Cutting-edge regeneration technologies for saturated adsorbents: a systematic review on pathways to circular...

Adsorption seemed like an excellent physicochemical process employed for wastewater treatment. In the last few decades, significant improvements have been made in efficiency and economy to remove contaminants from wastewater using several adsorbents. However, less attention was paid to the regeneration of used adsorbents. Aside from the adsorbent's high adsorption performance, the disposal of spent adsorbents is an environmental concern. Regeneration is an important aspect to stimulate the adsorption efficiency of the spent adsorbent for wastewater treatment. This article reviews the various regeneration techniques like electrochemical regeneration, biological regeneration, thermal regeneration, ultrasound regeneration, and chemical regeneration in detail that have been performed for the renewal of saturated adsorbents. In the ultrasonic regeneration technique, Fe₃O₄-loaded coffee waste hydrochar adsorbent showed 100% regeneration efficiency (RE) after 1.3 h at the power consumption of 300 W/L. Electrochemical regeneration of granular activated carbon, Nyex, graphene and titanium dioxide composite, and Nyex 1000 showed 100% RE after 3, 0.16, 0.12, and 1.5 h, respectively, with electrolyte Na₂SO₄ and NaCl. In the regeneration technique, powdered activated carbon showed 90% RE after 48-72 h. Immobilized fungal biomass (Rhizopus nigricans) adsorbent showed 111-115% RE with base (0.01 N NaOH, NaHCO₃, and Na₂CO₃) solvent. The present study addresses issues including waste generation, adsorbent potential and efficiency, eco-friendly techniques, and the release of adsorbed pollutants in regenerating saturated adsorbents. The mechanisms of adsorbent regeneration were thoroughly examined, highlighting the significance of the regeneration process in adsorption. Furthermore, this review discusses the advantages of hybrid regeneration techniques like microwave-activated ultraviolet-advanced oxidation, electro-peroxide approach, electrochemical and electrothermal methods, and the secondary use of spent adsorbents as catalysts, fertilizer, cementitious materials, secondary adsorbent bio-fuels, etc. Using saturated adsorbents is a practical technology for sustainable wastewater treatment that has the potential to minimize pollution and promote a circular economy. This review concludes with a discussion of the present challenges in the regeneration of the used adsorbents, as well as future directions for ensuring the system's feasibility from an economic and environmental standpoint for use on an industrial scale.