„Pilot Plant“ test of Adsorptive Micellar Flocculation

A critical review, by Federico Talens-Alesson

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

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.

The claim by the authors

AMFPROCESS

INPUT:

0.015M Al2(SO4)3

0.02M SDS

OUTPUT(residual):

0.0004M Al3+

0.0005M SDS

OUTPUT(removed):

0.0296M Al3+

0.0195M SDS

The mass balances for SDS and Al3+ as claimed by the authors are presented in flux diagram form:

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:

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

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

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.

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)28]7+, Al(OH)3 or Al(OH)4-

Therefore, for an apparent charge of 0.66, the prevailing species should be [Al13O4(OH)28]7+, 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.

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