Abstract
Hemp stalks, readily available bio-waste in northern India, are good source of cellulose nanofibers (CNFs) with exceptional strength and stiffness. In this study, a facile route was envisaged to affix disulphide linkages on hemp derived CNFs (CNF-S-S-CNF) using 3,3-dithiopropionic acid. CNF-S-S-CNF with two sulphur groups on the cellulose backbone scored high selectivity for mercury ions (distribution coefficient of 5 × 105 mL.g−1) owing to soft–soft interactions, besides exhibiting affinity for safranin O (SO) due to strong electrostatic attractions. Thermal stability up to 700 °C advocated high temperature applications of CNF-S-S-CNF. Response surface methodology (RSM) with Box Behnken Design (BBD) was applied using Design Expert software 13 for tuning the process parameters. Statistical analysis of variance (ANOVA) established a maximum adsorption capacity, qe of 100% for Hg(II) ions achieved at optimum conditions of pH 5.5, Hg(II) concentration of 100 mg L-1 with adsorbent dosage of 100 mg and contact time of 9.50 min, while for SO, 100% adsorption was achieved at pH 6.5, SO concentration of 1 mg L-1 with adsorbent dosage of 60 mg and contact time of 9.50 min, in accord with the experimental results. Both, linear and non-linear kinetics and isotherm models were fitted to further understand the adsorption mechanism. The non-linear pseudo-second order demonstrated better adsorption kinetics for Hg(II), while SO fitted better to the linear pseudo-second order model. Ultra-rapid adsorption kinetics was achieved with pseudo second order rate constant, K2 of 0.197 g.mg−1.min−1 and 100% removal within 5 s (Co = 10 mg L-1) of Hg(II) ions. FTIR and XPS confirmed strong interactions of CNF-S-S-CNF with Hg(II) ions accomplishing a maximum adsorption capacity, qe exp of 828.5 mg/g. Regeneration of adsorbent for 8 cycles with retention of 96% and 80% efficacy for Hg(II) ions and SO, respectively endorse its outstanding potential in waste-water remediation.
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