Synthesis of magnetic chitosan beads and its cross-linked derivatives for sorption of zinc ions from water samples of Yamuna and Hindon rivers in North India

The magnetic chitosan beads (MCB) were synthesized using a co-precipitation method and made to react with epichlorohydrin (ECH) to get the cross-linked derivative (ECH-MCB). In the FTIR spectrum of chitosan (CS) and magnetic chitosan beads, band appeared at 606 cm⁻¹ attributed to Fe–O group in case of MCB. The strong adsorption band at 3438 cm⁻¹, due to –OH stretching vibration, indicates that the hydroxyl groups in chitosan molecules were involved in the cross-linking of ECH-MCB. The surface of magnetic chitosan beads is smooth as compared to the rough surface of cross-linked beads as established by scanning electron microscopy (SEM) proving the better adsorption. The EDX of epichlorohydrin cross-linked magnetic chitosan beads gives the composition of elements present in terms of weight and atomic percentages. Thermal gravimetric analysis (TGA) establishes its three-stage degradation for MCB and ECH-MCB. Two endothermic peaks were observed at 96.28 °C for MCB and at 99.61 °C for ECH-MCB using differential scanning calorimetry (DSC). It is found that zinc metal ion sorption efficiency in terms of adsorption capacity of ECH-MCB (q_max is 0.059 mg/g) is significantly higher than MCB (q_max 0.0488 mg/g). Various factors affecting the uptake behavior of metal ions such as pH, adsorbent dosage, contact time and temperature effects were investigated. The correlation coefficient R² values lie between 0.9 and 1.0 and fitted well with the Langmuir and Freundlich isotherms. The equilibrium parameter R_L values support that the adsorption (0 < R_L < 1) is a favorable and spontaneous process. The thermodynamic parameters confirm that it is an endothermic reaction which results in an increase in the randomness of the adsorption process. The beads were regenerated by using easily available, economical chelating agent, ethylenediaminetetraacetic acid (EDTA) which forms considerably stable five-membered complex with Zn⁺² ions on desorption. These beads prove as promising materials for the removal of pollutants from industrial wastewater. In the case of Hindon water sample, the Zn(II) level was within permissible limits of 1.01 mg L⁻¹ from initial concentration of 4.34 mg L⁻¹ after 20 min of adsorption, and for Yamuna water samples, it was 1.00 mg L⁻¹ from 9.69 mg L⁻¹ after 40 min of adsorption.