„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 AMF PROCESS INPUT: 0.015M Al2(SO4)3 0.02M SDS OUTPUT(residual): M Al M SDS OUTPUT(removed): M Al M 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: Al 3+ :0.03 mole l-1 SO 4 2- :0.045 mole l-1 Na + (from SDS):0.02 mole l-1 DS mole l-1 The outputs (residual) are: Al3+: mole l-1 DS-: 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 = q l-1 Charge for SO42-:-2q x mole l-1 = q l-1 Charge for Na+ (from SDS):+q x 0.02 mole l-1 = q l-1 Charge for DS--q x 0.02 mole l-1 = 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 mole l-1 = q l-1 Charge for DS-:-q x mole l-1 = q l-1 Charge for SO42-:-2q x mole l-1 = q l-1 Charge for Na+ (from SDS):+q x 0.02 mole l-1 = q l-1 Total Output charge : q l-1

Analysis of the results (IV) If the inputs are 0.02M SDS and 0.015M Al 2 (SO 4 ) 3 (or 0.03M Al3+) and the outputs are M SDS and M Al3+, then the flocs contain 0.02M – M = M DS - and 0.03M – M = M Al 3+ 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 Al 3+ in the floc. With q l-1 from DS -, it would require each Al 3+ to have an effective charge of / = 0.658, meaning it should be present in a different form than Al 3+. We will look into this next.

Analysis of the results (V) Aluminum ions may be present in solution as Al 3+, but also as Al(OH) 2+, Al(OH) 2 +, [Al 13 O 4 (OH) 28 ] 7+, Al(OH) 3 or Al(OH) 4 - Therefore, for an apparent charge of 0.66, the prevailing species should be [Al 13 O 4 (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 M Al2(SO4)3 there is enough flocculant left to cause flocculation after adding enough surfactant to make the mixture 0.02M SDS again.previous work Therefore, it is not possible that the residual Al3+ concentration can be M, which is well below the minimum requirement to cause flocculation of SDS