Sub-100 nm resorcinol-formaldehyde hollow spheres to remove heavy metals from water

Developing adsorptive materials with high removal efficiency is essential for treating heavy metals in water systems and mitigating their severe environmental and health impacts. This study synthesised three-dimensional resorcinol–formaldehyde hollow spheres (RF-HSs) with micro- and sub-100-nm particle sizes for the adsorptive removal of cadmium (Cd) and lead (Pb). The materials were prepared via an extended Stöber method and a water-in-oil microemulsion method to produce micrometre- and nanometre-sized spheres, respectively. Batch adsorption experiments were conducted to determine optimal operating conditions, yielding an adsorbent dose of 2 mg, a contact time of 10 min, and a solution pH of 5.5. The nano-sized RF-HSs exhibited significantly higher removal performance for Cd and Pb than their micro-sized counterparts. Maximum adsorption capacities of 90 mg/g (Cd) and 271 mg/g (Pb) were achieved within a 20 min contact time. In complex real-water matrices, RF-HS nanoparticles achieved removal efficiencies of 85–97%. Furthermore, the nanospheres could be regenerated and reused for up to 10 cycles without any measurable loss in removal efficiency, thereby reducing the adsorption costs to $6.12 USD/g for Cd and $2.03 USD/g for Pb. Overall, the results indicate that RF-HS nanoparticles are a sustainable, cost-effective, and high-performance adsorbent for removing heavy metals from contaminated water systems.