Synthesis and applications of zinc oxide for removal of various pollutants: A review

The versatility of zinc oxide applications in the removal of various pollutants has attracted a wide interest of researchers in the past decade. Numerous studies reported on zinc oxide synthesis pathways and resulting nanoparticle morphologies, applications, formation mechanisms and synthesis parameters. In this review the reported zinc oxide synthesis methods are classified into chemical, physical and biological routes; they are evaluated in terms of the required chemicals, synthesis conditions and the resulting morphologies and properties of zinc oxide. The chemical route of zinc oxide synthesis covers precipitation, micro-emulsion, solgel, solvothermal and hydrothermal paths. The physical route includes laser ablation and high energy ball milling, while the biological route covers plant extracts and microbe mediated synthesis. The mechanisms of zinc oxide formation of the mentioned routes are based on one or more of the following processes: particle nucleation, diffusional growth, Ostwald ripening, particle aggregation and sintering. The most influencing synthesis parameters overall are temperature, drying duration and additives’ effect. Higher temperatures (>200°C) commonly produce larger particles of zinc oxide (> 80 nm); the prolong duration (> 60 min) often results in the agglomeration and sintering of zinc oxide particles. However, additives may mitigate agglomeration extent. Overall, the chemical route is more preferable due to its flexibility that is also linked to the greater variability of zinc oxide particles. The physical method produces more consistent zinc oxide particles but requires higher energy inputs. The biological method is very promising and associated with low chemicals consumptions and good quality of zinc oxide.