Spectroscopy Techniques for Monitoring the Composting Process: Comparison
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Please note this is a comparison between Version 2 by Lindsay Dong and Version 1 by Gustavo Curaqueo.

Composting is described as a sustainable alternative to organic waste reuse from the agricultural and household sectors. The organic matter degradation and stabilization product presents great variability due to the waste composition used. 

  • compost
  • UV-Vis spectroscopy
  • IR spectroscopy
  • fluorescence spectroscopy

1. Introduction

The worldwide population growth is estimated at around 26% by 2050 [1]. This would imply an increase in food production of almost 70% through agricultural systems [2]. This scenario represents a challenge regarding soil nutrition, the capacity to support the intensification level and ecosystem pressure, and the increased organic wastes generated by the industrial and/or household sector [3]. Therefore, an increase in agricultural production could impact soil degradation due to the application of agrochemicals and intensive management [4]. Thus, from the crop’s perspective, the nutrient uptake from the soil system can become a limiting factor [5]. Hence, sustainable alternatives are required that are capable of addressing the recycling of nutrients while minimizing the environmental impact [6].
In this sense, composting is an economic and controlled method for handling and converting organic matter from waste. This is defined as an aerobic biological control process of the decomposition of organic matter in a stable amendment with high fertilization value [7]. This occurs as a natural process that can be enhanced by adding different types of organic or biological substrates that optimize the development of microbial communities and favor the degradation of organic matter [8]. This generates a product high in bioavailable nutrients, free of pathogenic microorganisms and phytotoxicity with potential agronomic use [9], which is considered a natural alternative to chemical fertilizers, involving the concept of the circular economy [10].
Furthermore, due to the variability and characteristics of the residues used in the composting process, different parameters are analyzed to determine the stability, maturity, and quality of the composted product in each bio-oxidative and maturity stage [11]. This includes various analytical methodologies such as chromatography techniques, spectroscopy, or microbiological sequencing associated with the metabolic response, which allow the evaluation of their physical-chemical and biochemical process [12,13][12][13]. However, using a single method to investigate the decomposition process is insufficient. Integrating spectroscopic techniques to complement traditional methods is recommended as a better way to characterize the humification degree and organic matter stability [14]. Thus, spectroscopy methods have been incorporated due to their multiple advantages, such as short analysis time, minimal sample quantity, non-destructive sample, environmentally friendly, and experimental reproducibility [15]. These analyses include UV-Vis spectroscopy to evaluate the organic matter humification degree [16]. Infra-red (IR) spectroscopy for the monitoring of functional groups associated with the transformation of organic substances [17]. Fluorescence spectroscopy for the analysis of fluorescent compounds such as amino acids, proteins, and humic and fulvic substances [18]. Nuclear magnetic resonance (NMR) spectroscopy is associated with the study of carbon structures to analyze the transformation of aliphatic compounds and aromaticity degree [19]. Thus, spectroscopy methods are established as comparative and descriptive methods that allow the complementation of results for monitoring the composting process and its different residues [15].

2. Composting Process

Composting has become an important alternative worldwide for the reduction and reuse of waste generated in the industrial and household sectors. Highlighted as a sustainable, economical, and controlled method for managing organic matter degradation [24][20] to increase eco-friendly practices and circular economy within productive processes [25][21]. It is defined as an aerobic biological process of organic matter decomposition to a stable form with a high content of humic and fulvic substances [21][22]. A transformation that occurs naturally and can be enhanced or accelerated by adding different organic waste types and inoculums that optimize microbial growth and favor substrate degradation [26,27][23][24]. On the other hand, the process effectiveness and product quality will be related to the development of each stage. First, the bio-oxidative stage consists of three phases (mesophilic, thermophilic, and cooling) differentiated by the temperature reached during the decomposition process of simple and complex organic matter [28][25]. Then, it ends with the maturity stage, where the reorganization and condensation of organic matter into stabilized compounds such as humic and fulvic substances is highlighted [29][26]. Thus, among the characteristics provided by mature compost to the soil matrix are its physical properties. It highlights that facilitating soil management, generating an adequate porosity that improves water retention and gas exchange, reduces the risks of erosion, improves the physicochemical characteristics of the soil, enriches the movement of nutrients, helps regulate soil temperature, and reduces water evaporation, which regulates soil moisture by maintaining a stable balance of microorganisms [7]. Optimal development of these properties requires monitoring some key parameters associated with maturity and stability parameters in the composting process (Figure 1). Factors such as pH and temperature are the most important characteristics during composting [26][23]. These parameters define the metabolic processes and the succession of microbial communities at each stage [35][27]. Initially, vegetable or food waste is characterized by lowering the pH in the pile due to the release of organic acids in the early stages of degradation [36][28]. Also, incorrect storage of these wastes can cause the accumulation of acidic substances due to fermentation processes. Consequently, low pH levels in the early stages of composting can inhibit the degradation of organic matter [37][29] and decrease the temperature of the pile, delaying the transition to the thermophilic stage.
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Figure 1.
General scheme of physicochemical parameters associated with the composting process.
In turn, organic matter is a key factor to consider during the composting process, a parameter related to the C/N ratio that controls the evolution of the microbial community and humification processes [5]. Optimal initial C/N values range from 20 to 30, which avoids N mineralization and immobilization processes [38][30]. Consequently, the decrease in organic matter content, CO2 formation, N ammonification processes, and volatilization as NH3 form generate a variation in the C/N ratio during the composting process [29][26].
On the other hand, electrical conductivity reflects the waste mineralization process, and this is associated with salt formation due to the organic matter transformation into inorganic matter by the action of microorganisms [39][31]. Moreover, cation exchange capacity increases due to the content of polar functional groups such as hydroxyl, carboxyl, and methoxy by the action of raw material biological oxidation [40][32].
Furthermore, Figure 2 summarizes the main products obtained by the degradation of microbial communities at each stage of the composting process. These results have been adapted from [21,42][22][33] in which different ways of formation of humic compounds are proposed considering the environmental variations. The synthesis of these substances is characterized by their particular resistance to degradation and their beneficial properties of agronomic interest in soil quality and against environmental phytotoxicity [21][22].
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Figure 2. Transformation mechanism of organic matter in each stage of the composting process [21,42].

3. Ultraviolet-Visible Spectroscopy (UV-Vis)

UV-Vis spectroscopy in composting allows for the detection of the variation in the absorption bands during the process. These bands are related to the degradation of organic matter. They are interpreted as the change of absorption at a given wavelength by the effect of the variation of the electronic structures of the compounds [46]. The UV-Vis spectra have different absorption regions: 280 nm indicates the beginning of the transformation of aliphatic compounds and lignins, 465 nm indicates the beginning of the humification process by depolymerization of complex organic molecules, and 665 nm indicates process humification with high oxygen content and aromatic groups [47,48].
These parameters have also been interpreted as the relationship ratio between the regions described. The first one indicates the ratio between the beginning of the organic matter transformation process and the humification degree. It is expected that the latter will increase at the end of the process, and the transformation process will decrease over time [52]. Thus, research reports the relationship E250/E203 as a measure to evaluate the substitution of aromatic rings and the presence of aliphatic groups. Therefore, polar functional groups are found at higher rates in aromatic rings, such as hydroxyl, carbonyl, and carboxyl groups [53,54].
In turn, another parameter has been described to evaluate organic matter transformation. Specific ultraviolet absorbance (SUVA) is defined as absorbance at a given wavelength normalized by dissolved organic carbon concentration (DOC) [58]. Aromaticity degree can be associated with ranges between 250–280 nm, and SUVA
Transformation mechanism of organic matter in each stage of the composting process [22][33].

3. Ultraviolet-Visible Spectroscopy (UV-Vis)

UV-Vis spectroscopy in composting allows for the detection of the variation in the absorption bands during the process. These bands are related to the degradation of organic matter. They are interpreted as the change of absorption at a given wavelength by the effect of the variation of the electronic structures of the compounds [34]. The UV-Vis spectra have different absorption regions: 280 nm indicates the beginning of the transformation of aliphatic compounds and lignins, 465 nm indicates the beginning of the humification process by depolymerization of complex organic molecules, and 665 nm indicates process humification with high oxygen content and aromatic groups [35][36].
These parameters have also been interpreted as the relationship ratio between the regions described. The first one indicates the ratio between the beginning of the organic matter transformation process and the humification degree. It is expected that the latter will increase at the end of the process, and the transformation process will decrease over time [37]. Thus, research reports the relationship E250/E203 as a measure to evaluate the substitution of aromatic rings and the presence of aliphatic groups. Therefore, polar functional groups are found at higher rates in aromatic rings, such as hydroxyl, carbonyl, and carboxyl groups [38][39].
In turn, another parameter has been described to evaluate organic matter transformation. Specific ultraviolet absorbance (SUVA) is defined as absorbance at a given wavelength normalized by dissolved organic carbon concentration (DOC) [40]. Aromaticity degree can be associated with ranges between 250–280 nm, and SUVA
254 is directly related to the percentage of aromaticity of humic substances and molecular weight [59]. 
is directly related to the percentage of aromaticity of humic substances and molecular weight [41]. 

4. Infrared Spectroscopy (IR)

IR spectroscopy is obtained from the molecular vibration generated by the different functional groups [61]. In the characterization of the composting process, peaks have been described that can contribute to monitoring the conversion stages of organic matter.
IR spectroscopy is obtained from the molecular vibration generated by the different functional groups [42]. In the characterization of the composting process, peaks have been described that can contribute to monitoring the conversion stages of organic matter.
Table 2 describes the main functional groups attributed to these bands and their vibrations. The bands 2964 to 2930 cm
1 describes the main functional groups attributed to these bands and their vibrations. The bands 2964 to 2930 cm
−1 describe aliphatic carbon groups attributed to the presence of fatty acids [62]. The constant presence of this signal may be due to the concentration of biodegradable plant material composed of lignins, cutins, and suberins [63].
describe aliphatic carbon groups attributed to the presence of fatty acids [43]. The constant presence of this signal may be due to the concentration of biodegradable plant material composed of lignins, cutins, and suberins [44].
The signal between 1590 to 1504 cm
−1 corresponds to the presence of amide II groups and the lignin and cellulose present in residues containing plant materials such as plants or wood, which are rich in this compound [64]. Another typical band of lignin is caused by the vibration of the aromatic (C=C) skeleton [65]. On the other hand, a band of protein origin can be found in nitrogen-rich composts associated with amide II groups [66].
corresponds to the presence of amide II groups and the lignin and cellulose present in residues containing plant materials such as plants or wood, which are rich in this compound [45]. Another typical band of lignin is caused by the vibration of the aromatic (C=C) skeleton [46]. On the other hand, a band of protein origin can be found in nitrogen-rich composts associated with amide II groups [47].
Table 21. Summary wavenumber described for the evaluation of the composting process.
Wavenumber (cm−1) Description Reference
3437–3263 This occurs due to stretching vibration produced by OH groups of alcohols, phenols, and organic acids. [28,52,53,57,66][25][37][38][47][48]
2964–2930 Bands corresponding to C-H stretching and asymmetric vibrations in aliphatic structures.
1652–1642 Bands produced by stretching vibrations of C=C bonds in aromatic structures by ketone groups such as quinones and amide groups (C-N).
1590–1500 Obtained due to the deformation and vibration stretching of amide II groups (N-H) and (C-N) of secondary amides, respectively.
1408 Intensity assigned to the vibration asymmetric stretching of carboxyl groups (C-O).
1387 Described by the deformation of phenolic OH groups and aromatic alcohols, by the asymmetric stretching of carboxyl ions (COO−) of disubstituted aromatic rings, and by the presence of inorganic nitrogen as nitrates.
1116–1003 Characteristics of C-O bond stretching vibrations in structures such as polysaccharides, ethers, and secondary alcohols.

5. Fluorescence Spectroscopy

Fluorescence spectroscopy is a nondestructive method for characterizing organic matter in relation to its important fluorescent compounds, which include humic-like, fulvic-like, and proteinaceous substances [69]. This method is based on the signal interpretation by light emission effect in the process of electronic de-excitation of conjugated systems [70]. It is used to evaluate the humification degree of organic matter in the composting process. Fluorescence in organic matter is related to the condensed aromatic rings presence of unsaturated aliphatic carbon chains [71]. Fluorescence bands at long wavelengths have been described by high molecular weight and complex structural compounds with a high conjugation degree, and fluorescence bands at shorter wavelengths have been described for simple structures and small degrees of conjugated chromophores due to a low degree of aromatic polycondensation [72]. Therefore, the stability of organic matter and the increase in its condensation degree is associated with a chemically stable structure with greater residence in the environment, which contributes to enhancing the structure and fertility of the soil [73].

5.1. Emission and Excitation Spectra

Emission spectra are generated due to the sample's exposure to a specific excitation wavelength and are represented by one or more emission bands. The excitation wavelength for obtaining emission spectra in the composting process analysis is 254 nm. The results reported a fluorescence peak at 340 nm in the initial phase, changing to 430 nm during the process, indicating an increase in the aromatic group condensation due to the maturation of the compost [55]. Thus, the relationship between emission spectra at 450 and 500 nm with an excitation wavelength of 370 nm (F
Fluorescence spectroscopy is a nondestructive method for characterizing organic matter in relation to its important fluorescent compounds, which include humic-like, fulvic-like, and proteinaceous substances [49]. This method is based on the signal interpretation by light emission effect in the process of electronic de-excitation of conjugated systems [50]. It is used to evaluate the humification degree of organic matter in the composting process. Fluorescence in organic matter is related to the condensed aromatic rings presence of unsaturated aliphatic carbon chains [51]. Fluorescence bands at long wavelengths have been described by high molecular weight and complex structural compounds with a high conjugation degree, and fluorescence bands at shorter wavelengths have been described for simple structures and small degrees of conjugated chromophores due to a low degree of aromatic polycondensation [52]. Therefore, the stability of organic matter and the increase in its condensation degree is associated with a chemically stable structure with greater residence in the environment, which contributes to enhancing the structure and fertility of the soil [53].

5.1. Emission and Excitation Spectra

Emission spectra are generated due to the sample's exposure to a specific excitation wavelength and are represented by one or more emission bands. The excitation wavelength for obtaining emission spectra in the composting process analysis is 254 nm. The results reported a fluorescence peak at 340 nm in the initial phase, changing to 430 nm during the process, indicating an increase in the aromatic group condensation due to the maturation of the compost [54]. Thus, the relationship between emission spectra at 450 and 500 nm with an excitation wavelength of 370 nm (F
450
/F
500) has been described. This index investigates the organic matter sources [74] and correlates with humic substances origin. A value less than 1.4 indicates terrestrially derived humic substances, and larger than 1.90 indicates microbially derived humic substances [75].
) has been described. This index investigates the organic matter sources [55] and correlates with humic substances origin. A value less than 1.4 indicates terrestrially derived humic substances, and larger than 1.90 indicates microbially derived humic substances [56].
Contrary to emission spectra, excitation spectra are determined using a fixed emission wavelength and the excitation scan at different wavelengths. Moreover, the results obtained can be assimilated to the UV-Vis absorption spectra, the information from which can be used to generate the emission spectra [78][57]. Studies [14] describe a composting of municipal solid waste, where they evaluate an excitation spectrum of 300 to 500 nm using an emission wavelength of 520 nm. The results reported four major peaks, of which only two were representative. One of these, around 436 nm, was attributed to aromatic rings bearing electron donor groups, and the second at 383 nm, possibly due to fluorophores originating from the polycondensation of carbonyl groups and lignin-derived phenolic structures [79][58]. These two peaks were associated through the fluorescence ratio determination (I436/I383) that showed an inverse correlation to the SUVA254 indicator and the emission at 351 nm, which are related to the abundance of aromatic carbons and the organic matter humification, respectively [80,81][59][60]. 

5.2. Synchronous-Scan Spectra

5.2. Synchronous-Scan Spectra

Synchronous-scan spectra are generated from the simultaneous detection of excitation and emission scans at a predetermined wavelength difference, favoring signal amplification in trace compounds [82][61]. In turn, compared with other fluorescence analyses, it is possible to obtain a unique and clear spectrum associated with specific functional groups and chemical structures [83][62]. Synchronous fluorescence represents the spectra summation of different fluorophores in dissolved organic matter and exhibits better resolution than excitation and emission spectra [14]. This analysis reports the presence of characteristic peaks for the composting process associated with organic matter biodegradation and biosynthesis of humic-like substances [28][25]. Furthermore, a Peak B was reported between 308–365 nm attributed to the presence of fulvic-like indicates the presence of polycyclic aromatics with three to four fused benzene rings and two to three conjugated systems in unsaturated aliphatic structures [84,85,86][63][64][65]. The identification of humic-like substances has been reported in Peak C 363–595 nm [14,28][14][25] related to the presence of polycyclic aromatic compounds with approximately 5–7 fused benzene rings tend to increase in intensity as the compost matures [85,87][64][66]. 

5.3. Excitation-Emission Matrix

Excitation–emission matrix (EEM) spectra are obtained by subsequently scanning the emission spectra range by increasing the excitation wavelength. Figure 3 compiles the Excitation-Emission matrix results reported by [89][67]. These results are represented in a graph divided into five regions. The dissolved organic matter of each region represents particular structures, in the case of the composting process, which can increase or decrease the intensity of fluorescence as it moves towards its maturity stage [68]. These peaks have been mainly associated with organic compounds such as humic and fulvic acid-like amino acids, proteins, or phenolic structures [90][69].
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Figure 3. Location of structures for each region in an Excitation-Emission Matrix (EEM) of organic matter (adapted from [89]).
Location of structures for each region in an Excitation-Emission Matrix (EEM) of organic matter (adapted from [67]).
In this sense, Regions I and II are related to amino acids derived from simple aromatic proteins such as tyrosine and tryptophan with peaks at shorter excitation-emission wavelengths (<250 nm/<350 nm); Region III is described by peaks at shorter excitation wavelengths and longer emission wavelengths (<250 nm/>380 nm) are related to fulvic substances; Region IV is associated with microbial by-product-like material as a result of microbial protein degradation, reporting peaks at intermediate excitation, and shorter emission wavelengths (250~280 nm/<380 nm); Region V includes a longer excitation range and longer emission wavelengths (>280 nm/>380 nm) and is directly associated with humic substances and may even contain part of fulvic substances fluorescence, depending on the intensity of these [14,68,91,92][14][68][70][71].

6. Nuclear Magnetic Resonance (13C NMR)

Nuclear magnetic resonance (NMR) is produced by the reorganization of nuclear spin due to the application of a magnetic field to the nuclei at the low-energy level, which will absorb electromagnetic energy and transit to the high-energy level. When the electromagnetic field disappears, the nucleus releases the absorbed energy, returning to its low-energy state in a process called relaxation. This energy is detected as radiofrequency detects the presence of carbon and its relationship with the functional groups [97][72].
During the composting process, chemical shifts have been described to monitor the organic matter decomposition and to evaluate structures stabilized over time. Figure 4 summarizes the main chemical shifts reported in the literature, corresponding to carbon monitoring as Alkyl-C (45-0 ppm), methoxyl C (60–45 ppm), aliphatic C substituted with O or N (110–60 ppm), aromatic C (110–145 ppm), phenolic C and aromatic ether substituted with O or N (160–145 ppm) and carbonyl C, carboxyl C, quinones, ester, and amides carbon (210–160 ppm) [19,52,98,99][19][37][73][74].
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Figure 4. Signal distribution 13C NMR about the chemical shift described in the composting process [19,52].

7. Conclusions

In conclusion, the composting process has been established as a sustainable alternative within the circular economy for the reduction and treatment of organic waste. Also, it is a useful method for generating soil amendments capable of recycling nutrients from organic waste generated by the agricultural and household sectors. The stabilization of organic matter and correct degradation of complex structures allows the generation of an optimal product, minimizing the probability of environmental impact due to the persistence of phytopathogenic agents or the presence of toxic substances in concentrations that affect soil quality. Spectroscopy techniques such as UV-Vis, IR, Fluorescence, and 13C NMR allow monitoring of the different stages of the composting, showing the stabilization and degradation of organic matter in highly condensed compounds. Each technique establishes a relationship concerning the functional groups and elucidates the part of the structures that are formed as a result of the action of microorganisms on organic waste. Therefore, it is considered that these methodologies establish reliable and complementary quantitative indicators to establish the necessary quality criteria required by a compost.
Signal distribution 13C NMR about the chemical shift described in the composting process [19][37].

7. Conclusions

In conclusion, the composting process has been established as a sustainable alternative within the circular economy for the reduction and treatment of organic waste. Also, it is a useful method for generating soil amendments capable of recycling nutrients from organic waste generated by the agricultural and household sectors. The stabilization of organic matter and correct degradation of complex structures allows the generation of an optimal product, minimizing the probability of environmental impact due to the persistence of phytopathogenic agents or the presence of toxic substances in concentrations that affect soil quality. Spectroscopy techniques such as UV-Vis, IR, Fluorescence, and 13C NMR allow monitoring of the different stages of the composting, showing the stabilization and degradation of organic matter in highly condensed compounds. Each technique establishes a relationship concerning the functional groups and elucidates the part of the structures that are formed as a result of the action of microorganisms on organic waste. Therefore, it is considered that these methodologies establish reliable and complementary quantitative indicators to establish the necessary quality criteria required by a compost.

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