agriculture Article Utilization of Sewage Sludge-Derived Pyrogenic Material as a Promising Soil Amendment Vladimír Frišták 1, * , Martin Pipíška 1 , Dominika Koperová 1 , Reinhard Jagerhofer 2,3 , Gerhard Soja 2,3 and Stephen M. Bell 4 1 Department of Chemistry, Trnava University in Trnava, 91843 Trnava, Slovakia; martin.pipiska@truni.sk (M.P.); dominika.koperova@tvu.sk (D.K.) 2 Energy Department, Austrian Institute of Technology GmbH, 3430 Tulln, Austria; reinhard@stin.at (R.J.); gerhard.soja@ait.ac.at (G.S.) 3 Institute for Chemical and Energy Engineering, University of Natural Resources and Life Sciences Vienna, 1190 Vienna, Austria 4 Institute of Environmental Science and Technology (ICTA-UAB), Universitat Autónoma de Barcelona, 08193 Barcelona, Spain; stephen.bell@uab.cat * Correspondence: vladimir.fristak@truni.sk or fristak.vladimir.jr@gmail.com; Tel.: +421-33-592-1459 Abstract: The use of advanced treatment technologies such as pyrolysis and carbonization of bio- waste materials has the promise to develop coupled solutions for organic C and P problems in the future of food processing. Sewage sludge-derived pyrogenic materials that are applied to the soil system contain notable levels of P (1–20%). However, only a small portion of total phosphorus is available for plants. Therefore, our study assessed the pretreatment of sewage sludge with Na2 CO3 and subsequent pyrolysis at 400 ◦ C and 500 ◦ C for the production of MPCM400 and MPCM500 as  potential alternatives for inorganic P fertilizers. Non-pretreated sewage sludge-derived pyrogenic  carbonaceous materials (PCM400 and PCM500) were produced as controls. The derived materials Citation: Frišták, V.; Pipíška, M.; obtained by slow pyrolysis at a residence time of 120 min and pyrolysis temperatures of 400 ◦ C and Koperová, D.; Jagerhofer, R.; Soja, G.; 500 ◦ C, were analyzed by determination of electrical conductivity (EC), pH, elemental analysis of Bell, S.M. Utilization of Sewage Sludge-Derived Pyrogenic Material total C, H, N, scanning electron microscopy (SEM) and total concentrations of P, Cd, Fe, Pb and Zn. as a Promising Soil Amendment. PCM400, PCM500, MPCM400 and MPCM500 were characterized by analysis of total 16 US-EPA Agriculture 2022, 12, 360. https:// (U.S. Environmental Protection Agency) PAHs by a toluene extraction procedure. Additionally, doi.org/10.3390/agriculture12030360 PCM400, PCM500, MPCM400 and MPCM500 were tested by a germination test with cress seeds (Lepidium sativum L.) and a short-term rye seedling test. Total C concentrations were raised in the order: MPCM500 < MPCM400 < PCM500 < PCM500. Concentration of Σ16 US-EPA PAHs was almost Academic Editor: Vasileios Antoniadis five times higher at PCM500 (21.87 mg/kg) compared to PCM400 (4.38 mg/kg) and three times higher at MPCM500 (23.12 mg/kg) compared to MPCM400 (7.55 mg/kg) with a dominant role of Received: 3 February 2022 two and three-ring aromatic structures. Total P-concentrations in rye biomass increased in the order Accepted: 1 March 2022 for the controls: (2.43 ± 0.95 mg/g) < PCM400 (3.57 ± 0.27 mg/g) < PCM500 (4.04 ± 0.24 mg/g) < Published: 3 March 2022 MPCM400 (5.23 ± 0.09 mg/g) < MPCM500 (5.57 ± 0.70 mg/g) < IF (7.53 ± 2.65 mg/g). Obtained Publisher’s Note: MDPI stays neutral results showed that pyrolysis materials produced from sewage sludge represent a potential alternative with regard to jurisdictional claims in of conventional P inorganic fertilizers and organic C suppliers. published maps and institutional affil- iations. Keywords: sewage sludge; pyrolysis; phosphorus; Na2 CO3 ; feedstock pretreatment Copyright: © 2022 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. This article is an open access article Intensification of industrial activities and subsequent anthropogenic emissions of distributed under the terms and greenhouse gases will bring increases in global mean temperatures and climate changes. A conditions of the Creative Commons full understanding of how soil organic matter will respond to these changes in the climate Attribution (CC BY) license (https:// system is crucial. This is because organic matter is a multi-functional component of soil that creativecommons.org/licenses/by/ provides vital ecosystem services, including support of primary production [1]. Further- 4.0/). more, soil organic matter provides one of the Earth’s richest C reservoirs, holding more than Agriculture 2022, 12, 360. https://doi.org/10.3390/agriculture12030360 https://www.mdpi.com/journal/agriculture Agriculture 2022, 12, 360 2 of 13 three-fold the amount of C that is currently in the atmosphere as CO2 and has implications for the mitigation or exacerbation of climate change. Recently the international initiative “4 per 1000” emphasised the importance in not only deceleration of climate changes but mainly carbon stabilization in soil by promoting of agro-environmental techniques that increase the amount of organic matter in soils and meet the 4% per year target [2]. Farmland around the world is threatened by the loss of organic carbon due to intensive tillage and inappropriate cropping practices. Approximately 45% of all mineral soils in Europe have poor organic C content (0–2%) [3]. This enhances their vulnerability to biodiversity loss, soil erosion, compaction, reduced fertility and low water retention capacity. The relevance of soil organic C stems from its function as the primary component of soil organic matter, which has a stable moiety known as humus. Furthermore, sustainable soil fertility is not feasible without an adequate flow of nutrients to vegetation. Among the principal elements required for vegetation growth, P is one of the most important macronutrients. Inorganic P is extracted from rock phosphate and constitutes a limited resource that is believed to become exhausted during the current or next century and that is characterized by a very patchy geographical spreading [4]. The quality of the remaining rock phosphate resources is continuously decreasing because the concentrations of undesirable companion elements, such as cadmium and uranium, are increasing. Therefore, recycling of phosphorus-rich materials, such as sewage sludge, is a necessary strategy to alleviate the pressures on the residual resources and to improve the efficiency of current phosphorus input. Although the reduction of phosphorus use, the improvement of bioavailability and support for mycorrhizal activity in the soil are necessary, supplementary strategies to conserve the remaining phosphate resource, innovative recycling strategies for phosphorus recovery from wastes will be a main pillar of the future phosphorus economy. Existing phosphorus precipitation processes in wastewater treatment plants require significant amounts of input chemicals, such as MgCl2 , FeCl3 , FeSO4 or Al-salts, but the use of the resulting precipitates or sewage sludge as direct fertilizers is tightly regulated and frequently not legal. However, large amounts of sewage sludge are burnt or deposited in landfills where phosphates are withdrawn from any further use. Besides, residual phosphate concentrations in wastewater treatment plant effluents contribute to the eutrophication of surface waters. The use of advanced treatment technologies such as pyrolysis and carbonization of bio-waste materials has the capacity to provide coupled solutions for organic C and P problems in the future of food processing. Pyrolysis refers to a carbonization technique in which biomass or other organic materials are thermochemically transformed into essentially stable, recalcitrant organic C compounds, with the process requiring an oxygen-poor or oxygen-free inert atmosphere, pyrolysis temperatures of 300–1000 ◦ C and residence time ranging from a few seconds to a few hours [5]. Pyrolysis materials produced have increased (two to three times) content of total C and other elements (P, Zn and Ca) compared to the input feedstock. Sewage sludge composition is highly heterogenous. Sludge is mainly derived in the sewage network of cities by a combination of sewage, rainwater with sand particles, industrial and municipal wastewaters. Therefore, the characteristics of the sludge itself are a combination of the properties of these components. In an applied material such as sewage sludge potential ecotoxicity based on heavy metal (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn) mobility and organic pollutants (polycyclic aromatic hydrocarbons, pharmaceuticals, per- and polyfluoroalkyl substances), availability must be closely monitored [6]. Therefore, the effect of thermochemical processes on phosphorus bioavailability, carbon content and potential hazardous substances concentrations is crucial and will be mediated by pyrolysis optimization, the pretreatment of feedstock or the preparation of combined feedstock. Producing a fertilizer product that considers both the nutrients’ demand of crops and soil organic carbon gap will be a highly effective soil amendment with characteristics that cannot readily be imitated by traditional organic or mineral fertilizers. The deployment of such innovative materials demands feedstock-specific pyrolysis steps that will alter the labile C of wastes and residues into stable organic C compounds with mean soil residence times ranging from decades to centuries [7]. On the one hand, these amendments allow Agriculture 2022, 12, 360 3 of 13 for a long-term carbon sequestration in soil as a component of climate change mitigation strategies; on the other hand, positive side effects on physicochemical soil characteristics will be entrained. Thus, the usage of carbonization products as soil amendments or fertilizers is highly efficient in terms of resources (multi-purpose use of organic residues or wastes) and energy (exothermic carbonization process for production). Sewage sludge-derived pyrogenic materials that are applied to the soil system contain notable levels of P (1–20%). However only a small portion of total phosphorus is available for plants [8]. Transformation of apatite phosphorus (mainly Ca/Mg bound P) to non- apatite forms of inorganic phosphorus during thermochemical treatment of sewage sludge has been reported by several authors [9–11]. The bioavailability of this form of P in sludge-derived pyrogenic materials has shown lower intensity compared to feedstock materials and variability based on sludge type. This kind of slow phosphorus-releasing fertilizer represents an important step in P-loss mitigation and subsequent environmental problems. On the other hand, better availability of P in pyrogenic materials is often a limiting factor for pyrolysis application in sewage sludge treatment [12]. Therefore, suitable sludge modification or pretreatment and co-pyrolysis represent options on how to improve nutrient bioavailability. There have been several studies on sewage sludge pretreatment with CaO [13], MgCl2 [14], NaCl [14], CaCl2 [14], KCl [15] and K2 SO4 [15], but only limited information on Na2 CO3 effects. Based on these scenarios, the main research objective of this study was to investigate the effect of Na2 CO3 as an additive agent on P solubility and bioavailability in sewage sludge-derived pyrogenic carbonaceous material. We attempted to characterize modified and unmodified pyrogenic carbonaceous materials produced by slow pyrolysis of munic- ipal sewage sludge at 400 ◦ C and 500 ◦ C as potential soil additives for agriculture (i.e., industrial crops) and green infrastructure (i.e., green roofs, green walls) 2. Materials and Methods 2.1. Simulation Experiment For the assessment of the Na2 CO3 pretreatment effect on P bioavailability, a simulation experiment was conducted based on the work by Wzorek et al. [16]. Briefly, the iron and aluminium phosphates, as main forms of P in municipal sewage sludge (FePO4 ·2H2 O and Al(H2 PO4 )3 , Sigma-Aldrich, Taufkirchen, Germany), were mixed with Na2 CO3 (Sigma- Aldrich, Germany) in molar ratios of 1:1 and 1:2. After homogenization (2 h, lab-shaker), 10 g of each mixture was weighed into a container and pyrolyzed at 400 ◦ C and 500 ◦ C by a modified lab-scale pyrolysis reactor for 120 min under N2 atmosphere (heating rate 25–30 ◦ C/min, N2 gas flow 2 L/min). As a control, the pure forms of chemicals were pyrolyzed under the same conditions. For P availability evaluation, prepared products were homogenized and the obtained representative sample was extracted by deionized water (0.04 µS/cm) in ratio 1/20 (w/w) on a laboratory shaker at 225 rpm, 120 min and 22 ◦ C. Obtained extracts were filtered using syringe filters (pore size 0.45 µm) and analyzed for total P content by traditional colorimetric vanadate/molybdate method (yellow method) at λ = 430 nm by spectrophotometry (UV-VIS Shimadzu, 160A, Shimadzu, Kyoto, Japan) [17]. The filters were tested for P retention before each application. 2.2. Sewage Sludge Pretreatment and Pyrolysis Treatment Sewage sludge (BSS) was obtained from municipal WWTP in Seibersdorf (Austria). After, the collection of BSS was dewatered and oven-dried at 60 ◦ C for 96 h. For the Na2 CO3 pretreatment study, the one part of sludge was modified with Na2 CO3 (wsludge /wNa2CO3 = 100/1) and the suspension was oven-dried at 60 ◦ C for 96 h. Prepared pretreated and non-pretreated feedstock BSS was pyrolyzed at 400 ◦ C and 500 ◦ C by a mod- ified lab-scale pyrolysis unit for 120 min under N2 atmosphere (heating rate 25–30 ◦ C/min, N2 gas flow 2 L/min, thermal heating). The produced Na2 CO3 -modified pyrogenic car- bonaceous material (MPCM400 and MPCM500) and unmodified pyrogenic carbonaceous Agriculture 2022, 12, 360 4 of 13 material (PCM400 and PCM500) were ground and sieved to obtain uniform fractions of 0.5–1 mm for characterization and cultivation experiments. 2.3. Physicochemical Characterization Prior to each analysis, PCM400, PCM500, MPCM400 and MPCM500 were thoroughly mixed in a laboratory stirrer. Characterization analyses were performed for each studied material in three replicates. The pH values of the samples were determined by pH meter (inoLab pH level 2P, Weilheim, Germany) after mixing the samples with a solution of 0.01 mol/L CaCl2 (ratio 1:10 m/v) for 1 h and 1 h of equilibration. The electrical conduc- tivities (EC) of the samples were determined by conductivity meter (inoLab pH level 2P, Weilheim, Germany) in deionized water (1:10 m/v) after 24 h of stirring. The total C, H and N contents of the samples were quantified by an elemental analyzer (CHNS-O EA 1108, Carlo Erba Instruments, Milan, Italy). The morphological structures of PCM and MPCM were recorded by scanning electron microscopy (SEM) using JEOL JSM7500F scanning electron microscope (Tokyo, Japan). The method described in our previous work [18] was used to determine ash content. Total Cd, Fe, P, Pb and Zn concentrations were determined in PCM and MPCM samples by ICP-MS (Perkin Elmer, Elan DRCe 9000, Shelton, CT, USA) and ET-AAS (PerkinElmer, AA 400, Shelton, CT, USA) methods after previous digestion of the samples using the modified HNO3 method [19]. The limits of detection (LOD) for selected analytes were defined by 3σ criterion. The total content of 16 PAHs classified by the U.S. Federal Environmental Federal Environmental Protection Agency (US-EPA) were determined in PCM400, PCM500, MPCM400 and MPCM500 by HPLC (Agilent 1100 with column GRACE/Vydac C18 polymeric reversed-phase, 4.6 × 250 mm, 5 µm; and GRACE/Vydac C18 guard column, 4.6 × 7.5 mm, 5 µm, Santa Clara, CA, USA) after previous Soxhlet extraction by toluene as an extraction agent (solid/liquid: 1/15, 36 h, 110 ◦ C). Detection was conducted using a diode array HPLC detector DAD (λ = 310 nm) and fluorescence detector FLD (Ex = 260 nm, Em = 350 nm, 420 nm, 440 nm and 500 nm). The extraction procedure and analytical quantification of 16 PAHs were carried out as reported by [20,21]. 2.4. Germination Test The germination test with cress seeds (Lepidium sativum L.) was performed according to the scheme developed by Buss and Mašek [22]. The advantage of this test variant is that the germination of cress can be investigated in three different ways in one pot. Briefly, the experimental pot was divided into three levels in order to determine the different influences of contaminants in the three different forms (gaseous, dissolved and attached to PCM400, PCM500, MPCM400 and MPCM500) on cress seed germination. Therefore, three levels of seed contact system were prepared. On the first level, the effect of volatile compounds: PCM400, PCM500, MPCM400 and MPCM500 only was tested. The second level was focused on the combined effect of volatile and leachable (dissolved) compounds (in water): PCM400, PCM500, MPCM400 and MPCM500. The last level was targeted on volatile compounds, leachable compounds and direct contact with PCM400, PCM500, MPCM400 and MPCM500. Crushed samples of PCM and MPCM were mixed with quartz sand to ensure total P content equal to 40 mg P/40 mg sand in each aluminium container. There were 30 cress seeds per level, but only the third level had direct contact with the coals. The whole pot was covered with cling foil for 96 h in order to promote germination. Figure 1 shows the experimental design with the three levels. After 96 h, the number of geminated seeds was counted for each level separately. A germination test was conducted in triplicates for each tested material and control samples (non-amended sand). For the statistical evaluation, the normality and Tukey’s post-hoc tests were used (Statistica 8, StatSoft 2007, Tulsa, OK, USA). Agriculture 2022, 12, x FOR PEER REVIEW 5 statistical evaluation, the normality and Tukey’s post-hoc tests were used (Statisti Agriculture 2022, 12, 360 StatSoft 2007, Tulsa, OK, USA). 5 of 13 Figure 1. Experimental design for the Figure 1. Experimental germination design test for assessing for the germination test forthe effect ofthe assessing direct contact, effect of direct contac leased released volatile volatile and and dissolved dissolved in compounds compounds water. in water. 2.5. Neubauer2.5. TestNeubauer Test A Neubauer A test [23] as atest Neubauer standard [23] asrye seedling rye a standard test seedling to determine plant-available test to determine plant-avai phosphorus fraction from PCM and MPCM was used. The seedling phosphorus fraction from PCM and MPCM was used. The seedling test lasted 17test days lasted 17 and was carriedand out was in greenhouse carried out in conditions greenhouse (12 h light/12 conditions (12 hh dark, constant light/12 h dark, humidity, constant humidit 25 ◦ C). Rye seeds (Secale cereale L.) were treated in a 70% ethanol solution for about 5 s °C). Rye seeds (Secale cereale L.) were treated in a 70% ethanol solution for about 5 s and then washed then thoroughly washedwith deionized thoroughly water. with Experimental deionized pots were fulfilled water. Experimental withfulfilled pots were fine with silica sand (washed 3 times with solution 0.01 mol/L HCl and deionized water (electrical silica sand (washed 3 times with solution 0.01 mol/L HCl and deionized water (elect conductivity EC < 0.04 µS/cm)) and PCM400, PCM500, MPCM400 and MPCM500, and a conductivity EC < 0.04 µS/cm)) and PCM400, PCM500, MPCM400 and MPCM500, a commercial inorganic P fertilizer (IF, 42% P2 O5 , Glatz, Vienna, Austria) in a ratio to achieve commercial inorganic P fertilizer (IF, 42% P2O5, Glatz, Vienna, Austria) in a ratio to ach per pot total amount of 40 mg P/400 g sand. Non-amended sand was used as a control. In per pot total amount of 40 mg P/400 g sand. Non-amended sand was used as a contro each experimental pot, exactly 100 rye seeds in 0.5–1 cm department were positioned. To each experimental pot, exactly 100 rye seeds in 0.5–1 cm department were positioned accelerate germination and growth, the pots were covered with a plastic film during the accelerate germination and growth, the pots were covered with a plastic film during first three days. Pots were irrigated with Hoagland’s solution without P, including all other first three days. Pots were irrigated with Hoagland’s solution without P, includin nutrients, to maintain water holding capacity at a constant value over the experimental other nutrients, to maintain water holding capacity at a constant value over the ex period. The experiment was arranged in 4 blocks to keep the scattering of results due to mental period. The experiment was arranged in 4 blocks to keep the scattering of re external influences (light, heat). The location of replicate samples within each experimental due to external influences (light, heat). The location of replicate samples within each block was randomized. The harvesting of leaves and roots was carried out after 17 days of perimental cultivation. The block was plant materials wererandomized. The harvesting washed in deionized of leaves from water, separated and sand roots and was carried PCM and MPCM particles, and oven-dried for 4 days at 60 ◦ C. Subsequently, dried leaves water, s after 17 days of cultivation. The plant materials were washed in deionized and roots were rated fromand ground sand and PCM digested withand HNOMPCM particles, and oven-dried for 4 days at 60 °C. 3 /HClO4 (v/v 5:1) for P, Cd, Fe, Pb and Zn sequently, dried leaves and roots concentration by ICP-MS (Perkin Elmer, Elan DRCe 9000, were ground and digested Shelton, CT, USA) with andHNO 3/HClO4 (v/v ET-AAS (PerkinElmer, AA 400, Shelton, CT, USA). For the statistical evaluation, the normality and9000, She for P, Cd, Fe, Pb and Zn concentration by ICP-MS (Perkin Elmer, Elan DRCe CT, USA) Tukey’s post-hoc and ET-AAS tests were (PerkinElmer, used (Statistica AA2007, 8, StatSoft 400, Tulsa, Shelton,OK,CT, USA). For the statistical e USA). uation, the normality and Tukey’s post-hoc tests were used (Statistica 8, StatSoft 2 3. Results Tulsa, OK, USA). 3.1. Chemicals Pyrolysis 3. Results A high number of wastewater treatment plants in Europe still use precipitation with 3.1. Fe2 (SO4 )3 -FeCl Chemicals Pyrolysis 3 (sometimes, but rarely, FeSO4 ), Al2 (SO4 )3 -14H2 O and Ca as lime milk to remove phosphorus from wastewater [16,24]. After A high number of wastewater this type treatment of in plants chemical Europe precipitation, still use precipitation phosphorus isFepresent in sludge in the form of poorly soluble and weak 2(SO4)3-FeCl3 (sometimes, but rarely, FeSO4), Al2(SO4)3-14H bioavailable 2O and Ca com- as lime mi pounds. Subsequently, when sewage sludge is used as a feedstock for pyrolysis treatment, a sintering process occurs, which completes the chemical stability and relatively low bioavail- ability of phosphorus. The use of multiple additions and the use of copyrolysis of materials has proven to be an effective option in several cases [13–15]. Our experiment aimed at Agriculture 2022, 12, 360 6 of 13 performing a simulation of an Na2 CO3 effect on P-extractability from products obtained by the pyrolysis of soda ash with FePO4 or Al(H2 PO4 )3 and finding the optimal molar ratio of Na2 CO3 and FePO4 or Al(H2 PO4 )3 . The influence of Na2 CO3 on the extractable phosphorus content in the various tests is shown in Table 1. In all experiments, it was found that higher extractable phosphorus contents can be achieved at a pyrolysis temperature of 500 ◦ C. At combination of Na2 CO3 and FePO4 ·2H2 O with a molar ratio 1:1, at a pyrolysis temperature of 500 ◦ C, the highest phosphorus yield was 58.670 mg/g. The second highest extractable phosphorus concentration of 31.881 mg/g was achieved by a combination of Na2 CO3 and FePO4 ·2H2 O with a 2:1 molar ratio followed by a combination of Na2 CO3 and Al(H2 PO4 )3 with a 2:1 molar ratio and a concentration of 26.242 mg/g. The work by Wzorek et al. [16] showed that in thermochemical processes, phosphorus availability can be increased by adding Na2 CO3 to chemicals. The authors demonstrated that the addition of soda ash significantly increased the availability of phosphorus compounds in Na2 CO3 /FePO4 ·2H2 O with a molar ratio range from 1 to 5. The importance of the molar ratio in the mean crystalline phase type formation was emphasized. The shift to the hematite formation during thermochemical conversion and more mobile P form synthesis was discussed. Our results confirmed a positive effect of Na2 CO3 addition on P availability in both ratios compared to the control (pyrolyzed pure FePO4 or Al(H2 PO4 )3 ). Additionally, obtained results were used to predict the amount of Na2 CO3 required for sewage sludge modification based on the Fe content. Table 1. Chemical constitution and phosphorus extractability of tested samples. Sample Pyrolysis Temperature (◦ C) Molar Ratio Pextractable (mg/g) FePO4 ·2H2 O + Na2 CO3 400 1 mol/1 mol 9.078 ± 0.05 e FePO4 ·2H2 O + Na2 CO3 500 1 mol/1 mol 58.670 ± 0.81 a Al(H2 PO4 )3 + Na2 CO3 400 1 mol/1 mol 4.406 ± 0.15 g Al(H2 PO4 )3 + Na2 CO3 500 1 mol/1 mol 5.585 ± 0.15 fg FePO4 ·2H2 O 400 1 mol/1 mol 0.073 ± 0.01 i FePO4 ·2H2 O 500 1 mol/1 mol 0.167 ± 0.01 i Al(H2 PO4 )3 400 1 mol/1 mol 0.960 ± 0.03 h Al(H2 PO4 )3 500 1 mol/1 mol 50% for PCM400, >42% for PCM500, >88% for MPCM400 and >40% for MPCM500 of total Σ16 US-EPA PAHs. The effect of thermochemical conversion on raised total concentration of PAHs in PCM and MPCM was confirmed. Especially, pyrolysis at a higher temperature conditioned the formation of two and three-ring aromatic structures in sewage sludge. The main substrates for their formation can be dead biomass of microorganisms, free organic matter or hardly degradable polymers [26]. Bucheli et al. [27] showed pyrosynthesis as a crucial process of gaseous hydrocarbon radicals’ generation by organic matter crack- ing at a temperature < 500 ◦ C. Zielinska and Olesczuk [28] showed correlation between pyrolysis temperature and individual PAHs structure formation during the pyrolysis of sewage sludge. They confirmed an increased trend of low molecular weight compounds’ formation with increasing temperature. A similar effect of thermochemical conversion on PAHs concentration in sewage sludge-derived pyrogenic carbonaceous material has been found by several scientific works [29–31]. The effect of carbonates on PAHs concentration has been studied in detail by Kwon et al. [32]. The authors confirmed a positive effect of calcium carbonate on the reduction of PAHs concentrations in sewage sludge-derived pyrolysis products produced at a temperature > 650 ◦ C. The surface structure and morphology of PCM and MPCM obtained by scanning microscopy (Figure 2) showed amorphous particles with combined character. The original smooth surfaces of sewage sludge feedstock were transformed into rough and partly porous structures. The mineral character was more pronounced in the MPCM sample compared to the PCM. Similar structures were characterized in our previous study [33]. (PAH) (mg/kg) (mg/kg) (mg/kg) (mg/kg) Naphthalene 2.27 ± 0.16 9.40 ± 0.05 6.71 ± 0.31 9.29 ± 0.41 Acenaphtene ND ND ND ND Acenaphthylene 0.72 ± 0.03 4.41 ± 0.2 0.43 ± 0.02 5.81 ± 0.2 Agriculture 2022, 12, 360 Phenanthrene 0.37 ± 0.02 1.05 ± 0.05 0.14 ± 0.01 1.06 ± 0.47 8 of 13 Fluorene ND 2.59 ± 0.11 ND 1.85 ± 0.06 Anthracene 0.49 ± 0.01 2.08 ± 0.2 0.27 ± 0.01 2.06 ± 0.2 Benz[a]anthracene Table 3. Total NDconcentrations 0.52(mean ± 0.01± SD) of naphthalene, ND acenaphtene, 0.65 acenaphthy- ± 0.01 Chrysene lene, phenanthrene, ND fluorene, anthracene, 0.82 ± 0.01 benz[a]anthracene, ND chrysene, pyrene,1.14 ± 0.05 fluoran- Pyrene thene, benzo[b]fluoranthene, ND benzo[k]fluoranthene, 0.36 ± 0.02 benzo[a]pyrene, ND indeno[1,2,3-cd]pyrene, 0.33 ± 0.02 Fluoranthene benzo[ghi]perylene and dibenz[ah]anthracene 0.53 ± 0.02 0.25 ± 0.01in PCM400, PCM500,ND MPCM400 and0.37 MPCM500 ± 0.01 deter- mined by Soxhlet extraction with toluene (solid/liquid: 1/15, 36 h, 110 ◦ C). Benzo[b]fluoranthene ND 0.23 ± 0.01 ND 0.26 ± 0.01 Benzo[k]fluoranthene Polycyclic Aromatic Hydrocarbon (PAH) PCM400ND(mg/kg) PCM500 ND (mg/kg) MPCM400ND (mg/kg) ND(mg/kg) MPCM500 Benzo[a]pyrene Naphthalene ND 2.27 ± 0.16 0.16 ± 0.01 9.40 ± 0.05 ND 6.71 ± 0.31 0.30 9.29 ±± 0.41 0.01 Acenaphtene Indeno[1,2,3-cd]pyrene ND ND ND ND ND ND ND ND Acenaphthylene 0.72 ± 0.03 4.41 ± 0.2 0.43 ± 0.02 5.81 ± 0.2 Benzo[ghi]perylene Phenanthrene 0.37 ND ± 0.02 1.05ND ± 0.05 0.14ND ± 0.01 1.06ND ± 0.47 Fluorene ND 2.59 ± 0.11 ND 1.85 ± 0.06 Dibenz[ah]anthracene Anthracene ND 0.49 ± 0.01 ND 2.08 ± 0.2 ND 0.27 ± 0.01 ND 2.06 ± 0.2 Benz[a]anthracene ND 0.52 ± 0.01 ND 0.65 ± 0.01 ∑16 PAHs Chrysene 4.38 ND 21.87 0.82 ± 0.01 7.55 ND 23.12 1.14 ± 0.05 Pyrene ND = non-detected, ND 500 ◦ C. Pyrophosphates (P2 O7 4− ) can be found in biochars pro- duced mainly from plant residues by pyrolysis at >650 ◦ C [36]. The main P minerals present in sewage sludge-derived pyrogenic carbonaceous materials represent whitlockite ((Ca,Mg)3 (PO4 )2 , dehydrated struvite (NH4 MgPO4 ) nad hydroxyapatite (Ca5 (PO4 )3 (OH)). The stabilization mechanisms of P during pyrolysis have been shown to become impaired when adding Na2 CO3 to the feedstock [37]. The authors showed a positive effect of soda on P solubility in a canola cake biochar. Our results confirmed the hypothesis that the Na2 CO3 addition to sludge improves P bioavailability from produced pyrogenic materials. We assumed that during the pyrolysis process, Na2 CO3 as an agent digests organic forms of P and converts them to CaNaPO4 or Na3 PO4 that are better available for plants. The application of PCM400, PCM500, MPCM400 and MPCM500 as a soil amendment in the rye seedling test showed negligible changes in total concentrations of Pb and Zn in Agriculture 2022, 12, 360 11 of 13 plant tissues compared to the control or IF (Table 4). Obtained concentrations of Pb were under LOD (0.22 µg/L). Determination of Zn in plant digests confirmed concentrations of 0.020 ± 0.003 mg/kg for all amendments. The PCM400 and PCM500 caused a reduction in Cd uptake by rye. The Cd content in the plant biomass obtained from pots amended by MPCM400 and MPCM500 showed comparable values to the control unamended sample (0.027 ± 0.002 mg/kg). On the other hand, application of MPCM400 and MPCM500 led to a statistically significant increase in total Fe content in rye tissues. PCM400 and PCM500 did not affect the Fe uptake by rye. Similar results were obtained in our previous studies where pyrolysis materials based on sewage sludge caused no-significant increase in concentrations of heavy metals [6,20]. Table 4. Total concentration of Cd, Fe, Pb and Zn in rye tissues obtained from pots (mean ± SD) amended by PCM400, PCM500, MPCM400, MPCM500, inorganic fertilizer (IF) and non-amended soil (control). Cd (mg/kg) Fe (mg/kg) Pb (mg/kg) Zn (mg/kg) PCM400