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Humin

Humin refers to carbon-based macromolecular substances that are either the insoluble fraction of soil organic matter (SOM) remaining after extraction of base-soluble components such as humic and fulvic acids, defined operationally as material not extractable with dilute alkali or acid solutions,^[1] or insoluble by-products formed during acid-catalyzed processing of biomass-derived carbohydrates in industrial biorefineries.^[2] In the context of soil, it constitutes approximately 50% of the organic carbon in mineral soils and over 70% in lithified sediments, serving as a major contributor to long-term carbon sequestration in terrestrial ecosystems, with an estimated 2344 gigatons of organic carbon stored in the top 3 meters of global soils.^[1] As the most recalcitrant and least understood portion of SOM, humin is highly resistant to microbial decomposition due to its strong associations with clay minerals and metal ions.^[3]^[4] Recent advances in analytical techniques, including nuclear magnetic resonance (NMR) spectroscopy and solvent systems like acidified dimethylsulfoxide, have revealed humin's composition to be predominantly aliphatic hydrocarbons derived from plant lipids, waxes, cuticular materials, cutin/cutan, and suberin/suberan.^[1] Minor components include small amounts of carbohydrates (possibly from cellulose), peptides, and peptidoglycans, with notably little contribution from lignin-derived structures, distinguishing humin from the more aromatic base-soluble humic substances.^[1] Its molecular weight can range from about 100,000 to 10,000,000 daltons, forming a complex, amorphous network that binds soil particles.^[5] In soil ecosystems, humin plays critical roles in enhancing physical and chemical properties, including improving water-holding capacity, promoting soil aggregation and stability, and acting as a cation exchange system to boost fertility.^[5]^[3] By cementing primary particles, plant roots, and other organic residues into microaggregates (less than 250 micrometers), it contributes to soil structure formation and resistance to erosion.^[3] Humin's stability makes it essential for sustainable agriculture and environmental management, as it supports nutrient retention and carbon persistence in soils amid climate change pressures.^[1]

Introduction

Definition and Classification

Humin refers to a class of carbon-based, macromolecular organic polymers that are characteristically insoluble in aqueous solutions across a wide pH range. Traditionally, these substances are thought to arise through complex biochemical processes involving the polymerization of phenolic compounds and complexation with other organic and mineral components during the decomposition of plant and animal residues.^[6] However, recent studies indicate that humin is predominantly composed of aliphatic materials derived from plant lipids and cuticular components, with minimal contribution from lignin-derived phenolics.^[7] The term "humin" derives from the Latin word humus, meaning "earth" or "soil," reflecting its origins in terrestrial organic matter and its amorphous, heterogeneous composition.^[8] In soil science, humin is classified as one of the three primary fractions of humic substances, alongside humic acid and fulvic acid, with distinctions based on solubility in water as a function of pH. Humic acid is soluble in alkaline conditions but precipitates in acidic environments (pH < 2), fulvic acid remains soluble across all pH levels, and humin is insoluble in both acidic and alkaline solutions, representing the alkali-insoluble residue after extraction procedures.^[9] This operational classification underscores humin's role as the most recalcitrant and structurally complex component of soil organic matter.^[7] Natural humin, derived from soil humification processes, typically constitutes 50-90% of total soil organic carbon in mineral soils, contributing significantly to long-term carbon sequestration.^[7] In contrast, industrial humin emerges as a byproduct during acid-catalyzed conversions of biomass-derived carbohydrates, such as in the production of platform chemicals like hydroxymethylfurfural, sharing similar macromolecular and insoluble characteristics but with greater variability in composition due to process conditions.^[10] This distinction highlights humin's dual occurrence in environmental and anthropogenic contexts, though both types exhibit resistance to degradation and potential for structural applications.

Historical Background

The concept of humus in soil science traces its roots to the late 18th century, when early chemists began investigating the dark, organic components of soil and peat. In 1786, German chemist Franz Karl Achard pioneered the alkaline extraction of humic materials from peat bogs, developing methods that separated soluble fractions from insoluble residues, laying the groundwork for later distinctions in humic substances.^[11] This approach highlighted the complexity of soil organic matter, though Achard's work primarily focused on extraction techniques rather than detailed classification.^[12] The 19th century saw further refinement in understanding these substances, with German chemist Carl Sprengel advancing the classification in 1826 by distinguishing humic acid through alkali extractions and emphasizing its role in plant nutrition.^[13] Building on this, Dutch chemist Gerrit Jan Mulder advanced fractionation of soil humus in the 1840s, identifying the alkali-insoluble residue as "humin," a stable, dark component resistant to common solvents and distinct from soluble humic and fulvic acids.^[13] Mulder's observations underscored humin's persistence in soil, influencing early views on its inert nature.^[14] In the 20th century, microbiological perspectives transformed the study of humin, particularly through the work of American microbiologist Selman Waksman in the 1930s and 1950s, who linked its formation to microbial decomposition of plant residues.^[15] Waksman's seminal 1936 publication, Humus: Origin, Chemical Composition, and Importance in Nature, emphasized humin's critical role in soil fertility by stabilizing organic matter against further breakdown, integrating biological processes into chemical classifications.^[16] This era culminated in the 1980s with the establishment of the International Humic Substances Society (IHSS) in 1981, which standardized definitions, classifying humin as the alkali-insoluble fraction of humic substances to facilitate consistent research.^[17] The late 20th century marked the emergence of the industrial humin concept, distinct from natural soil variants, as byproducts from biomass processing gained attention in biorefinery contexts. Initial reports in the 1990s documented humin formation during acid-catalyzed dehydration of sugars, such as glucose and fructose, highlighting it as an unavoidable solid residue in processes aimed at producing platform chemicals like 5-hydroxymethylfurfural.^[18] These findings, exemplified by studies on sugar conversions, spurred interest in valorizing industrial humins for applications beyond waste management.

Natural Humins in Soil

Formation and Sources

Humin primarily forms in soil through the microbial decomposition of plant residues, including lipids, waxes, cuticular materials, and carbohydrates, carried out by diverse soil microorganisms such as fungi and bacteria under both aerobic and anaerobic conditions.^[19] This process begins with the breakdown of fresh organic inputs from plant litter, roots, and exudates, where resistant plant biopolymers like cutin/cutan and suberin/suberan serve as major precursors due to their recalcitrance.^[1] Fungi and bacteria contribute to the transformation of these materials into reactive intermediates, with minimal incorporation of lignin-derived structures.^[1] Anaerobic conditions, common in waterlogged soils, slow decomposition but favor the incorporation of partially degraded residues into stable humin pools.^[20] Central to humin formation are biochemical condensation reactions between aliphatic compounds from plant lipids and waxes, amino acids from protein breakdown, and sugars from carbohydrate hydrolysis, leading to the polymerization of insoluble macromolecules.^[21] These reactions generate cross-linked networks that resist further microbial attack, predominantly featuring aliphatic structures.^[1] Non-enzymatic Maillard reactions, involving the browning and polymerization of reducing sugars with amino acids, further contribute to this process by producing melanoidin-like polymers that integrate into humin.^[22] Plant residues contribute significantly to humin's carbon, underscoring the dominant role of terrestrial inputs in its composition.^[23] Environmental factors significantly modulate humin formation and stability. Acidic soil pH (typically below 5) promotes humin insolubility by suppressing the dissociation of functional groups, preventing solubilization and favoring retention in the solid phase.^[1] Moderate temperatures (10-25°C) and adequate moisture enhance microbial activity and reaction kinetics, accelerating precursor formation, while extreme conditions slow accumulation.^[24] Interactions with soil minerals, such as clay particles, are critical; humin binds tightly to these via organo-mineral complexes, stabilizing it against decomposition and promoting long-term sequestration.^[25] Humin accumulates gradually over centuries through continuous inputs and minimal turnover, constituting the most recalcitrant fraction of soil organic matter with residence times often spanning centuries to millennia.^[26] Its long-term stability reflects strong protection by chemical complexity and mineral associations, which buffer it from environmental perturbations.^[26]

Chemical Composition and Structure

Natural soil humins exhibit an elemental composition dominated by carbon, typically ranging from 50% to 60% by weight, with nitrogen at 2% to 5%, oxygen at 30% to 40%, hydrogen at 3% to 6%, and trace amounts of sulfur and metals such as iron and aluminum.^[1] This high carbon content reflects the recalcitrant nature of humins, while the presence of nitrogen primarily stems from incorporated microbial residues and peptides. Aromatic carbon constitutes a minor portion of the total carbon, with predominantly aliphatic structures derived from plant materials.^[1] The structural architecture of natural humins is characterized as a heterogeneous, three-dimensional network comprising predominantly aliphatic chains from plant lipids, waxes, cutin/cutan, and suberin/suberan, interconnected by ether and ester bridges.^[1] No definitive molecular formula exists due to this polydispersity, but humins are depicted as supramolecular assemblies rather than discrete macromolecules, with minor aromatic components amid flexible alkyl linkages.^[1] These models, informed by techniques like solid-state NMR, highlight the aliphatic dominance.^[1] Key structural elements include extensive cross-linking via covalent bonds, featuring carboxyl (-\ce{COOH}), alcoholic hydroxyl, and ketone (-\ce{C=O}) functional groups that enhance rigidity and resistance to biodegradation. Insolubility in both acidic and alkaline media arises from this cross-linked polymeric nature, high molecular weight exceeding 10,000 Da, and hydrophobic aliphatic domains.^[27] These attributes position humins as the most stable fraction of soil organic matter, comprising up to 50% of total organic carbon in mineral soils.^[1] Structural variability in humins is pronounced across soil types, influenced by vegetation and environmental conditions; for instance, forest soils tend to yield more aliphatic-rich humins due to inputs from decaying plant litter, whereas grassland soils may have slightly higher aromatic content from root-derived materials. This heterogeneity affects decomposition rates and carbon sequestration potential, with aliphatic dominance correlating to persistence.^[28] A critical aspect of humin stability involves organo-mineral associations, where humic polymers bind to iron (Fe) and aluminum (Al) oxides through ligand exchange and coordination, forming protective coatings that inhibit microbial access and enzymatic breakdown.^[29] These interactions, prevalent in clay-rich subsoils, can account for 50% or more of stabilized carbon, underscoring humins' role in long-term soil carbon dynamics.

Role in Soil Ecosystems

Humin constitutes the most recalcitrant and abundant fraction of soil organic matter, serving as a primary reservoir for carbon sequestration in terrestrial ecosystems. Its hydrophobic nature and resistance to microbial decomposition enable it to stabilize approximately 50% of soil organic carbon in mineral soils, preventing rapid turnover and mineralization that would otherwise release CO₂ into the atmosphere.^[1] This stabilization is essential for mitigating climate change, as humin-bound carbon persists for centuries, contributing to long-term carbon storage in soils worldwide.^[1] Globally, the humin fraction accounts for a major portion of the estimated 1,500 Pg of soil organic carbon to 1 m depth, forming the largest pool and influencing atmospheric CO₂ dynamics through enhanced carbon persistence.^[30] In diverse soil types, from forest floors to agricultural fields, humin's role in carbon cycling supports ecosystem resilience against environmental perturbations, such as temperature increases that accelerate organic matter breakdown.^[31] Beyond carbon dynamics, humin facilitates nutrient retention by providing cation exchange capacity (CEC), driven by its carboxyl, phenolic, and other functional groups. This CEC allows humin to adsorb and retain essential cations including K⁺, Ca²⁺, and NH₄⁺, minimizing leaching losses during rainfall and ensuring sustained nutrient availability for plant roots and soil biota. Such retention mechanisms are particularly vital in leached or sandy soils, where humin acts as a natural buffer against nutrient deficiencies.^[5] Humin also enhances soil physical structure by binding clay and silt particles into stable aggregates, which improves water infiltration, retention, and aeration while reducing erosion susceptibility. These aggregates create protected microhabitats that foster microbial communities, further promoting organic matter decomposition and nutrient cycling within the soil matrix. By stabilizing soil architecture, humin mitigates compaction and supports root penetration, contributing to overall ecosystem productivity.^[5]^[32] In terms of environmental significance, humin sorbs pollutants like heavy metals (e.g., Cd, Pb) and pesticides through hydrophobic partitioning and ionic bonding, limiting their transport to groundwater or uptake by plants. This sorption capacity underscores humin's role as a key indicator of soil fertility, where higher humin content correlates with reduced contaminant mobility and enhanced soil health.^[33]^[34]^[35]

Industrial Humins from Biomass

Production Methods

Humins are primarily produced as byproducts during the acid-catalyzed dehydration of carbohydrates derived from lignocellulosic biomass, targeting platform chemicals such as 5-hydroxymethylfurfural (HMF) and levulinic acid. This process involves the hydrolysis and subsequent dehydration of monomeric or polymeric sugars under acidic conditions, leading to the formation of insoluble, dark-colored humin polymers alongside the desired furanic or carboxylic acid products.^[2] Common feedstocks include sugars extracted from lignocellulosic materials like corn stover, wood chips, and sugarcane bagasse, which are pretreated to release glucose, fructose, or other hexoses. Typical catalysts are homogeneous Brønsted acids such as sulfuric acid (H₂SO₄) or hydrochloric acid (HCl) at concentrations of 3.5–10 wt%, or solid acids like sulfonated resins; reactions occur at temperatures of 140–200°C, often in aqueous media under atmospheric or elevated pressure for 0.5–24 hours.^[2] Humin yields typically range from 10–50% on a carbon basis, depending on reaction conditions, feedstock composition, and catalyst type; for instance, in HMF production from fructose, humins precipitate as solid residues or tars comprising 16–36 mol% of the output. Higher temperatures and longer residence times promote greater humin formation due to enhanced polymerization.^[2] Process variations aim to control or minimize humin byproduct formation while maximizing platform chemical yields. Biphasic systems, using water-organic solvent mixtures (e.g., with toluene or methyl isobutyl ketone), extract HMF in the organic phase to reduce rehydration and polymerization leading to humins. Microwave-assisted heating accelerates dehydration at lower temperatures, improving selectivity, while enzymatic pretreatments of biomass hydrolyze polysaccharides prior to acid catalysis, yielding cleaner sugar feeds and tunable humin outputs.^[2]^[36] Industrial scale-up of humin production has advanced since the 2010s, with companies like Avantium and Origin Materials operating pilot, demonstration, and commercial plants that generate multi-ton quantities annually as waste from bio-based chemical facilities, such as those producing FDCA via fructose dehydration or chloromethylfurfural (CMF) from biomass.^[2]

Formation Mechanisms

The formation of industrial humins initiates with the acid-catalyzed dehydration of biomass-derived C6 sugars, such as fructose, to furanics like 5-hydroxymethylfurfural (HMF). This process proceeds through key intermediates, including 3-deoxyglucosone, which forms via enolization and dehydration steps under acidic conditions.^[37]^[38] Subsequent polymerization of HMF leads to humins through pathways involving aldol condensation and electrophilic aromatic substitution. In aldol condensation, HMF reacts with rehydration products like 2,5-dioxo-6-hydroxyhexanal (DHH), forming enol intermediates that couple to yield oligomers with furan rings linked by methylene bridges. Electrophilic substitution occurs at the α- or β-positions of the furan ring, promoting cross-linking and insolubilization. Side reactions, such as HMF hydrolysis to levulinic acid and formic acid, further contribute to the reaction network by generating additional reactive species.^[39]^[40]^[41] A simplified equation for HMF self-polymerization captures this process:

n \ce{C6H6O3} \rightarrow (\ce{C6H4O2})_n + \text{byproducts}

Kinetic studies indicate activation energies ranging from 90–100 kJ/mol for aldol addition.^[42]^[39] Factors influencing these mechanisms include pH and solvent composition; pH values below 2 accelerate humin formation by enhancing protonation and dehydration rates, while the presence of water favors fragmentation and rehydration over polymerization, leading to higher byproduct yields. The role of pseudofuran derivatives as precursors remains debated, with recent models from the 2020s suggesting radical mechanisms under specific catalysts, such as metal oxides, that could initiate chain growth via electron transfer. Overall, humins arise from these uncontrolled side reactions, typically reducing target product yields, such as HMF, by 20–30 wt%.^[41]^[39]^[43]

Chemical Structure and Variability

Industrial humins are primarily composed of furan-based oligomers and polymers, featuring a network rich in furanic units derived from 5-hydroxymethylfurfural (HMF) intermediates during acid-catalyzed biomass conversion. These structures typically consist of furan rings connected via aliphatic bridges such as -CH=CH- linkages from aldol condensations or -CH2- and ether bonds, forming branched architectures. The molecular weight of these humins generally ranges from 1,000 to 10,000 Da for oligomeric fractions, though insoluble polymeric components can exceed this, contributing to their macromolecular nature.^[44] Key functional groups in industrial humins include carbonyls (aldehydes and ketones) at approximately 6.6 wt%, alongside hydroxyl groups estimated at 2.5 mmol/g (around 4-5 wt%). These oxygen-containing moieties, along with ether and acetal linkages, impart reactivity and polarity to the material.^[45]^[2]^[44] The chemical structure of industrial humins exhibits significant variability influenced by production parameters. Elevated temperatures above 180°C promote increased cross-linking, dehydration, and aromaticity, as evidenced by decreasing O/C and H/C atomic ratios, leading to more condensed networks. Feedstock choice also affects composition; for instance, glucose-derived humins incorporate more aliphatic chains and intermediates compared to fructose, which yields higher furan content due to efficient HMF formation.^[41] In contrast to natural soil humins, industrial variants are more homogeneous owing to controlled synthetic conditions from carbohydrate feedstocks, typically containing 50-60 wt% carbon and low nitrogen (<2 wt%) absent from proteinaceous sources. A seminal model from studies on HMF-derived humins proposes branched furan networks arising from HMF trimerization via intermediates like 2,5-dioxo-6-hydroxyhexanal, highlighting nucleophilic additions and aldol pathways that build the oligomeric framework.^[2]^[44]

Properties and Characterization

Physical Properties

Natural soil humins appear as dark brown to black amorphous solids, forming heterogeneous conglomerates tightly associated with mineral soil components.^[5]^[46] These materials exhibit high resistance to decomposition, persisting for centuries in soil environments due to their stable structure.^[5] A defining physical characteristic of soil humins is their insolubility in water, acids, and bases across a wide pH range, including neutral conditions, which distinguishes them from other humic fractions like humic and fulvic acids.^[5]^[46] This insolubility arises from strong associations with soil minerals and their highly condensed macromolecular nature, contributing to long-term carbon sequestration.^[46] Soil humins also display hydrophobic traits, with elemental analysis showing 77-81% hydrophobic moieties, enhancing soil aggregate stability and water repellency in certain contexts.^[46] Industrial humins, derived from biomass conversion processes, typically present as dark brown to black viscous tars, syrups, or powders, depending on production conditions such as solvent use and temperature.^[47]^[48] For instance, crude forms are often highly viscous liquids, while purified fractions yield powdered solids with irregular, agglomerated particles ranging from 10 to 100 μm in size.^[47] Solubility of industrial humins is limited in water and common aqueous media but increases in polar aprotic organic solvents like dimethyl sulfoxide (DMSO), where partial dissolution occurs, facilitating further processing.^[47]^[47] They show slight solubility in acetone or hot water (around 5%), but remain largely insoluble under neutral pH conditions.^[48] Thermally, industrial humins demonstrate decomposition onset above 200°C, with significant mass loss (e.g., 47 wt%) occurring between 200 and 800°C, and full thermo-oxidative degradation around 500°C.^[2]^[49] Their glass transition temperature (Tg) typically ranges from 100 to 150°C after thermal treatment, as seen in resin-like forms reaching ~125°C, which enables applications such as foam formation through controlled heating and cross-linking.^[50]^[2] Upon heating, industrial humins can develop macroporous structures, with some purified forms exhibiting porous particles and foams achieving porosities up to 98% and tunable cell sizes from 0.2 mm to several millimeters.^[47]^[51] Bulk densities for resulting foams are low, around 0.055 to 0.092 g/cm³, reflecting their lightweight, expanded nature.^[51] High hydrophobicity is evident in these materials, stemming from aromatic domains, with modified humins showing reduced water uptake (e.g., 3-4 wt% moisture) compared to unmodified substrates.^[50]^[52]

Chemical Properties and Reactivity

Humins exhibit a range of reactive functional groups, primarily phenolic hydroxyl (OH) groups and carboxyl groups, which dictate their chemical behavior in various environments.^[53] The phenolic OH groups are particularly reactive in redox processes, serving as electron donors that facilitate oxidation to quinone moieties, thereby enabling electron transfer in biogeochemical cycles.^[54] Meanwhile, carboxyl groups promote metal chelation, forming stable complexes with ions such as chromium and arsenic through coordination bonding.^[55] The chemical stability of humins is notable for their resistance to hydrolysis under ambient conditions, owing to their highly cross-linked, polyphenolic structure that requires elevated temperatures (170–250 °C) for significant bond cleavage.^[56] However, they are vulnerable to degradation by strong oxidants, such as hydrogen peroxide (H₂O₂), which can break down the macromolecular network into smaller carboxylic acids like acetic and formic acid.^[57] A key modification reaction involves the esterification of carboxyl groups with alcohols under acid catalysis, which alters the solubility and processability of humins by converting polar COOH functionalities into less hydrophilic ester linkages. This reaction proceeds as follows:

\text{R-COOH + R'-OH} \xrightarrow{\text{acid}} \text{R-COO-R' + H}_2\text{O}

^[58] Such derivatization enhances thermal stability and enables applications in polymer blending. Differences in reactivity arise between natural soil humins and industrial humins derived from biomass processing. Soil humins, integrated into soil organic matter, are more susceptible to slow microbial degradation by aerobic and anaerobic bacteria, which target peripheral functional groups over time.^[59] In contrast, industrial humins display heightened thermal reactivity, facilitating auto-cross-linking at elevated temperatures (e.g., 120 °C) through condensation of aldehyde and hydroxyl groups, leading to denser networks suitable for thermoset materials.^[60] Electron paramagnetic resonance (EPR) spectroscopy reveals the presence of persistent free radicals in humins, often in the form of semiquinones, which contribute to their redox stability and electron mediation capabilities.^[61] These radicals correlate strongly with humin content in soil samples, underscoring their role in long-term structural integrity.^[62]

Analytical Techniques

The isolation of humins from soil typically involves sequential extraction to separate soluble humic and fulvic acids, leaving the insoluble humin fraction. The standard procedure recommended by the International Humic Substances Society (IHSS) employs alkaline extraction with 0.1 M NaOH under an inert atmosphere (e.g., N₂) at a soil-to-extractant ratio of 1:10, followed by centrifugation and acidification of the supernatant to pH 1-2 with HCl to precipitate humic acids; the residual solid is then washed and dried to yield humin.^[35] For industrial humins derived from biomass conversion processes, such as acid-catalyzed dehydration of carbohydrates, isolation occurs via filtration of the reaction mixture, followed by solvent washing (e.g., with water or ethanol) to remove residual catalysts like H₂SO₄ or HCl and soluble byproducts.^[2] Spectroscopic techniques are essential for characterizing the carbon skeleton and functional groups in humins due to their insolubility in common solvents. Solid-state ^{13}C NMR, particularly cross-polarization magic-angle spinning (CP/MAS), quantifies aromatic versus aliphatic carbon ratios, with signals in the 50-70 ppm range indicating carbohydrate-derived O-alkyl carbons and broader aromatic peaks at 110-160 ppm reflecting polyphenolic structures.^[63] Fourier-transform infrared (FTIR) spectroscopy identifies key functional groups, such as carbonyl (C=O) stretches at approximately $1700 \ \text{cm}^{-1}, alongside O-H and C-O bands in the 1000-1200 cm^{-1} region, providing insights into oxygenated moieties.^[64] Advanced analytical methods address the structural complexity of humins beyond basic spectroscopy. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) thermally degrades humins at 500-600°C, releasing volatile monomers like levoglucosan (from carbohydrate units) and phenolic compounds, which are separated and identified to infer precursor origins.^[65] X-ray photoelectron spectroscopy (XPS) probes surface elemental composition, revealing C/O/N ratios and binding energies that highlight oxygenated surface functionalities, often showing higher oxygen content (20-30 at.%) compared to bulk analysis.^[66] Recent advancements include matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), which, after solubilization in polar matrices, reveals molar mass distributions with prominent peaks between 500 and 2000 Da, indicating oligomeric fragments.^[67] The inherent heterogeneity of humins—arising from diverse precursors and diagenetic processes—necessitates multimodal approaches for comprehensive characterization, as no single technique captures their full structural variability. For soil humins, IHSS standards emphasize combined ^{13}C NMR and elemental (C/H/N/O) analysis to certify purity and composition, ensuring ash content below 5% and consistent carbon signatures.^[68]

Applications and Safety

Environmental and Agricultural Uses

Natural humins, as the stable and insoluble component of soil organic matter, are incorporated into agricultural amendments primarily through compost and organic matter additions to improve soil quality. These amendments enhance the soil's cation exchange capacity (CEC) by providing negatively charged sites for nutrient retention and increase water-holding capacity, thereby mitigating drought stress in degraded soils. Studies indicate that soils with adequate humin content support healthier plant growth and higher crop yields in degraded or low-fertility conditions.^[69]^[70]^[71] In carbon sequestration strategies, conservation practices such as no-till farming encourage the accumulation of humin, which represents the most recalcitrant fraction of soil organic carbon and contributes to long-term storage. These practices can promote humin buildup, with sequestration rates of 0.1-0.5 t C/ha/year for soil organic carbon, depending on soil type and management intensity. Humin's stability helps sustain soil carbon pools over decades, supporting climate mitigation efforts in agriculture.^[1]^[72] Humin plays a key role in environmental remediation by sorbing heavy metal contaminants through complexation with its functional groups, such as carboxyl and phenolic hydroxyl sites. For instance, humin from peat soils adsorbs Cd²⁺ effectively via ion exchange and surface complexation, achieving high removal efficiencies in contaminated systems; representative studies show up to 90% sorption under optimal conditions. Additionally, humin is incorporated into bioremediation filters to immobilize pollutants and facilitate microbial degradation in affected soils.^[73]^[74] The content of humin in soil serves as a reliable proxy for assessing overall soil health and organic matter stability in environmental monitoring programs and sustainability evaluations. Higher humin levels correlate with improved soil resilience and reduced erosion risk, aiding in the certification of sustainable land management practices.^[1] In organic farming systems, humin-rich amendments promote nutrient retention and cycling, allowing for reduced reliance on synthetic fertilizers while maintaining or enhancing productivity. This efficiency stems from humin's ability to buffer nutrient availability and minimize leaching losses.^[75]^[76]

Industrial Applications

Industrial humins, as byproducts from biomass conversion processes, have gained attention for their potential in materials science due to their furanic and aromatic structure and thermal stability. In biocomposites and bioplastics, humins serve as reinforcing fillers, typically at loadings of 5-20 wt%, enhancing mechanical properties such as tensile strength through improved interfacial bonding and reduced polymer chain mobility.^[2] For instance, humin-flax fiber composites exhibit an elastic modulus of approximately 1.5 GPa, demonstrating their viability for structural applications in automotive and construction sectors.^[2] Additionally, humin-based thermoset resins are produced via cross-linking reactions, often with epoxies or furfuryl alcohol, yielding materials with improved tensile strength and Young's moduli reaching 1.8 GPa, suitable for durable coatings and laminates.^[2]^[77] In energy and catalysis sectors, humins are carbonized to produce activated carbons with high surface areas exceeding 1000 m²/g via KOH activation, enabling their use as electrode materials in supercapacitors.^[2]^[78] These carbons also function as supports for heterogeneous catalysts, such as palladium-immobilized humin-like resins, which facilitate hydrogenation reactions like the selective reduction of nitroarenes to anilines with high yields.^[79] Such applications leverage the porous structure of humins-derived materials to improve catalyst dispersion and recyclability, reducing reliance on fossil-based supports. Other industrial uses include the production of insulating foams and adhesives. Humin-based foams, synthesized through self-blowing processes, achieve low thermal conductivities at low densities, making them competitive with polyurethane foams for building insulation while offering biodegradability.^[51] In adhesives, humins are incorporated into wood composites, such as phenol-formaldehyde resins, enhancing bond strength in particleboards.^[2] Emerging research in the 2020s has explored humin-derived biofuels through pyrolysis and hydrotreatment, yielding bio-oil rich in aromatics and phenols, with proof-of-concept studies extending to 3D printing filaments via humin-polyolefin blends for sustainable prototyping. As of 2025, ongoing advancements in biorefinery processes continue to expand humin valorization.^[2]^[80]^[2] A notable example is Avantium's YXY process, which generates humins during the production of furandicarboxylic acid (FDCA) for polyethylene furanoate (PEF) polymers; these humins are repurposed into value-added products like wood-modifying resins, contributing to reduced fossil fuel dependency in bioplastics supply chains.^[81]^[82]

Safety Considerations

Humins exhibit low acute toxicity based on regulatory assessments.^[83] Industrial variants, produced via acid-catalyzed processes, may retain residual acids, necessitating neutralization prior to handling to mitigate potential irritation risks.^[84] Handling powdered forms of humins requires precautions against dust inhalation, which could cause respiratory irritation; personal protective equipment (PPE) such as respirators, gloves, and eye protection is recommended during processing.^[84] Thermal processing releases volatile organic compounds (VOCs), but humins remain non-flammable up to 400°C, with fire risks comparable to cellulosic materials like wood.^[84] Environmentally, humins are biodegradable over decades in soil, exhibiting no bioaccumulation potential (log Kow approximately 3-4) and posing minimal risk to aquatic or terrestrial ecosystems at typical exposure levels.^[85] They are safe for soil applications at concentrations below 10% w/w, supporting sustainable agricultural integration without adverse effects on plant growth or microbial activity.^[83] Under the Globally Harmonized System (GHS), humins are classified as non-hazardous, reflecting their low toxicity profile.^[84] Industrial variants have undergone EU REACH evaluations, complying with requirements for environmental and health hazard assessments.^[83] A key advantage is that heating industrial humins produces stable foams without toxic byproducts, distinguishing them from certain biomass tars that generate harmful emissions.^[84]

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[PDF] ORIGIN, CHEMICAL COMPOSITION, AND IMPORTANCE IN NATURE
... insoluble in water, although a part of it may go into colloidal solution in ... MULDER. "Humus" and "humic acids." The term "humus" has had varied.
[16]
Humus: Origin, Chemical Composition, and Importance in Nature
... decomposed decomposition decomposition of plant determined dopplerite fertility forest soils formation of humus formed fraction fungi green manures groups ...
[17]
[PDF] 30 YEARS OF IHSS - International Humic Substances Society
Franz Karl Achard (1753 – 1821) was a German chemist and biologist. In 1786 he published a study, where he treated peat with potassium hydroxide.Missing: discovery | Show results with:discovery
[18]
Latest development in the fabrication and use of lignin-derived ...
Mar 7, 2023 · Humic substances (HS) are originated from naturally decaying biomass. The main products of HS are humic acids, fulvic acids, and humins.
[19]
Contribution of Microbial Residues Obtained from Lignin and ... - MDPI
Sep 2, 2019 · The contribution of microbial residues formed on lignin and cellulose to the formation of humus (HS) was investigated.
[20]
Lignin Degradation and Humus Formation in Alluvial Soils and ... - NIH
The contribution of different groups of microorganisms, as well as nutrients and clay content, may influence humic-substance formation in such environments.Missing: plant | Show results with:plant
[21]
(PDF) Lignin Degradation and Humus Formation in Alluvial Soils ...
Aug 6, 2025 · The contribution of lignin to the formation of humic compounds was examined in different environments of the terrestrial-aquatic interface ...
[22]
Effect of different fractions of weathered pumice in the formation of ...
Dec 1, 2011 · Polycondensation reactions between amino acids and phenols are one of the pathways for the formation of humic substances, and clay minerals ...Abstract · Isolation Of Hlas From The... · Analysis Of Hlas
[23]
Chemical composition of soil humin in an organic soil profile
A broad signal at 150 ppm for O-substituted aromatic C indicates phenolic groups or aromatic C–O–CH3, common structural elements in lignin (Angst et al., 2021; ...
[24]
Role of clay minerals on soil organic matter stabilization and ...
This research demonstrates that Maillard reactions may be a major pathway for the incorporation of new C and N into SOM and for the formation of new humic ...Missing: humin | Show results with:humin
[25]
Increased temperature enhances microbial-mediated lignin ...
Apr 1, 2025 · Our study demonstrated that the temperature increasing can increase the rate of lignin/vanillin decomposition via affecting the activity of lignin-decomposing ...
[26]
characterization of bulk soil humin and its alkaline-soluble ... - SciELO
Humin has poor solubility mainly because it is tightly bonded to inorganic soil colloids. By breaking the linkage between humin and inorganic soil colloids ...<|separator|>
[27]
(PDF) Humin: Its Composition and Importance in Soil Organic Matter
Aug 10, 2025 · Humin is the insoluble component of soil organic matter (SOM) that remains after extraction of the other components of SOM that are soluble in aqueous base.
[28]
Stability of black carbon in soils across a climatic gradient - Cheng
Jun 4, 2008 · Assuming a Q10 of 3.4 as estimated above, the half-life of BC at a site with 10°C MAT may be as high as 925 years. Due to systematic ...
[29]
https://www.nature.com/articles/srep11214
[30]
https://www.tandfonline.com/doi/full/10.4155/cmt.13.77
[31]
https://ui.adsabs.harvard.edu/abs/2021EGUGA..23.8456J/abstract
[32]
Selective stabilization of aliphatic organic carbon by iron oxide
Jun 10, 2015 · However, ferric minerals are subject to reduction, potentially releasing iron and decreasing the stability of iron-bound organic carbon.
[33]
[PDF] INTERPRETING SOIL ORGANIC MATTER TESTS - AESL
Aug 1, 1995 · Soil humus makes up about. 60-80% of SOM. The rest is less stable and ... (100+ years); and (3) Humin which is high molecular weight ...
[34]
Global soil carbon: understanding and managing the largest ...
Apr 10, 2014 · Global soil organic carbon stocks. ▫ Global soil organic carbon (SOC) stocks estimated to be about 1500 PgC. ▫ Considerable uncertainty ...Missing: humin | Show results with:humin
[35]
Selected properties of the humin fraction isolated from Chernozems ...
... soil. Humin from this soil indicated the lowest content of carbon ( ... elemental composition of humic substances. Org. Geochem, 17(5), 635-648 ...
[36]
[PDF] Cation Exchange Capacity (CEC)
The capacity of the soil to hold on to these cations called the cation exchange capacity (CEC). These cations are held by the negatively charged clay and ...Missing: humin | Show results with:humin
[37]
[PDF] Amending subsoil with composted poultry litter
The soil cation exchange capacity (CEC) was increased by 186% in sodded ... rates of compost (CEC of organic matter = 100-300 cmolc kg-1, Evangelou, 1998) and the.Missing: humin | Show results with:humin
[38]
Effects of Humic Substances and Polysaccharides on Soil Aggregation
It plays a prominent role in sustaining soil fertility, and hence ... It also improves soil structure and its water holding capacity with minimum leaching.<|control11|><|separator|>
[39]
The Interaction of Pesticides with Humin Fractions and Their ... - MDPI
Oct 18, 2023 · The aim of this research was to assess the chemical behavior of pesticides (flufenacet, pendimethalin, α-cypermethrin, metazachlor, acetamiprid) toward stable ...
[40]
Roles of different humin and heavy-metal resistant bacteria from ...
Composting humin with higher humification had better sorption of heavy metals. · Heavy metal resistant bacteria in composting could bind more metal ions than ...
[41]
Optimized isolation method of humin fraction from mineral soil material
The whole isolation of humin takes about 9–10 weeks, depending on the SOM content. The extraction time of HA and FA clearly depended on the organic matter ...
[42]
https://pubs.acs.org/doi/10.1021/acsami.3c04613
[43]
A microwave-assisted process for the in-situ production of 5 ...
Jun 20, 2018 · In this study, a biphasic reaction system was developed for the microwave-assisted production of 5-hydroxymethylfurfural (HMF) and furfural.Missing: variations | Show results with:variations
[44]
Occurrence, Formation from d-Fructose and 3-Deoxyglucosone, and ...
Jun 3, 2021 · The mechanism of formation of βCs 1–3 occurs from the sugar intermediate 3-deoxyglucosone that reacts with tryptophan affording these carbohydrate-derived βCs.
[45]
Enhancements in the production of 5-HMF from glucose: A review ...
This process includes the following: (1) the conversion of glucose to 5-HMF through dehydration, (2) the direct decomposition of glucose into formic acid and ...Review Article · 2.4. Biphasic Solvents · 2.5. Mixed Solvents
[46]
Mechanistic Investigation into the Formation of Humins in Acid ...
Nov 28, 2022 · Elemental analyses indicate that humins are formed through a dehydration pathway, with humin formation and levulinic acid yields strongly ...
[47]
Molecular structure, morphology and growth mechanisms and rates ...
Nov 17, 2015 · Initially soluble oligomeric humins form via aldol condensation and/or etherification reactions in which HMF reacts with DHH-like intermediates ...
[48]
Advances in understanding the humins: Formation , prevention and ...
Humins is defined as byproducts formed in catalytic conversion of cellulose and its catalyzed derivatives (including glucose, fructose, HMF, LA, etc.) below ...
[49]
Humin Formation on SBA-15-pr-SO 3 H Catalysts during the ...
May 15, 2023 · Elemental analyses indicate that humins are formed through a dehydration pathway, with humin formation and levulinic acid yields strongly ...
[50]
Influence of the pH Value on the Hydrothermal Degradation of ...
Aug 26, 2019 · It is demonstrated that at lower pH values mainly 5-hydroxymethylfurfural (HMF), levulinic acid and humin are generated, while lactic acid and ...
[51]
Molecular structure, morphology and growth mechanisms and rates ...
Molecular structure, morphology and growth mechanisms and rates of 5-hydroxymethyl furfural (HMF) derived humins† · Supplementary files · Article information.Missing: trimerization branched furan
[52]
Quantification and Classification of Carbonyls in Industrial Humins and Lignins by 19F NMR
### Quantification of Carbonyls and Functional Groups in Industrial Humins and Lignins
[53]
https://www.sciencedirect.com/science/article/abs/pii/S0048969718340488
[54]
Molecular structure and composition elucidation of an industrial ...
Humins, (side-)products of the acid-catalysed dehydration of carbohydrates, will be produced in substantial quantities with the development of industrial ...
[55]
Humins as promising material for producing sustainable ...
May 15, 2017 · The chemical structure of humins consists of furfural and hydroxymethylfurfural moieties also with carbohydrate, levulinate and ...
[56]
[PDF] Building new tomorrows for humins as biorefinery by product ...
Jun 25, 2021 · ... glass transition temperature (Tg) in thermal treated humins, which ... surface functionality, porosity, and thermal and chemical stability.
[57]
[PDF] Valorisation of biorefinery-derived humins: towards the development ...
May 12, 2020 · The production of chemicals via acid-catalysed dehydration of sugars is inevitably related to the formation of co-products.Missing: byproducts | Show results with:byproducts
[58]
Investigating the properties of humins foams, the porous ...
The bulk density of the foams increases with the carbonization temperature, from 0.055 g cm−3 for F250, to 0.070 g cm−3 for F2500, and 0.092 g cm−3 for F2900 ( ...Missing: cm³ | Show results with:cm³
[59]
Molecular structure-reactivity correlations of humic acid and humin ...
Feb 15, 2019 · It has been reported that carboxyl, phenol and hydroxyl groups might act as the main electron donors for Cr(VI) reduction by HA (Hsu et al., ...
[60]
Redox Characteristics of Humins and Their Coupling With Potential ...
Furthermore, quinones are likely to be the functional groups that shuttle electrons during PCB dechlorination. ... Oxidation-Reduction; Polychlorinated ...
[61]
Arsenite adsorption and oxidation affected by soil humin
Apr 15, 2024 · Phenolic hydroxyl and carboxyl groups in humin participate in the As complexation. Phenolic hydroxyl groups and EPFRs of humin play main roles ...<|separator|>
[62]
[PDF] Full, Reactive Solubilization of Humin Byproducts by Alkaline ...
Jan 30, 2015 · 17−20 For example, the solubility of humic acid and fulvic acid fractions from SOM is improved by hydrolysis of C−O bonds at 170−250 °C ...<|separator|>
[63]
Selective catalytic oxidation of humins to carboxylic acids using the ...
Mar 5, 2024 · The development of an optimized reaction system for the selective catalytic oxidation (SCO) of water-insoluble and highly complex humins to short-chain ...Missing: pKa | Show results with:pKa
[64]
(PDF) Evaluation of pKa Values of Soil Humic Acids and their ...
Aug 7, 2025 · ... pKa values (3.1 and 7.5) for humic acid from the 15 cm depth soil have been found. These acids contain carboxylic and/or hydroxylic acid groups.
[65]
Upcycling Humins via Esterification Reactions of Hydroxyl Groups ...
May 27, 2024 · As for the functional groups, the main motifs were found to be based on aliphatic and conjugated hydroxyls, aldehydes, and ketones (Figure 1).Missing: percentage | Show results with:percentage
[66]
Humic substances, their microbial interactions and effects on ...
There is evidence that they can be degraded not only by aerobic but also by anaerobic bacteria. HS presence alters biological transformations of organic ...
[67]
[PDF] Humins from biorefineries as thermo-reactive macromolecular systems
May 25, 2020 · The structure and composition of humins is very diverse and depends on the thermochemical conditions. Upon sufficient heating, humins will ...Missing: variability | Show results with:variability
[68]
(PDF) Studies of semiquinone free radicals by ESR in the whole soil ...
Aug 6, 2025 · The results showed that the amount and line width of semiquinone free radical from whole soil samples had good correlation with humin fraction ...
[69]
[PDF] Studies of Semiquinone Free Radicals by ESR in the Whole Soil, HA ...
The amount of semiquinone free radical of the whole soil samples had direct correlation to humin fraction but no correlation to humic and fulvic acids (Figure 3) ...
[70]
Isolation of IHSS Soil Fulvic and Humic Acids
Neutralize the soil residue with 1 M NaOH to pH = 7 then add 0.1 M NaOH under an atmosphere of N2 to give a final extractant to soil ratio of 10:1. Extract the ...
[71]
13C NMR Estimates of Carbon Distribution | IHSS
The peak areas from which these percentages are calculated were obtained from solid-state CPMAS 13C NMR spectra which were measured by Dr. Patrick Hatcher ...Missing: humins ratios 50-70<|control11|><|separator|>
[72]
Quantitative Fourier Transform Infrared spectroscopic investigation ...
The technique provides compositional information on the functional groups present in humic substances. The FTIR technique produces information similar to 13C ...
[73]
[PDF] Journal of Analytical and Applied Pyrolysis - Radboud Repository
Jul 16, 2013 · To gain further insights in the pyrolytic behaviour of humins and product formation, analytical pyrolysis GC–MS/FID of the two humin samples was ...
[74]
XPS and two-dimensional FTIR correlation analysis on the binding ...
Jul 1, 2020 · XPS and two-dimensional FTIR correlation analysis on the binding characteristics of humic acid onto kaolinite surface. Author links open ...
[75]
Investigation of the Formation, Characterization, and Oxidative ... - NIH
Apr 4, 2023 · However, the efficiency of these processes was reduced by the formation of insoluble solid waste products called humins. Herein, the formation ...
[76]
Elemental Compositions and Stable Isotopic Ratios of IHSS Samples
Ash is the %(w/w) of inorganic residue in a dry sample. C, H, O, N, S, and P are the elemental composition in %(w/w) of a dry, ash-free sample.Missing: humin certification
[77]
Humic Substance - an overview | ScienceDirect Topics
Humic substances are defined as significant organic materials in soil that are characterized by their ubiquity, stability, and high molecular weight.
[78]
Changes in Soil Humin Macromolecular Structure Resulting from ...
The humification of organic matter is usually associated with a variation in the elemental composition of the molecules of humic substances, which is expressed ...
[79]
The Value of Humic Substances in the Carbon Lifecycle of Crops
Jun 11, 2019 · Humins are very large molecules (molecular weight of 100,000 to 10,000,000 Da) that are not soluble in water at any pH level and are, ...Missing: insolubility | Show results with:insolubility
[80]
Carbon Sequestration in Soil Humic Substances Under Long-Term ...
Aug 6, 2025 · Even though majority of the organic materials are transformed into humin in soils amended with various type of organic materials [23,34, 35] , ...Missing: remediation | Show results with:remediation
[81]
Characteristics of Adsorption Interactions of Cadmium(II) onto Humin ...
Aug 6, 2025 · The present study examined Cd solubility in pH 2–12 fresh and seawater media with and without humin to determine Cd species composition.<|control11|><|separator|>
[82]
Heavy metal pollution migration and its ecological impact on ...
Apr 28, 2025 · Studies suggest that higher concentrations of humic acid and humin in soil are associated with lower concentrations of certain heavy metals.
[83]
Understanding the Role of Humic Acids on Crop Performance and ...
Mar 1, 2022 · Soil pH. Soil pH influences nutrient availability. The ability of HA to affect soil pH changes depends on the amount of carboxylic and ...
[84]
Frequently asked questions about organic humic acid and fulvic acid ...
Humic acid has generated yield gains between 5% – 20% across crops (e.g., wheat, alfalfa, soybeans, potatoes, sugar beets, cotton, corn) based on the global A ...
[85]
A Structural Elucidation of Humins and Triglycidyl Ether of ...
Nov 1, 2019 · Each humin–TGPh formulation contains a ratio of 5% by the weight of the catalyst: BDMA, the accelerator, and (BDMA + accelerator). In the ...
[86]
Microporous Humins Prepared from Sugars and Bio-Based ...
Biomass conversion processes often form solid byproducts known as humins. This work shows that humins can be highly porous and useful toward applications.
[87]
Humins‐Like Solid Support for Palladium Immobilization: Highly ...
May 18, 2022 · A new bio-based resin, made from DHMF and maleic anhydride, is used as a solid support for palladium immobilization, and is effective and ...
[88]
Valorization of humin type byproducts from pyrolytic sugar ...
The obtained humin sample is soluble in organic solvents (dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and isopropanol (IPA)), in contrast to typical more ...Missing: pyridine | Show results with:pyridine
[89]
Avantium's YXY® process for PEF production generates two major ...
Avantium's YXY® process for PEF production generates two major side products: methyl levulinate and humins.
[90]
Avantium grants Kebony rights to use its patented technologies on ...
Sep 23, 2020 · Avantium Renewable Polymers has granted leading wood modification company Kebony the right to use its patented technologies related to humins.Missing: repurposed | Show results with:repurposed
[91]
https://doi.org/10.1002/bbb.1964
[92]
https://doi.org/10.1021/acssuschemeng.8b03971
[93]
Researchers at the Fraunhofer WKI develop humin-based binders ...
Nov 14, 2024 · Humins are a promising component for increasing the proportion of renewable raw materials in binders for wood-based materials.