Spectroscopic characterization of the adsorbent
FT-IR spectra of pendimethalin and paraquat dichloride (Fig. 1a–c) before and after adsorption show five important peaks which include O–H broad peak at 3117 cm−1, C\(\equiv\)C vibration at 2322 cm−1, 2084 cm−1 and 2113 cm−1, peaks at 1994 cm−1 and 1871 cm−1 which can be assigned to C=O group; such assignments can be affected by neighboring aromatic C–C ring stretched vibrations. At 1562 and 1063 cm−1, the following stretch assignments of C=C and C–O were observed. The peaks at 970 cm−1, 873 cm−1 and 676 cm−1 correspond to C–H in alkenes and C–H in aromatic rings. There was a minor difference that was observed prior to and after adsorption, which resulted in a shift and broadening of adsorption peaks. The shift of the –OH peak from 3117 to 3339 cm−1 for pendimethalin and 3117 cm−1 to 3119 cm−1 for paraquat dichloride indicates the involvement of the hydroxyl groups in the adsorption of the herbicides (Abbas and Ahmed 2016; Jawad et al. 2018).
The micrograph of the surface of ACBGNS before adsorption (Fig. 2a) shows the morphology of the surface adsorbent having pores, cracks, holes which are expected to enhance adsorption. After adsorption of the two herbicides differently (Fig. 2b, c), the surfaces showed the formation of clusters, patches, with filled cracks on the ACBGNS surfaces which shows the participation of ACBGNS in the adsorption of PE and PQ.
Some of the obtained physical properties of the activated carbon produced 11.2 moisture content (%), 0.242 density (g/cm3) and 1.78 pore volume (cm3). The value of the moisture content pore volume and the bulk density of the produced activated carbon revealed that it had good adsorptive properties. It was noticed from the properties of the activated carbon that though it might not give up to 100% adsorption, it will be good for adsorption of organic and inorganic materials to a large extent (Fito et al. 2019). Maximum weight loss was observed in the present study; 51.82% of the activated carbon was obtained after activation.
Effect of pH variation on PE and PQ adsorption
Figure 3 shows the variation in PE and PQ amount adsorbed onto ACBGNS with solution pH. Removal of PE and PQ by ACBGNS is observed to be at optimum pH of 6 and 8. It can be observed from the plots that ACBGNS posses high removal of PE at lower pH, while PQ was more adsorbed at higher pH condition. The presence of pectin on the surface of ACBGNS favored the adsorption of PQ which is cationic in nature; this is as a result of negatively charged ions increase in alkaline medium (Bello et al. 2017), while the surface of ACBGNS which also contains phenolic or carboxylic functional groups may be the reason for increased adsorption of PE at low pH (Bedin et al. 2015).
Effect of PE and PQ adsorption duration
Variation in the amount of PE and PQ molecules adsorbed with respect to duration is presented in Fig. 4. It was found that the adsorption rate of PE and PQ by ACBGNS was initially fast reaching equilibrium within 80 min. PE and PQ molecules were firmly attached onto the ACBGNS resulting in a higher rate of adsorption. At a later stage, the rate of PE and PQ removal was found to be constant. The available surfaces for binding the molecules became almost exhausted, and therefore, repulsive forces either between PE or PQ with the ACBGNS surface made the herbicide molecules difficult to occupy the vacant sites (Mondal and Kar 2018).
Effect of adsorbent amount on adsorption of PE and PQ
Figure 5 reveals the variation in equilibrium amount of PE and PQ adsorbed with adsorbent dosage (0.2–1.2 g). From the plots, the amount of PE and PQ adsorbed increased with an increase in the adsorbent amount used, while the amount adsorbed for a given amount of the adsorbent decreased considerably for both PE and PQ. This decrease observed for the herbicides in terms of adsorption per unit mass with respect to increasing adsorbent dosage could be attributed to possible overlapping of adsorption sites with an increase in adsorbent dosage. This is an indication that ACBGNS has a high adsorption capacity at low dosage.
Effect of initial herbicide concentration on adsorption of PE and PQ
Figure 6 presents an increase in amount per gram of PE and PQ adsorbed as their initial concentration increases. Driving forces of molecules in solution are low at a lower concentration, therefore resulting in a lower adsorption rate, while when concentration increases, it is expected that the PE and PQ would have higher driving forces which enable them to easily occupy the active sites on the ACBGNS surface from the aqueous phase. This was reported by other researchers (Mondal and Kar 2018; Lee et al. 2019).
Adsorption isotherm
Three different isotherm models, Temkin, Freundlich and Langmuir, were used to treat the generated isotherm data with the intention of proposing adsorption process mechanism. The Langmuir’s model has been used to describe the adsorption of monolayer which occurs on the outer surface of an adsorbent with infinite number of active adsorption sites.
$$\frac{1}{{q_{{\text{e}}} }} = \frac{1}{{Q_{{\text{o}}} }} + \frac{1}{{Q_{{{\text{oKLC}}_{{\text{e}}} }} }}$$
(10)
Ce is the concentration of PE or PQ in mg/L at equilibrium, qe, the amount of herbicide adsorbed per gram of the adsorbent at equilibrium (mg/g), Qo is maximum monolayer coverage capacity (mg/g), KL is the isotherm constant (L/mg) for Langmuir. From the slope and intercept, the values of qmax and KL were evaluated through extrapolation of the Langmuir plot of \(\frac{1}{qe}\) versus \(\frac{1}{Ce}.\) The Langmuir adsorption parameters obtained from the plots (figures not shown) are reported in Table 1. From the presented results, the value of Qo is higher for PQ than for PE. Qo is generally equivalent to the coverage of maximum adsorption capacity, indicating an increasing Qo value toward better adsorption. Hence, higher Qo obtained for PQ confirms that ACBGNS adsorbed PQ better than PE. Freundlich isotherm model’s empirical Eq. (11) can be used to describe the adsorption over heterogeneous surface of herbicides (Guo and Wang 2019).
$$\log Q_{e} = \log kf + \frac{1}{n}\log C_{e}$$
(11)
KF in the Freundlich’s equation describes adsorption capacity constant. The adsorption intensity’s empirical parameter can be defined by 1/n. From this study, it is clear that the capacity adsorption can depend on major factors including the nature of adsorbent and the type of herbicide used. The calculated values of RL for the two herbicides are given in Table 1 which shows PE and PQ adsorption onto ACBNGS surface is favorable. As tabulated in table, values of n ranged between 0 and 1 qualifying a favorable adsorption.
When the surface coverage is related to the adsorption free energy during the interaction of the herbicides with the adsorbent surface, we consider the isotherm described by the Temkin model. In addition, the heat of adsorption associated with the herbicide molecules on the layer is found to decrease proportionately because of adsorbent–adsorbate interaction. This type of adsorbate–adsorbent interaction is associated with uniformly associated binding energies, to reach a maximum value (Al-Saad et al. 2019). This statement is as presented by the following relation in Eq. (12):
$$q_{e} = \frac{{{\text{RT}}}}{{b_{T} }}\ln A_{T} + \left( {\frac{{{\text{RT}}}}{{b_{T} }}} \right)\ln C_{e}$$
(12)
The Temkin constant B = RT/b is related to the heat of adsorption, the molar gas constant is R (J/mol K), the temperature is T(K), the adsorption energy variation is b(J/mol) and the binding constant at equilibrium is bT (L/mg) which is equivalent to binding energy at maximum value (Araujo et al. 2018). Table 1 contains the summary of all parameters calculated for Temkin isotherm. From Table 1, both bT and B values for PE and PQ were reported as 223.81 and 11.07, respectively. The regression equation and R2 value observed for this model favorably described the adsorption process of the herbicides onto ACBGNS, across the experimental variables applied. When the values of the correlation coefficient, R2, were compared for the three tested isotherms, relatively, the Langmuir model fitted better among others for the adsorption data of herbicide onto ACBGNS.
Kinetic study
The kinetic studies of systems involved in adsorption describe how fast the adsorbate uptake onto the adsorbent is which controls the amount of equilibration time. The adsorption kinetic curves of the PE and PQ molecules on ACBGNS were obtained by subjecting the experimental data obtained to three models of kinetics comprising pseudo-first and second-order and intraparticle diffusion.
-
(a)
Kinetic model of the pseudo-first-order
Equation (13) can be used to define the model (Benkaddour et al. 2018).
$${\text{log }}(q_{e} - q_{t} ) = {\text{log}}(q_{e} ) - \frac{{K_{1} }}{ 2.303}t$$
(13)
k1 is the rate constant and qe is the amounts (mg/g) of PQ while qt is that of PE adsorbed at equilibrium and time t, respectively. log (qe − qt) was plotted against t, and values associated with k1 were evaluated from the graph’s slope. R2 values, calculated and experimental qe values obtained for PE and PQ adsorption onto ACBGNS are presented in Table 2. R2 was observed to be very low coupled with a reasonable difference established between the adsorption capacity (qe) values for experimental and calculated, showing failure in experimental data interpretation.
-
(b)
Kinetic model of the pseudo-second-order
This type of second order has been widely to predict kinetics of adsorption processes and the expression is as in Eq. (14) (Hameed et al. 2009).
$$\frac{t}{{q_{t} }} = \frac{1}{{K_{{2 q_{e}^{2} }} }} + \frac{1}{q}_{e} \left( t \right)$$
(14)
Adsorption rate constant is given by k2 (g/mg min), while t/qt values were plotted against t (figure not shown). According to the kinetic model of the pseudo-second order, parameters of adsorption of herbicides onto ACBGNS are given in Table 2. From the results, parameters obtained were found to fit the equation with the R2 values close to unity. There was good correlation between the calculated and experimental values. The use of this model in adsorption process describing the adsorption of PE and PQ herbicides onto ACBGNS was proved based on such comparison.
-
(iii)
Intraparticle diffusion model
Weber–Moris Eq. (15) predicts the possibility of the sole mechanism through the intra particle diffusion model (Raul et al. 2011).
$$q_{e} = C + K_{{{\text{int}}}}^{{t{1}/{2}}}$$
(15)
The constant kint (mg/g min 0.5) describes rate of diffusion, while the boundary layer thickness is defined by C. If the rate-limiting step is only through intra particle diffusion, then a linear plot would be obtained when qt versus t1/2 is extrapolated to the origin. If not, there is involvement of other mechanism. The values of the constant C from Table 2 were found to 15.74 and 11.28 for PE and PQ, respectively. It can be concluded that PQ and PE adsorption by the ACBGNS cannot be predicted by this model.
Thermodynamic adsorption study
Notably parameters obtained from thermodynamic evaluations give vital information about the adsorption nature with respect to feasibility and spontaneity of the process. Temperature effects associated with PQ and PE adsorption onto ACBGNS were studied by temperature variation from 303 to 333 K while keeping all other parameters (pH = 6 and 8, respectively, for PE and PE, adsorbent dosage 0.2 g, initial PQ concentration = 60 mg/l, t = 80 min) at optimized values. The thermodynamic constants for adsorption which can be used to describe the spontaneity and feasibility of an adsorption process are described entropy (∆S), free energy (∆G) and enthalpy (∆H) changes. Equations (16) and (17) describe them thus:
$$\Delta G = - {\text{RT}}\ln K_{{\text{c}}}$$
(16)
$$\ln k_{{\text{c}}} = - (\Delta H/{\text{RT}}) + (\Delta S/R) = - \Delta G/{\text{RT}}$$
(17)
the temperature being T(K), the gas constant, R, 8.314 J/mol K and kc the constant of equilibrium which can be obtained from the relation in Eq. (18) (Calvete et al. 2010).
$$k_{{\text{c}}} = C_{{\text{a}}} /C_{{\text{e}}}$$
(18)
where Ca is the adsorbed herbicide in mg/L and the equilibrium herbicide concentration is Ce (mg/L). The slope from the van’t Hoff’s plot and intercept and the values of ∆S and ∆H were evaluated, and Table 3 contains all the calculated values. ΔH values which were found to be positive show that an endothermic process is involved fort the adsorption of the herbicides onto ACBGNS. The positive value of ΔS indicates high randomness at PQ and PE molecules adsorption interphase onto ACBGNS (Gunay et al. 2007). Physical adsorption has values in the range of 2.1–20.9 kJ/mol, which describes the type of adsorption for both herbicides involved in this research, while higher range of 80–200 kJ/mol can be used to qualify chemisorption mechanism. ΔG values which are negative show the adsorption is thermodynamically feasible and spontaneous (Mustapha et al. 2019).
Computational study
Quantum chemical parameters of PE and PQ molecules calculated are presented in Table 4 which shows smaller energy gap (∆E) of PE as compared to PQ. This gap in energy (ΔE) can be used to describe softness and hardness of a molecule. Lower ΔE (and vice versa) describes soft molecules with tendency of better adsorption compared to hard molecules. Therefore, lower values of ΔE correlate well with the ease of adsorption of molecules onto an adsorbent. Eigen values of EHOMO and ELUMO as presented in Table 4 describe that PQ herbicide adsorption is higher than that of PE which may be due to the presence of chloride ion in solution and lone pairs of nitrogen increase its activity which is in good agreement with that earlier reported here experimentally. Relationship between adsorption and dipole moment is yet to be fully established in literature with most adsorption processes reported to favor lower μ as shown in Table 4. The μ for PQ is lower than that of PE and therefore better adsorbed. The adsorption ability of a chemical compound can be controlled by its binding energy (EB), surface volume (SV), surface area (SA). The higher the values of SV, SA, EB obtained, then the process of protonation, hydration and subsequent adsorption on a surface for a given molecule becomes more difficult. Therefore, lower values of these parameters associated with PQ herbicide show that it is better protonated, hydrated and adsorbed on ACBGNS than PE herbicide.
In the presence of charge atmosphere, polarizability can be defined as a measure of the behavior of molecular species. Tendency of the ease of a molecule to be adsorbed is related to better response. This explains why PQ herbicide is better adsorbed on ACBGNS than PE herbicide. Better adsorption of PQ is also favored by their SV and SA values which are low. It is established that molecules compete with difficulty to fit into the adsorption matrix of the adsorbent when the values of surface area and volume of the adsorbate are large.
An index, the function of Fukui can be used to assess the local reactivity (hence adsorption) of a molecule on the basis of the constituent atoms which are electronic in nature (Alokdut et al. 2017). Based on electrophilic and nucleophilic nature of the chemical reactivity of a molecule, Fukui indices provide useful information about the reactive centers (Mohan and Joseph 2018; Zohdy et al. 2021). Threshold values of \({f}_{k}^{+}\mathrm{ and} {f}_{k}^{-}\) control nucleophilic and electrophilic centers of in a given molecule. PE and PQ herbicide’s Fukui functions are shown in Table 5. F− for PE and PQ molecule have their highest Mulliken and Hirshfeld charges on N(1) and H(13), C(5) while for the nucleophilic (F+), PE herbicide on O(17), O(18) and PQ herbicide are on the aromatic ring N(5) shown in Table 5. Therefore, the adsorption of PE on ACBGNS occurs through the nitrogen atom while that of PQ occurs via the aromatic ring of the molecule. Optimized geometry, Frontier orbital diagrams of PE and PQ herbicides are reported in Fig. 7. Within the diagrams, red loop shows positive sites while negative charges are displayed in blue loops on the molecules.