Arsenic Removal by Ion Exchange

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Arsenic Removal by Ion Exchange

• Joe Chwirka - Camp Dresser McKee
  (chwirkajd_at_cdm.com)
• Bruce Thomson - University of New Mexico
  (bthomson_at_unm.edu)
• with contributions from Jerry Lowry and Steve
  Reiber

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Introduction

• Arsenic removal by Ion Exchange (IX) is effective
  for many applications
• Presentation will discuss
• Chemistry of the process
• Variables affecting process performance
• Especially competing ions and resin selectivity
• Salt use brine production and disposal
• Design considerations
• Two example systems

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Arsenic Treatment Technologies

• Likely Technologies
• Coagulation/ Microfiltration
• Fixed Bed Adsorption Technologies
• Coagulation/ Filtration
• POU or POE
• Unlikely Technologies
• Ion Exchange
• Activated Alumina or Regenerable Media
• NF or RO or EDR
• Enhanced Softening

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Ion Exchange Reactions

• Cation Exchange (water softening)
• 2 (R-Na) Ca2 R2-Ca 2 Na
• Anion Exchange (arsenic removal)
• R-Cl H2AsO4- R-H2AsO4 Cl-
• Will only remove ionic species
• Regenerate resin with concentrated NaCl brine

R Resin site
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Softening Processes

• BACKWASH
• TO WASTE

RAW WATER
Ion Exchange System
RINSE TO WASTE
BRINE TANK (NaCI)
TREATED WATER
RAW WATER FOR BACKWASH
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Point of Entry Softening
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Cation and Anion Exchange System
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Acid-Base Chemistry of As(V)
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Acid-Base Chemistry of As(III)
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Resin Selectivity Sequence

• Strong Base Type II Anion Resin selectivity
  sequence
• UO2(CO3)34- gt SO42- gt HAsO42- gt NO3- gt
  H2AsO4- gt Cl- gt HCO3-
• All SO42- must be removed before As is removed
• Other anions may also have higher selectivity
  than As (i.e. VO43-)

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Concentration Profiles in IX Column

• All SO42- is removed before HAsO42- is removed
• Can result in chromatographic peaking
• SO42- elutes HAsO42- from resin
• Effluent As concentrations are much higher than
  influent concentrations

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Chromatographic Peaking
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Chromatographic Peaking
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Chromatographic Peaking
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SEPARATION BREAKTHROUGH

• Kinetics EBCT
• Bed Capacity SO42-
• Leakage
• As N03- Peaking
• Alkalinity, pH/Corrosivity

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Kinetics Empty Bed Contact Time

• Exchange kinetics are rapid and internal mass
  transport limitations are small
• Empty Bed Contact Time (EBCT)
• EBCTmin 1.5 min
• Breakthrough is sharp leakage is low

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Bed Capacity SO42-
Sulfate Removal Technology!
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IX System - Unity, ME

• 11 Years Old
• As(III) at 100 µg/L
• Trace of Sulfide Fe
• 65 gpm Design Flow
• Filox Oxidizing Filter
• SB II Anion Resin

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FJ School IX System

• Filox IX
• 1200 gpd
• As 25-60 µg/L (sol)
• SO42- 25 mg/L
• pH 7.6
• Capable of lt 1.0 µg/L
• treated water

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EPA Study at Medical Facility - Arsenic
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EPA Study at Medical Facility - pH Alkalinity
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Design

• EBCT 1.5 min
• operated to regen. BEFORE SO42- causes
  breakthrough
• down flow service regeneration
• single or multiple beds
• single or multiple brine use

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Process Design

• Small Systems
• single or double vessels
• EBCT gt 1.5 min and long service cycle
• intermittent flow
• single use of brine
• may not be feasible w/o POTW for brine disposal

• Large Systems
• multiple parallel vessels
• staged regeneration
• possible reuse of brine
• may not be cost effective w/o POTW for brine
  disposal

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Regeneration

• Small Systems
• use a standard softener control small brine
  tank
• 8-15 lb salt used/cf of resin
• backwash, brine, slow rinse, fast rinse

• Large Systems
• parallel vessels and staged regeneration with
  salt storage brine maker
• 5.4 lb salt used/cf of resin
• backwash, brine, rinse steps

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Brine Disposal Options

• Municipal sewer
• Likely only for very small systems
• Systems near coast where salinity isnt problem
• Evaporation ponds
• Deep well injection

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IX Brine Recycling
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Fruitland, Idaho

• Water Chemistry
• As 34 micrograms/L
• Nitrate - 14.6 mg/L
• Sulfate - 51 mg/L
• pH - 7.3

1,500 X sulfate 429 X nitrate 1,929
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10 gpm water loss
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Floor Drain To POTW
10 gpm Water loss
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Plan Ah e a d
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• Sequentially regenerated
• pH
• Alkalinity
• 6 8,000 lb salt/week _at_ 0.10/lb
• 50 production loss for 5 hours (85 gpm)
• Regenerate 1/30 hours

Floor Drain To POTW
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Rimrock, Arizona

• Basin Water IX

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per acre-foot
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• Ten in parallel
• Seven in production
• pH
• Alkalinity
• Chromatographic peaking

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Truck fill
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