Hybrid polysaccharide beads for enhancing adsorption of Cr(VI) ions

Alginate (Alg) beads were modified by the adsorption of poly(4-vinyl-N-pentyl pyridinium bromide (QVPV) polycation, aiming towards the adsorption of Cr(VI) ions from aqueous solution. The adsorption isotherm of QPVP onto Alg beads fitted well the Langmuir adsorption model, yielding maximum adsorption capacity (qmax) of 91 mg g−1 and high constant affinity of 7.53 × 107 L mol−1. X-ray photoelectron spectroscopy and elemental analysis indicated that the QPVP chains were mainly on the Alg beads surface. The adsorption of Cr(VI) in the dilute range (0.5 mg L−1 to 10 mg L−1) on Alg-QPVP beads in batch system was investigated by means of flame atomic absorption spectrometry. Under optimal adsorption conditions (HNO3, pH 2, 6 mg of dried beads, 15 mL of solution, 30 min contact time) the nonlinear fitting with Langmuir model yielded qmax of 18 mg g−1 and constant affinity of 124 L mol−1, whereas the qmax of 156 mg g−1 and adsorption energy of 9.8 kJ mol−1 were obtained from linear fitting with Dubinin-Radushkevitch model. The breakthrough curves obtained for Cr(VI) at 100 mg L−1 in the fixed bed column system were fitted to the Adams-Bohart and Thomas models. Fitting parameters indicated a fast dynamic adsorption process, which takes place mainly on the beads surface. The reduction of adsorbed Cr(VI) ions to Cr(III) by ascorbic acid, a non-toxic biological reductant, allowed the fast and complete recycling of the Alg-QPVP beads for at least 20 times without losing performance. Novel Alg-QPVP beads presented attractive features for dynamic process, such as quickness (k > 1 × 10−3 L mg−1 min−1) and high adsorption capacity in the dilute range, low cost and recycling possibility.