4.7 Article

Aging microplastic aggravates the pollution of heavy metals in rhizosphere biofilms

Journal

SCIENCE OF THE TOTAL ENVIRONMENT
Volume 890, Issue -, Pages -

Publisher

ELSEVIER
DOI: 10.1016/j.scitotenv.2023.164177

Keywords

Aged microplastics; Adsorption; Heavy metals; Rhizosphere biofilms

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The adsorption behavior of Cd(II) on biofilms and pristine/aged polyethylene (PE/APE) was analyzed and quantified in this study. It was found that Cd(II) had a higher adsorption amount on APE compared to PE due to the oxygen-containing functional groups of APE that provided binding sites. Density functional theory calculations revealed that the binding energy of Cd(II) onto APE was much stronger than that of PE due to hydrogen bonding interactions and oxygen atom-metal interactions. APE increased the adsorption capacity of Cd(II) in MP biofilms by 4.7% compared to PE.
Heavy metals (HMs) and microplastics (MPs) are ubiquitous in agricultural soils. Rhizosphere biofilms are important sites for HM adsorption, and biofilms are easily disturbed by soil MPs. However, the adsorption of HMs on rhizosphere biofilms induced by aged MPs is not clear. In this study, the adsorption behavior of Cd(II) on biofilms and pristine/aged polyethylene (PE/APE) was analyzed and quantified. The results showed that the adsorption amount of Cd(II) on APE was greater than that on PE, in which the oxygen-containing functional groups of APE could provide binding sites to increase the adsorption of HMs. Density functional theory (DFT) calculations revealed that the binding energy of Cd(II) onto APE (-6.00 kcal center dot mol-1) was much stronger than that of PE (7.11 kcal center dot mol-1) due to hydrogen bonding interactions and oxygen atom-metal interactions. For HM adsorption on MP biofilms, APE increased the adsorption capacity of Cd(II) by 4.7 % relative to PE. The pseudo-second-order kinetic and Langmuir models suitably described the adsorption kinetics and isothermal adsorption of Cd(II), respectively (R2 > 80 %), indicating that monolayer chemisorption dominated. However, the hysteresis indices of Cd(II) in the Cd(II)-Pb(II) system (< 1) were higher than those in the single system (> 1) due to the competitive adsorption of HMs. Overall, this study clarifies the effect of MPs on the adsorption of HMs in rhizosphere biofilms and will help researchers assess the ecological risks of HMs in soils.

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