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.