Cetyl trimethyl ammonium bromide-modified magnetic biochar-integrated sand filter for microplastics removal from secondary-treated sewage effluents: Performance evaluation and mechanistic insights

Microplastics (MPs) discharge from effluent of sewage treatment plants (STPs) has prompted the need for effective treatment techniques. In this context, the present study provides insights on the preparation and use of cetyl trimethyl ammonium bromide (CTAB)-modified magnetic biochar (MBC-CTAB) as an adsorbent and integrated it in a sand filter for improved MPs removal from aqueous matrices. Rice husk biochars were synthesized, surface modified with FeCl3 and CTAB, and assessed for their MPs removal efficiency. Effects of various factors were analyzed from synthetic water spiked with 1 μm sized polystyrene MPs during batch and fixed-bed column studies. Optimal conditions yielded a maximum MPs removal of >97 % with MBC-CTAB at 7 mg/50 mL, pH 4, mixing speed of 180 rpm for 3 min during batch studies. Adsorption kinetics followed the pseudo-second-order model (R2 = 0.91), and the Langmuir model (R2 = 0.94) described the adsorption isotherm with a maximum capacity of 247 mg/g. Column studies removed ∼98 % MPs in real-time secondary sewage effluents using MBC-CTAB-integrated sand filter at flow rate: 5 mL/min, bed height: 4 cm, and MPs concentration: 7 mg/L. Electrostatic and hydrophobic interactions, hydrogen bonding, and pore-filling were the key MPs removal mechanisms. The application of the developed technique in efficient MPs removal highlights its practical relevance as sustainable approach for removing MPs from sewage effluents, which is a shared global concern.