1 Treating Water by Degrading Oxyanions Using Metallic 2 Nanostructures 3 Yiyuan B. Yinab◆, Sujin Guobc◆, Kimberly N. Heckab, Chelsea A. Clarkab, Christian L. 4 Coonrodab, and Michael S. Wong*abcde 5 aDepartment of Chemical and Biomolecular Engineering, Rice University, Houston, TX 6 77005, United States 7 bNanosystems Engineering Research Center for Nanotechnology-Enabled Water 8 Treatment, Rice University, Houston, TX,77005, United States 9 cDepartment of Civil and Environmental Engineering, Rice University, Houston, TX 10 77005, United States 11 dDepartment of Chemistry, Rice University, Houston, TX 77005, United States 12 eDepartment of Materials Science & Nanoengineering, Rice University, Houston, TX 13 77005, United States 14 ◆(Y.Y., S.G.) These authors contributed equally to the study. 15 *To whom correspondence should be addressed Email: [email protected] 16 Mailing address: 6100 Main Street, MS-362, Rice University, Houston, TX 77005 1 17 Abstract 18 Consideration of the water-energy-food nexus is critical to sustainable development, as 19 demand continues to grow along with global population growth. Cost-effective, 20 sustainable technologies to clean water of toxic contaminants are needed. Oxyanions 21 comprise one common class of water contaminants, with many species carrying 22 significant human health risks. The United States Environmental Protection Agency (US 23 EPA) regulates the concentration of oxyanion contaminants in drinking water via the 24 National Primary Drinking Water Regulations (NPDWR). Degrading oxyanions into 25 innocuous compounds through catalytic chemistry is a well-studied approach that does 26 not generate additional waste, which is a significant advantage over adsorption and 27 separation methods. Noble metal nanostructures, e.g., Au, Pd and Pt, are particularly 28 effective for degrading certain species, and recent literature indicates there are common 29 features and challenges. In this Perspective, we identify the underlying principles of - - 30 metal catalytic reduction chemistries, using oxyanions of nitrogen (NO2 , NO3 ), 2- - - - - 31 chromium (CrO4 ), chlorine (ClO2 , ClO3 , ClO4 ), and bromine (BrO3 ) as examples. We 32 provide an assessment of practical implementation issues, and highlight additional 33 opportunities for metal nanostructures to contribute to improved quality and sustainability 34 of water resources. 35 36 Keywords: metallic nanostructure, oxyanions, catalyst, contaminants, nitrate, chromate, 37 bromate, chlorite 38 2 39 Abstract Graphic - - NO2 /NO3 N2 - ClO2 / - BrO - ClO3 / 3 - ClO4 Cl- Br- 2- 3+ CrO4 Cr 40 41 Synopsis 42 The removal of toxic and prevalent oxyanion contaminants from water can be carried out 43 through catalytic degradation using metallic nanostructures. This Perspective highlights 44 the commonalities in reduction chemistry, and the challenges specific to oxyanions and 45 their unique reactivities. 3 46 Table of Content 47 1. Introduction 48 1.1. Sustainability of water 49 1.2. Toxic oxyanion contamination 50 1.3. Metallic nanostructure based catalytic water treatment as a sustainable process 51 2. Catalytic Detoxification of Oxyanions Using Metallic Nanostructure - - 52 2.1. Nitrogen oxyanions (nitrate or NO3 , nitrite or NO2 ) 2- 53 2.2. Chromium oxyanions (chromate CrO4 ) - - - 54 2.3. Halogen oxyanions (bromate BrO3 , chlorite ClO2 , chlorate ClO3 , perchlorite - 55 ClO4 ) 56 3. Perspective and Research Opportunities 57 3.1. Comparison of reduction catalytic activity for the different oxyanions 58 Applicability of catalytic remediation to other oxyanions 59 3.2. Applicability of catalytic remediation to other oxyanions 60 3.3. Possible catalytic water treatment scenarios using metallic nanostructures 61 3.4. Roadmap for deployment of catalysts for oxyanion treatments 62 3.5. Practical implementation issues of catalytic water treatments 63 4. Conclusion 64 4 65 Introduction 66 Sustainability of water 67 Fresh water is essential for life and is a key element of food and energy production, yet it 68 makes up for only 3% of all earthly water.1 Roughly ~30% of fresh water is readily 69 usable, with the rest locked in ice caps and glaciers.2 In 2014, an estimated 346 billion 70 gallons per day of fresh surface and groundwater were consumed in the U.S. for energy 71 production, agriculture, and other needs.3 As a consequence of anthropogenic activities, 72 millions of tons of toxic chemicals are unfortunately released into the water supply every 73 year, negatively impacting water quality.4 Clean fresh water is critical to both human 74 health and industrial development, and proper management is required for its sustainable 75 use.5 76 Toxic oxyanion contamination 77 When in the water environment, many elements speciate into oxyanions depending on pH 6,7 z 78 and electrochemical potential. Their molecular formulae can be generalized as AxOy , 79 where A represents a chemical element, O represents oxygen, and z is the overall charge 80 of the ion.8,9 Oxyanions are highly soluble and mobile in water, and are widespread in 81 drinking water sources such as surface and groundwater.8 82 Figure 1 shows elements that commonly form thermodynamically and/or 83 kinetically stable oxyanions at pH 6.5~8.5 and at electrochemical potential (Eh) values of 84 0.1~0.4, which are the conditions commonly found in drinking water system.10 An 85 oxyanion of a given element is considered thermodynamically stable if it is the lowest 86 free energy state compared with other species of the element.11 Pourbaix diagrams map 87 the most thermodynamically stable species for a given element as a function of pH and 5 12 2- 88 electrochemical potential. The sulfate (SO4 ) anion, for example, is the 89 thermodynamically stable species of sulfur at pH 6.5~8.5 and Eh 0.1~0.4 V as shown in 90 the Figure 2a.13 H He B C N Li Be 3- 2- - O F Ne (BO3 ) (CO3 ) (NO3 ) Na Mg Si P S Cl + + Al 2- 3- 2- - Ar (Na ) (Mg ) (SiO3 ) (PO4 ) (SO4 ) (ClO2 ) Cr Mn Fe Cu Zn As Se Br K Ca Sc Ti V 2- - 3+ Co Ni 2+ 3+ Ga Ge 3- 2- - Kr (CrO4 ) (MnO4 ) (Fe ) (Cu ) (Fe ) (AsO4 ) (SeO4 ) (BrO3 ) Sb Te I Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn - - - Xe (Sb(OH)6 ) (TeO4 ) (IO3 ) Cs Ba La-Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn (Pb2+) Fr Ra Ac-Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 91 92 Figure 1 Periodic table showing the most common oxyanion and cation species (marked 93 by color) under drinking water conditions (pH 6.5~8.5, Eh 0.1~0.4 V). 94 Thermodynamically stable oxyanions are peach-colored, kinetically stable oxyanions are 95 in red, and cationic elements are in blue. 96 97 Oxyanions that exist in the environment but are not at the lowest energy state of 98 the element are kinetically stable.14 Their conversion to a lower free energy state requires 15 - 99 additional energy to overcome an activation barrier. For example, perchlorate (ClO4 ) is 100 an environmentally stable oxyanion of chlorine in water, but chloride is the lowest energy 101 form of chlorine. Chloride (Cl-), and not perchlorate, is shown within the water stability 102 range in the Pourbaix diagram for chlorine (Figure 2b).16 While most non-metal 103 elements can form common oxyanions in water at pH 6.5~8.5 and Eh 0.1~0.4 V, metal 104 elements tend to form cations instead of oxyanions, or they precipitate as insoluble 105 oxides. For example, iron forms ferrous cation (Fe2+) in water at pH ~6.5 and Eh~0.1 V 106 (Figure 2c).17 6 (a) (b) 2.0 (c) 2.0 1.2 O 2 1.6 HClO ClO- 1.6 Cl 3+ 2 Fe 2- 1.2 FeO4 H2O 1.2 0.8 O2 - 0.8 HSO4 2- 0.8 O SO4 2 0.4 Fe O 0.4 0.4 2 3 Eh (V) Eh Eh (V) 2+ - -0.0 Fe H O Eh (V) Cl 2 S 0.0 -0.4 Fe3O4 H2O 0.0 Fe(OH) -0.8 2 H2S -0.4 HS- -1.2 Fe -0.4 -0.8 H2 H2 H2 2 4 6 8 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 107 pH pH pH 108 Figure 2 Pourbaix diagram of (a) sulfur (modified from Vermeulen et al.13), (b) chlorine 109 (modified from Radepont et al.16), and (c) iron (modified from Tolouei et al.17). The blue 110 dashed lines indicate the water electrochemical potential window. 111 2- + 112 Some oxyanions (e.g. carbonate CO3 ) and cations (e.g. sodium Na ) are benign 113 to the environment and human health, and are not subject to guidelines or regulations at 114 either the federal or state level. However, other oxyanions and cations are toxic and can 18 - - 2- 115 cause severe health problems. Nitrogen (e.g. NO3 /NO2 ), chromium (e.g. CrO4 ), - - 116 bromine (e.g. BrO3 ), and chlorine (e.g. ClO2 ) oxyanions as well as certain cations (e.g. 117 Cu2+, Cd2+, Pb2+ and Hg2+) are regulated by the National Primary Drinking Water 118 Regulations (NPDWRs) with a maximum contaminant level (MCL) as set by the US 119 Environment Protection Agency (EPA).19 U.S. states often have stricter regulations. For - 120 example, the state of California has an MCL of 6 ppb for perchlorate (ClO4 ) in drinking 121 water, but there is not yet a federal MCL.20 122 Many technologies have been developed for the removal of oxyanions from water 123 including adsorption,21 ion exchange (IX),22 and reverse osmosis (RO).23 However, these 124 methods and processes do not degrade the compounds, but instead transfer the 125 contaminant into a secondary waste stream requiring disposal.