A multicomponent polysaccharide obtained from dried tubers of certain natural terrestrial orchids was chemically modified by sulfonation using chlorosulfonic acid-dimethylformamide (HClSO3-DMF) complex as a reagent. For a structural characterization of salep sulfate H-1 nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectra, and Thermogravimetric analysis (TGA) curves were recorded. The sulfate content of modified salep was determined using elemental analysis. This modified biopolymer was used to prepare a new environment-friendly heavy metal ion adsorbent, salep sulfate-graft-polyacrylic acid hydrogel (SS-g-PAA). Swelling rate and equilibrium water absorbency in various pH and saline solutions were investigated to study the effect of salep sulfate on swelling behavior of the hydrogel. In addition, the effect of sulfate content on heavy metal ion adsorption from aqueous solution was investigated. The results show that SS-g-PAA can effectively remove heavy metal ions (Co2+, Zn2+, Cu2+) from aqueous solution and swelling behavior of the hydrogels highly dependent on the amount of sulfate group on corresponding modified polysaccharide. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3001-3008, 2013