Formation of Humic-Like Substances during the Technological Process of Lignohumate® Synthesis as a Function of Time

Olga Yakimenko,Andrey Stepanov,Olesya Osipova,Daria Khundzhua,Svetlana Patsaeva,Oleg Gladkov

doi:10.3390/separations8070096

Abstract

The composition, structure, and biological activity of humic-like substances (HLS) synthesized in the process of lignosulfonate conversion for the production of the humic product Lignohumate® (LH) were examined. It is shown that during the hydrolytic-oxidative process, the transformation of raw material and accumulation of HLS occur. Data on the chemical (elemental content, functional groups, FTIR) and spectral (absorbance and fluorescence) parameters and biological activity (in phytotest) combined with PCA show that the LH samples can be divided into three groups, depending on the duration of synthesis: initial raw material (0-time sample); “young” HLS (15–30 min), and “mature” HLS in 45–120 min of treatment. During the first 30 min, reactions similar to the ones that occur during lignin humification in nature take place: depolymerization, oxidative carboxylation, and further polycondensation with the formation and accumulation of HLS. After 45–60 min, the share of HLS reaches a maximum, and its composition stabilizes. Biological activity reaches a maximum after 45–60 min of treatment, and at that stage, the further synthesis process can be stopped. Further processing (up to 2 h and more) does not provide any added value to the humic product.

Highlights

Humic–based amendments are increasingly used in agriculture [1,2,3,4,5,6] and are being currently considered as a major category of biostimulants [7,8]
Data on the chemical and spectral parameters and biological activity combined with Principal Component Analysis (PCA) show that the LH samples can be divided into three groups, depending on the duration of synthesis: initial raw material (0-time sample); “young” humic-like substances (HLS) (15–30 min), and “mature” HLS in 45–120 min of treatment
We focused on the possibility of changing the composition of the synthesized humic-like biologically active formulations of LH, based on changing only one of the technological parameters of the process: the duration of synthesis

Summary

Introduction

Humic–based amendments (humic product, HP) are increasingly used in agriculture [1,2,3,4,5,6] and are being currently considered as a major category of biostimulants [7,8]. HPs are manufactured by industrial companies from various humic resources (mostly lignite, peat, and composts), as well as lignocellulose wastes [9,10,11,12]. The chemical composition and effectiveness of HPs as plant growth stimulators vary depending on the organic matter source, extraction processes (KOH extraction for lignite, cavitation for peat, wood bisulfite extraction for lignosulfonates, etc.), and modification technologies used to obtain the products [13,14,15]. Lignosulfonate (LS), the wood-processing byproduct, is one of the possible source organic materials for HP production. By all means, LS is a valuable organic raw material, sharing similar properties with humic substances in terms of an aromatic core with carboxylic and phenolic functional groups. The appropriate technological treatment of LS can result in the production of qualitative HP and contribute to the important environmental task of converting lignin-containing organic wastes [23,24,25]

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