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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 7: 1018 - 1029

DOI: https://doi.org/10.7536/PC221203

Review

Interactions Between Humic Acid and Co-Existing Substances in Aquatic Environments

  • Chundi Zhou 1 ,
  • Minghao Sui , 1, 2, *
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  • 1 State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University,Shanghai 200092, China
  • 2 Shanghai Institute of Pollution Control and Ecological Security,Shanghai 200092, China
* Corresponding author e-mail:

Received date: 2022-12-10

  Revised date: 2023-04-13

  Online published: 2023-06-15

Supported by

National Key R&D Program of China(2019YFC0408801)

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Abstract

Humic acid (HA) has attracted significant attention in the field of environmental remediation due to its occurrence characteristics and unique chemical reactivity. It is worth noting that in co-existing reaction systems, HA inevitably interacts with co-existing substances, making the reaction system complex and leading to unexpected results. Therefore, studying the interaction between HA and co-existing substances is of great significance for a correct understanding of the complexity of environmental water pollution and the development of new environmental functional materials with cooperative treatment of co-existing substances. This article reviews the synergistic/antagonistic removal effects of target pollutants in co-existing pollution systems involving HA, including inorganic co-existing pollutant systems, organic co-existing pollutant systems, and microbial co-existing systems. Based on the structural characteristics and physicochemical properties of HA itself, the interaction mechanisms between HA and co-existing pollutants are systematically analyzed. These mechanisms mainly involve coordination, electrostatic interactions, adsorption, hydrophobic interactions, π-π interactions, and oxidation-reduction reaction (REDOX). Finally, the challenges and future research directions for the removal of target pollutants by HA in co-existing pollution systems are discussed.

Contents

1 Introduction

2 Removal of target contaminant in different co-existing systems

2.1 HA with co-inorganic contaminant system

2.2 HA with co-organic contaminant system

2.3 HA with co-microbial system

2.4 Quantitative comparison of the removal effectiveness in HA co-existing contaminant systems

3 Interaction mechanisms between HA and co-existing contaminant

3.1 HA with co-inorganic contaminant system

3.2 HA with co-organic contaminant system

3.3 HA with co-microbial system

3.4 Characteristics of interaction mechanism in HA co-existing contaminant systems

4 Conclusion and Outlook

Key words： humic acid; environmental water pollution; co-existing substances; interaction; co-existing systems

Cite this article

Chundi Zhou , Minghao Sui . Interactions Between Humic Acid and Co-Existing Substances in Aquatic Environments[J]. Progress in Chemistry, 2023 , 35(7) : 1018 -1029 . DOI: 10.7536/PC221203

1 Introduction

Humus is a kind of macromolecular organic matter formed by animals and plants through complex biochemical processes of bacteria, algae and other microorganisms in the environment, which is ubiquitous in natural ecosystems[1,2]. Natural humus is mainly divided into three main components: humic acid (HA), fulvic acid (FA) and Humin. HA is mainly composed of carbon, hydrogen, oxygen and a small amount of nitrogen, sulfur and other elements, which is a complex macromolecular organic matter. In contrast, FA is composed of multiple aromatic carboxylic acid units with similar structures and is a relatively simple organic substance[1,2]. The solubility of the three is different, HA is soluble only under neutral or alkaline conditions, while FA is soluble at all pH values, and the humic fraction is insoluble in water[3]. However, in the actual water treatment process, it is difficult to achieve the purpose of HA separation by changing the pH value of water. Moreover, in natural water and sedimentary environments, HA accounts for a large proportion of dissolved organic matter, with concentrations ranging from 20 to 250 mg/L and masses ranging from 1 to 200 kDa[4⇓⇓~7]. In the mass range of HA, it is usually divided into macromolecular HA and micromolecular HA according to its molecular weight. In general, the mass range of macromolecular HA is usually greater than 10 kDa, while the mass range of small molecular HA is usually between 1 and 10 kDa[7]. The parent structure of HA is usually composed of an aromatic ring and an alicyclic ring, on which a variety of active functional groups are attached, such as hydroxyl (phenolic hydroxyl, alcoholic hydroxyl), carboxyl, amino, quinone, semiquinone, enol, sulfonic acid, etc. Fig. 1 gives the possible structural units in HA as well as the different types of functional groups[8].
图1 HA的结构单元及官能团种类[8]

Fig.1 Structural units and functional groups of HA[8] Copyright 2011, Springer Nature

Although HA has no special toxicity, it has indirectly brought many problems to human production and life because of its large reserves in the natural environment and its active chemical properties[1,3,4]. On the one hand, HA is an important precursor of Disinfection By-Products (DBPs) produced By water plant Disinfection process[9]. Gao et al. Showed that the interaction between free radicals and amino groups in HA in halide-containing disinfection system was the cause of inducing nitrogen-containing DBPs[10]. This is similar to the results of recent studies by Huang et al[11]. On the other hand, HA contains abundant aromatic components (such as phenols, anilines, benzoic acids, etc.), which can act as ultraviolet quenching substances and seriously interfere with the disinfection process of pathogenic microorganisms in water by ultraviolet disinfection process[12]. In the process of water delivery in urban pipe network, the small molecular HA remaining in the water body can provide carbon source for the microorganisms attached to the pipe wall, which promotes the growth of microorganisms and leads to the deterioration of water quality. The negatively charged group of HA can also react with the iron element in the pipe wall, which aggravates the corrosion and aging of the pipe network. In addition, some studies have confirmed that HA can interact with other coexisting substances in the environment, control the physical and chemical behavior of coexisting trace elements, reduce the bioavailability of pollutants and the fate of environmental migration[13]. For example, HA affects the environmental transport of tylosin and sulfamethoxazine through cation exchange and functional group adsorption[14]. Jia et al. Found that HA can reduce the toxicity and activity of Cd (Ⅱ) in surface water through coordination[15]. Kraiem et al. Proposed that increasing the concentration of HA in the environment could reduce the nitrogen removal rate in the bioreactor[16]. These studies imply that HA plays an important role in the coexistence system in the field of water treatment. Therefore, it is necessary to further clarify the interaction between HA and coexisting pollutants.
There are many kinds of environmental pollutants coexisting with HA, such as heavy metal ions, inorganic non-metal salts, antibiotics, phenolic organic compounds, microplastics, biological macromolecular organic compounds and so on. There are many interactions between HA and these pollutants, which leads to the great difference between the removal effect of target pollutants in a single system and the experimental results in the coexistence system. Xie and Shang designed experiments to investigate the role of HA in the reduction of bromic acid by zero valent-iron (ZVI)[17]. It was found that the introduction of HA reduced the reduction ability of zero-valent iron compared with the reduction performance of ZVI for bromic acid in a single system. Zhang et al. Found that PO 4 3 - and CO 3 2 - in the coexisting system significantly inhibited HA removal[18]. Qin et al. Proposed that HA inhibited the removal of nitrophenol by reducing the reaction rate constant in the system[19]. It has also been suggested that the carboxyl functional group in HA inhibits the removal of phenolic homologues by UV Fenton technology[20]. However, other studies have emphasized the synergistic effect of HA in the removal of coexisting pollutants. For example, Tsang et al. Confirmed that HA can provide additional hydrophobic areas for trichloroethylene, which significantly enhances the removal effect of iron-based materials on trichloroethylene[21]. Liu et al. Found that HA can act as an electron mediator in the critical layer of environmental functional materials and water, which significantly improves the precipitation of HA and heavy metals[22]. Xu et al. Proposed that HA combined with iron could enhance the oxidation process of organic pollutants by accelerating the redox reaction between Fe (Ⅲ) and Fe (Ⅱ)[23]. These seemingly contradictory results may be due to the complex molecular structure of HA, which triggers different mechanisms of action under different environmental conditions. These mechanisms always affect the removal of the target pollutant together with one or more other types of mechanisms[1]. Based on this, this paper systematically summarizes the important role of HA in the removal of target pollutants in a variety of environmental coexistence systems in the field of water treatment, including inorganic pollutants coexistence system, organic pollutants coexistence system and microbial coexistence system. The treatment effects of target pollutants in different coexistence systems were quantitatively compared, and the interaction mechanism between HA and coexisting substances was clarified, which provided a theoretical reference for achieving the treatment goal of removing coexisting pollutants in environmental water synergistically and efficiently.

2 Removal of Target Pollutants in Different Coexistence Systems

2.1 HA-inorganic pollutant coexisting system

In the reaction system where HA and inorganic pollutants coexist, their existing forms in the environment are often changed due to interaction. Feng et Al. Found that 0.1 M salt ions (Na (Ⅰ)), alkaline earth metal ions (Mg (Ⅱ), Ca (Ⅱ)) and hydrolytic metal ions (Al (Ⅲ), Fe (Ⅲ)) improved the removal efficiency of HA by activated sludge[24]. They emphasized the formation of metal cation bridges and the generation of HA co-precipitates with metal ions as the key to improve HA removal. Wang et al. Also confirmed that with the increase of plasma intensity of Na (Ⅰ), K (Ⅰ), Mg (Ⅱ) and Ca (Ⅱ), the electrostatic repulsion in the coexistence system decreased, thus increasing the adsorption capacity of the adsorbent for HA[25]. Wu et al. Found that HA significantly improved the affinity of aquatic plants for Cd (II) in aqueous solution and reduced the concentration of Cd (II) in the environment[26]. The interaction between HA and iron minerals also has an important impact on the environmental transport of metal ions. For example, Yang et al. Demonstrated that the presence of HA in the coexistence system was more favorable for the removal of Cu (II) by iron oxides[27]. Liu et al. Successfully synthesized a composite material of HA-coated magnetite, which showed synergistic effect on the removal of Cu (Ⅱ), Pb (Ⅱ), Hg (Ⅱ) and Cd (II)[28]. Among them, the composite material has the best removal effect on Cu (Ⅱ), and the removal performance is increased by 66. 67% compared with that without HA treatment. Du et al. Found that the adsorption degree of Cd (Ⅱ) by ferrihydrite and HA coprecipitation system was controlled by HA concentration, and the adsorption capacity increased with the increase of HA concentration[29]. However, existing studies have also found that HA in the coexistence system has different degrees of inhibition on the removal of heavy metal ions. Some studies have shown that the combination of HA and iron oxides reduces the adsorption capacity of iron oxides for pollutants[30]. On the one hand, the adsorption of negatively charged HA on the mineral surface can change its surface charge and colloidal stability. On the other hand, excessive macromolecular HA can mask the original active sites on the mineral surface. Saito et al. Found the characteristics of charge change and potential distribution during the adsorption of HA on the surface of iron oxides, emphasizing the importance of electrostatic interaction in the adsorption process[31]. Wang et al. Synthesized Ferrihydrite (FHCs) with different HA loadings to explore the potential interaction of HA in the adsorption of Cr (Ⅵ)[32]. The results showed that HA loaded on the surface of FHCs had antagonistic effect on the adsorption of Cr (Ⅵ), and with the increase of HA loading, the less active sites exposed on the surface of FHCs, the more obvious the antagonistic effect. Hajd Hajdú et al. Found that magnetite nanoparticles coated with HA were stabilized by the combined effect of steric effect and electrostatic effect, which may lead to the loss of adsorption of magnetite to inorganic pollutants in the surrounding environment[33]. Weng et al. Have shown that negatively charged HA inhibits the electrostatic adsorption process of As (Ⅴ) on iron oxides[34]. Mak et al. Further found in the study that HA not only has electrostatic repulsion with As (Ⅴ), but also greatly reduces the removal effect of As (Ⅴ) in the coexistence system by reducing the co-precipitation of iron corrosion products and As (Ⅴ)[35].

2.2 HA-organic pollutant coexisting system

At present, the removal of organic pollutants mostly depends on redox technology. However, HA in the coexistence system can often induce or shield the formation of reactive oxygen species, thereby affecting the removal of organic pollutants[36]. For example, Tratnyek et al. Showed that HA increased the reduction rate of ZVI and could cooperate with ZVI to reduce organic pollutants[37]. Fang et al. Found that quinone-rich HA could effectively activate persulfate to degrade trichlorobiphenyl[38]. However, the experimental results of Li et al. Show a completely different phenomenon[39]. In this system, HA not only failed to activate persulfate, but also competed with indomethacin for reactive oxygen species, which greatly reduced the removal rate of pollutants. The reason for the different effects may be due to the different experimental environments and HA types in the two studies. For example, Fang et al. Used HA rich in quinone groups to carry out organic matter degradation experiments at pH = 7.4, while Li et al. Used HA rich in quinone groups at pH = 4.5, and did not mention the presence of quinone groups in the HA used[38][39]. Therefore, the use of quinone-rich HA as a catalytic component in the organic degradation reaction system may be a feasible strategy to improve the removal efficiency of pollutants in Fenton-like system.
HA plays an important role in the photochemical degradation of coexisting pollutants. On the one hand, HA can act as a photocatalyst to enhance the rate and effect of the photodegradation reaction. On the other hand, HA can also adsorb the target pollutant and promote the contact between it and the photocatalyst, thus promoting the photocatalytic reaction. Therefore, photoFenton disinfection technology has been widely concerned by researchers in water treatment, including solar disinfection of drinking water, photocatalysis, photoFenton and other photochemical advanced oxidation technologies[40,41][42,43][44,45]. For example, Zhang et al. Found that HA improved the photosensitivity of Fe3S4[46]. The kinetic analysis further showed that HA could enhance the degradation efficiency of trichlorophenol by Fe3S4-HA in persulfate system. Carlos et al. Found that humus accelerated the photodegradation of carbamazepine in aqueous solution[47]. According to the mechanism analysis, the main photodegradation pathway is the oxidation process mediated by the triple excited state of HA, but the role of other active species can not be completely excluded. Porras et al. Confirmed that HA could significantly enhance the photoconversion process of ciprofloxacin[48]. It is emphasized that this enhancement is attributed to the ability of HA to activate electrons during photoconversion. When the concentration of HA was more than 5 mg/L, the degradation of ciprofloxacin was inhibited. They believe that high concentrations of HA may block light exposure and reduce the energy intake of the reaction system from the outside. Previous studies also suggested the possibility that high concentrations of HA would hinder the uptake of light energy into the reaction system, but neither study designed relevant experiments or characterization methods to prove this conjecture[49].
HA can bind iron minerals in the environment and indirectly affect the degradation of organic pollutants by promoting or preventing the transfer of electrons from iron to the target pollutants. Niu et al. Prepared HA-coated magnetite nanoparticles by a chemical method for the removal of sulfathiazole from water by Fenton process[50]. Due to the high electron transfer efficiency between Fe (II) -HA or Fe (III) -HA, the rapid regeneration of the active component Fe (II) and hydroxyl radical (· OH) is maintained throughout the reaction process, which greatly improves the removal efficiency of sulfathiazole by magnetite nanoparticles. Although this study speculated that the efficient electron transfer between Fe (Ⅱ) -HA and Fe (Ⅲ) -HA may be the main reason for the enhanced catalytic performance, the conversion relationship between Fe (Ⅲ) and Fe (Ⅱ) and the specific mechanism of Fe-HA coprecipitate-mediated Fenton-like system need to be further confirmed. In addition, persulfate (PS) has been widely used for in-situ chemical oxidation remediation of organic pollutants in groundwater. Li et al. Studied the effect of different kinds of HA on the degradation of dinitrotoluene in groundwater by PS/Fe (Ⅱ)[51]. The results showed that the complexation of HA with Fe (Ⅲ) was beneficial to the degradation of dinitrotoluene. Synchronous fluorescence analysis showed that the synergistic degradation process may be due to the coordination between HA and iron ions, which led to the rapid decomposition of PS. It has been reported that HA may inhibit the degradation of organic pollutants in Fenton-like systems, which may depend on the form of HA in different systems. For example, Hu et al. Found that dissolved HA could compete with sulfamethoxazole (SMX) for · OH in solution, thereby inhibiting the degradation of SMX in a Fenton-like system[49]. Lin et al. Found that the high concentration of humus attached to goethite significantly reduced the formation of · OH, thus inhibiting the degradation of trichlorobiphenyl by goethite[52]. The above two studies show that HA in solution will compete with the target pollutant for the active oxygen species in the reaction system, while HA adsorbed on the solid surface will occupy the active sites on the surface of the attachment and inhibit the occurrence of heterogeneous Fenton reaction.

2.3 HA-microbe coexisting system

Studies have shown that HA affects the toxicity and bioavailability of trace pollutants to aquatic organisms. Kim et al. Found that the addition of HA to the study system significantly reduced the inhibitory effect of pharmaceutical active compounds and endocrine disruptors on the growth of Lepidothrix sp[53]. Ding et al. Also observed that the toxicity of triclosan to diatoms was greatly reduced when HA was present in the environment[54]. HA can also affect chemical transformation behavior in a range of soils. For example, Guo et al. Systematically studied the effect of long-term fertilization on HA[55]. The results showed that HA effectively promoted the iron reduction and nitrate reduction of Shewanella putrefaciens. The electrochemical analysis confirmed that the C = C and C = O bonds as well as the carboxyl and phenolic functional groups in HA may be the important reasons for the enhanced electron transfer ability. Piepenbrock et al. Proposed that HA can be used as an electron mediator for the biochemical behavior of microorganisms and iron minerals, thereby enhancing the reduction rate of microorganisms to iron minerals[56]. In addition, Kraiem et al. First demonstrated that ANAMMOX activity was affected by the presence of HA[57]. HA was used as a carbon source by heterotrophic bacteria, promoting heterotrophic denitrifiers to dominate the reactor. When HA exceeded the threshold concentration of 100 mg/L, it could inhibit the ANAMMOX process and reduce the nitrogen removal rate by 50%.

2.4 Effect of HA from different sources on the removal of coexisting substances

Differences in the precursor species of HA may differentially affect the removal of the coexisting species. The study showed that there were significant differences in the removal efficiency of HA from different sources when it formed complexes with coexisting substances. For example, Du et Al. Compared the interaction of Suwannee River Standard humic acid Standard II (SRHA) and algae-derived Humic Acid (ADHA) with heavy metals La and Al[58]. Both SRHA and ADHA interact with La/Al through complexation and coprecipitation reactions. The results showed that the interaction between SRHA mainly composed of polyphenols and La/Al was stronger than that between ADHA mainly composed of proteins and La/Al. Sohn et al. Extracted six kinds of HA from soil and water sediments to explore the effect of HA from different sources on the adsorption of nano-TiO2[59]. The results showed that HA extracted from soil had the highest adsorption constant for nano TiO2, while HA extracted from water sediment had the lowest adsorption constant for nano TiO2. The experimental data were analyzed by synchronous scanning fluorescence spectrometry, which showed that soil sediments were rich in more highly conjugated HA molecules, so they preferentially adsorbed nano-TiO2. The results emphasize that the effects of HA from different sources on the migration and transformation of nano TiO2 in the environment are quite different.

2.5 Quantitative comparison of the effects of HA and coexisting pollutants

At present, there are many studies on the relationship between HA and coexisting substances, and the nature and structure of HA, the type of pollutants, the concentration and environmental conditions in each research system are different, which leads to the great difference in the removal effect of target pollutants in each reaction system. This makes it difficult to directly compare the synergistic or antagonistic effects of HA on the removal of coexisting pollutants in each study. In order to quantify the effect, 18 representative environmental pollutants were selected, and the enhancement or inhibition factor of HA on the removal of target pollutants in their coexistence system was designed and calculated, and the formula is as follows:[4,17,25,30,32,33,49,50,52,56,60⇓⇓⇓~64]
R=[(1-C*/C0)-(1-C/C0)]/(1-C/C0 )
Where, C is the residual concentration of pollutant reaction without HA in the system; The C* is the residual concentration of pollutant reaction with the participation of HA; C0 is the initial concentration of pollutant; R is the enhancement or inhibition factor of HA on the removal of target pollutants in the system. Fig. 2 shows the R value of pollutant removal in the system with HA participation. It can be seen from Figure 2 that HA in the coexistence system mostly promotes the removal of target pollutants. Among them, the removal effect of Cr (Ⅵ) is the best, which is 6 times higher than that of single Cr (Ⅵ) removal[32]. The R values of other pollutants are mostly between 0 and 1. The outstanding contribution of HA to the removal of Cr (Ⅵ) may be because the redox potential (E0) of Cr (Ⅵ) is the highest among these inorganic metal ions (Table 1), and HA can better play the role of active electron as an electron mediator and donor in the reaction system to promote the reductive removal of Cr (VI), which is feasible in thermodynamic analysis[65]. In addition, electrostatic repulsion, blocking of adsorption sites and competition of active species often occur in the coexistence system with antagonistic effect, which interferes with the selectivity of environmental treatment technology for the removal of target pollutants[4,17,62,64]. Therefore, the actual treatment of water pollution must take into account the mechanism of HA and coexisting pollutants, in order to use the characteristics of HA to remove the coexisting pollutants in environmental water.
图2 HA参与共存体系下污染物的去除效果

Fig.2 The removal synergistic or antagonistic effect of contaminants with HA in co-existing system

表1 重金属氧化还原电位

Table 1 Standard redox potential of heavy metal ions

Cr
(Ⅵ)		 Fe
(Ⅲ)		 Pb
(Ⅱ)		 Cd
(Ⅱ)		 Mg
(Ⅱ)		 Na
(Ⅰ)		 Ca
(Ⅱ)		 K
(Ⅰ)
E0 (Ⅴ) 1.33 0.77 -0.13 -0.40 -2.37 -2.71 -2.87 -2.93

3 Interaction mechanism between HA and coexisting pollutants

3.1 Mechanism of action between HA and inorganic coexisting pollutants

HA can form organic complexes with coexisting pollutants through various types of functional groups (carboxyl, hydroxyl, carbonyl, methoxy, etc.) in the molecule, and can also act as an electron mediator to promote the removal of high-valence pollutants[46,61,66]. Generally speaking, the interactions between HA and coexisting inorganic pollutants include coordination, electrostatic interaction, adsorption, redox, ion exchange, etc[25]. However, the reaction mechanism dominated by HA is quite different in different environmental systems. Therefore, it is necessary to analyze the interaction of HA in the coexistence system. In this paper, several typical interaction mechanisms are listed, including adsorption, coordination, coordination-electrostatic interaction, and coordination-redox interaction.

3.1.1 Adsorption

Adsorption of HA on natural minerals is an important geochemical process affecting their chemical composition and properties. However, inorganic salt ions in natural water, such as phosphate, can compete with HA containing aromatic hydrocarbons, chromophores and acidic groups for binding sites on minerals, thus inhibiting the adsorption process of HA on minerals. Similarly, the presence of HA also inhibits the adsorption of phosphate ions by minerals, and the degree of inhibition depends on the chemical composition of HA[58]. Li et al. Used two-dimensional spectral analysis, potentiometric titration and model calculation to systematically study the effect of phosphate on humus adsorption at the ferrihydrite interface[67]. The results showed that phosphate increased the concentration of total organic carbon, aromatic components, chromophores and acidic groups, indicating that phosphate could preferentially compete for carboxyl and phenolic groups on the surface of ferrihydrite, thus inhibiting the adsorption and mass transfer of HA on the surface of ferrihydrite. It can be seen that the interaction between environmental organic matter and inorganic anions greatly affects the environmental ecological changes. In addition, the interaction between HA and phosphorus is also closely related to the trophic status of the lake. For example, Wen et al. Found that phosphate can replace HA adsorbed in lake sediments, and HA containing polysaccharides and aromatics is more easily replaced by phosphate than HA containing carboxyl groups[68].

3.1.2 Coordination

The coordination between HA and heavy metal ions changes the existing forms of heavy metals in the environment and reduces the mobility of heavy metals[69,70]. Du et al. Used Artificial humic acid (A-HA) coated porous magnetic biochar to remove Pb (Ⅱ) in their study[60]. The A-HA on the surface of the composite is easy to form coordination bonds with Pb (Ⅱ), which enhances the removal performance of the composite for Pb (Ⅱ). In the same year, they used A-HA modified zero-valent iron materials. As shown in Fig. 3, A-HA is combined with zero-valent iron through C-O-Fe coordination bond, which makes the material have good dispersion and oxidation resistance as a whole. The abundant functional groups on the surface of A-HA can also adsorb, coordinate and precipitate with Pb (Ⅱ), which together affect the removal effect of Pb (Ⅱ)[70].
图3 人工腐殖酸稳定高分散零价铁去除铅离子的机制[70]

Fig.3 The possible mechanism of high-dispersion zero-valent iron particles stabilized by artificial humic acid for lead ion removal[70]. Copyright 2019, Elsevier

However, it was not mentioned in that study whether the C-O-Fe coordination bond stability formed in the material would be changed by environmental factors. This was confirmed in the study of Li et al., which pointed out that the coordination ability of HA with Fe under acidic and neutral conditions was higher than that under alkaline conditions[36]. In addition, it has been reported that the aromatic affinity site of HA plays an important role in the organic coordination process of metal ions. For example, the content of carboxylic acid and phenol components of aromatic structure in HA is positively correlated with the coordination ability of Cu (II) and Zn (II)[71].
Cd is a common heavy metal pollutant in environmental water, and recent studies have confirmed that HA plays an important role in the removal of Cd (Ⅱ)[55,72]. Qu et al. Studied the binding mechanism of Cd with iron oxide-HA composite at the molecular scale[73]. The results show that HA mainly promotes the clustering of iron oxides through ligand exchange reaction, which facilitates the precipitation of bidentate surface ligands (>(FeO)2Cd). Unlike the cation Cd (Ⅱ), As often exists in the form of anionic salts. The formation of As-HA complex has an important effect on the toxicity, migration and bioavailability of As[74]. Fakour and Lin were the first to describe the effect of HA concentration on the applicability of the As ligand binding model[75]. The experimental results show that the strong binding site of HA to As may come from the N-containing functional group with strong coordination ability of HA, while the carboxyl and phenolic hydroxyl groups are relatively weak binding sites. They emphasized that the affinity of HA for arsenate is higher than that for arsenous acid. However, negatively charged HA may also compete with As for adsorption sites and electrostatic repulsion, thus affecting the adsorption stability of As in the environment[34,76]. In contrast, the carboxyl and hydroxyl groups of HA can combine with iron in the water environment to form Fe-HA complexes, and then Fe acts As a cationic bridge to promote the combination of As and HA, thus forming a stable As-Fe-HA ternary complex, which greatly reduces the risk of As leaching from the solid phase to the environmental water[77,78]. The difference of physical and chemical properties of Cd and As means that it is difficult to remove them at the same time. In order to solve this problem, Guo et al. Synthesized a new ternary material HA/Fe-Mn oxide supported biochar[55]. The maximum adsorption capacity of the composite for Cd (Ⅱ) and As (Ⅴ) was 67. 11 mg/G and 35. 59 mg/G, respectively, which was significantly higher than that of the original biochar (11. 06 mg/G and 0 mg/G, respectively). Fourier transform infrared spectroscopy showed that the carboxyl group disappeared after the reaction, and the Cd-O/As-O bond was formed accordingly. The results of X-ray photoelectron spectroscopy (XPS) showed that the surface of the composite had more oxygen-containing functional groups, which meant that there were more active sites on the surface, which was an important reason for the high adsorption performance of the composite for Cd (Ⅱ) and As (Ⅴ).
It has been reported that HA can form stable uncharged organometallic colloids through complexation. The migration of nuclides is attributed to the formation of such organic colloids[79]. Zhao et al. Proposed that Cs (Ⅰ), Sr (Ⅱ) and Eu (Ⅲ) mainly form complexes with HA, a small molecule rich in phenolic hydroxyl groups[80]. In another study, Zhao et al. Found that the coordination ability of monovalent cation nuclides such as Cs (Ⅰ) with HA is weak and the adsorption is not significant, while the coordination ability of high-valence nuclides such as Eu (Ⅲ) with HA is much stronger than that of Cs (Ⅰ)[66]. There are few studies on the interaction between rare earth metal elements and HA[81,82]. Existing studies have shown that HA can act as an effective ligand for rare earth metal elements and control the speciation of rare earth metal elements in natural waters[83]. The main binding sites of HA with rare earth metal elements are carboxyl and phenolic hydroxyl groups, while a small number of soft Lewis base groups (such as N and S-containing groups) also contribute to the coordination of HA with rare earth metal elements[3,84,85]. Wang et al. Proposed that increasing the concentration of HA in the solution can increase the bond energy of the coordination bond between the rare earth metal element and HA[3]. This discovery has greatly deepened the understanding of the fate of environmental migration of rare earth metals.

3.1.3 Coordination-electrostatic interaction

Based on the traditional theory, it is found that coordination and electrostatic interaction play an important role in the coagulation process[86]. Song et al. Systematically studied the effect of in-situ hydrolysis of inorganic aluminum salt on HA removal by coagulation[87]. The results show that inorganic aluminum salts can be hydrolyzed into a variety of Al species with molecular nuclei from Al1 to Al20. The electrostatic repulsion between the special Al13 and HA is stronger, and the removal rate of HA is lower. In addition, Al13 will gradually transform into an octahedral space structure during coagulation, and its coordination ability with HA will decrease, which is not conducive to coagulation treatment. Chen et al. Found that the C = O bond (carboxylic acid, amide, quinone or ketone) and C — O bond (phenol, aliphatic C — OH and polysaccharide) of HA play an important role in the adsorption coordination process with titanium dioxide particles[88]. The adsorption reaction is also largely influenced by the electrostatic interaction on the particle surface.

3.1.4 Coordination-redox

Ag nanoparticles (AgNPs) released into the aquatic environment are easily decomposed into silver ions, which can disrupt normal biological functions at the cellular level and thus disrupt the ecosystem[89,90]. Because the toxicity of Ag (Ⅰ) is greater than that of metal Ag, the reduction reaction is expected to reduce the environmental risk of Ag (Ⅰ). Lu et al. Reported that the conversion of Ag (Ⅰ) to AgNPs under light irradiation can be achieved through organic ligand-metal charge transfer and photogenerated superoxide reduction[91]. The same finding was found in the study of Ag (Ⅰ) in the dark. For example, Dong et al. Suggested that the carboxyl group played an important role in the reduction of Ag (Ⅰ)[92]. As shown in Fig. 4, Ag (Ⅰ) first binds to the carboxyl group in HA to form Ag-OOC-ligand, which is subsequently reduced to AgNPs. Finally, the Ag-OOC-ligand or the reduction product of AgNPs can be gradually converted into stable Ag-S by the thiol group in HA. The reduction of Cr (Ⅵ), which is highly carcinogenic, also helps to reduce the environmental toxicity of Cr (Ⅵ). It has been reported that the hydroxyl group in HA can be used as the main electron donor for the reduction of Cr (Ⅵ), while the carboxyl and carbonyl groups are mainly involved in the coordination reaction of Cr (Ⅵ), but the specific reason is not clear[93]. Therefore, Wang et al. Synthesized ferrihydrite composites with different HA loadings to explore the reaction mechanism of HA in the process of adsorption and reduction of Cr (Ⅵ)[32]. The results of this study show that low molecular weight HA has a significant effect on the removal of Cr (Ⅵ). Glycerol was selected as the model substance of HA, and the electron cloud density around the glycerol atom was calculated before and after the reaction with Cr (Ⅵ), and it was found that the electron cloud density around α-C was the largest. This phenomenon indicates that the hydroxyl groups of primary and secondary alcohols in HA molecule have strong electron-withdrawing ability and show strong reduction to Cr (Ⅵ).
图4 黑暗条件下腐殖酸介导银离子向银纳米颗粒转化的机制[92]

Fig.4 The possible mechanism of transformation of silver ions to silver nanoparticles mediated by humic acid under dark conditions[92]. Copyright 2019, Elsevier

3.2 Mechanism of action between HA and organic coexisting pollutants

Molecular mechanics and dynamics simulations show that HA has a clear reticular spatial structure[94]. These network structures and their surface functional groups can net and adsorb a large number of organic pollutants in the water environment. Therefore, HA is the main carrier affecting the migration of organic pollutants in the environment. Generally speaking, HA can bind organic pollutants through non-specific interactions, such as electrostatic interaction, hydrogen bonding, hydrophobic interaction, π-π interaction and so on. For example, Xie et al. Analyzed the interaction mechanism of HA with different kinds of functional groups[1]. Among them, the interactions between HA and —NH2, —CH3, — OH are mainly electrostatic, hydrophobic and hydrogen bonding. It has also been found that there is an obvious electrostatic attraction between organic cations represented by toluidine blue dye and HA molecules[95]. In addition, HA itself can act as an electron mediator or electron donor to excite oxygen reactive species such as singlet oxygen (1O2), · OH, and HA triplet excited state to promote the degradation of target pollutants. It will also compete with organic pollutants for active oxygen species and inhibit the degradation of pollutants in the system[96]. This section summarizes the mechanism of HA in the coexistence system of HA with microplastics, antibiotics, other organic pollutants and microorganisms.

3.2.1 Coexistence Mechanism of HA and Microplastics

The occurrence of microplastics (MPs) as emerging pollutants in the environment can cause changes in water quality and sediment characteristics, thus affecting the geochemical cycle process. Therefore, a deeper understanding of the interaction between HA and MPs is essential to assess the environmental impact of MPs on the ecosystem[97,98]. Abdurahman et al. Focused on the adsorption process of HA on polystyrene microplastics[97]. It is proposed that HA first occupies the high-energy adsorption sites on polystyrene microplastics through hydrophobic interaction and π-π conjugation, and then diffuses to the low-energy adsorption sites. Chen et al. Combined spectral methods with chemometric analysis to reveal the molecular level mechanism of the interaction between HA and MPs[99]. The results show that MPs first interact with the aromatic structure of HA through π-π conjugation, and then are encapsulated in the organic polymer by carboxyl and carbonyl groups to form a highly conjugated copolymer with high electron density. Zhu et al. And Li et al. Emphasized that the adsorption of HA on MPs may change the potential of MPs to carry POPs in the environment[100][67]. In their study, they found that HA would compete with organic pollutants for the adsorption sites on MPs, thus significantly reducing the adsorption capacity of MPs for organic pollutants. Microplastic fibres (MPFs) are one of the most common types of microplastics. Pan et al. Systematically studied the adsorption behavior and mechanism of Thiacloprid (THIA) on HA at the MPFs/H2O interface[101]. As shown in Fig. 5, low concentration of HA mainly affected the adsorption behavior of THIA at the MPFs/H2O interface through competitive adsorption, while high concentration of HA affected the adsorption reaction process through the bridging effect. In the competitive adsorption process, the phenolic hydroxyl and carboxyl groups in HA molecules weaken the binding of THIA to MPFs through intermolecular hydrogen bonds and van der Waals forces. During the adsorption reaction dominated by the bridging effect, phenolic hydroxyl and carboxyl groups provide bridging sites for MPFs and THIA, thus forming the "MPFs-HA-THIA" complex.
图5 HA影响MPFs对THIA吸附的机理示意图[101]

Fig.5 Schematic diagram of the possible mechanism of HA affecting the adsorption of THIA on MPFs[101]. Copyright 2022, Elsevier

3.2.2 Mechanism of HA coexisting with antibiotics

In recent years, the concentration and fate of migration and transformation of residual antibiotics in the global environment have become a hot issue of public concern[96]. Antibiotics are a class of compounds with specific chemical structures and biological activities that usually kill or inhibit the growth and reproduction of bacteria[102,103]. The use of antibiotics can lead to the development of bacterial resistance, which is the result of natural selection. Resistant bacteria can survive and multiply under the pressure of antibiotics, thus forming new pathogenic strains and posing a greater challenge to treatment. Therefore, it is very important to explore the interaction between HA and antibiotics in environmental water. However, the environmental coexistence system is complex, and the different reaction conditions and target pollutants may lead to great differences in experimental results. Antilen et al. And Urdiales et al. Demonstrated that HA macromolecules can effectively adsorb residual ciprofloxacin antibiotics in the environment[102][103]. Zhang et al. Demonstrated that π-π conjugation between HA and organic aromatic structures enhanced the removal of oxytetracycline from solution[104]. However, Li et al. Found that HA competed with sulfamethoxazole (SMX) for the · OH produced in the degradation system, resulting in a decrease in the degradation efficiency of SMX[4]. Yu et al. Prepared coprecipitates of goethite and HA with different C: Fe molar ratios by biological method, and first investigated their activation effect on H2O2 and their ability to catalyze the degradation of sulfanilamide[61]. As shown in fig. 6, HA can act as an electron transfer mediator to promote efficient conversion between Fe (III)/Fe (II). The coprecipitate of goethite and HA has higher carbon-based stability and lower microporous specific surface area, which reduces the competitive effect of HA on hydroxyl and improves the degradation efficiency of sulfonamide. In addition, Niu et al. Found that natural dissolved organic matter showed dual effects in the photodegradation of norfloxacin, which could not only inhibit the redox reaction mediated by 1O2, but also enhance the defluorination pathway[105]. Zheng et al. Focused on the interaction between HA and tetracycline (TC) during "aging"[106]. In this study, HA was coated with CaCO3 precipitates to simulate the aging process of HA in the natural environment. The adsorption kinetics and isotherm results show that HA without modification with CaCO3 has a faster uptake rate and a larger adsorption capacity for TC, which means that aging affects the mass transfer process of TC from the solution to the outer surface of HA and from the outer surface of HA to its internal pores. The results of mechanism analysis showed that the five functional groups carried by HA promoted the adsorption of TC, and the binding affinity of HA for TC was in the order of — COO > N — H > — C — C > C — O > O — H. Among them, — COO is the most sensitive adsorption site. However, the — COO content of aged HA decreased significantly, which was the main reason for the decrease of TC adsorption capacity of aged HA. Yang et al also confirmed that the carboxyl functional group of HA is preferentially combined with TC, which is beneficial to the removal of TC[96].
图6 Gt-HA促进SA类芬顿降解的机制[61]

Fig.6 Potential mechanism of Gt-HA promoted Fenton-like degradation of SA[61]. Copyright 2020, Elsevier

3.2.3 Mechanism of HA coexisting with other organics

The interaction between humus and lysozyme, tyrosinase and bovine serum albumin has been reported in many literatures, and the importance of hydrophobic and electrostatic interaction between humus and macromolecular organic matter has been pointed out[107⇓⇓⇓~111]. Inhibition of tyrosinase activity can control fruit browning and maintain the flavor, color and nutritional value of fruit. Taherkhani et al. Found that HA did not significantly destroy the secondary structure of tyrosinase, but had a greater impact on enzyme activity[112]. HA can bind to tyrosinase through hydrophobic interaction and significantly inhibit tyrosinase activity. In addition, the traditional adsorption theory suggests that the hydrophobic interaction between HA and hydrophobic organic compounds (HOCs) is the main mechanism for the removal of HOCs[113]. The adsorption of HOCs by HA followed a linear relationship, and the adsorption capacity was related to the solubility of HOCs and the content of HA[90,114]. Jin et al. Also found that the adsorption affinity of HA to HOCs mainly depends on the concentration, polarity and aromaticity of HA[3,115]. In addition, the effect of HA on the removal of organic pollutants by zero-valent iron system has attracted more and more attention. Tsang et al. Found that the inner sphere coordination and hydrophobic interaction of HA oxygen-containing functional groups contributed to the formation of HA macromolecular polymers, and the newly formed hydrophobic region enhanced the removal of trichloroethylene by zero-valent iron[21].

3.3 Mechanism of HA coexisting with microorganisms

The presence of HA in the environmental system can cause changes in the microbial community. Porras et al. Used HA extracted from coal cinder to enhance the solar disinfection process[116]. The results showed that the degree of bacterial inactivation during solar disinfection was negatively correlated with the amount of metals carried in HA. However, when H2O2 is added to the system, the disinfection effect is greatly enhanced and the inactivation time of the disinfection process is reduced by inducing a photo-Fenton chemical process to generate a Fenton reaction and a copper-based Fenton-like reaction-assisted inactivation pathway. Ryu et al. Studied the function of microbial community and understood the activity of microbial degradation of HA in membrane reactor with HA as substrate[117]. Among them, the dominant bacteria changed from Actinobacteria to Proteobacteria, showing a significant increase in the number of β-Proteobacteria, γ-Proteobacteria and δ-proteobacteria, indicating that HA provided abundant nutrients for these bacteria and promoted their reproduction and growth in the reactor. It can be seen that the biological treatment process involving HA is more complex, and only by clarifying the interaction between HA and microorganisms, can the environmental effects brought by HA-microorganism coexistence system be better utilized or avoided.

3.4 Characteristics of interaction mechanism of HA in coexisting system

Due to the complexity of environmental pollution, the interaction mechanism dominated by HA in different coexistence systems is different. Generally speaking, HA is easy to interact with charged pollutants in the coexistence system because of its negative charge. Therefore, electrostatic interactions are ubiquitous in both inorganic and organic coexisting systems. In these two coexistence systems, the interaction mechanism between HA and coexisting substances is quite different due to the different physical and chemical properties of the target pollutants. Simply speaking, in the inorganic coexistence system, HA often coordinates with pollutants, which affects the existence form and environmental migration fate of inorganic pollutants in the environment. In the organic coexistence system, the unique network space structure of HA can adsorb macromolecular organic pollutants, and this mechanism plays an important role in the removal of target pollutants in the organic coexistence system. It is worth noting that there are also many unique interaction mechanisms in organic coexistence systems. For example, the aromatic ring or unsaturated bond in HA molecule forms π-π conjugation with organic pollutants, which strengthens the interaction between HA and coexisting organic pollutants. Moreover, the hydrogen elements with strong electronegativity such as N, O and F between organic molecules form hydrogen bonds with electron-rich groups, which also increases the attraction between HA and coexisting organic pollutants. The hydrophobic interaction caused by the involution and bending of the hydrophobic group significantly improves the removal of hydrophobic organic pollutants. These interactions mean that HA is more active and the reaction mechanism is more complex in the organic coexistence system. At the same time, the effect of HA on the function of microbial community in the environment can not be ignored.

4 Conclusion and prospect

HA plays an important role in the process of environmental water treatment because of its huge natural reserves, complex molecular structure and active physical and chemical properties. With the further understanding of the complex structure of HA, it is particularly important to further clarify the interaction between HA and coexisting pollutants, and to clarify the interaction mechanism between HA and pollutants in the coexisting system. On this basis, the key points of future research are as follows:
(1) There are many kinds of HA with complex structure. In order to scientifically and effectively evaluate the role of HA in environmental governance, the broad-spectrum studies in different systems should be compared with each other, and the quantitative analysis and research work should be carried out by mass spectrometry, fluorescence spectrometry, infrared spectrometry, nuclear magnetic resonance and ultraviolet spectrometry.
(2) HA and coexisting pollutants have different interaction mechanisms in different coexistence systems. Follow-up studies need to further develop the corresponding micro-characterization technology to deeply understand the interaction mechanism between HA and the surface of environmental pollutants at the nanoscale.
(3) In the study of HA coexistence system, there are few reports on the interaction between various pollutants, so future research should continue to explore the interaction between HA and various complex pollutant systems, focusing on the use of HA's unique properties to remove persistent and difficult pollutants in the environment.
(4) Most of the existing studies focus on the removal effect of specific adsorbents on target pollutants in HA coexistence system. The universality of mechanism research should be promoted, and the removal of coexisting pollutants should be considered from the perspective of self-interaction of pollutants.
(5) HA and protein have many similarities in structure, function and biological significance, so we can refer to the research work of protein to promote the research of HA. Methods such as mass spectrometry, nuclear magnetic resonance, and X-ray crystallography in protein research have been shown to be applicable to the analysis of HA. At the same time, the knowledge of protein interaction with other biomolecules in protein research can also be applied to the study of HA. In addition, theoretical computational methods such as molecular dynamics simulations and quantum mechanical calculations can also be used to study the structure and properties of HA. These research results can help us better understand the coexistence of HA and biomolecules, thus providing more possibilities for its application in biomedical and pharmaceutical fields.

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