Adsorptive removal of heavy metals and dyes from wastewater using Azadirachta indica biomass

This review article encircles the adsorptive characteristics of different parts of neem biomass for the removal of heavy metal ions and dyes from the aqueous medium. Neem is easily available and exhibits different functional groups such as carboxylic and hydroxyl groups. These groups are involved in the extraction of heavy metal ions and dyes from the aqueous medium due to established electrostatic interactions. Change in the reaction conditions such as pH, adsorbate concentration, adsorbent dosage, contact time, adsorbent particles size and temperature of medium also influence the adsorptive capacity of the biomass. Due to ionization of functional groups with the change of pH, pollutants uptake capacity of biomass also changes depending upon the nature of adsorbate and adsorbent. Small particles size provides large surface area for the adsorption of pollutants from the aqueous medium due to the availability of large number of exposed active sites, while temperature change tunes the thermal properties of biomass and alternatively tunes their adsorption capacity. Among different isotherm models, neem biomass-metal adsorption phenomenon mostly follows the Langmuir and Freundlich isotherm models. Adsorption of metal ions or dyes on the surface of neem biomass follows the pseudo-second-order kinetics which illustrates the dependence of adsorption rate on the concentration of adsorbate and adsorbent. Yet, this review article also describes that neem biomass can be modified by different chemical methods to enhance the number of functional groups or active sites to increase its adsorptive potential for the pollutants.