich isotherm models best fitted the adsorption phenomenon while pseudo-first and pseudo-second order kinetic models well described the adsorption mechanism. Furthermore, the adsorption speed constant (α) of the Elovich kinetic model being higher than the desorption coefficient (β) implies chemisorption was the dominant mechanism in the adsorption process. The composite shows 14.67% higher in retention capacity of the IC dye than the pristine carbon. Conclusively, the expanding of activated carbon/volcanic ash composite represents a potentially viable and powerful tool, leading to the plausible improvement of environmental preservation.
Keywords
Non-linear modelling
Adsorption
Indigo Carmine
Volcanic ash
Activated Carbon
Composite
1 Introduction
Human developmental achievements on earth commensurate to its growth parameters, has never been evaluated without remarkable indicators of negatively induced impacts in neigbouring communities, especially close to the industrial zones. Since many industries consume substantial volumes of water and generates a considerable amount of wastewater unabated, the pollution burden in developing industrialized countries has been on the rise and of major concern (Lee and Muhammad, 2019). Increased concern by communities, multinational co-operations, governments, non-governmental organizations, universities and scientific institutions and other environmental stakeholders, are seeking for means and ways to avert these anthropogenic pollution by industrial wastewater and effluent; containing non-biodegradable heavy metals, organic pollutants and dyes associated with contaminant and pollutant effects on human and environmental health (Aremu et al., 2020).
Many pollutants have been or continue to be used in large quantities. Due to their environmental persistence, these pollutants have the ability to bioaccumulate and biomagnify, impacting their toxicity directly on humans and aquatic life, as well as indirectly through the food chain based on their resident time (Ritter et al., 2002). The result has been an exacerbation of pollutant load in the abiotic media (Michael et al., 2021), overwhelming the carrier streams.
Indigo carmine a water-soluble semi-synthetic organic blue acid dye (indigo disulphonic acid) also known as ‘Saxon Blue’, has for a longtime been used extensively in various industries like textile, paper, plastic, food, printing, cosmetic, and pharmaceutical industries (Ibrahim et al., 2010; Ngaha, et al, 2018). However, its discharge from industries into receiving water bodies as effluent, pollutes the streams and its degradation products are also highly toxic, mutagenic, carcinogenic, and allergenic, seriously causing various health and environmental problems (kotlewska and Chojnowska, 2017; Ismail et al 2019).
Methods that have been used for the removal of IC dye includes non-thermic plasma gildard, photocatalysis (Lekshmi et al, 2017), adsorption (Ankoro et al., 2016) reverse osmosis (Caprarescu et al, 2016), membrane filtration (Gopi et al, 2017), electrochemical destruction (Lécuyer et al., 2021), irradiation (Zaouak et al, 2018), ozonation (Ortiz et al, 2016) and microbial biodegradation (Li et al, 2015) etc. Amongst the aforementioned methods, adsorption is one of the promising technic. It is economical feasible, flexible and easy to carry out especially in third world countries (Mehrabi et al, 2015). Selection of the best materials for adsorption treatment is a complex task, considering a number of factors, such as the quality standards to be met, and the efficiency as well as the cost (Oller et al., 2011).
Cost considerations can make it expedient to use local raw agricultural materials such as Palm kernel shell and natural material such as volcanic ash, as precursors for the production of adsorbents for removal of Indigo carmine. The performance of these materials may not always be optimal, but their immediate availability and combination in synergy can makes them an attractive choice (Liu et al., 2020; Hao et al., 2020). It is important for us to produce materials with properties that can effectively eliminate this pollutant using electrostatic interactions (physisorption) with bonds that are easy to break and lead to an easier and complete recovery without the adsorbent being damaged (Qiu et al., 2020).
The mount Cameroon (4°13′02.6364″ N and 9°10′21.8929″ E) is an active volcanic mountain, and in this light, it is very rich in volcanic ash (Kylling et al., 2014). In this work, palm kernel shell (lignocellucic material) from Mondoni oil mill (4°11′10.2444″ N and 9°28′1308″ E) was chosen because of its abundance for the production of the activated carbon. Activated carbon is an important adsorbent with its unique characteristics of high surface area, high porosity and surface functional groups (Reza, 2020) while volcanic ash contains chemicals substances such as Silicon oxide, aluminium sulphate and ferric chloride which are used as flocculants (Lee and Muhammad, 2019; Yao et al., 2020; Hu et al., 2021). These are the attractive characteristic attributes of the precursors to be coupled as a composite for the purpose of adsorption (Aremu et al., 2020). For this reason, Palm kernel shell activated carbon (PKSAC) and the Palm kernel shell activated carbon/Black Volcanic Ash composite (PKSAC/BVA) were prepared and characterized.
In the past, linear least-square method was widely used by transforming the equation of into a conventional linear form to calculate or predict the isotherms/kinetics parameters or the most fitted models. These models are principally subjected to their goodness of fit to the experimental data with the magnitude of coefficient of determination that are closed to unity (Ngakou et al., 2019). Linear methods have been proven to have significant limitation related to linearized form of isotherms/kinetics equations by producing a vast number of different outcomes, which implicitly alter the error structure, violates the error variance and normality assumptions of standard least squares leading to the bias of the adsorption data (Asuquo and Alastair, 2016). Due to the inherent bias from linearization, non-linear regression will be used in this work to provide a mathematical rigorous method for determining the isotherm/kinetic parameters using the original form of the equations. Therefore, the aim of this study is to model the adsorption isotherms and kinetics data obtained from the adsorption of Indigo Carmine dye on PKSAC and PKSAC/BVA and the resulting constants from these models will be used to discuss the adsorption mechanism.
2 Materials and methods
2.1 Preparation of the activated carbon
The PKS were pre-carbonized in a multistage process at the rate of 10 °C/min from room temperature and the recurrent residence time of 30 min and ramped to 500 °C, washed and dried. 10 g of the pre-carbonized PKS was mixed with 2% (wt/wt) of KOH as mild activating agent. The mixture was agitated and dried. The dried sample was then carbonized/activate at the same multistage process and ramped to700°C for 2.5 h to obtain the activated carbon. The activated carbon obtained was washed to neutral pH, dried at 110 °C in an oven, and the sample denoted PKSAC.
2.2 Preparation of the volcanic ash/activated carbon composites
The volcanic ash was calcined at 600 °C for 2 h and the sample was mixed with activated carbon in the ratio 1:2 and 100 mL of 6 M HNO3 acid was added into the mixture. The resulting mixture obtained was allowed for 24 h at room temperature and pressure. After which it was later washed to neutral pH to leach the excess of acid. The sample obtained was termed PKSAC/BVA composites.
2.3 Characterization of the PKSAC and PKSAC/BVA composite
The surface morphology of PKSAC and PKSAC/BVA were observed and photographed by using a scanning Electron Microscopy equipped with Energy Dispersive Spectrometer (SEM/EDS, LEO 1455 VP). The structural analysis of the samples was carried out by a powder X-ray diffractometer (PANalytical X’ Pert Pro) with Cu-Ka radiation. FTIR measurements were also carried out in the absorbance mode ranging from 400 to 4000 cm−1 using Universal ATR, Crystal: Platinum, Diamond, Bounces: 1, Solvent: ethanol for surface functional group determination. The particle sizes of the PKSAC and PKSAC/BVA samples were measured by using MALVERN Zeta sizer NanoZS90 at 25° C. Thermogravimetric analysis (TGA) and Differential Scanning Colorimetry (DSC) of PKS and BVA-AC were evaluated using thermogravimetric analyzer, TGA/DSC (LENSEIS STA PT-1000 thermal analyzer). The temperature range of the analysis was room temperature to 1200 °C with the heating rate of 10 °C/min under an inert atmosphere.
2.4 Batch adsorption experiments
Batch adsorption tests were carried out by mechanical agitation at room temperature. For each run, 20.0 mL of IC dye of known initial concentration was treated with a known weight of adsorbents (PKSAC and PKSAC/BVA). After agitation using Multipurpose Flask Shaker TT12F from Techmel and Techmel USA at a speed of 160 rotation per minutes for a fixed period of time. The solution was filtered and the filtrate was subsequently analyzed for IC dye concentration by UV/Vis spectrophotometer, model S23A from Techmel and Techmel USA at a wavelength of 612 nm. Similar measurements were also carried out at various adsorbents doses (0.02 to 0.07 g), pH (2 to 8) and initial concentrations of IC dye solution (10 to 20 mg/L). The amount of the IC dye (Qe) adsorbed per unit mass of adsorbent were calculated by using the following expressions.
3 Results and discussion
3.1 Characterization of PKSAC and PKSAC/BVA composite materials
3.1.1 Thermogravimetric analysis
The TGA diagram (Fig. 1a) of the PKS shows three thermal accidents while that of BVA (Fig. 1b) shows four thermal accidents. The first thermal decomposition took place at 83 °C and 81 °C for PKS and BVA respectively. This could be attributed to the departure of free water on the surface of the respective materials leading to a percentage loss of 7.8 % and 4.77% respectively. For the PKS, the second thermal weight loss is at 302 °C given a weight loss of 51.8% in an exothermic process. This may be attributed to degradation of hemicellulose and cellulose (holocellulose). The third is around 428 °C with a weight loss of 40.5% still in an exothermic process which is due to the degradation of lignin present in the PKS. From these observations of the thermogram of PKS it shows that the good temperature for preparation of activated carbon have to start from 500 °C. In the case of the BVA, the second and third thermal accident at 223 °C and 406 °C can be attributed to the departure of organic matter while the last thermal accident at 693 °C can be due to the degradation of possible carbonates present in the material. Similar trend for BVA sample was obtained by Celik et al., 2019.
Thermogravimetric Spectrogram of PKS (a) and BVA (b).
3.1.2 Fourier transformed infrared spectra
The FTIR spectra of PKSAC and PKSAC/BVA before and after IC adsorption are displayed on Fig. 2. The spectrum of PKSAC/BVA materials before and after IC adsorption shows two peaks located at 1644 and 1790 cm−1 characteristic of the stretching vibrations of the C⚌C and C⚌O double bonds of the aromatic rings and acid anhydrides respectively (Djamila et al., 2020). This spectrum is identical to that obtained after adsorption of IC. This suggests an adsorption of physical nature (physisorption) of the IC on the surface of this material.
Fourier transform infra-red spectroscopy of PKSAC, PKSAC/BVA , PKSAC-IC and PKSAC/BVA-IC.
On the spectrum of the carbon before and after IC adsorption (PKSAC, PKSAC-ICwe observe bands relative to the two starting materials. For example, the band located around 1574 cm−1 corresponds to the stretching vibration of the C⚌C double bond of the aromatic rings of the activated carbon. for the composite material, the band located at 1093 cm−1 and the peaks located at 447 and 415 cm−1 correspond respectively to the stretching vibration of the Si-O bond and the bending vibrations of the Si-O-Mg and Si-O-Si bonds inherent to the BVA material (M’leyeh et al., 2002). The same trend is observed on the spectrum of the PKSAC/BVA-IC material. Evidence that the IC is adsorbed on this material by a physisorption process.
3.1.3 Morphological analysis
In order to study the morphology of the different materials, SEM analyses were performed on the different samples. Fig. 3 below shows the pictures of the base materials and the composite Materials:
SEM images of a) PKSAC b) BVA and c) PKSAC/BVA.
Fig. 3a shows the PKSAC micrograph morphology. We observe that this material is formed of diverse array of granules of variable sizes, with a surface covered with small holes, which are similar to pores. These granules, however, have a spongy appearance. In Fig. 3b related to BVA, it is seen that the material is formed of blocks similar to non-uniform ashlars with macro-fractures. Each block, whatever its size, has a rough aspect, which is in perfect harmony with the nature of the material, since it comes from volcanic ash which is mainly made of inorganic matter, largely associated metals or oxides. The composite material image (see Fig. 3c) shows the characteristics of both PKSAC and BVA materials. Large particles of various sizes can be observed, with some having a spongy appearance with fairly developed porosity and others having a rough appearance. It is also observed on PKSAC/BVA composite that some particles have lamellar shapes while others similar to stone blocks have more complex shapes. The difference in color between the different particles can be attributed to the difference in nature between the different particles: some from carbonized PKS and others from BVA.
3.1.4 Energy dispersive x-ray analysis (EDX)
The relative compositions to the nature of the species that make up the materials were carried out by EDX analysis. The results of the different elemental compositions of the materials as well as their concentration within the materials are presented in Table 1 and the spectra relating to each material are given in Fig. 4 below.
| PKSAC | PKSAC/BVA | |
|---|---|---|
| Elements symbol | Atomic conc. | Atomic conc. |
| C | 87.17 | 72.79 |
| O | 9.34 | 20.09 |
| N | 2.61 | 4.94 |
| Si | 0.36 | 0.90 |
| Na | 0.16 | 0.20 |
| Al | 0.10 | 0.28 |
| Mg | 0.10 | 0.30 |
| P | 0.07 | – |
| Ca | 0.05 | 0.25 |
| S | 0.04 | 0.05 |
| Ti | – | 0.07 |
| Fe | – | 0.13 |
- EDX spectrum of different materials.
The results of the analyses show that PKSAC is mainly composed of carbon (87.17%), oxygen (9.34%) and nitrogen (2.61%). This high percentage of carbon makes this material an excellent AC. The other elements present (trace elements) are present in very small quantities, especially the metallic elements. The latter represent only 0.88% of the sample. The composite material has a high percentage of carbon (72.79%) which comes from the carbonization of the PKS and oxygen (20.09%) which comes from the surface functional groups of AC and metal oxides from BVA. It contains a higher amount of nitrogen element (4.94%) than the precursor materials (2.61%). This can be attributed to the fact that this element is found in the internal structure of the final material which does not allow its volatilization even at the carbonization temperature and also from the oxidation process using HNO3 acid which adds to the nitrogen content. Metals within the composite material are only 2.28%. This confirms that the matrix in the composite is AC and the reinforcement is BVA.
3.1.5 Structural analysis (XRD)
X-ray diffractometric analysis was performed on all samples to determine the different crystal phases and the different probable chemical compositions of the PKSAC and PKSAC/BVA composite materials. Fig. 5 shows the diffractogramme of the materials used in this work:
Diffractogramme of PKSAC and PKSAC/BVA materials.
On the spectrum of the PKSAC, we see the presence of two domes centered respectively at 24.24° and 43.47°, characteristic of the amorphous material. It does not contain any crystalline phase that is well in accordance with others previous works. Observation of the spectrum of the PKSAC/BVA composite material allows the identification of a semi-crystalline material. On one hand, for small angles, a large dome is observed between 15° and 28°, centered around 25° corresponding to the amorphous phase of the activated carbon from PKSAC. On the other hand, one observes formation for the angles 2θ equal to 22.06°, 23.72° and 27.87°, corresponding respectively to the values of interreticular distance d = 4.03, 3.75 and 3.20 Ǻ. These peaks are characteristic of the reflection of the crystalline phases of Albite (NaAlSi3O8) according to JCPDS 09–0466 file. While the peaks formed at 29.88°, 35.42° and 39.22°, corresponding respectively to the values of interreticular distance d = 2.99, 2.53 and 2.30 Ǻ correspond to the diffraction planes of Augite ((Si, Al)2O6(Ca, Mg, Fe, Ti, Al)2 belonging to the BVA in consistence with PFUFF ID R061108 file. The XRD analysis is as well in accordance with SEM, which proved the presence of both PKSAC and BVA materials in the composite.
3.1.6 Particle size distribution of the composites
In order to determine particle diameter values more accurately and conveniently, PKSAC/BVA materials was subjected to zeta sizer, which measures particle size by the process of quasi-elastic light scattering. The different diameter values for the sample studied are shown in Fig. 6 below:
particles diameter for PKSAC/BVA composite material.
The results observed for the composite material show a bimodal mode of distribution, with particle diameters ranging from 80 to 100 nm and centered at 93 nm. On the other hand, a second size distribution with particle diameters between 200 and 500 nm and centered at 230 nm is observed in the figure of the composite material. This double size distribution is attributed to the presence of particles of different nature. This result confirms the presence of a composite material and not a hybrid material with a monomodal particle size distribution.
3.1.7 Textural analysis
The analysis of the texture of the different materials was carried out by adsorption of nitrogen at 77 K. The results relating to porosity and pore volume are shown in Fig. 7 and in Table 2 below.
Pores volume in function of pores diameter for a) PKSAC and b) PKSAC/BVA.
| Materials | PKSAC | PKSAC/BVA |
|---|---|---|
| Specific surface area (m2.g−1) | 574.500 | 331.899 |
| Pores volume (cm3.g−1) | 0.281 | 0.197 |
| Pores diameter (nm) | 2.108 | 2.446 |
It can be seen in this figure that the variation in pore volume is found in the pore diameter range from 1.6 to 6.0 nm. However, it can be seen that these curves are not centered at the same point as the Lorentz curves. This suggests that the different materials analyzed do not have the same texture. This can be attributed to their different nature and/or to the preparation that modifies the texture of the base materials. Indeed, it is observed that the pore volume of PKSAC is greater than that of PKSAC/BVA composite VPKS-AC > VBVA-AC. The pore volume of the composite material is less than that of the Palm kernel shell activated carbon. This may be due to the arrangement of the particles of the base materials which reciprocally screen the pores during the preparation of the composite material. On the other hand, a larger pore diameter is observed for the composite material. This can be attributed to the method of preparation of this material which favors the opening of the pores. This could be beneficial for the adsorption of large molecules like Indigo Carmine. The pore diameters for all materials analyzed are between 2 and 50 nm, suggesting a dominant mesoporosity for the both materials. In addition, the specific surface area of the base material PKSAC is greater than that of the composite material. This implies that, the comment mentioned for the reduction in pore volume can also be made for the pore diameter.
3.2 Adsorption studies
3.2.1 Effect of pH
One of the key parameters acting in the adsorption process is a pH since it affects both the degree of ionization of the IC dye and the surface chemistry of the adsorbent materials. In this work, the effect of pH of the IC solution on the adsorbed quantity of dye was investigated under the condition of contact time of 1 h, adsorbent dose of 0.02 g and initial concentration of 20 mg/L. The results are shown on Fig. 8. On both PKSAC and PKSAC/BVA, the increasing in pH leads to the reduction of adsorbed quantity. The maximum adsorption quantity is obtained at lower pH 2. It is generally known that, due to the abundant presence of H+ ions, the anionic dyes like IC are preferentially adsorbed by the adsorbent at lower pH. At that particular pH 2, the surfaced functional groups on PKSAC and that on PKSAC/BVAare protonated and the electrostatic attraction between IC ion molecule is favorable leading to the increasing of adsorbed quantity (Doke et al., 2016). The reduction of the positive charge on the adsorbent surface due to the increasing of pH lead to the reduction of electrostatic forces between adsorbent and adsorbents and equally reduce the adsorption quantity. Moreover, at higher pH, there is an excess of OH– ions which compete with negatively charge IC molecule ions resulting to the reduction of adsorbed quantity.
Effect of pH on the removal of IC by PKSAC and PKSAC/BVA (C0 = 20 mg/L, adsorbent dose = 0.02 g and contact time of 60 min).
3.2.2 Effect of contact time
The effect of contact time on the adsorption of IC dye by PKSAC and PKSAC/BVAwas investigated at pH 2 with initial concentration of 20 mg/L and adsorbent dose of 0.02 from 0 to 60 min. Fig. 9 reveals the results achieved for the two adsorbents and it can be seen that the rate of adsorption is fast for the PKSAC and PKSAC/BVA within the five first minutes of the adsorption process. Then slowed down leading to one equilibrium state. For PKSAC/BVA, the the adsorbent-adsorbate equilibrium is rapidly established after 20 min while for the PKAAC the equilibrium is achieved a little bit later (30 min). The rapid increase of the adsorbed quantity of IC during the initial time of adsorption process may be attributed to the availability of large number of adsorption sites. The slowdown and the establishment of equilibrium are due to saturation of adsorption site (Ankoro et al., 2020). The adsorbed quantity at equilibrium of PKSAC and PKSAC/BVA are respectively 10.158 mg/g and 12.462 mg/g. This significant difference (18.488 % from PKSAC to PKSAC/BVA) in the adsorption capacity of the two adsorbent materials could be result of the difference in the physic-chemical properties as a result of obtention of a new composite material with better adsorption properties than activated carbon alone. This can be as a result of the increase in the oxygen conten (from 9.34 to 20.09 % for PKSAC and PKSAC/BVA respectively). In addition, the large specific surface area and pore size distribution on composite PKSAC/BVA is favorable for liquid phase adsorption of organic pollutants such as dyes in terms of both efficiency and economic consideration.
Effect of contact time on the removal of IC by PKSAC and PKSAC/BVA (C0 = 20 mg/L, adsorbent dose = 0.02 g and pH = 2).
3.2.3 Effect of adsorbent dose
Fig. 10 shows IC adsorption-based removal versus the mass of produced adsorbents under the conditions of contact time of 30 min, initial concentration of 20 mg/L and pH 2. The adsorption of IC decreases with an increase in adsorbent dose. The rate of the decrease is more prominent at higher amounts of adsorbent. Significant adsorption of the IC dyes is achieved at PSKAC and PKSAC/BVA mass of 0.02 g. These observations could be basically due to the reason that at lower mass of adsorbent the adsorption site are available while at higher mass, there is an agglomeration of particles of adsorbent reducing the available specific surface area and the increase in the diffusional path length (Lekene et al., 2015).
Effect of adsorbent dose on the removal of IC by PKSAC and PKSAC/BVA (pH = 2; C0 = 20 mg/L and contact time of 30 min).
3.2.4 Effect of initial concentration
The variation of the initial concentration of IC was carry out in the range of 10 to 20 mg/L at pH 2 using 0.02 g of adsorbent during 20 and 30 min respectively for PKSAC/BVA and PKSAC. Fig. 11 exhibits the adsorption capacities of the two adsorbents. The results showed that, as the initial concentration of IC increases, the adsorbed quantity of per unit of mass of adsorbent also increase. For PKSAC the adsorbed quantity increases from 5.861 to 11.025 mg/g while that of PKSAC/BVA run from 7.609 to 12.642 mg/g. The increasing of adsorbed quantity is mainly due to the increase of effective collisions in solution between the IC dye and the adsorbents. These increase ineffective collision provides a powerful driving force to overcome the mass transfer resistance between the aqueous and solid phases (Ankoro et al., 2020; Lekene et al., 2015)
Effect of initial concentration on the removal of IC by PKSAC and PKSAC/BVA (pH = 2, adsorbent dose = 0.02 g and contact time of 30 min).
3.3 Non-linear adsorption isotherms and kinetic modelling
3.3.1 Isotherm studies
To understand the adsorbate-adsorbent interaction, the adsorption isotherms were use. In this study, the Langmuir, Freundlich (Kausar et al., 2019), Temkin (Rezaei et al., 2016) and Dubinin-Kaganer-Radushkevich (D-K-R) (Nazir et al., 2019); isotherms were used to analyses the experimental data and their non-linear form are respectively given by the following Eqs. (2) to (5).
Where Ce (mg/L) and qe (mg/g) are the concentration and the adsorbed quantity of IC at equilibrium, qm is the maximum adsorbed quantity, KL (L/mg) is the Langmuir adsorption constant, KF ((mg/g)(L/mg)1/n) Freundlich adsorption constant related to adsorption capacity, n energetic heterogeneity of surface, KT is the equilibrium binding constant (L.mg−1)corresponding to the maximum binding energy and constant b (J/mol) is related to the heat of adsorption, K (mol2.k/J) is a constant related to the adsorption energy, R (8.314 J/mol.K) is the universal gas constant, andT (K) is the temperature at which the adsorption took place.
The validity of different models was evaluate using correlation coefficient (R2), root mean square error (RMSE) and Chi-square test (χ2) which are given by the following Eqs. 6, 7 and 8 (Lekene et al., 2018):
Where, qe.exp and qe.pre are experimental and predicted equilibrium adsorption capacities.
The best fitting and similarity of a model with experimental data is decided by largest value of R2 (or smallest values of RMSE and χ2). The plots of the four isotherms for IC adsorption onto PKSAC and PKSAC/BVA are exhibited on Fig. 12 and the values of their calculated parameters are reported in Table 3. According to the obtained results, the Langmuir and Freundlich isotherms were good suitability to the adsorption of IC owing to its highest R2 value (smallest Χ2 value). The monolayer adsorption capacity of the Langmuir model was found to be 69.723 and 20.515 mg/g for PKSAC and PKSAC/BVA respectively. In view of the value of adsorption capacity for monolayer coverage given by the Langmuir model which is so far from the experimental value, the best model which well fit experimental data was Freundlich model. On the other hand, to determine whether the adsorption process is favorable, a dimensionless constant separation factor (RL) was calculated according to
. The value of RL was found to be equal to 0.700 and 0.183 respectively for PKSAC and PKSAC/BVA indicating that the type of isotherm is favorable (Nunell et al., 2015). The Freundlich model have favorable characteristics due to the fact that n value is between 1 and 10 for the both materials and also the high value of KF (Table 3). It is also known that the Langmuir and Freundlich adsorption isotherms have shown their limit to explain whether the adsorption process is going through the physical or chemical process. The D-K-R isotherm explain better the type of adsorption process through the determination of mean free energy E (kJ/mol) of sorption which is the free energy that changes when one mole of IC dye is fixed on the surface of the adsorbent and it is be calculated from the following formula
. According to the obtained results of adsorption of IC on PKSAC and PKSAC/BVA, it was found that the adsorption process was governed by the physical adsorption since the free energy E (0.391 and 0.745 kJ/mol for PKSAC and PKSAC/BVA respectively) was found to be lesser than 8 kJ/mol, (Nazir et al., 2019, Mehrabi et al, 2015). This could be attributable to the Colombians interaction between materials surface functional groups with those of IC. This is confirmed by the value of KT Temkin constant which possess values less than 8 kJ/mol for the both two materials. This result confirms the Freundlich model as the best to explain adsorption process, because physical process is suitable for multilayer adsorption, unlike the Langmuir model which advocates a monolayer adsorption.
Isotherm models plot for PKSAC and PKSAC/BVA.
| Models | Adsorbent | ||
|---|---|---|---|
| Parameters | PKSAC | PKSAC/BVA | |
| Langmuir | Qm | 69.723 | 20.515 |
| KL | 0.021 | 0.224 | |
| RL | 0.700 | 0.183 | |
| R2 | 0.969 | 0.908 | |
| RMSE | 0.395 | 0.691 | |
| χ2 | 0.071 | 0.204 | |
| Freundlich | KF | 1.645 | 4.823 |
| 1/n | 0.877 | 0.494 | |
| R2 | 0.969 | 0.909 | |
| RMSE | 0.396 | 0.685 | |
| χ2 | 0.070 | 0.191 | |
| Tempkin | KT | 0.522 | 1.806 |
| b | 351.129 | 507.346 | |
| R2 | 0.956 | 0.904 | |
| RMSE | 0.472 | 0.705 | |
| χ2 | 0.112 | 0.210 | |
| D-K-R | Qs | 13.476 | 13,289 |
| K | 3.270E-06 | 9,011E-07 | |
| E | 390.736 | 744,897 | |
| R2 | 0.922 | 0,827 | |
| RMSE | 0.632 | 0,946 | |
| χ2 | 0.211 | 0,391 |
3.3.2 Kinetics studies
In order to describe the mechanism of the adsorption process and the rate controlling step of IC by PKSAC and PKSAC/BVA, four kinetic models: pseudo-first-order, pseudo-second-order (Hu et al., 2015), Elovich (Tran et al., 2017) and intraparticle diffusion (Rezaei et al., 2016) were used to correlate the experimental data and their non-linear forms are given respectively by the following Eqs. (9), 10, 11 and 12.
The kinetic curves are shown in in Fig. 13 while the all the kinetic parameters are gathered in Table 4. The lower values of RMSE and that of χ2 coupled to the high value of R2 of pseudo-first-order and pseudo-second-order makes these kinetic models suitable to correlate kinetic data of both PKSAC and that of PKSAC/BVA. On other hand, the calculated values of adsorbed IC quantities from these two models were found to be 10.326 and 12.536 mg/g for PKSAC and PKSAC/BVA respectively, and were close to the experimental data values (10.175 and 12.642 mg/g for PKSAC and PKSAC/BVA respectively) implying that there is an competition the in adsorption process between physisorption and chemisorption. The Elovich model suggest that, since the initial constant rate (α)is greater to desorption coefficient (β), which is the case for PKSAC and PKSAC/BVA, it lead to the assertion that the adsorption process is governed by the chemisorption. As observed in Table 4, low R2 value (0.914 and 0.922 for PKSAC and PKSAC/BVA respectively) and high values of RMSE and χ2 (0.420 and 0.192 for PKSAC and 3.251 and 18.419 for PKSAC/BVA) of the intraparticle diffusion model shows that that pore diffusion was not the rate controlling step. Moreover, the value of the intraparticular diffusion constant Ci was found to be different to zero (4.245 and 5.069 mg/g for PKSAC and PKSAC/BVA), reinforcing the postulate that intraparticle diffusion was not the only controlling step for IC dye adsorption and the process is also controlled by boundary layer diffusion (Rezaei et al., 2016).
Adsorption kinetics model plots for PKSAC and PKSAC/BVA.
| Models | Adsorbent | ||
|---|---|---|---|
| Parameters | PSKAC | PKSAC/BVA | |
| Pseudo-first-order | Qe (exp) | 10.175 | 12.642 |
| Qe (cal) | 10.326 | 12.536 | |
| K1 | 0.498 | 2.268 | |
| R2 | 0.989 | 0.997 | |
| RMSE | 0.314 | 0.187 | |
| χ2 | 0.106 | 0.031 | |
| Pseudo-second-order | Qe (cal) | 10.494 | 12.539 |
| K2 | 0.276 | 1.409 | |
| R2 | 0.984 | 0.998 | |
| RMSE | 0.372 | 0.168 | |
| χ2 | 0.169 | 0.029 | |
| Elovich | α | 1.150E + 25 | 2.153 |
| β | 6.130 | 0.268 | |
| R2 | 0.980 | 0.470 | |
| RMSE | 0.420 | 3.251 | |
| χ2 | 0.192 | 18.419 | |
| Intraparticule diffusion | Kip | 1.159 | 1.431 |
| C | 4.245 | 5.069 | |
| R2 | 0.914 | 0.922 | |
| RMSE | 2.022 | 2.369 | |
| χ2 | 4.861 | 5.546 |
4 Conclusion
Non-linear modeling of the uptake capacity of IC dye onto PKSAC and PKSAV/BVA composite materials was investigated in this present research work. This paper further highlighted the promising role of PKSAC/BVA composite material in the removal of water pollutants with IC dye as case study. It went as far as demonstrating the fact that adsorption dose not only depends on the specific surface area but also on the surface functional groups. The FTIR spectral of the PKSAC and PKSAV/BVA composite materials shows peaks located at 447 and 415 cm−1 correspond respectively to the stretching vibration of the Si-O bond and the bending vibrations of the Si-O-Mg and Si-O-Si bonds inherent to the BVA material implying the successful incorporation of the BVA into the PKSAC during the formation of the PKSAV/BVA composite material. This was also confirmed by the EDX analysis showing high percentages of oxygen (20.09%), nitrogen (4.94%) and silicon (0.9%) in the composite than the palm kernel shells activated carbon. Also, the XRD analysis shows a large dome is observed between 15° and 28°, centered around 25° corresponding to the amorphous phase of the activated carbon from PKSAC while angles for 2θ equal to 22.06°, 23.72° and 27.87°, corresponding respectively to the values of interreticular distance d = 4.03, 3.75 and 3.20 Ǻ relating to crystalline phases of Albite (NaAlSi3O8) in the composite. Furthermore, peaks were observed at 29.88°, 35.42° and 39.22°, corresponding respectively to values of interreticular distance d = 2.99, 2.53 and 2.30 Ǻ relating to diffraction planes of Augite ((Si, Al)2O6(Ca, Mg, Fe, Ti, Al)2. The Albite and augite are thought to be coming from BVA confirming once more the copulation process in the composite preparation. The prepared samples were used to retained hazardous IC dye from aqueous solution. Despite the fact that the PKSAC had higher surface are than the PKSAC/BVA composite the composite shows 14.67% higher in retention capacity of the IC dye than the pristine carbon. The optimum IC uptake was 11.025 and 12.642 mg/g for PKSAC and PKSAC/BVA composites adsorbent respectively. This was thought to be on one hand as a result of the incorporation process of the porous volcanic ash and on the other hand as result of the oxidation with HNO3 which creates more surface functional groups. The two stages combined together enhance the adsorption process of the composite. The isotherm and kinetic modeling of the obtained data were done using non-linear regression and the results shows that, for both PKSAC and PKSAC/BVA the Langmuir and the Freundlich isotherm model and the pseudo-first and pseudo-second order kinetic model well described. It was also found that. The adsorption mechanism of IC was best described as dominant chemisorption with higher adsorption speed constant (α) of the Elovich kinetic model.
CRediT authorship contribution statement
Godwin Agbor Tabi: Methodology, Investigation, Writing – original draft. Abega Aime Victoire: Validation.
Acknowledgments
The authors are most grateful to Dr. Paschal Okiroro Iniaghe, Department of Chemistry, Federal University Otuoke, Nigeria, for his Technical assistants, and Dr. Begho Obale for supporting this research with funds and materials. The authors also sincerely wish to thank the Research Unit ‘Adsorption and Surface’ of the Applied Physical and Analytical Chemistry Laboratory of the University of Yaoundé́ I.
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