Removal of the Pigment Congo Red from Synthetic Wastewater with a Novel and Inexpensive Adsorbent Generated from Powdered Foeniculum Vulgare Seeds

Abstract

In this research, powdered Foeniculum vulgare seed (FVSP) was treated separately with H₂C₂O₄, ZnCl₂, and a mixture of ZnCl₂-CuS. The characteristics of the treated and untreated FVSP samples, as well as their abilities to eliminate Congo red (CR) from solutions, were investigated. The influences of the empirical circumstances on CR adsorption by the ideal adsorbent were studied. The thermodynamic, isothermal, and dynamic constants of this adsorption were also inspected. The ideal adsorbent was found to be the FVSP sample treated with a ZnCl₂-CuS mixture, which eliminated 96.80% of the CR dye. The empirical outcomes proved that this adsorption was significantly affected by the empirical circumstances, and the second-order dynamic model as well as the Langmuir isotherm model fit the empirical data better than the first-order model and the Freundlich model. The values of E_a (15.3 kJ/mol) and ∆H^o (32.767 kJ/mol ≤ ∆H^o ≤ 35.495 kJ/mol) evidence that CR anions were endothermally adsorbed on Zn/Cu-FVSP via the ionic exchange mechanism. The superior Q_max values (434.78, 625.00, 833.33 mg/g), along with the cheapness and stability of the adsorbent used in this work, are evidence to confirm that this adsorbent will receive special interest in the field of contaminated water purification.

1. Introduction

The industrial products of plastic, rubber, paper, pulp, textiles, pharmaceuticals, cosmetics, and leather are usually colored by dyes [1,2]. As a result, wastewater generated from these industries and the manufacturing of dyes is polluted by various types of dyes [3]. According to a survey, there are more than 10⁵ different dyes commercially accessible; the annual production of these dyes is higher than 700,000 tons; and around 5 to 10% of these dyes are lost in industrial discharges [4,5,6,7]. The direct disposal of the liquid waste of these industries without suitable treatment into rivers and streams causes serious problems in the ecological system due to their toxicity, ability to increase organic loading, and pollution by color, which reduces photosynthesis in aquatic life [8,9]. Additionally, dyes are carcinogenic, mutagenic, and toxic [10,11,12]. Congo red (CR), a mutagen, carcinogen, and poisonous compound, has allergy and platelet aggregation effects, lowers protein serum concentrations, induces thrombocytopenia, and is a hazardous as well as a destructive dye [13,14]. CR, similarly to any synthetic dye, has high solubility in water and high optical and biological stability due to its complicated aromatic forms [15,16]. Thus, it is essential to eliminate this dye from the industries’ effluents before their disposal into the medium of water [17]. For this reason, the fungus of white rot [18], foam of polyurethane, and foam of polypropylene [19] have been used for the biodegradation of CR dissolved in some industries’ sewage. Nanoparticles of some metal oxides, such as ZnO [20], NiO [21], and CoFeO [22], were also applied in the photocatalytic degradation of CR. ZnO and Ni-doped ZnO were also used for methylene blue degradation [23]. It was stated that the effectiveness of both the photocatalytic and biodegradability of CR is unsatisfactory due to the different and complicated aromatic forms of this dye [24]. Consequently, coagulation [25], filtering [26], precipitation [27], and some other conventional methods were also employed for the elution of CR from its solutions. The high expense and low efficiency of these conventional methods confirm that their application is infeasible.

Contrarily, it was stated that the most highly effective and easiest method to design and operate is CR adsorption by clay [28], activated carbons of periwinkle shells [29], coffee waste [30], and aloe vera leaf shells [31], and nanoparticles of NiO [32], Co₃O₄, CuO, and Mn₂O₃ [33], MgO [34], etc. As a result, in this investigation, the selected method for eliminating CR from artificial aqueous solutions was adsorption.

Despite the excellent adsorptive ability of CR on activated carbon and nanoparticles of metallic oxides, the usage of these effective adsorbents has become undesirable in recent years due to the elevated costs of their manufacturing [35]. Thus, widely available, efficient, and inexpensive bio-adsorbents, such as Teucrium polium [35], waste from tea [36], shrimp shells [37], shiitake mushrooms [38], pine bark [39], and others, were recently used for eliminating CR from its aqueous solutions. Surfactant-modified waste ash was also used to purify wastewater from nitro and chlorophenol [39]. The findings of these investigations have shown that there are significant variations among these inexpensive adsorbents towards CR adsorption. Therefore, additional studies are needed on CR adsorption by other cheap and effective biosorbents.

The plant of Foeniculum vulgare is referred to as fennel in various countries and shamr in Saudi Arabia and some other Arabian countries. The seeds of this plant are essentially used as a flavoring for tea, drinking water, and food. They are also used as antioxidants [40], antimicrobials [41], and antitumor compounds [42]. According to the findings of clinical trials and animal experiments, the use of the plant Foeniculum vulgare on a regular basis is not toxic or harmful [43].

Gold nanoparticles were extracted from the seeds of fennel and used as catalysts for the photo-degradation of methylene blue and rhodamine B dyes [44]. The stem powder of fennel was also used for the extraction of V₂O₅–Fe₂O₃ nanocomposites used for 4-nitrophenol reduction [45]. It was also reported by Bani-Atta [46] that the modified Foeniculum vulgare seed powder (FVSP), in addition to its availability in huge quantities in different countries and its cheapness, has effective adsorptive activity for the removal of potassium permanganate from aqueous solutions.

Therefore, the adsorption effectiveness of CR by the modified FVSP will be investigated for the first time in this research. Oxalic acid (H₂C₂O₄), zinc chloride (ZnCl₂), and copper sulfide (CuS) were used for the modification of FVSP. The characterization and uptake performance of the prepared adsorbents were investigated to determine the best sample for CR adsorption.

Then, for the best adsorbent prepared and evaluated in this study, the impact of the experimental circumstances, as well as the constants of thermodynamics, isotherms, and kinetics, were studied for CR adsorption.

2. Methodology

2.1. Modification of FVSP

The Foeniculum vulgare seeds (FVS) were bought from a local herb shop in Tabuk, KSA, rinsed three times with distilled water, dried for 7 h in an oven at 125 °C, and crushed into powder by a grinder. The powder that resulted was sieved to produce fine particles ranging in size from 150 to 212 µm and was identified as FVSP. In a 2000 mL round-bottom flask, 150 g of FVSP was combined with 1500 mL of 25% w/w ZnCl₂ (purity 97%) and refluxed at boiling point for 3 h before being cooled to room temperature. The solid portion of the cooled blend was separated by filtration and boiled with 300 mL of 3 M HCl for an hour to eliminate the excess quantity of ZnCl₂. After filtering the new blend (FNSP + HCl), the solid filtrate was repetitively rinsed with distilled water and dehydrated in a 125 °C oven. Eventually, the dried solid was ground, sieved to obtain uniform particles, and then named Zn-FVSP.

The same procedures and circumstances were used for the modification of 150 g of FVSP with 75 g of CuS (purity 99.99%) and 1500 mL of 25% w/w HO₂CCO₂H (purity 99.99%) to obtain the other two adsorbents, which were named Zn/CuS-FVSP and Ox-FVSP, respectively.

FVSP was modified by ZnCl₂, HO₂CCO₂H, and CuS to improve the porosity and surface area, surface functional groups, and doping of the adsorbent surface with CuS nanoparticles, all of which have a positive impact on the adsorbent’s effectiveness.

2.2. Adsorbent Characterizations

The FVSP, Zn-FVSP, Zn/Cu-FVSP, and Ox-FVSP adsorbents were scanned by both FT-IR (Nicolet iS5 of Thermo Scientific, Waltham, MA, USA) and SEM (at 10 kV), respectively, to recognize the functional groups and morphology of the surfaces of these adsorbents. By using the BET technique at 77.350 K, the porosity and surface area of each of these adsorbents were also estimated.

Furthermore, a pH meter and 1 M, 0.1 M, and 0.01 M HCl and NaOH solutions were used to prepare five 0.04 M Na₂CO₃ solutions with varying initial pH values (pH_i) (2–10). Then, 0.6 g of Zn/Cu-FVSP, as the best adsorbent (ideal adsorbent), was blended with 50 mL of each Na₂CO₃ solution in 100 mL plastic bottles, and then these bottles were sealed and agitated at 155 rpm and 27 °C for 20 h in a shaker incubator. After this, the Zn/Cu-FVSP was removed from each of the five solutions by filtration, and the pH meter was applied again to measure the pH_f (final pH) of each solution. Ultimately, the pH_i–pH_f values were calculated and graphed as pH_i to determine the pH_ZPC of Zn/Cu-FVSP. This method was previously applied by Bani-Atta [46] for the determination of the pH_ZPC of modified FVSP.

2.3. Experiments of Adsorption

2.3.1. Identifying the Ideal Adsorbent

To recognize the idealistic adsorbent amongst the four samples that were prepared in this study for CR adsorption, 30 mg of each prepared sample was added individually into four amber glass bottles (50 mL) containing 25 mL of 150 mg/L CR solution. These bottles were then closed and stirred in a shaker incubator for 25 h at a temperature of 27 °C. The mixture in each bottle was then filtered to separate the solid fraction, and a Jenway 6800 UV–Vis spectrophotometer was used to measure the amount of CR residual in the liquid portion at 500 nm. Equation (1) was used to calculate the percentage of CR that had been successfully removed from solutions (%R) by each adsorbent.

$$\%\mathit{R}=\frac{\left({\mathit{C}}_{\mathit{i}}-{\mathit{C}}_{\mathit{f}}\right)}{{\mathit{C}}_{\mathit{i}}}\times \mathbf{100}$$

(1)

where C_i and C_f stand for the CR solution’s starting and final concentrations, respectively.

2.3.2. Kinetic and Equilibrium Experiments

All the batch adsorption tests using Zn/Cu-FVSP as the ideal adsorbent and 15 mL of CR solutions were carried out in amber glass bottles (30 mL) and a shaker incubator (155 rpm) under the various experimental conditions described in

Table 1. Empirical circumstances of adsorption.

Type of Empirical Condition	Concentration of CR (mg/L)	Contact Time (hour)	Adsorbent Dose (g)	pH	Temperature (°C)
Adsorbent dosage	60	25	0.005–0.03	7	27
Adsorption time and temp	150	0–7	0.015	7	27–57
Solution pH	300	25	0.015	5.5–11.5	27
CR conc	20–1000	9	0.015	7	27–57

. After the predetermined time had passed for each of these experiments, the Zn/Cu-FVSP was removed by filtration, and then the residual concentrations of CR in the liquid phases were measured as described in Section 2.3.1. Equations (1)–(3) were utilized to calculate the values of % R, Q_t (mg/L), and Q_e (mg/L), respectively.

$$Q_t=\left(C_i-C_t\right)\frac{V}{m}$$

(2)

$$Q_e=\frac{V}{m}\left(C_i-C_e\right)$$

(3)

Q_t and Q_e: uptake amount at time (t) and equilibrium, correspondingly. V: solution volume of CR (L); m: mass of Zn/Cu-FVSP (g); C_i: CR initial concentration; C_e: CR final concentration, and C_t: CR concentration at any time t.

The impact of pH variation on this adsorption was considered only in the range of 5.5 to 11.5, as the CR solution becomes blue at pH levels below 5. The effects of the Zn/Cu-FVSP dose, adsorption period, CR solution concentration, and temperature were investigated at various ranges, such as (0.005–0.03 g), (0–7 h), (20–1000 mg/L), and (27–57 °C), correspondingly.

The practical outcomes for the adsorption of 150 mg/L of CR solution by Zn/Cu-FVSP at 27, 42, and 57 °C and different periods (0–7 h) were analyzed by the linear dynamic Equations (4)–(6) of the 1st and 2nd orders and intra-particle diffusion, respectively.

$$\mathit{l}\mathit{o}\mathit{g}\left({\mathit{Q}}_{\mathit{e}}-{\mathit{Q}}_{\mathit{t}}\right)=\mathit{l}\mathit{o}\mathit{g}({\mathit{Q}}_{\mathit{e}})-{\mathit{K}}_{\mathbf{1}}\frac{\mathit{t}}{\mathbf{2.303}}$$

(4)

$$\frac{\mathit{t}}{{\mathit{Q}}_{\mathit{t}}}=\frac{\mathbf{1}}{{\mathit{K}}_{\mathbf{2}}{\left({\mathit{Q}}_{\mathit{e}}\right)}^{\mathbf{2}}}+\frac{\mathit{t}}{{\mathit{Q}}_{\mathit{e}}}$$

(5)

$${\mathit{Q}}_{\mathit{t}}={\mathit{K}}_{\mathit{d}\mathit{i}\mathit{f}}\sqrt{\mathit{t} }+\mathit{B}$$

(6)

t: adsorption time (min); K₁ and K₂: rate constant of the 1st and 2nd order, correspondingly. K_dif and B are the constants of the intra-particle model.

The practical outcomes for the adsorption of CR (20–1000 mg/L) on Zn/Cu-FVSP (0.015 g) at different temperatures (27–57 °C) were examined using the linear versions of the Langmuir (7) and Freundlich (8) models. Equation (9) was also applied to determine the values of the Langmuir separation factor.

$$ln\left({\mathit{Q}}_{\mathit{e}}\right)=ln\left({\mathit{K}}_{\mathit{F}}\right)+\frac{\mathbf{1}}{\mathit{n}}ln\left({\mathit{C}}_{\mathit{e}}\right)$$

(7)

$$\frac{{\mathit{C}}_{\mathit{e}}}{{\mathit{Q}}_{\mathit{e}}}=\frac{\mathbf{1}}{{\mathit{Q}}_{\mathit{m}\mathit{a}\mathit{x}}{\mathit{K}}_{\mathit{L}}}+\frac{{\mathit{C}}_{\mathit{e}}}{{\mathit{Q}}_{\mathit{m}\mathit{a}\mathit{x}}}$$

(8)

$$S_f=\frac{\mathbf{1}}{\mathbf{1}+K_L{C}_i}$$

(9)

Q_max: maximum uptake capability; K_L and K_F: Langmuir and Freundlich constants; n: uptake intensity constant; S_f: factor of separation; C_i: the highest initial concentration of CR.

Equations (10) and (11) were used to test the accuracy of the mathematical equations used to evaluate the dynamics and isotherm characteristics of this adsorption.

$$\mathit{CFEF}={\displaystyle \sum }_{\mathit{n}=\mathbf{1}}^{\mathit{n}}\left(\frac{{\left({\mathit{Q}}_{\mathit{e}\mathit{.}\mathit{exp}}+{\mathit{Q}}_{\mathit{e}\mathit{.}\mathit{cal}}\right)}^{\mathbf{2}}}{{\mathit{Q}}_{\mathit{exp}}}\right)$$

(10)

$$X^{\mathbf{2}}={\displaystyle \sum }_{\mathit{n}=\mathbf{1}}^{\mathit{n}}\left(\frac{{\left({\mathit{Q}}_{\mathit{e}\mathit{.}\mathit{exp}}+{\mathit{Q}}_{\mathit{e}\mathit{.}\mathit{cal}}\right)}^{\mathbf{2}}}{{\mathit{Q}}_{\mathit{e}\mathit{.}\mathit{cal}}}\right)$$

(11)

CFEF: fractional error function; X²: chi-square statistic; n: number of experimentations; Q_e.cal and Q_e.exp: calculated and experimental uptake capacity values, respectively.

By using Equations (12) and (13), it was possible to calculate the coefficients of the thermodynamical (∆G^o, ∆S^o, ∆H^o) for CR adsorption from its solutions (300, 400, 500 mg/L) on the surface of Zn/Cu-FVSP (0.015 g).

$$\ln \left(\frac{Q_e}{C_e}\right)=\frac{∆H^o}{RT}+\frac{∆S^o}{R}$$

(12)

$$∆G^o=∆H^o-T ∆S^o$$

(13)

∆G^o, ∆S^o, and ∆H^o: the free energy, entropy, and enthalpy changes, respectively. T (°C): temperature; R: the universal gas constant (0.008314 KJ/K mol).

The linear equation of Arrhenius (Equation (14)) was also applied to estimate the activation energy for the uptake of 150 mg/L of CR solution by Zn/Cu-FVSP at 27, 42, and 57 °C and different periods (0–7 h).

$$\ln \left(K\right)=lnA-\frac{E_a}{RT}$$

(14)

A: factor of Arrhenius; Ea: activation energy (KJ/ mol).

3. Results and Discussion

3.1. Characterization of the FVSP Adsorbents

Figure 1 displays the images produced by scanning FVSP, Ox-FVSP, Zn-FVSP, and Zn/Cu-FVSP under an electron microscope. These images show that the structures of the FVSP have been broken, crumpled, and melted after modification by these three chemical activation agents. Figure 1 also shows that the gaps are almost absent on the surfaces of FVSP, Ox-FVSP, and Zn-FVSP, but there are various cavities on the surfaces of Zn/Cu-FVSP. Instead of pores, these cavities can be thought of as channels on the Zn/Cu-FVSP surface, which suggests that adsorption has occurred both on the surface and inside the cavities.

The FT-IR results of these adsorbents (FVSP, Ox-FVSP, Zn-FVSP, and Zn/Cu-FVSP) are summarized in

Table 2. FT-IR results.

FT-IR Peak	FVSP		Ox-FVSP		Zn-FVSP		Zn/Cu-FVSP
	Wavenumber (cm−1)	Assignment	Wavenumber (cm−1)	Assignment	Wavenumber (cm−1)	Assignment	Wavenumber (cm−1)	Assignment
1	3302.49	Stretching of O-H, hydrogen bond	3363.67	Stretching of O-H, hydrogen bond	3365.41	Stretching of O-H, hydrogen bond	3364.91	Stretching of O-H, hydrogen bond
2	2922.38	Stretching of C-H (alkyl)	2922.41	Stretching of C-H (alkyl)	2922.09	Stretching of C-H (alkyl)	2921.60	Stretching of C-H (alkyl)
3	2853.30	Stretching of C-H	2853.46	Stretching of C-H	2853.24	Stretching of C-H	2852.82	Stretching of C-H
4	1742.92	Stretching of C=O (aliphatic aldehydes)	1743.17	Stretching of C=O (aliphatic aldehydes	1743.11	Stretching of C=O (aliphatic aldehydes	1738.88	Stretching of C=O (aliphatic aldehydes
5	1603.69	C=C stretching	1620.36	N-H (1°-amide) II band	1629.16	N-H (1°-amide) II band	1625.67	N-H (1°-amide) II band
6	-------	--------	1457.37	C-H scissoring	1457.52	C-H scissoring	--------	--------
7	1375.62	NO2 stretching	1317.52	Stretching of C-O	1370.97	NO2 stretching	--------	--------
8	1029.09	Bending of C-H in-plane	1064.92	C-O-C (ether)	1064.94	C-O-C (ether)	-------	-----------
9	--------	--------	779.46	Bending of C-H out-of-plane	--------	--------	--------	--------

, and the spectra are displayed in Figure 2.

Five identical absorption peaks with a slight shift can be observed for each one of these four adsorbents (Figure 2) (Table 2). These peaks were associated with the functional groups O-H (stretching of hydrogen bond), stretching of C-H, C-H (alkyl), C=O (stretching of aliphatic aldehyde), and stretching of C=C. The absorption peaks of C-H scissoring and C-O-C (ether) can be observed in the cases of Ox-FVSP and Zn-FVSP only, while the peak associated with NO₂ stretching appeared in the spectra of FVSP and Zn-FVSP. Moreover, the spectra of Ox-FVSP, FVSP, and Zn-FVSP, respectively, show C-O stretching, C-H out-of-plane bending, as well as in-plane bending. Thus, the effectiveness of the adsorbent towards the adsorbate is positively affected by some functional groups.

Both camphora of Cinnamomum [47] and carbon prepared from the powder of waste toner [48], which were used to adsorb some selected heavy metals and organic contaminants, included most of the functional groups found in these adsorbents.

It was found from the findings of the surface analysis of BET that the surface areas of FVSP, Ox-FVSP, Zn-FVSP, and Zn/Cu-FVSP are 11.7 m²/g, 25.0 m²/g, 68.1 m²/g, and 398.0 m²/g, correspondingly. This means that the surface area is Zn/Cu-FVSP ˃ Zn-FVSP ˃ Ox-FVSP ˃ FVSP. This demonstrates that the hydrothermal treatment of FVSP with CuS and ZnCl₂ has a significant impact in increasing the FVSP’s surface area.

Figure 3 displays the correlation between the (pH_i–pH_f) and pH_i values. This figure indicates that the value of pH_i–pH_f equals zero when the pH solution is 7.3. This means that the surface of the ideal adsorbent (Zn/Cu-FVSP) will be uncharged at a pH solution value of 7.3. Therefore, 7.3 is the pH_ZPC for this adsorbent. Bani-Atta [46] reported that the pH_ZPC for Foeniculum vulgare seed powder is 7.2.

3.2. Outcomes of Adsorption

3.2.1. Effectiveness of Adsorbents

The percentages of CR eliminated from solutions by the adsorbents FSVP, Zn-FSVP, Zn/Cu-FSVP, and Ox-FSVP were found to be 34.48%, 65.56%, 96.80%, and 49.13%, respectively (Figure 4). This reveals that the efficacy of these adsorbents decreases as follows:

Zn/Cu-FVSP ˃ Zn-FVSP ˃ OX-FVSP ˃ FSVP

This further demonstrates the superiority (96.80%) of Zn/Cu-FVSP, which also has the largest surface area and porosity (Section 3.1). Moreover, the absence of the functional group of NO₂ from the sample modified by ZnCl₂ and CuS (Zn/Cu-FVSP) may be the real reason for the superiority of this sample. As a result, Zn/Cu-FVSP was determined to be the ideal adsorbent for CR adsorption.

3.2.2. Impact of the Empirical Circumstances

To determine the ideal mass of Zn/Cu-FVSP required for this adsorption and to analyze the dosage affect, the percentages of CR dye eliminated (% R) from liquids were graphed vs. Zn/Cu-FVSP dosages (Figure 5a). This graph displays that the % R values were significantly increased when the Zn/Cu-FVSP dose rose from 0.005 g to 0.015 g and nearly stabilized when this adsorbent’s mass reached over 0.015 g. When the mass of Zn/Cu-FVSP was augmented between 0.005 and 0.015 g, the effective sites on the Zn/Cu-FVSP surface were enhanced, which increased the % R values [49]. When the Zn/Cu-FVSP mass was increased above 0.015 g, the cluster assembly of Zn/Cu-FVSP particles was responsible for the stabilization of % R [50]. The optimal dose in this study was therefore decided to be 0.015 g of Zn/Cu-FVSP.

It is crucial to examine the effect of the pH solution on this adsorption, because both the surface charge of Zn/Cu-FVSP and the ionization degree of the CR molecules are significantly controlled by changing the pH solution [51], where the adsorbent surface is positively, neutrally, or negatively charged at solution pH values less than, equal to, or greater than the adsorbent’s pH_ZPC [51]. Moreover, the adsorbate will be in the form of molecules and ions at a pH less than or higher than the adsorbate pK_a value, correspondingly. In this work, the pH_ZPC of Zn/Cu-FVSP was found to be 7.3, and 4 is the pK_a value of CR. Then, the Q_e (mg/g) values were determined at various pH levels (5.5, 6.5, 7.5, 8.5, 9.5, 10.5, and 11.5) and graphed against the pH (Figure 5b). As displayed in Figure 5b, Q_e was lightly and significantly decreased when the pH rose in the ranges of (5.5–7.3) and (7.3–11.5), in that order. In these two ranges of pH (5.5–7.3) and (7.3–11.5), CR will be in the form of ions as its pK_a = 4. Thus, the slight drop in Q_e can be attributed to the diminishing positive surface charges of Zn/Cu-FVSP (pH_ZPC = 7.3), which attract the anions of CR (pK_a = 4) [51]. Meanwhile, the large increase in the negative surface charges of Zn/Cu-FVSP, which repel the anions of CR, is responsible for the abrupt drop in Q_e when the pH is over 7.5 [51].

Comparable outcomes were noted for the adsorption of CR by soybean curd [52].

The effects of temperature and time on Zn/Cu-CR FVSP’s adsorption are shown in Figure 5c. This figure indicates that the CR uptake was progressively enhanced when enhancing the time of agitation in the period of 0 to 90 min, and there was no change in the uptake values after 90 min at each of these three temperatures. This is because all the effective uptake sites were originally empty before progressively accumulating CR anions during the adsorption period, which lasted from 0 to 90 min. Following this, none of the sites were available to adsorb more anions of CR. Figure 5c proves that this adsorption’s equilibrium was reached in 1.5 h, but the other tests were conducted at 9 h to ensure that all uptake constants had essentially been investigated at equilibrium.

Comparable results were stated for the adsorption of CR by Teucrium polium [35].

The plots in Figure 5c also show that this adsorption is positively impacted by temperature. This is a result of the lowering of the CR solution’s viscosity and the increasing transfer rate of CR anions in its solution as the temperature rises [53,54].

Figure 5d proves how the primary CR concentration affects the adsorption of Zn/Cu-FVSP. As displayed in this figure, Q_e was considerably and slightly augmented when the CR concentration rose from 20 to 400 mg/L and from 400 to 1000 mg/L, respectively. The first increase can be attributed to the availability of many adsorptive sites, while the second increase was due to the partial saturation of these effective sites.

Additionally, the solid particles of Zn/Cu-FVSP cannot be seen in the CR solutions by the human eye after filtration in each experiment. This proves the high stability of Zn/Cu-FVSP in the solution of CR.

3.2.3. Kinetics of Adsorption

Figure 6a–c provide the plots of log (Q_e-Q_t) vs. t (pseudo-first-order), plots of t/Q_t vs. t (pseudo-first-order), and plots of Q_t vs t^1/2 (intra-particle diffusion), respectively. The parameters of these three dynamic models, such as Q_e, K₁, K₂, K_dif, and B, were calculated by utilizing the intercepts and slopes of these linear plots. To evaluate the accuracy of these two mathematical kinetic models (first- and second-order modes), the values of the statistic parameters CFEF and X² were also computed using Equations (10) and (11). The calculated values of the kinetic and statistical parameters, along with the values of the R² (coefficient of correlation) and the experimental values of Q_e, are included in

Table 3. Dynamic constant for CR adsorption onto Zn/Cu-FVSP.

	Temperature (°C)
	27	42	57
Qe.exp (mg/g)	77.616	101.6075	124.926
Dynamic model of 1st order
Qe.cal (mg/g)	40.766	35.213	35.221
K1 (1/h)	0.0173	0.0138	0.0136
R2	0.761	0.784	0.801
CFEF	17.495	43.385	64.414
X2	33.310	125.190	228.473
Dynamic model of 2nd order
Qe.cal (mg/g)	82.645	105.263	128.205
K2 (g/mg. h)	0.00057	0.00079	0.00099
R2	0.998	0.998	0.999
Rate	0.047	0.083	0.127
CFEF	0.326	0.132	0.086
X2	0.306	0.127	0.084
Dynamic model of intra-particle diffusion (first region)
Kdif	7.395	6.508	7.402
B	0.347	30.216	48.423
R2	0.991	0.972	0.976
Dynamic model of intra-particle diffusion (second region)
Kdif	0.175	0.186	0.212
B	74.227	97.894	120.680
R2	0.877	0.976	0.970

. As shown in Table 3, the R² values in the first order (0.760 ˂ R² ˂ 0.802) are significantly lower than those in the second order (R² ≥ 0.998), and the experimental Q_e (Q_e.exp) values are close to those predicted by the dynamic model of the second order (Q_e.cal) but far from those predicted by the dynamic model of the first order. Additionally, the second-order model’s statistical parameter values (CFEF and X²) are much lower than those of the first-order model. These findings strongly support the success of the second-order model in analyzing the kinetic experimental data of this adsorption, as well as the failure of the first-order model in analyzing them.

These data reveal that the rate of CR adsorption on Zn/Cu-FVSP may either be controlled by chemical interactions such as electronic sharing or by an ionic exchange process between CR and Zn/Cu-FVSP [54]. The final evidence for the type of this adsorption mechanism in terms of whether it is a chemical interaction such as electronic sharing or an ionic exchange process will be obtained from the thermodynamic data.

Comparable outcomes for CR adsorption by Teucrium polium [35], the adsorption of methylene blue by both Azadirachta indica [55] and Nitraria retusa [56], and the adsorption of methylene blue by Ocimum basilicum [57] have also been reported.

Figure 6c shows that each of these plots has two linear sections with significant R² values but is not linear for the entire time span, with none passing from the origin point. These findings show that intra-particle diffusion and some other mechanisms control this adsorption. Additionally, the low values of the B constant (Table 3), which are related to the thickness of the adsorbent layer, demonstrate that the external diffusion of CR molecules has a minor role in controlling this adsorption rate.

3.2.4. Adsorption Isotherms

The isotherm curves resulting from the plotting of C_e/Q_e vs. C_e (Langmuir equation) and ln (Q_e) vs. ln (C_e) (Freundlich equation) for the adsorption of CR onto Zn/Cu-FVSP are shown in Figure 7a,b, respectively. The slopes and intercepts of the curves in Figure 7a,b, respectively, were used to calculate the constants of the Langmuir (Q_max and K_L) and Freundlich (K_F and n) equations. Along with the isotherm constants, the values of S_f, X², and CFEF were also computed and are summarized in

Table 4. Isotherm constants for CR adsorption onto Zn/Cu-FVSP.

	Temperature (°C)
	27	42	57
Qe.exp (mg/g)	313.92	443.70	609.57
Langmuir
Qmax (mg/g)	434.78	625.00	833.33
Qe (mg/g)	318.567	427.732	595.062
KL (L/mg)	0.003996	0.003898	0.006397
Sf	0.2002	0.2042	0.1352
R2	0.998	0.994	0.999
CFEF	0.069	0.575	0.345
X2	0.068	0.596	0.354
Freundlich
Qe (mg/g)	655.05	661.46	831.86
KF (mg/g)(L/mg)1/n	4.602	4.015	8.681
n	1.317	1.238	1.308
1/n	0.7592	0.8075	0.7646
R2	0.974	0.984	0.980
CFEF	370.714	106.877	81.058
X2	177.655	71.691	59.398

. As demonstrated by Table 4, R² in the case of the Langmuir model has values that are slightly higher than those of the Freundlich model. Moreover, the Freundlich model’s statistical parameter (X² and CFEF) values are significantly greater than the Langmuir model’s. These results strongly support the success of the Langmuir model in describing the isothermal properties of this adsorption. This confirms that the anions of CR were adsorbed onto the homogeneous surface of Zn/Cu-FVSP as a monolayer, with no interaction effects between them [32]. According to the values of 1/n and S_f that are less than one and higher than zero (Table 4), the empirical circumstances applied in this work were appropriate for the adsorption of CR onto Zn/Cu-FVSP [58].

Table 4 also shows that the computed Q_max values at 27, 42, and 57 °C are 434.78, 625.00, and 833.33 mg/g, in that order. These higher adsorption capacities were due to the higher surface area and the presence of effective surface functional groups on the surface of Zn/Cu-FVSP.

These high values of Q_max indicate the superior effectiveness of Zn/Cu-FVSP towards the adsorption of CR. The low cost and superior effectiveness of this adsorbent further demonstrate that Zn/Cu-FVSP will be of special interest in the field of contaminated water purification.

3.2.5. Thermodynamics of CR Adsorption

The linear plots in Figure 8 were used to calculate ∆S^o and ∆H^o for this uptake process, which were then used to calculate the ∆G^o values using Equation (13). The values of these thermodynamical constants (∆S^o, ∆G^o, and ∆H^o) are summarized in

Table 5. Thermodynamic constants for CR adsorption onto Zn/Cu-FVSP.

CR Concentration (mg/L)	∆Ho (kJ/mol)	∆So (kJ/mol)	∆Go (KJ/mol)			R2
			27 °C	42 °C	57 °C
300	32.767	0.1098	−0.184	−1.831	−3.479	0.942
400	34.903	0.1153	−0.323	−1.406	−3.135	0.936
500	35.495	0.1161	−0.673	−1.068	−2.809	0.957

. At each of these three CR concentrations, the values of ∆G^o are negative and increase with increasing temperature, as shown in Table 5. This is sufficient evidence that the anions of CR can feasibly and spontaneously adsorb on Zn/Cu-FVSP, and that this adsorption would reduce the free energy of Gibbs [37]. Moreover, the ∆H^o (32.767 kJ/mol ≤ ∆H^o ≤ 35.495 kJ/mol) values (Table 5) are in the 24–38 kJ/mol range, confirming that CR anions were adsorbed on Zn/Cu-FVSP via the ionic exchange mechanism [59,60]. The values of ∆S^o are also positive, which proves that the driving force behind this uptake is significantly more inflected by ∆S^o than ∆H^o [61].

Figure 9 displays the values of lnK plotted vs. 1/T. The slope of the linear curve shown in this figure (Figure 9) was used to estimate the energy of activation (E_a, KJ/mol) for this adsorption. The obtained E_a value (15.3 kJ/mol) from this work is in the 0.2–38 kJ/mol range. This is another example that confirms that this adsorption is caused by an ionic exchange mechanism [62].

3.3. The Performance of Cheap Adsorbents toward CR

Table 6. Q_max values of CR adsorption onto Zn/Cu-FVSP and other low-cost adsorbents used in the prior literature.

Adsorbents	Qmax (mg/g)		References
Zn/Cu-FVSP	434.78 625.00 833.33	27 °C 42 °C 57 °C	This study
Cattail rot	38.79		[1]
Pine bark	3.90	50 °C	[2]
Shrimp shell powder	232.00 264.60 288.20	30 °C 40 °C 50 °C	[37]
Modified shiitake mushroom	217.90 211.90 209.40	20 °C 35 °C 50 °C	[38]
Soybean curd	69.00 69.20 69.90	25 °C 40 °C 55 °C	[52]
Roots of Eichhornia crassipes	1.60		[54]
Egyptian hyacinth cellulose	230.00		[63]
Tea waste	32.30		[36]
Charcoal of Eichhornia	56.8		[64]
Charcoal of groundnut	117.6		[64]

contains the Q_max values for Zn/Cu-FVSP and the other inexpensive adsorbents utilized in the prior literature to adsorb CR. This table shows that the Zn/Cu-FVSP developed and used in this study had the highest Q_max. The high stability, cheapness, and exceptional efficacy of the Zn/Cu-FVSP utilized in this study confirm the fact that this adsorbent will receive special interest in the field of contaminated water purification.

4. Conclusions

In this investigation, H₂C₂O₄, ZnCl₂, and CuS were used as chemical agents to modify powdered Foeniculum vulgare seed (FVSP). To select the best adsorbent, unmodified and modified samples were examined for their ability to remove CR from solutions. It was noted that Zn/Cu-FSVP gave the highest percent CR removal (96.80%). Along with the thermodynamics, kinetics, and isotherms, the influences of the empirical conditions on the adsorption of CR by Zn/Cu-FVSP were also examined. In this adsorption, it was revealed that the ideal Zn/Cu-FVSP dose, ideal solution pH, ideal temperature, ideal contact time, and ideal concentration of CR are 0.015 g, 7.3, 57 °C, 90 min, and 1000 mg/L, respectively. The obtained outcomes indicate that the second-order dynamic model and Langmuir isotherm model fit the empirical data better than the first-order model and Freundlich model, respectively. The thermodynamic results revealed that anions of CR spontaneously adsorb on Zn/Cu-FVSP. Furthermore, the values of E_a and ∆H^o prove that the anions of CR were endothermally adsorbed on Zn/Cu-FVSP via the ionic exchange mechanism. The superior computed values of Q_max (434.78, 625.00, and 833.33 mg/g), along with the cheapness and stability of the adsorbent used in this work, are sufficient evidence to confirm that Zn/Cu-FVSP will receive special interest in the field of contaminated water purification.