Abstract
A novel tannic acid–reduced graphene oxide (TAG) composite absorbent was prepared by using a simple one-step method. The prepared TAG was characterized by various analytical techniques including FTIR, Brunauer–Emmett–Teller (BET), scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Tannic acid (TA) not only can prevent graphene sheets from agglomerating by suppressing the hydrophobic stacking interactions but also can introduce a large number of phenol hydroxyls. The adsorption studies indicate that the TAG composite showed an excellent uptake capacity (179.22 mg/g) for Cr(VI) under the optimized conditions (viz., adsorbent dosages of 0.75 g/L, solution pH of 2.5, contact time of 360 min, and temperature of 60 °C), which is higher than that of most graphene oxide– and TA-based adsorbents. The BET specific surface area of TAG was calculated to be 118.3 m2/g; the pore diameter of Tannin acid-reduced graphene hydrogel (TAGH) was mainly 3–6 nm, with pore size peak at 5.0 nm. The kinetics of Cr(VI) adsorption on TAG followed the pseudo-second-order model. The isotherm studies confirmed that the Cr(VI) chemically adsorbed on TAG in a monolayer fashion, which was attributed to the electrostatic interaction between chromium ions and oxygen negative ions. The process of Cr(VI) adsorption was found to be thermodynamically spontaneous and endothermic.
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