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Optimal isotherm model and explanatory characteristics associated with metal ion adsorption on humic acids isolated from forest soils

  • Soils, Sec 1 • Soil Organic Matter Dynamics and Nutrient Cycling • Research Article
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Understanding the mechanisms of interaction between humic acids (HAs) and metal ions in soil media is essential for integrated environmental and agricultural management. This study aims to select the best isotherm model and explore the correlation between physicochemical properties of humic acids isolated from forest soils and metal ion sorption parameters.

Materials and methods

The HA fractions of ten forest soils were initially isolated and well characterized. Afterward, the sorption of four metal ions (Cu, Fe, Mn, and Zn) on the isolated HAs was investigated. The experimental data was initially fit to the three isotherm models to find the optimal model. Subsequently, the correlation between physicochemical HA properties and metal ion sorption parameters was explored using correlation and clustering analysis.

Results and discussion

Results showed that despite employing identical extraction procedures, isolated HAs from soils differed statistically in their descriptive characteristics. The adsorption data generally fit the Freundlich isotherm model best. However, isotherm model performance was affected by both metal ion and HA types. Six descriptive HA attributes including H/C and O/C molar ratio, the internal oxidation degree (ω), carboxylic acidity, E4/E6 ratio, and the Δlog-K were closely associated with sorption parameters.


Since the performance of isotherm models was affected by both metal ion and HA type, the isotherm model should be selected with regard to target metal ions and HA type. Highly correlated variables with sorption parameters of HA-metal ions screened in this study could be reliably used to develop sorption predictive models for HA-metal ions in soil and aquifers.

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The electronic supplementary material can be seen in the Supplementary information of this article.



Analysis of variance


Carboxylic acidity


Coefficient of variation


Fulvic acids


Humic acid

K1 :

Temkin isotherm constant relevant to the maximum binding energy

K2 :

Temkin isotherm constant relevant to the heat of adsorption

KF :

Freundlich isotherm constant relevant to adsorption capacity

KL :

Langmuir equilibrium constant


A constant indicative of the intensity of adsorption in Freundlich isotherm model


Phenolic acidity

qm :

Langmuir monolayer sorption capacity


Pearson correlation coefficient

R2 :

Coefficient of determination


Total acidity

η2 :

Partial eta squared


Cohen’s kappa statistic


Internal oxidation


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This research was supported and financed by the post-graduate office of the University of Guilan.

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Authors and Affiliations



Mahmood Fazeli, Sangani, Conceptualization, methodology, software, validation, data curation, writing (original draft), writing (review and editing), visualization, project administration. Akbar Forghani, Conceptualization, methodology, validation, supervision, funding acquisition. Patrycja Boguta, Validation, investigation, data curation, writing–review and editing. Marjan Anoosha, Conceptualization, methodology, lab analysis, investigation, resources, data curation. Gary Owens, Validation, investigation, resources, data curation, writing–review and editing.

Corresponding author

Correspondence to Mahmood Fazeli Sangani.

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Conflict of interest

The authors declare no competing interests.

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Responsible editor: Anja Miltner

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Supplementary Information

Below is the link to the electronic supplementary material.

Supplementary file1 (DOCX 128 KB)

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Fazeli Sangani, M., Forghani, A., Boguta, P. et al. Optimal isotherm model and explanatory characteristics associated with metal ion adsorption on humic acids isolated from forest soils. J Soils Sediments 22, 2392–2405 (2022).

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