A New waste-free integrated approach for sapropel processing using supercritical fluid extraction

Highlights

• Possibilities and strategies of rationality use of sapropels are shown.

• Extraction of bioactive substances from sapropel by supercritical CO₂ is suggested.

• Treatment by supercritical CO₂ largely improves further sapropels conversion.

• Solid residues after CO₂ extraction are used to obtain carbon-mineral materials.

Abstract

An integrated approach to the processing of sapropel with the use of supercritical CO₂ extraction of bioactive substances with carbon dioxide at the first step of the process is discussed. The dependences of the yield and composition of extracts (8−35 MPa, 50 °C) and features of sapropel are established. Further carbonization of the solid residue remaining after extraction results in organic-rich products (phenol and its mono, dialkyl and methoxy derivatives, hydrocarbons C₁₄-C₂₂) and carbon-mineral materials with a total pore volume of 0.681–1.211 cm³ g⁻¹. It was found that the introduction of the initial stage of treatment with supercritical CO₂ allows not only extracting valuable amino acids, but also affects the yield, composition and properties of liquid and solid sapropel transformation products at subsequent stages of heat treatment.

Introduction

Harmonious exploitation of natural resources is based on the principle of comprehensive processing of natural raw materials, which consists in successive non-waste production of various valuable products with the minimum anthropogenic impact on the environment. Sapropel is among the accessible types of renewable natural organic raw materials that require the application of “green” processing technologies. Sapropel is a centuries-old bottom deposit that was formed from dead aquatic vegetation and debris of living organisms, plankton and humus particles under fresh and sea water, with an organic matter (OM) content of 15–90 % [[1], [2], [3]].

Detailed studies of marine sapropel deposits were carried out in the water areas of Mediterranean [[4], [5], [6], [7]], White [8], Baltic [9,10], Caspian [11] and Black [12,13] seas to elucidate the role of various environmental factors in the emergence and accumulation of sapropels as a source of oil formation. Mining and processing of marine sapropel are technologically difficult: the deposits should be mined from a depth of about 2 km; this is followed by labor-consuming steps of hydrogen sulfide removal and desalination. This is why marine sapropels are used mostly in balneology (as applications).

Freshwater sapropels of lakes and rivers are formed due to vital activity of bacteria in the absence of oxygen or at its low content, which allow suppressing the oxidation and decomposition of valuable organo-mineral components. Intensive formation of sapropels occurs in lakes in the middle zone of Europe and Asia [14]. In the American continent, sapropelic deposits are concentrated in Canada and the USA [15,16] and are confined to the area of the Great Lakes. In Western Europe, sapropel resources are severely depleted. The reservoirs containing sapropels were in Germany, Poland, Scandinavian countries and to a lesser extent in France and Great Britain. A large number of sapropel deposits are in Lithuania, Latvia, Ukraine and Republic of Belarus [17,18]. In the Russian Federation, which takes one of the leading places in the world (225 billion m³), the main deposits are located in the central part of Russia and also in the territory of Western Siberia and Far East [[19], [20], [21]].

The use of freshwater bottom deposits as fertilizers has long been known; however, owing to compositional diversity and production simplicity of lake sapropels, they can be considered as a valuable organo-mineral raw material for various applications [[22], [23], [24]]. Noteworthy is the fact that the recovery of sapropel deposits does not deteriorate the ecology of water reservoirs; on the contrary, it facilitates purification of water and improves its quality.

Studies of freshwater sapropels [25,26] are largely aimed to reveal biogeochemical features of their formation and elucidate the composition and structure of OM macromolecules. Only several works deal with the chemical processing of sapropels for obtaining valuable organic products and condensed carbon materials [27,28].

Studies on thermal processing (carbonization) showed that thermal destruction of lake sapropels at temperatures up to 600 °C results in the formation of 12–50 % liquid products [29,30]. They comprise oxygen-containing (acetone, acetic acid, phenol, cresols, xylenols) and nitrogen organic (pyridines, pyrazines, aminopyridines, pyrroles, urea) compounds [30], which provide high antioxidant activity. This is why liquid products of carbonization are of interest to medicine, balneology, cosmetology and veterinary medicine [31]. Solid carbon-mineral products of carbonization, the content of which varies in the range of 26.0–69.0 wt%, have a developed pore structure (especially after their treatment with water vapor) with the total pore volume 0.7–1.4 сm³ g⁻¹ and pore radius 200–1000 nm [30,32].

Other authors [33] demonstrated that carbonization of sapropel leads to porous materials with specific surface area up to 618 m² g⁻¹ and sorption activity with respect to iodine and methylene blue 58.9 and 189.1 mg g⁻¹, respectively. Such materials are promising as organo-mineral sorbents in wastewater treatment to remove heavy metal ions, and in air purification to remove low-molecular hydrocarbon vapor. It was shown that carbon-mineral materials obtained by carbonization can be used as the supports of hydrocracking catalysts [34] and heterogeneous biocatalysts for sucrose inversion [35,36].

It is known that the specific microflora that appears in freshwater sapropels enriches them not only with calcium, iron and phosphorus, but also with biologically active substances (BAS), particularly, carotins, chlorophyll, xanthophylls, sterols, organic acids, alcohols, enzyme-like substances, amino acids, and group B vitamins (B1, B2, B3, B6, B12), C, E [1,3,37]. BAS are thermally unstable and dissociate upon carbonization. So, the modern processing scheme of this natural raw material should include a step of their preliminary extraction.

There are some studies devoted to the extraction recovery of BAS from sapropels using liquid solvents (water, diluted mineral acids, alcohols, mixtures of alcohol and benzene, acetone, esters, gasoline) [[38], [39], [40]]. To cleave the bonds in OM macromolecule and increase the BAS yield, extraction is carried out at elevated temperatures and/or with the preliminary use of physicochemical techniques like microwave, autoclaving, osmotic shock, bead-beating, homogenization, freeze-drying, French press, grinding and sonication [[40], [41], [42]]. For example, the authors have managed to increase the extraction degree of lipids in water phase from Botryococcus sp. microalgae from 7.7 to 28.6 % (g lipid/g dry weight) using a 5 min microwave pretreatment [43].

However, there is a more efficient and sparing method for the recovery of natural BAS, which does not change their structure, – supercritical fluid extraction [44,45]. This method supports the “green” chemistry approaches and makes it possible to avoid organic solvents in the extraction technologies and separation of products. In supercritical fluid border between liquid and gaseous phases disappears. The physical properties of supercritical fluids are between the values for a liquid and gas-density similar to liquids, diffusivity and viscosity similar to gases. Since gas-liquid equilibrium does not exist in a supercritical state, the surface tension of supercritical fluids is zero. This property enables supercritical fluids to easily penetrate a solid matrix and extraction of components. In this way, a high mass transfer rate is achieved under supercritical conditions. So the use of supercritical fluids becomes economically and ecologically reasonable. For mild extraction of biologically active components from natural raw materials, the operating temperatures should be within the range from 20 to 100 °C, which allows applying the extraction by supercritical carbon dioxide (SC-CO₂) [[46], [47], [48]]. Low temperature SC-CO₂ (31.1 °C) is attractive for the extraction of heat sensitive compounds. In addition it is an inert, non-ammable, non-explosive, inexpensive, odourless, colourless, clean solvent that leaves no solvent residue in the product, it is also non-toxic and is generally accepted as a harmless. There are works on the recovery of BAS and valuable organic products by CO₂ supercritical extraction from raw materials of plant [[49], [50], [51]] and animal origin [51], algae [[52], [53], [54]], peats [55] and coals [56]. However, the literature and patent sources provide virtually no information on processing of the organic matter of lake sapropels by extraction using pure SC-CO₂. Recently, we published the results of a study on the use of a mixture of CO₂ and C₂H₅OH in supercritical conditions for the extraction of bioactive compounds from organomineral raw natural material sapropel [57]. It was shown that the introduction of a co-solvent C₂H₅OH allowed achieving a higher yield of the SC-substrate at lower pressures, with the increase in the proportion of the hydrophilic phase in the SCE, while the yield of amino acids in the extracts obtained in the presence of C₂H₅OH did not exceed 1395 mg kg⁻¹.

In this paper, we present more extensive research. We propose a new scheme of complex and virtually wasteless processing of sapropel to obtain the maximum amount of useful products; the first step of this scheme is the recovery of BAS using supercritical carbon dioxide (SC-CO₂) (Fig. 1).

It was necessary to estimate the effect of extraction conditions on the yield and composition of the extract. In the second step, the solid residue of sapropel after extraction of BAS was subjected to thermal pretreatment (carbonization), where the target products were the liquid products containing valuable organic compounds and carbon-mineral solid residue. The study was aimed to reveal the effect of the first step – treatment of the primary sapropels with supercritical carbon dioxide – on the yield and composition of liquid carbonization products and properties of the carbon-mineral residue.