Kinetic studies for Cd(II) biosorption from treated urban effluents by native grapefruit biomass (Citrus paradisi L.): The competitive effect of Pb(II), Cu(II) and Ni(II)

This paper describes the ability of native grapefruit biomass (NGB), a by-product of the food industry, to adsorb Cd(II) ions from treated urban effluents (TUE) through batch experiments in competitive conditions, after having been demonstrated its effectiveness in aqueous solutions. No statistically significant differences could be observed for Cd(II) biosorption according to the particle size of the sorbent (experiments E1), although a clear decrease in the percentage of Cd(II) retrieved was due to the presence of Ca2+ and Na+ ions in the TUE (experiments E2). However, when batch experiments were carried out in a multi-ion system (experiments E3), the uptake capacity of the NGB for Cd(II) was clearly inhibited by the presence of Pb(II), Cu(II) and Ni(II) ions, decreasing down to 24.05%. The affinity order for the four heavy metals was Pb(II) > Cu(II) > Ni(II) > Cd(II), despite a much lower Pb(II) concentration in the TUE (3 mg/L) in comparison to the other three heavy metals (100 mg/L). Different factors such as ionic radius, electroaffinity, hydrated radius and atomic weight can explain these results, which almost suppressed Cd(II) removal from TUE by NGB. Although the biomass showed a relative selectivity toward Pb(II) ions, the intraparticle diffusion mechanisms proved to be the rate-controlling factor in the removal of Cu(II) ions over Ni(II) and Cd(II). There was also a decrease in Cd(II) biosorption due to the presence of Ca2+ and Na+ ions together with the four heavy metals (experiments E4), although this difference was not statistically significant. Common ions F−, C2O4 and Ca2+ were desorbed and NH4+ biosorbed while heavy metals were uptaken by NGB. Although H2SO4, HCl and HNO3 showed the highest desorption efficiencies, they severely damaged the biosorbent and could not be used in repeated adsorption–desorption cycles.

► Cd(II) removal decreased from aqueous solutions to treated urban effluents. ► Despite an initial lower concentration, Pb(II) ions were efficiently removed. ► Cu(II) ions displayed the highest intraparticle diffusion rate constant. ► The biosorption mechanism for Pb(II) was surface adsorption. ► The biosorption order under competitive conditions was Pb > Cu > Ni > Cd.