Using chemically modified Ocimum tenuiflorum as an efficient and low-cost biosorbent for removing Congo red from aqueous solutions

Abstract

The powder of Ocimum tenuiflorum leaves (OtLP) was modified by ZnCl₂, H₂C₂O₄, or CuS. The modified powders were characterized and applied as adsorbents for the first time to adsorb Congo red (CR). The effects of the most potent experimental variables on the CR adsorption by the chosen adsorbent were examined. Kinetic, isotherm, and thermodynamic constants were investigated. The best adsorbent was discovered to be the powder modified by ZnCl₂ and CuS, which removed 97.82% of the CR and had a pore size of 1349.0 Å, pore volume of 2.1211 cm³.g⁻¹, and a 281.16 m².g⁻¹ surface area. This adsorption followed models of second order and Langmuir for its kinetics and isotherms, respectively. Thermodynamic analyses proved that this adsorption is both heat-absorbing and spontaneous. The high uptake capacities (625.000, 714.286, and 769.231 mg.g⁻¹) obtained in this study prove that this adsorbent will gain a significant attention in the field of water decontamination.

KEYWORDS:

• Congo red
• Ocimum tenuiflorum
• adsorption
• isotherms
• thermodynamics
• kinetics

1. Introduction

Congo red (CR) is an anionic diazo dye that metabolizes into benzidine, which is recognized as carcinogenic to humans [1]. Furthermore, benzidine resulting from the metabolization of CR has health risks like gastrointestinal, ocular, and skin irritation [2]. Blood clotting, respiratory difficulties, and somnolence are the other side effects of benzidine [2]. CR and other dyes are generally used in the paper and textile industries to dye paper and fabrics such as silk, hemp, and cotton [3]. These dyes also applied in a variety of other industries, like cosmetics, plastics, and the medical industries [4]. It is estimated that 10–20% of CR or any other dyes will be lost and discharged with the effluent during industrial operations [5]. As a result, discharging sewage from these industries into the surrounding aquatic environment will alter the odour and colour of the water and strongly impact the photosynthetic process of the aquatic life [6]. Long-term contact to sewage containing CR can affect the hematopoiesis, liver, and the circulatory system of humans, resulting in diarrhoea, nausea, breathing problems, and vomiting [7]. Furthermore, wastewater containing less than 1 ppm of CR, or any dye is very visible, unwanted, and must be removed before wastewater can be released into the environment [8].

Therefore, biodegrades such as polypropylene and polyurethane foams [9] and white rot fungus [10] have been employed for the degradation of CR contained in the effluent of those industries. Graphene oxide-zeolite and Laccase enzyme immobilized on Titania nanoparticles were also employed as a biocatalyst for degradation of Direct Red 23, 31 dyes and others [11,12]. The process of Fenton was used for the degradation of methylene blue and other poisonous dyes by using ions of lanthanide doped with magnesium ferrite [13] and biochar doped with iron [14], respectively. Moreover, nanoparticles of CoFeO [15], NiO [16], ZnO [17] and others were applied in the catalytic degradation of CR. Due to the complicated and heterocyclic structure of this dye (CR), its bio or photo degradability has been reported to be inadequate [18]. As a result, additional methods for removing CR from industrial effluent have been used, including coagulation [19], electrocoagulation [20], precipitation [21], and filtering [22]. These techniques were considered unsuccessful due to their low removal efficacy and superior cost.

On the contrary, the adsorption of CR by clay [23], activated carbon of some raw materials [24–26], nanoparticles of some metallic oxides like MgO [27], SnO₂ [28], NiO [29], and other adsorbents was not only the most effective but also the simplest to operate and design. As a result, the chosen method for removing CR from solutions in this investigation was adsorption. The expensive cost of manufacture has limited the use of these adsorbents, despite their great adsorptive performance. Therefore, the recent objective is to substitute these adsorbents with biosorbents that are cheap, efficient, and widely available. For this, several studies have been conducted to eliminate CR from solutions using chemically modified low-cost biosorbents like shiitake mushrooms hydrothermally treated [30], pine bark [31], shell powder of shrimp [32], Eichhornia crassipes roots [33], Egyptian water hyacinth [34], soybean curd xerogels [35], Teucrium polium [36], and others. The results of these investigations showed that there are significant disparities in the adsorption abilities of these low-cost biosorbents when it comes to CR. As a result, more researches on CR adsorption by other various low-cost biosorbents are required.

Ocimum tenuiflorum (Ot) is considered the world’s most holy plant due to its extensive medicinal and effective disease-prevention properties [37]. For example, this herb’s stems, leaves, roots, seeds, and flowers were used for several medicinal purposes in Greek and Roman times [38]. Most of the anticancer, analgesic, antimetic, antiasthmatic, antifertility, antistress, diaphoretic, epatoprotective, antidiabetic, hypolipidmic, and antistress are also extracted from OT [39–41]. This plant is widely cultivated all over the world. Therefore, the effective inhibitors of zinc [42] and aluminium [43] corrosion have been extracted from the leaves of this cheap and readily available herb (Ot). While the inhibitor extracted from the roots of Ot was used for inhibiting carbon steel corrosion [44]. Ot extracts and silver nanoparticles were also found to have a potent inhibitory effect on both gram-positive and gram-negative bacteria [45]. In the field of adsorption, the extract of Ot has been used for synthesis a novel silver-nanocomposites for reactive turquoise blue dye adsorption [46], leaves powder of Ot was applied as an effective and cheap biosorbent for removing fluoride from drinking water [47], and hexavalent chromium adsorption from solutions on Ot-based activated carbon was reported [48].

Despite the adsorptive effectiveness, low cost and widespread availability of the environmentally friendly Ocimum tenuiflorum in many countries, particularly in Tabuk and other parts of Saudi Arabia, no effort has been made to date to produce a new adsorbent for the removal of Congo red from wastewater using the leaf powder of Ocimum tenuiflorum (OtLP) as a raw material. As a result, the main purpose of this study is to evaluate the efficacy of CR adsorption using a new adsorbent prepared from the available and inexpensive OtLP. The leaf powders of Neem [49,50], Lamiaceae [51], Nitraria retusa [52], Ocimum basilicum [53,54] modified by ZnCl₂ were discovered to have a significant ability for KMnO₄ and methylene blue uptake. The effects of ZnCl₂, C₂H₂O₄, and CuS on the uptake performance of Teucrium polium [36] and seeds of Foeniculum vulgare [55] toward Congo red, and seeds of Foeniculum vulgare toward potassium permanganate [56] were also studied, and it was discovered that these three chemical agents have significant effects on these adsorbents’ adsorption ability. Thus, to attain the main goal of this work, zinc chloride (ZnCl₂), oxalic acid (C₂H₂O₄), and copper sulphide (CuS) were used to modify this plant’s leaf powder. The adsorptive capabilities of CR by OtLP and the other three modified samples were compared to identify the best adsorbent. Finally, the effects of experimental circumstances as well as thermodynamic, isotherm, and kinetic constants on the CR adsorption by the best adsorbent produced and evaluated in this study were investigated.

2. Methodology

2.1. Adsorbents preparation

The leaves of Ocimum tenuiflorum that had been bought at a medicinal herb market in Tabuk, Saudi Arabia were washed four times in distilled water and then left at 115 °C in an oven for 10 h to dry. After that, the dehydrated leaves were pulverized into powder with an electric grinder and designated as OtLP.

120 g of dried powder (OtLP) was re-condensed for 4 h in 1 L of ZnCl₂ (25% w/w). After that, the mixture was cooled down to room temperature. To remove any residual ZnCl₂ in the solid, the solid portion of this cold mixture was filtered and rinsed once in 200 mL of 1.5 M HCl. Finally, the obtained solid was washed multiple times with distilled water, then dried in a 115 °C oven for 21 h, pulverized, and sieved before being labelled as OtLP-Zn.

The same techniques and conditions were applied to a combination of OtLP (120 g) and CuS (60 g), and the yielding adsorbent was named OtLP-ZN/Cu.

Using the same conditions and processes as previously, a combination of OtLP (120 g) and CuS (60 g) was re-condensed using 1L of H₂C₂O₄ solution (25% w/w). The obtained adsorbent was labelled as OtLP-Ox/Cu.

120 g of OtLP were also separately modified by refluxing this powder with 500 mL (30% w/w) of NaOH, KOH, KCl, and H₂SO₄ solutions for 4 h.

2.2. Adsorbents characterization

The surface functional groups of the OtLP and the other three modified adsorbents before and after CR adsorption were qualitatively recognized using FT-IR (Nicolet iS5 of Thermo Scientific FT-IR, USA). The morphology of these adsorbents, on the other hand, was identified only before adsorption using SEM at a 10 kV accelerating voltage. The surface area along with the porosity of these adsorbents were then estimated using BET (NOVA-2200 Ver. 6.11) technology at 77.35 K for 22 h. In addition, five 0.05 M Na₂CO₃ solutions with pH starting values ranging from 2 to 10 (pH_i) were prepared. In a 100 mL plastic container, 50 mL of each solution were combined with 0.4 g of the optimum adsorbent. The bottles were shaken for 23 h at 27°C and 185 rpm in shaker incubators. A pH metre was applied for measuring the pH_f (final pH) of these solutions after each solution was separated by filtering. Finally, to determine the pH_ZPC of this adsorbent, the pH_i–pH_f were calculated and plotted against the pH_i.

2.3. Adsorption experiments

2.3.1. Selecting the perfect adsorbent

To select the most efficient adsorbent among the four adsorbents prepared and developed in this research, a fixed mass (0.035 g) of OtLP, OtLP-Zn, OtLP-ZN/CuS, and OtLP-Ox/Cu were added separately to four amber bottles containing 10 mL of 100 mg.L⁻¹ CR solution. Those four bottles were then sealed and agitated for 23 h at 27^oC and 175 rpm in a thermal shaker. Then the solid portion of the mixture of each bottle was separated by filtration, and the CR residual in the liquid part was quantified by a Jenway 6800 UV-Vis’s spectrophotometre at 500 nm. The percent of the CR that has been eliminated from solutions (% R) by each adsorbent was computed using Equation (1). (1) $\mathrm{\%}R=\frac{(C_{\circ }-C_e)}{C_{\circ }}\times 100$(1) Where C₀ and C_e are the CR solution’s starting and ultimate concentrations, respectively.

2.3.2. Equilibrium and kinetic experiments

Batch adsorption experiments of 30 mL of CR solutions by OtLP-Zn/Cu as a perfect adsorbent were conducted in amber bottles (50 mL) and a thermal shaker (175 rpm) under various experimental circumstances that are listed in Table 1. After the selected time of each of these experiments, the adsorbents were withdrawn, and the residual CR concentration in the supernatant was determined as stated in Section 2.3.1. The quantity of CR adsorbed at time t (q_t) and equilibrium (q_e) were computed using Equations (2) and (3). (2) $\begin{array}{rl}{q}_t& =\frac{V}{m}(C_{\circ }-C_t)\end{array}$(2) (3) $\begin{array}{rl}{q}_e& =\frac{V}{m}(C_{\circ }-C_e)\end{array}$(3) Where C_t stands for CR concentrations (mg.L⁻¹) at contact time t, m for OtLP-Zn/Cu mass (g), and V for CR solution volume (L).

Table 1. Experimental conditions for uptake of Congo red by OtLP-Zn/Cu.

Experiment type	Concentration of CR (mg.L^−1)	Contact time (h)	Adsorbent mass (g)	pH	Temperature (°C)
Influence of dosage	60	23	0.005-0.045	7.03	27 ± 1
Influence of time	100, 200, 300	0–7	0.035	7.03	27 ± 1
Influence of pH	300	23	0.035	5.5-11.5	27 ± 1
Influence of temp and CR conc	20–1000	23	0.035	7.03	27 ± 1-57 ± 1

The effects of changing the CR concentration (20-1000 mg.L⁻¹), mass of OtLP-Zn/Cu (0.005-0.045 g), pH (5.5-11.5), and agitation time (0–7 h) on this adsorption have been studied to determine the best experimental conditions. The impact of pH was inspected only in the range of 5.5–11.5, since at pH < 5, the CR solution colour changes to blue. The experimental data for the adsorption of 100, 200, and 300 mg.L⁻¹of CR solutions at 27 ± 1°C by OtLP-Zn/Cu were examined using the dynamic models indicated in Table 2.

Table 2. Used kinetic models in this study.

Kinetic model	Linear equation	Plot	Parameters
Pseudo 1st order	$\text{log}(q_e-q_t)=\text{log}(q_e)-K_1{\displaystyle \frac{t}{2.303}}$	$\text{log}(q_e-q_t)\mathrm{vs}.t$	k1 = −slope qe = exp (intercept)
Pseudo 2nd order	${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{K_2{(q_e)}^2}}+{\displaystyle \frac{t}{q_e}}$	${\displaystyle \frac{t}{q_t}}\mathrm{vs}.t$	k2 = (slope)^2/intercept qe = (slope)^−1

q_t and q_e (mg.g⁻¹): amount of CR adsorbed at time t and equilibrium (min), K₁ (1/min) and K₂ (g.mg⁻¹.min⁻¹) rate constants of the kinetic models of 1st and 2nd order, correspondingly.

The linear Equations (4) (Freundlich) and (5) (Langmuir) along with Equation (6) were used for analysing the isotherm practical results for the adsorption of CR solutions (20–1000 mg.L⁻¹) by OtLP-Zn/Cu (0.035 g) at a contact time of 23 h and a variety of temperatures (27 ± 1, 42 ± 1, and 57 ± 1°C). (4) $\begin{array}{rl}\ln (q_e)& =\ln (K_F)+\frac{1}{n}\ln (C_e)\end{array}$(4) (5) $\begin{array}{rl}\frac{C_e}{q_e}& =\frac{1}{q_{max}{K}_L}+\frac{C_e}{q_{max}}\end{array}$(5) (6) $\begin{array}{rl}{S}_f& =\frac{1}{1+K_L{C}_0}\end{array}$(6) where q_max (mg.g⁻¹) denotes maximum uptake, K_F and K_L denote Freundlich and Langmuir constants, respectively. n: constants relating to uptake intensity. S_f: a separating factor. C₀ is the maximum CR starting concentration.

To assess the accuracy of the models of mathematics used to analyse the kinetic and isotherm experimental data for CR adsorption by OtLP-ZN/Cu, the fractional error function (CFEF) Equation (7) and the statistic of chi-square (x²) Equation (7) have been applied. (7) $\begin{array}{rl}\mathrm{CFEF}& =\sum _{n=1}^n\left(\frac{(q_{e.\exp }+q_{e.\mathrm{cal}}{)}^2}{q_{\exp }}\right)\end{array}$(7) (8) $\begin{array}{rl}{X}^2& =\sum _{n=1}^n\left(\frac{{(q_{e.\exp }+q_{e.\mathrm{cal}})}^2}{q_{e.\mathrm{cal}}}\right)\end{array}$(8) where n is the number of experiments, and q_e,cal and q_e,exp are the values of the calculated and experimental uptake capacity, respectively.

Based on Equations (7) and (8), the thermodynamical coefficients like ΔS^o (entropy change), ΔH^o (enthalpy change), and ΔG^o (standard free energy change) were calculated from the results of the adsorption of 40, 60, 200, 500, and 700 mg.L⁻¹ CR solutions. (9) $\begin{array}{rl}\ln \left(\frac{q_e}{C_e}\right)& =\frac{\mathrm{\Delta }{H}^o}{\mathrm{RT}}+\frac{\mathrm{\Delta }{S}^o}{R}\end{array}$(9) (10) $\begin{array}{rl}\mathrm{\Delta }{G}^o& =\mathrm{\Delta }{H}^o-T\mathrm{\Delta }{S}^o\end{array}$(10) R (8.314 J.K⁻¹ mol⁻¹) stands for the universal gases constant, and T stands for the adsorption temperature (K).

Additionally, to prevent the photodegradation of CR, all the batch adsorption tests were conducted in a dark setting utilizing amber bottles and a thermal shaker.

3. Results and discussion

3.1. Characterization of OtLP and its derived adsorbents

Figure 1(a) shows a typical FTIR spectrum of the four adsorbents (OtLP, OtLP-Zn/Cu, OtLP-Zn, OtLP-Ox/Cu) before adsorption of CR. The OtLP sample exhibits six primary bands that are attributed to the O–H stretching mode at 3304.51 cm⁻¹, C–H stretching (alkyl) at 2922.63 cm⁻¹, the N–H group at 1604.52 cm⁻¹, strong C–H stretching at 1373.14 cm⁻¹, C–O stretching at 1243.58 cm⁻¹, and alkyl amine at 1024.79 cm⁻¹ (Figure 1). In the case of the three modified adsorbents (OtLP-Zn/Cu, OtLP-Zn, OtLP-Ox/Cu), some of these absorption peaks with a slight shifting appeared and some of them vanished. For instance, in the OtLP-Zn/Cu sample, the peaks at 3304.51, 1373.14, and 1243.58 cm⁻¹ cannot be observed along with the peak at 1373.14 cm⁻¹ in the case of the OtLP-Zn sample. Moreover, the peaks at 3304.51 and 1243.58 cm⁻¹ disappeared from the spectrum related to the OtLP-Ox/Cu sample. Most of the functional groups discovered in these adsorbents were also present in Cinnamomum camphora, which was employed for the adsorption of some heavy metals [57] and carbon produced from the powder of waste toner, which was used to degrade certain organic pollutants [58].

Figure 1. FT-IR spectra of adsorbents before adsorption (a) and after adsorption (b).

Figure 1(b) demonstrates the FTIR spectra of the adsorbents scanned after adsorption of CR. It can be seen that all absorption peaks in the FTIR spectra of the adsorbents before adsorption (Figure 1(a)) have been shifted to other positions after adsorption (Figure 1(b)). The changes in peak positions observed following adsorption confirm CR adsorption onto these adsorbents. Some new peaks in the range of 1600 cm⁻¹–800 cm⁻¹ after CR adsorption can also be observed (Figure 1(b)). It can be supposed that these new peaks were brought about by the adsorbed anions of CR.

The images of a, b, c, and d demonstrated in Figure 2 are the SEM images of OtLP, OtLP-Zn/Cu, OtLP-Zn, and OtLP-Ox/Cu, respectively. The images in this figure reveal that the structures and folds of OtLP were considerably affected by the chemical agents (ZnCl₂, ZnCl2/CuS, H₂C₂O₄/CuS) applied in this study. Where the folds and structures of OtLP were transformed to tiny and fine-scattered masses due to its modification by ZnCl₂/CuS (Image b) and H₂C₂O₄/CuS (image d). Whereas the treatment of OtLP by ZnCl₂ (image c) converted it into huge masses woven together in one or two pieces.

Figure 2. SEM images of OtLP (a), OtLP-Zn/Cu (b), OtLP-Zn (c), OtLP-Ox/Cu (d).

The porosity and surface area of OtLP-Zn/Cu and OtLP-Ox/Cu are expected to be greater than that of OtLP and OtLP-Zn based on these findings.

The BET surface analyser results indicate that (315.0 Å, 0.1715 cm³.g⁻¹, 6.05 m².g⁻¹), (1349.0 Å, 2.1211 cm³.g⁻¹, 281.16 m².g⁻¹), (69.0 Å, 0.0855 cm³.g⁻¹, 5.75 m².g⁻¹), and (581.3 Å, 1.3012 cm³.g⁻¹, 160.07 m².g⁻¹) are the pore size, pore volume, and surface area of OtLP, OtLp-Zn/Cu, OtLp-Zn, and OtLp-Ox/Cu, correspondingly. This means that the porosity (pores volume and size) and surface area of OtLp-Zn < OtLP < OtLp-Ox/Cu < OtLp-Zn/Cu. The outcomes of this part are as predicted from SEM results. This demonstrates that the treatment of OtLP with ZnCl₂ and CuS has a significant impact on the porosity and surface area of this new and cheap raw substance (OtLP).

The plot of the pH_i against (pH_i–pH_f) values is shown in Figure 3. This ideal adsorbent’s surface will be neutrally charged at a pH of 7.4 in a solution.

Figure 3. pH_ZPC of the OtLP- Zn/Cu adsorbent.

3.2. Adsorption experiments

3.2.1. Performance of adsorbents

The percentage values for removing CR from the experimental solutions and the quantities of the CR adsorbed by these various adsorbents were calculated using Equations (1) and (3), respectively. The obtained values were demonstrated in Figure 4(a and b). This figure shows that the adsorbents’ performance efficiency decreases in the following order: $\begin{array}{rl}& \mathrm{OtLP}-\mathrm{Zn}/\mathrm{Cu}>\mathrm{OtLP}-\mathrm{Ox}/\mathrm{Cu}>\mathrm{OtLP}\\ & >\mathrm{OtLP}-\mathrm{Zn}\end{array}$

Figure 4. Percentages of CR removed (a) and from solutions and the quantities of the adsorbate adsorbed (b) by the adsorbents of OtLP, OtLP-Zn, OtLP-Ox/Cu, and OtLP-Zn/Cu.

This may be explained by the fact that these adsorbents’ surface areas and pore properties both decrease in the same sequence as mentioned in section 3.1. In comparison between the efficiency of these adsorbents in terms of CR elimination, OtLP-Zn/Cu has the highest percentage removal (97.82%). Therefore, OtLP-Zn/Cu was exclusively used in this study to conduct the other adsorption trials. These findings also suggest that the surface morphological characteristics (surface area and porosity) of these adsorbents play a decisive role in the behaviour of this adsorption.

For the modification of 120 g of OtLP, 250 g of ZnCl₂ and 60 g of CuS were used. 27.49 US dollars and 10 US dollars are the price of 1 kg of ZnCl₂ and CuS, respectively. Therefore, modification of 120 g of OtLP will cost around 9.37 US dollars (6.87 for ZnCl₂ and 2.50 for CuS). This indicates that the effective OtLP-ZN/Cu adsorbent produced and used in this work is inexpensive.

It was also found that there were no significant changes in the concentration of CR after the adsorption process in the case of OtLP modified by NaOH, KOH, KCl, and H₂SO₄ solutions.

3.2.2. Factors influencing adsorption

Adsorbent dosage is an important factor that influences adsorption. As a result, the effect of OtLP-Zn/Cu dose variations on the CR uptake from solution was investigated in this work to identify the optimum dosage thereafter. The outcomes of this investigation are presented in Figure 5(a), which demonstrates that when the mass of OtLP-Zn/Cu increased from 0.005 g to 0.035 g, the CR uptake increased from 19.06% to 94.47%. This increase was due to increasing the availability adsorption effective sites [56]. Figure 5(a) also shows that increasing the mass of OtLP-Zn/Cu above 0.035 g has no discernible influence on CR uptake; hence, 0.035 g has been chosen as the optimal mass for the remaining experiments. The adsorption of permanganate ions by copper sulphide exhibited similar results [59].

Figure 5. Factors influencing adsorption [influence the mass of OtLP- Zn/Cu (a), influence of pH (b), contact time influence (c), influence of temperature and CR concentration (d)].

The uptake of any adsorbate is considerably influenced by pH because the degree of adsorbate ionization, as well as the type and density of the adsorbent charge, is all dependent on the pH value of the solution [60]. As a result, the impact of pH on CR uptake was also examined, and the results have been shown in Figure 5(b). As demonstrated, when the pH is increased from 5.5–7.5, the quantity of CR uptake decreases somewhat due to a decrease in the number of charges (+ charges) on the surface of OtLP-Zn/Cu (pH_ZPC = 7.4) with increasing pH in this range. As a result, the force of attraction between CR anions (pK_a = 4) and the positive charges of OtLP-Zn/Cu will be decreased in the light form, resulting in a modest reduction in CR uptake. While the quantity of CR uptake was sharply decreased when pH climbed above 7.4 because of the force of repulsion between the anions of this adsorbate and the surface of OtLP-Zn/Cu, which became a negatively charged surface at a pH higher than 7.4. The same outcomes have been obtained for CR adsorption by the powdered shrimp shell [32].

Figure 5(c) depicts the effect of agitation duration time on the uptake of CR anions from aqueous solutions (100, 200, and 300 mg.L⁻¹) on 0.035 g of OtLP-Zn/Cu. This figure indicates that the amount of CR uptake gradually climbed with time and almost remained constant above 60 min because there is no empty site for any further adsorption after this time (60 min). Although adsorption achieved equilibrium in 1 h, the other further experiments were carried out over a period of 23 h to verify that all adsorption parameters were evaluated at equilibrium. Practically identical results were observed for CR adsorption by Teucrium polium [36] and waste of tea [56].

Figure 5(d) illustrates how the initial CR concentration influences OtLP-Zn/Cu uptake capacity (q_e) at various temperatures. The preliminary concentration and temperature of the CR solution have been shown to have a favourable impact on q_e. The favourable effect of the CR concentration was caused by augmenting the number of successful collisions between OtLP-Zn/Cu and CR cations [61], as well as the force of driving that defeats the resistance of CR cation mass transfer between solid and liquid [61,62]. Since, increasing the preliminary adsorbate concentration typically increases both driving force and the successful collision rate [61,62]. The fact that rising temperature increases the kinetic energy of CR cations and decreases the viscosity of the CR solution explains the observed positive influence of temperature on q_e [63]. Moreover, the outcomes of this part reveal that the CR anion adsorption by OtPL-Zn/Cu is endothermic.

Furthermore, each of the adsorptions mentioned above resulted in crystal-clear CR solutions and the absence of any solid particles visible to the human eye, confirming the stability of this adsorbent (OtLP-Zn/Cu) in CR solutions [64].

3.2.3. Kinetics outcomes

The experimental results for the adsorption of 100, 200, and 300 mg.L⁻¹ of CR solutions at 27 °C by OtLP-Zn/Cu were analysed using the linear equations of the 1st and 2nd order dynamic models provided in Table 2. Figures 6 and 7 show plots derived by using first- and second-order kinetic models, respectively. The intercepts and slopes of the plots of these two figures were used to derive the values of the rate constants and other parameters of these two kinetic models, which are reported along with values of CFEF and X² in Table 3. This table shows that R² values (correlation coefficient values) computed by the 2nd order dynamic model are equivalent to or nearly equal to one, and vice versa for the 1st order model. The experimental q_e values are nearly identical to q_e values estimated using the 2nd order model and considerably different from q_e values computed using the 1st order model. Moreover, the values of X² and CFEF of the 2nd order model are so much smaller than those of the 1st order model. Thus, the experimental dynamic data can be adequately described by the dynamic model of the 2nd order, confirming that CR anions were adsorbed on the OtLP-Zn/Cu surface by chemical bonds [65]. This chemisorption may take place via the mechanism of electron sharing or exchange between CR anions and the surface functional groups of OtLP-Zn/Cu [66]. The thermodynamic results will provide the conclusive evidence for the type of this adsorption mechanism, namely whether it is a chemical interaction such as sharing of electrons or an ionic exchange activity. For CR adsorption by the stem of papaya, nearly identical results were obtained [67].

Figure 6. Kinetic model of the 1st order for Congo red uptake by OtLP- Zn/Cu.

Figure 7. Kinetic model of the 2nd order for Congo red uptake by OtLP- Zn/Cu.

Table 3. Constants of the 1st and 2nd-order rate models for uptake of Congo red by OtLP- Zn/CuS.

		Kinetic model
		1st order					2nd order
C0 (mg.L^−1)	qe,exp (mg.g^−1)	qe1, cal (mg.g^−1)	K1 (h^−1)	R^2	CFEF	X^2	qe2, cal (mg.g^−1)	K2 (g/mg.h)	R^2	rate	CFEF	X^2
100	66.850	13.289	0.0226	0.951	42.91	215.87	67.568	0.0055	1.000	0.371	0.00770	0.00762
200	130.964	20.063	0.0182	0.864	93.91	613.01	131.579	0.0029	1.000	0.386	0.00289	0.00288
300	190.800	28.563	0.0152	0.727	137.95	921.51	192.308	0.0014	0.999	0.268	0.01191	0.01182

3.2.4. Isotherms outcomes

Figures 8 and 9 illustrate the plots that were produced after applying the linear forms of the Freundlich and Langmuir isotherm models to the practical data of this adsorption. The intercepts and slopes of the plots of these two figures were used to compute the parameters and other constants of these two isotherm models, which are listed along with the values of X² and CFEF in Table 4. The best isotherm model used in this study is the Langmuir model, which is proved by the linearity of the plots of these two figures, the values of R², the values of X² and CFEF (Table 4). This demonstrates that the adsorption of CR takes place by a monolayer, the OtLP-Zn/Cu surface is a homogenous, the energies of adsorption are equivalent, and there are no interactions between the adsorbed CR cations [67]. Furthermore, S_f (factor of separation) values that are below 1 (Table 4) show that this adsorption is a favourable process under the experimental circumstances used in the study [68]. Nearly identical results were obtained for CR adsorption by shiitake mushrooms [30] and Teucrium polium [36].

Figure 8. Freundlich isotherm for uptake of Congo red by OtLP- Zn/Cu.

Figure 9. Langmuir isotherm for uptake of Congo red by OtLP- Zn/Cu.

Table 4. Isotherm constants for the OtLP-Zn/Cu adsorption of Congo red.

	Isotherm model
	Langmuir parameters						Freundlich parameters
Temperature	qmax (mg.g^−1)	KL (L.mg^−1)	Sf	R^2	CFEF	X^2	KF (mg.g^−1) (L.mg^−1)^1/n	1/n	n	R^2	CFEF	X^2
27 °C	625.000	0.00546	0.109	0.998	66.88	48.29	6.172	0.744	1.345	0.981	198.12	32099.33
42 °C	714.286	0.00927	0.067	0.994	40.66	32.05	11.044	0.727	1.375	0.977	41.18	2099.02
57 °C	769.231	0.01903	0.034	0.997	27.61	22.85	21.626	0.683	1.465	0.967	594.14	17491.20

The OtLP-Zn/Cu is an inexpensive and effective adsorbent for the removal of CR from the liquid phase, as evidenced by the high values of the q_max (625.000, 714.286, and 769.231 mg.g⁻¹) in Table 4. This demonstrates that OtLP-Zn/Cu is an inexpensive and efficient adsorbent that will be of particular importance in the techniques used to purify wastewater and water from CR and other pollutants.

3.2.5. Thermodynamic characteristics

The calculated ln(q_e/C_e) values of CR adsorption by OtLP-Zn/Cu were plotted against 1/T (K⁻¹) (Equation (9)) as shown in Figure 10. The values of ΔS⁰ and ΔH⁰ were calculated, respectively, using the intercept and slope of the straight lines in Figure 9. ΔG⁰ values were computed using Equation (10) and then listed in Table 5 along with ΔS⁰ and ΔH⁰ values. According to Table 5, all ΔH⁰ values are positive, proving that CR cations were endothermically adsorbed by the surface of OtLP-Zn/Cu [68]. This is agreed well with the outcomes of the temperature impact (section 3.2.2). The mechanism of adsorption can be estimated via the ΔH⁰ magnitude. ΔH⁰ values less than 30 kJ.mol⁻¹, kJ.mol⁻¹, 5 kJ.mol⁻¹, 24–43 kJ.mol⁻¹, 2–29 kJ.mol⁻¹, and greater than 80 kJ.mol⁻¹, respectively, indicate hydrogen bonding, van der Waals forces, hydrophobic bond forces, ionic exchange, dipole bond forces, and chemical bond forces [69]. The ΔH⁰ values (32.567–42.168 kJ.mol⁻¹) found in this study demonstrate that ionic exchange and hydrogen bonding between the OtLP-Zn/Cu functional groups and anions of CR are the mechanisms via which CR adsorbs to OtLP-Zn/Cu. Increasing the randomness of the system due to adsorption of CR anions on the OtLP-Zn/Cu can be proved by the positive ΔS⁰ values [70]. Table 5 further reveals that values of ΔG⁰ are negative and become further negative with elevating temperatures, indicating that the CR anion adsorption by OtLP-Zn/Cu is spontaneous and favoured by rising temperatures [6], respectively.

Figure 10. Thermodynamic parameters for uptake of Congo red by OtLP- Zn/Cu.

Table 5. Thermodynamic parameters for the OtLP-Zn/Cu adsorption of Congo red

			ΔG^o (KJ.Mol^−1)
Concentration (mg. L^−1)	ΔH^o (kJ. Mol^−1)	ΔS^o (kJ.Mol^−1)	300K	315K	330K	R^2
40	42.168	0.150	−2.835	−5.085	−7.335	0.987
60	40.567	0.145	−2.954	−5.130	−7.306	0.998
200	39.658	0.140	−2.427	−4.531	−6.635	0.992
500	35.814	0.124	−1.347	−3.205	−5.063	0.997
700	32.567	0.111	−0.763	−2.430	−4.096	0.998

3.3. Adsorption performance comparison

The uptake capacities of OtLP-Zn/Cu and many of the inexpensive adsorbents lately employed for CR adsorption from liquid media have been summarized in Table 6. This table reveals that OtLP-Zn/Cu has greater uptake capacities than the other adsorbents (625.000, 714.286, and 769.231 mg.g⁻¹), which can be attributed to the considerable surface areas, suitable pores, and chemical activity of OtLP-Zn/Cu. The novelty and significance of this adsorbent among the various adsorbents earlier utilized for eliminating CR from effluents are therefore confirmed by the cheap cost of OtLP-Zn/Cu, its superior adsorptive performance, and the fact that it can be easily separated from the aqueous solutions after adsorption.

Table 6. Adsorption capacity of Congo red on OtLP-Zn/Cu and other various adsorbents

Adsorbents	qmax (mg.g^−1)		Sources
OtLP-Zn/Cu	625.0	27°C	This research
	714.3	42°C
	769.2	57°C
OtLP-Zn/Cu	625.0	27°C	This research
	714.3	42°C
	769.2	57°C
Bottom ash	1.1		[2]
Deoiled soya	1.1		[2]
Groundnut charcoal	117.6		[6]
Eichhornia charcoal	56.8		[6]
Montmorillonite	714.3		[26]
Treated shiitake mushroom	217.9	20°C	[30]
	211.9	35°C
	209.4	50°C
Pine bark	3.9	50°C	[31]
Treated Shrimp Shell	232.0	30°C	[32]
	264.6	40°C
	288.2	50°C
Raw Shrimp Shell	182.1	30°C	[32]
	182.1	40°C
	256.4	50°C
Eichhornia crassipes roots	1.6		[33]
Cellulose of Egyptian water hyacinth	230.0		[34]
Soybean curd	69.0	25°C	[35]
	69.2	40°C	[35]
	69.9	55°C
Seed of Foeniculum vulgare	313.92	27°C	[55]
	443.70	42°C
	609.57	57°C

4. Conclusions

In this study, CuS, ZnCl₂, and H₂C₂O₄ were utilized to modify the raw powder of Ocimum tenuiflorum leaves (OtLP). To choose the most effective one of these four adsorbents, the raw and modified powders of this herb’s leaves were characterized and examined for their ability to adsorb Congo red (CR). It was discovered that the powder modified by a combination of CuS and ZnCl₂ (OtLP-Zn/Cu) had the best percentage removal for CR (97.82%) owing to its suitable porosity (pore size of 1349.0 Å and pore volume of 2.1211 cm³.g⁻¹) and its high surface area (281.16 m².g⁻¹). As a result, OtLP-Zn/Cu was identified as the best adsorbent in this study for CR adsorption from solutions. The impacts of the most potent experimental variables on this adsorption were investigated. It was demonstrated that the increasing temperature from 27 °C to 57 °C, CR concentration from 20 mg.L⁻¹ to 1000 mg.L⁻¹, mass of OtLP-Zn/Cu from 0.005 g to 0.035 g, and time from 2 min to 45 min increase the uptake of CR. When the mass of OtLP-Zn/Cu and adsorption time were respectively increased over 0.035 g and 45 min, the capacity of this adsorption was almost invariable. The amount of CR uptake reduced gradually as the pH increased from 5.5 to 7.5 and significantly when the pH increased above 7.4. This adsorption followed the 2nd order model and the Langmuir model for its kinetics and isotherms, respectively. Thermodynamic analyses proved that this adsorption is both heat-absorbing and spontaneous. The high uptake capacities (625.000, 714.286, and 769.231 mg.g⁻¹) obtained in this study prove that the OtLP-Zn/Cu will be a particularly attractive adsorbent in the field of wastewater remediation. Moreover, after adsorption, this adsorbent was easily and thoroughly separated from the aqueous solutions.

Acknowledgments

The author is grateful to the Nanotechnology Research Unit, Faculty of Science, University of Tabuk, for providing him with the devices He needed to complete this research.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability

The data collected or investigated through this research were entirely included in the manuscript that was submitted.