Water pollution is the most challenging issue facing mankind nowadays as a result of ever-increasing population and steadily improving life standards. More and more wastewater without effective treatments is arbitrarily discharged into the aquatic environment, which causes irreversible damage and incalculable loss. Especially, the industrial wastewater containing heavy metal ions, such as copper (Cu), cobalt (Co), and manganese (Mn), are toxic to human and living organisms even at low concentrations. A highly efficient and selective removal method has been long sought for heavy metal ions wastewater treatment. In this study, a highly monodispersed polyethylenimine-chitosan (PEI-CS) biosorbent, synthesized by an integrated process with facile microfluidic emulsion, chemical crosslinking, solvent extraction and chemical modification, was used as an efficient adsorbent to remove heavy metal ion from contaminated water. The as-prepared biosorbents demonstrated ultra-high adsorption capacity of 146 mg g−1 towards Cu ions, almost triple the performance reported in literature. Besides, PEI-CS biosorbents were also endowed with good mechanical strength, excellent adsorption selectivity towards targeted ion in the presence of other metal ions with different valence states, and high reusability. Therefore, the newly developed PEI-CS biosorbents is a highly promising candidate for wastewater treatment with heavy metal ions.