2.2. Characteristics of the extracted lignin.
Elemental analysis
Due to the varied structure of lignin, which varies depending on the source and geographical and climatic conditions, it is an interesting resource for industry. The elemental analysis by CHN elemental analyser and EDX of the extracted FLA was shown in Table 1 and Fig. S1, respectively. As expected, the high carbon and oxygen concentrations of FLA are coupled with traces of inorganic materials, as shown in Fig. S1. The carbon composition in extracted FLA is over 45 % (46.5 for elemental and 45.37 for EDX) which is in agreement with the literature.20 Because of the functional groups attached to the carbon, the adsorption effectiveness is increased due to the high carbon and oxygen concentration.
Table 1
Elemental composition of the extracted FLA by CHNs.
Element
|
Composition (wt.%)
|
C
|
46.5
|
H
|
4.7
|
N
|
0.3
|
S
|
3.2
|
O (by difference)
|
45.3
|
C/H
|
9.8
|
Lignocellulosic substances
The extractive components are of interest, according to the procedure mentioned in Sect. 1.2 in Supplementary information. Four substances were studied and the composition of FLA is presented in Table 2. FLA contained a larger part of lignin (64.8 %) than cellulose (24.16%) with small amount of ash and hemicellulose.
Table 2
Composition of hemicellulose, cellulose, lignin and ash.
composition
|
Mass (gram)
|
Percentage (%)
|
Hemicellulose
|
0.0736
|
7.36
|
Cellulose
|
0.2416
|
24.16
|
Lignin
|
0.6480
|
64.80
|
Ash
|
0.0368
|
3.68
|
Determination of acidic surface functional groups
Table 3 presents the results of Boehm's acid-base titration. According to the literature, after modification by H2SO4, the oxidation of the produced lignin has mainly high carboxyl content, resulting in a more hydrophilic and acidic lignin.21 Simultaneously, the surface of the basic group was found to have a lower possibility than the acidic surface group, indicating that increasing oxygen content lowers the electronic density of basal lines and, as a result, lowers the Lewis types of basic sites associated with the -electron-rich region found on basal lines .22
Acidic functional groups attached with the positive charges of dye by adsorption. As a result, several scientists have attempted to enhance the overall number of surface functional groups that are favorable for dye adsorption.
Table 3
Surface functionalities concentration on FLA.
Functionalities
|
Concentration (mmol / g)
|
Surface acidity
|
5.00
|
Carboxylic
|
1.25
|
Lactonic
|
0.15
|
Phenolic
|
3.60
|
Surface basicity
|
0.05
|
Point of zero charge (pHpzc)
The pHpzc is an essential physicochemical factor for FLA, and it can help estimate the surface charges of the RG-19 dye at various pH levels. As a result, the pHpzc was calculated and depicted in Fig. 1 (a). The pHpzc value was found to be at pH = 5.64. FLA's surface becomes neutral at this pH. On the other hand, the surface of FLA in acidic solution (i.e. pH 2 to 5) has positive values. The hydroxyl groups of FLA are present in FLA-OH2+ form at this pH range. The production of FLA species causes the surface charge to become negative as pH increases from 6 to 9. The surface of FLA becomes positively charged at lower pH (pH < pHpzc) and attracts negatively charged RG-19 species by electrostatic interaction which ultimately enhances the separation efficiency.
Thermal stability
The thermal stability of FLA sample is evaluated by TGA, and the derivative of mass loss, DTG curves that shown in Fig. 1 (b). Lignin thermal degradation happens slowly across a large temperature range, from ambient to 900 oC, which an agreement to the literature .23 This effect is caused by lignin's complicated structure, which includes Cα-Cβ-Cγ side chains on the aromatic ring as well as a variety of functional groups .24
For lignin identification, there are three stages of degradation. The weight loss reached 9.87% in the first stage, when the temperature reached 100 ºC, due to the valorization of water in the lignin. The weight loss in the second degradation stage till 290 ºC was 22.09 %, due to the decomposition of low molecular weight, such as CO2, CO, and CH4 that resulted from degradation of methyl-aryl ether groups in FLA .25 The third degradation stage occurs at temperatures ranging from 290 to 900 ºC, with a weight loss of 57.59 % due to decomposition reaction and condensation processes of aromatic rings.25 Breakage of methyl-aryl ether bonds is observed at temperatures below 400°C, although condensation or breakdown of aromatic rings occurs at temperatures between 400 and 600°C (weight loss; 20.71 %). So, up to 400 oC, the deterioration is caused by the fission of methyl-aryl ether linkages.26 The aromatic structures were destroyed by the breakage of C-C bonds in the rings at temperatures exceeding 400°C. After heating to 800°C, all lignin samples remain unvolatized due to the production of a highly condensed aromatic structure.27 Because of the condensed structure of lignin, the char is roughly 10.45 % at the end of this test .28
In addition, Fig. 1 shows the curve of the derivative of lignin mass loss as a function of temperature superimposed on the TGA curve (b). There are multiple peaks on the DTG. According to the literature, the peaks following dehydration are related to the breakage of β-O-4 bonds, the breakdown of C-C bonds, the cleavage of β-β bonds and to condensation and polymerization reactions, and are relative to moderate to high temperatures .29 The first is related to dehydration at a very low temperature of 51 oC. Second, the relatively sharp peak peaks at 282 oC, which corresponds to the cleavage of the β-O-4 bond. The resolution and strength of the peak related to the β-O-4 breakdown bond are proportional to the proportion of these bonds in the lignin. The lignin cleavage C-C bonds may have caused the modest shoulder peak at 350 oC .26 As for the lignin in which the proportion of the β-O-4 bonds is higher than that of the C-C bonds has a fairly higher intensity than that of the peak relative to the C-C bonds. The peak at 748 ºC is related to the condensation and polymerization reactions.
The differential scanning calorimetry (DSC) curve of lignin is provided in Fig. 2(c) due to the difficulty of determining the glass transition temperature (Tg) of lignin due to the vast molecular weight distribution and heterogeneity of the lignin chemistry .30 The extracted lignin DSC curve revealed a significant endothermic activity with a maximum temperature of approximately 115 oC, which is associated to lignin degradation. The Tg of FLA is 96.67 ºC. Tg values in the literature do not surpass 150 oC .31 As a result, the low value of Tg found in our study could be attributed to the low molecular mass of lignin in FLA.
Particle size analysis
Figure 1 (d) shows the particle size distribution (PSD) of the extracted FLA. This figure showed unimodal size distribution with average particle size was about 156 nm.
Textural characteristics
BET analysis was used to evaluate the textural features of FLA. The FLA has a surface area of 1.055 m2/g, a total pore volume of 1.5612×10− 2 cm3/g, and a pore diameter of 59.191 nm, according to the findings. The nitrogen sorption isotherm of FLA, according to the IUPAC classification, has a type IV form with an H3 hysteresis loop and substantial N2 adsorption at P/P0 > 0.9 (Fig. 1 (e)). It shows a plot of FLA's BJH pore size distribution, which indicates that the particles are basically mesoporous (data not shown) .32 FLA's high pore capacity increases internal mass transfer, which improves adsorption properties and improves the interaction between FLA and the RG-19 dye.
2.3. Removal of RG-19 dye by FLA.
It shows a plot of FLA's BJH pore size distribution, which indicates that the particles are basically mesoporous (data not shown) .32 FLA's high pore capacity increases internal mass transfer, which improves adsorption properties and improves the interaction between FLA and the RG-19 dye. The RG-19 dye molecule has two primary bands: one at 675 nm, which is due to the azo bond chromophore, and another at 350 nm, which is due to the aromatic rings in the dye molecule.
The broad band at λmax = 675 nm is responsible for the green color due to the π-π* transition of the hyper-conjugated system connected by the two N = N groups. Another band (λ = 350 nm) in RG-19 dye molecule can be attributed to the n-π* and π-π* transitions related to the aromatic ring attached to the –N = N– group and –N = N–.33 A considerable drop in the absorption peak at 675 nm was detected during the adsorption process, indicating the loss of conjugation and, as a result, color elimination.
An exploratory adsorption study by FTIR and SEM
FTIR spectroscopy can help the creation of the proposed interaction hypothesis by comparing the respective spectra before and after adsorption (Fig. 2 (a)). By comparing the respective spectra before and after adsorption (Fig. 2 (a)), FTIR spectroscopy can aid in the design of the proposed interaction hypothesis. The FLA sample exhibited basic absorption peaks of lignin, indicating that the skeleton structure of lignin had been successfully formed. For example, (i) the –OH stretching vibrations of phenolic and aliphatic groups, as well as hydrogen-bonded stretching vibrations, are attributed to a broad band at 3433 cm− 1, whereas the peaks observed at 2904 cm− 1 are attributed to the stretching vibration of C-H bands assigned to aromatic methoxy groups, methyl, and methylene groups of the side chain; 34 (ii) a stretching vibration band for -C = C- in the aromatic ring at 1661 cm− 1 ; 17 (iii) symmetric bending of ‒O‒CH3 at 1434 cm− 1; (v) C‒O‒C stretching in α-O-4 and β-O-4 linkages of FLA at 1268 cm− 1; 35 and (vi) a peak at 1073 cm− 1 referring to the characteristic of C-O stretching of carbohydrate material. 31 These bands clearly show that the functionalized lignin generated is nearly pure and free of polysaccharide contamination (Fig. 2 (a) A). As shown in Fig. 2 (a)B, all of the prior peaks were displaced and/or their intensity was reduced after adsorption, indicating an effective interaction between FLA and RG-19 dye and that the structure of lignin remained mostly unchanged during the adsorption process.
When comparing the FTIR spectra of FLA before and after adsorption, the significant differences in the FTIR spectrum are the intensity of the typical bands changing, as well as the emergence of eight additional bands at 2842, 1605, 1509, 1361, 1129, 842, 610, and 462 cm− 1 (Fig. 2(a) B). These changes could be the result of interactions between the functional groups at FLA and the RG-19 dye molecule, or they could be connected to the RG-19 dye's structure. In details, these new bands which are as follows; (i) The symmetrical stretching vibration of aliphatic –CH3 and –CH2 was assigned to the peak at 2842 cm− 1; 36 (ii) The RG-19 dye has many functional groups, which peak at 1605 cm− 1 (corresponding to –C-O aromatic, carboxylic, and alcoholic groups), while the symmetric C–O stretching or secondary aromatic amines peak at 1509 cm− 1; 37 (iii) The stretching vibration of C-N and the C-NH2 in-plane and out-of-plane bending modes of the interaction between FLA and RG-19, which are seen at 462 cm− 1 in IR, are attributed to the peak at 1361 cm− 1. 32 (v) The peak at 1129 cm− 1 for asymmetric C-O stretching [37]; (iv) The broad band at 610 cm− 1 is the characteristic absorption of –CH2Cl group and/ or out-of-plane bending of the –OH group; 30 and (iiv) The absorption peak at 842 cm− 1 is the out of plane bending vibration peak of N-H bond or = C-H bond of the benzene ring after tetra-substituted at 1,2,3,and 5 positions in RG-19 dye. 16
Another bands related to the structure of RG-19 without any interaction with FLA like (i) The peak at 1462 cm− 1 is assigned to the stretching vibration absorption of the C − Cl bond and in-plane bending vibration absorption of the C − H bond in the − CH2Cl group; (ii) the characteristic bands at 1268 cm− 1 may be attributed to the stretching vibrations of SO3H groups or amide group; 16 (iii) the peak represented at 610 cm− 1, after adsorption, was observed to be attributable to the sulphonic groups that did not contribute the adsorption process.
After adsorption, he bending vibration of –NH in the primary amine group from RG-19 dye caused the greater intensity of the peak at 1647 cm− 1. 37 The π-electron cloud moving with the bands originating at wavelengths of 1647 and 842 cm− 1 also affects the aromatic ring stretching. Furthermore, after RG loading, changes in FLA surface features, such as shifts in functional group bands and changes in all peak heights, suggest that groups such as hydroxyl and amine groups are involved in the adsorption process as expected. Peak shifts in the aromatic groups on the lignin surface were found, which were attributable to RG-19 adsorption. As a result, the modification occurred only in the side chains of the lignin, rather than in the core structure. These findings demonstrated that the RG-19 dye molecule was adsorbed on the FLA surface. However, there were some noteworthy changes in RG-19 adsorption in the regions 3407, 1045, and 400–600 cm− 1, which could be due to differences in RG-19 electron distribution.
The surface morphology of FLA before and after adsorption is shown in Figs. 2 (b) and (c). The SEM revealed the significant differences in surface topography between FLA and FLA-loaded RG-19. The activation process has modified the quantities of available of functional groups and their interactions, resulting in this deviation in the surface morphology. In details, the extracted lignin's surface is linked to the stacked structure of fragments with visible holes and spaces. This could be owing to the water-absorbing groups on lignin's surface, such as the hydroxyl and amine groups. The predominant porous structure is responsible for the high developed surface area. Moreover, the homogenous porous structure contributes significantly the adsorption of dye. After adsorption, the shape did not change associated with the lower number of pores than before adsorption and the structure became denser (Fig. 2 (c)).
In conclusion, the FT-IR and SEM analysis confirm that dye adsorption occurs possibly through electrostatic attractions/surface exchanges until the functional sites are completely unavailable; RG-19 dye molecules then diffuse into FLA particles for additional interactions, possibly through π-π interaction and hydrogen bonding.
Effect of initial pH and its relationship with point of zero charge.
The adsorbent FLA's adsorption potential is particularly susceptible to the changes in the pH of RG-19 because it is influenced by the chemical structure of FLA and RG-19, the surface charge and hydrolysis of FLA, degree of ionization, and the speciation of RG-19. Shifts in the pH affect the total charge on the surface of FLA which affect its’ interaction with RG-19 dye.
Figure 3 (a) shows the rejection efficiency at different pH values covering a range from 2 to 9. Generally, FLA displayed the highest adsorption potential at the lowest pH of 2 and steadily declined in adsorption capacity before pH 9, indicating the lowest removal of RG-19 dye. A point of inflection occurred between pH 6 and 7 where the adsorption capacity reached a second (smaller) peak and then began to decrease again from pH 7 to pH 9.
Around pH = 2, the highest separation effectiveness of RG-19 dye onto FLA was observed. Because the protonated sulfonate groups have a pKa value less than zero, the RG-19 dye stays negatively charged at basic circumstances and even at strongly acidic conditions. 38 At a molecular level, it is reasonable to infer that the functional group –NH2 or –NH- of RG-19 interacts with the O atom of FLA via hydrogen bond interaction. 34 The ionization of amine breaks the H-bonding because the amino groups of RG-19 are protonated at low pH and deprotonated at high pH. In contrast, when the pH approaches neutral, more of the –NH2 and –NH– groups are released from protonation, resulting in the strongest hydrogen bond between the amino groups of RG19. As a result, pH changes have a significant impact on adsorption capabilities. The protonation/deprotonation processes of the RG-19 amine groups with various pHs are represented by Eqs. (1)–(2).
RG-19-NH-C-NH2 + 2 H+ ↔ RGD-NH2+-C-NH3+ (1)
RG-19-NH2+-C-NH3+ + 2 OH− ↔ RGD-NH-C-NH2 (2)
The explanation for the lower rejection capabilities at higher pH was that the constant ionized RG-19 and the high OH- ions competed heavily for FLA adsorption sites. 39 However, as the pH reduced, the decline OH- ions weakened the competition, allowing more ionized RG-19 to be taken up.
The adsorption capability decreases when the pH level rises from 7 to 9. This could be because FLA's carboxyl and hydroxyl groups provide electrostatic repulsive force, preventing FLA and RG-19 from forming, resulting in limited rejection capability. The carboxyl and hydroxyl ionization of FLA results in improved the separation efficiency when the pH is less than 6. As a result, the significant variation trend under pH impact could be attributed to increased electrostatic attraction between the FLA and the RG-19.
The ionizable group and charge density in the molecular structure of the FLA as an adsorbent may be linked to the influence of pH on adsorption performance. This behavior can also be characterized in terms of pHpzc (Fig. 1(a)), where any pH above 5.64 results in a net positively charged surface of FLA, while any pH below 5.64 results in a net negatively charged surface. The maximum adsorption capacity at pH = 2 insinuates that the adsorbents highest affinity for the dye occurs when the surface of the adsorbent is primarily negatively charged which coincides with the dye itself being predominantly positively charged.
In an aqueous solution, RG-19 molecules dissociate and obtain negative charges, according to their structure. As a result, at pHpzc, more H + is available, causing FLA chains to absorb more H+, increasing the electrostatic contact of RG-19 molecules with FLA chains. The sulfonic acid groups of RG-19 are protonated when the pH is less than 5.64, resulting in a reduction in the negative charge of the anionic groups of these anionic dyes and altering the electrostatic adsorption of dyes by FLA. Because the FLA chains are negatively charged at pH > pHpzc, increasing electrostatic repulsion between electronegative RG-19 species and FLA particles would result in a decrease in rejection capability of RG-19. The charge of FLA chains is virtually nil at neutral conditions, while the hydrogen bond interaction between FLA and RG-19 rose considerably from neutral to positive charges.
Effect of various salts on the removal of RG-19 dye.
Figure 3 (a) shows that the behavior of the pH effect varies, indicating that other mechanisms beside the electrostatic interaction are involved in the adsorption of RG-19 dye onto FLA. The effect of ionic strength on the removal of RG-19 dye was explored by adding various salts (NaCl, LiCl, CaCl2, and Na2SO4) with the findings given in Fig. 3 (b). The type of anions and cations has a significant impact on the separation efficiency of RG-19 dye.
Generally, the rejection capability of RG-19 dye declined with addition of various inorganic salts. This could be because FLA's active sites are inhibited, making it difficult for RG-19 dye molecules to combine with FLA's surface. As a result, the removal rate of RG-19 dye is decreasing. 40 This investigation is supporting the strong contribution of electrostatic interaction mechanism of this adsorption process. On the other hands, the increasing of ionic strength leads to the decreasing the removal efficiency of RG-19 dye that mainly because the increasing of pH.
Figure 3 (b) also shows the influence of various cations and anions on the removal efficiency of RG-19 dye. It depicts the effect of various cations and anions on the rejection capability of RG-19 dye. The rejection rate of RG-19 dye in the presence of NaCl is higher than that of Na2SO4 in the same ionic strength, as for the different anions. While for various cations, the adsorption efficiency is much higher of adding NaCl than adding LiCl. This could be owing to the surface characteristic of FLA changing in the order Na+ > Li+. Na+ has a hydrated radius of 2.76 Å, which is smaller than Li+'s hydration radius of 3.4 Å. This explains why RG-19+ and Na+ are more competitive. 35
Furthermore, the separation efficiency of RG-19 in the presence of CaCl2 (2 − 1) is lower than that of NaCl (1–1) at the same ionic strength for varied valence of cations. When it comes to anions, the higher valence salt (Na2SO4) has a significantly lower removal rate than the lower valence salt (NaCl). This could indicate that the valence of cations and anions has a favorable impact on the rejection capability of RG-19 dye. As a result, more specific investigations must be conducted.
Effect of dose
The adsorbent dose determines FLA's ability to remove RG-19 dye. Figure 3(c) depicts the influence of FLA dosage on the extent of RG dye elimination. The separation efficiency of dye was increased linearly with the dose of FLA, which could be due to the enhanced surface area and availability of more active sites available for adsorption and hydrogen bonding. 41 The dye removal reached 66% at 0.5 g/L and increased to 72% with 1.5 g/L of FLA. The slight increase in the separation efficiency is due to the remaining unsaturated adsorption sites being filled during the adsorption process. 35
Effect of contact time and adsorption kinetics
Figure 4 (a) shows percent removal of the dye over the span of 4.5 hours. It was found that the adsorption of RG-19 onto FLA proceeds into two steps; the first step was the fastest 40 min., and then the second step was the growth rate became slow until the adsorption equilibrium was reached at about 1.5 hours. The large number of effective sites on the surface of FLA could explain the rapid separation via adsorption. 42 In details, the interaction of RG-19 dye molecules with the vacant adsorption sites of the FLA surface was the first stage in the adsorption process. Besides, the electrostatic interaction between hydroxyl and carbonyl groups was most likely the primary cause of the fast adsorption. 35 The lack of significant changes of the second step may likely be due to the surface of FLA having reached a point of saturation of the major pore spaces. The portion in the graph does show small increases in percent removal after 40 minutes, this may be explained such that rapid removal is a consequence of the occupation of major pore spaces and slow removal is a consequence of the slow filling of minor pore spaces.
Different kinetic models, such as the pseudo first order kinetic model (PFO), pseudo second order kinetic model (PSO), Elovich model, and intraparticle diffusion model, have been used to elucidate the potential rate-controlling steps and predict the adsorption mechanism based on the data obtained from these experiments. The kinetics of RG-19 dye onto the extracted FLA were described using the linear forms of these models (Table S-2). As can be observed, PSO (Fig. 4 (c)) has a larger R2 value than PFO (Fig. 4 (b)), indicating that the adsorption process is dominated by chemical adsorption rather than diffusion. In addition, when comparing the estimated values of adsorption capacity to the experimental data, the PSO kinetic model was shown to have the most satisfactory linear fitting. It meant that chemisorption was the controlling stage, as evidenced by the R2 values of the Elovich model (Fig. 4 (d)).
The RG-19 dye adsorption mechanism on FLA can be divided into four major steps; (i) RG-19 molecule transport from the bulk solution to the surface of FLA by bulk diffusion through the boundary as a result of random molecular motion of the individual molecules (ii) film diffusion; (iii) intra-particle or pore diffusion of the FLA (iv) chemical reaction or complex formation. As a result, it appears that the mechanism of RG-19 absorption by FLA involves three stages, each of which is linked to three steps (ii), (iii), and (iv) (iv).
Although the PSO model has the best matched order, identifying other controlling steps such as mass transfer or intraparticle diffusion using the PSO model is extremely difficult. As a result, the PSO results were unable to determine the process of RG-19 diffusion mechanism onto FLA. The trend did not pass through the origin in Fig. 4 (e), showing that intraparticle diffusion was not the only rate-limiting phase and that the adsorption process is governed by multiple steps. Figure 4 (e) depicts the multi-stage adsorption of RG-19 into FLA using multi-linearity. The larger the intercept, the greater the contribution of surface adsorption in the rate-limiting step, is as shown in Table 4. Furthermore, increasing kint along with increasing RG-19 concentration enhances RG-19 intra-particle diffusion onto FLA.
From Table 4, it was observed that larger the intercept, the greater the contribution of the surface adsorption in the rate-limiting step. Furthermore, increasing kint along with increasing RG-19 concentration enhances RG-19 intra-particle diffusion onto FLA. The adsorption rate is affected by two intra-particle diffusion mechanisms: one is diffusion within the pore volume, known as pore diffusion or intra-particle diffusion, and the other is diffusion along the surface of the pores, known as surface diffusion. The rate-limiting step in the first phase could involve surface diffusion rather than intra-particle diffusion.34 This is because the thickness of the boundary layer, as shown by the value of a (Table 4), which was greater than zero and denoted the RG-19 dye adsorption, can be influenced to some extent by boundary layer diffusion.
Table 4 shows the values of the approaching equilibrium factor, Rw, in the adsorption of RG-19 into the FLA. As the concentration was enhanced from 20 to 80 ppm, Rw values lowered from 7.6 10− 6 to 1.85 10− 5, respectively. These values, according to literature, 43 were in zone III, under the pseudo-rectangular kinetic curved, and drastically close to equilibrium. A similar observation was made for the removal of dye using pretreated FLA. 44 This meant that the equilibrium strategy would be more effective in rejection RG-19 from solution.
Table 4
Estimated constants of the kinetic parameters of RG-19 dye on FLA.
Kinetic Model
|
|
|
20
ppm
|
40
ppm
|
50
ppm
|
60
ppm
|
80
ppm
|
Pseudo-first-order (PFO)
|
R2
|
0.9948
|
0.9985
|
0.9937
|
0.9959
|
0.9920
|
qe−estimated (mg/g)
|
3.335
|
6.408
|
7.460
|
9.618
|
12.44
|
qe−experimental (mg/g)
|
3.48
|
6.83
|
7.74
|
9.85
|
3.0
|
K1 (min− 1)
|
0.9893
|
1.792
|
2.21
|
2.823
|
3.701
|
Pseudo-second-order (PSO)
|
R2
|
0.9976
|
0.9987
|
0.9965
|
0.9987
|
0.9956
|
qe−estimated (mg/g)
|
3.436
|
6.68
|
7.686
|
9.953
|
12.81
|
qe−experimental (mg/g)
|
3.48
|
6.83
|
7.74
|
9.85
|
13.0
|
K2 (g mg− 1 min− 1)
|
17.96
|
13.12
|
18.04
|
14.72
|
19.79
|
Elovich
|
R2
|
0.9022
|
0.9174
|
0.8941
|
0.9177
|
0.7732
|
α × 107 (mg/g min)
|
3.377
|
1.671
|
4.937
|
2.263
|
24.320
|
β (g/mg)
|
5.948
|
1.874
|
2.598
|
1.908
|
1.646
|
Intraparticle-diffusion
|
R2
|
0.963
|
0.922
|
0.842
|
0.846
|
0.961
|
a (mg/g)
|
2.86
|
4.94
|
6.41
|
8.46
|
10.38
|
Kint
(mg/g min0.5)
|
0.313
|
0.985
|
0.711
|
0.763
|
1.39
|
Rw
|
7.6 × 10− 6
|
5.31×
10− 5
|
3.41×
10− 5
|
3.28×
10− 5
|
1.85×
10− 5
|
Effect of initial dye concentration and equilibrium isotherms.
The concentration of RG-19 dye decreases with the removal efficiency increasing as shown in Fig. 5 (a). So, the dye adsorption per unit mass of adsorbent will decrease. It might be due to the excess active sites and the strong electrostatic attractive, chelating force for mass transfer. 15
As demonstrated in Fig. 5, the concentration of RG-19 dye lowers as the separation efficiency rises (a). As a result, dye adsorption per unit mass of adsorbent will decline. It could be because of the large number of active sites and the strong electrostatic interaction and chelating force for mass transfer.
The isotherm is a vital aspect to discuss the relationship between the adsorption capacity and concentration when an equilibrium conditions are met. Also, the adsorption isotherms are important for further study the adsorption mechanism. The experimental data from batch adsorption were analyzed in the light of six well-known models: Langmuir, Freundlich, Temkin, Dubinin-Radushkevich (D-R), Harkins-Jura (H-J), and Generalized adsorption isotherm models to gain an insight into the separation behavior of RG-19 dye onto FLA and gain the optimal fitting of theoretical model. The linear plotting of these isotherms and their plots are given in Table S-2 and Fig. 5. In Table 5, the respective parameters of these isotherm models are listed.
As evidenced by their strong correlation coefficient values in Table 5 and Fig. 5, the Langmuir adsorption isotherm satisfactorily represented the sorption of RG-19 dye from aqueous solutions onto FLA at temperatures ranging from 20 to 50 oC. (b). When there is no transmigration of RG-19 in the plane of the surface or in the inner surface of the FLA, Langmuir assumes equivalent energies of adsorption onto the surface. With increasing temperature, the Langmuir parameters qm and KL drop continuously. The essential features of the Langmuir isotherm may be expressed in terms of equilibrium parameter, which is a dimensionless constant referred to as separation factor RL.
Where Co is the initial concentration and RL indicates the trend of the isotherm. All of these values are between 0 and 1, showing that the adsorption process is beneficial, especially at higher temperatures (Fig. 5 (c)). The high adsorption with a predisposition to chemical adsorption can be explained by lower RL values. 45 The adsorption of RG-19 dye in water was aided by the uniformity on the surface or pores of FLA.
The Freundlich isotherm was also followed to a smaller extent than the Langmuir isotherm, and homogeneous adsorption rather than heterogeneous adsorption occurred. From Table 5 and Fig. 5 (d), as the temperature increases, the values of heterogeneity parameter 1/nf decreased and the adsorption was predicted to be more favorable is in the agreement with the interpretation of the RL values. 46 Also, the constants Kf and nf vary as the temperature rises, reflecting the empirical finding that the quantity adsorbed rises more slowly and larger pressures are necessary to saturate the surface.
The adsorption capacity of RG-19 dye is in good agreement with the Langumir rather than Freundlich model. This suggests that the RG-19 dye's adsorption behavior on the surface of FLA is homogeneous, and that the adsorption may be classified as a monolayer with high adsorption capacity, allowing it to be used as a low-cost adsorbent for the removal of organic contaminants from water.
According to Temkin isotherm, the values of R2 in the range of 30 to 50 oC are within 0.98–0.99 (Table 5), indicating that the adsorption system based on heat and the adsorption process were characterized by a uniform distribution of binding energies. The binding capacity is higher at higher temperatures and heat of adsorption increases as the temperature increase, indicating strongly exothermic reaction. The maximum binding energy for interactions between FLA at different temperatures was found to be 1.334–2.499 L mg− 1 according to the same model.
Dubinin–Radushkevich (D-R) isotherm is a term used to describe a physical or chemical adsorption mechanism on a heterogeneous surface with a Gaussian energy distribution. 47 The D-R model's constant can be used to compute the average free energy E (kJ mol− 1) of the adsorption process, the formula is as follows:
The predicted qm increases with increasing temperature conditions. The average free energy (E) showed a decrease in free energy when adsorption occurred at higher temperature conditions. According to the data from Table 5 and the linear plot of D-R model in Fig. 5 (f), qmax was ranged from 2.7 to 6.9 mg/g, the mean free energy ranged from 1.029 to 0.154 KJ/mol when increasing from 20 to 50 ºC. The values of R2 are in the range 0.822–0.888 which is lower than that of Temkin. Furthermore, the mean free energy, E, is less than 8 kJ/mole, indicating that the adsorption process can be classified as physical adsorption. On the basis of this information, it can be stated that physical adsorption will play a significant role in the adsorption of RG dye onto FLA.
According to Harkins-Jura (H-J) equation, 48 the values of R2 are located in the range of 0.81–0.88 (Table 5), which supports the multilayer adsorption of RG-19 onto FLA. The experimental data agree fairly with the H-J adsorption equation at all temperatures and this proves its validity to solute adsorption.
The values for the constants of the generalized adsorption isotherm shown in Table 5 were calculated using the slope and intercept of the best-fit line shown in Fig. 5 (h). It can be deduced that the generalized model fit well for RG-19 dye adsorption onto FLA.
Error analysis
In this work, Root Mean Squared Error (RMSE) has been determined. RMSE is the square root of the mean of squared errors of actual output by Eq. 5
The statistical comparison of RMSE values in Table 5 revealed that the error of the Freundlich isotherm is much less than that of the Langmuir isotherm. This suggests that the Freundlich model is better suited to explaining the RG-19 adsorption process on the FLA surface than the Langmuir model. This represents the FLA surface's diverse distribution of active sites. The R2 value of the type linear and non-linear coefficients, on the other hand, firmly rejected the D-R model. Freundlich, H-J, and generalized isotherm models are used to estimate the lowest error values. As a result, these isotherms are thought to be the best for explaining the RG-19 dye adsorption equilibrium on the FLA surface.
Table 5
Adsorption equilibrium parameters for RG-19 dye adsorption onto FLA.
Isotherm Model
|
|
20 ºC
|
30 ºC
|
40 ºC
|
50 ºC
|
Langmuir
|
qm (mgg− 1)
|
29.586
|
25.974
|
19.493
|
19.048
|
b (L mg− 1)
|
0.0504
|
0.0312
|
0.0432
|
0.0378
|
R2
|
0.95
|
0.896
|
0.967
|
0.963
|
RL
|
0.2857
|
0.3175
|
0.3509
|
0.3922
|
|
RMSE
|
0.0355
|
0.0874
|
0.0732
|
0.0732
|
|
X2
|
0.9998
|
0.9999
|
0.9999
|
0.9999
|
Freundlich
|
Kf (mg/g)
|
1.116
|
1.032
|
1.141
|
1.022
|
|
nf
|
1.017
|
1.308
|
1.443
|
1.443
|
|
R2
|
0.922
|
0.972
|
0.967
|
0.969
|
|
RMSE
|
0.0799
|
0.0319
|
0.0331
|
0.0378
|
|
X2
|
0.9999
|
0.9999
|
0.9999
|
0.9999
|
Temkin
|
KT (L mg− 1)
|
1.334
|
2.500
|
2.255
|
2.499
|
B
|
4.661
|
4.727
|
4.143
|
3.974
|
R2
|
0.833
|
0.98
|
0.996
|
0.994
|
|
RMSE
|
1.299
|
0.376
|
0.154
|
0.182
|
|
X2
|
0.9332
|
0.9989
|
0.9989
|
0.9989
|
D-R
|
qmax (mgg− 1)
|
2.696
|
5.515
|
5.966
|
6.859
|
KD.R
|
0.472
|
1.115
|
1.135
|
1.308
|
ε (kJ mol− 1)
|
1.029
|
0.669
|
0.154
|
0.154
|
R2
|
0.888
|
0.839
|
0.822
|
0.837
|
|
RMSE
|
0.1378
|
0.1600
|
0.1710
|
0.1788
|
|
X2
|
0.9878
|
0.9505
|
0.9352
|
0.9485
|
H-J
|
A
|
10.256
|
7.485
|
7.943
|
7.348
|
B
|
1.168
|
0.853
|
0.905
|
0.837
|
R2
|
0.88
|
0.825
|
0.812
|
0.811
|
RMSE
|
0.0091
|
0.0165
|
0.0176
|
0.0191
|
X2
|
0.9932
|
0.9999
|
0.9999
|
0.9999
|
Generalized
|
KG (mg L− 1)
|
3.485
|
4.577
|
4.018
|
4.211
|
nb
|
0.899
|
1.017
|
1.024
|
1.015
|
R2
|
0.901
|
0.984
|
0.990
|
0.990
|
RMSE
|
0.0808
|
0.0323
|
0.0265
|
0.0267
|
X2
|
0.9982
|
0.9999
|
0.9999
|
0.9999
|
Effect of Temperature and adsorption thermodynamics
It is apparent that as the temperature rises, the rejection efficiency of RG-19 rises as well. Figure 6 (a) depicts the initial concentration and temperature. The amount of RG-19 adsorbed onto the FLA adsorbent increases as the temperature lowered, demonstrating that the adsorption is exothermic. This decline is associated with the decreasing the attraction forces between FLA and RG-19 dye.
The thermodynamic parameters are mainly to study the direction and limitation of the adsorption reaction. 45 The Van't Hoff plot of ln Kc versus 1/T, as shown in Fig. 6 (b) gives a straight line with acceptable correlation coefficient (R2). The slope and intercept of theses plots, the ∆Sº and ∆Hº can be estimated, respectively. Moreover, the activation energy, Ea, is calculated according to the following equation
Ea = ∆Hº + RT (6)
The calculated Gibbs free energy change (∆Gº), adsorption entropy change (∆Sº), adsorption enthalpy change (∆Hº) and Ea for sorption of RG-19 dye at conc. (50 ppm) onto the FLA at different solution temperatures are listed in Table 6. The decrease in KL values from Langmuir with increasing the temperature (Table 5) as well as the negative ∆Hº confirming the exothermic behavior of the adsorption of RG-19 onto FLA that resulted in the release of energy. In other words, when ∆Hº is between 2 and 40 kJ/ mol, the adsorption process is governed by physical forces such as van der Waals forces, dipole bond forces, hydrogen bonding, and/or coordination exchange. 49 The negative value of ∆Sº is indicating the adsorption of RG-19 dye onto FLA was an energetically more favorable ordered structure and the magnitude of energy increases with increasing the RG-19 dye concentration. Furthermore, the negative sign of ∆Gº and ∆Sº suggest the spontaneous nature of RG-19 adsorption onto the FLA and its feasibility for the process at low temperature. The Ea values in this investigation are less than 42 kJ/ mol, indicating that the diffusion process is the primary control for the separation of RG-19 dye by the extracted FLA at different temperatures. Also, it is indicating that the main mechanism is the external surface of the FLA with low possibility of the contribution of pore diffusivity that supported from the results from intra-particle diffusion model and also the literature. 45
Table 6
Thermodynamic parameters for the adsorption of RG-19 dye onto FLA.
Conc(ppm)
|
T(K)
|
∆G0(kJ mol− 1)
|
∆H0 (kJ mol− 1)
|
∆S0(J mol− 1 k− 1)
|
Ea (KJ/mol)
|
50
|
293
|
-2.9954
|
-9.8
|
-22.82
|
7.37
|
303
|
-3.01528
|
7.28
|
313
|
-2.77067
|
7.20
|
323
|
-2.29446
|
7.12
|
Adsorption mechanism
There are two types of adsorption between FLA and RG-19: physical and chemical. According to the kinetic models, the experimental data fit well with the PSO and the intra-particle diffusion. The RG-19 dye is first adsorbs to the outer and inner binding sites; subsequently, concentration differences are created between the solution close and far from the FLA's surface, driving the RG-19 ions to diffuse towards FLA. According to kinetic studies, the electrostatic interaction of the cations controls the adsorption process in the beginning. As the system approaches equilibrium, intra-particle diffusion limits RG-19 adsorption by FLA.
Because the adsorption process is quite complex and may involve multiple mechanisms, it is critical to understand the chemical adsorption mechanism [50]. At the interface between RG-19 molecules and FLA, the predicted primary interactions include electrostatic interactions, hydrogen bonding, and - stacking between aromatic rings. Taleb et al. 51 in a work similar to the present study, confirmed that these three types of interactions can occur in the adsorption of dyes by modified lignin. 51 The proposed adsorption mechanism is illustrated in Fig. 7.
The FT-IR analysis of the FLA indicated a number of oxygenated functional groups such as hydroxyl, carboxylic, phenolic, carbonyl, and ether groups, which are likely to have a role in the adsorption of RG-19 dye, as shown in Fig. 2(a). FLA has negative charges due to its multiple functional groups, which include alcohol, phenol, and ketone. Despite the fact that the RG-19 dye is an anionic molecule with an anionic form, the FLA's negative charged dissociated carboxyl and oxygen prefer a strong electrostatic connection with the RG-19 dye's positive charged nitrogen (N). Taking into account, the electrostatic interaction in neutral form may have a minor impact on adsorption, whereas the interaction between N in RG-19 dye and O in oxygenated functional groups in FLA is mostly driven by van der Waals forces and hydrogen bonds. 52
Furthermore, the aromatic charcter of the FLA surface suggests either electron donor acceptor (EDA) connections or cation-bonding that can be produced via conjugation in the rings of RG-19 dye molecules. 53 These numerous interactions support the premise of adsorption heterogeneity, implying the Freundlich, Harkins-Jura (J-H) isotherm model (Fig. 5 (d) and (g) respectively).
On the other hand, the existence of numerous coupled variables may play a role in the RG-19 dye adsorption process by the FLA developed in this study. As a result, a consortium process where surface adsorption and intra-particle diffusion can coexist is presented.
The carbonyl and hydroxyl groups were protonated, leading in additional positive charges, and the RG-19 dye displayed anionic forms; as a result, the electrostatic interaction between the positive charges on FLA and the SO3− present in the dye molecule occurred, as indicated in the impact of pH. Interestingly, Figure (2(a)) shows that after adsorption or combinative functionalization, the number of carbonyl groups used as adsorptive sites increased.
Furthermore, because the adsorption tests were conducted under conditions where the acidic pH was lower than the pHpzc (Fig. 1(a)), we can assume that electrostatic interactions were also involved in the adsorption. The FLA has a positive charge density under these conditions, which favors the adsorption of anionic species like RG-19.
Based on the above discussions and with refereeing to the literatures, a possible adsorption mechanism is proposed and illustrated in Fig. 7.
Extensive review on performance of adsorbents-based lignin.
When compared to lignin-based adsorbents previously reported in the literature, it is clear that the functionalized lignin-based adsorbents (FLA) developed in this study have a high potential for removing reactive green 19 (RG-19) dye from aqueous solutions, with a maximum adsorption capacity (qm) of 29.58 mg/g. Table 7 shows that the produced FLA's better adsorption behavior was corroborated in the literature.54–61 As a result, the FLA indicates a low-cost, high-efficiency adsorbent material for color decontamination from polluted wastewater.
Table 7
Comparable investigation for different pollutants onto the various lignin-based adsorbents.
Pollutant
|
Maximum adsorption capacity (mg/g)
|
Ref.
|
Methylene Blue (MB) dye
|
8.1
|
54
|
brilliant black dye
|
15.8
|
55
|
Prednisolone
|
3.32
|
56
|
3,4-dichloroaniline (3,4-DCA)
|
28.71
|
57
|
Pb (II)
|
16.60
|
57
|
Brilliant Red HE-3B
|
10.173
|
58
|
Methylene Blue (MB) dye
|
20.62
|
59
|
Cu (II)
|
5.94
|
60
|
Ni (II)
|
7.95
|
60
|
Cu (II)
|
17.8
|
61
|
Reactive Green-19 (RG-19) dye
|
29.58
|
Present Study
|