Pyrolytic Modification of Avocado (Persea americana) Peel for the Enhancement of Cadmium(II) and Chromium(VI) Sorption Properties

Abstract

The sorption process of chromium(VI) and cadmium(II) onto avocado peel (AP) and its pyrolyzed version (PAP) was carried out. The pyrolysis process was investigated in a tube furnace under inert gas conditions (argon) using a temperature equal to 750 °C. A constant mass of used materials and metal solution volume of 0.5 g and 20 cm³, respectively, were chosen for the investigation of the sorption process. Different isotherm models were fitted to describe the process parameters. According to the obtained results and the model that provided the best fit according to the correlation coefficient R², the removal process is best described by the D-R model (R² = 0.993 and 0.918; q_d = 5.78 and 6.02 mg/g) for Cr(VI) and the Freundlich for Cd(II) ions (R² = 0.999 and 0.911; K_f = 0.2712 and 0.2952 (mg^1−(1/n)(dm^(31/ng−1))) for AP and PAP, respectively). The highest level of adsorption capacities reached 6.0 (AP)–7.1 (PAP) and 9.7 (AP)–10.3 (PAP) mg/g for chromium and cadmium ions, respectively. The kinetic modeling showed that in all of the adsorption processes, the best-fitting model was the pseudo-second-order kinetic model, suggesting the occurrence of a chemical reaction between ions and the surface of the used materials.

1. Introduction

Nowadays, the increase in challenges and the growing need for new, sustainable technologies has led to a rise in the attention paid to the development of advanced supplies for pollution removal and the recovery of natural resources [1,2]. Environmental contamination by heavy metals, such as cadmium and chromium, has become an overwhelming problem due to their toxic effects on the environment and human health [3,4]. Both of these metal ions are among the most prevalent and hazardous pollutants originating from industrial, agricultural, and urban activities. Therefore, the development of efficient and sustainable approaches for their removal from contaminated environments like groundwater is imperative. Cadmium and chromium ions pollution has been linked to various adverse health effects, including carcinogenicity, teratogenicity, and mutagenicity, making their removal from water and soil a top priority in environmental remediation efforts [5,6]. Traditional methods for metal ion remediation, such as chemical precipitation and ion exchange, often suffer from limitations related to cost, sludge generation, and the potential release of secondary pollutants. Therefore, there is an increasing interest in studying innovative and sustainable technologies that can address these challenges effectively.

Pyrolysis is a thermochemical process involving the decomposition of organic materials at increased temperatures in the environment without an oxygen presence [7,8,9]. This method has emerged as a promising avenue for the modification of organic sorbent materials such as peat, coconut fiber, fruit peel, food waste, etc. Through pyrolytic modification, organic precursors can be transformed into materials with tailored surface properties, enhanced surface area, and improved sorption capabilities. Recent studies have shown that biochar (pyrolyzed organic matter) was successfully used in water (polluted with metal ions) due to its various sophisticated properties like surface area and charge or hydrophobicity [2,10,11]. Wei-Hsin et al., in their work, reviewed various biomass-derived biochars to see whether they were good materials for the removal of toxic ions [12]. Maia et al., in their work, used banana peel waste for producing biochar via pyrolysis for the removal of dye (methylene blue) from aqueous solutions [9]. A similar work was proposed by Lu et al., where the authors used roots of Ipomoea aquatica for methyl violet dye removal [13]. Suhaimi et al. carried out a test with the use of biochar produced from Gigantochloa bamboo for methylene blue dye remediation [14]. By controlling pyrolytic parameters such as temperature, time of contact, and composition, it is possible to manipulate the physicochemical properties of organic materials, resulting in sorbents that exhibit high affinity, selectivity, and capacity for metal ions [15].

This paper explores how pyrolytic modification can be applied to avocado peel to create new sorbent for the efficient remediation of Cd and Cr ions from wastewater and compares it to unmodified peel waste. The results were additionally evaluated by studying different isotherm and kinetic models showing the mechanisms concerning the sorption of Cd and Cr ions on both unmodified and pyrolytically modified avocado peel. Additionally, an assessment of the applicability of these materials in bioremediation processes is presented. In the future, the authors plan to use chemical modification of chosen fruit-peel-derived biochar to check adsorption properties against various heavy metal ions present in wastewater.

2. Materials and Methods

2.1. Chemicals and Biomass

The avocado peel waste was acquired from the local market (Kraków, Poland). The material was rinsed three times with deionized water and proceeded to be placed in an air dryer for 24 h at 105 °C. In the next step, the dry mass was ground and screened to acquire parts between 0.2 and 0.5 mm. All of the chemicals used in the study were of analytical grade and were obtained from Sigma-Aldrich (Steinheim, Germany). The solutions of chromium and cadmium ions, 50–100 mg/dm³, were prepared by dissolving prepared weights of potassium dichromate (K₂Cr₂O₇) and cadmium(II) sulphate (CdSO₄), respectively, in 250 cm³ of deionized water.

2.2. General Methods

To learn about the morphological structure of avocado peel and its pyrolyzed version prior to and after the Cd(II) and Cr(VI) ions sorption process, a scanning electron microscope (Hitachi TM-3000, Tokyo, Japan) equipped with an X-ray dispersion spectrometer was used. Additional information regarding the occurrence of different chemical groups was obtained using the FTiR-ATR method (Thermo Scientific, Nicolet iS5 spectrometer with the ATR iD7 attachment, Waltham, MA, USA). Every sample was checked between 400 and 4000 cm⁻¹, with a resolution equal to 0.5 cm⁻¹. The concentrations of resisting metal ions in the solutions were directly measured by means of AAS (atomic absorption spectrophotometry, Perkin-Elmer AAS Waltham, MA, USA). The elemental analysis CHN was performed using a Perkin Elmer CHN analyzer type 2400 (Waltham, MA, USA). A surface analysis was conducted with the use of Macrometrics ASAP 2010 (Norcross, GA, USA). In the first step of the process, the materials were dried at 110 °C under helium conditions for 8 h and then at 100 °C in a vacuum of 0.0001 Torr for 8 h.

The thermal stability of the sample was checked with the use of an EXSTAR SII TG/DTA 7300 model under an argon atmosphere. The heating rate was equal to 20 °C/min in a temperature range between 20 and 1000 °C.

The point of zero charge (PZC) values for AP and PAP were ascertained using the pH drift technique. To achieve this, a 0.01 M NaCl solution was prepared in 100 cm³ Erlenmeyer flasks, and its initial pH was adjusted within the range of 2 to 11 by employing 0.1 M HCl and 0.1 M NaOH. Additionally, 50 mg of PA and PAP were introduced into separate flasks and subjected to agitation for 48 h using an orbital shaker set at 150 rpm. The pH of the samples was then measured at specified intervals. Subsequently, these pH measurements were plotted against the initial pH values, and the PZC point was determined as the initial pH at which no change in the measured pH occurred.

2.3. Pyrolysis Process

The pyrolysis process was carried out in the laboratory tube furnace (Strohlein, Ofen 85, Germany). The amount of avocado peel used for the pyrolysis was equal to 100 g. The prepared sample was placed in a quartz tube with an external diameter of 100 mm and closed in an electric heating jacket and insulating materials employed to sustain the specified temperature of 750 °C throughout the pyrolysis procedure. To ensure the pyrolytic conditions, an inert gas (argon) was used through the whole process. The gas flow (10 L/h) was controlled with the flowmeter to provide stable environment. The process was carried out for approximately 4 h and then the temperature was gradually lowered until it reached the room temperature ~25 °C [11,16]. The mass of the obtained biochar was equal to 21.4 g. The biochar yield in this particular case is equal to 21.4%.

2.4. Sorption Process

The metal ion removal by the means of adsorption process was prepared in a batch system—60 cm³ polypropylene flasks. Approximately 0.5 g of AP and PAP and 20 cm³ of Cr(VI) and Cd(II) ions in different concentrations (50, 100, 150, 200, 300 mg/dm³, respectively) were used for each test in different variants. After 5, 10, 15, 30, 60 and 90 min of shaking on the rotary shaker (150 rpm) after the start of sorption, the portion of the solution was filtered through analytical paper filter and the concentration of residual chromium(VI) and cadmium(II) ions in the filtrate were examined with an AAS method. The maximum sorption capacity was calculated using equation:

$$q_e=\frac{C_0-C_e·V}{m_e·1000},$$

(1)

where V is the volume of solution (cm³) and m_e is the mass of sample (g). The tests were conducted in triplicates and results averaged—the mean standard error was not higher than 5%.

2.5. Sorption Equilibrium

In this work, 4 isotherm models were calculated to obtain the best fit to the acquired values from the experiments. The linearized equations used for their calculations are presented in

Table 1. Linearized equations of Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherm models.

Model	Equation
Langmuir	$\frac{C_e}{q_e}=\frac{C_e}{q_{max}}+\frac{1}{{b·q}_{max}}$	(2)
Freundlich	$log{q}_e=log{K}_f+\frac{1}{n}log{C}_e$	(3)
Temkin	$q_e$ = B ln $K_t$ + B ln $C_e$	(4)
	B =$\frac{RT}{b_t}$	(5)
Dubinin–Radushkevich	$q_e=q_d\exp \left(-K_{ad}{\varepsilon }^2\right)$	(6)
	$\varepsilon =RTln(1+\frac{1}{C_e}$)	(7)

Where C_e—metal ion concentration in equilibrium (mg/dm³); q_e—sorption capacity in equilibrium (mg/g); q_max—maximum sorption capacity (mg/g); b—Langmuir constant (dm³/mg); K_f—Freundlich constant (mg^1−(1/n)(dm³)^1/ng⁻¹); n—heterogeneity coefficient; K_t—constant binding equilibrium responsible for the maximum binding energy (dm³/g); B—constant associated with the sorption heat (J/mol); R—gas constant (8.314 J mol/K); T—temperature (K); b_t—Temkin isotherm constant; q_d—theoretical isotherm saturation capacity (mg/g); K_ad—Dubinin–Radushkevich isotherm constant (mol²/kJ²); ɛ—D-R isotherm constant.

.

2.6. Sorption Kinetic Modeling

The possibility of acquiring knowledge on the mechanism of the adsorption is possible by calculation of kinetics. This study presents the use of 3 different models such as the pseudo-first-order, pseudo-second-order, and Weber–Morris model. The above-mentioned models were carried out by differentiating the time of contact—5, 10, 15, 30, 60 and 90 min of Cr and Cd ions with sorbents. The linearized versions of the model equations are shown in

Table 2. Linearized equation of sorption kinetic models.

Model	Equation
Pseudo first-rate order	$\mathit{log}\left(q_e-q_t\right)={log}_{qe}-\frac{k_1}{2.303}t$	(8)
Pseudo second-rate order	$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}$	(9)
Weber–Morris	$q_t$$=K_{id}{t}^{0.5}+I$	(10)

Where k₁—pseudo-first-order kinetics constant (1/min); k₂—pseudo-second-order kinetics constant; K_id—the intra-particle diffusion rate constant (mg/g min^0.5); I—intercept of the line in the Weber–Morris model.

.

2.7. Desorption Studies

The possibility to use sorbent more than once is a very important parameter. To obtain the information about the reusability of the sorbent 3 cycles of sorption/desorption were carried out. The acetic acid, citric acid, sodium chloride, and potassium chloride at a concentration of 0.1 M were used as eluents (additionally water as a control). The test was prepared as follows: To Erlenmeyer 250 cm³ flasks containing 1 g of AP and PAP, 100 cm³ of Cr(VI) and Cd(II) solutions (500 mg/dm³) were added, respectively. After 24 h, the probes were filtered and dried. Subsequently, the obtained samples were treated with the chosen eluent for 90 min. After a certain time, the ion concentration in the solution was checked by means of AAS. With the use of the equation presented below, the desorption degree was calculated:

$$R_{des}=\frac{M_{sol}}{M_{sor}}·100\%$$

(11)

where M_sol—metal content in solution after desorption (mg); M_sor—metal content in sorbent after desorption (mg).

3. Results and Discussion

In Figure 1 one can see the SEM microphotographs of the AP and PAP samples before and after the sorption process. As can be noticed, the materials are characterized by the presence of various pores and diversified surfaces. The PAP compared to AP has a cavernous structure due to the changes during the temperature decomposition of organic compounds.

Studying the EDS (energy dispersive spectroscopy) analysis, the main occurring elements in the materials are oxygen and carbon, with potassium, silicon and phosphorus are also present. Additionally, an analysis of the materials after the sorption process shows the presence of chromium (Figure 1A–C) and cadmium ions (Figure 1E,F) on the material surface. This result is proof of adsorption occurrence on both used materials and ions.

Providing the CHN (carbon, hydrogen, nitrogen) analysis, it was possible to calculate the percentage of the occurring main elements. Using the dry state of the sample, the results for the unmodified avocado peel are presented in

Table 3. Percentage of Carbon, Hydrogen, Nitrogen, and Oxygen occurring in the AP and PAP.

AP (%)
C	H	N	O
10.5	2.8	1.1	85.6
PAP (%)
C	H	N	O
76.2	1.8	3.4	18.6

.

As can be seen, the carbonization process led to an increase in the mass fraction of carbon and a decrease in the mass fraction of oxygen in the sample, which is in accordance with literature data [8,17,18].

To gain the information connected with the specific surface area, a BET (Brunauer–Emmett–Teller isotherm) analysis was performed. The surface area of the avocado peel was equal to 4.57 m²/g and 9.70 m²/g for the pyrolyzed material. Due to the pyrolysis time, there is a possibility that the present micropores widened up, leading to the low value of the specific surface area, which is in accordance with the microporous structure of the PAP [19].

The FTIR (Fourier transform infrared spectroscopy) spectrum provided in Figure 2 of PA prior and after metal ion sorption exhibits various characteristic peaks of the cellulous structure that are absent in the PAP version of the material, suggesting that the pyrolysis process transformed avocado peel into biochar. Additionally, changes in the peaks suggest the occurrence of bonding between the material surface and the metal ions after the adsorption process.

Analyzing the FTIR scheme provided in Figure 2, it can be seen that there is a presence of a band in 3300–3500 cm⁻¹ (AP) that is related to the stretching vibrations of hydroxyl (OH) groups, which are common in the carbohydrates and cellulose found in plant material. The band in the range 2800–3000 cm⁻¹ is associated with the stretching vibrations of aliphatic C–H bonds, which are present in lipids and carbohydrates. Region 1700–1750 cm⁻¹ corresponds to the stretching vibrations of carbonyl (C=O) groups, often found in esters, ketones, and aldehydes. C–O stretch (1000–1200 cm⁻¹): this region includes stretching vibrations of C–O bonds in various functional groups like alcohols, ethers, and esters, common in plant polysaccharides. Cellulose peaks (around 1200–1100 cm⁻¹): specific peaks in this region are indicative of cellulose, a major component of plant cell walls. The FTIR spectrum of avocado peel after pyrolysis shows changes in the above-mentioned bands due to the thermal degradation and conversion of organic matter into char. Decrease in O–H and C–H bands: the broad O–H and C–H bands decrease in intensity or shift due to the removal of volatile organic compounds during pyrolysis. Increase in C=O bands: An increase in the intensity of C=O bands is observed due to the formation of carbonyl-containing compounds during pyrolysis. The peaks associated with cellulose decreased in intensity, indicating the breakdown of cellulose during the pyrolysis process. Studying the FTIR scheme, it can be observed that after the sorption process of both Cd and Cr ions, there are significant differences in peak size and shifting, showing that in the process of removal, chemical bonding took place.

The chemical composition of used biomass is the main factor by which pyrolysis is affected. The thermal analysis of avocado peel was conducted using TGA (Figure 3). The TGA and DTG curves of AP show two main stages of weight loss: The first one starts in the 15th minute of the process, which corresponds to the temperature range between 300 and 400 °C. The second one starts after 20 min, which equals 450–500 °C and corresponds to the oxidation of solid carbon material and residual volatile matter. These results are similar to those obtained by Young-min Kim et al. In their manuscript, the authors used Citrus unshiu peel for the thermogravimetric analysis [20]. Chen et al. reported that the pectin and hemicellulose decomposition is in the range between 250 and 350 °C, which shows similarities to the results obtained in this manuscript [21].

To acquire information about the pH for highest sorption capacity and metal ion removal, the PZC (point of zero charge) was computed for both AP and PAP material. The point of zero charge (PZC) is defined as the pH value at which the surface charge of the adsorbent becomes neutral. To determine the PZC, a graph is created by plotting the pH variations in the solution against the initial pH. The PZC is identified as the pH at which this plot intersects the pH value of 0. In this case, we found that the point of zero charge for AP was around 6.81 and 9.84 for PAP (Figure 4). These results show that there were basic groups resulting in the presence of a positive charge on the biochar surface. When the pH of the solution is below the PZC, the material’s surface carries a positive charge resulting from the protonation of acidic groups. This leads to reduced metal sorption because of the electrostatic repulsion between the ions and the functional groups on the surface [22].

3.1. Adsorption Isotherms and Kinetics

The sorption process of two metal ions and two materials was determined with the use of Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherms. According to the obtained results, the highest correlation coefficient was fitted to the D-R isotherm model for both AP and PAP and Cr(VI) ions (R² = 0.99 and 0.91, respectively) and the Freundlich model for Cd(II) ions (R² = 0.99, and 0.91, respectively). The obtained results for the isotherm models are presented in

Table 4. Parameters of sorption isotherm models.

Parameter	AP		PAP
	Cr(VI)	Cd(II)	Cr(VI)	Cd(II)
Freundlich
R2	0.837	0.999	0.710	0.911
KF(mg1−(1/n)(dm3)1/ng−1)	0.2201	0.2712	0.3669	0.2952
1/n	0.7392	0.8861	0.7047	0.9684
Dubinin–Radushkevich
R2	0.993	0.828	0.918	0.8305
qd [mg/g]	5.78	6.91	6.02	7.50
Kd [mol2/kJ2]	−0.0001	0.0000	−0.0006	−0.0007
Langmuir
R2	0.6326	0.7885	0.4803	0.0850
Kl	0.0104	0.0077	0.0139	0.00421
Temkin
R2	0.958	0.9412	0.8335	0.9023
B	2.2488	4.0309	2.5940	4.7241
Kt	0.1413	0.1549	0.2001	0.1854

. Additionally, it can be seen that the 1/n parameter is lower than 1 in each of the experiments, suggesting that the adsorption process is of a chemical nature.

To acquire knowledge about the characteristics of the mechanism of the removal process by the means of adsorption, the kinetics studies were carried out. To compare different kinetic models, the correlation coefficient was calculated for each chosen model. Three mainly used models were used: the pseudo-first-order, pseudo-second-order, and Weber–Morris model. According to the obtained results, the highest fit was with the PSO model (pseudo second rate). On Figure 5 and

Table 5. Results obtained for the pseudo-second-order rate kinetic model.

Parameter	AP					PAP
	Cr(VI) concentration [mg/dm3]
	50	100	150	200	300	50	100	150	200	300
	PSO
qe	1.4430	2.4084	4.4075	4.7665	7.5937	0.5624	1.8898	5.2575	5.3936	7.4058
k2	0.0127	0.0822	0.3779	0.0939	0.0082	0.2353	0.1326	0.2197	0.0762	0.0154
R2	0.9652	0.9815	1.0000	0.9956	0.9904	0.8931	0.8499	0.9998	0.9951	0.9969
Parameter	AP					PAP
	Cd(II) concentration [mg/dm3]
	50	100	150	200	300	50	100	150	200	300
	PSO
qe	3.4473	4.8813	5.9513	7.5885	11.6958	3.7922	4.7208	5.7733	7.8693	12.0486
k2	0.8273	0.4368	0.1721	0.0624	0.0194	0.4991	0.1493	0.1044	0.0290	0.0051
R2	1.0000	1.0000	1.0000	0.9992	0.9970	1.0000	0.9998	0.9999	0.9923	0.9928
Parameter	AP					PAP
	Cr(VI) concentration [mg/dm3]
	50	100	150	200	300	50	100	150	200	300
	PFO
qe	0.739	3.915	2.447	13.946	9.801	0.354	0.562	0.983	1.330	5.278
k1	0.0226	0.1864	0.2441	0.3552	0.1214	0.0652	0.0284	0.1040	0.0410	0.0722
R2	0.9332	0.9481	0.9609	0.7967	0.9556	0.9865	0.7661	0.8491	0.8755	0.9836
Parameter	AP					PAP
	Cd(II) concentration [mg/dm3]
	50	100	150	200	300	50	100	150	200	300
	PFO
qe	0.374	0.344	1.046	2.535	6.659	0.396	0.875	1.401	3.548	8.718
k1	0.1246	0.0569	0.0785	0.0922	0.0916	0.0849	0.0537	0.0663	0.0854	0.0605
R2	0.9755	0.9396	0.9568	0.8085	0.8717	0.8487	0.9830	0.9882	0.7688	0.9738
Parameter	AP					PAP
	Cr(VI) concentration [mg/dm3]
	50	100	150	200	300	50	100	150	200	300
	Weber–Morris
I	-0.1463	0.9495	3.8555	2.8876	0.4110	0.1596	1.1081	4.4020	3.6807	1.7692
kid	0.1179	0.1829	0.0754	0.2579	0.7734	0.0476	0.0870	0.1140	0.2057	0.6525
R2	0.9544	0.8246	0.7231	0.6011	0.8856	0.8938	0.9413	0.7135	0.9407	0.9043
Parameter	AP					PAP
	Cd(II) concentration [mg/dm3]
	50	100	150	200	300	50	100	150	200	300
	Weber–Morris
I	3.1875	4.4919	4.9155	4.8134	5.2114	3.3878	3.7154	4.3112	3.4715	0.7694
kid	0.0356	0.0495	0.1359	0.3756	0.8196	0.0546	0.1248	0.1840	0.5716	1.1932
R2	0.6735	0.8712	0.7760	0.5949	0.7525	0.7082	0.9407	0.9098	0.5612	0.9178

, the PSO scheme and parameters for all used concentrations for both Cr(VI) and Cd(III) and AP and PAP are presented. Studying the obtained results, one can see that the rate-limiting step is due to the surface adsorption–chemical binding of ions on the surface of chosen materials both before and after the pyrolysis process. This thesis is also supported by the changes in infrared spectra after ion removal. The lower values for the PSO constant can be connected with the high affinition of metal ions to the functional groups on the material surface [23]. Moreover, the calculated values of maximum sorption capacity q_e are close to those obtained from the experiments. This fact proves the correctness of the pseudo second-rate order model for this phenomenon. Obey et al. in their work used biochar derived from the non-customized matamba fruit shell for the wastewater treatment, where the PSO demonstrated the highest fit [24]. Ahmad and colleagues presented a study of methylene blue removal with the use of rice husk, cow dung, and sludge biochar, also providing information that the pseudo-second-order had the highest correlation coefficient [25].

The adsorption mechanism can be connected with the surface complexation in which the pH value is slightly reduced after the removal process due to the displaced protons from the carbon surface. On the other hand, in this study, we did not see lowered pH of the solutions after interacting with sorbents. Thus, this phenomenon is probably not a complexation especially when studying the FTIR spectrum. Peaks corresponding to the C=C and C=O stretching vibration are lowered in both cases of Cr and Cd ions, which might be responsible for the delocalization of π electrons resulting in adsorption through Cπ-cation interactions. In

Table 6. Comparison of sorption capacities of biochar derived from various biosorbents.

Biochar Material	Sorption Capacity (mg/g)
	Cr(VI)	Cd(II)
PAP	7.5	12
Coconut shells	2 [26]
Sawdust	3.2 [27]
Modified date seed biochar	27 [28]
Coconut tree sawdust	3.46 [29]
Oakwood char		0.37 [30]
Miscanthus sacchariflorus		11 [31]
Buffalo weed		11.63 [32]
Pig manure biochar		107 [33]

, one can see the sorption capacities of other biosorbents used for the removal of toxic metal ions from wastewater.

3.2. Sorption/Desorption

The reusability of materials is crucial for bioremediation technology; thus, sorption and desorption studies were performed. Three cycles were conducted and the results are presented in

Table 7. Percentage of Cr(VI) and Cd(II) elution from AP and PAP with the use of different eluents, respectively.

Elution [%]
Eluent	Cycle 1		Cycle 2		Cycle 3
AP
	Cr(VI)	Cd(II)	Cr(VI)	Cd(II)	Cr(VI)	Cd(II)
Citric acid	75	81	79	74	71	78
Acetic acid	52	64	61	66	55	53
Sodium chloride	24	31	27	29	21	26
Water	0.4	0.3	0.1	0.2	0.1	0.1
PAP
Citric acid	77	79	75	82	81	72
Acetic acid	55	58	57	63	61	55
Sodium chloride	20	19	22	21	27	19
Water	0.2	0.1	0.3	0.2	0.2	0.3

, as it is well known that the choice of eluent has a high impact on the ions desorption percentage. In this particular study, citric acid had the highest efficiency for the removal both of Cr(VI) and Cd(II) ions from AP and PAP material. Water was used as a control eluent and no desorption of ions was observed, which also proves the chemical nature of the adsorption process.

4. Conclusions

The biochar derived from the avocado peel effectively removed both Cr(VI) and Cd(II) metal ions from the aqueous solutions. This process is best described by the pseudo-second-order kinetic model for both used materials and ions. The equilibrium adsorption isotherms with the highest fit are Freundlich for Cr(VI) ions and Dubinin–Radushkevich for Cr(II) ions. The process was very rapid at the beginning and most of the metal ions were removed in the first minutes of the process for both AP and PAP. The maximum sorption capacity q_e values for Cr(VI) and Cd(II) were equal to 6 mg/g; 9.7 mg/g for AP and 7.1 mg/g; 10.28 mg/g for PAP, showing a slightly higher capacity of biochar compared to untreated peel. According to the kinetics, the mechanism adsorption of both used metal ions and materials might involve chemisorption and Cπ-cation interaction. The values obtained from sorption/desorption studies show that this material can be used multiple times without significant loss in its sorption capacity. We conclude that this material both before and after the pyrolysis process can be used as an effective, low-cost, and environmentally friendly sorbent for heavy metal removal from aqueous solutions.