Optimized isolation method of humin fraction from mineral soil material

Jerzy Weber,Maria Jerzykiewicz,Irmina Ćwieląg-Piasecka,Elżbieta Jamroz,Lilla Mielnik,Jakub Bekier,Andrzej Kocowicz,Magdalena Debicka,Aleksandra Ukalska-Jaruga,Romualda Bejger

doi:10.1007/s10653-021-01037-3

Abstract

Humic substances, including humin fraction, play a key role in the fate of organic and inorganic xenobiotics contaminating the environment. Humin is an important fraction of humic substances, which has been the least studied to date. This is due to the difficulties connected with its isolation that pose a number of methodological problems. Methods of humin fraction isolation can be divided into following main groups: (1) digestion of mineral soil components with HF/HCl followed by alkali extraction of HA and FA; (2) alkali extraction of HA and FA followed by extraction of humin by different organic solvents; and (3) alkali extraction of HA and FA followed by HF/HCl digestion of mineral soil components. Nevertheless, each of these methods has different limitations. We described in detail a useful procedure of humin isolation, in which this fraction was not extracted, but isolated from the soil by removing its soluble organic and mineral components. A modified method of HA and FA extraction with 0.1 M NaOH, according to the International Humic Substances Society, was used in the first step. Then, the mineral components in the residue were digested with the 10% HF/HCl. Unlike the procedures oriented to increase the concentration of organic matter, samples were treated several times with the HF/HCl mixture until the mineral fraction was almost completely digested. The main assumption of the method modification was to obtain the highest yield with the lowest possible ash content, but without affecting humin chemical structure. The results showed that the proposed procedure is characterized by a high efficiency and recovery and, therefore, it can be used to isolate high amounts of humin from soil.

Highlights

Contaminants entering the soil can be accumulated in the upper soil horizon—from where they can be taken up by plants—or they can be moved to deeper genetic horizons as well as to groundwater
The results showed that the proposed procedure is characterized by a high efficiency and recovery and, it can be used to isolate high amounts of humin from soil
Their fate depends on the soil properties, especially the quantity and quality of soil organic matter (SOM), a complex mixture of chemically and physically nonhomogeneous organic compounds derived from decomposition of plant and animal remnants as well as macromolecular colloidal products of transformation of these constituents

Summary

Introduction

Contaminants entering the soil can be accumulated in the upper soil horizon—from where they can be taken up by plants—or they can be moved to deeper genetic horizons as well as to groundwater. Their fate depends on the soil properties, especially the quantity and quality of soil organic matter (SOM), a complex mixture of chemically and physically nonhomogeneous organic compounds derived from decomposition of plant and animal remnants as well as macromolecular colloidal products of transformation of these constituents. The quantity and properties of SOM, among which HS play a key role in determining xenobiotics fate, are an important element of any study on soil contamination (Cwielag-Piasecka et al, 2018; Loffredo & Senesi, 2006).

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