Subcritical water is an innovative way of using a single green solvent for the valorisation of biomass.
The World population has been increasing dramatically. As a result, there is a demand for more, and more resources. The three mains resources that are in critical need are: food, water and energy. To ensure that future generations will have access to these resources, which are essential to human life, new paradigms must be found.
Until recently, society followed a linear economy, where raw materials were transformed into products that are then disposed of after their lifetime. This behaviour, which is prevalent in developed countries, has led to a fast depletion of resources and at the same time, to the production of a huge amount of waste. The environmental load of a linear economy is unsustainable. The depletion of raw materials, the environmental load of production processes, the storage of waste or its conversion into energy by combustion processes, have created both direct and indirect environmental problems that compromise our current way of living.
Circular economy appeared as an answer to this problem. In a circular economy approach, all by-products, wastes and even end-of-life products, are viewed as a resource for other applications, either as raw materials, or intermediates that can be incorporated in the same production process where they were generated. To implement a truly sustainable circular economy, it is first necessary to assess the remaining value of by-products and wastes, and to decide how these should be processed in order to realise their value.
In 2016, over 20 million tons of waste were generated in the EU by agriculture, forestry and fishing sectors (source: Eurostat). The waste generated by these sectors have an important economic potential, due to the value that can be created from its recovery and transformation. Agriculture and forestry wastes are mainly lignocellulosic materials, meaning that their main constituents are cellulose, hemicellulose, and lignin. Cellulose is the dominant structural polysaccharide and is a regular, linear homopolymer, made of D-glucose monomers linked by β-(1,4) glycosidic bonds. Whereas, hemicellulose is a polysaccharide made of various sugar monomer units (such as xylose, galactose, mannose, arabinose), which makes it non-crystalline. The function of hemicellulose is binding cellulose and lignin. Lignin is a random, three-dimensional phenyl-propanoic polyphenol. Lignin found in nature is made of three monomers: coniferyl alcohol, sinapyl alcohol and p-coumaryl alcohol, arranged in an irregular structure that provides strength and resistance.
Following a biorefinery concept, these three main compounds can be separated and depolymerised by hydrolysis into their building blocks, in order to generate a diversity of products. While lignin can be processed to generate, for example, natural adhesives and polymeric additives. Cellulose and hemicellulose can be processed into the so-called C5/C6 platform to produce biofuels, chemicals and polymers. Moreover, depending on the source of waste, there can be some potential in extractives, such as polyphenols with antioxidant activity. The challenge remains in finding a truly viable and sustainable process that can selectively extract and hydrolyse these compounds, while avoiding chemical degradation. The most common methods for the hydrolysis of lignocellulosic waste are chemical (which are effective but with a high environmental load), and biological/enzymatic which are environmentally accepted and highly selective but require long reaction times and strict reaction parameter control.
Subcritical water as an alternative
An alternative is subcritical water (SCW), also called hot compressed water (HCW). These designations are used for liquid water at temperatures above its normal point of 100°C, and below its critical temperature of 374°C. To keep water in the liquid state at such temperatures, pressure must be applied, therefore avoiding water vaporisation. SCW is a highly promising, energy-efficient, and environmentally benign solvent for the extraction of many compounds (such as polyphenols) as well as for hydrolysis reactions. Water is nonflammable, nontoxic, readily available, safe, and a truly environmentally friendly solvent.
The properties of SCW are very sensitive to changes in temperature. The dielectric constant of SCW decreases with increasing temperature, due to hydrogen bond dissociation. Thus, the solubility of ionic molecules decreases, whilst the solubility of hydrophobic species increases. This enables the extraction of compounds that are normally extracted with organic solvents such as ethanol or methanol. In addition to this, ionic strength is another property of water that changes. As temperature increases, the ionic product of water, KW, increases by three orders of magnitude from KW = 10-14 at 25 °C, to KW = 10-11 at 300°C. This means that the concentration of H+ and OH- ions is higher than in water at room conditions, making water a more reactive medium. As a result of this, water then behaves as reagent, catalyst, and solvent, in so-called auto-hydrolysis processes.
Because the properties of SCW can be tuned with temperature, by using only water, it is possible to selectively extract components of interest from many agroindustrial waste feedstocks. In the case of hemicellulose and cellulose, and depending on the temperatures applied, SCW can hydrolyse the large, insoluble polysaccharides to smaller and soluble sugars, namely oligosaccharides and monosaccharides. The soluble oligosaccharides can then be enzymatically converted to the respective monosaccharides more easily and then, can be used as building blocks in the C5/C6 platform.
Hemicellulose, a heteropolysaccharide, is depolymerised more easily than cellulose, a homopolysaccharide made up of glucose units. Lignin is a biopolymer composed by phenolic alcohols, and can be obtained in different ways, depending on the type of residue. Lignin can be extracted via an organosolv method using, for example, ethanol before applying SCW. It can also be separated from cellulose after removal of hemicellulose, when using enzymes to convert cellulose to glucose units. On the other hand, more resilient lignin can be obtained in a highly pure state at the end of the SCW process. Extractives, such as polyphenols, soluble sugars and some protein can be extracted at lower temperatures where SCW is still not capable of hydrolysing biopolymers. Therefore, a stepwise process can be devised in order to fractionate the main components of agroindustrial waste (Fig.1).
Thus, at temperatures below 140°C, it is possible to extract compounds such as soluble sugars, polyphenols and soluble proteins that are available in the matrix for direct extraction without the need for disrupting the structure of the feed material. Due to its amorphous nature, hemicellulose requires less energy to be hydrolysed into water soluble sugars. This hydrolysis/extraction process can be performed at temperatures up to 200°C. Because of its crystallinity, cellulose requires more energy for hydrolysis into soluble glucose oligomers and glucose. This normally only happens at temperatures above 250°C.
An efficient semi-continuous process was devised (Fig. 2) so that a stepwise temperature program is applied, and different fractions are recovered continuously. In such a process, the feed material (agroindustrial waste) is loaded into a reactor, and pressurised liquid water is pumped through the reactor as it starts to be heated. The outlet stream is depressurised, cooled, and then recovered. A first fraction is recovered as the reactor is heated from room temperature until about 140°C (polyphenols and other extractives) and a second one between 140° and 200°C (hemicellulose). Heating is resumed up to 250°C (or higher, depending on the feed material) to solubilise cellulose.
In the Biocatalysis & Bioenergy group headed by Prof Susana Barreiros, Prof Pedro Simões, and Dr Alexandre Paiva, the valorisation of different agroindustrial waste feedstocks (e.g. grape pomace, spent coffee grounds, olive pomace) at lab and semi-pilot scale has been studied. Also, the extracts obtained were successfully applied in cosmetic products, as well as alternative carbon source for the microbial production of carotenoids and biosurfactants.
LAQV@REQUIMTE – Faculty of Sciences and Technology/NOVA