Humic Substances' Macromolecular Architecture and Dyes/Metals Adsorptive Removal

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This entry is adapted from the peer-reviewed paper 10.3390/agronomy13122926

Humic substances are naturally occurring materials composed of complex biogenic mixtures of substituted aromatic and aliphatic hydrocarbon core materials derived from the degradation and decomposition of dead plant and animal matter. They are ubiquitous in both terrestrial and aquatic systems constituting biotic pools and are characterized by unique properties; they are amphiphilic redox compounds with exceptional chelating features. Humic substances play a crucial role in both agriculture and the environment as carbon sequestrators, soil improvers, plant health promoters, as well as stabilizers of soil aggregates and regulators of organic/inorganic nutrients bioavailability.

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1. Introduction

Humic substances (HS), a decomposition product of plant and animal tissues in the environment, are classified as a unique chemical class of organic compounds ^[1]. According to the International Humic Substances Society (IHSS), HS are complex and heterogeneous mixtures of polydispersed materials formed in soils, sediments, and natural waters by biochemical and chemical reactions during the decay and transformation of plant and microbial remains; humic substances are major components of natural organic matter (NOM) and can be divided into three fractions. Humic acids (HA), insoluble in acidic environments (pH < 2) but soluble at higher pH values, and fulvic acids (FA)—soluble in water at all pH values—are their main fractions, and humin is the fraction that is water-insoluble at all pHs (Figure 1), solubility reflecting the traditional alkali-extraction separating methods from the original material. Several chemically reactive groups (e.g., carboxyl and hydroxyl) are present. Humic substances exhibit a wide range of molecular weights and sizes ^[1], from a few hundred Da to several hundred kDa; FA are of lower molecular mass than HA, and soil-derived molecules are larger than those from aquatic NOM, also composed of mixtures containing humic substances ^[1]^[2]. Macromolecular-structure models of HS have been developed based on size exclusion chromatography (SEC) ^[3]^[4].

Figure 1. Traditional scheme of natural organic matter components separation.

Small and heterogeneous humic molecules ^[5] are self-assembled in supramolecular conformations, bound by weak van der Waals, π–π, CH–π, and hydrogen forces. However, as HS tend to aggregate in aqueous systems through physical and chemical crosslinking ^[6], different intra- and intermolecular interactions are observed; also, HS are negatively charged, accumulate at interfaces, and contain both hydrophobic and hydrophilic functional groups attached to their aliphatic–aromatic backbones ^[7], forming micelles that solubilize otherwise insoluble organic compounds ^[8]. As a result, due to their molecular complexity, it seems difficult to identify HS molecules and structures ^[6] by size exclusion chromatography, light scattering (LS), or viscosity measurements that commonly provide precise information on polymers.

Although polymer concepts such as the glass transition temperature (T_g) have been applied to support the presence of both glassy and viscoelastic domains within the HS ^[9], the available evidence does not support the formation of large-molecular-size and persistent HS in soils. Instead, soil organic matter (SOM) seems to be a continuum of progressively decomposing organic compounds ^[10]. These initial degradation compounds and microbially-produced small molecules undergo condensation reactions and hydrolytic or oxidative processes to generate humic substances (i.e., refractory materials that resist breakdown by the organisms themselves), the formation of which may involve chemical alterations of biomolecules, e.g., lignin, occurring randomly and leading to an increased system heterogeneity ^[11]. Subsequently, experiments relying on the alkali and acid extraction and functional-group chemistry to describe the prevalence of humic and fulvic acids and humin in the soil have led to the conclusion that HS, comprising complex and bulk macromolecules, are the largest and most stable SOM fraction. However, these components represent only a small fraction of total organic matter (TOM); direct observations verify the existence of smaller, simpler molecules ^[12], and the reported molecular masses are in the region of low hundred and low thousand Da. Thus, HS do not appear to be true polymers but molecular species with a tendency to congregate, supermixtures, and supramolecules. Ultrafiltration has been used to fractionate HS into ranges that refer to congregations rather than to large covalently linked molecules ^[13].

2. Structure and Characterization of Humic Substances

2.1. Macromolecular Nature and Chemical Retrosynthesis

Natural HA are functional polymers with uniform, independent of their source and origin, unusual for a polymer structural properties (e.g., pH and redox buffering effectiveness, strong binding of water and ions, surface activity, interaction with minerals, hydrophobic and hydrophilic species carrying, as well as improvement of soil performance and plant growth by enhancing microbial and fungal activity) ^[14]. Structural and functional characteristics of different HA matrices are crucial for the reactivity with heavy metals and organic contaminants, e.g., dyes, to be determined ^[15]. The size of HA molecules depends on the number of solvent-accessible or surface functional groups, altering the HA chemistry by affecting the density of reactive sites.

Thermal investigation of the NOM molecular structure provides evidence of glass transition phenomena in peat HA and stream FA identified with temperature modulated differential scanning calorimetry (TMDSC) and thermal mechanical analysis (TMA). The T_g of both soil- and stream-derived humic substances suggests a macromolecular structure for HA and FA in NOM. A more aromatic humic acid shows a higher T_g than a river fulvic acid. Thermal analysis techniques may determine thermodynamic parameters related to molecular-scale interactions between NOM and contaminants ^[16]. As already stated, HS may not be regarded as polymers, but rather as supramolecular associations of relatively small heterogeneous molecules. Such supramolecules (either aggregates or self-assemblies) are clearly sizable; in any case, HA fractions that exhibit the highest molar masses (in the region of 15–25 kDa ^[17] obtained from HPSEC) are expected to be of macromolecular construction. As water molecules tend to exclude from humic molecular aggregations, the latter are held together and stabilized by dispersive—mainly hydrophobic—binding forces ^[17].

Synthetic humic acids (SHA), made by hydrothermal polycondensation ^[14] or in the presence of peroxidases and phenoloxidases ^[18]^[19]^[20], are also useful for technical applications. Natural HA and SHA have been examined using fluorescence spectroscopy (FS) and atomic force microscopy (AFM) ^[5]. MnO₂ and O₂ catalysts promote the polyphenol–amino acid polymerization, and accelerate both the transformation of FA to HA and the evolution of HS ^[21]. Consistent with the view of 3-D macromolecular HS matrix, the Flory–Huggins theory has been used as the preferred thermodynamic framework for describing hydrophobic organic compounds (HOC) binding to dissolved HS ^[22]. Mimicking natural humification, fast and efficient chemical processes produce SHA that optimize desirable properties (e.g., amphiphilicity and redox potential); cheap and sustainable artificial humic polymers made from biomass waste, having a negative carbon footprint, i.e., lowering atmospheric carbon dioxide, can be exploited in industry and agriculture. Esterification and copolymerization with flexible monomers may improve the plasticity properties or optimize the advantages of these specialty polymers in environmental remediation ^[14].

2.2. Characterization Techniques, Supramolecules, and Molecular Weights

Many analytical methods of qualitative and quantitative HS characterization have been applied; analysis of HS has been focused on standardization (e.g., elemental compositions, or ash and water contents of IHSS standard humic and fulvic acids). Physicochemical and elemental analysis, titration, UV–vis and fluorescence spectrophotometries, NMR spectroscopy, mass spectrometry, fractionation methods (i.e., gel permeation chromatography and flow field flow fractionation), as well as degradation methods (e.g., oxidation, pyrolysis, and hydrolysis) have been employed.

In HPSEC systems, a variable wavelength UV–vis detector is used to detect the chromophoric composition of HS, which contain chromophores with unique molar absorptivities for a given wavelength; thus, molecular weights and their distributions ^[23] may depend on the wavelength chosen. Molecular weights of selected HA and FA increase with increasing detector wavelength, and, as the number average molecular weight (M_n) is more sensitive to wavelength changes than the weight average molecular weight (M_w), HS appear less polydisperse at higher wavelengths ^[23]. Lower light absorption due to a change in solution compositions is attributed to HS disaggregation and is confirmed by UV–vis spectrophotometry at wavelengths from 250 to 450 nm ^[17].

Nuclear magnetic resonance spectroscopy, pyrolysis studies, X-ray absorption near-edge structure spectroscopy, and electrospray ionization mass spectrometry show that soil HS are dynamic associations organized into micellar structures in aqueous media. Molecules associated with HS (e.g., biomolecules bound in humic fractions) are, therefore, humic components, as they cannot be separated by physicochemical techniques ^[24]. These constituents are linked together via intermolecular interactions to form supramolecules, segregate on the scale of nm, and display a unique molecular motion ^[24]. Readily aggregated at low pHs ^[25] and dispersed (as functional groups dissociate) at higher pHs, HA and FA—behaving like linear, flexible polyelectrolytes—are associations of phenolic and benzene-carboxylic molecules (i.e., building blocks that originate from microbes, polyphenols, lignin, and condensed lignin) held together by weak linkages ^[25].

Size exclusion chromatography has been used to characterize HA in natural waters, domestic and treated wastewaters, effluent from night soils, landfill leachates, pig slurry, sediments, soils, and oxidized coal ^[26]. Molecular weights determined by HPSEC are consistent with those from vapor pressure osmometry (VPO) and field flow fractionation (FFF). Analyses indicated that aquatic HS are smaller with a lower polydispersity than believed in the past. Both spectroscopy and molecular weight measurements provide information on the bulk properties of aquatic humics from different sources ^[27]^[28]. A comparison of ultrafiltration fractionation (UF) and HPSEC has revealed that neither technique leads to absolute molecular weight values ^[29].

Physical parameters of HA fractions—isolated by basic (HAb) and pyrophosphate (HAp) extractions from several types of peat—have been analyzed with UV–vis, fluorescent, infrared (IR), and electron paramagnetic resonance (EPR) spectroscopies. The average molecular weights of fractions range from 17.2 to 39.7 kDa, and their humification index (HIX) varies from 0.49 to 1.21. The HAp fractions show higher contents of both aromatic structures and phenolic OH groups compared to the HAb fractions.

2.3. Structural and Compositional Architecture of Size Fractions

UV–vis and fluorescence spectroscopic indices, e.g., A₄₆₅/A₆₆₅ (E₄/E₆) ^[30] and A₂₅₀/A₃₆₅ (E₂/E₃) ratios, have been correlated with basic HS properties such as molecular weight, acidity, and aromaticity ^[2]. In samples of HA—derived from original coal oxidized by H₂O₂—the A₄₄₆-to-A₆₆₅ ratio, used to estimate the molecular weight and aromaticity of HA, has been determined by UV–vis spectrophotometry at 446 nm and 665 nm. ¹³C nuclear magnetic resonance (¹³C NMR) spectroscopy clarified the detailed characteristics of the HA carbon structure, and Fourier transform infrared (FTIR) spectra showed both the existence and the types of HA bonds ^[31]. Infrared (IR) analysis reveals the active groups and, also, the macromolecular structure of HS. The spectra of FA (typically containing larger amounts of carboxyl groups) differ from those of HA, which contain more aromatic rings ^[32]. Quantification of coal-derived humic components from mixtures in aqueous solutions has been carried out via ATR FTIR spectroscopy by exploiting the carboxylate bands at 1570 and 1383 cm⁻¹ ^[33]. Peak intensities of diffuse reflectance infrared Fourier transform (DRIFT) spectra are not useful to calculate HS concentrations.

Aquatic HS contain both aromatic and aliphatic components, as well as polynuclear aromatic and fused-ring structures. The major aliphatic segments are composed of two to four short saturated chains, i.e., methylene units, and the aromatic rings bear three to six alkyl substituents; branched structures are also present ^[34]. Structural and functional properties of sodium humate (SH) and commercial lignohumate (LH) have been studied by UV–vis, FTIR, steady-state fluorescence, and ¹³C NMR spectroscopies. Samples of SH isolated from brown soil, compost, or lignite are poly-condensed, humified, unsaturated, oxidized, and aromatic while LH samples are characterized by low levels of conjugated chromophores and fluorophores, are less condensed, less humified, and contain simple heterogeneous structural components of low molecular weight and size ^[35].

NMR is a powerful technique useful in the characterization of heterogeneous matrices such as HS, mainly through its wide range of applications in liquid, semisolid, and solid states. NMR is primarily carried out in the solid state via cross-polarization ¹³C MAS NMR spectroscopy; on this basis, a variety of carbons in similar chemical environments are determined altogether ^[36] at the corresponding aliphatic, aromatic, phenolic, carboxylic, and quinone regions ^[37]^[38]. The composition of HA has been examined using solid-state ¹³C NMR. Direct-polarization magic-angle spinning (DPMAS) ¹³C NMR spectra peak areas, corrected by cross-polarization spin-lattice relaxation time experiments with total sideband suppression (CP/T₁–TOSS), have been used to obtain quantitative intensities ^[39]; the HA investigated consisted of more than 60% aromatic, carbonyl, and carboxyl carbons.

UV–vis and FTIR spectroscopies, as well as elemental analysis ^[40], have been applied to characterize the HA and FA fractions of HS. The spectra showed humification, the generation of negative charges related to the cation exchange capacity (CEC), and the complexation with metal ions. Degree of humification depends on the C/N and E₄/E₆ ratios ^[41]. FTIR, UV–vis, and fluorescence spectroscopies indicate qualitative differences between resin- and alkali-extracted humic acid fractions isolated from bog soil. Resin-extracted HA fractions contain lesser amounts of aliphatics, carbohydrates, aromatics, and amides; are rich in carboxyl and phenolic groups; have higher A₂₅₄/A₄₃₆ and E₄/E₆ ratios; and exhibit a higher fluorescence intensity and longer wavelengths of emission maxima ^[42]. Potentiometric titration has been used to study the acid–base properties of size-fractionated HA. The acidic group contents increase as the molecular mass decreases and are related to the HA aromaticity ^[43]. In HA divided by ultrafiltration into fractions of 300, 100–300, 50–100, 10–50, and 1–10 kDa, and characterized by IR and ¹³C CPMAS NMR spectroscopies, the molecules of ≥100 kDa fraction are aliphatic while those of the ≥10 kDa fraction are aromatic, and an increase in carboxyl groups is observed as the molecular size decreases ^[44].

The HA size fractions differ in their chemistry and composition; smaller fractions contain more chargeable functional groups and a higher amount of aromatic carbon, while aliphatic carbon increases with the molecular weight. Furthermore, the smallest HA fractions and FA fractions of similar molecular weights and carboxyl carbon contents have different chemical compositions; i.e., HA contain more aromatic and less aliphatic carbon ^[45].

Reverse osmosis (RO) combined with XAD-8/XAD-4 resin adsorption may be used for the identification of water NOM constituents, e.g., the humic material, which can be fractionated on the basis of its hydrophobic and hydrophilic properties. Results have shown that hydrophobicity is associated with a higher molecular mass and increased aromatic contents, while the hydrophilic character corresponds to higher amounts of nitrogen and oxygenated functional groups ^[46].

3. Interaction of Humic Substances with Soils and Pollutants

3.1. Soil Improvers, Chemical Regulators, and Chelating Agents

Humic substances ^[47] constitute ~75% of the organic matter weight in most soils and ~50% of the organic carbon in surface waters. The contents of HS vary with the geographic area and soil type; in the case of infertile sandy soils, humic substances are expected to be reduced.

Humic substances perform multiple dynamic and interactive vital roles—protective, accumulative, and ameliorative—in soil physics, chemistry, and biology; they create a framework for sustainable agriculture by enhancing plant growth and nutrition, improving soil fertility, and maintaining edaphic quality. Also, HS increase the buffering and water-holding capacities, shear strength, bulk density, porosity, and aeration of soils. The dark brown color of HS enhances sunlight absorption and improves soil thermal properties, playing a crucial role during the plant growing period. Their organic groups, being mostly carboxyl and phenolic, deprotonate in neutral and alkaline environments, increasing the cation exchange capacity of soils and offering anti-inflammatory properties ^[48]^[49].

Additionally, HS promote chemical interactions and surface phenomena (i.e., adsorption of macro- and micronutrients), remove pollutants from waters and soils, act as antioxidants, protect against UV radiation ^[48], and they may be soil bactericides ^[50] or plant fungicides ^[51]; on the other hand, they provide a stabilized environment for microorganisms beneficial to soil fertility. Humic substances may act as stress alleviators, increasing the resistance of plants to both biotic and abiotic factors, e.g., salinity, heat, drought, or reactive oxygen species (ROS); they behave as phytohormones, inducing structural, physiological, and biochemical changes; they accelerate seed germination; and they stimulate the growth of plants. Humic substances have also been characterized as nutrient facilitators for N and P; the dissociation of carboxyl functional groups renders soil pH slightly acidic, favoring the solubility of these micronutrients.

Bearing a plethora of organic groups, e.g., –NH₂, –OH, –COOH, –CONH₂, and –SH ^[52], HS are expected to be highly susceptible toward complex formation with both organic and inorganic compounds—polar and nonpolar—including anthropogenic organic chemicals and other biocidal substances; also, HS exhibit interesting specific properties, e.g., they are universal amphiphiles with ion-binding capacity nearly 20% of that of a battery storage material ^[14]. Both the colloidal character and the high surface functionality of HS facilitate the retention of either ionic or molecular pollutants regulating their mobilization and/or immobilization in natural environments ^[53]^[54]^[55].

3.2. Complexation with Nutrient and Contaminant Metals

A major contribution of HS to soil is their role as chelators associating with soil metal ions (both micronutrients and heavy metals). Metal uptake by HS depends on the type of soil, the metal, and the concentration of HS; it changes the speciation of metals in soils and regulates the mobility, bioavailability, and translocation of nutrients and toxics, the HS acting as geochemical barriers to the acute toxicity of the pollutants ^[56].

The presence of heavy metals in soils as a result of urbanization and industrialization constitutes a prime danger to the environment and to human health. Heavy metals, toxic and nonbiodegradable, can easily find their way into the environment and the food chain, threatening ecosystems and humans ^[57].

Owing to its simplicity and high efficiency, soil leaching is considered an important cleanup technique for contaminated soils. In this case, heavy metals are removed via their complexation with washing agents ^[57]. HA are perhaps the most widely encountered natural complexing ligand. Almost all metals detected in soil (both the natural components and those induced as pollutants) can interact with HS, including major and trace elements, alkaline earth and transition metals, as well as actinides. Metal ion–HS interactions are omnipresent in both aquatic and terrestrial ecosystems; in addition to complex formation and/or chelation (attachment of metal ions to HS surface groups and π-binding), they may include weaker interactions involving, e.g., electrostatic attraction. Complexation of the metal ions with the various nitrogen- and oxygen-containing HS functional groups, as well as carbon–cation π-association between HS aromatic rings, e.g., quinones, and metals enhance the effectiveness of metal immobilization and alter the electroactivity of HS. As a result, metal ions may remain dissolved or become coprecipitated with HS, especially when scavenged by humic-modified mineral surfaces (Figure 2).

Figure 2. Association of metal ions and dye molecules with humic substances.

The changes in HA electroactivity after complexation with metal ions have been correlated with a different biological role of HA; in addition to reducing the toxicity and bioavailability of pollutants, HA may also catalyze their oxidation by both anaerobic and aerobic microorganisms by acting as electron-shuttling compounds. In this case, HA transfer electrons from humic-reducing bacteria to distant electron acceptors using the quinone and phenolic hydroxyl groups, as well as the N- and S-containing moieties ^[58].

4. Dyestuff and Color Removal by Humic Substances

4.1. Dye Adsorption

Adsorption of contaminants on humics has been extensively investigated; e.g., polycyclic aromatic hydrocarbons (PAH) can undergo rapid sorption onto surfaces of organic matter (i.e., HA and FA) in soil matrix, resulting in a decreased PAH bioavailability ^[59]. Humic substances, presenting polyelectrolytic characteristics with structural units involving condensed aromatic rings connected with phenol groups and attached chelant groups ^[52], interact with all classes of ecotoxicants, including azo dyes ^[60]. The synthetic-dye industry and dyeing processes are amongst the top ten sources of pollution ^[61]. The presence of synthetic organic dyes in groundwater, soil, and surface water has been reported; dyes are considered to be micropollutants, are visible to the eye at very low concentration, i.e., 1 mg L⁻¹, in aquatic environments ^[62], and can poison living organisms (e.g., bacteria) at solution concentrations as low as 0.5 mg L⁻¹ ^[63]. Dyes from effluents of dyestuff manufacturing industries and the residual unfixed dyes present in exhausted aqueous dyebaths result in environmental pollution after their discharge into the receiving waters. Colorant removal is of crucial significance in relation to water of the desired quality. Retention experiments have demonstrated that HS (Figure 2) are efficient dye adsorbents. Therefore, HS–dye association leads to the removal of the sorbed dyestuff from both soils and waters; this process appears to be a promising remediation strategy ^[64].

Commercial HS have been found to associate rapidly with toluidine blue to produce a dye–HS complex peaking at 630 nm, used for HS determination at microgram levels in natural waters. At increased pH values (pH = 11), more dye molecules are bound to the HA matrices ^[65]. By application of the Langmuir model to the adsorption data, a total number of 1.45 ± 0.04 mmol g⁻¹ binding sites has been calculated ^[66]. A dye–HS complex is also formed when HA taken from an Andisol interact with an oxazine dye (oxazine-1 perchlorate). The amount of bound dye depends strongly on the ionic strength and less on the solution acidity. However, the number of adsorption sites in humic acid for binding oxazine increases with pH and decreases with the electrolyte concentration, affecting the amount of dye retained ^[67]. The complexation of C.I. Basic Blue 3 with commercial potassium humate on surface sites available for deprotonation and potential adsorption of cations has been verified by spectroscopic techniques ^[64]; the dye–HS interactions involve attraction between the positively charged dye and the dissociated carboxyl groups of HS favored at pH = 4.

Certainly, the changes observed in the UV–vis spectrum of a dye in the presence of humic matter are the most valid proof of dye–humics association; it is worth noting that when brilliant blue (or C.I. Acid Blue 9) is sprayed onto turf grass, a green color is imparted to the field. The color shift from blue to green has been attributed to the interactions between the acid dye and the humic material present ^[68]. Additionally, to select suitable fluorescent dye tracers in hydrology and aquatic studies, the fluorescence quenching impacted by HS on polycyclic aromatic compounds has been employed to investigate the interactions of dissolved IHSS standard HS (humic and fulvic acids) with three fluorescent dyes, these being xanthene derivatives, i.e., fluorescein (C.I. Acid Yellow 73), rhodamine 6G (C.I. Basic Red 1), and rhodamine B (C.I. Basic Violet 10) ^[69].

4.2. Humic-Matrix Hybrid Materials in the Removal of Dyes

Hybrid organo-inorganic materials have been prepared and used for colored wastewater remediation. In this context, Fe₃O₄ and chitosan, conventional adsorbents used for the retention of dyes and heavy metals, have been superficially modified with HS obtained from dry horse dung powder to achieve higher efficiency. The new adsorbent proved successful in retaining both methylene blue and Pb(II) over the pH range of 5 to 6 via ion exchange and chemical adsorption, respectively ^[70]. Humic acid (sodium salt) has been, also, immobilized onto Fe₃O₄ using the eco-friendly coprecipitation route (reaction of HA with FeCl₃/FeSO₄) to produce high-surface functionalized nanosorbents bearing extended magnetic saturation, applied to remove the carcinogenic dye malachite green from water. The highest dye removal has been achieved at pH = 6; sorption has been characterized as chemisorption, endothermic, feasible, spontaneous in nature, and involving the formation of multilayers ^[71]. With the use of the coprecipitation technique, humic acid extracted from the peat soil of South Sumatra has been deposited onto Fe₃O₄ and utilized in malachite green removal ^[72]^[73]; in fact, from the mixture of dyes employed—containing malachite green, methylene blue, and rhodamine B—the adsorbent demonstrated selectivity toward malachite green, attributed to the smaller size of this dye. Compared with the parent HA, HA/Fe₃O₄ is stable, with enhanced adsorption capacity, and can be regenerated ^[74]. Perlite, an inorganic aluminosilicate eco-friendly material, has been superficially modified with commercially available HA and tested for methylene blue retention via a reaction between acylated HA and amino-bonded perlite. The deposition of HA enhances dye adsorption compared to the original expanded perlite. Factors affecting the adsorption process are pH, ionic strength, and contact time ^[75].

Surface-active silanized humic derivatives immobilized onto silica gel have been synthesized in aqueous solution; potassium humate from leonardite and low moor peat have been employed. These humic materials are natural hyperbranched polyelectrolytes (with self-assembling properties to form humic adlayers), capable of ionic, hydrophobic, and donor–acceptor interactions due to their surface functionality and amphiphilic properties. Silanized derivatives proved successful in azo dye (C.I. Direct Brown 1) and plutonium removal from contaminated waters ^[76]. Distinct HA (one commercial and the other isolated from Brazilian peat soil) have been attached onto aminopropyltrimethoxysilane-modified silica gel to produce two new adsorbents for indigo carmine (C.I. Acid Blue 74) uptake ^[52]. Owing to their polyfunctionality, HS can be immobilized onto various types of surfaces ^[77].

5. Conclusions

Humic substances are widely applied in agriculture as plant growth promoters exhibiting bactericidal and fungicidal properties—supporting both the sustainability of natural ecosystems and novel strategies for environmental protection—, in medicine, pharmaceutical, and cosmetic areas as solubilizing agents and to allow the transport of hydrophobic active compounds. Apart from that, the average properties of structurally heterogeneous HS, mostly their amphiphilic character and chelating functionality, have not yet been fully exploited and require further investigation. Future research may involve efforts to improve methods for the global standardization and chemical identification of HS. More importantly, much work should be undertaken to correlate the structural characteristics of HS molecules and fractions with the dye retention capacity; possible mechanisms for dye adsorption onto HS have not been thoroughly studied and must be confirmed on a case-by-case basis to establish specific tailored dye-pollution management systems. The diverse chemical and physical composition of HS is responsible for the facts that a single method cannot be utilized for the different HS–dye systems and that there is no universal mechanism that describes the manner in which HS–dye complexes are formed.

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