Removal of Arsenic by Ferric Hydroxide

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Removal of Arsenic by Ferric Hydroxide

• Aimee Howard1, Fabiola Brusciotti2,
• Andrew Lee2, Nada Assaf-Anid1 and Paul Duby2
• Columbia University, Earth and Environmental
  Engineering
• Manhattan College, Chemical Engineering
• Prepared for presentation at
• "Arsenic in Drinking Water An International
  Conference at
• Columbia University", New York, 26-27 November
  2001.

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Abstract

• Arsenic removal from a 50 ppm solution of
  arsenic(III) by co-precipitation with ferric
  hydroxide was studied in batch and continuous
  flow reactors. Ferric hydroxide was prepared by
  mixing ferric chloride with sodium hydroxide
  added in a stoichiometric excess of 15.
  Experiments were performed at two arsenic to iron
  ratios of 15 and 110. Co-precipitation occurred
  within 1 minute and the final pH of the treated
  water was about 9. Arsenic concentration was
  measured by Inductively Coupled Plasma (ICP) and
  Graphite Furnace Atomic Absorption
  Spectrophotometry (GFAAS). The percent removal
  reached 99.8 and led to final As concentrations
  in the 100 ppb range.

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Introduction

• Arsenic is present in nature as component of
  several minerals (ex. FeAsS)
• As mobility is controlled by pH and Eh of the
  groundwater (see fig.1)
• Most common valence states in water are As(III)
  and As(V), present as ions.

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(No Transcript)
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Predominance diagram for scorodite when the
activity of As dissolved compounds is equal to
10-0.7 and activity of Fe dissolved compounds is
equal to 1.
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Background

• As removal depends upon the ionic forms present
  and water chemistry
• Most technologies remove As(V) more effectively
  than As(III)
• In case As(III) is present, oxidation to As(V)
  may be required
• Co-occurrence of other compounds (e.g. sulfate)
  and suspended solids (e.g. silica), can interfere
  with As removal

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Removal Technologies

• Precipitation Processes
• Coagulation/Filtration (C/F)
• Lime softening (LS)
• Adsorption Processes
• Activated Alumina (AA)
• Ion Exchange (IE)
• Natural and artificial zeolite
• Membrane Processes
• Nanofiltration (NF)
• Reverse Osmosis (RO)
• Electrodialysis Reversal (EDR)

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Experimental Setup (batch)
Stirrer
pH meter
Volt meter
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Experimental Procedure (batch)

• Initial solution (1L)
• As(III) 50 ppm
• Fe as FeCl3, so that Fe/As 10 or 5
• NaOH in excess of 15 and 20 of the
  stoichiometric amount to form Fe(OH)3
• Stirrer 50 rpm
• pH and Eh measurements
• ICP and GFAAS analysis

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Experimental Setup (column)
Initial As(III) solution
Fe(OH)3
Pump
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Experimental Procedure (column)

• Initial solution (1L)
• As(III) 50 ppm (recirculated)
• Column filling
• Fe(OH)3, formed by adding FeCl3 (0.5g Fe) and
  NaOH (15 excess), and settled overnight
• Flow rate 1 Liter / 30 min.

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Results (batch)
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Results (batch)
Fe/As 10 NaOH 15 excess
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Results (column)
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Conclusions

• Batch
• 99 removal is obtained within 1 minute
• Final As conc is 150 ppb for Fe/As 10, and
  450 ppb for Fe/As 5
• Final pH is about 9
• Eh0.6 V when removal of As starts and it
  decreases during As precipitation
• Column
• About 95 of the arsenic is removed after three
  passes, and then up to 97.5 removal after six
  passes

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Future Work

• Experiments with lower initial concentrations of
  Arsenic
• Varying experimental conditions (pH, Fe/As,
  duration, etc) to achieve lower final
  concentrations of As
• Optimization of column setup, varying dimensions
  and flow rate
• Tests of natural As contaminated water