Abstract
In this study, gallic acid was separated by triphenylphosphine oxide in the presence of conventional solvents. Triphenylphosphine oxide is an organophosphorus extractant and highly selective towards carboxylic acids. Reactive extraction results were compared with physical extraction results. The extraction efficiencies reached up to 61, 76, 86, 67, and 84 % in the presence of triphenylphosphine oxide with oleyl alcohol, dimethyl adipate, isobutanol, methyl isopropyl ketone, and methyl ethyl ketone, respectively. Further, the number of theoretical units and the solvent to feed ratio were calculated for the practical design of a liquid-liquid extraction column. Roughly 2 to 4 theoretical units were calculated to meet the targeted extraction efficiencies. Gradient boosting algorithm showed a good performance to predict the results. This study is the first to investigate the reactive extraction of gallic acid by triphenylphosphine oxide, and include fundamental information for the recovery of gallic acid.
Key words: Reactive extraction, gallic acid, organophosphorus extractant, triphenylphosphine oxide.
Introduction
Fossil fuel sources are finite and the consumption of them is directly associated with environmental pollution and climate change. During the last years, a global effort has been made to replace fossil fuels with renewable energy sources. Biomass is one of the promising renewable energy sources and can be converted into valuable products [1]. Great progresses have been made in science and technology for bio-based carboxylic acid production. Carboxylic acids are the best known type of organic acids that have at least one carboxylic group in the structure [2]. They are broadly used in the chemical, pharmaceutical, food and fuel industries. They are versatile building blocks for the synthesis of chemicals, pharmaceuticals, cosmetics, bioplastics, biopolymers and biofuels [3]. There are several publications addressing carboxylic acid production from biomass. Various carboxylic acids were produced via fermentation of biomass (carbohydrates, glucose, sucrose, cellulose, lignin etc.) by using engineered cultures [2, 4–6].
Gallic acid (3,4,5 trihydroxybenzoic acid) is a carboxylic acid of great interest. It has antimicrobial, antioxidative, anticancer, and antidiabetic properties [7]. Gallic acid is broadly used in chemical and pharmaceutical industries [8]. The production of gallic acid (HGA) is possible by microbial fermentation [9–13]. The separation of HGA from the fermentation broth and aqueous solutions has been the subject of several publications. HGA was adsorbed onto Na-montmorillonite [14], and a coal-based activated carbon [15]. HGA was extracted by molecular imprinted polymers [16], ethanol [17], tributyl phosphate [18, 19], trioctyl amine and aliquat 336 [20, 21]. All of these separation methods have their advantages and disadvantages in terms of selectivity, simplicity, performance, and cost [22, 23].
The recovery of carboxylic acids from the fermentation broth is cost intensive and time consuming [6]. Separation is the major cost driver in the total cost for the downstream process [22, 23]. The most common acid recovery techniques are liquid-liquid extraction, adsorption, distillation, reverse osmosis, electro dialysis, and reactive extraction. Purity, high extent of recovery, low energy consumption, little waste generation, and modest investment cost are the critics of the separation process selection [5]. Reactive extraction process meets all these requirements. In the reactive extraction process, reactive extractants are used with the solvents to recover the acid. Since the reactive extractants are viscous, solvents are used to regulate their physical properties (viscosity, surface tension, and density). Alcohols, ketones, esters etc. are the conventional solvents used with the reactive extractants to recover acids. In the reactive extraction process, acid reacts with the extractant molecule, and so an acid-extractant complex forms. Further, the formed complex solubilizes in the solvent. Organophosphorus extractants and aliphatic amines are the most efficient type of reactive extractants [24–26]. When compared to the other techniques, reactive extraction presents several advantages. Extractants are selective towards acids. In-situ acid removal is possible without damaging the microorganisms. The nature of the reaction is reversible so extractants can be back-extracted to the system.
To the best of our knowledge, there has been no paper reported so far dealing with the separation of HGA by triphenylphosphine oxide (Ph3PO), which belongs to the family of organophosphorus compounds. The current study addressed this gap in the literature. For the solvent screen, five solvents were chosen from different categories. Reactive extraction results were evaluated regarding the distribution coefficient (KD) of the acid, extraction efficiency (E %), and loading factor (Z). The acid-extractant complexations were predicted by Mass Action Law Model. A temperature study was executed to determine the thermodynamic parameters. This study provides useful guidelines for the separation of HGA by reactive extractant of Ph3PO.
Method and Materials
All chemicals used in the experiments are listed in Table 1. The chemicals were of reagent grade and they were used without further purification. The mother stock HGA solution was prepared by distilled water. The initial concentration of HGA was 0.06 mole kg-1. It was prepared based on its concentration in the fermentation broth. pH of initial HGA solution was read on a pH meter (Mettler Toledo, SevenMulti). For the reactive extraction experiments, 5 mL aqueous phase (0.06 mole kg-1) was mixed with 5 mL organic phase in a conical flask. Ph3PO concentration was varied in the solvents from 12 to 44 % by volume to see its effect on the extraction efficiency. The mixture was shaken in a temperature-controlled shaker (Nüve, ST30) at 120 rpm for 2 hours. Based on the preliminary experiments, 2 hours shaking was found to be sufficient. Phase separation was done by centrifugation (Nüve, ST200) at 4000 rpm for 10 minutes. After phase separation, aqueous phase was taken by a syringe. The residual acid concentration was analyzed by a UV-vis spectrometer at the maximum adsorption wavelength of 260 nm. Since Ph3PO has a low affinity to water, the water co-extraction to the organic phase was found to be negligible. The extracted HGA concentration was calculated by mass balance. The physical extraction experiments were performed with pure solvents alone. The thermodynamic study was conducted from 298.2 K to 318.2 K to estimate the thermodynamic parameters. All the experiments were repeated in triplicate in order to ensure the consistency. The distribution coefficient and the extraction efficiency were calculated to evaluate the extraction performance.
KD =\(\frac{{[HGA]}_{\text{org}}}{{[HGA]}_{\text{aq}}}\)(1)
E % = \(\frac{K_{D}}{1+\ K_{D}}\) 100 (2)
where [HGA]org is the HGA concentration in the organic (extract) phase, [HGA]aq is the HGA concentration in the aqueous (raffinate) phase.