Graphene oxide was synthesized from graphite using the MH (modified Hummer s method) 30 and

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1 Supplemental Information Synthesis of Graphene Oxide from Graphite Graphene oxide was synthesized from graphite using the MH (modified Hummer s method) 30 and the Tour methods 31. For the MH method, SP-1 graphite powder (1.0 g) was dispersed in concentrated sulfuric acid (5 ml) and pre-oxidized using K2S2O8 (1.0 g) and P2O5 (1.0 g). This suspension was maintained at 80 ºC for 4.5 h, poured into 160 ml of DI water, and allowed to rest and cool to room temperature overnight. The pre-oxidized graphite powder was collected by vacuum filtration on a 0.45 µm Polytetrafluoroethylene (PTFE) membrane (Millipore), washed extensively with deionized (DI) water, and dried overnight at room temperature. The pre-oxidized graphite was then placed in concentrated sulfuric acid (40 ml), and KMnO4 (5.0 g) was slowly added to the graphite suspension, with the temperature closely monitored to avoid an increase above 10 ºC by using an ice bath. After the KMnO4 addition, the mixture was slowly heated to 35 ºC and left to react for 2.5 h. DI water (77.0 ml) was then slowly added into the suspension, not allowing the temperature to exceed 50 ºC. After water addition, the mixture was left to react for an additional 2 h at room temperature. The solution was then poured into 240 ml of DI water, and 4.2 ml of H2O2 (30%) were added, which turned the solution a bright yellow color. The solution was kept at room temperature for 2 days, and the precipitate was recovered by centrifugation (12,000 x g, 30 min) and washed with 100 ml of a 1:10 HCl solution (2x) and DI water to remove residual chemicals. The resulting material was re-suspended in DI water and dialyzed with Fisherbrand dialysis tubing (molecular weight cut-off 3,500 Da) for 3 days for additional purification. The final dark brown graphite oxide suspension was dried by lyophilization (5 days) and stored at room temperature until use.

2 For the Tour method, graphite (1.0 g) was added to 200 ml of a 9:1 mixture of H2SO4:H3PO4 and bath sonicated (M3800 Branson Ultrasonic Bath, Emerson, Danbury, CT) for 5 min. The reaction vessel was placed in an ice bath, and KMnO4 (6.0 g) was added to the mixture under constant stirring. The solution was then slowly heated to 50 ºC and stirred for 12 h. The temperature was strictly kept at 50 ºC because Mn2O7, formed by adding KMnO4 to a concentrated sulfuric acid, can detonate at temperatures higher than 55 ºC. Next, the solution was cooled to room temperature overnight and poured into iced DI water (~400 ml) with 3 ml of H2O2. The precipitate was then washed in succession with 100 ml of DI water (2x), 100 ml of 1:10 HCl (2x), and 100 ml of DI water. After each washing step, the mixture was centrifuged (12,000 x g, 30 min) and the supernatant decanted until the ph was equivalent to that of the DI water (~4.0). Next, the material was purified by dialysis (3,500 Da membranes) for 72 h. Finally, the dark brown product was dried by lyophilization (5 days) and stored in a sealed Falcon tube at room temperature until use.

3 Oxidation of Powder Activated Carbon A suspension was made by combining six grams of powdered activated carbon (20B, Norit Americas Inc., Atlanta, GA, USA) and 150 ml of 15.7 N HNO3 solution in an Erlenmeyer flask. This suspension was then heated and stirred on a magnetic stirrer with hot plate at ~90 ºC for 1 h. The flask was removed from the hot plate and cooled down to room temperature. The suspension was then vacuum filtered to remove the PAC from the solution through 0.7 µm GF/F grade filter paper. The oxidized PAC accrued on the filter paper was rinsed with 500 ml of nanopure water several times until the ph of the rinse solution remained constant to ensure excess acid was removed. The oxidized and rinsed PAC was dried at 90 ºC under vacuum and stored in a sealed container at room temperature.

4 Silver Impregnation of Carbon Materials To impregnate the carbon materials with silver, each carbon (200 mg) wassoaked in 10 ml of 0.5 M AgNO3 solution. The carbon-silver slurries were stirred for two days at 150 rpm at room temperature. Then they were centrifuged for 30 minutes at 12,000 G. The supernatants were removed from the centrifuge tubes by Pasteur pipettes and replaced with 50 ml of nanopure water for rinsing. The carbon slurries with nanopure water were re-suspended with vortex mixer and separated by repeating the centrifuge step. To ensure removal of excess silver, each rinse was repeated three times. The silver impregnated carbons were dried at 90 ºC under vacuum and stored in a sealed container at room temperature.

5 Table S1. Colorado River (Central Arizona Project (CAP)) Surface Water Background Chemistry* ND is non-detect in sample. Detection limits are provided next to non-detect samples. General Chemistry Analytes Units Concentration (January 2017) Alkalinity as CaCO3 mg/l 125 Ammonia Nitrogen mg/l ND (DL 0.05) Barium, Total, ICAP/MS µg/l 140 Bromide µg/l 85 Calcium, Total, ICAP mg/l 76 Chloride mg/l 93 Copper, Total, ICAP/MS µg/l ND (DL 2.0) Iron, Dissolved, ICAP mg/l ND (DL 0.02) Iron, Total, ICAP mg/l 0.16 Magnesium, Total, ICAP mg/l 27 Manganese, Total, ICAP/MS µg/l 6.1 Nitrate as Nitrogen by IC mg/l 0.28 Orthophosphate as P mg/l 0.32 Potassium, Total, ICAP mg/l 4.8 Silica mg/l 9.9 Sodium, Total, ICAP mg/l 94 Specific Conductance (@25 ºC) µs/cm 1000 Strontium, ICAP mg/l 1.1 Sulfate mg/l 240 Total Dissolved Solids (TDS) mg/l 660 Total phosphorus as P mg/l ND (DL 0.02) Total Suspended Solids (TSS) mg/l ND (DL 10.0) Turbidity NTU 1.1 Temperature F 65 Dissolved Oxygen mg/l ph * Data from

6 Table S2. Batch Bottle Experimental Matrices Additives Synthetic Water Chloride-spiked Synthetic Water Chloride- and NOM-spiked Synthetic Water Natural CAP Water NaHCO3 (1 mm) X X X Bromide (200 µg/l) X X X X Chloride (20 mg/l) X X NOM (10 mg/l) X

7 Normalized intensity (a.u.) Intensity (a.u.) Figure S1. Raman Spectroscopy of Ag + Impregnated and Non-Impregnated Carbonaceous Additives 1.0 D Band G Band -- PAC -- MH -- Tour MH-GO Tour-GO PAC Raman shift (cm -1 ) D Band G Band Marc-Ag -- PAC-Ag Tung-Ag -- MH-Ag PAC-Ag -- Tour-Ag Raman shift (cm -1 )

8 Figure S2. Bromide Removal as a Function of Ag + Loading on Carbon Adsorbents. X in each dataset represents the mean for that dataset. % of Bromide Removed n = 12 n = 12 n = 12 n = Percent Silver Loaded onto Carbon Adsorbents

9 Figure S3. EDAX of Carbon Adsorbents Imaged on SEM. Experimental matrix is 1 mm NaHCO3 spiked with 200 µg/l Br - and 20 mg/l Cl -. PAC-Ag MH-Ag Tour-Ag

10 SEM/EDAX of Nylon Filters with no Carbon Adsorbents Present in Solution Bromide (Yellow)