•We investigated the efficiency of agricultural wastes to remove heavy metals from aqueous solutions as single and competitive systems.•We examined the effect of chemical activation technique in order to enhance plant residues capacity for Ni removal.•Sorption of heavy metals was a function of pH and contact time.•The Langmuir and pseudo second order models well described the isotherm and kinetic of sorption of heavy metals respectively.•Determination of pHzpc of plant residues was useful parameter in describing sorption mechanism.•The native and modified plant residues used in present study had high capacity for removal of heavy metals from aqueous solutions.•The sorption mechanisms such as electrostatic attraction and ion exchange as well as complexation seem to be the most occurring phenomena. In this research, the residues of sunflower, potato, canola, and walnut shell were used as sorbents to sorb heavy metals (Fe, Mn, Zn, Ni, Cu, and Cd) from aqueous solutions using batch experiments. The effect of pH and contact time was investigated. Maximum sorption of heavy metals by different sorbents was observed around pH 4–8. The optimal contact time was in the range of 20–600 min. The values of pHzpc (zero point of charge) measured for sunflower, potato, canola, and walnut shell residues were 6.06, 6.80, 6.15 and 5.85, respectively. The plant residues showed considerable capacity for removal of Fe, Mn, Zn, Cu, and Cd from aqueous solution but Ni sorption by residues was negligible. Chemical activation of sorbents with NaOH enhanced Ni sorption. Competitive sorption decreased the removal of heavy metals compared to the single system. The Langmuir isotherm model fitted well to experimental metals sorption. The sorption kinetic of heavy metals was described well by pseudo second order model. In our study, the sorption mechanisms such as electrostatic attraction and ion exchange as well as complexation seem to be the most occurring phenomena. This figure indicates competitive sorption of six elements by different agricultural wastes. The simultaneous presence of metals in solution resulted in competition between different metal ions for sorption on the sorbent surface. The order of sorption of other metals (except Zn and Cd) was the same for sunflower and potato residues. The metals were not sorbed on the basis of Pauling (ionic radius) order. In a competitive system, the metals which were sorbed more than others by sunflower, potato, canola and walnut shell residue were Zn, Cd, Cd and Fe, respectively. Display omitted