Advanced Reduction Processes for Water and Wastewater Treatment

Transcription

1 Advanced Reduction Processes for Water and Wastewater Treatment Ahmed Abdel Wahab Professor, Chemical Engineering Program Director, Qatar Sustainable Water and Energy Utilization Initiative (QWE) Texas A&M University at Qatar World Water Day Workshop Water Research Center, Sultan Qaboos University

2 Water Supply/Use versus Water Consumption Supply Consumptive use Evaporated during the process transpired by plants incorporated into products or crops consumed by people or livestock

3 Water Supply/Use versus Water Consumption Supply Inefficient, driven by demand Lost/Wasted Returned to source Lost by other means Network losses Infiltration Disposal of treated wastewater Disposal of industrial wastewater Loss/evaporation due to inefficient use Consumptive use

4 Ideal Supply 100% water efficiency Impossible Cost Practicality Uncontrolled losses Consumption

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6 Water Supply/Use versus Water Consumption Efficient Holistic approach to resources management Maximized reuse/recycle Minimum losses Optimized water consumption Optimized energy consumption Are we there yet?? Lost Consumptive use

7 Example of an efficient system with a holistic approach management Biosolid Urban channels Discharge Treatment Biosolid Harvested plants Food and Fibre Biogas Sludge Organic solid waste Wastewater District cooling Landscaping Industry Fuel or electricity Energy Urban Center Water supply

8 Areas of Improvement Fragmented management Risk-free approach Specific sectorial goals Lack of holistic approach to resources management Subsidizing the tariffs Inefficient irrigation systems Lack of regulatory framework/incentives for wastewater recycle/reuse Public perception with respect to wastewater reuse Don t put all your efforts on the backend!

9 Success Story- Singapore NEWater Project Introduced in NEWater plants can meet 30% of Singapore s water needs NEWater to meet 50% of Singapore s water demand by 2060 This project is supported by significant research effort for continuous advancement

10 Opportunities/Motivation Cost of wastewater treatment is less than one third of desalination cost Clustered industrial facilities offers the potential for macroscopic industrial water management Solar Energy is a sustainable source of energy for water production and treatment Hybrid systems can significantly reduce desalination/treatment cost Human capacity building and public awareness are key elements in achieving the sustainability goals

11 Advanced Reduction Processes for Water and Wastewater Treatment

12 Introduction Oxidation-reduction reactions are the primary method for destroying environmental contaminants. Every oxidation-reduction reaction must be feasible thermodynamically if it is to occur. However, the requirement to achieve desired levels of destruction within a reasonable time is often not met. Therefore, the main limitation in developing redox treatment processes is normally related to the process kinetics. Although a multitude of reactions are possible, only a few occur at sufficiently rapid rates to be used efficiently for treatment.

13 Advanced Oxidation Processes (AOPs) An example of a group of redox treatment processes that are able to meet kinetic limitations are Advanced Oxidation Processes (AOPs). AOPs have been applied to a number of water treatment problems where oxidation of contaminants is required. AOP is based on the formation of hydroxyl radicals (OH ) that act as oxidants for the target contaminants. The hydroxyl radical is an effective oxidant because it reacts rapidly with a large number of compounds by removing electrons from them.

14 Advanced Reduction Processes (ARPs) Applications of free radical chemistry have been limited almost entirely to applying oxidizing free radicals. However, reductive free radicals can also be formed and applied to treatment problems that require reductions. Examples of oxidized contaminants that are destroyed by reductive treatment include: chlorinated organics, perchlorate, chromate, nitrate, nitrite, arsenate, selenate, bromate, chlorate, and a number of radionuclides. Advanced Reductive Processes (ARPs) are based on an approach that is similar to that of many AOPs, i.e. combining reagents and activating methods to produce reactive free radicals. However, ARPs produce reducing, rather than oxidizing, free radicals.

15 Advanced Reduction Processes (ARPs) Reducing Agent Activating method Reductant Radical Sulfite Dithionite Sulfide Ferrous iron UV light E-Beam Ultrasound Microwave SO 2 - SO 3 - HS - e - aq H

16 Advanced Reduction Processes (ARPs) Dithionite: Dithionite (S 2 O 4-2 ) is known to have a long, weak S-S bond that can be broken to produce two sulfur dioxide radical anions S 2 O 4-2 = 2 SO 2 - It has an absorption peak in the ultraviolet near 315 nm It is a high production volume chemical with low price Sulfite: Sulfite solutions absorb UV light with a maximum near 276 nm. UV irradiation of sulfite solutions has been found to produce sulfite radical anion. The hydrated electron is another strong and rapid reductant and it has been found in sulfite solutions irradiated with UV light.

17 Advanced Reduction Processes (ARPs) Sulfide: Sulfide solutions absorb UV light with a maximum at 230 nm. Sulfide irradiation with UV has promoted formation of sulfide radicals and hydrogen. Ferrous Iron: Solutions of ferrous iron absorb UV light with a maximum at 220 nm and UV irradiation promotes formation of hydrogen and aqueous electrons.

18 Previous and ongoing ARP projects 1. Advanced reduction process for hazardous waste treatment, National Priorities Research Program (NPRP), Qatar National Research Fund (QNRF), Disinfection by products removal from water using advanced reduction process, NPRP, QNRF; Reductive immobilization and removal of arsenic and selenium from contaminated water using advanced reduction process, NPRP, QNRF; Solar-driven advanced reduction processes for destroying persistent contaminants in water, NPRP, QNRF;

19 Discovery Project (NPRP ) SO 3 2- UVL/UVM/UVB Targets S 2 O 4 2- Microwave VC Fe(II) Ultrasound 1,2-DCA S 2- Electron Beam

20 VC and 1,2-DCA waste streams from PVC and vinyl product manufacture facilities intermediate accumulation from reductive biodegradation of PCE/TCE accidental release Maximum Contamination Level (MCL) for drinking water MCL of VC=2 μ g/l MCL of 1,2-DCA= 5 μ g/l

21 Results of screening tests SO 3 2- Targets S 2 O 4 2- Fe(II) UVL/UVM/UVB VC 1,2-DCA S 2-

22 Direct Photolysis of VC Direct photolysis with UV-L with a peak at 254 nm wavelength resulted in VC destruction. VC degradation kinetics under direct photolysis was assumed to follow a pseudo-first-order decay model: At a UV light intensity of 2400 µw/cm 2 and initial VC concentration of 0.5 mg/l, the rate constants were found to be 0.012, 0.011, and min -1 at ph 3, 7, and 10, respectively.

23 Direct Photolysis of VC There are two possible pathways that could describe the transformation from VC to chloride.

24 Direct Photolysis of VC 1.4 ph 8.55 ph 7.93 VC+UV no buffer Model VC+UV no buffer 1.2 VC conc. (mg/l) ph 7.90 ph ph Irradiation Time (min)

25 Reagents screening VC VC degradation by various reagent/uv combination

26 Reagents screening 1,2-DCA 1,2 DCA degradation by various reagent/uv combination dithionite sulfite sulfide ferrous iron

27 Reagents screening Summary Most ARPs (reagent/uv combination) are effective in degrading VC and 1,2 DCA ph has great effect on the degradation rates The rapid degradations caused by reactive species that are produced when the reducing reagents receive UV irradiation dioxide radical (SO - 2 ) sulfite radical (SO - 3 ) hydrated electron (e aq- ) hydrogen atom (H)

28 Dechlorination efficiency Dechlorination efficiency: the fraction of chlorine atoms in VC or 1,2 DCA that was degraded and converted to chloride ions R dech = (C Cl, chloride ion released )/ (C Cl, in initial VC or 1,2 DCA C Cl, in final VC or 1,2 DCA ) Major influence factor: Solution ph At low or neutral ph ( 7), chloroethane (C 2 H 5 Cl) is the major organic product At higher ph ( 8.2), non chlorinated hydrocarbon (probably propane C 3 H 8 ) is the major organic product At ph 11, >90% dechlorination obtained (all organic Cl releases as Cl )

29 Degradations mechanisms Reactive species in the sulfite/uv ARP aqueous electron (e aq- ) and sulfite radical (SO 3 - ) which one is the major species that causes the degradation? Use e aq scavengers to test the mechanism e aq scavengers: NO 3 and N 2 O reaction rate of e - aq and NO 3- / N 2 O >> reaction rate of e - aq and VC / 1,2-DCA NO 3 or N 2 O does not scavenge SO 3

30 Degradations mechanisms Results VC + sulfite/uv + NO - 3 VC + sulfite/uv + N 2 O 1,2-DCA + sulfite/uv + NO - 3 1,2-DCA + sulfite/uv + N 2 O Degradation kinetics is not affected Degradation kinetics is not affected Degradation is completely inhibited Degradation is completely inhibited e aq is the major reactive species causing 1,2 DCA degradation SO 3 is the major reactive species causing VC degradation

31 Disinfection Byproducts Removal from Water Using Advanced Reduction Process (NPRP ) Bromate occurrence in high concentrations in desalinated water and in reject brine from desalination plants is a major concern in the GCC. High concentrations of bromide (~76 mg/l) promotes the formation of high concentrations of bromate. Published reports indicated that tests of drinking water samples from a GCC country showed bromate levels around 10 times the WHO's recommended guidelines. Chlorate is a disinfection byproduct resulting from the use of chlorine dioxide as a disinfectant. Once bromate or chlorate ions are formed in water, they are relatively stable in environmental conditions and very difficult to remove

32 Bromate reduction Direct photolysis Figure 3. Mechanisms of bromate reduction by sulfite/uv ARP. [14]

33 Bromate removal by sulfite/uv ARP UV-L (254 nm) UV-M ( nm) Normalized Concentration (C/C 0 ) (c) UV-L only bromate sulfite 1.56 mg/l sulfite 3.1 mg/l sulfite 7.8 mg/l sulfite 15.6 mg/l sulfite 31.3 mg/l Bromate Concentration ( M) (a) Only bromate Sulfite 1.56 mg L -1 (5 times) Sulfite 3.1mg L -1 (10 times) Sulfite 4.78 mg L -1 (15 times) Sulfite 7.8mg L -1 (25 times) Time (min) The effect of sulfite dose on bromate removal. Conditions: [bromate] 0 = 4 μm, UV-M irradiance ~ 3,500 μw/cm 2, and UV-L ~ 4,900 μw/cm 2 Ref. B. Jung et al. Chemosphere 117 (2014)

34 Chlorate (ClO 3- ) UV-L 254 nm UV-M nm UV-B nm Dithionite/UV ARP S 2 O hν 2SO 2 - Dithionite rapidly decomposes at acidic ph and forms various products. 2S 2 O H 2 O 2HSO 3- + S 2 O 3 2-2HSO 3- S 2 O H 2 O Decomposition products: Sulfite (SO 3 2- ), Bisulfite (HSO 3- ), Thiosulfate(S 2 O 3 2- ), Metabisulfite(S 2 O 5 2- ) Therefore, a decomposition product that is being activated by UV could be responsible for chlorate photodegradation. Ref. B. Jung et al., Int. J. Environ. Sci. Technol. 2016, DOI /s y 34

35 Summary UV-L UV-M UV-B Dithionite VC 1,2-DCA Nitrate Bromate 1,2-DCA Chlorate 1,2-DCA Sulfite Nitrate VC 1,2,-DCA Bromate Chlorate Bromate 1,2,-DCA 1,2-DCA Sulfide Nitrate VC 1,2-DCA 1,2-DCA Ferrous iron VC 35

36 Advanced Reduction Processes (selenium & arsenic) 2S 2 O H 2 SeO 3 + H 2 O Se + 4HSO 3 - S 2 O HSeO 3- Se + 2HSO 3- + OH

37 Solar-Driven Advanced Reduction Processes V vs. ph = Vis (A) (B) (C) (D) (E) CB TiO2 Red. Vis Ti 3+ TiO2 CB Red. Vis TiO2 CB Red. Fe(III) Red. 3.2 ev TiO2 CB Bi2S3 e ev h + Vis Ox. Red. 3.2 ev TiO2 CB MWCNT e - h + Vis Ox N VB Ox. VB Ox. VB Ox. VB VB Time % TCE removal (min.) TiO 2 + sunlight TiO 2 + sulfite + sunlight 30 45% 72% 60 63% 89% (A) (B) (C) (D) VLR-TiNP@Fe 3 O 4 Magnetic core VLR-TiSD Spindle MWCNT@VLR-TiSD MWCNT+VLR-TiNT Concentration of chlorate (mg/l) chlorate+tio2+solar chlorate+dithionite12mm+tio2+solar Time (min)

38 Solar-Driven Processes in the Dark!

39 Concluding Remarks A holistic approach to water resources management is a key for maximizing water efficiency More effort is required for maximizing wastewater reuse Advanced oxidation and reduction processes can destroy persistent contaminants and eliminate them from the environment Abundance of solar energy in the region makes it an attractive energy source for water and wastewater treatment

40 Acknowledgments

41 THANK YOU