Pilot Plant test of Adsorptive Micellar Flocculation

1
Pilot Plant test of Adsorptive Micellar
Flocculation

• A critical review, by Federico Talens-Alesson

2
Background

• The technique consists in mixing in aqueous
  solution anionic surfactant SDS in micellar form
  and Al3.
• The two chemicals will form large filterable
  particles and, in the process, capture molecules
  of a range of organic chemicals

3
Physical Chemistry

• The process is the combination of two phenomena
• electrostatic neutralisation of the charge of
  SDS micelles (anionic) by Al3 adsorbing on its
  surface. It leads therefore to electrically
  neutral flocs.
• Binding of organic compounds either through
  complexation with Al3 at the surface of the
  micelle or by solubilisation inside its core.

4
The claim by the authors
The mass balances for SDS and Al3 as claimed by
the authors are presented in flux diagram form
INPUT 0.015M Al2(SO4)3 0.02M SDS
OUTPUT(residual) 0.0004M Al3 0.0005M SDS
AMF PROCESS
OUTPUT(removed) 0.0296M Al3 0.0195M SDS
5
Analysis of the results (I)
The inputs are, therefore Al3 0.03 mole
l-1 SO42- 0.045 mole l-1 Na (from SDS) 0.02
mole l-1 DS- 0.02 mole l-1
The outputs (residual) are Al3 0.0004 mole
l-1 DS- 0.0005 mole l-1 SO42- 0.045 mole
l-1() Na (from SDS) 0.02 mole l-1()
()There is no involvement of Na or SO42- in the
flocculation process, these species are inert to
the process. Therefore, also as outputs
6
Analysis of the results (II)
Every stream, input or output, must be
electrically neutral. This is true from the
input, as indicated below.
Charge for Al3 3q x 0.03 mole l-1 0.09 q
l-1 Charge for SO42- -2q x 0.045 mole l-1 -
0.09 q l-1 Charge for Na (from SDS) q x 0.02
mole l-1 0.02 q l-1 Charge for DS- -q x
0.02 mole l-1 - 0.02 q l-1 Total Input charge
0
Note q is the charge of an electron
7
Analysis of the results (III)
However, this is not true from the residual
output
Charge for Al3 3q x 0.0004 mole l-1
0.0012 q l-1 Charge for DS- -q x 0.0005 mole
l-1 - 0.0005 q l-1 Charge for SO42- -2q x
0.045 mole l-1 - 0.09 q l-1 Charge for Na
(from SDS) q x 0.02 mole l-1 0.02 q
l-1 Total Output charge - 0.0693 q l-1
8
Analysis of the results (IV)
If the inputs are 0.02M SDS and 0.015M Al2(SO4)3
(or 0.03M Al3) and the outputs are 0.0005M SDS
and 0.0004M Al3, then the flocs contain 0.02M
0.0005M 0.0195 M DS- and 0.03M 0.0004 M
0.0296 M Al3 The floc must be essentially
neutral as it is caused by the neutralisation of
the surface charge of the micellar colloids.
Obviously this demands a match between the charge
of the DS- and Al3 in the floc. With - 0.0195 q
l-1 from DS-, it would require each Al3 to have
an effective charge of 0.0195/0.0296 0.658,
meaning it should be present in a different form
than Al3. We will look into this next.
9
Analysis of the results (V)
Aluminum ions may be present in solution as Al3,
but also as Al(OH)2, Al(OH)2, Al13O4(OH)287,
Al(OH)3 or Al(OH)4-
Therefore, for an apparent charge of 0.66, the
prevailing species should be Al13O4(OH)287,
with some minor contribution of Al(OH)2. This
would require a pH around 4.5. This could only be
achieved by adding alkali. Such addition of
alkali would also have the effect of lending the
(residual) effluent enough cations (e.g. Na from
NaOH) to provide an electrically neutral
effluent. There is no mention of such step in the
article.
10
Analysis of the results (VI)
In a previous work is was shown how after mixing
solutions of SDS and Al2(SO4)3 to produce a
mixture 0.02M SDS and 0.0125M Al2(SO4)3 there is
enough flocculant left to cause flocculation
after adding enough surfactant to make the
mixture 0.02M SDS again. Therefore, it is not
possible that the residual Al3 concentration can
be 0.0004M, which is well below the minimum
requirement to cause flocculation of SDS