Biosorption experiments were carried out in 250 mL Erlenmeyer
flasks comprising of 0.5 g of crab shell particles and 100 ml of metal solutions. The pH before and during the experiments were adjusted using 0.1MHCl or NaOH. The contents were agitated in a rotary shaker at 200 rpm (23±2 °C). After 8 h, the reaction mixture was filtered through a 0.45 μm PTFE membrane filter and analyzed for metal concentration using inductively coupled plasma-atomic emission spectrometer (ICP-AES; Perkin Elmer Optima 3000 DV). For pH edge experiments, initial Mn(II) and Zn(II) concentrations were fixed at 500 mg/L and the equilibrium pH was varied from 2 to 6. For isotherm experiments, initial Mn(II) and Zn(II) concentrations were varied from 10 to 1000 mg/L at equilibrium pH of 6. Kinetic experiments were conducted using the same method as above, except that the samples were collected at different time intervals to determine the time point at which biosorption equilibrium was attained. The metal uptake, the amount of metal ions biosorbed per gram of crab shell particles, was calculated using,