Coagulative removal of polystyrene microplastics from aqueous matrices using FeCl3-chitosan system: Experimental and artificial neural network modeling

Effluent from sewage treatment plants (STPs) is a significant source of microplastics (MPs) re-entry into the environment. Coagulation-flocculation-sedimentation (CFS) process as an initial tertiary treatment step requires investigation for coagulative MPs removal from secondary-treated sewage effluents. In this study, experiments were conducted on synthetic water containing 25 mg/L polystyrene (PS) MPs using varying dosages of FeCl3 (1-10 mg/L) and chitosan (0.25-9 mg/L) to assess the effect of process parameters, such as pH (4-8), stirring speed (0-200 rpm), and settling time (10-40 min). Results revealed that ~89.3% and 21.4% of PS removal were achieved by FeCl3 and chitosan, respectively. Further, their combination resulted in a maximum of 99.8% removal at favorable conditions: FeCl3: 2 mg/L, chitosan: 7 mg/L, pH: 6.3, stirring speed: 100 rpm, and settling time: 30 min, with a statistically significant (p<0.05) effect. Artificial neural network (ANN) validated the experimental results with RMSE=1.0643 and R2=0.9997. Charge neutralization, confirmed by zeta potential, and adsorption, ascertained by field-emission scanning electron microscope (FESEM) and Fourier-transform infrared spectroscopy (FTIR), were primary mechanisms for efficient PS removal. For practical considerations, the application of the FeCl3-chitosan system on the effluents from moving bed biofilm reactor (MBBR) and sequencing batch reactor (SBR)-based STPs, spiked with PS microbeads, showed >98% removal at favorable conditions. Sewage treatment plants are regarded as prominent source for re-entry of microplastics (MPs) as hazardous micro-pollutants into the aquatic environment. Coagulation-flocculation-sedimentation process as initial step for particles removal in tertiary treatment can be effective for MPs removal. However, coagulative MPs removal from secondary-treated sewage effluents by a combined system of an inorganic coagulant and a biopolymer as coagulant aid and their interactions with MPs has rarely been studied. Herein, this study deals with the coagulative MPs removal from secondary-treated effluents using FeCl3-chitosan system at environmentally relevant conditions based on the outcomes of batch experimentation on synthetic water containing polystyrene microbeads.