<p>
</p><p>Thanks to
its unique features, membrane distillation (MD) has been particularly applied
for desalination but also for niches applications with feed solutions
containing a mixture of volatile molecules. For such solutions, the complex
interplay of the solutes and solvent physicochemical and operating parameters
makes it challenging to predict the separation efficiency by MD. There is thus
a need for a better understanding of the behavior of volatile compounds in MD
as well as the influence of their physicochemical environment. This study aimed
at investigating the influence of different operating parameters on rejection
efficiency of air-gap MD towards carboxylic acids (formic, acetic and succinic
acids). Acid rejection was found to be highly dependent on the carboxylic acid
structure. In addition, it increased with the acid concentration, which could
be related to the formation of acid dimers in the feed solution. This behavior
is opposite to what is classically observed for pressure-driven membrane
processes thus suggesting that MD can be a suitable alternative to these
techniques for the concentration/separation of carboxylic acids. On the other
hand, acid rejection decreased with the increase of feed temperature which
could be explained by the calculation of the apparent energies of activation of
both the water and carboxylic acids using an Arrhenius-type model. Finally, the
acid dissociation rate played a key role in the acid rejection. Taking
advantage of this observation, it was demonstrated how a simple pH adjustment
can be used to successfully achieve the selective separation of ethanol
(compared to acetic acid) from an acetic acid/ethanol aqueous mixture (typical
case of the extraction of bioethanol from a fermentation broth).</p>
<p></p>