High-capacity adsorption of hexavalent chromium by a polyaniline-Ni(0) nanocomposite adsorbent: Expanding the Langmuir-Hinshelwood kinetic model

Abstract

A nanocomposite of 2-napthalenesulfonic acid doped-polyaniline decorated with zero valent nickel nanoparticles (PANI-NSA@Ni0), was considered for the removal of hexavalent chromium from synthetic effluent. Adsorption conditions (pH, dose) were optimised, and parameters necessary for the design of PANI-NSA@Ni0-based adsorption equipment (adsorbent capacity, kinetic constants, etc.) were estimated from batch adsorption experiments. PANI-NSA@Ni0 successfully achieves total hexavalent chromium removal at a dose of 0.6 g/L, corresponding to a Cr(VI) loading of 333.3 mg/g, and when 2 < pH < 3. The combined monolayer capacity of the adsorbent was found to be 820.5 mg/g, with equilibrium adsorption behaviour adequately described by a modified dual-site Langmuir isotherm model. The adsorption of Cr(VI) by PANI-NSA@Ni0 was found to be largely unaffected by the presence of competing ions. Following adsorption, successful recovery of 95.5% of adsorbed chromium was achieved by employing HNO3 followed by NaOH as desorption agents, however this resulted in ≈ 88% loss in adsorption capacity for the subsequent cycle. After redeposition of Ni0 on the desorbed material, the adsorbent’s capacity was restored to 92% of the original capacity. An adsorption-coupled reduction mechanism, followed by the precipitation of Cr(OH)3, is believed to be the major mechanistic process responsible for Cr(VI) removal. Based on the proposed mechanism, a modified Langmuir-Hinshelwood adsorption-coupled reduction kinetic model was used to successfully describe the adsorption kinetics. The modified Langmuir-Hinshelwood adsorption-coupled reduction kinetic model produces rate constants which are independent of operating conditions such as initial pollutant concentration and adsorbent dose, and adequately describes the system’s equilibrium using the same rate constants. Thermodynamic parameters calculated using the best fitting isotherm model and novel kinetic model were both in agreement, and revealed that the adsorption process proceeds spontaneously and endothermically, with an increase in randomness at the solid–liquid interface and potential changes in the structure of the adsorbent/adsorbate.