Nano-TiO₂-Enhanced Surface Functionalization of Recycled Concrete Aggregates for Improved Degradation Efficiency of Low-Concentration Sulfur Dioxide

Abstract

This study investigates the enhancement of recycled concrete aggregate (RCA) surfaces with nano-TiO₂ for an improved degradation of low-concentration sulfur dioxide (SO₂). Nano-TiO₂ particles, known for their photocatalytic properties, were uniformly deposited on RCA surfaces. Upon exposure to SO₂ under light irradiation, the functionalized RCA exhibited significantly improved degradation efficiency. This was attributed to the photo-induced oxidation of SO₂ by nano-TiO₂. Enhanced degradation was further observed under UV light due to increased photoactivation. The nano-TiO₂ coating also showed good durability and stability, ensuring long-term effectiveness. The experimental outcomes reveal that TiO₂-treated recycled aggregates exhibit an 85% retained photocatalytic activity post five cycles of reuse. Furthermore, the investigation employs a second-order polynomial-based mathematical fitting function to generate a three-dimensional trend surface, visually illustrating the inverse relationships between sulfur dioxide degradation and environmental variables, such as initial concentration and flow rates. Finally, this study demonstrates the potential of nano-TiO₂-modified RCA for mitigating the environmental impact of low-concentration SO₂, contributing to the development of more sustainable construction materials and broadening RCA’s applications in environmental remediation.

1. Introduction

In the pursuit of sustainable development and environmental stewardship, the construction industry has been confronted with the dual challenges of mitigating environmental impacts and enhancing resource efficiency [1,2,3]. Among the various strategies employed, the utilization of recycled aggregates (RAs) has emerged as a promising approach to reduce waste generation and conserve natural resources. Recycled aggregates, primarily derived from construction and demolition debris, offer a viable alternative to virgin aggregates, thereby mitigating the environmental footprint associated with mining and quarrying activities. However, the widespread adoption of RAs in construction materials has been hindered by concerns over their inferior physical and mechanical properties, particularly in comparison to their virgin counterparts [4,5,6].

One of the primary limitations of RAs lies in their surface properties, which often exhibit higher porosity, lower density, and a greater tendency for water and contaminant absorption. These characteristics can adversely affect the durability and performance of concrete and other construction materials incorporating RAs [7,8,9]. Consequently, research efforts have focused on developing innovative methods to enhance the surface properties of RAs, aiming to improve their compatibility and performance within various construction applications.

Among the various surface modification techniques, the incorporation of nanomaterials, particularly nano-TiO₂ (titanium dioxide nanoparticles), has garnered significant attention due to its unique physicochemical properties and environmental remediation capabilities. Nano-TiO₂ is renowned for its high surface area-to-volume ratio, excellent photocatalytic activity, and chemical stability, making it an ideal candidate for surface functionalization of RAs. When exposed to ultraviolet (UV) light, nano-TiO₂ generates reactive oxygen species (ROS) such as hydroxyl radicals and superoxide anions, which can effectively degrade a wide range of organic and inorganic pollutants [10,11].

SO₂, a major air pollutant emitted from industrial processes, power plants, and transportation, poses significant health risks and contributes to environmental degradation, particularly acid rain and air pollution. The degradation of low-concentration SO₂ is particularly challenging due to its high solubility in water and the formation of stable sulfite and sulfate ions [12,13,14]. Traditional methods for SO₂ removal, such as scrubbing and adsorption, often require complex equipment, high energy consumption, and frequent maintenance. Therefore, the development of cost-effective and environmentally friendly technologies for SO₂ degradation is of paramount importance.

The integration of nano-TiO₂ into the surface of RAs presents a novel approach to address this challenge. By enhancing the surface functionality of RAs with nano-TiO₂, it is possible to create a photocatalytic surface that can actively degrade low-concentration SO₂ in ambient conditions. This not only improves the environmental performance of RAs but also expands their potential applications in air purification systems, green infrastructure, and sustainable construction materials.

The present study aims to investigate the feasibility and effectiveness of nano-TiO₂-enhanced surface functionalization of RAs for improved degradation efficiency of low-concentration SO₂. Specifically, this study will explore the optimal conditions for nano-TiO₂ deposition on RA surfaces, evaluate the physicochemical changes induced by the modification process, and assess the photocatalytic performance of the modified RAs under controlled laboratory conditions. Furthermore, this study will delve into the underlying mechanisms governing the photocatalytic degradation of SO₂ by nano-TiO₂-modified RAs, providing valuable insights into the development of more efficient and sustainable air pollution control technologies.

The significance of this research lies in its potential to contribute to the advancement of sustainable construction practices by promoting the use of RAs and developing innovative solutions for air pollution control. By harnessing the unique properties of nano-TiO₂, this study aims to overcome the limitations associated with conventional RAs and pave the way for the development of high-performance, eco-friendly construction materials. Additionally, the findings of this research will have implications for the broader field of environmental engineering, particularly in the areas of air pollution control and sustainable materials development.

2. Results and Discussion

2.1. Characterization of Prepared TiO₂-Based Aggregates

Figure 1 depicts recycled concrete aggregates on the left and natural river sand aggregates on the right. As evident from the figure, the recycled aggregates exhibit a rough exterior surface coupled with a porous structure, whereas the natural river sand appears relatively smooth. Upon modification with nano-titanium dioxide (TiO₂), both the recycled aggregates and natural river sand surfaces are observed to be coated with white granular substances, which are confirmed through EDX spectroscopy to be nano-TiO₂. The porous structure of recycled aggregates can offer a larger comparative surface area, which facilitates the loading of nano-titanium dioxide and provides more active sites to promote the photocatalytic reaction. This indicates that the immersion method can successfully and relatively uniformly immobilize nano-TiO₂ onto the aggregate substrates.

Furthermore, the adoption of nano-TiO₂ modification enhances the surface properties of both aggregate types, imparting them with additional functionalities such as improved durability, enhanced photocatalytic activity (potentially aiding in self-cleaning and pollution degradation), and increased resistance to degradation processes. The rough and porous nature of the recycled aggregates, in particular, provides a larger surface area for nano-TiO₂ attachment, potentially amplifying the beneficial effects of the modification. This modification strategy not only addresses the environmental concerns associated with waste disposal from construction and demolition activities but also promotes the utilization of recycled materials in a high-performance and sustainable manner, thereby contributing to the circular economy.

2.2. Photocatalytic Degradation of Sulfur Dioxide

Figure 2 presents a comprehensive analysis of the photocatalytic degradation of sulfur dioxide (SO₂) facilitated by diverse aggregates, both untreated and modified with nano-TiO₂, alongside an assessment of the reusability potential of these photocatalytic aggregates. Notably, the untreated river sands exhibit a modest SO₂ degradation rate of 5.2%, attributable solely to the inherent degradation capacity of ultraviolet (UV) irradiation, as UV light possesses inherent pollutant degradation capabilities. However, the absence of nano-TiO₂ on the sand surface precludes the exploitation of photocatalytic effects. Upon modification with nano-TiO₂, a significant enhancement in SO₂ degradation is observed, with a rate soaring to 32.45%, representing a nearly sixfold increase compared to untreated river sands. This substantial boost stems from the photocatalytic activity imparted by the nano-TiO₂ particles adhered to the sand surface, which harnesses light energy to promote the degradation of SO₂.

Turning to recycled aggregates, their untreated form demonstrates a SO₂ degradation rate of 19.32%, still superior to untreated river sands but without the benefit of photocatalysis due to the absence of nano-TiO₂. This elevated performance can be attributed to the inherent alkaline nature of recycled aggregates, rich in calcium oxide or similar compounds, which facilitate the neutralization of acidic gases like SO₂. Remarkably, when recycled aggregates are treated with nano-TiO₂, their SO₂ degradation efficiency surpasses 43.67%, exceeding double that of untreated recycled aggregates and achieving a 15% improvement over nano-TiO₂-treated river sands. This remarkable enhancement is underpinned by the high porosity of recycled aggregates, which allows for the effective incorporation of nano-TiO₂ particles within their pores. The internal surfaces of these pores provide an expanded reaction area and an abundance of active sites, fostering more efficient photocatalytic reactions and, consequently, a heightened SO₂ degradation rate. The observation underscores the pivotal role of nano-TiO₂ in enhancing the photocatalytic degradation of SO₂, particularly when combined with aggregates possessing favorable physical properties such as high porosity. Furthermore, the analysis highlights the potential for recycled aggregates, even in their untreated state, to contribute to pollution mitigation through their inherent chemical properties.

In assessing the reusability of nano-TiO₂-modified aggregates for photocatalytic sulfur dioxide (SO₂) degradation, a rigorous cleaning and drying protocol was followed. Specifically, the aggregates were rinsed with deionized water for 2 min, subsequently oven-dried at 105 °C for 24 h, and finally reused for additional photocatalytic cycles. Figure 2b,c illustrate the degradation efficiency over multiple cycles, with each cycle represented by numbers 1–6.

A discernible trend emerges: as the number of reuse cycles increases, the SO₂ degradation rates for both nano-TiO₂-treated river sands and recycled aggregates decline. Nevertheless, the decrease in efficiency for nano-TiO₂-treated recycled aggregates is notably less pronounced compared to that of nano-TiO₂-treated river sands. Notably, even after five reuse cycles, the SO₂ degradation efficiency of nano-TiO₂-treated recycled aggregates maintains over 85% of its original capacity, whereas the corresponding value for nano-TiO₂-treated river sands falls below 70%. This disparity can be attributed to the unique properties of recycled aggregates. Their rough surface and porous structure offer several advantages. Firstly, their porous nature provides ample space for the accommodation of photocatalytic reaction products, minimizing the obstruction or blockage of active sites that might otherwise occur if these products were not effectively removed. Secondly, their porosity and roughness create a larger surface area, thereby enhancing the number of active sites available for photocatalytic reactions. Lastly, the rough surface enhances the adhesion between the recycled aggregates and nano-TiO₂ particles, mitigating the risk of detachment during reuse cycles. Collectively, these factors contribute to the superior reusability and stability of nano-TiO₂-treated recycled aggregates in photocatalytic SO₂ degradation applications, underscoring their potential as a sustainable and efficient material for environmental remediation.

The utilization of recycled aggregates in construction materials not only promotes sustainability but also offers unique opportunities for environmental remediation. When these aggregates are modified with nano-TiO₂, they become capable of degrading harmful air pollutants like sulfur dioxide (SO₂). This degradation process is multifaceted, involving both physical and chemical mechanisms, with alkaline calcium hydroxide (Ca(OH)₂) playing a pivotal role.

On the one hand is the initial physical absorption phase. The presence of Ca(OH)₂ within the porous structure of recycled aggregate creates an alkaline microenvironment [15,16,17] that favors the absorption of acidic gases like SO₂. As SO₂ molecules come into contact with the aggregate’s surface, they dissolve in the thin film of moisture or aqueous solution formed on the porous surface. This physical absorption is driven by concentration gradients and the inherent affinity of alkaline compounds for acidic gases. On the other hand is the role of nano-TiO₂ photocatalysis. Concurrently, the nano-TiO₂ particles embedded in or coated on the aggregate surface are activated by light, particularly ultraviolet (UV) radiation. Upon activation, these nanoparticles generate reactive oxygen species (ROS), such as hydroxyl radicals (OH•) and superoxide ions (O₂•-). These ROS diffuse into the porous structure of the aggregate, where they encounter the absorbed SO₂ molecules. The ROS then initiate a series of oxidation reactions, converting SO₂ into more stable and less toxic compounds, such as sulfates. This photocatalytic degradation process not only reduces the concentration of SO₂ but also regenerates the active sites on the aggregate’s surface, allowing for continuous absorption and degradation cycles (see Figure 3).

The physical absorption of SO₂ by Ca(OH)₂ and its photocatalytic degradation by nano-TiO₂ exhibit strong synergistic effects. The alkaline environment provided by Ca(OH)₂ enhances the solubility and absorption of SO₂, while the photocatalytic activity of nano-TiO₂ accelerates its degradation. This synergy ensures that a higher proportion of the absorbed SO₂ is efficiently converted into harmless compounds. The synergistic interactions and the associated enhanced performance are derived from surface modification. A typical surface modification technique, impregnation, is employed to disperse nano-TiO₂ particles uniformly on the recycled aggregate’s surface. The technique not only improves the dispersion and stability of the nanoparticles but also increases the overall surface area available for both absorption and photocatalysis. The larger surface area enhances the contact between SO₂ molecules and the active sites, leading to faster and more efficient degradation.

2.3. Effect of Initial Concentrations and Flow Rates on the Photocatalytic Degradation of Sulfur Dioxide Using Nano-TiO₂ Treated Recycled Aggregates

Figure 4 comprehensively illustrates the degradation behavior of sulfur dioxide (SO₂) under varying initial concentrations and flow rates, offering valuable insights into the complex interplay between these two parameters. Subgraph (a) specifically presents a three-dimensional (3-D) scatter plot of the experimental data, providing a visual representation of the data distribution and trends in the relationship between SO₂ abatement efficiency, initial SO₂ concentration, and gas flow rate. To deepen our understanding of how these variables impact SO₂ degradation, a predictive model was devised to construct a fitting trend surface. This approach enables the quantification of the non-linear relationships between the independent variables (initial concentration and flow rate) and the dependent variable (SO₂ degradation efficiency). A binary quadratic polynomial, known for its capability to capture non-linear trends in two-variable systems, was chosen as the mathematical framework for model construction (refer to Equation (2)). The coefficients of this polynomial, detailed in

Table 1. Coefficients of the parameters and 95% confidence bounds.

Coefficients	95% Confidence Bounds
p00 = 97.59	(90.64, 104.5)
p10 = −7.477	(−11.54, −3.419)
p01 = −0.06216	(−0.08018, −0.04413)
p20 = 0.2742	(−0.3598, 0.9082)
p11 = 0.002232	(−1.243 × 10−5, 0.004476)
p02 = 2.151 × 10−5	(6.811 × 10−6, 3.621 × 10−5)

, were determined through a rigorous statistical analysis to ensure optimal fit and predictive accuracy.

The fitting performance of the model is demonstrated in Figure 4d, which showcases a relatively strong correlation between the experimental data and the predicted values. The plot reveals that most of the (experimental, predicted) data pairs adhere closely to the benchmark line y = x, indicating a satisfactory degree of agreement between the model’s predictions and the observed experimental outcomes. This high level of fitting accuracy underscores the model’s effectiveness in capturing the essential dynamics of SO₂ degradation under varying operational conditions, thereby enhancing our ability to predict and optimize SO₂ removal processes for environmental and industrial applications. Furthermore, the utilization of a predictive model based on a binary quadratic polynomial not only enhances the scientific rigor of the analysis but also facilitates the identification of critical operational parameters and their respective thresholds for optimal SO₂ degradation. By examining the coefficients and their signs in Table 1, researchers can gain insights into the relative importance and directionality of the effects of initial concentration and flow rate on SO₂ abatement, paving the way for more targeted and efficient process design and optimization strategies.

$$z\left(x,y\right)=p00+p10x+p01y+p20x^2+p11xy+p02y^2$$

(1)

Leveraging the predictive model’s capabilities, a three-dimensional (3-D) trend surface is meticulously constructed, offering a comprehensive visualization of the sulfur dioxide (SO₂) degradation dynamics as a function of varying initial concentrations and flow rates. This trend surface, as depicted in subgraph (b), provides a holistic understanding of the intricate interplay between these parameters and their combined influence on SO₂ abatement. To further facilitate the interpretation of the model’s predictions, a contour map of the predicted SO₂ degradation efficiency is presented in subgraph (c). This contour map serves as a valuable tool for identifying regions of high and low efficiency, enabling researchers to pinpoint optimal operating conditions for maximum SO₂ removal.

It is noteworthy that the analysis reveals a consistent trend: an increase in both flow rates and initial SO₂ concentrations leads to a decrease in the predicted SO₂ degradation efficiency. This observation underscores the complex nature of the degradation process, where the positive effect of increased flow rates in facilitating mass transfer and contact between the SO₂ and the scrubbing medium is offset by the challenges posed by higher initial SO₂ concentrations, which require more intensive treatment. The identification of this inverse relationship highlights the importance of balancing these two parameters to achieve optimal SO₂ removal performance. By fine-tuning the operating conditions based on the predictive model’s insights, engineers and researchers can develop more efficient and cost-effective SO₂ abatement strategies, contributing to the mitigation of air pollution and the protection of public health.

The effect of the intricate interplay between initial SO₂ concentrations and flow rates on photocatalytic degradation efficiency was comprehensively illuminated by the results. At lower initial SO₂ concentrations, the system exhibited robust degradation efficiencies, with flow rate fluctuations having a negligible impact. This phenomenon can be rationalized by the ample availability of active sites on the nano-TiO₂ modified aggregates, which ensured efficient photocatalytic reactions even at moderate flow rates. Conversely, as the initial SO₂ concentration escalated, the degradation efficiency declined, presumably due to intensified competition among SO₂ molecules for active sites and the build-up of reaction intermediates that hindered further degradation.

In terms of flow rate, this study identified an optimal range that maximized degradation efficiency. At low flow rates, the extended residence time of SO₂ molecules within the reactor facilitated deeper degradation, yet excessively low flow rates could lead to saturation of active sites and reduced mass transfer rates, thereby compromising efficiency. Conversely, high flow rates, though enhancing mass transfer, shortened the residence time of SO₂ molecules, resulting in incomplete degradation and consequently, lower efficiencies. Therefore, striking the perfect balance between flow rate and residence time emerged as a pivotal factor in optimizing degradation performance.

Moreover, the unique properties of the recycled aggregates, including their rough surface and porous structure, played a pivotal role in enhancing the photocatalytic process. The porous architecture not only facilitated intimate contact between SO₂ molecules and nano-TiO₂ particles, amplifying the photocatalytic activity, but also provided ample space for the sequestration of reaction products, mitigating their inhibitory effects on the degradation process. This synergy between the photocatalytic properties of nano-TiO₂ and the structural advantages of the aggregates underscores the potential of this approach for efficient SO₂ abatement in various industrial and environmental applications.

3. Experimental Details

3.1. Materials

The raw materials primarily encompass commercial nano-sized titanium dioxide (Degussa P25), featuring an average particle size ranging from 20 to 50 nanometers, with a specific anatase-to-rutile ratio of 25:75. The aggregate composition is dominated by recycled fine concrete aggregates sourced from a local construction waste processing facility, complemented by commercial natural river sand. The aggregate particles are carefully selected within a size range of 2.36 to 4.75 mm.

3.2. Sample Preparation

The aggregates (comprising recycled aggregates and natural river sand) underwent an initial thorough cleaning process using deionized water, followed by immersion in a 1 mol/L sodium hydroxide (NaOH) solution to facilitate surface hydroxylation. This hydroxylation step was crucial for enhancing the adsorptive properties of the aggregates towards the polar nano-titanium dioxide particles. Subsequently, the modified aggregates were immersed in a nano-titanium dioxide (nano-TiO₂) solution, prepared at a ratio of 1 g of nano-TiO₂ per 80 g of aggregate, dispersed in 100 milliliters of deionized water. The immersion process was accompanied by ultrasonic treatment for 30 min, ensuring thorough dispersion and interaction between the nano-TiO₂ and aggregate surfaces. This mixture was then left undisturbed for 24 h to allow for optimal adsorption and bonding. After the 24 h soaking period, the nano-TiO₂-modified aggregates were transferred to an oven and dried at 105 °C for an additional 24 h. This drying step removed any residual moisture, leaving behind a stable and uniformly coated nano-TiO₂ modified recycled aggregate and nano-TiO₂ modified natural river sand aggregate. The methodology employed in this modification process was designed to not only ensure thorough coverage of the aggregate surfaces with nano-TiO₂ but also to promote strong interactions between the polar nanoparticles and the hydroxylated aggregate surfaces. The ultrasonic treatment facilitated the penetration of nano-TiO₂ into the pores and crevices of the aggregates, enhancing the durability and effectiveness of the modification. Furthermore, the precise control over the mixing ratios and drying conditions contributed to the production of high-quality, uniformly modified aggregates.

3.3. Testing

The prepared modified aggregates were placed within a sealed container, which was topped with an ultraviolet (UV)-transparent glass panel. Above this glass panel, two parallel UV lamps were positioned to provide a light intensity of 3 mW/cm² with a peak wavelength of 365 nm (see Figure 5). On the left side of the container, a source of commercial concentrated sulfur dioxide (SO₂) gas with a purity of 99.99% was connected, and its initial concentration was regulated at 1000 parts per billion (ppb) and flow rate maintained at 3 L per minute (L/min) through a controller. To the right of the container, a high-precision SO₂ analyzer was attached, capable of measuring SO₂ concentrations with an accuracy of 1 ppb and real-time readings every minute.

Prior to commencing the experiment, the UV lamps were kept off, and SO₂ gas was introduced into the container for 30 min to purge the air and saturate the container with SO₂. Following this, the UV lamps were switched on, and the entire container was enclosed within a light-tight hood to initiate the photocatalytic degradation process, which lasted for 30 min. Subsequently, the UV lamps were turned off, and the next sample was prepared for testing. To ensure the accuracy and reliability of the experiments, each set of tests was conducted in triplicate (three parallel experiments), and the mean value was taken as the experimental result. The degradation rate of SO₂ was calculated using Equation (1), which is a mathematically rigorous formula designed to quantify the efficiency of the photocatalytic process. This rigorous experimental setup, coupled with the utilization of high-precision instruments and thorough replication of tests, ensures that the data collected is of the utmost scientific quality, facilitating the drawing of robust conclusions regarding the performance of the nano-TiO₂ modified aggregates in the photocatalytic degradation of SO₂. MATLAB with the version of R2023b is used to conduct the 3-D scatter map and tread surface.

$$\begin{array}{c}D={\displaystyle \frac{{\left[{\mathrm{S}\mathrm{O}}_2\right]}_{\mathrm{m}\mathrm{a}\mathrm{x}}-{\left[{\mathrm{S}\mathrm{O}}_2\right]}_{\mathrm{m}\mathrm{i}\mathrm{n}}}{{\left[{\mathrm{S}\mathrm{O}}_2\right]}_{\mathrm{m}\mathrm{a}\mathrm{x}}}}\times 100\%\end{array}$$

(2)

where ${\left[{\mathrm{S}\mathrm{O}}_2\right]}_{\mathrm{m}\mathrm{a}\mathrm{x}}$ indicates the maximum concentration of the sulfur dioxide, whilst ${\left[{\mathrm{S}\mathrm{O}}_2\right]}_{\mathrm{m}\mathrm{i}\mathrm{n}}$ indicates the minimum concentration of the sulfur dioxide.

4. Conclusions

The following conclusions can be drawn from this study:

• Enhancement of RCA surface with nano-TiO₂ for SO₂ degradation: The uniform deposition of nano-TiO₂ particles on recycled concrete aggregate (RCA) surfaces significantly improved the degradation efficiency of low-concentration sulfur dioxide (SO₂). This enhancement is primarily attributed to the photocatalytic properties of nano-TiO₂, which promotes the photo-induced oxidation of SO₂ under light irradiation.
• Durability and stability of nano-TiO₂ coating: The nano-TiO₂ coating on RCA surfaces has demonstrated good durability and stability, as evidenced by the 85% retained photocatalytic activity after five cycles of reuse. This finding ensures the long-term effectiveness of the functionalized RCA for SO₂ degradation.
• Three-dimensional trend surface analysis: Employing a second-order polynomial-based mathematical fitting function, a three-dimensional trend surface was generated to visually present the inverse relationships between sulfur dioxide degradation and environmental variables, such as initial SO₂ concentration and flow rates. This analysis provides valuable insights into the optimization of operational conditions for enhanced SO₂ degradation.
• Potential for environmental impact mitigation: This study demonstrates the potential of nano-TiO₂-modified RCA as an effective material for mitigating the environmental impact of low-concentration SO₂. By promoting the degradation of this harmful gas, the functionalized RCA contributes to the development of more sustainable construction materials and expands the applications of RCA in environmental remediation.

In summary, this study underscores the importance of nano-TiO₂ surface functionalization of RCA for enhanced SO₂ degradation and highlights its potential for practical applications in promoting environmental sustainability.