Compost-enhanced humification of organic pollutants: Mechanisms, challenges, and opportunities

Abstract

Organic pollutants remain a persistent threat to ecosystems and human health. In soils, humification gradually converts these compounds into stable humic substances and attenuates their toxicity, but the transformation can take decades—far too slow to match current pollution loads. In this Perspective, we argue that mature compost offers a pragmatic means to accelerate this process: it delivers partially humified intermediates that can “seed” soil humification and shorten its timescale from decades to seasons. Spectroscopic evidence shows that compost-derived humus is enriched in aromatic backbones and reactive functional groups (–COOH, –OH) that both catalyze further condensation of organic matter and immobilise pollutants through π–π stacking, hydrogen bonding and covalent coupling. By merging these catalytic and sorptive functions, compost amendments provide a scalable, low-cost route to the long-term stabilization of organic contaminants. We outline the key mechanistic questions that now need resolution—particularly the reactivity of specific intermediates in situ—to guide field trials and unlock the full potential of compost-driven accelerated humification as an environmental remediation platform.

Graphical abstract

Highlights

• •The potential of compost to promote soil humification has been underestimated.
• •The application of compost can makes the soil humification process controllable.
• •The humification process of organic pollutants deserves the same attention.

1. Introduction

Rapid industrialization and urbanization have led to a large number of organic pollutants entering the soil environment, negatively impacting the ecological environment and human health. Consequently, organic pollution has emerged as a major environmental problem. Organic pollutants cannot be removed through natural self-purification and tend to exist in the environment for a long time, endangering the environment. Long-term exposure to these pollutants may lead to stunted physical development and affect physical and mental health, leading to neurological damage, mental decline, and cancer [1]. To control the current environmental situation, a variety of efficient experimental methods for removing organic pollutants have been developed, including adsorption [[2], [3], [4], [5]], photodegradation [6,7], photocatalysis [8], and advanced oxidation [9,10]. However, more experimental data and field tests are needed to support their feasibility. Removing organic pollutants is thus a long-term goal in developing and applying these methods.

Organic pollutants in the environment mainly undergo two processes: degradation and humification [[11], [12], [13], [14], [15]]. It is generally accepted that organic pollutants act as electron donors supporting oxidative respiration through reduction or as fermentation substrates facilitating fatty acid degradation and removing mineralization pathways [16,17]. In contrast, the humification process involves the generation of many intermediate substances, many of which contain functional groups such as –COOH, –OH, and aromatic structures. These intermediates have adsorption capabilities and can transform organic pollutants into more stable humic substances (HSs). Therefore, research on utilizing the humification process to reduce the toxicity of organic pollutants in soil should receive the same level of attention as research on degradation processes.

Composting technology, an environmentally friendly technology, involves a humification process that is inherently similar to soil humification. This process transforms organic pollutants into stable HSs through microbial metabolism and enzymatic reactions. Compost products significantly accelerate the humification of organic pollutants in soil, and composting has been widely applied in agriculture and ecosystem management. Notably, recent attention has increasingly focused on the degradation pathways of organic pollutants during the composting process as well as their role in providing nutrients to crops and improving soil fertility [[18], [19], [20]]. After microbial decomposition, organic matter in compost provides substantial nutrients, such as N, P, and K, that create a favorable crop growth environment and enhance soil fertility [21,22]. During the composting process, organic matter is broken down by microorganisms, which produce a large amount of humus precursors. When compost is applied to soil (Fig. 1), these precursors directly participate in the humification process, reducing the time required for the decomposition-transformation process of natural organic matter and promoting the stabilization of organic pollutants (e.g., polycyclic aromatic hydrocarbon, pentachlorophenol, antibiotics) within humus [[23], [24], [25], [26]]. This reduces the bioavailability and toxicity of pollutant and lowers the risk of pollutant migration. In contrast to natural humification, a process that takes decades to centuries, compost application facilitates humus formation in months to years [27]. Moreover, composting is a low-cost, sustainable treatment technology that promotes soil health and effectively reduces the negative environmental impacts of agricultural waste [28]. Therefore, compared to natural humification, compost products efficiently promote the humification of organic pollutants, improve soil quality, and facilitate the long-term stabilization of pollutants, which is important for environmental remediation. In this article, we discuss the trajectory of research on composting-enhanced humification of organic pollutants in soil and address the challenges and trends that may affect future research.

Fig. 1.

Compost products promote soil humification.

2. Promotion of soil humification by compost products

As a strategy for stabilizing organic pollutants, soil humification differs from other degradation methods, such as biodegradation, photodegradation, and chemical oxidation [6,7,10]. Humification primarily occurs through microbial metabolism, enzymatic reactions, and organic matter transformation [29,30]; pollutants are converted into HSs or bound with HSs, which reduces their toxicity and mobility. This process exhibits high efficiency with low concentrations of organic pollutants. Recalcitrant organic pollutants, such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and antibiotics, form stable associations with HSs via π–π interactions, hydrophobic adsorption, and covalent bonding [[31], [32], [33], [34], [35]], which reduce their toxicity. Humification is typically based on the natural organic matter process of humus formation, which improves soil fertility and offers a cost-effective advantage over complex and expensive equipment and reagents.

Although soil humification holds the potential for treating organic pollutants, it is a slow process with a formation cycle of several years or even decades, and the process of polymerization of organic pollutants is even longer [27]. The release rate of organic pollutants in the environment greatly exceeds the rate of soil humification. In addition, the unique molecular structures of organic pollutants and the environmental conditions of microbiota increase the difficulty of soil humification [36], making this process controversial.

In contrast, composting, which also involves soil humification, is receiving great attention. The composting process consists of four stages: mesophilic, thermophilic, ripening, and cooling. Humification mainly occurs in the ripening and cooling stages. Compost degrades organic pollutants into sugars (glucose), amino acids, and other small molecules that can form intermediates and polymerizes small molecules formed by high-temperature degradation into a large number of easily biodegradable aromatic structures, lignocellulosic, aliphatic compounds, and other intermediates; some of these pollutants get humified. When compost is applied to and enters the soil, the compost intermediate accelerates soil humification, polymerizes pollutants through the Maillard reaction, stabilizes them in HSs, and, in turn, promotes the polymerization of organic pollutants and HSs (Fig. 2). As composting progresses, the HS structure evolves. Studies have shown that the degree of aromaticity of HS, and the number of benzene rings and their (carboxyl, hydroxyl, and carbonyl groups) are positively correlated with the extent of humification during the composting process [[37], [38], [39]]. The humification process during composting can be indirectly assessed by analyzing the structural changes in the HSs. The ability to accelerate the degradation of organic matter by artificially controlling conditions such as temperature, humidity, and oxygen supply is an advantage of composting humification, and it reduces the time required for the decomposition and transformation of organic pollutants in the natural environment. This results in a humification rate significantly higher than that of natural humification. Furthermore, compost is rich in carbon and nitrogen sources and microorganisms with humification functions (e.g., Streptococcus, Bacillus, and Pseudomonas) [40]. These components enhance the activity of soil enzymes (such as catalase and ligninase) and provide a favorable ecological environment for microorganisms, thereby accelerating the humification of organic pollutants [29,30]. It has been proven that compost products rich in organic matter can increase the degree of aromaticity of humic acids in the soil and thus enhance their ability to aggregate pesticides [41]. Moreover, compost products rich in aromatic –COOH, –NH, and lignin structures exhibit strong adsorption capacities for organic pollutants (phenanthrene) [42,43].

Fig. 2.

The process of organic pollutants incorporation into humic substances by Maillard reaction.

The use of compost to promote the humification of organic pollutants is an important area of soil remediation. The application of composting is influenced by various factors, including the type and source of compost materials, temperature, humidity, oxygen content, and pH [[44], [45], [46], [47], [48]]. Materials with high levels of carbon content promote microbial activity and enhance the efficiency of humification [44]; high temperatures facilitate the degradation of lignin and promote the formation of phenolic and quinone structures [49]; alkaline pH values promote the synthesis of HSs, which are alkaline-soluble compounds [50]; appropriate aeration and composting treatments can result in high HS content [51]; and variations in soil moisture influence soil oxygen levels, microbial respiration, and enzyme activity, thereby affecting the humification process [52]. In addition, the role of microorganisms during the composting humification process cannot be overlooked. Microbial activity enhances the production of aromatic HS structures and stabilizes organic pollutants during HS humification. Among the various microbial groups, Bacillus species (Clostridium Ⅳ, Bacillus halophilus, Bacillus parahaemolyticus) exhibit the most prominent dominance. In compost inoculated with Bacillus species, the concentration of HSs increased by 43.4–57.4 %. [53,54]. Some studies have shown that enzymes such as laccase and lignin peroxidase can be used to catalyze the oxidation and self-coupling of organic pollutants, including phenol, sulfadimethoxine and triclosan, and cross-coupling with polymers to form HSs through C–O, C–N–C, and C–C bonds; these functional enzymes are synthesized by microorganisms [32,55,56]. Therefore, to ensure that composting effectively promotes the humification of organic pollutants in soil, the activity of functional enzymes needs to be maintained through the reasonable regulation of soil conditions and the optimization of compost materials and preparation processes [57,58]. This requires selecting raw materials rich in lignocellulose, which can enhance the stability of enzyme activity in compost products. When applied to soil, these raw materials continuously stimulate the production of important enzymes, such as lignin-degrading enzymes and cellulases, thereby increasing the efficiency of humus precursor production and promoting the humification of organic pollutants [44]. Adding biochar and other additives can also protect enzyme molecules, reducing their degradation [59]. Microorganisms (such as Bacillus and Actinobacteria) can be introduced into compost to further enhance enzyme activity [60,61]. In sum, regulating the soil environment, maintaining the pH in a neutral to weakly acidic range, and ensuring that humidity is between 60 % and 80 % can improve the water retention and aeration of soil upon compost application, thereby prolonging the duration of enzyme activity [62]. Moreover, it is important to control the application rate when using compost. Studies have shown that applying 20 tons of compost per hectare can balance nutrient supply and enzyme activity protection [63].

3. Compost accelerates the humification of organic pollutants

Compared to humification under natural conditions, compost humification is short and controllable. Research has shown that HSs formed by natural and compost humification are highly similar in structure and element composition [36]. Nevertheless, the advantages of compost HSs have led to the increasing use of compost to promote agricultural production and environmental pollution remediation.

It has been reported that compost stabilizes organochlorine insecticides in contaminated soil through its intermediates, specifically due to the combined action of their chemical affinities and the physical adsorption of the mesoporous structure [64]. Compared to other types of organic pollutants, organic pollutants containing stable aromatic ring structures are easily incorporated into HSs because of their strong affinity for the aromatic structures in HS, leading to stability between them. Some aromatic compounds contain polar functional groups (such as carboxyl and hydroxyl groups) that form hydrogen bonds with the functional groups in HSs (such as phenolic hydroxyl and carboxyl groups), further enhancing the binding capacity of HSs [[65], [66], [67], [68]]. Furthermore, due to their strong hydrophilicity and charge characteristics, antibiotics can aggregate during composting through interactions with the polar functional groups (such as carboxyl and phenolic hydroxyl groups) in HSs [32]. Phenanthrene, methylene blue, polychlorinated phenol, and antibiotics are thus polymerized or incorporated into HSs. For example, sulfonamides and antibiotics form stable covalent bonds with quinone groups in humic acids through nucleophilic addition reactions, during which laccase catalyzes the precursor substances and pollutants [32]. Studies have shown that applying compost products significantly enhances the humification capacity of pentachlorophenol in soil. For instance, the humification capacity of pentachlorophenol has been shown to increase by 54.8 % in soil with added humic acid [41], and the addition of composted humus has resulted in the 89.0 % removal of polycyclic aromatic hydrocarbons in diesel-contaminated soil [69]. Thus, the application of compost not only accelerates soil humification and facilitates the entry of organic pollutants into HSs but also combines with activated minerals, increases the function of hydroxyl groups, and has hydrophilic properties, thereby allowing the nutrients in the soil to combine with organic pollutants [70].

Composting can accelerate humification, requiring particular environmental conditions (i.e., temperature, moisture, and oxygen levels) [46,47]. Composting may fail if these conditions are not properly controlled, and harmful gases (e.g., NH₃, CH₄, and N₂O) may be produced [71]. Compared to natural processes, composting requires technical monitoring and management. Composting effectively improves soil fertility, promotes soil organic matter content, and provides nutrients in the short term, but its application in agriculture requires the investment of resources, including time, labor, materials, and management, which may not always be sustainable. Further, composted humus may not be able to fully replace natural humus in terms of long-term stability, soil structure improvement, and maintenance of microbial activity. Further research and improvements to composting technologies are required to render the compost production process more controllable and to provide a scientific basis for the accelerated humification of organic pollutants in soil.

4. Challenges and perspectives

Humification provides a promising approach to treating organic pollutants. When compost products are applied to soil, the precursor substances in the compost enter the soil and enhance the efficiency of humification. However, research on the accelerated humification process induced by compost humus is still in its early stages. The main reasons for the limited research include (1) the varying effects of compost products in different soil types and environments, (2) the need for further control and optimization of soil enzyme activity following compost application, and (3) the potential long-term impacts of continuous compost application on the soil ecosystem. Therefore, future research must address these issues to optimize the effectiveness of compost in promoting humification.

• (1)Synergistic mechanism between compost products and soil. Researchers should further explore the interactions between compost products and different types of soils, particularly their role in promoting the degradation of organic pollutants and humification [30]. They should investigate the diversity and functionality of microbial communities in compost as well as the relationship between enzyme activity in compost and the physical and chemical properties of soil. A systematic study of composting mechanisms under various soil conditions will allow for precise pollution management solutions based on soil type.
• (2)Maintaining and optimizing enzyme activity in soil. Future research should focus on techniques for the long-term preservation of enzyme activity in compost products and thus ensure their soil stability. Determining how to control temperature and humidity fluctuations during the composting process and how to apply appropriate enzyme inhibitors or activators is crucial for sustaining enzyme activity and enhancing the degradation of organic pollutants and humification. It is also important to study the sources, types, and adaptability of enzymes in different soil environments in depth.
• (3)Long-term effects of compost products on soil quality. Most of the research only focuses on the short-term effects of compost products on soil after application during the experimental cycle, such as humic acid formation and microbial community changes, etc., and long-term effects of compost application on soil remains weak. Future investigations should focus on comprehensively understanding the impacts of compost on soil fertility, microbial communities, plant growth, and the overall soil environment. The formation and stability of humus, as well as the influence of compost on soil structure, pH, and nutrient content, need to be studied. Researchers should also determine the optimal application rates, timing, and frequency of composting to minimize negative effects and enhance the sustainability of compost in pollution management.

CRediT authorship contribution statement

Dongyu Cui: Writing – review & editing, Supervision, Investigation, Formal analysis, Conceptualization. Yike Kang: Writing – original draft, Visualization, Investigation, Formal analysis, Conceptualization. Beidou Xi: Writing – review & editing, Supervision, Funding acquisition. Ying Yuan: Writing – review & editing, Supervision. Qiao Liu: Formal analysis, Conceptualization. Wenbing Tan: Writing – review & editing, Supervision.

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.

Dr. Beidou Xi, the Executive Editor of Environmental Science and Ecotechnology, and Dr. Wenbing Tan, the Editorial Board Member of Environmental Science and Ecotechnology, were not involved in the editorial review or the decision to publish this article.