Humin
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Humins are carbon-based macromolecular substances, that can be found in
soil chemistry Soil chemistry is the study of the chemical characteristics of soil. Soil chemistry is affected by mineral composition, organic matter and environmental factors. In the early 1850s a consulting chemist to the Royal Agricultural Society in England, ...
or as a by-product from
saccharide In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or may ...
-based biorefinery processes.


Humins in soil chemistry

Soil consists of both mineral (inorganic) and organic components. The organic components can be subdivided into fractions that are soluble, largely
humic acid Humic substances (HS) are organic compounds that are important components of humus, the major organic fraction of soil, peat, and coal (and also a constituent of many upland streams, dystrophic lakes, and ocean water). For a long era in the 19th ...
s, and insoluble, the humins. Humins make up about 50% of the organic matter in soil. Due to their very complex molecular structure, humic substances, including humin, do not correspond to pure substances but consist of a mixture of many compounds that remain very difficult to characterize even using modern analytical techniques.


Humins from biomass sources

Humins also produced during the
dehydration In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...
of sugars, as occurs during the conversion of lignocellulosic biomass to smaller, higher value organic compounds such as
5-hydroxymethylfurfural Hydroxymethylfurfural (HMF), also 5-(hydroxymethyl)furfural, is an organic compound formed by the dehydration of reducing sugars. It is a white low-melting solid (although commercial samples are often yellow) which is highly soluble in both water ...
(HMF). These humins can be in the form of either viscous liquids or solids depending on the process conditions used.


Humin structure and mechanism of formation

Both the structure of humins and the mechanism by which they are synthesized is at present not well defined as the formation and chemical properties of humins will change depending on the process conditions used. Generally, humins have a polymeric furanic-type structure, with
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
,
aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
and
ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...
functionalities. However, the structure is dependent on feedstock type (e.g.
xylose Xylose ( grc, ξύλον, , "wood") is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional gro ...
or
glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
) or concentration, reaction time, temperature, catalysts and many other parameters involved in the process. These parameters also influence the mechanism of formation which is still a matter of debate. Different pathways have been considered, including ring-opening
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
of HMF (believed to be the key intermediate for the formation of humins),
nucleophilic addition In organic chemistry, a nucleophilic addition reaction is an addition reaction where a chemical compound with an electrophilic double or triple bond reacts with a nucleophile, such that the double or triple bond is broken. Nucleophilic additions di ...
s, or via the formation of an aromatic intermediate. While there is no clear evidence to substantiate or exclude the mechanisms, general consensus is on a series of condensation reactions that reduce the efficiency of
biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
conversion strategies.


Safety aspects

Humins are not considered to be a dangerous substance according to officially recognized
hazardous material Dangerous goods, abbreviated DG, are substances that when transported are a risk to health, safety, property or the environment. Certain dangerous goods that pose risks even when not being transported are known as hazardous materials ( syllabi ...
classification systems based on physical-chemical properties such as flammability,Muralidhara, A., Tosi, P., Mija, A., Sbirrazzuoli, N., Len, C., Engelen, V., de Jong, E., Marlair, G., ACS Sustainable Chem. Eng., 2018, 6, 16692-16701 explosiveness, susceptibility to oxidation, corrosiveness or eco-toxicity. Heating of humins forms a macroporous material known as humins foams and also these materials did not present critical fire behaviour despite their highly porous structure.


Potential applications of humins

In the past, humins from biomass sources have been mostly considered as combustible materials to supply heat for biorefinery processes. However, high value applications have started to receive more attention, notably the use of humins in the preparation of
catalytic Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
materials and in material applications (e.g. plastic reinforcement and construction materials). Humins can also be subjected to thermal treatments in order to form interesting solid materials, such as lightweight and porous humin foams.Tosi, P., van Klink, G.P., Celzard, A., Fierro V., Vincent, L., de Jong, E., Mija, A., ChemSusChem, 2018, 11, 2797-2809 Overall, humins appear to improve the final properties of the materials although research is mainly at the proof-of-principle stage (early).


See also

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Organic matter Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
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Soil organic matter Soil organic matter (SOM) is the organic matter component of soil, consisting of plant and animal detritus at various stages of decomposition, cells and tissues of soil microbes, and substances that soil microbes synthesize. SOM provides numerous b ...
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Humus In classical soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Lati ...
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Humic substance Humic substances (HS) are organic compounds that are important components of humus, the major organic fraction of soil, peat, and coal (and also a constituent of many upland streams, dystrophic lakes, and ocean water). For a long era in the 19th an ...


References


Further reading

Singer, Michael J., and Donald N. Munns (2005). Soils: An Introduction (6th Edition). Upper Saddle River: Prentice Hall. {{ISBN, 978-0-13-119019-1. Plant nutrition Soil science