One-Stage Microwave-Assisted Carbonization and Phosphoric Acid Activation of Peanut Shell and Spruce Cone Biomass for Crystal Violet Adsorption

Abstract

This study focuses on a single-step microwave-assisted carbonization and activation method for biomasses derived from peanut shells and spruce cones. Using phosphoric acid as the activating agent, this process leads to carbon materials with a micro-mesoporous structure, favoring dye adsorption. Elemental and surface analyses confirmed that the physicochemical properties of the obtained carbons are strongly dependent on the biomass’ source. The carbon materials obtained in this way, differing in porous structure and the presence of functional groups on their surfaces, were used for static adsorption of hazardous dye crystal violet from water. The adsorption behavior of both materials fits well with the Langmuir and Freundlich isotherms, indicating a combination of monolayer and heterogeneous surface adsorption, driven primarily by physical interactions. Of these two materials, carbon derived from spruce cones was characterized by better porosity, higher surface functionality, and higher adsorption capacity, demonstrating its potential as a cost-effective and sustainable material for wastewater treatment applications.

1. Introduction

Water is a vital resource for human survival. It comes from various sources, including lakes, ponds, rivers, groundwater, and rainfall. However, the quality of this valuable resource is deteriorating due to pollution [1]. The main causes of this problem are related to industrial, commercial, and agricultural activities. Human activities generate a significant amount of organic and inorganic pollutants [2,3]. Among the various organic pollutants generated by industrial activities, dyes are considered toxic substances that pose a serious threat to ecosystems and human health [4,5,6].

Crystal violet (CV) is a cationic dye that is well known for its harmful effects on human health as a potent mitotic toxin and carcinogen and is linked to tumor development in certain fish species [7]. CV is widely used in textile manufactories for dying cotton and silk, and also in manufacturing paints and printing inks [8]. CV possess very well-known antibacterial properties against Escherichia coli, Staphylococcus aureus, Streptococcus faecalis, and Bacillus subtilis [9].

Since many organic dyes are harmful to humans, removal of color from process or waste effluents is important for the environment. Most of the wastewater containing dyes from various industries, mainly from dye production and textile finishing, is discharged into river streams. Even at low concentrations, dyes affect aquatic life and the food web [10].

Adsorption is a widely used method for the treatment of industrial wastewater containing dyes, heavy metals, and other organic and inorganic pollutants [11]. The advantages of this technique are its simplicity of operation, low cost (compared to other separation processes), and lack of sediment formation. As is already known, the textile industry is a major consumer of water and releases a large amount of dyes in its wastewater.

Activated carbon is the most widely used adsorbent for this purpose due to its large surface area, porous structure, high adsorption capacity, and high degree of surface reactivity. The only disadvantage of its use is its high cost, which sometimes limits its use. Hence, there is a constant search for alternative materials that are relatively inexpensive and which, at the same time, provide reasonable adsorption efficiency.

There are two different processes for preparing activated carbon. Compared to physical activation, chemical activation offers two important advantages: The first is the lower temperature at which the process is carried out. The second is that the overall efficiency of chemical activation is usually higher because no burnt charcoal is required.

Among the numerous dehydrating agents available, phosphoric acid in particular is a powerful and widely used chemical in the preparation of activated carbon. Chemical activation with H₃PO₄ improves the development of pores in the carbon structure, and, due to the influence of chemicals, the efficiency of the carbon is usually high [12,13,14,15].

The aim of the research was to obtain high-quality activated carbon as an adsorbent with high adsorption capacity towards to crystal violet dye. For this purpose, the organic precursor was first activated with H₃PO₄, followed by microwave treatment. The physicochemical, microstructural, and adsorption properties of carbons were evaluated. Furthermore, adsorption kinetics and adsorption mechanisms were also studied.

2. Materials and Methods

2.1. Materials

2.1.1. Preparation of Natural Precursors

As natural precursors, peanut shells (Arachis hypogaea L.) and spruce cones (Picea abies L.) were used and ground in a mill before carbonization. The obtained fractions were separated using a vibratory sieve shaker, and a sieve fraction of the powder, 100 μm–1.4 mm, was used for carbon preparation. The NEXOPART EML 200 Premium test sieve shaker from NEXOPART GmbH & Co. KG (Oelde, Germany) was applied to determine the fractions.

Both natural precursors are typical lignocellulosic materials containing lignin, cellulose, and hemicellulose [16,17]. The cones also have some condense tannins, resin acids, stilbenes, flavonoids, etc. [18].

2.1.2. Chemicals

Crystal violet (C₂₅H₃₀N₃Cl, CV, 4-{bis [4-(dimethylamino)phenyl]methylidene}-N,N-dimethylcyclohexa-2,5-dien-1-iminium chloride) cationic dye was obtained from Merck (Darmstadt, Germany) (Figure 1), whereas phosphoric acid (H₃PO₄) was obtained from POCh (Gliwice, Poland).

2.1.3. Microwave-Assisted Preparation of Carbon Adsorbents

The carbon adsorbents were obtained by microwave-assisted carbonization of natural precursors with the presence of phosphoric acid. The sample was exposed to microwave irradiation for 5 min with a power of 500 W. The natural precursors (10 g) were impregnated with 85% phosphoric acid at an impregnation ratio of 1:1 (w/w), placed in microwave kiln, and then carbonized in a microwave reactor [19]. The microwave irradiation was carried out using a MAS-II Plus Microwave Synthesis Workstation from SINEO Microwave Chemistry Technology Co., Ltd. (Shanghai, China). The biomass conversion efficiency was approximately 60%. The obtained carbon adsorbent from peanut shells (C-PS) and spruce cons (C-SC) were washed in a Soxhlet apparatus with distilled water until pH 7 was obtained and then dried at 100–110 °C. The pH was measured using a Radiometer PHM 64 pH meter (Copenhagen, Denmark).

2.2. Methods

2.2.1. Porous Structure

The porous structures of the C-PS and C-SC adsorbents were characterized using nitrogen adsorption–desorption measurements made using an adsorption analyzer ASAP 2420 (Micromeritics Inc.,Norcross, GA, USA). Determination was based on the measurements of nitrogen adsorption and desorption isotherms on the surface of the studied adsorbent at 77 K. The specific surface area (S_BET) was determined using the Brunauer–Emmett–Teller (BET) method, assuming that the area of a single nitrogen molecule is 0.162 nm². The total pore volume (V_total) was determined from a single point of the adsorption isotherm at a relative pressure p/p₀ of 0.99. The pore size (D) and the pore size distribution (PSD) were determined by the Barrett–Joyner–Halenda (BJH) method from adsorption and desorption branches. The micropore surface area (S_micro) and the micropore volume (V_micro) were calculated by the t-plot method. The microporosity was defined as the ratio of the micropore volume to the total pore volume. The sample degassing temperature was 185 °C.

2.2.2. Morphology

To determine the morphology of activated carbons (C-PS, C-SC), images were taken using a high-resolution Quanta 3D FEG scanning electron microscope from FEI Company (Hillsboro, OR, USA) at an acceleration voltage of 5 kV. Due to electrostatic charging during analysis, the sample was covered with a thin layer of Pd/Au.

2.2.3. Elemental X-Ray Microanalysis

Elemental composition analysis of the obtained carbons (C-PS, C-SC) was studied by SEM-EDS method using Quanta 3D FEG Microscopy with an EDX detector, FEI (Hillsboro, OR, USA). SEM EDX analysis combines Scanning Electron Microscopy (SEM) to provide high-resolution images of sample surfaces with Energy Dispersive X-ray Spectrometer EDX (Octane, Elect Plus) to identify and quantify elemental compositions. When the sample is bombarded with an electron beam in SEM, it emits characteristic X-rays detected by EDX, allowing for simultaneous morphological and chemical analysis from the micro- to nanoscale. The test was carried out at an accelerating voltage of 20 kV. The samples were analyzed in four replicates. The obtained values are presented as weight (wt.%) and atomic percentage (at.%).

2.2.4. X-Ray Photoelectron Spectroscopy

XPS spectra were obtained using an ultra-high vacuum (<2 × 10⁻⁸ Pa) multi-chamber UHV system (PREVAC, Rogów, Poland) equipped with Al Kα line excitation source VG Scienta SAX 100 (12 kV, 30 mA), monochromator VG Scienta XM 780, and hemispherical analyzer Scienta R4000 (VG Scienta AB, Uppsala, Sweden). The pass energy and energy steps were 200 eV and 0.5 eV at the survey spectra and 50 eV and 0.1 eV at the detailed spectra. The spectra were calibrated for a carbon C (1s) excitation at a binding energy of 284.8 eV.

2.2.5. Adsorption Study

The efficiency of C-PS and C-SC activated carbons for the adsorption of crystal violet (CV) was tested at room temperature (22 ± 2 °C) and in the batch mode by changing various parameters, including the contact time (CT, 20–120 min) and initial CV concentration (IC, 100–400 mg L⁻¹). The pH of the dye aqueous solutions measured using a Radiometer PHM 64 pH meter (Copenhagen, Denmark) was 7.

For static batch adsorption experiments and for calibration curve, the stock solution of crystal violet (1 g L⁻¹) was prepared. The water solutions were prepared by a dilution of stock solution to desired concentration.

Static batch adsorption experiments of crystal violet was performed at room temperature. In total, 100 mL of solution was transferred into the glass beaker containing approx. 1 g of carbon, and stirred at 200 rpm. After time t of adsorption, the absorbance of the supernatant solution was measured using a UV-vis spectrophotometer at the wavelength corresponding to the maximum absorbance (${\lambda }_{max}$  =  590 nm). The concentration of the CV dye remained in the solution was determined using a UV-visible spectrophotometer (UV-2550 Shimadzu, Kyoto, Japan). The concentration at time t ($C_t$) of the dye in the mixture was calculated using a calibration curve prepared from the known concentrations of the CV.

The CV dye adsorption capacity ($q_t$) and its percentage removal ($R_t$) were determined using Equations (1) and (2).

$$q_t={\displaystyle \frac{\left(C_0-C_t\right)}{W}}\times V$$

(1)

$$R_t={\displaystyle \frac{C_0-C_t}{C_0}}\times 100$$

(2)

where $q_t$—the adsorption capacity at time t (mg g⁻¹); $C_0$—the adsorbate concentration at the initial time (mg L⁻¹); $C_t$—the adsorbate concentration at time t (mg L⁻¹); $V$—the solution volume (L); $W$—dried carbon adsorbent mass (g); and $R_t$—the dye removal at time t (%).

Adsorption Isotherm Study

The isotherm studies were performed in batch mode at optimized conditions for different initial concentrations ranging from 100 to 400 mg L⁻¹ until equilibrium time (2 h). Langmuir, Freundlich, and Dubinin–Radushkevich adsorption isotherms were applied to the experimental data.

The form of Langmuir adsorption model was determined using Equation (3):

$$q_e={\displaystyle \frac{q_{max}·K_L·C_e}{1+K_L·C_e}}$$

(3)

where $q_e$—the equilibrium adsorption capacity (mg g⁻¹);$q_{max}$—the maximum adsorption capacity; $K_L$—the Langmuir constant related to the affinity of the adsorbent for the adsorbate; and $C_e$—the equilibrium concentration of adsorbate in solution (mg L⁻¹).

The form of Freundlich adsorption model was determined using Equation (4):

$$q_e=K_F·{\left(C_e\right)}^{1/n}$$

(4)

where $q_e$—the amount of adsorbed adsorbate (mg g⁻¹); $C_e$—the equilibrium concentration of adsorbate in solution (mg L⁻¹); $K_F$—the Freundlich constant (adsorption capacity); and 1/n—the adsorption intensity coefficient (the closer to 0; the more uneven the adsorption surface).

The form of Dubinin–Radushkevich (D–R) model was determined using Equation (5):

$$q_e=q_m{e}^{-\beta {\epsilon }^2}$$

(5)

where $q_e$—the amount of adsorbed adsorbate (mg g⁻¹); $q_m$—the maximum adsorption capacity (mg g⁻¹); $\beta$—the D–R isotherm constant related to the adsorption energy; and $\epsilon$—the Polanyi potential.

The Polanyi potential was determined using Equation (6):

$$\epsilon =RT\ln \left(1+{\displaystyle \frac{1}{C_e}}\right)$$

(6)

The adsorption energy ($E_{ads}$, J mol⁻¹) was determined using Equation (7):

$$E_{ads}={\displaystyle \frac{1}{\sqrt{2\beta }}}$$

(7)

Adsorption Kinetics Study

This study aimed to assess the influence of time on the adsorption performance of the crystal violet dye by the activated carbon adsorbents. The Lagergren’s pseudo-first-order (PSF) and Ho and McKay’s pseudo-second-order (PSO) models were applied to the experimental data and are expressed as Equations (8) and (9), respectively:

$${\displaystyle \frac{{\mathit{dq}}_t}{\mathit{dt}}}=k_1\left(q_e-q_t\right)\to \ln \left(q_e-q_t\right)=\ln \left(q_e\right)-k_{1}t$$

(8)

$${\displaystyle \frac{{dq}_t}{dt}}={k_2\left(q_e-q_t\right)}^2 \to {\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{t}{q_e}}$$

(9)

where $q_t$—the amount of adsorbate adsorbed at time t (from experimental data); $q_e$—the amount adsorbed at equilibrium (fitted parameter or experimental); $k$—the kinetics rate constant; and $t$—the time.

3. Results and Discussion

3.1. Carbon Preparation

A microwave kiln HotPot MAXI is a white, cylindrical, refractory tool consisting of a lid and a base. The tool has external dimensions of 195 mm × 110 mm (diameter × height) and internal dimensions of 135 mm × 45 mm (diameter × height). The top of the lid has a central breathable hole that allows for smoke to escape and also serves as a means of observation. It is also composed of two different materials. The white part is made of ceramic fiber, a material known for its fireproof properties, while the interior of the kiln is made of silicon carbide (SiC), primarily used to absorb microwave radiation in the microwave oven to achieve rapid heating effects, as shown in Figure 2. In normal operation, placing the microwave kiln in a medium-sized 1100 W microwave oven can quickly heat it to temperatures exceeding 900 °C in 5–10 min [20].

This tool offers significant advantages in the creation of carbons. In terms of both firing time and cooling time, it cuts the time by more than half and reduces electricity consumption compared to conventional kilns. The microwave kiln is a very practical tool for obtaining activated carbons in a very short time.

3.2. Carbon Characterization

Scanning Electron Microscopy (SEM) was employed to investigate the surface morphology of the activated carbons synthesized from two different lignocellulosic precursors: peanut shells (C-PS) and spruce cones (C-SC). The microstructural analysis of the SEM micrographs (Figure 3 and Figure 4) revealed significant morphological differences between the two carbon materials, highlighting the influence of precursor structure and composition on the physical characteristics of the resulting activated carbons. The SEM images for C-PS show clear porosity, numerous channels and layered and fibrous structures (Figure 3).

Figure 4 shows the SEM micrographs of the C-SC activated carbon, which exhibits a markedly different morphology. The material appears to have a slightly layered structure, a heterogeneous surface with numerous micropores and cracks. The sample’s appearance may suggest a directional structure. The observed differences underscore the critical role of precursor selection in tuning the morphological properties of activated carbons.

The porosity of activated carbons significantly influences their adsorption performance by affecting the surface accessibility, diffusion, and interaction of adsorbates like dyes. The carbons derived from peanut shells (C-PS) and spruce cones (C-SC), both activated with 85% phosphoric acid (1:1), were analyzed for specific surface area, pore volume, and pore structure via nitrogen adsorption–desorption isotherms and pore size distribution (PSD) analysis.

According to the IUPAC recommendation, the nitrogen adsorption–desorption isotherms of both carbons were assigned to Type IV with H4-type hysteresis loops, indicating the micro-mesoporous materials [21].

The rapid increase in nitrogen adsorption at very low p/p₀ results from adsorption in narrow mesopores, while the gradual increase in N₂ adsorption at medium and high relative pressures corresponds to monolayer and multilayer adsorption in mesopores. The phenomenon of capillary condensation occurs with the appearance of hysteresis loops. A H4-type loop is typically associated with narrow slit-like pores for aggregates of micro- and mesoporous carbons. The isotherms obtained for the tested materials are typical for carbons with a mixed micro- and mesoporous structure (Figure 5).

According to

Table 1. Porosity of activated carbons.

Sample		SBET, m2 g−1	Vtotal, cm3 g−1	Smicro, m2 g−1	Vmicro, cm3 g−1	Microporosity, %
peanut shells	C-PS	1145	0.757	391	0.169	22
spruce cones	C-SC	1156	0.740	430	0.186	25

, the porous structure obtained from nitrogen adsorption–desorption analysis showed that both carbons possessed high specific surface areas (S_BET approx. 1145–1156 m² g⁻¹) and total pore volumes (0.74–0.76 cm³ g⁻¹). The micropore surface area and volume were slightly higher for C-SC (430 m² g⁻¹ and 0.186 cm³ g⁻¹) compared to C-PS (391 m² g⁻¹ and 0.169 cm³ g⁻¹), contributing to better adsorption performance.

The pore size distribution obtained from nitrogen desorption represents the volume occupied by pores of different diameters. The space occupied by different pore sizes is represented as differential distribution curves dV/dlog (D) (Figure 6). It is worth noting that the pore size distributions derived from the adsorption and desorption branches differ significantly. Some variations are also observed between the carbon types, reflecting their different pore structures.

The pore size distribution for the studied carbons reveals a well-developed porous structure ranging from 2 to 150 nm, encompassing micropores, mesopores, and macropores. The proportion of pores with diameters greater than 50 nm is almost negligible, suggesting that the carbons are primarily micro- and mesoporous.

It is well known that hysteresis loops that appear in the multilayer range of physisorption isotherms are generally associated with the filling and emptying of mesopores.

In more complex pore structures, the desorption pathway often depends on network effects and various forms of pore blocking, which occur when wide pores have access to the external surface only through narrow, ink bottle-like pore necks. Wide pores are filled and remain filled during desorption until the narrow necks empty at lower vapor pressures.

Delayed condensation occurs in open-ended pores with cylindrical geometry, meaning that in such a pore complex, the adsorption branch of the hysteresis loop is not in thermodynamic equilibrium. Therefore, if the pores are filled with a liquid-like condensate, thermodynamic equilibrium is established on the desorption branch [21].

The desorption branch was previously preferred for analyzing mesopore size, but this practice is now considered questionable because the desorption pathway may depend on network percolation effects or pore diameter changes along single channels. On the other hand, the persistence of a metastable multilayer is likely to delay the condensation process on the adsorption branch, especially if the pores tend to adopt a slit-like shape [22].

From desorption branch, a maximum appears at 4 nm corresponding to mesopores. In this case, applying the BJH method to the adsorption and desorption branches of the isotherm gives a completely different result. According to Groen et al., the peak observed at around 4 nm does not accurately reflect the porous properties of the material [23].

The surface composition and chemical states of elements on phosphoric-acid-activated peanut shell (C-PS) and spruce cone (C-SC) carbons were analyzed using XPS and SEM-EDS techniques (

Table 2. Data from the SEM-EDS microanalysis of activated carbons.

Sample	Element
	C		O		P
	wt.%	at.%	wt.%	at.%	wt.%	at.%
C-PS	85.34	89.07	13.18	10.33	1.48	0.60
C-SC	84.03	88.26	13.74	10.83	2.23	0.91

and

Table 3. Data from the XPS of activated carbons.

Sample	Element, %At. Conc.
	C	O	P
C-PS	88.5	10.3	1.2
C-SC	88.1	10.5	1.4

). These analyses provide critical insights into the types and distribution of oxygenated groups and the heteroatom doping (mainly phosphorus) that influence adsorption performance, particularly in dye removal.

Elemental analysis by SEM-EDS and XPS revealed that the resulting porous carbons were composed primarily of carbon and oxygen. Both techniques report similar atomic percentages for carbon, with slightly lower values in XPS, likely reflecting the increased surface contribution of oxygen and phosphorus. The minimal difference suggests a uniform carbon distribution from bulk to surface. Oxygen levels are also comparable across both techniques, indicating that oxygen-containing groups are fairly evenly distributed, with no significant surface enrichment or depletion.

Phosphorus content is higher in XPS than EDS when comparing atomic percentages. This suggests that phosphorus is surface-enriched, particularly in sample C-SC, possibly due to the doping or activation process leading to phosphorus-containing functional groups (e.g., phosphate, phosphonate) migrating toward or forming at the surface. The elevated P content in XPS indicates that phosphorus functional groups are more concentrated at the outermost surface, which could enhance surface-related properties such as adsorption or hydrophilicity. Phosphorus groups may contribute to acidity and provide additional adsorption sites, further improving dye uptake.

From to XPS data, both materials show high carbon content, indicating good carbonization and structural integrity (C 1s, 284.8 eV). Oxygen content reflects surface functional groups that can enhance hydrophilicity and interact with polar dye molecules (O 1s, 533.1 eV). Phosphorus likely comes from residual phosphate groups introduced during activation, which may enhance surface acidity or electron-donating properties (P 2p, 134.1 eV).

According to the XPS spectra (Figure 7), the carbon C (1s) peak was deconvoluted into five main components: C=C sp² (284.5 eV), C–OH/O–C–O (286.6 eV), C=O (287.6 eV), COOR (288.8 eV), and π-π* shake-up (290.6 eV). The oxygen O (1s) peak was split into four main components: O=C (531.2 eV), Car–OH (533.6 eV), O=C-O-R/C-O-C/C-OH (532.5 eV), and O=C-O-R (534.5 eV).

Surface chemical characterization by XPS revealed that both carbons were predominantly composed of a well-developed graphitic carbon structure, indicating a degree of conjugation and structural order (C=C sp²), with contents of 90.3% for C-PS and 87.4% for C-SC (

Table 4. C (1s) data from the XPS of activated carbons. * show π-π* transition in XPS.

Sample	Name, %At. Conc.
	C=C sp2	C–OH/O-C-O	C=O	COOR	π-π* Shake-Up
C-PS	90.3	2.6	0.1	3.5	3.5
C-SC	87.4	3.8	1.5	3.4	4.0

). Carbon from spruce cons showed a higher proportion of oxygen-containing groups such as hydroxyl/ether (C–OH/O–C–O, 3.8%), carbonyls including ketones, aldehydes (C=O, 1.5%), and esters/carboxylates (COOR, 3.4%) compared to analog from peanut shells. These functional groups are known to facilitate adsorption via hydrogen bonding and electrostatic interactions. The spruce-based carbon is also characterized by a higher amount of π-π* shake-up satellite (4.0%). It occurs in conjugated systems (graphitic domains), indicating extended π-electron systems and structural order.

Based on XPS spectra of the O (1s), it was confirmed that spruce-based carbon had a greater abundance of carbonyl/quinone groups (O=C, 21.2%) and phenolic hydroxyl groups (Car–OH, 35.7%) than C-PS (

Table 5. O (1s) data from the XPS of activated carbons.

Sample	Name, %At. Conc.
	O=C	Car–OH	O=C-O-R/C-O-C/C-OH	O=C-O-R
C-PS	19.3	34.0	42.3	4.5
C-SC	21.2	35.7	36.6	6.5

). Slightly higher ester content in C-SC could support acid-base interactions during adsorption. The enhanced oxygen functionality in C-SC likely contributes to its higher affinity for crystal violet dye through polar interactions.

High-resolution XPS spectra of the P (2p) region were recorded for both activated carbon samples (C-PS and C-SC). The results reveal that the binding energies and spectral profiles are identical for both samples, indicating that phosphorus exists in the same chemical states, regardless of the precursor used.

The main peak at 133.3 eV (P 2p_3/2 A) is characteristic of phosphate species, where phosphorus is in the oxidation state (P⁵⁺). This indicates that phosphorus is primarily present in the form of phosphate groups at the surface. The secondary component at 136.0 eV (P 2p_3/2 B), with a much smaller contribution (~6%), likely corresponds to more oxidized phosphorus species, such as polyphosphates or pyrophosphates. The presence of these forms may result from oxidative surface treatments or the condensation of phosphate species during carbon activation.

3.3. Crystal Violet Adsorption

Due to their properties, porous activated carbons are widely used to purify water from dye contaminants [24,25,26,27,28].

The performance of activated carbons derived from peanut shells (C-PS) and spruce cones (C-SC) for the removal of crystal violet dye was evaluated across a range of initial dye concentrations (100–400 mg L⁻¹) and contact times (20–120 min). According to Guzel et al., crystal violet dye molecules have a size of 1.4 nm × 1.4 nm [29]. Abbasi et al. consider a more spatial structure with a triangular form of the distances between carbons at different nitrogens, which are, respectively, 0.9 nm × 1.0 nm × 1.1 nm [30]. In each case, the molecular size allows for the CV molecules to diffuse into the meso- and macropores. Furthermore, hydrophobic interactions may occur between the nonpolar, graphitic structure of the carbons surface and the less polar aromatic groups of the CV molecule. Therefore, the results demonstrate significant differences in the adsorption efficiency between the two carbon sources, particularly at lower concentrations.

For both C-PS and C-SC, dye removal increased with longer contact times, indicating that the adsorption process is time-dependent and likely controlled initially by surface adsorption followed by slower pore diffusion. C-PS showed a steady rise in removal efficiency over time, reaching a maximum of 84% at 120 min for the 100 mg L⁻¹ solution. In contrast, C-SC removal efficiency increased rapidly up to 90% and then plateaued, indicating a faster equilibrium.

C-SC consistently outperformed C-PS at all concentrations. At a concentration of 100 mg L⁻¹, C-PS achieved 84% elimination, compared to 91% for C-SC. This trend continued at higher concentrations, with both materials exhibiting a decrease in elimination efficiency with increasing concentration likely due to saturation of the active sites. C-PS maintained a significantly lower elimination percentage, particularly at a concentration of 400 mg L⁻¹ (29%) compared to C-SC (54%) (Figure 8).

The adsorption kinetics of crystal violet onto phosphoric-acid-activated carbons derived from peanut shells (C-PS) and spruce cones (C-SC) were evaluated using pseudo-first-order (PFO) and pseudo-second-order (PSO) models across four initial dye concentrations (100–400 mg L⁻¹) (

Table 6. Adsorption kinetics parameters of crystal violet.

${\mathit{C}}_0 = 100 \mathbf{m}\mathbf{g} {\mathbf{L}}^{-1}$$; \mathit{W} = 1 \mathbf{g}$$;\mathit{V} = 0.1 \mathbf{L}$
Sample	Exp.	PFO			PSO
	${\mathit{q}}_{\mathit{e}}$ (mg g−1)	${\mathit{q}}_{\mathit{e}}$ (mg g−1)	${\mathit{k}}_1$ (min−1)	${\mathit{R}}^2$	${\mathit{q}}_{\mathit{e}}$ (mg g−1)	${\mathit{k}}_2$ (mg g−1 min−1)	${\mathit{R}}^2$
C-PS	8.5	12.6	3.54 × 10−4	0.9971	11.5	2.24 × 10−3	0.9782
C-SC	9.1	9.8	3.48 × 10−4	0.9901	10.5	5.34 × 10−3	0.9988
${\mathit{C}}_{\mathbf{0}}=\mathbf{200} \mathbf{m}\mathbf{g} {\mathbf{L}}^{-\mathbf{1}}$;$\mathit{W}=\mathbf{1} \mathbf{g}$;$\mathit{V}=\mathbf{0.1} \mathbf{L}$
Sample	Exp.	PFO			PSO
	$q_e$ (mg g−1)	$q_e$ (mg g−1)	$k_1$ (min−1)	$R^2$	$q_e$ (mg g−1)	$k_2$ (mg g−1 min−1)	$R^2$
C-PS	10.3	29.3	4.79 × 10−4	0.9203	13.2	2.42 × 10−3	0.9945
C-SC	14.7	23.4	4.14 × 10−4	0.9827	17.8	2.41 × 10−3	0.9964
${\mathit{C}}_{\mathbf{0}}=\mathbf{300} \mathbf{m}\mathbf{g} {\mathbf{L}}^{-\mathbf{1}}$;$\mathit{W}=\mathbf{1} \mathbf{g}$;$\mathit{V}=\mathbf{0.1} \mathbf{L}$
Sample	Exp.	PFO			PSO
	$q_e$ (mg g−1)	$q_e$ (mg g−1)	$k_1$ (min−1)	$R^2$	$q_e$ (mg g−1)	$k_2$ (mg g−1 min−1)	$R^2$
C-PS	11.0	22.0	4.55 × 10−4	0.9748	13.6	2.92 × 10−4	0.9930
C-SC	18.7	31.3	3.85 × 10−4	0.9511	22.6	1.79 × 10−3	0.9978
${\mathit{C}}_{\mathbf{0}}=\mathbf{400} \mathbf{m}\mathbf{g} {\mathbf{L}}^{-\mathbf{1}}$;$\mathit{W}=\mathbf{1} \mathbf{g}$;$\mathit{V}=\mathbf{0.1} \mathbf{L}$
Sample	Exp.	PFO			PSO
	$q_e$ (mg g−1)	$q_e$ (mg g−1)	$k_1$ (min−1)	$R^2$	$q_e$ (mg g−1)	$k_2$ (mg g−1 min−1)	$R^2$
C-PS	11.6	23.6	5.00 × 10−4	0.9756	13.9	3.47 × 10−3	0.9952
C-SC	21.8	29.8	3.73 × 10−4	0.9774	25.8	1.85 × 10−3	0.9988

, Figure S1). These models help to clarify the rate-controlling mechanisms and the likely nature of adsorption.

Kinetic modeling further elucidated the adsorption mechanism. Both the PFO and PSO kinetic models were applied, but the PSO model showed better fitting (R² > 0.98), suggesting the existence of a chemisorption process.

The rate constants ($k_2$) for C-SC were generally higher than those of C-PS, indicating faster adsorption kinetics. The PSO model also predicted $q_e$ values closer match to the experimental adsorption capacity, particularly for C-SC, reinforcing its superior adsorption capability.

The equilibrium adsorption behavior of crystal violet on activated carbons derived from peanut shells (C-PS) and spruce cones (C-SC), both treated with phosphoric acid, was evaluated using the Langmuir, Freundlich, and Dubinin–Radushkevich (D-R) isotherm models (

Table 7. Equilibrium adsorption parameters for crystal violet on the carbon adsorbents.

Model	Parameter	C-PS	C-SC
Langmuir	$K_L$ (L mg−1)	0.1763	0.0571
	$q_{max}$ (mg g−1)	11.37	22.05
	$R^2$	0.9281	0.9821
Freundlich	$K_F$ (mg g−1)	6.25	4.49
	$n$	9.22	3.32
	$R^2$	0.9988	0.9997
Dubinin–Radushkevich	$q_m$ (mg g−1)	10.98	18.30
	$\beta$ (mol2 J−2)	69.42	71.05
	$E_{ads}$ (J mol−1)	0.0849	0.0839
	$\epsilon$ (J2 mol−2)	154.03 − 8.71	248.46 − 13.54
	$R^2$	0.8001	0.7914

, Figure S2). These models provide information on the nature of the adsorption process, surface characteristics, and the maximum adsorption capacity of the materials.

The Langmuir model assumes monolayer adsorption on a surface with a finite number of homogeneous sites and no interaction between the adsorbed molecules. The maximum capacities ($q_{max}$) of 11.37 mg g⁻¹ for C-PS and 22.05 mg g⁻¹ for C-SC confirm the improved superior adsorption efficiency that was observed experimentally.

C-SC provides more accessible or higher-energy adsorption sites, potentially due to better-developed porosity or favorable surface chemistry. C-PS had a higher Langmuir constant value (0.1763 L mg⁻¹) compared to C-SC (0.0571 L mg⁻¹), indicating a higher affinity for crystal violet at low concentrations, despite its lower capacity. Both materials fit well with the Langmuir model, but C-PS showed a slightly better correlation (R² = 0.9281) than C-SC (R² = 0.9821), indicating a more ideal monolayer adsorption behavior.

The Freundlich isotherm, which describes heterogeneous surface adsorption and multilayer formation, also fitted well (R² > 0.999). The Freundlich exponent $n$ reflects favorable adsorption properties, and both materials had $n$ > 1, indicating favorable adsorption. The Freundlich constants ($K_F$) and heterogeneity factor ($n$) showed that C-PS had a higher values of those parameters compared to C-SC, suggesting stronger surface heterogeneity and more favorable adsorption sites on C-PS. However, the higher $q_{max}$ values for C-SC indicate that, despite slightly lower heterogeneity, C-SC have a greater adsorption capacity, likely due to their enhanced porosity and surface chemistry.

The Dubinin–Radushkevich (D-R) model provides insights into the adsorption mechanism, whether physical or chemical, and the porosity of the adsorbent. The process is considered to be chemisorption- or ion-exchange-controlled when the value is between 8 and 16 kJ mol⁻¹ and to be controlled by physical adsorption or van der Waals interactions when the value is below 8 kJ mol⁻¹ [31]. The isotherm parameters revealed adsorption energies ($E_{ads}$) approx. 0.084 J mol⁻¹ for the obtained carbons, which are well below 8 kJ mol⁻¹, indicating the presence of a physisorption process driven primarily by specific interactions. The D-R model had the weakest correlation for both materials (C-PS = 0.8001; C-SC = 0.7914), suggesting that it may not adequately describe the adsorption mechanism compared to the Langmuir and Freundlich models.

For both adsorbents, the Polanyi potential values showed a clear decreasing trend with increasing concentration, reflecting the gradual occupation of adsorption sites with decreasing energy. At the lowest concentration, C-PS exhibited a Polanyi potential of approximately 154.03 J² mol⁻², while C-SC exhibited a noticeably higher value of approximately 248.46 J² mol⁻². This indicates that the second carbon initially offers adsorption sites with significantly higher energy compared to the first.

With increasing concentrations, $\epsilon$ decreased in both cases, reaching values approaching 8.71 J² mol⁻² and 13.54 J² mol⁻², respectively, at the highest concentrations studied. The decreasing difference between the two carbons at higher concentrations indicates the saturation of the highest-energy sites and adsorption onto lower-energy sites, which is typical for heterogeneous adsorbent surfaces.

The observed differences in Polanyi potential correlate well with the differences in microporosity between the two adsorbents, which can be directly related to their porous structure (Table 1).

Pore diffusion, π–π electron donor–acceptor interaction, and H-bonding, as well as electrostatic interactions, are different types of adsorption mechanisms [32].

Spruce-based carbon is characterized by a more oxidized functional group (XPS), higher phosphorus content (XPS and SEM-EDS), higher π–π* reaction intensity (aromatic interactions), more micropores, and higher adsorption capacity. All these features indicate enhanced physisorption, particularly through π–π stacking, hydrogen bonding, and electrostatic interactions with the cationic dye crystal violet.

Peanut-based carbon, although still rich in graphitic carbon and characterized by decent oxygenation, exhibits fewer surface functions, which explains the slightly lower adsorption capacity and a slight preference for physisorption at lower concentrations.

The superior adsorption performance of spruce-based carbon can be attributed to its balanced combination of a high surface area, enhanced microporosity, and rich surface chemistry with oxygen- and phosphorus-containing groups.

These properties enable efficient adsorption via multiple mechanisms (Figure 9): physical adsorption through micropores and mesopores, which provide a large surface area and pore volume; and specific interactions, including hydrogen bonding, as well as π–π interactions between the aromatic rings of the dye and graphitic domains on the carbon surface, supported by the π–π* shake-up signals observed in XPS. While peanut-based carbon also showed promising adsorption behavior, its slightly lower porosity and fewer functional groups resulted in reduced dye uptake and slower kinetics.

Adsorption studies have shown that both materials are effective, but C-SC offers a higher capacity, while C-PS is characterized by a higher affinity and faster surface interactions at lower dye concentrations.

The CV adsorption capacity values in this work are comparable, or even higher, to those found in the literature. Loulidi et al. suggested that the maximum adsorption capacity in a monolayer was 12.2 mg g⁻¹ [33], while Abbas et al. found that the maximum adsorption capacity was 20.95 mg g⁻¹ [34].

4. Conclusions

The use of microwave irradiation allows for the production of carbonaceous materials. The process of charring biomass using a microwave kiln requires the careful selection of parameters, and the type of biomass feedstock significantly influences the porous structure of the resulting activated carbon.

Pore size distribution analysis confirmed the presence of a micro-mesoporous structure, which is advantageous for adsorbing dye molecules like crystal violet by providing an accessible surface area and sufficient pore volume for diffusion and adsorption.

Elemental analysis using SEM-EDX and XPS showed that the chemical composition of the carbon is closely related to the type of biomass feedstock used, and the presence of phosphorus confirms the activation with phosphoric acid.

Both adsorbents follow the Freundlich and Langmuir models well, suggesting a combination of monolayer and heterogeneous surface adsorption. The adsorption mechanism is physical in nature for both carbons.

In conclusion, phosphoric acid-activated carbons derived from spruce cones (C-SC) exhibit superior physicochemical properties and enhanced adsorption performance compared to peanut-based carbons (C-PS). The combined influence of a well-developed porous structure, enriched surface oxygen and phosphorus functionalities, and graphitic carbon domains facilitate the efficient removal of crystal violet dye. These findings demonstrate the potential of C-SC as a cost-effective and sustainable adsorbent for wastewater treatment applications.