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The role of Humic acid and Sodium Alginate on the formation and

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The role of Humic acid and Sodium Alginate on the formation and Powered By Docstoc
					 The role of NOM in controlling
the size and oxidation state of Fe
         in natural waters

          Dr John Gaffney

              SEAES
                   Introduction

   Fe is both essential nutrient and terminal
    electron acceptor in respiration.
   Fe is ubiquitous and insoluble in circum-neutral
    pH surface waters.
   Production of particulate Fe in surface waters
    may be from mechanical erosion.
   Material of low SSA is likely from oxidation of
    Fe2+ entering surface waters via a groundwater
    source.
   Upon reaching higher pH (circumneutral)
    surface waters (oxic conditions) the following
    reactions occur:
                      Oxidation:
         4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H2O
                      Hydrolysis:
           Fe3+ + 3H2O → Fe(OH)3 + 3H+
                     Coagulation:
                Fe(OH)3 → (Fe(OH)3)n
   Fe forms insoluble Fe(OH)3 in surface waters.
   Insoluble Fe(OH)3 has a surface area, giving it a
    sorption capacity and hence control on soluble
    components of a system (e.g. contaminant
    metals Pb, Cu and Zn).
   Determination of Fe(OH)3 particle size is
    therefore essential for understanding
    contaminant movement and fate.
   Thus studying the formation of Fe(OH)3 from
    an Fe2+ starting point is essential.
   Previous work in this field has stated that the rate of
    Fe2+ oxidation is affected only by pH (Davison and
    Seed, 1983).
   More recently NOM and ionic strength have been
    found to have an affect (Pullin, 2004).
   NOM is also used in water treatment processes as a
    coagulant and has been shown to influence Fe(OH)3
    particle size.
   NOM has the potential to influence the rate of
    oxidation of Fe2+, thus the rate of formation of
    Fe(OH)3 and the stability of Fe(OH)3.
               Aim and objectives
    To investigate the role of NOM in controlling the rate of oxidation of
    Fe(II) in natural waters and the end products of the reaction.

Objectives
   Monitor the rate of Fe(II) oxidation in a laboratory simulation

                                     HA and AA

   Characterise the form of Fe present at the end point and determine the
    influence of HA and AA on these products.
   Determine the PSD of Fe and NOM at the end point of the reaction.
   Examine the form of Fe and the PSD of Fe and NOM in a natural system.
   Compare data from the laboratory and the natural system to check the
    validity of the simulation.
                             Method

   In order to simulate natural conditions laboratory
    simulations were to be constructed:

   Solution - synthetic river water (SRW).
   Fe2+ at environmental concentration (5mg/L).
   NOM – dominant types present in natural waters at
    environmental concentration. (Humic acid (HA) 30
    and 50mg/L and Alginic acid (AA) 30 and 50mg/L).
    pH 6.5 in sealed vessel.
                Fe(OH)3

 Determined by addition of NaOH.
 As Fe(OH)3 forms, 3 H+ are released.

 By maintaining pH 6.5 by addition of
  NaOH the amount of Fe(OH)3 formed
  can be quantified.
                 Free Fe2+

 Determined by ferrozine assay.
 A chelator of divalent Fe, which forms a
  stable magenta complex (Stookey, 1970)
  absorbs at 562nm.
 Sample left to stand for 30 seconds prior
  to measurement.
                       Hidden Fe2+

   Ferrozine is a strong chelator of Fe2+
   Therefore in this study, ferrozine was left to interact
    with the sample for prolonged periods of time.
   This allowed a competitive reaction to occur
    between ferrozine and other ligands (i.e. NOM).
   Subsequent colour development during this time
    period was attributed to Fe2+ stuck on other ligands
    being released and thus complexed with ferrozine.
                                        Results
Inorganic             1.0

                      0.8

                      0.6                                                       Fe2+


             Fe (%)
                      0.4                                                       Fe(OH)3
                      0.2                                                       Other Fe
                      0.0
                            0   2   6   10     40      971   DG   24hrs pt DG

                                         Time (mins)



30mg/L                1.0


Humic acid            0.8

                      0.6
             Fe (%)




                      0.4

                      0.2

                      0.0
                            0   2   6   10     40      300   DG   24hrs pt DG

                                         Time (mins)




50mg/L                1.0


Humic acid            0.8

                      0.6
             Fe (%)




                      0.4

                      0.2

                      0.0
                            0   2   6   10     40      300   DG   24hrs pt DG

                                         Time (mins)
 The rate of oxidation of Fe2+ not affected
  by the presence of HA.
 The rate of formation of Fe(OH)3 and
  total conversion to Fe(OH)3 is influenced
  by HA.
 Other Fe present during the titration may
  be attributed to Fe2+ being stuck on the
  HA rendering it not available for oxidation.
Inorganic               1.0

                        0.8

                        0.6
                                                                                   Fe2+




               Fe (%)
                        0.4
                                                                                   Fe(OH)3
                        0.2                                                        Other Fe
                        0.0
                              0   2   6   10     40      971    DG   24hrs pt DG

                                           Time (mins)




30mg/L                  1.0


Alginic acid            0.8

                        0.6
               Fe (%)




                        0.4

                        0.2

                        0.0
                              0   2   6   10     40      1380   DG   24hrs pt DG

                                           Time (mins)




50mg/L                  1.0


Alginic acid            0.8

                        0.6
               Fe (%)




                        0.4

                        0.2

                        0.0
                              0   2   6   10     40      2760   DG   24hrs pt DG

                                           Time (mins)
 The rate of Fe2+ oxidation is influenced by
  the presence of AA, with an increase in the
  concentration of AA resulting in a decrease
  in the rate of oxidation.
 The rate of formation of Fe(OH)3 is
  influenced by the presence of AA, increase
  in AA results in a decrease in the rate of
  formation.
 No Fe2+ bound to AA at the end point of
  the titration.
Location of Sites on Crowden Great Brook
Crowden Great Brook,
(Looking down on site
60)




Site 30
                     Conclusions
   Fe2+ is present in both laboratory simulations.
   NOM influences both the rate of oxidation of Fe2+,
    and formation of Fe(OH)3 (AA) and the total
    formation of Fe(OH)3 (HA).
   The longevity of Fe2+ present in oxygenated surface
    waters is dependant on the presence and concentration
    of NOM.
   These findings have implications for determination of
    the particle size distribution of Fe in natural waters and
    thus on the transport of contaminants such as Cu, Pb
    and Zn.

				
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