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 adsorptioncoupled
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.
