## 1 Introduction  
Lactic acid (2-hydroxypropionic acid) is an important commercial chemical due to its application in food, medical, pharmaceutical, packaging and textile industries. In the past decades, the demand for lactic acid has been significantly high, especially for its application in biodegradable polymers, i.e., polylactic acid (PLA), which has a potential to replace traditional commodity plastics. PLA-based plastics have some unique features such as high tensile strength, stiffness and resistance to fats and oils compared to traditional plastic materials; however, they require further improvement in some properties and characteristics such as viscosity, thermal stability and production cost [1]. The properties of PLA can be improved by blending with several other synthetic polymers (e.g.,polystyrene, polyethylene or polypropylene) and biopolymers (e.g., thermoplastic starch, rubbers or poly(3-hydroxybutyrate)) to obtain novel biodegradable materials with lower cost and higher durability compared to the traditional plastic materials [1; 2]. However, extraction, purification and continuous supply of lactic acid as raw material from fermentation processes is a major challenge because of its highly hydrophilic structure and the lack of efficient low cost purification techniques [3]. Lactic acid has a chiral carbon atom and exists in two isomeric forms, i.e., L-(+) and D-(-). Industrially preferred L-(+)lactic acid can be manufactured either by the chemical synthesis of acetaldehyde or by fermentation processes, where the latter accounts for \(\sim\)90% of the global lactic acid production [4]. Traditionally, lactic acid is produced by homolactic fermentation processes using pure cultures of lactic acid bacteria belonging to the genus _Lactobacillus_[5]. Recently, a heterolactic fermentation process with a new metabolic pathway named cappophilic (CO\({}_{2}\)-led) lactic fermentation (CLF) was found to be present in the hyperthermophilic marine bacterium _Thermotoga neapolitana_[6]. Under CLF conditions, both hydrogen and lactic acid are synthesized simultaneously unlike in the classic dark fermentation processes [6; 7; 8; 9]. _T. neapolitana_ fermentation takes place on a wide range of carbohydrate-rich substrates at 80 \({}^{\circ}\)C (353.15 K) and pH 6.5. However, the lactic acid concentration in the _T. neapolitana_ fermentation broth is in the lower range (2-10 g/L) compared to lactic acid fermentation (50-150 g/L lactic acid) by homolactic fermentation processes [10; 11]. Several downstream processing techniques have been used to recover lactic acid from the fermentation broth, e.g., calcium lactate crystallization, ion-exchange, adsorption, esterification-distillation/hydrolysis, membrane extraction and electro-dialysis [12; 13; 14; 15]. Among these techniques, adsorption by activated carbon and anionic resins are suitable for fermentation broth with low concentrations of lactic acid, especially from heterolactic fermentation [16; 17]. A wide range of anionic resins are commercially available with various functional groups and support matrices, which makes the adsorption process more flexible under various operating conditions. The adsorption capacity and efficiency are influenced by various factors such as the properties of the adsorbent (porosity, surface area, particle size and functional group), the adsorbate (polarity, \(pK_{a}\), molecular weight, structure and solute concentration) and the operating conditions (contact time, pH, temperature and mixing speed) [18]. Several anionic adsorbents such as Amberlite(r) IRA-67 (Sigma Aldrich SRL, Milan, Italy) [18], IRA-92 [19], IRA-400 [20], IRA-425 [21] and a non-polar adsorbent like activated carbon [18; 22] have been studied for lactic or organic acids adsorption from a fermentation broth at room temperature. Most of the anionic resins are non-toxic to microorganisms and can thus be used directly in the fermenter [23].