Enhancement of microplastics and nanoplastics removal via filtration method using surface-engineered palm kernel shell biochar

Microplastics (MP) and nanoplastics (NP) are major aquatic contaminants, raising concerns due to their strong affinity for other toxic substances. Filtration is widely employed for MP and NP removal due to its simplicity, efficiency and variety of available filtration media. In this study, the removal efficiency of MP and NP was investigated using surface-engineered biochar of palm kernel shell (PKS) origin, modified with cetyltrimethylammonium bromide (CTAB). The modified biochar demonstrated performance superior to the unmodified biochar, achieving 95.71 % and 96.12 % polyethylene MP (2–4 μm) removal efficiency as measured by turbidity and gravimetric methods, respectively, at an optimal CTAB concentration of 1.5CMC. The optimized biochar (PKS-1.5CMC) also improved the removal efficiencies for a range of other MP and NP particles varying in size (159 nm–48 μm), shape (irregular, spherical, fibrous) and polymer type (polyethylene, polyamide). The modification with CTAB increased the biochar's surface positive charge and hydrophobicity, resulting in stronger electrostatic attraction and hydrophobic interactions with MP and NP particles, which are negatively charged and hydrophobic by nature. In terms of MP and NP properties, higher removal efficiencies were obtained for (i) larger MPs due to easier retention, (ii) NPs due to their tendency to agglomerate, resulting in larger particle size, (iii) irregularly shaped particles, because of their surface roughness, providing more attachment sites and (iv) polyethylene MPs and NPs, owing to their higher hydrophobicity and lower negative zeta potential. Significant formation of a cake layer observed on the upper surface of the filter media suggested that filtration, rather than adsorption, was the dominant mechanism for the removal of MP and NP by biochar.