Abstract
In this study, Congo red anionic dye was removed from an aqueous solution using powdered Citrus limetta peel. The adsorbent was evaluated with the use of FTIR and SEM. The highest dye removal was achieved when the operating parameters were optimized, including pH = 6.0, adsorbent dose = 0.4 g, contact time = 90 min, initial adsorbate conc. = 10 ppm, and temperature = 60 °C. The pseudo-second-order model was investigated to have the best fit for the kinetics of the process, with R2 = 0.9918 and Qe(cal) = 0.206 mg g-1, which is very close to the experimental Qe(exp) = 0.191 mg g-1. These two models’ plots showed that both physical and chemical adsorption were feasible. ΔG and ΔH being negative suggest that the adsorption was thermodynamically favorable and spontaneous. Testing the suggested technique with groundwater resulted in an 82% CR adsorption efficiency. Due to the incredible removal capacity of CR dyes from industrial effluents, research suggests that CLPP can be used as substitute adsorbents for the treatment of wastewater from the weaving and dyeing industries.
HIGHLIGHTS
Citrus limetta peel powder is cheap, effective and potential adsorbent for Congo red adsorption from wastewater.
The protonated part of hydroxyl and carboxyl group of CLPP combine with negative part of Congo red dye for adsorption of dye molecules.
Adsorbent characterization, optimized parameters, and adsorbent recovery were studied.
Kinetics, isotherm models, and thermodynamics were evaluated.
INTRODUCTION
The availability of fresh, clean drinking water decreases as the population is increasing. It is impossible to overstate the importance of water for human consumption. The solubility of all contaminants or solutes makes water quality gradually decrease (Wang & Chu 2011; Imran et al. 2022). Water pollution causes worldwide harmful diseases and kills about 15,000 people every day. As a result, the first logical step in solving this enormous issue is the recovery of clean water from wastewater. According to the World Bank, the textile business is one of the most polluting industries on the planet (Khan & Malik 2014). The Comprehensive Regulation of Contaminant Services Regulations of South Korea, which is applicable to both water and air pollution, identifies workplaces that produce more than 80 tons of air pollutants annually or more than 2,000 
of water pollutants daily (Kim et al. 2022). It is common knowledge that dumping dyes into waterways lowers sunlight penetration, boosts biological (BOD) and chemical (COD) oxygen consumption, slows respiration, and restricts plant growth (Al-Tohamy et al. 2022). Synthetic dyes are resistant, bioaccumulative, poisonous, mutagenic, and carcinogenic substances (Lellis et al. 2019).
. The molar mass of CR is 
, 
= 496 nm, 497 nm. 
Structure of Congo red.
CR was formerly used to color fabrics, but lighter, stain-resistant dyes have now replaced it. It is still used in histology to stain tissues for microscopic study and as an acid-base indicator since it turns red in the presence of alkalies and blue in the presence of acids (Swan & Zaini 2019). Along with similar dyes from the textile, printing and dyeing, paper, rubber, and plastic sectors, it is a serious effluent concern. Consequently, CR has been prohibited because of its cancer-causing properties (El-Ahmady et al. 2020).
For the decolorization of effluents, different kinds of processes are available, including ozonation, membrane separation, coagulation/flocculation, co-precipitation, oxidation, electrolysis, microorganism degradation, photochemical, and adsorption employing various types of adsorbents (Sharma & Kaur 2018; Samsami et al. 2020). Because it is a cheap method for extracting pigments or decolorizing textile pollutants, adsorption has been found to be one of the most successful and established wastewater treatment processes in the textile industry (Chai et al. 2021), Agricultural wastes such as rice husk (Chuah et al. 2005), sugarcane (Sarker et al. 2017), orange peel (Alwared et al. 2021), banana peel (Akpomie & Conradie 2020), dried neem leaf (Bhattacharyya & Sharma 2004), and corncobs (Peñafiel et al. 2020), as well as byproducts such as sugarcane husk, bamboo sawdust, moss, mud, kaolin, red soil, alumina, leaf extract, wood pellets, powdered peanut shells powder, and other lingo-cellulosic wastes have been used to study biosorption (Sharma & Kaur 2018), The benefits of adopting these materials include their widespread availability and inexpensive cost, as well as the fact that they do not require regeneration.
Scheme of adsorption of Congo red onto Citrus limetta peels.
EXPERIMENTATION AND METHODS
Chemicals and equipment
Nitric acid 
and sodium hydroxide (NaOH) were utilized as chemicals in the research studies, while deionized water was used throughout the study. UV–Vis spectrophotometer, scanning electron microscope (SEM), Fourier-transform infrared (FTIR) spectrophotometer, centrifugation machine, microwave, pH meter, weighing machine, conical flasks, beakers, pipettes, and a hot plate are all required during the entire procedure.
Preparation of CR dye solution
CR dye with a molecular formula 
and molar mass of 696.665 g mol−1 was collected from a chemistry laboratory, and to make 1,000 
dye stock solutions, CR was dissolved in 500 ml of deionized water, and the solution was then further diluted as needed.
Collection of adsorbents
, were extracted via sieving and then kept in sealed storage containers for subsequent research. 
Citrus limetta dried peel and peel powder.
Adsorption batch studies
Batch adsorption analyses were utilized to optimize a number of adsorption characteristics, including temperature (0, 10, 20, 30, 40, 50, and 60 °C), initial concentration of adsorbate CR dye (5, 10–100 ppm), amount of adsorbent (C. limetta peel powder; CLPP) dose (0.1–1.0 g), and pH (1–12). Throughout the experiments, just one adsorption parameter was adjusted at a time, keeping the others unchanged. After shaking, centrifugation was performed, and the quantity of CR dye contained in the sample solution was quantified by measuring absorbance at 496 nm using a UV–Vis spectrophotometer.
is the initial absorbance before dye adsorption and 
is the final absorbance after dye adsorption. Adsorption capacity 
was determined by the formula given in the following equation:
is the initial concentration, 
is the equilibrium concentration of dye, V is the volume taken for the dye solution; and M is the adsorbent's molar mass.Identification of ![]()
45 mL of 0.1 M 
solutions were taken in several 100 mL conical flasks to identify the point of zero charge 
of the adsorbent, and 1 g of the adsorbent was added to each flask in a range of 1–12. Now, 0.1 M HCl/NaOH solutions were used to adjust the 
values of these solutions in the range of 1–12. Each flask's total amount of solution was exactly 50 mL. After 2 days, the final pH of the liquids in these flasks was tested. The difference between initial pH and final pH values 
was plotted against initial pH. The junction point of the curve of ΔpH versus 
was noted as 
of the CLPP.
CHARACTERIZATION OF ADSORBENT
FTIR spectroscopy analysis
Using a Perkin Elmer FTIR spectrophotometer, the position of different functional groups (located at the surface of CLPP) was identified. The FTIR analysis of the adsorbent was carried out using a 100 mg pallet of potassium bromide (KBr) with a spectrum range of 4,000–600 
(Tiernan et al. 2020).
Scanning electron microscopy (SEM) analysis
The adsorbent was investigated using an SEM both before and after adsorption (JEOL, JSM-6510LV, Japan). In order to eliminate the hazardous CR dye, the morphological characteristics and texture of C. limetta were determined by using an SEM.
RESULTS AND DISCUSSION
FTIR analysis of CLPP
in C. limetta peel was mostly attributable to a wide band appearance of about 3,337 
in the FTIR spectra. The peak at 2,917 
was attributed to methylene group C–H stretching. The CO bond of the carboxylic groups found in the parts of biosorbent (CLPP) was ascribed to the strong peaks at 1,690 and 1,647 
(Nandiyanto et al. 2019). The ring of aromatic molecules might be responsible for the peaks at 1,453 and 1,421 
. The CH group on the surface of the CLPP is referred to the band at 1,377 cm−1. The peak at 1,183 
refers to an irregular length of the COC ether group found in the CLPP surface. The CO stretch of primary alcohol is responsible for the band's appearance at 1,037
(Oyekanmi et al. 2021). Some shifts (
) were identified in the FTIR spectra of Figure 4, which correspond to the OH and COOH groups, showing that these compounds noticeably improve the adsorption of Congo red dye onto the powdered C. limetta peels. Small changes in other frequencies are also observed. Functional groups such as 
, 
, 
, 
, and aliphatic stretching were identified on the adsorbent surface, as shown in Table 1. FTIR spectrum analysis of CLPP
FTIR peak of CLPP ( ) | Assignment |
|---|---|
| 3,337 | O–H |
| 2,917 | CH3/C–H asymmetric stretching vibration |
| 1,690, 1,647 | C–O stretching of carboxylic bond |
| 1,453 and 1,421 | Aromatic compound |
| 1,377 | C–H groups |
| 1,183 | C–O–C asymmetric stretching |
| 1,037 | C–O stretching of alcohol |
| FTIR peak of CLPP () | Assignment |
|---|---|
| 3,337 | O–H |
| 2,917 | CH3/C–H asymmetric stretching vibration |
| 1,690, 1,647 | C–O stretching of carboxylic bond |
| 1,453 and 1,421 | Aromatic compound |
| 1,377 | C–H groups |
| 1,183 | C–O–C asymmetric stretching |
| 1,037 | C–O stretching of alcohol |
FTIR spectrum of Congo red dye before adsorption (a) and after adsorption (b) (Oyekanmi et al. 2021).
FTIR spectrum of Congo red dye before adsorption (a) and after adsorption (b) (Oyekanmi et al. 2021).
SEM analysis of CLPP
SEM image of CLPP biomass (a) biomass loaded with CR (b) (Reproduced with permission from Shakoor & Nasar (2016)).
SEM image of CLPP biomass (a) biomass loaded with CR (b) (Reproduced with permission from Shakoor & Nasar (2016)).
Effect of pH
(a) Effect of pH on adsorption of CR using CLPP. (b) Graph for determination of point of zero charge 
.
(a) Effect of pH on adsorption of CR using CLPP. (b) Graph for determination of point of zero charge .
The point of zero charge (
) provides the best explanation for how pH solution affects dye absorption. As a consequence of pH, it is an advantageous and useful surface characteristic for identifying whether the surface is positively or negatively charged. The point of zero charge (
) value for CLPP was 6.6 (Figure 6(b)). This indicates that the adsorbent (CLPP) surface is positively charged at pH values below 8, net zero at pH 8, and negatively charged at pH values above 8. So, any anionic dye, like CR, is better able to adhere to the surface of the CLPP in a solution with a pH lower than 8. Due to the electrostatic force of attraction, the surface of the CLPP adsorbent becomes positively charged, which enhances the absorption of anionic dye. The removal of anionic dyes tartrazine and methyl blue yielded similar results (Ali et al. 2022; Rani & Chaudhary 2022).
Effect of adsorbent dose
at 20 °C were studied. Figure 7 shows that as the adsorbent dose is increased, the percentage of dye removal increases until the quantity of dye removal is no longer increased up to a certain limit (i.e., 0.4 g), after which it becomes constant because dye molecules have taken up all of the active sites (Lafi et al. 2019; Guo et al. 2020). The cause of this might be linked to interparticle attraction or the loss of open surface area due to congestion. The results of the experiment are given in Supplementary Table S2. 
Effect of adsorbent dose on adsorption of CR using CLPP.
Effect of contact time
At 10 ppm initial CR concentrations, the effect of contact time of CLPP adsorption capacity was investigated. The adsorption capacity rises with contact time and attains equilibrium after 90 min, as shown in Figure 8. It mostly originates from the presence of these binding sites on the powdered C. limetta peel that hinders any additional adsorption. This may be due to the fact that there are initially quite enough sites on the surface, which makes adsorption relatively simple. However, as time passes, the adsorbent surface gets saturated, slowing down the rate of adsorption (Suryavanshi & Shukla 2010). However, within the first 50 min, the rise is relatively higher. Experimental results are given in Supplementary Table S3.
Effect of initial dye concentration
Effect of contact time on adsorption of CR onto CLPP.
Effect of initial dye concentration on adsorption of CR onto CLPP.
Effect of temperature
Effect of temperature on adsorption of CR using CLPP.
Adsorption kinetic models
The adsorption kinetic model is crucial in regulating the effectiveness of the adsorption process and the rate at which adsorbate is taken up, which regulates the residence duration at the solid surface interface. The adsorption of CR was investigated using kinetic models: pseudo-first-order and pseudo-second-order, intraparticle diffusion model, and liquid film diffusion model (Wekoye et al. 2020).
Pseudo-first-order kinetic model
is the pseudo-first-order rate constant, while 
and 
are the quantities (mol g−1) adsorbed at equilibrium and at time t, respectively (
). The slope and intercept of 
against t may be used to compute the rate constants 
and 
. If the estimated and observed 
values are equal, the adsorption follows pseudo-first-order kinetics. The output of the pseudo-first-order kinetic model is shown in Supplementary Table S6, and Figure 11 displays its plot. Values of 
and 
were calculated from the intercept and slope of the straight line of the plot, respectively. Values of the slope coefficient (0.9585), 
show that the dye's adsorption on CLPP did not follow a pseudo-first-order kinetic model, as there is a huge difference between these values (Yaneva & Georgieva 2012). 
Pseudo-first-order plot of CR removal onto CLPP.
Pseudo-second-order kinetic model
versus t should provide a straight line, from which the slope and intercepts may be used to get 
and 
(Felista et al. 2020). From the result (shown in Supplementary Table S7 and Figure 12), it indicates that the value of slope coefficient (0.9918) for the evaluation of the pseudo-second-order model is greater than that of the pseudo-first-order kinetic model, and a little difference between 
and 
values shows that data fits best to the pseudo-second-order kinetic model. 
Pseudo-second-order plot of CR removal onto CLPP.
Intraparticle diffusion model
denotes the quantity of dye adsorbed in (
) at time t, I is the layer thickness, and 
is the intraparticle diffusion constant in (
). A straight line is obtained by drawing a graph between 
and 
calculating the slope 
and intercept ‘I’. A linear plot indicates that the only model employed in the adsorption process is the intraparticle diffusion model, and a zero intercept indicates that this kinetic model is the rate-determining step.
value of the intraparticle diffusion model was 0.9795. 
Intraparticle diffusion plot of CR removal onto CLPP.
Liquid film diffusion model
The movement of adsorbate molecules through a liquid film that supports the solid adsorbent is considered to be the rate-determining step in the adsorption process.
) and 
is the film diffusion constant. The adsorption kinetics is regulated by diffusion through liquid film, according to a linear curve of –ln(1 − F) against time with zero intercept (Plazinski 2010). Supplementary Figure S9 depicts the experimental outcome plot. There is no evidence of linearity in the graph. 
is also less than unity. As a result given in Supplementary Table S9 and Figure 14, the liquid diffusion model is only partially applicable to the CR dye adsorption onto CLPP. 
Liquid film plot of CR removal onto CLPP.
Results from the intraparticle diffusion model, the pseudo-second-order kinetic model, the pseudo-first-order kinetic model, and the liquid film model are summarized in Table 2 in the appropriate order. Because it has the highest slope coefficient (0.9912) and closest to unity among the other models, the pseudo-second-order (PSO) kinetic model is used to explain the present adsorption process, as shown in Table 2. Furthermore, in the situation of the pseudo-second-order kinetics, there is a clear consensus between calculated and experimental 
demonstrating that the data fits well with the pseudo-second-order model.
Results of all discussed kinetic models
| Kinetic models | Parameters | Values |
|---|---|---|
| Pseudo-first-order |  | 0.312  |
 | 0.191  | |
 | 5.808  | |
 | 0.9585 | |
| Pseudo-second-order |  | 4.316 × 10−5 |
 | 0.191  | |
 | 0.206  | |
 | 0.9912 | |
| Intraparticle diffusion model |  | 0.0114  |
 | 0.9795 | |
| Liquid film model |  | 0.0319  |
 | 0.967 |
| Kinetic models | Parameters | Values |
|---|---|---|
| Pseudo-first-order | | 0.312 |
| | 0.191 | |
| | 5.808 | |
| | 0.9585 | |
| Pseudo-second-order | | 4.316 × 10−5 |
| | 0.191 | |
| | 0.206 | |
| | 0.9912 | |
| Intraparticle diffusion model | | 0.0114 |
| | 0.9795 | |
| Liquid film model | | 0.0319 |
| | 0.967 |
Adsorption isotherms
The adsorption process must be completely quantified before it can be used commercially. These mathematical models explain the interactions between the adsorbent and the adsorbate, as well as the adsorption capacity. The most suitable isotherm model in the adsorption process is done by analyzing concentration data from tests utilizing several isotherm models. Langmuir, Freundlich, D-R, and Temkin isotherms for CR adsorption onto CLPP adsorbent were used in this study.
Langmuir isotherm model
The solid adsorbent's capacity for adsorption is restricted. Only one molecule of a solute may interact with each active site, which is all identical (monolayer adsorption). The adsorbate molecules do not interact with one another (Chen 2015).
is the maximum monolayer adsorption capacities (
); 
is the Langmuir isotherm constant (
); 
is the equilibrium concentration (
); and 
is the amount of adsorbate (
).
(
)/
(
) and 
(
) from which slope and intercept may be calculated, a straight line is formed. The experiment's findings, which are presented in Supplementary Table S11 and Figure 15, show that the adsorption of CR onto CLPP proceeds favorably since the value of 
for the Langmuir isotherm was 0.9891 (almost one). The value of the adsorption capacity 
is 
(given in Table 3). It demonstrated that, for monolayer adsorption, the energy of the initial layer of molecules adsorbed on the adsorbent's surface is substantially equivalent to the potential of heat (Batool et al. 2021) The separation factor (
, also known as the equilibrium factor), which describes the major characteristics of the Langmuir isotherm, indicates whether the adsorption process is favorable (0 < 
< 1), unfavorable (
> 1), linear (
= 1), or irreversible (
= 0) (Batool et al. 2021).
(
) is the Langmuir constant related to the adsorption's energy and 
is initial dye concentration. Values of 
identifying the shape of isotherm were calculated by the Langmuir constant ‘
’ and adsorbate concentration. The experimental value of KL is 6.56 × 10−10 L/mg. From 
and 
, we can determine 
. From the results (Supplementary Table S10), it was observed that 
values equal to 1 confirm the linear uptake of CR adsorption. When using non-linear equations to calculate isotherm values, Wavemetrices IGOR Pro 6.1.2 software was used (Zafar et al. 2019). Figure 15(b) demonstrates the Langmuir isotherm's non-linear model for CR dye adsorption onto CLPP. Parameters of adsorption isotherm models
| Isotherm models | Parameters | Values |
|---|---|---|
| Langmuir isotherm |  |  |
 |  | |
 | 0.9891 | |
| Freundlich isotherm | N | 1.9  |
 | 5.2742  | |
 | 0.9902 | |
| Temkin isotherm | B |  |
 |  | |
 | 12.5 | |
 | 0.885 | |
| D-R isotherm | Β |  |
 |  | |
 | 0.99 |
| Isotherm models | Parameters | Values |
|---|---|---|
| Langmuir isotherm | | |
| | | |
| | 0.9891 | |
| Freundlich isotherm | N | 1.9 |
| | 5.2742 | |
| | 0.9902 | |
| Temkin isotherm | B | |
| | | |
| | 12.5 | |
| | 0.885 | |
| D-R isotherm | Β | |
| | | |
| | 0.99 |
(a) Langmuir isotherm's linear plot for removal of CR onto CLPP. (b) Langmuir isotherm's non-linear plot for removal of CR onto CLPP.
(a) Langmuir isotherm's linear plot for removal of CR onto CLPP. (b) Langmuir isotherm's non-linear plot for removal of CR onto CLPP.
Freundlich isotherm model
(
) is the quantity of adsorbate per unit adsorbent, 
(
) is the Freundlich constant where n is the slope, and 
(
) is the equilibrium concentration of CR dye in the solution. A straight-line graph is constructed by plotting 
versus 
; the slope, 1/n, gives us the value of n, and the intercept, 
, gives us the value of 
. When the value of ‘n’ is larger than one, more adsorption takes place (Anah & Astrini 2018; Felista et al. 2020). The value of n in the current study is 1.9 
, which is more than 1 and follows the Freundlich adsorption isotherm condition, i.e., 0 > n > 1, showing that cooperative adsorption of CR dye on CLPP occurs to a significant extent. Good adsorption is indicated by a value of n between 2 and 10; moderate adsorption is represented by a value of n between 1 and 2; and poor adsorption is indicated by a value of n less than 1 (Imran et al. 2022; Sultana et al. 2022). The result of the Freundlich isotherm is presented in Supplementary Table S12, the Freundlich isotherm's linear plot is given in Figure 16(a), and the non-linear plot in Figure 16(b). The value of 
in case of Freundlich isotherm was 5.2742 and that of 
was 0.9902 very close to unity. 
(a) Freundlich isotherm's linear plot for removal of CR onto CLPP. (b) Freundlich isotherm's non-linear plot for removal of CR onto CLPP.
(a) Freundlich isotherm's linear plot for removal of CR onto CLPP. (b) Freundlich isotherm's non-linear plot for removal of CR onto CLPP.
Temkin isotherm model
is the Temkin isotherm equilibrium binding constant (
); 
is the Temkin isotherm constant; 
is the equilibrium concentration (
); 
is the equilibrium quantity of adsorbate in the adsorbent (
); R is the general gas constant (
); and T is the temperature in kelvin (K) (Yaneva & Georgieva 2012).
is positive (Dada et al. 2012). The value of 
is the relatively low compared to the other isotherms adsorption models. As a result, it does not provide the greatest fitting for the study's experimental results. Supplementary Table S13 contains the experimental data, and Figure 17(a) and 17(b) shows the linear graph and the non-linear plots, respectively. 
(a) Temkin isotherm's linear plot for removal of CR onto CLPP. (b) Temkin isotherm's non-linear plot for removal of CR onto CLPP.
(a) Temkin isotherm's linear plot for removal of CR onto CLPP. (b) Temkin isotherm's non-linear plot for removal of CR onto CLPP.
Dubinin–Radushkevich isotherm model
is the equilibrium quantity of adsorbate in the adsorbent (
), D-R isotherm constant is represented by β (
); 
represents the hypothetical isotherm adsorption capacity (
); and ε is the Polanyi potential and is expressed in Equation (14):
), the value of β is used:
. Additionally, it appears that the exclusion of CR dye is the mechanism of physical adsorption demonstrating from the value of 
obtained from Equation (16) of the D-R isotherm (
= 0.064 kJ mol−1). The value of free sorption energy increases with the strength of the bond between the adsorbent and adsorbate. In most cases, the D-R model is employed to distinguish between physical and chemical adsorption mechanisms (Javed et al. 2017a). The straight-line plot and the correlation factor (
) demonstrate that experimental data (presented in Supplementary Table S14, and Figure 18(a)) obeyed the D-R adsorption isotherm model completely (Yaneva & Georgieva 2012). The D-R isotherm's non-linear plot for adsorption of CR dye molecules is presented in Figure 18(b). 
(a) Dubinin–Radushkevich isotherm's linear plot for removal of CR onto CLPP. (b) Dubinin–Radushkevich isotherm's non-linear plot for removal of CR onto CLPP.
(a) Dubinin–Radushkevich isotherm's linear plot for removal of CR onto CLPP. (b) Dubinin–Radushkevich isotherm's non-linear plot for removal of CR onto CLPP.
The experimental results are best explained by the Freundlich and D-R isotherms, as demonstrated by the determination coefficient in accordance with the graphical depiction and Table 3 listing of the parameters of different isotherms. The slope coefficient for the Langmuir adsorption isotherm model is 0.9891, which is less than the slope coefficient for the Freundlich isotherm model. This difference confirms that the data fits best the Freundlich isotherm model, according to which CR adsorption takes place on the heterogeneous surface of the CLPP. Additionally, a value of n larger than one implies that the procedure of CR adsorption onto CLPP is beneficial, and the calculated value of 
indicates the physical adsorption. The operating conditions used in the adsorption mechanism (pH, initial concentration, amount of adsorbent, etc.) and the characteristics of the adsorbent determined the adsorption capacity. A comparison of adsorption capacity of different adsorbents for the removal of CR is given in Table 4. The parameter employed for comparison is the adsorption capacity. The value of adsorption capacity, which is in good alignment with the results of the majority of previously published research, suggests that CR might be easily adsorbed on the CLPP developed in this study.
Comparison of adsorption capacity of various adsorbents for Congo red dye removal
| Adsorbent | Maximum adsorption capacity | Reference |
|---|---|---|
| Banana peel powder | 1.721 | Mondal & Kar (2018) |
| Durian peel powder | 107.52 | Kamsonlian et al. (2013) |
| Solanum tuberosum peel | 6.9 | Rehman et al. (2018) |
| Peanut shell | 15.09 | Abbas et al. (2012) |
| Sugarcane bagasse | 38.2 | Zhang et al. (2011) |
| Orange peel powder | 11.919 | Harnal et al. (2020) |
| Tea waste | 3 | Foroughi-dahr et al. (2016) |
| Powdered eggshell | 95.25 | Zulfikar & Setiyanto (2013) |
| Cedrus deodara sawdust | 182.5 | Muneer et al. (2021) |
| Aloe vera leaves shell | 1,850 | Khaniabadi et al. (2017) |
| Citrus limetta peel powder |  | Present study |
| Adsorbent | Maximum adsorption capacity | Reference |
|---|---|---|
| Banana peel powder | 1.721 | Mondal & Kar (2018) |
| Durian peel powder | 107.52 | Kamsonlian et al. (2013) |
| Solanum tuberosum peel | 6.9 | Rehman et al. (2018) |
| Peanut shell | 15.09 | Abbas et al. (2012) |
| Sugarcane bagasse | 38.2 | Zhang et al. (2011) |
| Orange peel powder | 11.919 | Harnal et al. (2020) |
| Tea waste | 3 | Foroughi-dahr et al. (2016) |
| Powdered eggshell | 95.25 | Zulfikar & Setiyanto (2013) |
| Cedrus deodara sawdust | 182.5 | Muneer et al. (2021) |
| Aloe vera leaves shell | 1,850 | Khaniabadi et al. (2017) |
| Citrus limetta peel powder | | Present study |
ARE error analysis
and 
are calculated and experimental values of initial adsorbate concentration in solid state, respectively and ‘n’ is the number of valuable observation. The larger the value of 
and the smaller the value of an adsorption isotherm or kinetic model's error function, the better fit that model is to adsorption data (Batool et al. 2018; Alrobei et al. 2021).The outcomes of ARE error analysis for various adsorption isotherms and adsorption kinetic models are presented in Table 5. The results showed that, compared to other adsorption isotherms and kinetic models, the values of the ARE error analysis for the Freundlich isotherm and the pseudo-second-order kinetic model are smaller (although the values of the regression coefficients, i.e., 
are greater). It was revealed that the Freundlich and pseudo-second-order kinetic models were appropriate for this adsorption process.
ARE error analysis values of adsorption isotherm and adsorption kinetic models
| Isotherm models | (i) Langmuir isotherm model |  |
| (ii) Freundlich isotherm model |  | |
| (iii) Temkin isotherm model |  | |
| (iv) Dubinin–Radushkevich isotherm |  | |
| Kinetic models | (i) Pseudo-first-order | −60.74977625 |
| (ii) Pseudo-second-order | −98.60786052 | |
| (iii) Intraparticle diffusion model | −97.20220512 | |
| (iv) Liquid film model | −97.8442112 |
| Isotherm models | (i) Langmuir isotherm model | |
| (ii) Freundlich isotherm model | | |
| (iii) Temkin isotherm model | | |
| (iv) Dubinin–Radushkevich isotherm | | |
| Kinetic models | (i) Pseudo-first-order | −60.74977625 |
| (ii) Pseudo-second-order | −98.60786052 | |
| (iii) Intraparticle diffusion model | −97.20220512 | |
| (iv) Liquid film model | −97.8442112 |
Thermodynamic of adsorption
The thermodynamic characteristics, such as Gibbs free energy, might be used to study the viability and kind of adsorption. By plotting 
in contrast to 1/T, a straight-line graph is obtained. The graph's slope is ΔH, while its intercept is ΔS. Where R denotes the general gas constant, and its value is 8.31 
, and T denotes the temperature in Kelvin.
and 
:
is the constant of equilibrium, for sorption, R is the universal gas constant, and T is the temperature in kelvin (K). The value 
can be obtained by the given relation:
) and enthalpy change 
. Enthalpy 
and entropy 
at various temperatures were calculated using the slope and intercept of the 
versus 1/T plot. Figure 19 depicts the Van't Hoff plot graph. As the temperature rises from 298 to 338 K, the −ve value of 
increases, indicating that the adsorption process is more feasible and spontaneous, resulting in better CR sorption capability on CLPP. Because the value of free energy (
) reduces with rising temperature, the adsorption process becomes more spontaneous (He et al. 2010). It is clear that the adsorption of CR on CLPP is an exothermic process with negative 
values since the values of adsorption capacity decrease as temperature rises (shown in Table 6). This may be due to reduced binding forces between the dye molecules and the active sites of adsorbent (Litefti et al. 2019). A positive 
value reveals that the adsorption method is based on a separation mechanism (Oyekanmi et al. 2021). Statistical results for the thermodynamic adsorption are summarized in Supplementary Table S15. Calculated thermodynamic adsorption parameters
  |   |   |
|---|---|---|
| 1.6375 | ||
| −0.0779 | ||
| −2.0062 | ||
| −3.0973 | −37.378 | 0.133 |
| −3.5164 | ||
| −5.1942 | ||
| −5.6659 |
| | |
|---|---|---|
| 1.6375 | ||
| −0.0779 | ||
| −2.0062 | ||
| −3.0973 | −37.378 | 0.133 |
| −3.5164 | ||
| −5.1942 | ||
| −5.6659 |
Van't Hoff plot for adsorption of CR onto CLPP.
Adaptation of the procedure with tap water
Tap water was used to test the suggested procedure's applicability (Javed et al. 2017b; Batool et al. 2021). The experiment was performed under optimized conditions (pH = 6.0, adsorbent dosage = 0.4 g, contact period = 90 min, starting dye concentration = 10 ppm, temperature = 60 °C) to investigate the potential of CLPP for the removal of Congo red dye. According to the results, 82% of the color was removed. The results proved that the employed process is applicable to normal water samples (Naushad et al. 2016).
BIOSORPTION MECHANISM
, 
, 
, and 
groups were prominent on the surface of the C. limetta peels, according to FTIR spectra. After the adsorption of CR dye, FTIR peaks move (
) in the direction of –OH and –COOH groups were found in the spectra, indicating that such groups were primarily applicable for CR dye adsorption onto the surface of C. limetta peels. Water molecules in which adsorbent and adsorbate is present dissociates into 
and 
ions. CR dye contain 
group, 
group, which dissociates from water electrostatically attracts sodium ions to form NaOH leaving 
ions. The carboxylic or hydroxylic groups present on the surface of CLPP consume 
by protonation, forming positively charged 
molecules that combine with 
group of CR dye through an electrostatic effect to adsorb CR dye (Yang et al. 2019).
The pH of the adsorbate has an impact on the active site of the biosorption system. The pH of the solution has a substantial impact on the physical interaction of dye molecules on the binding sites of the absorbent. The highest adsorption capacity of Congo red dyes on C. limetta peels was noticed in this research when the solution was acidic (pH 6). C. limetta peels had a positively charged surface characteristics at an acidic pH and many protonated sites (
) on the surfaces, which increased the electrostatic interaction between the negatively charged CR dye molecules and the positively charged surface of the CLPP. Additionally, at a basic pH, CLPP's surface had many hydroxyl ions (OH), which led to the deprotonation, of the –COOH and –OH groups. This may decrease the electrostatic force between the positively surface charge of the C. limetta peel and the negatively charged CR dye molecules. CR dye adsorption onto the CLPP favored H-bonding and 
interaction in addition to electrostatic attraction (Oyekanmi et al. 2021).
Isotherm studies revealed that the D-R and Freundlich models best illustrated experimental data regarding the adsorption of CR dye molecules on the surface of CLPP, suggesting that multilayer adsorption takes place on a miscellaneous surface enriched with negative ions for the adsorption process.
DESORPTION
Recovery of adsorbent (CLPP) and adsorbate (Alrobei et al. 2021) are salient features of wastewater treatment because it demonstrate that this technique is inexpensive. Using sodium hydroxide (NaOH) as a desorbing agent, desorption test was performed to determine whether the CLPP adsorbent can be reused (Javed et al. 2017a; Dai et al. 2019; Lafi et al. 2019). In the experiment, a specific quantity (0.4 g) of CLPP adsorbent containing dye was agitated with 0.01 M NaOH for 25 min. According to the experiment's results (Supplementary Table S16), 80% of the adsorbent was regenerated, making it suitable for use in further adsorption procedures.
CONCLUSION
CLPP has an efficient and appropriate adsorption process for removing harmful, toxic synthetic dyes. The adsorption process of CR was significant within the range investigated. The optimum conditions for 88.2% adsorption of CR were pH 6, 0.4 g of adsorbent dose, 90 min contact time, and initial concentration of 10 ppm within 60 °C.
Different kinetic models, such as pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, as well as the liquid film model, have been used and debated. Kinetic data was better fitted to the pseud-second-order kinetic model as the value of slope coefficient (
) is 0.9918 and 
, which is very close to the experimental 
.The appropriate isotherm model for the adsorption process was selected after examining concentration data from the experiment using a number of isotherm models. In this investigation, the Langmuir, Freundlich, Temkin, and D-R isotherm models were adopted. The Freundlich and D-R isotherm models best described the biosorption process since their slope coefficient (
) value is 0.99, which is close to unity, and their ARE error analysis has a lower value.Overall, the thermodynamic characteristics demonstrated that CR dye adsorption on CLPP was spontaneous, exothermic, and dependent on physical forces.
The FTIR analysis identified a transition in the –OH and –COOH groups, predicting that these groups were mainly in charge of CR dye adsorption C. limetta peel surfaces.
The established technique was 82% applicable with tap water, and the desorption experiment resulted in an 80% regeneration of the adsorbent, indicating that it may be reused for adsorption. C. limetta peels may be employed as a competitive adsorbent for the removal of pollutants from the textiles and coloring industries due to the remarkable removal capacity of CR dyes.
ACKNOWLEDGEMENTS
First and foremost, all praise to Allah, the Almighty, the Most Merciful, for His blessings given to me during my study and in completing this work. I would like to offer my heartfelt appreciation and sincere thanks to my beloved parents, family, and friends for their prayers, support, and encouragement. In addition, my greatest gratitude and appreciation are addressed to my supervisor, Dr Tariq Javed, Lecturer, Department of Chemistry, University of Sahiwal, Sahiwal, Punjab, Pakistan, who has given me his valuable guidance, advice, and encouragement so I could complete this work in time.
FUNDING
No funding was allocated for the study's accomplishment.
DATA TRANSPARENCY STATEMENT
The authors will assure data transparency.
AUTHORS’ CONTRIBUTIONS
All authors contributed equally to this research study. The final manuscript was evaluated and approved by all authors.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.
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