Modeling Biosorption Of Cadmium, Zinc And Lead Onto Native And Immobilized Citrus Peels In Batch And Fixed Bed Reactors

Abstract

Biosorption, i.e., the passive uptake of pollutants (heavy metals, dyes) from aqueous phase by biosorbents, obtained cheaply from natural sources or industrial/agricultural waste, can be a cost-effective alternative to conventional metal removal methods. Conventional methods such as chemical precipitation, membrane filtration or ion exchange are not suitable to treat large volumes of dilute discharge, such as mining effluent. This study is a continuation of previous research utilizing citrus peels for metal removal in batch reactors. Since fixed bed reactors feature better mass transfer and are typically used in water or waste water treatment using ion-exchange resins, this thesis focuses on packed bed columns. A number of fixed bed experiments were conducted by varying Cd inlet concentration (5-15 mg/L), bed height (24-75 cm) and flow rate (2-15.5 ml/min). Breakthrough and saturation uptake ranged between 14-29 mg/g and 42-45 mg/g respectively. An empty bed contact time of 10 minutes was required for optimum column operation. Breakthrough curves were described by mathematical models, whereby three popular models were shown to be mathematically identical. Citrus peels were immobilized within an alginate matrix to produce uniform granules with higher uptake capacity than raw peels. All breakthrough curves of native and immobilized peels were predicted using external and intra-particle mass transfer resistances from correlations and batch experiments, respectively. Several analogous mathematical models were identified; other frequently used models were shown to be the approximate derivatives of a single parent model. To determine the influence of competing metals, batch and fixed bed experiments were conducted in different binary combinations of Pb, Cd, Zn and Ca. Equilibrium data were analyzed by applying competitive, uncompetitive and partially competitive models. In column applications, high affinity Pb replaced previously bound Zn and Cd in Pb-Zn and Pb-Cd systems, respectively. However, the Cd-Zn system did not show any overshoot. Calcium, which is weakly bound, did not affect target metal binding as much as other metals. Saturated columns were desorbed with 0.1 N nitric acid to recover the metal, achieving concentration factors of 34-129. Finally, 5 g of citrus peels purified 5.40 L mining wastewater.