Removal of phosphorus (P) as a pollutant from aqueous solutions by the biological materials is a new approach by which plant and microbial biomass are used as sorbents. In this research, the residues of sunflower, potato, canola, and walnut shell in the form of native and chemically modified were used as biosorbents to remove P from aqueous solutions. Sorption of P by these sorbents was studied using batch technique. The effect of pH and contact time was also investigated. Optimum pH for P sorption by sunflower, potato, canola, and walnut shell residues was found to be 5, 3, 5, and 3.5, respectively. The values of zero point charge (pHzpc) measured for sunflower, potato, canola, and walnut shell residues were 6.06, 6.80, 6.15, and 5.85, respectively. Maximum sorption of P by native sorbents was found to be in the range of 2.8–4.3 mg g−1. Chemical modification of the sorbents was carried out by urea, FeCl3, and CaCl2. The modification of sorbents increased the pHzpc and consequently increased the P sorption. Phosphorus sorption isotherm was described well by Langmuir isotherm model. The speciation of P in solution was carried out by Visual Minteq program and indicated that the dominant species of P in solution of all sorbents was H2PO 4 . The P sorption kinetic followed the pseudo-second-order model. Scanning electron microscopy images revealed that modification of the sorbents resulted in the development of the porous surface with coarse fibrous texture making the surface more suitable for the attachment of reactive functional groups.
