Chemical Characteristics of Humic Substances in Nature

*Claudio Fernando Mahler, Nicoly Dal Santo Svierzoski and Cassiano Augusto Rolim Bernardino*

#### **Abstract**

Humic substances are the main constituents of natural organic matter, found in both aquatic and terrestrial environments. Humic substances are a complex, dispersed, and heterogeneous mixture of various organic compounds synthesized from organic matter residues, decomposed by microorganisms. Most scientists indicate that humic substances are as a supramolecular association of small heterogeneous molecules stabilized by weak intermolecular bonds. When these substances are presented in water intended for drinking or industrial use, it can have a significant impact on the treatability of this water and on the success of chemical disinfection processes, due to possible formation of organic compounds harmful to human health. Moreover, the humic substances can be used of several ways such as fertilizer to help in the development of plants, to improve soil erosion and to removal of organic compounds and metals from soils and waters. In addition, humic substances suggest an important role in mitigating areas degraded by the phytoremediation technique. The purpose of chapter is to provide an overview of humic substances and to discuss their concepts, chemical characteristics, ecological effects and technological applications for soils and aquatic systems.

**Keywords:** Humic substances, Humic acid, Fluvic acid, Humin, Organic matter, Soils, Aquatic system

#### **1. Introduction**

Humic substances are the main constituents of natural organic matter, found in both aquatic and terrestrial environments. Natural organic matter can be classified into aquagenic organic matter and paedogenic organic matter. Aquagenic organic matter occurring in ocean waters is formed by the excretion and decomposition of plankton and aquatic bacteria. Paedogenic organic matter is produced by the decomposition of land plants and microorganisms, including leached material from the soil anchored in the aquatic system. This type of organic matter is formed by the degradation of lignin, polysaccharides and proteins, which lead to the formation of organic compounds of carboxylic, phenolic, benzoic, and aliphatic compounds. In addition, humic substances are the main carbon reservoir in the biosphere and account for approximately 70 to 90% of soil organic matter [1].

According to the International Humic Substances Society (IHSS), humic substances are complex heterogeneous mixtures of polydisperse materials formed by a humification process, in which chemical and biochemical reactions occur during the decomposition of plant and microbial residues [2]. In [3] was also noted this new understanding, where humic substances are heterogeneous and relatively small molecular components of soil organic matter in supramolecular associations with a variety of organic compounds of biological origin and synthesized by abiotic and biotic reactions in soil. The first publications on humic substances were reported in 1786 with the extraction of humic acids from peat by Achard in Germany and by Vauquelin in 1797 with the extraction of plant residues. Between 1829 and 1837, studies to understand the origin and composition of humic substances began with the research conducted by Sprengel, reporting that more alkaline soils exhibited greater abundance of humic acids, making them more fertile [4]. In the 19th and 20th centuries, humic substances were often examined through the acid–base theory. In the mid-20th century studies were initiated in order to classify humic fractions. The author in [5] created the nomenclatures of fulvic acids, humic acids and humins. This classification is based on their solubility in aqueous medium. Then, fulvic acids are soluble at acidic or alkaline pH, humic acids are soluble at alkaline pH, and humins are insoluble at any pH [4, 6, 7]. Researchers have been discussing, in recent years, models in order to explain the chemical structure of humic substances. Currently, there are three widely discussed models - the macromolecular, the micellar, and the supramolecular structure models. The macromolecular model assumes that humic substances are a soil polymer and that humification is the process by which organic compounds from plants and animals are not fully oxidized to CO2 and H2O and accumulate in the soil as humus [8–10].

The micellar model states that humic substances consist of macro-structures of high molecular mass and that the macromolecular properties result from associations of small molecular species in micellar structures [11]. The author in [12] defines humic substances as a supramolecular association of small heterogeneous molecules stabilized by weak intermolecular bonds, which can be broken by small amounts of organic acids. Among the three main structural models (micellar, macromolecular and supramolecular) of humic substances, currently, the supramolecular model is the most widely accepted by researchers in the IHSS [2].

However, the scientific community has been discussing the macromolecular and supramolecular models, due to the undefined chemical structure of humic substances.

#### **2. Concept and characterization of humic substances**

Researchers in this area has been discussing the concept, origin, and composition of humic substances for the last decades, and these questions have not yet been clarified. According to [13, 14] humic substances are a complex, dispersed, and heterogeneous mixture of various organic compounds synthesized from organic matter residues, decomposed by microorganisms.

Humic substances exhibit a wide variety of structures and chemical compositions. As an example, a considerable part of these substances present the benzene ring (phenols and quinones), as shown in **Figure 1**. In addition, several functional groups are present in humic substances such as carboxylic, hydroxyl (phenolic and alcoholic), carbonyl, and amino groups [15]. The authors [16] list the substances identified in humic extracts, that include mono-, di- and trihydroxy acids, fatty acids, dicarboxylic acids, linear alcohols, phenolic acids, terpenoids, steroid compounds, carbohydrates and amino acids.

In [9], the author reports that the functional groups that contribute most to the surface charge and reactivity of humic substances are phenolic and carboxylic groups. Humic acids behave as mixtures of dibasic acids, with pKa value around 4

**141**

acids [17].

**Figure 1.**

*Chemical Characteristics of Humic Substances in Nature DOI: http://dx.doi.org/10.5772/intechopen.97414*

regulating the bioavailability of metallic ions [17].

to 14.2% hydrogen, 2.9 to 6.0% nitrogen and 0.1 to 0.9% sulfur).

for protonation of carboxylate groups and around 8 for protonation of phenolate groups. There is considerable overall similarity between the individual humic

*Molecular building blocks that form humic substances containing the benzene ring.*

For this reason, the pKa values measured for a given sample are mean values relative to the constituent species. The other important characteristic is charge density. Fulvic acids are defined as associations of small hydrophilic molecules which several acidic functional groups that form the fulvic clusters dispersed in solution at any pH. Humic acids are made by associations mainly of hydrophobic compounds which it are stabilized at neutral pH by hydrophobic dispersive forces such Van der Waals forces, π-π bonds, CH-π bonds. Their conformations grow progressively in size when intermolecular hydrogen bondings are increasingly, flocculate at lower pH [11]. The presence of carboxylate and phenolate groups gives humic acids the ability to form complexes with ions such as Mg2+, Ca2+, Fe2+ and Fe3+. Many humic acids have two or more of these groups arranged in such a way that allows the formation of chelate complexes [18]. The development of complexes (chelates) is an important aspect of the biological role of humic acids in

According to [19] the elemental composition of humic substances can be divided and vary as follows: fulvic acids (35.1 to 75.7% carbon, 16.9 to 55.8% oxygen, 0.4 to 7.9% hydrogen, 0.5 to 8.2% nitrogen and 0.1 to 3.6% sulfur), humic acids (37.2 to 75.8% carbon, 7.9 to 56.6% oxygen, 1.6 to 11.7% hydrogen, 0.5 to 10.5% nitrogen and 0.1 to 8.3% sulfur) and humin (48.3 to 61.6% carbon, 28.8 to 45.1% oxygen, 7.3

The presence and relative abundance of fulvic acids, humic acids, and humin is inferred by laboratory extraction, a process that modifies their original state. Humic and fulvic acids are extracted as a colloidal solution from the soil to other solid phase sources in a strongly basic aqueous solution of sodium or potassium hydroxide. Humic acids are precipitated into the solution by adjusting the pH to 1 with hydrochloric acid, keeping fulvic acids in solution. This is the operational difference between humic and fulvic acids. Humin is insoluble in diluted alkali. The alcohol-soluble portion of the humic fraction is generally referred to as humic acid. The named "gray humic acids" (GHA), according to [20], are soluble in alkaline media of low ionic strength; "brown humic acids" (BHA) are soluble under alkaline conditions regardless of ionic strength; and fulvic acids (FA) are soluble regardless of pH and ionic strength. Humic acid, as traditionally produced in the laboratory, is not a single acid; instead, it is a complex mixture of many different acids containing carboxylic and phenolic groups so that the mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid. Humic acid used for soil correction is manufactured using the same well-established procedures. Humic acids can form complexes with ions commonly found in the environment creating humic colloids. Humic acids are insoluble in water at acid pH, while fulvic acids are also derived

*Chemical Characteristics of Humic Substances in Nature DOI: http://dx.doi.org/10.5772/intechopen.97414*

#### **Figure 1.**

*Humic Substances*

the decomposition of plant and microbial residues [2]. In [3] was also noted this new understanding, where humic substances are heterogeneous and relatively small molecular components of soil organic matter in supramolecular associations with a variety of organic compounds of biological origin and synthesized by abiotic and biotic reactions in soil. The first publications on humic substances were reported in 1786 with the extraction of humic acids from peat by Achard in Germany and by Vauquelin in 1797 with the extraction of plant residues. Between 1829 and 1837, studies to understand the origin and composition of humic substances began with the research conducted by Sprengel, reporting that more alkaline soils exhibited greater abundance of humic acids, making them more fertile [4]. In the 19th and 20th centuries, humic substances were often examined through the acid–base theory. In the mid-20th century studies were initiated in order to classify humic fractions. The author in [5] created the nomenclatures of fulvic acids, humic acids and humins. This classification is based on their solubility in aqueous medium. Then, fulvic acids are soluble at acidic or alkaline pH, humic acids are soluble at alkaline pH, and humins are insoluble at any pH [4, 6, 7]. Researchers have been discussing, in recent years, models in order to explain the chemical structure of humic substances. Currently, there are three widely discussed models - the macromolecular, the micellar, and the supramolecular structure models. The macromolecular model assumes that humic substances are a soil polymer and that humification is the process by which organic compounds from plants and animals are not fully

oxidized to CO2 and H2O and accumulate in the soil as humus [8–10].

lecular model is the most widely accepted by researchers in the IHSS [2].

**2. Concept and characterization of humic substances**

matter residues, decomposed by microorganisms.

pounds, carbohydrates and amino acids.

However, the scientific community has been discussing the macromolecular and supramolecular models, due to the undefined chemical structure of humic

Researchers in this area has been discussing the concept, origin, and composition of humic substances for the last decades, and these questions have not yet been clarified. According to [13, 14] humic substances are a complex, dispersed, and heterogeneous mixture of various organic compounds synthesized from organic

Humic substances exhibit a wide variety of structures and chemical compositions. As an example, a considerable part of these substances present the benzene ring (phenols and quinones), as shown in **Figure 1**. In addition, several functional groups are present in humic substances such as carboxylic, hydroxyl (phenolic and alcoholic), carbonyl, and amino groups [15]. The authors [16] list the substances identified in humic extracts, that include mono-, di- and trihydroxy acids, fatty acids, dicarboxylic acids, linear alcohols, phenolic acids, terpenoids, steroid com-

In [9], the author reports that the functional groups that contribute most to the surface charge and reactivity of humic substances are phenolic and carboxylic groups. Humic acids behave as mixtures of dibasic acids, with pKa value around 4

The micellar model states that humic substances consist of macro-structures of high molecular mass and that the macromolecular properties result from associations of small molecular species in micellar structures [11]. The author in [12] defines humic substances as a supramolecular association of small heterogeneous molecules stabilized by weak intermolecular bonds, which can be broken by small amounts of organic acids. Among the three main structural models (micellar, macromolecular and supramolecular) of humic substances, currently, the supramo-

**140**

substances.

*Molecular building blocks that form humic substances containing the benzene ring.*

for protonation of carboxylate groups and around 8 for protonation of phenolate groups. There is considerable overall similarity between the individual humic acids [17].

For this reason, the pKa values measured for a given sample are mean values relative to the constituent species. The other important characteristic is charge density. Fulvic acids are defined as associations of small hydrophilic molecules which several acidic functional groups that form the fulvic clusters dispersed in solution at any pH. Humic acids are made by associations mainly of hydrophobic compounds which it are stabilized at neutral pH by hydrophobic dispersive forces such Van der Waals forces, π-π bonds, CH-π bonds. Their conformations grow progressively in size when intermolecular hydrogen bondings are increasingly, flocculate at lower pH [11]. The presence of carboxylate and phenolate groups gives humic acids the ability to form complexes with ions such as Mg2+, Ca2+, Fe2+ and Fe3+. Many humic acids have two or more of these groups arranged in such a way that allows the formation of chelate complexes [18]. The development of complexes (chelates) is an important aspect of the biological role of humic acids in regulating the bioavailability of metallic ions [17].

According to [19] the elemental composition of humic substances can be divided and vary as follows: fulvic acids (35.1 to 75.7% carbon, 16.9 to 55.8% oxygen, 0.4 to 7.9% hydrogen, 0.5 to 8.2% nitrogen and 0.1 to 3.6% sulfur), humic acids (37.2 to 75.8% carbon, 7.9 to 56.6% oxygen, 1.6 to 11.7% hydrogen, 0.5 to 10.5% nitrogen and 0.1 to 8.3% sulfur) and humin (48.3 to 61.6% carbon, 28.8 to 45.1% oxygen, 7.3 to 14.2% hydrogen, 2.9 to 6.0% nitrogen and 0.1 to 0.9% sulfur).

The presence and relative abundance of fulvic acids, humic acids, and humin is inferred by laboratory extraction, a process that modifies their original state. Humic and fulvic acids are extracted as a colloidal solution from the soil to other solid phase sources in a strongly basic aqueous solution of sodium or potassium hydroxide. Humic acids are precipitated into the solution by adjusting the pH to 1 with hydrochloric acid, keeping fulvic acids in solution. This is the operational difference between humic and fulvic acids. Humin is insoluble in diluted alkali. The alcohol-soluble portion of the humic fraction is generally referred to as humic acid. The named "gray humic acids" (GHA), according to [20], are soluble in alkaline media of low ionic strength; "brown humic acids" (BHA) are soluble under alkaline conditions regardless of ionic strength; and fulvic acids (FA) are soluble regardless of pH and ionic strength. Humic acid, as traditionally produced in the laboratory, is not a single acid; instead, it is a complex mixture of many different acids containing carboxylic and phenolic groups so that the mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid. Humic acid used for soil correction is manufactured using the same well-established procedures. Humic acids can form complexes with ions commonly found in the environment creating humic colloids. Humic acids are insoluble in water at acid pH, while fulvic acids are also derived

from humic substances, but are water soluble throughout the pH range [21]. Humic and fulvic acids are often used as a soil supplement in agriculture and, less commonly, as a human nutritional supplement. As a nutritional supplement, fulvic acid can be found in liquid form as a component of mineral colloids. Fulvic acids are polyelectrolytes and are unique colloids that diffuse easily through membranes, whereas all other colloids do not [22]. A sequential chemical fractionation called Humeomics can be used to isolate more homogeneous humic fractions and determine their molecular structures by advanced spectroscopic and chromatographic methods [23].
