Synthesis, Characterization, and Application as a Chromium(VI) Adsorbent of Amine-Modified Polyacrylamide-Grafted Coconut Coir Pith

Abstract

The amine-modified polyacrylamide-grafted coconut coir pith carrying −NH3+Cl- functional group at the chain end (PGCP−NH3+Cl-) was investigated as an adsorbent for its possible application for the removal of chromium(VI) from aqueous solution and wastewater. The infrared spectroscopy results were used to confirm the graft copolymer formation and −NH3+Cl- functional group. The grafting of polyacrylamide onto the coir pith improved the thermal stability of the adsorbent and enhanced the apparent activation energy for the thermal degradation of PGCP−NH3+Cl-. X-ray diffraction pattern and scanning electron microscopy (SEM) studies were carried out to investigate the crystallinity and morphology of the adsorbent. The decrease in crystalline domains in PGCP−NH3+Cl- results in the loss of tensile strength of the grafted chain and consequently enhances the free mobility of the grafted chain. Batch adsorption technique using PGCP−NH3+Cl- was applied for the removal of chromium(VI) anion from aqueous solution and wastewater. The maximum adsorption of 99.4% (12.43 mg/g) took place from an initial concentration of 25.0 mg/L Cr(VI) at 30 °C, pH 3.0, and an adsorbent dosage of 2.0 g/L. The kinetics of sorption of Cr(VI) ions were described by a pseudo-second-order kinetic model. The temperature dependence indicates the exothermic nature of the process. Equilibrium isotherms were determined for different temperatures and the results are analyzed using the Langmuir and Freundlich isotherm equations. Adsorption isotherm experiments were also conducted for comparison using a commercial chloride form Dowex, a strong base (quaternary amine functionality) anion exchanger. Quantitative removal of 22.7 mg/L Cr(VI) in 50 mL of electroplating industry wastewater by 125 mg of PGCP−NH3+Cl- was observed at pH 3.0. Alkali regeneration was also tried for several cycles with a view to recover the adsorbed metal ions and also to restore the sorbent to its original state.