**3. The Recovery from Apple Pomace Extraction for the Building and Construction Sectors**

The considerable quantities of apple pomace produced in the world have been forcing researchers to develop novel and modern methods for their effective use. It is commonly known that apple peel (the main component of the pomace) contains a much higher content of active substances—phenolic antioxidants—than the pulp of the fruit. The relatively low price (compared to the price of raw apples) and widespread availability of AP make them a raw material with great potential [65,66]. However, due to their high water and sugar contents, AP are easily perishable (biologically unstable) and require immediate processing, such as dehydration (drying), as a pretreatment, which is associated with high energy consumption and, hence, additional OPEX. On the other hand, as an adverse side effect, drying can cause the degradation of temperature-sensitive valuable phenolic antioxidants [67]. The extraction of active compounds from AP can be an attractive method of their reuse. In addition, the solid waste generated during the process can be further used in accordance with the ideas of sustainability, e.g., as substrates for energy production.

## *3.1. Green Extraction Techniques*

Green extraction techniques are methods for isolating active phenolic antioxidants from plant-based materials in an environmentally friendly manner. They rely on the utilization of alternative (green) solvents, eliminating the amount of synthetic and/or petroleum-based chemicals, and reducing energy costs and waste generation to obtain highquality plant extracts [68]. Among green solvents, bioethanol is the most often used one due to its high biodegradability and low price [69]. Water, which is known to be the most natural solvent on the Earth, is effective only for the extraction of polar compounds [70]. Nowadays, various green extraction techniques including ultrasound-assisted, microwaveassisted, enzyme-assisted, pulsed electric field extraction, supercritical fluid extraction or pressurised liquid extraction have been explored [71]. Among them, supercritical fluid extraction (SFE), pressurised hot water extraction (PHWE), ultrasound-assisted extraction (UAE) or a combination of assisted extraction techniques are widely used. Recovered in a green way, phytochemicals from AP can be further used in many industries, including, e.g., construction and building, as anticorrosion agents, wood protectors, preservatives, antioxidants and biopolymers.

The SFE is a relatively new extraction method that is performed in the presence of supercritical fluids (most often liquid carbon dioxide—CO2). The process is carried out in specialised high-pressure equipment where CO2 is compressed under high pressure. The simultaneous increase in temperature and the pressure of the system leads CO2 to reach a supercritical state; in that phase, CO2 behaves similarly to both a liquid and a gas and mass transfer limitations that slow down the liquid transport are overcome [72]. After extraction, there is no need for additional purification of the extract or CO2 removal, because gas expands and evaporates at normal temperature and pressure (25 ◦C and 1 atm). Moreover, the SFE technique does not require air access, which protects the substances contained in the extracted material against oxidation. Another advantage is that the carbon dioxide (extraction solvent) used is non-toxic, odourless, colourless, non-flammable, cheap and reaches a supercritical state at relatively low temperatures (above 31 ◦C) (Figure 3). Due to such a relatively low temperature, it is possible to obtain plant extracts without losing their properties (degradation of active compounds), which often takes place at higher temperatures [73,74]. However, the SFE method has some disadvantages, with the main ones being the high cost of the aperture and the limited range of substances that can be extracted with CO2 as the sole solvent due to its non-polar nature [75]. The application of the SFE technique for the extraction of antioxidants from apple pomace was investigated in the work of Giovanna et al. [76]. In that study, fresh, freeze-dried and oven-dried apple pomace was treated with (a) subcritical CO2 and (b) subcritical CO2 with ethanol (5%) as a co-solvent, at pressures of 20 and 30 MPa and temperatures of 45 ◦C and 55 ◦C. For the comparison, a conventional extraction technology, i.e., Soxhlet extraction with ethanol and boiling water maceration, was also performed. The results of their research showed that the freeze-dried apple pomace extract obtained using the SFE method (at 55 ◦C, 30 MPa) with the use of ethanol (5%) as a co-solvent had the highest total phenolic antioxidant content (TPC) measured by the Folin–Ciocalteu method (8.87 ± 0.10 mg GAE (gallic acid equivalent)/g of extract) (Table 2). In addition, this extract was also found to have the highest antioxidant activity measured by DPPH• assay (5.99 ± 0.11 mg TEA (Trolox equivalent antioxidant)/g of extract). For the extract obtained from SFE, carried out on freeze-dried apple pomace at the same conditions (55 ◦C, 30 MPa), but only with the subcritical CO2 as a solvent, the TPC was equal to 6.41 ± 0.19 mg GAE/g of extract. Much lower TPC was obtained for the freeze-dried extract obtained using the Soxhlet and boiling water maceration methods, with 4.13 ± 0.90 and 2.37 ± 0.01 mg GAE/g of extract obtained, respectively [76]. The optimal conditions for the SFE apple pomace extraction process were also investigated in the work of De la Peña Armada et al. [77]. The results of their research indicated that the optimal conditions for the SFE process could be established at a temperature of 46 ◦C and a pressure of 425 bar. Under these conditions, the obtained extracts were characterised by the highest concentration of triterpenic acids (betulinic acid, oleanolic acid, ursolic acid, uvaol, erythrodiol and lupeol) and the highest antioxidant activity tested by means of the ORAC (Oxygen Radical Absorbance Capacity) assay (609.17 ± 96.11 μmol TE (Trolox equivalent)/g extract). For

comparison, the extract obtained using the Soxhlet method showed a lower antioxidant capacity (ORAC: 565.95 ± 60.66 μmol TE/g extract) [77].

**Figure 3.** The phase diagram for CO2.

PHWE is a similar technique to SFE, but in this case, water is used as a solvent. By increasing the temperature and pressure, water obtains similar properties to ethanol; this causes an increase in the solubility of many medium-polar compounds in water and ensures the extraction efficiency. The appropriate temperature of the extractant (water) should be above the atmospheric boiling point (100 ◦C, 0.1 MPa), but below its critical point (374 ◦C, 22.1 MPa) [78]. The main advantages of this method are its low cost and environmental friendliness; PHWE limits the use of organic solvents. Besides, the process water can be disposed of without causing any major environmental problems [79]. The applicability of the PHWE technique for the extraction of antioxidants from apple pomace was investigated in the work of Plaza et al. [79]. Using a response surface methodology (RSM), scientists optimised the PHWE parameters by maximising the yield of phenolic antioxidants from AP while minimising the possible formation of undesirable substances (e.g., melanoidins—the final Maillard reaction products). They reported that the highest amount of phenolic compounds (1.8 μmol/g dry AP) was obtained at a temperature of 170 ◦C and 3 min of extraction time [80].

The UAE generates high-frequency pulses that increase the mass transfer of the extracted biocompounds to the used solvent. This is due to the presence of cavitation bubbles created by ultrasonic waves passing through the solvent. The rupture of cavitation bubbles on the analyte surface causes damage at the impact site and increases the rate of mass transfer of the extracted material to the solvent. UAE can be carried out using two types of device: ultrasonic (US) bath or probe-generating ultrasound (Figure 4). Both of them are equipped with one (US probe) or more (US bath) ultrasound generators called transducers. There is also a temperature control in the ultrasonic bath. Moreover, unlike extraction with a probe, several samples can be extracted simultaneously in an ultrasonic bath. Ultrasonic baths usually operate at frequencies from 37 to 45 kHz, while the ultrasound probes operate at a lower frequency of ca. 20 kHz. Lower frequencies lead to the formation of larger cavitation bubbles. The main disadvantage of this method is the possibility of partial degradation of the analyte compounds. In addition, after UAE, the obtained extract must be filtered to separate it from the extraction residues that sometimes require significant amounts of solvents and can lead to oxygen degradation of the extract. The UAE technique is often combined with other extraction methods, e.g., the Sono–Soxhlet approach involves the combination of UAE with Soxhlet extraction; other approaches

include UAE being combined with microwave-assisted extraction, and the combination of UAE and SFE [81,82]. There are several reports in the literature from recent years on the use of the UAE technique to recover active substances from apple pomace [83–86]. For example, in the work of Pollini et al. [86], the effect of the solvent on the TPC of apple pomace extract was investigated. In their research, the extract obtained through UAE, using the mixture of ethanol and water (50:50, vv) as a solvent, had the highest TPC value (1062.9 ± 59.80 μg GAE/g of fresh AP) compared to other solvents used (ethanol:water, 70:30 and 30:70, vv) [86]. Malinowska et al. [84], compared the effect of the solvent used (water and ethanol) and the source of AP on the efficiency of the UAE process. The results showed that AP water extract (from conventional crops) had a two-times-lower TPC value than the AP ethanolic extract and the AP water extract (from ecological crops) [84]. The temperature of the UAE process, time of extraction and ultrasound power (e.g., power intensity) also plays an important role. Overly high temperatures (e.g., much higher than room temperature), power intensities (a wide range of ultrasonic frequencies of 20–100 Hz are applied in the literature) and expanded extraction times (time longer than 30 min) can lead to the deconstruction of valuable compounds [87,88]. The influence of the mentioned extraction conditions (temperature in the range of 10 ◦C to 40 ◦C, and ultrasound intensity in the range of 0.764 W/cm2 to 0.335 W/cm2) was studied in the work of Pingret et al. [88]. The results of their research indicate that the optimal conditions for the water-extraction of phenolic antioxidants from apple pomace using the UAE method are 40 ◦C, 40 min and 0.764 W/cm2 (Table 2) [89].

**Figure 4.** Ultrasonically assisted solvent extraction: (**A**) in an ultrasonic bath and (**B**) with a probegenerating ultrasound.



Regarding the evaluation of the above-mentioned extraction processes for the recovery of active compounds from apple waste, the efficiency and costs have to be evaluated. In the case of the SFE and PHWE techniques, the costs of extractions are relatively expensive, due to the high cost of specialistic equipment (Figure 5). However, the advantage of these techniques is that they use environmentally friendly solvents, such as CO2 and H2O. On the other hand, the cost of the ultrasonic bath or probe-generating ultrasound used in the UAE technique is relatively low, but this method requires larger amounts of solvents than in SFE and PHWE. However, taking into account the production costs of some synthetic compounds, obtaining compounds from the extracts may be a cheaper solution. Natural compounds are also more desirable than synthetic ones [90,91].


**Figure 5.** Comparison of the SFE, PHWE and UAE extraction techniques.

3.1.1. Green Corrosion Inhibitors Active Compounds

Metals and their alloys have been widely used in the building and construction industries as base materials for various equipment (e.g., pipes and water tanks). However, factors such as moisture, salts, acidic and alkaline solutions, gases, etc. can lead to numerous damages to the material, known as the corrosion process [92]. The corrosion products (i.e., rust) significantly affect the construction elements and, hence, generate serious impacts on human safety and the overall economy of the construction process. Various methods are used to protect metal surfaces from corrosion. One of them is the use of substances that inhibit the corrosion process, i.e., corrosion inhibitors [93–95]. Recently, most of the commercially used corrosion inhibitors have been synthetic inorganic molecules containing, e.g., copper, zinc, arsenic, nickel or arsenic salts [96]. However, the use of most of them (e.g., toxic phosphate or chromates) raises concerns regarding the safety of living organisms and the natural environment (e.g., surface water) [94,97,98].

Nowadays, great emphasis in the construction industry is given to the use of natural, non-toxic, readily available and biodegradable products; therefore, new sources of substances that will be effective and inexpensive are considered [94]. Active molecules from AP extracts (phenolic compounds with antioxidant properties) are tested as potential corrosion inhibitors, due to their electron-donating properties and active sites [94]. Their anti-corrosion mechanism of action consists in the creation/adsorption of the protective film layer on the metal surface by blocking active sites on the metal surface (in response to the Langmuir adsorption isotherm). The inhibitory effectiveness is associated with the chemical composition of AP extracts and chemical structure of active phenolic compounds—the presence of heteroatoms, such as sulfur (S), oxygen (O), phosphorus (P) and nitrogen (N) in their polar functional groups (e.g., -OH, -COOH, -OCH3 -CN and -NO2). These heteroatoms favour the adsorption processes via an interaction between the metal surface and the π-electrons clouds in the conjugated system, or by the formation of the bonds with the non-bonding electron pairs of the heteroatoms [97,99–101]. Moreover, the corrosion inhibition efficiency (IE) of plant extracts is related to the electron density sites of the inhibitor molecules [96,97].

There are several reports in the literature on the use of AP extracts and individual active substances, that can be isolated from AP, as green anti-corrosion agents (Table 3) [97,100,102–106]. In the work of Vera et al. [100], the phenolic antioxidants occurring in the Fuji apple peel extract turned out to be highly effective (IE = 89.88% at an inhibitor/extract concentration of 1000 ppm) anticorrosive agents of carbon steel. The major components of the AP extract were 3,5,2 -trihydroxy-7,8,4 -trimethoxyflavone 5-glucosyl-(1- >2)-galactoside (44.33%), 5-methoxy-6,6-dimethyl-3 ,4 -methylenedioxypyrano (2,3,7,8) flavone (38.49%), quercetin-5-glucoside (3.27%) and quercetin-3-α-L-arabinopyranoside (3.15%). Other phenolic antioxidants, such as caffeic acid, chlorogenic acid, rutin, kaempherol and isoquercetin, were detected in lower concentrations [100]. In the study by Nazari et al. [97], an AP-based green inhibitor was found to exhibit high efficiency in reducing the carbon steel corrosion in 3.5% NaCl brine. 1-Linoleoyl-sn-glycero-3-phosphocholine (C26H50NO7P), containing N, P and O heteroatoms, was found to be a major constituent of AP extract (19.3 wt.%). The inhibition action mechanism of AP extract molecules was based on blocking the anode active sites on the steel surface and transforming Fe3O4 into a more corrosion-resistant Fe2O3. The highest IE (98%) was obtained on the seventh day of the measurement at the highest concentration of AP extract used (3%). In addition, the above-mentioned AP-derived inhibitor was synthesised without generating any waste [97]. In another study, pectin, which is abundant in AP, was used as an anti-corrosion coating for carbon steel. The protective effect (PE) increased with the increasing pectin concentration. For the lowest applied pectin concentration (100 ppm), PE = 83.62%, while for the highest (500 ppm), PE was equal 89.31% [102]. The influence of pectin on corrosion of metals in hydrochloric acid solution was also studied in the work of Fiori-Bimbi et al. [103]. In their work, the maximum value of pectin's mild steel corrosion inhibition efficiency was equal to 94.2% (T = 318 K, inhibitor concertation = 2 g L−1) [102]. Pectin may also be a promising anti-corrosion agent for carbon steel in a neutral aqueous solution. Prabakaran et al. [104] developed an inhibitor composed of pectin (250 ppm), propyl phosphonic acid (50 ppm) and Zn(II) ions (20 ppm). The corrosion IE value for this mixture was 94%, indicating an excellent synergistic effect of components [104]. Procyanidin B2 and quercetin are major AP components. Procyanidin B2 was reported to be an effective corrosion inhibitor of carbon steel in 1 M HCl. The corrosion IE reached 94.21% at 30 ◦C (500 mg/L) after 24 h [105]. However, 800 ppm of quercetin was found to reduce 92% of mild steel corrosion in 1 M HCl after 1 h [106].


**Table 3.** Selected green corrosion inhibitors from AP.
