Synergistic effect of W(VI) and Ni(II) uptakes on an MgAl-layered double hydroxide

The coadsorption of anionic and cationic pollutants on adsorbents holds considerable importance in the development of relevant removal technologies and the understanding of pollutant transport in complex environmental media. Herein, tungsten (W), an emerging contaminant, and nickel (Ni) were chosen as two differently charged inorganic pollutants to investigate their removal characteristics on a magnesium-aluminum layered double hydroxide (LDH) prepared via microwave radiation. In the single systems, the amount of adsorbed W on LDH was initially increased and then decreased with increasing initial W concentration. In concentrated W solutions, LDH dissolution was intensified, accompanied by the intercalation and polymerization of W in the interlayer space. Among the various oxyanions, phosphate showed the most substantial inhibition on W uptake. In contrast, uptake of Ni was enhanced with the rapider adsorption rate at higher concentrations, due to the isomorphic substitution and precipitation. Coexisting cations of similar sizes competed with Ni to substitute with Mg, resulting in reduced uptake, except in the case of Fe³⁺ which disintegrated the LDH structure. In a binary system, the uptakes of W and Ni increased by 2.65 and 1.80 times, respectively, compared to their corresponding single systems, indicating an intriguing synergistic effect. Furthermore, the presence of Ni restored the LDH's ability to remove W in the presence of coexisting H₂PO₄^-, SO₄^2-, and CrO₄^2-. However, due to the presence of W, the coexisting Co²⁺ and Zn²⁺ inhibited Ni uptake more significantly. The crystallinity decrease of LDH was induced and identified as the cause of the uptake synergy between W and Ni. These findings provide valuable insights for the development of efficient multifunctional adsorbents and enhance our understanding of the transfer dynamics of W in the presence of coexisting substances.