Interlayer Anions of Layered Double Hydroxides as Mobile Active Sites To Improve the Adsorptive Performance toward Cd2+

Layered double hydroxides (LDHs) have been considered important sinks for ionic contaminants in nature and effectively engineered adsorbents for environmental remediation. The availability of interlayer active sites of LDHs is critical for their adsorptive ability. However, inorganic LDHs generally have a nano-confined interlayer space of ca. 0.3-0.5 nm, and it is unclear how LDHs can utilize their interlayer active sites during the adsorption process. Thus, LDHs intercalated with SO₄^2-, PO₄^3-, NO₃^-, Cl^-, or CO₃^2- were taken as examples to reveal this unsolved problem during Cd²⁺ adsorption. New adsorption behaviors and pronounced differences in adsorption performance were observed. Specifically, SO₄^2-/PO₄^3- intercalated LDHs showed a maximum Cd²⁺ adsorption capacity of 19.2/9.8 times higher than other LDHs. The ligand exchange of H⁺ (on the surface -OH) by Cd²⁺ and formation of Cd-SO₄^2-/PO₄^3- complexes led to the efficient removal of Cd²⁺. Interestingly, interlayer SO₄^2- was demonstrated to be able to move to the edges/outer surfaces of LDHs, providing abundant movable adsorption sites for Cd²⁺. This novel phenomenon made the SO₄^2- intercalated LDH a superior adsorbent for Cd²⁺ among the tested LDHs, which also suggests that LDHs with a nano-confined interlayer space can also highly utilize their interlayer active sites based on the mobility of interlayer anions, offering a new method for constructing superior LDH adsorbents.