A sol-gel encapsulated biomass process for the removal ofcopper(II) ions from water and wastewater

Abstract

Industrial effluents laden with heavy metals must be pretreated before they are discharged into the aquatic environment. The chemical precipitation treatment method produces a large amount of hazardous sludge for disposal. Other conventional treatment methods such as electrochemical treatment and ion exchange are expensive and incapable of removing trace levels of copper(II) ions. Alternatively, biological materials can be applied for the removal and recovery of heavy metals due to their good performance and low cost. Micrococcus sp., which is a Gram-positive bacterium isolated from a local activated sludge process, was proved to be an effective biosorbent for copper(II) removal. However, the use of freely-suspended biomass for biosorption is impractical. This study therefore aimed to develop a novel immobilized cell process for copper(II) removal and recovery from industrial wastewater. The Micrococcus sp. suspended cells were first immobilized in sol-gel/PVA matrix and the immobilized cells were then applied in the biosorption and desorption processes for the removal and recovery of copper(II) ions. The novel sol-gel immobilization technique was used to entrap Micrococcus sp. cells in a sol-gel derived material of silica and polyvinyl alcohol, which has been shown to provide a biocompatible microenvironment for microorganisms. Immobilized biosorbents with both spherical and cylindrical shapes were prepared and their biosorption performances were compared. The optimum conditions for the preparation of immobilized biosorbents were determined. The results indicate that the copper(II) uptake capacity of the spherical biosorbent was higher than that of the cylindrical biosorbent. The biosorption of copper(II) ions was affected by the composition of the sol-gel/PVA matrix as well as the concentration of biomass in the biosorbent. The optimum drying time of the spherical and cylindrical biosorbents was two days and three days, respectively. The effects of pH, biomass dosage, metal concentration, contact time and agitation speed on copper(II) biosorption by the biosorbents were studied in a batch system. Copper(II) biosorption was highly dependent on the solution pH and a maximum biosorption capacity was generally reached at pH 5.0. The Langmuir and Freundlich isotherm models were applied to simulate mathematically the equilibrium data. The Langmuir model could better simulate the experimental data with r2 >= 0.99. The pseudo-second order kinetic model could simulate the kinetic data very well with r2 >= 0.96. The rate of copper(II) uptake increased significantly with an increase in the agitation speed from 30 rpm to 100 rpm, and a slight enhancement was observed with a further increase in the agitation speed. The surface morphology of the immobilized biosorbents before and after the copper(II) uptake was examined by a scanning electron microscopy and the surface morphology of the biosorbent changed after the copper(II) uptake. Desorption studies were then conducted to determine the metal recovery efficiency of the metal-laden biosorbent using nine different desorbing agents. The most effective desorbing agent for copper(II) recovery was 0.1 M NTA. The optimum desorption time for both the spherical and cylindrical biosorbents was 6 h. Spherical biosorbent was preferred to cylindrical biosorbent due to its faster copper(II) biosorption and desorption rates. The reusability of the suspended biomass and immobilized biosorbent was compared by performing repeated biosorption/desorption cycles. In contrast to suspended cells, immobilized biosorbent can solve the problem of cell loss in multiple biosorption/desorption cycle operations. Finally, a four-stage semi-continuous immobilized cell batch reactor system was applied for removing and recovering copper(II) from both synthetic and industrial wastewater. This process was capable of producing the effluent at low copper(II) concentration for both synthetic and industrial wastewater. The results indicate that the immobilized cell process can be further developed into practical reactor systems for advanced wastewater treatment.