The effect of leonardite on the adsorption and desorption characteristics of boron in calcareous soils

Document Type : Research Paper

Authors

• Reyhaneh Abbasi Esakan ¹
• Behnam Dovlati ²
• Ebrahim Sepehr ²
• Marziyeh Piri ²

¹ . Department of Soil Science, Faculty of Agriculture, University of Urmia, Urmia

² Department of Soil Science, Faculty of Agriculture, Urmia University, Urmia, Iran

Abstract

Keywords

• Boron
• Leonardite
• Adsorption isotherm
• Calcareous soil
• Desorption

Main Subjects

• ..

EXTENDED ABSTRACT

Introduction

The adsorption and desorption processes play a crucial role in the solubility of boron (B) in soil solutions. Boron toxicity is more pronounced in dry, semi-dry, and heavy soils. The surface adsorption process is one of the most significant chemical reactions of elements in the soil environment, so much so that the interaction of elements and various components of soils has attracted the attention of many researchers today. Water-soluble boron is easily absorbable by plants, while surface-adsorbed boron is considered a relatively long-term source of boron in the soil. Surface-adsorbed boron in the soil can be regarded as a factor that controls the toxicity and deficiency threshold of boron in the soil. The processes of adsorption and desorption of elements depend on a series of physical and chemical factors of the soil, including pH, concentration of soluble boron, soil texture, type of clay minerals, amount and type of organic matter, type of exchangeable ions, and also the amount of lime in the soil. A type of weathered lignite known as leonardite, which is a type of coal formed from the oxidation of lignin, contains large amounts of humic substances, carbon, and functional groups (carboxyl, hydroxyl, phenolic, and carbonyl) and is very effective in increasing the availability of nutrients for plants and the absorption of heavy metals. Leonardite is also an adsorbent for metals due to its high porosity and nanoscale particle size. Regarding the role of organic matter in the soil on the surface adsorption and desorption of boron, some researchers have reported that increasing organic matter in the soil leads to an increase in the surface adsorption of boron.

Methods

In this study, the effect of leonardite at levels of 0, 2, and 4 percent on the adsorption and desorption isotherms of boron in soils was investigated. 2.5 grams of sieved soil were mixed with different amounts of leonardite and 25 milliliters of boron solution at specified concentrations (0, 5, 10, 15, 20, 40, 60, 90, and 120 milligrams per liter) and shaken for 24 hours to reach equilibrium. The samples were then centrifuged, and the boron concentration in the supernatant was measured using the azomethine-H colorimetric method. The data were fitted using the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models with Sigma Plot 14.1 software.

Results and Discussion

The results showed that the application of 4 percent leonardite in soil 1, the q_max​ value decreased from 1268 to 867 milligrams per kilogram, representing the highest reduction, while in soil 4, the q_max​ decreased from 1275 to 983 milligrams per kilogram, indicating the lowest reduction. These differences may be attributed to the distinct physicochemical properties of the soils, including the amount of equivalent calcium carbonate, initial boron concentration, and cation exchange capacity. The amount of free energy of adsorption (E) in the presence of leonardite decreased in all four studied soils, indicating that boron adsorption follows a physical process. This is because if the value of E is between 8 to 16 kJ/mol, it can be inferred that the adsorption follows a chemical process. If the value of E is less than 8 kJ/mol, physical adsorption is considered the dominant process. Overall, increasing the amount of leonardite leads to an increase in q_max desorption, with the highest sorption observed in soil 2 and the lowest in soil 1.

Conclusions

Different leonardite treatments decreased boron adsorption and increased desorption compared to the control treatment. The effect of 4% leonardite on reducing the amount of adsorption was greater than that of 2%.

Conflict of interest

The authors declare no conflict of interest

Author Contributions

 “Conceptualization, Behnam Dovlati, Ebrahim Sepehr and Marziyeh Piri; methodology, Behnam Dovlati and Ebrahim Sepehr; software, Behnam Dovlati; validation, Behnam Dovlati, Ebrahim Sepehr; formal analysis, Behnam Dovlati; investigation, Reyhaneh Abbasi Esakan.; resources, Reyhaneh Abbasi Esakan and Marziyeh Piri; data curation, Behnam Dovlati and Ebrahim Sepehr; writing—original draft preparation, Marziyeh Piri and Reyhaneh Abbasi Esakan; writing—review and editing, Marziyeh Piri and Reyhaneh Abbasi Esakan; funding acquisition,  Urmia university. All authors have read and agreed to the published version of the manuscript.” All authors contributed equally to the conceptualization of the article and writing of the original and subsequent drafts.

Data Availability Statement

 “Not applicable”.

Acknowledgements

The authors would like to thank the Urmia University for the financial support for this research project and for equipping the soil chemistry laboratory. The authors would like to thank all participants of the present study.

Ethical considerations

The study was approved by the Ethics Committee of the Urmia University for Master's thesis. The authors avoided data fabrication, falsification, plagiarism, and misconduct.

Conflict of interest

The author declares no conflict of interest.