One-Pot Transformation of Technical Lignins into Humic-Like Plant Stimulants through Fenton-Based Advanced Oxidation: Accelerating Natural Fungus-Driven Humification

Abstract

Commercial humic acids mainly obtained from leonardite are in increasing demand in agronomy, and their market size is growing rapidly because these materials act as soil conditioners and direct stimulators of plant growth and development. In nature, fungus-driven nonspecific oxidations are believed to be a key to catabolizing recalcitrant plant lignins, resulting in lignin humification. Here we demonstrated the effective transformation of technical lignins derived from the Kraft processing of woody biomass into humic-like plant fertilizers through one-pot Fenton oxidations (i.e., artificially accelerated fungus reactions). The lignin variants resulting from the Fenton reaction, and manufactured using a few different ratios of FeSO4 to H2O2, successfully accelerated the germination of Arabidopsis thaliana seeds and increased the tolerance of this plant to NaCl-induced abiotic stress; moreover, the extent of the stimulation of the growth of this plant by these manufactured lignin variants was comparable or superior to that induced by commercial humic acids. The results of high-resolution (15 T) Fourier transform-ion cyclotron resonance mass spectrometry, electrostatic force microscopy, Fourier transform-infrared spectroscopy, and elemental analyses strongly indicated that oxygen-based functional groups were incorporated into the lignins. Moreover, analyses of the total phenolic contents of the lignins and their sedimentation kinetics in water media together with scanning electron microscopy- and Brunauer–Emmett–Teller-based surface characterizations further suggested that polymer fragmentation followed by modification of the phenolic groups on the lignin surfaces was crucial for the humic-like activity of the lignins. A high similarity between the lignin variants and commercial humic acids also resulted from autonomous deposition of iron species into lignin particles during the Fenton oxidation, although their short-term effects of plant stimulations were maintained whether the iron species were present or absent. Finally, we showed that lignins produced from an industrial-scale acid-induced hydrolysis of wood chips were transformed with the similar enhancements of the plant effects, indicating that our fungus-mimicking processes could be a universal way for achieving effective lignin humification.

Sorption behavior of malachite green onto pristine lignin to evaluate the possibility as a dye adsorbent by lignin

“…Some researchers have found that materials derived from the chemical modification of lignin can be utilized as raw materials for making plastics such as polyurethanes and polyesters in the manufacture of plastics [9,10], as well as raw materials for phenolic resins, epoxy resins and carbon fiber products. In terms of agriculture, lignin has a similar structure to humic acid, so that it is possible to utilize humified lignin as an agricultural fertilizer through chemical and biological modification processes [11]. However, lignin modification involves some technically demanding approaches and significant labor input, as well as coming with high costs.…”

Which Traits of Humic Substances Are Investigated to Improve Their Agronomical Value?

“…Moreover, natural or semi-synthesized HS analogs showing superior plant stimulatory actions than natural HSs could be suggested. In fact, plant lignin and hydrochar were found to be valuable materials for obtaining humic-like substances capable of stimulating plants [ 20 , 39 ]. Composting using vegetable, plant residues and animal manures was also proven to lead to the production of HSs [ 40 ].…”

“…Many research studies have investigated soil-derived, vermicomposted, or plant-derived HSs to evaluate HS structure–function relationships for plant stimulations [ 20 , 24 , 25 , 26 , 27 , 28 , 39 ]. However, in reality, it is difficult to use soil-extracted HSs for large-scale applications because the extractable HS quantity is relatively small and limited.…”

Quantitative Structure-Activity Relationship of Humic-Like Biostimulants Derived From Agro-Industrial Byproducts and Energy Crops

“…Additionally, longer alkaline hydrolysis times may be adopted to reach a more extensive lignin depolymerization in order to increase the concentration of free phenolic units in HLS (Guizani and Lachenal, 2017), although a selective oxidation of lignin is difficult to achieve. In order to overcome undesired reactions and control the HLS oxidation, the implementation of specific oxidative strategies are required (Cha et al, 2017;Guo et al, 2018;Jeong et al, 2018). Finally, a removal of alkyl moieties from HLS may be envisaged to reduce the amount of such inhibitory compounds, for example by extracting them from the lignocellulosic matrix.…”