Comparisons of two chelating adsorbents prepared by different ways for chromium(VI) adsorption from aqueous solution

Poly(catechol-diethylenetriamine- p -phenylenediamine) (PCEA1) and Poly(catechol-diethylenetriamine-melamine) (PCEA2), two kinds of similar chelating adsorbents with analogous structures, were used for the comparative study on Cr(VI) removal from aqueous solution. Many methods were employed to characterize these adsorbents, including scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectrometer (EDS). Batch experiments were carried out to investigate the adsorption performance of Cr(VI), such as pH value, adsorbent dosage, initial concentration, contact time and temperature. Furthermore, adsorption isotherm , intra-particle diffusion, kinetics and thermodynamics were studied completely and systematically. XPS technique confirmed that the reduction from toxic Cr(VI) to nontoxic Cr(III) existed in the adsorption process . After that, the removal of Cr(VI) in simulative wastewater caused by both adsorbents was nicely fitted with the Langmuir isotherm model and pseudo-second-order kinetic model . The maximum adsorption values of the PCEA1 and PCEA2 were 200   mg/g and 175.4   mg/g, respectively. The intra-particle diffusion study further demonstrated that adsorption processes all had two portions. The negative ΔG and positive ΔH indicated that the adsorption was spontaneous and endothermic. Tests on Cr(VI) removal indicated the potential applicability of the two adsorbents with a similar adsorption mechanism.