Sustainable and bio-based air purification materials have emerged as a compelling response to the growing demand for environmentally responsible solutions to indoor air pollution. These materials are derived from renewable resources, exhibit low energy requirements during production, and are often biodegradable or recyclable at the end of their lifecycle. Their integration into building systems not only reduces environmental impacts but also enhances the quality of indoor air.

Examplesinclude activated carbon produced from agricultural residues, biochar, and plant-based media such as moss and bamboo, which exhibit natural capabilities for capturing and degrading pollutants (Schripp et al., 2017). Such materials support passive purification by harnessing biological processes such as biofiltration and phytoremediation, allowing for the removal of volatile organic compounds (VOCs), carbon dioxide (CO₂), and particulate matter.

The increasing demand for energy-efficient buildings has further stimulated interest in air purification materials that complement sustainable architectural practices. These materials offer the dual benefits of reducing pollutant levels and supporting global objectives for emission reduction and environmental health (Pal et al., 2020). Their use aligns with contemporary green building frameworks and represents a significant shift towards passive, self-sustaining air purification mechanisms in the built environment.

IAQ plays a fundamental role in determining the health, comfort, and productivity of building occupants. Conventional air purification systems, which rely heavily on mechanical filtration and synthetic media, may not be energy-efficient or sustainable in the long term. In contrast, bio-based materials provide a natural, low-energy alternative that continuously filters air without intensive maintenance or power consumption.

Biological air purification systems—including green walls and bioactive indoor plants—function through natural absorption and metabolic degradation of pollutants such as CO₂, formaldehyde, and benzene (Wang et al., 2014). These systems not only improve air quality but also regulate indoor humidity, thereby contributing to thermal comfort and health. For instance, moss- or algae-based purification systems have demonstrated potential in absorbing substantial amounts of CO₂ while operating autonomously, with minimal energy input. Additionally, biofilters integrated into HVAC systems use biological agents—typically microorganisms like bacteria and fungi—to metabolize airborne pollutants. These systems are effective in decomposing VOCs, ammonia, and other common indoor contaminants (Gullón et al., 2017). Incorporating such purification strategies into architectural elements (e.g., walls, ceilings, ventilation ducts) enables a consistent and passive purification process, reducing reliance on synthetic filters and chemical agents.

Thus, sustainable and bio-based materials present a transformative approach to IAQ management, with implications for public health, energy-building performance, and environmental impact. Their scalability, cost-effectiveness, and ability to work under typical indoor conditions make them highly attractive for contemporary architectural applications.

Despite their potential, the adoption of sustainable and bio-based purification materials in buildings is accompanied by several technical and logistical challenges. These include high initial production and installation costs, variability in performance across environmental conditions, and concerns about long-term durability—particularly in living or biologically active systems. Furthermore, scalability remains a concern, as most bio-based solutions are still under pilot or small-scale applications, lacking integration into mainstream construction practices.

Nevertheless, these challenges are counterbalanced by a number of emerging opportunities. Advances in bioengineering and nanomaterials have enabled the design of more robust, high-performing bio-based adsorbents (Soreanu et al., 2013). Simultaneously, global demand for healthier indoor environments and greater occupant well-being has fueled investment in nature-based solutions. Policies promoting green construction and incentives tied to certification systems such as LEED and WELL have further supported this trend (Geng et al., 2019).

Importantly, the use of renewable and waste-derived feedstocks contributes to circular economy objectives and waste valorization strategies in construction (Nanda et al., 2016). Table 2 summarizes the principal challenges and opportunities associated with the use of bio-based purification materials in architectural contexts.

The first phase of this study focuses on the structural enhancement of puffed rice to serve as the primary substrate for bio-based air purification applications. The objective of this phase is to optimize the physical characteristics of the rice media—specifically porosity, surface area, and mechanical integrity—to facilitate efficient CO₂ adsorption in indoor environments.

Puffed rice, sourced from puffed waste rice, underwent a series of standardized pre-treatment and thermal activation procedures. The process draws upon conventional biomass activation techniques and is adapted to improve the performance of the material under ambient conditions (Koul et al., 2022; Reza et al., 2020; Ahmed et al., 2018).

The activation protocol consisted of the following sequential steps.

• Moisture Reduction: The cleaned rice was air-dried at ambient temperature until moisture content was reduced to below 16%. Maintaining controlled moisture levels was essential to facilitate uniform expansion and effective pore development during heating.

The outcome of Phase 1 established the optimal thermal processing parameters necessary to yield a structurally robust and porous substrate, ensuring high suitability for subsequent hydroxylation treatments aimed at enhancing CO₂ adsorption efficiency.

The second phase of the methodology involved chemical activation of the puffed rice media to improve its physicochemical properties—specifically pore volume, surface area, and functional group availability—all essential for optimizing gas-phase CO₂ adsorption. Alkaline activation was employed due to its well-established capacity to enhance adsorption potential through surface modification (Zhao et al., 2013).

Chemical activation processes are broadly classified as single-stage or dual-stage, depending on whether thermal carbonization precedes the chemical treatment (González-Martín et al., 2021). In this study, a single-stage activation approach was adopted, in which pre-expanded puffed rice samples were directly immersed in aqueous solutions of sodium hydroxide (NaOH). This choice was motivated by the objective of maintaining moderate processing conditions compatible with sustainable, low-energy material fabrication. To establish an alkaline surface environment favorable for CO₂ interaction—given its acidic molecular nature—the puffed rice samples were functionalized with hydroxyl groups. Sodium hydroxide solutions of varying molar concentrations (0.25 M–2.5 M) were prepared in 500 mL batches. Circular-shaped samples of puffed rice (8–10 cm diameter; 1.2 cm thickness) were immersed in each solution using a customized immersion apparatus that ensured consistent exposure and saturation (Figure 2).

Following the immersion treatment, the samples were rinsed and air-dried. Both treated and untreated samples were subjected to comparative characterization to quantify the chemical activation’s effectiveness.

• Fourier Transform Infrared Spectroscopy (FTIR): Conducted using an IRTracer-100 spectrometer (Shimadzu, Japan), FTIR analysis was used to identify changes in functional groups post-treatment, particularly the incorporation of hydroxyl (-OH) and carboxyl functionalities.

To determine the functional performance of the treated puffed rice media, a series of controlled experiments were conducted using a laboratory-scale CO₂ contactor system. This phase aimed to quantify the adsorption capacity of the bio-based material under standardized conditions simulating indoor air quality environments. The testing apparatus consisted of a transparent plexiglass column designed to facilitate vertical gas flow through a packed bed of the adsorbent material as shown in Figure 3.

A pre-mixed gas containing 10% CO₂ by volume was introduced at a constant flow rate from a certified pressurized cylinder. The puffed rice medium—either untreated or chemically functionalized with NaOH—was packed into the column to a height of 20 cm, accounting for approximately 20% of the total column volume. Each experimental run lasted 6 h, during which the concentration of CO₂ at the column outlet was continuously monitored using a calibrated infrared CO₂ gas analyzer. Separate tests were performed for each variant of NaOH-treated samples (0.25 M–2.5 M), along with untreated control samples. The objective was to evaluate the relationship between hydroxylation level and CO₂ adsorption capacity.

This phase enabled direct performance comparison across various treatment levels, confirming the effectiveness of chemical functionalization in enhancing the CO₂ capture capacity of the puffed rice-based purification medium under realistic environmental conditions.

To contextualize the performance and applicability of the developed puffed rice-based purification media, a comparative evaluation is undertaken against a range of benchmark activated bio-based materials widely utilized for carbon dioxide (CO₂) adsorption in indoor air quality enhancement. This analysis emphasizes a suite of physicochemical and operational metrics, including specific surface area, adsorption capacity, activation conditions, and sustainability considerations.

The critical parameters examined encompass.

• Surface Area (m ² /g): A primary determinant of adsorption performance, as higher surface areas facilitate greater pollutant contact and retention.

Table 3 presents a summary of typical performance ranges for these indicators among high-performing bio-based materials.

This section examines the impact of moisture content and thermal activation on the porosity and puffing efficiency of the treated puffed rice media, a crucial precursor for CO₂ adsorption in indoor air purification. Porosity—quantified here by the puffing percentage (i.e., thickness expansion)—directly influences the surface area and the availability of active adsorption sites, which are fundamental for enhancing gas uptake performance.

The experimental matrix consisted of five moisture content levels (12%, 14%, 16%, 18%, and 20%) across four thermal activation conditions (200°C, 220°C, 240°C, and 260°C). Puffing thickness (cm) was recorded as an indicator of porosity development (Figure 4). visualizes the distribution of puffing percentages for different moisture contents and temperatures.

To derive a systematic ranking of optimal conditions, a Multi-Criteria Decision Making (MCDM) approach was employed. The criteria weights were equally distributed across all temperature conditions. Table 4 summarizes the normalized puffing responses and resulting MCDM scores. The moisture content of 18% emerged as the optimal condition, achieving the highest aggregate ranking across all thermal levels.

The results demonstrate a strong interaction between moisture content and thermal treatment in governing the material’s expansion behavior. Moisture levels below 14% yielded suboptimal puffing due to insufficient internal vapor pressure for expansion, while levels above 18% caused structural collapse post-threshold, reducing porosity. At 260°C, the puffing effect was maximized for 18% moisture, reaching 1.2 cm in thickness. This analysis confirms that 18% moisture content at 260°C offers optimal structural porosity, thereby enhancing the material’s theoretical capacity for CO₂ adsorption. These findings form the basis for subsequent chemical functionalization and CO₂ capture evaluation phases presented in the following sections.

Following the physical activation phase, the optimal sample—identified based on its superior puffing percentage, structural integrity, and porosity—was selected for chemical functionalization. This sample exhibited the most favorable characteristics for adsorption applications and was therefore used in all subsequent chemical activation experiments.

To enhance the CO₂ adsorption potential of the media, chemical activation was carried out using aqueous NaOH solutions at varying concentrations: 0.25 M, 0.5 M, 1 M, 1.25 M, 2 M, and 2.5 M. Each unit of puffed rice media absorbed approximately 18–22 mL of NaOH solution. After 24 h of air drying under ambient conditions, the treated samples underwent noticeable physical changes: they darkened in color and adopted a pliable, rubber-like texture. These changes indicate successful surface modification and enhanced reactivity. The introduction of hydroxyl functional groups through NaOH treatment promoted both the physisorption and chemisorption of acidic gases such as CO₂, thereby improving the material’s overall capture efficiency. The next section details the structural and morphological characterization of the material before and after functionalization to explain the impact of NaOH treatment on its performance attributes.

A series of laboratory-scale evaluations were conducted to assess the CO₂ capture performance of NaOH-treated puffed rice media under controlled conditions. The experimental setup comprised vertically layered media samples (8 cm in diameter and 1–1.2 cm in thickness), occupying 20% of the contactor reactor volume. These media layers were exposed to varying NaOH concentrations ranging from 0.5 M to 2.5 M, while a simulated gas stream with an initial CO₂ concentration of 10% was continuously passed through the column. Under optimized conditions, the system achieved a maximum CO₂ removal of 0.37 moles, reducing the CO₂ concentration in the effluent stream to approximately 6.2%.

As illustrated in Figure 9, a pronounced enhancement in CO₂ uptake was observed with increasing NaOH concentration, with peak adsorption occurring at 2.0 M. The untreated puffed rice media exhibited a baseline adsorption efficiency of 22.4%. Treatment with 2.0 M NaOH elevated this efficiency to 38.0%, corresponding to an improvement of approximately 84% compared to the untreated control.

This increase in CO₂ capture capacity is primarily attributed to the chemical functionalization process, wherein hydroxyl groups introduced by NaOH treatment enhanced the material’s affinity toward acidic CO₂ molecules. This mechanism is in line with previous findings that highlight the role of alkaline functional groups in promoting acid-base interactions on bio-adsorbent surfaces. However, a decline in adsorption efficiency was noted at 2.5 M NaOH (0.3 moles), which may be ascribed to over-functionalization effects. Excessive chemical loading can obstruct micropores or induce partial structural collapse, thereby restricting the accessibility of active sites and hindering gas diffusion pathways. Similar effects have been reported in chemically over-activated biochars and carbon-based adsorbents.

To evaluate the practical efficacy of the developed puffed rice-based purification media, a comparative assessment was conducted against selected state-of-the-art bio-based and activated carbon materials widely reported in the literature for CO₂ adsorption. This comparison considered not only adsorption capacity, but also activation temperature, processing complexity, sustainability, and potential for architectural integration. Such multi-dimensional benchmarking is essential to ascertain the innovative contribution and applied value of the proposed material.

As summarized in Table 5, materials such as coconut shell activated carbon, bamboo-derived activated carbon, and agricultural biochar represent common standards in the field of low-cost CO₂ sorbents. Coconut-shell activated carbon, for instance, achieves a high adsorption range (4.5–6.2 mmol/g) due to its well-developed microporous structure, but requires high-energy thermal activation exceeding 900°C and often relies on chemical activation with KOH, which may introduce corrosivity concerns and post-treatment waste challenges (Yang et al., 2010).

Similarly, bamboo-based biochar activated at temperatures above 800°C offers a moderate adsorption performance (∼4.0 mmol/g), but its granular or powder form limits practical deployment in interior systems without additional binding or encapsulation technologies (Ahmad et al., 2007). Agricultural waste-derived biochars, while sustainable and low-cost, typically exhibit lower CO₂ uptake (∼2.5–3.5 mmol/g) unless enhanced through chemical activation, and their morphology often lacks the structural uniformity required for systematic deployment (Shafawi et al., 2021).

In contrast, the proposed puffed rice media activated through a two-step process—thermal expansion at 260°C followed by chemical functionalization with 2.0 M NaOH—demonstrated a CO₂ adsorption capacity of approximately 4.6 mmol/g (equivalent to 38% CO₂ removal under test conditions and the material mass). This performance is comparable to high-end bio-adsorbents, with the added advantages of lower thermal processing energy, material biodegradability, and ease of geometric adaptation into indoor paneling systems.

The lightweight nature (∼8 g per unit), moldability, and passive operation potential of the treated puffed rice panels suggest their high suitability for integration into indoor air purification strategies—particularly in resource-constrained or retrofit scenarios. Additionally, the material supports modular deployment without reliance on electricity or auxiliary equipment, aligning with current sustainability targets and net-zero emission goals in the built environment sector.