Buoyancy-enabled direct lithium extraction from brines using waste-derived Li-Al LDH-coated cenospheres

Direct lithium extraction (DLE) from salt-lake brines is challenged by the low concentration of Li⁺ and abundant competing ions, as well as the difficulty of separating adsorbents after adsorption. Here, we develop a buoyancy-enabled adsorption strategy using a self-floating solid-bubble adsorbent (SBA) composed of fly ash cenospheres coated with Li-Al layered double hydroxide (LDH). The low-density cenosphere core provides buoyancy and mechanical support, while the Li-Al LDH shell supplies vacancy sites for selective Li⁺ capture. The resulting SBA spontaneously floats at the brine-air interface, enabling interfacial Li⁺ adsorption and direct surface recovery without packed-column operation or subsequent solid-liquid separation. In simulated and authentic Qarhan brines, SBA exhibited high Li⁺ selectivity, with a Li⁺/Mg²⁺ separation coefficient of 699.59 and a Li⁺ uptake of 2.51 mg·g^-1 in authentic brine. After 10 adsorption-desorption cycles, the Li⁺ uptake remained 2.39 mg·g^-1, while 91.65% of the adsorbent remained afloat under dynamic disturbance. Mechanistic analysis indicates that Li⁺ selectivity arises mainly from size-compatible insertion into LDH vacancies, whereas competing ions are constrained by hydration and size mismatch. This work demonstrates a waste-derived, buoyancy-enabled approach that integrates selective Li⁺ capture with facile adsorbent recovery for lithium extraction from brines.