Aplikasi hidrogel poli(asam akrilat)-kitosan h asil iradiasi gamma sebagai adsorben ion pb(ii) dan cd(ii): studi isoterm, kinetika dan termodinamika adsorpsi = Application of poly acrylic acid chitosan hydrogel prepared by gamma irradiation as adsorben of pb(ii) and cd(ii) metal ions isotherms, kinetics and thermodynamic adsorption studies