Synthesis, characterization, and application as a mercury(II) sorbent of banana stalk (Musa paradisiaca) -: Polyacrylamide grafted copolymer bearing carboxyl groups

Acrylamide was graft polymerized onto banana stalk, BS (Musa paradisiaca), using the ferrous ammonium sulfate/H2O2 redox initiator system in an aqueous medium. The effects of reaction variables such as time, temperature, and monomer and initiator concentrations on the percentage grafting were studied. A new adsorbent carrying a carboxylate functional. group at the, chain end was synthesized by surface modification of polyacrylamide-grafted BS (PGBS-COOH). Infrared. spectroscopy and acid-base titration were used to confirm graft copolymer formation and carboxylate functionalization. X-ray diffraction and SEM studies were carried out to investigate the crystallinity and morphology of the adsorbents. XRD studies indicated that the grafting of polyacrylamide resulted in a consequent decrease in crystallinity. SEM studies of PGBS-COOH clearly indicated that polyacrylamide grafts deposited more on the surface of the unit cell than in the intercellular gaps. A probable mechanism for graft copolymerization and surface functionalization is also suggested. The use of this adsorbent material for the removal of Hg(II) from water and wastewater was investigated using the batch adsorption technique. The adsorption of Hg(II) on the adsorbent was found to be pH-, time-, concentration-,and temperature-dependent. The optimum pH range for the process was found to be 6.0-9.0. Maximum removals of 99.3 and 84.1% were observed at Hg(II) concentrations of 50 and 100 mg/L, respectively, in this pH range. Removal of Hg(II) is adversely affected by increasing initial Hg(II) concentration. The adsorption process follows pseudo-second-order kinetics. Kinetic parameters as functions of initial concentration. and temperature were, calculated. Hg(H) adsorption was found to decrease with increasing ionic strength. The L-type adsorption isotherm obtained for the adsorbent indicated a favorable process and fitted the Langmuir isotherm model well. The adsorption capacity for Hg(II), calculated using the Langmuir isotherm equation was 137.89 mg/g at 30 degreesC, which increased to 210.50 mg/g at 60 degreesC. Thermodynamic parameters such as the changes in free energy, enthalpy, and entropy were calculated to predict the nature of adsorption. The isosteric heat of adsorption was found to be 49.89 +/- 1.31 kJ/mol and was independent of surface coverage. Chlor-alkali industrial wastewater samples were treated with this adsorbent to demonstrate its efficiency in removing Hg(II) from industrial wastewater. Recovery,of Hg(II) after adsorption and regeneration of the adsorbent for several cycles can be carried out by treatment of the loaded adsorbent with 0.2 M HCl.