Hydrocolloid liquid-core capsules for the removal of heavy-metal cations from water

[Display omitted] •Novel liquid-core capsules with a non-crosslinked alginate core were produced.•Capsules demonstrated highest efficiency adsorption of ∼300mg Pb2+/g alginate.•Regeneration was carried out by suspending capsules in 1M HNO3 for 24h.•Adsorption capacities of the capsules followed the order: Pb2+>Cu2+>Cd2+>Ni2+. Liquid-core capsules with a non-crosslinked alginate fluidic core surrounded by a gellan membrane were produced in a single step to investigate their ability to adsorb heavy metal cations. The liquid-core gellan–alginate capsules, produced by dropping alginate solution with magnesium cations into gellan solution, were extremely efficient at adsorbing lead cations (267mg Pb2+/g dry alginate) at 25°C and pH 5.5. However, these capsules were very weak and brittle, and an external strengthening capsule was added by using magnesium cations. The membrane was then thinned with the surfactant lecithin, producing capsules with better adsorption attributes (316mg Pb+2/g dry alginate vs. 267mg Pb+2/g dry alginate without lecithin), most likely due to the thinner membrane and enhanced mass transfer. The capsules’ ability to adsorb other heavy-metal cations – copper (Cu2+), cadmium (Cd2+) and nickel (Ni2+) – was tested. Adsorption efficiencies were 219, 197 and 65mg/g, respectively, and were correlated with the cation’s affinity to alginate. Capsules with the sorbed heavy metals were regenerated by placing in a 1M nitric acid suspension for 24h. Capsules could undergo three regeneration cycles before becoming damaged.