A self-floating hydrogel for heavy metal recovery from water

Heavy metal pollution is a serious environmental problem endangering the ecosystem and human health. The removal of heavy metals becomes crucial for water treatment. However, from the point of view of thermodynamics, it is a big challenge to transform the heavy metals from the homogeneous state in bulk liquid to the heterogeneous state in a collection area. Here we show a facile strategy of collecting heavy metals from bulk liquid to water surface by engineering a self-floating hydrogel. We construct the hydrophobic surfaces on the polyacrylamide/polyvinylpyrrolidone double-network hydrogel by simply coating with silica nanoparticles (SiNPs). Both of our experiments and computation analysis indicate that the SiNPs-coated hydrogel can self-float stably on the water surface, due to the balance between the interfacial tension, buoyancy force, and gravitational force. We further show that the self-floating hydrogel can effectively adsorb various heavy metals, i.e., Cu²⁺, Fe³⁺, and Ni²⁺, from water and enrich them inside the hydrogel, which decrease the concentration of heavy metals in water. Our findings present new insights into the hydrophobicity-induced self-floatation of soft matters on bulk liquid and provide a novel strategy for heavy metal efficient recovery, promising great applications in chemical and biosensors, drug delivery and water purification.