Mesquite (Prosopis glandulosa) is a leguminous species noted for its rapid growth, capable of reaching heights of up to 2 m per year. The plant is highly tolerant of elevated temperatures and can fix atmospheric nitrogen. Mesquite often forms dense, thorny thickets on degraded or nutrient-poor soils, making it a suitable and sustainable feedstock for biochar production (Prasad and Tewari, 2024; Kosgei et al., 2022). This species is predominantly found in the Sonoran Desert of the southwestern United States, as well as across sub-Saharan Africa, South America, parts of the Middle East, and regions of Australia and South Asia (Morrison et al., 2023). In many ecosystems, mesquite is classified as an invasive species because it tends to outcompete native vegetation, thereby threatening local biodiversity and ecosystem productivity. Importantly, the utilisation of mesquite for biochar generation does not compete with land designated for agricultural production, enhancing its environmental and economic viability (López-Cano, et al., 2018).

Mesquite is a fast-growing tree species widely distributed across the arid and semi-arid lowlands of the southwestern United States. It flourishes in soils with alkaline, calcareous, clay-rich, and rocky compositions, demonstrating strong tolerance to poor soil conditions due to its extensive root system and allelopathic properties (Yang, et al., 2024). In addition, mesquite can rapidly colonise grassland habitats and riparian zones, often forming dense thickets that outcompete native vegetation and alter ecosystem dynamics under favourable environmental conditions (Marmiroli et al., 2018).

The global expansion of biochar use across agricultural and environmental sectors aligns closely with the principles of sustainable land management (Segura-Chavarría, 2018). Accordingly, this study evaluated the impacts of mesquite harvesting on landscape sustainability. To prevent disturbances similar to overgrazing-which can hinder mesquite regeneration and promote the spread of invasive non-native grasses and vegetation-it is recommended to remove approximately one out of every three mesquite trees (Laird et al., 2010). This selective harvesting strategy is also utilised in biomass collection for energy production, thereby reducing potential environmental impacts (Saleem, 2022). The sustainable and ecologically responsible management of mesquite resources is therefore essential for ensuring both effective biochar production and environmentally sound wastewater treatmen

The term “biochar” refers to a carbon-rich material produced under conditions of minimal or no oxygen (Wang et al., 2020). Growing global interest in biochar largely arises from its potential role in carbon sequestration and its effectiveness as a functional medium for wastewater treatment. In spite of the publication of more than 7,000 studies related to biochar in 2019 alone, it remains a relatively ill-defined and unregulated product (Alhassan et al., 2025). Definitions of biochar vary considerably across disciplines and industries, and the pyrolysis methods used for its production differ widely (Spreafico et al., 2021). Research exploring biochar’s potential as a bio-sorbent for water and wastewater treatment has continued for several decades, with significant advancements achieved over the past 10 years (Irewale et al., 2024). For example, a pilot-scale wastewater treatment facility in Pontevedra (Galicia) operating entirely with biochar has demonstrated exceptional performance in removing phosphorus and mineral pollutants (Eniola and Sizirici, 2023). Moreover, biochar has received regulatory approval from governing bodies in England (Environment Agency) and South Korea (Environmental Industry and Technology Institute) and is commercially available in several areas (Yekeen et al., 2023). Additionally, various airport authorities, including those at Portland International Jetport (Oregon) and Denver International Airport (Colorado), have implemented on-site wastewater treatment systems utilising biochar (Sillah et al., 2024).

Biochar can be synthesised through several thermochemical processes, including hydrothermal carbonisation, pyrolysis, gasification, and torrefaction (Ercan et al., 2023). Among these, pyrolysis is a complex thermal degradation process involving the breakdown and chemical transformation of biomass. The procedure encompasses multiple stages, such as drying, dehydration, depolymerisation, fragmentation, vaporisation, and carbonisation (Parrilla-Lahoz et al., 2025). Pyrolysis is typically categorised into slow, fast, and flash types, distinguished by variations in operating temperature, heating rate, and residence time (Ighalo et al., 2022). Slow pyrolysis is performed at 350 °C–700 °C, with a gradual heating rate (5 °C–20 °C min-1) and extended residence times ranging from minutes to hours, favouring the formation of char. Conversely, fast pyrolysis occurs at moderate temperatures between 400 °C and 600 °C, employing extremely high heating rates (>100 °C min-1) and short vapour residence times (1–3 s) (Boer et al., 2021), conditions that primarily facilitate bio-oil production. Flash pyrolysis, on the other hand, involves ultra-rapid heating and minimal residence times designed to maximise bio-oil yield (Rasaq et al., 2021). Both fast and flash pyrolysis are generally carried out at 450 °C–600 °C. Hydrothermal carbonisation refers to the conversion of biomass into biochar within an aqueous phase under specific temperature and pressure conditions (Lin et al., 2022). This approach is especially suitable for feedstocks with high moisture content (30–90 wt%), enabling the transformation of hemicellulose, cellulose, and lignin into carbon-rich solids with increased energy density (González-Arias et al., 2022). Gasification, another thermochemical technique, converts feedstock into synthesis gas and char under limited oxygen conditions, typically at 600 °C–1,200 °C with short heating and residence periods (Maitlo et al., 2022). In contrast, torrefaction is a milder form of pyrolysis in which biomass undergoes chemical alterations in an oxygen-deficient environment when heated between 200 °C and 320 °C (Piersa et al., 2022).

Table 1 provides physicochemical characteristics of biochar relevant to wastewater treatment. Biochar is a carbon-rich substance formed through the pyrolysis of biomass under oxygen-limited conditions, characterised by its porous structure, alkaline pH, and low bulk density, which collectively contribute to its strong adsorption performance (He et al., 2022). Its major advantages include the wide availability of feedstock materials, low production cost, and environmental sustainability in wastewater treatment. Biochar exhibits high efficacy in the removal of heavy metals, organic pollutants, and nutrients, and can undergo chemical or physical modification to improve adsorption capacity and selectivity. Additionally, it contributes to carbon sequestration and enhances soil structure, nutrient retention, and microbial activity (Ighalo et al., 2022). However, its moderate porosity can limit large-scale industrial applications, while adsorption efficiency is often affected by factors such as solution pH and water composition (González-Arias et al., 2022). Mesquite has emerged as a sustainable and abundant feedstock for biochar production, with pyrolysis parameters-namely temperature, heating rate, and particle size-playing a crucial role in determining its physicochemical properties (Prasad and Tewari, 2024). The primary applications of biochar include wastewater treatment, soil amendment, and energy production through its use as a fuel source or as a precursor for activated carbon synthesis (Kosgei et al., 2022). Research findings show that pyrolysis optimization methods affect both biochar production rates and surface characteristics which in turn affect the success of following modification steps because metal species stability depends on surface chemistry and pore structure (Lu et al., 2022).

Chemical-based modification has emerged as a dominant strategy for tailoring the properties of biochar, owing to its effectiveness and versatility. These treatments commonly involve exposing biochar to acidic or alkaline reagents, oxidizing substances, metal salts or oxides, and, in some cases, organic modifiers. Acid treatment is frequently applied to alter surface chemistry by introducing additional acidic functional groups and restructuring pore networks, which together enhance the availability of sites for cation interaction. However, excessive acid strength or prolonged exposure can compromise pore integrity, leading to structural deterioration. Variations in surface area following acid modification have been reported, largely influenced by the specific acid used and its concentration (Ambika et al., 2022; Wang and Wang, 2019).

In contrast, alkaline treatment primarily promotes surface activation through the formation of new functional groups and the expansion of pore volume. These changes improve the interaction between biochar and negatively charged contaminants, thereby increasing adsorption efficiency (Liu et al., 2020; Hafeez et al., 2022). Oxidative modification represents another effective pathway, as oxidants such as hydrogen peroxide and potassium permanganate introduce oxygen-rich functionalities onto the biochar surface. Such modifications have been shown to significantly enhance contaminant uptake when compared with untreated biochar materials (Li et al., 2022). Metal-based modification further extends the functionality of biochar by altering its surface charge, catalytic behavior, and, in some cases, magnetic response. Unmodified biochar typically exhibits limited affinity for anionic pollutants due to its negatively charged surface. Incorporation of metal species modifies these surface characteristics, resulting in improved adsorption performance toward negatively charged contaminants. Additionally, metal incorporation can impart catalytic properties that support redox-based degradation reactions. A practical challenge associated with biochar application in aqueous environments is its recovery after use, as fine particles are difficult to separate from treated water. This issue can be effectively addressed through iron-related modification, which introduces magnetic properties that enable straightforward recovery using external magnetic fields. The integration of magnetic iron oxides or zero-valent iron particles has been shown to enhance both the reusability of biochar and its capacity to remove heavy metals (Karunanayake et al., 2018).

The preparation of metal-enhanced biochar generally follows two distinct pathways. One approach involves combining metal precursors with biomass feedstock prior to pyrolysis, producing metal-containing biochar directly during thermal conversion. Alternatively, biochar may first be generated through pyrolysis and subsequently treated with metal ions or metal oxides under controlled impregnation conditions (Li et al., 2024; Ni et al., 2023). Metals commonly employed in these processes include iron, manganese, magnesium, and aluminum. Among these, iron-modified biochar has consistently demonstrated superior effectiveness in contaminant removal and degradation processes. This enhanced performance has been linked to the uniform distribution of iron species on the biochar surface, as evidenced by energy-dispersive X-ray spectroscopy (EDS) analysis (El-Bestawy et al., 2023). For example, Jiang et al. (Jiang et al., 2020) reported a degradation efficiency of 73.47% using a zero-valent iron–biochar composite. The improved reactivity of iron-loaded biochar is attributed to an increased density of active sites, which promotes more efficient electron transfer during catalytic reactions (Liang et al., 2024).

Physical modification techniques represented an important category of approaches used to tailor the surface characteristics and pore architecture of biochar. Common physical treatments included mechanical size reduction through ball milling, steam activation, and gasification processes. These methods were primarily applied to enhance surface area, modify pore structure, and improve the overall reactivity of biochar materials.

In recent years, ball milling had attracted considerable interest as an effective technique for producing fine and nano-scale materials due to its high operational efficiency and environmentally friendly nature (Honghong et al., 2017). During the milling process, high-energy collisions between rapidly moving grinding media and biochar particles mechanically reduced particle size, resulting in materials with increased surface area and improved structural uniformity. The simplicity, cost-effectiveness, and reproducibility of ball milling made it a widely adopted modification method in experimental investigations (Kumar et al., 2020). Previous studies reported that mechanically milled biochar exhibited enhanced surface functionality and greater specific surface area, which collectively contributed to improved adsorption performance toward a wide range of contaminants. Gas-based activation methods, including treatment with steam, oxygen, or carbon dioxide, were also extensively employed to modify biochar properties. These processes promoted the development of porous structures and generated additional reactive surface sites, thereby increasing adsorption capacity. Gas activation was typically conducted at elevated temperatures, often exceeding 700 °C, and required controlled amounts of activating gases. Although this approach was considered environmentally benign and avoided secondary pollution, it generally resulted in lower biochar yields compared with conventional pyrolysis. Furthermore, the high thermal and energy demands associated with gas activation limited its large-scale application, despite the significant improvements achieved in surface area and pore volume (Amusat et al., 2021; Shao et al., 2018; Pallarés et al., 2018).

Biological modification of biochar relied on the use of living organisms or biological materials, such as microorganisms or plants, to alter its surface characteristics and functional behavior. In this approach, microbial communities were deliberately introduced and allowed to develop on the biochar surface. Residual microbial biomass, including bacteria, fungi, algae, and yeasts, was reported to exhibit a strong affinity for heavy metals. Beyond metal accumulation, these microorganisms contributed to the adsorption and biodegradation of various organic and inorganic contaminants through biological activity and metabolic processes (Tao et al., 2021; Xu et al., 2022; Muhammad et al., 2021). Despite its potential benefits, biological modification had been applied less frequently than physical or chemical modification methods. This limited adoption was primarily associated with the complexity of biological systems and the extended time required to establish stable interactions between biochar and microorganisms or plants. Additional challenges, such as sensitivity to environmental conditions and difficulties in process control, further restricted the widespread implementation of this modification strategy.

Iron incorporation into biochar has become a key focus of recent studies aimed at improving materials used for environmental cleanup, carbon retention, and nutrient regulation. Biochar itself is produced via the thermal conversion of biomass under limited oxygen conditions and is characterized by a porous carbon matrix, extensive internal surface area, and a variety of surface functional sites. While these properties make biochar attractive for many applications, its unmodified form often shows inadequate interaction with certain contaminants, reducing its practical efficiency. To address these shortcomings, researchers have increasingly adopted iron-based modification techniques to tailor the surface chemistry and reactivity of biochar. Introducing iron species onto or into the biochar matrix significantly alters its physicochemical behavior, enabling stronger binding of heavy metals, improved immobilization of phosphorus compounds, and enhanced redox activity toward organic and inorganic pollutants. The effectiveness of these processes is closely linked to the nature of iron–carbon (Fe-C) interactions formed during modification. Such interactions govern adsorption mechanisms, catalytic reactions, and electron transfer processes, which collectively determine the performance of iron-modified biochar systems, as summarized in Table 1.

Figure 2 illustrates the transformation of original biochar into iron-modified biochar and highlights the key interaction mechanisms that arise from iron incorporation. Initially, the biochar precursor is shown with a porous carbon matrix containing surface functional groups such as hydroxyl (-OH) groups. These functional groups act as anchoring sites for iron species during the modification process. The central part of the figure presents the main iron modification routes. These include iron salt impregnation, co-precipitation using Fe²⁺ or Fe³⁺ ions, and thermal or combustion-assisted loading. Through these processes, iron species are introduced onto or into the biochar structure in different chemical forms, such as zero-valent iron (Fe⁰), iron oxides (e.g., Fe₂O₄), and iron oxyhydroxides (e.g., FeOOH). As a result, iron particles become uniformly dispersed within the biochar pores or attached to its surface. The final section of the figure emphasizes the iron–carbon interaction mechanisms that govern the enhanced performance of iron-modified biochar. Surface complexation occurs when iron species chemically bind with functional groups on the biochar surface. Redox reactions involve reversible changes in iron oxidation states, which play a crucial role in contaminant transformation. Electron transfer between iron particles and the carbon matrix further enhances catalytic activity. Collectively, these interactions explain why iron-modified biochar exhibits improved adsorption, catalytic reactivity, and environmental remediation potential compared to unmodified biochar.

A range of fabrication strategies has been employed to integrate iron species into biochar, each influencing the distribution, chemical form, and functionality of iron within the carbon matrix. Widely reported preparation routes include precursor-assisted thermal conversion, post-synthesis chemical deposition, hydrothermal treatment, and mechanical activation through ball milling. Among these, the precursor-assisted approach introduces iron compounds at an early stage by incorporating iron salts directly into the biomass prior to thermal processing. In this method, biomass feedstocks are treated with aqueous solutions containing divalent or trivalent iron ions, enabling iron to diffuse throughout the organic structure before carbonization. Subsequent pyrolysis promotes the formation of iron-based nanoparticles-such as iron oxides or metallic iron-embedded within the developing biochar framework (Zhang et al., 2012). During thermal treatment, iron species undergo transformation through interactions with reducing gases generated from biomass decomposition, resulting in the in-situ conversion of iron ions into stable iron phases (Yin et al., 2024). In contrast, post-synthesis modification methods involve treating already-produced biochar with iron-containing solutions. This approach is typically followed by thermal activation or chemical treatment, facilitating the attachment of iron oxides or zero-valent iron particles onto the biochar surface rather than within its interior structure (Tan et al., 2015).

Chemical precipitation represents another commonly applied strategy, in which biochar is immersed in metal salt solutions and subsequently exposed to precipitating agents that induce controlled deposition of iron compounds onto available surface sites (Wang et al., 2024). Hydrothermal processing further enables the controlled crystallization and homogeneous distribution of iron oxides within biochar pores under elevated temperature and pressure conditions, using water-based reaction environments (Peng et al., 2022). Mechanical ball milling provides a distinct modification route by subjecting biochar–iron mixtures to high-energy impacts, which generate structural defects, reduce particle size to the nanoscale, and enhance surface reactivity through intensified bond disruption and radical formation (Fang et al., 2025). More recently, advanced techniques such as sol-gel synthesis, microwave-assisted impregnation, and refined hydrothermal processes have been explored to achieve improved regulation of iron oxidation states, nanoparticle dispersion, and surface charge characteristics. These developments allow precise tailoring of pore structure and redox behavior, ultimately enhancing the environmental performance of iron-modified biochar materials (Qiu et al., 2022).

Despite notable progress in the development of iron-modified biochar, several fundamental research challenges remained unresolved, limiting its broader application potential. One major limitation concerned the long-term stability of these materials under realistic environmental conditions. In particular, the behavior of iron-modified biochar during prolonged exposure to variable pH levels, redox environments, and microbial activity had not been sufficiently characterized. Existing studies provided limited insight into how iron–carbon (Fe-C) interactions evolved over time and how such changes influenced contaminant retention, transformation, or release under field conditions. Another unresolved issue involved the detailed characterization of iron species at the nanoscale. While bulk analyses were commonly reported, fewer studies employed advanced spectroscopic and microscopic techniques capable of resolving iron oxidation states, coordination environments, and spatial distribution within the biochar matrix. Analytical tools such as X-ray absorption spectroscopy, Mössbauer spectroscopy, and transmission electron microscopy were therefore considered essential for establishing clear structure–function relationships between iron phases and biochar performance (Wang et al., 2025).

In addition, the synthesis of iron-modified biochar had not yet been sufficiently standardized for application-specific use. Many preparation methods lacked scalability or economic feasibility, and optimization for individual target contaminants-such as nitrate, arsenic, or per- and polyfluoroalkyl substances-remained limited. Achieving consistent material properties required careful selection of biomass feedstocks and iron precursors to tailor surface chemistry, redox behavior, and adsorption capacity. Environmental implications associated with iron modification also required more comprehensive assessment. Potential risks related to iron leaching, nanoparticle toxicity, and greenhouse gas emissions during production and application had not been fully evaluated. As a result, system-level life-cycle assessments were considered necessary to balance functional benefits against environmental trade-offs and to support sustainable deployment strategies (Meng et al., 2025).

Finally, although iron-modified biochar had attracted interest for use in engineered treatment systems-including constructed wetlands, permeable reactive barriers, and bioreactors-field-scale validation remained limited. Key aspects such as hydraulic performance, regeneration capability, and long-term pollutant removal kinetics had not been adequately examined under operational conditions (Masud et al., 2025). Overall, iron-modified biochar was recognized as a multifunctional material positioned at the intersection of iron chemistry and carbon science. Progress toward its practical implementation depended on improved mechanistic understanding of Fe-C interactions and the development of application-driven, environmentally responsible modification strategies. Addressing these gaps was expected to significantly advance the use of iron-modified biochar in pollution mitigation, nutrient management, and climate-resilient agricultural systems.

Research into modified biochar composites for wastewater treatment has increased during the last few years because these materials enable both pollutant absorption and catalytic breakdown of contaminants. The material known as iron-modified biochar shows great potential because its iron components create redox sites which enable both advanced oxidation reactions and traditional adsorption processes (Lu et al., 2022). The treatment system for wastewater uses iron-modified biochar as a substance which acts as both an adsorbent and a catalyst. The addition of iron species to biochar matrixes results in better photocatalytic performance because it enhances charge separation and decreases electron–hole recombination which produces higher degradation rates for organic pollutants (Zhang et al., 2021). The reversible redox process between Fe²⁺ and Fe³⁺ enables the production of reactive oxygen species which act as essential agents for breaking down persistent organic contaminants through oxidative reactions (Lu et al., 2022). The surface sites of biochar become more positive after iron modification which results in better adsorption of negatively charged contaminants including dyes and metal ions (Venkatesh et al., 2022; Dhila et al., 2025). The biochar matrix shows strong bonding with iron oxides which prevents particle clumping those results in better distribution of active sites and extended adsorption performance (Rana et al., 2024). The iron-modified biochar system performs adsorption as its primary function while also enabling photocatalytic degradation through light-induced charge transfer that produces reactive oxygen species. Research shows that iron-containing phases which rest on biochar surfaces enhance electron-hole separation to produce better organic pollutant degradation than systems without modifications (Zhang et al., 2021; Lu et al., 2022).

Biochar synthesised from mesquite pods exhibits a diverse array of surface functional groups which play a crucial role in governing adsorption mechanisms (Wang et al., 2020). The predominant interactions involved include: (a) π–π electron donor–acceptor interactions, (b) electrostatic interactions, and (c) hydrogen bonding (Guo et al., 2023) as shown in Figure 3. The surface of mesquite biochar is highly heterogeneous, consisting of carbonised polyaromatic and graphitic domains interspersed with non-carbonised regions enriched with reactive functional groups, thereby facilitating interactions with a wide spectrum of organic contaminants (Diaz-Uribe et al., 2022). The electrostatic behaviour is largely dictated by the charge characteristics of both the sorbate molecules and the biochar surface (Zhang et al., 2023). For instance, the cationic dye crystal violet associates with negatively charged deprotonated -COOH groups through electrostatic attraction, whereas the anionic dye indigo carmine interacts with protonated -NH³⁺ on the biochar surface (Rouahna et al., 2023). Additionally, hydrogen bonding plays a prominent role under certain conditions; for example, adsorption of the anionic dye brilliant blue onto deashing-treated mesquite biochar is significantly enhanced through hydrogen bond formation between the dye molecules and surface functional groups (Verma et al., 2025).

Kimbell et al. (2018) conducted an extensive investigation into the performance of three types of biosolid-derived biochar for the removal of triclosan from wastewater. Batch adsorption experiments were carried out at pH seven using deionised water, humic acid solutions, and model wastewater to elucidate the underlying sorption mechanisms. The presence of additional organic micropollutants and inorganic nutrients was found to reduce triclosan removal efficiency due to competitive adsorption effects. Despite these interferences, biosolid-based biochar demonstrated effective triclosan removal from secondary wastewater effluent, achieving efficiencies comparable to those of activated carbon, commercial biochar, and other benchmark sorbents. It is worth noting that the triclosan concentrations applied in this investigation were quite higher compared to those usually encountered in wastewater, signifying potential drawbacks for practical application at lower concentrations. Nonetheless, biosolid-derived biochar shows strong potential as a tertiary treatment option for the adsorption of triclosan and other trace organic micropollutants in wastewater effluents. Further research is required to assess the advantages of re-pyrolyzed biosolid biochar and to identify potential operational limitations. The integration of biochar into tertiary treatment processes could significantly mitigate the release of organic micropollutants into natural water bodies, particularly in ecologically sensitive areas or regions dependent on treated effluent. To determine the practical feasibility of employing fixed-bed filtration systems utilising biosolid-based biochar for wastewater reclamation, comprehensive pilot-scale investigations are recommended. Research shows that adding iron or other metal compounds to biochar creates a system that removes dyes through two mechanisms: adsorption and light-activated catalytic oxidation (Lu et al., 2022).

Biochar has attracted growing interest owing to its favourable physicochemical and adsorptive characteristics, which enable efficient sequestration of heavy metals and organic pollutants from wastewater (Wang et al., 2020). The presence of surface functional groups like hydroxyl (−OH), amino (-NH₂), and carboxyl (-COOH) facilitates diverse adsorption mechanisms including chemical bonding and physical interactions making biochar a highly effective material for adsorbent development (Tan et al., 2021). Its porous framework allows for the adsorption of up to 93.6% of six endocrine-disrupting compounds through multiple intermolecular processes, including surface complexation and intraparticle diffusion (Liao et al., 2022). The intrinsic properties and structure of biochar can be tailored to enhance adsorption capacity and selectivity through various modification techniques. These include increasing surface area and porosity via steam treatment or high-temperature pyrolysis, adjusting pH to optimise surface charge, incorporating metal dopants to form chemical complexes, and embedding nanoparticles such as hematite to bolster electrostatic attraction (Lee and Shin, 2021). Biochar-based remediation represents a sustainable approach for agricultural, industrial, and municipal wastewater treatment, serving as an environmentally friendly alternative to activated carbon (Gupta M. et al., 2022). Table 3 outlines the primary functions, merits, limitations, and applications of biochar in wastewater remediation. Biochar serves as an efficient adsorbent for heavy metals, organic pollutants, and nutrients, owing to its porous structure and active functional groups (-OH, -NH₂, -COOH) that promote chemical as well as physical adsorption (He et al., 2022; Wang et al., 2020; Kimbell et al., 2018; Khan et al., 2024). Its sustainability, low production cost, and potential for carbon sequestration and soil enhancement further reinforce its viability as a substitute for activated carbon (Segura-Chavarría, 2018; Singh et al., 2022; Andrés et al., 2019; Kaetzl et al., 2020). Nevertheless, restricted porosity and decreased performance under varying pH or water compositions may limit its large-scale applicability (Hussain et al., 2021; Kimbell et al., 2018; Ma et al., 2020). Mesquite-derived biochar, in particular, offers a renewable and abundant feedstock, with chemical modification especially alkaline activation-significantly improving its adsorptive efficiency. Furthermore, the integration of biochar with advanced oxidation processes or membrane filtration systems can enhance overall treatment performance. Biochar is also increasingly employed in tertiary wastewater treatment for the removal of micropollutants, nutrients, and pathogens (Verma et al., 2025; Wang et al., 2020; Maitlo et al., 2022; Tan et al., 2021; Liao et al., 2022; Lee and Shin, 2021; Khan et al., 2024; Das et al., 2022; Fawzy et al., 2021; Shalini et al., 2023).

Biochar contributes significantly to nitrogen removal in wastewater treatment systems, primarily by promoting microbial denitrification. It also enhances phosphate uptake through multiple mechanisms, including ion exchange, electrostatic attraction, complexation, and precipitation (Zhang et al., 2020). Biological nutrient removal systems are widely employed to lower nitrogen and phosphorus concentrations in wastewater, and process selection is crucial to achieving optimal cost-effectiveness and treatment efficiency (Parde et al., 2024). The A2N system, incorporating anoxic, anaerobic, and aerobic stages, is known for its low energy requirements and ability to produce high-quality effluent; however, residual nutrient levels can remain elevated following treatment (Chen P. et al., 2022). Activated sludge-derived biochar was produced under vacuum conditions via carbonisation at a heating rate of 10 °C/min up to ≤600 °C for 1 hour, followed by cooling, grinding, and characterisation using SEM, FTIR, X-ray diffraction (XRD), and BET analyses (Sierra et al., 2023). Laboratory-scale batch tests were subsequently performed to evaluate its nutrient removal performance using synthetic A₂N reactor influent. Within 30 min, the biochar exhibited substantial adsorption capacities for nitrogen (35.17–10.95 mg/g) and phosphorus (7.57–3.25 mg/g) (Cai and Ye, 2022). Reusability assessments demonstrated effective nutrient removal across five consecutive adsorption–desorption cycles. The dominant removal mechanisms included ion exchange, electrostatic attraction, surface complexation, and precipitation. The surface complexation and redox-mediated immobilisation properties of iron-modified biochars enhance their ability to remove metals and nutrients which results in better retention stability when water chemistry changes. Collectively, these findings confirm that activated biochar represents a cost-effective and efficient adsorbent for nutrient removal from biological nutrient removal system effluents.

Biochar was characterized by a notably higher carbon content compared to conventional activated carbon, which contributed to its increasing importance in environmental management and pollution mitigation efforts (Ravindiran et al., 2024). Numerous studies have demonstrated the ability of biochar to remove a wide range of contaminants from wastewater systems, highlighting its versatility as an adsorbent material (Li et al., 2022; Bayar et al., 2024). The following sections discuss the removal efficiency and adsorption performance of biochar for different categories of environmental pollutants.

Global agricultural yield is closely linked to the dynamics of aeolian desertification and deforestation. The expansion of urban areas onto arable land, combined with declining investment in the agricultural sector, has forced many crop producers to abandon farming opportunities. In southern Texas, New Mexico, and Mexico, rural households often experience some of the lowest income levels in the United States. The urbanisation of adjacent river valleys has, however, stimulated a commercial market for peri-urban agricultural production. The mesquite tree, a species well adapted to desert conditions, has emerged as a viable crop capable of generating employment for farming communities in southern New Mexico and other economically disadvantaged arid regions worldwide. Mesquite pods are nutritionally rich, containing approximately 12%–15% protein with lysine levels comparable to those found in soy protein. They also comprise 10%–15% soluble sugars and up to 60% insoluble sugars. The development of a sustainable mesquite-based industry requires efficient pod collection from trees located in areas where trichomes-needle-like axial hairs that can cause skin irritation-are either absent or removed during harvesting. Approximately 5%–7% of mesquite trees are non-productive, while the remaining majority produce commercially viable pods. To support this endeavour, a community seed cooperative was established in a desert region to process mesquite pods using a patented technology developed by New Mexico State University. This initiative provides an economic pathway for enhancing household income in both rural and urban settings while promoting the local production of high-protein flour. Mesquite pod flour represents a promising and sustainable alternative to wheat flour, as mesquite thrives in arid and semi-arid environments and offers considerable nutritional benefits. Mesquite trees employ biological nitrogen fixation and produce pods containing up to 36% sugar and up to 13% protein, as well as high levels of essential minerals such as calcium and zinc. As a result, mesquite flour holds substantial potential as a nutritious substitute for conventional flour, contributing to food security in water-scarce regions. Furthermore, this review discusses the nutritional composition of mesquite pods and flour, along with their positive implications for human health and soil conservation. Both unconverted and converted biochar derived from mesquite biomass have been shown to improve soil aggregation and structural stability by enhancing aggregate water resistance (Hussain et al., 2021).

Biochar can be synthesised in a cost-efficient manner through the use of basic technological methods while maintaining laboratory-level quality control (Verma et al., 2025). As a result, mesquite-derived charcoal presents itself as a practical and sustainable option for the tertiary treatment of municipal wastewater and secondary effluents. Filtration systems incorporating mesquite biochar offer an affordable and effective means of advanced treatment, simultaneously generating a beneficial by-product (Kaetzl et al., 2020). This residual biochar can subsequently be repurposed for agricultural enhancement or as a renewable energy resource.

A comprehensive cost-benefit analysis was undertaken to evaluate both the private and societal advantages, as well as the associated expenses, of incorporating biochar into wheat cultivation systems in the U.S. (Kulyk, 2015). This assessment integrates insights from existing literature, empirical findings from field trials, and informed assumptions concerning the current and projected dynamics of the U.S. biochar market. The principal aim is to ascertain whether the benefits derived from biochar application surpass the corresponding costs under specific conditions, while also outlining policy recommendations to encourage its widespread adoption. Biochar contributes to long-term carbon stabilisation, forming a measurable and durable carbon sink offering greater consistency compared to the variable outcomes of greenhouse gas mitigation through conventional land-use practices (Mosa et al., 2023). Furthermore, the distinctive physicochemical attributes of biochar enhance soil fertility and crop productivity. By functioning as a persistent carbon storage medium, biochar influences soil–root interactions, promoting improved nutrient and water utilisation efficiency, ultimately leading to higher agricultural yields (Singh et al., 2022). These attributes position biochar as a promising solution capable of simultaneously addressing climate change and global food insecurity; nonetheless, its incorporation into contemporary agricultural systems entails various challenges, uncertainties, and potential risks (Nyambo et al., 2021). Japan has formally recognised biochar as a soil amendment, whereas Australia and New Zealand are evaluating its integration into national climate mitigation frameworks (Lehmann et al., 2021). Moreover, 14 countries-including Australia, Costa Rica, and several African nations-have officially endorsed biochar as an approved climate mitigation technology (Kumar and Singh Maurya, 2022). In the U.S., legislative initiatives supporting biochar research, development, and application have been progressively enacted since 2008 (Rodriguez Franco et al., 2024).

Research about biochar economic viability at large production levels remains limited despite its classification as an affordable environmentally friendly adsorbent material. The treatment cost of a biochar-based downflow fixed-bed adsorption system reached €0.89 m^-3 according to a recent techno-economic assessment which demonstrated biochar could be an affordable solution for wastewater treatment (Baaloudj et al., 2025). The total treatment expenses depend heavily on four essential elements which include feedstock choices, pyrolysis methods, regeneration performance and system design (Hu et al., 2020; Aziz et al., 2024).

Biochar has been identified as a cost-effective and sustainable method for repurposing carbon-rich materials such as agricultural residues, biomass, and other carbon-laden wastes into valuable resources for wastewater treatment applications. Studies examining the scalability of biochar production indicate operational similarities with activated carbon manufacturing, highlighting biochar’s potential as a viable alternative for reducing waste while delivering effective wastewater remediation (Rammal et al., 2025). Furthermore, regulatory frameworks and economic assessments at national port levels emphasise the importance of integrating biochar utilisation within port-related industries that exhibit strong economic growth, thereby optimising waste management, reducing container traffic, and improving overall cost–benefit efficiency (Thengane et al., 2021). These findings are pivotal in guiding the development and implementation of biochar-based systems as engineered, sustainable solutions for mitigating wastewater contamination in port environments and other regions (Mbugua et al., 2025). Recent studies indicate that only a limited number of biochar wastewater treatment systems have advanced beyond the laboratory or bench scale. However, a pilot-scale biochar treatment facility has demonstrated notable success in removing phosphorus and mineral contaminants while concurrently recovering these minerals for use as fertiliser (Pap et al., 2022). Endorsed and regulated in the England, U.S., and South Korea, this system underscores the scalability and global relevance of biochar technology (Garcia et al., 2022). Despite these advancements, the pollutant removal efficiency of biochar especially in its unmodified form remains relatively modest. This limitation is attributed to the partial irreversibility observed in adsorption mechanisms for certain metal contaminants. To address this, various modification strategies, including acid, alkaline, oxidising, and salt treatments, have been explored to enhance biochar’s surface area, porosity, and overall adsorption capacity (Gęca et al., 2023). Pyrolysis conducted at higher temperatures, along with steam treatment, has been experimentally demonstrated to improve the pore structure and enlarge the surface area of biochar. Prolonged treatment times and elevated water flow rates have been observed to enhance the sorption efficiency for metals like copper, zinc, and cadmium (Djellouli et al., 2025; Esfandiar et al., 2022). Extended exposure during treatment generally results in an increase in surface area and micropore volume; however, steam treatment tends to exert minimal influence on the surface functional groups, primarily because of the inherently non-polar nature of steam (Desmurs et al., 2022).

Recently, mesquite biochar has gained attention as an effective sorbent in environmental applications. Owing to its extensive surface area and well-organised structure, mesquite biochar functions as a highly efficient super-adsorbent for contaminants such as nitrates, heavy metals, phosphates, ammonium, and fluorides in wastewater (Wang et al., 2020). Converting mesquite biomass into a powerful filtration medium offers a sustainable solution for wastewater treatment while simultaneously delivering socio-environmental benefits. The combination of mesquite biochar adsorption with complementary technologies-like advanced oxidation processes or membrane filtration-has emerged as a promising area of research (Villegas-Peralta et al., 2025). However, its relatively modest porosity poses challenges for industrial applications that require higher adsorption capacities. Ongoing investigations into optimising pyrolysis conditions are rapidly advancing to enhance material properties. Overall, mesquite biochar holds substantial potential to meet stringent pollutant removal demands in wastewater treatment, though future studies should prioritise the development of more porous and efficient mesquite-derived biochar for expanded applicability (Verma et al., 2025; Hussain et al., 2021). The evaluation of modified biochars for wastewater treatment requires additional research to determine their stability and regeneration abilities and treatment effectiveness in operating wastewater treatment systems at different scales.

Biochar can be applied in various physical forms; while earlier investigations primarily concentrated on powdered biochar, recent advancements have expanded its application to granular activated carbon formats (Zhang et al., 2012), as shown in Figure 4. Both configurations have been assessed independently and as components of composite filtration systems, such as biochar–sand mixtures (Bautista Quispe et al., 2022). The physical properties of biochar, shaped by its porous and heterogeneous structure, are strongly influenced by the feedstock type and pyrolysis conditions. Mesquite-derived biochar, in particular, exhibits meso- and macro-porous structures and possesses lower density and complexity than conventional granular activated carbon (Wang et al., 2020). These characteristics enable rapid surface and structural modifications to target specific pollutant removals (Solanki et al., 2023). Recent innovations have focused on refining raw mesquite biochar through chemical and physical treatments to enhance its adsorption efficiency for contaminants present in gaseous, liquid, and solid phases (Villegas-Peralta et al., 2025).

A considerable research gap persists regarding the long-term effects of biochar on soil ecosystems, particularly in relation to soil biota and their ecological functions. Although the stability of biochar as a soil carbon amendment has been relatively well-documented, its enduring impacts on soil biodiversity, microbial activity, and biogeochemical processes remain insufficiently understood (Das et al., 2022). Given the fundamental role that soil organisms play in regulating carbon turnover, nutrient cycling, and overall ecosystem health, further long-term investigations are essential to elucidate how biochar incorporation influences microbial community dynamics and ecosystem multifunctionality over time (Andrés et al., 2019).