Inverse Vulcanized Polymers for Sustainable Metal Remediation

Heavy metal exposure has an enormous burden on human health. Current metal removal technologies require substantial improvements in relation to their efficiency and environmental impact if this issue is to be addressed. Over the last decade, several new types of sulfur‐rich sorbents have been investigated. These polymers typically have high removal efficiencies for toxic metals such as mercury and are often made using sustainable and low‐cost reagents. This review surveys polymers made by inverse vulcanization that have been tested for metal capture. Focus is put on environmental impact, feedstock for sorbent synthesis, selectivity towards metal removal, toxicity studies, and the reusability of the polymers. Furthermore, this review discusses current limitations and the potential opportunities to use different comonomers for improved metal capture.

the industrial activity of a chemical company was released into the environment and eventually accumulated in freshwater and food supplies. The consumption of this mercury-contaminated food and water by the local population resulted in 2200 deaths and 12 000 cases with chronic symptoms. [5] Since then, the United Nations has implemented a global treaty-The Minamata Convention on Mercurywhich aims to regulate the anthropogenic release and trade of mercury and mercury compounds. [6] Another example of mass metal poisoning occurred more recently in Nigeria in 2010, where more than 400 children died from the effects of lead poisoning due to mining activities, despite the heavy regulations typically placed on the use and release of lead. [7] Metals such as chromium and cadmium can also cause severe adverse health effects in humans and are therefore also strictly regulated. [4,8] To meet these regulations, different technologies have been invented to help comply with these environmental obligations. These include chemical precipitation, coagulation, membrane technology, ion exchange, and electrochemical precipitation. [9] The effectiveness of these methods is influenced by factors such as the pH, temperature, ionic strength, and the concentration of organic matter in the contaminated water. Adjusting for these conditions can utilize a lot of energy and chemical reagents and if done so insufficiently, can lead to incomplete metal capture, and generate toxic sludges or other waste. Hence, new technology to remediate metal contamination is needed to keep up as regulatory frameworks become more strict.

Inverse Vulcanization
Over recent years, much focus has been put into removing metals from water, soil, or gas using polymers synthesized by inverse vulcanization. These kinds of polymers consist of chains of sulfur crosslinked with an organic comonomer, containing typically 30-80 wt.% of sulfur. One method of synthesizing these polymers involves heating the sulfur to a temperature >159 °C, promoting ring-opening polymerization of the elemental sulfur. [10] In this first stage of the reaction, elemental sulfur undergoes homolytic scission of an SS bond, generating a thiyl diradical that could react with another molecule of S 8 or polysulfide, forming polymeric sulfur. [10] The thiyl diradical could also react with CC double or triple bonds of unsaturated organic molecules, forming a CS bond (Figure 1). Without the presence of an unsaturated organic molecule, the polymeric sulfur can depolymerize back to S 8 . [10] The term "inverse vulcanization" was coined by Pyun and collaborators to highlight the concept of an unsaturated organic small molecule crosslinking polysulfide polymers made from elemental sulfur. [10] This contrasts with classic vulcanization in which a small amount of sulfur is used to crosslink a preformed organic polymer.
One characteristic that makes inverse vulcanization attractive for environmental remediation is its alignment with several tenets of green chemistry: no solvent is required, reactions have high atom economy, many sustainable organic comonomers have been reported, and in many cases byproducts are minimal. [11] In the case of the reagents, sulfur is a large-scale coproduct of the crude oil refining process produced in a large surplus and unsaturated organic molecules can be found in bio-based materials and byproducts of industrial processes. [11a] Therefore, reagents suitable for inverse vulcanization are often readily available and low cost. Additionally, new techniques have been developed to lower the energetic requirements for the synthesis of polysulfide polymers. These include the use of reaction accelerators or catalysts that lower temperature requirements to 100 -135 °C for synthesis, [12] nucleophilic activation of sulfur (110 -130 °C), [13] mechanochemical synthesis (room temperature), [14] and photoinduced inverse vulcanization. [15] Rapid and efficient heating, provided by microwaves, can also be used for some inverse vulcanization reactions. [16] The first polymer made by inverse vulcanization was reported by Pyun in which 1,3-diisopropenylbenzene was copolymerized with sulfur (Figure 2). [10] The resulting polymer was demonstrated to be suitable as a redox-active cathode material for LiS batteries. As sustainable energy storage is critical for the future, this application also aligns with the overarching goals of green and sustainable chemistry. Since then, many applications for polysulfide polymers made with different comonomers have been established. [11b] Some of those applications include dynamic and repairable materials, [17] infrared lenses for thermal imaging, [18] oil spill remediation, [19] and metal sorption. [20] All of these applications, in some way, address issues in sustainability. For example, repairable materials are important for recycling and lifecycle management, [21] sulfur-based thermal imaging replaces low-abundance elements like germanium [18a] and has use in wildlife surveillance and conservation, [22] and pollution control is clearly important for environmental stewardship.

Metal Remediation
Over the last 7 years, polymers made by inverse vulcanization have been investigated for their ability and efficiency to capture a range of metals. These metal-polymer interactions occur due to sulfur's ability to act as a binding site to soft metals. However, the presence of additional coordination sites within the polymer, such as carboxylates [23] and bipyridine, [24] have further improved the scope of metal capture.
The high proportion of sulfur in these polymers makes them hydrophobic and insoluble in common solvents, with a few exceptions. Metal remediation of contaminated water using polymers made by inverse vulcanization takes advantage of this water-insolubility since the captured metals bound to the polymer can be easily removed by simple filtration techniques. [20] Recently, the Chalker group introduced magnetic separation as an alternate technique for the extraction of these types of polymers by incorporating magnetic   within the polymeric material. [25] By doing so, they could separate the polymer from water post remediation using a simple magnet. Nevertheless, due to the insolubility of the polymers, the metal capture relies on the surface area of the polymer exposed to the contaminated media. Therefore, a higher surface of the sorbent would provide more metal binding sites and more efficient removal. In that context, many polymers made by inverse vulcanization used for metal capture were synthesized using a surface area enhancement technique. These include the coating of the polymer on silica gel, [12] polymer foaming using supercritical CO 2 , [26] or CO 2 generated during the reaction, [27] the use of NaCl crystals as a washable template during synthesis, [24,28] and forming high surface area fibers by electrospinning. [29] Here, the polymers made by inverse vulcanization are reviewed based on the metal species they can remove, focusing on their capacity, the percentage removed, and the time needed for maximum metal capture ( Table 1).

Mercury Remediation
Mercury remediation using polymers made by inverse vulcanization has been intensely studied. [43] The World Health Organization (WHO) recommends that the concentration of total mercury in drinking water should not exceed 0.001 mg L −1 (1 ppb) [44] and inorganic mercury should not exceed 0.006 mg L −1 . [45] It specifies that the term 'total mercury' includes inorganic mercury and organic mercury compounds. However, liquid elemental mercury and its vapor are also toxic and therefore regulated.

Ionic Mercury (Hg(II))
The first reported polysulfide for Hg(II) capture, made by inverse vulcanization, used an oligomer produced by direct reaction of d-limonene and sulfur (Figure 3). That material, reported by Chalker and co-workers, showed a 55% removal of Hg(II) from the aqueous solution after 24 h, but this was on a flat surface of the polymer and the material exhibited an intriguing chromogenic reaction with mercury for potential use in sensing applications. [38] Later, Hasell and co-workers took advantage of its solubility in organic solvents to coat this oligomer on silica gel, increasing the surface area exposed, and reaching 99% of Hg(II) removal in less than 8 h. [12] Recently, a full kinetic analysis across varying pH ranges was reported. In some cases, more than 99% of HgCl 2 could be removed in 5 min. [30] Currently, there are only three reported polymers made by inverse vulcanization that have shown to remove mercury below, or close to, 1 ppb. These polymers are made with sulfur and one of either 2-carboxyethyl acrylate, [23] castor oil, [35] or garlic oil blend [36] (Figure 4). The castor oil polymer synthesized by Chalker and co-workers removed mercury from a 107 ppm Hg(II) aqueous solution to 1.1 ppb (>99.99% removal). Whereas the polymers made with 2-carboxyethyl acrylate and garlic oil blend accounted for a final concentration of Hg(II) of 0.1 ppb (99.9%) and 0.2 ppb (99.0%), respectively, with lower initial concentrations (1.1 and 2 ppb). Kinetically, the Hg(II) adsorption is 10 times faster for the 2-carboxyethyl acrylate polymer, with the other two polymers requiring 24 h for 99% of Hg(II) removal. The biggest difference in these materials is presented in their adsorption capacities, with the 2-carboxyethyl acrylate polymer showing the highest of all to date, adsorbing 835 mg of Hg(II)/g of polymer. The other two polymers analyzed removed less than 10 mg of Hg(II)/g of polymer. This higher efficacy will also influence the affordability of the polymers for realworld applications, since the more they adsorb per gram, the less polymer is required. An important factor that influences the efficacy of these polymers for mercury uptake is the surface area exposed to the metal-contaminated water and their hydrophilicity or wettability. [23] Although the 2-carboxyethyl acrylate polymer displayed the most promising results, the organic monomer does not currently come from a sustainable source. Additionally, there is work yet to be done to study the effects of surface area enhancement on mercury adsorption rates of the polymers made with castor oil and garlic oil blend. Furthermore, there are many other reports of polymers that adsorb mercury with lower efficiencies.

Organomercury Compounds
Polymers made by inverse vulcanization have also been tested for their removal of organomercury compounds from water. These kinds of mercury pollutants are usually formed by microbial metabolism of Hg(II) in water, and some others are produced for their use as fungicides. [46] The polysulfide sorbent that left the lowest concentration of organomercury after treatment uses canola oil as a comonomer and has shown mercury uptake for different species of metal. [20] An aqueous solution containing 150 ppb of the fungicide 2-methoxyethylmercury chloride, after treatment with the canola oil polymer resulted in a final concentration of mercury of 3 ppb (98% removal). However, the initial concentration tested was low in comparison with other studies. Additionally, longer carbon chains in the comonomer have been shown to cause a higher organomercury removal due to the lipophilic nature of those compounds. [37] However, an excess of lipophilicity would decrease the wettability of the polymers in water which can slow mercury uptake in aqueous solution. Hence, it is necessary to balance the lipophilicity and hydrophilicity of these polymers for organomercury remediation.
In other work, Hasell and co-workers examined three inverse vulcanized polymers made with either diisopropenyl benzene, squalene, or perillyl alcohol for their uptake of a 2.5 ppm methylmercury or Hg(II) solution. [37] In that work, they determined that all polymers tested had a lower affinity to methylmercury than Hg(II) ions. The polymer made with squalene showed the highest adsorption of methyl mercury (≈30%) compared to perillyl alcohol (≈25%) and diispropenyl benzene (20%). The higher affinity of the polymer made with squalene compared to the others was attributed to its greater lipophilicity.

Cadmium(II) (Cd(II))
Cadmium is released into the environment by smelters and improper battery disposal. The WHO gives a guideline value of <3 ppb of Cd(II) in drinking water. [45] Cadmium(II) removal from water by polymers made by inverse vulcanization has been shown to be possible by Urbano and co-workers. In this work, the polymer was synthesized using castor oil as a comonomer. The polymer was shown to remove a maximum of >99% of Cd(II) in 24 h of exposure to a solution with a 100 mg L −1 concentration of Cd(II). [39] However, the high removal of Cd(II) is only achieved when using 100 g L −1 of the polymer, with only 20% removal observed for an initial sorbent dosage of 10 g L −1 . This low Cd(II) removal capacity makes this polymer limited in practical cadmium remediation, but it is an important benchmark study for using sulfur polymers in cadmium remediation.

Gold(III) (Au(III))
Although the presence of gold ions in water is not regulated due to their low toxicity, their capture from water would be useful for mining ore [49] or recovering this precious metal from electronic waste. [50] Given the known affinity of sulfur for gold, it is not surprising that polymers made from inverse vulcanization have been studied for gold uptake. [51] Mann and Chalker reported a very high selectivity for gold uptake of a polymer made from sulfur and canola oil. [52] In complex mixtures of metals, gold binding was preferential, even at concentrations relevant to mining. Remarkably, the gold was not just bound to the polymer, but partially reduced back to gold metal. In this way, the polymer is a redox-active sorbent. High gold recovery was reported by polymer incineration [52] or breaking down in a nucleophilic solvent such as pyridine. [53] Soon after, Hasell and co-workers demonstrated that the polymer made from sulfur and limonene, when coated on silica gel, could remove all gold from a 400 ppm solution in less than 1 h. [12] Sorbent capacities on the order of 40 mg g −1 were reported. While impressive in gold uptake rate and capacity, methods are still required for conversion back to gold metal and separation from the sorbent. Yang and co-workers made a polymer through inverse vulcanization by reacting sulfur with a mixture of 8 edible oils and tested its ability to bind to Au(III) ions. [42] They also found that Au(III) was reduced to gold metal during the adsorption process and that the removal was as high as >99%. Furthermore, they determined that the polymer made using a 50 wt.% of sulfur to oil had the highest gold adsorption capacity (21 mg g −1 ) and a drop in capacity was observed with a higher or lower wt.%.
Adv. Sustainable Syst. 2023, 7, 2300010   Figure 3. Reaction of sulfur and d-limonene to obtain a polysulfide oligomer. [38] Figure 4. Sulfur copolymers made with a) 2-carboxyethyl acrylate and its degradation products, [23] b) castor oil, [35] and c) garlic oil blend. [36] More recently, Jenkins and co-workers reported a water-soluble sulfur-rich polymer using a quaternary amine as a comonomer. This polymer removes Au(III) through a binding and precipitation mechanism (Figure 6). [40] The removal of Au(III) was around 94% in 0.5 h in a multi-metal solution containing calcium(II), copper(II), iron(II), potassium(I), magnesium(I), lead(II), and zinc(II). Despite this selectivity towards Au(III), this polymer also removed silver, lead, and cadmium, though to a lesser extent than gold. The high binding capacity was attributed to its improved solubility in water resulting in more metal binding sites available for metal capture. One potential disadvantage of this polymer was that after all the metal is bound, some polymers still remained in solution. The important conclusion, however, is that water solubility of metal sorbents generally improves sorption rates and capacity by having more binding sites available than if the polymer is hydrophobic or insoluble in water.

Iron (III) (Fe(III))
The presence of Fe(III) in water is unwanted since it promotes the growth of certain bacteria, it could clog drains, discolor plumbing fixtures, and it gives water a poor taste. [44] Its presence is harmful if its concentration is above 3 ppm in drinking water. [44] In an attempt to remove the cation from water, Chalker and co-workers synthesized a sulfur polymer using waste canola oil after being used in the food industry. The polymer captured 95% of Fe(III) in two hours. The removal was also not affected by the presence of hydrogen peroxide (usually found in iron removal processes for wastewater or stormwater). Later, it was reported that this polymer could be synthesized on a tonne scale, which bodes well for industrial uptake. [43] After treatment, the final concentration in an aqueous solution containing Fe(III) was below the limits specified by the WHO.
Another sulfur polymer synthesized by Tritto and co-workers was also explored in iron sorption, with the ability to remove 83% of Fe(III) in 24 h. [41] The polysulfide was made with dipentene, a mixture of monoterpenic olefins, and a diethyldithiocarbamate-based reaction accelerator. This product was further reacted post-polymerization with garlic oil, diallyl disulfide, and myrcene (Figure 7). The multi-step synthesis does increase the complexity of preparation of this sorbent, but the use of biobased and renewable organic comonomers is notable.

Lead(II) (Pb(II))
Toxic lead can be released into drinking water from old lead plumbing, solder, and alloys. Other environmental release is associated with smelting and industrial metallurgy. Its presence in water is regulated by WHO to 10 ppb. [44,45] One example of a polymer synthesized by inverse vulcanization that was shown to remove Pb(II) was the water-soluble polymer made by Jenkins and co-workers ( Figure 6). 30% of lead ions were removed from a complex solution containing multiple metal cations. [40] No reports were made on the efficiency of Pb(II) removal using this polymer from solutions only containing Pb(II) ions. In addition, Lin and co-workers demonstrated the effective removal of 93% of Pb(II) from a multi-metal solution using a polymer containing bipyridine as an additional metal binding site (Figure 8). [24] In other work, Urbano and co-workers tested Pb(II) ion uptake using a polysulfide polymer synthesized with castor oil. [39] The polymer removed 88% of Pb(II) from a 100 mgL −1 [Pb(NO 3 ) 2 ] solution and a dosage of 100 gL −1 was determined.

Palladium(II) (Pd(II))
Palladium is released into the environment from the catalytic converters exhausts on vehicles. [38] Due to the low toxicity of Pd(II), there are no guidelines for its concentration in drinking water by any international organization. However, the high cost of Pd(II), its low abundance, and usefulness as a catalyst make it desirable to be recovered from the environment. So far, the only report of a polymer made by inverse vulcanization for the use in palladium binding was the polymer made from d-limonene and sulfur. Unfortunately, low removal efficiency was observed when using a flat surface of the polymer (42% of Pd(II) recovered). [38] Increased surface area of the sorbent may improve the uptake of palladium for potential use in palladium binding, in remediation operations, mining, recycling, or catalyst retrieval from reaction mixtures.

Multi-Metal Removal
Contaminated water often contains a complex mixture of metals and other suspended or dissolved matter. Therefore, a sorbent capable of removing several metals from a mixed metal system would have large benefits. Towards this goal, a sulfur polymer made with monomers containing a bipyridine group Adv. Sustainable Syst. 2023, 7, 2300010 Figure 5. Mercury metal in soil reacts with a polymer made from canola oil and sulfur. A color change can also be observed in this process. Simple mechanical sieving separates the polymer-bound mercury from the remediated soil. Reproduced under terms of the CC-BY license. [20] Copyright 2017, the Authors, published by Wiley-VCH GmbH. Figure 6. Inverse vulcanization of a charged quaternary diallylammonium monomer to produce a water-soluble polymer that could remove gold selectively. [40] was prepared (Figure 8). The polymer was tested against a 3.5 ppm solution containing 16 metals (Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Al(III), Ga(III), Ag(I), Cd(II), In(III), Ba(II), Hg(II), Tl(III), Pb(II), and Bi(III)). This polymer was shown to remove up to 100% of Cu(II), Ag(I), Cd(II), Hg(II) and Bi(III), and more than 85% of Cr(III), Mn(II), Ni(II), In(III), Ba(II) in solution. However, the metals were originally from a multi-metal ICP standard solution and further validation on field samples containing other contaminants will be required. Nonetheless, the multi-modal binding of this polymer is an innovative approach to multi-metal sorption for complex remediation projects. [24]

Post Remediation
The environmental impact of the use of inverse vulcanized polymers would rely not only on their ability to remove metals from water but also on proper processing and stewardship of the spent sorbent. Ideally, this would involve polymer

Storage and Toxicity
If the polymer is to be stored long-term, or used directly in the environment, it is important it is not toxic and the metals bound to the polymer are not leached back into the environment. Unfortunately, little is known about the toxicity of polymers made by inverse vulcanization. The Chalker group has shown that the canola oil and d-limonene polysulfide, with or without adsorbed mercury, have no effect on the viability of selected human liver cell lines after 24 h of exposure. [20,38] These experiments highlight that the polymers do not release toxic chemicals nor do they leach bound mercury into the surrounding water. However, others have shown that polymers made by inverse vulcanization can display antimicrobial activity. [54] For example, a study by Hasell and co-workers examined the effects of inverse vulcanized polymers made with dicyclopentadiene or diisopropenyl benzene against Gram-positive (S. aureus) and Gram-negative (E. coli) bacteria. [54a] In this study, both types of polymers showed antibacterial activity against E. coli and the polymer made using diisopropenyl benzene was also active against S. aureus. Remarkably, the polysulfide made using diisopropenyl benzene was shown to kill over 99% of both bacterial cell lines. Although these studies open new pathways for polysulfides made through inverse vulcanization to be used as antimicrobial agents, they are also indicative of their potential for bioactivity.
Sulfur polymers have also shown potential to be used as a casing to encapsulate spent metal-contaminated sorbents to prevent leaching of the toxic metal back into the environment. Chalker and co-workers reported that the canola oil sulfur copolymer has potential to prevent the leaching of a spent sorbent made of powder-activated carbon. To do so, the Pb(II) containing activated carbon sorbent was encapsulated within the sulfur polymer, and physically contained by hot-pressing.
The polymer in the sorbent has a reactive surface that becomes the mortar that allows adhesion to the final polymer casing. The adhesion occurs through metathesis of the SS bonds in the polysulfide polymer (Figure 9). [21] The physically contained sorbent did not leach after prolonged exposure to water, making it safer to store and transport. [55] While this application of sulfur polymers is not in metal binding, it highlights the versatility in their use in remediation due to the unique SS bonds that can be broken and reformed.

Regeneration of Polymers after Metal Sorption
The positive environmental impact of sulfur polymers in metal capture will only be realized if the spent sorbents can be regenerated. Several studies have shown the desorption and the readsorption of the metal from these polymers by using acidic solutions of HCl or HNO 3 mixed with thiourea to release the captured metal. The first of these was reported by Chung and co-workers. In this work, the desorption of 100% of bound Hg(II) was observed from a micro fibrous mat functionalized with a sulfur copolymer. [23] This polymer could also be fashioned into a microbead morphology, and packed into a column for water filtration. [31] These studies found no reduction in metal binding performance after 5 adsorption/desorption cycles. Another example of metal desorption is described by Yang and co-workers using the sulfur polymer made with the mixture of 8 edible oils. In this report, the Au(III) adsorbed was released by using a solution of thiourea in HCl. The metal adsorption efficiency for this polymer dropped from 99.8% to 71.6% over 3 cycles. [42] If the bound metals are not removed from the polymer, the spent sorbent needs to be stored or repurposed. For example, mercury sorbents made from either sulfur and cottonseed oil [32] or sulfur and potato-starch [33] showed good adsorption capacities for Hg(II). However, in these cases, the rate of mercury uptake was reduced to 80% of the original rate after 5 adsorption-desorption cycles. In that case, the spent sorbent Adv. Sustainable Syst. 2023, 7, 2300010 Figure 9. Reactive compression molding was used to encapsulate Pb(II)-activated carbon sorbent in a polysulfide casing to prevent leaching of captured lead. SS metathesis during hot-pressing allows the sulfur polymer surfaces to react and bind together, forming a solid casing around the spent sorbent. Reproduced with permission. [55] Copyright 2020, Elsevier.
(with bound mercury) was repurposed as a catalyst. Here, the bound mercury is a catalyst and the polymer is a catalyst support. Conversion of phenylacetylene to acetophenone was demonstrated, with moderate to good yields (Figure 10). [33] While the use of mercury as a catalyst is not ideal from a safety and sustainability perspective, this study is an important concept in the stewardship of metal sorbents, demonstrating a new and valuable use after binding a metal.

Outlook
Polymers made from inverse vulcanization have shown great promise in applications related to metal capture. Their sustainable building blocks, simple preparation, and flexible deployment bring new advantages over current methods. Additionally, the low cost of the feedstocks (sulfur and sustainable unsaturated molecules) bode well for production and use in areas with few economic resources.
However, there is still much more to be investigated. For instance, most of the polymers presented lack metal binding sites other than sulfur; thus, their metal remediation capabilities rely only on the capture of soft metals, such as gold or mercury. Additionally, the sulfur copolymers with other functional groups such as carboxylates or amines address this issue in part, but additional feedstocks are required to ensure the monomer is sustainably sourced. With that in mind, more monomers or combinations of different monomers are yet to be explored that could give different functionalities and metal binding sites to the polymers. Some of the organic molecules still to be tested are, for instance, metal-binding compounds made in nature, such as chlorophyll, dyes, or pigments, which could even be functionalized before the reaction with sulfur. The development of new synthetic techniques to create sulfurrich polymers at room temperature will also be required in cases where monomers do not tolerate high temperatures typically used in the polymerization. Some recent progress in alternative sulfur sources is promising in this regard. [56] In the case of inverse vulcanized polymers with binding sites other than sulfur, it would be possible to trap other toxic metals such as arsenic or chromium, which are also environmentally threatening pollutants, or even low-toxic precious metals such as silver or platinum that could reduce the need for mining ore and facilitate recycling of various electronic waste.
Finally, a more thorough toxicity study should be performed to understand the effective use of sulfur polymers in metal remediation, their potential leaching, and their effect on organisms and ecosystems. The use of sulfur polymers for metal remediation in an industrial scale would generate large quantities of spent sorbent. Therefore, it is necessary to develop methods to recycle these sorbents to avoid storing waste that could potentially be released into the environment. The reaction mechanism with the metal bound to the polymer acting as a catalyst. [33]