Romanian Journal of Physics 65, 812 (2020)
AND POSSIBLE AGRICULTURAL CONSEQUENCES OF SEWAGE
SLUDGE FROM URBAN TREATMENT PLANTS
NARCIS MIHAI TANASE1,2, ION V. POPESCU3,4, CRISTIANA RADULESCU3,5*,
IOAN ALIN BUCURICA5*, IOANA DANIELA DULAMA5*, SOFIA TEODORESCU5,
RALUCA MARIA STIRBESCU5, GEORGIAN ALIN BARBOIU1,2 1 University of Bucharest, Doctoral School of Physics, 077125 Magurele, Romania
2 Water Company, 130055 Targoviste, Romania
3 Valahia University of Targoviste, Faculty of Sciences and Arts, 130004 Targoviste, Romania 4 Academy of Romanian Scientists, 050085 Bucharest, Romania
5 Valahia University of Targoviste, Institute of Multidisciplinary Research for Science and
Technology, 130004 Targoviste, Romania
*
Corresponding authors: radulescucristiana@yahoo.com; bucurica_alin@yahoo.com; dulama_id@yahoo.com.
Received June 23, 2020
Abstract. All water treatment plants (WWTPs) produce waste/residue known as sewage sludge (SS) during the purification of raw water. Typically, treatment procedures involve coagulation, flocculation, sedimentation, and filtration processes for removing colloidal as well as suspended solids from wastewater, then it follows chemical and biological processes. In this study, the sewage sludge resulted from urban WWTP is investigated for physicochemical characteristics. Withal, this article aims to determine the elemental composition of SS samples, in order to identify the major percentage of chemical components present in the sludge. Different analytical techniques such as Inductive Coupled Plasma – Mass Spectrometry (ICP–MS), Attenuated Total Reflectance – Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Raman spectrometry were used to obtain the first information about collected samples. Silica, alumina, ferric oxide and lime constitute the main constituents of analyzed samples. Some heavy metals are also found in the sewage sludge. It is required to explore a suitable option for developing sustainable sludge management strategies under stringent environmental norms. Based on the characteristics, the sewage sludge can be used in agriculture or in the manufacture of cement and cementitious materials, as a substitute for building materials, providing a safe disposal way. In this respect, one of the goals of this paper is to point whether sewage sludge from urban WWTP can be used in agriculture, according to Council Directive 86/278 / EEC Protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture. Key words: sewage sludge, treatment plant, heavy metals, ICP-MS, ATR-FTIR,
Raman spectrometry.
1. INTRODUCTION
in terms of heavy metals content by ICP-MS. The inorganic and organic compounds were investigated, using both FTIR and Raman spectroscopies. The content of obtained heavy metals was discussed, particularly in terms of Cd content, as an important factor in the use of SS for agricultural purposes.
2. MATERIALS AND METHODS
2.1. MATERIALS
Sewage sludge samples were collected from the wastewater treatment plant of Targoviste (area ca. 54 km2, 79610 inhabitants) located at 80 km SV by Bucharest
(Romania’s capital). This wastewater treatment plant can process about 33000 m3
of urban effluents daily, and it is connected to a biogas production line and an electricity generation station with an installed capacity of 6 kWh.
In November 2018, 6 sewage sludge samples from each unit of WWTP Targoviste (i.e., mechanic treatment unit – MT, biologic treatment unit – BT, chemical treatment unit – CT, and storage unit – SU) were collected. In 2019, between February 10th
and April 14th, 10 sewage sludge samples (i.e., S1-S10) were collected every 7 days
from the storage unit of WWTP Targoviste. 2.2. METHODS
2.2.1. Sample preparation
All samples were dried in an oven with forced airflow circulation at 40°C for 72 hours to determine the water content. After that, samples ground using LMWs vibratory disk mill (Testchem, Pszow, Poland) equipped with a stainless-steel disk. This procedure aimed to obtain a fine powder with a high level of homogeneity.
In the case of ATR-FTIR and Raman investigations, it was not required any supplementary sample preparation. For ICP-MS analysis, 200 mg samples were digested with aqua regia (i.e., a mixture of 3 mL nitric acid – HNO3 65%, Merck –
and 9 mL hydrochloric acid, HCl 37%, Merck) at 175°C [19, 27]. The mineralization process was performed using the TopWave microwave digestion system (Analytik Jena) according to the following table:
Table 1
The parameters for the mineralization process of the sludge samples
Parameter [measurement unit] Value
Temperature [°C] 175
Pressure [bar] 40
Power [%] 90
Ramp [min] 1
2.2.2. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)
The chemical composition of sewage sludge samples was determined by inductively coupled plasma-mass spectrometry (ICP-MS) using the iCAP Qc system (Thermo Fisher Scientific, Waltham – Massachusetts, USA). This instrument ensures the highest plasma robustness (e.g., due to flow nebulizers, cyclonic spray chambers and wide bore injectors) and the highest stability to sample injection (i.e., due to Peltier cooling system). Also, iCAP Qc is equipped with an ergonomically designed quadrupole and innovative RAPID (Right Angle Positive Ion Deflection) lens technology for separation of ions and neutrals. The quantitative analyses (using the standard mode – STD) were performed in triplicates and the results are expressed as mean value ± RSD. The QTegra software automatically corrects the well-known isobaric interferences. Before the analysis of the samples, the calibration curve procedure was performed using a stock standard solution (Merck). The calibration curves showed good linearity and the correlation coefficients (i.e., R2) ranged between 0.995 and
0.999. To verify the accuracy and traceability of the method, standard reference materials NIST SRM 2710a Montana Soil was used [21, 24, 29–33].
2.2.3. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR)
The chemical functional groups of inorganic compounds present in sewage sludge samples (i.e., powder samples) were identified by Fourier Transform Infrared spectroscopy using Vertex 80v spectrometer (Bruker, Ettlingen, Germany), equipped with Attenuated Total Reflectance (ATR) accessory. ATR-FTIR spectroscopy allows limited applications in quantitative research of inorganic groups of sediment, sludge, or other solid mineral samples because its penetration depth is of few microns. However, this technique is suitable for qualitative investigations [34]. The FTIR spectra were recorded in the range of 4000–400 cm–1, with 0.2 cm–1 spectral
resolution, 0.1% T accuracy and 32 scans/spectra.
2.2.4. Raman Spectroscopy
As a complementary technique for FTIR, Raman spectroscopy was used. The spectra were recorded using a portable Xantus-2TM Raman analyzer (Rigaku, Boston,
United States of America). It is equipped with two laser sources (i.e., 785 nm and 1064 nm) and two detectors (i.e., TE cooled CCD and TE cooled InGaAs). The scans were made in 2000–200 cm–1 spectral range with 15–18 cm–1 spectral resolution. For
the sewage sludge samples, the following parameters were used: 1064 nm excitation source, 30 mW laser power, 5000 ms integration time, and 20 scans/spectra.
3. RESULTS AND DISCUSSION
degree, and the treatment processes from urban WWTP. The physicochemical and biological treatments applied in WWTP lead to the complexation of a few heavy metals (76% of Cu, 59.1% of Fe and Mn, and 41.6% of Cr) [35] to the organic fraction of the sludge (mainly humic substances), while a part of organic pollutants (45–63%) partially are removed by biodegradation [36].
Fig. 1 – Water content of sewage sludge samples.
Starting from the fact that humic substances from sludge composition play a major role in controlling the behavior and mobility of pollution-derived heavy metals, must be taken into account the interaction between metals and humic material. Apart from this, the water from SS (Figure 1) has an important role in metal mobility and metal complexing process, marking different variations of metal concentration in sludge composition. Since there was a variety of water content in SS samples (Figure 1), with a maximum of 21.89% in S6 and a minimum of 17.62% in S5, it is explicable why the concentration of the metals in collected samples are different (Table 3 and Figure 3). Nevertheless, several ionic coordination compounds can occur, in which ligands provide an electron pair to bond selective to a metal atom (i.e., Li, Na, K, Rb, Mg, Ca, Sr, Ba). Basically, heavy metals have a lesser tendency to form ionic bonds and a strong tendency to form coordination bonds with humic ligands, which contain suitable atoms, particularly nitrogen and oxygen, and provide both electrons.
only for Pb and Cd (Table 2), while the CT and SU samples fall within the limit values allowed by Council Directive 86/278 / EEC.
Table 2
Heavy metals content in sewage sludge samples collected in November 2018 [mg/kg d.w.] comparative with limit values in accordance with Council Directive 86/278 / EEC [37]
Sample Ni Cu Zn Cd Pb MT 479.70 814.14 3465.44 96.07 1953.80 BT 227.61 390.78 2715.70 59.83 1245.50 CT 127.17 985.86 2742.10 37.69 921.20 SU 130.59 946.08 2441.32 29.79 928.90 RSD* [%] 3.08–7.72 1.85–6.08 2.87–7.69 2.75–6.08 3.02–8.01 86/278/CEE 300–400 1000–1750 2500–4000 20–40 750–1200
Instead, in the case of sludge samples collected during the period of February to April 2019, the amount of the heavy metals did not exceed the limit values set by the Directive 86/278/EEC except that from the sample P6, in which the concentration of Zn was found to be higher (4195.89 mg/kg) than the permissible value for sewage sludge intended for agricultural use (2500 mg/kg) and for reclamation (3500 mg/kg) as well as for Ni content in both samples P3 and P6, with values of 427.03 mg/kg and 4195.89 mg/kg, respectively, exceeding even the permissible values for sewage sludge land application (400 mg/kg) according to data presented in Table 3.
Table 3
Heavy metals content in sewage sludge samples collected in February – April 2019 [mg/kg d.w.] comparative with limit values in accordance with Council Directive 86/278 / EEC [37]
Sample Ni Cu Zn Cd Pb P1 264.34 791.95 2877.83 16.09 296.06 P2 276.40 869.80 1864.82 13.68 213.59 P3 427.03 1668.80 3801.50 15.51 745.79 P4 306.91 1388.00 3147.95 59.56 1225.99 P5 205.57 1014.39 2370.30 17.87 828.18 P6 459.63 1136.94 4195.89 16.85 755.06 P7 132.08 661.44 3115.83 28.78 1026.23 P8 137.72 899.23 2228.53 32.65 1216.48 P9 118.97 792.64 1858.71 29.90 1012.66 P10 140.47 943.21 2176.30 43.64 1225.72 RSD* [%] 4.22–8.02 2.50–5.81 1.55–8.02 3.45–5.98 4.02–7.08 86/278/CEE 300–400 1000–1750 2500–4000 20–40 750–1200
Fig. 2 – Elemental content of sewage sludge samples collected from each unit part of Targoviste WWTP.
It should be noted that only dried sewage sludge samples P1, P2 P5, P7, and P9 comply with the requirement of the European Directive, and thus, these could be used for fertilizer production as a suggestion. It can be concluded that the exceeding of limit values of heavy metal content (Figures 2, 3) in sewage sludge samples collected from WWTP of Targoviste City, according to European Directive, can be not so significant to forbid the use of this sludge in agricultural purposes.
As mentioned above, world strategies for sewage sludge usage should be focused on low economic costs, national regulations, resource recovery, and valuable eco-friendly purposes. The determining factor in the utilization of SS as a suitable material of fertilizers is the sludge quality. In this respect, the main reason that sewage sludge is difficult to use for agricultural purposes is Cd content.
According to organo-mineral fertilizer standards proposed by European Commission, 2016 [38] the Cd concentration of sewage sludge intended for organo-mineral fertilizer production cannot be higher than 9 mg/kg in order not to exceed limit value (5 mg/kg). Taking into consideration obtained data (Tables 3 and 4) it can be observed that not all the samples meet the requirement provided by European regulation.
Table 4
The maximum quantity of sewage sludge [T/ha/year] that can be introduced annually in agricultural land according to the Directive of the Council of the European
Communities 86/278 / EEC of 12 June 1986.
Ni Cu Zn Cd Pb Total quantity* P1 11.349 15.152 10.425 9.323 50.665 9.323 P2 10.854 13.796 16.087 10.965 70.228 10.854 P3 7.025 7.191 7.892 9.671 20.113 7.025 P4 9.775 8.646 9.530 2.518 12.235 2.518 P5 14.594 11.830 12.657 8.394 18.112 8.394 P6 6.527 10.555 7.150 8.902 19.866 6.527 P7 22.714 18.142 9.628 5.212 14.617 5.212 P8 21.783 13.345 13.462 4.594 12.331 4.594 P9 25.216 15.139 16.140 5.017 14.812 5.017 P10 21.357 12.723 13.785 3.437 12.238 3.437
* The total quantity was considered the lowest sewage sludge value for the analyzed elements, to comply with the values imposed for each metal.
Identification and quantification of key sources of heavy metals in sewage sludge are closely correlated with the accumulation mechanism of these metals through a chemical reaction (i.e., complexation, redox). The large contributors of Zn in SS were residences, galvanization process, and car washing; in the case of Ni may be mentioned the chemicals added to WWTPs and in the case of Cu various amalgams may be revealed. In the case of Cd, the main sources were car washing, traffic, industrial activities, smoking, and paints. It was concluded that sources of Ni, Cd and Pb were more poorly understood and difficult to track.
in the complexing process, as a consequence of heavy metals accumulation in sludge sewages. It is also worth noting that the spectra from FTIR and Raman analyses of most of the analyzed samples did not provide a full match with the provenience sources even after mechanical, chemical and biological treatments, due to the different organic substances used in domestic, agricultural and industrial activities.
Table 5
FTIR and Raman data for the sewage sludge samples and tentative vibrational assignments P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 Tentative vibrational assignments
Wavenumber [cm–1] & Relative intensity* [a.u.]
3278w 3260w 3280w 3273w 3274w 3283w 3265w 3283w 3261w 3279w OH groups of phenolic and carboxylic groups from humic material 2955w – – – – 2955w 2952w – 2958w – C-H aliphatic bond and Si-O from silicate,
respectively 2919w 2921w 2922w 2921w 2920w 2919w 2922w 2922w 2920w 2920w C-H of methyl and methylene groups of
humic compound 2850w 2851w 2851w 2852w 2851w 2850w 2852w 2852w 2851w 2850w stretching vibration of the CH
2 groups
1633m 1632w 1632w 1634m 1635w 1632m 1631m 1633m 1632m 1633m C=C bonds of alkene 1539m 1535w 1538w 1532m 1537w 1538m 1532m 1534m 1535m 1537m C=C aromatic stretching, amide II group 1415m 1415w 1410w 1416m 1415w 1416m 1410m 1414m 1416m 1415m C-O, C-H bonds of phenolic groups from
humic acid
1235w 1233w 1238w 1230w 1230w 1235w 1243w 1235w 1237w 1238w C-O, O-H bonds of phenolic and carboxylic groups from humic acid 1009s 1018s 1019s 1011s 1007s 1018s 1013s 1008s 1021s 1012s Si-OH from silicate 874w 874w 874w 874w 874w 874w 874w 874w 874w 874w C-H and C=C bonds from aromatic cycles
Raman shift [cm–1] & Relative intensity* [a.u.]
– 1885s 1899s 1892s 1851s 1857s 1892s 1850w – 1892w C=O anhydride bond 1588w 1588w – 1580w 1566m 1559w 1588w 1559w 1559w 1581w N-H band
1508s 1508w – 1508s – – 1508s – 1530w – C – N bond from aromatic cycles bind -NH- 1471m – – 1478s – – 1478s 1411w – – C-O, C-H bonds of phenolic groups from
humic acid – – – – – 1320m – 1320w 1320w – Calcium oxalate 1282w 1274w – – – – – – – – Amide III and II group 1243w 1243w – – 1227w – – – – – C-O, O-H bonds of phenolic and carboxylic
Table 5 (continued) P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 Tentative Vibrational Assignments
Raman shift [cm–1] & Relative intensity* [a.u.]
1084
m 1076w – 1076w – – 1076w – 1092w – Dolomite
1052w 1060w – 1052w – – 1052w – – – C-O-C- or -C-O; C-N stretching 1032w 1028w – – 1026w – – – – Si-O from silicate 1019w – – 1003w – – 1003w – – Si-OH from silicate
978m 978w – 978w 961w 961w 978w 961w 986w 970w Si-O from clay 936m 928w – 937w 944w 911w 927w 911w – – Si-OH from silicate
– 877w – 877w 885s 894s – 885w 885w – C-O (carbonate in calcite minerals) 825w – – – 834s 834s – 851w 834m 834w C-H and C=C bonds from aromatic cycles 782w – – – 799s 799s – 799m 782m 782w Si-O (quartz, clay minerals)
– – – – – – 764w – 756m 764w Clay mineral
720m 729m – 702s – – 702m – 720w 720w Ca-O bond (carbonate) 675s 675s – 666s 693w 693w 666m 693w 693w – Si-O (quartz)
– 630w – 639m 621s 621s 630w 621s 602s – Si-O-Si bending (quartz/illite) 584s 584s – 584s 565s 565s 584s 565s 556s 584w Clay mineral 537s 509s – 528s 527w 528w 509s 528w – 509s Clay mineral 499s 490s – 480s – – 490s – 499w 480s Silicate
– 442s – 461s – – – – – 452s SiO4 symmetric stretch (kaolin)
– – – – – – – – 413s – Ti-O bend (rutile)
* s-strong; m-medium; w-weak
Furthermore, the FTIR and Raman spectra libraries usually consist of spectra for pure substances, thus spectra obtained from sludge samples are expected to have low congruity compared to reference spectra.
The FTIR spectra of sewage sludge samples show absorption bands in the region of 3280–3260 cm−1 that correspond to the stretch of OH groups available
mainly in humic substances (phenolic, alcohol and carboxylic groups from humic material); 2963–2850 cm−1 and 1460 cm–1 peaks could be stretching of asymmetric
C–H aliphatic bonds probably present in humic material. The weak peaks around of 1230 cm−1 region are assigned to C-O and O-H of phenolic and carboxylic
groups from humic compounds. The medium and weak peaks around 1535 cm−1
are attributed to C=C aromatic stretching and amide II group. The medium signals recorded in the range of 1632 cm–1 revealed the C=C bonds in alkene, while the
weak signals around 2920 cm–1 highlighted the C-H of methyl and methylene
groups of humic compounds. The weak intensity of peaks at 2850 cm–1
corresponding to the symmetric and asymmetric stretching vibration of the CH2
humic substances to bind the cations of metals and to form complexes (i.e, chelates) makes them valuable for agriculture, in the transport of micronutrients from the soil to plants [22]. The recorded Raman data complete the FTIR results, emphasizing the presence of silicates, carbonates, sulphates, and other clay minerals (Table 5). The humic organic substances with a lower molecular weight have the highest number of phenolic and carboxylic groups and are therefore the structures that can bind the ions of metals efficiently. Previous studies of authors [20–25] reported that high concentrations of cations such as Cu2 +, Zn2+, Cd2+ are
efficiently bound in metallic complexes as monodentate and bidentate complexes. This can be an explanation regarding the metal accumulation in sludge sewages.
4. CONCLUSIONS
The most important practices to treat sewage sludge from WWTPs were closely examined at the European level. In the future, one of the most important goals for agricultural purpose is improving the chemical composition of sewage sludge with microelements and prolong the life cycle of sludge as fertilizer – biosolid compost (i.e. electro-osmosis process, that means the forcing water molecules to move out from sewage sludge when dragged by an applied electrical field, in order to bring SS up to 45–50% dry solid content [39]). Starting from these issues, this comprehensive study highlighted the chemical composition of sewage sludge samples collected from water treatment plant of Targoviste City using nondestructive (i.e., ATR-FTIR and Raman spectroscopies) and destructive (ICP-MS spectrometry) analytical techniques. The research showed that sewage sludge has a high concentration in Ni, Pb and Cd, which are considered toxic for soil and plant growth. However, the Cd content of most sewage sludge samples is more than 5 mg/kg of dry solids (excepted P4) and limits their use as organic fertilizers.
In conclusion, this research can be a valuable point of start for future studies regarding the establishment of real sources of toxic metals and organic pollutants in domestic, commercial, and urban run-off wastewater from WWTPs and to evaluate the percentage of inorganic and organic pollutants accumulated in SS, and the percentage of pollutants released in the environment with the treated effluents, as well.
REFERENCES
1. A. Kelessidis and A.S. Stasinakis, Waste Manage. 32(6), 1186–1195 (2012). 2. H. Hudcová, J. Vymazal, and M. Rozkošný, Soil Water Res. 14(2), 104–120 (2019). 3. J.S. Mtshali, A.T. Tiruneh and A.O. Fadiran, Resour. Environ. 4(4), 190–199 (2014).
4. C. Puchongkawarin, C. Gomez-Mont, D.C. Stuckey, and B. Chachuat, Chemosphere 140, 150–158 (2015).
6. P. Kosobucki, A. Chmarzynski and B. Buszewski, Pol. J. Environ. Stud. 9(4), 243–248 (2000). 7. A. Wolna-Maruwka, Pol. J. Environ. Stud. 18(2), 279–288 (2009).
8. Q. Wang, W. Wei, Y. Gong, Q. Yu, Q. Li, J. Sun, and Z. Yuan. Sci. Total Environ. 587, 510–521 (2017).
9. C. Maulini-Duran, A. Artola, X. Font, and A. Sánchez, Bioresour. Technol. 147, 43–51(2013). 10. A. Gherghel, C. Teodosiu and S. De Gisi. J. Clean. Prod. 228, 244–263 (2019).
11. A. Khursheed and A.A. Kazmi, Water Res. 45(15), 4287–4310 (2011).
12. H. Kominko, K. Gorazda, Z. Wzorek, and K. Wojtas, Waste Biomass Valorization 9, 1817–1826 (2018).
13. H. Kirchmann, G. Börjesson, T. Kätterer, and Y. Cohen, Ambio 46, 143–154 (2017). 14. D. Lipińska, Comp. Econ. Res. 21, 121–137 (2018).
15. United States Environmental Protection Agency, Priority Pollutant List, 2015. Available online at https://www.epa.gov/sites/production/files/2015-09/documents/priority-pollutant-list-epa.pdf, last accessed 15th June 2020.
16. X.Z. Meng, A.K. Venkatesan, Y.L. Ni, J.C. Steele, L.L. Wu, A. Bignert, A. Bergman, and R.U. Halden, Environ. Sci. Technol. 50, 5454–5466 (2016).
17. Q.Y. Cai, C.H. Mo, Q.T. Wu, Q.Y. Zeng, and A. Katsoyiannis, Chemosphere 68, 1751–176 (2007). 18. European Commission – Joint Research Centre, Institute for Environment and Sustainability,
Occurrence and levels of selected compounds in European sewage sludge samples, 2012. Available online at https://ec.europa.eu/jrc/sites/jrcsh/files/jrc76111_lb_na_25598_en_n.pdf, last accessed 15th June 2020.
19. C. Radulescu, I.D. Dulama, C. Stihi, I. Ionita, A. Chilian, C. Necula, and E.D. Chelarescu, Rom. J. Phys. 59(9–10), 1057–1066 (2014).
20. C. Radulescu, I.A. Bucurica, P. Bretcan, E.D. Chelarescu, D. Tanislav, I.D. Dulama, R.M. Stirbescu, and S. Teodorescu, Rom. J. Phys. 64(3–4), 809 (2019).
21. R.M. Stirbescu, C. Radulescu, C. Stihi, I.D. Dulama, E.D. Chelarescu, I.A. Bucurica, G. Pehoiu, Rom. Rep. Phys. 71(2), 705 (2019).
22. C. Radulescu, A. Pohoata, P. Bretcan, D. Tanislav, C. Stihi, and E.D. Chelarescu, Rom. Rep. Phys. 69(2), 705–717 (2017).
23. S. Iordache, D. Dunea, C. Ianache, C. Radulescu, and I.D. Dulama, Rev. Chimie (Bucharest)
68(4), 879–885 (2017).
24. I.D. Dulama, C. Radulescu, E.D. Chelarescu, I.A. Bucurica, S.Teodorescu, R.M. Stirbescu, I.V. Gurgu, D.D. Let, and N.M. Stirbescu, Rom. J. Phys. 62(5–6), 807–815 (2017).
25. G. Pehoiu, C. Radulescu, O. Murarescu, I.D. Dulama, I.A. Bucurica, R.M. Stirbescu, and S. Teodorescu, Bull. Environ. Contam. Toxicol. 102(4), 504–510 (2019).
26. C. Radulescu, S. Iordache, D. Dunea, C. Stihi, and I.D. Dulama, Rom. J. Phys. 60(9–10), 1171–1182 (2015).
27. C. Radulescu, C. Stihi, I.D. Dulama, E.D. Chelarescu, P. Bretcan, and D. Tanislav, Rom. J. Phys.
60(1–2), 246–256 (2015).
28. C. Radulescu, C. Stihi, L. Barbes, A. Chilian, and E.D. Chelarescu, Rev. Chimie (Bucharest)
64(7), 754–760 (2013).
29. A.A. Georgescu, A.F. Danet, C. Radulescu, C. Stihi, I.D. Dulama, and D.E. Chelarescu, Rom. J. Phys. 61(5–6), 1087–1097 (2016).
30. I. Manea, L. Manea, C. Radulescu, I.D. Dulama, S. Teodorescu, R.M. Stirbescu, E.D. Chelarescu, I.A. Bucurica, Rom. Rep. Phys. 69(4), 711 (2017).
31. C. Radulescu, C. Stihi, S. Iordache, D. Dunea, and I.D. Dulama, Rev. Chimie (Bucharest) 68(4), 805–810 (2017).
32. A.A. Georgescu, A.F. Danet, C. Radulescu, C. Stihi, I.D. Dulama, and C.L. Buruleanu, Rev. Chimie (Bucharest) 68(10), 2402–2406 (2017).
34. B.C. Smith, Fundamentals of Fourier Transform Infrared Spectroscopy. Taylor & Francis Group, Boca Raton, 2011.
35. K. Ignatowicz, Environ. Res. 156, 19–22 (2017).
36. Q. Wang, W. Wei, Y. Gong, Q. Yu, Q. Li, J. Sun, and Z. Yuan, Sci. Total Environ. 587–588, 510–521 (2017).
37. Council Directive 86/278/EEC of 12 June 1986 on the Protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture, 1986.
