CC BY
44
Primary questions about treatment of humus-rich ground water. Formation and properties of humic substances
1 2 A. Ivanitsky , E. Lakso
1 Laboratory of Industrial Ecology of the MSTU Scientific-Research Centre Laboratory of Hydraulics of the Oulu University
Abstract. The brown colour of water is usually a sign of high humus and iron content. End products of microbial degradation of plants and animals are called humus. Humic substances are a series of relatively high-molecular-weight, brown to black coloured substances formed by a secondary synthesis reaction (Christofer, 1996). A substantial fraction of the mass of humic substance is in carboxylic acid functional groups, which endow these molecules with an ability to chelate positively charged multivalent ions (Mg2+, Ca2+, Fe2+ and most other trace elements).
1. Introduction
Humic substances are ubiquitous in natural waters. They are macromolecular organic acids that can react with both trace metals and some organic contaminants. The high humic content is typical for northern groundwater. The consumption of KMnO4 gives us a picture of the humus amount in water. For example, in raw water (Finland, the Oulu region) the normal amount of KMnO4 in humus containing groundwater is about 20 mg/l, at the same time iron and manganese, in high concentration, are presented (about 0,5 mg/l for Mn and 2 - 3 mg/l for Fe). The amount of KMnO4 in drinking water after treatment according to Finnish standards has to be less than 12 mg/l. Some techniques remove humic substances from natural waters by various methods, chemical precipitation, using, for example, aluminium sulphate - is an effective way to remove humic substances from both: ground and surface waters, but presence of iron and manganese makes the treatment process more complicated. Before the working up a subject the origin, formation and properties of humic substances have to be known.
2. The formation of humic substances
The formation of humic substances is one of the least understood aspects of humic chemistry and one of the most intriguing (Weber, 1997). Several pathways exist for the formation of humic substances during the decay of plants and animals remained in soil, the main ones being shown in Fig.1. The classical theory, popularized by Waksman (in 1932), is that humic substances represent modified lignins (pathway 1), but the majority of present-day investigators favour a mechanism involving quinones (pathways 2 and 3). In practice all four pathways must be considered as likely mechanisms for the synthesis of humic and fulvic acids in nature, including sugar-amine condensation (pathway 4). These four pathways may operate in all soils, but not to the same extent or in the same order of importance. A lignin pathway may predominate in poorly drained soils and wet sediments (swamps, etc.) whereas synthesis from polyphenols may be of considerable importance in certain forest soils. Frequent and sharp fluctuations in temperature, moisture and irradiation in terrestrial surface soils under a harsh continental climate may favour humus synthesis by sugar-amine condensation.
Fig.1. Mechanism for the formation of humic substances. (Stevenson 1982)
Ivanitsky A., Lakso E. Primary questions about treatment of humus-rich ground water..
3. Properties of humic substances
Humic substances are divided into three fractions based on their solubility in aqueous acids and bases:
Humin - the fraction of humic substances that is not soluble in water at any pH value. Humins are black in colour.
Humic acid - the fraction of humic substances that is not soluble in water under acidic conditions (pH < 2), but is soluble at higher pH values. Humic acids are the major extractable component of humic substances. They are black to brown in colour.
Fulvic acid - the fraction of humic substances that is soluble in water under all pH conditions. They remain in solution after removal of humic acid by acidification. Fulvic acids are light yellow to yellow-brown in colour.
Each fraction is a heterogeneous mixture of organic substances having a wide range of molecular weights and negative charges. Differences between humic acids and fulvic acids can be explained by variations in molecular weight, numbers of functional groups (carboxyl, phenolic OH) and extent of polymerization (Stadnikov, 1937; Weber, 1997; Peterson, 1996). In Fig.2 it is shown that carbon and oxygen contents, acidity and degree of polymerization all changed systematically with increasing molecular weight. The low - molecular - weight fulvic acids have higher oxygen but lower carbon contents than the high - molecular - weight humic acids. Fulvic acids contain more functional groups of acidic nature, particularly COOH. The total acidity of fulvic acids (900 - 1400 meq/100 g) are considerably higher than for humic acids (400 - 870 meq/100 g).
Humic substances (pigmented polymers)
Fulvic acid Humic acid Humin
Light yellow Yellow brown Dark brown Grey-black Black
2 000 ■ 45% ■ 48% ■ 1 400 ■
increase in intensity of colour increase in degree of polymerization increase in molecular weight increase in carbon content decrease in oxygen content
decrease in exchange acidity decrease in degree of solubility
- 300 000 • 62% 30% 500
Fig.2. Chemical properties of humic substances. (Stevenson 1982)
Another important difference is that while the oxygen in fulvic acids can be accounted for largely in known functional groups (COOH, OH, C=O), a high portion of the oxygen in humic acids seems to occur as a structural component of the nucleus.
A chelate with metals is formed when two or more coordinate positions about the metal ion are occupied by donor groups of a single ligand to form an internal ring structure. The large organic molecules contain a variety of functional groups, of which the most important ones are carboxylic, phenolic and alcoholic groups (Kuharkina, 1953; Senn et al., 1996). The complexing ability of humic and fulvic acids results largely from their content of oxygen functional groups, such as COOH, phenolic OH and C=O group.
The acidic functional groups are ionised to a certain degree, depending on the pH of the water. Thus, in water of natural pH, humic substances have an ability to bind metals and adsorb onto surface of, for example, mineral particles, while organic pollutants mainly adsorb to humic compounds by hydrophobic interaction. The
dissolved humic substances become carriers of metals and organic pollutants. Humic complexes of trace metals can be more stable because they do not settle or sorb to settling particles.
4. Conclusion
Metals associated with colloids of inorganic or organic components do not settle under the impact of gravity (Bingman, 1996; Kronberg, 1987). A similar effect would be exerted by ligands of macromolecules (i.e. humic and fulvic acids) under conditions where they remain in solution. The association of trace metals with dissolved organic will also directly influence the behaviour of the metals by affecting precipitation process.
This makes the treatment technology more complicated and increases size and cost of operation units. During working up a project the best technology has to be found from the different points of view; such as cost, simplicity in operation and the best treatment results. Possible treatment process is ozone treatment to destroy humic substances and, thus, decreases the chemical oxygen consumption of water. Another possibility is the biological treatment with using specific bacteria.
At first, these technologies have to be tested in laboratory scale. Then a small-scale (1 m3/h) experimental treatment plant will be built. After that the methods are easy to apply for the planing and construction of normal scale treatment process.
Acknowledgement. This research work is supported by TEKES (Finnish State Scientific Centre of Research and Development).
References
Bingman C. Humus, humic acid and natural chelating agents. 2p., 1996.
http://www.coo.caltech.edu/~aquaria/krib/Chemistry/humic.html Christofer T.B.S. Humic substances. 2p., 1996.
http://www.agri.upm.edu.my/jst/resources/as/om_huicsubs.html Kronberg L. Mutagenic compounds in chlorinated humic and drinking water. Ebo Akademi, 54p., 1987. Kuharkina T.A. Research in the field of humic acids, coals and different stages of coal formation. Chemistry of
solid burning fossils, Abstracts of The First All-Soviet Union Conference, 789p., 1953. (in Russian) Peterson K. Properties of humic substances from groundwater and surface water. Linkoping University, Sweden,
Publ.House "Motala Ab", 209p., 1992. Senn T.L., Kingman A.R. and Codley W.C. A review of humus and humic acids. Chemson University, South Carolina, USA, 4p., 1996. http://www.humic.com/review-ha.html Stadnikov G.L. The oil and coal origins. Moscow, 132p., 1937. (in Russian) Weber J. Soil humic substances. Agricultural University of Wroclaw, Poland, 47p., 1997. http://www.ar.wroc.pl/~weber/humic.html
