Sand mulch-aided ambient-air fabrication of microporous cocoa waste derived-activated carbon for methylene blue adsorption

ABSTRACT Porous carbon materials for adsorption applications are regarded as one of the most viable water cleaning strategies. Cocoa pod husk (CPH) was treated with two-step catalytic pyrolysis employing hydroxides and carbonates of sodium and potassium to synthesise activated carbon for methylene blue (MB) decolourisation. Dye adsorption assessment of activated carbon prepared with various activators (NaOH, KOH, Na2CO3 and K2CO3) established KOH-derived activated carbon as a superior absorbent. Upon impregnation ratio optimisation with KOH, CPHAC-3K (CPH: KOH of 3) prepared at 800°C acquired a BET surface area, pore volume and pore size of 1688.4 m2/g, 0.76 cm3/g and 1.25 nm, respectively. The maximum MB adsorption capacity of CPHAC-3K was 445 mg/g, demonstrating an impressive adsorption property compared with literature. The dye adsorption of CPHAC-3K was mainly attributed to its excellent textural property (microporous) and numerous oxygen-containing functional groups on the surface, as shown by BET and FTIR studies, respectively. Adsorption investigations demonstrated that the Langmuir adsorption isotherm and pseudo-second-order kinetic model effectively represented the adsorption process, with a considerable contribution from electrostatic and π-π interactions, pore filling and hydrogen bonding. This work established that CPHAC-3K generated through an optimised preparation process and conditions may be used as an efficient adsorbent compared to some previously reported biomass-based activated carbon to remove cationic dyes from wastewater.


Introduction
Water is an integral part of the natural world and is critical to the well functioning of our natural ecosystems.Accessibility to drinking and safe water is now a daunting challenge in different world areas.Industrial drainage systems release approximately 30,000 to 150,000 tons of dyestuff into water bodies per year, posing environmental issues owing to their toxic effect and carcinogenicity [1,2].Dye degradation via photocatalysis [3][4][5], flocculation/coagulation, chemical precipitation process and activated carbon basedadsorption are the few amongst many dye removal techniques devised for water purification.Adsorption with the help of activated carbon is a straightforward and efficient method for removing a wide variety of dyes.In addition, there is no generation of harmful intermediates compared with other dye eliminating techniques.However, the high cost of activated carbon-based adsorption can be offset by using cheap and waste materials to prepare activated carbon.
Activated carbon is a popular descriptor employed to categorise carbon-based materials with a high surface area, an internal porous structure (comprising pores with a varying size distribution) and a wide variety of oxygen-bearing functionalities.Previously, activated carbon was mainly fabricated from coal, coir and pitch.However, in recent times, different arrays of precursors have proved suitable for activated carbon preparation due to abundance, renewability, and environmental friendliness.Industrial by-products, lignocellulosic materials, municipal waste and agricultural waste products have shot to the limelight as cheap precursors for the fabrication of activated carbon is concerned.As a result, researchers have directed much of their efforts in recent years on the manufacture of activated carbon utilising agro-residue and lignocellulosic resources, which are both practical and economical [6,7], such as palm kernel shells [8], foxnut shell [9], and macadamia nut shells [10] and others.
CPH, commonly yellow, is obtained by de-husking the cocoa fruit (removal of cocoa beans from cocoa pods).Roughly, the harvesting of 1 ton of dried cocoa bean is accompanied by the generation of 10 tons of CPH [11].Next to the Ivory Coast, Ghana is the world's second-largest cocoa producer.In 2019/2020, Ghana produced 850,000 tons of cocoa beans (thus 8.5 million tons of CPH).Many ways of utilising CPH waste has surfaced, including using CPH as animal feed, potassium-rich fertiliser and alkali-ash (potash) for soap production in Ghana.In addition, CPH is rich in carbon, which implies it may be employed as an alternative energy source via combustion, pyrolysis, or directly treating water as an adsorbent.
Apart from its textural features, surface functionalities on the surface of activated carbon are critical for adsorption applications.Notably, the acid oxygen-containing surface groups have a beneficial influence on the adsorption of heavy metals [12] and dye from aqueous solutions.Although several research articles [13,14] on chemical activation of various biomasses exist, information on the use of CPH as the precursor is few hence a limited review papers on CPH-based activated carbon.According to published data, activated carbon with a large specific surface area and an oxygen-rich surface can achieve remarkable dye adsorption [15,16].By adopting the cheap manufacturing protocol in our previous research [17], a sand covering was applied to create an air-depleted atmosphere for synthesising functional carbon materials.The basis for this preparation method originates from indigenous methods of producing charcoal from wood and lumber, wherein wood is packed into the ground and blanketed with sand-mulch to prevent complete combustion into ashes.This air-deprived method yields an oxygen-rich activated carbon from CPH waste, later subjected to MB adsorption.The practicality of this study for the production of activated carbon is based on its simplicity, safety, and possible scalability, which may pave the way for low-cost and efficient functional carbon material synthesis.

Collection and drying of CPH
First, the husk was rinsed with distilled water to eliminate any contaminants.The washed samples were dried for 4 h at 120°C in an oven.The materials were crushed with an electric blender and then sieved through a 0.074 mm screen to facilitate an accurate study of thermal behaviour by thermogravimetric analysis (TGA).A vast and uneven sample would reduce the heat transfer from the surface to its core leading to an ineffective pyrolysis profile and an erroneous TGA reading.The peels were subsequently oven-dried at 120°C for 24 h to minimise hydrolysis, which would cause decaying of the CPH, thus increasing the shelf life.

Double-crucible configuration
The double-crucible arrangement is a home-created device consisting of a 30 cm 3 alumina crucible (covered with its lid) and a 50 cm 3 crucible.The smaller crucible (30 cm 3 ) containing the carbonised sample or activated is placed in a bigger crucible (external crucible, 50 cm 3 ).Sieved sand (−60 mesh) is poured into the bigger crucible to bury the smaller crucible such that the thickness of sand above the lid of the inner crucible (smaller crucible) is 15 mm and finally covered with its lid and then introduced into the furnace for heating (Figure S1).

Carbonisation and activation of CPH
5 g of the sieved CPH (particle sizeof 0.074 mm) was first carbonised at 450°C with the double-crucible configuration (shown in Figure S1) in a muffle furnace, the resulting char (Cocoa pod husk biochar-CPHBC) was blended with solid KOH, NaOH, K 2 CO 3 and Na 2 CO 3 at fixed ratios of 1:1 (Precursor: activator) through ball milling (ball to sample percentage of 47:1, frequency: 20 Hz, time: 30 min).The milled CPHBC-KOH, CPHBC-NaOH, CPHBC-K 2 CO 3 and CPHBC-Na 2 CO 3 was introduced into the double-crucible configuration (Figure S1) and then heated in a tubular furnace at 800°C (heating rate:15°C/min, holding time: 2 h and N 2 flow of 50 cm 3 /min).The obtained char after catalytic pyrolysis was dissolved in 3 M HCl and then scrubbed with distilled water several times till the pH of the resulting mixture was 7.After this, the water-scrubbed activated carbon was oven dried for 24 h at 120°C.Samples prepared by this process is denoted as CPHAC-1K, CPHAC-1NA, CPHAC-1KC and CPHAC-1NAC (where CPHAC represents the cocoa pod husk-activated carbon; K, NA, KC, and NAC means KOH, NaOH, K 2 CO 3 and Na 2 CO 3 .The number in front of K, NA, KC and NAC represents the impregnation ratio of activator to precursor).After that, a preliminary study was carried out on the prepared activated carbons to ascertain the best activator that can render CPHBC more porous for dye adsorption.The prepared activated carbon was subjected to dye removal, and results showed that CPHAC-1K showed tremendous dye removal, as shown in Figure S3.KOH was chosen as an ideal activator for further investigation based on this outcome.Via the same experimental procedures and parameters, two new activated carbon with KOH as an activator was fabricated with ratios of 2:1(CPHAC-2K) and 3:1(CPHAC-3K).

Characterisation of activated carbon
The LECO CHNS-932 elemental analyzer was employed to evaluate the elemental makeup of materials (carbon, nitrogen, hydrogen, and sulfur).Proximate analysis was performed following ASTM D 3172-3175 test standards, and findings were reported as moisture, volatile matter, ash, and fixed carbon contents.The surface morphology of the samples was studied using Scanning electron microscopy (SEM) (Japan Hitachi S-4800).The thermogravimetric studies on a NETZSCH STA 409 PC/PG thermal analysis instrument was performed at a heating rate of 10°C/min in an N 2 environment.FTIR spectra were produced using a Nicolet iS50 FTIR spectrometer (Thermo scientific, USA) in the 4000-500 cm −1 range.Nitrogen adsorption-desorption isotherm was developed using Micromeritics Tristar II 3020, Micrometrics ASAP 2020 Plus, and Quantachrome NOVA2000e, using liquid nitrogen (@77 K) for porosimetry studies.Before testing, the materials were vacuum degassed at 300°C for 4 h.The Brunauer-Emmett-Teller (BET) and non-local density functional theory (NLDFT) techniques were utilised for determining the specific surface area (SSA) and the generation of the pore size distribution (PSD) curves, respectively.

Adsorption isotherms
To examine the adsorption nature of MB on CPHACs, adsorption assessments were undertaken under batch settings.Work solutions were prepared by diluting portions of a stock solution (1000 mg/L) with distilled water.Batch adsorption was carried out in a 150 mL conical flask using 0.1 g of CPHACs with the MB solution having an initial concentration of 100-500 mg/L.For 12 h, the conical flasks containing the mixture were agitated with a magnetic stirrer (model: MS-300, Henan Lanphan Industry Company Ltd) at an agitation speed of 120 rpm.A filter paper (Whatman grade 1 filter paper, Pore diameter: 90 mm) was used to separate the adsorbent from the MB aqueous.The adsorption capacity of the adsorbent was determined by Equation 3.
where q e (mg/g) = adsorption capacity of activated carbon; C o (mg/L) = initial concentration of MB solution; C e (mg/L) = final concentration of MB solution after equilibrium; V(L) = volume of MB solution; m (g) = mass of activated carbon dose used and R (%) = the removal percentage of MB by CPHACs.Linear Langmuir and Freundlich isotherms were adopted to fit the experimental isotherm data of MB adsorption on CPHACs.These equations are expressed as: where q L (mg/g) represents the maximum MB uptake on CPHAC-3K as per the Langmuir isotherm, K L (L/mg) represents the Langmuir constant, K F ((mg/g)(L/mg)1/ n) provides a measure of adsorption capacity and n the Freundlich constant.C o is the highest initial MB solution concentration, and R L factor shows whether the adsorption isotherm for a set of experiments is reversible, favourable, linear or unfavourable.To ensure the reproductivity of the results and reduce error margin, the adsorption experiments were repeated three times, and the calculated average was reported in this research.

Adsorption kinetics
Kinetic testing was conducted in the same manner as equilibrium tests.The aqueous samples were collected at predetermined time intervals, and the MB concentrations were determined in the same way as calculated in the equilibrium adsorption test.The quantity of MB adsorbed at time t, qt (mg/g), was determined as follows: Where C t (mg/L) is the liquid-phase concentration of MB solution at time t (min), the Pseudo-first-order model [18] and pseudo-second-order model [19] were used to analyse the kinetic data.These models can be expressed as Where qe(mg/g) and qt (mg/g) denote the MB capture by CPHAC at equilibrium and at specific time intervals t(min), respectively.The adsorption rate constant for the pseudo-first and second-order kinetic model is represented as K 1 (1/min) and K 2 (g/mg min), respectively.Apart from R 2, which shows the correlation extent between experimental data and adsorption model, the Δq(%) (normalised standard deviation) values can be used to determine the best fit kinetic model and can be expressed as:

Ambient-air fabrication of activated carbon
From Figure S1, the double-crucible configuration in the absence of N 2 gas is presented.During the carbonisation and activation in an ambient air-filled furnace, the generated volatiles serve as a blanketing medium (gas) that protects the sample from burning into ash.The sand-mulch (Figure S1) causes an appreciable in-built pressure difference between the inner crucible and the external environment, which prevents air penetration into the internal crucible.Therefore, it is sound to propose that an oxygen-deficient atmosphere is created in-situ due to the sand mulch during the heat treatment process [17,20].The key distinction in this fabrication method of preparing activated carbon is how an oxygen-free environment is created during the activation period.N 2 gas is commonly charged into the furnace during the pyrolysis and activation stages of the classic N 2 protection technology, which is nonrecyclable.This study employs cheap construction sand as a mulching medium for the as-proposed ambient-air approach [17].As such, the ambient-air strategy is more cost-effective than the N 2 -protection technique.Figure S1 depicts the experimental setups for carbonisation and activation.[21].Critical parameters affecting a material's feasibility as a precursor for activated carbon generation are a high lignin content and a low ash level; CPH meets these criteria and can be regarded as a promising precursor.The thermal behaviour of CPH at a heating rate of 20°C/min was investigated by thermogravimetric and derivative thermogravimetric (TG/DTG) analysis, as shown in Figure S2.This finding indicates that carbonisation temperatures greater than 400°C are necessary to generate a carbon-rich material for subsequent catalytic pyrolysis.

Assessment of MB dye adsorption capacities
Figure S3 presents single-point adsorption measurements of CPHACs synthesised with various activators.It can be shown from the figure that all CPHACs displayed substantial affinities of adsorption with regard to MB. CPHAC-1K, nonetheless, exhibited the highest binding affinity for the dye (MB), while CPHAC-1NA, CPHAC-1KC and CPHAC-1NAC all showed close uptakes for MB when compared with CPHAC-1 K (shown in Figure S3).The order of adsorption performance was CPHAC-1K (268 mg/g) > CPHAC-1NA (201 mg/g) > CPHAC-1KC (179 mg/g) > CPHAC-1NAC (120 mg/g).Disparities in patterns and extent of adsorption may result from pore structure and surface chemical character, which is affected by reaction processes between the distinct activators and the carbon precursor and intensity of activation.Carbon activation with metal hydroxides (KOH and NaOH), as per literature, involves the etching of carbon structures via a sequence of reduction and oxidation reactions that produce water, carbon monoxide and carbon dioxide through a series of carbon gasification processes.These activator/heat-induced redox reactions result in the development of metallic residues (Na or K) that form intercalate compounds in the carbon lattices [22].The intercalation expands the carbon lattices, increasing the porosity of the CPHACs.
Moreover, the carbonates of alkali metals (Na and K) thus K 2 CO 3 and Na 2 CO 3 react with carbon under gasification conditions in a series of reactions as shown below in Equations (R1) and (R3) as proposed by Otawa et al. [21].
These Equations (R1) and (R3) suggest that porosity growth with M 2 CO 3 (where M is the alkali metal: Na or K) is due to the reduction of M 2 CO 3 to generate M, M 2 O, CO 2 , and CO [23].M and M-based compounds developed during the activation step widens existing pores and generate new ones.[24,25].In addition, there is the generation of CO 2, signifying the etching of the carbon framework, which is the basis of pore formation.Pores and cavities can also be created from the evaporation of activation agents (M 2 CO 3 ) and their intermediates(M 2 O) that occupy specific locations in the carbon matrix [26].Furthermore, pores are also realised after thorough water cleaning and acid leaching.These generated pores and cavities provide adsorbate molecules access to the internal pore system of the activated carbon.[27].Following the preliminary adsorption performance results (Figure S3), KOH was chosen as an ideal activator for further exploration.

Elemental analysis of CPHACs
Table S2 covers the elemental analysis of CPHACs generated from CPH activation with KOH.The high carbon (C) and oxygen (O) content of CPH can be ascribed to its lignocellulosic component and ash concentration.Table S2 shows that the carbon percentage of the CPH precursor was 44.42 wt.%, which was lower than the carbon content of CPHACs (CPHAC-1K, CPHAC-2K, and CPHAC-3K: 67.86, 61.67 and 54.72 wt.%, respectively).The decreasing trend in the carbon content may be attributed to the strong etching effect at high impregnation ratios, which affects the structural integrity, causing a high volume of micropores.In addition, the carbon and oxygen elemental content of other KOH-derived AC was reported to be 58.57and 37.96 wt.% [11], respectively, which is synonymous with this work (Carbon% = 54.72 and Oxygen% = 41.99.)It is worth noting that the oxygen (O) content of the resulting CPHACs increased with a rising impregnation ratio, which can be due to the generation and retention of more potassium carbonates and oxides [11].

Textural analysis of CPHACs
Identifying the porous structure of activated carbon is a critical first step to designing its function.For this reason, an inert gas (popularly N 2 ) is used to generate nitrogen adsorption isotherms curves, which can characterise the textural properties and porosimetry of activated carbon.Figure 1(a) illustrates the N 2 adsorption/desorption isotherms of ACs prepared with various impregnation ratios at 800°C.According to the IUPAC classification, the adsorption isotherms of all CPHACs are type I isotherms [28].
The type I isotherm is attributed to microporous materials and possesses a limited external surface area.Another proof of the dominant presence of micropores in the prepared activated carbons is their impressive adsorption even at low pressures.Besides this, the significant N 2 absorption at the initial portion of the isotherm curve even at low relative pressures (P/Po <0.01) is consistent with N 2 adsorption in microporous activated carbons [29].The sequence of the volume of N 2 absorbed were recorded as CPHAC-3K > CPHAC-2K > CPHAC-1K > CPHBC.The BET surface area of the prepared activated carbon rose significantly 946 to 1688 m 2 /g as the impregnation ratio of KOH to char from 1 to 3. The amount of N 2 absorbed per gram of activated carbon at standard temperature and pressure (STP) increases from 100 to 500 cm 3 (volume values extrapolated at relative pressure value of 0.99) when the impregnation ratio is raised from 1 to 3. Consultation of other research articles [30,31] revealed the same trend as other biomass-based precursors undergoing chemical activation with increasing KOH impregnation ratio.The activated carbon pore volume increased from 0.18 to 0.76 cm 3 /g with an increased KOH/char impregnation weight ratio.The total pore volume increased (0.169 to 0.665 cm 3 /g) with an increasing impregnation ratio (1 to 3).Meanwhile, the average pore width decreased from 1.724 to 1.245 nm as the impregnation ratio of char to KOH were increased.In the sense of physisorption, IUPAC defined pore sizes as follows: (i) micropores have pore width < 2 nm, (ii) mesopores are 2 nm-50 nm in diameter, and (iii) macropore diameter > 50 nm [28].Based on this pore characterisation, all CPHACs can be defined as microporous after considering the average pore sizes shown in Table 2, and this is also backed by the pore distribution curves shown in Figure 1(c), which shows CPHACs with a major portion of their pore distribution curve in the microporous region (pore with < 2 nm).CPHAC-3K also presented ultra-microporosity, as shown in Figure 1(c).An appreciable portion of mesopores (13%) is shown for CPHAC-3K in Table 1.This mesopore content can be attributed to the etching effect caused by the increase in the impregnation ratio.

Structural and morphological analysis
Figure 2 reveals the morphology of the char and CPHAC-3K (before and after MB adsorption).The SEM micrograph is in complete conformity with the N 2 adsorption isotherm findings.The prepared char had a low surface area as presented in Table 1 and is evidenced with SEM images in Figure 2(a) as an irregular surface with limited pores.CPHAC-3K, on the other hand, is a heavily loaded with distributed, interconnected, hierarchical porous structure with a size range of 0.5-3 μm, as illustrated in Figure 2(b,  c).The development of porous activated carbon from a carbonaceous precursor with KOH as an activation agent has been discovered to proceed according to a series reaction.KOH activation of carbon occurs with three main stages [29].Briefly, initial pore formation was due to the release of H 2 O and CO 2 through carbon gasification, followed by the generation of K 2 CO 3 due to the reaction between carbon fragments and KOH at temperature 700-800°C.K 2 CO 3 is further transformed into K 2 O, which stimulate an etching effect on the carbon framework that generates more pores.Finally, the formation of metallic K at a temperature of approximately 800°C creates intercalations in the carbon framework (graphitic layers) that causes a substantial pore generation.

Surface chemistry
The FTIR spectra of CPH, CPHBC, and CPHAC-3K between the range of 4000-500 cm −1 is presented in Figure 3(a).All spectra clearly show a broad band peak that starts from 2973 to 3694 cm −1 , with a maximum peak point appearing at a wavenumber of 3292 cm −1 , which is consistent with the hydroxyl (O-H) functional groups and adsorbed water present in the sample.The appearance of a prominent peak at 2910 cm −1 represents the C-H stretching group [32].The peak at 1736 cm −1 can be attributed to the stretching of C = O in xylan related to hemicellulose present in the cocoa pod shells [33], and the Calculated surface area (S BET ) considering Brunauer-Emmett-Teller (BET) equation with relative pressure (P/Po) constraint between 0.05 to 0.30 Micropore volume (V meso ) and surface area (S micro ) calculated by the t-plot method.Total pore volume (V total ) derived at a corresponding relative pressure value of about 0.99.Mesopore volume (V meso ) and surface area (S meso ) calculated by (V total -V micro ) and (S BET -S micro ) respectively.Pore size (PS) determined by S BET and V total values.alcohols and phenols in the samples.The existence of methylene, hydroxyl and methyl functional groups and aromatic compounds demonstrates that CPH contains lignocellulosic structures, which is also seen in other materials such as karanji fruit hulls [35] and eucalyptus residues [36].
A comparison between raw material CPH and CPHAC-3K spectra show a reduction in band intensity (C-H: 2910 cm −1 , C = O: 1598 cm −1 , and C-O-C: 1027 cm −1 ) and, in some cases, a complete band elimination (C = O: 1736 cm −1 ) after the carbonisation process and activation signifying a decrease in aromaticity owing to the breaking of bonds.With a considerable number of peaks relating to oxygen-containing functional groups, it is therefore sound to conclude that the mulch-assisted ambient air protocol shown in Figure S1 can yield an oxygen-dense porous carbon material as proven in [17].

Adsorption isotherm
The adsorption isotherm illustrates how adsorbate/adsorbent interfaces interact with one another.Numerous pieces of literature frequently adopt the Langmuir and Freundlich isotherm models to fit experimental adsorption results [18,35].The Langmuir adsorption isotherm assumes that the adsorbate molecules form a monolayer on the adsorbent surface.Freundlich adsorption isotherm describes an adsorption process occurring on a heterogeneous surface through a multilayer adsorption mechanism.The experimental adsorption values for MB capture on CPHAC-3K were fitted using Langmuir and Freundlich isotherms, Equations ( 2) and (3), respectively (Figure S4).Table 2, generated from MB adsorption data conducted at room temperature, demonstrates that the Langmuir isotherm presents the strongest correlation, with high R 2 values of 0.9996, almost nearing unity.Zhang et al. [19], Patawat et al. [37] and Han et al. [36] all observed a similar pattern for the adsorption of MB on mangosteen peel activated carbon, Dipterocarpus alatus fruit activated carbon, and eucalyptus residue-derived activated carbon, respectively.
As observed in Table 2, the separation factor (R L ) related to the Langmuir isotherm for MB adsorption on CPHAC-3K is 0.036, demonstrating favourable adsorption of MB with a maximal adsorption capacity of 449.5 mg/g.Cazetta et al. [38] have shown that coconut shells treated with NaOH exhibit a maximal adsorption capacity of 996 mg/g with an R L value of 5.55 � 10 −5 (showing the adsorption process is favourable).A similar observation was observed by Pozeti et al [39].with buriti shells-based activated carbon, prepared by employing ZnCl 2 as an activator.The resulting activated carbon exhibited high adsorption capacity for MB molecule (274.62 mg/g) due to the well-developed porosity structure with R L value ranging from 6.36 � 10 −3 to 6.43 � 10 −4 also indicating adsorption favourability, while Islam et al. [40] found that the adsorption capacity of rattan-based AC for MB of 359 mg/g.The adsorption in this study was also proved favourable by the Freundlich model fitting results in Table 2.The magnitude of the exponent, 1/n: 0.2142, further indicates the favourability of adsorption.Value of (1/n) < 1 represents favourable adsorption condition [41].
The data in Table 2 demonstrate that the Langmuir isotherm correlates the data with a higher R 2 value than the Freundlich isotherm, indicating monolayer adsorption.This value (445.9 mg/g) is comparable to other published values for activated carbons prepared from various agricultural wastes (Table 3).Activated carbon prepared in this study is one of the most promising adsorbents for MB adsorption compared with previous literature.

Kinetics modelling
Figure 4 illustrates the impact of contact time on the adsorption potential of CPHAC-3K for MB at various initial MB concentrations.This figure demonstrates that the adsorption capacity of MB increases as the contact time increases and that the adsorption reached equilibrium in approximately 12 h.At a contact time of 9 h, an initial MB concentration of 500 mg/L, a pH value of 7, and a dosage of 1 g/L adsorbent, an adsorption capacity of 425.5 mg/g is obtained.This figure also shows that a rapid increase in capacity for MB is achieved during the first 1 h.The rapid adsorption (210 mg/g for the first 30 mins and 281 mg/g for the first hour) during the initial stage could be due to the higher driving force facilitating the transfer of MB molecules to the surface of CPHAC-3K owing to the vast difference in gradient between the particle surface and MB based aqueous solution.Another factor for fast adsorption is the availability of uncovered surface area and active sites on/in the adsorbent.Two kinetic models: pseudo-first-order and pseudo-second-order, depicted by Equations ( 6) and (7), respectively, were used to correlate the experimental kinetic data for MB obtained from Equation (5).Table 4 contains the calculated constants for the two kinetic equations and their corresponding R 2 values for various initial MB concentrations.As shown in Table 4, the linear plot of ln(qe-qt) versus t (Figure S5(a)) for the pseudo-firstorder equation has low R 2 values.This table also demonstrates a significant discrepancy between the experimental and calculated adsorption capacity values (determined by q% values), suggesting a weak fit to the experimental data by a pseudo-first-order model.The straight-line plot of t/qt against t (Figure S5(b)) for pseudo-second-order equations have high R 2 values (>0.999), as reported in Table 5, indicating that the pseudo-second-order kinetic model best represents the adsorption kinetics.Furthermore, the experimental and calculated adsorption capacity values are in good agreement, with few differences (Table 4), which indicates that MB adsorption on CPHAC-3K is governed by pseudosecond-order kinetics.
A similar result was reported by Aboua et al. [42] for the adsorption of MB onto macorederived activated carbon and also with NaOH-modified coconut activated carbon [38], where the adsorption kinetics fitted well with the pseudo-second-order model.

XPS analysis (Pre-and Post-MB dye adsorption)
XPS is a crucial characterisation tool for identifying the presence and composition of elements and bond types present in materials [43].Most importantly, XPS could provide essential information on the adsorption mechanisms of the dye molecules [22].Thus,  [22], and finally the π-π interactions present in the aromatic rings were detected (shown in Figure 3(b)).From Figure 3(c), it can be inferred that CPHAC-3K absorbed MB, due to the emergence of a new peak assigned to C-S/C = S (287.3 eV) formed from the bond between carbon from the activated carbon and the sulphur (S) present in the methylene blue dye.Furthermore, the pi-pi* excitation peak had not only shifted to a lower binding energy state from 293.40 to 292.6 eV [44] but had also experienced a significant reduction in intensity.The above observations pointed to the adsorption process likely involving mechanisms such as pi-pi interactions.In addition, the reduction of the pore width of CPHAC-3K from 1.25 nm to 1.10 nm (CPHAC-3K-MB in Table 1) after MB adsorption showed the MB was adsorbed into pore structure thereby reducing the pore size.

Proposed adsorption mechanism
The mechanism by which aromatic contaminants are captured on porous carbon materials in an aqueous solution is intricate and poorly understood.However, literature-based evidence has shown that the adsorption mechanism of dye molecules onto an activated carbon surface is championed by electrostatic attraction, van der Waals forces, hydrogen bonding, π-π interactions, hydrogen bonding, and pore-filling.The significant factors influencing dye molecule adsorption onto porous carbon surfaces include adsorption conditions, adsorbate structure, and surface chemistry.Based on SEM and BET data, it can be assumed that owing to the porous structure of CPHAC-3K (87% microporosity and 13% mesopores), MB adsorption features the pore filling mechanism.With the MB dye size predicted as 1.43 nm in length by [8], In the case of functional groups on the surface of CPHAC-3K, nitrogen atoms present in the MB molecule structure may create an attraction (hydrogen bond) with the hydrogen-containing functional groups.A possible interaction (Yoshida hydrogen bonding) between the aromatic rings of MB and the O-H functionalities on CPHAC-3K is depicted in Figure S6.Electrostatic attraction between OH − and oxygen-containing functional group and N atom present in the MB dye molecule contributed to the adsorption capacity of CPHAC-3K (Figure 4).Additionally, MB is a typical planar molecule with an aromatic backbone; therefore, it can be effectively captured onto CPHAC-3K surface through π-π electron interactions between the hexagonal skeleton of the carbon material and the aromatic backbone of MB.The pore-filling mechanism is evident in Figure 2(d), showing a layer of MB covering the pores in CPHAC-3K after MB adsorption.

Regeneration & cycling studies
The re-use of an adsorbent is a critical feature to consider when assessing its practical implementation.An effective sorbent should have a high adsorption efficiency and exceptional reusability and stability for practical application.The methods of regenerating active sites on an-already used adsorbent are critical for the subsequent removal of contaminants and the adsorbent's stability throughout re-use [45].For leaching out adsorbed MB molecules and restoring used AC-based adsorbents, various extractants have been used, including acids, bases, salts, and deionised water.CPHAC-3K was regenerated in this work by scrubbing with alcohol and hot deionised water until MB adsorption was complete, then dried in a vacuum oven.CPHAC-3K shows 79% adsorption efficiency after 3 cycles, with an adsorption capacity of 352 mg/g.(Figure S7)

Conclusions
CPHBC was utilised as a precursor to prepare activated carbon with KOH, NaOH, Na 2 CO 3 and K 2 CO 3 .During preliminary studies, KOH-catalysed activated carbon (CPHAC-1K) showed the highest MB single-point adsorption capacity.With KOH discovered as the best performing activator, CPHAC-2K and CPHAC-3K was prepared.Based on textural characterisation, CPHAC-3K was selected for MB adsorption studies.CPHAC-3K presented an adsorption efficiency of 445 mg/g for MB, with the equilibrium adsorption and experimental data agreeing well with the Langmuir isotherm and pseudo-second-order kinetic model, respectively.According to the surface chemical character (oxygen-rich functional groups), textural properties and porosimetry of CPHAC-3K, it was suggested that the adsorption mechanism included electrostatic attractions, π-π interactions, pore filling, and hydrogen bonding.The CPHAC adsorbent is an effective and sustainable functional material for removal of MB from an aqueous solution.
peak at 1598 cm −1 is attributed to the C = O group present in ketone or aldehyde or the C-C stretching in the benzene ring[33].1027 cm −1 band represents the C-O-C or C-O functional group existing in the phenol, ethers and alcohols[34].The minor peak band at 610 cm −1 in the spectra is due to the O-H functional showing the existence of

Table 2 .
Langmuir and Freundlich adsorption isotherms with respective corresponding constants.

Table 3 .
Comparison of CPHAC-3K with other adsorbents reported in previous research.

Table 4 .
[44]tic constants for pseudo-first and second-order models.3KandMBasrepresentative carbon and dye, respectively, the surface chemistry of the surface of the CPHAC-3K was detected through curve-fitting of the C 1s region.After XPS spectra deconvolution, five prominent peaks corresponding to C = C/ C-C present in aromatic C and aliphatic structures (285.4 eV)[41], C-O in alcohols and phenols (285.7 eV)[44], C = O in carbonyl, ketones and quinones (286.8 eV), O-C = O in carboxyl group (289.2 eV)