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Aging microplastic aggravates the pollution of heavy metals in rhizosphere biofilms

Shanying He1, Yufei Wei1, Zhiheng Li1

  • 1School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou, Zhejiang 310018, China.

The Science of the Total Environment
|May 25, 2023
PubMed

Insights

Aged microplastics (MPs) in soil enhance heavy metal (HM) adsorption onto root biofilms. Aged polyethylene showed stronger binding for cadmium (Cd(II)) due to oxygen groups, impacting soil contaminant risk assessment.

Area of Science:

  • Environmental Chemistry
  • Soil Science
  • Materials Science

Background:

  • Heavy metals (HMs) and microplastics (MPs) are prevalent contaminants in agricultural soils.
  • Rhizosphere biofilms play a crucial role in HM adsorption but are susceptible to disturbance by MPs.
  • The impact of aged MPs on HM adsorption within these biofilms remains poorly understood.

Purpose of the Study:

  • To investigate the adsorption behavior of cadmium (Cd(II)) on biofilms associated with pristine and aged polyethylene (PE/APE).
  • To quantify the influence of aged MPs on HM adsorption capacity in rhizosphere biofilms.
  • To elucidate the mechanisms governing HM adsorption on MPs and biofilms.

Main Methods:

  • Experimental analysis of Cd(II) adsorption on PE and APE.
  • Density Functional Theory (DFT) calculations to determine binding energies and interactions.
  • Kinetic and isotherm modeling (pseudo-second-order and Langmuir) to describe adsorption processes.
  • Investigation of competitive adsorption effects in a Cd(II)-Pb(II) system.

Main Results:

  • Aged polyethylene (APE) exhibited significantly higher Cd(II) adsorption than pristine PE, attributed to oxygen-containing functional groups.
  • DFT calculations confirmed stronger binding of Cd(II) to APE (-6.00 kcal·mol⁻¹) compared to PE (7.11 kcal·mol⁻¹), driven by hydrogen bonding and oxygen-metal interactions.
  • APE-associated biofilms increased Cd(II) adsorption capacity by 4.7% relative to PE biofilms.
  • Chemisorption dominated, following pseudo-second-order kinetics and Langmuir isotherms (R² > 80%).
  • Competitive adsorption in a Cd(II)-Pb(II) system led to higher hysteresis indices (>1) compared to single-solute systems (<1).

Conclusions:

  • Aged microplastics, particularly polyethylene with oxygen functional groups, enhance the adsorption of heavy metals like cadmium in soil rhizosphere biofilms.
  • The findings highlight the importance of considering MP aging and surface chemistry in assessing HM mobility and ecological risks in agricultural soils.
  • This research provides crucial insights for understanding contaminant fate and transport in MP-polluted environments.
Keywords:
AdsorptionAged microplasticsHeavy metalsRhizosphere biofilms

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