Supporting Information

Transcription

1 Supporting Information Iron-Anode Enhanced Sand Filter for Arsenic Removal from Tube Well Water Shiwei Xie, Songhu Yuan, *, Peng Liao, Man Tong,, Yiqun Gan,, Yanxin Wang, State Key Lab of Biogeology and Environmental Geology, China University of Geosciences, 388 Lumo Road, Wuhan , P. R. China School of Environmental Studies, China University of Geosciences, Wuhan, Hubei , P.R. China Supporting information includes 11 figures, 3 tables, filter operation, sequential extraction and operating cost estimation. S1

2 Figure S1 Photo of a household sand filter in the rural area in the Jianghan Plain, central China Figure S2 Photos of (a) the iron-anode enhanced sand filter and (b) the iron electrodes S2

3 Figure S3 Photo of the batch reactor used in the field Figure S4 Vertical profiles of DO concentrations along the filter during treatment without electricity. The flow rate in the filter was controlled at about 12 L/h. S3

4 Figure S5 Variation of DO concentrations in the well water and the sampling ports of the filter during the treatment of 140-L water. The flow rate in the filters was controlled at about 12 L/h with 0.6 A current applied to the iron anode. Figure S6 Variation of As(III) concentrations in the well water and the sampling ports of the filter during the treatment of 140-L water. The flow rate in the filters was controlled at about 12 L/h with 0.6 A current applied to the iron anode. S4

5 Figure S7 Variation of DO concentrations from the well water and two sampling ports during the 30-days intermittent operation. IR and IG were for the operating currents applied to the red bucket and green bucket respectively. The flow rate in the filters was controlled at 8-12 L/h except L and L (dropped to ~3 L/h). The top layer of sand was replaced at 500 L water yield. Figure S8 Variation of Fe(II) concentrations from the well water and two sampling ports during the 30-days intermittent operation. The conditions were the same as S5

6 indicated in Figure S7. Figure S9 Variation of As(III) concentrations in the well water and two sampling ports during the 30-days intermittent operation. The conditions were the same as indicated in Figure S7. Figure S10 ATR-IR spectra of the frozen dried sand grains collected from the top layer of sand in the red bucket and the bottom layer of sand in the green bucket after completion of the 30-day intermittent operation. S6

7 Figure S11 XRD patterns of the precipitates collected from the (a) top, (b) middle, and (c) bottom layer of sand in the green bucket at the end of 30-day intermittent operation. The patterns revealed the presence of quartz (Q) and calcium carbonate (C), but no Fe minerals were reflected. S7

8 Section S1: Filter Operation. For the tests with the yield of 140-L water, the groundwater from tube well 1 was pumped into the red bucket continuously by a peristaltic pump. Oxidation of Fe(II) was negligible during the course of pumping and the concentration of dissolved oxygen (DO) was <0.1 mg/l in the filter influent. The water level was maintained at 1 10 cm above the electrodes during the filtration. At this time, Fe(II) oxygenation happened due to the exposure to air. When L water was treated, the DO concentration in the effluent of red bucket decreased to less than 0.5 mg/l. To elevate DO, the filter was drained and maintained empty for at least 1 h. During the interval of two runs, the filter was drained and covered with lids. Filter sand was totally replaced between two tests. In the 30-day intermittent operation, the filter was run 3 5 days per week. The filter could produce L water per day, consistent with the usage habits of the local villager. The filter was drained and covered with lids in idle. During the test, the means of water feeding was changed at 220-L water yield. The water source was shifted from tube well 1 to tube well 2. Groundwater from the tube well 2 was pumped to a 50-L polyethylene bucket, and then was fed to the red bucket manually with a bailer. Oxidation of Fe(II) and increase in DO value (1 3 mg/l) was observed during the time of transportation and storage (<12 h). The water level was controlled at 1 10 cm above the electrodes during the filtration. The filter was drained for the treatment of every L groundwater. The flow rate in buckets was controlled at S8

9 8 12 L/h in most of the experiments by adjusting the tap, but gradually decreased to 3 L/h when clogging occurred. At 500-L water yield, about 4 cm thickness of top layer of sand in the red bucket was replaced with clean sand. The operating currents applied to the two buckets went through three stages according to the removal performance. In the first stage (0 400 L water yield), the electrodes in both buckets were operated at 0.6 A. In the second stage ( L water yield), the power applied to the green bucket was turned off. As the performance was stable, we further turned off the power applied to the red bucket in the third stage ( L water yield). Section S2: Sequential Extraction. The procedure for the analysis of speciation of Fe and As in the solid was referred to Neummann et al 1 with minor modifications. Wet solid samples ( g, water content: about 14%) were weighted, transferred into 50 ml polypropylene (PP) centrifuge tubes and added with 35 ml of the phosphate solution. The centrifuge tubes were closed tightly, stirred on a shaking table for 24 h in darkness, and centrifuged at g for 20 min. Then, the supernatant was drew out and filtered through a 0.22-μm cellulose acetate fiber membrane. The filtrates were acidified with concentrated HNO3 to ph 1, and subsamples were diluted for Fe, As and Mn analysis. For the following extraction steps, the solids in the tubes and extraction solutions were mixed, centrifuged, filtered and acidified as described above. All extractions were carried out in duplicates. The specific information of 4-step S9

10 extraction was presented in Table S1. Table S1 Sequential extraction procedure for As-bearing solid 1 Step Extractant Target speciation Possible mechanism 1- phosphate 1 M NaH 2PO 4, ph 5, 24 Adsorbed As and Fe Anion h, 25 C exchange 2-acetate 1 M CH 3COONa, ph Adsorbed As and Fe, Anion 4.5, 24 h, 25 C carbonates, easily dissolvable exchange, Fe (oxy)hydroxides protonation 3-oxalate 0.2 M sodium Amorphous Fe Complexation oxalate/oxalic acid, ph 3, (oxy)hydroxides 12 h, 25 C 4-oxalate/ascorbic 0.2 M sodium Reducible Fe oxides Reduction and acid oxalate/oxalic acid complexation M ascorbic acid, ph 3, 10 h, 25 C S10

11 Table S2 Trace element concentrations in the leachate from the sand in the filter by Toxicity Characteristic Leaching Procedure (TCLP) Element Ag As Ba Cd Cr Hg Pb Se Leachates (mg/l) regulatory level (mg/l) Note: means the measured data is below the detection limit. The extraction fluid was prepared by adding 5.7 ml of glacial acetic acid (CH 3CH 2OOH) to 64.3 ml of 1 M NaOH followed by dilution to 1 L with deionized water. The final ph is 4.93 ± The TCLP test was carried out with 10 g of wet sand and 195 ml of extraction fluid and was operated in duplicate. S11

12 Table S3 Sequential Extraction Data for the Solids from the Field Filters a extractant filter sample 1- phosphate 2-acetate 3-oxalate 4-oxalate/ascorbic acid 1-4 total extracted As mg/kg % mg/kg % mg/kg % mg/kg % mg/kg red bucket top b middle bottom green bucket top middle bottom extracted Fe mg/kg % mg/kg % mg/kg % mg/kg % mg/kg red bucket top b middle bottom green top S12

13 bucket middle bottom a Reduplicate samples were collected and mean values were presented. b The extracted As and Fe in the top layer of the red bucket were calculated as the sum of those from the present sand plus the replaced sand. Section S3: Operating Cost Estimation. In the field tests, the operating cost is comprised by the electrical energy applied for electrolysis and iron anode consumption. The electrical energy depends on the operating current and voltage across the electrodes. For the operating current of 0.6 A, the voltage across the electrode was maintained at a stable value of 4.8±0.5 V during most of time. The voltage sometimes rose to 6.5 V, but quickly dropped to V when the electrode polarity was reversed. Assuming a constant voltage of 4.8 V and a flow rate of 12 L/h, the total electrical energy consumption was 0.48 and 0.24 kwh m -3 when the power was applied to two and one bucket, respectively. Hence, the total electrical energy consumption was 0.31 kwh for 945-L water yield during the 30-day intermittent oepration, namely 0.33 kwh m -3 on average. The iron anode corrosion theoretically produced 52 mg/l Fe(II) at the 30-day intermittent operation, corresponding to an iron consumption of 0.07 kg m -3 on average. The cost for sand consumption was ignored for the local availability from the river bank. The prices of electricity and S45C steel plate in China were 0.57/kW h and 4/kg, respectively, so the operating S13

14 cost was estimated to be 0.47 m -3 or $0.07 m -3 (assuming an exchange rate of 6.7 for $1.0). REFERENCES (1) Neumann, A.; Kaegi, R.; Voegelin, A.; Hussam, A.; Munir, A. K.; Hug, S. J. Arsenic removal with composite iron matrix filters in Bangladesh: a field and laboratory study. Environ. Sci. Technol. 2013, 47 (9), S14