Abstract
Industrial pollution by heavy metal ions such as Hg2+ and Ag+ is a universal problem owing to the toxicity of heavy metals. In this study, a novel nano-adsorbent, i.e., poly-2-mercapto-1,3,4-thiadiazole (PTT), was synthesized and used to selectively adsorb mercury and silver ions from aqueous solutions. PTT nanoparticles were synthesized via chemical oxidative dehydrogenation polymerization under mild conditions. Oxidant species, medium, monomer concentration, oxidant/monomer molar ratio, and polymerization temperature were optimized to obtain optimum yields. The molecular structure and morphology of the nanoparticles were analyzed by ultraviolet-visible (UV-Vis), Fourier transform infrared (FT-IR), matrix-assisted laser desorption/ionization/time-of-flight (MALDI/TOF) mass and X-ray photoelectron (XPS) spectroscopies, wide-angle X-ray diffraction (WAXD), theoretical calculations and transmission electron microscopy (TEM), respectively. It was found that the polymerization of 2-mercapto-1,3,4-thiodiazole occurs through head-to-tail coupling between the S(2) and C(5) positions. The PTT nanoparticles having a peculiar synergic combination of four kinds of active groups, S–, –SH, N–N, and =N– with a small particle size of 30–200 nm exhibit ultrarapid initial adsorption rates of 1500 mg(Hg)·g−1·h−1 and 5364 mg(Ag)·g−1·h−1 and high adsorption capacities of up to 186.9 mg(Hg)·g−1 and 193.1 mg(Ag)·g−1, becoming ultrafast chelate nanosorbents with high adsorption capacities. Kinetic study indicates that the adsorption of Hg2+ and Ag+ follows the pseudo-second-order model, suggesting a chemical adsorption as the rate-limiting step during the adsorption process. The Hg2+ and Ag+-loaded PTT nanoparticles could be effectively regenerated with 0.1 mol·L−1 EDTA or 1 mol·L−1 HNO3 without significantly losing their adsorption capacities even after five adsorption–desorption cycles. With these impressive properties, PTT nanoparticles are very promising materials in the fields of water-treatment and precious metals recovery.
Highlights
Over the past few decades, heterocyclic and heterochain aromatic polymers have received considerable attention because of their wide use in the fields of electrode materials, sensors, adsorbents, cathode materials for rechargeable lithium-ion batteries and so on [1,2,3,4,5]
It was found that only CuCl2 with an reduction potential (RP) of 0.86 V could be used as the oxidant to synthesize successfully the poly-2-mercapto-1,3,4-thiadiazole (PTT) polymer
The PTT nanoparticles display a powerful adsorbability toward heavy metal ions due to the presence of a large number of –S, –SH, N–N and =N– groups containing lone pairs of electrons on the PTT macromolecular chains verified by the Fourier transform infrared (FT-IR) spectroscopy and MALDI/TOF mass
Summary
Over the past few decades, heterocyclic and heterochain aromatic polymers have received considerable attention because of their wide use in the fields of electrode materials, sensors, adsorbents, cathode materials for rechargeable lithium-ion batteries and so on [1,2,3,4,5]. Polymers 2018, 10, 150 aromatic heterocyclic polymer, the 1,3,4-thiadiazole-ring containing polymers have revealed a promising future in the fields of rechargeable lithium batteries, biological and chemical sensors, clinical diagnosis and pharmacological studies, optoelectronic devices, and heavy-metal-ion adsorbents because of their unique energy storage performance, electrocatalytic activity, and electron-rich properties [6,7,8]. Many sources including chloralkali plants, oil refineries, power plants, paper and pulp factories, rubber processing plants, fertilizer production plants, and similar industries are responsible for mercury discharge into the environment [19,20] Silver, as both a heavy metal and a precious metal, has some toxicity to humans and mammals. The adsorption performance for heavy metal ions and regeneration of the polymer were studied in detail
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