Perspective on Sustainable Solutions for Mitigating Off-Gassing of Volatile Organic Compounds in Asphalt Composites

Abstract

This perspective explores the use of biochar, a carbon-rich material derived from biomass, as a sustainable solution for mitigating volatile organic compounds (VOCs) emitted during asphalt production and use. VOCs from asphalt contribute to ozone formation and harmful secondary organic aerosols (SOAs), which negatively impact air quality and public health. Biochar, with its high surface area and capacity to adsorb VOCs, provides an effective means of addressing these challenges. By tailoring biochar’s surface chemistry, it can efficiently capture VOCs, while also offering long-term carbon sequestration benefits. Additionally, biochar enhances the durability of asphalt, extending road lifespan and reducing maintenance needs, making it a promising material for sustainable infrastructure. Despite these promising benefits, several challenges remain. Variations in biochar properties, driven by differences in feedstock and production methods, can affect its performance in asphalt. Moreover, the integration of biochar into existing plant operations requires the further development of methods to streamline the process and ensure consistency in biochar’s quality and cost-effectiveness. Standardizing production methods and addressing logistical hurdles will be crucial for biochar’s widespread adoption. Research into improving its long-term stability in asphalt is also needed to ensure sustained efficacy over time. Overcoming these challenges will be essential for fully realizing biochar’s potential in sustainable infrastructure development

1. Introduction

Biochar is recognized as a solid and porous carbon material produced from the thermal decomposition of biomass feedstock in oxygen-limited conditions [1,2]. While advanced carbon materials, like graphite, graphene, and carbon nanotubes, have significantly impacted material science and technology, their production typically relies on various fossil fuel-based precursors, raising significant sustainability concerns. In contrast, biochar represents a unique class of carbonaceous adsorbents, sourced from biological materials, including plants, animals, and microorganisms. Biochar’s tunable morphology and surface chemistry can be engineered from biowaste. Algae, for example, contain a range of organic components, such as carbohydrates, lipids, and proteins, which can serve as excellent carbon precursors for nanoparticle formation. These attributes position biochar as an eco-friendly, sustainable alternative to conventional carbon materials.

Compared to other carbonaceous materials, such as activated carbon, carbon nanotubes, and carbon dots, which pose significant competition to biochar as air pollutant adsorbents, biochar is produced at relatively lower temperatures and at a cost nearly one-sixth that of activated carbon [3,4]. The additional benefits of biochar production and commercialization include its straightforward preparation process, a wide range of biomass feedstock options, and its reusability for various applications [5]. Although biochar offers several advantages, its ability to remove contaminants is more limited than other carbonaceous materials, often requiring preparation or pre- or post-treatment processes to enhance its performance [6].

Biochar can be synthesized through several thermal processes, including pyrolysis, gasification, hydrothermal carbonization, and flash carbonization [7]. Key variables, such as pressure conditions, residence time, heating rate, temperature, and the type of reactor, significantly affect the physicochemical properties and yield of bio-oil, biochar, and condensed gases generated during biomass decomposition [8]. For instance, higher temperatures and prolonged durations often result in smaller particle sizes and more graphitic structures in the pyrolysis production of biochar [9]. The choice of a specific thermal process plays a crucial role in determining the characteristics of the final carbon materials. In pyrolysis, biomass is exposed to high temperatures (300–900 °C) in an inert atmosphere, causing the organic components to break down into smaller carbonaceous structures. Hydrothermal carbonization, on the other hand, heats biomass in water at moderate temperatures (150–250 °C) under pressure, producing carbon nanoparticles with unique surface functionalities [10]. This method also enables the modification of surface chemistry by adjusting the reaction conditions, such as pH, temperature, and reaction duration, facilitating the incorporation of oxygen, nitrogen, or sulfur-containing groups [11].

Among various strategies to remove environmental pollutants, sorption and catalysis stand out as particularly attractive due to their ease of operation and low cost on a large scale. Biochar, in particular, has proven its efficiency in effectively adsorbing a wide range of contaminants [12,13,14]. Biochar has been widely utilized as an essential adsorbent in various sustainability initiatives, such as carbon capture and sequestration [15,16,17], heavy metal detoxification [18,19,20,21,22], and the mitigation of atmospheric pollution [4,23,24,25,26].

Additionally, recent research studies have actively explored biochar’s roles in reducing greenhouse gas emissions [27,28,29], sequestering carbon [30], and adsorbing volatile organic compounds emitted from the surfaces [31]. The ability of biochar as an adsorbent is mainly attributed to its physical characteristics, such as a large specific surface area and high porosity, along with abundant functional groups, such as C-O, C=O, -COOH, and -OH. These characteristics make it an ideal candidate as an adsorbent for air pollutants.

Among gaseous air pollutants, volatile organic compounds emitted from asphalt pavement are noteworthy; the use of biochar in bituminous composites has emerged as a promising avenue to both mitigate emissions and enhance colloidal stability, thereby extending asphalt service life [32,33]. Asphalt continues to emit pollutants long after the manufacturing, installation, and compression stages are complete. These emissions persist throughout the lifespan of the asphalt, especially as it ages, even when exposed to normal environmental temperatures [34]. However, in warm months, higher temperatures contribute to a rise in emissions from the surface of aged asphalt. With over 2.5 million miles of asphalt roads in the U.S. and global production exceeding 62 million tons in 2020, about 80% of which was used for road paving [35], emissions from asphalt surfaces have become a growing concern, particularly during warmer seasons, due to their potential impacts on human health and the environment.

The laboratory simulation studies conducted by Li et al. [36] demonstrated VOC emission concentrations ranging from 4.24 mg/m³ to 104.16 mg/m³ for the following three modeled scenarios: mixture plant, transportation, and paving. The study revealed that alkenes (C_nH_2n, n ≤ 4), aldehydes, alkanes (C_nH_2n+2, n ≥ 6), and alkylbenzenes, despite their lower concentrations, were the main contributors to ozone formation potential (OFP) and secondary organic aerosol (SOA) formation, accounting for more than 62% and 97%, respectively [36]. Other studies have demonstrated that VOCs are released from asphalt surfaces, including polycyclic aromatic hydrocarbons (PAHs), aliphatic hydrocarbon chains, chlorinated organic solvents, and PM_2.5, all of which contribute to various health effects [37,38]. PAH-containing VOCs are emitted from asphalt manufacturing plants and are subsequently discharged into the surrounding air, soil, and water [39].

Recent studies have highlighted the benefits of using biochar to adsorb volatile and semi-volatile organic compounds emitted from asphalt-surfaced areas, thereby reducing the negative impacts of prolonged sunlight and elevated temperatures on air quality [32,40,41,42].

Adding biochar can increase asphalt’s mechanical resilience, enhancing its durability and resistance to deformation and cracking [43]. However, several challenges should be resolved to completely realize the potential application of biochar in construction materials. Key issues include ensuring biochar’s compatibility with asphalt and cement, maintaining consistency and quality, and achieving economic feasibility. Additional obstacles involve the technical integration of biochar at an industrial scale, the need to update regulatory standards, and the assurance of long-term stability and safety of biochar-enhanced materials.

Lehmann et al. [44] reviewed biochar’s impact on climate change mitigation, focusing on its effects on soil emissions, plant growth, and CO₂ uptake. They discussed the trade-offs between energy generation and carbon sequestration, influenced by feedstock and production choices. This involves deciding how much focus should be placed on producing energy from biochar versus using biochar to store carbon in the soil. Studies have highlighted biochar’s role in carbon capture, particularly analyzing how variations in its characteristics and the composition of feed gases affect CO₂ uptake [45]. While numerous biochar reviews have focused on its applications as soil enhancers, bio-adsorbents to reduce soil greenhouse gas emissions, and fertilizers, a few recent studies have explored non-soil applications of biochar [46,47]. Most reviews to date have been on biochar synthesis, characterization analysis, modifications, and major applications, such as purification of soil, water, and wastewater by removing contaminants [48,49,50,51]. Kumar-Mishra and Mohanty [47] reviewed biochar’s applications in energy storage and conversion, covering conversion technologies, biochar formation mechanisms, surface chemistry modifications, catalysts, and its use in energy storage devices like supercapacitors and nanotubes, as well as in bio-based and inorganic composite materials. Lin et al. explored advancements in applications of biochar-supported composites, including biochar clays, biochar microorganisms, and biochar enzymes [52]. It is worth noting that there have also been excellent reviews in recent years on biochar’s applications as a catalyst in the catalysis-related field [53,54].

Aligned with other non-soil applications of biochar, the objective of this study is to provide a literature review on the recent advancements and applications of biochar in asphalt and highlight research gaps.

1.1. Assessment of the Physicochemical Properties of Biochar, Activated Carbon, and Carbon Nanotubes

Comparing different carbonaceous adsorbents is not entirely sensible in many respects, as they are distinct materials influenced by numerous factors, including variations in product type, feedstock, production methods, and the limitations of comparison methodologies. However, a basic comparison between biochar and other widely used carbonaceous adsorbents, such as activated carbon and carbon nanotubes, can still offer valuable insights into their relative adsorption capacities and environmental impact.

Table 1. Comparison of selected physical properties of biochar (BC), carbon nanotubes (CNT), and activated carbon (AC), including two biochars (BC1 and BC2), a multi-walled carbon nanotube (MWCNT), a single-walled carbon nanotube (SWCNT), a powdered activated carbon (PAC), and a granular activated carbon (GAC).

Property	BC1	BC2	MWCNT	SWCNT	PAC	GAC
Surface Area (m2/g)	85	142	175	557	1255	1354
Pore Volume (cm3/g)	0.057	0.185	0.664	1.043	0.757	0.778
Micropores (%)	13.4	11.5	7.0	16.1	37.0	42.6
Mesopores (%)	75.7	69.8	65.4	77.8	61.9	56.4
Macropores (%)	10.9	18.7	27.6	6.1	1.1	1.0
Oxygen Content (%)	24.27	21.95	2.99	4.94	28.01	21.92
pH PZC (pristine)	3.3	4.1	7.5	6.7	3.2	4.1

presents the physicochemical properties of biochar, including surface area, pore structure, oxygen content, and surface charge, alongside those of carbon nanotubes and activated carbons. The data, reported by Jiang et al. [55], were originally used to evaluate the adsorption behavior of two estrogenic contaminants (17β-estradiol and 17α-ethynyl estradiol) by graphene nanomaterials and compared with a multi-walled carbon nanotube (MWCNT), a single-walled carbon nanotube (SWCNT), two biochars (BC1 and BC2), a powdered activated carbon (PAC), and a granular activated carbon (GAC).

The results in Table 1 show that activated carbons (GAC and PAC) exhibit the highest surface areas (up to 1354 m²/g) and dominant microporosity, which are beneficial for adsorbing small molecules. Activated carbon is a highly porous carbonaceous material characterized by an exceptionally large surface area and a well-developed pore structure, making it highly effective for adsorption-based applications. Activated carbon is widely used in wastewater treatment due to its strong capacity to remove pollutants, such as dyes, pharmaceuticals, heavy metals, and organic contaminants [56,57]. Notably, biochar itself can serve as a precursor to activated carbon. Biochar is typically produced via pyrolysis and used directly or with minimal processing, whereas activated carbon undergoes further chemical or physical treatment that significantly enhances its adsorption potential. Activation methods include physical activation, using oxidizing gases like steam or CO₂ at high temperatures [58], and chemical activation, either through a one-step or two-step process involving activating agents, such as KOH, ZnCl₂, or NaOH [59,60]. These treatments greatly increase microporosity and surface reactivity, contributing to the superior adsorption performance of ACs.

Carbon nanotubes, particularly SWCNTs, also demonstrate high surface areas (up to 557 m²/g) and are dominated by mesoporous structures, while MWCNTs are rich in macropores (Table 1). Carbon nanotubes offer chemically inert surfaces suitable for physical adsorption and possess high specific surface areas comparable to those of activated carbons. However, unlike activated carbons, carbon nanotubes feature a more uniform and well-defined atomic structure, enabling strong interactions with various pollutants, including small molecules, heavy metals, and organics, in both gas and liquid phases [61]. Despite their high cost, carbon nanotubes offer strong potential for selective and efficient pollutant removal.

In contrast, biochars (BC1 and BC2) have significantly lower surface areas (85–142 m²/g) and pore volumes but possess the most heterogeneous pore structures, spanning micro-, meso-, and macropores (Table 1). Despite their lower specific surface area, biochars show relatively high oxygen contents (21.95–24.27%) and acidic surface characteristics (pH PZC ~3.3–4.1), which can enhance their interaction with polar contaminants. The point of zero charge (PZC) governs surface charge and thereby controls electrostatic interactions with ions and polar molecules. Surface functionalization, particularly oxygen-containing groups, can further enhance adsorption through improved interaction with contaminants.

Jiang et al. [55] emphasize that the presence of larger pores in biochars and carbon nanotubes limits their surface areas compared to activated carbons, but the functional groups and structural diversity of biochars make them viable candidates for pollutant adsorption, especially under realistic environmental conditions where surface chemistry plays a critical role.

1.2. Comparative Environmental Impact: Activated Carbon vs. Biochar

While the physicochemical comparison highlights key performance differences, understanding the broader environmental implications could be important, particularly between biochar and activated carbon, which is the most direct commercial competitor in carbon-based adsorbents.

In a comprehensive meta-analysis (

Table 2. Comparative energy demand and global warming potential of activated carbon and various biochar types derived from different feedstocks.

Material Category	Material Example	Energy Demand (MJ/kg)	Global Warming Potential (kg CO2eq/kg)
Activated carbon	Virgin (hard coal)	44	3–11
	Olive waste-based	170	11
	Recycled	-	1.2
	Granular	79.8	9.3
Fossil waste biochar	Dense refuse-derived fuel	1.8	−0.3
Manure biochar	Poultry litter	1.1	−0.2 to −0.5
Crop residue biochar	Barley, wheat, corn stover, straw	1–3	−0.9 to −2.1
Food and paper waste biochar	Food waste, cardboard, paper sludge	1.1–1.8	−0.1 to −1.1
Woody biomass biochar	Forestry, wood waste, poplar	1.4–16	−1.3 to −0.1
Green/yard waste biochar	Green waste, yard waste	1.8–3	−1.1 to −0.3
Energy crops biochar	Miscanthus, switchgrass	1.4–11	−3.5 to 0
Sewage-based Biochar	Sewage sludge	1.8	−0.8

), data from 84 life cycle assessment (LCA) studies were collected and evaluated to compare the environmental performance of biochar and activated carbon across different feedstock sources [62]. By harmonizing the data under a common functional unit, the adsorption of heavy metals, the authors ensured a fair basis for evaluating energy demand and global warming potential across diverse materials and production methods.

Their findings reveal a sharp contrast in environmental impact between biochar and activated carbon, particularly when produced from virgin fossil-based feedstocks, such as coal, indicating substantially higher energy requirements, reaching up to 170 MJ/kg, and positive global warming potential (GWP) values as high as 11 kg CO₂ eq/kg. These impacts are largely due to the energy-intensive activation processes and fossil fuel inputs during production. Conversely, biochar, especially when derived from agricultural residues, forestry waste, or organic byproducts, consistently exhibits lower energy demands, often below 3 MJ/kg, and in many cases, delivers net-negative GWP values (e.g., −4 to −0.7 kg CO₂ eq/kg), indicating carbon sequestration benefits.

2. Biochar’s Multi-Faceted Mechanisms in Capturing Air Pollutants

A deeper understanding of biochar’s physicochemical properties provides a foundation for exploring the mechanisms by which it captures air pollutants and prevents their release into the environment. In a general definition, the absorption of VOCs on carbonaceous composites is primarily attributed to physicochemical processes. Chemical adsorption occurs mainly through functional groups and stacking interactions, which are reversible, rather than through irreversible chemical reactions. In a study conducted by Clurman et al. [63] on the removal of acetaminophen (C₈H₉NO₂) using biochar, adsorption isotherms and kinetic models indicated that the process is controlled by chemisorption, where a monolayer of acetaminophen is chemisorbed onto the biochar surface. Highlighting the role of functional groups in chemical adsorption, Clurman et al. [63] demonstrated that the increase in compounds featuring oxygen groups on the modified biochars’ surface (e.g., increasing C=O groups from 47.8 a.u. to 152 a.u.) and changes in their crystalline structure enhance the removal efficiency. However, the primary mechanisms of organic compound sorption by biochar involve physical adsorption rather than chemical adsorption. The different mechanisms involved in the organic pollutants’ adsorption are shown in Figure 1.

The review article by Zhao et al. [26] provides an interesting examination of the effectiveness, underlying mechanisms, and factors influencing biochar’s ability to eliminate groups of air pollutants, including SO₂, H₂S, CO₂, Hg⁰, VOCs, and NH₃. The work presented by Zhao et al. [26] illustrates the various mechanisms by which biochar removes the six above-mentioned air pollutants, each with distinct properties. For example, they showed that the removal of SO₂ by biochar9 occurs through a combination of oxidation and physical adsorption, including electrostatic and van der Waals interactions [26]. The dominant mechanism depends on the functional groups on the surface, the specific surface area, and the biochar’s pore configuration [26].

While the mechanisms of VOC capture or degradation in asphalt matrices are not extensively discussed, various studies have examined how biochar removes VOCs in other media. Abbass et al. [64] investigated the mechanisms by which biochar facilitates the degradation of organic contaminants in wastewater. Since some of these mechanisms may also apply to the performance of biochar in asphalt matrices, we will review the general mechanisms involved in this process.

In the partitioning process, VOCs or other organic pollutants diffuse into the voids of the biochar’s non-carbonized part. This part of the biochar, which has not been fully converted to carbon during pyrolysis, retains more of the original organic structure and functional groups. These features allow for easier interaction with the organic adsorbate, facilitating its sorption onto the biochar. The non-carbonized portion may contain amorphous carbon or crystalline carbon with an organized lattice structure. The balance between these carbonized and non-carbonized regions, as well as their specific structural characteristics, determines the overall adsorption capacity and efficiency of the biochar for removing organic pollutants from the environment.

Pore filling mechanism is highly influenced by the physical structure and porosity of the biochar, particularly the presence of micropores (pores smaller than 2 nm) and mesopores (pores ranging from 2 to 50 nm) [65,66]. The type of feedstock used to produce biochar and the pyrolysis conditions significantly influence the pore structure and surface chemistry. While low pyrolysis temperatures yield biochar that is effective in adsorbing inorganic contaminants, higher pyrolysis temperatures promote the adsorption of organic contaminants by expanding the biochar’s surface area and microporosity [64]. In the latter case, pore filling, along with hydrophobic and electrostatic interactions, becomes the predominant adsorption mechanism. The pore-filling mechanism occurs quickly, resulting in a higher initial removal rate; however, once the biochar pores are occupied by hydrophobic compounds, active pore clogging takes place [67].

Electrostatic and electron donor-acceptor interactions include Coulombic forces of attraction and repulsion at charged locations on the adsorbent (biochar) [67]. In conjunction with these electrostatic interactions, they may also be influenced by weaker forces that act upon electrically neutral molecules, including London van der Waals forces, hydrogen bonding, and the hydrophobic effect [68,69].

π–π interactions represent a major class of noncovalent forces, playing a crucial role in the structure of biomolecules, the behavior of chemical bonds, and the properties of π-conjugated materials, such as biochar and carbon nanotubes (CNTs). These interactions involve π structures, such as benzene or other aromatic rings [70].

Research evidence indicates that the presence of oxygen-containing functional groups enhances the adsorption of chemical contaminants on the surface of carbon materials, including biochar. This is partially attributed to π–π electron donor–acceptor interactions [71,72,73]. Surface carboxylic acid (-COOH), nitro (-NO₂), and ketonic (-C=O) groups of carbon materials can act as electron acceptors, facilitating π–π electron donor–acceptor interactions with aromatic molecules, thereby enhancing sorption [74]. N-containing groups, such as amide, pyridine, and quinoline present in carbon materials can serve as π-electron-donor sites.

Ahmed et al. [75] investigated the adsorption of five organic pollutants with hydrophobic characteristics (estrone, estriol, 17β-estradiol, 17α-ethynyl estradiol, and bisphenol A) onto functionalized biochar. The research focused on how π-electron donors, like phenanthrene, and π-electron acceptors, such as 1,3-dinitrobenzene and p-amino benzoic acid, in the biochar affect the adsorption process. The results indicated that H-bonding and π-π electron donor–acceptor interactions were the main mechanisms driving the adsorption of these contaminants onto the biochar.

In another example, Cheng et al. [76] synthesized a cellulose-based biochar infused with nitrogen, exhibiting a high adsorption capacity of 103.59 mg/g for atrazine (ATZ) removal. Based on the material characterizations and theoretical calculations, including density functional theory (DFT), the good adsorption performance of the target biochar for ATZ was mainly due to chemisorption and π-π electron donor–acceptor (EDA) interactions, which were enhanced by the high graphitization of the biochar. Pyridinic and graphitic nitrogen improved adsorption through hydrophobic effects and π-π EDA interactions, while pyrrolic nitrogen and other surface groups (-COOH, -OH) facilitated H-bonding.

Hydrophobic interactions are more prominently associated with aqueous environments, where the biochar surface repels water and attracts hydrophobic contaminants. Therefore, it is essential to include this mechanism in our discussion of organic contaminant removal. The hydrophobic interactions alongside other mechanisms, such as pore-filling, and the π-π interactions contribute to the overall sorption capacity of biochar, highlighting its versatility and effectiveness in removing a wide range of organic contaminants [77,78].

As biochar ages, oxygen- and nitrogen-containing functional groups on carbon surfaces generally reduce hydrophobicity by introducing polar sites that attract water molecules. Functional groups, such as hydroxyl (-OH), carbonyl (C=O), carboxyl (-COOH), amine (-NH₂), and amide (-CONH₂), create active sites on the carbon surface where hydrogen bonding and dipole interactions with water can occur. This increases the carbon’s affinity for water and other polar molecules, making it more hydrophilic. However, freshly produced (unaged) biochar with minimal surface oxidation tends to be hydrophobic, effectively capturing hydrophobic organic substances through a combination of hydrophobic interactions, the neutral ionization of organic compounds, and partitioning mechanisms [13,79,80].

The rapid extraction of atrazine from aqueous solutions could be attributed to the high hydrophobicity of the biochar, which selectively captures hydrophobic solutes from water, while the adsorption of sulfamethoxazole is less efficient due to its lower hydrophobicity [81,82]. In a study conducted by Li et al. [83], hydrophobic interactions were identified as the primary driving force behind the adsorption of ionizable organic pollutants, such as benzoic acid, o-chlorobenzoic acid, and p-chlorobenzoic acid.

The solubility of the pollutants is crucial in determining how effectively they adsorb onto the hydrophobic biochar. Pollutants that are more soluble in water generally exhibit lower adsorption due to their tendency to remain in the aqueous phase. However, if these soluble pollutants contain hydrophobic functional groups, such as methyl groups, they can interact more favorably with the hydrophobic surfaces of biochar [84].

• Atmospheric Aerosols and Greenhouse Gases

Atmospheric aerosols, comprising primary organic aerosols (POAs), secondary organic aerosols (SOAs), and particulate matter (PM), play a critical role in air quality, human health, and climate dynamics. POA refers to non-volatile organic compounds emitted in particulate form, typically smaller than 1 µm in size, from both naturally occurring (biogenic) processes and human-generated (anthropogenic) sources. Biogenic sources include natural phenomena, such as volcanic eruptions, as well as emissions from vegetation, including plants and trees, and activities involving microorganisms. Anthropogenic sources encompass emissions generated by industrial activities and the incineration of fossil fuels and field biomass, such as agricultural straw burning and wildfires [85,86]. POAs do not undergo any atmospheric process other than dilution and deposition.

Unlike POAs, which are emitted directly from biogenic or anthropogenic sources in particulate form, secondary organic aerosols (SOAs) are generated through various atmospheric processes involving the oxidation and subsequent condensation of gaseous precursors. These gaseous precursors are semi-volatile organic compounds (SVOCs) or volatile organic compounds (VOCs) that are also emitted from both natural sources and anthropogenic activities and undergo successive atmospheric oxidation by ozone (O₃), nitrate radicals (NO₃·), or hydroxyl radicals (OH·) [87,88]. This oxidation process leads to the formation of low-volatility compounds that condense on pre-existing particles or undergo homogeneous nucleation [89], contributing to the aerosol burden in the atmosphere. Therefore, the distinction between POAs and SOAs stems from their respective formation mechanisms. While both can originate from similar natural and anthropogenic sources, POAs are emitted directly as particles, whereas VOCs (precursors to SOAs) undergo atmospheric oxidation processes to form aerosol particles, resulting in SOAs.

Together, POAs and SOAs make up a significant portion of atmospheric particulate matter, categorized by size into PM_2.5 and PM₁₀. These particles not only influence climate by affecting the Earth’s radiative balance and cloud formation but also have severe health implications, particularly fine particles (PM_2.5). These particles can reach the bloodstream by passing deep into the lungs, which can significantly increase the risk of developing respiratory and cardiovascular diseases.

In addition to VOCs, other emissions, such as carbon dioxide (CO₂) and methane (CH₄), also play crucial roles in atmospheric processes and climate change.

CO₂ plays a pivotal role in global warming as the primary anthropogenic greenhouse gas. CO₂, primarily a byproduct of combustion, is emitted alongside VOCs, SVOCs, and particulate matter from fossil fuel burning, biomass burning, and industrial activities. CO₂ functions as a greenhouse gas by trapping heat in the atmosphere, limiting the amount of infrared radiation that can escape back into space. This process, known as the greenhouse effect, leads to a rise in global temperatures, driving climate change. CO₂ concentration in the atmosphere had significantly increased from ~280 to ~421 ppm by 2023 due to human activities. Model predictions indicate that the global temperature will rise by approximately 2 °C if the CO₂ concentration reaches around 570 ppm by 2100 [90,91].

In addition to fossil fuel combustion, natural and human-induced events like wildland fires contribute significantly to greenhouse gas emissions. For example, approximately one-third to one-half of the world’s carbon monoxide (CO) emissions and around 20% of nitrogen oxide (NO_x) emissions stem from wildland fires [92,93]. These fires are also significant sources of greenhouse gases, such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) [94]. Although a substantial portion of the emitted CO₂ is absorbed by plants as they regrow, a notable portion remains in the atmosphere for years, and possibly even centuries [95].

Methane (CH₄), classified as a toxic and flammable gas, plays a crucial role in atmospheric chemistry and climate dynamics. Although methane is present in lower concentrations than carbon dioxide (CO₂), it is the most abundant VOC in the atmosphere and has a much stronger warming effect, with a global warming potential up to 83 times greater than that of CO₂ over a 20-year period [96]. This effectiveness is due to methane’s strong radiative forcing and high reactivity, making it highly efficient at absorbing infrared radiation and thus significantly contributing to atmospheric warming.

Despite its hazardous nature, methane’s impact extends beyond its direct effects. Through atmospheric oxidation, CH₄ produces formaldehyde (HCHO) and other VOCs, which can contribute to the formation of secondary organic aerosols (SOAs). This oxidation process also affects the atmospheric oxidative capacity by increasing the concentration of hydroxyl radicals (OH·), which play a key role in converting VOCs into SOAs. Over time, the atmospheric concentration of CH₄ has increased approximately 2.5-fold, from 731 parts per billion (ppb) in 1750, the preindustrial era [97], to 1890 ppb in 2020 [98].

3. Asphalt Emissions: A Critical Non-Combustion Contributor to Air Pollution

While much attention is rightly placed on combustion sources, such as vehicle exhaust and industrial emissions, non-combustion sources also play a notable role in degrading air quality. Recent discoveries have shed light on an overlooked source of volatile and semi-volatile organic compounds (SVOCs) that are discharged into the atmosphere as either gas molecules or aerosols. Asphalt mix and bitumen are notable sources of VOCs and SVOCs released from non-combustion processes, yet their impact on air pollution has often been overlooked [99,100,101]. These organic/inorganic compounds have the potential to contribute to the formation of fine airborne particles, ozone, and other secondary air pollutants [85,89]. Asphalt-related emissions consist of a wide range of chemical species that vary in both particle size and toxicological properties, posing potential risks to human health [102].

While emissions from high-temperature asphalt production are regulated and managed to some degree at the plant, the VOCs and SVOCs released during loading, transportation, and paving processes are often less rigorously controlled. More importantly, based on the recent findings, emissions from asphalt-surfaced areas extend beyond the stages of the asphalt formulation process, transportation, and installation at the construction site. These emissions continue throughout the asphalt’s service life and are particularly pronounced under solar radiation and during warmer seasons as the temperature rises [34]. According to recent research, asphalt emissions can double with a temperature increase from 40 °C to 60 °C, reaching their peak at temperatures exceeding 140 °C. This finding implies that these compounds are more likely to be emitted into the atmosphere during the warmer summer months [34]. Therefore, implementing strategies to prevent these chemicals from being released into the environment could be a significant step in effectively addressing air pollution.

Asphalt releases a variety of chemical substances, each with distinct toxicological characteristics that could potentially threaten human health [103].

Despite their significance, asphalt emissions are often overlooked in air quality management strategies and regulatory frameworks. Identifying the origins and properties of asphalt emissions is crucial for formulating effective strategies to minimize their effects on air quality and population well-being. By addressing both combustion and non-combustion sources, like asphalt, policymakers and urban planners can work towards improving overall air quality and creating healthier environments for communities worldwide.

The Main Components of Asphalt VOCs

Understanding the characteristics and composition of asphalt VOCs is essential for evaluating their environmental impact and developing effective mitigation strategies. Li et al. [104] have thoroughly reviewed the characterization methods of asphalt VOCs, highlighting both field and laboratory approaches. Field sampling captures authentic VOC compositions near emission sources, while lab simulations offer a stable and customizable platform. Both methods can complement each other, but variations in materials, devices, and conditions lead to discrepancies in results.

VOC samples obtained from asphalt materials require qualitative and quantitative analyses to determine their chemical composition and potential environmental effects. Several analytical techniques are used for asphalt VOC analysis, including gravimetric analysis, ultraviolet–visible (UV-vis) spectroscopy, electronic nose, and gas chromatography mass spectrometry (GC-MS). Li et al. have effectively compared the advantages and limitations of various analytical methods for VOCs [104].

GC–MS is recognized as one of the most powerful techniques for characterizing complex chemical mixtures, such as asphalt VOCs [105]. Researchers have studied the compositional characteristics of VOCs emitted from various asphalt sources using analytical techniques, particularly GC-MS [106]. In a GC-MS study, Espinoza et al. [107] analyzed VOCs and SVOCs emitted from hot mix asphalt and warm mix asphalts containing natural zeolite and RAP, at 155 °C. They identified 57–81 organic compounds in the fumes released from these specific asphalt mixtures. In this study, saturated aliphatic hydrocarbons were the most abundant, making up over 43.7% of the total. Aromatic compounds (6.1% to 10.5%) and alkenes (4.5% to 8.1%) were followed as the next most significant constituents. Zhou et al. [106] argue that benzene derivatives and aldehydes among VOCs contribute most significantly to ozone formation, while only alkanes, benzene derivatives, and olefins contribute to the formation of SOAs.

Specifically, ten primary VOCs have been associated with ozone formation and the generation of secondary organic aerosols (SOAs) during various stages of asphalt production, including mixing, transportation, and application. Emission concentrations range from 4.24 mg/m³ to 104.16 mg/m³ across these three stages [36]. They showed that alkenes, aldehydes, alkanes, and alkylbenzenes, with relatively lower concentrations, were the main sources of ozone formation (62%) and SOA (97%) generation. As shown in

Table 3. Top 10 priority-controlled pollutant (VOC) allowances based on weight ratio calculation (ref: [36]).

	Mixture Plant Process		Transportation Process		Paving Process
1	Benzene	1000.000	1,3-Butadiene	1000.000	Trichloroethylene	1000.000
2	1,3-Butadiene	963.555	Trichloroethylene	926.417	1,3-Butadiene	712.823
3	Toluene	531.012	Toluene	568.961	Toluene	645.573
4	Propionaldehyde	381.854	Propylene	467.649	Benzene	645.573
5	Propylene	356.978	Benzene	452.333	Propionaldehyde	582.964
6	m-/p-Xylene	290.172	2,2,4-Trimethylpentane	380.699	m-/p-Xylene	391.040
7	2,2,4-Trimethylpentane	248.160	Propionaldehyde	342.968	Propylene	338.007
8	Butanal	229.527	Ethylene	290.900	Butanal	337.763
9	Ethylene	193.049	m-/p-Xylene	229.481	2,2,4-Trimethylpentane	292.806
10	Pentanal	152.576	Butanal	200.007	Ethylene	228.143

, the main components of VOCs in the three scenarios vary significantly. In the mixture plant process, the primary VOCs are benzene, 1,3-butadiene, and toluene, while benzene has the highest allowance of 1000.000. During the transportation process, the most significant VOCs are 1,3-butadiene and trichloroethylene, followed by toluene. In the paving process, trichloroethylene is the predominant VOC with an allowance of 1000.000, while 1,3-butadiene and toluene also have high allowances, indicating their substantial impact at this stage.

Xiu et al. [101] investigated organic chemical emissions from asphalt composites modified with crumb rubber, quantifying VOCs, PAHs, and total suspended particulate matter. This group categorized 77 asphalt VOCs into the following five main categories: aromatic hydrocarbons, aliphatic hydrocarbons, compounds containing nitrogen and oxygen, halogenated hydrocarbon compounds, and carbon disulfide (CS₂).

Mousavi et al. [32] identified VOCs released from untreated aged asphalt binder at 150 °C for 24 h using GC-MS analysis. This temperature replicates the effects of short-term aging and the emissions associated with the asphalt production processes. Aromatics accounted for the largest portion of total emissions, comprising 6.74% of the overall peak, which aligns with their abundance in bitumen (30–45 wt.%). The temperature used in the study (150 °C) was insufficient to induce thermal cracking of several hazardous PAHs or to volatilize polar aromatics (resins), indicating that these compounds are less susceptible to volatilization under this aging condition.

The chemical composition of asphalt, along with other influencing factors, plays a major role in the types of VOCs it releases, as demonstrated by the different VOC profiles reported by various research groups (

Table 4. Key components of asphalt VOCs at different temperatures from various studies.

T (°C)	No. of VOCs	Some of the Main VOCs Identified	References	Year
150	31	benzene derivatives, alkanes (nonane, heptane, octane), alkenes (1-pentene, 2-methyl), alkynes (3-octyne, 5-methyl-)	Mousavi et al. [32,110]	2023
160–180	14	styrene, toluene, ethylbenzene, O/M-xylene, P-diethylbenzene	Zhou et al. [106]	2023
120–180	10	benzene, toluene, 1,3-butadiene, propionaldehyde	Li et al. [36]	2020
155	81	trichloromethane, heptane, octane, dimethyl heptane, nonane	Espinoza et al. [107]	2020
165	34	trichloromethane, benzene, toluene, etc.	Wang et al. [111]	2020
165	77	anthracene, fluorene, pyrene, etc.	Xiu et al. [101]	2020
160	12	12 types of PAHs	Mo et al. [108]	2019
175	41	benzene, toluene, trichloromethane, ethylbenzene, etc.)	Lin et al. [109]	2016
180	44	toluene, N-butyraldehyde, ethane, etc.	Boczkaj et al. [112]	2014
180	25	xylene, anthracene, naphthalene, etc.	Gasthauer et al. [113]	2008

). Furthermore, elements such as the production technique, temperature, and duration of heating, along with the methods used for detection, can greatly impact the characteristics of VOCs [108,109]. In a general classification, asphalt VOCs can be divided into approximately ten primary categories of components, including aldehydes and their derivatives, alkanes and their derivatives, benzene and its derivatives, aliphatic hydrocarbons, PAH, nitrogen oxides, sulfur compounds, carbon oxides, halogenated hydrocarbons, and other VOCs [99].

4. Asphalt Aging: Evaporation of Lightweight Components

The alteration of the colloidal structure of bitumen with aging significantly influences its mechanical properties and overall performance. Bitumen stability depends on a delicate balance within its three-component system—asphaltenes, aromatics (polar and non-polar), and saturates [114,115]. Aging alters this composition, disrupting colloidal stability and deviating bitumen’s initial composition. These changes lead to instability or incompatibility among bitumen’s primary SARA fractions (saturates, aromatics, resins, and asphaltenes), driven by aging processes that change molecular configurations, the polarity of the compounds, and bonding interactions. For instance, irreversible oxidation leads to the global hardening and embrittlement of bitumen, associated with increased viscosity [116]. Along with increasing viscosity, the aging process leads to significant changes in the softening point and stiffness of bitumen [117].

Asphalt aging is influenced by intrinsic and extrinsic factors, as well as the duration of exposure [118]. Intrinsic factors include the chemical composition and origin of bitumen, aggregate properties, permeability and porosity, and the thickness of the bitumen layer coating the aggregates [119]. Extrinsic factors impact asphalt during production (short-term aging) and through environmental exposure throughout its service life (long-term aging) [119,120].

Short-term aging, also known as thermal–oxidative aging, occurs during asphalt mix production, transport, and compaction at the construction site, typically within hours. This process is characterized by high temperatures (>130 °C) that promote chemical oxidation [121]. The primary effect of short-term aging is an increase in bitumen viscosity, driven by two factors—the evaporation of lightweight components and the irreversible oxidation and polymerization of reactive hydrocarbons and polar components [122,123]. As a result, short-term aging leads to the hardening of bitumen, increasing the risk of embrittlement and cracking [124]. During short-term aging, aromatics in bitumen are converted into polar aromatics (resins), which then evolve into larger, more complex molecules, known as asphaltenes. This transformation enriches aged bitumen with asphaltenes, further increasing its viscosity and making it more prone to embrittlement and cracking [124].

Bitumen experiences long-term aging over time, primarily due to progressive mass loss and oxidation due to prolonged exposure to environmental conditions and sunlight-induced photo-oxidation [125,126,127]. This aligns with our earlier discussion that UV-exposed bitumen emits submicron atmospheric aerosols even at ambient temperatures that not only accelerate bitumen’s aging but also impact human health and air quality [34]. Prolonged exposure to solar UV radiation significantly degrades bitumen by inducing the formation of free radicals, which initiate oxidative processes, altering the molecular structure and composition of bitumen over time [128,129]. UV exposure to bitumen results in a more pronounced increase in carbonyl and sulfoxide compounds compared to thermal oxidation [130,131]. Irreversible changes to the original composition of bitumen occur due to the generation of free radicals in conjunction with processes such as polar functionalization, aromatization, carbonization, and chain scission [119,132,133]. While the negative impact of aging caused by volatilization and mass loss of bitumen is significantly less pronounced compared to the changes in asphalt composition due to oxidation, the significance of bitumen’s mass loss on its overall performance remains undeniable.

Using molecular modeling in the framework of density functional theory (DFT), We have previously demonstrated how resin molecules contribute to stabilizing the colloidal suspension of crude oil and bitumen by reinforcing the structure of asphaltene stacks [134]. The loss of resins, either through volatilization or their transformation into asphaltenes, along with the growth in the level of asphaltene content during thermal aging, results in the destabilization of the colloidal suspension. This destabilization can result in increased viscosity and the formation of a more solid-like bitumen structure, negatively impacting the flow properties and processing of bitumen.

Recently, we investigated the impact of losing lightweight aromatics and saturated hydrocarbons on the association behavior of asphaltene stacks, impacting the colloidal stability of asphaltenes in the matrix of bitumen. It is worth noting that the increased stiffness and viscosity of bitumen observed during aging are primarily driven by a higher concentration of asphaltenes within its matrix. Prior to aging, asphaltenes, which typically consist of two or three planes, are uniformly dispersed within a medium of resins, aromatics, and saturates. Aging significantly disrupts this uniform and consistent distribution, leading to the agglomeration of asphaltenes within the bitumen matrix.

The DFT analysis indicates that the presence of aromatic and saturated molecules on asphaltene surfaces creates steric repulsion, which is crucial for keeping the asphaltene planes apart and preventing their aggregation. The hindrance caused by lightweight small molecules can interfere with the π-π physical interactions at the interface of asphaltene sheets, facilitating their de-agglomeration [110]. Consequently, the mass loss of bitumen from volatilization directly disrupts the distribution of asphaltenes within the bitumen, promoting their aggregation.

5. Adsorption Performance of Biochar in Asphalt: Impact on Air Quality and Asphalt Durability

As discussed, the volatilization-induced mass loss in asphalt binder disrupts the uniform dispersion of asphaltenes within the binder, leading to increased asphaltene aggregation. We have recently shown the benefits of adding biochar to asphalt, effectively minimizing its mass loss [110]. Asphalt modified with biochars derived from various biomass sources, such as walnut shell, peanut shell, Douglas fir, pine bark, birch, and algae, has been shown to effectively lower the emission of volatile compounds from the asphalt. This modification helps maintain the stability of maltene composition and preserves the colloidal balance of the asphalt [110]. The outcomes of this study highlight the importance of reducing emissions from bitumen to delay its aging process. These results support existing findings that highlight the positive impact of biochar on enhancing the durability of asphalt. Additionally, the performance of biochar as an adsorbent to retain asphalt volatile compounds reduces these emissions during asphalt’s service life, resulting in improved air quality and fostering more sustainable and eco-friendly infrastructures.

Biochar’s Impact on Asphalt: Delaying Aging and Enhancing Performance

Asphalt aging significantly impacts its performance, primarily due to the oxidation of binder components. When oxygen interacts with the asphalt binder, it causes chemical changes that increase viscosity and stiffness, resulting in reduced flexibility and resilience. This oxidative process leads to the formation of brittle compounds, increasing the asphalt’s susceptibility to cracking, fatigue, and raveling [135] and weakening adhesion between the binder and aggregates.

While biochar helps in evenly distributing asphaltenes by decreasing the volatilization of small organic molecules, its primary benefit in asphalt is enhancing aging resistance by mitigating the oxidative aging of binder components [136]. Due to its fibrous and porous nature, biochar can significantly amplify the durability and lifespan of asphalt pavement. It enhances resistance against cracking, rutting, and degradation caused by temperature changes and the stresses of traffic loads [137,138,139].

UV rays generate free radicals that accelerate the aging of asphalt composites. Ghasemi et al. [140] hypothesized that carbonaceous particles grafted with bio-derived molecules like amines and amides can scavenge these radicals and delay aging. They integrated findings from molecular modeling and laboratory experiments to examine biochars derived from woody biomass, which contained a higher concentration of phenolic functional groups, and algal biomass, which had an increased level of amine and amide functional groups. Based on the results, biochar from algal biomass is considerably more efficient at slowing down the aging process compared to biochar derived from woody biomass. This enhanced effectiveness was attributed to the presence of surface-bound functional groups, such as amines and amides, which improve the asphalt’s ability to protect against the diffusion of free radicals.

Rajib et al. [138] demonstrated that adding biochar to asphalt enhances its resistance to different levels of aging, including oxidation and UV-induced aging. They showed biochar delays the rheological and chemical aging, based on the changes in crossover modulus index and functional groups like carbonyl and sulfoxide, respectively. These findings are supported by other researchers, such as Celauro et al. [141], who demonstrated changes in the accumulation of carbonyl and hydroxyl functional groups over different exposure times. Their research also highlighted the positive role of biochar in mitigating asphalt aging related to photo-oxidation. In a comparison between neat and rubberized asphalt, Rajib et al. [138] showed that biochar was more effective for oxidation aging in rubberized asphalt, while it provided greater benefits against UV (photo-oxidation) aging in neat asphalt. This is because biochar acts as a UV blocker and free radical scavenger, and rubberized asphalt already has carbon black that offers inherent UV resistance. Zhou-Adhikari [142] also showed that pyrolysis biochar, rich in carbon, can protect asphalt surfaces from UV light, reducing photo-oxidative aging. They concluded that biochar preserves asphalt properties and improves its high-temperature stability by protecting it from UV radiation.

As discussed, high temperatures play a crucial role in the thermal oxidation of asphalt, leading to hardening and cracking. Additionally, thermal instability at elevated temperatures can cause other common issues, such as rutting, where the pavement deforms permanently under traffic loads; bleeding, where softened asphalt rises to the surface, creating a slippery layer; and a loss of stiffness, reducing the pavement’s load-bearing capacity. Biochar is among the cost-effective additives that can enhance asphalt’s high-temperature performance [143].

Building on earlier research in this area, Dong et al. [136] have continued to highlight the enhanced thermal stability of asphalt at high temperatures when biochar is incorporated. This enhancement helps stabilize the performance of road surfaces against temperature fluctuations, evidenced by the increased dynamic shear modulus and greater resistance to the permanent deformation of the asphalt surface at the post-aging phase. Similarly, in another study conducted by Ma et al. [43], the influence of biochar as an asphalt modifier was evaluated for its effect on improving the thermal stability of asphalt. They recorded increases of up to 35% in complex modulus and 36.5% in penetration, significantly enhancing the material’s resistance to permanent deformation. Improvements in the rutting factor and viscosity–temperature index were also observed, contributing to better temperature sensitivity and anti-rutting properties. It is worth noting that Ma et al. [43] mentioned the negative effect of biochar on the binder’s low-temperature properties; however, they indicated that this issue can be mitigated by controlling the biochar content.

In another study, Zhang et al. [144] investigated the viscoelasticity, rutting, and fatigue resistance of modified asphalt by comparing the enhancement effects of wood-based biochar and graphite as asphalt modifiers. They used RTFO (Rolling Thin Film Oven), PAV (Pressure Aging Vessel), and DSR (Dynamic Shear Rheometer) tests for their evaluations, and SEM (Scanning Electron Microscope) was employed to analyze the detailed microstructural features of both biochar and graphite [144]. The rationale for this comparison originates from the use of various carbon-based materials, such as carbon fiber, carbon black, and graphite, in asphalt modification. Notably, carbon fiber has demonstrated its ability to improve asphalt’s resistance to rutting under elevated temperatures, while also reducing its susceptibility to oxidation [145]. Moreover, carbon fiber has been shown to prolong asphalt’s resistance to fatigue failure [146], strengthen its flexural properties, and promote the self-healing potential of asphalt concrete [147]. According to Zhang et al. [144], increasing the biochar content from 2% to 8% resulted in greater elasticity and significantly improved the high-temperature stability of modified asphalt against rutting. They explained that the rougher surface and greater porosity of biochar provide a larger specific surface area compared to the relatively smooth surface of flake graphite. They attributed this to biochar’s rougher texture and higher porosity, which provide a larger specific surface area in contrast to the smoother surface of flake graphite. Consequently, biochar bonds more effectively with asphalt to form a stable network. This results in biochar-modified asphalt exhibiting better rutting resistance at high temperatures than graphite-modified asphalt, particularly when using small particle-sized biochar.

6. Biochar as a VOC Adsorbent in Asphalt

As previously mentioned, emissions from asphalt are a significant yet often overlooked source of precursors for secondary organic aerosols (SOAs). Unlike traditional sources, such as vehicle exhaust or industrial processes, asphalt-related emissions, particularly at ambient conditions, have not received as much attention in discussions about air pollution. Consequently, studies on the performance of biochar in capturing VOCs emitted from asphalt, before they are discharged into the environment, are not as extensive as those focusing on biochar’s role in enhancing the mechanical and physical properties of asphalt. Therefore, we will focus on the latest findings regarding biochar’s performance as a VOC adsorbent, particularly for emissions from asphalt surfaces.

Zhou et al. investigated the efficiency of biochar derived from materials such as waste wood, straw, and, and swine manure in mitigating various emissions from asphalt, including alkanes, polycyclic aromatic hydrocarbons, and sulfides [31]. The structural analysis indicated that biochar sourced from waste wood and pig manure was mainly composed of silicon dioxide (SiO₂), amorphous carbon, calcium carbonate (CaCO₃), graphite, and multiple polymer substances. In contrast, the straw-based biochar exhibited a well-defined crystal structure. Furthermore, the biochar effectively adsorbed VOC compounds from asphalt, including C₁₅H₃₀, C₁₆H₃₂, C₁₉H₄₀, and C₂₁H₄₄. Notably, biochar originating from waste wood and straw sources reduced VOC emissions by half. Additionally, the study found that biochar could effectively adsorb saturates (C₈H₁₈) and aromatics (C₁₀H₈); although, its adsorption efficiency for the hazardous compound dichloroethane was relatively low.

Mousavi et al. conducted multiple studies emphasizing the advantages of using different types of biochar to selectively capture certain harmful asphalt emissions, mitigating the negative impact of prolonged sun exposure and elevated temperatures on air quality [32,40,41,42,110].

Mousavi et al. used algal-derived biochar to adsorb compounds contributing to secondary organic aerosols [40]. The biochar utilized in this research, obtained via the thermochemical liquefaction of a feedstock derived from algae, exhibited a high nitrogen content due to its abundance of proteins and nucleic acids. It also featured oxygen-based functional groups, including -COOH, -OH, and C=O. Given its specific composition, the authors referred to this biochar as inherently functionalized carbon (IFC). It is worth noting that a primary difference between biomass derived from algae and that from land plants is the notably greater protein concentration found in algal biomass. This elevated protein level leads to the formation of amino acids and carboxylic acids during the hydrolysis process.

Mousavi et al. employed quantum-based molecular modeling, using density functional theory (DFT), to simulate the adsorption behavior of six organic compounds released from asphalt (Figure 2a) [40]. This set included three oxygen-containing structures, benzoic acid, benzofuran, and hexanal, and three sulfur-containing structures, dibenzothiophene, 3-pentylthiophene, and hexanethiol. The interactions between these compounds and the active centers on the biochar model surface were subsequently evaluated. As illustrated in Figure 2b, these active centers include nitrogen-containing functional groups, such as amides, amines, pyrroles, and pyridines. Based on the DFT results, benzoic acid and dibenzothiophene exhibited the highest average adsorption energy at 18.1 kcal/mol, while benzofuran had the lowest average adsorption energy, recorded at 8.3 kcal/mol. Remarkably, even though the strongest interactions were observed, there was no evidence of hydrogen transfer, simple ion pair formation, or amidization. This suggests that the interactions between the nitrogen-rich surface and the selected atmospheric contaminants are primarily governed by physical forces, such as dispersion forces, hydrogen bonding, and dipole–dipole interactions, rather than chemical reactions. While benzofuran displayed the least interaction with the IFC model surface, dibenzo-thiophene showed the highest level of interaction. The findings from the dynamic vapor sorption experiment revealed that the overall adsorption of benzofuran molecules on the IFC, 2.448%w, was significantly greater compared to that of dibenzo-thiophene, which was only 0.018%w. Mousavi et al. [40] suggested that this phenomenon arises from the less pronounced interaction between benzofuran and the active sites on the IFC surface, which enhances its mobility and allows for greater diffusion into the pores. In contrast to benzofuran, the stronger affinity of dibenzothiophene towards the IFC surface restricts its mobility and pore penetration, resulting in a lower overall adsorption capacity for dibenzothiophene.

Mousavi et al. also demonstrated the metal-catalyzed reactions in biochar and their impact on reducing emissions from asphalt surfaces. In a comparative study, they evaluated the efficacy of biochars with varying metal content in minimizing the hazardous VOC emissions from asphalt surfaces [42]. They compared the effectiveness of an acacia-derived biochar with that of a low-metal-content biochar made from silver grass. The elevated levels of metals within the acacia biochar were associated with its enhanced effectiveness in reducing VOC emissions from asphalt. Specifically, asphalt containing acacia biochar emitted 16.9% VOCs, compared to 21.2% VOC emissions from asphalt using silver grass biochar.

Mousavi et al. used DFT-based molecular modeling to analyze the adsorption properties of the following three major metals identified in acacia biochar: calcium (~8 wt.%), aluminum (~6.9 wt.%), and iron (~4.4 wt.%). For this purpose, each gaseous pollutant molecule interacted with the areas of the biochar surface, including functional groups composed of nitrogen and metals (Ca, Al, Fe) (Figure 2b). As an example, Figure 3 depicts the interaction of benzoic acid with a designated nitrogen area of biochar (pyrrole) that includes a calcium atom. Similar calculations were conducted to evaluate the interaction of benzoic acid with three other N-zones (amide, amine, and pyridine), all featuring the same metal atom (Ca). Parallel analyses were conducted to evaluate the interactions of other gaseous air pollutants with various nitrogen zones containing Ca, Al, and Fe atoms. Based on the DFT results focusing on adsorption energies and bond distances, Fe consistently demonstrated the strongest interactions with VOCs across all nitrogen-containing regions (pyridine, pyrrole, amine, and amide), followed by Ca, Al, and areas lacking metal, in the hierarchy of Fe > Ca > Al > no metal. For example, the adsorption energy associated with the interaction between benzoic acid and the Fe-pyrrole zone demonstrated considerable stabilization, measuring E = −104.8 kcal/mol. This value is five times greater than the adsorption energy measured without an Fe atom (−20.1 kcal/mol) and approximately twice that of the Ca-pyrrole zone interaction (E = −50.7 kcal/mol).

Another study by Mousavi et al. also highlighted the effective role of iron in biochar for reducing asphalt emissions [32]. Their research demonstrated that an iron-rich biochar, produced through the hydrothermal processing of a blend feedstock consisting of C. merolae algal biomass and swine manure, significantly reduced asphalt emissions [32]. Introducing iron-rich biochar to asphalt resulted in a 76% reduction in emissions, significantly higher than the 59% reduction achieved with low-iron biochar.

The combined biomass feedstock (C. merolae and swine manure) used in this research was rich in nitrogen, offering numerous nitrogen sites capable of coordinating with iron (or other metals) to create C-N-Fe (or C-N-metal) bonds. This nitrogen-rich property is particularly beneficial, because introducing iron and nitrogen compounds during biochar production creates Fe/N co-doped biochar, which has been shown to break down and remove organic pollutants effectively [148,149,150,151]. DFT calculations revealed that the active sites of Fe-modified biochar, containing -N-Fe functional groups, adsorb VOCs 8 to 10 times more effectively than unmodified biochar, containing only nitrogen groups

To emphasize the contribution of metals in enhancing the ability of biochar to reduce VOC emissions, it is important to note that incorporating metals, like Ca, Mg, La, Al, Cu, and Fe, into biochar has been shown to significantly improve its adsorptive performance. This enhancement occurs through alterations in surface properties, including functional groups, pore size, surface charge, and specific surface area, of the engineered biochar [152,153,154,155]. Although introducing iron sources such as iron salts (e.g., FeCl₃), iron oxides (e.g., Fe₃O₄), or nano zero-valent iron is a common pre-treatment method for enhancing the Fe content of biochar [148,149,150,156], these approaches have drawbacks, including pore blockage, reduced porosity, and a decrease in the biochar’s surface area [155,157]. Therefore, developing new approaches to enhance the Fe content of biochar without relying on iron precursors would be highly beneficial. In this context, studies focused on developing inherently Fe-rich biochar offer promising alternatives [32,158].

The effect of phenolic compounds on biochar’s capacity to capture VOCs released from asphalt has also been explored. Given the role of preparation techniques on biochar’s chemical composition, a comparative study was conducted to examine the chemical composition of acacia biochar produced through two different preparation techniques—a one-stage pyrolysis process in a fixed-bed reactor and a two-stage pyrolysis process involving an auger reactor connected to a fluidized-bed reactor [9]. The results showed that the fixed-bed reactor produces biochar with a higher number of surface functional groups, suggesting greater potential for adsorption [9]. FTIR analysis further revealed a higher concentration of phenolic OH groups. Additionally, the fixed-bed biochar exhibited a higher concentration of inorganic elements, including Ca, Al, K, and Fe.

DFT-based assessment of adsorption energies confirmed that phenolic groups and phenoxy radicals play a crucial role in the interaction of metal sites present on biochar with target VOCs including benzoic acid, benzofuran, and dibenzothiophene. While the biochar featuring phenoxyl radicals was found to be less effective than the one with phenolic OH groups, it still outperformed the original unmodified biochar, which lacked both phenolic OH groups and phenoxyl radicals [9,41]. The UV-Vis absorption analyses supported the DFT outcomes, demonstrating that biochar with a high content of phenolic OH functional groups could adsorb greater amounts of VOC air pollutants.

In UV-Vis absorption spectroscopy, biochar typically shows a broad, featureless spectrum, where the absorbance steadily declines at longer wavelengths [159]. When exposed to VOCs, the biochar sample may not display significant spectral changes that reflect its capacity to adsorb these compounds. In such cases, signal–concentration plots are adopted, where the signal from the analytical device correlates directly with the sample concentration. This facilitates the estimation of the unknown concentration within the sample. For biochar–VOC interactions, the biochar samples were exposed to known concentrations of VOCs. After filtration, the final VOC concentration in the supernatant was analyzed to quantify the amount of VOCs retained by the biochar [41].

Biochar as a Carbon Negative Adsorbent

Biochar contributes to carbon sequestration by storing carbon that is captured from atmospheric carbon dioxide. During photosynthesis, plants absorb CO₂, converting it into organic carbon, which becomes part of the biochar when the biomass is pyrolyzed. Ideally, if the biochar production process is entirely emissions-free, the net CO₂ equivalent would reflect the precise amount of carbon stored in the biochar. This situation would signify a true reduction in atmospheric CO₂ levels, thereby enhancing efforts to combat climate change. However, the pyrolytic conversion of biomass into biochar is an energy-intensive process that generates CO₂ emissions. Therefore, to accurately determine the carbon footprint, the CO₂ emissions from the production process should be deducted from the overall CO₂ absorbed by the plants.

The carbon footprint of biochar varies based on the geographical origin and composition of the feedstocks, as well as the specific purposes for which biochar is applied. Variables such as the type of biomass, manufacturing techniques, and transportation logistics can all impact the overall carbon footprint. A comprehensive cradle-to-grave life cycle assessment (LCA) is usually performed to evaluate the carbon footprint of biochar derived from different biomass feedstocks. As an example, Leppäkoski et al. [160] calculated the carbon emissions associated with willow-based biochar production in Finland and evaluated the potential of marginal lands to offset these emissions when used for willow biochar production. Recent literature indicates that the carbon footprint of biochar ranges from −2.0 kg to −3.3 kg of CO₂- eq per kg (equivalent per kilogram) of biochar [161]. This negative footprint shows that biochar production and application can result in net carbon sequestration, removing more CO₂ from the atmosphere than it emits.

7. Research Gaps and Future Directions

Considering the promising role of biochar in reducing VOC emissions and improving asphalt durability, future research is warranted to address the following research gaps to facilitate biochar adoption.

Optimization of Biochar Properties: There is a significant gap in understanding how key biochar properties, such as pore size distribution, surface functional groups, and metal content, can be optimized to enhance the retention of VOCs in asphalt. Pre- and post-treatment strategies, such as doping biochar with Fe and N compounds, have shown great promise in enhancing its adsorption capabilities [162,163,164,165]. However, further research should focus on developing new methods, such as creating inherently Fe-rich biochar to enhance its adsorption properties specifically for asphalt applications.

Ecological Impact and Life Cycle Analysis: The integration of biochar into various matrices, including asphalt, offers numerous environmental benefits [166], yet there is a significant gap in understanding the full ecological implications of its use. A comprehensive life cycle assessment (LCA) is crucial to evaluate the overall sustainability of biochar-modified asphalt, from its production through its end-of-life phase [167].

Economic Viability and Scalability: The profitability and scalability of biochar production and use in asphalt applications vary based on the geographic location, raw material, production scale, pyrolysis parameters, biochar cost, and the variety of crops grown [168]. To facilitate implementation, a comprehensive techno-economic analysis is required.

8. Conclusions

As global efforts to mitigate climate change and protect public health intensify, biochar offers a promising solution for reducing asphalt-induced air pollution [169] and enhancing infrastructure sustainability [140]. By incorporating biochar into asphalt, significant reductions in volatile organic compound (VOC) emissions can be achieved [170], addressing a major source of ground-level ozone and secondary organic aerosols (SOAs). This not only improves air quality but also mitigates the environmental impact of asphalt production throughout its lifecycle [41]. Biochar also enhances the performance of asphalt. It increases resistance to rutting, fatigue cracking, and oxidative aging [139], leading to longer-lasting roadways that require less frequent maintenance and repaving. Additionally, biochar helps preserve the color of asphalt and reduces surface degradation from solar radiation, contributing to the aesthetic and functional longevity of roads [137]. From a climate perspective, biochar is a carbon-negative material, with an embodied carbon as low as ~ −200 kg/ton [42]. This means biochar captures more carbon than is emitted during its production, further supporting climate change mitigation. The cost of biochar is approximately USD 600/ton [171], which, when coupled with the long-term savings from reduced maintenance and fewer re-paving, makes it a cost-effective material for sustainable infrastructure development. To facilitate the widespread adoption of biochar-modified asphalt, it is essential to streamline its integration into plant operations. Standardizing biochar production methods, ensuring consistent properties, such as particle size and surface chemistry, and developing efficient blending techniques will be key to overcoming logistical challenges and enhancing cost-effectiveness.