Use of Brushite as Adsorbent for the Removal of Anionic and Cationic Dyes Present in Aqueous Solutions

Abstract

Currently, water pollution caused by dyes is a serious problem since they are toxic and carcinogenic to living beings. To reduce the presence of these contaminants, natural adsorbents have been considered as they are easy to obtain, inexpensive, and have high removal efficiency. In this work, the adsorption process using natural brushite (nDCPD) was studied for the removal of phenol red (PR), achieving a removal rate of 99.15% and an adsorption capacity of 82.24 mg/g, and gentian violet (GV), achieving a removal rate of 97.03% and an adsorption capacity of 74.22 mg/g. Equilibrium adsorption occurs for both dyes in multiple layers on the surface. The adsorption process is spontaneous for both dyes. The kinetics of the adsorption process involve using a single active site on the surface for PR adsorption, while for GV, two active sites on the surface are required. Analysis via FTIR, EDS, and XRD revealed various mechanisms that intervene in the adsorption process of both dyes on the surface of nDCPD, such as electrostatic forces, functional groups, physisorption, and ion exchange.

1. Introduction

Due to the rapid growth of anthropogenic activities, water pollution has accelerated to worrying levels in recent years [1,2,3]. Discharges of pollutants from industries such as the paint, plastic, paper, rubber, leather, dyeing, pharmaceutical, medicine, chemical, and textile industries cause a decrease in water quality because the effluents contain heavy metals and dyes [4,5,6]. These substances can cause serious health problems, as well as severe environmental damage, causing bioaccumulation in living beings [7,8,9]. A significant portion of pollutants generated are dyes, with an estimated 20% of the world’s dye production being discharged into drainage systems, rivers, and lakes [10,11,12,13]. Pollution from these effluents can be highly visible, even at low concentrations [14,15,16,17]. The presence of these dyes in the water not only destroys the water’s aesthetic quality but also prevents the penetration of sunlight, affecting the existing food web in aquatic ecosystems [18,19,20]. Different types of dyes are used in various industries, including acid dyes, reactive dyes, basic dyes, azo dyes, direct dyes, and disperse dyes, most of which contain traces of metals and are soluble in water [21,22,23]. Dyes are broadly classified into cationic, anionic, and non-ionic categories. Anionic dyes have a net negative charge, which includes acidic, reactive, azo, and direct dyes, while cationic dyes have a positive charge and are basic [12,20,24]. Among the dyes present in these effluents are phenol red (PR) and gentian violet (GV) (Figure 1), which are anionic and cationic dyes, respectively. These dyes can cause several diseases upon ingestion or skin adsorption, such as severe damage to the gastrointestinal tract, carcinogenicity, permanent blindness, and respiratory and kidney failure [25,26,27].

To preserve the environment, various separation methods have been developed and employed, including solvent extraction, ion exchange, coagulation–flocculation, filtration, and dialysis, among others, to remove these heavy metals in different concentration ranges [28,29,30,31,32,33]. Despite their high removal efficiency, these methods present significant challenges, such as high sludge generation, high energy consumption, and elevated operating and maintenance costs [34,35,36,37]. Adsorption emerges as a promising method for dye removal due to its key advantages: low cost, simple implementation, non-toxicity, low maintenance costs, minimal or no production of polluting sludge, and high efficiency in contaminant removal compared to other separation methods [37,38,39,40]. However, the use of commercial adsorbents like activated carbon and zeolites makes this method less feasible [39,40,41,42]. Therefore, other types of adsorbents with similar dye adsorption capacity, obtained from renewable sources with high availability and low cost, have been investigated [33,34,37]. Waste derived from the agricultural industry and non-commercial industrial processes generate large quantities of waste that are typically discarded without further use, posing an environmental issue since they are mainly disposed of on the ground [22,43].

Among these wastes is bovine bone, from which apatites are obtained. These are minerals formed from phosphate and calcium, including hydroxyapatite (HA) and brushite (nDCPD), which are abundant in nature and are capable of forming solid solutions, accepting anions and cations as substitutes, and thereby allowing for the retention and elimination of contaminants [44,45,46]. Brushite, a bioadsorbent, has been used to remove certain heavy metals and dyes, showing a performance comparable to other adsorbents [45,46,47]. Considering that nDCPD may have potential as an adsorbent of dyes present in effluents, it is necessary to analyze how it interacts with PR and GV during the removal process, describing the interaction mechanisms between the surface and the dye. For this purpose, a study was conducted on various parameters in the adsorption process, such as contact time, adsorbent mass, initial dye concentration, and temperature, to evaluate the behavior of nDCPD using equilibrium, kinetic, and thermodynamic studies, taking into account the characterization of the material before and after dye removal to understand the adsorption mechanism.

2. Materials and Methods

2.1. Reagents

The dyes used in this project are phenol red (PR; λ_max = 435 nm) and gentian violet (GV; λ_max = 590 nm) from Química Meyer^® (Mexico City, Mexico). Deionized water was used for all experimental solutions [48,49].

2.2. Development of the Bioadsorbent

The nDCPD used in this project was obtained from bovine bone, which was washed with previously heated water and subsequently dried in a convection oven (Shel Lab CE-5F, Cornelius, OR, USA) at 80 °C for 24 h. It was then crushed and sieved to a particle size of 0.045 mm to obtain a powder. This power was treated with a 0.1 M HCl solution (Química Meyer^®, Mexico City, Mexico) then washed and added to a 0.1 M NaOH solution [45,47].

2.3. Obtaining the PR and GV Adsorption Isotherms

For the adsorption isotherm experiments of each dye, 1 g of nDCPD was added to 50 mL of the dye solution at different concentrations. The solutions were placed in a shaking incubator (ZHWY-200D, Shanghai, China) at 200 rpm and maintained at temperatures of 25 °C, 35 °C, and 45 °C for a period of 24 h. The concentration of the PR and GV dyes was measured using a UV–visible spectrophotometer (Velab VE-5000, Mexico City, Mexico) to determine the amount of dye removed by the nDCPD adsorbent [47,48].

$$\mathrm{q}={\displaystyle \frac{\mathrm{V}\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)}{\mathrm{m}}},$$

(1)

where C₀ and C_e are the initial and final concentration in mg/L, V is the volume of solution in L, and m is the mass of the adsorbent in g. To determine the removal percentage, the following equation was used [46,47]:

$$\mathrm{\%}\mathrm{R}={\displaystyle \frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)}{{\mathrm{C}}_0}}\ast 100.$$

(2)

The different isotherm models used to fit the experimental data obtained can be seen in the

Table 1. Non-linear adsorption isotherm models [33,40,45,49].

Model	Equation	Parameters
Langmuir	${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{m}}{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}}$	qm = maximum adsorbed capacity KL = Langmuir constant
Freundlich	${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}^{{\displaystyle \frac{1}{\mathrm{n}}}}$	n = adsorption intensity measurement KF = Freundlich constant related to adsorption capacity
Temkin	${\mathrm{q}}_{\mathrm{e}}=\mathrm{A}+\mathrm{B}\mathrm{l}\mathrm{n}\left({\mathrm{C}}_{\mathrm{e}}\right)$	A = constant related to the heat of adsorption B = bonding equilibrium constant (maximum binding energy)
Dubinin–Radushkevich (DR)	${\mathrm{q}}_{\mathrm{e}}={\mathrm{q}}_{\mathrm{m}}\mathrm{e}\mathrm{x}\mathrm{p}\left(-{\mathrm{k}}_{\mathrm{D}\mathrm{R}}{\mathsf{\epsilon }}^2\right)$	qm = maximum adsorbed capacity kDR = rate constant ε = DR model parameter $\mathsf{\epsilon }=\mathrm{R}\mathrm{T}\ln \left(1+{\displaystyle \frac{1}{{\mathrm{C}}_{\mathrm{e}}}}\right)$
Sips	${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{m}}{\left({\mathrm{K}}_{\mathrm{s}}{\mathrm{C}}_{\mathrm{e}}\right)}^{\mathrm{n}\mathrm{s}}}{1+{\left({\mathrm{K}}_{\mathrm{s}}{\mathrm{C}}_{\mathrm{e}}\right)}^{\mathrm{n}\mathrm{s}}}}$	KS = constant related to adsorption energy ns = dimensionless parameter of SIPS model
Redlich–Peterson (RP)	${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{K}}_{\mathrm{R}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{a}}_{\mathrm{R}}{\mathrm{C}}_{\mathrm{e}}^{\mathsf{\beta }}}}$	KR, aR, β = Redlich–Peterson constant

[33,40,45,46].

To compare the efficiency of different equilibrium models, the determination coefficient (R²) and the standardized deviation were used as criteria [48]:

$$∆\mathrm{q}=100\ast \sqrt{{{\displaystyle \frac{\left(\frac{{\mathrm{q}}_{\mathrm{e}\mathrm{x}\mathrm{p}}-{\mathrm{q}}_{\mathrm{c}\mathrm{a}\mathrm{l}}}{{\mathrm{q}}_{\mathrm{e}\mathrm{x}\mathrm{p}}}\right)}{\mathrm{N}-1}}}^2},$$

(3)

where N is the number of data, and q_exp (mg/g) and q_cal (mg/g) denote the experimental and calculated data using the models of the removed dyes. The separation factor, R_L, was used to predict the affinity between the absorbed and absorbent, using the following equation [48,49,50]:

$${\mathrm{R}}_{\mathrm{L}}={\displaystyle \frac{1}{1+{\mathrm{K}}_{\mathrm{K}}{\mathrm{C}}_0}},$$

(4)

where 0 < R_L< 1 for favorable, R_L > 1 for unfavorable, R_L = 1 for linear, and R_L = 0 for irreversible adsorption process [48,49,50]. To determine the type of adsorption process, the free energy of adsorption, E (kJ/mol), was calculated using the following equation [20,48,49,50]:

$$\mathrm{E}={\displaystyle \frac{1}{\sqrt{2{\mathrm{K}}_{\mathrm{D}\mathrm{R}}}}}.$$

(5)

2.4. Thermodynamic Analysis

To understand the nature of the adsorption process, the thermodynamic parameters—∆G (kJ/mol), ∆H (kJ/mol), and ∆S (kJ/mol K)—were determined, allowing for an understanding of the process and mechanism of adhesion of metal ions [45,51]:

$$\mathsf{\Delta }\mathrm{G}=-\mathrm{R}\mathrm{T} \ln \left(55.5{\mathrm{K}}_{\mathrm{L}}\right),$$

(6)

where K_L (L/mol) is the Langmuir constant, R (kJ/mol K) is the gas constant, and T (K) is the process temperature. Using the van’t Hoff equation, the other parameters can be determined using the following equation (Equation (7)):

$$\mathsf{\Delta }\mathrm{G}=∆\mathrm{H}-\mathrm{T}∆\mathrm{S}.$$

(7)

2.5. Dye Removal Kinetics

For the removal kinetics of dyes, the mass of the adsorbent varied from 0 to 1 g in 50 mL of solution and was stirred for 9 h at a speed of 200 rpm at different temperatures (25, 35, and 45 °C). An aliquot of the aqueous solution was taken and subjected to centrifugation (Thomas ICZC8, Madrid, España) at 6000 rpm for 10 min. The concentration of each dye was measured using a spectrophotometer (Velab VE-5000, Mexico City, Mexico). To analyze the experimental data, the adsorption kinetic models were fitted as shown in the

Table 2. Adsorption kinetic models used in the analysis of the experimental data [45,48,50].

Model	Equation	Parameters
Pseudo First Orden (PFO)	${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}\left[1-\mathrm{e}\mathrm{x}\mathrm{p}\left(-{\mathrm{k}}_1\mathrm{t}\right)\right]$	qmax = maximum adsorbed capacity k1 = PPO rate constant
Pseudo Second Orden (PSO)	${\mathrm{q}}_{\mathrm{t}}={\displaystyle \frac{\mathrm{t}}{\frac{1}{{\mathrm{k}}_2\ast {\mathrm{q}}_{\mathrm{e}}^2}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}}}$	k2 = PSO rate constant
Elovich	${\mathrm{q}}_{\mathrm{t}}={\displaystyle \frac{1}{\mathsf{\beta }}}\left[\mathrm{l}\mathrm{n}\left(\mathrm{t}\right)+\mathrm{l}\mathrm{n}\left(\mathrm{\alpha }\ast \mathrm{\beta }\right)\right]$	α = initial adsorption rate β = It is related to the covered surface and activation energy by chemiadsorption
Intraparticular Diffusion (ID)	${\mathrm{q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{I}\mathrm{n}\mathrm{t}}{\mathrm{t}}^{0.5}+\mathrm{C}$	kint = ID rate constant C = indicate the diffusional phenomena control the sorption rate
External Diffusion (ED)	${\mathrm{q}}_{\mathrm{t}}={\displaystyle \frac{{\mathrm{C}}_0\mathrm{V}}{\mathrm{m}}}\left[1-\mathrm{e}\mathrm{x}\mathrm{p}\left(-{\mathrm{k}}_{\mathrm{e}\mathrm{x}\mathrm{t}}\ast \mathrm{t}\right)\right]$	kExt = ED rate constant C0 = initial concentration of dye in solution V = dye solution volume m = adsorbent mass
Avrami	${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}\left[1-\mathrm{e}\mathrm{x}\mathrm{p}{\left(-{\mathrm{k}}_{\mathrm{A}}\mathrm{t}\right)}^{{\mathrm{n}}_{\mathrm{A}}}\right]$	kA = cinetic constante nA = Avrami of exponent

.

2.6. Brushite Characterization

Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) was used to analyze the biosorbent before and after adsorption. The infrared spectrum was analyzed in a wavenumber range of 4000–400 cm⁻¹ using a Thermo Scientific Nicolet iS10 analyzer (Waltham, MA, USA). A total of 32 scans were obtained with a resolution of 4 cm⁻¹. In addition, X-ray diffraction (XRD) patterns were obtained using a diffractometer (Ultima IV Rigaku, Cedar Park, TX, USA), measuring from 4° to 80° in 2θ with a step size of 0.03°. The isoelectric point was determined using the mass titration method: a sample of the adsorbent in water with an initial mass of 0.05 g was stirred at 200 rpm for 24 h in 50 mL of water to determine its pH with a potentiometer (Science Med SM-25CW, Helsinki, Finland). An additional 0.05 g was added every 24 h until the pH remained unchanged [45]. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS-EDX) values were obtained using a JOEL spectrometer (6510 pus, Peabody, MA, USA).

3. Results and Discussion

3.1. Effect of Initial PR and GV Concentration on the Equilibrium Adsorption Process

In adsorption, parameters important for the elimination of dyes present in water must be considered to understand their influence on the overall process [48,52,53]. The removal of PR and GV at different concentrations and temperatures is shown in Figure 2. It can be observed that the adsorption process has a direct relationship with temperature. As the temperature increases, the removal percentage increases from 78.4%, 90.6%, and 95.5% at 25 °C, 35 °C, and 45 °C, respectively, for GV adsorption. In contrast, for PR, the removal decreases from 92%, 75.8%, and 69.6% as the temperature increases from 25 °C, 35 °C, and 45 °C, respectively. This implies that the adsorption capacity of GV in nDCPD increases by approximately 18% with the increasing temperature, while for PR, the adsorption capacity decreases by about 24% with the increasing temperature. This behavior in the adsorption process can be directly linked to the pretreatment performed, which can cause a change in the available sites on the surface of the bioadsorbent where both dyes are captured. This change can explain the reduced adsorption of PR compared to the adsorption capacity for GV [34,54]. Studies have shown an increase in the adsorption capacity of PR with an increase in the dye concentration when using onion peel [39], E. crassipe [48], and shrimp waste [54]. Similarly, for GV, the same behavior is obtained when using SCB-CaCO₃ [31], charred lime peel [8], E. crassipe [22,48], date palm biochar [19], rice husk [40], fish scales [9], active charcoal [28], pineapple and lime leaves [23], and Planta stachys [10].

This increase in the adsorption capacity may be due to the rise in collisions between dye molecules, which facilitates the adsorption process and results in a significant decrease in mass transfer between phases. Additionally, this has an effect on the removal percentage, which decreases directly with the increase in the initial concentration of both dyes [52,53,54,55]. This parameter is very important in the dye removal process, considering the other parameters, due to its relationship with mass transfer.

3.2. Adsorption of GV and PR in Equilibrium

The data obtained for the equilibrium adsorption curves at different temperatures for both dyes are shown in Figure 3 and Figure 4. It can be observed that the maximum adsorption capacity of PR at the different temperatures is 82.25 mg/g, 75.82 mg/g, and 69.63 mg/g at 25 °C, 35 °C, and 45 °C, respectively. This indicates that increasing the temperature results in a decrease in the adsorption capacity of 7.82% and 15.35% compared to the maximum adsorption of PR. In the case of GV adsorption, an opposite effect is observed with increasing temperature, showing an increase in the adsorption capacity of 59.99 mg/g, 67.32 mg/g, and 82.47 mg/g at 25 °C, 35 °C, and 45 °C, respectively. This represents an increase in the adsorption capacity of 12.22% and 27.25% relative to the maximum adsorption capacity obtained. These results indicate that the adsorption process on the surface of nDCPD differs based on the nature of each dye, as GV is a cationic dye with a pH_sol = 3.5, while PR is an anionic dye with a pH_sol of 8.35. This implies that the adsorption mechanism of the dyes onto the surface of nDCPD depends on electrostatic forces, given that the isoelectric point of the absorbent surface is at pH_pzc = 6.3, so the surface of the adsorbent is negatively charged when pH_sol > pH_pzc or positively charged when pH_sol < pH_pzc [24,52,54,55].

Table 3. Parameters of the adjustments of the different isotherm models for the dyes.

	GV			PR
Models	25 °C	35 °C	45 °C	25 °C	35 °C	45 °C
Langmuir
q, mg/g	147.22	98.451	94.915	121.39	121.265	93.382
KL, L/mg	0.2863	0.1151	0.0464	0.0833	0.2956	0.8870
RL	0.68–0.19	0.46–0.09	0.26–0.04	0.70–0.29	0.40–0.10	0.18–0.09
R2	0.8646	0.9038	0.9709	0.9679	0.9515	0.9003
%∆q	59.372	18.6051	10.5370	19.432	11.946	9.4557
Freundlich
KF, mg/g*(L/mg)1/n	5.0144	15.037	22.819	41.1945	29.858	22.931
n	1.3143	1.9495	2.1727	2.1616	1.8854	2.0943
R2	0.8376	0.8361	0.9179	0.9356	0.9064	0.8313
%∆q	36.0772	38.1780	50.2946	3.4831	5.1723	4.4106
Temkin
A, L/mg	0.0752	0.0981	19.5284	38.4691	24.4304	13.8894
B, kJ/mol	15.6876	23.8596	25.8703	30.1609	30.1140	24.5567
R2	0.7469	0.5244	0.6726	0.4666	0.6629	0.5164
%∆q	4.4532	32.5152	21.1038	7.0570	4.4712	4.1241
Sips
q, mg/g	61.207	67.138	73.592	90.033	81.973	64.485
Ks, L/mg	0.1057	0.2732	0.5676	0.8041	0.5809	0.4723
nS	5.2506	3.5830	2.2223	1.8911	2.2110	3.4634
R2	0.9935	0.9960	0.9988	0.9881	0.9921	0.9950
%∆q	0.7052	8.8839	5.1608	3.8651	3.1381	3.2333
RP
KR, L/g	3.8825	12.7834	27.1742	71.8030	38.8517	32.1360
aR, (L/mg)β	0.0264	0.1515	0.2863	0.8018	0.3942	0.5797
β	1.0000	0.9530	0.9235	0.8510	0.9100	0.9523
R2	0.8646	0.8989	0.9709	0.9593	0.9425	0.8766
%∆q	3.5108	8.9011	21.6620	6.7514	3.9836	13.3026
DR
q, mg/g	70.996	71.519	73.870	88.165	82.259	64.240
kDR, mol2/J2	0.1379	0.0203	0.0046	0.0030	0.0048	0.0069
E, kJ/mol	7.3721	3.5093	1.3446	6.0633	7.2169	9.1287
R2	0.9532	0.9834	0.9955	0.9640	0.9746	0.9717
%∆q	7.4907	9.0599	5.4908	2.9383	3.0269	5.9102

presents the parameters obtained from each of the adjustments made to the experimental data of the proposed models, considering the normalized standard deviation (Δq%) and the determination coefficient (R²) to select the best fit. Based on these criteria, the adsorption of both dyes is attributed to the heterogeneous surface present (Sips), where the process occurs physically (n_s > 1). This is confirmed by the Freundlich model, which indicates a value of n > 1, revealing that the process takes place on the surface of nDCPD [27,38,40]. The Langmuir model suggests favorable adsorption (0 < R_L< 1) [32,37,54,55]. Additionally, the adsorption energy determined using the DR model (E < 8 kJ/mol) for both dyes confirms a physical adsorption process on a heterogeneous surface [16,29,41,49]. However, for PR, the energy increases with the increasing temperature, while for GV adsorption, this energy decreases with the rising temperature, indicating a difference in the nature of the adsorption process involving more than one dye removal mechanism [28,38,54,55].

Considering the results reported in the literature, the Langmuir model provides the best fit for PR adsorption in onion peel (q = 71.64 mg/g) [39], graphene oxide (q = 657.89 mg/g) [21], and bentonite (q = 169.4 mg/g) [3]. For coconut shell removal (q = 10.5 mg/g) [2] and biochar (q = 29 mg/g) [26], the Freundlich model is assumed to be the best fit. In the case of GV, the Langmuir model fits best for graphene (q = 85 mg/g) [18], date palm (q = 226 mg/g) [19], rice husk (q = 90 mg/g) [36], active charcoal (q = 90.91 mg/g) [27], pineapple leaves (q = 9.77 mg/g) [23], and poplar seeds (q = 153.8 mg/g) [11]. The Freundlich model best represents adsorption with coffee husks (q = 5 mg/g) [25], chitosan extracted (q = 7.57) Freundlich [5], E. crassipes (q = 25 mg/g) [22], fish scales (q = 90.61 mg/g) [9], and shrimp waste (q = 61.74 mg/g) [54]. The Temkin model fits best for carbonized lime peel (q = 740.7 mg/g) [8].

In our study, both tints fit better with the Sips model, which is a combination of the Langmuir and Freundlich models [45,49], which indicates that the behavior in the adsorption isotherm is sufficient to understand what occurs during the adsorption process. Furthermore, nDCPD appears to be a good candidate for the removal of these dyes from industrial effluents as our results fall within the range of adsorption capacities reported in the literature.

3.3. Thermodynamic Analysis of the Adsorption Process of Both Dyes in nDCPD

To understand the nature of the adsorption process of dyes with nDCPD, the thermodynamic parameters shown in

Table 4. Thermodynamic parameters for PR and GV adsorption on nDCPD.

	Phenol Red
T, K	−∆G, kJ/mol	∆H, kJ/mol	∆S, kJ/mol K
298.15	−35.50	−93.2	0.174
308.15	−39.94
318.15	−44.14
	Gentian Violet
298.15	−38.02	70.1	0.348
308.15	−37.01
318.15	−35.86

were analyzed. For both dyes, the process is spontaneous (ΔG < 0). This observation aligns with what was observed in Figure 3, where the adsorption capacity increases with temperature, supported by the positive enthalpy values (ΔH > 0) in the case of GV. Conversely, for PR adsorption, the opposite occurs: the adsorption capacity decreases with increasing temperature, supported by the negative enthalpy values (ΔH < 0) [45,52,55]. Apparently, the forces present on the surface of the adsorbent participate significantly in the process along with the interaction in the solid–liquid interface [16,21,38]. There is also a decrease in the randomness of the solid–liquid interface for PR (ΔS < 0). In the case of GV adsorption on nDCPD, the adsorption process increases due to the increase in the randomness of the solid–liquid interface (ΔS > 0) [45,52,55]. Given the values of ΔH begin less than 8 kJ/mol, it can be inferred that the adsorption process takes place on the surface of nDCPD [16,21,38].

3.4. Effect of Bioadsorbent Concentration on the Adsorption Process of Both Dyes in nDCPD

For the practical application and high efficiency of dye removal, it is necessary to know the contact time required to carry out the adsorption process, which is a key parameter in establishing the kinetic mechanism in heterogeneous systems. This is observed in equilibrium due to the accumulation or saturation of the sites available to adsorb the dyes on the surface of nDCPD [36,55,56]. In Figure 5 and Figure 6, the effect of the adsorbent concentration (Cads) on the adsorption of PR and GV was observed by performing the analysis using a concentration range of 4 to 20 mg/L with a PR at 30 ppm and 70 ppm for GV.

The figures show that by increasing the concentration of the adsorbent, the adsorption capacity of the dye decreases, regardless of the temperature. This may be due to the blocking of the active sites present on the surface. Additionally, there is a greater availability of the adsorption process area until equilibrium is reached, which was reached at approximately 2 h for all temperatures in the case of PR adsorption. For GV, the same equilibrium time is obtained at 25 °C and 35 °C, but at 45 °C, equilibrium is reached only after 7.5 h. This may be due to the fact that the sites on the surface of nDCPD no longer allow for the transfer of dyes from the solution to the solid [46,56,57]. This behavior has been reported for PR and GV in the literature for various adsorbents. This analysis allowed us to understand the limitations of mass transfer, so this parameter must be considered particularly important [27,43,49].

3.5. Adsorption Kinetics of PR and GV on nDCPD

To understand the dynamic adsorption process of the dyes with respect to time, the experimental data obtained were adapted to the kinetic models proposed above (Table 2), allowing us to understand the control mechanism for the removal of both dyes. To choose the most suitable model, the same criteria previously used in the adsorption isotherm models were used (R² and ∆q%).

Table 5. Kinetic parameters of PR adsorption on nDCPD.

Cads, g/L	4			8			12			16			20
Models	25 °C	35 °C	45 °C	25 °C	35 °C	45 °C	25 °C	35 °C	45 °C	25 °C	35 °C	45 °C	25 °C	35 °C	45 °C
PFO
Qe, mg/g	78.526	73.841	60.258	40.733	37.107	29.697	26.652	24.689	19.888	19.859	18.474	15.893	15.824	14.776	11.916
k1, h−1	3.2034	2.9505	3.0503	4.2126	2.8987	2.1128	3.2403	3.2645	2.2513	4.2380	3.6972	2.0938	12699	3.6363	2.4530
R2	0.9985	0.9997	0.9998	0.9989	0.9999	0.9981	0.9991	0.9999	0.9990	0.9987	1.0000	0.9987	0.9985	0.9999	0.9992
∆%q	9.5612	7.1489	9.5607	8.9845	9.3256	7.6723	8.4509	8.2306	7.4781	8.4367	8.2145	9.5671	10.3209	6.4231	7.6893
PSO
Qe, mg/g	78.954	74.353	60.637	40.991	37.344	30.358	26.886	24.926	20.236	20.121	18.728	16.229	16.089	15.030	12.129
k2, g/mg*h	0.5024	0.4065	0.5565	1.0000	1.0000	0.2971	1.0000	1.0000	0.5663	1.0000	1.0000	0.5721	1.0000	1.0000	1.0000
R2	0.9986	0.9998	0.9999	0.9988	1.0000	0.9993	0.9990	0.9999	0.9997	0.9982	0.9994	0.9997	0.9995	0.9993	0.9997
∆%q	2.0134	1.9832	2.7461	3.9035	2.4531	3.0087	2.7650	2.2561	1.8760	3.9823	2.7690	2.1356	2.7832	4.0021	3.0997
Elovich
β, mg/g*h	0.2828	0.3029	0.3893	0.5463	0.8456	0.7800	0.8356	0.9500	1.1810	1.1190	1.2691	1.4762	1.4055	1.5867	1.9709
α, g/mg	3 × 109	4 × 109	9 × 109	2 × 109	3 × 109	4 × 109	1 × 109	3 × 109	5 × 109	9 × 108	2 × 109	2 × 109	7 × 109	2 × 109	1 × 106
R2	0.9957	0.9971	0.9973	0.9961	1.0000	0.9996	0.9953	0.9969	0.9993	0.9941	0.9960	0.9997	0.9925	0.9963	0.9988
∆%q	32.754	35.9823	29.8791	25.673	44.5120	23.510	37.8932	23.189	17.8634	20.0025	25.731	23.4876	32.8912	37.3241	29.453
ID
kID, mg/g*h0.5	23.702	22.2698	18.1502	12.1643	11.1644	9.1538	7.9966	7.4206	6.0854	5.9402	5.5234	4.8837	4.7041	4.4269	3.6283
	22.996	20.9595	17.1622	11.851	10.5779	7.9209	7.6603	7.0721	5.4137	5.7598	5.3566	4.2642	4.6482	4.2661	3.2938
C, mg/g	0.0.6766	0.6772	0.6751	0.6612	0.6743	0.7127	0.6689	0.6717	0.7013	0.6632	0.6639	0.7090	0.6546	0.6668	0.6927
R2	32.875	25.6478	23.1278	22.890	25.6721	24.614	22.1278	18.769	16.5489	17.6532	23.675	12.6734	52.6370	33.2178	21.4367
∆%q
ED
kext, h−1	0.0412	0.0382	0.0302	0.0430	0.0384	0.0297	0.0420	0.0384	0.0299	0.0417	0.0382	0.0321	0.0415	0.0382	0.0298
R2	0.4561	0.1469	0.1856	0.2312	0.1563	0.1045	0.3421	0.1612	0.1302	0.0456	0.1783	0.1051	0.1803	0.1719	0.1490
∆%q	39.567	22.9215	15.4361	19.635	26.8320	29.870	32.1233	33.658	25.6341	16.3490	21.492	25.9571	25.8031	31.2256	31.6782
Avrami
qe, mg/g	78.526	73.841	60.258	40.733	37.107	29.698	26.652	24.689	19.888	19.856	18.474	15.893	15.824	14.776	11.916
kA, h−1	1.7927	1.6339	1.6658	1.9507	1.6232	1.3569	1.7054	1.7261	1.4093	1.9589	1.8416	1.3529	2.4531	1.8254	1.4790
nA	1.8980	1.8058	1.8312	2.1595	1.7859	1.5571	1.9004	1.8913	1.5975	2.1635	2.0076	1.5477	2.4523	1.9921	1.6585
R2	0.9985	0.9997	0.9998	0.9989	0.9999	0.9981	0.9991	0.9999	0.9990	0.9987	1.0000	0.9987	0.9985	0.9999	0.9992
∆%q	12.563	10.5632	11.4532	9.0875	17.5326	22.678	21.7854	23.760	14.2521	15.9704	9.5480	21.4523	10.8324	11.2562	8.7520

shows the parameters of PR adsorption on nDCPD, where it can be observed that the PSO model best represents the experimental data. This indicates that two active sites are needed to remove a dye molecule from the solution because the sites where adsorption takes place are widely available on the surface. There may be a dimensional growth of the sites due to the Avrami model, where n_A > 1, indicating that these mechanisms of the adsorption process are due to the growth of the active sites in a one-dimensional and homogeneous manner [6,19,21,30]. The adsorption capacity at 25 °C, 35 °C, and 45 °C was 80.95 mg/g, 74.54 mg/g, and 60.6 mg/g, respectively, representing a decrease of 7.92% and 25% with increasing temperature, indicating that the operating condition with the higher adsorption capacity is at 25 °C.

In the case of GV adsorption on nDCPD, the kinetic parameters are shown in

Table 6. Kinetic parameters of GV adsorption on nDCPD.

Cads, g/L	4			8			12			16			20
Models	25 °C	35 °C	45 °C	25 °C	35 °C	45 °C	25 °C	35 °C	45 °C	25 °C	35 °C	45 °C	25 °C	35 °C	45 °C
PFO
qe, mg/g	39.531	71.156	73.350	14.049	23.808	43.459	9.8194	20.316	31.264	13.465	8.7534	24.824	6.6049	7.0169	19.965
k1, h−1	0.2712	0.1586	2.0481	0.4931	0.2714	2.6024	0.2887	0.1318	2.0602	0.1008	0.3326	0.5964	0.3231	0.3308	2.0607
R2	0.9358	0.9069	0.9992	0.9530	0.9519	0.9997	0.9753	0.9224	0.9983	0.9550	0.9618	0.9997	0.9743	0.9432	0.9992
∆%q	2.7166	1.4302	2.7649	0.9832	1.2312	2.2289	3.2217	2.1287	1.94787	2.9532	3.8903	2.5432	1.7245	4.0021	3.2121
PSO
qe, mg/g	51.564	100.98	74.871	16.776	31.590	43.837	13.061	30.068	31.903	22.276	11.188	25.091	8.5879	8.9012	20.264
k2, g/mg*h	0.0050	0.0013	0.1234	0.0357	0.0079	0.5068	0.0201	0.0032	0.2942	0.0028	0.0298	1.0000	0.0359	0.0381	0.4688
R2	0.9477	0.9127	0.9999	0.9721	0.9593	0.9998	0.9830	0.9259	0.9989	0.9550	0.9738	0.9986	0.9797	0.9583	0.9997
∆%q	15.1183	13.7320	3.6107	4.2108	2.6589	20.6711	3.68721	8.6205	5.6218	4.7689	9.5471	4.5521	6.7892	5.3240	6.2314
Elovich
β, mg/g*h	0.0915	0.0552	0.3060	0.2967	0.1458	0.5150	0.3362	0.2056	0.7033	0.3166	0.4054	0.8998	0.5161	0.5176	1.1204
α, g/mg	31.360	33.449	33.878	26.486	17.610	20.814	7.1366	8.0390	9.3688	3.7827	8.2037	2.170	5.5979	6.8414	8.578
R2	0.9501	0.8983	0.9996	0.9818	0.9586	0.9974	0.9847	0.9020	0.9986	0.9224	0.9784	0.9922	0.9802	0.9650	0.9994
∆%q	37.8945	23.5621	21.675	15.789	32.780	20.0056	56.8921	23.541	7.9856	21.223	32.8741	19.5632	15.6891	27.892	10.6732
ID
kID, mg/g*h0.5	12.2228	17.4541	22.485	4.7647	7.4133	13.1152	3.387	4.4598	9.6042	2.4640	2.9530	7.3178	2.1504	2.2775	6.1186
	9.7736	0.0000	19.749	1.2655	0.3244	12.2672	0.1120	0.0000	8.3831	0.0000	0.3093	7.4088	0.1813	0.2653	5.3810
C, mg/g	0.9699	0.9275	0.9860	0.9537	0.9727	0.9485	0.9905	0.9305	0.9087	0.9202	0.9836	0.9279	0.9808	0.9737	0.9178
R2	43.5478	33.7651	52.895	45.327	21.225	33.4621	44.7891	22.548	43.6781	30.378	25.6210	41.0956	32.0867	27.980	29.0648
∆%q
ED
kext, h−1	0.0146	0.0210	0.0379	0.0120	0.0178	0.0465	0.0110	0.0158	0.0091	0.0115	0.0138	0.0385	0.0128	0.0138	0.0553
R2	0.7871	0.8630	0.0898	0.5671	0.7986	0.1139	0.7999	0.8888	0.9334	0.9325	0.7436	0.8245	0.7477	0.7368	0.7023
∆%q	73.2522	48.5231	54.320	33.101	42.986	41.9571	29.0786	39.760	46.5908	22.410	29.6741	36.7832	47.3210	20.045	56.4310
Avrami
qe, mg/g	39.531	71.156	73.350	14.049	23.808	43.459	9.8194	20.316	31.264	13.465	8.7534	24.824	6.6049	7.0169	19.965
kA, h−1	0.3600	0.1991	1.3384	0.5329	0.3537	1.5311	0.3139	0.1651	1.3355	0.1293	0.3976	0.5453	0.3870	0.4013	1.3407
nA	0.7535	0.7965	1.5303	0.9254	0.7675	1.6997	0.9198	0.7989	1.5427	0.7804	0.8365	0.9621	0.8347	0.8249	1.5370
R2	0.9358	0.9069	0.9992	0.9530	0.9519	0.9997	0.9753	0.9224	0.9983	0.9550	0.9618	0.9997	0.9743	0.9432	0.9992
∆%q	42.6254	38.5478	39.674	35.783	32.786	29.6560	26.5219	22.337	19.4389	25.674	28.6512	32.6743	33.8901	31.974	28.3678

, where it can be observed that the best fit is presented with the PFO model, which reveals that only one active site is needed for the adsorption of a GV molecule in solution. This may imply that a greater number of adsorption sites are available on the surface of nDCPD. It was also found that n_A < 1 (Avrami model), which indicates that the dye adsorption mechanism tends to change throughout the process [8,9,10,55]. The adsorption capacity was 36.91 mg/g, 55.36 mg/g, and 74.08 mg/g at 25 °C, 35 °C, and 45 °C, respectively, which indicates that increasing the temperature from 25 °C to 45 °C increases the adsorption capacity from 33.33% to 50%, respectively. Although the ID and ED models are not the most suitable, they were taken into account to determine whether there are limitations due to mass transfer [28,43,53].

For this bioadsorbent, the same adsorption mechanisms were found for dye removal with nDCPD. In the removal process, a surface site is required for each dye molecule, and there are no limitations due to internal factors or external mass transfer. With the model obtained in this work, this could be explained by the use of different pre-treatments and the nature of the dye, allowing for a greater interaction between the surface of the bioadsorbent and the solution because of the electrostatic forces created between the two; on the other hand, if the surface is negatively charged, this creates repulsion forces between the dye and the surface, as in our case, where the treatment was carried out on nDCPD [24,27,55,56].

It has been reported in the literature that the adsorption capacity of PR varies from 26.49 mg/g to 142.9 mg/g when using straw biocarbon, rice husk, graphene oxide, bentonite, onion husk, and E. crassipes [6,19,21,45]. In the case of GV adsorption, adsorbents such as lime peel, fish scales, Stachys plant, treated hydroxyapatite, palm tree, pineapple leaves, and shrimp waste have been used, resulting in an adsorption capacity ranging from 2.97 mg/g to 284.1 mg/g [8,13,19,22,54]. Moreover, the surface of nDCPD can be negatively or positively charged depending on the anionic and cationic nature of the dye, facilitating the removal process due to electrostatic attraction [45,46]. Additionally, based on the adjustments of the kinetic models, the removal of both dyes is not only influenced by these forces but also by other physical adsorption processes, ion exchange, or a combination of these [19,55]. This allows for an appreciable adsorption capacity of the dyes, making nDCPD a potential candidate for dye removal from solutions due to its high availability, versatility, cost-effectiveness, and easy access.

3.6. Mass Transfer Limitation Analysis

Within the adsorption process, it is important to study the limitations of mass transfer. A broader analysis was conducted on this phenomenon for the dye removal process. Figure 7 shows the case for PR, where three stages in the dye adsorption process can be observed. At the beginning of the PR removal, all the available sites on the nDCPD surface are free, allowing for instantaneous diffusion. After this stage, gradual adsorption of the dye occurs because the sites become saturated, which slows down the process until it reaches the equilibrium stage. At this point, there are limitations due to mass transfer in the adsorption process [27,45,53].

In performing the analysis of the influence of mass transfer in the GV adsorption process in nDCPD, Figure 8 shows that there are only two stages. The initial presence of the straight line suggests that the adsorption of the dye is accelerated in such a way that equilibrium is reached without the need to go through a slow adsorption, which indicates that mass transfer limitations are present in a large part of the GV removal process, which significantly influences the overall speed of the adsorption process [27,45,53].

Table 7. Kinetic constants for diffusion processes in the adsorption of PR and GV.

Adsorbent	Cads, g/L	kp1, mg/g h0.5	kp2, mg/g h0.5	kp3, mg/g h0.5
nDCPD-GV, 25 °C	4	63.59	------	11.89
	8	33.20		5.302
	12	21.59		2.482
	18	16.19		2.236
	20	10.56		2.230
nDCPD-GV, 35 °C	4	63.50		0.977
	8	29.42		0.198
	12	20.01		0.114
	18	15.02		0.081
	20	12.03		0.076
nDCPD-GV, 45 °C	4	48.70		0.996
	8	23.89		0.707
	12	15.71		0.553
	18	12.44		0.356
	20	9.49		0.341
nDCPD-PR, 25 °C	4	15.56	12.73	0.990
	8	7.37	4.36	0.302
	12	3.26	1.92	0.151
	18	2.53	2.74	‘0.036
	20	2.22	4.89	0.031
nDCPD-PR, 35 °C	4	19.12	8.43	0.765
	8	8.86	5.96	0.383
	12	4.81	3.48	0.153
	18	3.59	2.37	0.104
	20	2.98	1.61	0.038
nDCPD-PR, 45 °C	4	57.23	2.83	0.996
	8	34.74	2.21	0.194
	12	24.38	0.575	0.005
	18	20.56	0.144	0.006
	20	15.58	0.122	0.001

shows the kinetic constants of external (k_p1) and internal (k_p2) diffusion; mainly, in the case of the GV adsorption process in nDCPD, it can be observed that there are no limitations due to internal diffusion because this step in the removal process is not present, from which it can be inferred that the limitations due to mass transfer are only due to external diffusion. For the adsorption of PR, having both kinetic constants of the mass transfer processes, the criterion k_p1/k_p2 > 1 is used, with which it was determined that the process that dominates the overall rate of mass transfer is external diffusion [27,43,45]. These results reveal that the adsorption of dyes is dominated by external diffusion, indicating that the dyes take time to reach the surface of the adsorbent from the core of the solution; therefore, more energy is needed for them to adsorb on the surface of nDCPD, which is directly related to the adsorption capacity [27,43,45].

3.7. Characterization of nDCPD in PR and GV Adsorption

The SEM image of nDCPD (Figure 9a) shows a well-defined monoclinic disk morphology and a rough surface where irregular features can be observed, which are characteristic of brushite [13,18,40,43,50]. In the micrographs of the adsorbent after the removal process, it should be noted that there is no significant change in the morphology of nDCPD (Figure 9b,c); thus, it can be inferred that the temperature and time in which the adsorption of the dyes takes place produces no effect [13,18,40,43,50].

The elemental analysis of the adsorption of both dyes was carried out via X-ray dispersion spectroscopy (EDS), and the results are shown in

Table 8. Elemental analysis of nDCPD before and after adsorption of dyes.

Sample	wt, %
	C	O	Si	Ca	P	Ca/P
nDCPD	37.59	35.28	0.28	15.89	10.96	1.45
RF-nDCPD	25.13	37.92	0.34	21.67	14.94	1.44
GV-nDCPD	20.34	34.92	0.40	26.24	18.10	1.46

, where the C/O, Ca/Si, and Ca/P ratios of the adsorbent can be observed before and after the removal of the dyes. For nDCPD, the C/O ratio is 1.07, which decreases by 38.3% and 45.8% for PR and GV adsorption, respectively; while the Ca/Si ratio is 59.75 for nDCPD, which increases after PR and GV adsorption by 12.3% and ~16%, respectively. These results indicate that there will be a great adsorption capacity due to the participation of functional groups that contain C and O atoms. The slight increase in the presence of Ca and Si atoms in the adsorbent after the removal does not significantly affect the adsorption capacity of nDCPD.

This reveals to us that the anionic and cationic nature of the dyes, together with these functional groups, are responsible for promoting better removal, allowing us to suggest that there could be more than one adsorption mechanism involved, whether via the intervention of electrostatic forces or the participation of the functional groups present on the surface or ion exchange [45,50]. In the case of the Ca/P ratio in all materials, the value referring to brushite is maintained (Ca/P ≈ 1.45), which indicates that the adsorption of the dyes does not affect the crystalline structure of the nDCPD [13,18,40,43,46].

The diffractograms of nDCPD before and after the adsorption of the dyes are shown in Figure 9, containing the peaks at 26° (0 0 2), 28.7° (2 1 0), 32.2° (3 0 0), 34.1° (2 0 2), 40° (3 1 0), 46.7°(2 2 2), 48.3°, 49.7° (2 1 3), and 53.3° (0 0 4), which match the JCPDS (International Center for Diffraction Data Cards) database with respect to brushite [13,18,40,43,58]. The characteristic peaks at 26°, 32.2°, and 34.1 refer to the presence of phosphate groups with an amorphous or semi-crystalline structure for all samples, which confirms what was found in the EDS study, where the particular Ca/P ratio of brushite does not show any change [13,18,40,58].

The infrared spectrum of PR adsorption with nDCPD (Figure 10) shows a band at 3415 cm⁻¹ and a shoulder to 3049 cm⁻¹, which can be attributed to the stretching of the OH- groups of the water molecule and to the surface bonds of the Si-OH groups in brushite, respectively [13,18,40,53,56]. At 2928 cm⁻¹, there is the band corresponding to the stretching of the PO-H bonds that occurs in the HPO groups [13,18,40,53,57,58]. The bands present at 1551 cm⁻¹ and 1449 cm⁻¹ are related to the CO₃²⁻ groups [13,18,40,53,57,58]. The vibration at 1654 cm⁻¹ is related to the physical bonding modes of water, while 1241 cm⁻¹ is due to bending of the O-H plane and P-O-H [13,18,40,53,57,58]. The bands at 1064 cm⁻¹ and 1032 cm⁻¹ are attributed to the stretching of the P-O groups, while the band at 861 cm⁻¹ is related to P-OH stretching in the HPO₄²⁻ groups. The vibrations of the phosphate groups of nDCPD were observed at 598 cm⁻¹ and 555 cm⁻¹ [13,18,40,55,56,57,58]. The infrared spectrum of the adsorption of GV with nDCPD (Figure 11) shows similar bands with respect to those observed in the adsorption of PR, having a variation where the bands appear such that 3294 cm⁻¹ and a shoulder at 3066 cm⁻¹ are attributed to the stretching of the OH- groups of the water molecule and the surface bonds of the Si-OH groups in nDCPD, respectively [13,18,40,55,56,57,58]. At 2921 cm⁻¹, we have the band corresponding to the stretching of the PO-H bonds that occurs in the HPO groups [13,18,40,55,56,57,58]. The band at 1637 cm⁻¹ is related to the physical bonding mode of water, while at 1245 cm⁻¹, it is due to the bending of the O-H and P-O-H plane [13,18,40,55,56,57,58]. The bands present at 1545 cm⁻¹, 1451 cm⁻¹, and 1409 cm⁻¹ are related to CO₃²⁻ groups [13,18,40,55,56,57,58]. The band at 1008 cm⁻¹ is attributed to the stretching of the P-O groups. The band at 954 cm⁻¹ is the band related to the asymmetric extension ν1 of the P-O vibrations of the PO₄³⁻ group, while the band at 861 cm⁻¹ is related to the P-OH stretching in the HPO₄²⁻ groups [13,18,40,55,56,57,58]. The vibrations of the phosphate groups of nDCPD were observed at 685 cm⁻¹ [13,18,40,55,56,57,58].

Within the spectra of Figure 11 and Figure 12, it was observed that several vibrations disappear or decrease their intensity compared to those of nDCPD. Among them are the vibrations assigned to the OH– groups and those related to the phosphate and silicate groups, which confirms that the adsorption mechanism of Pr and GV is carried out by the intervention of these functional groups present on the surface of the adsorbent, which demonstrates that by belonging to the family of nDCPD apatites, it can adsorb these dyes on its surface.

4. Conclusions

This study established that brushite obtained from bovine bone (nDCPD) is a potential candidate to be used as a bioadsorbent in the removal of phenol red and gentian violet from aqueous solutions. The adsorption of these dyes was carried out on a heterogeneous surface using more than one monolayer, removing around 92% of PR and 96% of GV from the solution. PR removal decreases significantly when the temperature it as its maximum adsorption capacity of 82.3 mg/g at 25 °C. In contrast, GV removal has an adsorption capacity of 82.5 mg/g at 45 °C, indicating that dye removal depends on the chemical structure of the dye and its interaction with the nDCPD surface, considering that both processes are spontaneous in nature. Similarly, the kinetic analysis of the PR removal process uses two sites for each dye molecule in the adsorbent to carry out the removal, taking into account that there is a growth of the sites present over the adsorption time (n_A > 1), and in the case of GV removal, only a single site is required to carry out the removal of the dye from the solution, taking into account that there is not only the presence of electrostatic attraction to the nDCPD surface but also more than one adsorption process involved (n_A < 1).

The characterization of nDCPD before and after adsorption allowed for the establishment of a complete overview of the dye removal process, considering the isoelectric point, demonstrating that there are electrostatic forces due to the nature of the dye (anionic and cationic) and a process of ion exchange which involves the functional groups present on the surface, such as phosphate groups, without significantly modifying the crystalline structure and microstructure of nDCPD, which means that it can be inferred that this material is a suitable candidate for the removal of anionic and cationic dyes present in aqueous solutions.

In this study, it was established that nDCPD is a potential candidate for the removal of PR and GV, achieving rates of approximately 92% and 96%, respectively, on the heterogeneous surface of the bioadsorbent. The removal of PR decreases with the increasing temperature, while the removal of GV increases with the rising temperature, indicating that dye removal depends on the chemical structure of the dye. In the kinetic process of PR removal, two sites are utilized, whereas for GV removal, only one site is required, suggesting the involvement of additional adsorption mechanisms. The characterization of nDCPD revealed the presence of electrostatic forces due to the nature of the dye and ionic exchange involving functional groups on the surface (phosphate groups), without significantly altering the crystalline structure and microstructure of nDCPD. This indicates that nDCPD is a suitable candidate for dye removal.