# Recent Advances in Muconic Acid Extraction Process
## 4 Muconic Acid Extraction
### Reactive Extraction Systems Used for MA Separation
#### 4.2.2 Reactive Extraction Influencing Factors  
The main factors investigated in the reactive extraction of muonic acid were pH, temperature, extractant concentration, and time [49; 50; 51; 52]. It was demonstrated that the pH of the aqueous phase has a significant impact on carboxylic acid reactive extraction [15; 16; 17; 18; 19]. In reactive extraction, MA can interact with the extractant in one of two ways: (I) by hydrogen-bonding with the undissociated acid molecule, or (II) by the formation of ion pairs when MA is dissociated (pH > pKa), Figure 5.  
Figure 4: Representation of muonic acid–extractant complex (1:1, for TBP,TOPO and TOA and 2:1 for DOA) formed through H-bonding. The aqueous solution's pH, the acid's pKa, the concentrations of the extractant and the acid, and the extractant's basicity with regard to the acid, all affect how the extract is extracted. The first mechanism (I) can be used to explain the extraction of carboxylic acid by phosphorous-based extractants (TBP, TOPO, etc. ), and both mechanisms can be used to explain the extraction of amine-based extractants (TOA, Aliquat 336). According to [PERSON] et al. (2017), pH has a significant impact on muonic acid's reactivity in aqueous media: cis,cis-MA deprotonates to the corresponding muonate dianion in an alkaline environment (dianion does not isomerize and is stable over a long length of time). On the other hand, in acidic environments, cis,cis-muonic acid easily isomerizes to its cis,trans isomer. Moreover, the carboxylic group of the acid interacts with alkene functionalities, and prolonged heating initiates intramolecular cyclization [55]. This is extremely important as all the reactive extraction studies have been carried out on cis,cis-MA, and none took into account its conversion into a cis,trans-MA isomer [55]. Regarding the temperature influence on MA reactive extraction, [PERSON] et al. [49] found a positive correlation between the temperature increase and the extraction efficiency until 35 \({}^{\circ}\)C (between 20-30 \({}^{\circ}\)C the extraction process is spontaneous); after this value, MA extraction using organophosphorus extractants (TBP and TOPO) was negatively impacted by the temperature increase (Table 4). [PERSON] et al. (2015) examined the effects of time in an interval between 1 min (shortest time required to achieve thorough mixing of both phases) and 24 h, on reactive extraction for three amines (DOA, THA, and TOA--amine concentrations in ethyl oleate: 0.5 mol/kg \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Solvent**} & \multirow{2}{*}{**Extractant**} & \multicolumn{2}{c}{**Extractant Conc.**, **mol/kg**} & \multirow{2}{*}{**T, \({}^{\circ}\)C**} & \multirow{2}{*}{**K**} & **Extraction Efficiency, \%** \\ \hline \multirow{4}{*}{Hexane} & & 0.89 & 20 & 14.66 & 93.61 \\  & & 0.89 & 25 & 16.33 & 94.23 \\  & TOPO & 0.71 & 30 & 22.98 & 95.83 \\  & & 0.77 & 35 & 21.39 & 95.53 \\  & & 0.77 & 40 & 18.61 & 94.90 \\  & & 1.95 & 20 & 2.25 & 88.43 \\  & & 1.96 & 25 & 3.04 & 89.50 \\ \multirow{2}{*}{Ethyl propionate} & \multirow{2}{*}{TBP} & 1.95 & 30 & 3.48 & 90.07 \\  & & 2.17 & 35 & 2.89 & 89.73 \\  & & 2.18 & 40 & 2.22 & 89.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Temperature influence on MA (initial concentration 0.007 mol/kg) reactive extraction using organophosphorus extractants in hexane (diluent) [49]. Figure 5: pH influence on extraction efficiency. and 0.1 mol/kg). The authors could not observe any effect of this parameter on extraction efficiency. The maximum extraction efficiency for all systems was attained after 1 min and remained constant throughout the whole reaction time, showing an instantaneous process [52]. The amount of extractant in the organic phase has a significant impact on the distribution and extraction efficiency of the acid [18; 19; 56; 57; 58]; typically, more extractant in the organic phase improves extraction efficiency because there is more of one reaction participant present.