Synthesis, assessment, and application of two-dimensional ferromagnetic nanocomposites for the removal of microplastics from drinking water and wastewater effluent

Abstract

The increase in microplastics (MPs) concentration in water matrices continues to pose a serious threat to aquatic ecosystems, and subsequently human health as a result of bioaccumulation within these aquatic organisms. Significant number of MPs have been detected from different water matrices including surface water and wastewater treatment effluent, globally. Recent reports from South Africa have also indicated their presence in tap water in low concentrations. Water treatment plants globally, are not optimised or designed for the removal of MPs. Therefore, technological innovations are required to alleviate the limitations embedded in these treatment systems. Nanotechnology has emerged as a pivotal technology to address a wide range of environmental challenges through performance enhancement. Therefore, this study investigated the removal of MPs from aquatic environments using magnetic nanocomposites (MNCs). This is the first study to the best of our knowledge that explicitly evaluates the performance of MNCs g-C3N4@Fe3O4 and BNNS@Fe3O4 for the removal of MPs. Herein, the application of each MNC is assessed for their removal efficiency of individual and multitudinous combinations of polystyrene (PS) and polyethylene (PE) MPs, including different size ranges. Additionally, these MNCs are applied in drinking water and real domestic wastewater effluent to determine their effectiveness against a combination of PS and PE MPs. Furthermore, a phytotoxicity study was also conducted to assess the toxicity of g-C3N4@Fe3O4 and BNNS@Fe3O4 on various common agricultural crops (Hordeum vulgare L. (barley wheat), Cicer arietinum (black chickpea) and Vigna radiatus L. (moong)). The recyclability study was performed for five successive rounds of reuse and each of the MNCs magnetic stability was assessed via VSM. Finally, a cost assessment analysis for MPs removal with the best performing MNC (g-C3N4@Fe3O4) was also conducted as well as the mechanistic insights of the interactions between PE/PS MPs and the MNC was postulated. The MNCs (g-C3N4@Fe3O4, BNNS@Fe3O4) were synthesized by conventional co precipitation. Synthesized MNCs were characterised by various analytical techniques such as, XRD, TGA, FTIR, BET, XPS, SEM-EDX, TEM, VSM, and Zeta potential. The optimization of various parameters (pH, time, MNC dose, MP dose) were done through gradient experiments. Optimised parameters were applied in batch experiments to investigate g C3N4@Fe3O4 and BNNS@Fe3O4 for the removal of different types and sizes of MPs (PE (125 µM), PS (25-180 µM), PE+PS combo (PE=125 and 25-180 µM) and PS (180-500 µM) from Milli-Q water, wastewater effluent and drinking water. The maximum removal of PE (96.16%, size 125 µM), PS (92.5% , size 25-180 µM), PE+PS combo (94.89%, size PE=125 and 25-180 µM) and PS (45.62%, size 180-500 µM) were noticed with MNC g-C3N4@Fe3O4 under optimum operating conditions ([pH]= 4; time= 5 h; [MNC]= 1.2 g/L; [MP]= 0.5 g/L) from Milli-Q water. The maximum removal efficiency of PE (94.44%, size 125 µM), PS (85.96%, size 25-180 µM), PE+PS combo (88.28%, size PE=125 and 25-180 µM) and PS (38.77%, size 180-500 µM) were observed with MNC BNNS@Fe3O4 under optimum operating conditions ([pH] = 3; time= 12 h; [MNC]= 0.9 g/L; [MP] = 0.5 g/L) from Milli-Q water. A direct correlation was observed between the removal rate and the size of the MPs. The investigation of MNC removal efficiency in different water matrices yielded 93.7 and 86.56% from drinking water via g-C3N4@Fe3O4 and BNNS@Fe3O4, respectively. A removal rate of 91.91 and 83.78% was observed from domestic wastewater effluent filtered with a 0.22 µM filter for g-C3N4@Fe3O4 and BNNS@Fe3O4, respectively, whilst a removal rate of 90.28 and 82.23% was observed from the same domestic wastewater effluent (unfiltered) for g-C3N4@Fe3O4 and BNNS@Fe3O4, respectively. The results for filtered and unfiltered wastewater effluent are similar indicating that filtering plays no significant role in improving the removal efficiency. The reusability study revealed that both MNCs retained a removal efficiency of more than 50% after 5 cycles whilst g-C3N4@Fe3O4 retained a removal efficiency of almost 80% after 3 cycles. The VSM results exhibited that both MNCs possess superparamagnetic behaviour which indicates that both BNNS@Fe3O4 and g-C3N4@Fe3O4 have excellent magnetic properties, enabling their application in practical settings. This was further confirmed by the after-treatment results wherein both MNCs retained their superparamagnetic properties after adsorption of the MPs, allowing for effective magnetic separation. MNCs' phytotoxicity on common agricultural crops was assessed to investigate any potential ecotoxic effects on the crops. The phytotoxicity of domestic raw wastewater influent, final treated effluent, g-C3N4@Fe3O4, BNNS@Fe3O4, g-C3N4@Fe3O4 filtrate and BNNS@Fe3O4 filtrate were assessed through seed germination indices (G.I.%). The MNC filtrate of g C3N4@Fe3O4 and BNNS@Fe3O4 revealed mild toxicity (approaching non-toxic) and no toxicity, respectively. The operating cost of g-C3N4@Fe3O4 for MPs removal from domestic wastewater effluent was approximately 41.09$/m3 . This makes it is a cost-effective treatment when compared to literature. The four main potential interactions postulated to occur between the PS/PE MPs and g-C3N4@Fe3O4 and BNNS@Fe3O4 MNCs are: electrostatic interaction, π π interaction, Van Der Waals forces and hydrogen bonding. In conclusion, this thesis demonstrates the promising potential of MNCs (g-C3N4@Fe3O4 and BNNS@Fe3O4) for the efficient removal of MPs from various water matrices, including drinking water and final treated wastewater effluent. The remarkable removal efficiency and superparamagnetic properties of these materials, coupled with their low environmental toxicity and cost-effectiveness, highlight their feasibility for practical applications. This thesis further confers the understanding of g-C3N4@Fe3O4 and BNNS@Fe3O4 MNCs recyclability, therein promoting a circular economy and a sustainable approach for wastewater treatment. These findings contribute to advancing sustainable wastewater treatment solutions and address the global challenge of MP pollution