(196r) Polysulfide-Based Nanofiber Prepared Via Inverse Vulcanization and Electrospinning for Effective Mercury (II) Sequestration

Recent progress on inverse vulcanization has opened various opportunities to develop advanced materials with useful electrochemical and optical properties which have been investigated as Li-S batteries, for IR imaging technology, and as self-healing materials. Herein, the first extensive study on the preparation of an Hg²⁺ adsorbent based on inverse vulcanized polysulfide (PS) is presented.

Sulfur-rich poly(sulfur-random-acrylate) (poly(S-r-Ac)) was successfully prepared via direct addition of β-carboxyethyl acrylate to molten sulfur as confirmed via FTIR and ¹H and ¹³C NMR analyses. The viscoelastic nature (i.e. 0< phase angle (δ) <90°) and thermal properties of (poly(S-r-Ac) can be directly manipulated by adjusting molar ratio of components (i.e. S:CEA molar ratio). That is, increasing sulfur content decreased the viscosity, and T_g, and % weight remaining in simultaneous thermal analysis profiles of the PS. This trend agrees with the hypothesis that with greater amount of sulfur, sulfur rank in poly(S-r-Ac) is longer which was also confirmed by higher S-S XPS peak area of PS in S:CEA = 2:1 molar ratio. Given that S-S (226 kJ mol^-1) bond energy is weaker than other intramolecular bonds present such as C-S (272 kJ mol^-1), C-C (346 kJ mol^-1), C-O (358 kJ mol^-1), and C=O (799 kJ mol^-1), it is expected that poly(S-r-Ac) with greater amount of sulfur (i.e. S:CEA=2:1) degrades at relatively lower temperature and has lower T_g than those with lesser amount of sulfur component.

Poly(S-r-Ac) is soluble to most common solvents which include dimethyl formamide (DMF), enabling it to form homogenous solution with polyacrylonitrile (PAN). Polysides blended in polyacrylonitrile (PAN) were prepared as dope solutions in DMF and were electrospun into nanofibers (NFs). The NFs with different PS loadings were characterized and were evaluated as an adsorbent for Hg²⁺ sequestration. Batch adsorption experiments showed that the PS (S:CEA=2:1) follows a monolayer Langmuir-type adsorption isotherm with a maximum adsorption capacity of 612 mg g^-1. Kinetics study showed that the PS efficiently sequestered Hg²⁺ attaining adsorption equilibrium within 1.5 hrs. Lastly, prepared PS exhibited selectivity towards Hg²⁺ having highest concentration factor (CF=1.79 L g^-1) and distribution coefficient (K_d =1.95 x 10⁶ mL g^-1) despite having the least concentration amidst other competing ions. Little affinity towards Cr²⁺, Zn²⁺, Pb²⁺, and Cu²⁺ and no affinity towards Cd²⁺ and Ni²⁺ were observed. Reusability test exhibited the ability of the prepared PS NF to be recycled using 6 M HCl as an eluent to sequester adsorbed Hg²⁺ in the NF.

This work demonstrates new insight for the preparation of polysulfide-based adsorbents from inverse vulcanization. It also presents new approach for easy, cheap, and effective preparation of adsorbents for Hg²⁺ sequestration from contaminated water or wastewater. This research was supported by NRF funded by the Ministry of Science, ICT and future Planning (2017R1A2B2002109) and the Ministry of Education (2009-0093816).