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Interaction of fluorene and its analogs with high-density polyethylene microplastics: An assessment of the adsorption mechanism to establish the effects of heteroatoms in the molecule
Summary
This study examined how the chemical structure of organic pollutants — specifically whether they contain sulfur, oxygen, or nitrogen heteroatoms — determines how strongly they adsorb onto high-density polyethylene microplastics in water. Sulfur-containing dibenzothiophene adsorbed most strongly, while nitrogen-containing carbazole showed almost no interaction, demonstrating that heteroatom identity and hydrophobicity largely govern the process. Understanding these adsorption differences matters because microplastics that carry adsorbed pollutants act as 'Trojan horses,' delivering concentrated toxins into aquatic organisms that ingest them.
The threat of microplastics (MP) pollution in aquatic ecosystems can be even more severe for they are able to interact with organic pollutants that can migrate to adjacent environments. The presence of heteroatoms in organic pollutants can directly influence adsorption onto MP. This research evaluated the adsorption of fluorene (FLN) and its heteroatom analogs dibenzothiophene (DBT), dibenzofuran (DBF) and carbazole (CBZ) onto high-density polyethylene (HDPE) MP from residual (HDPE) and commercial (HDPE) sources. The Langmuir isotherm showed a better fit, while DBT showed higher maximum adsorption capacity (19.2 and 15.8 μmol g) followed by FLN (13.4 and 11.7 μmol g), and DBF (13.5 and 10.3 μmol g) to the HDPE and HDPE respectively, which indicates a direct correlation with the hydrophobicity of the molecules determined by Log K. In contrast, CBZ showed no significant interaction with MP, due to their polar characteristic, thus, no kinetic and thermodynamic parameters could be determined. The adsorption process of all PAH was determined to be exothermic and spontaneous, with low temperatures favoring the process. The pseudo-second-order kinetic models have fitted to the adsorption onto both HDPE; intraparticle diffusion was also observed. Computational studies, physical characterization techniques and batch adsorption experiments demonstrated that the mechanism is governed by hydrophobic interactions, with van der Waals forces as a secondary effect in the adsorption of FLN, DBT and DBF onto HDPE and HDPE. Thus, allowing a deeper understanding of the interactions between HDPE MP and FLN as well with its derivatives.