Adsorption of neutral organic compounds on polar and nonpolar microplastics: Prediction and insight into mechanisms based on pp-LFERs

Adsorption of 18 neutral organic compounds (OCs) on polar (polybutylene succinate (PBS) and polycaprolactone (PCL)) and nonpolar (low-density polyethylene (LDPE) and polystyrene (PS)) microplastics (MPs) were investigated. The adsorption coefficients (K-d) varied with ranges of 130-42,002, 124-27,768, 6.40-10,713, and 1.52-10,332 L kg(-1) for adsorption on PCL, PBS, LDPE, and PS MPs, respectively. The polar MPs showed greater adsorption capacities than nonpolar MPs. Non-specific interaction, i.e. hydrophobic partition played a crucial role in the adsorption of OCs on all MPs, while polar interactions also contributed significantly to the greater adsorption on polar MPs. Poly-parameter linear free energy relationships (pp-LFERs) with multiple linear regression (MLR) and feedforward network (FN) were then employed to model the adsorption of OCs on MPs so as to obtain deep insights into adsorption mechanisms. The MLR models achieved R-adj(2) of 0.90-0.97 and root mean square error (RMSE) of 0.13-0.38 log units, while the FN models achieved R-adj(2) of 0.85-0.90 and RMSE of 0.21-0.60 log units. The MLR models are more accurate under selected equilibrium concentrations while FN models are capable of making predictions under varying equilibrium concentrations. Lastly, both MLR and FN models showed good prediction on literature adsorption data on nonpolar MPs.

Automated summary

The study found that polar microplastics have higher adsorption capacities for organic compounds than nonpolar microplastics, with adsorption mechanisms mainly involving non-specific interactions and polar interactions. Poly-parameter linear free energy relationships (pp-LFERs) with multiple linear regression (MLR) and feedforward network (FN) models were used to model the adsorption of OCs on MPs, with MLR models being more accurate at selected equilibrium concentrations and FN models capable of predicting under varying equilibrium concentrations. Both MLR and FN models showed good prediction accuracy on literature data for adsorption on nonpolar MPs.