Biological Antagonism Inspired Detoxification: Removal of Toxic

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Biological Antagonism Inspired Detoxification: Removal of Toxic Elements by Porous Polymer Networks Liang Feng, Wen-Miao Chen, Jialuo Li, Gregory S. Day, Hannah Drake, Elizabeth Joseph, and Hong-Cai Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02826 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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Biological Antagonism Inspired Detoxification: Removal of Toxic Elements by Porous Polymer Networks Liang Feng†, Wen-Miao Chen†, Jia-Luo Li†, Gregory Day†, Hannah Drake†, Elizabeth Joseph†, Hong-Cai Zhou*†§ † Department

of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States

§ Department

of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843-3003,

United States

Keywords: water treatment; biological antagonism; mercury; selenite and selenite; porous polymer; environmental science

ABSTRACT: Water contamination by toxic heavy elements is becoming an urgent problem in environmental science and separation technologies. However, the design of sophisticated absorbents with high stability and outstanding removal efficacy for ion co-adsorption is still a technical challenge. Herein, inspired by biological Hg/Se antagonism detoxification, we have designed the first porous polymer network (PPN) for the concurrent removal of Hg/Se species in aqueous solutions. Remarkably, the MoS42- functionalized PPN-150-MoS4 exhibits a rapid and highly efficient simultaneous removal of toxic anions (SeO42- and SeO32-) and metals (Hg2+). The high thiophilicity of Hg2+ leads to 99.9 % removal within minutes. More importantly, selenite and selenate, typically known for being difficult to remove from aqueous environments, can be removed by PPN-150-MoS4, > 99% removal within minutes when in the presence of Hg2+. At the same time, the removal efficiency for Se(IV) and Se(VI) oxoanions in the absence of Hg2+ is very low, reaching only 14% removal. Overall, PPN-150MoS4 exhibits one of the highest adsorption capacities toward SeO32- (124 mg/g), making it a promising and cheap sorbent material for water remediation applications. This work provides a fresh route for detoxification and remediation strategies that aim to regulate the presence of toxic ions in nature. The material herein shall guide the state-of-art design of efficient water treatment techniques through a combination of biological antagonism and materials chemistry.

INTRODUCTION Trace amounts of selenium are essential for nutritional needs of humans and animals; however, at high concentrations (> 400 μg/day) it becomes extremely toxic.13 Unfortunately there is growing concern regarding the concentration of selenium in the environment, with the main forms of environmental pollutant selenium being the inorganic species, SeO42- and SeO32-, due to their high water solubility. The World Health Organization (WHO) and the United States Environmental Protection Agency (USEPA) have set the maximum acceptable concentration as 40 and 50 ppb, respectively.4-5 As a consequence, finding methods of maintaining the concentration below those targets is of the utmost concern. In addition to selenium, other major water pollutant include heavy metals, in particular, mercury (Hg) pollution is extremely harmful, not only because it can poison aquatic life, but also because it accumulates along the food chain, eventually leading to endangerment of human health.6-8 Currently, various approaches based on adsorption, precipitation filtration, and ion-exchange have been developed for the removal of toxic anions and cations from water.9-10 For example, metal oxides (FeOx, Al2O3, TiO2 and so on) and layered double hydroxides (LDHs) have been

utilized for the adsorption of selenium and heavy metals.1114 Although cost-effective, they usually show lack the porosity and surface areas needed to ensure high uptake capacities. In addition, carbon materials (nanotubes and other composites), silicas, water-stable Zr metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs), all with high and accessible surface areas, have demonstrated high efficiency towards the removal of toxic species in water.12, 15-22 Unfortunately these materials are usually expensive, which limits their practical applications in environmental remediation. Therefore, it is urgent to find water-stable candidates combining the advantages of high surface area and cost-effectiveness for toxic species removal. Compared to conventional materials used for selenium capture, porous polymer networks (PPNs) have the potential to meet both the cost and removal efficiency requirement.23-29 Although amorphous, they can achieve low-cost, facile, and large-scale syntheses. Networks constructed from covalent bonds will also allow for high water-stability of the PPN, making them suitable for various applications.30 Furthermore, the organic PPN backbone and pore environment can also be easily functionalized through post-synthetic modifications to

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achieve targeted applications.31-32 Herein, we chose a mesoporous melamine-formaldehyde polymer, dubbed PPN-150, due to the low cost and high abundance of the starting monomers. Inspired by the Hg–Se antagonism that has long been observed in biota, we have designed an absorbent system that can utilize this synergistic detoxification mechanism, which has been intensively studied over the past 70 years by biologists.33-38 Results have shown that Se can protect animals from the toxicity of Hg species, although Se itself can also be highly toxic when at high concentrations within most animal species. Research into Hg-Se antagonism has shown that an approximately 1:1 molar ratio of Hg and Se can be considered as biologically inert. Taking advantage of such molecular-level understanding of Hg–Se antagonism, a modified PPN-150 with sulfur motifs that mimics cysteine containing enzymes was designed and utilized for Hg-Se co-adsorption. In order to introduce strong affinity towards heavy metals (Hg) and selenium anions, PPN-150 was treated with (NH4)2MoS4 solutions to allow for the incorporation of sulfide-containing groups into the PPN. The judicious choice of MoS42- can help to enhance the stability of sulfur-functionalized materials and simplify the

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99% removal within several minutes, with an outstanding adsorption capacity, 51.5 mg/g for SeO42- and 124 mg/g for SeO32- (Figure 1c). These features make it a promising and cheap sorbent material for water remediation applications. The results here offer new opportunities for water purification by using cheap and mesoporous PPNs as efficient sorbent materials.

RESULTS AND DISCUSSION Preparation of MoS42- Functionalized Porous Polymer Networks. We first chose a mesoporous poly(melamine-formaldehyde resin), mPMF39-40 or PPN150, because it is cost-effective, easy to scale-up, hierarchically porous, exceptionally stable, and biocompatible. Compared to traditional sorbent materials such as MOFs, which are known to be sensitive to harsh conditions (F- rich environments and wide pH ranges), PPNs show high chemical stability due to the robust nature of their covalent bonds.41 PPN-150 is prepared through a polymerization reaction between melamine and paraformaldehyde in DMSO.40 This reaction can easily be performed at both the lab scale

Figure 1. Preparation of PPN-150-MoS4 via post-synthetic installation. (a) The high thiophilicity of Hg2+ leads to rapid and complete removal of toxic Hg2+ using PPN-150-MoS4. (b) Traditional ion exchange-based techniques show poor adsorption kinetics and removal efficiency for toxic SeO32-/SeO42- anion removal; (c) Utilizing the synergistic effects of Hg and Se, PPN150-MoS4 shows outstanding removal efficiency and kinetics for Hg2+ and SeO32-/SeO42- co-adsorption. Inserted pictures show the color change from white to brown after the successful installation of MoS4 in PPN-150.

storage problems typically associated with traditionally reported sulfur-based compounds, such as those that utilize -SH and Sx2- groups, which can easily oxidize, losing their removal efficiency. We demonstrate that the high thiophilicity of Hg2+ leads to a 99.9 % removal within minutes (Figure 1a). The material also shows a high capacity towards Hg2+, achieving 303 mg/g uptake. Not only does PPN-150-MoS4 allow for the rapid removal of toxic metals, but it can also achieve the highly efficient concurrent removal of SeO42-/SeO32- and Hg2+, exhibiting >

in vials and multi-hundred gram scales in multi-liter glass reactors. The IR spectrum of PPN-150 reveals peaks at 3358 cm-1, 2949 cm-1 and 1537/1480 cm-1, associated with ν(N-H), ν(-CH2-) and β(N-H) modes, respectively (Figure S4). TEM reveals the amorphous and porous nature of PPN-150 (Figure 2c). The N2 sorption isotherms at 77 K demonstrate the formation of a hierarchically porous structure, possessing both micro- and mesopores (Figure 2a-b). The highly porous features of PPN-150 are expected

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to provide enough room for post-synthetic modification of

indicated by TEM images, PPN-150-MoS4 display an amorphous morphology with defective mesopores randomly distributed throughout the particle. The thermal stability of PPN-150 and PPN-150-MoS4 was determined via thermogravimetric analysis (TGA, Figure S1). TGA plots under N2 indicate the high thermal stability of the materials, up to 330 oC and 350 oC, respectively. The chemical stability of PPN-150-MoS4 was investigated using various aqueous solutions with pH varying from 0 to 12. As indicated by Figure S2, there is no notable leaching of MoS42- after dispersing PPN-150-MoS4 into the solutions (pH 0-12) for 24 h. The high chemical stability makes the functionalized polymer suitable for environmental remediation applications, as most environmentally polluted water streams tend to be at either neutral or acidic pH. Due to this outstanding chemical and thermal stability, this material was believed to be the ideal platform for further studies into toxic element removal.

Figure 2. Porosity analysis of PPN-150 before and after MoS4 loading. (a) N2 isotherms and (b) pore size distributions of PPN-150 and PPN-150-MoS4, indicating the presence of micropores and mesopores in both samples; (c) TEM images of PPN-150 and (d) PPN-150-MoS4, demonstrating the hierarchically porous structure of PPN absorbents.

the system, giving further benefit for guest loading.

The MoS42- modified PPN-150, PPN-150-MoS4, was prepared by simply soaking PPN-150 powders in (NH4)2MoS4 aqueous solution while stirring at room temperature, which is a facile and green synthesis approach to achieve efficient immobilization (Figure 1). Under low pH conditions, the amine groups within PPN150 can be protonated, producing cationic ammonium groups that can tether the anionic MoS42-. The successful loading of MoS42- was verified by XPS and SEM-EDS. The oxidation state of Mo and S in PPN-150-MoS4 were characterized by XPS, indicating that MoS42- remains stable after the immobilization process (Figure 3a-b). The loading ratio of MoS42- was 25.9 wt%, calculated by ICPMS. The homogenous dispersion of MoS42- throughout the amorphous network was confirmed by the SEM-EDS mapping (Figure 3f). The successful loading of MoS42- in PPN-150 can also be verified by IR spectroscopy (Figure S4). As indicated by Figure S4, peaks at 484 cm-1 associated with Mo-S bending can be found in PPN-150-MoS4 while these peaks are absent from the IR spectrum of PPN-150. The N2 isotherm of PPN-150 after functionalization shows a decrease in the surface area from 844 to 503 m2g-1, and a decrease in total uptake from 796 to 447 cm3g-1 (Figure 2a). The pore size distribution also reveals that the pore volume is reduced after modification, due to the introduction of guest molecules within the pores. However, the structure maintains its hierarchical porosity, making PPN-150-MoS4 still capable of achieving efficient mass transfer during ion capture processes (Figure 2b). The hierarchical porosity of PPN-150 and PPN-150-MoS4 can also be corroborated by TEM images (Figure 2d). As

Toxic Metal Removal by PPN-150-MoS4. The uptake ability of PPN-150-MoS4 towards toxic Hg2+ was firstly tested via a series of Hg2+ aqueous solution from 5 to 500 ppm. 10 mg PPN-150-MoS4 was emerged in the solutions for 24 h to reach adsorption equilibrium. Subsequently, the supernatant was analyzed by ICP-MS to determine the remaining concentration of Hg2+ in the supernatant. Figure 4a revealed that the adsorb amounts of Hg2+ in the polymer increase as the initial concentrations of Hg2+ increase. As indicated by the sorption isotherms of Hg2+, the maximum capacity was calculated as 303 mg/g (Figure 4a and Table S4). Langmuir isotherms were used to describe the experimental adsorption data (Figure S5 and Table S17). The affinity of PPN-150-MoS4 toward Hg was calculated, as indicated by the distribution coefficient Kd, which can reach as high as 1.00 × 107 mL/g when the initial Hg concentration is between 5 to 20 ppm (similar to the practical conditions). For example, the concentration of Hg2+ after undergoing adsorption in PPN-150-MoS4 for 24 h decreased from the original 20 ppm to 4 ppb, achieving 99.9% removal and a Kd value of 5.00×106 mL/g. It should be noted that materials with Kd values of >104 mL/g are regarded as exceptional sorbents. The kinetic behavior for Hg2+ was subsequently investigated (Figure 4b and Table S5). Interestingly, PPN150-MoS4 exhibits rapid removal, > 96%, of Hg2+ within 7 mins, and complete removal, >99.9%, of Hg2+ within 3 hours. The outstanding Hg removal efficiency places PPN150-MoS4 as a promising porous absorbent. We contribute this high affinity toward Hg2+ to the soft sulfur-based MoS42- anions immobilized in PPN-150, which is very selective for soft metal ions, while the amine groups in PPN-150 get protonated and lose the capture ability towards metal ions. Several common metal ions (Co2+, Ni2+, Cu2+, Zn2+ and Hg2+) existing in water pollutants with different thiophilicities were mixed for selectivity test. The affinity of PPN-150-MoS4 toward these metal species, as indicated by the distribution coefficient Kd, has been summarized in

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Figure 3. Characterization of PPN-150-MoS4 before and after adsorption. (a-e) XPS showing the (a) Mo 3d peaks and (b) S 2p peaks of PPN-150-MoS4; (c) XPS showing the Hg 4f peaks of PPN-150-MoS4-Hg; (d) XPS showing the Se 3d peaks of PPN150-MoS4-Hg-Se(VI); (e) XPS showing the Se 3d peaks of PPN-150-MoS4-Hg-Se(IV); (f-i) SEM images and corresponding EDS element mapping of (f) PPN-150-MoS4, (g) PPN-150-MoS4-Hg, (h) PPN-150-MoS4-Se(VI), and (i) PPN-150-MoS4-Hg-Se(VI).

Figure 4. Metal removal performance by PPN-150-MoS4. (a) Sorption isotherms for Hg2+ by PPN-150-MoS4. (b) Sorption kinetics curves for Hg2+ by PPN-150-MoS4. (c) Removal efficiency of five mixed metal ion systems (Co2+, Ni2+, Cu2+, Zn2+ and Hg2+) by PPN-150-MoS4. (d) Removal efficiency of five individual metal ions (Co2+, Ni2+, Cu2+, Zn2+ and Hg2+) by PPN-150-MoS4.

Figure 4c and Table S3. Interestingly, PPN-150-MoS4 shows much higher adsorption ability toward Hg2+ than other ions. From Figure 4b and Table S3, PPN-150-MoS4 exhibits exceptional uptakes of Hg2+ with Kd values in the order of 105 mL/g, followed by Cu2+ with Kd of 757 mL/g and negligible adsorption capacity (< 3% removal) toward other metal ions including Co2+, Ni2+ and Zn2+. The uptake of these metal ions was further measured from their corresponding individual aqueous solutions by PPN-150MoS4, which showed the similar trend (Table S2). The trend was observed as Co2+, Ni2+, Zn2+ 95% removal in just a matter of minutes, while traditional ion exchange based Se(VI)/Se(IV) oxoanions can only reach about 14% removal of oxoanions after 24 hours. The exceptional capacity for Hg2+ and Se(VI)/Se(IV) oxoanions places PPN-150-MoS4 as a promising and scalable absorbent material for water treatment. Most importantly, this work provides fresh insights into the biological antagonism inspired sophisticated absorbent design, which may shed light on new opportunities for a wide range of materials for ion detoxification in water remediation.

METHOD Synthesis of MoS42- functionalized porous polymer networks. PPN-150 was prepared according to a previous work.40 MoS42- functionalized PPN-150 was synthesized based on a facile stirring process. Typically, 0.2 g PPN-150 and 0.2 g (NH4)2MoS4 were dispersed in 10 mL degassed

deionized water, and the obtained suspension was allowed to stir under ambient conditions for 24 h. The resulting brown solid was filtered, washed with deionized water and then acetone, and finally air-dried, giving 0.24 g of product with a yield of ~93%. The MoS42− loading capacity within PPN-150 was verified by the ICP measurements after nitric acid digestion. Single-component heavy metal uptake experiments. The stock solution of metal ions (Co2+, Ni2+, Cu2+, Zn2 and Hg2+) was prepared by dissolving the corresponding nitrate salts in water directly. In a typical adsorption, PPN-150-MoS4 (10 mg) was added into a glass bottle containing 10 ml stock solution of metal ions at a concentration of 10 ppm. The mixture was kept at room temperature for 24 h and the metal concentration in the supernatant solutions were determined by ICP-MS. Multi-component heavy metal uptake experiments. To distinguish the selectivity of PPN-150-MoS4 towards various ions, a solution of 10 ppm for each ion (Co2+, Ni2+, Cu2+, Zn2+ and Hg2+) was mixed with 10 mg PPN-150-MoS4 at room temperature for 24 h. The supernatant solutions were then analyzed by ICP-MS. Adsorption isotherm measurement. PPN-150-MoS4 (10 mg) was mixed with a 10 ml stock solution of heavy metal ions or selenite/selenite with various concentrations. The mixture was kept at room temperature for 24 h to ensure complete adsorption. The metal concentration in the supernatant solutions were determined by ICP-MS. Adsorption kinetics measurement. PPN-150-MoS4 (10 mg) was mixed with a 10 ml stock solution of heavy metal ions and/or selenite/selenite at a concentration of 20 ppm. The mixture was kept at room temperature for 24 h. During the adsorption process, the metal concentration in the supernatant solutions were analyzed by ICP-MS. Stability test of PPN-150-MoS4. PPN-150-MoS4 (10 mg) was mixed with aqueous solution of different pH values (012), respectively, and kept for 24 h. ICP-MS analysis. Calibration standards were prepared from certified reference standards from RICCA Chemical Company. Samples were further analyzed with a Perkin Elmer NexION® 300D ICP-MS. Additionally, in order to maintain accuracy, quality control samples from certified reference standards and internal standards were utilized. Thermogravimetric Analysis. For thermogravimetric analysis, about 10 mg of the sample was heated on a TGA Q500 thermogravimetric analyzer from room temperature to 600 °C at a rate of 5 °C·min-1 under an N2 flow of 20 mL·min-1.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxxxxx. Materials and methods, synthesis and characterization details (PDF).

AUTHOR INFORMATION

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Corresponding Author *[email protected]

Notes The authors declare no competing financial interest. Use of toxic solutions containing SeO42-, SeO32- or Hg2+ requires special attention and careful procedures. No other unexpected or unusually high safety hazards were encountered.

ACKNOWLEDGMENT This work was supported as part of the Center for Gas Separations, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-SC0001015), U.S. Department of Energy Office of Fossil Energy, National Energy Technology Laboratory (DE-FE0026472), Robert A. Welch Foundation through a Welch Endowed Chair to HJZ (A-0030) and National Science Foundation Small Business Innovation Research (NSF-SBIR) under Grant No. (1632486). The authors also acknowledge the fruitful discussion and assistance from Dr. Tomlin at Elemental Analysis Laboratory, Texas A&M University.

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