Synergistic Effect of Microplastic Type and Particle Size on Microplastic Removal from Water using Rice Husk Ash

This research aimed to examine the potential use of fly ash (FA) and bottom ash (BA) derived from rice husk ash (RHA), as adsorbents for the removal of polyethylene (PE) and polypropylene (PP) microplastics (MPs) from water. The study investigated the influence of particle size and type of MPs on the adsorption capacity of BA and FA through adsorption kinetic and isotherm studies. BA and FA's physical and chemical properties, including elemental composition, pH value, pH_pzc, surface area, and pore volume, were analysed. Batch sorption kinetic studies for PE and PP removal by both BA and FA were preliminarily studied to obtain the equilibrium time. The kinetic experiments were conducted separately at an initial PE or PP concentration of 60 mg/L, BA or FA dosage of 1.2 g/L, and shaken at 150 rpm and 28 ± 0.5 °C for 24 h. Preliminary results showed that the equilibrium times of PE and PP removal from water by BA and FA were achieved within 2 h. After that, the effect of particle sizes (50-75 µm and 100-125 µm) of PE and PP on their removal efficiencies by BA and FA were investigated. To determine the adsorption capacity (Q_e) of BA and FA towards PE and PP, batch sorption isotherm experiments were conducted at initial PE or PP concentrations ranging from 0 to 100 mg/L and shaken for 2 h (equilibrium time) using an incubator shaker, and other conditions were similar to the previous experiments. It was found that BA was more efficient at removing PP from water compared to FA at both PP particle sizes (50-75 µm and 100-125 µm). The Q_e of BA (6.63 mg/g) for PP was almost 2 times higher than that of FA (3.37 mg/g) for the small PP particle size (50-75 µm). In addition, there were higher removal efficiencies of PP and PE by both BA and FA for the smaller particle size (50-75 µm). However, the Q_e of FA for PE and PP at the larger particle size (100-125 µm) showed slight differences. This finding highlights that BA and FA could be used as promising adsorbents for PP and PE removal from water. Despite both adsorbents being derived from RHA, there were differences in their ability to remove MPs, with BA outperforming FA. Additionally, different types and particle sizes of MPs induced differences in their adsorption capacities by the RHA.