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Article

Characterization of Rice Husk-Based Adsorbent for Iodine and Methylene Blue Solutions

by
María Lorena Cadme Arévalo
1,
Raisha Lorena Campisi Cadme
2,
Thais Sarah Arreaga Cadme
3,
Ronald Oswaldo Villamar-Torres
4,
Javier Fernández González
3,
José Benavente Herrera
5,
Alda Geijo López
3,
Sesan Abiodun Aransiola
6 and
Naga Raju Maddela
7,*
1
Faculty of Agricultural and Forestry Sciences, “La María” Campus, Quevedo State Technical University, Quevedo 121250, Los Ríos, Ecuador
2
George Washington University, Washington, DC 20006, USA
3
Department of Materials Science and Physical Chemistry, University of Barcelona, Gran Vía de las Cortes Catalanas 585, 08007 Barcelona, Catalonia, Spain
4
Carrera Agropecuaria, Facultad de Ciencias Pecuarias y Biológicas, “La María” Campus, Quevedo State Technical University, Quevedo 121250, Los Ríos, Ecuador
5
Department of Geodynamics, Faculty of Sciences, University of Granada, 18071 Granada, Granada, Spain
6
Department of Microbiology, Faculty of Science, University of Abuja, Abuja PMB 117, Nigeria
7
Department of Biological Sciences, Faculty of Health Sciences, Universidad Técnica de Manabí, Portoviejo 130105, Los Rios, Ecuador
*
Author to whom correspondence should be addressed.
Processes 2025, 13(9), 2748; https://doi.org/10.3390/pr13092748
Submission received: 25 July 2025 / Revised: 25 August 2025 / Accepted: 26 August 2025 / Published: 28 August 2025
(This article belongs to the Section Separation Processes)
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Versions Notes

Abstract

This study focused on the comprehensive characterization of the adsorbent obtained from rice husk, which was selected for its high adsorption capacity in iodine solution (IS) and methylene blue solution (MBS). This was achieved with adsorbents prepared by a combined treatment involving calcium carbonate prior to carbonization and activation with phosphoric acid. Characterization was performed using advanced techniques, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), laser light diffraction and energy-dispersive X-ray spectroscopy (EDS), which allowed for the evaluation of the adsorbent’s microstructure and composition. The results revealed a complex structure of the adsorbents with interconnected pores, which facilitates efficient adsorption in IS and MBS and the standard indicators to evaluate adsorption capacity. The novelty of this study lies in the application of advanced characterization techniques to optimize the adsorbent properties and understand how preparation conditions affect the adsorbent’s microstructure. The characterized adsorbent materials in this study presented great potential for applications in water treatment and industrial processes, offering an economical and environmentally sustainable solution. Promoting the use of rice husks in the production of adsorbents contributes to the circular economy, reducing production costs and environmental pollution. The results suggested that these materials are effective in the removal of pollutants, which make them relevant for practical applications in water and soil bioremediation.

1. Introduction

The efficient removal of contaminants in aquatic and terrestrial environments is a critical environmental challenge due to the persistence and toxicity of organic and inorganic compounds that affect human health and ecosystems. Various technologies have been developed for environmental remediation, including oxidation processes, photocatalysis, electrocatalysis, adsorption, biological methods, and advanced oxidation, each distinguished by its specific mechanisms and applications [1,2,3,4]. Among these options, adsorption stands out for its operational simplicity, high efficiency, and low cost, which facilitate its application for the selective removal of contaminants in wastewater. Adsorption capacity can be adjusted by optimizing adsorbent materials, with biomass derivatives being particularly attractive due to their sustainability and low cost [5]. In particular, biochar produced from rice husks (hereafter, this material is presented as rice husk-based activated carbon, RHAC) has demonstrated high porosity, stability, and efficiency in removing organic and inorganic contaminants, establishing itself as a promising alternative in environmental management [6]. Research into RHAC has advanced substantially, focusing on improving its adsorption properties through traditional chemical and physical activation; however, most studies have not sufficiently explored innovative pretreatment methods that modify adsorbent microstructure and chemical functionality to optimize adsorption [7,8]. This work introduces a key innovation consisting of a combined pretreatment with calcium carbonate (CaCO3), followed by carbonization and chemical activation with phosphoric acid (H3PO4), which improves the porous structure and functional groups of the adsorbent. This approach increases the efficiency of the selective retention of contaminants in aqueous solutions, expanding the potential applications of RHAC and contributing to the development of more effective and sustainable adsorbents.
Rice is a staple food in many parts of the world [9], and its cultivation generates approximately 107 million tons of husk annually [10], which is inadequately managed. Thus, this agricultural residue results in serious environmental problems, including the emission of organic and inorganic pollutants that affect air, water and soil quality [11,12]. Despite efforts to valorize this agricultural residue, most current strategies do not fully exploit its potential as a raw material to produce a high-quality adsorbent. Rice husk is mainly composed of carbon, hydrogen, nitrogen, oxygen, sulfur, sulfate, chlorine and several oxides (Al, Ti, K, Na, Mg, Si and Ca), whose composition can vary according to its moisture content and specific characteristics [13,14,15,16].
The production of an adsorbent from rice husk is a promising approach for wastewater treatment but faces challenges in terms of efficiency and selectivity. The current literature shows that RHAC adsorbent has been widely investigated, but most studies focused on chemical or physical activation without innovative pretreatment procedures [7,8]. The use of a combined treatment involving CaCO3 prior to carbonization and activation with H3PO4, which could significantly improve the adsorptive properties of this material, was the high point of this study. The process applied in this research was based on consolidated methodologies widely reported in the scientific literature, which uses controlled techniques of carbonization and chemical activation of rice husks at temperatures ranging from 400 to 700 °C, with durations of 1 to 3 h, ensuring the reproducibility and comparability of the results. Relevant data indicate that the use of chemical activating agents such as H3PO4 and other compounds significantly improves the adsorption capacity of RHAC [17], allowing its application in an environment with different pH values [18,19]. Furthermore, modification of RHAC with surfactants has shown an increase in the removal efficiency of dyes such as methylene blue and crystal violet [20,21,22]. RHAC is particularly attractive due to its wide availability and low price, making it more accessible than other feedstocks such as charcoal or sugarcane [21]. In addition, RHAC has been extensively investigated for the removal of contaminants such as arsenic, hexavalent chromium, surfactants and dyes, demonstrating high adsorption efficiency [23,24,25,26,27,28,29,30,31,32].
Combined treatment with CaCO3 prior to carbonization and activation with H3PO4, allows for the modification and optimization of the microstructure and adsorbent functionality of the material. Such a procedure can improve its performance in removing specific contaminants in aqueous solutions, contributing to expanding the knowledge and applications of RHAC. Though the adsorption results in methylene blue solution (MBS) and iodine solution (IS) showed similar capacities among the materials obtained, they are subtle and not directly reflected in marked differences in adsorbent capacity in this research. Consistent results with reports where CaCO3 contributes to improvements in the porous structure and chemical functionality of the carbon are key factors that determine the interaction and retention of iodine and methylene blue adsorbates. When the materials obtained have similar sorption properties, it is difficult to distinguish the relative influence of each synthesis condition on functional performance. Therefore, detailed structural characterization and appropriate statistical methods are essential to differentiate how each variable contributes to adsorptive performance. Thus, even when adsorption capacities appear similar, in-depth analysis reveals how microstructure and surface chemistry, modulated by the synthesis process, optimize the functionality of activated carbon for specific applications.
The synthesized RHAC was characterized using a scanning electron microscope (SEM), atomic force microscope (AFM), laser light diffraction instrument, and energy-dispersive X-ray spectrometer (EDS), which enabled a detailed analysis of the microstructure, morphology, elemental composition, and particle size distribution, which are the fundamental aspects that determine their adsorbent properties. With the current state of knowledge, the integration of in situ studies, which allow for real-time observation of the adsorption process under near-real operating conditions, provides key information on the dynamics, mechanisms, and structural modifications of the material during use, complementing traditional ex situ analyses. This comprehensive approach allows for a precise understanding of how preparation conditions affect the microstructure and adsorbent properties of RHAC, which is essential for optimizing its design and application in environmental and industrial settings, contributing to the development of more efficient adsorbents adapted to contemporary practical needs. Nevertheless, the results obtained justify the applicability of RHAC as an effective bio-adsorbent for the removal of pollutants, contributing to the circular economy and environmental sustainability by promoting the use of agricultural waste to produce high-added-value materials.

2. Materials and Methods

2.1. Carbonization and Activation

The adsorbent material was manufactured at the Materials Science Laboratory of the University of Barcelona (Spain) and the Soil Laboratory of the State Technical University of Quevedo (Ecuador). The material was prepared from ground rice husk desilicated with CaCO3 in a carbon solution ratio of 5:1, at 120 °C for 2 h, before carbonization in a tube furnace under nitrogen (N2). It was then activated with 85% H3PO4 followed by heat treatment in carbon dioxide (CO2). To evaluate the feasibility of the different methods, two additional approaches were compared during carbonization and activation: one using a muffle furnace in the oxidizing atmosphere and the other via pyrolysis. In addition, the effects of particle size (125 µm, 250 µm and 500 µm), temperature (500 °C, 600 °C and 700 °C), time (1, 2 and 3 h) and adsorption capacity of the adsorbent were also investigated. The details of the process and its optimization are described in our previous publication [33].
The treatments and nomenclature established for the carbonized samples were identified (Table 1), considering the origin of the raw material treated: desilicated charcoal (CaD), process used for carbonization (H = tube furnace; M = muffle and CART: artisanal coal), particle size (1 = 0–125 µm; 2 = 126–250 µm; and 5 = 251–500 µm), temperature (5 = 500 °C, 6 = 600 °C, and 7 = 700 °C for tube furnace; 4 = 400 °C, 5 = 500 °C, 6 = 600 °C in muffle), time (1 = 1 h, 2 = 2 h, 3 = 3 h). For example, the desilicated and carbonized scale of 125 µm at 600 °C over 1 h corresponds to the code of ‘CaDH161’. When activated, the letters CAD (desilicated activated husk) were added followed by the letter corresponding to the equipment (H: artisanal oven and M: muffle), then the number corresponding to the temperature and the number corresponding to the time. When passing to the activation process at 700 °C for 2 h, the code corresponding to this is CaDH161-CADH72. Samples were coded considering initial particle size, carbonization activation temperature and time. For instance, CADM542-CADM62 was identified as 251–500 µm scale desilicated with CaCO3, carbonized in a muffle furnace at 400 °C for 2 h and activated at 600 °C for 2 h, and CaDH252-CADH52, 126–250 µm scale was carbonized in a tube furnace at 500 °C for 2 h and activated at 500 °C for 2 h. In the case of artisanal charcoal, the husk was pyrolyzed directly, without modification (desilicated and activated) and then crushed in a manual mill and sieved until the required particle sizes were obtained. The response variable used was yield (%). Each experimental unit started with 3 g of rice husk to obtain adsorbent.
The carbons selected for the carbonization and activation phases were chosen based on the process’s yield per weight, which is considered a key indicator of the conversion efficiency and potential quality of the resulting RHAC, as well as the effective utilization of the raw material. Maximizing yield per weight indirectly implies reducing unwanted interactions with oxygen during thermal processes, which favors the formation of a more compact and oxidation-resistant carbonaceous structure. This strategic selection ensures that the materials analyzed did present optimal structural and chemical characteristics for adsorption applications, guaranteeing both productive efficiency and functional performance.

2.2. Physical Method

This study was carried out at the CCiTUB (Center for Science and Technology of the University of Barcelona, Spain). For the analysis of the uniformity of the particle size distribution, the Laser Diffraction Particle Size Analyzer 13 320 (PNB05577AB ©2011 Beckman Coulter, Inc., Brea, CA, USA) was used. This instrument allowed for a generic calculation and determined the cumulative distribution (average of the coals) identified at what distance the points D10 and D90 were located with respect to D50, using Equation (1). According to the literature [34,35,36], the establishment of ranges of the RSF value “Relative Span Factor” is a statistical parameter determined by laser scattering to describe the particle size distribution in a sample. They are defined as presented in Table 2, and details of the process are described in our previous publication [33].
R S F =   D 90 −   D 10 D 50
In laser diffraction particle size analysis, the D10, D50 and D90 values were statistical parameters that described the particle size distribution of a sample. D10 represented the size below which the smallest 10% of the particles were found, D50 was the average size where 50% of the particles were smaller and 50% were larger, and D90 indicated the size below which 90% of the particles were found. These parameters were crucial for assessing the uniformity and range of particle sizes, which influenced properties such as surface area and reactivity of the material. In the context of laser diffraction analysis, these values allow for a better understanding of the particle size distribution, essential for characterizing and optimizing material properties.

2.3. Adsorption Method

The materials with the best performance (calculated as statistical means) in carbonization and activation were tested to determine the adsorption capacity of the adsorbent by measuring the iodine and methylene blue index, considered as standard indicators of the quality and efficiency of the adsorbent material to retain molecules of different sizes. Iodine is indicative of microporosity and methylene blue of mesoporosity, applying the standards established by the European Federations of Manufacturers that are on par with ASTM (ASTM D2866-11 2018) (https://store.astm.org/d2866-11r18.html (accessed on 2 January 2025). The most adsorbent carbons are presented in Table 1. The results obtained were previously reported [33], where the efficiency of the adsorbent samples in the removal of these pollutants was highlighted. These results coincided with those reported elsewhere [37], indicating iodine values above 900 mg/g with a considerable specific surface area and the presence of smaller micropores, which allowed for the adsorption of iodine molecules with a cross-sectional area of 0.4 nm2.

2.4. Physical Method to Characterize Adsorbent Structure, Morphology and Composition

The morphology and surface structure of the material were analyzed via SEM, resulting in high-resolution images. The samples were coated with a gold layer to facilitate SEM observation using a JEOL J8M-5310, (brand JEOL, model J8M-5310, Tokyo, Japan) equipped with an EDS working at 20 kV. Topography and surface roughness were analyzed using AFM. It was performed using the Multimode8 with Nanoscope V electronics (Bruker, Billerica, MA, USA), taking measurements in several central areas to avoid edge effects. AFM images were obtained in two dimensions (2D) and three dimensions (3D).

2.5. Data Analysis

The data obtained via the different techniques were analyzed using the equipment-specific software for each method. The results were integrated to provide a holistic view of the adsorbent properties. Comparisons between the different characterized parameters (size, morphology and composition) were performed to evaluate their relationship with the adsorptive capacities observed in previous tests. Finally, a validation process was carried out by comparison with previous studies in the scientific literature, thus ensuring the reliability and reproducibility of the results obtained. Statgraphics Centurion software was used to evaluate the effects of the treatments under a completely randomized design. A Tukey test with a significance level of p ≤ 0.05 was applied to detect significant differences between means.

3. Results

3.1. Characterization of Adsorbents

Laser Scattering Analysis

The laser scattering analysis (Table 3) shows that the particles of the samples analyzed did not maintain uniformity, in relation to particle size, that is, how displaced the points D10 and D90 of D50 were using Equation (1). A particle size distribution with a heterogeneous mixture of fine and coarse particles was identified, which might influence the properties such as the surface area and reactivity of the material. The CaDH162-CADH62 material, with an initial particle size of 0–125 µm, was carbonized at 600 °C for 2 h and subsequently activated in a tube furnace under the same conditions. This material recorded the highest RSF value, with an average particle size of 6.7 µm. This result indicated that this adsorbent had a remarkably uniform particle size distribution, probably associated with a lower dispersion in the distribution values. The homogeneity in particle size was evidenced by the low variability between the D10, D50 and D90 percentiles, which confirmed a more homogeneous distribution. However, particles with sizes larger than 6.5 µm tend to have low adsorption capacity and filtration efficiency due to their smaller surface area. These particles tend to have a predominance of mesopores and a limited number of micropores, influencing their ability to adsorb smaller compounds. Despite this, the CaDH162-CADH62 material achieved significant values in MBS (171.8 mg/g) and IS (1001.9 mg/g), suggesting that its porous structure and particle size distribution are suitable for certain adsorption applications.
The efficient adsorption features of RHAC are desirable in industrial and scientific applications with the possibility of use in adsorption processes or in the manufacture of composite materials. CaDM541-CADM53 (starting material of 251–500 µm carbonized at 400 °C for 1 h and activated at 500 °C for 3 h in a muffle furnace presented an RSF value of 1.1 µm, particle size 0.7–2.5 µm) showed a less uniform distribution or greater variability in particle size. It reached a value of 1013.8 mg/g in SI and <171.8 mg/g in MBS. Comparing the results of CaDH252-CADH53 and CaDH252-CADH52 corresponding to the same initial matrix (126–250 µm), carbonized at 500 °C for 2 h and activated at 500 °C for 2 and 3 h, respectively, the same RSF value (4.8 µm) was obtained, corresponding to the range 4.6–6.5 µm, with a medium–large particle size, indicating that the times to which the samples were subjected did not have a great influence on this result; however, it did influence the adsorption capacity. The first material, CaDH252-CADH53, showed an adsorption of 1031.2 mg/g in IS and 176.8 mg/g in MBS, and the second material, CaDH252-CADH52, achieved adsorption of 1003.5 mg/g in IS and <171.8 mg/g in MBS. It was observed that the samples prepared in the tube furnace and muffle furnace showed differences within the range of experimental error or normal variability in preparation and measurement processes (Table 3, Figure 1).
According to the data presented in Table 2 and Table 3, CaDH162-CADH62, with particle size > 6.5 µm, had a smaller surface area, very low adsorption capacity and low filtration efficiency. CaDM541-CADM53, with particles of 0.7–2.5 µm, had higher surface area, high adsorption capacity and slow flow filtration efficiency. They generally exhibited a specific surface area, with intermediate to large size, low adsorption capacity and moderate filtration efficiency to remove certain contaminants. Materials in the 2.6–4.5 µm range have moderate surface area, with small to medium particle size, considerable adsorption capacity and moderate filtration efficiency, which favors interaction with contaminants during adsorption.
On the other hand, the ratio between D50 and D90 can indicate the degree of dispersion of the particle sizes, being a more dispersed distribution. When analyzing CaDH252-CADH53 with D50 121.9 µm and D90 639.4 µm, the D90/D50 ratio indicated a value of 5.24 and determined a particle size distribution with wide variability. The value obtained, greater than 1, indicates a greater dispersion in particle sizes, since there is a greater proportion of larger particles compared to smaller ones. Similar results could be observed in the case of CaDM542-CADH53, with the D50 28.0 µm and D90 58.2 µm D90/D50 ratio indicating a value of 2.08 and, in CaDM541-CADM53, with D50 8.9 µm and D90 13.2 µm D90/D50 ratio with a value of 1.48.
It is worth noting that rice husks, due to their high cellulose and silica content, exhibit thermal behavior that can generate agglomerates during carbonization, especially in fine particles with a high surface-to-volume ratio [22,38]. This phenomenon can affect the porous structure and, therefore, the adsorption capacity of the material. Likewise, small particles tend to adsorb a thin film of water on their surface, acting as a binder and increasing the apparent size of the particles. This character directly influences pore size distribution and the accessibility of active sites, which are fundamental aspects for maximizing adsorption capacity in thermal and adsorption processes, making the behavior of these particles more complex but decisive for the efficiency of the material.

3.2. Analysis Using Scanning Electron Microscopy (SEM)

The analyzed material presented a complex structure of interconnected micropores and mesopores, with an irregular and random distribution of pore sizes. This structure forms interconnected fractures and cavities, which facilitated access and diffusion of adsorbed molecules through the material. The observed surface roughness and honeycomb shape contributed significantly to the adsorption capacity of the material, as they increased the surface area available for interaction with adsorbates. In addition, the cracks separating the honeycomb structures allowed greater accessibility to the pores, which improved adsorption efficiency by allowing molecules to effectively move and trap in the pore spaces. These structural features are fundamental to explain the results obtained in the adsorption of iodine and methylene blue, where the material showed high efficiency in the removal of these dyes (Figure 2).
The adsorbent materials were obtained from desiliconized rice husks through treatment with CaCO3 to reduce their silica content. Microstructural characterization, carried out with techniques such as SEM and laser scattering analysis, made it possible to identify and quantify the presence of micropores and mesopores, which are essential for adsorption. In particular, CaDM263-CADM61 showed a predominance of micropores, reflected in high iodine adsorption (IS) and low methylene blue adsorption (MBS), limiting its use to small molecules. CaDM541-CADM43 presented a balanced structure of micropores and mesopores, with minimal residual silica. Such features are suitable for applications under demanding environmental conditions such as thermal variations and the presence of contaminants. CaDM162-CADM53 stood out for its high porosity and stability, favoring regeneration and reuse in cyclic processes, with balanced adsorptions in IS and MBS. CaDM263-CADM43 showed a mixed amorphous–crystalline structure, attributed to silica residues, combining micropores and mesopores, effective for the adsorption of volatile organic compounds and complex molecules. These results confirmed the viability of rice husk as a source for adsorbents with adjustable properties according to the microstructure and composition.
The SEM results show that CaDH252-CADH53 had a rough surface with a high presence of micropores (confirmed by AFM technique: 270–750 nm), suggesting a high capacity to adsorb volatile organic pollutants and heavy metals. This material reached adsorption values of 1031.2 mg/g in SI and 176.8 mg/g in MBS. On the other hand, CaDH252-CADH52 and CaDH152-CADH61 presented heterogeneous surfaces with macroscopic pores of moderate size (i.e., 75–500 nm and 1000–2000 nm, respectively) and distribution, forming interconnected cavities that enhance adsorption. CaDH252-CADH52 and CaDH152-CADH61 materials showed adsorptions of 1003.5 mg/g and 1001.9 mg/g, respectively, in SI, and values below 171.8 mg/g in MBS. CaDH162-CADH162 exhibited densely packed and interconnected macropores of different sizes (800–1500 nm), facilitating the adsorption of dyes in aqueous solutions, with an adsorption value of 171.8 mg/g in SI. These structural characteristics, together with the adsorption values obtained, indicate that the presence and distribution of micropores and mesopores are key determinants in the adsorption efficiency of these materials for organic pollutants and heavy metals.
CaDH252-CADH51, with sizes between 130 and 700 µm, was formed of a rough surface and a network of channels that facilitated the diffusion of adsorbed molecules. It presented high surface area, favoring adsorption, with a value of 989.7 mg/g in iodine solution (IS) and less than 171.8 mg/g in methylene blue (MBS), indicating selectivity towards small molecules due to the predominance of micropores. On the other hand, CaDH152-CADH53 had larger particles (600–1200 µm), with rough structure and irregular porosity, forming agglomerates, increasing total porosity and surface area, maintaining adsorption values of 973 µm in IS and 186.3 in MBS. This morphology facilitated the adsorption of volatile organic compounds and complex molecules, being suitable for applications requiring high adsorption and regeneration capacity, such as the treatment of contaminated air and water.
CaDH252-CADH53 was characterized by a notable presence of micropores, which favors the efficient adsorption of volatile organic compounds and heavy metals attributed to its high surface area and compact internal structure. In contrast, CaDH252-CADH52 and CaDH152-CADH61 exhibited macroscopic pores of moderate size and distribution that formed interconnected cavities, facilitating the transport and retention of larger molecules. This more open porous configuration suggests high potential for applications requiring the adsorption of bulky compounds, due to greater accessibility and diffusion within the matrix. Therefore, although all adsorbent samples showed complex porous structures, differences in pore scale and organization could determine their adsorption capacities for different types of contaminants under operating conditions. The results obtained in this study are consistent with those reported by Alba et al. [39] who emphasized that the shape and size of the material’s pores were decisive for its effectiveness in remediating environmental contaminants. According to the literature [40,41,42,43,44], surface roughness and structural porosity play a crucial role in the manufacture of biocompatible materials, as increasing these characteristics improves physical contact between the adsorbents.
The specific properties of the RHAC analyzed in this study directly affected their adsorption capacity. According to Nunes and Guerreiro [37] and Baçaoui et al. [45], iodine values above 900 mg/g indicated an adequate surface area and the presence of small micropores capable of adsorbing iodine molecules with a cross-sectional area of approximately 0.4 nm2. In contrast, larger molecules such as methylene blue (cross-sectional area > 2.08 nm2) require adsorbents that have both micropores and larger mesopores for effective adsorption. In line with this, Ebadollahzadeh and Zabihi [46] suggested that the optimization of activated carbon, aimed at maximizing the available surface area, is essential for improving its adsorbent capacity towards different molecules.

3.3. Elemental Composition Analysis Using Energy-Dispersive Spectroscopy (EDS)

In the results shown in Figure 3, an external zone of the coating characterized by wavy surfaces recurrently presented in the RHAC samples analyzed in the different tests was identified. To precisely delimit the curvature of these surfaces, a complementary analysis based on the determination of the gold content present in the particles was used, which allowed for improved visualization and contrast in the images. This study revealed that the RHACs examined share a similar morphology, although the presence of contamination attributable to leaching processes was detected. A considerable presence of C, O was identified along with traces of other elements, such as Mg, Fe, K, P, Al, S, Si, Ca and Cl (Table 4), from the biomass used, which could influence the formation and stability of the porosity, the interaction with contaminants and the adsorbent properties of the material. CaDH252-CADH52 presented a carbon content of 91.4%, while CaDH152-CADH61 showed 71.7%. In contrast, CaDM263-CADM43 had a low carbon content (11.6%) and a high proportion of silicon (54.1%), which is a characteristic component of rice husk and lignocellulosic materials that are not completely desilicated. Oxygen varied between 8.2% and 55.4% depending on the sample. Although gold coating was performed, this result was not presented since it was not part of the composition of the material. In general, variability in elemental composition is directly related to the origin and treatment of the biomass, and these differences can significantly influence the adsorptive capacity and stability of the material.
Valverde and Sarria [15] indicated that the values detected in the elemental composition of RHACs could be attributed to the previous treatments applied to rice husks, which include desilication with CaCO3 and activation with H3PO4. These processes can influence the leaching of certain elements during desilication and washing after activation, as well as causing volatilization and transformation of compounds. Thermochemical studies of rice husks conducted by Díaz Tobar [47] and Beidaghy Dizaji et al. [48] reported the presence of oxides, such as MgO (0.7%), Al2O3 (0.3%), K2O (4.3%), P2O5 (1.1%), and CaO (1.2%). According to several investigations [49,50,51,52,53], Mg, Ca, P, and others are considered “foreign elements” or “natural contaminants” in activated carbon, as they originate from the raw material and can affect both the final properties of the material and its adsorption capacity. Alba et al. [39] reported that the distribution of silica within the carbon structure could improve the structural stability of the material, making it more resistant to degradation during adsorption/desorption cycles. Although silica does not necessarily increase adsorption capacity, its presence can influence pore accessibility and interaction with the adsorbate. However, in general, silica does not reduce the efficiency of adsorbate removal; rather, it can improve the stability of the material, which is beneficial for applications that require multiple cycles of use.

3.4. Analysis of the Physical Composition via Atomic Force Microscopy (AFM)

The largest visible pore size was observed in CaDH152-CADH61 (1.8–3.5 µm), followed by CaDM263-CADM43 (1.5–3.0 µm), while CaDM263-CADM61 presented the smallest pores (<0.0017 µm), with surface roughness of 4.1 nm, asymmetry close to zero (−0.000399) and kurtosis of 3.13. CaDM542-CADM62 showed pores between 0.035 and 0.070 µm, indicating the possible presence of micropores not detected in the analyzed sample. The lowest roughness was recorded in CaDM541-CADM43 (528 nm).
The analysis of the relationship between specific surface area and pore size range provides fundamental information about the adsorbent properties of the material. The results obtained were consistent with the reported values for specific surface area and pore size distribution and were reflected in the adsorption capacity observed in these activated carbons. The relevance of these findings was supported by previous studies demonstrating that materials with similar characteristics are highly effective in applications requiring high adsorption capacity at the molecular level, such as in filtration and separation processes. A larger surface area was directly associated with an increase in adsorption capacity, due to the greater availability of active sites for interaction with dyes, which is key in water remediation applications. This was consistent with the findings of Sevilla [54], who highlighted that micropores, with a porosity greater than 0.2 cm3/g and high internal surface area values, allowed carbon with a larger specific area to have more space to effectively adsorb adsorbates (Table 5).
Kurtosis, which reflects the concentration of peaks and valleys on the surface, varied from high values, i.e., 44, as in CaDH252-CADH52, indicating surfaces with sharp peaks and deep valleys favoring adsorption, to low values as in CaDM542-CADM62 (i.e., 2.73), suggesting flatter and less active surfaces. Asymmetry showed a predominance of peaks in CaDH252-CADH52 (i.e., 5.2) and valleys in CaDM541-CADM43 (i.e., –0.626).
The AFM study identified the suitable microscopic characteristics of the adsorbent material, suggesting its applicability as an adsorbent material for a variety of contaminants. In the present study, most of the RHAC samples exhibited negative asymmetry, suggesting a surface topography characterized by a higher abundance of deep valleys compared to peaks. This morphology may enhance adsorptive capacity, since the valleys act as effective anchor sites for the capture of contaminant molecules, increasing the retention and efficiency of the material. For example, sample CaDH252-CADH53 presented the most pronounced negative asymmetry (i.e., –0.705) in an analyzed area of 10 µm × 550 nm, accompanied by a high adsorption capacity, with values of 1031.2 mg/g in IS and 176.8 mg/g in MBS. These findings suggest that the predominance of deep valleys on the surface is correlated with high adsorptive efficiency against molecules of different sizes. This information provides an in-depth understanding of activated carbon texture and its influence on adsorptive performance, providing a valuable tool for the optimization and design of application-specific adsorbent materials.
In this study, kurtosis was used to evaluate the distribution of heights on the activated carbon surface, reflecting the concentration and shape of peaks and valleys. High kurtosis values (leptokurtic) indicate surfaces with sharp peaks and deep valleys, while low values (platykurtic) correspond to flatter and more uniform surfaces. All samples showed positive kurtosis, with a maximum value of 8.65 in CaDM542-CADM53 (3 × 0.7 µm area) and a minimum of 2.5 in CaDM561-CADM52 (3 × 0.5 µm). These results suggested that the activation process and the pretreatment with CaCO3, which reduced the silica content, influenced the formation of a more heterogeneous and rougher microstructure and favored the appearance of marked peaks and valleys on the surface. This texture could improve the adsorptive capacity by increasing the active surface area and the sites available for dye retention. Importantly, the distribution of surface heights provides a better understanding of topography and its relationship to adsorptive performance (Figure 4).

4. Discussion

4.1. Laser Scattering Analysis

The increase in particle size observed in the samples compared to the initial material was mainly due to agglomeration during sieving, favored by ambient humidity and the entanglement of small particles, as evidenced in AFM images; this could be attributed to particle size but not pore size [55]. Although CaCO3 pretreatment seeks to reduce silica, residual content may remain that affects the morphology [56]. Furthermore, the laser diffraction technique is sensitive to the optical properties of the particles, which can influence the accuracy of the measurements. Factors such as concentration and dispersion medium also affect the dispersion and detection of smaller sizes. These limitations include the characteristics of the techniques and the material, without implying errors in the results.

4.2. Analysis by Scanning Electron Microscope (SEM)

SEM analysis revealed spectral lines with peaks characteristic of the production process and composition of the coals, maintaining a shape similar to a Gaussian curve [57]. The presence of elements, such as C, O, Mg, Fe, K, P, Al, S, Si, Ca and Cl, agreed with previous reports [49,50,51,52,53], which point out that the surface of biochar is enriched with various organic functional groups and inorganic elements that influenced their chemical and physical properties. Thermochemical studies of rice husk confirmed the presence of different oxides in different concentrations, e.g., MgO, 0.7%; Al2O3, 0.3%; K2O, 4.3%; P2O5, 1.1%; and CaO, 1.2%, which can affect the formation and stability of porosity in activated carbons. The results were consistent with the literature [48,58]. This elemental composition, combined with the activation process, contributed to the generation of differentiated porous structures among the samples, directly influencing their adsorptive capacity and specific applications.

4.3. Elemental Composition Analysis by Energy-Dispersive X-Ray Spectrometer (EDS)

The analysis and interpretation of activated carbons were complex due to their marked heterogeneity and the presence of multiple phases, which could generate variability in the results depending on the particles selected for study. In general, activated carbons obtained from lignocellulosic materials usually contain elements other than carbon, derived from the original components of the biomass used as raw material. These elements, considered “natural contaminants” of coal, contribute to the formation of ash after combustion [59]. As reported in several studies [49,50,51,52,53], the surface of biochar is enriched with numerous organic functional groups and inorganic elements, which confer specific characteristics to lignocellulosic materials and can influence their adsorptive capacity and structural stability.
In this study, the carbon values obtained ranged from 11.6% to 91.4%, in line with what has been reported in the literature [60,61]. In addition, studies of the thermochemical composition of rice husks were conducted, showing how thermal processes decompose the organic matter present, with exothermic peaks around 339 °C and 444 °C associated with the oxidation of compounds and the formation of amorphous silica, mainly SiO2, which constituted most of the inorganic fraction of the material. These transformations are key to understanding the structural and adsorbent properties of activated carbon derived from this biomass, as they influence the microstructure and final chemical composition of the adsorbent.

4.4. Physical Composition Analysis by Atomic Force Microscope (AFM)

Micropores with porosity greater than 0.2 cm3/g and high internal specific surface area values suggest that a carbon with higher surface area has more space and, thus, a greater capacity to adsorb the adsorbate. It is worth noting that surface roughness is a key variable in quality control and fluid circulation, as it directly influences the transfer of aqueous solutions during the adsorption process [62]. In fact, higher roughness indicates better molecular adhesion [63], facilitating adsorption on both iodine (IS) and methylene blue (MBS). These findings are in agreement with the results of current research, where samples with higher surface area and roughness presented superior adsorptive capacity in both methods, validating the importance of these properties in the performance of activated carbon. AFM analysis revealed a predominance of macropores in the surface topography of the studied samples. However, to understand the adsorption capacity of iodine, it is crucial to consider its molecular size. Given the 0.4 nm2 ratio of iodine, its entry into the adsorbent material was facilitated by the presence of both micropores (diameters < 2 nm, essential for the adsorption of small molecules) and macropores (diameters > 50 nm, which act as transport pathways).
The results obtained are in line with studies indicating that high temperatures and long activation times promoted greater loss of volatile material, which reduced the performance of activated carbon [8,64,65]. Surculento Villalobos et al. [66] showed that carbonization at 500 °C optimized performance by preserving volatile components, while Valverde and Sarria [15] highlighted that between 200 and 300 °C, combustion caused loss of volatile compounds. At a temperature of 500 °C, glycosidic bonds were broken and cellulose was partially depolymerized, which were essential for the porous structure [67]. Likewise, chemical activation with phosphoric acid modified the residual acidity and affected the quality and performance of activated carbon [68,69], while Farrera et al. [70] related the activation method with the amount of ash produced. Based on these observations, it can be summarized that porous structure and physicochemical properties vary depending on the activation method and conditions, which emphasizes the importance of controlling these parameters to optimize the adsorptive characteristics of rice husk-derived biochar.
Our results suggested that, despite the surface morphological variability observed by AFM, the high adsorption capacity of iodine in various samples implied a porous network that guarantees accessibility to the micropores. This aligns with the literature indicating that iodine can efficiently permeate structures with a combination of micropores and macropores [37]. The consistency of this high iodine adsorption in most of the studied samples in this investigation, regardless of slight variations reported in thermal treatments, suggested that the carbonization and activation processes were effective in creating a porous structure suitable for adsorption of small- and medium-sized molecules. The main factors influencing the adsorption were the particle size of the initial material, the temperature and time of carbonization and activation, which affected the formation and development of the microstructure of activated carbon, specifically the specific surface area, porosity, and surface chemical composition. The mechanism underlying the observed differences lies in how these factors modulate the physical and chemical structure of the adsorbent: smaller particle size improves homogeneity and surface area; appropriate temperatures and times optimize thermal decomposition, the generation of micropores and mesopores, as well as the retention of active functional groups. These structural and chemical variations determined the quantity and accessibility of active sites for adsorption, directly impacting the efficiency of the process. The macropores facilitated the transport of adsorbate into the micropores, where most of the adsorption occurred. Methylene blue maintained an area of 2.08 nm2, making it easier for it to enter the adsorbent material, influencing the values obtained in the adsorption tests on IS and MBS. These observations were in agreement with the literature [71], where it was mentioned that the pore size distribution in the final adsorbent was strongly influenced by the degree of impregnation, that is, the higher the impregnation, the larger the pore diameter.

5. Conclusions

  • The analyzed adsorbent materials exhibited a complex structure characterized by a diversity of interconnected pore sizes distributed in an irregular (roughness) and random (asymmetry) manner, forming fractures and interconnected alveolar cavities, as well as cracks that separate the different regions. These structural features facilitated the mobilization and trapping of the adsorbate through the pore spaces, which was validated by the adsorption results in IS and MBS. The results suggested that these materials have a considerable adsorbent capacity for various contaminating elements, which makes them potentially applicable in adsorption processes.
  • The results obtained provided reliable information on the characterization techniques to confirm the adsorption information of the analyzed materials. This made their uses viable to reduce contaminants of interest in the production of filters on a domestic and commercial scale. In addition, the application of these materials in adsorption processes could be beneficial for the removal of contaminants in various environmental matrices.
  • This study revealed that the materials analyzed have a complex porous structure, characterized mainly by a predominance of macropores, together with a limited presence of micropores and mesopores. This porous configuration, together with a high surface roughness, suggested a significant capacity to adsorb contaminants, especially those of larger molecular size. The abundance of macropores facilitates the transport and access of molecules to the active adsorption sites, while micropores, although less abundant, contribute to the retention of small molecules. Despite the demonstrated potential for contaminant removal, the large-scale application of these materials requires consideration of practical aspects such as cost-effectiveness, availability of raw materials and the necessary infrastructure for their production and industrial implementation. In addition, it is essential to evaluate the stability of the material under conditions of prolonged use and its regeneration capacity, aspects that directly impact its efficiency and useful life.
  • Throughout the investigation, several challenges and limitations were encountered that were crucial for the development of this study. These included the complexity in optimizing the experimental conditions and interpreting the adsorption data, especially due to the complex porous structure of the materials. Despite these challenges, the research managed to provide valuable information on the potential of the materials for contaminant removal. Future research should focus on overcoming these limitations to improve the efficiency and applicability of these materials in practical contexts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13092748/s1, Figure S1: Images of laser scattering lines. (a) CADM542-CAD62; (b) CaDM263-CAD61; (c) CaDM541-CAD43; (d) CaDM263-CAD43; (e) CaDH252-CAD53; (f) CaDM541-CAD53; (g) CaDH252-CAD52; (h) CaDH4152-CAD61; (i) CaDH162-CD62; (j) CaDH252-CAD51; (k) CaDH152-CAD53; (l) CaDM542-CAD53; Figure S2: Scanning electron microscope (SEM) images of different adsorbents at four different magnifications; Figure S3: Images of different rice husk-based activated carbon (RHACs) adsorbents’ elemental composition as determined by Energy Dispersive X-ray Spectroscopy (EDS); Figure S4: Images of Physical composition analysis of adsorbents by Atomic Force Microscopy (AFM).

Author Contributions

Conceptualization, J.F.G.; methodology, M.L.C.A.; validation, A.G.L. and J.F.G.; formal analysis, R.O.V.-T.; investigation, M.L.C.A. and R.L.C.C.; resources, M.L.C.A.; data curation, M.L.C.A. and J.B.H.; writing—original draft preparation, M.L.C.A., R.L.C.C. and T.S.A.C.; writing—review and editing, J.F.G., S.A.A. and N.R.M.; supervision, J.F.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article. The original contributions presented in this study are included in the article.

Acknowledgments

We thank the authorities of the State Technical University of Quevedo, Javier Fernández (University of Barcelona) as tutor and José Banavente (University of Granada) as co-tutor, for their trust and support in providing facilities, equipment, analysis, among others, which allowed us to develop the research work entitled “Rice husk activated carbon for the removal of pollutants in deep well water”, prior to obtaining the title of doctor in Earth Sciences.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Adsorbent’s particle sizes determined by laser scattering lines. (a) CADM542-CAD62; (b) CaDM263-CAD61; (c) CaDM541-CAD43; (d) CaDM263-CAD43; (e) CaDH252-CAD53; (f) CaDM541-CAD53; (g) CaDH252-CAD52; (h) CaDH4152-CAD61; (i) CaDH162-CD62; (j) CaDH252-CAD51; (k) CaDH152-CAD53; (l) CaDM542-CAD53. Raw images are provided in Supplementary Figure S1.
Figure 1. Adsorbent’s particle sizes determined by laser scattering lines. (a) CADM542-CAD62; (b) CaDM263-CAD61; (c) CaDM541-CAD43; (d) CaDM263-CAD43; (e) CaDH252-CAD53; (f) CaDM541-CAD53; (g) CaDH252-CAD52; (h) CaDH4152-CAD61; (i) CaDH162-CD62; (j) CaDH252-CAD51; (k) CaDH152-CAD53; (l) CaDM542-CAD53. Raw images are provided in Supplementary Figure S1.
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Figure 2. Scanning electron microscope (SEM) images of different adsorbents at four different magnifications. Values (µm) inside each image panel indicate scale bar (yellow arrow) values. Raw SEM images are provided in Supplementary Figure S2.
Figure 2. Scanning electron microscope (SEM) images of different adsorbents at four different magnifications. Values (µm) inside each image panel indicate scale bar (yellow arrow) values. Raw SEM images are provided in Supplementary Figure S2.
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Figure 3. Images of different rice husk-based activated carbon (RHACs) adsorbents’ elemental composition as determined by energy-dispersive X-ray spectroscopy (EDS). Values (µm) in each photograph indicate scale bar (yellow arrow) values. Raw images are supplied in Supplementary Figure S3.
Figure 3. Images of different rice husk-based activated carbon (RHACs) adsorbents’ elemental composition as determined by energy-dispersive X-ray spectroscopy (EDS). Values (µm) in each photograph indicate scale bar (yellow arrow) values. Raw images are supplied in Supplementary Figure S3.
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Figure 4. Physical composition analysis of adsorbents by atomic force microscopy (AFM). Enlarged images of each panel are supplied in Supplementary Figure S4. These surface properties, linked to the activation method and porous structure, directly influence the adsorptive capacity and selectivity against contaminants. Overall, the analysis of kurtosis and asymmetry allowed us to evaluate and compare the surface texture of the activated carbons, relating their physical characteristics to their performance in the removal of contaminants.
Figure 4. Physical composition analysis of adsorbents by atomic force microscopy (AFM). Enlarged images of each panel are supplied in Supplementary Figure S4. These surface properties, linked to the activation method and porous structure, directly influence the adsorptive capacity and selectivity against contaminants. Overall, the analysis of kurtosis and asymmetry allowed us to evaluate and compare the surface texture of the activated carbons, relating their physical characteristics to their performance in the removal of contaminants.
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Table 1. Activated carbon with greater adsorption in solutions of iodine and methylene blue [33].
Table 1. Activated carbon with greater adsorption in solutions of iodine and methylene blue [33].
IDIodine Solution (IS)Methylene Blue Solution (MBS)
mAC * (g)Volume (mL)Iodine (mg/g)IDmAC (g)Volume (mL)Value (%)Adsorption Capacity ‘q’ (mg/g)
CaDH162-CADH531.185.81094.8CaDM561-CADM520.5100096.8193.7
CaDM542-CADM621.195.81085.6CaDM542-CADM530.5100094.6189.3
CaDM263-CADM611.156.71074.0CaDM541-CADM430.5100093.9187.8
CaDM541-CADM431.216.11051.5CaDH152-CADH530.5100093.2186.3
CaDM162-CADM531.167.11043.2CaDM162-CADM530.5100092.3184.6
CaDM263-CADM431.206.51039.1CaDH162-CADH530.5100091.2182.5
CaDH252-CADH531.05.11031.2CaDM542-CADM620.5100088.7177.4
CaDM541-CADM531.235.51013.8CaDH252-CADH530.5100088.4176.8
CaDH252-CADH521.207.21003.5CaDM263-CADM430.5100088.0176.0
CaDH152-CADH611.404.31001.9CART10.5100087.8175.7
CaDH162-CAD1621.276.11001.9CaDH162-CADH620.5100085.9171.8
CaDH252-CADH511.178989.7----------
CaDH152-CADH531.198973----------
CaDM542-CADM531.208964----------
* Activated carbon mass.
Table 2. Relative span factor (RSF) range of particle size via laser scattering and characteristics of adsorbent material.
Table 2. Relative span factor (RSF) range of particle size via laser scattering and characteristics of adsorbent material.
RSF Range (µm)Feature
0.7–2.5Very small particle size, high adsorption capacity and filtration efficiency. Greater surface area per unit volume. They increase the pressure drop in filtration systems and hinder the flow of liquid through the activated carbon bed. They are inefficient in filtration with high flow. At slow flow, it improves adsorption kinetics and speed.
2.6–4.5Size from small to medium. Moderate adsorption capacity and filtration efficiency. They facilitate interaction with contaminants during adsorption. They have a moderate surface area (500–1000 m2/g). It maintains a greater effective adsorption capacity, but less than that of very small particles. They present a combination of micropores (<2 nm) and mesopores (2–50 nm).
4.6–6.5Intermediate to large size, low adsorption capacity and low filtration efficiency. It limits its effectiveness in eliminating certain contaminants. They usually have a specific surface area of 500–1000 m2/g with good adsorption capacity. It has micropores of <2 nm and mesopores of 250 nm.
>6.5Very large particle size, very low adsorption capacity and low filtration efficiency. They have a lower surface area, with a predominance of mesopores and a limited number of micropores (<2 nm), affecting their adsorption capacity of smaller compounds.
Table 3. Relative span factor (RSF) value of particle size via laser scattering.
Table 3. Relative span factor (RSF) value of particle size via laser scattering.
Adsorbent CodeD10 (µm)D50 (µm)D90 (µm)RSF (µm)
CADM542-CADM6211.660.9131.52.0
CaDM263-CADM6177.6232.7570.32.1
CaDM541-CADM324.973.3502.46.5
CaDM263-CADM4318.2153.5403.12.5
CaDH252-CADH5350.2121.9639.44.8
CaDM541-CADM533.18.913.21.1
CaDH252-CADH5252.4137.6714.14.8
CaDH152-CADH6148.881.0419.44.6
CaDH162-CADH6246.971.51524.76.7
CaDH252-CADH5141.565.985.20.7
CaDH152-CADH5349.197.6403.53.6
CaDM542-CADH534.928.058.21.9
Table 4. Elemental composition of different rice husk-based activated carbon (RHACs) adsorbents as determined by energy-dispersive X-ray spectroscopy (EDS).
Table 4. Elemental composition of different rice husk-based activated carbon (RHACs) adsorbents as determined by energy-dispersive X-ray spectroscopy (EDS).
RHAC IDAverage Weight (%)Activation Performance (%)
CONaMgSiKPCa
CaDM542-CADM6212.3152.354.00.5628.02-1.351.4261.42
CaDM263-CADM6170.6020.88-0.765.7--2.5092.83
CaDM541-CADM4336.1143.071.740.3317.43-0.620.7067.73
CaDM263-CADM4311.6334.19--54.18---60.36
CaDH252-CADH5370.2424.090.22-4.13-0.450.8794.86
CaDM541-CADM5315.6250.251.134.2115.26--13.5379.57
CaDH252-CADH5291.428.160.040.100.06--0.2197.08
CaDH152-CADH6171.7226.541.05-0.290.06-0.3591.36
CaDH162-CADH6255.6732.521.25-10.56---95.47
CaDH252-CADH5139.3255.420.050.094.850.21-0.0794.49
CaDH152-CADH5348.9036.65--13.820.63--96.52
CaDM542-CADM5317.3848.506.222.1319.21--6.5690.66
Table 5. Physical composition of different rice husk-based activated carbon (RHAC) adsorbents as determined via atomic force microscopy (AFM).
Table 5. Physical composition of different rice husk-based activated carbon (RHAC) adsorbents as determined via atomic force microscopy (AFM).
RHAC Id3D Image2D Image
Survey Area (µm)Pore Size (µm)Survey Area (µm)Average (nm)Standard Deviation (nm)Surface (µm2)Projected Surface (µm2)Rugosity (nm)AsymmetryKurtosis
CaDM542-CADM623.0 × 3.00.035–0.0703.0 × 0.2274.417.50.01980.0093179.50.005832.73
CaDM263-CADM611.0 × 1.00.0017–0.01.0 × 1.70.00740.43764.0744.14.1−0003993.13
CaDM541-CADM435.0 × 5.00.2–0.45.0 × 0.510119.20.06250.0422528−0.6262.62
CaDM263-CADM434.0 × 3.81.5–3.04.0 × 3.026937.80.10400570215−0.5863.74
CaDH252-CADH53|5.0 × 5.00.38–0.755.0 × 0.7581.538.40.6450.5321740.4743.34
CaDM541-CADM5310.0 × 7.00.37–0.7510.0 × 3.05321990.1210.0510488−0.5496.38
CaDH252-CADH5210.0 × 12.00.25–0.510.0 × 0.521.810.32.602.531165.244
CaDH152-CADH6110.0 × 10.01.0–5.010.0 × 57743590.6160.2391206−1.095.29
CaDH162-CADH6210.0 × 10.00.8–1.510.0 × 1.53421250.8100.3305860.2433.55
CaDH252-CADH515.0 × 5.00.35–0.710 × 0.717331.90.5840.5081800.3163.13
CaDH152-CADH5310.0 × 10.00.6–1.23.0 × 1.218959.50.3210.217292−0.1642.81
CaDM542-CADM5310.0 × 6.00.35–0.73.0 × 0.777.215.70.1180.10580.7−1.668.65
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Cadme Arévalo, M.L.; Campisi Cadme, R.L.; Arreaga Cadme, T.S.; Villamar-Torres, R.O.; Fernández González, J.; Benavente Herrera, J.; Geijo López, A.; Aransiola, S.A.; Maddela, N.R. Characterization of Rice Husk-Based Adsorbent for Iodine and Methylene Blue Solutions. Processes 2025, 13, 2748. https://doi.org/10.3390/pr13092748

AMA Style

Cadme Arévalo ML, Campisi Cadme RL, Arreaga Cadme TS, Villamar-Torres RO, Fernández González J, Benavente Herrera J, Geijo López A, Aransiola SA, Maddela NR. Characterization of Rice Husk-Based Adsorbent for Iodine and Methylene Blue Solutions. Processes. 2025; 13(9):2748. https://doi.org/10.3390/pr13092748

Chicago/Turabian Style

Cadme Arévalo, María Lorena, Raisha Lorena Campisi Cadme, Thais Sarah Arreaga Cadme, Ronald Oswaldo Villamar-Torres, Javier Fernández González, José Benavente Herrera, Alda Geijo López, Sesan Abiodun Aransiola, and Naga Raju Maddela. 2025. "Characterization of Rice Husk-Based Adsorbent for Iodine and Methylene Blue Solutions" Processes 13, no. 9: 2748. https://doi.org/10.3390/pr13092748

APA Style

Cadme Arévalo, M. L., Campisi Cadme, R. L., Arreaga Cadme, T. S., Villamar-Torres, R. O., Fernández González, J., Benavente Herrera, J., Geijo López, A., Aransiola, S. A., & Maddela, N. R. (2025). Characterization of Rice Husk-Based Adsorbent for Iodine and Methylene Blue Solutions. Processes, 13(9), 2748. https://doi.org/10.3390/pr13092748

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