## 4 Discussion  
### Influence of Operating Parameters on the Adsorption of Lactic Acid onto GAC and Anionic Resins  
#### 4.1.1 Effect of pH and Adsorbent Dose on the Lactic Acid Adsorption Mechanisms  
The study shows that GAC, RLX425 and AMB400 can be used to adsorb lactic acid from a model fermentation broth used for _T. neapolitana_ fermentation when it contains low concentration of lactic acid (<5 g/L). The adsorption capacity for GAC (38.2 mg lactic acid/g adsorbent) was found to be the highest, followed by AMB400 (31.2 mg lactic acid/g adsorbent) and RLX425 (17.2 mg lactic acid/g adsorbent) with 10% (\(w/v\)) adsorbent dose and an initial lactic acid concentration of 4.5 g/L at 303.15 K. This study also showed that the lactic acid recovery efficiency is better for the GAC and the AMB400 resin compared to the efficiencies reported by previous studies (Table 6). Both GAC and the weakly basic anion exchange resin (RLX425) were effective in adsorbing lactic acid at a pH below the \(pK_{a}\) (3.86) of lactic acid, whereas the strongly basic anion exchange resin (AMB400) was effective at a pH above the \(pK_{a}\) (3.86) value (Figure 1b). The results show that GAC and RLX425 are effective at pH 2.0 and AMB400 is effective at pH 5.0 (Figure 1b). The adsorption of lactic acid onto GAC is due to a physical adsorption process and depends on electronic polarizability, the decreased physical adsorption of lactic acid at high pH (e.g., 5.0) is due to decreased dipole-dipole interactions [22]. Lactic acid is usually recovered from the fermentation broth via hydrophobic interactions with a neutral poly(styrene-co-DVB) resin at pH 2.0, as reported by [PERSON] and [PERSON] [34]. In addition, [PERSON] et al. [23] found that the adsorption of organic acids onto GAC or weakly basic anionic resins like Amberlite(r) IRA-67 is dominated at a pH below the \(pK_{a}\) value of the acids, which was also observed for the RLX425 resin in this study (Figure 1b). RLX425 is a weakly-basic resin that has a pyridine functional group and the lone electron pair of the nitrogen atom allows it to form hydrogen bonds with the lactate ion [12]. Both GAC and the RLX425 resin are effective at a low pH (2.0), thus their use would require a post fermentation process with acidification of the medium below the standard pH value (4.5-6.5) that occurs during dark and capnophilic lactic fermentation [18; 25]. On the other hand, a strongly basic adsorbent like AMB400, which is effective at pH 5.0, is more recommended for in situ extraction of lactic acid from a fermenter [20]. The results from this study were further validated by the observations from previous studies, where GAC as well as the weakly basic resins were effective in adsorbing lactic acid at a pH 2.0 [16; 18; 22; 25] and strongly basic resins were effective at a pH 5.0 [20; 35]. AMB400 is a strongly basic resin that has a quaternary ammonium as functional group with a positive charge and can thus form ionic bonds with the lactate ion. The binding of lactic acid to AMB400 predominantly occurs as: \(\mathrm{R_{rm}^{+}Cl^{-}+L^{-}\longleftrightarrow R_{rm}^{+}L^{-}+Cl^{-}}\), where \(L\) is the lactate ion and \(\mathrm{R_{rm}}\) is the resin matrix with a functional cationic group [36]. The \(\mathrm{Cl^{-}}\) form of the quaternary ammonium of the AMB400 resin has a weak basic property. This facilitates lactic acid adsorption through Lewis acid-base interactions with the partially positively-charged hydroxyl (\(\mathrm{OH^{-}}\)) and acidic protons (\(\mathrm{H^{+}}\)) of lactic acid [12; 13]. The low recovery efficiency (\(\sim\)50%) of lactic acid with the AMB400 resin is attributed to the positively charged quaternary ammonium functional group which cannot be approached by the partially negatively-charged oxygen of the acid due to the steric hindrance provided by the bulk alkyl groups on the nitrogen as well as due to competing counter ion species (i.e., \(\mathrm{Cl^{-}}\) and \(\mathrm{L^{-}}\)) [13]. The lactic acid adsorption by the AMB400 resin occurs through an ion exchange mechanism [37].