Abstract:

Objective The strong adsorption ability of seed hemp on metal ions makes it impossible for seed hemp pulp to spin due to the high iron content, which leads to serious waste of resources restricting the effective utilization of bioresources and the achievement of China's carbon peak and neutrality goals. To address this problem, the adsorption solution (AS) including chitosan (CS) and polyvinyl alcohol (PVA) was designed to adsorb iron ions and therefore reduce iron content followed by study on the removal mechanism, which aim to provide basis theoretical support for the innovation and industrialization of iron ion removal technology of seed hemp.
Method Based on the metal chelation of CS and the emulsification and stabilization effects of PVA, AS was firstly prepared and used to deal with seed hemp pulp in this study, in which the removal effect, optimal removal conditions were explored by evaluating the spinnability of seed hemp pulp and the physical properties of the resulting fiber. In addition, the chelation mechanism of CS on iron ions was analyzed by density functional theory (DFT) employing the DMol³ module in Materials Studio software.
Results AS exhibited effective adsorption on iron ions where CS played a major role and PVA an assisted role. The adsorption effect of AS on iron ions was excellent under the condition of pH = 6.5 but the effect decreased obviously when pH value increased to 8 (Fig. 2), indicating the powerful influence of pH value on adsorption effect. It is observed that temperature has little effect on the adsorption effect of iron ions in the range between 30 and 50 ℃ (Fig. 3). Furthermore, adsorption time showed little effect on the adsorption effect of iron ions and the adsorption equilibrium was achieved with adsorption time of 30 min (Fig. 4).After the treatment with AS on the seed hemp pulp, the required aging rate during the glue making process was decreased and the properties containing polymerization degree, maturity, nickel mesh value and viscosity of the obtained spinning solution were enhanced (Tab. 4). In addition, the spinnability of the spinning solution was significantly improved showing the similar spinnability performance to that of the spinning solution prepared from wood pulp (Tab. 5). Through wet spinning, both the dry and wet breaking strengths of the obtained fibers were enhanced while the number of fiber defects were also significantly reduced (Tab. 6).DFT Calculation results show that all the binding configurations between citric acid (CA), ethylene diamine tetra acetic acid (EDTA), CS and FeCl₂, FeCl₃ were exothermic and form stable chelate. It is worth noting that CS represents stronger chelating ability to iron ions than CA and EDTA, especially the chelating ability to Fe²⁺ was stronger than that to Fe³⁺ (Fig. 5). The reason for the strong chelating ability for CS was contributed to the long distance between coordination atoms and the ease to rotate for coordination atoms leading to the formation of regular octahedron by the coordination of chitosan with metal ions (Fig. 6).
Conclusion Both the spinnability of the seed hemp pulp and the physical properties of the seed hemp viscose fiber are improved after AS treatment. The results show that AS with the mass ratio 1∶2 of CS and PVA, the mass fraction 3.6% adsorbent and pH =6.5 will provide the best removal effect for iron ions at 45 ℃ for 30 min, which can decrease the concentration of iron ions from 52 mg/kg to 35 mg/kg and the obtained seed hemp pulp has favorable spinnability. The adsorption and removal of iron ions in seed hemp pulp is achieved by the chelation of CS with iron ions to form complex in which PVA plays an auxiliary role in emulsification and stability, enhancing the chelation effect of CS and preventing the secondary deposition of iron ions.

Key words: seed hemp pulp, iron ion, chitosan, polyvinyl alcohol, density functional theory