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

SO4 (2-) inthe nano-confinedinterlayer space of LDHs can act as movable active sites for adsorptionof Cd2+, while PO4 (3-) couldnot. The adsorption of Cd2+ by SO4-LDH or PO4-LDH was owing to the combination of replacement of H+ by Cd2+ and formation of SO4 (2-)-Cd2+ outer-sphere complexes or phosphate-Cd2+ inner-sphere complexes, and the replacement of H+ and formation of complexes happened asynchronously. Layered double hydroxides (LDHs) have been consideredimportantsinks for ionic contaminants in nature and effectively engineeredadsorbents for environmental remediation. The availability of interlayeractive sites of LDHs is critical for their adsorptive ability. However,inorganic LDHs generally have a nano-confined interlayer space ofca. 0.3-0.5 nm, and it is unclear how LDHs can utilize theirinterlayer active sites during the adsorption process. Thus, LDHsintercalated with SO4 (2-), PO4 (3-), NO3 (-), Cl-, or CO3 (2-) were taken as examples toreveal this unsolved problem during Cd2+ adsorption. Newadsorption behaviors and pronounced differences in adsorption performancewere observed. Specifically, SO4 (2-)/PO4 (3-) intercalated LDHs showed a maximum Cd2+ adsorption capacity of 19.2/9.8 times higher than otherLDHs. The ligand exchange of H+ (on the surface -OH)by Cd2+ and formation of Cd-SO4 (2-)/PO4 (3-) complexes led to the efficientremoval of Cd2+. Interestingly, interlayer SO4 (2-) was demonstrated to be able to move to the edges/outersurfaces of LDHs, providing abundant movable adsorption sites forCd(2+). This novel phenomenon made the SO4 (2-) intercalated LDH a superior adsorbent for Cd2+ among the tested LDHs, which also suggests that LDHs witha nano-confined interlayer space can also highly utilize their interlayeractive sites based on the mobility of interlayer anions, offeringa new method for constructing superior LDH adsorbents.