From waste to wholesome: An innovative material with biocidal and pollutant abatement ability partly made with red mud waste

Wednesday 21 January 2026

Abstract

This research produces construction materials with photocatalytic activity that can abate pollutants and airborne micro-organisms responsible for sickness and infections. Photocatalysis is a natural process where light, water and oxygen, activate certain materials to cause chemical reactions that break down harmful substances.

Studies demonstrated that the photocatalytic activity of TiO₂ transforms pollutants like nitrogen oxides (NO_x), sulphur oxides (SO_x) and volatile organic compounds into harmless substances. However, its effectiveness is restricted to the ultraviolet (UV) spectrum of the light. This study explores the potential of red mud (RM), a bauxite refining waste, as a photocatalytic material. Red mud is rich in iron oxide; particularly hematite (α-Fe₂O₃) which has been reported to exhibit photocatalytic activity.

RM is used as partial Portland cement replacement which lowers the carbon print of the materials and recycles waste contributing to the circular economy. The results indicate that RM calcined at 600°C has photocatalytic activity, facilitating the degradation of NO_x and organic pollutants including methylene blue (MB) into harmless products.

The photocatalytic activity of RM is not restricted to UV light but expands into the visible light spectrum. The RM also showed a time-dependent antibacterial ability. Tests in light and dark conditions demonstrated enhanced bacterial inactivation with light due to combined UV-induced, DNA damage and RM photocatalysis.

Introduction

Air pollution and airborne microorganisms are major contributors to disease. Airborne microorganisms are associated with hospital-acquired infections which affect approximately one in 10 hospital patients and are responsible for significant mortality and increased financial burden.

Air pollutants such as NO_x and SO_x, and small organic molecules (formaldehyde and toluene) also pose risks to human health. NO_x contributes to the formation of smog, a mixture of hazardous chemicals produced when sunlight interacts with atmospheric pollutants and, together with SOx, leads to acid rain which causes emphysema and bronchitis in humans (Harrison 2004).

TiO₂ is a semiconductor that, when exposed to UV radiation, initiates chemical reactions that generate highly reactive oxygen species which transform air pollutants (NOₓ, SOₓ and volatile organic compounds) and microorganisms into harmless substances such as carbon dioxide and water.

Numerous studies have demonstrated that photocatalysis improves air quality (Zhu et al. 2004; Guerrini and Peccati 2007; Chen and Lin 2007; Pillai et al. 2014) TiO₂ exhibits high photocatalytic activity primarily under UV irradiation, whereas the semiconductor α-Fe₂O₃, with a lower band gap (~2.2 eV), can absorb a substantial portion of the visible (Vis) light spectrum up to approximately 600 nm (Mishra et al. 2015).

RM is a waste produced when refining bauxite to produce alumina. Its management is a burden that requires vast land and careful handling. However, the abundance of iron and aluminium make RM a valuable material which has attracted attention for decades. RM has been used in many applications including calcium sulfoaluminate cement production, acidic soil neutralisation and treatment for iron deficient soil (Hind et al. 1999).

When used in PC production, it contributes to the formation of clinker phases C₃A and C₄AF, and it acts as a flux reducing melting temperature (clinker formation) by about 200°C (Pontikes and Angelopoulos 2013). More recently, RM has been used for low carbon cement and geopolymer production (Pavia et al. 2023; Alelweet and Pavia 2023).

RM has moderate to low pozzolanicity; calcination at low temperature can increase pozzolanic activity, and the main active phases are pozzolanic transition aluminas and Al hydroxide oxide, zeolites and feldespathoids (Alelweet et al. 2021).

This investigation examines the photocatalytic potential of RM and its effectiveness in inactivating Escherichia coli (E. coli) when incorporated into PC mortars.

Materials and methods

CEM III/A 42.5 N/SRC and CEN standard sand were used to prepare mortars with 1:3 binder-to-sand ratio; 0.45 w/b; cured at 20 ± 2°C and above 95% RH. RM sourced from Alcoa, Spain, was calcined at 600°C. The resulting RM-600 was used to replace up to 15% CEM III by weight in 5% increments, designated as R5, R10 and R15.

Blocks measuring 40.5 × 40.5 x 20.5 mm were cast for NO_x abatement and antimicrobial testing. The 28-day compressive strength for R5, R10 and R15 are 31 MPa, 29.70 and 25.41 MPa respectively (the control CEM III mortar reached 37 MPa).

The RM’s chemical composition, determined with X-ray fluorescence (XRF) appears in Table 1. Scanning electron microscope (SEM) analyses revealed that RM-600 consists of fine, irregular, highly porous particles (Figure 1). The mineral composition, determined with X‑ray diffraction (Figure 2), evidenced hematite as the dominant phase.

The nitric oxide (NO) degradation ability of RM-600 under light was assessed for 60 minutes following UNE 127197-1. The degradation of MB by RM-600 under light was assessed using UV–Vis spectroscopy (200–800 nm) over a 3-hour period. Solar irradiation was simulated using an OSRAM Ultra-Vitalux 300 W lamp, which emits a broad spectrum of light consisting of roughly 16% UV and mainly VS light.

A bacterial suspension of E. coli K12, grown overnight in Luria-Bertani broth at 37°C with an optical density of 0.08 at 600 nm, was applied to RM mortars and exposed to light/dark conditions up to 60 minutes under an OSRAM lamp, at 30 ± 1°C ambient temperature. At 30 and 60 minutes, samples were diluted, plated on nutrient agar, and the resulting colony counts were used to quantify bacterial survival in colony forming unit (CFU/mL).

Figure 1: SEM image of RM-600.

Antibacterial action of RM-600-based mortars. Figure 5 illustrates the bacterial inactivation kinetics of RM-600/PC mortars represented as the log₁₀(CFU/mL) versus time. A negative slope of curves denotes a decline in viable cell count, indicating progressive bacterial inactivation over time.

The steeper slope of R10 reflects a more rapid reduction in viable bacterial counts, hence enhanced microbial inactivation. Figure 6 compares the E. coli inactivation performance of R15 under dark and illuminated conditions after 30 and 60 min.

As illustrated in Figure 6a, no significant reduction in E. coli counts was observed in dark conditions, implying that the RM-600 mortars do not inactivate bacteria in the absence of light. In contrast, under illumination (Figure 6b–c), a marked decrease in bacterial viability exists which represents enhanced inactivation. Damage to the bacterial DNA caused by UV-B radiation (280–315 nm), combined with the light-induced activity of RM-600, disrupts cellular structures and accelerates microbial killing.

Results and discussion

Photocatalytic activity of RM-600. The efficiency of RM-600 on removing nitrogen pollutants, measured under solar radiation, appears in Figure 3. Figure 4 shows how much ultraviolet (UV) and visible light (Vis) RM absorbs at different wavelengths (200-800 nm) during NO abatement: the absorption peak at 585 nm gradually fades and a new peak appears at 373 nm, suggesting the formation of new chemical species likely due to pollutant degradation. Additional changes in the visible region at 618 nm and 660 nm further confirm pollutant degradation (Table 2).

Antibacterial action of RM-600-based mortars. Figure 5 illustrates the bacterial inactivation kinetics of RM-600/PC mortars represented as the log₁₀(CFU/mL) versus time. A negative slope of curves denotes a decline in viable cell count, indicating progressive bacterial inactivation over time. The steeper slope of R10 reflects a more rapid reduction in viable bacterial counts, hence enhanced microbial inactivation. Figure 6 compares the E. coli inactivation performance of R15 under dark and illuminated conditions after 30 and 60 min. As illustrated in Figure 6a, no significant reduction in E. coli counts was observed in dark conditions, implying that the RM-600 mortars do not inactivate bacteria in the absence of light. In contrast, under illumination (Figure 6b–c), a marked decrease in bacterial viability exists which represents enhanced inactivation. Damage to the bacterial DNA caused by UV-B radiation (280–315 nm), combined with the light-induced activity of RM-600, disrupts cellular structures and accelerates microbial killing.

Figure 6: Antibacterial activity of RM-600-bases mortars (R15) at different dilutions under dark (a) after 60 min, and light (b, c) conditions after 30 and 60 min of irradiation.

Conclusion

Bauxite refining waste (RM-600) has photocatalytic activity, caused by the activation of its hematite content under light exposure, as shown by the tests confirming pollutant transformation and effective NO abatement.

RM-600/PC mortars inactivate bacteria under light exposure, and this ability becomes progressively stronger with time. Tests under light and dark conditions confirm that the enhanced performance over time results is due to UV-induced DNA damage combined with the photocatalytic activity of the RM.

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