Mechanisms of lead removal from aqueous solutions using a novel tannin gel adsorbent synthesized from natural condensed tannin

Condensed tannins are ubiquitous plant polyphenols and have many potential applications. A novel tannin gel adsorbent was synthesized from natural condensed tannin and its adsorption mechanisms and properties for lead (Pb) removal from aqueous solutions were investigated. It was found that lead could be removed through three adsorption mechanisms, i.e., ion exchange, hydrolytic adsorption and surface precipitation. When the initial concentration of lead was varied from 20 mg/l to 80 mg/l, ion exchange took place preferentially in the low pH range of 3.0 to 6.0, and the stoichiometric equation could be described as 2RH + Pb²⁺ = R₂Pb + 2H⁺. On the other hand, when pH > 6.0, surface precipitation and hydrolytic adsorption took the predominant parts in the adsorption. The adsorption was notably affected by pH of solutions. The adsorption isotherm corresponding to the ion exchange predominating stage was correlated well to Langmuir Equation and the maximum removal capacity in the stage was 0.15-0.19 mmol Pb/g dry tannin gel adsorbent (30-40 mg Pb/g dry adsorbent). This study has the potential to develop an effective natural adsorbent for removal and recovery of heavy metals.