Stalagmites in karst cave inspired construction: lotus root-type adsorbent with porous surface derived from CO2-in-water Pickering emulsion for selective and ultrafast uranium extraction.

Simultaneous construction of porous and hollow adsorbent, especially from gas-in-water Pickering emulsion (PE) reactor, is vital for improving mass transfer kinetics and uptake amount. Inspired by the formation process of stalagmites in karst cave, amino and amidoxime bifunctionalized lotus root-type microsphere with porous surface (NH2@AO-PLRMS) is prepared by the silica nanoparticles (SPs)-stabilized CO2-in-water Pickering emulsion reactor and subsequent two-step grafting polymerization. The important roles of SPs acting as Pickering emulsifier, surface pore-forming agent, and adjusting internal lotus root structure are confirmed. Lotus root-type pores are dependent on the interface intensity and the permeability for compressed CO2 bubbles in PE droplets. Benefitting from the lotus root-type structure and abundant affinity sites, the maximum uranium adsorption capacity of NH2@AO-PLRMS is 1214.5 mg·g-1 at 298 k, and an ultrafast uptake process can be achieved in the first 30 min. Both thermodynamic and kinetic studies indicate a spontaneous, entropy increased, and exothermic chemisorption process, and the synergies of amidoxime and amino groups can enhance the adsorption selectivity. Remarkably, NH2@AO-PLRMS displays a high uranium adsorption capacity and desorption efficiency after seven cycles. These findings provide a way to obtain adsorbents with enhanced uranium extraction performance from gas-in-water PE reactor.