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.
