Synthesis of silica nanoparticles (SiNPs) from agro-wastes for removal of heavy metals from an aqueous medium - a mini review

Heavy metals are micropollutants, persist in the environment and accumulate in organisms, causing adverse effects to ecosystem. Synthetic inorganic minerals, polymers, activated carbon, ash, char, biomass powders, shells, etc. are used as adsorbents to remove the heavy metals in aqueous medium. Advancements in nanotechnology and green chemistry focus on novel materials for efficient removal of heavy metals. Among the different interesting nanomaterials, Silica nanoparticles (SiNPs) have recently emerged as an important nanomaterial used for environmental remediation due to their tractable pore size, distinct surface area, surface reactivity, outstanding biocompatibility, structural flexibility, stability, low toxicity, and adoptable surface modification. Due to the huge availability of biomass, SiNPs synthesis from crop residues and agro-industries waste by-products has been widely suggested. The SiNPs could be synthesized from agro-waste through calcination, sol-gel, ball milling, etc. Some of the SiNPs produced from certain agro-waste recorded a specific surface area of 5 to 653 m 2 g -1 with a particle size of sub-50 nm. These SiNPs remove a wide range of heavy metals, viz., Cr, Ni, Pb, Cd, As, Hg, Zn, etc., from aqueous solutions with varied concentrations. This review briefly discusses various agro-waste sources for SiNPs green synthesis and SiNPs heavy metal adsorption.