Persimmon tannins as biosorbents for precious and heavy metal adsorption in wastewater: a review

Over the years, adsorption has proved to be an effectual technique in the recovery of a variety of substances from aqueous solution. In response to the call for environmental and economic sustainability, much attention has been placed on biosorbents. Persimmon tannin-based adsorbents (PTBAs), in particular, have shown promise in terms of recovering metals from wastewater. PTBAs are universally occurring, low-cost biological polymers that can be easily obtained and transformed into non-soluble tannin gels or immobilized onto various carriers. This study provides an overview of available literature (2002-2020) regarding the application of PTBA for remediating water containing heavy and precious metals. The adsorption capacity of several PTBAs (modified and unmodified) was summarized and this reflected the efficacy of PTBAs as a biosorbent. Gold was the most common target of PTBAs with the highest recorded maximum adsorption capacity of 15.36 mol/kg of Au3+. The uptake mechanisms of precious and heavy metals Au, Pd, Pt, V, and Cr(VI) unto PBTAs were also discussed. Furthermore, the factors influencing adsorption of metals by PTBAs, particularly pH, were identified. The effect of pH on metal adsorption onto PTBAs from previous studies is compiled and discussed.