Anthill Clay Activated Ocimum gratissimum Extract for Effective Adsorption of Dyes and Heavy Metals from Wastewater

Anthill clay activated Ocimum gratissimum extract for effective adsorption of methylene blue and chromium (VI) ion from wastewater: Insights into the adsorption isotherms, kinetics, thermodynamics, and mechanisms

Lukman Shehu Mustapha

^a,b,1,*

, Samuel Oluwaseun Kolade

^c

, Sodiq Olayemi Durosinmi

^b

, Inn Shi Tan

^a

, Sie Yon Lau

^a

, Kehinde Shola Obayomi

^a,c,d,1,**

aDepartment of Chemical Engineering, Curtin University, CDT 250, 98009 Miri, Sarawak, Malaysia

bDepartment of Chemical Engineering, Federal University of Technology Minna Niger State, Nigeria

cZuckerberg Institute for Water Research (ZIWR), The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion 84990, Israel

dInstitute for Sustainable Industries and Liveable Cities, Victoria University, Werribee, VIC 3030, Australia

A R T I C L E I N F O Editor: Wenshan Guo Keywords:

Anthill clay Ocimum gratissimum Green activated Adsorption mechanism Reusability

A B S T R A C T

The removal of methylene blue (MB) dye and hexavalent chromium (Cr(Vl)) ions from water bodies have necessitated the search for a promising adsorbent material with high adsorptive performance, facile operation, and economical. Here, a novel adsorbent was prepared by activating anthill clay (AC) with Ocimum gratissimum leaf extract (OG@AC) by ultrasonic strategy method to eliminate MB and Cr(Vl) from the aqueous environment;

however, the activation strategy serves as an important factor that influenced the adsorptive performance to- wards MB and Cr(Vl). Furthermore, a high BET surface area of 120.48 m²/g was measured for OG@AC which is attributed majorly to the green activation strategy. The SEM micrograph of OG@AC was seen to have displayed a leaf-like plate formed by quartz and kaolin. In the batch experimental study, the MB and Cr(Vl) adsorption capacities towards OG@AC were determined to be 227.52 and 143.45 mg/g, respectively. Also, the adsorption kinetics and isotherms models results revealed that the adsorption process of OG@AC towards MB and Cr(Vl) was best described by pseudo-first-order and Freundlich models, suggesting that the adsorption process involves a multilayer physisorption. Moreso, thermodynamic results revealed the adsorption process is feasible, sponta- neous, and endothermic in nature. The OG@AC exhibited an outstanding regeneration ability and stability to- wards MB and Cr(Vl) by maintaining a removal efficiency of 79.11 % and 81.20 % even at the tenth successive adsorption-desorption cycles. Overall, this research sheds more insight on how anthill clay is activated using a feasible design and green chemical (Ocimum gratissimum leaf extract) to enhance its adsorptive performance towards MB and Cr(Vl), which can broaden its wide application in the real world.

1. Introduction

Environmental pollution is significantly worsened by the combined effects of urbanization and industrialization because of biological and industrial wastewater discharged into the environment [1,2]. The aforementioned circumstances have given rise to several concerns regarding the contamination of groundwater resources, water quality, and a surge in human health-related problems [3–5]. Currently, there

are many industries with human activity, such as printing, textiles, medicines, paint, smelting, coal mining, and electroplating. However, without any treatment, chemicals from these industries are continuously released into natural water sources and wastewater which increases the worldwide threat of a significant scarcity of clean water supply [6,7].

Chromium is an inorganic heavy metal that occurs in the forms of (Cr (VI)) and (Cr(III)) respectively. The hexavalent chromium (Cr(VI)) is present in aqueous solutions in the form of a tetrahedral chromate ion

* Correspondence to: L.S. Mustapha, Department of Chemical Engineering, Curtin University, CDT 250, 98009 Miri, Sarawak, Malaysia.

** Correspondence to: K.S. Obayomi, Zuckerberg Institute for Water Research (ZIWR), The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion 84990, Israel.

E-mail addresses: Mustaphalukman523@gmail.com(L.S. Mustapha), Obayomikehindeshola@gmail.com(K.S. Obayomi).

1The first author and the last author have equal contribution.

Contents lists available at ScienceDirect

Journal of Water Process Engineering

journal homepage: www.elsevier.com/locate/jwpe

https://doi.org/10.1016/j.jwpe.2024.106286

Received 28 August 2024; Received in revised form 2 October 2024; Accepted 3 October 2024

Journal of Water Process Engineering 67 (2024) 106286

Available online 4 October 2024

2214-7144/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ).

(CrO42−) [8]. Approximately 35 % of the chromium utilized is released into effluents. Hexavalent chromium exhibits an oxidizing effect and also generates free radicals which facilitate the conversion of Cr(VI) to trivalent chromium (Cr(III)). According to the World Health Organiza- tion (WHO), the acceptable limit for Cr(VI) is 0.05 mg/L and prolonged exposure to Cr(Vl) haves has severe health implications such as liver damage, cancers, neurological system, and birth abnormalities [2,9].

Methylene blue (MB), a cationic dye primarily employed in the paper, leather, and textile processes, is one of the most hazardous pollutants due to its high solubility in water and noteworthy toxicity [10–14].

In recent times, several techniques have been employed to eliminate contaminants from effluents prior to their recycling or discharge into the environment [15]. These methods encompass physical techniques such as photocatalysis, electrochemical processes, membrane separation, reverse osmosis, and adsorption [16]. However, many of these tech- niques are associated with energy-intensive operations and high costs.

Nevertheless, due to its outstanding adsorptive efficiency, ease of regeneration, cost-effectiveness, environmental friendliness, simplicity, and reusability, the adsorption technique has been thought to be a viable method for remediating pollutants from wastewater [17–19]. However, the use of effective adsorbent materials with widely accessible, abun- dant surface functions, well-developed porous structures, and a high specific surface area is necessary for the adsorption treatment to be successful. Despite the study of a variety of adsorbent materials, a novel adsorbent with excellent adsorption performance is still needed [20–22].

Anthill Clay minerals have exhibited notable adsorption abilities when it comes to effectively capturing toxic elements discharged by industrial wastewater. Moreover, they also possess favorable transport characteristics and demonstrate high exchange and capture capacities.

Clays are cost-effective, and abundant, and consist of hydrated silicates, aluminum, magnesium, and iron silicates. They possess a distinct surface composition characterized by the presence of both anionic and cationic ions [23,24]. The anthill clay morphology is defined by two-dimensional sheets composed of tetrahedral (SiO4) and octahedral (Al2O3) crystals.

These sheets have a particle size smaller than 2 μm [7]. Recent studies have provided evidence of the remarkable adsorption capabilities of clays for natural dyes and heavy metals. These properties of adsorption are influenced by various factors such as the clay type, surface area, composition, and surface charge characteristics [25]. However, the use of different kinds of clay whether modified or unmodified for waste- water treatment applications has been reported in previous studies.

Gupta et al., [26] developed a bio-adsorbent material by grafting CeO2

quantum dots over Areca nutshell biochar using Saccharum officinarum extract for methylene blue removal from synthetic effluent. Ashour et al., [27] examined the use of activated natural clay via acid and thermal treatment for chromium (VI) removal. The authors report ach- ieved an uptake of 66 % with an adsorption capacity of 14.3 mg/g. Also, Yakkerimath et al., [28] in their findings, also explored the use of un- modified natural clay for Cr(Vl) treatment. With a maximum adsorption capacity of 12.3 mg/g. Lastly, Ouaddari et al., [36] reported the use of natural and purified clays; smooth clay (SC), rigorous clay (RC), and bentonite clay (BC) for the elimination of MB. The authors revealed that the purified clays showed an enhanced adsorption capacity for MB compared to pristine ones: 147.64; 257.69 and 1383 mg/g versus 128.75; 193.4 and 681.85 mg/g for SC, RC, and BC respectively. This suggests that enhanced adsorption capacities can be attributed to elec- trostatic interactions and ion exchange capacity.

With great promise for the development of novel wastewater treat- ment techniques, the biotreatment of anthill clay materials has remained a largely unexplored field so far. The present study develops a novel adsorbent material using a cost-effective, and eco-friendly green chemical (Ocimum gratissimum leaf extract) to enhance the adsorptive performance of anthill clay (OG@AC) for the elimination of MB and Cr (Vl) from an aqueous environment which has not been reported previous studies. Furthermore, functional properties, textural, crystallographic,

thermal stability, and surface characteristics of the bioactive molecules incorporated into anthill clay were analyzed using different analytical techniques (FTIR, SEM, XRD, TGA, and BET). Also, several factors, such as adsorbent dosage, solution pH, contact time, initial concentration, and temperature that influence the adsorption process of OG@AC to- wards MB and Cr(Vl) were examined. The adsorption experimental data were fitted to the isotherms, kinetics, and thermodynamics models.

Finally, the ability of the adsorbent material to be reused at different adsorption-desorption cycles and adsorption mechanisms were also studied.

2. Materials and method 2.1. Materials

The anthill clay used in the research was obtained from the Asa-dam area, Kwara State in the North-central part of Nigeria. All the chemicals used in this study including sodium hydroxide (NaOH), chromium (VI) nitrate (Cr(NO3)6), sulfuric acid (H2SO4), and methylene blue (C16H18CIN3S) were purchased from Zayo- Sigma, Lagos State, Nigeria and were of analytical grade with a purity level ranging between 98.5 and 99.8 %.

2.2. Ocimum gratissimum leaf extract preparation

Ocimum gratissimum leaves were obtained from Ilorin, Kwara State, Nigeria, and were dried at 60 ^◦C in an oven overnight. The dried leaves were then crushed and reduced to a fine powder with the aid of a mortar and pestle. Thereafter, 100 g of the Ocimum gratissimum leaves powder was dissolved in 500 mL of distilled water and was stirred continuously for 4 h at 60 ^◦C. Finally, the mixture was left to cool at room temperature and filtered using Whatman (90 mm) filter paper.

2.3. Adsorbent preparation

In order to attain a uniform particle size of 5 μm, the anthill clay was first crushed into a fine powder using a pestle and mortar and then sieved. Subsequently, 30 g of sieved-powdered anthill clay was weighed and transferred into a 500 mL Erlenmeyer flask containing 300 mL of deionized water (DI). Afterward, the mixture was subjected to 4 h of intermittent sonication at 50 ^◦C. Following this, the mixture was allowed to settle for a period of 48 h. Furthermore, a centrifugation process was carried out for the separation between the resulting particles and the solution for 30 min at 6000 rpm, dried in an oven at 90 ^◦C, and stored in an air-tight plastic bag for further use. Again, the as-synthesized treated anthill clay was activated using a green synthesis method. 300 mL of Ocimum gratissimum leaf extract was measured and introduced into a 500 mL Erlenmeyer flask and was stirred continuously on a magnetic stirrer for 5 min at 800 rpm. Thereafter, 20 g of the dried powdered anthill clay was also measured and transferred into the solution under continuous stirring for another 30 min. Thereafter, the beaker contain- ing the mixture was then placed in an ultrasonic bath and allowed for 2 h at 40 ^◦C. Subsequently, the mixture was then allowed for 24 h before being centrifuged at 6000 rpm for 30 min. Furthermore, the activated clay material after centrifugation was dried in an oven at 60 ^◦C for 5 days and stored in an air-tight plastic bag for further use. The as- synthesized activated anthill clay with Ocimum gratissimum leaves extract was labeled as OG@AC.

2.4. Adsorbent characterization

The developed OG@AC adsorbent material was characterized to examine their crystallinity, thermal stability, functional group, textural and surface properties using X-ray diffractometer (XRD), thermog- ravimetry analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), and Brunauer-Emmett-

Teller (BET). The XRD technique was equipped with a pixel goniometer and Heidenheim encoder measuring 2θ angles at a scanning speed of 3^o/ min, ranging from 10⁰ to 90⁰ was employed to analyze the material crystallinity. The TGA machine (Digital PerkinElmer, TGA 8000) used a weight loss technique, and the stability was heated over a temperature range of 200–900 ^◦C at 20 ^◦C/min. Also, FTIR measurement was employed to measure the adsorbent using a spectrum wavelength range of 400-4000 cm⁻¹. The SEM analysis was employed to analyze the morphological structure (SEM-EDS, VE-9800, Keyence). Lastly, the OG@AC pore diameter, pore volume, and surface area were evaluated through N2 adsorption analysis using the BET measurement (NOVA 4200, UK). Overall, the process of titration was used to determine the zero-point charge (pHpzc) of OG@AC.

2.5. Adsorption experiment

In the present study, a batch experimental adsorption test was car- ried out to examine the adsorptive performance of OG@AC towards MB and Cr(Vl) ions. 0.025 g/L of OG@AC was measured and introduced into a set of 500-mL beakers containing 50 mg/L (200 mL) concentrations of MB and Cr(Vl). Then, the mixture was placed in a water bath isothermal shaker at a shaking speed of 120 rpm at 25 ^◦C for 60 min adsorption time and pH 6. Prior to equilibrium attainment, samples of the adsorbed MB and Cr(Vl) were taken at regular time intervals, filtered using a What- man filter paper (90 mm), and measured using atomic absorption spectroscopy (AAS-900) and UV–visible spectrophotometry (Shimadzu UV-1800, MB was measured at a wavelength of 664 nm). The MB and Cr (Vl) adsorb percentage removal was calculated using Eqs. (1)–(3) respectively.

R,%= (C0− Ct

Ct

)

×100% (1)

qt=(C0− Ct)V

M (2)

qe=(C0− Ce)V

M (3)

where, Co, Ct and Ce (mg/L) represent the initial, equilibrium, and concentration at time (t); V is the volume of the adsorbate solution (L);

and M is the mass of OG@AC measured in mg.

However, during the batch adsorption study, isotherms, kinetics, and thermodynamics and the influence of adsorption parameters such as contact time (5, 10, 20, 30, 40, 50, 60 min), pH (1, 2, 3, 4, 5, 6, 7, 8), adsorbent dosage (0.005, 0.010, 0.015, 0.020, 0.025, 0.030 g/L), initial concentrations of Cr(VI) and MB (50, 100, 150, 200, 250 mg/L), and temperatures (25, 30, 35, 40, 45 and 50 ^◦C) were all examined.

However, the capacity for multiple usages aimed to enhance the cost- effectiveness of the adsorption process was assessed with the reusability of OG@AC. First, the desorption of OG@AC adsorbent material was processed with a 0.15 mol/L solution of Na2CO3 as an eluent. After the adsorption of MB and Cr(Vl), the spent OG@AC catalyst was mixed with O.15 mol/L of Na2CO3 under, agitation at 180 ^◦C for 2 h at room tem- perature and dried at 80 ^◦C for 12 h. Furthermore, after the desorption experiment, the reusability experiment was carried out under equilib- rium conditions. 50 mg/L of Cr(VI) and MB (100 mg/L) in a 100 mL beaker was mixed with 20 mg/L, pH 6, equilibrium time of 50 min at a temperature of 45 ^◦C.

2.6. Kinetics, isotherms, and thermodynamics studies

The adsorption data were fitted to the pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion models (IDM) to examine the adsorption process rate-limiting steps using the following equations;

qt=qe

(1− e^−k1t)

(4)

qt= qe2K2t

1+qeK2t (5)

qt=Kidt^0.5+C (6)

where qt and qe (mg/g) are the adsorption capacities at a time ‘t’ and equilibrium denote; k₁, k_2, and k_id represent (1/min), (g/mg min) and (mg/g⋅min^1/2) representing PFO, PSO, and IDM kinetic models; and C is the intraparticle diffusion constant.

The adsorption isotherm study was conducted by fitting the equi- librium adsorption data to the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) models using Eqs. (7), (8), (9), and (10) respectively.

qe=QmaxKLCe

1+KLCe (7)

qe=KFCe

1n (8)

qe=RT bTln(

KTCeq

) (9)

qe=qme− KDRε² (10)

where Qmax is the maximum adsorption capacity (mg/g); Ce is the equilibrium concentration (mg/L); qe is the equilibrium adsorption ca- pacity (mg/g); KL is the Langmuir constant (L/mg); KF is the Freundlich constant measured in (mg/g)/(L/mg)^(1/n)), KDR is D-R energy adsorption (in mol²/kJ²); and RL is the Langmuir dimensionless constant. R is the gas constant (8.314 J/Kmol); T is the temperature (K).

The thermodynamic parameters including Gibbs free energy change (ΔG^◦) (J/mol), the enthalpy change (ΔH^◦) (J/mol), and entropy change (ΔS) (J/(mol. K) were examined at different temperatures using Eqs.

(11)–(13).

ΔG^o= − RTlnK0 (11)

Keq=Cad

Ce (12)

logkeq= − (ΔH⁰) 2.303RT+

(ΔS⁰)

2.303R (13)

where ΔG^◦is the Gibbs free energy change (J/mol); ΔH^◦is the enthalpy change (J/mol); ΔS is the entropy change (J/(mol.K); R is the gas con- stant (8.314 J/mol.K); and T is the temperature(K).

The error function measurement was calculated using the Chi-square test (X²) and the sum of square error (SSE) methods as presented in Eqs.

(14) and (15).

SSE=∑ⁿ

i=1

(qe,exp− qe,cal

)₂

(14)

χ²=

∑

(qe,exp− qcal

)₂

qe,cal (15)

3. Results and discussion 3.1. OG@AC analysis

The XRD analysis of OG@AC was measured and presented in Fig. 1 (a). The XRD results revealed that the major phases identified in OG@AC were quartz (Q) and kaolinite (K) which are common components of kaolin clay [29–31]. The XRD patterns of OG@AC matching the JCPDS

files for SiO2 and Al2(Si2O5)(OH)4 were observed at specific 2θ values of 12.42, 20.68, 25.38, 27.54, 38.31, 42.36, 47.53, 50.05, 59.86, and 62.30^o. The recorded 2θ values corresponded to the crystallographic orientations of (001), (020), (002), (021), (200), (205), (240), (150), (311) and (340) respectively. The obtained results demonstrated that the extract's leaf biogenic molecules enhanced the attachment of hy- droxyl groups to the edges of OG@AC.

The thermogravimetric analysis depicted in Fig. 1(b) shows that a weight reduction of 0.88 in OG@AC which falls between 50 ^◦C and 200 ^◦C is attributed majorly to the evaporation of adsorbed water and the decomposition of volatile organic matter [32]. Also, a weight reduction of 80.55 % was noticed when the temperature increased from 200 to 320 ^◦C suggesting the transformation of Al(OH)3 into Al2O3 [33].

Finally, the material was observed to attain thermal stability between 320 and 580 ^◦C. The TGA result from this study was seen to be consistent with research works on the decomposition behavior of clay materials based on kaolin [34,35].

The FTIR spectra of OG@AC depicted in Fig. 1(c) show that the Peaks at 580.11 and 601.23 cm⁻¹ are assigned to vibrations from the distortion of Si-O-Mg and Si-O-Al bonds [36]. Also, the peaks observed at 912.34, and 1045.29 cm⁻¹ were assigned to the stretching Al-Al-OH and vi- brations of Si–O [37,38]. Furthermore, peaks at 1105.14 and 1547.62 cm⁻¹ correspond to the H-O-H group deformation likely due to the presence of biomolecules on the OG@AC [39,40]. Moreover, the broad peak at 2385.39 cm⁻¹ corresponds to the elongation of O–H bonds indicating the presence of a significant amount of physiosorbed H2O on the surface of OG@AC [41]. However, the peaks detected at 3625.30 cm⁻¹ indicate the O–H stretching of Si-OH and Al-OH groups where the signals overlap with the O–H water molecules [42,43]. These OH

groups are located within the octahedral and tetrahedral sheets [44].

The functional groups identified via FTIR analysis demonstrate the successful integration of biomolecules providing valuable insights into how OG@AC adsorbs Cr(VI) ion and MB.

The BET surface measurement is presented in Fig. 1(d). The adsorption-desorption isotherm curve observed exhibited a type IV behavior, as described by the IUPAC classification, suggesting that the material is dominated by a mesoporous structure [45]. The outcome suggests that the OG@AC displays wide distribution within the pores.

The OG@AC surface area was 120.48 m²/g, 0.026 cm³/g pore volume, and a pore diameter of 38.26 nm. In general, OG@AC displays a mes- oporous architecture at the nanoscale providing a better pore volume and surface area that facilitate the formation of OG@AC active site for the Cr(VI) ion and MB removal [46,47].

The micrograph structure examination of the anthill and OG@AC is displayed in Fig. 2(a-b). As observed in Fig. 2(a), the anthill comprises a mixture of kaolinite and quartz forming compact aggregates, hexagonal structures, and leaf-like edges. Also, the micrograph of OG@AC depicted in Fig. 2(b) shows a distinct clustered spheroidal crystal shape following the biotreatment which has made the surface be coated with biological molecules introducing fresh active sites that enhance its capacity to adsorb contaminants such as Cr(VI) ion and MB from wastewater.

3.2. Batch adsorption study 3.2.1. Solution pH impact

The pH solution is a crucial aspect of the adsorption process as it influences the charges on the surface of the adsorbent. Also, the mo- lecular structure of the adsorbate, the functional group properties, and Fig. 1.(a)XRD spectra; (b)TGA plot; (c) FTIR and (d) N2 adsorption and desorption isotherm.

the ionization level make it a very important aspect to consider [48–50]

However, MB and Cr(VI) adsorption by OG@AC was investigated over a pH range of 1–8 at an initial concentration of 100 mg/L, adsorbent dosage of 0.025 g/L, and 50 min contact time at temperature of 45 ^◦C.

Also, a broad range of pH was chosen for this experiment due to the potential degradation of MB molecules in alkaline conditions and the ionic nature of Cr(VI) [51]. As depicted in Fig. 3(a), it was seen that the adsorptive capacity of OG@AC for Cr(VI) ion and MB significantly increased from 38.27 to 168.21 and 24.01 to 151.76 mg/g respectively with increasing pH from 1 to 6. However, increasing the solution pH beyond this point resulted in no significant change in the adsorption capacities of MB and Cr(Vl). Cr(VI) behaves as an ionic metal at pH <7 while MB is termed as cationic dye that exists in the form of molecules.

At pH values (<4), the amino in the MB becomes protonated converting MB to MB²⁺[52]. At lower pH levels (<4) the binding sites' occupation could be attributed to the high concentration of H⁺ions making the OG@AC surface become more positively charged and promoting elec- trostatic repulsion between the positively charged MB and Cr(Vl), thereby promoting a decrease in the adsorption percentage removal and adsorption capacities. On the other hand, increasing the pH solution (pH > 4) resulted in the formation of OH⁻, making the adsorbent OG@AC surface more negatively charged which favors the adsorption of positively charged MB and Cr(Vl) via electrostatic attraction, promoting an increase in the adsorption capacities and uptake of MB and Cr(Vl) respectively, [53,54]. This finding is supported by the point of zero charge (pHpzc) measurement as depicted in Fig. 3(b). The OG@AC sur- face charge was measured to be 4.0 using the zeta potential. This sug- gests that the OG@AC surface becomes positively charged at pH <pHpzc

and becomes negatively charged at pH >pHpzc [55]. Therefore, at pH <

pHpzc, a notable electrostatic repulsion arises between the positively

charged Cr⁶⁺and MB while at pH >pHpzc, the adsorption of MB and Cr⁶⁺is more favored through the electrostatic attraction [56].

3.2.2. Impact of contact time

The effect of adsorption time on MB and Cr(Vl) adsorption towards OG@AC was examined at varied time intervals ranging from 0 to 60 min using 100 mg/L initial concentrations of MB and Cr(Vl), an adsorbent dosage of 0.025 g/L, pH of 6, and at temperature of 45 ^◦C. Based on the results displayed in Fig. 4(a), the adsorption of MB and Cr(VI) was achieved after 50 min with adsorption capacities recorded at 38.65 for Cr(VI) and 21.05 mg/g for MB. The rapid adsorption observed during this period might be attributed to the presence of more adsorption-active sites made available by the presence of significant electron-donating functional groups [57]. However, after 50 min, the adsorption of MB and Cr(Vl) towards OG@AC was observed to be constant due to the accumulation of the adsorbent sites of OG@AC with the adsorbed MB and Cr(VI) [58]. Furthermore, the OG@AC exhibited a greater capacity for adsorbing Cr(VI) when compared to MB which can be attributed to the fact that there are more positive ions on Cr(Vl) than in MB [59,60].

3.2.3. Impact of initial concentration

The research explored how varying initial concentrations of Cr(VI) and MB ranging from 50 to 250 mg/L, influenced the adsorptive per- formance of OG@AC. Throughout the Cr(VI), experiments, a contact duration of 50 min, pH of 6, and adsorbent dosage of 0.025 g/L, and at a temperature of 45 ^◦C were maintained. The plots presented in Fig. 4(b-c) show the influence of initial MB and Cr(VI) concentration on the adsorption removal and adsorption capacity of OG@AC. Fig. 4(b) revealed that the MB removal percentage decreased from 97.46 to 52.13

% while the adsorption capacity increased from 100.21 to 270.05 mg/g, Fig. 2.Micrograph structures of (a) Anthill clay, and (b) activated anthill clay (OG@AC).

Fig. 3.Impact of (a) pH on adsorption capacity (b) pHpzc curve of OG@AC.

However, a similar trend was observed for Cr(VI) ion from Fig. 4(c), where the removal percentage also decreased from 98.21 to 56.34 % while an increase in adsorption performance was observed from 110.03 to 240.14 mg/g respectively [61]. However, the decrease in both MB and Cr(Vl) efficiency at higher concentrations can be attributed to the competition among the formation of more MB and Cr(Vl). Additionally, the adsorption process involves different steps such as diffusion into the

pores of the adsorbent, and movement of molecules to the external adsorbent surface. The increase in adsorption capacity at higher con- centrations can be ascribed to MB and Cr(VI) molecules interaction. This favors a higher adsorption capacity of MB and Cr(Vl). Moreover, higher MB and Cr(Vl) concentrations provide more driving force for the diffu- sion of MB and Cr(Vl) molecules onto the adsorption sites of OG@AC [62,63].

Fig. 4. (a) Effect of contact time (b) MB concentration at initial; (c) Cr(VI) concentration at initial; (d) OG@AC dosage on MB; (e) OG@AC dosage on Cr(VI) and (f) temperature on adsorption capacity.

3.2.4. Impact of OG@AC dosage

In order to ascertain the most effective OG@AC dosage for removing Cr(VI) and MB from wastewater, the influence of OG@AC dosage ranging from 0.005 to 0.030 g/L under equilibrium conditions of 50 min contact time, pH 6, temperature at 45 ^◦C, and MB and Cr(Vl) initial concentration of 100 mg/L were examined. The results of this study are illustrated in Fig. 4(d). In Fig. 4(d), the plot depicted that the removal percentage increased with a decrease in adsorption capacity as the dosage increased. However, the increase in mass of adsorbent dosage from 0.005 to 0.02 g/L resulted in 94.65 % removal while further in- crease beyond 0.02 g/L OG@AC dosage showed a minimal impact on the efficiency removal of MB. Also, an increase in the percentage removal was observed for Cr(Vl) with increasing OG@AC dosage. The Cr(Vl) removal efficiency was observed to have increased from 67.21 to 98.07

%. The increase in the percentage removal of MB and Cr(Vl) with increasing OG@AC dosage could be attributed to the availability of more active sites as the dosage increases, thereby promoting the uptake of MB and Cr(Vl). However, increasing the adsorbent dosage beyond 0.02 g/L resulted in no significant change in the adsorption capacity and percentage removal of MB and Cr(Vl). The observation could be attributed to OG@AC adsorption site blockage resulting from surface agglomeration on the adsorbent surface hindering the adsorption of MB and Cr(Vl) molecules [64–66].

3.2.5. Effect of temperature

The effect of temperature on the adsorption process of Cr(VI) and MB by OG@AC was examined at varied temperatures ranging from 25 to 50 ^◦C while maintaining a constant contact time of 50 min, pH of 6, a dosage of 0.020 g/L, and an initial MB and Cr(Vl) concentration of 100 mg/L. It was observed that the temperature curve displayed in Fig. 4(e) showed an increasing trend in the adsorption capacities of Cr(VI) and MB as the temperature rises. The adsorption capacity of MB increased from 25 to 45 ^◦C which resulted in 158.57 to 270.39 mg/g and Cr(VI) 179.05 to 310.25 mg/g. Therefore, the temperature observed suggests an endothermic reaction process of Cr(VI) and MB onto OG@AC and this phenomenon enhanced the interaction between the MB and Cr(VI) ion molecules and the OG@AC active surface sites providing enough ki- netics energy needed for the Cr(VI) and MB molecules movement within

the adsorbent surface.

3.3. Adsorption kinetics

This study examined the step that limits the rate of the adsorption process of methylene blue on OG@AC. The experimental data were analyzed using the pseudo-first-order, pseudo-second-order, and intra- particle diffusion for fitting. Fig. 5(a-b) and Table 1display the non- linear kinetic curves and the associated kinetic parameters. Analyzing the kinetic model parameters using their R² values and sum of square errors (SSE) from Table 1, it was evident that the adsorption mechanism of Cr(VI) and MB on OG@AC was best fitted to experimental data of the

Fig. 5. Non-linear plot of Cr(VI) and MB comprising pseudo-first order, pseudo second order and intra particles at pH of 6 temperature of 45 ^◦C and 0.020 g/

L OG@AC.

Table 1

Adsorption kinetics parameters for MB and Cr(VI) adsorption on OG@AC.

Kinetics OG@AC

Cr(VI) MB

Pseudo-first order

K1(min⁻¹) 0.013 0.148

qe, exp.(mg/g) 6.27 5.83

qe, cal(mg/g) 4.75 3.17

R² 0.999 0.998

SSE 0.421 0.563

ᵪ² 1.061 2.500

Pseudo-second order

K2(g/mg/min) 0.056 0.258

qe, exp.(mg/g) 8.311 6.271

qe, cal(mg/g) 12.36 10.32

R² 0.969 0.978

SSE 0.104 3.031

ᵪ² 0.190 0.260

Intraparticle

Kid(g/mg/min) 0.786 0.284

qe, exp.(mg/g) 8.311 6.271

C (mg/g) 0.366 4.019

R² 0.955 0.857

SSE 0.178 0.219

ᵪ² 0.696 0.724

pseudo-first-order kinetic by lower values of 0.421 and 0.563 SSE as well as lower sum of square error values (X²) of 1.061 and 2.500. Addition- ally, the higher R² values approaching unity for Cr(VI) 0.999 and MB 0.980 indicated the superior suitability of the PFO model when compared to the PSO and intraparticle models. Furthermore, there is close agreement between the calculated adsorption and experimental adsorption capacity values (qe, cal) for Cr(VI) (4.75) and MB (3.17 mg/

g) derived from the PFO and the qe values support the claim that the PFO model offers the most accurate depiction of the adsorption kinetics for Cr(VI) ion. However, the suitability of the PFO to the adsorption process indicates that physical adsorption and diffusion play a significant role in the adsorption process [67]. Also, the PSO kinetic model resulted in 0.969 for and 0.978 value of R² for MB and Cr(Vl), indicating a satis- factory fit and hinting at the likelihood of a process of chemisorption in the adsorption process [14]. Moreover, analysis of the intraparticle diffusion plot revealed that the experimental data fitting did not fit in well with experimental data suggesting that the adsorption process was not solely governed by intraparticle diffusion but potentially involved external diffusion as well. The primary stage implies the migration of adsorbate molecules to the surface of the adsorbent via movement of mass, while the subsequent stage showcases the adsorbate molecules' infiltration into the adsorbent through the layer boundary. Importantly, it was observed that the IDM curves did not start from the starting point, indicating other factors than IDM were also impacting the adsorption rate [68]. Consequently, the decreased diffusion rate of Cr(VI) (0.786) compared to MB (0.28 mg/g min^0.5) within the OG@AC's porous matrix may be linked to the presence of oxygen-based functional and hydroxyl groups on the interactions of π-electron and OG@AC's exterior which are facilitated by forces of electrostatic [6].

3.4. Adsorption isotherm

In an equilibrium adsorption state, the allocation of adsorption molecules between the liquid and solid phases is governed by the adsorption isotherm [69]. Furthermore, adsorption reaches equilibrium when, under constant temperature, there is an equal distribution of the adsorbate between the liquid phase and the adsorbent surface, with no additional net adsorption taking place. Despite the development of numerous isotherm models to suit experimental data, none are exact, and each involves underlying assumptions [70]. The experimental data collected from Cr(VI) and MB adsorption on the OG@AC was analyzed using eight distinct isotherm models: Langmuir, Freundlich, Temkin, Dubinin–Radushkevich. The equations and curves for fitting the isotherm models are presented in Table 2and Fig. 6(a-b). Based on the model parameters and their respective values (Table 2), it was observed that the adsorption of methylene blue and Cr(VI) on the OG@AC surface was most accurately described by the Freundlich isotherm. The deter- mination was made considering their elevated coefficient of determi- nation (R²) alongside the diminished sum of square errors (SSE) and X² values. It is evident that the R² of 0.9653 for Cr(VI) and 0.9778 for MB in Freundlich model exhibited a higher correlation coefficient compared to the Temkin, Langmiur and D-R isotherm models. The strong agreement of the Freundlich model with the adsorption mechanism implies that the adsorption of Cr(VI) and methylene blue on the OG@AC is primarily governed by multilayer adsorption on a surface with varying properties.

Significantly, the Freundlich isotherm describe heterogeneous adsor- bent surfaces for both Cr(VI) and MB and adsorption on multilayer [71,72]. Also, the values of nf (3.47 and 8.19) for the adsorption of MB and Cr(VI) by OG@AC indicated favorability, as they fell within the 1–10 range. In general, n_f values >1 signify enhanced adsorption effi- ciency, whereas nf values below 1 indicate an outstanding adsorption performance [13,73]. Significantly, the quantity of adsorption sites (nf) for Cr(VI) adsorption by OG@AC surpassed that of MB, suggesting a more robust interaction in adsorption between Cr(VI) and OG@AC.

Moreover, the Langmuir model yielded maximum adsorption capacities of (143.45) and MB (227.52 mg/g). The significant adsorption capacity

may be attributed to the following factors: initially, the porous nature and extensive surface area of the OG@AC facilitate the diffusion of Cr (VI) and methylene blue molecules enabling them to reach and occupy the pore locations within the adsorbent surface. Additionally, the plentiful negatively charged surface of the OG@AC enables the adsorption of the positively charged cationic on MB and Cr(VI) ion via robust electrostatic interactions. Furthermore, the experimental data fitting order of Cr(VI) & MB adsorption on OG@AC are Freundlich >

Langmuir >Temkin >D-R.

3.5. Thermodynamic evaluation

In order to understand the role of temperature in the adsorption process, the thermodynamics study of Cr(VI) and MB adsorption by OG@AC were examined at different temperatures of 298, 303, 308, 313, 318, and 323 K. The thermodynamics plot and the corresponding pa- rameters are presented in Table 3and Fig. 7(a). The ΔG^◦values shown in Table 3were negative and decreased with rising temperatures, sug- gesting that the Cr(VI) and MB adsorption were feasible, spontaneous, and favored increased temperatures. The ΔH^o positive values signify that the adsorption process of Cr(VI) and MB adsorption on OG@AC is endothermic in nature. In general, physisorption is described when ΔH^o values lower than 40 kJ/mol, whereas values higher than 40 kJ/mol indicate an adsorption process controlled by chemisorption [74]. Hence, the ΔH^o values of 318.17 and 613.42 kJ/mol for MB and Cr(Vl) suggest that the adsorption process is favored by chemisorption, which is characterized by interactions such as ion exchange, electrostatic force, and van der Waals forces. In contrast to MB, a greater ΔH^o value was recorded for Cr(VI), possibly due to the increased attraction between Cr (VI) and OG@AC. The positive ΔS^o values indicate a rise in randomness at the OG@AC-Cr(VI)/MB interface once equilibrium adsorption has been reached.

Table 2

Adsorption isotherms parameter for MB and Cr(VI) adsorption towards OG@AC.

Isotherm OG@AC

Cr(VI) MB

Langmuir

qo(mg/g) 143.45 227.52

KL(L/min) 0.107 0.068

RL 1.49 ×10⁻² 6.19 ×10⁻³

R² 0.8523 0.9327

SSE 37.9 12.2

ᵪ² 20.1 16.31

Freundlich

nf 8.19 3.47

KF(mg/g) 0.3089 0.469

R² 0.9653 0.9778

SSE 2.641 3.521

ᵪ² 3.620 2.167

Temkin

BT(kJ/mol) 0.138 3.590

AT 0.338 4.526

R² 0.9206 0.8725

SSE 4.75 7.08

ᵪ² 6.37 6.16

D-R model

Qm(mg/g) 38.463 24.752

KDR(L/mg) 66.31 288.38

R 1 1

T 1 1

R² 0.6702 0.8121

SSE 8.61 5.01

ᵪ² 31.02 15.0

3.6. Reusability study

Regeneration is crucial when choosing an adsorbent for industrial use and commercial purposes as it provides valuable information on the performance upon reuse and stability of OG@AC, thereby leading to potential cost reductions. The Na2CO3 solution (0.10 mol/L) was employed as an eluent due to acidic nature of the surface of OG@AC for adsorption /desorption analysis. The reusability investigation of OG@AC for adsorbing Cr(VI) and MB was conducted over tenth cycles, as illustrated in Fig. 7(b). The capacity of OG@AC to remove Cr(VI) and MB gradually diminished with each successive cycle of reuse. In particular, the percentages adsorption for MB and Cr(VI) decreased to 79.11 % and 81.20 % throughout the cycle. The decrease in effectiveness of adsorption of the adsorbates may be attributed to the release of Cr(VI) and MB molecules from the surface OG@AC during desorption, resulting in a reduction in the accessibility of adsorption sites [75]. Also, as the adsorption-desorption cycles keeps increasing, the effectiveness, stabil- ity and performance of the adsorbent material keep decreasing, pro- moting a lower uptake of the MB and Cr(Vl). The efficiency percentage removal of above 75 % for MB and Cr(VI) after 10th cycle suggesting the potential and effective use of OG@AC in an industrial scale treatment.

3.7. Influence of co-existing ions analysis

Industrial and natural wastewater from diverse sources comprises

different inorganic salts which have the potential to influence the contaminant adsorption efficiency. Hence, it is essential to investigate how the existence of coexisting ions affects the adsorption characteris- tics of MB and Cr(VI) when employed on OG@AC. This study includes the introduction of anions and cations into the wastewater solution to evaluate their effects. As depicted in Fig. 7(c), the cation and anion were found to enhance the adsorption of MB and Cr(VI). Specifically, the impact of anions followed the sequence: SO42− >Cl⁻ >NO3− in terms of adsorption of Cr(VI), while the cations exhibited the following hierarchy of influence considering MB adsorption Na⁺ < Ca²⁺. The pattern observed for anions was consistent with that seen in adsorption of Cr (VI). The anions with higher valence tend to compete more effectively for adsorption sites because of their increased ionic strength in the so- lution. However, it is noteworthy that the adsorptive removal of Cr(VI) by OG@AC showed an increment reaching 91 % for Na⁺, 92.38 % for Ca²⁺, 97.21 % for Cl⁻, NO3− had 97.02 % and SO42−99.37 % respectively.

Also, the cations presence led to an increase in adsorption of MB by OG@AC, with 67.10 % for Na⁺and 69.24 for Ca²⁺. Conversely, the presence of anions resulted in a notable percentage removal reaching 65.02 %, 64.99 and 71.16 % for NO3−, Cl⁻, and SO42−. Afterward, different ion coexistence tends to form cluster upon interaction with organic compounds. This process leads to the conversion of numerous small molecules into larger colloidal particles, thereby leading to removal of MB and Cr(VI) ion. Consequently, anions possessing a higher valency configuration can easily permeate the porous framework of OG@AC leading to a more robust electronic interaction. Particularly, SO₄²⁻ generate complexes with the hydroxyl units on the OG@AC which boosts its binding potential to the adsorbent when compared to Cl⁻ and NO3− within a system of coexisting ions. Furthermore, the density on the surface of OG@AC increases the negative charge due to the presence of SO42− among coexisting ions, thereby increasing the electrostatic attraction between the MB and Cr(VI). The ionic radii of 0.332 nm for NO3− and 0.335 nm for Cl⁻ assist the monovalent anions while that of Cl- exhibits a greater ability to penetrate the OG@AC surface.

3.8. Proposed adsorption mechanism for MB and Cr(VI) adsorption The characterization and adsorption process revealed that the bio- molecules from the OG extract generated an alkaline environment, improving the adsorption capabilities of the Si-O-Si and Si-O-Al covalent bond between OG@AC which showcases its excellent adsorption prop- erties. The adsorption mechanism showing the interaction between Fig. 6. Non-linear plot of Cr(VI) and MB comprising Langmuir, Freundlich, Temkin, and D-R at pH of 6, 0.020 g/L dosage, and temperature of 45 ^◦C.

Table 3

Adsorption thermodynamic parameters for MB and Cr(Vl) adsorption by OG@AC.

Adsorbate Temp (^o K) ΔH^o(kJmol⁻¹) ΔS^o(kJmol⁻¹ K⁻¹) ΔG^o(kJmol⁻¹)

Cr(VI)

298 613.42 0.4370 −2291.41

303 −2140.90

308 −2047.11

313 −1963.37

318 −1832.46

323 −1702.14

MB

298 318.17 0.3216 −4124.23

303 −3837.72

308 −3740.12

313 −3572.01

318 −3108.02

323 −3072.51

OG@AC and the adsorbates (MB and Cr(VI)) was presented in Fig. 8.

From the findings, the BET and SEM analysis offered valuable details regarding the structural characteristics of OG@AC, revealing a rough surface contours formation and a notable abundance of leaf-like parti- cles. OG@AC is visibly characterized by its intricate pore size, sub- stantial surface area, and mesoporous structure, presenting numerous active sites for the adsorption of MB and Cr(VI). The analysis by FTIR confirmed that the main functional groups attached to OG@AC were harmonious with their function as an adsorbent during the process of adsorption. OG@AC's fundamental components include hexagonal structures of aluminum-oxygen and silicon-oxygen defining its meso- porous design. This encompasses the interlinked pore structures, aluminosilicate framework, the existence of H2O molecules and cations within the voids [62,76]. As both MB and Cr(VI) are molecules with polarity, generating a dipole moment from distinct positive and negative charges at opposite poles, the expectation is for electrostatic interactions to take place between the structures of OG@AC and these adsorbates.

The notable adsorption capacity of OG@AC was attributed to its high levels of hydroxyl, aluminosilicates, and carboxylic groups. These functional groups, particularly the hydroxyl and carboxyl groups formed chelates with Cr(VI), thereby enhancing the mechanism of physical

adsorption. Cr(VI) is a divalent cation with positive charge. MB is clas- sified as a cationic dye with the potential position on either the nitrogen atom or central carbon atom.

Due to their positive charges, both types of pollutants engage with the OG@AC groups such as -OH and –COOH, serving as the predominant factors influencing the adsorption process. Furthermore, the adsorption and elimination of Cr(VI) may involve coordination mechanisms that potentially enhance OG@AC's ability to adsorb Cr(VI) compared to other substances. Kinetics, thermodynamic and Isotherm results of the parameters adsorption and curve fitting for the MB and Cr(VI) elimi- nation by OG@AC revealed that chemical adsorption governs the uptake process primarily with physical adsorption, providing additional sup- port. In the varied surfaces of OG@AC, the complex process of multi- layer molecular adsorption and intraparticle diffusion are key factors that significantly influence the overall adsorption mechanism. The interaction between Cr(VI)/MB solution within OG@AC involves ion exchange contributing to the process of adsorption. However, the mechanism of the adsorbing of the pollutants is aided by electrostatic attraction between molecules and van der Waals forces. For instance, Al- OH, Si-OH, and H-OH in OG@AC can establish robust hydrogen bonds with Cr(VI) and MB molecules. In a similar vein, the π-electrons within Fig. 7.Van't Hoff plot (a) and (b) Number of cycles on adsorption efficiency and (c) Co-exist ion effect on MB and Cr(VI) removal at pH of 6, 50 ^◦C temperature and 20 mg/L dosage OG@AC.

layers of carbon can capture the Cr(VI) and MB molecules, as outlined by Ouyang et al., [77]. These actions occur in tandem during the adsorption process, culminating in OG@AC's outstanding capability to adsorb Cr (VI) and MB. Also, findings from kinetics, thermodynamics, and the isotherm on the suggested adsorption mechanism play a crucial role in the elimination of Cr(VI) and MB by OG@AC. A closer fit towards the Freundlich isotherm fit experimental data best as compared to alterna- tive models for the adsorption suggests that the microporous regions within a diverse OG@AC predominantly drive the adsorption process.

This suggests chemical and physical adsorption mechanism participa- tion. It emphasizes the role played by pores of the adsorbent and the dynamics interaction between the adsorbent and adsorbate in the elimination of contaminants by OG@AC. Furthermore, a closer match of the adsorption kinetic data on Cr(VI) and MB with the PFO model in- dicates the prevalence of adsorption forces that greatly affect the initial adsorbate concentration. This highlights an adsorption process that moves towards equilibrium. Moreover, the thermodynamic parameters signify the spontaneity of the adsorption process with the temperature's influence, providing additional understanding of the adsorption's na- ture. All the combined results propose that OG@AC stands as a viable material for the removal of toxic substances from water solutions, of- fering direction on enhancing conditions for maximal efficiency.

3.9. Adsorbents comparison

Table 4presents a comparison of different materials, detailing their adsorption capacities, optimal adsorption conditions, and potential for reusability in removing Cr(VI) and MB. Even though certain materials showed quicker adsorption times, OG@AC remained competitive regarding environmental aspects and its adsorption capacity. OG@AC's reusability showcased its ability to outperform other different materials, considering the cycle number it could sustain. Furthermore, OG@AC functioned as an eco-friendly adsorbent with remarkable adsorptive capacities for Cr(VI) and MB because of its biocompatibility.

4. Conclusion

Herein, the biomolecules from Ocimum gratissimum leave extract were employed to successfully enhance the adsorptive performance of anthill clay (OG@AC) via an ultrasonic technique for the adsorption of MB and Cr(VI). The material's crystalline and micrograph structures, thermal stability, functional groups, and textural properties were

examined using state-of-art techniques such as XRD, SEM, TGA, FTIR, and BET. The BET surface area of the developed OG@AC was 120.48 m²/g, suggesting that the adsorptive performance of the material was successfully improved. The batch adsorption experimental study revealed that the adsorption process was largely influenced by initial concentration, adsorbent dosage, pH, temperature, contact time, and competing ions. The adsorption isotherms examination revealed that the adsorption process of MB and Cr(Vl) onto OG@AC is well explained by the Freundlich model with maximum adsorption capacities of 227.53 and 143.45 mg/g for MB and Cr(Vl), suggesting the adsorption process occurs on a multilayer surface of OG@AC. The adsorption kinetics revealed that the adsorption process followed a pseudo-first order, fa- voring a physisorption mechanism. Furthermore, the thermodynamics evaluation showed that the process is feasible, spontaneous, and endo- thermic in nature with an increase in the degree of freedom between the adsorbate (MB and Cr(Vl)) and OG@AC interface. The reusability test showed an outstanding percentage removal of MB and Cr(Vl) (>75 %) after the tenth cycle, indicating their stability and their potential in an industrial treatment scale. The adsorption mechanism study revealed that the interaction between the adsorbed MB and Cr(Vl) and OG@AC was governed by ion exchange, electrostatic attraction, surface and pore filling, surface complexation, and hydrogen bonding. In conclusion, the developed green-activated OG@AC showed an outstanding potential and applicability to be employed as an eco-friendly and effective adsorbent for pollutants both in small and industrial-scale wastewater treatment.

CRediT authorship contribution statement

Lukman Shehu Mustapha: Writing – original draft, Validation, Software, Methodology, Formal analysis, Data curation, Conceptuali- zation. Samuel Oluwaseun Kolade: Formal analysis, Conceptualiza- tion. Sodiq Olayemi Durosinmi: Supervision, Formal analysis. Inn Shi Tan: Writing – review & editing. Sie Yon Lau: Writing – review &

editing. Kehinde Shola Obayomi: Writing – review & editing, Writing – original draft, Supervision, Methodology, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 8.Proposed adsorption mechanism for MB and Cr(VI) adsorption onto OG@AC.