CEE 371 Water and Wastewater Systems

Transcription

1 Updated: 22 November 2009 CEE 371 Water and Wastewater Systems Print version Lecture #23 Drinking Water Treatment: Ion Exchange, Adsorption & Arsenic Reading: Chapter 7, pp David Reckhow CEE 371 L#23 1 Adsorption and Ion Exchange Description The physical and/or chemical process in which a substance (solute) accumulates at a solid-liquid interface (adsorbent) Types of adsorbents Natural solids (soil, sediments, aquifer; zeolites) Anthropogenic (activated carbon; polymeric anion exchangers) Types of adsorption mechanisms Classical adsorption : dominated by hydrophobic forces and specific chemical attraction Ion exchange: attachment of positive or negative ions on a surface with release of other ions of equivalent charge David Reckhow CEE 371 L#23 2 Lecture #23 Dave Reckhow 1

2 Ion Exchange Characteristics Breakthrough eventually; Requires regeneration Can be used for many years More highly charged ions are often better retained Common uses Softening Sodium replace hardness ions Regenerated with a strong NaCl solution Nitrate removal Arsenic removal R-Na 2 + Ca +2 R-Ca + 2Na + R-Na 2 + Mg +2 R-Mg + 2Na + Zeolites & MIEX David Reckhow CEE 371 L#23 3 Cation & Anion Resins Strong acid cation exchange The resins are highly ionized in both the acid (R-SO 3 H) and salt (R-SO 3 Na) form. They can convert a metal salt to the corresponding acid by the reaction: 2(R-SO 3 H)+ NiCl 2 (R-SO 4 ),Ni+ 2HCI Strong base anion exchange These resins are used in the hydroxide (OH) form for These resins are used in the hydroxide (OH) form for water deionization. They will react with anions in solution and can convert an acid solution to pure water: R--NH 3 OH+ HCl R-NH 3 Cl + HOH Lecture #23 Dave Reckhow 2

3 Selectivity of ion Exchange Resins Order of Decreasing Preference Strong acid cation exchanger 1. Lead 2. Calcium 3. Nickel 4. Cadmium 5. Copper 6. Zinc 7. Magnesium 8. Potassium 9. Hydrogen 10. Ammonia 11. Sodium Strong base anion exchanger 1. Iodide 2. Nitrate 3. Bisulfite 4. Chloride 5. Cyanide 6. Sulfate 7. Hydroxide 8. Fluoride 9. Bicarbonate IX Regeneration Process 1. The column is backwashed to remove suspended solids collected by the bed during the service cycle cle and to eliminate channels that may have formed during this cycle. The back-wash flow fluidizes the bed, releases trapped particles, and reorients the resin particles according to size. 2. The resin bed is brought in contact with the regenerant solution. In the case of the cation resin, acid elutes the collected ions and converts the bed to the hydrogen form. A slow water rinse then removes any residual acid. 3. The bed is brought in contact with a sodium hydroxide solution to convert the resin to the sodium form. Again, a slow water rinse is used to remove residual caustic. The slow rinse pushes the last of the regenerant through the column. 4. The resin bed is subjected to a fast rinse that removes the last traces of the regenerant solution and ensures good flow characteristics. 5. The column is returned to service. Lecture #23 Dave Reckhow 3

4 Resin Use: rough calculations I Cation columns 1. Take your water TDS (total dissolved solids) analysis. 2. Look at Calcium, Magnesium and Sodium concentrations in ppm. 3. Add up the total ppm and divide by 17.1 (i.e., 250 ppm/17.1 = 14.6 grams/gal). 4. Assuming a conservative 16 kilogram/cu. ft. capacity with an economical regeneration, now divide 16,000 by 14.6 (16000/14.6 = 1095). 5. You will get over a thousand gallons of deionized water per cubic foot of resin. Resin Use: rough calculations II Anion columns 1. Next add up the anions, they should be equal to the cations plus silica (SiO 2 ) and dissolved CO Add up the anions plus silica and carbon dioxide in ppm. Lets say total carbonate and silica is 20 ppm. 3. Take the total anion ppm and divide by 17.1 (270 (250+20)ppm /17.1 = 15.8 g/gal..). 4. Divide ide 14 kilogram/ft 3 by your answer (14,000/15.8 = 886 gallons per cubic foot). 5. A 5 gpm system would require about 2.5 cu.ft. of cation resin and about 3 cubic feet of anion resin, resulting in about 2500 gallons of DI water between regenerations. Lecture #23 Dave Reckhow 4

5 Nitrate removal by IX Resin selective for nitrate So sulfate doesn t cause massive release of nitrate Complete breakthrough at right showing influent concentrations removal release David Reckhow CEE 371 L#23 9 IX Unit design 2-5 ft of resin Product water is typically 1-2 mg/l Blending with raw water is often done because 10 mg/l is standard Brine wastewater About 3-4% of total t lflow Options include: sewer disposal, evaporation ponds, ocean disposal, and deep injection wells David Reckhow CEE 371 L#23 10 Lecture #23 Dave Reckhow 5

6 Mapping Arsenic in Groundwater by Sarah Ryker County map US EPA MCL = 10 μg/l David Reckhow CEE 371 L#23 11 Arsenic Removal Most Arsenic removal technologies require that it be in the oxidized d form As(+V) rather than As(+III) which is most common in GW Oxidation Ozonation will oxidize 95% of As (+III) in <15 sec (Ghurye & Clifford, 2001 [EPA report]) Sulfide and TOC compete for ozone, but do not limit overall As oxidation Chlorine and permanganate are also effective, chlorine dioxide, chloramines and UV are not Subsequent As(+V) removal Ion Exchange & Activated Alumina (Wang et al., 2000 [EPA report]) Some removal expected during coagulation (McNeill & Edwards, 1997) David Reckhow CEE 371 L#23 12 Lecture #23 Dave Reckhow 6

7 MIEX Ion Exchange Process- Magnetic Ion Exchange Continuous-flow MIEX process Raw water Contactors Settler Treated water Regeneration tank 1-15% 85-99% 1-15% Freshly regenerated resin From: Philip C. Singer Waste brine Lecture #23 Dave Reckhow 7

8 Sorption Naphthalene: Aqueous System with Sediment Re eactive Surface Sites Adsorption Partitioning Coating of organic matter Solid Sediments David Reckhow CEE 371 L#23 15 Isotherms Freundlich Multi-layer adsorption q = KC 1 n q (mg/g) Amount Adsorbed 1/n < 1.0 1/n = 1.0 1/n > 1.0 Amount Dissolved In Water C (mg/l) David Reckhow CEE 371 L#23 16 Lecture #23 Dave Reckhow 8

9 Isotherms (cont.) Simple partitioning When 1/n = 1.0 q = KC Incorporating organic carbon layer K oc = K/f oc Octanol/water partition coefficients [A] A K ow = [ A] octanol water Good correlation with K oc Relatively easy to measure David Reckhow CEE 371 L#23 17 Adsorption Removal of Dissolved compounds industrial solvents, pesticides taste & odor compounds chlorination byproducts biodegradable substances (biological filtration) doesn t require regeneration Several Applications for activated carbon granular (GAC) in a fixed bed powdered (PAC) in a rapid mix Can be expensive when used strictly as an adsorbent Almost always requires regeneration or replacement David Reckhow CEE 371 L#23 18 Lecture #23 Dave Reckhow 9

10 Adsorption model Langmuir Isotherm R = k R ad de = k ad de M c s M v s d c ' d c ad M s k ad k de ( v v ) m Maximum surface coverage Actual Surface coverage Particle David Reckhow CEE 371 L#23 19 Langmuir model At equilibrium, R ad=r de kde ( k ) + cd Which gives the Langmuir Isotherm v m v = v m ad c d v K d = v m k ad /k de c d David Reckhow CEE 371 L#23 20 Lecture #23 Dave Reckhow 10

11 Fixed Bed Adsorber v=v max Saturated Bed v<v max Mass Transfer Zone v~0 Clean Bed Adsorbed Conc. David Reckhow CEE 371 L#23 21 Filtration Removing media David Reckhow CEE 371 L#23 22 Lecture #23 Dave Reckhow 11

12 To next lecture David Reckhow CEE 371 L#23 23 Lecture #23 Dave Reckhow 12