Carbon Nanocomposite for Purification of Man-Made Polluted Waters

Among the main man-made water pollutants that pose a danger to the environment are oil products, heavy metals and radionuclides, as well as micro- and nanoplastics formed as a result of the destruction of polymeric materials. A characteristic feature of contaminated waters nowadays is their multicomponent and multiphase nature. To purify such waters, it is necessary to use a combination of several advanced methods, with sorption being one of them. The aim of this work is to develop a nanocomposite sorbent comprising magnetically responsive thermally expanded graphite (TEG) and the natural clay bentonite and assess its ability to purify man-made contaminated waters. In the course of the research, the methods of scanning electron microscopy, optical microscopy, dynamic light scattering, and atomic absorption spectrophotometry were used. To obtain the nanocomposite, magnetoresponsive TEG containing micro- and nanoparticles of metallic iron and its oxides as a magnetic component, and bentonite with a montmorillonite content of at least 70% and the particle size of less than 100 μm were used. Given the complex chemical nature of the surface of montmorillonite and magnetoresponsive TEG particles, the interaction of the hydrophobic centers of bentonite with the surface of TEG particles during mechanical activation leads to the formation of loose aggregates capable of sorbing particles of micro- and nanoplastics and non-polar hydrocarbons. The sorption properties of the nanocomposite are dependent on the hydrophobic centers mainly located on the surface of oxidized graphene layers in thermally expanded graphite. The hydrophilic properties of the nanocomposite are due to the presence of aluminol and silanol groups, as well as the charge on the surface of montmorillonite nanocrystals and the Brønsted centers on the surface of TEG particles. The use of the nanocomposite for purification of a nuclear power plant (NPP) radioactively contaminated water simulant containing stable isotopes of cesium, strontium, cobalt, manganese in the presence of hydrophilic and hydrophobic organic substances reduced the content of organic substances by 10-15 times, and the degree of extraction of heavy metals from water was for cesium - 81.4%, strontium – 89.9%, cobalt – 92.4%, and manganese – 98.8%. The use of a carbon nanocomposite for purification of real radioactively contaminated water obtained from the object “Shelter” (“Ukryttya” in Ukrainian), in the Chernobyl Exclusion Zone, Ukraine) with an activity of 137Cs – 3.3∙107 Bq/dm3, 90Sr – 4.9∙106 Bq/dm3, containing, in addition to radionuclides, organic substances, including micro- and nanoplastics, reduced the radioactivity by three orders of magnitude. The filtrate obtained after purification was free from suspended particles, including colloidal ones. The use of cesium-selective sorbents for additional purification of the filtrate allowed further decontamination of radioactively contaminated water with an efficiency of 99.99%.