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Abstract

This study used neem fruit biochar, an inexpensive and environmentally friendly material made from mature neem trees, which was activated chemically using H₃PO₄ (50% v/v) for three hours at 80°C. It was then pyrolyzed for 1, 3 and 5 h at 500°C. The samples bore the labels NFBC1, NFBC3, and NFBC5, respectively. This adsorbent was used as bioadsorbent to remove the Cr(VI) ion from aqueous solutions. This biochar was characterized using Brunauer–Emmett–Teller, Fourier-transform infrared spectroscopy, scanning electron microscope, X-ray diffraction, DTG, and pH_pzc techniques in order to ascertain their fundamental characteristics, including their chemical structure, porosity, and surface characteristics. It was found that activation increased the bioadsorbent's ability to adsorb pollutants and resulted in a more amorphous shape. The percentage removal of Cr(VI) ions was examined in relation to adsorption parameters, including temperature (30, 40 and 50°C), initial concentration of Cr(VI) ions (10–100 mg/L), contact time (5–180 min), and biosorbent dosage (0.01–0.06 g/50 mL). While isotherm investigations fit with Langmuir and Freundlich isotherm models, kinetics studies shown that the adsorption mechanism of Cr(VI) ion employing NFBC1, NFBC3, and NFBC5 follows pseudo-second order. Based on the Langmuir model, the maximum adsorption capacities at 30°C were 84.746, 87.470 and 102.696 mg/g, for NFBC1, NFBC3 and NFBC5, respectively. The enthalpy ( $Δ H$ ) and free energy ( $Δ G$ ) changes that were computed suggested that the adsorption process was exothermic and spontaneous. The solid/solution interface's reduced unpredictability during adsorption was indicated by the negative value of ΔS. According to the results, the modified bioadsorbent is a useful and affordable substitute for removing Cr(VI) ions from contaminated water.

Keywords

bioadsorption biochar Cr(VI)neem fruit

Introduction

Due to the release of solid waste, heavy metals, and gaseous pollutants, the growth of companies and the acceleration of the production of various consumer goods have resulted in major environmental issues. In the end, practically all living beings and nonliving objects are seriously threatened by these contaminants (Thangagiri et al., 2022).

Water is the characteristic asset needed for all living things. The level of purity of the water being consumed is very crucial since it has a direct effect on health (Javeda et al., 2024; Raza et al., 2022). Increased amounts of untreated sewage, combined with agricultural runoff and industrial discharge have degraded water quality and contaminated water resources around the world. For instance, the heavy metals (such as chromium) released by industries cause severe health issues. Particularly, the heavy metals, chromium is used extensively in paint production, leather tanning, stainless steel, chrome plating, battery production, etc., is discharged in such high amounts can easily enter in to the food chain. Inhalation of Cr(VI) could cause ulcer, sneezing, running nose, nosebleeds, carcinogenicity, mutagenicity, affect circulatory system, kidney damage, and threaten human life (Abu-Zurayk et al., 2024, 2015; Amakua et al., 2023; Kumar et al., 2020).

Heavy metal-containing wastewater can be treated using a variety of physicochemical methods, including photocatalytic reduction, solvent extraction, ion exchange, chemical precipitation, electroreduction, reverse osmosis, chemical reduction, chemical oxidation, electrochemical precipitation, microbial reduction, and adsorption (Abu-Zurayk et al., 2024, 2015; Al-Dalahmeh et al., 2024). However, these methods are expensive and ineffective for treating a variety of wastewater that contains heavy metals (Amakua et al., 2023). Nonetheless, adsorption techniques are the most efficient way to remove chromium from the aqueous phase due to their high efficiency under a variety of operating settings, low operating costs, and convenience of use, especially at low adsorbates concentrations (Halder et al., 2015; Thangagiri et al., 2022).

Usually produced by slowly heating, carbonizing, and gasifying pure biomass materials like wood, grass, or leaves, biochar is a pyrogenic substance rich in carbon. Activated carbon is, by far, one of the oldest and most employed adsorbents for the removal of many heavy metals from solutions (Amakua et al., 2023). Nevertheless, the expensive nature of commercially available activated carbon restricts its continuous use. This has prompted researchers to investigate the potential of several widely accessible, reasonably priced, unconventional adsorbents made from various agricultural and forestry byproducts to act as efficient heavy metals adsorbents (Arami et al., 2005; Dursun et al., 2007; Franca et al., 2009; Thangagiri et al., 2022; Uddin et al., 2009).

Many agricultural and forestry byproducts have lately acquired appeal as substitutes for the production of activated carbon due to its accessibility, affordability, and sustainability (Abdelwahab et al., 2005; Ahmad et al. 2022; Soudani et al., 2022). One agricultural product that is commonly grown in tropical and subtropical regions is the neem tree (Azadirachta indica) (Tan et al., 2010). Neem is known as Azadirachta indica in its botanical name and it belongs to the Meliaceae family. It is a common plant found throughout the world with many therapeutic benefits (Garba and Afidah, 2014). The products from neem tree (bark, leaves, etc.) were widely studied for the removal of many pollutants, because it is a green adsorbent that is efficient, cheap, and readily available to serve as an alternative adsorbent for the removal of the heavy metals from wastewater.

In order to remove Cr(VI) from aqueous solution, neem leaves were used as a potential bioadsorbent (Zafar et al., 2013). As much as 87% of Cr(VI) could be extracted in 300 min from a solution containing 14.1 mg/L at 300 K using a little amount of neem leaf powder (1.6 g/L). Hatiya et al. (2022) employed neem biomass, which is a blend of neem leaves and bark, as a bioadsorbent to remove Pb(II). It was found that neem activation enhanced the adsorption capacity. Neem leaves are an effective adsorbent material for removing chromium from wastewater, according to Pandhram and Nimbalkar (2013). At the ideal parameter values, the bioadsorbent made from neem leaves showed the highest removal effectiveness of up to 85%. Bhattacharyya and Sharma (2004) found the same observation. Sharma and Bhattacharyya (2005) reported that, under ideal process conditions, activated carbon made from neem seed husk has an adsorption capacity that could remove 99.75% Cr(VI) (Danmallam et al., 2020). With activated charcoal made from powdered neem leaves, the high maximum Langmuir adsorption capacity was reached for Pb, Cu, Cd, Zn, Ni, and Cr, removal (Patel, 2020). According to Thangagiri et al. (2022), neem biochar is prepared, characterized, and used to remove Cr(VI) from synthetic waste water. Adsorption tests at different pH values verified that neem biochar eliminated 58.54 mg/g of Cr(VI) at pH 2.

In the present work, the details of the preparation and characterization of neem biochar (NBC) after soaking the wet neem fruit (NF) in phosphoric acid and the use of the prepared NBC to remove Cr(VI) ion from a synthetic waste water is investigated. It is observed that no work has been done on NF to consider the effect solution pH, contact time, biosorbent dosages, initial metal ion concentration and temperature on the Cr(IV) ion removal were investigated using the prepared activated biosorbent. Furthermore, the adsorption isotherm and kinetics model of chemically modified NF biomass were examined.

Materials and characterization

Materials

NFs were gathered from the area's wild plants. Sigma Aldrich Chemicals Company provided H₃PO₄, K₂Cr₂O₇, NaOH, HCl, NaCl, and 1,5-diphenylcarbazide with a purity of greater than 99%, which were utilized without additional purification. In this investigation, deionized water was utilized.

Characterizations

Thermo-Nicolet NEXUS 670 Fourier-transform infrared spectroscopy (FTIR) spectrophotometer (Waltham, MA USA 02451) was used to record the materials’ FTIR spectra. Using a NETZSCH STA 409 PC/PG Thermal Analyzer (Germany), the dried samples were subjected to thermal gravimetric analysis at a heating rate of 20°C per minute in the temperature range of 25°C to 900°C. An X-ray powder diffractometer using CuKα radiation (λ = 1.5418 Å) was used to investigate the samples. A GFL-1083 shaker with a thermostat was used to shake the samples. Using the NOVA 2200E surface area and pore size analyzer from Quantachrome Corporation, Boynton Beach, Florida, (USA). Brunauer–Emmett–Teller (BET) was used to calculate the total pore volume from the near uptake of saturation (P/Po = 0.99). The Barret, Joyner, and Halenda (BJH) method was used to measure the mesopore length, mesopore surface area, and pore size distribution. The specific surface area was calculated using the BET equation. Visible and ultraviolet spectroscopy, was used to assess the concentration of Cr(VI). The pH of the prepared biochar was measured with a Metrohm pH Meter and the measurements were carried out in triplicates at 25°C.

The following formulas were used to calculate the weight difference between the proper analysis ash and moisture content (Gaya et al., 2015): $Moisture content (%) = \frac{W_{1} - W_{3}}{W_{2} - W_{1}} \times 100$ (1) $Ash content (%) = \frac{W_{2} - W_{3}}{W_{2} - W_{1}} \times 100$ (2)where W₁ is the crucible's weight, W₂ is the crucible's initial weight with a sample, and W₃ is the crucible's end weight with a sample. The average of three tests was used to calculate the ± standard deviations for the percentage yield, moisture content, and ash content of activated carbon.

Biochar preparation

Wet NF were harvested from the tree and dried for 24 h at 40°C in a hot air oven. The dried fruits (1500 g) were then activated by soaking in 50% w/w phosphoric acid for 4 h. Following the decantation of the phosphoric acid, the NF biomass (NFBM) was repeatedly rinsed with distilled water until the biomass was free of phosphoric acid. It was then mashed with a pestle and mortar and sieved on 45 µg for use in the production of biochar after being dried for 24 h at 80°C in a hot air oven. The NFBM was pyrolyzed for 1, 3, and 5 h at a temperature of 500°C while oxygen was kept at a minimum (Tan et al., 2010).

Labeled as NFBC1, NFBC3, and NFBC5, respectively, dried crushed NFBM was wrapped in aluminum foil and kept in a muffle furnace at 500°C for 1, 3, and 5 h while being kept in an oxygen-free environment (Patel, 2020). 600 g of biochar (40%) was the amount that was obtained. After allowing the biochar to cool to ambient temperature, it was crushed with a pestle and mortar. A 0.25 mm screen mesh was used to filter the ground biochar. To remove Cr(VI) ion from the artificial wastewater, samples taken from the mesh were utilized as a biosorbent.

Determination of pH_pzc

50 mL of KNO₃ was transferred to a series of 100 mL conical flasks in order to use the solid addition method (ASTM, 2005) to estimate the adsorbents’ pH at point zero charge (pH_PZC). The initial pH (pH_i) of this solution was adjusted using either 0.1 M HNO₃ or 0.1 M NaOH solutions to roughly range from 2 to 10. 0.1 g of adsorbent was added to each flask after the pH_i of the solution was accurately measured. For a full day, the flasks were allowed to balance while being shaken occasionally by hand. The final pH (pH_f) values of the liquid supernatant were measured. The difference between pH_i and pH_f readings was plotted against pH_i. The difference between pH = 0 and pH_PZC is indicated by the intersection points of the generated curve.

Adsorption experiments

A batch equilibration method was used to determine the adsorption of chromium (VI) on NFBC1, NFBC3 and NFBC5. In a typical adsorption run, 0.15 g of NFBC1, NFBC3 or NFBC5 was equilibrated with 25.0 mL of metal ions solutions in the range of 10–100 mg/L. in a stoppered 100 mL Erlenmeyer flask in a thermostatic water bath and stirring speed was 150 r/min at a constant temperature of 25°C. The pH of solution was adjusted until the equilibrium was achieved. At the end of the desired equilibrium period, the contents of the Erlenmeyer flask were filtered, centrifuged for 10 min at 3500 r/min and the supernatant was subsequently analyzed for residual concentration of Th(IV) and U(VI) ions. It was done by combining 0.5 mL of 1,5-diphenylcarbazide, 0.25 mL H₂SO₄, and 0.018 mL H₃PO₄ mL were added to 1.25 mL of a residual concentration of Cr(IV) ions. This was done in accordance with Onchoke and Sasu’s (2016) method for determining the amount of Cr(VI) in a sample using visible and ultraviolet spectrophotometer operating at 540 nm wavelength. The reproducibility of data varied in the range of $\pm$ 1.5%. The following formulas were used to determine the amounts of Chrome (q_e) and the percentage removal of adsorption (%R) adsorbed by the three samples: $Adsorption efficiency = q_{e} = \frac{(C_{o} - C_{e}) V}{M}$ (3) $% R = \frac{(C_{o} - C_{e})}{C_{o}} \times 100 %$ (4)where q_e (mg/g) is the amount of Cr (VI) adsorbed by NFBC1, NFBC3 and NFBC5, $C_{o}$ and $C_{e}$ (mg/L) are the initial and equilibrium concentration of Cr(VI), respectively, V (L) the initial volume of Cr(VI) solution and M (g) the weight of the adsorbent.

Adsorption isotherms

Ten solutions were prepared with 25.0 mL volume from Cr(VI) ion solution with different initial concentrations C_o (10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mg/L) were added separately to stoppered volumetric flasks containing (0.03 g) of each adsorbent at a preadjusted temperature in a thermostat shaker for 3 h at 150 r/min. After that, each mixture was allowed to settle for 2 min and the clear aqueous phase was withdrawn, centrifuged and filtered by using 0.45 µm filter.

Kinetic studies

For adsorption kinetics, set of experiments were carried out in which 25 mL of 50 mg/L solutions of Cr(VI) ions solution were adsorbed onto 0.03 g of NFBC1, NFBC3, or NFBC5 over a time period of 5–180 min following the procedure of batch experiment. The kinetic experiments were carried out at three temperatures (30, 40, and 50°C) in order to find the kinetic parameters of adsorption.

Thermodynamic studies

The thermodynamic was studied by agitating 50 mg/L solution of Cr(VI) ions solution with 0.0.3 g of adsorbents at temperature 30, 40, and 50°C for a time period of 3 h.

Results and discussion

Characterization of biosorbents

Table 1 listed a comprehensive summary of the impact of chemical activation on the physicochemical characteristics of NFBC1, NFBC3, and NFBC5. These characteristics include biochar yield, moisture content, ash content, pH, and pH_pzc. The outcomes demonstrated that the characteristics of the biochar were significantly impacted by the pyrolysis settings. Table 1 illustrates the inverse relationship between the heating time and the biochar output. The shortest time duration of one hour produced the maximum yield% (78.23%). This can be linked to the application of the triggering agent H₃PO₄ content (Kumar and Jena, 2016) as well as the thermal breakdown of the cellulose and hemicellulose contents (Pellera et al., 2012). Moreover, these outcomes align with the findings for additional forms of biomass and organic substances (Ates and Un, 2013; Pellera et al., 2012). In line with the literature, ash content and moisture percentage rose as pyrolysis time increased (Shamsuddin et al., 2016). The rise in the proportion of inorganic materials in these samples explains this. The enhanced ash content (10.65–26.18) as a result of activation is consistent with the pH value of these samples increasing from 3.18 to 3.44. This study's pH range agrees with other researchers’ findings (Som et al., 2012; Yargicoglu et al., 2015) that biochar made from neem tree parts has an acidic pH. These findings demonstrate that the three adsorbents have demonstrated optimal levels of ash content (%), moisture content (%), and negatively charged surface. As a result, each is capable of removing Cr(VI).

Table 1.

Physicochemical properties of NFBC1, NFBC3, and NFBC5 adsorbents.

Biochar	Yield %	Moisture %	Ash%	pH_PZC	pH
NFBC1	78.23	2.13	10.65	2.5	3.18
NFBC3	72.10	1.66	15.24	2.6	3.34
NFBC5	53.93	1.00	26.18	2.8	3.44

The BET analysis of the NFBC1, NFBC3, and NFBC5 produced isotherms (Figure not shown) that had very similar adsorption and desorption patterns typical of type III isotherms, indicating a multilayer adsorption mechanism. Similar patterns have also been noted in the research conducted by Amakua et al. (2023). Table 2 lists the BET surface area (SA_BET), The BJH surface area (SA_BJH), total pore volume (V_total), average pore diameter (D_BET), and pore diameter (Dv(d)) of NFBC1, NFBC3, and NFBC5. The findings indicated that a slight increase in surface area, total pore volume and average pore diameter with increasing pyrolysis time would be related to bore obstruction caused by H₃PO₄ activation. For all three adsorbents, the pore structures are mesoporous.

Table 2.

Textural properties of NFBC1, NFBC3, and NFBC5 adsorbents.

Sample	SA_BET (m²/g)	SA_meso (m²/g)	V_total (cm³/g)	V_meso (cm³/g)	D_BET (nm)	D_ap (nm)
NFBC1	3.045	0.985	0.007	0.002	9.778	3.647
NFBC3	3.254	2.438	0.009	0.003	11.06	3.527
NFBC5	3.521	1.489	0.011	0.005	11.945	3.591

Scanning electron microscope

Scanning electron microscope (SEM) micrographs of NFBC1, NFBC3 and NFBC5 showed a natural pour as a part of vascular tunnels with a solid wall (Figure 1A). The shape and structure were deteriorated and the surface became more irregular and agglomerated as the calcination time increased due to the increase in pyrolysis duration (Figure 1B and 1C) which were same as observed during the studies performed by Kumar and Jena (2016).

Figure 1.

SEM micrographs of biochar (A) NFBC1, (B) NFBC3 and (C) NFBC5 in SE mode with a magnification of 2000X.

X-ray diffraction

Figure 2 shows the X-ray diffraction (XRD) pattern for NFBC1, NFBC3, and NFBC5. XRD results showed an amorphous structure for all samples. The amorphous character of the carbon in the biochar, where tiny carbon crystallites exist perpendicular to the (002) crystal plane, is responsible for the broad diffraction peak at 2θ = 13.78 to 31.86° (Thangagiri et al., 2022). As the heating time increased the material crystallinity is partially improved and the shape of broad peak became narrower with existence of new peaks at 2θ = 19.19, 23.7, 24.2, 25.0, and 3.10°, respectively.

Figure 2.

X-ray diffractogram pattern of biochar NFBC1, NFBC3, and NFBC5 adsorbent.

FTIR

The surface functional group analysis performed using FTIR. The FTIR spectra recorded between 400 and 3900 cm⁻¹ are depicted in Figure 3. The peaks at 1106 to 3445 cm⁻¹ indicating ester C–O stretch, ether C–O stretch, nitro groups of N = O bend, H-C-H alkane asymmetric and symmetric stretch, and N-H (Hatiya et al., 2022). The band at 2650 cm⁻¹ is assigned to the aliphatic –CH₂– stretching vibrations. The band at 1581 cm⁻¹ is assigned to the carbonyl groups present on the surface of the biochar. The aromatic C = C vibrational frequency is observed at 1435 cm⁻¹. The band at 950 cm⁻¹ corresponds to the C–H bending vibrations of the aromatic ring (Manoharan et al., 2022). The bands at 1157, 1063, and 474 cm⁻¹ is assigned to P-O-P, P-OH, P-O, and P = O stretching vibration modes, respectively (Maity and Ray, 2018).

Figure 3.

The FTIR spectrum for NFBC1, NFBC3, and NFBC5.

Effect of pH solution

Determining the pH at which maximum adsorption of NFBC1, NFBC3, and NFBC5 will take place is crucial since the adsorption process is controlled by the pH of the aqueous solution. Before the influence of solution pH experiment, the pH at which the surface of the adsorbents (NFBC1, NFBC3, and NFBC5) will be negatively charged (pH_PZC) was established. To gain a better understanding of the possible involvement of electrostatic interaction in the adsorption of Cr(VI) onto the materials, the pH_PZC of the adsorbents under investigation which are listed in Table 1 was determined. At a pH higher than the pH_pzc, the three samples’ surfaces have a negative charge. This suggests that at pH values below this, these adsorbents’ surfaces will be negatively charged (Amakua et al., 2023; Pandhram and Nimbalkar, 2013). Depending on our previous publication (Al-Dalahmeh et al., 2024) the present set of experiments employed aqueous solutions of Cr(VI) with a pH of 2.0. Consequently, the NFBC1, NFBC3, and NFBC5 particle surfaces can be regarded as almost acidic, meaning that Cr(VI) adsorbed primarily in the cationic form.

Adsorption isotherm models

Adsorption isotherm refers to the relationship between a substance's concentration in equilibrium solution and the amount adsorbed at a constant temperature. The experimental adsorption data were analyzed using equations for equilibrium adsorption isotherms. The parameters of these models are essential for determining the adsorption mechanism and surface characteristics. In this work three isotherm models are examined in the following manner (Mukherjee and Halder, 2018).

The Langmuir isotherm model

The following is the Langmuir isotherm equation (Langmuir, 1918): $q_{e} = \frac{q_{max} K_{L} C_{e}}{1 + K_{L} C_{e}}$ (5)where C_e is the equilibrium concentration (mg/L) and q_max and K_L are the Langmuir constants, which correspond to energy and adsorption efficiency, respectively.

The adsorption process is classified as favorable (0 < R_L < 1), unfavorable (R_L > 1), linear (R_L = 1), or irreversible (R_L = 0) based on a dimensionless constant “separation factor” parameter R_L, (Harnal et al., 2020). The formula: $R_{L} = 1 / 1 + C_{o} K_{L}$ (6)

The equilibrium constant K_d at the temperature where Cr(VI) ions dissolve can be estimated as a dimensionless parameter by multiplying the Langmuir constant (K_L) by the atomic mass of Cr(VI) (51.99 g/mol), by 1000, and then by 55.55 (the number of moles of pure water per liter) (Tran et al., 2017).

The Freundlich isotherm

The Freundlich isotherm model (Freundlich, 1906) consists of the following empirical equation for nonideal sorption with heterogeneous adsorption: $q_{e} = K_{F} C_{e}^{1 / n}$ (7)in which n and KF stand for Freundlich constants.

The Dubinin–Radushkevich isotherm

Dubinin–Radushkevich (D-R) isotherm model is commonly used to examine the adsorption mechanism on a heterogeneous surface with Gaussian energy distribution. The D-R model works best with intermediate concentrations and strong solute action. The D-R equation can be expressed as follows (Dubinin et al., 1947; Redlich and Peterson, 1959): $q_{e} = q_{max} \exp (- K_{D R} ε^{2})$ (8)where ε is the polanyi potential equivalent to $R T \ln (1 + 1 / C_{e})$ , $K_{D R}$ is the mean free adsorption energy per adsorbate molecule, R is the universal gas constant (8.314 J/mol. K), while T is the absolute temperature, $E_{D R}$ (kJ/mol) is the mean free adsorption energy per adsorbent molecule when transported from infinity to the solid surface in the solution, $E_{D R}$ (kJ/mol), the energy of the adsorption process, may be computed using the value of $K_{D R}$ : $E_{D R} = (2 K_{D R})^{- 1 / 2}$ (9)

When moving from infinity to the solid surface in the solution, the mean free adsorption energy per adsorbent molecule is denoted by $E_{D R}$ (kJ/mol). In general, the D-R model is employed to distinguish between chemisorption and physisorption.

Three different error analyses models, namely, the linear correlation coefficient (R²) (El Hanandeha et al., 2021), the applicability of each model for the respective adsorption scheme chi-square (χ²) (Silva et al., 2020) and the Marquardt's percent standard deviation (MPSD) (Ng et al., 2002) were applied to isotherm and kinetic data to determine the best fit model, and are listed in Supplemental Table S1. On the basis of the lowest value of various error functions (χ² and MPSD) calculated and from R² value, that is, the isotherm giving the R² value closest to unity (Table 3), the fitting is D-R < Freundlich < Langmuir. As shown in Figure 4 clearly indicates that the Langmuir model and Freundlich models can be used to describe the adsorption process of Cr(VI) by NFBC1, NFBC3, and NFBC5.

Figure 4.

Shows the equilibrium isotherm for the nonlinear regression approach of Cr(VI) adsorption on NFBC1, NFBC3, and NFBC5 adsorbents. The initial Cr(VI) concentration was 10–100 mg/L, the agitation speed was 150 r/min, the pH was 2.0, the adsorbent dosage was 0.03 g/25 mL, the contact period was 180 min, and the temperature was 30°C.

Table 3.

Langmuir, Freundlich, and D-R isotherm constants for the adsorption of Cr(VI) ions on NFBC1, NFBC3, and NFBC5.

Isotherm model	Parameter	NFBC1	NFBC3	NFBC5
Langmuir	q_max (mg/g)	84.746	87.470	102.696
	K_L (L/mg)	0.742	0.573	0.591
	R_L	0.078	0.017	0.016
	R ²	0.9935	0.9965	0.9970
	$χ^{2}$	0.556	0.615	0.139
	MPSD	5.795	8.571	2.700
Freundlich	K_F (L/g)	0.309	0.272	3.748
	N	1.801	1.965	1.778
	R ²	0.9839	0.9992	0.9902
	$χ^{2}$	1.819	0.116	0.754
	MPSD	11.581	3.189	5.814
D-R	q_max (mg/g)	53.958	57.399	86.841
	E (kJ/mol)	2.644	2.454	2.040
	R ²	0.7813	0.7550	0.8725
	$χ^{2}$	13.443	15.418	6.957
	MPSD	24.269	25.678	15.559

MPSD: Marquardt's percent standard deviation; D-R: Dubinin–Radushkevich model.

Table 3 lists the Langmuir, Freundlich and D-R isotherm parameters. The Langmuir monolayer adsorption capacity (q_max) was found to be 84.746, 87.470 and 102.696 mg/g, for NFBC1, NFBC3, and NFBC5, respectively. The dimensionless parameter, R_L, remained between 0.078 and 0.016 (0 < R_L< 1) consistent with the requirement for a favorable adsorption process. Table 4 compares the adsorption capabilities of various adsorbents for the adsorption of Cr(VI). Several studies used different adsorbents to study the adsorption of Cr(VI). This comparative study of the adsorption capacity showed that the investigated adsorbent was either competitive or more efficient than other adsorbents, supporting the usage of chemically activated NF as an attractive and economical solution for the removal of Cr(VI). Neem leaf modification, however, has the potential to increase the adsorption capacity of unmodified neem leaves (to a greater extent).

Table 4.

Comparison between results recorded for Cr(VI) adsorption onto NFBC1, NFBC3, and NFBC5 to the results of previous studies on neem tree parts.

Adsorbent	Conditions		q_max (mg/g)	References
Adsorbent	pH	T/°C	q_max (mg/g)	References
Neem leaf powder	5.0	27	19.49	(Sharma and Bhattacharyya, 2005)
Activated neem leaves	2.0	25	10.4	(Gupta and Babua, 2006)
Neem leaves powder	4.1	25	6.75	(Pandhram and Nimbalkar, 2013)
Neam leaves powder	5.5	25	18.25	(Rahul et al., 2014)
Neem seed husk powder	2.0	27	1.89	(Danmallam et al., 2020)
Activated neem leaf	5.0	35	111.5	(Patel, 2020)
Neem leaves biochar	2.0	25	58.54	(Thangagiri et al., 2022).
Neem stem bark	2.0	45	57.64	(Amakua et al. 2023)
Modified neem stem bark	2.0	45	76.05	(Amakua et al. 2023)
NFBC1	2.0	30	84.74	This work
NFBC3	2.0	30	87.47	This work
NFBC5	2.0	30	102.69	This work

The bonding energy is associated with the Freundlich equilibrium constants K_F and n. The degree of nonlinearity between adsorption and solution concentration is indicated by the n value, which can be expressed as follows: adsorption is linear if n = 1, a chemical process if n < 1, or a physical process if n > 1. The physical biosorption of Cr(VI) is indicated by the n value in the Freundlich equation, which was determined to be between 1 and 10 (Sharma and Bhattacharyya, 2005).

The D-R model is typically used to differentiate between physisorption and chemisorption. The above-mentioned n constant of the Freundlich isotherm, which demonstrates that n values range from 1 to 10, supports the computed E_DR value, which reveals that the process is physisorption in nature.

Effect of contact time and adsorption kinetics

At a given Cr(VI) concentration of 50 mg/L, and adsorbent dosage of 0.03 g/25 mL solution, an agitation speed of 150 r/min, and a temperature of 30°C, the contact time for Cr(VI) on the NFBC1, NFBC3, and NFBC5 was calculated to range from 5 to 180 min (Figure 5). The findings demonstrated a two-stage kinetic behavior associated with chrome adsorption on NFBC during 1, 3, and 5 h: With time, the adsorption curve rises sharply, indicating that chrome adsorption rates are very high and that there are plenty of accessible sites. Chrome adsorption first increased significantly up to the first 90 min, and then there was a second stage with a much lower adsorption rate at the initial stage of 90 to 180 min. The adsorption reaches a maximum, with a capacity of 81.359, 84.505, and 100.094 mg/g at 90 min for NFBC1, NFBC3, and NFBC5, respectively, after that remains unchanged with further time. Hence, the optimum contact time was selected as 180 min for further experiments.

Figure 5.

Effect of contact time for the removal of Cr(VI) by NFBC1, NFBC3, and NFBC5. The initial concentration of Cr(VI) = 50 mg/L, agitation speed 150 r/min, pH = 2.0, adsorbent dosage = 0.03 g/25 mL, contact time 180 min and 30°C.

Experimental data obtained from the time-dependent investigation were fitted into three kinetic models using nonlinear least square analysis in order to determine the mechanism and the rate-determining step in charge of the adsorption of Cr(VI) onto NFBC1, NFBC3, and NFBC5 (Figure 6).

Figure 6.

Nonlinear regression kinetic models for Cr(VI) adsorption on NFBC1, NFBC3, and NFBC5 adsorbents. The initial Cr(VI) concentration was 50 mg/L, the agitation speed was 150 r/min, the pH was 2.0, the adsorbent dosage was 0.03 g/25 mL, the contact period was 180 min, and the temperature was 30°C.

The kinetic models of the adsorption process were tested employing the following familiar models:

(i) Lagergren (1898) pseudo-first-order kinetics: $q_{t} = q_{e} (1 - e^{- k_{1} t})$ (10)

(ii) Pseudo-second-order kinetics (HO, 2006): $q_{t} = \frac{k_{2} q_{e}^{2} t}{1 + k_{2} q_{e} t}$ (11)

(iii) The intraparticle diffusion model (Weber and Morris, 1963) $q_{t} = k_{i n t} t^{0.5} + C$ (12)where k₁ is the first-pseudo-order constant (min⁻¹), k₂ is the pseudo-second order constant (g/mg·min), k_int is the intraparticle diffusion rate constant (mg/g·min^1/2) and C is a value of intercept constant.

The aforementioned models were used to fit the experimental data, and nonlinear regression analysis and constant calculations were performed (Figure 6). Table 5 displays an overview of the kinetic parameters for each of the three models. Lower values of χ² and MPSD, along with R² values that are extremely near to unity, suggest that the adsorption process adhered to pseudo-second order kinetics models. In addition, whose predicted values, q_e._cal and q_max, from the experimental data are almost equal. Similar findings were observed by other researchers when examining the adsorption of heavy metal ions on biochar (Ding et al., 2014). Although intraparticle diffusion is important, it is not always the rate-controlling stage in the adsorption process, as indicated by the plot of the square root of time versus adsorption capacity (Figure not shown).

Table 5.

Shows the predicted q_e,cal, and observed q_e,exp values for the adsorption of Cr(VI) onto NFBC1, NFBC3, and NFBC5, as well as pseudo-first order, pseudo-second order, and intraparticle diffusion adsorption rate constants.

System	NFBC1	NFBC3	NFBC5
$q_{e, e x p}$ (mg/g)	84.746	87.470	102.696
Pseudo-first order
$q_{e, c a l}$ (mg/g)	61.029	68.161	75.805
$k_{1}$ (1/min)	0.054	0.061	0.056
$R^{2}$	0.7882	0.7159	0.7053
$χ^{2}$	216.837	97.280	290.859
MPSD	86.44	54.56	94.102
Pseudo-second order
$q_{e, c a l}$ (mg/g)	84.389	87.719	102.041
$k_{2}$ (g/mg·min)	0.003	0.002	0.003
$R^{2}$	0.9953	0.9961	0.9949
$χ^{2}$	0.393	0.519	0.668
MPSD	2.10	3.01	2.91
Intraparticle diffusion equation
$k_{i d}$	2.677	3.232	2.639
$C$	45.164	48.690	71.410
$R^{2}$	0.9544	0.9791	0.9789
$χ^{2}$	4.057	1.818	2.612
MPSD	6.83	4.55	4.27

MPSD: Marquardt's percent standard deviation.

Effect of temperature and thermodynamics parameters

By conducting the adsorption experiments at three distinct temperatures (30, 40, and 50°C) and applying the following equations, the standard Gibbs free energy change ( $Δ G \circ$ ), enthalpy change ( $Δ H \circ$ ), and entropy change ( $Δ S \circ$ ) were calculated in order to assess the thermodynamic criteria for the adsorption process: $Δ G \circ = - R T \ln K_{d}$ (13) $Δ G \circ = Δ H \circ - T Δ S \circ$ (14) $\ln K_{d} = - \frac{Δ G \circ}{R T} = - \frac{Δ H}{R T} + \frac{Δ S \circ}{R}$ (15)where K_d is the equilibrium constant, T is the absolute temperature (K), and R is the gas constant (8.314 J/mol.K).

Equation (13) was used to calculate the $Δ G \circ$ values. The slope and the intercept of the plots of ln K_d vs 1/T (Figure 7) were used to calculate the values of $Δ H \circ$ and entropy change $Δ S \circ$ . The outcomes are shown in Table 6. The drop in Gibbs energy, $Δ G \circ$ , which ranged from −37.018 to −33.727 kJ/mol in the temperature range (303–318 K), indicated the spontaneity of the adsorption process. In the Cr(VI) concentration range of 10–100 mg/L, $Δ H \circ$ ranged between −33.067 and −65.702 kJ/mol, whereas the entropy change, $Δ S \circ$ , varied between −22.289 and −89.737 J/mol.K. The enthalpy drop was consistent with the adsorption process's exothermic and spontaneous characteristics. In contrast to Cr(VI) ions attached to the surface of NFBC1, NFBC3, and NFBC5, the distribution of Cr(VI) ions in solution was clearly more ordered, which led to a net decrease in entropy.

Figure 7.

Thermodynamic study for Cr(VI) removal by the NFBC1, NFBC3, and NFBC5. The initial concentration of Cr(VI) = 50 mg/L, agitation speed 150 r/min, pH = 2.0, adsorbent dosage = 0.03 g/25 mL, contact time 180 min.

Table 6.

Thermodynamic parameters for the biosorption of Cr(VI) ions onto NFBC1, NFBC3, and NFBC5.

Adsorbent	$- Δ G \circ$ /kJ/mol	$- Δ H \circ$ /kJ/mol	$- Δ S \circ$ /J/K.mol
NFBC1	37.018	33.067	12.281
	36.981
	36.742
NFBC3	35.741	65.702	98.662
	34.982
	33.727
NFBC5	36.16835.80435.330	48.851	41.779

Conclusions

This work presents the effect of biosorbent NF biochar on the removal of Cr(VI) ions from an aqueous solution. It was concluded that activation of biochar with H₃PO₄ was effective to obtaining porous biosorbent and changed the surface chemical structure of the NFBC1, NFBC3 and NFBC5 which was confirmed with FTIR, SEM, XRD, and BET. The highest adsorption capacity was displayed by the biochar NFBC5 which had a maximum adsorption capacity of 102.696 mg/g at the ambient temperature of 30°C, pH = 2, contact time 90 min and biosorbent dosage 0.03 g/25 mL. The results of the D-R isotherm model suggest that the reaction was dominated by physiosorption mechanisms. The reaction was spontaneous and exothermic with equilibrium attained within the first 120 min. The Pseudo-second order kinetic model best fit the experimental data.

Supplemental Material

sj-docx-1-adt-10.1177_02636174251332165 - Supplemental material for Kinetic and thermodynamic investigations of neem fruit (Azadirachta indica) activated with H₃PO₄ for adsorption of hexavalent chromium

Supplemental material, sj-docx-1-adt-10.1177_02636174251332165 for Kinetic and thermodynamic investigations of neem fruit (Azadirachta indica) activated with H₃PO₄ for adsorption of hexavalent chromium by Imad Hamadneh, Abeer Alsharafat, Maisaa Alzawahrah, Akram Abu Shawer, Hana Hammad, Ahmed Al-Mobydeen, Sawsan Qanadilo and Ammar H Al-Dujaili in Adsorption Science & Technology

Footnotes

Acknowledgments

The authors would like to thank the Deanship of Scientific Research at the University of Jordan for its scientific support.

Data availability

The original contributions presented in the study are included in the article,further inquiries can be directed to the corresponding author.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

This research was funded by the Scientific Research and Innovation Support Fund,Ministry of Higher Education,Jordan (Grant No. WE/1/18/2021).

ORCID iDs

Imad Hamadneh

Ammar H Al-Dujaili

Supplemental material

Supplemental material for this article is available online.

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Supplementary Material

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Kinetic and thermodynamic investigations of neem fruit ( Azadirachta indica ) activated with H 3 PO 4 for adsorption of hexavalent chromium

Abstract

Keywords

Introduction

Materials and characterization

Materials

Characterizations

Biochar preparation

Determination of pHpzc

Adsorption experiments

Adsorption isotherms

Kinetic studies

Thermodynamic studies

Results and discussion

Characterization of biosorbents

Scanning electron microscope

X-ray diffraction

FTIR

Effect of pH solution

Adsorption isotherm models

The Langmuir isotherm model

The Freundlich isotherm

The Dubinin–Radushkevich isotherm

Effect of contact time and adsorption kinetics

Effect of temperature and thermodynamics parameters

Conclusions

Supplemental Material

sj-docx-1-adt-10.1177_02636174251332165 - Supplemental material for Kinetic and thermodynamic investigations of neem fruit (Azadirachta indica) activated with H3PO4 for adsorption of hexavalent chromium

Footnotes

Acknowledgments

Data availability

Declaration of conflicting interests

Funding

ORCID iDs

Supplemental material

References

Supplementary Material

Determination of pH_pzc

sj-docx-1-adt-10.1177_02636174251332165 - Supplemental material for Kinetic and thermodynamic investigations of neem fruit (Azadirachta indica) activated with H₃PO₄ for adsorption of hexavalent chromium