Development of Sustainable Construction Materials for Photocatalytic Treatment of Bio-Aerosols

Abstract

Given the urgency of the global air pollution crisis, there is a pressing need to rethink construction materials as participants in environmental remediation. The integration of functionalized building materials, such as photocatalytic surfaces, self-cleaning coatings, and antimicrobial composites, proves crucial for improving air quality in indoor and outdoor environments. However, while promising, these conventional photocatalytic construction materials encounter limitations such as inadequate performance under visible light spectrum or low-light conditions and issues related to long-term durability. Also, extending their photo-activity to the visible light spectrum presents significant challenges, such as compromising the technique's sustainability and low energy consumption aspects, accompanied by financial and technological costs. Thus, the research area concerning using eco-friendly and low-cost visibly-active photocatalytic construction materials for indoor microbial disinfection remains underexplored and limited. Implementing such next-generation construction materials will enable buildings to function as passive enclosures and play an active role in pollution control, occupant health, and workplace safety. In light of this, the present study introduces a novel inherent and visibly-active heterojunction formation concept to fabricate a sustainable antimicrobial cementitious material. This approach seeks to fabricate a visibly active Fe2TiO5 heterostructure for cementitious composites that demonstrates photocatalytic and antimicrobial characteristics under the visible spectrum of light. To achieve this, cement mortar incorporating IWPs, i.e., FA, FS, and BS, was outer-coated with photocatalyst TiO2. The proposed technique aims to develop a sustainable, cost-effective, and novel method for degrading the urban indoor microbial environment, incorporating circular economy principles. Primarily, the study evaluated the structural suitability of the developed iron-rich cementitious material in terms of its durability and strength properties. Under this, seven mortar mixtures (including the control mixture) were designed with a constant 0.5 w/c ratio. FA and BS were replaced at 10% (constant) and 5–30% (varying by 5%) by the weight of cement, and the FS replaced sand at a constant rate of 15% by the weight of sand while designing different mortar mixtures, respectively. To assess thel performance of the developed iron-rich cementitious material, different physical (porosity, real, bulk, and dry density), durability (water absorption by immersion), and strength (compressive strength, flexural strength, and split tensile strength) properties were performed. The fresh properties of the different IWP-modified mortar mixtures indicated that higher levels of BS replacement increased slump value in IWP-modified mortar mixtures, enhancing workability and cohesiveness, while the fresh density decreased slightly. The results indicated that the utilized IWPs, i.e., FA, BS, and FS, substantially impact the physical and mechanical properties of the developed iron-rich cementitious composite material. The strength property results indicated that the cementitious mortar mix C85S85 exhibited superior compressive strength, with values of 36.2 N/mm² at 28 days and 50.8 N/mm² at 56 days. The 5% BS replacement mix, specifically C85S85, surpassed the reference C100S100 mortar mix, achieving split tensile strengths of 1.98 N/mm² at 7 days and 3.8 N/mm² at 28 days. Furthermore, the results indicated that different levels of BS replacement influence the flexural strength of IWPs-modified cementitious mortar specimens in a manner analogous to their impacts on compressive and split tensile strength. Thus, the mix C85S85 yielded the most favorable results, exhibiting flexural strengths of 3.58 N/mm² and 4.69 N/mm² at 7 and 28 days, respectively. The physical property results demonstrated that the reference mix C100S100 attains a 28-day porosity value of 16.31%, whereas a rise in BS replacement elevates this value. Whereas the 28-day real, bulk, and dry density decreased as BS replacement increased, confirming the suitability of different IWPs-modified mixes for standard cement mortar class. Consequently, the findings indicate that the formulated mortar mixtures can provide low-density construction materials. The study on durability properties indicated that increased BS substitution resulted in higher water absorption values through immersion, with the C85S85 mix attaining the optimal value of 5.84%. This suggests that the compressive strength results of the mix C85S85 in the present study align well with its durability and physical properties results. Therefore, the mortar mix C85S85 demonstrated superior durability and strength properties than the reference mix C100S100. The microstructure analysis indicated that including IWPs in the mortar mix improves the microstructure and compactness at all curing ages. SEM analysis demonstrated that the improved compressive strength and reduced porosity of mix C85S85 are attributed to the increased formation of CSH gel relative to other IWPs-modified mortar mix. XRD results indicate crystalline behavior at 28 and 56 days, with no notable phase transition observed as BS replacement levels increase. A significant peak of quartz was generated, accompanied by other hydrated phases such as calcium hydroxide, calcium carbonate, calcium silicate hydrate, and gismondine. The TGA data demonstrated a consistent mass loss pattern with increasing temperature across all IWPs-modified mortar samples. Therefore, the mortar mix C85S85 is recommended to effectively use FS, BS, and FA as substitutes for sand and cement in sustainable building materials. The results demonstrate that the iron-rich cementitious material enhances environmental sustainability by adopting a circular economy and green building practices. Following that, the cementitious specimens underwent TiO2 coating. The resulting Fe2TiO5 heterostructure for cementitious composite was then analyzed using various analytical approaches. The analytical examination confirmed the presence of a surface-active Fe2TiO5 heterojunction oxide layer. This was validated using multiple techniques, including XRD, XPS, FTIR, Raman spectra, and EDS with elemental mapping. XRD analysis revealed ITO peaks, confirming the incorporation of iron into the TiO2 lattice and forming a surface-active Fe2TiO5 heterojunction oxide layer on the composite's outer surface. FTIR spectra indicated the formation of Ti–O–Fe vibrational bonds, thereby further confirming the presence of Fe in the composite. EDS, elemental mapping, and XPS revealed the presence of elements including Ti, O, Fe, Na, Si, and Fe on the outer surface of the Fe2TiO5 heterostructure. The crystallite sizes of Fe2TiO5 nanoparticles were measured at 31.203 nm for the anatase phase and 34.95 nm for the rutile phase. The determined lattice parameters for the Fe2TiO5 nanoparticles were in close agreement with the reference TiO2, suggesting that Fe has been incorporated into the TiO2 lattice without modifying the average unit cell dimension. TEM images demonstrated a highly crystalline structure and non-spherical morphology, with an average particle size between 13 and 34.5 nm. The UV-DRS results indicate that the Fe2TiO5 heterostructure operates efficiently under visible light, exhibiting an energy band gap of 2.57 eV. The BET analysis showed that the Fe2TiO5 heterostructure possesses a surface area of 73.757 m² g⁻¹ and a pore volume of 9.1x10⁻² cm³g-1. The PL spectrum analysis indicated a reduction in the (e-/h+) recombination rate and increased charge separation efficiency of the Fe2TiO5 heterostructure. Also, the developed Fe2TiO5 heterostructure underwent a thorough photocatalytic and antibacterial property check, confirming its stability and potency as a photocatalytic and antimicrobial construction material under the visible light spectrum. Under this, the present study evaluated the photocatalytic efficacy of the Fe2TiO5 heterostructure in reducing MB in aqueous solutions under solar-visible light exposure. The Fe2TiO5 heterostructure demonstrated enhanced decolorization, evidenced by a decrease in the primary absorption peak and a reduction in the color intensity of the MB solution. The Fe2TiO5 heterostructure demonstrated a 93.1% removal rate of MB at 60 min, in contrast to the 68% observed for simple P25-TiO2 coated specimens. The kinetics of MB degradation utilizing Fe2TiO5 catalysts demonstrated a linear correlation, with a notable increase in the reaction rate constant (0.029 min-1) compared to the TiO2-coated cementitious composite (0.015 min-1). Further, the antibacterial activity of the Fe2TiO5 heterostructure was determined using an E. coli aliquot in a batch reactor under solar-visible light exposure. The Fe2TiO5 heterostructure demonstrated a bacterial inactivation efficiency of 99%, achieving a log reduction of 2.301 ± 0.04 after 60 min of experimentation. The Fe2TiO5 heterostructure exhibited a rate constant of 0.038 min-1, surpassing the TiO2-assisted photocatalysis-induced antibacterial process, which had a rate constant of 0.0083 min-1. This indicates its potential for practical applications. The trapping experiments demonstrated that •O2− and •OH are the most active oxidative species in the Fe2TiO5 heterostructure for the photocatalytic disinfection process. Thus, the findings indicated the durability and efficacy of the Fe2TiO5 heterostructure as a photocatalytic construction material within the visible light spectrum. Furthermore, the indoor antimicrobial efficacy of the Fe2TiO5 heterostructure was evaluated by neutralizing E. coli bioaerosols under visible light in a glass reactor. The optimized experimental conditions resulted in a 1.312 log reduction of E. coli bioaerosols over a 60 min reaction period. The kinetic parameters, such as k of the log-linear model and δ from the double Weibull model, were employed to analyze the bioaerosols inactivation results of Fe2TiO5 heterostructure. The findings indicated that the inactivation rate was significantly higher under visible light conditions with a k value of 0.04 min-1 and δ1 and δ2 values of 34.29 and 32.86, respectively. Also, the Fe2TiO5 heterostructure displayed enhanced antimicrobial properties relative to uncoated-C85S85 and simple TiO2-coated cementitious specimens. Its antimicrobial efficiency reached 69.33% in the dark and 90-95% under visible light, in contrast to the simple TiO2-coated cementitious specimen, which exhibited an antibacterial efficiency of only 23% under visible light and no activity in dark conditions. It also demonstrated a high rate constant (k) value of 0.022 min-1, in contrast to the simple-TiO2 coated (0.0021 min-1) and uncoated-C85S85 (0.0016 min-1) cementitious composites. The overall material cost dropped from 6.69% for C85S85 to 14.08% for C60S85 compared to the control mix C100S100 while preserving the strength and durability of the iron-rich cementitious materials. Thus, the broader applications of the novel Fe2TiO5 heterostructure for disinfecting indoor biological contaminants support its potential for commercial-scale implementation. The Fe2TiO5 heterostructure exhibited remarkable durability and recyclability, exceeding 45 cycles, as verified by multiple analytical methods, highlighting its potential for commercial applications due to its enhanced stability. Thus, the findings reveal that the Fe2TiO5 heterostructure executes excellent antimicrobial properties in inactivating E. coli bioaerosols under dark and visible light conditions. Hence, it can be concluded that the fabricated Fe2TiO5 heterostructure on cementitious composite is economically, ecologically, and sustainably viable to be utilized as a construction material for the remediation of various indoor microbial contaminants.