**3. Comparison of conventional and non-conventional adsorbents**

For the adsorption process to be efficient, selection of the most appropriate adsorbent is a crucial step. The main basis of selection of an adsorbent include low cost, high adsorption capacity, effective for a broad spectrum of pollutants and having a low footprint [208, 209]. There has been extensive research in the field of conventional and non-conventional adsorption performances and mechanisms. Different adsorbents follow varied mechanisms because of difference in raw material and adsorbent production conditions. Mainly four mechanisms have been identified for efficient adsorption of pollutants; chemisorption, physisorption, ion-exchange and precipitation [210, 211]. Davis et al. stated that ion-exchange does not necessarily describe the mechanism of adsorption but a lot of other factors and mechanisms co-aid to make the process successful [210]. Some other researchers also explained the adsorption mechanisms [212–215]. Literature evidently points

out that activated carbons have proven themselves as brilliant adsorbents due to their high specific surface area, mechanical and structural surface morphology and presence of functional groups which can also be modified. However, non-conventional adsorbents are increasingly employed as low-cost and effectual adsorbents. Their commercialization remains a task but there as upcoming as their available in abundant. More focused research into their engineering and modification can bring them at par some commercial solid adsorbents.
