Generally, plant biomass-based carbon materials carry a specific appeal for academics and scientists. The activated carbon (AC) produced through plant-biomass carbonization demonstrates better chemical attributes, ample resources, low cost, biocompatibility, and matured porous structures over conventional petroleum-based and coal-based carbon materials. Simultaneously, addition of nitrogen atom within the carbon structure offers ample functional groups, sturdier adsorption ability and greater chemical solidity (Shi et al., 2012; Pei et al., 2018). The N-enriched ACs (NEACs) might possess enhanced properties like better-formed surfaces and pore morphologies, which could offer a reaction medium transfer channel and enhance the usage domain of carbon-based substances (Wang et al., 2012; Li et al., 2019; Bumajdad and Khan, 2021). Hence, a comprehensive and integrated study concerning AC production using several plant species and development of N-atom insertion into AC skeleton and associated applications concerning the elimination of many wastewater pollutants through adsorption is an interesting scientific domain. Several studies have shown that nitrogen enrichment has a significant impact on the surface attributes like surface basic sites, surface polarity, electrical conductivity, and electron donor. As far as adsorption is concerned, nitrogen doping is helpful in enhancing the hydrophilicity that improves the diffusion of the hydrophilic adsorbents within the aqueous media (Lorenc-Grabowska et al., 2013; Lupul et al., 2015; El-Shafey et al., 2016). Additionally, nitrogen doping may cause the development of surface charges that enhance electrostatic interactions between the adsorbate and sorbent; consequently, sorption ability is enhanced.

Of late, many studies (Jiang et al., 2011; Lim et al., 2012; Sakaushi and Antonietti, 2015; Deng et al., 2016) have shown that NEACs have demonstrated improved capability for the sequestration of extensively ranged contaminants from aquatic medium. Increased adsorption efficacy was associated with the electron releasing influence by the nitrogen-based functional groups introduced into ACs (Lorenc-Grabowska et al., 2013). The N-species treatment enhance the development of nitrogen-based functional groups on the porous surface of the carbon substrate; moreover, it influences the effective use of heteroatom synergistic characteristics and may results in fast adsorption of assimilated materials, thereby improving cycling stability. Hence, a precise understanding concerning nitrogen enrichment processes, reaction processes, and essential reaction factors influencing adsorption are critical for developing efficacious adsorbent materials, experimental design, enhancing process scale, and decision-making.

The process of thermochemical manufacturing of ACs from plant biomasses and their N-fortification has been schematically demonstrated in Figure 1. Generally, plants biomasses are heated up in a passive environment for evading oxidation. This process comprises the use of highly pure nitrogen (N₂) gas for purging the reaction chamber in order to eliminate oxygen, air, and other emanating gases that might cause biomass oxidation leading to ash formation instead of the required ACs. The procedure also makes sure that biomass experiences devolatilization and drying concurrently which causes the creation of a superior heating value and better porous ACs. Chemical enrichment, usually called doping, with foreign atoms is an effectual technique for attaining the above-stated characteristics of carbon-based material. Several dopants, other than nitrogen, have been introduced into the carbon matrix, including phosphorous, boron, sulphur, and oxygen. Of the various available alternatives, nitrogen doping of carbon materials has drawn the attention of academics engaged in the adsorption methods because of its good ability to improve the significant attributes for optimal adsorption in the liquid stage (Wood et al., 2014; Hussain et al., 2018). Apart from the heteroatom outcome, the formed ACs’ properties like specific surface area and porosity are determined by plant species, nature, and plant parts like root, trunk, leaves, stem, etc. Furthermore, heating properties such as duration, rate, and temperature also affect AC characteristics.

However, scaling-up adsorption by NEACs remain restricted because of the self-aggregated substrate-free characteristics that cause recycling and isolation challenges. Nevertheless, some research works have noted that, nitrogen-comprising adsorbents are not quite effective because of inferior nitrogen enrichment and intricacy of the operating processes that reduce their competitiveness. Hence, in order to enhance NEAC adsorption efficacy, extensive research has been conducted concerning N-development on carbon substrates to address the specified challenges and exploit the remarkable characteristics like greater surface area, appropriate surface characteristics, and appropriate pore dimensions.

Liquid phase-specific adsorption is a complicated process, and the underlying aspects are not understood as much. A vital aspect specific to the use of N-enriched ACs for treating an aqueous environment is that organic and inorganic material must be concurrently removed in order to purify water (Yang et al., 2019; Zuo et al., 2019). Hence, identifying better processes for increasing nitrogen content in adsorbents for increasing pollution elimination efficacy is desirable but difficult. Even with up to date progresses, the cost-efficient and larger scale production of NEACs is still very difficult-to develop, although low-cost functionalized ACs production has been a remarkable research domain in the field of material science and engineering. The superior material characters, like, larger specific surface area, fine particle size distribution, unique electrochemical properties, greater energy density and chemical stability are the key techno-economical points to receive enormous investments in the development of industrial-scale production plant for NEACs production (Jiang et al., 2013; Zhang S. et al., 2019). To date, a detailed assessment concerning the innovations in nitrogen-enriched biomass-based activated carbon (NEAC) and adsorption characteristics remains to be conducted. This review presents three critical technical roadblocks, (i) the N-enrichment procedures onto the biomass-derived AC along with the reaction mechanism (ii) characteristic transformations due to N-enrichment and (iii) efficacy of NEACs in eliminating pollutants from wastewater along with latest research initiatives to develop cost-efficient process development. Lastly, the review suggests future recommendations and research gaps concerning this domain.

Improvement of quality ACs having high nitrogen content is among the latest research trends specific to the adsorption field (Ilnicka and Lukaszewicz, 2015; Ilnicka et al., 2018). Considering the extensive use of AC-based substances for several environmental and experimental uses, it is vital to ascertain the comprehensive reaction phenomena that lead to the creation of nitrogenous functional groups on AC surface and their effects concerning aqueous phase adsorption. Nevertheless, there is information scarcity concerning the creation of N-functionalities through the use of valuable nitrogen-based reagents. Information concerning this domain is expected to significantly enhance biomass resource usage efficiency. Hence, a comprehensive assessment concerning N-enrichment processes for plant-based ACs is required (Chen et al., 2018).

The literature comprises several ways to enrich porous carbon with nitrogen. The commonly-used heat-treatment techniques may be divided into three highly used processes: (i) a two-step technique using nitrogen functionalisation and subsequent chemical activation (pre N-enrichment); (ii) a two-step technique comprising chemical activation and subsequent nitrogen functionalisation (post N-enrichment); and (iii) a single-stage technique comprising subjecting the biomass to a nitrogen donor and chemical activator to direct heat treatment (also called in-situ N-enrichment) (Liu Y. et al., 2019). The first method, which comprising pre-nitrogen enrichment, can leads to eliminating nitrogen functional groups due to the chemical etching that should follow the activation steps (Chen et al., 2018); thus, this method is not a rational option for increasing the amount of nitrogen content. In contrast, the post nitrogen enrichment-based (technique ii) causes activation to produce a porous morphology that facilitates deep penetration of the nitrogenous substance, thereby leading to effective doping of N. Nevertheless, the free radicals specific to the nitrogenous substances have a reaction affinity for surface functional groups having oxygen; therefore, there is a typical need for a reasonable degree of surface oxidation (Laheäär et al., 2014). The third technique, i.e., in-situ preparation, comprises chemical activation leading to interactions with the nitrogenous substances; consequently, modification phenomena lead to presently unknown effects. Figure 2 summaries typical chemical- and heat-based N-enrichment processes.

Nitrogen enrichment by means of the so-called ‘doping’ approach has been extensively recommended since past decade. Doping is described generally as the procedure of deliberate adding of tiny impurities to a material to adjust its attributes. The quality of the impurity (dopant) describes the attributes of the material post accomplishment of the doping procedure. Explicitly, the chemistry concerning this process is the heteroatom nitrogen dopants enhancing capacitive characteristics by augmenting carbon-surface charge density; additionally, basicity and wettability of carbon substrates are also enhanced (Shin et al., 2011) because of the enhancement of the electron density of the N-based dopant. It provides adsorbents with distinct molecular properties and several functional groups that cause a change in efficiency (Chen et al., 2012; Zhao, Y. et al., 2012; Pan et al., 2012; Nahar et al., 2013). Moreover, most studies assessing this subject use isolated phases, namely, carbonizing biomass at initial stage using appropriate activators, and nitrogen atom insertion through nitrogen doping. It is established that oxygen from biomass influences doping and activation (Houshmand et al., 2011). Therefore, it is appropriate to merge the three steps. Consequently, there is a need for further research specific to two areas: degree of reactivity of nitrogen-providing reagents and pore forming agents, for which pore tailoring and nitrogen dopant processes can be affected. Secondly, since different activating chemicals cause unique interactions when carbonized with plant-based biomasses, this must be another research target.

The acid/base characteristics of AC materials are subject, to a large degree, not only to the raw materials or their porosity and the structure of their surfaces but also to the heteroatoms incorporated in their structures. The most significant and extensively introduced heteroatoms on the surface of AC are nitrogen and oxygen. The acidic nature of porous carbon surfaces is closely associated with their oxygen-based surface groups which are chiefly present on the external surface or at the boundary of the basal plane and is contributory to the chemical character of the carbon. Conversely, the basicity of AC is related to: (i) resonating-electrons of aromatic rings of carbon that attract protons, and (ii) basic functionalities of surface that are able to bind with protons. Nitrogen functionalities usually provide basic characteristics, which can increase the interactions between the acid molecules and carbon surfaces by introducing dipole-dipole, covalent bonding, hydrogen bonding, etc. (Wenzhong et al., 2008; Shen and Fan, 2013). Different kinds of chemical compounds have been recommended to be used in the production of ordered micro-to meso-porous nitrogen-rich carbons; for instance, compounds having amine groups (ethylamine, ethylmethylamine, propylamine, trimethylamine, etc.), quaternary ammonium salts and surfactants, N-substituted amides, etc. Such appropriate N-doping materials undergo a reaction with plant biomasses and lead to a notable fraction of nitrogen in the ACs (Qu et al., 2015; Shi et al., 2015; Watanabe et al., 2015). Physiochemically created NEACs generally possess microporous carbon and sufficient surface area (Pietrzak et al., 2006; Zhao et al., 2014). The subsections that follow will address the evolution of the nitrogen enrichment techniques, reaction mechanisms along with the changes in the chemistry of the surface of the ACs.

The use of carbon-based adsorbents relies on their physiochemical properties, such as their surface area per Gram of the adsorbent, shape/extent of their porosity, nature, extent, location of their functional groups, and their morphologies. Thus, a thorough surface physiochemical review of ACs must involve the characterisations of the physical constitution. The following paragraphs will highlight the materials’ properties changes when N-enrichment technique applied on plant-biomass derived ACs.

The process of thermal carbonization produces a consistent structure, with a simple and semi-developed pore structure (Osman et al., 2016), which must be widened and extended properly by chemical or physical activation. Baur et al. (2018) functionalized the pores of activated carbon fibre with amine (diethylenetriamine, DETA) via wet impregnation. After drying it was found that although the S _BET decreases as a result of the reduction in porosity, the pore width was constant. The performance of the diethylenetriamine loading into the ACs was excellent and enhanced the removal of formaldehyde as much 100-fold if compared with the unloaded ACs.

So, the N-enrichment of ACs not only would affect its surface chemical property, but also its porous nature, in a form and to an extent that depended on the starting AC, the used N-reagent, and the employed experimental technique. Development of the surface area and microporosity inside the carbon structure has often been reported when plant biomasses thermally treated with NH₃(g) (Stöhr et al., 1991). For example, the increase in surface area distributed in pores larger than 1 nm (determined by DFT model assuming slit-shape pores geometry) was as high as 151% for a wood-derived commercial AC that was subjected to high temperature NH₃(g) treatment after oxidization, compared to the oxidized only predecessor (Dastgheib et al., 2004). There have been some reports of the porous structure remaining unchanged (Jansen and van Bekkum, 1994; Abe et al., 2000) or of a decrease in the microporosity and surface area (Xie, F. et al., 2000; Przepiórski, 2006) but such reduction might not be due to only the amination since the process accompanied with high temperature thermal treatment.

In case of NH₃(g), the structure of pore remained virtually unaffected (Jansen and van Bekkum, 1994), and the surface region became greatly developed (He et al., 2010) or reduced/improved depending on the employed ammoxidation stages (Nowicki et al., 2009). With nitrogenating compounds other than NH₃(g) (like amides and amines), reductions in the surface area and volume of micropores were also observed but such reductions might be also due to the employed thermal treatment (Seredych et al., 2008). Numbers of plants that have been taken under consideration by the researchers for producing NEACs, by applying wide varieties of nitrogen enrichment agents at different thermal conditions, are summarized in Table 1. Beside selecting nitrogen enrichment agents, the carbonization temperature, reaction retention time and employed experimental procedure also played a vital role to avail anticipated surface area. Researchers conducted their experiments at temperature ranged from 450°C to 1,110°C and achieved the S _BET as small as 159.2 m²/g and as high as 2,995 m²/g as per Table 1.

Because the chemical functional groups formed on the surface of the NEACs plays a very significant role in the liquid phase adsorption, the intensity and nature of such functionalities are the primary factor to categorise the NEACs as prospective adsorbents. The FTIR technique is regarded as one of the most effective qualitative bulk characterisation methods for identifying the inorganic or organic functional groups developed on the ACs. Hence it has been used to identify the variety of functional groups onto the NEACs (Rehman et al., 2019).

A comparison study is always regarded as an optimum approach when two different types of materials properties needed to be differentiated. In this regard, Dilokekunakul et al. (2020) prepared nitrogen-enriched AC from bamboo waste and compare its performance toward CO₂ adsorption with that of a pristine ACs. The unmodified carbonaceous materials (AC) were carbonized at 600°C and activated with CO₂ gas at 900°C while the modified ACs were further treated with KOH and urea (N-reagent) with thermal treatment in air at 650°C (AC-UK). The oxidation treatment was also carried out without KOH and urea (AC-Air). The findings associated with this nitrogen enriched plant-derived ACs and none-NEAC, are displayed in Figure 10. A type I isotherm along with an H4 hysteresis loop was found to be demonstrated by all samples (NEACs and non-nitrogen enriched, see Figure 10A) which signify that microporous volume of all samples is greater than mesoporous volume. As observed in Figure 10B, pore size distributions attained via N₂ sorptiometry indicate that there is a distinct fluctuation in the micropore volume of all the prepared samples. For NEAC (AC-UK), the surface area was smaller than the other two samples and the incremental micropore volume was seen average up to 1.47 nm (smaller than that of the AC and AC-Air). This means that the drop in surface area for AC-UK is due to less accumulative pore volume. For the FTIR spectra shown in Figure 10C, at 1,036 and 472 cm⁻¹ bands, the FTIR peaks signify C–O stretching, while C–C stretching vibrations could be observed in every kind of ACs. Additional peaks at 3,467 and 1,638 cm⁻¹ are demonstrated for the AC-UK, which could be due to N–H/O–H stretching as well as C=C stretching, respectively, which were minimal in AC and AC-Air. In spite of the lower S _BET, at low pressure (<10 kPa), the CO₂ adsorption ability at 0°C and 25°C were found to be the best for the AC enriched with nitrogen, i.e., sample AC-UK). When increasing the pressure above 20 kPa, it was found that the CO₂ adsorption for AC-UK at 0°C is the lowest. This behaviour was referred, by the authors, to combined factors, namely, the nature and extent of N-functionalities, as depicted in Figure 10D, and pore size distribution (see Figure 10C). This chemical phenomena could lead to development of enhanced porous structure into the AC skeleton to improve adsorption (Dilokekunakul et al., 2020).

X-ray photoelectron spectroscopy (XPS), also referred to as electron spectroscopy for chemical analysis (ESCA), is broadly employed to study carbon-based materials’ surface structures. It allows analysing the first 10 or 20 atomic layers and provides information pertaining to both elemental concentration as well as composition. Due to the sensitivity of the technique to the atom environment, XPS can be used to study the identity of functional groups presents on the surface. XPS spectra are extensively employed to study the NEACs’ surface structures as these offers both qualitative information regarding the types of surface ions/atoms presents in the materials as well as the relative surface amount (in atomic%) pertaining to each type. With regards to N-enriched AC samples, one interest would be the nitrogen core-electron (Nls) binding energy region (393–405 eV). The Nls spectrum can be defined as a composite curve that can be deconvoluted to several individual curves that signify the relative amounts and types of the functional states wherein the carbon exists. As described earlier, the treating the ACs and/or their precursors with N-reagents may considerably increase the content of nitrogen in the biomass-derived ACs. At the same time such treatment may reduce the oxygen content on the surface because of the preferential reaction between oxygen-containing functional groups and N-compounds (Hulicova-Jurcakova et al., 2009a). Figures 11A, B and display a typical XPS spectrum pertaining to the nitrogen-doped ACs. Deconvolution of the N 1s spectra can be result in four main peaks: pyridinic-N (N-6, 398.7 eV), pyrrolic/pyridine-N (N-5, 400.3 eV), quaternary (graphitic)-N (N-Q, 401.8 eV), and pyridine-N-oxide (N-X, 403.0 eV) (Sun et al., 2014). Some researchers assign the N 1s peak around 400 eV for pyrrolic/pyridone-N (Xu, B. et al., 2012; Hulicova-Jurcakova et al., 2009a; Hulicova-Jurcakova et al., 2009b). Overall, pyridinic-N and pyrrolic/pyridone-N are regarded to be more dominant, which is advantageous since these two functional groups help in conferring pseudo-capacitive behaviour pertaining to adsorbent (Xu, B. et al., 2012; Hulicova-Jurcakova et al., 2009a; Hulicova-Jurcakova et al., 2009b). These functional groups could be attributed to reaction of the N-agents and their decompositions (e.g., NH₃, cyanic acid, etc.) with carboxyls and hydroxyls, while quaternary-N could result due to transformation of pyridinic-N into a graphitic substrate (Zhao et al., 2015). Interestingly, researchers identified that adding pore forming agents could surge the presence of certain N-functional groups such as pyridone-N-oxide and pyrrolic/pyridone-N, while reducing the quaternary-N content, and had no evident influence on the pyridinic-N amount (Chen et al., 2016).

In aqueous phase adsorption of organic molecules, surface chemistry and pore structure play significant roles and need to be investigated thoroughly (Moreno-Castilla, 2004). As mentioned earlier, recent research trends have demonstrated the tendency to use NEACs to eliminate wide-ranged pollutants from wastewater because of its greater adsorption capacity (Rao et al., 2008). Adsorption pertaining to an adsorbate by adsorbents can be defined in two aspects: adsorption capacity and affinity. Adsorption capacity may be limited by the available intermolecular space belong to a sorbent available for adsorption for a given adsorbate, while adsorption affinity relies on the level of strength possessed by attractive forces that exist between adsorbent and adsorbate. Several studies suggest that the mechanism of adsorption from solution can be determined not only by the physical characteristics associated with ACs structure (pore size distribution, specific surface area, microchannels, etc.), but also carbons’ surface and intra-porous chemical characteristics (Wright, 1967; Dil et al., 2017; Mazaheri et al., 2017; Rehman et al., 2019).

Adsorption by NEACs can 1) impact the mobility as well as fate of both general organic matters and xenobiotic pollutants (e.g., amino acids, humic acid and proteins, dyes, heavy metals, hydrophobic); 2) modify the surface properties relating to adsorbents via surface chemistry alteration by various treatments to eliminate or incorporate functional groups; and 3) be crucial for potential adsorbent applications pertaining to water treatment technologies (Yue et al., 2009; Rivera-Utrilla et al., 2011). It is imperative to mention that a key role is played by the types of the chemical functional groups with regards to the adsorption of targeted pollutants. Acid functional groups improve the removal process for cationic species, while basic functional groups help sequester anionic species from aqueous solutions (Rivera-Utrilla et al., 2011). As an example, in this review we discuss the adsorption of the hexavalent chromium ions, in order to explain the adsorption mechanism. Figure 12 presents a schematic pertaining to Cr(VI) adsorption onto the NEAC.

The reaction can occur via four different mechanisms based on the interaction between various functional groups of AC (Figure 12) and the chromium species (hexavalent and trivalent Cr) during the adsorption process. The first mechanism (I) is associated with the anionic adsorption of Cr(VI) on to basic functional groups. The second mechanism (II) refers to the reduction of Cr(VI) to Cr(III) due to contact with certain functional groups that behave as electron donors. This second mechanism is related to these adsorbents that possess more acid functional groups when compared with basic sites, for example, the oxidized ACs. A three steps surface interaction is established by the third mechanism (III): (a) through anion adsorption, Cr(VI) is adsorbed on basic sites; however, (b) via an adjacent electron donor, it could be reduced to Cr(III) and then (c) via electrostatic repulsion, Cr(III) is released to the solution. Lastly, the fourth mechanism (IV) includes adsorption of Cr(III) (cation exchange) on acid sites, which is a coupled reduction-adsorption mechanism; thus, the fourth mechanism follows the mechanisms II and III. This adsorption mechanism is regarded to be optimum for adsorbents that have more acid sites available during adsorption processes (e.g., oxidized ACs); nevertheless, most adsorbents include acid functional groups that enable the chances of a combined reduction-adsorption interaction. In this case, the occurrence of adsorption mechanisms could be because of: (a) polar adsorption pertaining to the adsorbate via the adsorbent’s (hydrophilic shell) functional groups, (b) adsorbate surface accumulation, (c) initially with functional groups versus surface/polar charges or (d) π−π attraction occurring between the layers of the adsorbent (hydrophilic core) and the cyclic adsorbate. Essentially, the surface of NEACs provide adsorption spots enriched with high density of N-functional groups, thus multilayer of molecules of NEACs, comprised of piled chemilayers, have a much higher likelihood of entering aquatic surroundings because of the difficult and complex process of separating single-layer. It has been reported that N-functional groups perform adsorption via hydrogen bonds, π−π interactions and Lewis acid−base (Lian et al., 2016; Niu et al., 2022) which may enhance the adsorption of organic contaminants to be higher than that of the non-modified ACs.

Because of the nature of mixed polarized organic contaminant molecules in water, scholars considered NEACs for improved adsorption to eliminate organic toxins released from hospitals wastewater such as the toxins reported in (Emmanuel et al., 2005; Kist et al., 2013). Reviewing the literature revealed that there was a scarcity of research papers that explained the adsorption characteristics and reaction process during elimination of mixed polarized organic pollutant molecules from wastewater (Lloyd et al., 2015). Some scholars just put their attention on adsorption kinetics to eliminate pollutants from wastewater by the use of NEACs. For instance, Tanada et al. (1999) altered commercial ACs with amino groups for effective removal of formaldehydes from wastewater produced in a hospital. They considered that the adsorption follows monolayer adsorption. Lloyd et al. (2015) tried acid-amine modification of sawdust-based mesoporous AC to eliminate mixed polarized glutaraldehyde (GA) molecules from wastewater of the hospital and found strong enhancement of the adsorption due to the acid-amine modification. During the solution stage, GA monomers of hydroxyl functionality are predominating. Thus, for pH < 7 (NEACs carries net positive charge), adsorption interactions could happen through complexation that involve covalent carboxyl-hydroxyl bonding condensation as per Figure 13A. In case of basic condition (i.e., pH > 7), the surface of NEAC assumed net negative charge because of deprotonation of surface acidic and basic groups. During the solution stage, GA polymers have mixed hydroxyl, aldehyde, and carboxyl functionalities. Thus, in basic condition, it may be conjectured that particular adsorption interactions could happen through covalent bonding reactions like carboxyl-hydroxyl shown in Figure 13B; carboxyl-aldehyde shown in Figure 13C; hydroxyl-aldehyde shown in Figure 13D; and amine-aldehyde (Schiff bases formation) shown in Figure 13E. Furthermore, physical electrostatic pull could take place through amine-hydroxyl as shown in Figure 13F and amine-carboxyl (shown in Figure 13G). Therefore, the high GA adsorption on NEAC, in comparison with non-nitrogen modified AC, could be attributed to the anchored amine groups as well as the high acidic surface groups on the non-nitrogen modified AC.

When the aldehyde is of small size, like acetaldehyde, it was found that its adsorption on the polar heterogeneous surface of NEAC, controlled largely by the size and extent of small micropores with some contribution of the larger pores (El-Sayed and Bandosz, 2002).

Notwithstanding a wide difference in polarity, charge, size, among other relevant characteristics affecting the process of adsorption, for anions contaminants, the adsorption usually improved on NEACs compared to virgin ACs (recent published example is shown in Figure 14). Detailed explanation can be seen in the referred article.

In concise, the effectiveness to eliminate pollutant substances from waters by process of adsorption was related to: the carbon surface’s acid–base character (Xie, R. et al., 2017), and also with the permeable carbon structure, i.e., its surface area (Dastgheib et al., 2004), volume of micropore (Przepiórski, 2006; Wang, J. et al., 2021), and pore size distribution (Chingombe et al., 2006). Due to such complexity, after amination of ACs, adsorption of pollutants might improve significantly, like, cyanide increased around five times (Monser and Adhoum, 2002) and Mercury adsorption enhanced more than two times (Zhu et al., 2009), but also it might decrease as well as in the case benzoic acid adsorption after AC amination (Abe et al., 2000). Table 2 summarise some of the practical applications of NRAC in removal of water contaminants.

Cost is a vital aspect for technoeconomical evaluation of the sorbent materials. Plant biomasses have been on the top of the list as a ‘low-cost’ feedstock in the preparation of porous carbons (Babel et al., 2012; Kalyani and Anitha, 2013). Up to now, natural resources, for example, agricultural residues, plant biomasses, and other industrial bio-residues, have been considered as cheap precursors for ACs production and applied as wide-ranged industrial applications, especially, as adsorbents and electrodes manufacturing (Mohamed et al., 2010; Paul et al., 2019).

As per reaction engineering concept, the development of a cost-effective carbon material is not only depends on the source material but also on the design of process, where energy consumption and usages of number of chemical/s involved (Bumajdad and Khan, 2021; Zhang, Z. et al., 2019b). Hence, the development of renewable and green strategies may not only save energy resources for NEACs preparation, but also develop carbon structure with ample hetero atoms doping and improved microchannels in lower cost. Among the available doping strategies, the nitrogen doping is beneficial among other metallic and non-metallic doping as it can stimulate electronic transmission, possesses better stability and reasonable capacitances (Luo et al., 2022). Additionally, the developed porous network configuration and better surface area can provide lodgings more electrolytic ions onto the pore surface and channels of NEACs, and consequently elevate electrolyte diffusion and internal electron transfer during physiochemical reactions. Generated micropores in classified porous grid can intensely increase absorption/desorption efficiency, elude ion-sifting effects and hence increase the surface capacitance (Borghei et al., 2018). The immense necessity for research initiatives to propose a sustainable financial analysis in adsorbents production, regeneration and applications in water treatment was understood since 1990s (Bailey et al., 1999), but up to now, this is considered as one of the major technoeconomical gap in the literature (Ahmed et al., 2016). Just few articles (e.g., (Dickinson et al., 2015; Jirka and Tomlinson, 2015)) conducted cost analysis on biochar adsorbents production in certain scale; although the necessity of inclusive cost benefit analysis mentioned to be fully analyzed. An extensive variety of low-cost sorbents production techniques examined where numbers of reactants, precursors and reaction parameters had been justified and finally their effects on the adsorption performance mentioned. However, the information of the material-balance (raw materials chemicals), material costing and energy consumption not been mentioned until now, although it is highly important and indispensable for proposing any financial and cost-benefit analysis (Ahmed et al., 2016). Thus, analogies of the development of functionalized NEACs are challenging due to inconsistencies in data. Theoretical models based on financial parameters covering from raw materials collection costs to the achieving “break-even point (BEP)” for carbon based adsorbent production in larger scale has not also been carried out. Very few researchers have so far been able to accumulate published technical articles from literature containing the necessary information on the process cost analysis related to adsorbents modification and production methodologies. They have just considered the lab-scale researches, hence, neither mentioned any data on the production cost of closed-loop manufacturing of adsorbent nor predicted commercial scale costing (Bailey et al., 1999; Jirka and Tomlinson, 2013).

N-enriched activated carbons produced by thermochemical treatment of plant biomasses was reviewed in this work with focus on reaction mechanisms, the factors affecting reactions, improved materials’ characters, and pollutants removal efficiency, etc. Based on that, overall recommendations concerning the shortcomings on NEAC production and academic gaps are suggested.

i) Reaction Engineering

As specified previously, the nature of reactions existing between N-enriching substances and plant-based biomass is complex. The literature indicates that the present research trends seldom address real-time models for such reactions; moreover, there is no simulation-specific data regardless of its significance in regulating N-enrichment quantity and output quality. Computer simulations are critical for establishing the correlation between atomic-scale molecular characteristics and physically measurable characteristics; this is specifically pronounced typically to the surface science domain and, explicitly, carbon surfaces. N-substances and unmodified/modified biomass exhibit numerous organic reactions, of which several reactions are due to carbon-based surfaces. It is exceptionally difficult to assess the effects of particular functional groups using empirical techniques. Molecular simulation can address all such issues and can be used to assess adsorption data pertaining to extrinsic molecules having random functional groups and AC of different pore distributions at all pressure and temperature values. Researchers (An et al., 2019) formulated a slit-sheet AC framework to assess AC adsorption effectiveness using the solution of the kinetic expression using the Grand Canonical Monte Carlo (GCMC) simulation technique, whose schematics are depicted in Figure 15.

As indicated in Figure 15, they assessed four functional groups, namely, hydrogen (−H), amine (−NH₂), hydroxyl (−OH), and carboxyl (−COOH), for evaluating the influence of functional groups on adsorption characteristics of the substance. Functional groups were contrasted by using predetermined densities. It should be noted that the simulation computed molecular structure created post adsorption; it could also establish that nitrogen functional groups were present.

The present work is comprehensive review of reaction dynamics concerning nitrogen enrichment of plant-based biomass used for producing nitrogen-enriched activated carbon (NEAC). It has been observed that several amides and amines, were utilized so far for producing plant-biomass based NEACs. The amide family is the single most efficacious nitrogen-based substances offering significant N-functional groups for the carbon substrate. Amines and N-based surfactants are not as efficacious. It was indicated that imides, amides, amines, and nitriles functionalities are dominant at low temperatures (<600°C). Concurrently, thermally strong aromatic rings like pyridine or pyrrole imitating functions were more prevalent at higher temperatures (>600°C). The cause of thermal strength of amide functional groups and manageable changes from higher to lighter molecular weights, N-based groups provided distinct molecular characteristics corresponding to NEAC surfaces. Root, stem, bark, and leaves from live and dumped plants were used for AC and NEAC production. Among the plant parts used for biomass creation, leaves were found to have relatively more nitrogen content after carbonization; it may be attributed to the chlorophyll content present inside leaves. The significant changes pertaining to material characteristics concerning functional groups and the increase in nitrogen presence have been presented and contrasted using material characteristics-based outcomes. NEACs exhibited enhanced presence of C-N, N-H, N-O, N-CH_n, while a majority of the ACs prepared without nitrogen-enhancing agents indicated the presence of only C-H, C-O and C-C bonds. NEAC wastewater pollutant elimination efficacy has been contrasted and summarized. Outcomes from the comparative assessment indicated that the use of NEACs allow more efficient wastewater pollution elimination if compared to pure ACs. A major research gap of coherent information of cost benefit studies and cost divergences analysis have been observed. Extensive research on the financial forecasting on the cost-effective chemical process development is severely necessary for clear understanding of economically viable adsorption processes and to explain the technology. Decision-makers and scholars pursuing additional research in this field are expected to benefit from the recommendations and indicated gaps.