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Relating Cd2+ binding by humic acids to molecular weight: A modeling and spectroscopic study
Hongcheng Bai1, Zhenmao Jiang1, Minjin He1
1College of Resources and Environment, Department of Environment Science and Engineering, Southwest University, Chongqing 400715, China; Chongqing Key Laboratory of Agricultural Resources and Environment, Chongqing 400715, China; China Key Laboratory of the Three Gorges Reservoir Region's Eco-Environment, Ministry of Education, Chongqing 400715, China.
Journal of Environmental Sciences (China)
|July 25, 2018
Summary
Molecular weight fractions of humic acids (HAs) influence heavy metal binding. Lower molecular weight fractions with more phenolic groups show higher cadmium binding capacity, impacting environmental mobility.
Area of Science:
- Environmental Chemistry
- Soil Science
- Geochemistry
Background:
- Molecular weight (Mw) of humic acids (HAs) is crucial for understanding heavy metal mobility and speciation in the environment.
- Soil HAs play a significant role in complexing and sequestering environmental contaminants like cadmium (Cd2+).
Purpose of the Study:
- To characterize different molecular weight fractions of soil humic acids (HAs).
- To elucidate the binding characteristics of cadmium (Cd2+) with individual HA Mw fractions using advanced modeling and spectroscopic techniques.
- To understand the influence of HA Mw and functional groups on heavy metal binding.
Main Methods:
- Ultrafiltration of soil humic acid (HA) into four molecular weight (Mw) fractions (<5 kDa, 5-10 kDa, 10-30 kDa, >30 kDa).
- Characterization of HA fractions using elemental analysis and Fourier-transform infrared (FT-IR) spectroscopy.
- Investigation of cadmium (Cd2+) complexation using titration experiments, Langmuir and non-ideal competitive adsorption-Donnan (NICA-Donnan) models, and fluorescence excitation-emission matrix (EEM) quenching.
Main Results:
- Acidic functional group concentration, particularly phenolic groups, decreased with increasing HA Mw.
- Humification degree and aliphaticity increased with increasing HA Mw.
- Cadmium (Cd2+) binding capacity to HA fractions followed the order: UF1 (<5 kDa) > UF2 (5-10 kDa) > UF4 (>30 kDa) > UF3 (10-30 kDa), with phenolic groups being key.
- Binding capacity was governed by functional group content, while binding affinity was influenced by structural factors.
Conclusions:
- Different molecular weight fractions of humic acids exhibit distinct binding capacities for cadmium, primarily due to variations in phenolic group content.
- Understanding the role of HA Mw fractions is vital for predicting heavy metal migration and bioavailability in soil ecosystems.
- This study provides novel insights into HA-metal interactions, informing strategies for heavy metal pollution control.
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