Sulfur Copolymerization with Hydrophilic Comonomers as Polysulfides in Microbeads for Highly Efficient Hg²⁺ Removal from Wastewater

Abstract

Repurposing of sulfur, a petroleum industry byproduct, will not only address safety issues of its storage but also advance its valorization. Herein, sulfur was directly copolymerized with 2-carboxyethyl acrylate (CEA) to synthesize a S-rich hydrophilic polysulfide (pS-CEA) for Hg2+ sequestration. Physicochemical properties such as the hydration number and S-content were correlated with the ability of pS-CEAs to capture Hg2+ in terms of the binding capacity (q) and site utilization. Results reveal that the S content in pS-CEA must be balanced carefully with its wettability to ensure an efficient Hg2+ sequestration. At an optimized S/CEA molar ratio, pS-CEA (6:1) exhibited an extremely high affinity towards Hg2+ with a distribution coefficient (K d ∼ 26 095 mL g–1). It attained a superior adsorption capacity q m ∼ 989 mg Hg2+ g–1 derived from the Langmuir isotherm model and followed a pseudo-second-order kinetics. pS-CEA was conveniently configured into microbeads via agar gelation and the drop-in-oil method. At its highest acceptable pS-CEA loading (150 wt %), pS-CEA/agar microbeads attained a q m = 527 mg g–1. The microbeads were configured into a packed bed column and successfully filtered out 99.89% Hg2+ from atypically high-concentration feeds (C o = 50 mg L–1). The filtration column can be easily regenerated by passing a stripping solution (0.5 M HCl/1 M thiourea) with nearly 100% Hg2+ elution efficiency. This work demonstrates the effective design and benefits of the developed polysulfide, which can be synthesized from sulfur waste as a functional material for Hg2+ pollution remediation.

Polymers Made by Inverse Vulcanization for Use as Mercury Sorbents

“…62 Alternatively, mercury sorbent beads could be fashioned by agar gelation. 63 The base sulfur polymers were excellent in mercury sorption, with Langmuir sorption capacities approaching 1000 mg Hg 2þ /g in some cases. 63 Tikoalu et al also recently noted some subtle effects on the rate of mercury uptake and how it may depend on the hydrophilicity of the organic crosslinker (Figure 8).…”

“…63 The base sulfur polymers were excellent in mercury sorption, with Langmuir sorption capacities approaching 1000 mg Hg 2þ /g in some cases. 63 Tikoalu et al also recently noted some subtle effects on the rate of mercury uptake and how it may depend on the hydrophilicity of the organic crosslinker (Figure 8). 66 For example, castor oil contains a hydroxylated fatty acid ester that improved the initial rate of Hg(II) uptake in a copolymer made from the castor oil triglyceride and sulfur.…”

“…These sulfur polymers improved mercury uptake by virtue of their hydrophilicity, in comparison to hydrophobic sulfur polymers. [62][63] These sulfur polymers were soluble in DMF and could be processed by electrospinning into micro-and nanofibrous mats when a polyacrylonitrile (PAN) carrier polymer was used, with the amount of PAN varying from 33% to 66% of the total polymer mixture. 62 These mats exhibited exceptional mercury binding capacities (qm = 634-835 mg Hg / g sulfur polymer, depending on the composition) and the fibrous membranes could be used for removal of mercury from water via filtration.…”

“…In their copolymerizations of sulfur and hydrophilic monomers such as methacrylic acid and 2-carboxyethyl Template for Organic Materials © Thieme Stuttgart • New York 2021-05-06 page 6 of 10 acrylated, significant hydrophilicity was imparted to the sorbent (Figure 8). [62][63] The carboxylate functionality and polysulfide groups were both proposed to bind to mercury as well. These polymers could be integrated into nanofibers by electrospinning, as discussed previously.…”

“…Hydrophilic groups in polymers made by inverse vulcanization can improve wettability and increase the rate of mercury removal from water. [62][63][65][66] Polar groups that increase hydrophilicity and/or chelation to mercury can also be introduced post-polymerization. For instance, Chen et al showed that a polymer made from unsaturated triglycerides such as cottonseed oil and sulfur can be functionalized after inverse vulcanization (Figure 9).…”