Hybrid Nylon-6/Pumice Nonwoven Composites as Nature-Based Adsorbents for Methylene Blue Dye-Contaminated Wastewater: Insights into Monolayer and Multilayer Adsorption Mechanisms

Abstract

The contamination of water bodies by dye effluents from micro-scale in-house denim laundries remains a significant environmental concern in central México, particularly in the Atoyac River, where conventional treatment methods are not economically viable. This study develops and evaluates Nylon-6/pumice powder (PPw) nonwoven composites as hybrid adsorptive membranes for the removal of methylene blue (MB) from aqueous solutions. Pumice, a locally abundant siliceous mineral, was incorporated into Nylon-6 through melt-compounding and melt-blown fiber processing at 1 wt% and 5 wt% loadings. SEM, XRD, and TGA confirmed even filler distribution, structural stability, and the development of a porous, layered structure. Batch adsorption tests revealed a rapid initial dye adsorption, followed by a slower diffusion-controlled phase, with equilibrium achieved within 15 min for PPw and within 30 min for the composites. The data fitted both Langmuir and Freundlich isotherms, indicating that MB adsorption involved a combined mechanism: monolayer adsorption on uniform silanol/aluminol sites and multilayer physical adsorption at the polymer–mineral interfaces. Higher PPw content increased adsorption capacity (q_max = 1.1460 mg/g) and surface uniformity, resulting in favorable Freundlich exponents (n = 2). Finally, it was found that adsorption proceeds via chemisorption, where the pumice powder provides reactive sites. These findings demonstrate that Nylon-6/PPw nonwoven composites combine the strength of a synthetic material with the surface reactivity of a natural mineral, providing an effective and scalable Nature-Based Solution for decentralized dye removal, aligned with Sustainable Development Goals 6 and 12.

1. Introduction

Safe and clean water is a fundamental human right recognized by Resolution 64/292 [1] of the United Nations, connected to Sustainable Development Goal 6 (SDG 6), which “seeks to ensure safe drinking water and sanitation for all, focusing on the sustainable management of water resources, wastewater and ecosystems, and acknowledging the importance of an enabling environment”. However, nowadays, polluted water harms both human and environmental health in many parts of the world [2,3,4]. This is also true for the Atoyac Basin, including the Atoyac River, located in the east–central region (Puebla, Tlaxcala, and Oaxaca) of México, which is considered an environmental hell [5,6]. Originating in the Iztaccíhuatl–Popocatépetl volcanic range, the Atoyac River was once a vital source of irrigation and drinking water; however, years of industrial, agricultural, and domestic waste have rendered it one of the most contaminated rivers in the country.

Within this basin, the community of San Mateo Ayecac in Tepetitla de Lardizábal, Tlaxcala, has become a critical pollution hotspot, partly due to the presence of over thirty micro-scale in-house denim laundries that perform stonewashing and bleaching on raw denim using medium- to large-capacity washing machines. These micro-scale laundries discharge untreated wastewater directly into urban drains that flow into the Atoyac River. The effluent typically contains synthetic dyes, such as indigo and methylene blue (MB), at concentrations below 10 ppm and with a neutral pH, which are removed during washing, along with detergents, surfactants, bleaching agents, and suspended solids from pumice stones and textile fibers. The water turns intensely blue, with high chemical and biological oxygen demand (COD and BOD), leading to oxygen depletion in water bodies, loss of biodiversity, and soil contamination [7].

Conventional wastewater treatments include coagulation–flocculation, advanced oxidation, membrane filtration, and activated sludge systems [8]. These types of treatments are technically efficient but impractical for micro-scale laundries due to their complex infrastructure and the need for qualified personnel; in addition, they produce secondary waste that must be disposed of. As a result, they are neither accessible nor sustainable. Therefore, small-scale, low-cost wastewater systems are needed that can be appropriated by the community to address dye pollution at its source using materials available in the region.

An alternative to conventional wastewater treatment is Nature-Based Solutions, which use natural materials and processes to address environmental challenges [9,10,11,12]. The International Union for Conservation of Nature (IUCN, 2016) defines NbSs as [10] “actions to protect, sustainably manage, and restore natural or modified ecosystems that address societal challenges effectively and adaptively.” In the case of wastewater treatment, NbS methods include constructed wetlands [13], phytoremediation [14], and biochar adsorption [15], among others. We consider that pumice, a volcanic mineral with large deposits in Central México [16], can be used as a natural material that contributes to mitigating environmental contamination in the Atoyac River and, as such, qualifies as an NbS [17]. Pumice powder (PPw) is mainly composed of silicon and aluminum oxide, with interconnected porosity, a high specific surface area, and a negative charge surface due to the presence of Si-OH (silanol) and Al-OH (aluminol), which gives it a strong affinity to cationic dyes such as MB [18,19].

However, the use of pumice powder in column filters is complicated due to the agglomeration of the powder in the presence of water, probably due to capillary inflow of the water into the pores, increasing the density of the powder, and to capillary forces that create cohesive forces (liquid bridges), limiting its function [20,21]. To overcome this challenge, the PPw can be incorporated into a nonwoven polymer, allowing better handling, high permeability, large surface area, and flexibility of design, ideal for modular filters [22]. Nylon-6 is a potential candidate due to its excellent chemical resistance and processability into fibers and nonwoven fibers [23,24]. Combining Nylon-6 with pumice powder can therefore create a composite medium that combines the adsorptive qualities of a natural material with the structural durability of a synthetic polymer.

This research aims to develop and evaluate Nylon-6/pumice powder (PPw) nonwoven composites as a hybrid adsorptive medium for removing methylene blue from aqueous solutions representative of textile wastewater. The work aims to (i) characterize the morphology, crystalline structure, and thermal stability of the composites; (ii) quantify their adsorption capacity and model their equilibrium behavior using Langmuir and Freundlich isotherms; and (iii) assess their potential as a Nature-Based Solution for decentralized wastewater treatment applicable to micro-scale laundries in the Atoyac River basin. By combining advanced polymer processing with locally available volcanic materials, this study connects material science innovation with sustainable water management and community-level environmental remediation. This study will focus on low MB concentrations, and the effect of pH will not be addressed. This is based on our on-site studies of the physicochemical parameters of wastewater from micro-scale laundries. The results revealed that immediate wastewater neutralization is a common practice, resulting in a pH near 7 (7.39 ± 0.77) and an MB concentration below 10 ppm.

2. Materials and Methods

2.1. Materials

Mineral pumice stone was obtained from a natural deposit in the state of Veracruz, México. The dye methylene blue C₁₆H₁₈N₃SCl.3H₂O was purchased from Jalmek, San Nicolás de los Garza, Nuevo León, México, catalog number A8500, 98% purity (dry basis). Hirlon S.A. de C.V., Ciudad de México, México, provided pellets of Nylon-6 resin. All reagents were of analytical grade and used without further purification. The adsorption solutions were prepared using distilled water (resistivity ≥ 18.2 MΩ·cm).

2.2. Preparation of Nylon-6/Pumice Nonwoven Fabrics

The manufacturing of pumice-loaded Nylon-6 (Ny-6/PPw) nonwoven fabrics (NWF) involved several steps, including pretreatment of materials, melt-compounding extrusion, and melt-blown nonwoven fabrics formation. A detailed description of each step is provided as follows.

2.2.1. Pre-Treatment of Materials

Nylon-6 pellets were vacuum-dried at 80 °C for 12 h to prevent hydrolytic degradation during processing. Pumice stones, averaging 4 inches in size, were ground using a knife mill, model CONAIR JC-1016, Pittsburgh, PA, USA, with a mesh size of +350. This pumice powder (PPw) was used for adsorption tests and to develop nonwoven composite fabric membranes of Ny-6/PPw. The powder used in this work was neither activated nor chemically or physically modified.

2.2.2. Melt-Compounding Extrusion

For processing Ny-6/PPw compounds, a Thermo Scientific TSE-24 MC twin-screw extruder, Waltham, MA, USA with a 24 mm diameter and an L/D ratio of 40:1 was used, featuring two intensive mixing zones. For masterbatch preparation, a coupling with a variable-frequency sonicator operating at frequencies between 15–50 kHz and delivering 750 W of power was installed at the extruder outlet [25].

To prepare the 10% by weight PPw masterbatch, 200 mL of isopropyl alcohol was first added to 150 g of pumice powder, which was then thoroughly mixed. The resulting mixture was immediately added to 1350 g of Ny-6 resin, ensuring uniform mixing. The mixture was then placed in a tray at 90 °C for 12 h to remove the solvent and promote the adhesion of the PP to the pellets. The mixture was then processed in a twin-screw extruder at a temperature of 215 °C, with a 10% feed rate equivalent to 5 kg/h of material, and a mixing speed of 100 rpm. To obtain the 1% and 5% dilutions by weight of pumice powder, 75 g of the masterbatch are mixed with 675 g of Ny-6, and 375 g of the masterbatch with 375 g of Ny-6, respectively.

2.2.3. Nonwoven Fabrics (NWF)

For producing Ny-6/PPw NWF, a combined fiber extrusion line from Fiber Extrusion Technology (Leeds, UK) was used, which includes the FET-100 Extrusion and FET-102 nonwoven modules. This line has a single-screw extruder with a 20 mm screw diameter and an L/D ratio of 30.

The Ny-6/PPw composites were first dried at 120 °C for 12 h to remove moisture, prevent hydrolytic degradation, and promote recrystallization. They were then processed in order of increasing concentration, using a temperature profile of 230–250 °C in the extruder, metering pump, die, and spinneret. An air flow of 1500 L/min at 250 °C was applied to stretch the filaments and form the nonwoven fabric. The NWF was deposited on a conveyor belt moving at 0.6 mpm and finally collected on a cardboard core in a winder. A straight spinneret with 41 holes, each 0.250 mm in diameter and 2.4 mm long, was used. The pressure inside the barrel was maintained at 60 bar, with a flow rate of 4 rpm in the metering pump.

2.3. Characterization

2.3.1. Scanning Electron Microscopy (SEM)

The morphology of the membranes and the dispersion of pumice particles within the fiber matrix were examined using a TOPCON SM-510 Scanning Electron Microscope, Tokyo, Japan, at an accelerating voltage of 5–10 kV, with energy dispersive X-ray microanalysis. The samples were sputter-coated with gold before imaging.

2.3.2. X-Ray Diffraction (XRD)

Crystallinity and phase identification of pumice and composite membranes were performed using XRD (Cu Kα radiation, λ = 1.5406 Å) over a 2θ range of 5–60°, with a step size of 0.02° and 1 s per step, under 40 kV voltage and 25 mA, using a D8 Advanced ECO BRUKER diffractometer, Billerica, MA, USA.

2.3.3. Thermogravimetric Analysis (TGA)

TGA was conducted using a TA Instruments Q500 analyzer, New Castle, DE, USA, in a nitrogen atmosphere from 25 °C to 600 °C at a heating rate of 10 °C/min to evaluate thermal stability and estimate filler content.

2.3.4. Adsorption Assays

Methylene blue solutions in distilled water were prepared at concentrations ranging from 10 to 50 mg/L to determine the adsorption capacity of pumice and its composites. Adsorption tests were conducted at room temperature in a beaker containing a 50 mL MB solution, stirred at 250 rpm, with different amounts of added adsorbents: pumice powder, NWF of Nylon-6, Nylon-6/PPw (1 wt% PPw), and Nylon-6/PPw (5 wt% PPw). For nonwoven fabrics, quantities of 2.5 g were used. Samples were taken every 10 min using a 30 G insulin syringe on a 96-well plate, and adsorption was recorded at 664 nm using an Agilent Biotek Epoch 2 Microplate Spectrophotometer, Santa Clara, CA, USA. The concentration at time t (C_t) was determined from calibration curves. This process was repeated 3 times, and the average values were used for analysis. Data were analyzed using one-way analysis (ANOVA) followed by Tuckey’s test. The significance level was set at 5%, and OriginPro software (version 2021) was used for the analysis (See Supporting Information file).

The percentage removal of dye (%R) and the adsorption capacity q_t were determined using:

$$\% R = {\displaystyle \frac{\left(C_0-C_t\right)}{C_0}} \times 100$$

(1)

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

(2)

where q_t (mg/g) is the adsorption capacity at time t, C₀ and C_t are the initial and time-dependent concentrations (mg/L), V is the volume of solution (L), and m is the membrane mass (g).

2.3.5. Adsorption Analysis

The adsorption results were analyzed using the Langmuir and the Freundlich adsorption isotherms [26,27]. In the first case, it is assumed that adsorption occurs at specific homogeneous sites within the adsorbent, where each site can accommodate only one molecule (monolayer coverage). The equation in this case is

$$q_e={\displaystyle \frac{q_{max}{K}_L{C}_e}{1+K_L{C}_e}}$$

(3)

where $q_e$ is the amount adsorbed at equilibrium (mg/g), $C_e$ is the equilibrium concentration of adsorbate in solution (mg/L), $q_{max}$ is the maximum adsorption capacity (mg/g), and $K_L$ is the Langmuir constant related to affinity (L/mg). The common linear form is:

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

(4)

In the latter, the Freundlich model, it is assumed that adsorption occurs on heterogeneous surfaces, with adsorption sites having different energies, and that multilayer adsorption is possible. In this case, the equation is

$$q_e=K_F{C}_e^{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$n$}\right.}}$$

(5)

where $K_F$ is the Freundlich constant ((mg/g)(L/mg)^1⁄n) indicating adsorption capacity, and $1/n$ is the heterogeneity factor (adsorption intensity). In this case, the linearized form is:

$$ln{q}_e=ln{K}_F+ {\displaystyle \frac{1}{n}}ln{C}_e$$

(6)

In addition, the pseudo-first-order [28] and pseudo-second-order kinetic [29] models were used to determine the adsorption mechanism. The pseudo-first-order model describes physisorption, and assumes that the adsorption rate is proportional to the difference between the adsorption capacity in equilibrium, $q_e$, and the adsorption capacity at time t, $q_t$:

$${\displaystyle \frac{d{q}_t}{dt}}=k_1\left(q_e-q_t\right)$$

(7)

Integrating:

$$ln\left(q_e-q_t\right)=ln{q}_e-k_{1}t$$

(8)

where $k_1$ is the pseudo-first-order constant rate, (1/min).

On the other hand, chemisorption is described by the pseudo-second-order model, which assumes that the kinetics depend on the square difference between the adsorption capacity in equilibrium and the adsorption capacity at time t:

$${\displaystyle \frac{d{q}_t}{dt}}=k_2{\left(q_e-q_t\right)}^2$$

(9)

Integrating:

$${\displaystyle \frac{t}{q_t}}= {\displaystyle \frac{1}{k_2{q}_e^2}}+ {\displaystyle \frac{t}{q_e}}$$

(10)

where $k_2$ is the pseudo-second-order constant rate, (g/(mg min)).

3. Results

3.1. Morphology and PPw Content

3.1.1. Morphology

Figure 1a shows the SEM micrograph of pumice powder magnified 1000 times. Different aggregates were observed, characterized by irregular shapes and varying sizes. The chemical composition, based on EDX results, indicates that oxygen (O) is the most abundant element at 37.18%, followed by silicon (Si) at 32.06%, and carbon (C) at 12.12%. The nonwoven fabric membranes of Nylon-6 are shown in Figure 1b. In this case, the fibers exhibit uniform morphology but vary in diameter. In the micrograph, some fine filaments can be seen along the fiber. When 10 wt% PPw was incorporated into Nylon-6, the fibers became 3 or 4 times thicker (Figure 1c), with clear pumice aggregates visible on the fiber surface, as confirmed by the presence of Si in the EDX results. Additionally, in this case, the fabrics are very fragile and break easily when touched.

3.1.2. X-Ray Diffraction Analysis (XRD)

X-ray diffraction (XRD) patterns of PPw, Ny-6, and the Ny-6/PPw composite nonwoven fabrics are shown in Figure 2. The pumice powder exhibited a broad hump centered around 20–30°, characteristic of its amorphous silicate structure, with weak diffraction peaks. The crystalline pattern corresponds to the standard JCPDS code 00-041-1481, with the main component of pumice being anorthite, a type of natural zeolite composed of sodium, calcium, aluminum, and silicate [30]. The main crystalline plane detected is the (002) plane. The Nylon-6 nonwoven fabric showed two diffraction peaks at 2θ = 20.1° and 23.2°, corresponding to the (200) and (002 + 202) crystal planes of the stable α-crystalline phase of Nylon-6 [31]. Upon incorporation of pumice powder with 1 and 5 wt%, α-crystals remain, but their intensity decreases. Moreover, other crystalline peaks appeared at 2θ = 21.3° and 22.0°, corresponding to the (001) and (200 + 20-1) crystal planes of the less stable γ-crystalline phase of Nylon-6 in the masterbatch, suggesting that 10 wt% pumice powder affects the crystal formation and polymer chain packing of Nylon-6. A similar polymorphic behavior was observed by Andrade-Guel et al. [32].

3.1.3. Pumice Powder Content

The pumice powder content in the fibers was determined using TGA. The thermograms of the samples are shown in Figure 3. In Figure 3a, the thermogram of Nylon-6 showed a gradual decrease in the plateau up to 400 °C. Then, a significant weight loss occurs between 400 and 460 °C, corresponding to the thermal degradation of the polymer backbone. Overall, the degradation behavior of Nylon-6 was not altered by the presence of PPw. Ny-6/PPw nonwoven composite fabrics exhibited a similar weight loss pattern as Nylon-6; however, a second plateau appears after 460 °C, representing the thermally stable inorganic residue (pumice powder). The residual mass at 600 °C increased with higher PPw content, confirming successful incorporation (see Figure 3b). For example, membranes with 0, 1, 5, and 10 wt% pumice powder showed residual masses of approximately 1.01%, 0.91%, 5.27%, and 7.93%, respectively, closely matching the expected theoretical values, especially for the samples with 5 and 10 wt%. In the cases of pure Nylon-6 and the sample with 1 wt% PPw, the difference is unclear, likely due to the small amount of PPw in the polymer matrix.

3.2. Methylene Blue Adsorption Analysis

3.2.1. Removal of Dye

Figure 4 illustrates how the concentration of methylene blue (MB) in solution, C_t, changes over time for various initial dye concentrations and adsorbents: pumice powder (PPw), nonwoven fabrics of Nylon-6, Ny-6/PPw with 1 wt% PPw, and Ny-6/PPw with 5 wt% PPw. In all cases, there is a rapid decrease in C_t during the first few minutes of contact, suggesting that most MB molecules quickly adsorb onto accessible surface sites. Afterward, the adsorption rate gradually slows down until equilibrium is reached, indicating a two-stage kinetic process: initially dominated by surface adsorption, followed by intraparticle diffusion control.

For PPw, equilibrium was achieved within approximately 10–15 min, indicating a high availability of active sites and strong electrostatic interaction between MB⁺ cations and negatively charged silanol (Si–OH) or aluminol (Al–OH) groups. Nylon-6, in contrast, showed limited adsorption with slower kinetics and higher residual dye concentration, attributed to its relatively hydrophobic surface and lack of abundant ionic sites. The hybrid materials with 1 and 5 wt% PPw increased the removal efficiency of MB, thus having a strong adsorption performance and reduced the equilibrium time compared with Nylon-6. This confirms that pumice particles introduce active binding sites and increase surface heterogeneity within the Nylon-6 matrix.

These findings are consistent with the dye removal percentages (%R) shown in Figure 5, where PPw and the Nylon-6/pumice composites exhibited the highest color removal efficiencies at equilibrium. Increasing the initial MB concentration decreased the percentage of R slightly, reflecting site saturation effects at higher surface coverage.

3.2.2. Adsorption Capacity

The adsorption capacity as a function of contact time, q_t, for each material is shown in Figure 6. A steep increase in q_t was observed during the early stage, confirming fast uptake driven by surface diffusion. The curves reached a plateau at equilibrium, corresponding to the maximum adsorption capacity q_e. PPw and the Nylon-6/pumice composites showed significantly higher q_e values than neat Nylon-6, confirming the beneficial effect of the mineral phase on sorption performance. The evolution of q_e as a function of C₀ (Figure 7) demonstrates that adsorption increases with the initial dye concentration, consistent with the progressive filling of active sites.

The equilibrium data were fitted to the linearized Langmuir and the Freundlich models (Equations (4) and (6)) to determine the main adsorption mechanism. The Langmuir model assumes monolayer adsorption on uniformly energetic sites, while the Freundlich model describes adsorption on a heterogeneous surface. The linearized Langmuir plots (C_e/q_e vs. C_e) are shown in Figure S1, and the linearized Freundlich plots (ln q_e vs. ln C_e) are shown in Figure S2. The corresponding fitting parameters are summarized in

Table 1. Fitting parameters obtained using Langmuir and the Freundlich models.

Sample	Langmuir			Freundlich
	qmax	KL	R2	n	KF	R2
PPw	0.9718 ±0.0007	2.9707 ±0.0093	0.9830	1.9923 ±0.0021	0.5496 ±0.0003	0.9637
NWF Ny-6	−14.977 ±0.4151	−261.36 ±6.9033	0.1816	0.9657 ±0.0007	0.2844 ±0.0002	0.9981
NWF Ny-6/PPw, 1% PPw	1.0631 ±0.0026	1.2885 ±0.0145	0.9644	2.1214 ±0.0159	0.6877 ±0.0012	0.9389
NWF Ny-6/PPw, 5% PPw	1.1469 ±0.0014	1.2213 ±0.0076	0.9414	1.9606 ±0.0037	0.7202 ±0.0005	0.9195

.

As summarized in Table 1, PPw showed high correlation coefficients for both models (R² = 0.9830 for the Langmuir model and 0.9637 for the Freundlich model), indicating that MB adsorption mainly occurs through monolayer coverage on a nearly homogeneous surface with moderate heterogeneity. The q_max = 0.9718 mg/g and n = 1.9923 > 1 suggest a favorable and spontaneous adsorption process, while the K_F = 0.5496 confirms moderate affinity typical of aluminosilicate surfaces.

For Nylon-6, the Langmuir correlation was very poor (R² = 0.1816), with physically unrealistic negative parameters (q_max < 0), confirming that the Langmuir model does not apply. The Freundlich model provided an excellent fit (R² = 0.9981), with n = 0.9657, indicating weak, heterogeneous physisorption due to the low polarity of the polymer and limited surface charge.

The Ny-6/PPw (1 wt% PPw) nonwoven fabric showed good correlation, with R² values of 0.9644 (Langmuir) and 0.9381 (Freundlich), confirming that dye adsorption occurs on both the polymer surface and pumice inclusions. In the case of the Freundlich model, it suggests favorable adsorption on a heterogeneous surface involving both electrostatic and physical interactions, indicated by the values of n = 2.1214 and K_F = 0.6877.

The highest adsorption capacity (q_max = 1.1469 mg/g) was obtained for Ny-6/PPw (5 wt% PPw) nonwoven fabric. In addition, a good correlation was observed with both models, suggesting that increasing pumice content increases the number of active sites, resulting in a more uniform surface. Moreover, the value of n = 1.9606 in the Freundlich model also indicates favorable sorption

It is worth noting that, in the case of the Langmuir model, R² and q_max values with higher PPw loading suggests that its addition transforms a weak adsorbent (Nylon-6) into an effective hybrid material that tends to monolayer adsorption. In summary, the proposed adsorption mechanisms of MB with for the different adsorbents are shown in

Table 2. Proposed adsorption mechanisms for the different adsorbents.

Sample	Langmuir	Freundlich
PPw	Langmuir-Freundlich	Monolayer adsorption on silanol/aluminosilicate sites; electrostatic interactions
NWF Ny-6	Freundlich	Weak physisorption on a heterogeneous surface; limited electrostatic affinity
NWF Ny-6/PPw, 1% PPw	Langmuir-Freundlich	Heterogeneous multilayer adsorption
NWF Ny-6/PPw, 5% PPw	Langmuir-Freundlich	Near-monolayer adsorption; surface homogenization via higher pumice content

.

Two kinetic models were used to determine the adsorption mechanism, the pseudo-first-order and the pseudo-second-order models, using their respective linear equations (Equations (8) and (10)) [33]. The results of the fitting for each model for the different samples with varying initial concentrations are shown in

Table 3. Fitting parameters obtained using pseudo-first-order and pseudo-second-order kinetic models.

Sample	qe	Pseudo-First-Order		Pseudo-Second-Order
		k1	R2	k2	R2
PPw
C0, 10 mg/L	0.1862	0.0585	0.0393	0.0065	0.9993
C0, 20 mg/L	0.3674	0.0536	0.0606	0.0493	0.9999
C0, 30 mg/L	0.5274	0.0513	0.2429	0.1458	0.9999
C0, 40 mg/L	0.6476	0.0412	0.0068	0.2650	0.9981
C0, 50 mg/L	0.7918	0.1152	0.1288	0.5083	0.9998
NWF Ny-6
C0, 10 mg/L	0.1473	0.0081	0.4149	0.0032	0.9928
C0, 20 mg/L	0.3001	0.0007	0.5277	0.0269	0.9937
C0, 30 mg/L	0.4506	0.0097	0.6268	0.0906	0.9822
C0, 40 mg/L	0.6027	0.0078	0.5277	0.2522	0.9890
C0, 50 mg/L	0.7447	0.7447	0.5426	0.4008	0.9914
NWF Ny-6/PPw, 1% PPw
C0, 10 mg/L	0.1959	0.0209	0.8513	0.0075	1.0000
C0, 20 mg/L	0.3787	0.0173	0.9445	0.0546	0.9999
C0, 30 mg/L	0.5665	0.0181	0.7401	0.1827	0.9999
C0, 40 mg/L	0.7575	0.0125	0.9149	0.4373	1.0000
C0, 50 mg/L	0.8777	0.0079	0.7871	0.6747	0.9993
NWF Ny-6/PPw, 5% PPw
C0, 10 mg/L	0.1962	0.0276	0.8195	0.0076	1.0000
C0, 20 mg/L	0.3811	0.0185	0.8950	0.0557	1.0000
C0, 30 mg/L	0.5745	0.0166	0.8859	0.1909	1.0000
C0, 40 mg/L	0.7683	0.0049	0.2181	0.4485	0.9998
C0, 50 mg/L	0.9069	0.0148	0.9574	0.7483	0.9999

. It can be seen that the pseudo-second-order kinetic model is the one that better describes the kinetic data, since most of the R² are very close to 1, while the correlation coefficient obtained using the pseudo-first-order model is very low, even though in some cases it is above 0.9.

4. Discussion

4.1. Interpretation of Adsorption Behavior

The adsorption performance of pumice powder, Nylon-6, and Nylon-6/pumice nonwoven composites was examined through kinetic and equilibrium analyses (Figure 4, Figure 5, Figure 6 and Figure 7). The results clearly show different adsorption mechanisms driven by both surface chemistry and morphological features.

The sharp decline in MB concentration (C_t) at early times (Figure 4) indicates that adsorption occurs quickly initially due to the availability of numerous surface sites and strong electrostatic attraction between the cationic MB⁺ molecules and negatively charged sites on the adsorbent surface. This is followed by a slower approach to equilibrium, indicating that intraparticle diffusion and surface rearrangement gradually become the primary rate-controlling processes [34]. This two-stage kinetic behavior has also been observed in natural and composite adsorbents that contain zeolite-based materials, a type of porous silica-aluminate [35].

For PPw, equilibrium was reached within 15 min, confirming a high density of active sites and a strong affinity for MB. Similar kinetic profiles have been reported for pumice, where silanol and aluminol groups dominate surface interactions, enabling the monolayer adsorption of cationic dyes [36]. The Langmuir correlation coefficient (R² = 0.9830) and the Freundlich exponent n = 1.9902 indicate that adsorption is both favorable and primarily occurs as a monolayer.

In contrast, Nylon-6 showed a weak adsorption capacity and a poor fit with the Langmuir model (R² = 0.1816), resulting in negative parameter estimates (q_max and K_L) that lack physical meaning. On the contrary, the excellent fit with the Freundlich model and n ca. 1 suggests heterogeneous physisorption with weak van der Waals and hydrogen bonding interactions. This limited MB-Nylon-6 affinity [37,38] can also be related to the minimal surface charge at a neutral pH.

A significant increase in MB adsorption is shown with the incorporation of PPw into the nonwoven Nylon-6 fabrics. For Ny-6/PPw (1 wt% PPw) the correlation for the Langmuir model improved to R² = 0.9644. On the other hand, the Freundlich model shows good correlation and a higher n value close to 2, suggesting heterogeneous multilayer adsorption. The enhancement results from the presence of silanol and aluminol functional groups on the composite surface, which provide electrostatic adsorption sites for MB⁺ ions [35].

For Ny-6/PPw (5 wt% PPw), the Langmuir (R² = 0.9414) and Freundlich (R² = 0.9195) fits remained satisfactory, indicating a tendency toward homogeneous monolayer adsorption. The high q_max (1.1460 mg/g) and n (ca. 2) suggest favorable adsorption at nearly uniform sites. It is suggested that increasing the filler content enhances site accessibility and uniformity of surface energy.

The improvement in performance with pumice loading aligns with reports on bi-component polymeric membranes [39], where fillers enhance material performance by increasing surface adhesion and interphase interactions.

4.2. Adsorption Mechanisms

The equilibrium isotherm data clearly differentiate between the adsorption mechanisms of the different materials tested. Nylon-6 (physisorption) < PPw (monolayer electrostatic) < Ny-6/PPw (1 wt% PPw) (multilayer heterogeneous) < Ny-6/PPw (5 wt% PPw) (near-monolayer uniform adsorption). The shift from Freundlich to Langmuir behavior with increasing PPw content suggests a gradual homogenization of the adsorption surface due to PPw dispersion on the Nylon-6 surface, thereby decreasing the heterogeneity of the adsorption sites [39]. Silanol and aluminol groups increase the surface charge density, promoting the adsorption of MB through ion exchange, electrostatic interactions, and π–π stacking between aromatic dye rings and amide groups on Nylon-6. Furthermore, the fact that all n values are higher than 1 for all pumice powder-containing samples indicates that adsorption is both favorable and spontaneous. Adsorption affinity is also supported by the increase in K_F values with PPw content.

The fit to a pseudo-second-order model indicates that MB adsorption is primarily governed by surface interactions. This behavior is particularly noticeable in samples containing PPw and is consistent with the presence of silanol and aluminol sites, which can interact with MB cations through electrostatic attraction. In addition, Nylon-6 contributes with amide groups capable of hydrogen bonding and polar interactions. These mechanisms suggest that adsorption proceeds via chemisorption, where the pumice powder provides reactive sites, resulting in efficient dye removal.

4.3. Adsorption Comparison

It is widely recognized that the intrinsic properties of the adsorbent directly influence the adsorption. Characteristics such as specific surface area, porosity, surface charge, polarity, and adsorbent dosage play a pivotal role in determining the extent to which a contaminant can be effectively captured [40]. These parameters not only govern the interaction mechanisms between the adsorbate and the adsorbent but also define the practical efficiency of the materials employed in real treatment scenarios. In this context, the comparatively lower adsorption capacities obtained in this study, relative to several high-performance engineered adsorbents reported in the literature (

Table 4. Maximum methylene blue adsorption capacity of various adsorbents.

Material	Experimental Conditions *	Maximum MB Adsorption Capacity (mg/g)	Ref
Medical Waste Incineration Fly Ash (MWIFA)	40 min, pH 10–12,132 Pt-Co, 7.5 g/L, 300 rpm, room temperature	48.78	[42]
Fig leaf- activated carbon (FLAC-3)	60 min, pH 7, 80 ppm, 0.032 g/L, --, room temperature	41.7	[43]
Chestnut Shell-AC	600 min, pH 12, 1476 ppm, --, --, 25 °C	1191	[44]
Methacryloyloxyethyl -Fe3O4-GO	50 min, pH 11, --, 1.5 g/L, --, --	205	[45]
Soybean hulls	180 min, pH 7, 50 ppm, 1 g/L, 120 rpm, 25 °C	169.90	[46]
Algerian kaolinite (acid treatment)	24 h, unadjusted, 250 ppm, 100 g/L, 200 rpm, ~25 °C	64.58	[47]
Shrimp shells	60 min, pH 8, 50 ppm, 10 g/L, --, 30 °C.	17.6	[48]
Acrylic polymers loaded with magnetic iron manganese oxides (AP/MIMO)	120 min, pH 4–10, 1000 ppm, 0.5 g/L, 600 rpm, 50 °C.	2611.23	[49]
Magnetic clinoptilolite powder (Alg/Clin/Fe3O4)	60 min, pH 10, 10 ppm, 1 g/L, --, 25 °C	12.484	[50]
PPw	20 min, pH 7, 50 ppm, 50 g/L, 250 rpm, 25 °C	0.9718	This study
NWF Ny-6	11 h, pH 7, 50 ppm, 50 g/L, 250 rpm, 25 °C	−14.9977
NWF Ny-6/PPw, 1% PPw	12 h, pH 7, 50 ppm, 50 g/L, 250 rpm, 25 °C	1.0631
NWF Ny-6/PPw, 5% PPw	12 h, pH 7, 50 ppm, 50 g/L, 250 rpm, 25 °C	1.1460

* Experimental conditions order: Contact time, pH, MB initial concentration, adsorbent dosage, stirring rate (rpm), temperature (°C).

), are consistent with a broader shift in current research priorities. Recent efforts have increasingly emphasized the development of sustainable adsorption technologies based on naturally abundant, low-cost materials (see Supporting Information file), aiming to minimize both environmental footprint and economic burden [41], aligning with the NbS approach. Materials such as pumice-based powders and Nylon-derived nonwoven fabrics align with this approach, offering advantages in terms of availability, ease of processing, and scalability. Despite their modest maximum adsorption capacities, the materials evaluated here exhibit performance levels that are adequate for addressing real contamination scenarios, such as small-scale laundries. Notably, the MB concentrations typically observed in effluents discharged into the Atoyac River in Mexico can be effectively treated using the adsorbent dosages and contact times applied in this study. These results demonstrate the practical potential of readily accessible natural materials as feasible alternatives for mitigating dye pollution in this region.

5. Conclusions

The integration of pumice powder into a Nylon-6 nonwoven matrix produced a hybrid adsorbent that rapidly and efficiently removed methylene blue from aqueous media. The overall performance of the Nylon-6/pumice powder nonwoven composites results from synergistic effects, in which pumice powder contributed active hydroxyl groups that enhanced electrostatic interactions and surface uniformity. At the same time, Nylon-6 provided ease of handling and structural integrity. Adsorption occurred quickly in the early stages, followed by a diffusion-controlled phase that reached equilibrium within minutes. The experimental data fit both Langmuir and Freundlich models, revealing a mixed monolayer–multilayer mechanism influenced by pumice content. Moreover, the excellent correlation coefficient obtained when fitting the kinetic data to the pseudo-second-order model suggests that chemisorption is the main adsorption mechanism. The feasibility of a practical route to decentralized, sustainable treatment of dye-contaminated wastewater, consistent with the principles of Nature-Based Solutions, was explored. This study contributes to the design of scalable, low-cost adsorbents derived from locally available materials, thereby strengthening community-based actions for treating textile wastewater and supporting SDGs 6 and 12 by utilizing sustainable materials for water treatment.