This study aimed to develop agro-based engineered biochars for adsorbing low concentration nitrate with economic and effective characteristics. Two kinds of wheat or corn-straw based biochars were prepared at different pyrolysis temperatures using the acid/iron modification method. The performance of adsorption and desorption, mechanism (adsorption model, biochar morphology, zeta potential), influencing factors (pH, co-existence of anions and organic matter) and stability were comprehensively evaluated. The results showed that the modification by HCl impregnation for corn straw-based biochars (CH500) or HCl-FeCl3 for wheat-straw based biochar pyrolyzed at 500 °C (WFe500) could reach a NO3− removal efficiency of 85–91% within 24 - 48 h at a low concentration of 20 mg N L−1. The maximum adsorption capacity (Qm) from Langmuir model for the CH500 (1958 mg kg−1) was 20% greater than that of WFe500 (1651 mg kg −1). After the kinetic adsorption reached the equilibrium, there was no clear desorption by DI water. The pH had no significant effect on the adsorption, and the adsorption efficiency of NO3− could be kept at ~ 60% under the co-existence of anions as well. After acid or iron modification, polarity index of H/C further decreased, and the index of (O + N)/C or O/C further increased. The zeta potential (ζ), the BET surface area and pore volume of CH500 and WFe500 also increased. It was suggested that the modification increased the positive charge loads of H+ and Fe3+ on the biochar surface, which enhanced polarity and electrostatic attractions and led to increased effective contacts between the biochar and low concentration NO3−. The NO3− accumulated at the charge sites on the surface and inside the biochar via electrostatic attraction could replace other anion previously fixed on the biochar (such as Cl− loaded by modification) via ion exchange. The increased volumes/areas of internal pores by the modification were crucial for adsorption and retention of NO3− ions after the effective contact. The modification method, pollutant characteristics and biochar feed stock characteristics should be considered together as primary influencing factors for preparing high performance biochar. Graphical Abstract

Development of Agro-Based Engineered Biochars for Low Concentration Nitrate Adsorption: Modification Selection, Performance, Factors, Mechanism, and Reliability

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发表日期：

2025年4月21日

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Wenjia Han,
Ai Chen,
Zhuocan Li,
Rong Zhou,
Kai Zhang,
Wei Zhao,
Xiaotao Bi,
Xiali Zhang

摘要

This study aimed to develop agro-based engineered biochars for adsorbing low concentration nitrate with economic and effective characteristics. Two kinds of wheat or corn-straw based biochars were prepared at different pyrolysis temperatures using the acid/iron modification method. The performance of adsorption and desorption, mechanism (adsorption model, biochar morphology, zeta potential), influencing factors (pH, co-existence of anions and organic matter) and stability were comprehensively evaluated. The results showed that the modification by HCl impregnation for corn straw-based biochars (CH500) or HCl-FeCl₃ for wheat-straw based biochar pyrolyzed at 500 °C (WFe500) could reach a NO₃⁻ removal efficiency of 85–91% within 24 - 48 h at a low concentration of 20 mg N L⁻¹. The maximum adsorption capacity (Q_m) from Langmuir model for the CH500 (1958 mg kg⁻¹) was 20% greater than that of WFe500 (1651 mg kg ⁻¹). After the kinetic adsorption reached the equilibrium, there was no clear desorption by DI water. The pH had no significant effect on the adsorption, and the adsorption efficiency of NO₃⁻ could be kept at ~ 60% under the co-existence of anions as well. After acid or iron modification, polarity index of H/C further decreased, and the index of (O + N)/C or O/C further increased. The zeta potential (ζ), the BET surface area and pore volume of CH500 and WFe500 also increased. It was suggested that the modification increased the positive charge loads of H⁺ and Fe³⁺ on the biochar surface, which enhanced polarity and electrostatic attractions and led to increased effective contacts between the biochar and low concentration NO₃⁻. The NO₃⁻ accumulated at the charge sites on the surface and inside the biochar via electrostatic attraction could replace other anion previously fixed on the biochar (such as Cl⁻ loaded by modification) via ion exchange. The increased volumes/areas of internal pores by the modification were crucial for adsorption and retention of NO₃⁻ ions after the effective contact. The modification method, pollutant characteristics and biochar feed stock characteristics should be considered together as primary influencing factors for preparing high performance biochar.

Graphical Abstract