Filtration of microplastic spheres by biochar: removal efficiency and immobilisation mechanisms

Extensive presence of microplastic pollution in the aquatic environment has recently been identified as a critical global challenge. A large proportion of the microplastic in aquatic environments originates from the effluent discharges from wastewater treatment plants and urban runoff. We present an experimental study on the removal of microplastic spheres using biochar as potential low-cost material for integration in sand filter systems to improve their efficiency for removing microbeads in wastewater treatment plants. Based on the results of a series of filtration tests and microscopic characterisation, the major mechanisms of interactions between the microplastic spheres and biochar and immobilisation processes are presented. The results of leaching column tests on three biochar samples produced at three different temperatures from corn straw and a hardwood biochar are compared. The results show that the biochar filters provide significant capacity for the removal and immobilisation of 10 µm diameter microplastic spheres (above 95%) which is much larger than that of similar grain-sized sand filter studied. The extensive ESEM microscopic examination on the samples retrieved after the leaching tests show that the microplastic spheres were immobilised through three morphologically controlled mechanisms which are conceptualised to be 'Stuck', 'Trapped' and 'Entangled' whilst the microplastic spheres only 'Stuck' in sand filter. The presence of abundant honeycomb structures and thin chips to the high removal and immobilisation capacity of corn straw biochar produced at 500 °C and the hardwood biochar. In this study, we demonstrate that biochar can offer extensive potential for immobilisation of microplastic spheres (microbeads). This capacity can in principle be investigated and utilised to improve the efficiency of sand filters to remove microplastic in wastewater treatment plants.