Separation and purification technologies in polyhydroxyalkanoate (PHA) manufacturing: A review
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Abstract
Polyhydroxyalkanoate (PHA), a biopolymer synthesized by various microorganisms, is a sustainable alternative to petroleum-derived plastics, which is a biodegradable, biocompatible, and non-toxic biopolymer. In the PHA downstream process, recovery methods are generally categorized into biochemical and mechanical approaches, and the separation methods include precipitation, centrifugation, and evaporation. PHA exhibits considerable variability based on the type of cell culture, recovery or separation method, and material used. On the other hand, most high-purity PHA is obtained by combining biochemical recovery and precipitation using solvents or chemicals (> 95.4% on average). In particular, additional purification, such as redissolution or pretreatment, is essential to meet the purity standards for medical use. Endotoxin levels below 0.5 of 0.06 endotoxin units/mL are required. Consequently, most PHA downstream processes rely on chemicals with negative economic and environmental impacts because of their handling and treatment. Therefore, analyzing these manufacturing processes through a life cycle assessment is important for understanding their impacts. This paper discusses the overall technique for the downstream process of PHA and highlights the need to develop and optimize environmentally friendly and cost-effective methods, such as mechanical approaches, to produce high-value products to meet the required standards for various applications.
Abstract
Graphical Abstract
1 Introduction
Plastics are mostly derived from petroleum and used widely for packaging, storage, and construction because of their superior performance with high tensile and tear strength, flexibility, formability, and lightweight [1,2]. On the other hand, petroleum-derived plastics have low biodegradability, which poses a major threat to marine and terrestrial environments. Current global plastic consumption has exceeded 200 million tons with an annual rate of 5% [1,2], but most waste has accumulated in landfills (40%) and has been discarded in dump sites (27.4%–30.3%). Only 14% of waste is recycled, and 1.7%–4.6% is found in the marine environment [3,4].
Biodegradable plastics have attracted considerable attention as promising materials to replace petroleum-derived plastics owing to their high biodegradability by primary microorganisms such as bacteria, fungi, and algae, helping reduce plastic pollution [5]. Representative biodegradable plastics include polyhydroxyalkanoate (PHA; the abbreviation listed in Table S1), polylactic acid (PLA), and starch-based and cellulose-based polymers. In particular, PLA is comparable to petroleum-derived plastics in terms of its mechanical properties and is used in industry because of its biodegradability, recyclability, and large-scale production capacity [6]. Nevertheless, the reliance on agricultural byproducts for PLA production could potentially promote environmentally harmful agricultural practices. PLA requires high temperatures and pressures for effective degradation [7]. Starch-based and cellulose-based polymers derived from renewable plant-based resources exhibit excellent biodegradability. On the other hand, several inherent disadvantages of these materials have been identified. In particular, they are highly vulnerable to the ingress of moisture, which can deteriorate their mechanical properties when exposed to water [8]. In addition, the complexity of the manufacturing processes is exacerbated by the thermal instability and tendency for retrogradation of these materials, necessitating further chemical modification to enhance their properties [9].
PHA decomposes completely naturally under ambient conditions without harming the environment [10,11]. In addition, PHA has unique properties, such as piezoelectricity, excellent biocompatibility, non-toxicity, and superior barrier properties of oxygen and water vapor [12]. Some varieties of PHA exhibit greater flexibility and ductility compared to PLA and show compatibility with biological tissues, making them suitable for medical applications without adverse effects [13]. Despite the current high production costs and less optimal mechanical properties of PHA, further developments may enhance its viability as a substitute for petroleum-derived plastics [8,9]. The global market for PHA is expected to increase to 7.7 million metric tons by 2026, representing an average annual growth rate of up to 10% between 2023 and 2033 [14].
The PHA manufacturing process consists of upstream and downstream processes. The upstream process involves cultivating a single strain of microorganism (i.e., pure culture) or recovering the cell mass from mixed microbial culture (MMC) from the waste resources (e.g., wastewater sludge and leachate). The downstream process encompasses the separation and purification of PHA. A significant portion of production costs swere attributed to the raw materials used during the upstream process and the difficulty of separating these materials during the downstream process [15,16]. The lower titer issues and cost of raw materials in the upstream process have been investigated, with the potential solution of using genetically engineered microorganisms for pure culture [17]. On the other hand, MMC-based PHA recovery from the waste resources (e.g., food waste, waste-activated sludge, and grey water) incurs minimal upstream costs but challenges in recovery with additional pretreatment or separation steps [18,19].
The downstream process for PHA production is a critical stage in determining the final purity and yield of products, with its efficiency being influenced by the techniques used, affecting the overall economics of the process [16]. For example, highly purified PHA is required for pharmaceuticals and biomedical applications [20]. The downstream process comprises recovery, separation, and purification steps similar to the general biopharmaceutical production process. The recovery step involves cell lysis and the extraction of PHA using chemicals, enzymes, or physical forces. The separation step uses techniques, such as centrifugation, precipitation, evaporation, and filtration, to isolate PHA from other impurities. The purification step further enhances the purity of the separated PHA through processes, such as re-dissolution or ozonation, and pretreatment with chemicals. They can be used to improve the final purity of the product but with a reduction in product yield. Consequently, this study compared the yield and purity achieved through various recovery and separation methods and evaluated the impact of pretreatment and purification steps on the downstream PHA manufacturing process. An economic evaluation via life cycle assessment (LCA) is also included. This review provides essential insights for the design of efficient downstream processes tailored to specific PHA applications.
2 Types, Properties and Applications of PHA
PHA, a group of microbial polyesters, is synthesized and accumulated intracellular granules by microorganisms. The biosynthesis of PHA is typically promoted in the cultivation medium when the amount of essential nutrients for growth, particularly the N source, is limited while carbon is abundant. Synthesized PHA has similar mechanical properties to synthetic plastics, including thermoplastic and mechanical properties. Their excellent biocompatibility and non-toxicity render them ideal for biomedical applications, prompting increased research interests in recent years [21–26]. The type, structure, and physical properties of PHA can be influenced by the microbial strain and carbon source used during the synthesis process [27].
PHA can be produced by a diverse strain of microorganisms, ranging from the wild-type to genetically engineered cells. Wild-type strains used for production include Cupriavidus necator, Azohydromonas lata, Pseudomonas spp., and Aeromonas hydrophila. In addition, their genes encoding enzymes catalyzing PHA synthesis can be introduced into Escherichia coli via genetic engineering for enhanced production efficiency. Each strain requires different growth conditions for high levels of PHA accumulation, with the supplied carbon source playing a crucial role in determining the type of PHA. For example, C. necator uses propionic acid or propionate with glucose to produce poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV); P. putida, putida KT2442, and entomophila L48 consume fatty acids with sugars to produce medium chain length (MCL)-PHA; A. hydrophila utilizes fatty acids, such as lauric acid or oleic acid to produce poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx). A. latus produces poly(3-hydroxybutyrate) (PHB) or PHBV under nitrogen-limited conditions. Recombinant E. coli use glucose and fatty acids to synthesize short-chain length (SCL)-PHA and MCL-PHA. These strains exhibit a range of PHA contents, reaching up to 88 wt.% in wild-type strains and over 90 wt.% in genetically engineered strains [28].
PHA is composed of different monomeric units, such as 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV), 3-hydroxyoctanoate (3HO), and 3-hydroxyhexanoate (3HHx). Fig. 1 presents the general structures of these compounds. The type of PHA is determined by the type and composition of the monomeric units. For example, PHB and poly(3-hydroxyoctanoate) (PHO) are homopolymers that are polymerized via the ester bonds of 3HB (Fig. 1a) and 3HO (Fig. 1b), respectively. Similarly, PHBV and PHBHx are copolymers of repeated alternating bonds of 3HB with 3HV (Fig. 1c) and 3HHx (Fig. 1d), respectively. These PHA can be classified into two main groups based on the number of carbon atoms in each monomeric unit: SCL-PHA and MCL-PHA. SCL-PHA consists of monomeric units of 3–5 carbon atoms, including PHB and PHBV. In contrast, MCL-PHA comprises 6–14 carbon atoms, including PHO and PHBHx [29,30].
Basic chemical structure of a) 3-hydroxybutyrate (3HB), b) 3-hydroxyoctanoate (3HO), c) 3-hydroxyvalerate (3HV), and d) 3-hydroxyhexanoate (3HHx).
The properties of each type of PHA are determined by the composition of the monomeric units, which determines their applications. PHB, PHBV, PHO, and PHBHx are the most commonly produced types of PHA in current [29,31,32]. Table 1 lists their physical properties and applications. PHB is the most widely used variant in industry because of its piezoelectric, thermoplastic nature, and high crystallinity with melting points (Tm) up to 180°C. Compared to polypropylene, PHB has greater rigidity and heat resistance but less flexibility [33]. These properties of PHB make it ideal for use in packaging containers, disposable articles, and agricultural supplies, such as mulch films, agricultural nets, and grow bags [34–36]. PHBV and PHBHx exhibited enhanced flexibility but lower Tm and crystallinity than PHB. These changes in physical properties were attributed to differences in the mole percentage of the monomeric units comprising the copolymer. Although 3HB is a high crystalline unit imparting hard and brittle properties, 3HV and 3HHx are elastomeric units, which contribute to the improved elasticity, softness, and flexibility of the copolymer as the mole percentage increases [36]. Ultimately, their copolymerization decreases tensile strength, Young’s modulus, melting temperature (Tm), and glass transition temperatures (Tg) but an improvement in elongation. They have a wide range of applications depending on their properties, with rigid PHBV and flexible PHBV used for packaging containers and hard tissue engineering scaffolds (e.g., bone), respectively [37–39]. PHBHx has superior thermal stability to PHB and PHBV, expanding the processing window and minimizing thermal degradation because of its 3HHx units exhibiting greater elasticity. These properties make PHBHx suitable for use as material for processing such as thermoforming, 3D printing, paper, and fertilizer coatings [40]. High-purity PHBHx can also be used in nanofibers, such as medical sutures for hemostasis, wound-healing, and tissue fixation, and has been reported to breakdown into non-toxic products in vivo and degrade within 60 days [41,42]. PHO is an extremely elastic material, composed of a single elastomeric unit of 3HO, resulting in poor mechanical properties but soft, flexible, and elastomeric, which is suitable for soft scaffolds of tissue engineering such as skin, heart valves, other vascular applications, nerve conduits, biomaterials for cardiac patch, and nanofibers [39,43]. Furthermore, they can be used as drug carriers, infection-resistance biomaterials, and scaffolds to initiate and promote chondrogenic and osteogenic activities [44].
3 Downstream Process for PHA
The main downstream process of PHA consists of recovery and separation steps. The recovery step involves releasing PHA granules from inside the cells, which is accomplished by disrupting the cells through various means, including using solvents, digestion, or mechanical disruption. After recovery, the separation step separates the PHA granules via precipitation, centrifugation, evaporation, or filtration. The quality of PHA is improved further by a purification step (i.e., re-dissolution and ozonation) and the pretreatment step.
3.1. Recovery Methods
Achieving high recovery of PHA is challenging because of factors, including cell wall fragility, type of PHA, required purity of polymer, PHA molecular mass, and environmental impacts [57]. Intracellularly accumulated PHA varies widely in size, ranging from 50 to 20,000 kDa [58], making efficient separation difficult [59]. Addressing these challenges requires considering these factors when determining PHA recovery methods. These methods aim to remove impurities, such as cell debris and non-PHA cell mass (NPCM) and can be classified as biochemical and mechanical recovery [60].
Biochemical recovery processes include solvent and digestion recovery methods. Solvent recovery, the most effective method for achieving high purity, involves using organic solvents, such as chloroform, ethyl acetate (EA), butyl acetate (BA), and dimethyl carbonate to dissolve and recover the internal PHA [18]. Chloroform is used widely as a control solvent for comparative assessments of product recovery with other solvents. The solvents are incorporated into the wet biomass suspension, followed by agitation and heating. The bacterial bodies are lysed by the solvent, resulting in the recovery of the intracellular PHA into the solvent phase [61]. In contrast, digestion recovery aims to recover the high purity of PHA from NPCM using oxidants, alkaline or acid compounds, and surfactants or enzymes [18,62,63]. These chemicals are introduced into the bioreactor, where they react with the dried PHA-containing biomass. The bacterial bodies are then digested in the bioreactor, resulting in the release of the PHA and the intracellular matrix, which comprises proteins and sugars [64]. Although digestion recovery has a higher yield than solvent recovery, it presents a risk of severe product degradation. Enzyme digestion maintains the molecular weight of the products largely intact but adds complexity and cost to the process, requiring additional energy, time, and resources for treatment.
Mechanical recovery, including bead mill, supercritical fluid (SC-fluid), high-pressure homogenization (HPH), and ultrasonication, provides environmentally friendly alternatives for extracting products from the microbial cell mass. These methods offer lower process costs and shorter processing times than biochemical recovery. They rely on physical forces, such as solid or liquid shear forces generated by the methods mentioned above, to disrupt the cell wall and release the desired products [65]. The bead mill agitates grinding media (beads) to grind or disperse minute particles in a cylindrical vessel. The bead size affects the milling energy and frequency, which is crucial for recovery performance, allowing temperature control and scalability [66,67]. SC-fluids are maintained above the critical pressure and temperature and are dissolved by gas diffusivity and liquid solubility. SC CO2 is used widely for PHA extraction because of its high density, low viscosity [68], non-toxic, non-flammable nature, high solubility, and economic benefit [69]. Controlling the applied pressure is critical to optimizing the efficiency of SC-fluid recovery processes. HPH extracts or preserves bioactive compounds and phytochemicals by applying uniform and transmit pressure to a sample [70,71], depending on the distribution of shear stress across a product [70]. Ultrasonication relies on the mechanical energy converted from ultrasonic energy and enhances PHA recovery by improving the mass transfer process in highly viscous and immiscible fluids [72]. The operating efficiency depends significantly on the ultrasonic energy source and operating time [73].
3.2. Separation Methods
Separation methods include precipitation, centrifugation, evaporation, and filtration, influenced by the previous recovery method. Precipitation, evaporation, and filtration are commonly used after solvent recovery, while centrifugation is conducted after digestion and mechanical recovery methods. The production yield and purity are crucial factors intimately related to production efficiency and the decision of its field of application. In particular, chemical purity is essential when used as a biomaterial to prevent inflammatory and other negative reactions in the human body. PHA is quantified by chromatography, such as high-performance liquid chromatography or gas chromatography. The purity and yield of PHA can be calculated by substituting the quantified results into the following Eqs. (1), (2) [74]. Table 2 lists the product yield and purity after recovery and separation methods.
PHA yield and purity depending on differences in recovery methods and subsequent separation methods.
3.2.1. Precipitation
In the PHA separation process, precipitation is conducted to concentrate and separate products from the solutions in biomaterial manufacturing [75]. Precipitation is used to obtain purified PHA granules in the PHA-dissolved suspension using anti-solvents, including alcoholic solvents (e.g., methanol, ethanol, and acetone) and non-polar organic solvents (e.g., n-hexane and n-heptane) [72,74,76–82]. Methanol precipitation resulted in 77.3% yield and 96.5% purity. Ethanol and acetone produced yields of 90.0 and 90.8%, with purities of 94.9 and 89.6%, respectively. These results suggest that alcoholic solvents can provide a high purity PHA (> 93.0%), with yields varying according to the type of anti-solvent. In addition, non-polar organic solvents can achieve even higher purity (~ 98.0%) of PHA than alcoholic solvents but with lower yields.
3.2.2. Centrifugation
Centrifugation, involving mechanical separation of particles contained in a solution through centrifugal force, is used widely and has similarities with precipitation in PHA separation [83]. This process produced a high yield of 93.7% and 90.8% when combined with mechanical and digestion recovery method, respectively, while the purity remains 93.0% and 89.1%. Centrifugation after mechanical recovery results in more than two-fold enhancement in product yield compared to other separation methods [84–88].
3.2.3. Evaporation
Solid PHA can be recovered by evaporation from the PHA-dissolved solution by heating. This technique is particularly effective for biochemical recovery using a halogenated solvent with a high purity and yield of over 95.0% compared to non-halogenated solvent or mechanical methods. Although this method offers the potential for high yield and purity under certain conditions, it is currently a less favorable process than other processes because of the remaining non-evaporable polymer matrix and high energy consumption [60]. On the other hand, limited data on comparisons between digestion and mechanical recovery methods indicate that further investigation is needed to understand the efficiencies and challenges of these techniques.
Consequently, of the 51 total PHA manufacturing processes were analyzed, the pure cell culture was the most common cell type, accounting for 40 cases. In particular, mechanical recovery methods have not been widely employed for MMC. The pure cell cultures typically achieved approximately 7.8% and 13.3% higher yield and purity of PHA than MMC, respectively. This is likely because pure cell cultures contain fewer impurities and are grown under more controlled conditions, resulting in more consistent and efficient processing. Despite the variations in yield and purity among different PHA types, these differences are negligible, suggesting that the PHA types have little effect on both parameters. Among the various PHA types, PHB and PHBV are mostly produced by the two culture types, accounting for 39. Regarding the recovery method, biochemical accounts for 46 out of the total, with organic solvents accounting for the primary material share at 30. On the other hand, biochemical methods carry the risk of dissolving unwanted byproducts and pose environmental hazards because of the use of chemicals and organic solvents. Although mechanical recovery is inherently less favorable than biochemical recovery because of its lower production efficiency, this can be mitigated by combining centrifugation, which offers significant time-saving benefits. For example, solvent recovery using chloroform with precipitation achieved maximum yield and purity of 78.0% and 99.0% within 60 h, respectively, while mechanical recovery using HPH with sodium dodecyl sulfate (SDS) followed by centrifugation achieved 98.0% and 95.0% within just 1 h, respectively [87]. A bead mill with centrifugation resulted in an excellent yield of 100%, suggesting that mechanical methods combined with centrifugation can be highly efficient in terms of time while achieving comparable quality [86].
The evaporation method has the highest average yield of 90.5% compared to other separation methods, but the precipitation method produced the highest purity of 85.4%. Centrifugation has an average yield of 89.2% and purity of 88.1%, with a large variation between the data. The most efficient yield and purity for each method are as follows: Precipitation using n-hexane combined with biochemical recovery using ethyl acetate achieved 99% yield and 100% purity [89]. The centrifugation combined with HPH using SDS had a 98% yield and 95% purity [87]. The evaporation combined with biochemical recovery using chloroform has a 96% yield and 95% purity [77]. In addition, finding efficient production conditions is vital because the combination of materials and specific methods used in the recovery and separation processes can result in lower yields of 12% to 57% or lower purities of 42% to 50% [76,78,79,88,89,90]. For example, even the same combinations of chloroform recovery with acetone precipitation have different values depending on the recovery conditions (e.g., temperature) [61,82]. In contrast, the same recovery and separation conditions can also provide different results depending on the materials used [74]. Different reflux methods in precipitation under the same process conditions doubled the yield while maintaining similar purity [79].
In addition, membrane filtration removes cell debris and allows solid PHA to be obtained by additional subsequent processes. Membrane filtration has several advantages over other techniques. It can separate desired substances based on size with high throughput. On the other hand, this technique requires relatively high set-up costs and a combination of one or more technologies depending on the application. Moreover, it faces challenges related to membrane fouling [91]. Biochemical using dimethyl carbonate with membrane filtration in PHBV production achieved a yield and purity of 49.0% and 98.0%, respectively [92].
3.3. Effects of Purification Process
PHA is purified to improve the purity of products and remove undesired impurities, such as hydrophobic lipids and proteins, after separation. Various methods include re-dissolution, ozonation, and activated charcoal filtration, depending on the type of solvent, the PHA being recovered, and the release method [60,81,86]. Re-dissolution is a favorable method for improving purity by eliminating residual impurities [45]. Its effectiveness in purification depends on the solvent used. 1-Butanol and 2-propanol increased the purity by 5.9% and 3%, respectively (Fig. 2), but decreased the yield by 0.4% and 0.0%, respectively [76,100]. Furthermore, 2-propanol reduced the endotoxin level to below 2 endotoxin units per gram in PHO. On the other hand, NaOCl increased the purity by 31.5% but decreased the yield by 29.9% [101]. These changes were attributed to the potent ability of NaClO to degrade the amorphous PHA granules and NPCM, ultimately leading to a loss of yield but high purity.
In addition, ozonation and activated charcoal filtration are other methods to improve the final purity of PHA. Ozone is a strong oxidant that eliminates contaminants such as bacteria, viruses, and metals. It facilitates impurity removal in aqueous polymer suspensions or latexes because of its bleaching, deodorization, and solubilization of impurities [60,102]. On the other hand, one or more functional additives, such as surfactants, detergents, emulsifiers, dispersants, anti-foaming agents, defoaming agents, biocides, viscosity modifiers, and pH control agents, are required for ozonation [102]. Activated charcoal, modified to reduce the internal pore size and increase the surface area at high temperatures, effectively removes endotoxin and other impurities as the PHA-dissolved solution passes through it as the mobile phase [86]. Generally, although the purity increased after purification, the yield tended to decrease, with the extent varying according to the purification method. Therefore, further research will be needed to develop methods that maintain PHA integrity while increasing the final purity.
3.4. Effects of Pretreatment
Pretreatment is applied before cell recovery to remove lipids and weaken cell envelopes [103], facilitating easier extraction of intracellular PHA. This is conducted using various techniques such as thermal, chemical, mechanical, biological, physical, and combinations of these methods [104,105]. It also affects the product yield and purity in subsequent downstream processes that enhance the recovery efficiency, as summarized in Table 3. NaOCl, which is a favorable used chemical for pretreatment, shows promising improvements in yield and purity. For example, NaOCl pretreatment at 100°C increased the purity by 6% but reduced the yield by 8% because of harsher oxidation or bleaching [92]. The effects of higher NaOCl concentrations at 37°C were attributed to the increased cell permeability by hypochlorite and the removal of cell debris and residual lipid, with higher yields [61].
Changes in the yield and purity of PHA using different pretreatment methods.
After pretreatment, various recovery methods with chemicals, such as SDS, NH4-Laurate, and NH4OH, were used. This treatment significantly enhanced the purity to 92 – 100%, whereas the yield decreased from 58% to 74% because of its strong oxidizing properties [90]. Another pretreatment using NaOH and NaOH with NaCl combined with supercritical fluids [84] achieved yields of 85 and 93%, showing that NaOH pretreatment combined with NaCl has a higher yield. This result was attributed to the change in cell wall strength before disruption. These findings suggest that although NaOCl is effective for purity, optimizing the temperature and concentration is crucial for achieving an optimal balance between yield and purity. NaOH, particularly with NaCl, offers a robust higher yield recovery, underscoring the need for tailored pretreatment strategies to maximize the PHA recovery efficiency. In addition, the effects of heat treatment and pH adjustment of biomass for solvent recovery were examined using 1,2-propylene carbonate [77], which exerted a negligible effect on the purity and yield of PHA.
4 Economic Evaluation and LCA
Conducting an economic evaluation and environmental impact assessment of the entire PHA manufacturing process using LCA and helping assess the holistic impact of bio-based plastics and products are essential for determining the viability of replacing existing petroleum-derived plastics with PHA. The economic potential should be assessed, including extraction protocol in terms of extraction costs and recyclability of the solvents and reagents involved. The extraction protocols are evaluated from three perspectives: material costs (e.g., extraction solvents, anti-solvents, digestion agents, and purification reagents), processing costs (e.g., the energy requirements of the extraction operation and process), and environmental aspects (e.g., disposal management, hazards and risks, and carbon footprint) [99,106].
Regarding economic and environmental aspects, the processes using solvent or anti-solvent are deemed less favorable because of the associated environmental concerns, such as wastewater issues [78,100]. The results of an LCA-based comparison of the three manufacturing for producing PHB from wastewater using alkali-surfactant, surfactant-hypochlorite, and dichloromethane solvent showed that manufacturing using a solvent resulted in the highest cost and environmental burden: 1.95 €/kg PHB, 4.26 kg CO2-eq/kg PHB, and 156 MJ/kg PHB [107]. The assessment suggested that digestion recovery is more attractive than solvent recovery in terms of environmental and economic aspects. Nevertheless, enhancing the purity and yield is necessary if digestion recovery is conducted because solvent recovery showed higher PHA yields with quality comparable to commercial polymers [107]. The availability of PHA in its application is affected by its purity.
For example, PHA with a purity above 90% is suitable for commercial applications [108], whereas a purity greater than 99.9% and meeting endotoxin standards are necessary for biomedical applications [109]. This review showed that solvent recovery can achieve 95% purity without further purification, while digestion recovery requires additional purification due to its purity of 88%. The process cost for additional solvent recovery is ultimately required because solvent re-dissolution is generally used in purification to achieve high purity, indicating that solvent recovery is more economical than digestion recovery when the targeted purity is above 90%. On the other hand, digestion recovery can generally achieve higher yields than solvent recovery, making it economically feasible if the purification method used barely impacts the yield. For biomedical applications, it is crucial to meet high purity (i.e., > 99.9%) and endotoxin standards (i.e., < 0.5 or < 0.06 endotoxin unit/mL) [110] because PHA is mainly derived from Gram-negative microorganisms, which may contain lipopolysaccharides and other complexes that can cause a toxic reaction in the human body, necessitating through purification process [109]. PHB is a potential biopharmaceutical owing to its degradation product 3HB with anti-inflammatory, anti-cancer, and antioxidant properties, highlighting the importance of achieving high purity and low endotoxin levels. Nevertheless, the challenge is that more separation and purification steps increase purity but decrease yield. Furthermore, additional purification processes are expensive, increasing the total downstream process cost and resulting in higher product prices. Currently, the mechanism of PHA synthesis is incompletely understood, and there is no clear solution to reduce the manufacturing process costs. Nevertheless, continued study into process development with local and governmental efforts to regulate the use of petroleum-derived plastics and encourage biodegradable plastics is essential to meet the growing demand for these materials.
5 Conclusions
PHA is emerging as a viable alternative to petroleum-derived plastics because of its biodegradability under natural conditions and sustainable microbial production. PHA is suitable for various applications, including packaging, agriculture, and coatings. In addition, it is being explored for use in high-value products, such as biomedical and biofuel applications, owing to its distinctive properties. The aforementioned high-value products can be manufactured using an appropriate downstream process, including recovery, separation, and purification steps aimed at achieving high purity. This paper showed that the solvent recovery, used primarily in conjunction with precipitation techniques, is effective in achieving a high degree of purity and yield. Combining digestion and mechanical recovery methods with centrifugation resulted in substantial efficiency. In addition, mechanical recovery methods, such as HPH and bead milling, combined with centrifugation, are highlighted as scalable and environmentally friendly alternatives that offer reduced processing times and lower environmental impact. The highest yields and purities achieved by each method were as follows: 99.0% and 100.0% for solvent recovery with precipitation, 96.9% and 96.6% for digestion recovery with centrifugation, and 98.0% and 95.0% for mechanical recovery with centrifugation. These findings highlight the significant impact of recovery and separation methods on the final product quality.
The economic feasibility of PHA is contingent upon the methods used at each phase of the manufacturing process. The LCA evaluation indicates solvent recovery is more economically efficient for maintaining high purity. On the other hand, for biomedical applications, the solvent and digestion methods require additional purification to achieve the requisite purity. However, environmentally friendly mechanical recovery methods, such as SC-fluid recovery, show considerable potential as a viable and sustainable alternative. The exploration of hybrid methods that integrate biochemical and mechanical recovery methods could play a key role in refining and improving processes, underscoring the need to develop downstream processes that efficiently and economically produce valuable PHA. Future research should focus on developing strategies that facilitate the production of high-purity PHA with minimal downstream processing steps. This includes the exploration of renewable green solvents such as 1,3-dioxolane, dimethyl carbonate and butyl acetate [111–113], the optimization of recovery parameters for various PHA types, and the development of robust pretreatment methods to improve process efficiency. Such approaches are essential to ensure the sustainability and economic feasibility of biodegradable plastics. The optimization of recovery, separation, and purification methods has the potential to enhance the economic and environmental viability of PHA production, facilitating its use as a replacement for conventional plastics.
Supplementary Information
Notes
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (2022R1A4A3029607).
Conflict-of-Interest Statement
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Author Contributions
E.K. (Master’s student) conducted conceptualization, investigation, and data curation of the original draft, and wrote and edited the manuscript. H.R. (Master’s student) reviewed and edited the manuscript. J.W. (Professor) supervised the methodology of the original draft and revised the manuscript. Y.B. (Professor) supervised the conceptualization and methodology of the original draft and revised the manuscript.