Recycling and regeneration of carbonaceous and porous materials through thermal or solvent treatment

Abstract

Recyclability is a prerequisite for successful commercialization of new adsorbent materials. For the recycling, adsorbents should have high desorption efficiency at low economic and energy costs. Here, a comprehensive review is offered to describe the strategies for thermal or solvent-assisted (chemical) regeneration of carbonaceous materials derived from diverse bioresources (e.g., activated carbon, biochar, and hydrochar) along with other common or novel adsorbents (e.g., natural/synthetic zeolites and metal-organic frameworks). Our study reveals thermal regeneration as the most common recycling manoeuvre due to the simplicity of reactor design. However, the use of high temperatures during thermal treatments perturbs key surface attributes of adsorbents through oxidation, polymerization (of functional groups/entities), or framework breakdown. In contrast, solvent-assisted methods generate waste solvents and causes chemical degradation. The polarity of the solvents plays a pivotal role in performance of regenerated sorbents or regeneration potential due to strong adsorbatead-sorbent interaction (e.g., electrostatic and pi-pi interaction). Note that, for the selection of any adsorbent, there is inherently contradictory structural requirement between selective adsorption (favored by smaller pore space) and efficient desorption (favored by larger pores). Nonetheless, recent introduction of 'smart adsorbents' strikes the right balance between such opposing characteristics. Overall, a vast assemblage of adsorbents and hybrid regeneration techniques developed in recent times (2012-2018) signals the emergence of application-directed synthesis, self-assembly, and desorption-degradation coupling to make adsorptive removal process environmentally benign and scalable.