Sorption Behavior and Removal Performance of Polypropylene Microplastics Using Bamboo Biochar and Rice Husk Ash in Aqueous Solution

This study investigated the sorption behavior and removal efficiency of virgin polypropylene (PPV) and aged polypropylene (PPA) microplastics using bamboo biochar (BBC) and rice husk ash (RHA) in aqueous solution. The physicochemical properties and surface morphology of PPA, BBC, and RHA were characterized via scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, X-ray fluorescence (XRF), Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD). Furthermore, batch adsorption kinetic and isotherm experiments were conducted. The pH of the point of zero charge (pHₚzc) of PPV (5.45), BBC (8.05), and RHA (9.7) indicated that PPV surfaces show a net negative charge, whereas BBC and RHA exhibit net positive charges at neutral pH, promoting different electrostatic mechanisms between polypropylene microplastics and the adsorbents. PPA exhibited a higher removal efficiency than PPV, likely due to aging in surface water that induced surface modifications. Kinetic analysis revealed that PPV and PPA adsorption onto BBC follows a pseudo-second-order (PSO) model (R2=0.887-0.944), suggesting chemisorption as the dominant mechanism, with similar rate constants (k2=0.002 g/mg min). In contrast, PPV and PPA sorption by RHA was best described by a pseudo-first-order (PFO) model (R2=0.876-0.966), indicating physisorption as the primary mechanism, with a higher kinetic rate for PPA (k1=0.145 min-1) compared to PPV (k1=0.078 min-1). Equilibrium was achieved within 24 h for both PPV and PPA. In addition, the adsorption isotherms of PPV and PPA on both BBC and RHA aligned well with the Freundlich model (R2=0.984-0.993), indicating multilayer adsorption on heterogeneous surfaces. Overall, these findings demonstrate that BBC and RHA exhibit distinct sorption mechanisms toward polypropylene microplastics, with BBC favoring chemisorption and exhibiting a higher affinity for aged particles, while RHA primarily facilitates physisorption. Aging plays a key role in modifying microplastic surfaces, thereby enhancing the sorption behavior and removal performance.