Steam activation of porous concave polymer nanospheres for high-efficient chromium and cadmium removal

[Display omitted] •Activated concave APF nanospheres with high surface area and abundant functional groups (e.g., amines, imines) were successfully synthesized.•The efficient uptake of Cr(VI) and Cd(II) ions from aqueous medium were 1054 and 342 mg g−1, respectively.•Approximately 71.5% of Cr(VI) was simultaneously reduced to Cr(III).•The adsorption mechanism and reduction mechanism were explored. The issue of heavy metal contamination in water is a global concern, and the development of highly efficient adsorbent materials is crucial for the removal and detoxification of heavy metals. Polymer-based materials have emerged as a promising class of adsorbents due to their ability to capture heavy metal pollutants and reduce them to less toxic forms. The limited surface area of conventional polymer adsorbents makes them less effective for high-capacity adsorption. Herein, we present a low-temperature steam activation approach to address this challenge. This activation approach leads to a remarkable increase of over 20 times in the surface area of concave aminophenol-formaldehyde (APF) polymer nanospheres (from 45 to 961 m2/g) while preserving their reductive functional groups. The activated concave APF nanospheres were evaluated for their adsorption capabilities towards two typical heavy metal ions (i.e., Cr(VI) and Cd(II)) in aqueous solutions. The maximum adsorption capacities achieved were 1054 mg g−1 for Cr(VI) and 342 mg g−1 for Cd(II), which are among the highest performances reported in the literature and are much higher than the capacities of the non-activated APF nanospheres. Additionally, approximately 71.5 % of Cr(VI) was simultaneously reduced to Cr(III) through the benzenoid amine pathway during adsorption, highlighting the crucial role of the steam activation strategy in enhancing the capability of polymer adsorbents.