Amine–modified polyacrylamide–bentonite composite for the adsorption of humic acid in aqueous solutions

In this study a new polyacrylamide-bentonite composite with amine functionality (Am–PAA–B) was prepared by direct intercalation polymerization technique. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD) pattern, surface area analysis and potentiometric titration elucidated the characteristics of Am–PAA–B. To apply the Am–PAA–B for the removal of humic acid (HA) commonly found in wastewaters, batch tests were conducted to evaluate the adsorption capacity. The effect of process parameters such as agitation time, concentration of HA, pH, ionic strength, adsorbent dose and temperature on the extent of HA adsorption from solutions was investigated. The pH dependent sorption of HA was found maximum (>99.9%) at pH range 4.0–7.0 for an initial concentration of 25 μmol/L. The removal mechanism of HA by Am–PAA–B was mainly through the complexation of the carboxyl groups in HA with the protonated NH2 groups in Am–PAA–B. The kinetics of adsorption follows pseudo-second-order model and the rate constant increases with increase in temperature indicating endothermic nature of adsorption. The equilibrium conditions were achieved within 3 h under the mixing conditions employed. The adsorption was endothermic and the rate-limiting step was found to be film diffusion process. The equilibrium data were well fitted to the Langmuir isotherm. The monolayer adsorption capacity was found to be 103.91 μmol/g at 30 °C. Regenerative efficiency of Am–PAA–B was assessed for four consecutive adsorption/desorption cycles and found to be favorable. Conclusively, we found that the Am–PAA–B was significantly effective for the removal of HA from aqueous solutions.