Isolation, Characterization, and Environmental Application of Bio-Based Materials as Auxiliaries in Photocatalytic Processes

Sustainable alternative substrates for advanced applications represent an increasing field of research that attracts the attention of worldwide experts (in accordance with green chemistry principles). In this context, bio-based substances (BBS) isolated from urban composted biowaste were purified and characterized. Additionally, these materials were tested as auxiliaries in advanced oxidizing photocatalytic processes for the abatement of organic contaminants in aqueous medium. Results highlighted the capability of these substances to enhance efficiency in water remediation treatments under mild conditions, favoring the entire light-driven photocatalytic process.


Introduction
The exploitation of renewable resources as an alternative feedstock for the production of value-added bio-based chemicals and materials has dramatically increased in recent decades [1][2][3][4][5][6][7][8][9][10]. Among the different substrates, the use of an organic fraction from urban and/or agricultural waste has attracted the attention of experts driven by the principles of a circular (and sustainable) economy ( [11,12] and references therein). Furthermore, the current principal environmental exploitation of biowaste (intended as: biodegradable garden and park waste, food and kitchen waste from households, restaurants, caterers, and retail premises, and comparable waste from food processing plants) in the European Union (EU) is the production of methane (a greenhouse gas) from decomposing processes in landfills [3,13]. In 1999, the Landfill Directive of the European Community (1999/31/EC) forced all EU member states to diminish the amount of biodegradable municipal waste left in landfill, with the aim of promoting their re-use and recycling [14].
Seeking further biowaste valorization, waste-derived bio-based substances (BBS in the following) have been isolated from urban-composted biowaste; BBS are supramolecular aggregates with a complex lignin-derived structure (similar to soil humic substances) containing several functionalities (namely, acid and basic functional groups bonded to aromatic and aliphatic chains) [31][32][33][34][35]. The presence of both hydrophilic and hydrophobic functionalities forming the chemical structure of such BBS confers to these substances amphiphilic properties, as testified by the literature on analogous substrates [36][37][38]. As reported in our previous study [39], these BBS can be used in several industrial applications (as emulsifiers, foaming agents, in detergent formulations or as textiles auxiliaries, for the production of nanostructured materials, in animal husbandry, as fertilizers, or for the production of biofuels and bioplastics). Among the potential applications of such BBS, the most striking one is the "waste for cleaning waste" [40]. This approach implies the use of BBS as chemical auxiliaries for the photochemical degradation of pollutants in aqueous environment. According to the literature, these BBS have been used (either in homogeneous or heterogeneous phase) to sensitize the photodegradation of standard contaminants as well as emerging pollutants (EPs), whose dangerousness has only recently been recognized [41,42].
Advanced oxidation processes (AOPs) have been widely studied as a sustainable alternative treatment for the purification of the contaminated aqueous environment [43][44][45][46]. Among these, (photo)-Fenton processes (in the presence of a Fe(II) source, H 2 O 2 , and eventually UV irradiation) have been widely studied as a greener alternative to the more conventional methods for remediation of contaminated wastewaters [47]. The Fenton reaction provides the degradation of organic contaminants by means of oxidizing species (principally hydroxyl radicals (OH)) formed by H 2 O 2 decomposition catalyzed by Fe(II). Additionally, the Fenton process can be accelerated by UV-Visible irradiation (i.e., λ < 500 nm, thus becoming a photo-Fenton process) [48,49]. The introduction of humic-like substances in (photo)-Fenton processes has been reported to enhance the degradation of organic pollutants by direct formation of different O-containing reactive species (i.e., hydroxyl radicals, superoxide species, and singlet oxygen) or by involving the triplet-excited states of the functionalities still present in the humic-like structure [50,51]. On the other hand, humic-like substances can accelerate the Fe(III)-to-Fe(II) redox mechanism, thus regenerating the catalyst [52].
Therefore, this study aims at isolating humic-and fulvic-like substances (BBS-HA and BBS-FA) obtained from green composted biowaste (GC in the following) and characterizing the physicochemical properties in order to elucidate the structural organization and the surfactant behavior as well as their photocatalytic performance on homogeneous degradation processes, simulating the remediation of a contaminated wastewater.

Spectroscopic and Analytical Characterization of BBS-HA
The two fractions obtained from the compost, BBS-HA and BBS-FA, were characterized by spectroscopic and chemical analyses. Specific ultraviolet absorbance (SUVA) values, obtained by normalizing the absorbance at 254 nm by the DOC content, were 4.71 and 4.54 L mg −1 m −1 for BBS-HA and BBS-FA, respectively, indicating that both components have a high aromatic character [53,54].
FT-infrared spectra of BBS-HA and BBS-FA ( Figure 1) present the following bands: (i) a broad peak at ca. 3400 cm −1 attributable to O-H stretching vibrational mode; (ii) shoulder peaks at ca. 2940-2920 cm −1 and 2850 cm −1 due to both asymmetric and symmetric aliphatic C-H stretching mode; (iii) a band at ca. 1750-1700 cm −1 due to C=O stretching mode of carbonyl functionalities (mainly carboxylic acids, see Figure 1, red circles), more pronounced in BBS-FA; (iv) signals at ca. 1650-1630 cm −1 attributable to aromatic C=C skeletal vibrations (Figure 1, black squares), more pronounced in BBS-HA than in BBS-FA; (v) a broad band at 1420-1410 cm −1 assigned to both O-H deformation and C-O stretching modes of phenolic moieties (Figure 1, red circles); (vi) an intense band at 1230-1220 cm −1 due to both C-O stretching and O-H deformation modes of COOH groups (Figure 1, red circles). The band in the 1040-1030 cm −1 range is attributed to C-O stretching of polysaccharides and polysaccharide-like impurities (i.e., residues of the non-humic fraction, Figure 1, white circles) [55]. Interestingly, the BBS-HA presents also a shoulder at ca. 1545-1540 cm −1 belonging to the aromatic C=C stretching mode and/or to secondary amide [56] (Figure 1, black squares), and a sharp (but weak) signal at ca. 1130 cm −1 attributable to C-O stretching vibration modes due to alcoholic and ether functionalities ( Figure 1, black star). The interpretation of spectra was according to the literature [55,56]. Thus, BBS-HA appeared to be characterized by a higher aromatic character, in agreement with SUVA findings, and by the incorporation of proteinaceous residues. As expected by the extraction method, the BBS-FA fraction has a higher amount of carboxyl groups [55,56].  (Figure 1, red circles). The band in the 1040-1030 cm −1 range is attributed to C-O stretching of polysaccharides and polysaccharide-like impurities (i.e., residues of the non-humic fraction, Figure 1, white circles) [55]. Interestingly, the BBS-HA presents also a shoulder at ca. 1545-1540 cm −1 belonging to the aromatic C=C stretching mode and/or to secondary amide [56] (Figure 1, black squares), and a sharp (but weak) signal at ca. 1130 cm −1 attributable to C-O stretching vibration modes due to alcoholic and ether functionalities ( Figure 1, black star). The interpretation of spectra was according to the literature [55,56]. Thus, BBS-HA appeared to be characterized by a higher aromatic character, in agreement with SUVA findings, and by the incorporation of proteinaceous residues. As expected by the extraction method, the BBS-FA fraction has a higher amount of carboxyl groups [55,56]. 13 C solid-state nuclear magnetic resonance (NMR) spectroscopy offers the possibility of direct chemical characterization of organic materials in terms of content of different functional groups [57][58][59]. In the 13 C CPMAS solid-state NMR spectrum of BBS-HA ( Figure 2 together with the deducted abundance of functional groups), the region between 160 and 220 ppm is dominated by a signal at 173 ppm, due to carboxyl C. The aromatic and phenolic region (110-160 ppm) represented 30% of C. Here, the signal at 129 ppm, indicating a relatively larger presence of non-substituted aromatic rings, may be due to aromatic lignin [60]. Weaker signals occurred in the 45-110 ppm region, which included only 13% of C. The signal at 71 ppm is associated to the simultaneous resonance of C-2, C-3, and C-5 of pyranoside rings in cellulose and hemicellulose [56], while the weak signals at 65 ppm (C-6) and 105 ppm (anomeric C) indicated a low presence of crystalline cellulose. The high signal at 56 ppm is attributable to lignin methoxyl C and highlights the selective enrichment of lignin-derived compounds with composting as a result of cellulose decomposition. The 0-45 ppm region is characterized by two signals at 30 and 24 ppm, most likely attributable to paraffinic C of lipids and waxes. Their integration area almost equals that of the aromatic region (28.8%) and again derives from the selective enrichment of recalcitrant components [60]. 13 C solid-state nuclear magnetic resonance (NMR) spectroscopy offers the possibility of direct chemical characterization of organic materials in terms of content of different functional groups [57][58][59].
In the 13 C CPMAS solid-state NMR spectrum of BBS-HA ( Figure 2 together with the deducted abundance of functional groups), the region between 160 and 220 ppm is dominated by a signal at 173 ppm, due to carboxyl C. The aromatic and phenolic region (110-160 ppm) represented 30% of C. Here, the signal at 129 ppm, indicating a relatively larger presence of non-substituted aromatic rings, may be due to aromatic lignin [60]. Weaker signals occurred in the 45-110 ppm region, which included only 13% of C. The signal at 71 ppm is associated to the simultaneous resonance of C-2, C-3, and C-5 of pyranoside rings in cellulose and hemicellulose [56], while the weak signals at 65 ppm (C-6) and 105 ppm (anomeric C) indicated a low presence of crystalline cellulose. The high signal at 56 ppm is attributable to lignin methoxyl C and highlights the selective enrichment of lignin-derived compounds with composting as a result of cellulose decomposition. The 0-45 ppm region is characterized by two signals at 30 and 24 ppm, most likely attributable to paraffinic C of lipids and waxes. Their integration area almost equals that of the aromatic region (28.8%) and again derives from the selective enrichment of recalcitrant components [60]. Surface tension measurements of BBS-HA in the 0.2 to 5.0 g L −1 concentration range evidenced a trend to decrease when increasing the BBS-HA concentration, thus clearly indicating that BBS-HA had marked surfactant behavior ( Figure 3). Nonetheless, conversely to the behavior reported for analogous substances [61], no evident discontinuities were observed, attributable to a variation of the microstructure, following the formation of micellar aggregates. Micelles could influence solute properties depending on the substrate partitioning between the bulk solution and the micellar phase; a net change of the surface reactivity with solute species, due to the formation of micellar aggregates, has been reported for humic acids structurally similar to BBS-HA [62]. Moreover, the presence of a surfactant, even if below its critical micellar concentration, could enhance the solubility in water hydrophobic substrates. In the present case we do not expect any peculiar behavior related to a specific BBS-HA microstructure depending on the concentration; on the other hand, the high UV-VIS light absorption coefficient of BBS-HA could result in a screen effect for its photochemical application (see BBS-HA UV-Vis spectrum reported in Figure 4). Based on this latter aspect, an upper BBS-HA concentration limit of 1.0 g L −1 was chosen to study the photocatalytic performance of BBS-HA.  Surface tension measurements of BBS-HA in the 0.2 to 5.0 g L −1 concentration range evidenced a trend to decrease when increasing the BBS-HA concentration, thus clearly indicating that BBS-HA had marked surfactant behavior ( Figure 3). Nonetheless, conversely to the behavior reported for analogous substances [61], no evident discontinuities were observed, attributable to a variation of the microstructure, following the formation of micellar aggregates. Micelles could influence solute properties depending on the substrate partitioning between the bulk solution and the micellar phase; a net change of the surface reactivity with solute species, due to the formation of micellar aggregates, has been reported for humic acids structurally similar to BBS-HA [62]. Moreover, the presence of a surfactant, even if below its critical micellar concentration, could enhance the solubility in water hydrophobic substrates. In the present case we do not expect any peculiar behavior related to a specific BBS-HA microstructure depending on the concentration; on the other hand, the high UV-VIS light absorption coefficient of BBS-HA could result in a screen effect for its photochemical application (see BBS-HA UV-Vis spectrum reported in Figure 4). Based on this latter aspect, an upper BBS-HA concentration limit of 1.0 g L −1 was chosen to study the photocatalytic performance of BBS-HA. Surface tension measurements of BBS-HA in the 0.2 to 5.0 g L −1 concentration range evidenced a trend to decrease when increasing the BBS-HA concentration, thus clearly indicating that BBS-HA had marked surfactant behavior ( Figure 3). Nonetheless, conversely to the behavior reported for analogous substances [61], no evident discontinuities were observed, attributable to a variation of the microstructure, following the formation of micellar aggregates. Micelles could influence solute properties depending on the substrate partitioning between the bulk solution and the micellar phase; a net change of the surface reactivity with solute species, due to the formation of micellar aggregates, has been reported for humic acids structurally similar to BBS-HA [62]. Moreover, the presence of a surfactant, even if below its critical micellar concentration, could enhance the solubility in water hydrophobic substrates. In the present case we do not expect any peculiar behavior related to a specific BBS-HA microstructure depending on the concentration; on the other hand, the high UV-VIS light absorption coefficient of BBS-HA could result in a screen effect for its photochemical application (see BBS-HA UV-Vis spectrum reported in Figure 4). Based on this latter aspect, an upper BBS-HA concentration limit of 1.0 g L −1 was chosen to study the photocatalytic performance of BBS-HA.

Photocatalytic Performances of BBS-HA in Homogeneous Phase
The capability of BBS-HA to promote the photodegradation of organic pollutants has been tested using caffeine, a substance naturally occurring in some beverages and also used as a pharmacological agent, as a probe molecule, being included in the list of contaminants of emerging concerns (CECs). Preliminary experiments were performed by irradiating an aqueous solution of caffeine (5 mg L −1 ) in the presence of different amounts of dissolved BBS-HA (from 0.1 to 1.0 g L −1 ). The pH was not modified and was in the range 8.0-8.5; this slightly basic pH originates from the preparation of the BBS-HA solutions at different concentrations, obtained by diluting the BBS-HA stock solution having a pH value in the range 9.0-9.5 (see Section 3.3). The results indicated that even after several hours of irradiation (up to 6 h), the maximum yield of caffeine degradation reached 10%. Based on these data, further experiments were performed lowering the pH at a value of 5.5, and comparing the results obtained by irradiating the caffeine in the presence of (i) BBS-HA alone; (ii) BBS-HA and hydrogen peroxide; (iii) BBS-HA, hydrogen peroxide and Fe(II) (photo-Fenton like). This mild, circumneutral pH was chosen on the basis of encouraging results reported in analogous experiments performed in the presence of other waste-sourced biosubstances [63]. Figure 5 reports the obtained results; in the panel A, the relative caffeine concentration evolution with irradiation time is reported for the three experimental conditions explored. When the irradiation is performed in the presence of BBS-HA alone, a very low caffeine decrease can be observed, yielding about 10% of caffeine abatement after 6 h of irradiation. It is reasonable to attribute this low but not negligible degradation to the BBS-HA capability of generating reactive oxygenated species upon irradiation, similar to what has been previously reported [64]. Upon the addition of hydrogen peroxide, a significant enhancement of the caffeine degradation rate occurs, and after 6 h of irradiation, ca. 75% of caffeine has been degraded. However, the photo-Fenton process performed in the presence of BBS-HA is dramatically faster; for this reason, in this latter condition, a shorter reaction time was explored and the observed caffeine degradation profile is reported in panel B ( Figure 5): The complete disappearance of caffeine was already observed after 10 min; this result is in agreement with the literature [63], where a group of emerging contaminants was subjected to photo-Fenton treatment in the presence of a similar BBS. The positive effect of the present BBS-HA on the photo-Fenton process can be ascribed to the formation of Fe(II)-BBS-HA complexes, which may react more rapidly with H2O2 than Fe(II)-aquo complexes, leading to a higher rate of HO• production. Voelker and Sulzberger [65] observed the same mechanism with fulvic acids, concluding that Fe(II)-fulvate complexes formed at pH = 5 accelerated the Fenton reaction. Using iron complexing substances in order to promote photo-Fenton processes under milder pH values is indeed a topic attracting attention from many researchers and recently the use of Ethylenediamine-N,N′-disuccinic acid (EDDS) was proposed [66].

Photocatalytic Performances of BBS-HA in Homogeneous Phase
The capability of BBS-HA to promote the photodegradation of organic pollutants has been tested using caffeine, a substance naturally occurring in some beverages and also used as a pharmacological agent, as a probe molecule, being included in the list of contaminants of emerging concerns (CECs). Preliminary experiments were performed by irradiating an aqueous solution of caffeine (5 mg L −1 ) in the presence of different amounts of dissolved BBS-HA (from 0.1 to 1.0 g L −1 ). The pH was not modified and was in the range 8.0-8.5; this slightly basic pH originates from the preparation of the BBS-HA solutions at different concentrations, obtained by diluting the BBS-HA stock solution having a pH value in the range 9.0-9.5 (see Section 3.3). The results indicated that even after several hours of irradiation (up to 6 h), the maximum yield of caffeine degradation reached 10%. Based on these data, further experiments were performed lowering the pH at a value of 5.5, and comparing the results obtained by irradiating the caffeine in the presence of (i) BBS-HA alone; (ii) BBS-HA and hydrogen peroxide; (iii) BBS-HA, hydrogen peroxide and Fe(II) (photo-Fenton like). This mild, circumneutral pH was chosen on the basis of encouraging results reported in analogous experiments performed in the presence of other waste-sourced biosubstances [63]. Figure 5 reports the obtained results; in the panel A, the relative caffeine concentration evolution with irradiation time is reported for the three experimental conditions explored. When the irradiation is performed in the presence of BBS-HA alone, a very low caffeine decrease can be observed, yielding about 10% of caffeine abatement after 6 h of irradiation. It is reasonable to attribute this low but not negligible degradation to the BBS-HA capability of generating reactive oxygenated species upon irradiation, similar to what has been previously reported [64]. Upon the addition of hydrogen peroxide, a significant enhancement of the caffeine degradation rate occurs, and after 6 h of irradiation, ca. 75% of caffeine has been degraded. However, the photo-Fenton process performed in the presence of BBS-HA is dramatically faster; for this reason, in this latter condition, a shorter reaction time was explored and the observed caffeine degradation profile is reported in panel B ( Figure 5): The complete disappearance of caffeine was already observed after 10 min; this result is in agreement with the literature [63], where a group of emerging contaminants was subjected to photo-Fenton treatment in the presence of a similar BBS. The positive effect of the present BBS-HA on the photo-Fenton process can be ascribed to the formation of Fe(II)-BBS-HA complexes, which may react more rapidly with H 2 O 2 than Fe(II)-aquo complexes, leading to a higher rate of HO• production. Voelker and Sulzberger [65] observed the same mechanism with fulvic acids, concluding that Fe(II)-fulvate complexes formed at pH = 5 accelerated the Fenton reaction. Using iron complexing substances in order to promote photo-Fenton processes under milder pH values is indeed a topic attracting attention from many researchers and recently the use of Ethylenediamine-N,N -disuccinic acid (EDDS) was proposed [66].

Materials
Bio-based substances (BBS) were isolated from commercially composted urban biowaste, obtained from the ACEA Pinerolese Industriale S.p.A. waste treatment plant in Pinerolo (Pinerolo, Italy). The composted biowaste is derived from urban public park trimmings and home gardening residues aged for more than 180 days. During this time, the fraction of aromatic C and -COOH functionalities increase, while the fraction of aliphatic, easily degradable organic molecules decreases [33]. The process is an advanced system that comprises specific technological facilities, developed by ACEA Pinerolese Industriale S.p.A, and under European validation [67]. The physicochemical properties of the starting compost are reported in Table 1.

Compost Fractionation and BBS Recovery
The compost fractionation is a modified version of the procedure adopted for the separation of humic and fulvic acids from soils and organic matrices, proposed by the International Humic

Materials
Bio-based substances (BBS) were isolated from commercially composted urban biowaste, obtained from the ACEA Pinerolese Industriale S.p.A. waste treatment plant in Pinerolo (Pinerolo, Italy). The composted biowaste is derived from urban public park trimmings and home gardening residues aged for more than 180 days. During this time, the fraction of aromatic C and -COOH functionalities increase, while the fraction of aliphatic, easily degradable organic molecules decreases [33]. The process is an advanced system that comprises specific technological facilities, developed by ACEA Pinerolese Industriale S.p.A, and under European validation [67]. The physicochemical properties of the starting compost are reported in Table 1.  Sigma-Aldrich, Saint Louis, MO, USA). All aqueous solutions were prepared using ultrapure water Millipore Milli-Q TM . All chemicals were used without further purification.

Compost Fractionation and BBS Recovery
The compost fractionation is a modified version of the procedure adopted for the separation of humic and fulvic acids from soils and organic matrices, proposed by the International Humic Substance Society [68]. In detail, the compost was dispersed into a 0.5 M NaOH solution in polyethylene (PE) bottles with 1:10 solid:solution ratio, and left under mechanical shaking in the dark overnight under an atmosphere of N 2 . The supernatant (that contains the BBS fractions) was then separated from the solid residue by centrifugation at 3000 rpm for 15 min, filtered and newly centrifuged at 10,000 rpm for 10 min to finely remove the solid particles still present in the supernatant. The latter was again filtered at 2.7 µm. The pH was corrected at pH < 1 by adding HCl conc. to promote the flocculation of the humic-like substances and the flask maintained in the dark overnight. The day after, the suspension was again centrifuged at 10,000 rpm for 20 min to separate the precipitate (humic-like substances) from the supernatant (still containing the fulvic-like substances). Finally, the supernatant was filtered again, collected in PE bottle, and kept under N 2 atmosphere at 4 • C.
The humic-like fraction was purified from the NaCl content (due to the previous acid-base treatments and pH adjustments) by cyclic washing with deionized H 2 O, followed by centrifugation (rate: 10,000 rpm, time: 10 min), until pH 3 was reached. The humic-like fraction was then dialyzed against deionized water in tubular membranes (Spectra/Por ® 6, cut-off: 1000 Da, thus recovering only BBS with a M W higher than 1000 Da) for 3-4 days with daily water replenishments until reaching an electrical conductivity below 10 µS cm −1 . The suspension was then freeze-dried and kept as such for further analysis. The obtained fraction was coded as BBS-HA, where HA stands for Humic-Acid.
The fulvic-like fraction present in the supernatant was separated from the non-humic soluble components (peptides and carbohydrates residues), through a first column purification with PVP (elution rate: 1-2 mL min −1 ) that retained the fulvic-like substances, discharging the non-humic fraction. The fulvic like substances were eluted from the column with 0.1 N NaOH, and then purified through elution in an amberlite column (H + -saturated cation-exchange resin) for the removal of Na + . In order to obtain a solid substrate, the fulvic-like fraction was freeze dried and then completely dried in an oven at 40 • C. This type of BBS were coded as BBS-FA, where FA stands for Fulvic-Acid.
This procedure yielded 22 g of BBS-HA and 1 g of BBS-FA every 1000 g of compost. Thus, due to the very low separation yield of BBS-FA from this compost, BBS-HA were the only bio-based material considered as photocatalytic auxiliaries for testing possible applications in the photodegradation of organic molecules.

Physicochemical and Photocatalytic Characterization
A stock 5 g L −1 BBS-HA aqueous solution was prepared by taking up solid BBS-HA with MilliQ ® water, stirring 1 h, then adding aliquots 0.2 M NaOH to keep pH in the 9.0-9.5 range until the complete solid dissolution occurred. The solution was finally filtered through a 0.45 µm Millex-HA membrane (Millipore, Merk, Darmstadt, Germany) and brought to the required volume with MilliQ ® water. The stock solution was kept frozen before use. Aliquots of the stock solution were diluted with MilliQ ® water to obtain solutions containing variable amounts of BBS-HA. UV-visible absorbance measurements were performed on a Helios (Thermo Electron, Waltham, MA, USA) spectrophotometer with deionized water as the blank. A quartz cell with 1.0 cm path length was used, and analyses were performed at room temperature. Dissolved organic carbon (DOC) measurements were performed using Pt-catalyzed, high-temperature combustion (850 • C) followed by infrared detection of CO 2 by means of a VarioTOC (Elementar, Hanau, Germany) [53]. Specific ultraviolet absorbance (SUVA) values were determined by normalizing the UV absorbance measured at λ = 254 nm by the DOC concentration and are reported in the units of liter per milligram carbon per meter (L mg −1 m −1 ) [54].
Fourier transform infrared (FTIR) spectra were recorded in transmission mode by means of a Bruker Vector 22 spectrophotometer (Billerica, MA, USA) equipped with Globar source, DTGS detector, and working with 128 scans at 4 cm −1 resolution in the 4000-400 cm −1 range. Samples were mixed with KBr (1:20 wt. ratio).
Solid-state NMR spectrum of BBS-HA was collected on a Bruker Avance II 400 Ultra Shield instrument, working at 400.23 and 100.63 MHz for 1 H and 13 C, respectively. The sample was packed in a cylindrical zirconia rotor (4 mm o.d.), with a sample volume of 80 µL and spun at 12 kHz to minimize the spinning sidebands. The experiment employed the ramp CP pulse sequence ( 1 H 90 • pulse = 3.6 µs; contact time = 1 ms) with the TPPM 1 H decoupling (rf field = 69.4 kHz) during the acquisition period. 400,000 scans were accumulated with an optimized relaxation delay of 0.2 s. The spectrum was processed with a line broadening of 50 Hz. The 13 C chemical shift scale was referenced with the resonance of α-glycine ( 13 C methylene signal at 43.5 ppm), which was used as an external standard.
Surface tension measurements were performed with a digital tensiometer (K10, Krüss, Hamburg, Germany) equipped with a platinum Wilhelmy plate.
Photodegradation experiments were carried out by irradiating, under continuous stirring, 10 mL of aqueous samples in a closed Pyrex ® cell with a Xenon (1500 W) lamp (Solarbox, CO.FO.ME.GRA S.r.l., Milan, Italy) equipped with a 340 nm cut-off filter. The irradiance of the lamp, measured with a UV-Multimeter system, was 26.7 W m −2 . The caffeine degradation was monitored by HPLC, employing a Merck-Hitachi instrument, equipped with Lichrospher RP-C18 (125 mm × 4 mm i.d., particle diameter = 5 µm, from Merck, Darmstadt, Germany), L-6200 pumps and UV/Vis L-4200 detector. The selected eluent was a 1% H 3 PO 4 in 50/50 (vol %) methanol/water mixture at 0.8 mL min

Conclusions
Composted waste-derived bio-based substances (BBS-HA) were isolated and characterized physicochemically. The obtained materials, showing amphiphilic properties, were used as auxiliaries in photodegradation processes for the abatement of caffeine (the target contaminant) in aqueous medium. Results obtained evidenced the promising capability of these substances to drive photo-Fenton-like processes for water treatments under mild conditions, making the exploitation of these biowaste-derived substances extremely powerful and interesting from the industrial viewpoint. In fact, the introduction of such BBS in light-driven photocatalytic processes provides an overall enhancement of the environmental and economic sustainability of the entire process, in accordance to the "waste cleaning waste" philosophy and the green chemistry principles. Based on these preliminary encouraging results, further investigation aiming to give insights into the key reactive species and the key degradation reactions deserves to be undertaken.