Tannins for wastewater treatment

Abstract

Tannins, non-toxic natural polyphenol biomolecules widely recognized as secondary plant metabolites categorized under polyphenols and are potential candidates for biosorbent manufacturing. The acquit of toxic wastewater from commerce to open habitat causes relentless unceasing pollution of the environment, air, water resources due to the latency of soluble metal ions. Adsorption is the most developed and efficient process for extracting a multitude of substances from aqueous solutions. Tannin based adsorbents (TBAs) are highly proficient in the removal of heavy metals (density > 5 g/cm3), dyes, surfactants, and chemical products from polluted waters. TBA exploits the extraction and removal of heavy metals (mercury, uranium, lead, and chromium) under its proficient natural affinity to uptake heavy metals by forming chelates with metals due to the presence of a high number of adjacent hydroxyl groups in its molecule. Electrostatic attraction schemed to be the underlying mechanism, cationic heavy metals assimilated on to the anionic surface grabbed into the porous framework with a low surface area and toxic heavy metal ion permutation effectively attained at HNO3 or HCL (0.1 mol/L) solution. Moreover, tannins upon chemical modification/inclusion with different substances produce adsorbents with superior adsorption capability towards targeted metal ions. The removal of cationic contaminants favored at basic pH levels, and while analyzing mimosa bark tannin foams reveals 12.5% of Cu(II), and pine bark tannin foam 20.1% of Pb(II) adsorption. The TBA adsorption rate remained unlateralized even after four consecutive cycles. Herein this review provides an in-depth study on tannin-based adsorbents capable of removing the toxic heavy metal ions from wastewater and their formulation, extraction methods, effect if the interference of competitive ions, possibility of lixiviation of constituent particle to water and some possible future prospects regarding their efficient separation.