Control of Microplastics and Nanoplastics Discharge via Biochar-Based Filtration: Optimization Using Central Composite Design (CCD) and Identification of Column Fouling Mechanism

Abstract

Microplastics (MPs) and nanoplastics (NPs) are emerging aquatic pollutants of significant environmental concern due to their pervasive hazards. Filtration using filter media is a common approach for mitigating MP and NP contamination; however, the optimization of process parameters and the underlying column fouling mechanisms remains insufficiently explored. This study investigates the optimization of MP and NP removal using surface-engineered biochar in a continuous-flow column system via response surface methodology (RSM) employing central composite design (CCD). Four operating parameters were evaluated: pH (3–11), MP and NP concentration (0.01–0.09 g/L), flow rate (5–9 mL/min), and biochar bed depth (5–15 cm). Optimal removal efficiency was achieved at pH 7, MP and NP concentration of 0.01 g/L, 7 mL/min flow rate, and 10 cm biochar bed depth, yielding removal efficiencies of 93.75% (measured by turbidity method) and 93.07% (estimated by gravimetric method). Analysis of variance (ANOVA) confirmed the model's significance, with a high coefficient of determination (R²) observed between predicted and actual data. All tested parameters and two interacting parameters, (i) concentration-flow rate and (ii) flow rate-biochar bed depth, significantly influenced MP and NP removal efficiency. Prolonged operation under optimal conditions induced fouling of biochar-packed bed, and an evaluation using Hermia's model, assuming uniform bed porosity and filtration as the main removal mechanism, indicated the presence of standard blocking, intermediate blocking, and cake filtration as primary fouling mechanisms. This study highlights the potential of surface-engineered biochar as a promising filter media for efficient MP and NP removal while providing insights into the column fouling dynamics.