For circumventing the cumbersome and expensive multifunctional and multipolymer adsorbents for high-performance removals of hazardous water-contaminant(s), chitosan-g-[2-acrylamido-2-methyl-1-propanoic acid (AMPS)-co-2-(3-acrylamidopropanamido)-2-methylpropane-1-sulfonic acid (APAMPS)-co-2-(N-(3-amino-3-oxopropyl)acrylamido)-2-methylpropane-1-sulfonic acid (NAOPAMPS)-co-acrylamide (AM)] (i.e., chitosan-g-tetrapolymer), a multifunctional scalable and reusable hydrogel, was synthesized by grafting of chitosan and in situ attachments of N–H functionalized NAOPAMPS and APAMPS hydrophilic acrylamido-monomers during free-radical solution-polymerization of the two ex situ added AMPS and AM monomers in water. The response surface methodology was employed to synthesize one hydrogel envisaging the optimum balance between swelling and stability for the superadsorption of Cu(II), Cd(II), Pb(II), Cr2O72−, and HPO42−. The in situ attachments of NAOPAMPS and APAMPS, grafting of chitosan into tetrapolymer, structures and properties, pH-responsive abilities, superadsorption mechanism, and reusability were understood via in depth microstructural analyses of adsorbed and/or unadsorbed chitosan-g-tetrapolymer(s) through 1H/13C NMR, FTIR, XPS, TGA, XRD, DLS, and pHPZC. The maximum adsorption capacities of Cd(II), Cu(II), Pb(II), Cr2O72−, and HPO42− were 1374.41, 1521.08, 1554.08, 47.76, and 32.76 mg g−1, respectively. © 2019 Elsevier Inc.