Alginate-immobilized exopolymeric substances from bacteria for Lead (Pb) removal: biosorption and reusability studies

In this study, exopolymeric substances (EPS) from Bacillus cereus were immobilized in alginate to form alginate-EPS beads. The surface characteristics of the alginate-EPS beads were examined using Scanning Electron Microscopy (SEM) and Fourier-Transform Infrared Spectroscopy (FTIR). The mechanisms of Pb removal was determined based on equilibrium and kinetic models. Packed-bed biosorption studies using Pb solutions (750 mg L^-1 concentration) evaluated their efficacy for Pb uptake and removal, under the influence of initial pH of metal solution and the size of biosorbents. The reusability of the alginate-EPS beads was also tested by examining the column regeneration, exhaustion time and sorption-desorption activities of the Pb-loaded beads. Results revealed that the alginate-EPS beads produced have uneven surface with various functional groups (hydroxyl, amide, carboxyl, phosphates) detected. Their Pb removal efficacy was highest in Pb solutions with pH 4, recording 51.52% Pb removal, followed by pH 6 (48.62%) and pH 8 (46.78%). For bead size, alginate-EPS beads measuring 3 mm diameter size was more efficient in Pb removal (51.52%), compared to 2 mm- (39.23%) and 5 mm-sized beads (28.52%). All alginate-EPS beads complied with the Langmuir isotherm (R² = 0.992) and pseudo-second order kinetic (R² = 0.995), suggesting monolayer adsorption is likely to have occurred on their homogenous surface. The alginate-EPS beads were successfully used for five adsorption cycles, with consistent Pb removal (41.40-50.59%). This study recommends the use of 3 mm bead size for Pb removal (pH 4 solutions) due to greater Pb removal (51.5%) and uptake (13.29 mg g^-1), and reasonable exhaustion time (540 min). There is potential of applying alginate-EPS beads in packed-bed systems as a feasible and effective method for the removal of heavy metals from wastewater.