Bamboo raw materials (Moso bamboo, Phyllostachys edulis) were sourced from Fujian Province, China. The bamboo used in this study was approximately 3 years old, which is commonly used for biomass carbon preparation due to its stable lignocellulosic composition. Potassium hydroxide (KOH), potassium carbonate (K₂CO₃), potassium bicarbonate (KHCO₃), and hydrochloric acid (HCl, 36%–38%) were of analytical grade and obtained from Tianjin Zhiyuan Chemical Reagent Co., Ltd. All reagents were used without further purification.

Fresh bamboo segments were washed thoroughly and oven-dried at 103°C until constant weight. The dried bamboo was then ground and sieved through a 200-mesh screen to obtain uniform bamboo powder as the carbon precursor. A total of 3.0 g of the pre-carbonized bamboo powder (denoted as C-MB) was carbonized at 500°C for 1 h under nitrogen atmosphere (flow rate: ~500 mL/min) using a muffle furnace to obtain the base carbon material. The resulting C-MB was mixed with KOH, KHCO₃, or K₂CO₃ at different mass ratios (1:1, 1:2, 1:3, and 1:4), placed in a nickel crucible, and activated in a tubular furnace by heating to 800°C at 5°C/min under N₂. The sample was held at 800°C for 2 h and allowed to cool naturally. After activation, the products were washed with 1 mol/L HCl and then rinsed with deionized water until the pH stabilized. To ensure high purity, Soxhlet extraction with deionized water was performed for 48 h. Finally, the samples were dried at 105°C for 12 h. The resulting activated carbon samples were denoted as AC-MB@X(Y), where X represents the activator (KOH, KHCO₃, or K₂CO₃) and Y indicates the mass ratio (Fig. 1).

The textural properties of the samples were analyzed using N₂ adsorption-desorption isotherms at 77 K with a BET surface area analyzer (ASAP 2020, Micromeritics, USA). Specific surface area (S_BET) was determined by the Brunauer-Emmett-Teller (BET) method, and pore volume and distribution were calculated via the Density Functional Theory (DFT) model, yielding total pore volume (V_0.99), micropore volume (V_micro), and mesopore volume (V_meso). The morphology and microstructure of the samples were observed using scanning electron microscopy (SEM, Regulus 3400, Hitachi, Japan), while the elemental composition and distribution were obtained via energy-dispersive X-ray spectroscopy (EDS).

The formaldehyde adsorption performance was evaluated using a sealed-chamber method (Fig. 2). Formaldehyde vapor was generated by adding a measured volume of formaldehyde solution (37 wt%) to a Petri dish inside an 8.5 L airtight chamber. After equilibration, a pre-weighed adsorbent was introduced into the chamber through a sample release device, initiating the adsorption process. It should be noted that the present adsorption experiments were conducted using a sealed-chamber removal test designed to simulate indoor air purification conditions. Therefore, the obtained results mainly reflect removal efficiency under dilute gas-phase conditions rather than equilibrium adsorption isotherms.

Formaldehyde concentration was continuously monitored using a real-time detector until equilibrium was reached. The adsorption capacity and removal efficiency were calculated using the following equations:qe=(C0−Ct)V/m. R=C0−CtC0×100% .where:

q is the adsorption capacity (mg/g),

R is the formaldehyde removal efficiency (%),

C₀ and C_t are the initial and final formaldehyde concentrations (mg/m³),

V is the chamber volume (m³),

m is the mass of the adsorbent (g).

To elucidate the intrinsic relationship between the structural features of bamboo-derived activated carbons and their formaldehyde removal performance, correlation analyses were conducted between removal efficiency and key pore structure parameters, as shown in Fig. 9. The results show that the formaldehyde removal rate exhibits a strong linear relationship with specific surface area (R² = 0.88), indicating that larger accessible surface areas contribute significantly to enhancing adsorption. Similarly, a notable correlation was found with total pore volume (R² = 0.83), suggesting that increased internal porosity facilitates molecular diffusion and retention. More importantly, micropore volume demonstrated a high degree of correlation (R² = 0.87), underscoring its dominant role in trapping formaldehyde molecules. Considering the kinetic diameter of formaldehyde (~0.24 nm), the sub-nanometer pores generated during activation act as effective molecular sieves that selectively capture formaldehyde gas. In contrast, mesopore volume exhibited a slightly weaker correlation (R² = 0.76), indicating that while mesopores aid in gas transport and structural buffering, they play a secondary role in direct adsorption.

Integrating the correlation data from Fig. 9 with the pore characteristics summarized in Tables 1–3, it is evident that KOH activation most effectively constructs hierarchical porous networks rich in accessible micropores. Compared with KHCO₃ and K₂CO₃, the strong etching and gas-evolving nature of KOH promotes the formation of narrowly distributed micropores, which are well-suited to the molecular size of formaldehyde. This optimized pore architecture results in superior adsorption performance. Overall, the formaldehyde adsorption capacity of bamboo-derived activated carbons is fundamentally governed by micropore content and total porosity. A high-density microporous structure provides sufficient active sites, while the mesoporous framework ensures rapid transport and diffusion of formaldehyde gas molecules [53,54]. These findings highlight KOH as an efficient activating agent for designing advanced porous adsorbents for indoor air purification and building-related environmental remediation.

To comprehensively evaluate the influence of different alkaline activating agents on the pore structure and adsorption performance of bamboo-derived activated carbons, three representative activators-KOH, KHCO₃, and K₂CO₃-were employed under a fixed alkali-to-carbon ratio of 3:1 for cross-agent comparison. The structural parameters and adsorption results are summarized in Table 4. Among the three samples, AC-MB@KOH(3) exhibited the most outstanding performance, delivering a specific surface area of 2141.77 m²/g, a formaldehyde adsorption capacity of 0.34 mg/g, and a removal efficiency of 90%. These values are significantly higher than those obtained for AC-MB@KHCO₃(3) (0.25 mg/g, 73%) and AC-MB@K₂CO₃(3) (0.27 mg/g, 81%). The results clearly demonstrate that the choice of activating agent plays a decisive role in regulating pore development and adsorption performance. Under identical precursor and activation conditions, KOH at A/C = 3:1 provides the most effective pore-structure engineering route among the three potassium-based activators evaluated in this work. Similar trends have been reported in previous studies on chemically activated carbons, where stronger activating agents typically induce more intensive carbon etching and promote the formation of highly developed microporous structures [9,45,55,56]. It should be noted that adsorption capacities reported in different studies may vary significantly due to differences in experimental conditions such as initial concentration, humidity, and evaluation methods. Therefore, direct quantitative comparison of adsorption capacities across different studies is not always appropriate. Nevertheless, the present results remain consistent with the widely reported trend that stronger alkaline activation favors the development of microporous structures and enhances gas-phase adsorption performance [20,25].

To further evaluate the adsorption kinetics of the three activated carbons under identical conditions, the time-dependent formaldehyde removal efficiencies are presented in Fig. 10. All samples exhibit a rapid increase in removal efficiency during the initial stage, followed by a gradual approach toward equilibrium. The fast initial adsorption stage can be attributed to the large number of accessible adsorption sites and rapid diffusion of formaldehyde molecules into the porous network. Among the three materials, AC-MB@KOH(3) shows the fastest adsorption rate and the highest equilibrium removal efficiency (~90%), whereas AC-MB@K₂CO₃(3) and AC-MB@KHCO₃(3) exhibit slower adsorption kinetics and lower final efficiencies. This trend is consistent with the pore structure characteristics summarized in Table 4, indicating that the enhanced micropore development induced by strong KOH activation significantly improves both adsorption capacity and adsorption kinetics for gas-phase formaldehyde capture.

The superior performance of the KOH-activated carbon can be further understood from the perspective of activation chemistry. As a strong alkaline activator, KOH reacts vigorously with carbon at elevated temperatures, generating potassium-containing intermediates and gaseous species such as H₂, CO, and CO₂. The in situ release and diffusion of these gases within the carbon framework create a pore formation effect that promotes extensive carbon etching and pore expansion, resulting in the formation of abundant micropores and interconnected porous networks. In contrast, KHCO₃ and K₂CO₃ exhibit milder activation behavior. KHCO₃ first decomposes into K₂CO₃ during heating and then participates in subsequent activation reactions, while the gas release during this process is relatively slow and insufficient to induce intensive pore development. K₂CO₃ activation typically requires higher temperatures to initiate effective carbon etching and may cause localized sintering or partial structural collapse at excessive loadings.

Beyond the activation chemistry, the enhanced formaldehyde removal performance can also be attributed to the adsorption mechanism associated with the developed pore structure and surface chemistry. Micropores in the range of approximately 0.5–2 nm provide a size-matching confinement environment for small formaldehyde molecules (kinetic diameter ≈ 0.24 nm), enabling efficient adsorption through van der Waals interactions and pore-filling effects [57]. Meanwhile, oxygen- and nitrogen-containing functional groups on the carbon surface can enhance surface polarity, thereby strengthening dipole–dipole interactions and weak chemisorption with formaldehyde molecules. Furthermore, the coexistence of micropores and mesopores forms a hierarchical pore architecture that facilitates rapid gas diffusion and improves accessibility to internal adsorption sites. This synergistic combination of micropore adsorption, surface polarity, and improved mass transport ultimately results in the superior adsorption performance observed for AC-MB@KOH(3).

To further verify the structural characteristics responsible for the superior adsorption performance, AC-MB@KOH(3) was selected for scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis. As shown in Fig. 11, the low-magnification SEM image (Fig. 11a) reveals that the activated carbon largely preserves the macroscopic morphology of the bamboo precursor, with visible grooves and surface textures. The high-magnification image (Fig. 11b) clearly displays numerous uniformly distributed pores, suggesting that the strong etching effect induced by high-temperature KOH activation effectively facilitates pore formation and surface roughening. In addition, the EDS elemental mapping (Fig. 11c) confirms that the carbon framework is primarily composed of C, with minor amounts of O and N uniformly distributed on the surface. These heteroatoms likely originate from the intrinsic composition of bamboo and residual functional groups generated during carbonization and activation, which may contribute to additional physical adsorption or weak chemical interactions with formaldehyde molecules [58].

In practical indoor environments, formaldehyde often coexists with moisture and other VOC molecules, which may introduce competitive adsorption effects. In particular, water vapor may partially occupy adsorption sites or influence molecular diffusion within micropores under humid conditions. Nevertheless, the well-developed microporous structure of the present bamboo-derived carbons is expected to maintain effective adsorption of small formaldehyde molecules. From an application perspective, regeneration and post-use handling of the adsorbent are also important considerations for sustainable utilization. Adsorbed formaldehyde can potentially be removed through thermal or vacuum-assisted desorption processes, allowing the adsorbent to be reused. Moreover, bamboo-derived carbons exhibit good thermal stability and may be further regenerated by mild heat treatment or repurposed for other carbon-based applications. These characteristics indicate that the present bamboo-based adsorbents are compatible with sustainable material utilization strategies and practical indoor air purification applications.