Removal of polystyrene nanosphere and fragment from aqueous solutions by magnetic biochar derived from crab shell

Adsorption by biochar is widely recognized as a promising strategy for micro/nanoplastics removal from aqueous systems. Magnetic biochar derived from crab shell (M-CSBC) was successfully prepared by pyrolysis and subsequent ball milling with magnetic modification, and it was pioneeringly applied for the effective removal of spherical polystyrene nanoplastics (PSNPs) and fragmental polystyrene microplastics (PSMPs) from aquatic environment. M-CSBC exhibited excellent removal for PSNPs/PSMPs with different shape and size. The adsorption process fitted Langmuir model well, and the maximum removal capacities of PSNPs (100 nm) and PSMPs (0.35-0.45 mm) were 90.090 mg/g and 14.472 g/g, respectively. The removal of PSNPs/PSMPs was significantly influenced by variations in pH and salinity, with a more pronounced effect observed for PSNPs compared to PSMPs. M-CSBC was generally more suitable for the neutral acidic, saline aquatic systems. 500 nm NPs were easier to be removed compared with 100 nm NPs, while the removal efficiency of 0.35-0.45 mm PS fragments was larger than that of 0.45-0.60 mm fragments. The removal of PSNPs/PSMPs by M-CSBC were controlled through a combination of physical trapping, chemical adsorption (electrostatic interactions, chemical bonding, complexation, cation-bridging) as well as the magnetization separation. Especially, the real-time observations revealed that both the dispersed M-CSBC and the mixture of M-CSBC and plastic particles behaved akin to a "magnetic mop", efficiently sweeping up and removing the other plastic particles without enough M-CSBC adsorption. The present findings demonstrate the immense potential of M-CSBC as a highly promising candidate for the efficient microplastics and nanoplastics remediation.