Adsorption and enrichment of low-concentration heavy metal complexes by retractable claws made of polyelectrolyte brushes

The removal of heavy metal complexes from water at low concentrations poses a difficult challenge as the cost of chemical or energy input is often disproportional to the benefits gained. Herein, we propose a novel strategy by employing polyelectrolyte brushes as the "retractable claws" for adsorption and enrichment of heavy metal complexes. The polyelectrolyte brush constructed by grafting poly[2-(methacryloyloxy)ethyl trimethylammonium chloride] on the reduced graphene oxide (rGO-PMTC), exhibits a high adsorption capacity of 4.80 mg g-1 for 4.96 mg L-1 of Cu-ethylenediaminetetraacetic acid (Cu-EDTA), which is nearly 30 times larger than that of parent graphene oxide. Moreover, a very rapid adsorption rate can be achieved as 7.17 g mg -1 min -1, which is up to 3 orders of magnitude higher than many reported adsorbents. Molecular dynamics simulations reveal that during the adsorption process, the flexible polyelectrolyte brushes are fully extended to form the "claws" for quickly capturing Cu-EDTA complexes by strong electrostatic attraction. Subsequently, the polyelectrolyte brushes shrink with Cu-EDTA-mediated bridging and wrap the heavy metal complexes inside. In addition, the adsorbed Cu-EDTA complexes can be completely released from the polyelectrolyte brushes by simply introducing sodium chloride, resulting in an almost 20-fold enrichment of metal concentration for recycling. After ten adsorption-desorption cycles, the polyelectrolyte brushes only have 10 % reduction in the adsorption efficiency. The novel adsorbent can be extended to the effective removal of other heavy metal complexes with a low concentration and real electroplating wastewater.