Graphitic nitride-catalyzed advanced oxidation processes (AOPs) for landfill leachate treatment: A mini review.

Landfill leachate poses significant risks to public health via the release of high-toxicity contaminants, including refractory organic compounds, ammonia-nitrogen compounds, and heavy metals. Significant efforts have been made to develop useful methods for leachate disposition and treatment. Advanced oxidation processes (AOPs) are one of the most promising methods, because they can rapidly degrade diverse pollutants and significantly improve the biodegradability of leachate. Graphitic carbon nitride (g-C3N4), a fascinating conjugated polymer, has become a hot topic in AOP research due to its metal-free benefits and high photosensitivity. Thus, combining AOPs with g-C3N4 achieves excellent degradation of refractory pollutants in leachate. Since the composition of leachate is complex in the practical conditions, the information reported by current studies of using g-C3N4 as a remediator is still incomplete and fragmented. Thus, in this review, the recent status of leachate treatment and approaches for its disposal has been summarized and some conclusions have been drawn. In addition, a brief introduction to g-C3N4 and its application in AOPs for leachate treatment have been critically discussed and with its future outlook assessed. Although the development of g-C3N4 in AOPs for leachate treatment is highly efficient and practical, comprehensive study about its application and technology expansion is urgently needed, based on the complex operating conditions. Perspectives on the treatment of leachate using g-C3N4-AOPs are also included. The information and perspectives provided in this review will provide guidance and novel understanding to accelerate the development of g-C3N4-based AOPs for leachate treatment.

Introduction

Landfills, the ultimate destination of most artificial wastes, contain huge amounts of pollutants derived from industrial and municipal solids. Due to naturally-occurring processes like weathering and runoff, high-concentration liquid leachates are released and performed direct and potential threats to surface and subsurface water as well as flora, fauna, and human health, especially for the unlined or old landfills localized near watercourses or in wetlands (Ford et al., 2011; Lorah et al., 2009). Therefore, successful treatment of toxic substances in landfill leachate is a significant step towards the sustainable management of solid waste. Leachates must be carefully treated before being released into the surroundings or transferred to a secondary treatment plant. Unfortunately, in many countries and regions, leachates have been overlooked. Ford et al. conducted a field investigation to discover the source of arsenic at a Superfund site in Massachusetts (USA) and detected the presence of leachate-derived arsenic in groundwater and sediment (Ford et al., 2011). In addition, the presence of commonly used pesticide residues in concentrated leachate poses a high acute and chronic risk to the environment (Wang et al., 2019).

Different environmental factors and different properties of the parent wastes lead to leachates with very diverse compositions (Qasim, 2017). Generally, typical landfill leachates include a large amount of organic matter, humic and fulvic acids, and xenobiotic organic compounds (such as aromatic hydrocarbons, phenolics, chlorinated aliphatics, pesticides, and plastizers). The dissolved organic carbon (DOC) and chemical oxygen demand (COD) in leachate can be as high as 1341−3100 mg L−1 and 3385−5000 mg O2 L −1, respectively. The common characteristics of landfill leachate are low biodegradability (five-day biochemical oxygen demand to chemical oxygen demand (BOD5/COD) ratios of 0.07−0.27). Leachate also contains a high content of damaging inorganic ions (such as 2124−6200 mg L−1 chloride and 1125−3200 mg L−1 ammonium), a high conductivity (12770–25000 S cm−1), and toxic heavy metals (cadmium, chromium, copper, lead, nickel and zinc) (Anglada et al., 2011; Cabeza et al., 2007; Chiang et al., 1995; Moreira et al., 2015; Urtiaga et al., 2009; Zhang et al., 2010a).

Numerous attempts have been made to develop efficient techniques for leachate treatment, including biodegradation (Miao et al., 2019; Rani et al., 2019; Song et al., 2019), coagulation (Xu et al., 2019), adsorption (Lam et al., 2019), and membrane filtration (Zhou et al., 2020). However, most conventional approaches have limitations and do not eliminate all the negative impacts of the leachate on the environment. For example, landfill aging processes and the low B/C ratio of leachate can decrease the efficacy of biodegradation. In addition, adsorption techniques have problems with the recovery and disposal of irreversible adsorbents. Membrane technologies have high operating costs and challenging problems with fouling.

AOPs are an efficient and powerful treatment for landfill leachate. AOPs are classified into different types, including photocatalytic oxidation, Fenton or Fenton-like processes (Gode et al., 2019), electrochemical oxidation (Nie et al., 2016), and persulfate-based AOPs (Roy and Moholkar, 2019; Zhou et al., 2019). However, technical challenges limit their widespread applications. For example, although AOPs based on ozone activation have a high oxidizing capacity, the low mass transfer efficiency of ozone from the gas phase to the liquid phase and the release of excess ozone have negative impacts on the surrounding environment. In the Fenton oxidation process, the consumption of ferrous species increases the high cost of the subsequent sludge treatment, while the Fenton photo-assisted process is only effective in a narrow pH range. Electrochemical advanced oxidation requires large amounts of energy and expensive metal electrodes. Among these techniques, semiconductor-induced photocatalysis has attracted significant attention because it produces large number of active radicals for eliminating recalcitrant organic pollutants and uses economical and renewable solar energy. Thus, selection of a suitable semiconductor material is essential for photocatalysis-assisted AOPs.

Graphitic carbon nitride (g-C3N4), a metal-free polymer, has attracted extensive interest in various scientific applications due to its graphite-like structure, specific electronic structure, excellent chemical stability, and large surface area. Fortunately, the properties of stabilization and low toxicity render g-C3N4 as a relatively environment-friendly material with minor ecological or toxicological concerns. Coupling g-C3N4 and AOPs enhances the effectiveness of both materials and shows excellent results in degrading high concentration refractory contaminants in leachate. Herein, we present a brief review focusing on the development of graphitic carbon nitride as a promising catalyst for landfill leachate treatment. Although only a limited number of systematic and comprehensive studies using actual waste leachate have been performed, the potentials of AOPs combined with g-C3N4 for degrading the possible pollutants in leachate is also examined. In addition, the possible practical applications to leachate treatment are explained and concretely analyzed via a comparative discussion. The references provided in this review also provide further insights to extend the study of g-C3N4-based AOPs for leachate treatment.

Graphitic carbon nitride (g-C3N4)

Brief introduction to g-C3N4

Graphitic carbon nitride has attracted much interest in the scientific community due to its unique structures and remarkable properties. The history of the discovery and development of this polymer dates back to the 1990s (Sudhaik et al., 2018). Among several developed allotropes, g-C3N4 demonstrates outstanding advantages due to its graphite-like structure, huge surface area, specific electronic structure, excellent chemical stability and high electron mobility (Kong et al., 2019; Thomas et al., 2008; Zhang et al., 2011). Recently, research on g-C3N4 has mainly focused on energy generation and storage, heterogeneous catalysis, organic synthesis, sensors, and environmental applications (Mamba and Mishra, 2016; Mishra et al., 2019; Patnaik et al., 2018).

The high performance and application of g-C3N4 is highly structure-dependent. There are two main allotropes of g-C3N4 (Fig. 1 (a) and Fig. 1(b)), periodic arrangements containing subunits of triazine (C3N3 ring) and tri-s-triazine (C6N7 tri-ring), respectively (Zambon et al., 2016; Zhao et al., 2015). Theoretical calculations have shown that the g-C3N4 allotrope with C6N7 segments is more thermodynamically stable than the allotrope based on C3N3 (Kroke et al., 2002). Thus, tri-s-triazine is widely regarded as a promising candidate for g-C3N4-based applications (Kong et al., 2019). Unlike conventional semiconductors (e.g. SiC, GaN, ZnO, etc.), g-C3N4 has unique stability with respect to heat treatment and chemical resistance. Thermal gravimetric analysis (TGA) of g-C3N4 suggested that g-C3N4 is not volatile at 600 °C, while it can be completely decomposed at temperatures greater than 700 °C (Lei et al., 2015; Li et al., 2010; Gillan, 2000). g-C3N4 also has high stability in extremely acidic and basic media and is insoluble in conventional solvents, including ethanol, water, tetrahydrofuran, toluene and diethyl ether. The structural stability of g-C3N4 may be due to the Van der Waals forces between the stacked graphitic layers. In addition, polymeric g-C3N4 has also attracted growing interests as semiconducting photocatalyst and has been named as the “holy grail’’ for highly efficient photocatalytic hydrogen evolution reaction (HER) due to its facile synthesis, fascinating electronic band structure, reliable functionalization and unique visible-light responsiveness (Liu et al., 2017). The g-C3N4 prepared by the microwave-assisted molten-salt strategy presents remarkable photocatalytic activity with considerable quantum yield (up to 10.7 % at 420 nm), suitable alignment of energy bandgap structures, convenient carrier transfer path, and more efficient separation of photo-generated electron-hole pairs. Furthermore, the low cost, low toxicity, facile and large-scale fabrication of g-C3N4 improve the possibility of its application in various types of AOPs.

Fig. 1

Required temperatures for synthesizing g-C3N4 using different precursors and details of the thermal polymerization pathway for the formation of g-C3N4 using cyanamide (CA) as the precursor. (a) Triazine and (b) Tri-s-triazine based structures are two main allotropes of g-C3N4.

Preparation methods

g-C3N4 can be prepared via several different approaches and the resulting products exhibit different physiochemical properties. Common methods of synthesizing g-C3N4 include thermal polymerization, solvothermal and sol–gel methods, template-assisted methods, and chemical functionalization. Among these, thermal polymerization is the most popular and commonly used method for g-C3N4 preparation, due to its outstanding technical simplicity and low costs. Therefore, the thermal polymerization method is highlighted and emphasized in this review. The thermal polymerization process is mainly achieved by thermally polymerizing nitrogen- and carbon-rich precursors. The most frequently used organic precursors include urea, melamine, trithiocyanuric acid, dicyandiamide, cyanamide, triazoles, thiourea, guanidine thiocyanate, and guanidine hydrochloride. Formation of melamine by polyaddition and the removal of amino groups by polycondensation are the necessary processes during the formation of the g-C3N4 polymer network (Sudhaik et al., 2018). The thermal conditions play a crucial role in preparing the target g-C3N4, and different precursors require different thermal conditions for polymerization. As has been effectively summarized by Sudhaik et al. (Fig. 1), the fabrication of g-C3N4 using diverse precursors requires different thermal decomposition temperatures (Sudhaik et al., 2018). In addition, the thermal polymerization of cyanamide (CA) to synthesize g-C3N4 was discussed as an example. Different poly-condensates can coexist over a wide temperature range. When the temperature is higher than 600 °C, g-C3N4 becomes unstable and partially or fully decomposes, generating NH3 and CxNyHz. Thus, thermal polymerization temperature between 550 and 600 °C is generally chosen as the optimal temperature for g-C3N4 synthesis. Also, the properties of the target g-C3N4, including the surface area, porosity, absorption, photoluminescence, C/N ratio and nanostructure, can be strongly affected by the technical parameters of the thermal polymerization.

g-C3N4-based nanocomposites

Even though the suitable structure, low cost, and highly stability of g-C3N4 make it a superior photocatalyst, many challenges still remain for its practical applications. Some significant disadvantages, such as poor electrical conductivity, insufficient visible light absorption, fast recombination of photoinduced charge carriers, and low specific surface area, also significantly hinder the broad application of g-C3N4 (Chen et al., 2015a; Zhang et al., 2019b, b). Consequently, considerable efforts have been directed to developing g-C3N4 composites with high numbers of active sites and high stability.

Carbonaceous nanomaterial-based g-C3N4

Tailoring g-C3N4 to obtain particular properties has been a target in recent studies. Combining g-C3N4 with carbonaceous nanoparticles, nanosheets, and nanoribbons has attracted considerable interests due to the excellent properties of the composites, such as high specific surface area, remarkable adsorption properties, and high plasticity. Coupling g-C3N4 with nanocarbon materials is a good strategy to exploit the synergistic effect of both materials and broaden the applications of g-C3N4. Benchmark carbon materials, such as graphene oxide (RGO), carbon nanotubes (CNTs), and carbon nanospheres (CNS), have been extensively studied. Graphene is a potential candidate for constructing g-C3N4-based nanocomposites. For example, coupling graphene with g-C3N4 produces nanocomposites with new features (Bai and Shen, 2011; Chen et al., 2013). Graphene@g-C3N4 hybrids have been shown to be good catalysts under visible light (Ai et al., 2015; Liao et al., 2012; Ong et al., 2015a, b; Tong et al., 2015), with synergistically improved photo-response of both materials in the visible region, prolonged life of the photogenerated charge carriers, enhanced adsorption properties and increased specific surface area. The coupling of g-C3N4 with CNTs also improved the efficiency of the photocatalytic degradation of aqueous pollutants (Ma et al., 2014; Pei et al., 2015; Wu et al., 2015). Other g-C3N4-based nanocomposites have also been fabricated with properties enhancement, such as C60/g-C3N4 (fullerene/g-C3N4)(Bai et al., 2014; Chai et al., 2014), carbon quantum dots (CQDs)/g-C3N4 hybrid nanocomposite(Li et al., 2016b; Liu et al., 2019; Zhao et al., 2020) and biochar/ g-C3N4 (BC-g-C3N4)(Li et al., 2019a, b). Hard and soft-templating procedures based on carbonaceous nanomaterials have both been used to remarkably improve the surface area of g-C3N4 by introducing pores into its structure (Chen et al., 2015b; Mo et al., 2014). Despite the successful optimization of its specific surface area, the low conductivity, high recombination rate, and low photocatalytic efficiency of g-C3N4 limit its applications.

Metal-doped g-C3N4

Rational design of g-C3N4 with highly catalytic activity is significant for its extensive application. Metal-doping has been proved as a typical strategy to design the advanced g-C3N4 with boosting activity. Metal deposition, introducing heteroatoms, and coupling g-C3N4 with other semiconductors can further enhance the catalytic performance and extend the scope of the potential applications of g-C3N4. As reported, with metals addition, the active species in localized space could be enriched by deposition of metal nanoparticles into pores and channels, while the electronic structure of g-C3N4 could be changed for enhancing H2O adsorption and oxidation (Huang et al., 2020; Li et al., 2020). The above modulation of metals could regulate the tunes band edges and thus motivate the generation of hydroxyl radical (·OH). Moreover, the formed metal nanostructures could also attribute a portion of catalytic activity of g-C3N4. Metals that are commonly used for decoration or doping include alkali metals, transition metals, rare earth metals and noble metals (Mamba and Mishra, 2016). For example, sodium-doped g-C3N4 showed a higher photocatalytic activity for decomposing organic chemicals under visible light illumination compared to pure g-C3N4, due to the effects of sodium on the band structure, narrowing the band gap and efficiently separating the charge carriers (Zhou et al., 2011). Similarly, doping potassium into g-C3N4 tuned the band structure and led a higher photocatalytic activity (Zhang et al., 2015a). 0.5 % Fe(III) loading also improved the catalytic degradation activity by improving the visible light absorption and inhibiting charge carrier recombination. Zirconium, molybdenum, cerium and europium have also been doped into g-C3N4 (Jin et al., 2015; Wang et al., 2015b, 2016; Xu et al., 2013).

Moreover, noble metal deposition has become a popular approach for improving the photocatalytic performance of g-C3N4. Li et al. found that Ag loading improved the photocatalytic properties of g-C3N4 nanosheets by a factor of more than seven due to the efficient utilization of visible light and improved charge separation efficiency (Li et al., 2015). Other studies of Ag-doped g-C3N4 have reported the removal of a series of environmental pollutants including toluene, E. coli and S. aureus biofilms, and MB (Bing et al., 2015; Fontelles-Carceller et al., 2015; Xiaojuan et al., 2014).

Non-metal-doped g-C3N4

Non-metal heteroatoms, such as S, O and P, have been shown to improve the degradation of organic pollutants by improving the visible light absorption, altering the band gap, and enhancing charge carrier separation and transfer(Chai et al., 2017; Wang et al., 2015a; Wu et al., 2018a). Moreover, doping non-metals into g-C3N4 has been explored to overcome the risks of secondary pollution. Besides, to overcome the limitations of applying g-C3N4 in photocatalysis, strategies involving coupling two or more semiconductors to build heterojunctions have emerged as one of the most promising ways for tailoring its photocatalytic properties (Mamba and Mishra, 2016; Wu et al., 2018b). These attempts to design heterostructures provide new ideas for g-C3N4 applications.

Advanced oxidation processes (AOPs)

AOPs

AOPs have been proved to be the important and useful methods of water treatment because of their high reactivity, rapid treatment rate, and low environmental risk (Glaze et al., 1987). During the AOP, organic chemicals are efficiently degraded by generating various reactive oxygen species (ROS). The popular ROS employed by different types of AOPs included hydroxyl radicals (·OH), superoxide anions (O•–), sulfate radicals (SO4 •–), and singlet oxygen (1O2). Since g-C3N4 is a fascinating material for environmental remediation, much interest has been directed to expanding the scope of its application. In view of the features of g-C3N4, its application in AOPs was expected to yield positive results. Recently, the application of g-C3N4 in AOPs has been applied to the treatment of landfill leachate.

AOPs for leachate treatment

Reactive species generated by AOPs, such as hydroxyl radicals (·OH), act as powerful oxidizers in the degradation of refractory pollutants, with high rate constants of ∼108-1010 M−1 s−1 (Gautam et al., 2019; Stasinakis, 2008). Landfill leachate contains high concentrations of toxic and bio-refractory compounds, which cannot be completely removed by conventional biological or physiochemical treatment processes. Hence, AOPs are expected to be the very applicable to the treatment of landfill leachate.

The most popular ROS, ·OH derived from the Fenton reaction, exhibits low efficiency in the removal of refractory organics and ammonia because it is rapidly quenched by untargeted compounds. SR-AOP (sulfate radical-advanced oxidation process) has been shown to be more effective than the Fenton process, since SO4 •– has a longer half-life than ·OH in the simultaneous decomposition of refractory organics and ammonia (Guo et al., 2020; Jiang et al., 2018). Thus, most of the refractory pollutants in leachate can be degraded by coupling with various AOPs (Chen et al., 2019; Deng and Ezyske, 2011). Recent research has focused on developing efficient AOPs for leachate treatment with low chemical use and energy consumption. The available AOPs that have been studied for leachate treatment can be categorized into ozone-based, photo-assisted, electrochemical, and miscellaneous processes, as shown in Fig. 2 . The different AOPs had different advantages during the treatment, but no single process delivered the desired removal performance (Gautam et al., 2019). Details of various AOPs for leachate treatment were listed as Table 1 .

Fig. 2

Advanced oxidation processes for leachate treatment.

Table 1

. Details of various AOPs for leachate treatment.

AOPs	Mechanism	Advantages	Disadvantages	Effectiveness
Photo-assistedprocess	light adsorption andradical generation	Operation at ambient conditions with possible use of solar irradiation;No mass transfer limitations;Oxidizing organic compounds into harmless ones such as CO2 and H2O;	Energy reduction as declining opacity by excessive catalyst addition;pH has a complex effect on photocatalytic oxidation;	Most of the biodegradable compounds were destroyed or/and fewer biodegradable intermediates were formed;
Electrochemical process	indirect oxidation (oxidation through the mediator);direct oxidation (oxidation at anode surface);	Bipolar electrochemical reactors are easy to operate and control;Operating conditions under ambient temperature and pressure prevents discharge and volatilization of unreacted wastes;Not chemicals;	Energy intensive process and formation of chlorinated organics;High operating costs caused by expensive metallic electrodes;	More significance in removing color in comparison to COD;Organic pollutants could be converted into carbon dioxide and water;
Ozone-based process	Molecular ozone reactions;·OH radical reactions;	Enhance ozone consumption;Increase efficiency of ozonation by controlling radical generation;·OH radical generation at lower pH;Promote surface reactions between the adsorbed ozone and pollutants;	Cannot meet the discharge standards alone;High ozone dose makes energy intensive;Low mass transfer from gas to a liquid;	Alter the molecular structure of the organic compounds and oxidize them to more biodegradable compounds;Reduce COD and BOD in leachate extensively; a good option only for pre- or post-treatment of leachate;
Miscellaneous process	(Including Wet air oxidation (WAO); ultrasound assisted process, persulfate oxidation and hydrodynamic cavitation (HC), etc.The outcomes of different AOPs were summarized as: (a) organic substances in leachate were oxidized into the highest stable oxidation states; (b) biodegradability of recalcitrant compounds becomes more compatible with the subsequent biological treatment;Mechanism, advantages and disadvantages of each process were omitted here.)

Hazardous leachate can be effectively treated by altering the molecular structure of organic compounds and oxidizing them into biodegradable species using the high oxidative activity of ozone. However, ozone-based AOPs have limitations due to the low mass transfer efficiency of ozone from the gas phase to the liquid phase (El-Din and Smith, 2001; Gautam et al., 2019). Moreover, partial leaching of the ozone may negatively impact the surrounding environment. Because leachate has a complex matrix with a high content of organic compounds, a high dose of ozone is required to achieve the desired treatment efficiency. To a large extent, their high energy consumption and massive emission of ozone limit the application of ozone-based AOPs to the pre-treatment or post-treatment of leachate. Critical issues for application of the Fenton oxidation process include the narrow operating pH range (< 4), the rapid consumption of Fe2+, and the high cost of follow-up sludge treatment. Moreover, a high concentration of iron contaminants is produced and the suspended solids in leachate could block or absorb the UV energy during the photo-assisted Fenton process. Although electrochemical AOPs are easy to operate, needing for large amounts of energy and the expensive metal electrodes dominated the main drawback. In addition to these issues with application of the AOPs discussed above, the characteristics of leachate (e.g. the high concentration of contaminants and presence of various interfering substances) undoubtedly increase the difficulty of efficient treatment.

Combining AOPs with other methods has been shown to improve the treatment of leachate. 1,4-dioxane, one of the common organic chemicals in leachate, was not significantly degraded by single AOPs due to the presence of other interfering substances. Pre-adsorption on activated carbon (AC) in photocatalysis increased the degradation of 1,4-dioxane and decreased the production of toxic intermediates (Nomura et al., 2020). Poblete et al. further suggested that the use of adsorption as a pretreatment method result in a high removal rate for organic compounds and heavy metals during photocatalysis (Poblete et al., 2019). Ultrasound is an efficient assistance method, because it produces tiny bubbles that expand and implode in the liquid, coupled with the effects of turbulence and liquid circulation, and helped cleave H2O molecules to generate more hydroxyl radicals. Joshi and Saurabh found that combining ultrasonication with the Fenton reaction resulted in the efficient treatment of leachate (Joshi and Gogate, 2019). Anfruns et al. proposed a useful approach for efficient leachate treatment that combines AOPs with bio-anaerobic ammonium oxidation (anammox), although it had a high nitrogen content and low BOD/N ratio. This anammox-assisted AOPs achieved a high nitrogen removal efficiency of up to 90 % (Anfruns et al., 2013). Furthermore, combining the Fenton reaction with membrane separation was very useful for decreasing membrane fouling and provided complementary treatment for leachate and produced effluent with contaminant contents that met the required regulatory levels for environmental release (Moravia et al., 2013).

Application of AOPs with g-C3N4 for leachate treatment

Efficient practices implied that AOPs are technically achievable for the degradation and treatment of leachate. Among AOP methods, semiconductor-assisted photocatalysis has attracted tremendous attention because it enhances the production of reactive species by employing low cost, renewable solar energy.

The main concerns of catalyst-based AOPs for the leachate treatment are structural characterization and performance evaluation (Hou et al., 2020; Kong et al., 2019; Lu et al., 2020; Mousavi et al., 2018; Shao et al., 2019). Structural characterization features can be represented by the spectroscopic or morphological methods. Catalytic degradation performance as well as the cycle stability need to be highly considered (Kumar et al., 2018; Li et al., 2016a; Liu et al., 2016). The involved mechanisms mainly include the identification of intermediate and end products as well as the relevant analysis of degradation pathways.

The selection of a suitable semiconductor material is essential for photocatalysis, consequently. Among the available semiconductors, including TiO2, ZnO, CdS, SiC, WO3, and WS2, TiO2 is the most widely applied and studied semiconductor material. For landfill leachate treatment, the first study using photocatalysis involved in application of TiO2 as a photocatalyst on a pilot-plant scale. In this study, humic acid (HA), a model refractory organic compound for leachate pollutants, was rapidly decolorized and degraded under sunlight (Wiszniowski et al., 2004). These results showed that the photo-induced AOPs approach significantly shortens the subsequent biodegradation process (10 min per sample) and decreases the BOD5 (5 days). Even though TiO2 has the advantages of long-term stability and excellent results in combination with AOPs, its wide band gap, low utilization of solar energy, and fast electron-hole recombination limit its practical application. Many subsequent studies have been conducted to improve these properties, including by photocatalyst modification (decoration with metals, metal doping or heterojunction construction), technical improvements, and combination with other processes (Rocha et al., 2011; Vilar et al., 2011a, a; Vilar et al., 2011b, b). Moreover, exploring alternative semiconductor photocatalysts is another important way to improve photocatalytic leachate treatment.

g-C3N4 has attracted wide interest as a promising photo-catalyst because it is inexpensive, easily available, robust, and has wide visible-light activity. The known photodegradation processes performed by g-C3N4 are shown in Fig. 3 , and provide theoretical evidences that g-C3N4 can generate free radicals and potentially efficiently treat leachate. As shown, during photocatalysis g-C3N4 is excited by absorbing photons, generating photo-induced electron-hole pairs due to the transfer of an electron from the valence band (VB) to the conduction band (CB) (Ge et al., 2011; Zhang et al., 2019b). The absorbed energy (hν) must be larger than the band gap (Eg) of g-C3N4 (∼2.7 eV) (Wen et al., 2017; Zhang et al., 2010b). In general, during the electron transfer process, the surface electrons (e−) and holes (h+) serve as electron donors and acceptors for producing strongly oxidizing or reducing radical species which can directly degrade the various organic contaminants in leachate into CO2, H2O and other small molecule products. Take phenol removal by the Fe-g-C3N4 graphene hydrogel (rGH/Fe-g-C3N4) 3D structure as an example, in the presence of visible light, electrons and holes could be generated by rGH/Fe-g-C3N4. Partial electrons would quickly transfer to Fe3+ for Fe2+ formation, while other electrons could transfer to graphene via π-π conjugation, and thus directly decompose H2O2 to form ·OH. Generated Fe2+ can further react with H2O2 to produce ·OH, which can firstly attack the ortho and para-carbons within benzene ring to form catechol and hydroquinone, subsequently open the ring to form the chain compounds and are finally mineralized into CO2 and H2O (Hu et al., 2020).

Fig. 3

Schematic illustration of the photodegradation processes performed by g-C3N4 for leachate treatment.

The application of g-C3N4-based materials in leachate treatment is currently under development and there are only a limited number of reported studies. We will do our best to summarize the existing studies and the potential of C3N4-induced AOPs. Although there have been no specific studies of the practical application of g-C3N4 in leachate treatment, there are sufficient available mechanisms to eliminate the contaminants that are present in leachate. Employing g-C3N4 as a photocatalyst results in high degradation performance for the recalcitrant organic pollutants in leachate under visible light (Wang et al., 2009). Further combining fungal degradation (using white rot fungi) with photocatalytic g-C3N4 produced superior performance for the degradation of organic compounds in leachate under visible light (Hu et al., 2017). The white rot fungi that were used efficiently on pre-biodegradation of natural lignocellulosic substrates and various organic pollutants by secreting extracellular enzymes, and their spatial structure provided a large number of tiny interspaces and active reaction sites for degradation reactions. Ultimately, this combination not only achieved 74.99 % organic carbon (TOC) removal (the initial TOC concentration was 100 mg L−1), but also showed excellent degradation potential for almost all the organic compounds present in leachate (Hu et al., 2017). Other studies have also shown that the microbial degradation of dye-containing wastewater by photosynthetic bacteria was improved significantly due to a synergistic effect between the C16 alkane degradation performed by JLS1 bacteria and simultaneous photocatalysis by g-C3N4, (Xu et al., 2017; Zhang et al., 2017).

In addition, the microbial contaminations as well as the highly-concerned Novel Coronavirus (SARS-CoV-2) or other viruses appeared in landfill leachate may pose severe pathogenic risks to humans. Water disinfection and microbial control (e.g. of bacteria, viruses, and microalgae) using g-C3N4-based photocatalysts provide complementary treatment for leachate in addition to traditional water disinfection techniques. Previous studies showed that g-C3N4 possessed both excellent antibacterial and antiviral activities (Zhang et al., 2019a), which may provide a low-cost and environmental-friendly alternative for the sustainable leachate treatment (Ong et al., 2016). Yu et al. displayed that the high inactivation efficiency of antibiotic-resistant bacteria (tetracycline-resistant Escherichia coli, TC-E. coli) in secondary effluents could be achieved by using Ag/AgBr/g-C3N4 as the photocatalyst (Yu et al., 2020). In addition, Zhang et al. also found a water-surface-floating photocatalytic g-C3N4 composites (g-C3N4/EP-520) can exhibit an excellent antibacterial/ antiviral activity under visible-light irradiation, extending the application of g-C3N4-based material for water disinfection (Zhang et al., 2018). To better meet the real demand, further studies toward an in-depth understanding of the microbial inactivation process are encouraged, while the researches about lethal action of photocatalysts and the vulnerable components of microbes for maximizing the inactivation efficiency are also required. Several studies have demonstrated the theoretical and technical feasibility of the application of g-C3N4 in combination with AOPS in leachate treatment. More related studies are still required to improve the understanding of the application of AOPs to leachate treatment. Table 2 presents in detail the relevant studies of the potential application of g-C3N4 in leachate treatment.

Table 2

Relevant studies of the potential application of g-C3N4 to the treatment of substances in leachate.

Categories	Substances	Photocatalysts	Efficiency	Ref. (Year)
Organics	all organic compounds in leachate, especially volatile fatty acids and long-chain hydrocarbons	a combination of Phanerochaete chrysosporium and photocatalysis with g-C3N4	74.99 % TOC removal efficiency (72 h, initial concentration of 100 mg L−1)	(Hu et al., 2017)
Heavy metals and salts	Cr(VI)	Ti3+-TiO2/g-C3N4	65 %/39 %	(Lu et al., 2015)
	As(III)	urea-derived g-C3N4	70 %	(Kim et al., 2018)
	nitrate reduction	TiO2/g-C3N4	40.3 % (16 h)	(Zhang et al., 2020)
High molecular weight compounds	amoxicillin (AMX)	A magnetic fluorinated mesoporous g-C3N4	90 %	(Mirzaei et al., 2019)
	bisphenol A (BPA)	Pd/g-C3N4 nanoparticles	91 % BPA with an initial concentration of 20 mg L−1 in 60 min	(Wang et al., 2017)
	trace polychlorinated biphenyls (PCBs)	magnetic carbon nitride nanocomposites	–	(Li et al., 2017)
	degradation of 2-chlorophenol	Cr2O3/g-C3N4	94 %	(Anjum et al., 2018)
	tetracycline hydrochloride (TC-HCl)	Polymeric carbonnitride foam (CNF)	18.9 % (pharmaceutical wastewater) to 78.9 % (natural seawater)	(Wang et al., 2018)
Other	disinfection and microbial control	g-C3N4	bacteria were broken in 12 h; virus died after 6 h; all microalgae ruptured after 6 h	(Zhang et al., 2019a) (Ong et al., 2016)

The high nitrogen load of landfill leachate increases environmental stress on rivers and streams. Photocatalytic denitrification of leachate by g-C3N4 has become an available option. The photocatalytically generated electrons and holes can be used for reduction and oxidation, respectively. Photocatalytic denitrification using g-C3N4 also can avoid the limitations of applying biological denitrification, due to insufficient electron donors. TiO2/g-C3N4 photocatalysts coupled with in-situ cultivated biofilms have been shown to enhance nitrate reduction in water, with 40.3 % removal of the nitrate after 16 h. This approach provides a new strategy for enhancing nitrate reduction in leachate and insights into the reduction mechanism (Zhang et al., 2020). Moreover, the hazards of releasing large molecular weight organic compounds into the environment are also an important issue in leachate treatment. A magnetic fluorinated mesoporous g-C3N4 has been employed for the photocatalytic degradation of the antibiotic amoxicillin (AMX) in leachate, and achieved a higher degree of mineralization with a lower accumulation of toxic by-products (Mirzaei et al., 2019). Moreover, AOPs based on sulfate radicals (SO4 •–), which have a high oxidation potential, can be combined with g-C3N4. Sulfate radicals are usually generated by the activation of persulfate (PS) through heating, UV irradiation, bases, or transition metals. g-C3N4 displayed high catalytic activity for the activation of persulfate in persulfate-based AOPs. The results reported by Wang et al. indicated that Pd nanoparticles anchored on g-C3N4 were very active for the degradation of bisphenol A (BPA) by efficiently activating PMS (Wang et al., 2017). The composite photocatalyst Cr2O3/C3N4 achieved the rapid photodegradation of 2-chlorophenol under visible light with a removal rate of 94 % at the optimal pH (Anjum et al., 2018).

Conclusion

In this review, current development and progress on the application of g-C3N4 to leachate treatment have been showcased. The application of g-C3N4 as a photocatalyst in AOPs for the treatment of landfill leachate has been comprehensively reviewed, providing theoretical and technical possibilities for the wide application of g-C3N4-based AOPs for leachate treatment. Notably, although there have been a large number of studies dedicated to the application of g-C3N4-based photocatalysts in the evolution of H2 and the degradation of pollutants, direct practical applications to landfill leachate have received less attention. Most studies have dealt with the degradation of single components in leachate. The development of a high-performance g-C3N4 catalyst would be key for the remediation of complex wastewater using AOPs. Based on laboratory tests and theoretical studies, and considering the characteristics of waste leachate like its complex, high concentration components, high ammonia nitrogen content, and the presence of unknown interfering substances, it is necessary to expand this application to resolve the practical issues of leachate pollution. Coupling AOPs with other methods and technologies should be actively explored and applied in practical problem solving. Furthermore, leachate treatment using g-C3N4 should also be extended to the remediation of secondary sites, such as leachate sediment and the surrounding soil. Hopefully, this review will encourage more research into applying the robust and inexpensive g-C3N4 in photocatalytic AOPs for leachate treatment.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.