A review on regeneration of adsorbent and recovery of metals: Adsorbent disposal and regeneration mechanism

Adsorption is a reliable and cost-effective technique for removing contaminants from wastewater. However, the major issue with the adsorption process is the regeneration and recovery of spent adsorbents. This review focuses on the Regeneration and recovery of pollutants from saturated adsorbent using acid (such as HCl, HNO₃, H₂SO₄ and organic acids), alkali (NaOH) or other chemicals (HNO₃, KCl, NaCl and NH₄Cl) (chemical regeneration), heat (thermal regeneration), micro-wave energy (microwave-assisted regeneration), electrical energy (electrochemical regeneration) and ultrasonic power (Ultrasound regeneration). The maximum desorption efficiencies observed were 99.5%, 92.6%, 284%, 150% and 66.61% in chemical, thermal, micro-wave-assisted, electrochemical and ultrasound regeneration techniques. The number of regeneration cycles performed was nearly in the range of 1-10 cycles. In the final step, waste is disposed of therefore incineration and landfill disposal have been discussed. However, among all these techniques, the Chemical regeneration technique has consumed the highest energy i.e. 6.6 kWh/kg. The current challenges in the regeneration and recovery of saturated adsorbent such as operational cost, waste generation, development of eco-friendly technique, maintaining potential and efficiency of adsorbent and release of adsorbed pollutants, were also covered. Additionally, several aspects of the adsorption process such as applications of saturated adsorbents (antimicrobial agents or disinfectants, materials for civil construction, as a catalyst and fertilizers) were explained in detail. Mechanisms of adsorbents regeneration were also discussed in detail and emphasis has been drawn to the importance of adsorbent regeneration in the adsorption process. Therefore the novelty of this article is in overcoming the adsorption challenges and also focusing on metal recovery and adsorbent regeneration. Further studies should elucidate the techno-economic and environmental aspects.