An insight into the green synthesis of SiO2 nanostructures as a novel adsorbent for removal of toxic water pollutants

Green synthesis of nanomaterials is a sustainable, biologically safe, reliable, and eco-friendly approach. Green synthesis is beneficial to reduce the devastating effects of the traditional chemical synthesis methods and particularly aims at decreasing the usage of toxic chemicals. This review deals with the green synthesis of silica nanoparticles (SiO2 NPs) with emphasis on the engineering surface properties for enhanced adsorption capability and their applications as novel nano-adsorbents for water pollutants removal. Green synthesized SiO2 NPs have shown excellent adsorption properties with higher adsorption capacity of 150-200 mg/g and more than 95% removal for various toxic water pollutants along with reusability for more than 5 cycles. These SiO2 NPs show fascinating physical and chemical properties i.e. tunable size (5 nm to more than 100 nm), low toxicity, biocompatibility, high porosity, higher specific surface area (500--700 m2/g) making them attractive/suitable for several applications in biomedical, agriculture, catalysis, construction, water treatment, etc. Commonly, highly pure SiO2 NPs are synthesized from organic chemicals (very expensive and highly toxic in nature) as a precursor that led to high production costs, high energy consumption, and environmental hazards. On the other hand, green synthesis of SiO2 NPs from natural resources like biomass that includes rice husk, bamboo leaves/stem, sugarcane bagasse, corn cobs, wheat straw, etc. is cost-effective, less toxic, and eco-friendly which has been discussed in detail. Furthermore, the effect of key synthesis parameters (i.e., temperature, time, concentration, pH, etc.) on the morphology, size, purity, and specific surface area of SiO2 NPs have been summarized. Finally, the applications of SiO2 NPs as nano-adsorbents for the removal of toxic water pollutants (i.e., heavy metal cations, anions, dyes, etc.) including the adsorption mechanisms along with the future scope, challenges, and suggestions have been discussed.