Physicochemical characterization and mechanism analysis of native and protonated grapefruit peels adsorbing cadmium

By-products such as citrus peels are low-cost biosorbents for heavy metal removal from industrial wastewaters. The cadmium-binding mechanism for native and protonated grapefruit peels was investigated. Potentiometric titrations were described well by a two-site model based on carboxyl and hydroxyl sites with pK(a) values of 3.9 and 11.1, respectively. A one-site model excellently described increasing metal binding from pH 1 to pH 6 due to decreasing competition with protons. Sorption isotherms showed a maximum Cd2+ uptake of 1.7 and 2.2 meq/g for native and protonated peels, respectively. An isotherm model with a 1:2 stoichiometry, where one divalent metal binds to two monoprotic sites, was superior to a 1:1 stoichiometry. For protonated peels, mainly protons were exchanged with Cd2+; for native peels, light metal ions were exchanged. Cd2+ sorption was completely reversible by acidic desorption, maintaining the sorption capacity during several cycles. Esterification of carboxyl groups, which reduced Cd2+ binding by 80%, showed their importance in metal binding. Formaldehyde treatment to block amine and hydroxyl groups reduced Cd2+ binding by a smaller extent. Fourier transform infrared spectra indicated the involvement of carboxyl and hydroxyl groups in metal binding. Grapefruit waste is a promising biosorbent with high capacity and stability.