From green chemistry and nature-like technologies towards ecoadaptive chemistry and technology

Introduction

The amazingly quick and almost unanimous adoption of the Green Chemistry principles, which were developed by Anastas and Warner [1] at the end of the 20^th century, both by the scientific and industrial chemistry communities, is an unambiguous indicator of the societal preparedness to the change towards the greener and more sustainable world. The need for that was the bigger, the worse the image of Chemistry and chemists was getting inducing public anxiety or fear of Chemistry [2]. Such a perception was, on one side, the result of growing awareness of the general public with regard to the real danger associated with the chemical production, but on the other side, it was heated up by non fair advertising and the “bad news first” attitude of the media leading to development of chemophobia in the society, which was defined by IUPAC as “irrational fear of chemicals” [3]. The more timely was the philosophy of environmentally benign, or green, chemistry [4] offering bright alternatives both for the society and professional chemists. As a logical development of green chemistry philosophy, the biomimetic, or nature-inspired technologies came recently to stage aimed at mimicking and reproducing chemical tools and materials of nature at the molecular level [5]. This is a substantial step from the industrial chemistry oriented principles of green chemistry. Still, for harmonization of chemistry and nature, not only nature-mimicking, but nature-adaptive approaches are needed which can be developed following the principles of “ecoadaptive” chemistry and technology.

Ecoadaptive chemistry can be defined as a field of science that uses the methods and results of chemistry to understand molecular mechanisms of self-organization which underlay sustainability of resilient and adaptive biogeochemical systems, and to reproduce them by designing nature-like systems, materials, and technological processes. This notion relates and adopts the principles of adaptive chemistry formulated by Jean-Marie Lehn – Nobel Laureate in Chemistry (1982) in his visionary work “Perspectives in Chemistry: From Supramolecular Chemistry to Constitutional Dynamic Chemistry towards Adaptive Chemistry” [6]. In this perspective he writes: “The merging of the features: – information and programmability, – dynamics and reversibility, – constitution and structural diversity, points towards the emergence of adaptive and evolutive chemistry” [6]. By saying this, Lehn stresses the major role of Chemistry in the science of complex matter.

In this work, we demonstrate how the merging of structural diversity and programmability gives rise to ecoadaptive design and self-organizing complex matter on the example of products and technologies based on a use of natural complex systems – humic substances (HS). HS are the supramixtures, or supramolecular assemblies, of the products of abiotic and biotic transformations of biomacromolecules comprising the remains of plants and other living organisms [7], [8]. HS penetrate throughout the entire environment and represent from 50 to 90% of organic matter in soil and water ecosystems and play multiple life-sustaining functions on Earth [7], [9]. To name a few, HS regulate transport and availability of biogenic elements to plants, immobilize and mitigate toxicity of hazardous elements in the contaminated ecosystems, protect plants from non-specific abiotic stresses, play a key role for soil fertility determining water-retention and structure of soils [9]. The in-depth understanding of the chemical mechanisms which underlay these vital functions of HS can lay the grounds for ecoadaptive design of the nature-inspired humic products capable of self-assembly aimed at their use for soil and water clean up, carbon sequestration, soil fertility restoration, plant nutrition. In this work, we present these ecoadaptive approaches on the example of manufacturing humic derivatives and supramolecular complexes with acquired new properties.

The unique chemical feature of HS is the extreme structural heterogeneity facilitating their resistance to biodegradation [7]. The complexity of HS composition can be clearly seen in Fig. 1, which shows the typical mass spectrum of HS measured with a use of high resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) [10], [11]. FTICR MS identifies dozens of thousands molecular formulae in the typical sample of HS [12].

Fig. 1:

The schematic representation of supramolecular ensemble of HS composed of oxidized biomacromolecular precursors highlighted in different colors based on the original Kleinhempel formula (a); the typical FTICR mass spectrum of HS (b); chemical space of coal and peat HS projected onto Van Krevelen diagram (c and d, respectively), and Van Krevelen diagram binned into 20 cells using cell-based partitioning approach, which are assigned to seven major classes of the biomolecular precursors of HS – condensed tannins, lignins (phenylisopropanoids), terpenoids, other lipids, proteins, carbohydrates, and hydrolyzed tannins. The respective molecular compositions were extracted from the CACTUS database for CHO molecules with molecular masses from 200 to 600 Da (e). Credit to A. Zherebker for graphical work on Van Krevelen diagrams of coal and peat HS.

The projection of assigned molecular compositions onto the 2D van Krevelen diagram (relationship of the H/C versus O/C ratios of the identified molecular formulae) enables visualization of immense chemical diversity of HS [13]. Chemical space of HS occupies a very substantial portion of the combinatorial space of all feasible CHO molecules [12]. Figure 1c,d show examples of chemical space of HS from coal and peat, respectively [14], [15]. The mapping of Van Krevelen diagram with respect to the compound classes which occupy its space is frequently used for comparison of different samples [16], [17], [18]. The specific feature of mapping proposed in this work (Fig. 1e) is application of cell-based partitioning approach for binning the entire space of Van Krevelen diagram into 20 cells. The latter are assigned to molecular compositions of different compound classes extracted from the CACTUS database as it is shown in Fig. 1e. In this case, occupation density (D_i) of each cell by experimental points can be calculated as D_i=N_i/N, where D_i is the population density of the i^th cell (i=1–20); N_i is the number of data points (molecular formulas) within the ith cell, N is the total number of data points plotted in Van Krevelen diagram. It can be also calculated as an intensity-weighted density. The calculated densities can be used as descriptors of the molecular space which is occupied by HS samples or by any other complex mixture. These descriptors can be used for generation of QSAR models for HS, for classification analysis, and for comparison of different HS samples. So, comparison of the synthetic VK diagram (Fig. 1e) with the experimental ones of coal and peat HS (Fig. 1c,d) show that coal is characterized with much higher population density of cells assigned to lipids, lignins, and condensed tannins occupying cells 1–5, 6–9, 11–12, 16–17, whereas peat HS have much higher contributions of cellulose (carbohydrates), peptides, and hydrolized tannins (cells 1, 2, 7–10, 12–14, 17–20). These compositional differences are indicative of essentially different properties of coal and peat HS: coal HS are more hydrophobic as compared to more hydrophilic peat HS. It is of particular importance because coal and peat are major industrial resources of HS.

The largest resource of HS is lignite with the HS content from 50 to 80%. Its richest (the most oxidized) variety – leonardite contains up to 95% of HS [19]. This is why a large-scale production of humics-based chemicals and materials from lignite could provide a long sought alternative for coal industry: from burning coal for power – to processing it for production of green humics-based chemicals. The largest renewable resource of HS is peat whose reserves are estimated on the level of 120 bln tonns [20]. The content of HS in peat is much less as compared to lignite, and varies from 20 to 40%. A use of peat for chemicals production might be arguable, while the swamp ecosystems possess the highest biodiversity and are protected by some countries as natural refugees [21]. However, the countries where peat occupies large territory (e.g. Finland, Canada, USA, Russia) use it intensively for different industrial applications [22]. The major difference in coal and peat is humification degree of the biomass remains, which are much more processed – coalified – in lignite as compared to peat. This is reflected in the more hydrophobic properties of coal HS as compared to the peat ones as it was suggested above.

On the contrary to mineral oil, both coal and peat are solid minerals, so rectification is not applicable for their processing. At the moment the regular practice is to extract humic components both from coal and peat with a use of the alkali extraction [23]. This practice is based on the operational definition of HS as “alkali extractable organic matter” from soil and fossil organic rocks [8], [24]. The obtained alkaline extract is called “humate” (from “humic substances”) or potassium or sodium humate if the producer would like to specify the alkaline metal used for extraction. The production scheme of humates is shown in Fig. 2.

Fig. 2:

Production of humates (upper row), market niches and market size estimates (bottom row). For the humate market estimates see a link: https://www.gminsights.com/industry-analysis/humic-acid-market.

It is this “all-humic” extract, which can be further fractionated into humic and fulvic acid fractions according to the most adopted fractionation scheme of HS [8]. It is based on different solubility of HS components in alkaline and acidic media and implies acidification of the whole alkaline humic extract until pH 1–2. The acid-insoluble fraction, which precipitates at this pH, is called humic acid (HA), and the fraction, which remains in solution, is called fulvic acid (FA) [8]. The common industrial approach is to use the entire alkali extract from the raw material – coal or peat – for further drying. Separation of humic and fulvic acids usually proceeds only in the lab, while for the basic industry it remains prohibitively expensive. However, along with the merge of fulvates as the new industrial products (Fig. 2), the producers started opposing them to humates by positioning the humates as the salts of only humic acids, which is far from the reality given the production scheme of the industrial humates. Given high expense of the fulvic acid production, the rapidly growing market of fulvic acids raises a question with regard to how fulvic they are, in a sense, whether they conform to the classical definition of fulvic acids as an acid-soluble fraction of humic substances extracted with the alkali from soil and organic rocks. In the reality, the producers take the definition “acid soluble”, but forget about “fraction of the humic materials extracted from the soil or organic rocks”, and sell acid-soluble lignosulfonates under the name of fulvic acids. Even the large-scale production of the “real” fulvic acid by Vitens (the Netherlands) [25], which is a byproduct of drinking water treatment, is only a partial truth. Strictly speaking, again, this product is an anion-exchanging resin isolate of the natural organic matter (NOM) present in the underground water passing through the peat layers. Again, this product is not separated from the humic acids, – it is just stated that it is a fulvic acid.

Hence, until today the major product of basic humic industry is the “humate” which is a non-fractionated continuum of humic compounds. There are two main strategies for increasing efficiency and application range of this product: deep fractionation and directed modification. Both of them imply drastic reduction in structural heterogeneity of HS, but the approaches and tools are very different. The former should be based on in-depth fractionation of HS similar to oil refinery. Its implication might boost in the near future the new humics-based chemistry similar to petroleum-based chemistry. However, the efficient and technologically sound fractionation approaches are still to be discovered. The less technologically demanding way might be to use humic materials as the polyfunctional matrix and incorporate into it the reactive centers which will provide for reproducibility of the needed properties. This would enable production of humics by design, or “designer-humics”. This approach was proposed by our working group in collaboration with Dr. Kirk Hatfield (University of Florida, USA) more than 10 years ago [26], – and its realization lead us finally to formulation of the notion of ecoadaptive chemistry.

Towards ecoadaptive design of humics-based products mimicking their functions in nature

The most attractive feature of HS as a source material for chemicals production is their excellent compatibility with the principles of green chemistry. First of all, they do not expose harmful effects to living organisms in the whole range of natural contents in the environment [27], [28]. Secondly, both humates and fulvates are soluble in water and surface active, which makes them applicable for water-based sol-gel technologies as well as for other water-based chemistries such as stabilization of nanoparticles, surface modification, immobilization, etc. [29], [30], [31], [32]. In addition, the polyelectrolytic character facilitates their chemical modification in water using interpolyelectrolyte assemblies [33]. Another important property is amphiphilic character of humic molecular ensemble [34] making them suitable as carriers for hydrophobic components (e.g. drugs), etc. It is constellation of these properties that provides for their multiple life-sustaining functions in the environment, – and few of them we tried to reproduce in the lab. The first one refers to accumulative function of HS provided by their capability to form geochemical barriers which can accumulate the elements and retain organic compounds, the second one refers to maintaining soil structure and supporting its water retention capacity – this is one of the most important function of HS; the third one refers to their “depot and supplies” function – they store the microelements in the bioavailable form and supply the plants with micronutrients. As follows from these functions, the HS based products can be used for developing nature-inspired “green” agricultural and remediation technologies.

Design of self-adhesive humics-silsesquioxane systems capable of forming organomineral networks on solid support

In nature, HS are capable of forming geochemical barriers which function like scavengers by removing dissolved components from the mobile aqueous phase [35]. This occurs due to formation of complexes (with metals) or surface adducts (in case of organic compounds) with the HS coatings immobilized onto the solid phase [35], [36]. Of particular importance is that binding to HS not only retards migration, but reduces toxicity of metals (of heavy metals, in particular) and bioaccumulation of hydrophobic contaminants as well [37], [38]. Hence, the humic coatings can be used both for removal and detoxification of waterborne contaminants. The most straightforward and needed technological implication of this natural process can be in situ clean up of groundwater via installation of humic permeable reactive barriers (“HPRB”) within the contaminated aquifer. The traditional “PRB” technology implies a placement of a deep trench perpendicular to the movement of groundwater and replacement of the contaminated aquifer material with the permeable reactive media capable of interception and retention of the contaminants [39], [40]. Along the efficiency, the major drawback of this technology is a necessity to treat the excavated contaminated ground which makes it prohibitively expensive. A viable alternative to this approach might be in situ installation of injectable reactive barriers using liquid injection materials capable of phase switch upon contact with the solid support – with the aquifer ground [41].

For designing an injection material with the desired properties, we have used the humic material as a non-toxic, environmentally compatible matrix, and modified it by introduction of surface active silanol groups via reaction with aminoorganosilanes as it is described in [42], [43] and shown in Fig. 3. This provided for high adhesive affinity of HS for mineral surfaces due to formation of covalent Si–O–Si bonds. The self-assembling of the designed 3-aminopropyltriethoxysilane (APTES) – HS system in aqueous phase was studied using small angle X-ray scattering and transmission electron microscopy [43]. The key parameters which were capable of switching this system from soluble to immobilized state were a ratio of APTES:HS, concentration, presence of Ca ions, and pH. The enhanced values of the first three parameters facilitated condensation of silanetriols followed by formation of silsesquioxane network. Further cross-linking of silsesquioxane-HS network resulted in growth of fractal dimension of the particles until they switched from mass to surface fractal type (Fig. 3).

Fig. 3:

Formation of HS-APTES polyelectrolyte complexes followed up by HS-silsesquioxane particles growth shown as a reaction scheme (two upper rows) and the images of transmission electron spectroscopy (TEM). Credit to A. Volikov for graphical representation.

The follow up column experiments have shown that this switch was responsible for functional characteristics of APTES-HS system with regard to its capabilities of forming permeable network onto the water-saturated solid support. At the same time, pH dependence had an extreme shape with minimum switch rate at pH 6 [43]. The proof of the concept studies were conducted using the column tests on in situ installation of injectable humic PRBs (“HPRBs”). They confirmed the relevance of the discovered molecular switchers which enabled successful in situ installation of “HPRBs” [44]. The performance of these model “HPRBs” was assessed with respect to a model contaminant – the diazodye Direct Red 81. The dye interception by the model “HPRB” is shown in Fig. 4. The best results were achieved with APTES:HS ratio of 2:1 which provided for positive charge of the barrier and facilitated retention of negatively charged dye molecules (Fig. 4).

Fig. 4:

Installation of humic permeable reactive barrier (HPRB) in the sand-packed column (a) using injection of 2:1 APTES: HS composition at a concentration of 5 g/L and pH 8 at the flow rate of 1.3 mL min⁻¹ (b) followed by removal of non-absorbed humic material with distilled water (c) and the use of HPRB for interception of azodye Direct Red 81 (d–f) (concentration of the Direct Red81 was 25 mg L⁻¹, the absorbed amount was 14.92 mg). The details can be found in [44].

The obtained results were instructive for refining the conceptual design of the in situ installation of “HPRB” using a fence raw of boreholes as it was proposed by the team of Kirk Hatfield from the University of Florida, USA [38], [39]. The above studies on the APTES-HS system dynamics showed a way for passive injection of APTES-HS system via the prior adjustment of the governing parameters: APTES:HS ratio, concentration, Ca content and pH, to the environmental conditions of the contaminated aquifer (Fig. 5). To advance these studies to the pilot scale and, in perspective, to validation studies in the field, the deployment strategy for installing injectable “HPRB” is to be developed for different plume architectures and aquifer properties. To demonstrate the developed technology, dynamic microcosm experiments can be conducted with intact cores of contaminated subsurface material gathered from a representative field site. This will lay grounds for the future deployments of the developed nature-like technology in the field. The installed barriers can be used for groundwater clean up via sorption of heavy metals and other contaminants on humic adlayers, which can be further removed by washing with weak alkaline solutions and pumping out the obtained washings. In case of landfills, the HPRBs can be also used for immobilization of microorganisms which readily populate humic coatings providing them solid support and protection from the harsh toxic environment. There are numerous data which report substantial reduction in toxicity of metals and other toxic compounds in the presence of HS [45], [46]. Given their redox shuttling properties they can be also used for manipulating metabolism of metanogenic consortia, e.g. for suppressing methane emission.

Fig. 5:

The conceptual diagram of in situ passive installation of HPRB using injection of HS-APTES composition via a fence row of boreholes. Credit to A. Volikov for graphical work.

Of importance is that the same dynamic APTES-HS systems can be used as soil conditioners for restoration of degraded soils. This is because in nature, HS enhance formation of soil aggregates serving a dual role – as a soil glue via binding mineral particles, and as a surface modifier, which increases hydrophobicity of mineral surfaces. Given the main function of soil aggregate as a water container, we have suggested that soils can be considered as inverse-phased Pickering emulsion where water (dispersed phase) is stabilized against air (dispersing medium) with solid particles (clays) carrying hydrophobic modifiers (humic and other organic materials) [47]. Such a concept allows for setting the distinct demands to the soil conditioners: they should be capable of restoring domination of hydrophobic over hydrophylic domains in the soil particles, while due to hydrophobic character of dispersing medium (air) the emulsion stabilizer should be also hydrophobic – otherwise emulsion will be destroyed. And this is exactly the case when the virgin soil is ploughed, the soil particles are exposed to air which oxidize soil organic matter shifting the balance towards overall hydrophilic surfaces. The particles with hydrophilic surfaces cannot stabilize emulsions of water in air [48]. This concept is also in sync with the reported data on the best performance of hydrophobic soil conditioners [49], [50]. For proving it, we have used the developed APTES-HS systems as soil conditioners as shown in Fig. 6.

Fig. 6:

Evaluation of ameliorating capacity of silsesquioxane-HS complexes with regard to soil structure followed by determination of water stability of soil aggregates in the soil amended with silsesquioxane-HS complexes.

Our experiments have shown that application of APTES-HS systems to degraded soils brought about an increase in the amount of water-stable aggregates as well in the content of HS in the treated soil layer [47]. This gives good perspectives for directed design of nature-inspired HS-based soil-conditioners.

Ecoadaptive design of iron fertilizers based on HS-stabilized iron (hydr)oxides

Another type of humics-based products is iron-containing humics-stabilized nanoparticles [51], [52], [53], which can be used as a source for plants nutrition instead of synthetic iron chelates. The idea is based on the natural phenomenon that in soils, the waterborne sols of iron-containing NPs are formed due to complexing with HS, which can bind large amounts of poorly ordered iron (hydr)oxides providing for stabilization of colloidal iron in the form of NPs [54]. Improved acquisition of iron by plants in the presence of HS has been numerously reported [55], [56], [57]. The hypothesis on the key role of HS in inhibiting crystallinity of iron (hydr)oxides in soils was formulated by Schwertmann back to 1960s of the 20^th century [58]. However, there have been no systematic studies so far published with respect to quantitative relationships between size and crystallinity of humics-stabilized iron-containing NPs and their availability to plants. A lack of these studies might be the source of inconsistency of the reported results on iron bioavailability from the so called iron humates [59], [60]. The results vary drastically depending both on the iron source used and on the kind of plants which were used for testing [61], [62]. This is why we paid particular attention to characterization of iron-containing NPs with regard to size and crystallinity in the HS-stabilized iron sources obtained from different iron precursors (Fig. 7).

Fig. 7:

Preparation scheme of Fe-HS products from different Fe(II) precursors.

Our first study implied a use of iron(II) sulfate as an iron source. We tried to stabilize oxidation degree of ferrous iron by acidification of initial solution of iron sulfate with H₂SO₄ or by addition of ascorbic acid (AA). Two commercial potassium humates were used for this synthesis (CHP and CHS), and the synthesis was run at three different pH. The resultant samples were assigned as Fe-CHP-01 (10, AA), Fe-CHS-02 (11,5), Fe-CHS-03 (9,5, AA), Fe-CHS-04 (9,5, AA), Fe-CHS-05 (10, H₂SO₄). The details of this study are reported by Sorkina et al. [52]. The thorough investigation of the obtained iron humates with a use of X-ray diffraction analysis (XRD), Moessbauer spectroscopy, transmission electron microscopy (TEM) and synchrotron X-ray absorption spectroscopy (XANES and EXAFS) revealed substantial heterogeneity of crystalline iron phases within the iron humates obtained (Fig. 8).

Fig. 8:

TEM images of five samples of the iron humates synthesized as described in [52] and Fourier-transformed EXAFS spectra of these samples and of reference standards selected (FeSal3 stands for a complex of Fe3+ with salicylic acid). Credit to T. Sorkina for graphical work.

Despite the undertaken precautions with regard to stabilization of ferrous ion, its content in the iron humates did not exceed 10%, the predominant oxidation degree of Fe was +3. The humate was not able to preserve ferrous iron from oxidation. Another major question was whether the ferrous iron exists in the humates as complexes (similar to chelates), or it is incorporated in the iron-containing NPs. The corresponding EXAFS studies showed a lack of chelated iron in the iron humates: they were composed of low-crystalline iron containing NPs stabilized within humic matrix. This was in agreement with the data of TEM shown in Fig. 8. Hence the synthesized preparations of iron humates could be considered as heterogeneous mixtures of poorly crystallized iron (hydr)oxide NPs stabilized by HS. Bioavailability of these iron humates was tested using plants with different iron acquisition strategy: wheat and cucumber. Wheat plant is a monocot and it acquires iron via release of complexing agents – siderophores which are taken up by the plant [63]. The cucumber acidifies the rizoshere improving iron (3+) dissolution, it reduces the released ferric ions to ferrous ones and take them up [64]. The vegetation experiments were conducted in the modified Hogland medium amended by the equal amount of Fe (25 μM) in the form of iron humates or iron chelate – FeEDDHA as described by Sorkina et al. [52]. The obtained results are summarized in Fig. 9.

Fig. 9:

The bioassay results of the five iron humates obtained under conditions described in [52] using plants with different iron acquisition strategies: strategy I – cucumber (Cucumis sativus L. cv. “Dalnevostochny”) (a), and strategy II – wheat plant (Triticum aestivum) (b). Credit to T. Sorkina for graphical work.

The presented data show, first of all, that the plant with strategy I used in this study (cucumber) was much more successful in acquiring iron from the iron humates as compared to the strategy II plant (wheat): the former could accumulate three to four times iron in the leaves as compared to control, whereas accumulation factor for the latter did not exceed 1.5. The uptake of iron chelate in both experiments was larger as compared to iron humate, in particular, for the cucumber: the accumulation factor for FeEDDHA was 7.5. The corresponding value for a wheat plant was 1.7. Hence the plants of both strategies could acquire iron from the iron humates, however the efficiency of acquisition varied drastically: the dicot (cucumber) accumulated much more iron as compared to the monocot (wheat). Of interest is that photosynthetic activity of plants (the content of chlorophyll) did not increase proportionally to the iron content in the leaves. It was the highest in case of Fe-EDDHA, whereas for iron humates it did not differ from the parent humate (cucumber) or was substantially higher (in case of wheat). This might put a question of how functional is the iron which was taken up from the iron-containing NPs versus chelated iron.

Our further studies were directed towards much better control of size and crystallinity of the iron-containing NPs stabilized by HS in the pursuit of defining the role of these parameters in bioavailability of iron to plants. For this purpose, we conducted HS-assisted synthesis of nanosized feroxyhite as described by Polyakov et al. [51] and synthesized Fe complexes with HS as described by Garcia-Mina et al. [65]. This was to obtain HS-stabilized iron sources differing greatly in size and crystallinity, which was supported by physical-chemical characterization of the obtained samples described in detail in [53]. We have shown that the synthesized feroxyhyte NPs were crystalline and had the sizes between 20 and 30 nm, whereas the second preparation was identified as NPs of ferric polymers with very small sizes (<11 nm) and very poor crystallinity. Bioavailability studies on these samples were conducted only using wheat plants. They have shown that the large and crystalline feroxyhyte NPs were sorbed on the roots and were not translocated in the leaves, whereas the small and amorphous NPs of ferric complexes stabilized with HS were taken up by wheat on the level comparable to FeEDDHA.

The conducted studies confirmed the importance of size and crystallinity as driving factors for HS-stabilized iron-containing NPs, but they did not allow for making them out: we still do not know if the crystalline particle of <5 nm in size will be taken up by plant or not. These kind of studies are to be conducted. They could shed light on the properties of iron-containing NPs which are most adequate for producing nanofertilizers, in particular, iron containing fertilizers.

Conclusions and outlook

The results obtained for humic complex systems make us to believe that merge of green chemistry and nature-inspired technology can give rise to the new direction in science – ecoadaptive chemistry and technology, which implies manipulation with and reproduction of the complex matter and systems of nature (Fig. 10).

Fig. 10:

Ecoadaptive chemistry as a merge of green chemistry and nature-like technology at the crossing of chemistry, complex systems, and sustainability.

So, the presented data demonstrate good prospects for a use of plentiful humic resources both for production of green chemicals and green materials. Particular advantages of their application can be seen in agriculture, where the HS-based products are already used as soil conditioners, growth stimulators, organic fertilizers, etc. Added-value of the humics-based products can be gained via ecoadaptive design, which enables a use of this product for reproduction of life-sustaining functions inherent within humic substances in nature. It means that the resultant product is not only suited to the specific category of agrochemicals or remedial agents, but its application enables reproduction of the process, which is facilitated in nature by humic substances. To demonstrate how this approach works, we have designed humics-based injection material, whose injection into aquifer results in formation of humic permeable reactive barrier and can be used for interception of metals or other waterborne compounds similar to geochemical barriers in nature. We have shown that application of silanol-rich HS to degraded soil induces self-assembly of water stable aggregates which is a reproduction of another life-sustaining function of HS – maintaining soil structure. Hence, application of silanol-rich humics-based conditioners might have a long-term effect by replenishing same materials and tools which were used by nature – and it can be further sustained by nature. The similar approach was applied to ecoadaptive design of iron fertilizers based on a use of humic matrix for stabilization of poorly crystalline nanosized fraction of iron (hydr)oxides – same way the nature work to keep bioavailable iron under aerobic soil conditions. The Fe-HS products were not so readily available for the plant uptake as compared to the synthetic iron chelates being rather slow release iron fertilizers which is, again, much closer to iron nutrition in nature. Hence the results obtained for the humic complex systems were supportive of the proposed concept of ecoadaptive chemistry and technology.