Selective Pb²⁺ removal and electrochemical regeneration of fresh and recycled FeOOH

Heavy metal pollution is a key environmental problem. Selectively extracting heavy metals could accomplish water purification and resource recycling simultaneously. Adsorption is a promising approach with a facile process, adaptability for the broad concentration of feed water, and high selectivity. However, the adsorption method faces challenges in synthesizing high-performance sorbents and regenerating adsorbents effectively. FeOOH is an environmentally friendly sorbent with low-cost production on a large scale. Nevertheless, the selectivity behavior and regeneration of FeOOH are seldom studied. Therefore, we investigated the selectivity of FeOOH in a mixed solution of Co²⁺, Ni²⁺, and Pb²⁺ and proposed to enhance the capacity of FeOOH and regenerate it by using external charges. Without charge, the FeOOH electrode shows a Pb²⁺ uptake capacity of 20 mg/g. After applying a voltage of −0.2/+0.8 V, the uptake capacity increases to a maximum of 42 mg/g and the desorption ratio is 70%–80%. In 35 cycles, FeOOH shows a superior selectivity towards Pb²⁺ compared with Co²⁺ and Ni²⁺, with a purity of 97% ± 3% in the extracts. The high selectivity is attributed to the lower activation energy for Pb²⁺ sorption. The capacity retentions at the 5^th and the 35^th cycles are ca. 80% and ca. 50%, respectively, comparable to the chemical regeneration method. With industrially exhausted granular ferric hydroxide as the electrode material, the system exhibits a Pb²⁺ uptake capacity of 37.4 mg/g with high selectivity. Our work demonstrates the feasibility of regenerating FeOOH by charge and provides a new approach for recycling and upcycling FeOOH sorbent.