Processes of remobilization in subsurface arsenic removal for drinking water treatment

1
Processes of remobilization in subsurface arsenic
removal for drinking water treatment
Doris van Halem1,2 Marcel Tielemans3 Weren de
Vet1,4 Gary Amy1,2 Hans van Dijk1 1 Delft
University of Technology, the Netherlands 2
UNESCO-IHE Institute for Water Education, the
Netherlands 3 Het Waterlaboratorium, the
Netherlands 4 Oasen Drinking Water Company, the
Netherlands Contact d.vanhalem_at_TUdelft.nl/drink
water.tudelft.nl
2
Subsurface or in-situ iron removal

• Numerous applications in Europe Van Beek, 1983
  Rott and Friedle, 1985 Braester and Martinell,
  1988 Rott and Lamberth, 1993 Mettler et al.,
  2001 Appelo et al. 1999 etc.
• Injection-production single well or multiple
  injection wells (VyredoxTM method)

Injection wells
Production well
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Subsurface or in-situ iron removal (contd)

• Subsurface treatment in short
• injection of aerated water into anaerobic
  aquifer
• (adsorbed) iron(II) is oxidized to iron(III)
• retardation of the oxygen front.

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Subsurface or in-situ iron removal (contd)

• Subsurface treatment in short
• iron (oxy)hydroxides promote iron(II) adsorption
• abstraction of groundwater with decreased iron
  levels
• also adsorption of anions such as arsenic and
  phosphate

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Subsurface or in-situ iron removal (contd)

• Drinking water treatment plant in the Netherlands
• operates subsurface aeration at multiple sites
• for enhancement of nitrification, so not for iron
  or arsenic removal
• a single well for injection and abstraction.
• in short
• - arsenic concentrations very low (lt10 ug/L)
• - iron concentrations approx. 5-6 mg/L
• - 1 volume injection of volume abstraction
• - duration of a cycle approximately 1 month

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Subsurface or in-situ iron removal (contd)
Start subsurface iron removal
Source Oasen Drinking Water Company
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Arsenic occurrence and geochemistry

• Arsenate (V)
• typically in aerobic water (surface water)
• negatively charged
• Arsenite (III)
• typically in anoxic water (groundwater)
• uncharged
• more mobile than As(V)

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Arsenic occurrence and geochemistry (contd)
Source www.worldbank.org

• Arsenic a worldwide problem!

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Subsurface arsenic removal

• Oxidation-adsorption mechanism
• Arsenic adsorption onto iron hydroxides
• pH dependency
• competing anions (e.g. PO43-)
• high FeAs ratio required

Source Sharma (2001)
Comparison of As (V) and As(III) sorption edges
on (A) ferrihydrite and (B) goethite. Total
arsenic concentrations are 100 µM (circles) and
50 µM (squares). Open symbols As(V) closed
symbols As(III). Experimental conditions 0.01 M
NaClO4 0.03 g L-1 ferrihydrite or 0.5 g L-1
goethite.
Source Dixit and Hering (2003)
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Subsurface arsenic removal (contd)
Source Oasen Drinking Water Company
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Subsurface arsenic removal (contd)
Does remobilization occur?
Source Oasen Drinking Water Company

• Is there a risk of remobilization once injection
  is stopped?
• gt especially relevant for operation in rural
  areas of Bangladesh.

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Processes of remobilization

• Reductive dissolution of iron hydroxides due to
  onset of reducing conditions once injection of
  aerated water is stopped
• Desorption of arsenic due to transformation of
  freshly precipitated iron hydroxides
  (ferrihydrite) to iron hydroxides of higher
  crystallinity (e.g. goethite).
• Desorption due to changing groundwater quality
  (e.g. pH, phosphate)

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Aging of freshly precipitated ferrihydrite

• Behaviour of arsenic(V) and arsenic(III) in the
  presence of freshly precipitated iron hydroxides
• Aging of iron hydroxides in the oven at 60C to
  accelerate process of crystallization
• No significant desorption of arsenic observed -gt
  experiments repeated at 20C

Experimental conditions pH 7, iron (0.03mM)
and arsenic (0.01mM)
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Aging of freshly precipitated ferrihydrite
(contd)
Arsenic(V)
Arsenic(III)
Experimental conditions pH 7, iron (0.03mM)
and arsenic (0.01mM)
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Ongoing arsenic research

• Simulation in the laboratory regeneration of the
  iron surface
• (ii) Arsenic remobilization from accumulated iron
  precipitates
• (iii) Field study in rural Bangladesh

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Processes of remobilization in subsurface arsenic
removal for drinking water treatment
Doris van Halem1,2 Marcel Tielemans3 Weren de
Vet1,4 Gary Amy1,2 Hans van Dijk1 1 Delft
University of Technology, the Netherlands 2
UNESCO-IHE Institute for Water Education, the
Netherlands 3 Het Waterlaboratorium, the
Netherlands 4 Oasen Drinking Water Company, the
Netherlands Contact d.vanhalem_at_TUdelft.nl/drink
water.tudelft.nl
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Water quality in reference wells
Table 1 Water quality parameters in reference
wells around subsurface treatment well (LS-P08)
a average 1992-2006 b average 1994-2005 c average
2003-2005