Onion Peel: A Promising, Economical, and Eco-Friendly Alternative for the Removal of Divalent Cobalt from Aqueous Solutions

: There is a growing need for an economical and efficient method capable of removing heavy metals from residual water. The current contribution aimed to evaluate the capacity of onion peel, an abundant agroindustrial waste product, to remove divalent cobalt (Co 2+ ) from aqueous solutions. Onion peel was submitted to proximal chemical analysis, and various operational factors involved in biosorption were tested. The most suitable temperature (30 ◦ C), pH (7.0), and biosorbent particle size (300–800 µ m) were found. With an initial Co 2+ concentration of 380 mg L − 1 , the maximum capacity of Co 2+ removal was 59.88 mg g − 1 in 120 min. The pseudo-second order and Langmuir models provided the best fit to the experimental kinetics and equilibrium of Co 2+ biosorption, respectively. The thermodynamic study evidenced an exothermic, non-spontaneous, and favorable reaction ( ∆ H 0 = − 5.78 kJ mol − 1 ; ∆ S 0 = − 21.13 J mol − 1 K − 1 ), suggesting the formation of stable bonds in the biosorbent-Co 2+ complex. The carbonyl and hydroxyl groups apparently play a fundamental role in Co 2+ removal, and electrostatic attraction, ion exchange, and chemisorption are the principal mechanisms. Thus, the biosorption of Co 2+ by onion peel has potential as an economical, eco-friendly, efficient, and sustainable treatment for wastewater.


Introduction
The urban, industrial, and technological development of human beings has brought many advantages as well as many challenges.One of the challenges is the greater quantity of toxic residues that accompanies the ever-increasing production of goods.Many residues are not collected and processed adequately before being discharged into the environment, especially in developing countries [1].
Of the different types of industrial waste, heavy metals are particularly harmful to ecosystems and public health because they are highly toxic and are not biodegradable.Moreover, only some of them can be biotransformed into less toxic compounds [2].Heavy metals easily leach through the soil and into the groundwater, at which point they are taken up by diverse plants and animals and accumulate in the food chain [1,3].
To avoid this chain reaction of contamination, it is necessary to treat residual water containing heavy metals.There are various methods for the removal of toxic heavy metals from wastewater, such as ultrafiltration, reverse osmosis, electrodialysis, precipitation, and ion exchange.Each of these processes has one or more important disadvantages, including a high cost, a large amount of toxic residues or sludge generated, the incomplete removal of the heavy metals, and/or a high energy demand for the processes involved [4].In the last few decades, adsorption and biosorption have been considered the most potent and profitable processes for the removal of heavy metals from contaminated residual water [5] due to their low cost, efficiency, selectivity, versatility, flexibility, and tolerance to the toxicity of heavy metals [6].
One toxic heavy metal, cobalt (Co), is a trace element in the human body (constituting approximately 0.021 µg/g) under the conditions of natural equilibrium.It forms part of the structure of vitamin B 12 (cyanocobalamin).Cobalt deficiency is manifested as a loss of appetite, irritability, cell lysis, susceptibility to disease, hyperbilirubinemia, and macrocytic anemia [7].As a consequence of exposure to cobalt-contaminated food or environments, an excess of cobalt may accumulate in the human organism and cause allergy, lung conditions, lung cancer, auditory and visual impairment, vascular and endocrine dysfunction, interference in the absorption of iodine by the thyroid gland, the induction of genetic effects through oxidative damage to DNA by reactive oxygen species (ROS), and alterations in the process of DNA repair [8,9].
Cobalt and its salts are utilized to furnish color to different products, to elaborate steel, alloys (e.g., cobalt-chrome), and superalloys (e.g., nickel-cobalt) [10,11], and to manufacture cement, detergent, cleaning products, cosmetics, varnish, dyes, polyester resins (used in the fabrication of plastic), and the metallic components of clothes and shoes.It also serves to polish diamonds and marble.In the food industry, it serves as a stabilizer of beer suds [8,12].Lithium-cobalt batteries are components of cell phones, computers, and other electronic apparatuses, as well as a key constituent of electric vehicles (which are held to be more environmentally friendly than cars and trucks fueled by gasoline and diesel).As cobalt has become a heavy metal of vital importance in industry, the amount of associated toxic residues and environmental pollution has increased dramatically [11,13].
The lack of proper treatment of residual water containing cobalt has caused some bodies of surface water to contain levels of this metal reaching up to 0.5 mg L −1 , which is far above the maximum level of 0.1 mg L −1 recommended by the World Health Organization [14].Even though the concentration of cobalt rarely exceeds 5 µg L −1 in drinking water [15], it varies greatly in industrial wastewater.For example, residual water from cobalt mines contains anywhere from <10 mg L −1 [16] to 34,400 mg L −1 [17], while residual water from the electroplating industry has been found with 743.4 mg L −1 (12.6 mM) [18] to 5000 mg L −1 [19].Surface sediments near nickel-cadmium battery plants have approximately 700 mg kg −1 .The danger of such sediments is represented by leachates that reach bodies of water [20].
In groundwater, cobalt exists mainly in its divalent form (Co 2+ ).Since there is no worldwide standard for the acceptable cobalt concentration in water, each country has made its own regulations.The United States Agency for Toxic Substances and Disease Registry (ATSDR) has established a range of 1-107 µg/L as acceptable [10].Canada has set a maximum limit of 1 µg/L for cobalt in water with a hardness of 100 mg/L [21].Australia and New Zealand have defined 90 µg/L and 1 µg/L as the maximum limit for fresh and sea water, respectively [22].To remove cobalt, residual water is usually treated with chemical agents to precipitate the metal as hydroxides.Alternatively, it is passed through ion exchange resins [10].Although precipitation is an economical method and can be employed on a large scale with an efficiency of 99%, it is unspecific and becomes inefficient in the event of a low concentration of cobalt.More importantly, it generates abundant chemical sludge that is difficult to handle and costly to treat.Ion exchange, on the other hand, is a fast and efficient method with great specificity but cannot be used on a large scale.Furthermore, the resins involved in its processes are expensive, and their regeneration produces secondary pollution [23].
In the search for biotechnological alternatives, biosorption could be a desirable option for removing cobalt from residual water because it is both specific and economical.Moreover, it is possible to carry out biosorption with agroindustrial waste materials.Some of these are so abundant that they represent a problem for adequate disposal.Indeed, when not collected opportunely in local markets, they often cause plagues of rodents and insects.Whereas agroindustrial waste material can sometimes serve as compost, it may have other more valuable applications, including biosorption to clean heavy metals from polluted water, thus contributing to a circular economy (which is based on resource efficiency and the promotion of sustainability).
There are reports on the capacity of adsorption of cobalt in aqueous solutions by banana peel (2.55 mg Co 2+ g −1 ), orange peel (1.82 mg Co 2+ g −1 ), and lemon peel (22.0 mg Co 2+ g −1 ) [24,25].Of the types of agroindustrial waste materials described in the literature as biosorbents, the best capacity has been shown by the later.However, its adsorbent value is still relatively low compared to that of resins and zeolite (in the range of ~70-200 mg Co 2+ g −1 ) [26][27][28][29].Furthermore, there are many other agroindustrial wastes available in large quantities in markets, as is, for example, onion peel, which has biosorbent potential.
According to the Food and Agriculture Organization of the United Nations (FAO), onions were the second most abundant vegetable crop in the world in 2022 (after tomatoes), constituting ~111 million tons (onions and shallots, excluding their dehydrated products).The countries with the most abundant cultivation of onions are India and China [30].Mexico has an overall harvest of ~1.5 million metric tons of onions, being among the top 20 producers worldwide [31,32].As with any extremely large food crop, a tremendous amount of waste material is generated.
The primary residue of onions is its peel, which has been the subject of research to explore possible applications.Onion peel is rich in carbohydrates (up to 88.56% by weight) [33] and bioactive components such as flavonoids, fructooligosaccharides, phenolic compounds, and organosulfur compounds [34].Consequently, it might serve as a component in animal feed [35], human food products [36,37], and fertilizer [38], or as a source of phytochemicals beneficial for health [34,39,40].Additionally, onion peel may be suitable as an adsorbent of some contaminants of water (e.g., nitrates) [41], and has been studied as a source of food colorants [42], bioethanol [43], and other products derived from biorefineries [44].
Although an enormous volume of onion peel waste material is generated every year, it has not yet been reported, to our knowledge, as a biosorbent to remove cobalt ions from water.Due to the abundant availability of onion peel and the vital necessity of treating residual water with heavy metals such as Co 2+ , the current contribution aimed to explore the potential of this agroindustrial waste material as a biosorbent for the removal of Co 2+ from aqueous solutions.Onion peel was characterized physicochemically, microscopically, and spectroscopically.The effect of various factors on the kinetics of the biosorption of Co 2+ as well as the equilibrium (sorption isotherm) and the thermodynamics of the removal of the metal were analyzed.Finally, different eluent solutions were assayed for Co 2+ desorption and recovery.Based on the results, onion peel is suitable for the removal of Co 2+ from aqueous solutions.Therefore, future research is needed on its application as a biosorbent in aqueous solutions of other mono-metal systems and of multi-metal systems.

Biosorbent
The peel from bulb onions (Allium cepa L.), collected in a local market in Mexico City, Mexico (19.516793415202162, −99.13332227283549), was washed with an abundance of tap water, rinsed with type II deionized water (0.7-1.0 mmhos cm −1 ), and dried in a lab forced air drying oven (Luzeren DHG-9070A, Luzeren, Hangzhou, China) at 60 • C for 48 h.The resulting biomass was ground with a hammer mill (DCFH 48, Glen Creston, London, UK) and sifted to homogenize the size of the particles between 0.3 and 0.8 mm.The sifted material was stored in hermetically sealed glass containers to keep it dry until needed.
For some of the present evaluations, onion peel was subjected to a process of saturation with cobalt.Accordingly, 1 g L −1 of dry onion peel was placed in a solution with 400 mg L −1 Co 2+ .After the suspension was left under constant agitation for 3 h, the liquid was poured out to separate it from the saturated biosorbent.The latter was washed with an abundance of type II water to eliminate the excess metal.Subsequently, the saturated biosorbent was left to dry for 48 h in a lab oven at 60 • C and then stored in hermetically sealed glass jars.

Co 2+ Quantification
The cobalt in solution was quantified with a spectrophotometer by the dimethylglyoxime (DMG, JT Baker, Xalostoc, Mexico State, Mexico) method.The absorbance of the yellow DMG-Co 2+ complex is proportional to the concentration of divalent cobalt and can be read on a UV-VIS spectrophotometer (Hinotek 752N, Ningbo Hinotek Technology, Ningbo, China) at 400 nm [45].

Effect of Some Operational Parameters on the Biosorption of Co 2+ by Onion Peel
For each of the operational parameters herein analyzed, simultaneous controls free of the biosorbent were run under the same experimental conditions in order to verify that the concentration of cobalt in the solution was not lowered by abiotic factors, including precipitation or adsorption of cobalt on the glass walls of the container.Given the absence of any decrease in the concentration of Co 2+ in the abiotic controls, the Co 2+ removal observed in the distinct experiments was due only to biosorption by onion peel.Controls free of cobalt were also run to examine whether any colored compound from onion peel was being leached.In such a case, the colored compound could possibly interfere with the spectrophotometric quantification of residual cobalt.Since no colored compound was leached from the onion peel, there was no interference in the spectrophotometric determination.

Effect of the Size of the Particles of Onion Peel
Three size intervals of ground onion peel (300-500 µm, 500-800 µm, and 300-800 µm) were tested to assess the differences in effect on the biosorption of Co 2+ .Accordingly, a solution of CoCl 2 •6H 2 O (JT Baker, Xalostoc, Mexico State, Mexico) was prepared at a concentration of 100 ppm of Co 2+ and was adjusted to pH 7 at room temperature (rt = ~30 • C).The concentration of Co 2+ (C 0 ) was measured at time zero (t 0 ) in a sample of the solution.A known volume of the solution was placed in a given Erlenmeyer flask, and one of the three particle sizes of ground onion peel was added at a concentration of 1 g (dry weight) L −1 .The suspensions were kept under constant agitation at 120 rpm for 6 h, during which time the pH was maintained at a constant level and adjusted when necessary with a diluted solution of HCl or NaOH (JT Baker, Xalostoc, Mexico State, Mexico; purity > 99%).
Samples of the suspensions were taken at various time points to examine the kinetics of biosorption.They were immediately filtered through Whatman No. 42 filter paper (Cytiva, St. Louis, MO, USA), and 1 mL of the filtrate was taken to quantify the concentration of residual Co 2+ in order to enable the calculation of the biosorption capacity (q t , mg g −1 ) with Equation (1): where C 0 is the initial concentration of Co 2+ (mg L −1 ) at t = 0, C res is the residual concentration of Co 2+ (mg L −1 ) at time t (min), and X is the concentration of the biosorbent (g L −1 ).At equilibrium, C res = C e (mg L −1 ) and q t = q e (mg g −1 ).The results were analyzed to understand the kinetics of biosorption by onion peel and to identify the most suitable particle size of the material for the remainder of the experiment.

Effect of pH
Utilizing the particle size selected, a similar procedure was followed to evaluate the effect of pH on biosorption.The kinetics of biosorption was assessed at ~30 • C with an initial concentration of 100 mg L −1 Co 2+ and solutions adjusted to different pH values (2.0, 3.0, 4.0, 5.0, 6.0, and 7.0).The highest pH tested was 7.0 in order to avoid the precipitation of Co 2+ in the form of hydroxide [46].The pH was kept constant with a solution of diluted HCl or NaOH.Thus, the pH best suited for Co 2+ removal by onion peel was determined.

Effect of the Contact Time and Initial Concentration of Co 2+ in the Solution
Once again, a similar procedure was carried out to evaluate the effect on biosorption when using a range of initial concentrations of Co 2+ (from 20-400 mg L −1 ) with the previously defined onion peel particle size and pH of the solution.The kinetics of the removal of the metal over time was modeled based on the experimental data.With the values of the residual concentration at equilibrium (C e ) and the equilibrium capacity (q e ), the model of sorption isotherms was constructed.For each concentration of Co 2+ , the contact time required to reach equilibrium (t eq , min) was established.

Effect of Temperature
Solutions of Co 2+ from 20 to 400 mg L −1 were put into different flasks.The pH was adjusted to 7.0 and kept constant.After the initial concentration of Co 2+ (C 0 ) was quantified for each flask, the selected particle size of onion peel was added (X = 1 g L −1 ) and each suspension was left under constant agitation at 30 • C, 50 • C, or 60 • C. Samples were taken to determine the residual Co 2+ at equilibrium (C e ) and the capacity of biosorption at equilibrium (q e ).Hence, the sorption isotherm was ascertained at each distinct temperature, finding the effect of this parameter on the capacity of sorption of Co 2+ by onion peel.

Kinetic and Equilibrium Modeling of Co 2+ Removal
The kinetic behavior of onion peel as a biosorbent to remove Co 2+ over time was modeled with equations for pseudo-first order and pseudo-second order kinetics (Equations ( 2) and ( 3)) [47].
Pseudo-first-order (PFO) Pseudo-second-order (PSO) where q t is the biosorption capacity (mg g −1 ) at time t (min), q e1 and q e2 represent the predicted capacity of the removal of the metal (mg g −1 ) at equilibrium in accordance with the PFO and PSO models, respectively, and k is the rate constant of the kinetic model (k 1 , min −1 ; k 2 , g mg −1 min −1 ).
The sorption isotherms at the different temperatures assayed were calculated with the equations given by Langmuir, Freundlich, and Sips (Equations ( 4)-( 6)) [48]. Langmuir Freundlich Sips where C e is the concentration of cobalt in the solution (mg L −1 ) at equilibrium, q e is the experimental capacity of biosorption (mg g −1 ) at equilibrium, q m is the maximum predicted capacity of Co 2+ removal (mg g −1 ), n is the exponent, and k is the constant of the corresponding model (k L , L mg −1 ; k F , mg g −1 (mg L −1 ) −1/nF ; ks, (mg L −1 ) −nS ).The kinetic and equilibrium parameters for all three models were obtained on Graph-Pad Prism software version 10.2.2 (GraphPad Software, Boston, MA, USA) by non-linear regression.After feeding experimental data into the models, the similarity of the resulting tendencies was analyzed with the greatest correlation coefficient (R 2 ) and the least root mean square error (RMSE).

Evaluation of the Thermodynamic Parameters
The thermodynamic parameters were determined based on the method proposed by Guo et al. [49] and Zhao et al. [50].The distribution coefficients (K d , L g −1 ) were calculated for each concentration of cobalt at equilibrium at the different temperatures assayed (30, 50, and 60 • C) by means of Equation (7).
Subsequently, the graph was constructed for ln K d = f(C e ) at each temperature.The intersection at the y-axis is considered ln K 0 , where K 0 is the equilibrium constant of sorption.Its substitution in Equation ( 8) provides the change in Gibbs free energy (∆G 0 , J mol −1 ) for each temperature.
where R is the constant of ideal gases (8.3145 J mol −1 K −1 ) and T is the temperature in Kelvin (K).

Evaluation of the Desorption of Co 2+
The desorption of Co 2+ from onion peel was carried out by exposing 1 g L −1 of material saturated with the metal to different eluent solutions at rt and under constant agitation at 120 rpm for 3 h.The eluents assayed were H 2 SO 4 (0.1 M), HCl (0.1 M), HNO 3 (0.1 M), C 2 H 2 O 4 (0.1 M), NaCl (0.1 M), NaOH (0.1 M), acetone (1%), and deionized water at pH 7. The concentration of Co 2+ in the filtered solution was determined and then the desorption capacity (q D , mg g −1 ) and desorption efficiency (E D , %) were calculated at 3 h with Equation (11) and Equation (12), respectively.
where C sol and C i are the concentration of Co 2+ in the solution (mg L −1 ) at the final (t f = 3 h) and initial (t i = 0 h) time of desorption, respectively.The concentration in the solution of the biosorbent saturated with Co 2+ (g L −1 ) is represented by X sat and the maximum experimental capacity of biosorption of Co 2+ (59.88 mg g −1 ) at equilibrium and rt by q e max exp .

Characterization of the Biosorbent 2.7.1. Proximal Chemical Analysis
The proximal chemical analysis of onion peel was performed to discover the percentage of humidity, crude fiber, crude protein, ether extract, and ash by following the appropriate methods of the AOAC [51]: 934.01, 2001.11,920.39, 942.05, and 962.09, respectively.The nitrogen-free extract was found by subtracting the sum of the previously mentioned quantities from 100%.

Specific Surface Area, Size, and Volume of the Pores of the Biosorbent
The specific surface area, size, and volume of the pores of the biosorbent were assessed on Quantachrome ASiQwin Automated Gas Sorption Data Acquisition and Reduction (Quantachrome Instruments, Boynton Beach, FL, USA).The isotherms of adsorp-tion/desorption of degasified liquid nitrogen were determined with a vacuum pump at 77 K.The samples were pretreated with nitrogen at 373 K.The surface area was calculated by the multipoint BET method (Brunauer, Emett and Teller) and the distribution of the size of the pore and its volume by the BJH method (Barret, Joyner and Halenda) [52,53].

Evaluation of the Zeta Potential
Solutions of deionized water were adjusted to pH values between 1.5 and 7 with a diluted solution of HCl or NaOH.To each solution was added 1 g L −1 of biosorbent and the measurement of the zeta potential of the biosorbent surface was made at rt with a Zetasizer Nano-25 apparatus (Malvern Instruments Ltd., Malvern, UK) [54].

Analysis of the Morphology and the Elemental Composition of the Biosorbent Surface
Micrographs were taken of the biosorbent before and after biosorption of Co 2+ and following desorption in order to analyze its microstructure and explore possible changes in its morphological characteristics.The micrographs were acquired on a scanning electron microscope coupled to energy-dispersive X-ray spectroscopy (SEM/EDS).The samples were dried at 60 • C for 24 h and then covered with gold prior to being observed under a JEOL high-resolution scanning electron microscope (model JSM7800F, Jeol Ltd., Tokyo, Japan) with an acceleration voltage of 5 kV [45].

Fourier-Transform Infrared Spectroscopy (FTIR)
On the biosorbent surface, the main functional groups involved in the biosorption of Co 2+ were identified.Accordingly, samples of onion peel before and after biosorption and following desorption were submitted to Fourier-transform infrared spectroscopy (FTIR) on a Frontier spectrophotometer (PerkinElmer, Waltham, MA, USA) equipped with an attenuated total reflection (ATR) accessory with a SeZn crystal (Pike Technologies, Madison, WI, USA).The FTIR-ATR spectra were acquired with 64 scans in the interval of 4000-500 cm −1 with a resolution of 4 cm −1 [54].

Statistical Analysis
All assays were performed at least twice (independently) and the determinations were made at least three times.Data are expressed as the average ± standard deviation.The statistical analysis was carried out on GraphPad Prism software Ver.10.2.2 (GraphPad Software, Boston, MA, USA).Significant differences were established with two-way ANOVA followed by Tukey's test to create confidence intervals.Significance was considered at α = 0.05.

Effect of Biosorbent Particle Size
The kinetics of the effect of the particle size of onion peel on the biosorption of Co 2+ can be appreciated in Figure 1.The particles of 300-500 µm and 500-800 µm had a very similar capacity of biosorption at experimental equilibrium (q e exp ) (36.96 ± 0.17 mg g −1 and 40.54 ± 0.11 mg g −1 , respectively), although the rate of Co 2+ removal during the first 20 min was greater with the smaller particles.
40.54 ± 0.11 mg g −1 , respectively), although the rate of Co 2+ removal during the first was greater with the smaller particles.According to diverse reports, the smaller the particle size, the better the capac rate of removal of the sorbate, which owes itself to a greater surface area and conseq an increase in surface contact [45,[55][56][57].Due to the proximity of the two intervals ticle size (300-500 µm and 500-800 µm), a mixture was made of both sizes of m (300-800 µm).Compared to the individual fractions, the mixture reached an interm capacity of biosorption (38.06 ± 0.32 mg g −1 ) at experimental equilibrium (qe exp), a time taken to reach equilibrium was the same as the 300-500 µm size (45 min).Th a significant difference (p < 0.0001) between the two individual fractions (300-500 µ 500-800 µm) in the biosorption at experimental equilibrium (36.96 ≤ qe exp ≤ 40.54 and the time to reach equilibrium (45 ≤ teq ≤ 60 min) (Table 1).Nevertheless, the va qe exp were very similar, probably because the particle size of the two fractions was Other studies that have tested distinct sizes of particles have presented similar fi with relatively minor differences in biosorbent capacity at equilibrium and a grea ference in relation to the rate of removal [45,58].Based on the aforementioned resu mixture of the two fractions (300-800 µm) was used for posterior experiments, w advantage of a larger quantity of onion peel being available for biosorption.
The kinetic parameters obtained by the pseudo-first order and pseudo-secon models are portrayed in Table 1.The pseudo-second order model showed a bett the experimental data, as evidenced by the closeness of the theoretical calculations as the greater value of R 2 (≥0.993) and the lower value of RMSE (≤0.989).According to diverse reports, the smaller the particle size, the better the capacity and rate of removal of the sorbate, which owes itself to a greater surface area and consequently an increase in surface contact [45,[55][56][57].Due to the proximity of the two intervals of particle size (300-500 µm and 500-800 µm), a mixture was made of both sizes of material (300-800 µm).Compared to the individual fractions, the mixture reached an intermediate capacity of biosorption (38.06 ± 0.32 mg g −1 ) at experimental equilibrium (q e exp ), and the time taken to reach equilibrium was the same as the 300-500 µm size (45 min).There was a significant difference (p < 0.0001) between the two individual fractions (300-500 µm and 500-800 µm) in the biosorption at experimental equilibrium (36.96 ≤ q e exp ≤ 40.54 mg g −1 ) and the time to reach equilibrium (45 ≤ t eq ≤ 60 min) (Table 1).Nevertheless, the values of q e exp were very similar, probably because the particle size of the two fractions was similar.Other studies that have tested distinct sizes of particles have presented similar findings, with relatively minor differences in biosorbent capacity at equilibrium and a greater difference in relation to the rate of removal [45,58].Based on the aforementioned results, the mixture of the two fractions (300-800 µm) was used for posterior experiments, with the advantage of a larger quantity of onion peel being available for biosorption.
The kinetic parameters obtained by the pseudo-first order and pseudo-second order models are portrayed in Table 1.The pseudo-second order model showed a better fit to the experimental data, as evidenced by the closeness of the theoretical calculations as well as the greater value of R 2 (≥0.993) and the lower value of RMSE (≤0.989).A different letter marking the removal capacity at experimental equilibrium indicates a significant difference caused by the effect of particle size (two-way ANOVA for multiple comparisons with Tukey's test to create confidence intervals; n = 6; significance was considered at α = 0.05).

Effect of the pH of the Solution
The higher the pH (from 1.5 to 7), the greater was the capacity of onion peel to remove cobalt from the solution (Figure 2).The removal capacity was almost null at a pH of 2.0 (q e exp = 2.72 mg g −1 ) because of the elevated concentration of H + ions in the acidic solution.These ions compete with the cation Co 2+ for the sorption sites.Moreover, it is likely that at an acid pH, the sorption sites are protonated, providing a biosorbent surface with a positive charge, which inhibits the adsorption of Co 2+ by an electrostatic repulsive force [55,59].A different letter marking the removal capacity at experimental equilibrium indicates a significant difference caused by the effect of particle size (two-way ANOVA for multiple comparisons with Tukey's test to create confidence intervals; n = 6; significance was considered at α = 0.05).

Effect of the pH of the Solution
The higher the pH (from 1.5 to 7), the greater was the capacity of onion peel to remove cobalt from the solution (Figure 2).The removal capacity was almost null at a pH of 2.0 (qe exp = 2.72 mg g −1 ) because of the elevated concentration of H + ions in the acidic solution.These ions compete with the cation Co 2+ for the sorption sites.Moreover, it is likely that at an acid pH, the sorption sites are protonated, providing a biosorbent surface with a positive charge, which inhibits the adsorption of Co 2+ by an electrostatic repulsive force [55,59].The increase in pH from 2.0 to 7.0 caused a significant enhancement of the capacity of onion peel to remove Co 2+ at equilibrium (from 2.72 ± 0.07 to 38.06 ± 0.32 mg g −1 ).This The increase in pH from 2.0 to 7.0 caused a significant enhancement of the capacity of onion peel to remove Co 2+ at equilibrium (from 2.72 ± 0.07 to 38.06 ± 0.32 mg g −1 ).This may owe itself to the change from a positive to a greater and greater negative charge of the biosorbent with higher pH values, thus favoring an electrostatic attraction for Co 2+ at the sorption sites.According to the aforementioned findings, electrostatic attraction (a mechanism of physisorption) is one of the principal mechanisms contributing to the biosorption of Co 2+ by onion peel.
However, the notable biosorption as of a pH of 3.0 (25.64 ± 0.18 mg g −1 ) strongly suggests the likelihood of other mechanisms being involved in the removal of the metal from the solution.One possibility is ion exchange, which consists of the substitution of a cation in the solution by a cation on the surface of the biosorbent.Indeed, research carried out on the capacity of Hami melon peels to remove Ni 2+ and Pb 2+ led to similar conclusions in relation to the effect of the pH of the solution.Ion exchange was found to be responsible for biosorption at a low pH (2.0) and electrostatic interaction at a higher level (4.0 to 10.0) [60,61].
As the pH in the current study rose from 4.0 to 7.0, the time to reach equilibrium (t eq ) became shorter (Figure 2), perhaps because the sorption sites are more negatively charged at pH 7.0 than pH 4.0.The experimental data of the kinetic parameters of the assays with distinct levels of pH are displayed in Table 2, along with the values predicted by the pseudo-first order and pseudo-second order models.The latter model provided the best fit to the experimental kinetic behavior of Co 2+ removal by onion peel, considering a pH of 4.0 to 7.0.Since the greatest biosorption capacity was obtained at a pH of 7.0, this value was chosen for the remainder of the experiments.A different letter marking the removal capacity at experimental equilibrium indicates a significant difference caused by the effect of pH (two-way ANOVA for multiple comparisons with Tukey's test to create confidence intervals; n = 6; significance was considered at α = 0.05).

Effect of the Contact Time and Initial Concentration of Co 2+ in the Solution
The time of contact of onion peel with Co 2+ had a considerable effect on biosorption at all of the different initial concentrations of the metal (20-380 mg L −1 ) tested in the aqueous solution (Figure 3).The biosorption capacity increased rapidly during the initial stage of biosorption, then slowly as the experimental time increased until it finally reached a maximum constant value corresponding to the equilibrium biosorption capacity.
As the concentration of Co 2+ in the solution went from 20 to 380 mg L −1 , the capacity of biosorption by onion peel rose from 10.82 ± 0.11 to 59.88 ± 0.08 mg g −1 , while the percentage of Co 2+ removal decreased from 54.1 to 15.76% (Figure 3).This is easily understood, since an increase in the concentration of the metal implies a greater probability that the sorbate will make contact with an available sorption site [45].Moreover, the gradient of the concentration of the sorbate constitutes the driving force for promoting the transfer of sorbate mass from the aqueous phase (the solution) to the solid phase (the biosorbent).On the other hand, with a higher concentration of sorbate in the solution, the active sites of sorption on the surface of the biosorbent become occupied more quickly, thus saturating the biosorbent and lowering the percentage of Co 2+ removal [62].
of the concentration of the sorbate constitutes the driving force for promoting the transfer of sorbate mass from the aqueous phase (the solution) to the solid phase (the biosorbent).On the other hand, with a higher concentration of sorbate in the solution, the active sites of sorption on the surface of the biosorbent become occupied more quickly, thus saturating the biosorbent and lowering the percentage of Co 2+ removal [62].The experimental data on the Co 2+ removal capacity and time to reach equilibrium are presented for different concentrations of the metal in the solution (Table 3), along with the predicted values of these kinetic parameters when using the pseudo-first order and pseudo-second order models.Of the two models, the latter furnished the better fit to the experimental results.As the concentration of Co 2+ increases, the rate constant (k2) of second-order adsorption tends to decrease while the time to reach equilibrium becomes longer (from 45 to 120 min).The diminished rate of biosorption is likely due to a reduced availability of vacant sorption sites as the onion peel becomes saturated, implying a lower probability of ions finding available sites [45,63].The experimental data on the Co 2+ removal capacity and time to reach equilibrium are presented for different concentrations of the metal in the solution (Table 3), along with the predicted values of these kinetic parameters when using the pseudo-first order and pseudo-second order models.Of the two models, the latter furnished the better fit to the experimental results.As the concentration of Co 2+ increases, the rate constant (k 2 ) of second-order adsorption tends to decrease while the time to reach equilibrium becomes longer (from 45 to 120 min).The diminished rate of biosorption is likely due to a reduced availability of vacant sorption sites as the onion peel becomes saturated, implying a lower probability of ions finding available sites [45,63].
The reported Co 2+ removal capacity of various materials is described in Tables 4 and 5.In these studies, the best range of pH for divalent cobalt removal was from 4.0 to 9.0.At higher pH values, Co 2+ precipitates as cobalt hydroxide [46].Compared to the other biosorbent materials used without pretreatment or chemical modification, the onion peel of the current contribution showed a great capacity of biosorption of the metal (Table 4).Additionally, it exhibited a very good removal capacity in relation to some resins of ion exchange and is much less costly.Finally, the onion peel displayed a capacity of Co 2+ removal superior to some adsorbent minerals listed in Table 5.
The methods of adsorption, biosorption, and ion exchange are all efficient, and they are all affected by the same factors.The most important ones are the pH, the concentration of the metal, the temperature, and the time of contact [76].Each method has advantages and disadvantages.For instance, the resins of ion exchange are expensive, but can be regenerated many times.However, the process of regeneration produces secondary contamination.Activated carbon is able to adsorb heavy metals from aqueous solutions on a large scale, but is unspecific and is becoming more scarce and, thus, more expensive [77].For this reason, other types of adsorbent carbon have been studied, such as those derived from biological materials or natural minerals (e.g., zeolite, dolomite, and bentonite).Unfortunately, these materials lose efficiency after several cycles [23,78].Agroindustrial waste material is, by nature, economical.Although its regeneration potential is limited, it can be reused as a substrate of fermentation for the production of bioethanol or biogas, as has been proposed in the search for clean technology and sustainable energy [79,80].With a greater initial Co 2+ concentration (C 0 ), the general tendency was an enhanced biosorption capacity of onion peel at equilibrium (qe) (Figure 4).Interestingly, there were actually two tendencies, as shown by the analysis of the effect of temperature together with the initial concentration of the metal in solution.When C 0 < 65 mg L −1 , the biosorption capacity (q e ) improves slightly as the temperature increases.Conversely, when C 0 ≥ 100 mg L −1 , the biosorption capacity (q e ) decreases as the temperature increases.The maximum Co 2+ removal capacity (q e max ) was reached at 30 • C, perhaps because of a greater tendency of the metal ions to migrate from the surface of the biosorbent to the solution at a higher temperature [68].The methods of adsorption, biosorption, and ion exchange are all efficient, and they are all affected by the same factors.The most important ones are the pH, the concentration of the metal, the temperature, and the time of contact [76].Each method has advantages and disadvantages.For instance, the resins of ion exchange are expensive, but can be regenerated many times.However, the process of regeneration produces secondary contamination.Activated carbon is able to adsorb heavy metals from aqueous solutions on a large scale, but is unspecific and is becoming more scarce and, thus, more expensive [77].For this reason, other types of adsorbent carbon have been studied, such as those derived from biological materials or natural minerals (e.g., zeolite, dolomite, and bentonite).Unfortunately, these materials lose efficiency after several cycles [23,78].Agroindustrial waste material is, by nature, economical.Although its regeneration potential is limited, it can be reused as a substrate of fermentation for the production of bioethanol or biogas, as has been proposed in the search for clean technology and sustainable energy [79,80].

Effect of Temperature on the Biosorption of Co 2+
With a greater initial Co 2+ concentration (C0), the general tendency was an enhanced biosorption capacity of onion peel at equilibrium (qe) (Figure 4).Interestingly, there were actually two tendencies, as shown by the analysis of the effect of temperature together with the initial concentration of the metal in solution.When C0 < 65 mg L −1 , the biosorption capacity (qe) improves slightly as the temperature increases.Conversely, when C0 ≥ 100 mg L −1 , the biosorption capacity (qe) decreases as the temperature increases.The maximum Co 2+ removal capacity (qe max) was reached at 30 °C, perhaps because of a greater tendency of the metal ions to migrate from the surface of the biosorbent to the solution at a higher temperature [68].The present findings coincide with those reported in relation to Co 2+ removal by other materials such as activated carbon from the leaves of Citrus limetta, lemon peel, spent green tea leaves, spent coffee, and Fe3O4-chitosan nanocomposite [25,68,69,72,81].Nonetheless, The present findings coincide with those reported in relation to Co 2+ removal by other materials such as activated carbon from the leaves of Citrus limetta, lemon peel, spent green tea leaves, spent coffee, and Fe 3 O 4 -chitosan nanocomposite [25,68,69,72,81].Nonetheless, some studies have described an improved capacity for Co 2+ removal with higher temperatures [45].
The effect of temperature is indicative of the thermic nature of the process of biosorption, and the current results suggest the exothermic nature of the biosorption of Co 2+ by onion peel.Research on the biosorption of Co 2+ by different materials has found examples of endothermic and exothermic biosorption, as well as cases in which the temperature has no effect (Tables 4 and 5).

Biosorption at Equilibrium
The experimental isotherms of sorption of Co 2+ by onion peel at 30, 50, and 60 • C are illustrated in Figure 5a-c, as are those predicted by the Langmuir, Freundlich, and Sips models.As aforementioned, when C 0 ≥ 100 mg L −1 , an increase in temperature leads to a decrease in the maximum capacity of adsorption (q e max exp ) of Co 2+ by onion peel (from 58.46 to 38.32 mg g −1 ).The isotherm predicted by the Langmuir model is the closest of the three models to the experimental data at all temperatures tested.Considering the values of k L (0.032 to 0.095 L mg −1 ), the Langmuir model best fits the actual behavior of the biosorption of Co 2+ by onion peel at equilibrium and reveals a favorable biosorption (0 < k L < 1).This model assumes that sorption is carried out by a monolayer on a homogenous surface with a finite number of sorption sites and an absence of interactions between the adsorbed species [49].The data of the error functions (R 2 and RMSE) could appear to point to the Sips model as the best fit for the experimental results at all temperatures.According to the Sips model, however, the theoretical values of the biosorption capacity at 30 and 50 • C (71.11 ± 3.22 and 80.94 ± 9.03 mg g −1 , respectively) are well above the experimental values (58.46 ± 2.26 and 50.29 ± 1.66 mg g −1 , respectively) and do not reflect the experimentally observed tendency to decrease with an increase in temperature (Table 6).At a temperature of 60 • C, the Sips model predicts a value of the maximum biosorption capacity closer to the one generated by the Langmuir model (q mS ≈ q mL ≈ 39.0 mg g −1 ) and to the experimental results (Figure 5c).This occurs because the exponent n s approaches one unit (n s = 0.996).The parameters of biosorption calculated by the three models are shown in Table 6.Constant units: k L (L mg −1 ), q mL (mg g −1 ), k F (mg g −1 (mg L −1 ) −1/nF ), n F (dimensionless), ks (mg L −1 ) −nS , q mS (mg g −1 ), n S (dimensionless).A different letter marking the removal capacity at experimental equilibrium indicates a significant difference due to the effect of temperature (two-way ANOVA for multiple comparisons with Tukey's test to create confidence intervals; n = 6; significance was considered at α = 0.05).

Thermodynamic Study
Since there was only a slight tendency of enhanced biosorption with an increase in temperature when C 0 < 65 mg L −1 , and a very notable tendency of decreased biosorption with an increase in temperature when C 0 ≥ 100 mg L −1 , the latter data were employed for the thermodynamic study.The graphs of ln K d versus C e (Figure 6a) and ∆G 0 versus T (Figure 6b) allowed for the determination of the thermodynamic parameters that characterize the biosorption of Co 2+ by onion peel (Table 7), carried out in accordance with the methods proposed by Guo et al. [49] and Zhao et al. [50].Constant units: kL (L mg −1 ), qmL (mg g −1 ), kF (mg g −1 (mg L −1 ) −1/nF ), nF (dimensionless), ks (mg L −1 ) −nS , qmS (mg g −1 ), nS (dimensionless).A different letter marking the removal capacity at experimental equilibrium indicates a significant difference due to the effect of temperature (two-way ANOVA for multiple comparisons with Tukey's test to create confidence intervals; n = 6; significance was considered at α = 0.05).

Thermodynamic Study
Since there was only a slight tendency of enhanced biosorption with an increase in temperature when C0 < 65 mg L −1 , and a very notable tendency of decreased biosorption with an increase in temperature when C0 ≥ 100 mg L −1 , the latter data were employed for the thermodynamic study.The graphs of ln Kd versus Ce (Figure 6a) and ΔG 0 versus T (Figure 6b) allowed for the determination of the thermodynamic parameters that characterize the biosorption of Co 2+ by onion peel (Table 7), carried out in accordance with the methods proposed by Guo et al. [49] and Zhao et al. [50].The values herein found for the free energy of Gibbs (ΔG 0 ) were positive, indicating that the process is not spontaneous [82].The values of ΔG 0 increase with a higher temperature of the aqueous solution, implying a greater energy barrier for biosorption.There- The values herein found for the free energy of Gibbs (∆G 0 ) were positive, indicating that the process is not spontaneous [82].The values of ∆G 0 increase with a higher temperature of the aqueous solution, implying a greater energy barrier for biosorption.Therefore, the adsorption of Co 2+ becomes less spontaneous as the temperature of the solution rises [69].The negative value of enthalpy (∆H 0 ) confirms the exothermic nature of the process of Co 2+ removal by onion peel.On the other hand, the negative value of entropy (∆S 0 ) suggests the formation of stable bonds and a reduction in the randomness of the system, and consequently a favorable reaction [69].Hence, a part of the Co 2+ seems to be adsorbed on the surface of onion peel by chemisorption.

T ln K
Various reports on the use of onion peel without any pretreatment for the removal of diverse types of metals and other contaminants are listed in Table 8.In the majority of the works cited, the pseudo-second order model provided the best fit with the experimental kinetic behavior of biosorption.In many of them, the Langmuir model was suitable for predicting the process at equilibrium.The majority of the authors cited (Table 8) have established a physical process as the predominant mechanism of adsorption.Interestingly, some studies on onion peel have found an endothermic biosorption while others describe an exothermic biosorption, even in relation to the removal of the same ion (Cd 2+ ) [56,62].The disparity in results may owe itself to differences between varieties of onion and in sorption conditions.Table 8.Some adsorbates for which onion peel has been evaluated as an adsorbent without pretreatment.

Sorbate
q e (mg g −

Desorption
The desorption of Co 2+ was carried out in various eluent solutions (Figure 7), finding the best capacity and efficiency with the solutions of strong acids.The most effective desorption of Co 2+ occurred with HCl (43.32 mg g −1 ) and HNO 3 (41.95mg g −1 ), exhibiting an efficiency of 73.44% and 72.27%, respectively.This could be related to an increase in the hydrogen ions (H + ) in the solution, which protonate the surface of the biosorbent and thus promote ion competition between Co 2+ and H + for the same binding sites, leading to the release of cobalt ions from the biosorbent surface [87,88].Other researchers have also reported a better desorption efficiency for cobalt with the use of acidic solutions, especially with HCl [45,[88][89][90][91].The NaCl solution possibly favored ion exchange between Na + and the Co 2+ adsorbed on the biosorbent [92], reaching a recovery of 44.8% of the metal (26.40 mg g −1 ).The other four solutions that are not strong acids exhibited a lower efficiency of desorption.The water at pH 7.0 showed 0% of elution of Co 2+ .
The results of desorption with acidic solutions and NaCl reinforce the hypothesis that electrostatic attraction and ion exchange are the main mechanisms of removal of Co 2+ by onion peel.However, the lack of complete desorption of Co 2+ suggests the formation of covalent bonds, which are known to exist in complexes with great stability [60].Indeed, the negative value of entropy (−ΔS 0 ) found in the thermodynamic analysis also points to the stability of the biosorbent-Co 2+ complex.Hence, chemisorption is an additional mechanism involved in the biosorption of cobalt by onion peel, as aforementioned.

Proximal Chemical Analysis
The data from the proximal chemical analysis of onion peel and the intervals of the corresponding values reported in the literature are provided in Table 9.Since onions have been consumed for at least 4000 years and their cultivation has spread around the world, there are many varieties.These can be divided into at least three categories based on the color of the skin (white, yellow, and red) of the mature bulbs.The composition, size, flavor, and pungency of onions depend on the genotype and environmental factors, including the temperature, type of soil, fertilization, geographic zone, and harvest season.The same factors may affect the composition of onion peel [34,97], as The NaCl solution possibly favored ion exchange between Na + and the Co 2+ adsorbed on the biosorbent [92], reaching a recovery of 44.8% of the metal (26.40 mg g −1 ).The other four solutions that are not strong acids exhibited a lower efficiency of desorption.The water at pH 7.0 showed 0% of elution of Co 2+ .
The results of desorption with acidic solutions and NaCl reinforce the hypothesis that electrostatic attraction and ion exchange are the main mechanisms of removal of Co 2+ by onion peel.However, the lack of complete desorption of Co 2+ suggests the formation of covalent bonds, which are known to exist in complexes with great stability [60].Indeed, the negative value of entropy (−∆S 0 ) found in the thermodynamic analysis also points to the stability of the biosorbent-Co 2+ complex.Hence, chemisorption is an additional mechanism involved in the biosorption of cobalt by onion peel, as aforementioned.

Proximal Chemical Analysis
The data from the proximal chemical analysis of onion peel and the intervals of the corresponding values reported in the literature are provided in Table 9.Since onions have been consumed for at least 4000 years and their cultivation has spread around the world, there are many varieties.These can be divided into at least three categories based on the color of the skin (white, yellow, and red) of the mature bulbs.The composition, size, flavor, and pungency of onions depend on the genotype and environmental factors, including the temperature, type of soil, fertilization, geographic zone, and harvest season.The same factors may affect the composition of onion peel [34,97], as indicated by the wide intervals of values described by different authors for its components (Table 9).
The onion peel utilized presently was hermetically conserved in dry form, as reflected in the low humidity content (4.56%), which is close to the 3% humidity detected by Ismail et al. [94].The humidity of the material depends on factors such as the temperature, drying time, and relative humidity in the atmosphere [98].The current results of the protein (3.9-5.83%) and ash content (6.9-12.24%)are similar to those documented by Sagar et al. [95].The principal minerals in onion peel are potassium, phosphorous, and calcium, according to this author [95].The percentage of fat (ether extract) was higher in the present sample (6.5%) than that found in several studies (0.04-0.47%) [33,34,95], but lower than the value (15.13%) published by Bello et al. [93].
Some researchers have identified crude fiber as the main component of onion peel, constituting up to 62.09% [36].Among the most abundant components of crude fiber are α-cellulose (41%), hemicellulose (16%), and lignin (38.9%) [99].The value of fiber in this study was 26.97%, lower than the content of carbohydrates (53.35%).Diverse publications mention that onion peel is rich in carbohydrates, comprising up to 88.56% on a dry weight basis [33,34].Pectin, galacturonic acid, galactose, rhamnose, and arabinose are carbohydrates found in onion peel, according to various publications [100].

Zeta Potential
The current maximum zeta potential (−3.6 mV) for onion peel was found at a pH of 2.0 and the minimum level of this parameter (−27.85 mV) was found at a pH of 7.0 (Figure 9).Due to the lack of a positive value of the zeta potential at any pH (1.5 to 7.0), the pH of zero charge (PZc) could not be determined.Other researchers have described a PZc of onion peel from 4.02 to 6.34 [41,63,84].

Zeta Potential
The current maximum zeta potential (−3.6 mV) for onion peel was found at a pH of 2.0 and the minimum level of this parameter (−27.85 mV) was found at a pH of 7.0 (Figure 9).Due to the lack of a positive value of the zeta potential at any pH (1.5 to 7.0), the pH of zero charge (PZc) could not be determined.Other researchers have described a PZc of onion peel from 4.02 to 6.34 [41,63,84].The surface charge is related to the composition of the onion peel, which is known to contain a high quantity of fiber and carbohydrates.The functional groups (e.g., COO − , OH − , and COOH − ) in these compounds provide binding sites for Co 2+ .In the biosorbent, the greater the number of available binding sites with a negative charge, the more negative will be the zeta potential and the better will be the adsorption of the metal if one of the principal mechanisms of removal is electrostatic attraction [45], as is currently the case.At a pH of 4, 5, and 6, the corresponding zeta potential values showed no significant difference, while the biosorption capacities were very similar (35.45, 36.07, and 36.25 mg g −1 , respectively).
The zeta potential of onion peel was higher following biosorption (Figure 9) because the binding of Co 2+ to functional groups on its surface left less negatively charged sorption sites.Similar results have been reported subsequent to the saturation of other adsorbents with cations such as Pb 2+ , Cd 2+ , Ni 2+ , Cu 2+ , and Cr 3+ [103,104].This change in zeta potential confirms one of the conclusions reached during the evaluation of the effect of the pH of the solution and Co 2+ desorption: electrostatic attraction is one of the important mechanisms involved in Co 2+ removal by onion peel.

SEM/EDS Analysis
According to the EDS analysis, the main metals in the onion peel before biosorption were Na + , Mg 2+ , K + , and Ca 2+ (Co 2+ was not detected) (Figure 10a).Contrarily, after saturation was reached, Co 2+ was found but not Na + , Mg 2+ , or K + , probably because the latter metals were displaced by the Co 2+ ions (Figure 10b).Hence, ion exchange was likely another mechanism governing the biosorption of Co 2+ by onion peel.
The EDS spectrum of onion peel following desorption with 0.1 M HCl (Figure 10c) reveals the complete elimination of Ca 2+ as well as the continuing presence of some Co 2+ ions.The peak associated with Co 2+ ions was less intense subsequent to desorption (versus post-biosorption).The lack of complete desorption of Co 2+ may be due to the very stable chemical bonds, such as covalent bonds, formed during biosorption.The spectrum displays a peak corresponding to Au + , which is the material used to cover the onion peel.The surface charge is related to the composition of the onion peel, which is known to contain a high quantity of fiber and carbohydrates.The functional groups (e.g., COO − , OH − , and COOH − ) in these compounds provide binding sites for Co 2+ .In the biosorbent, the greater the number of available binding sites with a negative charge, the more negative will be the zeta potential and the better will be the adsorption of the metal if one of the principal mechanisms of removal is electrostatic attraction [45], as is currently the case.At a pH of 4, 5, and 6, the corresponding zeta potential values showed no significant difference, while the biosorption capacities were very similar (35.45, 36.07, and 36.25 mg g −1 , respectively).
The zeta potential of onion peel was higher following biosorption (Figure 9) because the binding of Co 2+ to functional groups on its surface left less negatively charged sorption sites.Similar results have been reported subsequent to the saturation of other adsorbents with cations such as Pb 2+ , Cd 2+ , Ni 2+ , Cu 2+ , and Cr 3+ [103,104].This change in zeta potential confirms one of the conclusions reached during the evaluation of the effect of the pH of the solution and Co 2+ desorption: electrostatic attraction is one of the important mechanisms involved in Co 2+ removal by onion peel.

SEM/EDS Analysis
According to the EDS analysis, the main metals in the onion peel before biosorption were Na + , Mg 2+ , K + , and Ca 2+ (Co 2+ was not detected) (Figure 10a).Contrarily, after saturation was reached, Co 2+ was found but not Na + , Mg 2+ , or K + , probably because the latter metals were displaced by the Co 2+ ions (Figure 10b).Hence, ion exchange was likely another mechanism governing the biosorption of Co 2+ by onion peel.Before biosorption, the SEM micrographs of the onion peel exhibit a heterogenous material with an irregular and porous surface on the outer layer and other layers underneath (Figure 10d).After biosorption, the surface appears soft and more regular, possibly because of the saturation of the material with Co 2+ (Figure 10e).Following desorption, the surface looks heterogeneous again and with fiber (Figure 10f).There did not seem to be any damage produced by the eluent solution.The EDS spectrum of onion peel following desorption with 0.1 M HCl (Figure 10c) reveals the complete elimination of Ca 2+ as well as the continuing presence of some Co 2+ ions.The peak associated with Co 2+ ions was less intense subsequent to desorption (versus post-biosorption).The lack of complete desorption of Co 2+ may be due to the very stable chemical bonds, such as covalent bonds, formed during biosorption.The spectrum displays a peak corresponding to Au + , which is the material used to cover the onion peel.
Before biosorption, the SEM micrographs of the onion peel exhibit a heterogenous material with an irregular and porous surface on the outer layer and other layers underneath (Figure 10d).After biosorption, the surface appears soft and more regular, possibly because of the saturation of the material with Co 2+ (Figure 10e).Following desorption, the surface looks heterogeneous again and with fiber (Figure 10f).There did not seem to be any damage produced by the eluent solution.

FTIR-ATR Analysis
The FTIR-ATR spectrum of the onion peel before biosorption shows a wide signal band assigned to the stretching frequency of the -OH groups at around 3255 cm −1 (Figure 11a), generated by the hydrogen bridges formed between organic molecules in the biomaterial [105].The -OH groups could derive from phenols and flavonoids, which provide onion peel with its antioxidant capacity [38].The content of such compounds is lower for white versus yellow and red onion peel [106].The FTIR-ATR spectrum of the onion peel before biosorption shows a wide signal band assigned to the stretching frequency of the -OH groups at around 3255 cm −1 (Figure 11a), generated by the hydrogen bridges formed between organic molecules in the biomaterial [105].The -OH groups could derive from phenols and flavonoids, which provide onion peel with its antioxidant capacity [38].The content of such compounds is lower for white versus yellow and red onion peel [106].The -CH stretching frequency between 2905 and 2848 cm −1 is attributed to the -CH2 and -CH3 groups.The signal for the carbonyl group (C=O) in the compounds, which contain carboxyls and aldehydes, is found at 1720, 1591, and 1335 cm −1 .The band at 1227 cm −1 corresponds to the C-O stretching frequency of the ethers, and the bands at 1178 cm −1 and 1004 cm −1 to the signals inside and outside the C-H plane and to the stretching frequency of the C-O bonds in the carbohydrates, respectively.Below 800 cm −1 is the fingerprint signal of the biomaterial [62,83,85,86,105,107].
The fingerprint signal is distinct for the three conditions of onion peel herein tested: unsaturated (Figure 11a), saturated (Figure 11b), and desorbed (Figure 11c).The differences can be explained by the structural modifications resulting from biosorption and desorption.One of the biggest changes caused by biosorption is the reduction in intensity of the band at 3500-3000 cm −1 ascribed to the hydrogen bridges between the -OH groups, thus evidencing the interaction of Co 2+ with the functional groups that contain -OH.Other differences induced by saturation with Co 2+ are the peaks from 1700 to 1000 cm −1 (the shaded part in Figure 11).A decrease in intensity is displayed for the bands at 1720, 1335, 1178, and 1004 cm −1 , while the signal at 1591 was displaced to 1574 cm −1 .These signals belong to the C=O stretching frequency of the carbonyl group.Additionally, two new bands appeared at 1436 and 1203 cm −1 , signals representing the deformation of the O-H and the C-O stretching frequency.Overall, the findings point to the crucial role in Co 2+ removal by onion peel played by the hydroxyl groups (-OH) and carbonyl groups (C=O) in the carboxyls, aldehydes, and ketones.This conclusion coincides with two reports, one The -CH stretching frequency between 2905 and 2848 cm −1 is attributed to the -CH 2 and -CH 3 groups.The signal for the carbonyl group (C=O) in the compounds, which contain carboxyls and aldehydes, is found at 1720, 1591, and 1335 cm −1 .The band at 1227 cm −1 corresponds to the C-O stretching frequency of the ethers, and the bands at 1178 cm −1 and 1004 cm −1 to the signals inside and outside the C-H plane and to the stretching frequency of the C-O bonds in the carbohydrates, respectively.Below 800 cm −1 is the fingerprint signal of the biomaterial [62,83,85,86,105,107].
The fingerprint signal is distinct for the three conditions of onion peel herein tested: unsaturated (Figure 11a), saturated (Figure 11b), and desorbed (Figure 11c).The differences can be explained by the structural modifications resulting from biosorption and desorption.One of the biggest changes caused by biosorption is the reduction in intensity of the band at 3500-3000 cm −1 ascribed to the hydrogen bridges between the -OH groups, thus evidencing the interaction of Co 2+ with the functional groups that contain -OH.Other differences induced by saturation with Co 2+ are the peaks from 1700 to 1000 cm −1 (the shaded part in Figure 11).A decrease in intensity is displayed for the bands at 1720, 1335, 1178, and 1004 cm −1 , while the signal at 1591 was displaced to 1574 cm −1 .These signals belong to the C=O stretching frequency of the carbonyl group.Additionally, two new bands appeared at 1436 and 1203 cm −1 , signals representing the deformation of the O-H and the C-O stretching frequency.Overall, the findings point to the crucial role in Co 2+ removal by onion peel played by the hydroxyl groups (-OH) and carbonyl groups (C=O) in the carboxyls, aldehydes, and ketones.This conclusion coincides with two reports, one describing the fundamental role of the carboxyl groups in the adsorption of cationic metals by onion peel [63], and another demonstrating that carboxyl and hydroxyl groups in the cellulose, hemicellulose, and lignin of lemon peel are the principal factors involved in Co 2+ removal [25].
When comparing the spectra of desorbed and saturated onion peel, the former displays a wider band at 3500-3000 cm −1 (Figure 11c), which is narrower than the corresponding band in the unsaturated onion peel.There is a greater similarity between the spectra of the desorbed and saturated onion peel than between the desorbed and unsaturated material.In relation to the unsaturated peel, the desorbed material exhibits a displacement of the bands afforded by the signal of the C=O group at 1684, 1589, and 1319 cm −1 , of the C-O of ether at 1194 cm −1 , and of the -OH group at 1410 cm −1 .Furthermore, a marked decrease is found in the intensity of the signal at 1589 cm −1 and of the bands at 1173 and 1004 cm −1 assigned to the carbohydrates.The similarity of desorbed and saturated onion peel probably owes itself to the bonding of some of the hydrogen bridges and C=O groups to the remaining Co 2+ in the former material, thus confirming the importance of the -OH and C=O groups and the mechanism of chemisorption in the removal of Co 2+ by onion peel.
According to the current results, onion peel is capable of removing Co 2+ from residual water in an economical, efficient, and environmentally friendly manner.Hence, a new value can be given to this agroindustrial waste material, which is so abundantly available that it often causes environmental problems due to improper disposal.The opportunity to turn an environmental problem into an environmental asset represents an enormous benefit of the use of onion peel as a biosorbent of heavy metals.Future research is needed on the capacity of this material as a biosorbent in other mono-metal systems and in multi-metal systems.

Conclusions
With resources becoming more limited and water increasingly contaminated by heavy metals due to the ever-greater amount of production in the economy, an abundant agroindustrial waste material like onion peel could provide a solution to the need for water treatment.Onion peel offers an economical alternative and has a very good capacity to remove Co 2+ from aqueous solutions without causing harm to the environment.The onion peel under study is a mesoporous material rich in carbohydrates.The latter contain C=O and O-H groups responsible for the biosorption of cobalt.The outer monolayer of the biosorbent becomes coated with Co 2+ until reaching saturation in an exothermic, non-spontaneous, and partially reversible process.The factors capable of influencing the interaction of the sorbate (Co 2+ ) with the biosorbent are the size of the particles of onion peel, the initial concentration of Co 2+ , the pH and temperature of the solution, and the time of contact.According to the present findings, Co 2+ removal by onion peel is a process of physical and chemical sorption.The main mechanisms involved in Co 2+ removal are electrostatic attraction, ion exchange, and chemisorption.

Figure 1 .
Figure 1.Effect of the particle size on Co 2+ biosorption by onion peel (pH = 7.0, C0 = 100 mg 1 g L −1 , and T = 30 °C).The continuous lines portray the kinetic behavior predicted by th with the best fit to experimental data.

Figure 1 .
Figure 1.Effect of the particle size on Co 2+ biosorption by onion peel (pH = 7.0, C 0 = 100 mg L −1 , X = 1 g L −1 , and T = 30 • C).The continuous lines portray the kinetic behavior predicted by the model with the best fit to experimental data.

Figure 2 .
Figure 2. Effect of pH on Co 2+ biosorption by onion peel (particle size = 300-800 µm, C0 = 100 mg L −1 , X = 1 g L −1 , and T = 30 °C).The continuous lines portray the kinetic behavior predicted by the model with the best fit to experimental data.

Figure 2 .
Figure 2. Effect of pH on Co 2+ biosorption by onion peel (particle size = 300-800 µm, C 0 = 100 mg L −1 , X = 1 g L −1 , and T = 30 • C).The continuous lines portray the kinetic behavior predicted by the model with the best fit to experimental data.

Figure 3 .
Figure 3.Effect of pH on Co 2+ biosorption by onion peel (particle size = 300-800 µm, pH = 7.0, X = 1 g L −1 , and T = 30 °C).The continuous lines portray the kinetic behavior predicted by the model with the best fit to experimental data.

Figure 3 .
Figure 3.Effect of pH on Co 2+ biosorption by onion peel (particle size = 300-800 µm, pH = 7.0, X = 1 g L −1 , and T = 30 • C).The continuous lines portray the kinetic behavior predicted by the model with the best fit to experimental data.

Figure 4 .
Figure 4. Effect of the temperature and initial concentration of Co 2+ on the biosorption of the metal by onion peel (particle size = 300-800 µm, pH = 7.0, X = 1 g L −1 ).

Figure 4 .
Figure 4. Effect of the temperature and initial concentration of Co 2+ on the biosorption of the metal by onion peel (particle size = 300-800 µm, pH = 7.0, X = 1 g L −1 ).

Figure 7 .
Figure 7. Capacity (q D ) and efficiency (E D ) of the desorption of Co 2+ .

Processes 2024 , 29 Figure 8 .
Figure 8.(a) BET isotherm, (b) multi-point BET plot, and (c) BJH pore size distribution for onion peel.W = number of gas molecules adsorbed at a given relative pressure (P/Po); dV = differential of the pore volume; r = pore radius; STP = standard temperature and pressure.

Figure 8 .
Figure 8.(a) BET isotherm, (b) multi-point BET plot, and (c) BJH pore size distribution for onion peel.W = number of gas molecules adsorbed at a given relative pressure (P/Po); dV = differential of the pore volume; r = pore radius; STP = standard temperature and pressure.

Table 1 .
For three onion peel particle size intervals, the experimental data and kinetic parameters describing the capacity of Co 2+ removal are listed.

Table 1 .
For three onion peel particle size intervals, the experimental data and kinetic parameters describing the capacity of Co 2+ removal are listed.

Table 2 .
For different levels of pH of the solution, the experimental data and predicted values of the capacity of Co 2+ removal by onion peel and time to reach equilibrium are shown.

Table 3 .
For distinct initial concentrations of Co 2+ , the experimental data and kinetic parameters for Co 2+ removal by onion peel are provided.

Table 3 .
For distinct initial concentrations of Co 2+ , the experimental data and kinetic parameters for Co 2+ removal by onion peel are provided.

Table 4 .
Capacity of adsorption of Co 2+ by various materials of biological origin.

Table 5 .
Materials not of biological origin that have been evaluated for their capacity to remove Co 2+ from aqueous solutions.

Table 6 .
Parameters of the isotherms of sorption of Co 2+ by onion peel at different temperatures.

Table 6 .
Parameters of the isotherms of sorption of Co 2+ by onion peel at different temperatures.

Table 7 .
Thermodynamic parameters determined for the biosorption of Co 2+ by onion peel.

Table 7 .
Thermodynamic parameters determined for the of Co 2+ by onion peel.

Table 9 .
Proximal chemical analysis of onion peel.

Table 9 .
Proximal chemical analysis of onion peel.