Optimization of Engineering Parameters for Continuous Mode Biosorption

Abstract

The discharge of heavy metals into aquatic ecosystems has become a matter of concern over the last few decades. They are extremely toxic elements, which can seriously affect plants and animals and have been involved in causing a large number of afflictions. Due to the high costs of commercial adsorbents, biosorption is an economically feasible way to perform the treatment of potentially toxic species-containing effluents. It is the sequestration of metals by the dead natural material called biosorbent. This research presents experimental optimization of different engineering parameters such as Biosorbent type, Immobilization method, initial solution pH, flow rate, Initial metal concentration and Desorbing agent for the removal of Cu(II). Three types of wastes namely Crop wastes (CW) including Bagasse, Rice Husk, Rice Straw, Wheat Straw, Waste leaves (WL) including Peeple Leaves, Semal Leaves and Dry fruit Waste (DW) including Almond Shell and Walnut shell were selected as they are abundantly available in local areas. First of all, each biosorbent was prepared through washing, crushing and grinding followed by size analysis through seven ASTM screens. Detailed size analysis was performed and presented for the first time in literature to link it with biosorption. This data was used to calculate different mean diameters namely volume surface mean, mass mean and length mean diameters. Size frequency curve was also plotted. For almond shell the volume surface mean diameter showed close resemblance to the median obtained from the size frequency curve while for all the remaining biosorbents, mass mean diameter came out to be representative diameter. Scanning Electron Microscopy was performed to establish different aspects (Physical and chemical) of biosorbent structure. CW showed needle like particles with complex flaky or porous structure. LW showed cylindrical particles with holes while DW showed aggregatesof small particles. FTIR analysis showed typical lignocellulosic profile for each biomass showing the presence of multiple functional groups including –OH, -COOH. XRD showed amorphous nature of the biosorbents. These characterizations showed that selected biosorbent possess structure suitable for biosorption. Selection of the optimum biosorbent was performed through typical kinetic and equilibrium experiments. In these experiments WL showed highest uptake of Cu(II) while CW and DW showed intermediate to low uptakes. Biosorption generally takes place in acidic environment. Therefore, stability of the selected biosorbents (Wheat Straw, Peeple leaves, Seemal Leaves, Almond Shell and Rice Husk) in 0.1M HNO 3 was tested. Almond Shell showed best and WL showed worst results. Thus, Almond Shell (AS) was selected as optimum Biosorbent. This approach has been applied for the first time to select an optimum biosorbent. Among different options of continuous contactors, fixed bed column is preferred due to its effective utilization of biomass bed and subsequent separation from the adsorbate solution. The most important parameter in the fixed bed operation is the nature of immobilization of the biomass. Generally, entrapment of the biosorbent is being performed to apply it in the fixed bed column. However, this process results in addition of a resistive layer of immobilizing media which hinders the mass transfer. Therefore, a novel surface immobilization mm and have complex structure. Braummer-Emmett-Teller (BET) area of beads showed a suitable adsorption area 0.8094 m 2 /g. For fixed bed experiments, optimum pH was estimated to be 4.6. It was an intermediate pH between two extremes i.e., point of zero charge (pH = 3.8) and point of precipitation (pH =5.1). Up flow arrangement was selected due to its inherent capability to avoid channelling. Bed height was selected to 20 cm to avoid axial dispersion of the Cu(II). Column experiments were performed at 19.7mL/min and 36 mL/min to determine optimum flow rate at 10 ppm Cu(II) concentration. Time required to achieve 50 % break through was greater in case of 19.7 mL/min than 36 mL/min. 19.7 mL/min flow rate corresponded to about 3 min residence time in the column and taken as optimum flow rate. In order to determine the optimum influent concentration three column experiments were performed at different initial concentration namely 10 ppm, 50 ppm and 100 ppm. As, expected very quick breakthrough took place at 50 ppm and 100 ppm. Hence, 10 ppm was selected an optimum initial concentration. HNO 3 and NaOH were selected to determine the optimum desorbent. These chemicals were selected due to the fact that H + and Na + ’ metals. Experiments showed that HNO 3 recovered much more Cu(II) ions than NaOH. Also, in case of NaOH, a notable deterioration of immobilized beads was found. Almond shell was identified as an optimum biosorbent as it showed moderate uptake and highest acid resistance amongst selected biosorbent. Kinetic modelling showed that, pseudo 2 nd order kinetic best described biosorption of Cu(II) by AS. Freundlich model enumerated the equilibrium behaviour of Cu(II) removal by AS. Application of Dubinin-Radushkevic (D- R) model suggested the physio-sorption while Boyd's model showed liquid film diffusion as the rate controlling step in the biosorption of Cu(II) by AS. Powdered AS was immobilized on the surface of beads which showed moderate BET area. Beads showed best performance (50% break through) at lower flow rate (19.7 mL/min) and low Cu(II) concentration (10 ppm). It was possible to regenerate beads saturated with Cu(II) ions using 0.1M HNO 3 without any damage. Thomas model was applied to describe the break through curves. It isrecommended to further explore the surface immobilization of different biosorbents to test their effect on the breakthrough of different pollutants (binary and tertiary) in fixed bed column.