A novel adsorbent was prepared and its adsorption properties for Cr(VI) were studied. The iron-(III) complex of a carboxylated polyacrylamide-grafted sawdust has been found to be an effective adsorbent for the removal of Cr(VI) from aqueous systems. Experiments were carried out as a function of concentration of Cr(VI), agitation period, agitation speed, pH, ionic strength, and temperature. Maximum removal (>99.0%) was observed at an initial concentration of 25.0 mg/L in the pH range 2.0-3.0. Coordination unsaturated sites for the iron(III) complex of polymer were considered to be the adsorption sites for Cr(VI) species, with the predominant species being HCrO 4 -. Rate constants as a function of concentration and temperature were evaluated with the help of a proposed second-order kinetic model. The percentage removal of Cr(VI) decreased with increasing ionic strength. The L-type adsorption isotherm obtained for the adsorbent indicated a favorable process. Adsorption isothermal data could be interpreted by the Langmuir and Freundlich equations. The uptake of Cr(VI) on adsorbent increased from 144.20 mg/g at 20 °C to 172.74 mg/g at 60 °C. Thermodynamic parameters such as ∆G°, ∆H°, and ∆S°for the adsorption process were calculated. The isosteric heat of adsorption was also determined at various surface loadings of the adsorbent used. Simulated Cr(VI) electroplating wastewaters were also treated by the sorbent to demonstrate its efficiency in removing Cr(VI) from wastewater with other ions. Desorption studies showed that over 95.5% of Cr(VI) can be desorbed from the adsorbent using 0.1 M NaOH.

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.

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Iron(III)-loaded carboxylated polyacrylamide-grafted sawdust was investigated as an adsorbent for the removal of phosphate from water and wastewater. The carboxylated polyacrylamide-grafted sawdust was prepared by graft copolymerization of acrylamide and N,NЈ-methylenebisacrylamide onto sawdust in the presence of an initiator, potassium peroxydisulfate. Iron(III) was strongly attached to the carboxylic acid moiety of the adsorbent. The adsorbent material exhibits a very high adsorption potential for phosphate ions. The coordinated unsaturated sites of the iron(III) complex of polymerized sawdust were considered to be the adsorption sites for phosphate ions, the predominating species being H 2 PO 4 Ϫ ions. Maximum removal of 97.6 and 90.3% with 2 g L Ϫ1 of the adsorbent was observed at pH 2.5 for an initial phosphate concentration of 100 and 250 mol L Ϫ1 , respectively. The adsorption process follows secondorder kinetics. Adsorption rate constants as a function of concentration and temperature and kinetic parameters, such as ⌬G Ϯ , ⌬H Ϯ , and ⌬S Ϯ , were calculated to predict the nature of adsorption. The L-type adsorption isotherm obtained in the sorbent indicated a favorable process and fitted the Langmuir equation model well. The adsorption capacity calculated by the Langmuir adsorption isotherm gave 3.03 ϫ 10 Ϫ4 mol g Ϫ1 of phosphate removal at 30 o C and pH 2.5. The isosteric heat of adsorption was also determined at various surface loadings of the adsorbent. The adsorption efficiency toward phosphate removal was tested using industrial wastewater. Different reagents were tested for extracting phosphate ions from the spent adsorbent. About 98.2% of phosphate can be recovered from the adsorbent using 0.1M NaOH. Alkali regeneration was tried for several cycles with a view to recover the adsorbed phosphate and also to restore the adsorbent to its original state.

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