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 ma ...

-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 acids, 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 (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 hydro ...

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 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 g ...

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, u ...

) 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, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...

of HMF (believed to be the key intermediate for the formation of humins), nucleophilic additions, 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 conversion strategies.

Safety aspects

Humins are not considered to be a dangerous substance according to officially recognized hazardous material 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).

Humins in soil chemistry

Humins from biomass sources

Humin structure and mechanism of formation

Safety aspects

Potential applications of humins

See also

References

Further reading