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Organic matter humification in olive oil mill wastewater by abiotic catalysis with manganese(IV) oxide
Bioresource Technology 99 (2008) 8528–8531
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Organic matter humification in olive oil mill wastewater by abiotic catalysis with manganese(IV) oxide G. Brunetti a,*, N. Senesi a, C. Plaza b a b
Dipartimento di Biologia e Chimica Agroforestale ed Ambientale, University of Bari, Via Amendola 165/A, 70126 Bari, Italy Centro de Ciencias Medioambientales, Consejo Superior de Investigaciones Científicas, Serrano 115 dpdo., 28006 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 14 February 2007 Received in revised form 21 February 2008 Accepted 28 February 2008 Available online 11 April 2008 Keywords: Olive oil mill wastewater Humification Humic acids Abiotic catalysis Manganese(IV) oxide
a b s t r a c t The chemical changes occurring in an olive oil mill wastewater (OMW) sample digested catalytically with MnO2 for 30 and 60 days were evaluated comparatively with those occurring in the same OMW left standing for the same time in an open-air lagoon. Both treatments increased the pH and electrical conductivity and decreased the contents of dry matter, total organic C and total N, and C/N ratio of OMW. The humic acid (HA)-like fraction isolated from the fresh OMW was characterized by a marked aliphatic character, small O and acidic functional group contents, marked presence of proteinaceous materials, partially modified lignin moieties and polysaccharides-like structures, extended molecular heterogeneity, and small degrees of aromatic ring polycondensation, polymerization and humification. With increasing the time of either lagooning or catalytic digestion, a loss of aliphatic materials and an increase of extraction yield, oxygenation, acidic functional groups, carbohydrates and aromaticity occurred in the HA-like fractions. The more evident changes measured for the HA-like fractions from catalytically-digested OMW, with respect to those from lagooned OMW, indicated that MnO2 was able to catalyze organic matter humification in OMW. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The disposal of olive oil mill wastewater (OMW), which consists of the aqueous fraction of olive juice and the water added during the different phases of olive oil processing, represents a major social, economic, and environmental problem in Mediterranean olive oil producing countries (Bas et al., 2001; Plaza et al., 2005). The OMW is rich of dissolved and suspended organic substances such as sugars, phenols, nitrogenated compounds, organic acids, polyalcohols, and a residual oil emulsion (García-Gómez et al., 2003). The current technologies used to reduce the large load of potentially toxic organic substances present in OMW in order to allow its discharge in natural water bodies are expensive and/or unreliable (Azbar et al., 2004). Therefore, the need arises to adopt more economically and environmentally sustainable solutions for OMW disposal. The presence of organic matter and plant nutrients qualifies OMW as a potentially useful material for use as a soil liquid amendment (Paredes et al., 1999). The OMW is often accumulated in open-air lagoons and applied to soil without any further treatment (Sierra et al., 2001). However, this practice may cause several adverse effects on soil due to the presence of insufficiently mature and unstable organic matter in OMW. These effects include the in* Corresponding author. Tel.: +39 080 5442953; fax: +39 080 5442850. E-mail address: [email protected] (G. Brunetti). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.02.047
crease of the mineralization rate of native soil organic C through extended microbial oxidation, the induction of anaerobic conditions by mineralization of large amounts of non-stabilized organic C with associated extended O2-consumption, and the alteration of soil pH (Senesi and Brunetti, 1996). For these reasons, the transformation of fresh organic matter contained in OMW into stabilized organic compounds, the so-called humic-like substances chemically resembling native soil humic substances, is highly recommended before OMW application to soil. Humic substances are the most important components of soil organic matter. These substances are formed by secondary synthesis reactions (humification) of plant, animal, and microbial decay products (Stevenson, 1994). Manganese oxides, which are common soil mineral constituents, are proven to be able to promote the transformation in laboratory conditions of sugars, phenols, and amino compounds to humic-like substances by acting as catalysts of various condensation, oxidative polymerization, ring cleavage, decarboxylation, and dealkylation reactions (Huang, 2000; Jokic et al., 2001, 2004). In particular, MnOx are shown to be catalysts more reactive than other soil mineral constituents such as AlOx and FeOx in the polymerization of phenolic compounds by virtue of their high oxidation potential and high specific surface reactivity (Shindo and Huang, 1984; Wang and Huang, 2000). Further, MnOx may act as Lewis acids by accepting electrons from phenolic compounds and form semiquinones that, in turn, may undergo oxidative polymerization and generate humic materials (Huang, 2004).
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The objective of this work was to investigate the catalytic effect of MnO2 on the humification of organic matter contained in OMW. To reach this objective, the chemical changes occuring in an OMW sample and its humic acid (HA)-like fraction subjected to either catalytic oxidation with MnO2 or open-air lagooning were evaluated comparatively by use of chemical methods and spectroscopic techniques. 2. Methods The fresh OMW (FOMW) sample was collected from an olive oil mill located in the Bari province, Southern Italy, which employs a three-phase decanter centrifuge system for oil separation. Aliquots of 2000 L of FOMW were either left standing for 60 days in an open-air lagoon (LOMW) or catalytically treated with 2 kg of MnO2 for 60 days in a pilot reactor with continuous air bubbling from the bottom and mechanical stirring (COMW). In both treatments, the temperature ranged from 13 °C to 21 °C. Representative samples of LOMW and COMW were collected after 30 and 60 days of each treatment. The principal properties of OMW samples were determined in triplicate by conventional methods (Clesceri et al., 1998). Further, the HA-like fraction was isolated from each air-dried and 0.5mm-sieved OMW using a conventional procedure (Schnitzer, 1982), and analyzed for elemental and acidic functional group composition, and by ultraviolet/visible, Fourier transform infrared (FT IR), and fluorescence spectroscopies. The chemical methods of substrate analyses, the HA isolation procedure, and the methods of HA analyses are described in detail elsewhere (Plaza et al., 2005). 3. Results and discussion 3.1. Olive oil mill wastewaters Table 1 shows the main chemical characteristics of FOMW and LOMW and COMW after 30 and 60 days of treatment. The chemical properties of FOMW generally fall within the ranges commonly reported in the literature for this effluent (Paredes et al., 1999). With respect to FOMW, after 30 and, especially, 60 days of lagooning or catalytic treatment, the contents of dry matter, total phenols, TOC, and total N and the C/N ratio decrease, the values of pH and electrical conductivity increase, and the contents of ash, total P and total K remain almost constant. After 60 days of treatment the COMW sample, as compared to the LOMW sample, shows slightly larger values of pH, electrical conductivity, dry matter, TOC and total N, almost unvariate contents of ash, total P and total K, and smaller total phenol content and C/N ratio. The decrease of dry matter and TOC contents measured after either lagooning or catalytic digestion suggests the occurrence of a net organic matter mineralization in the OMW subjected to either treatment. The pH increase may be attributed to the miner-
alization of proteinaceous materials to yield alkaline ammonia and/or to loss of volatile acids, whereas the increase of electrical conductivity may be ascribed to formation of soluble salts during the mineralization processes. The decrease of total phenol content may be due to polymerization of low molecular weight phenols (Paredes et al., 1999). The decrease of total N content may be mostly attributed to volatilization of ammonia N due to the increased pH of treated OMW, and, secondarily, to the occurrence of nitrification and denitrification processes. Finally, the slight decrease of C/N ratio may reflect the loss of C-rich materials with mineralization. Further, the smaller total phenol content and C/N ratio achieved in COMW, especially after 60 days, with respect to LOMW, may be ascribed to a larger degree of stabilization and maturity and a more extended synthesis of organic components recalcitrant to microbial degradation (i.e., humification) possibly occurred in COMW (Paredes et al., 1999). 3.2. Humic acids 3.2.1. Yield and elemental and functional group composition The yield, elemental composition and atomic ratios and the acidic functional group composition of HA-like fractions isolated from the various OMWs examined, with respect to those of soil HA, are shown, respectively, in Tables 2 and 3. In general, with increasing the treatment time, the extraction yield, the O and acidic functional group contents and C/H and O/C ratios tend to increase, and C and H contents and C/N ratio tend to decrease, with respect to the corresponding values of the FOMW–HA. In particular, the HA-like fraction isolated from COMW, with respect to that isolated from LOMW, especially after 60 days of treatment, shows a much larger extraction yield, larger contents of O, total acidity and COOH and phenolic OH groups, smaller C and H contents and a slightly smaller C/N ratio. These results suggest that, with increasing the time of lagooning, and especially catalytic digestion, oxidation processes possibly involving methoxyl and alcoholic groups of lignin residues and oxidative polymerization of phenols would occur in HA-like fractions. The more evident effects observed for the HA-like fractions isolated from COMW than from LOMW suggest the efficient catalytic action of MnO2 in the oxidation reactions leading to organic matter humification in OMW. 3.2.2. FT IR spectra The main features of the FT IR spectra of HA-like fractions isolated from the various OMWs examined, and their corresponding assignments are the following: (a) an intense broad band centered between 3400 and 3360 cm 1, which is usually attributed to O–H stretching and, secondarily, to N–H stretching of various functional groups; (b) a faint absorption at about 3005 cm 1 generally assigned to olefinic C–H stretching; (c) two sharp bands at 2924 cm 1 and 2854 cm 1 due to aliphatic C–H group stretching,
Table 1 Main chemical properties (± standard errors of three laboratory replicates) of fresh olive oil mill wastewater (FOMW) and OMW either left standing in an open-air lagoon or catalytically treated for 30 and 60 days (LOMW30 and LOMW60, and COMW30 and COMW60, respectively) Property 1
Dry matter (g L ) Ash (g L 1) pH Electrical conductivity (dS m Total phenol (g L 1) Total organic C (TOC, g L 1) Total N (g L 1) Total P (g L 1) Total K (g L 1) C/N ratio
1
)
FOMW
LOMW30
LOMW60
COMW30
COMW60
54.5 ± 0.5 9.3 ± 0.4 4.0 ± 0.1 8.2 ± 0.1 7.0 ± 0.1 31.4 ± 0.3 0.36 ± 0.01 0.24 ± 0.01 1.91 ± 0.05 88
46.7 ± 0.3 9.2 ± 0.3 4.2 ± 0.1 9.1 ± 0.1 6.7 ± 0.1 25.9 ± 0.3 0.30 ± 0.02 0.24 ± 0.01 1.99 ± 0.08 88
43.2 ± 0.2 9.2 ± 0.2 4.8 ± 0.1 10.0 ± 0.1 6.6 ± 0.1 23.6 ± 0.2 0.28 ± 0.01 0.24 ± 0.02 1.99 ± 0.07 86
52.0 ± 0.4 9.2 ± 0.3 4.6 ± 0.1 10.1 ± 0.1 5.5 ± 0.1 28.8 ± 0.3 0.34 ± 0.02 0.23 ± 0.01 1.98 ± 0.07 85
46.4 ± 0.3 9.2 ± 0.1 5.2 ± 0.1 11.2 ± 0.1 5.0 ± 0.1 24.6 ± 0.2 0.31 ± 0.01 0.24 ± 0.01 1.96 ± 0.5 80
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Table 2 Yield, elemental composition (± standard errors of three laboratory replicates, moisture- and ash-free), and atomic ratios of humic acid-like fractions (HA) isolated from fresh olive oil mill wastewater (FOMW) and OMW either left standing in an open-air lagoon or catalytically treated for 30 and 60 days (LOMW30 and LOMW60, and COMW30 and COMW60, respectively), compared to mean values of native soil humic acids Origin of HA samples
Yield (g kg
FOMW LOMW30 LOMW60 COMW30 COMW60 Soila
0.9 2.6 5.5 7.9 16.9 –
a
1
)
C (g kg 669 ± 5 656 ± 6 647 ± 3 642 ± 4 616 ± 7 562
1
)
H (g kg 99 ± 2 93 ± 1 83 ± 2 88 ± 0 75 ± 1 48
1
)
N (g kg 20 ± 1 21 ± 1 21 ± 0 21 ± 1 21 ± 0 32
1
)
S (g kg
1
)
1±0 1±0 1±0 1±0 2±1 8
O (g kg 210 ± 6 229 ± 6 247 ± 4 248 ± 4 287 ± 5 355
1
)
C/N (atomic ratios)
C/H (atomic ratios)
O/C (atomic ratios)
38.3 36.8 35.8 36.5 34.8 20.5
0.6 0.6 0.7 0.6 0.7 1.0
0.2 0.3 0.3 0.3 0.4 0.5
From Schnitzer (1978).
Table 3 Acidic functional group contents (± standard errors of three laboratory replicates, moisture- and ash-free) and relative fluorescence intensity (RFI) of humic acid-like fractions (HA) isolated from fresh olive oil mill wastewater (FOMW) and OMW either left standing in an open-air lagoon or catalytically treated for 30 and 60 days (LOMW30 and LOMW60, and COMW30 and COMW60, respectively), compared to mean values of native soil humic acids Origin of HA samples
Total acidity (mmol g
FOMW LOMW30 LOMW60 COMW30 COMW60 Soila
5.0 ± 0.1 5.1 ± 0.2 5.3 ± 0.1 5.8 ± 0.2 6.8 ± 0.2 6.7 ± 0.1
a
1
)
COOH (mmol g 1.8 ± 0.1 1.9 ± 0.2 2.0 ± 0.1 2.4 ± 0.1 2.9 ± 0.2 3.6 ± 0.1
1
)
Phenolic OH (mmol g 3.2 ± 0.1 3.2 ± 0.1 3.3 ± 0.2 3.4 ± 0.1 3.9 ± 0.1 3.9 ± 0.1
1
)
RFI (arbitrary units) 2.4 2.0 1.7 1.5 0.9 –
From Schnitzer (1978).
whose relative intensity decreases slightly with the time of lagooning or catalytic digestion, and is slightly weaker for COMW–HAs than for LOMW–HAs; (d) an absorption at 1743 cm 1 due to C@O stretching of esters, whose relative intensity decreases in LOMW–HAs and disappears in COMW–HAs; (e) a sharp band at 1711 cm 1 due to C@O stretching of COOH and other carbonyl groups; (f) a band in the 1640–1650 cm 1 region generally attributed to absorptions of several groups including aromatic C@C, C@O stretching of amide groups (amide I band), quinonic C@O and/or C@O of H-bonded conjugated ketones, which tends to become more intense in COMW–HAs; (g) a band at about 1540 cm 1 preferentially ascribed to N–H deformation and C@N stretching of amides (amide II band), whose intensity decreases slightly in LOMW–HAs and COMW–HAs; (h) a sharp peak at about 1460 cm 1 attributed to aliphatic C–H deformation; (i) a broad band with three faint peaks centered at about 1240–1230 cm 1, generally ascribed to C–O stretching and O–H deformation of carboxyls and C–O stretching of aryl ethers, whose relative intensity increases with time in LOMW–HAs and COMW–HAs; and (j) an absorption at about 1070–1090 cm 1, generally attributed to C–O stretching of polysaccharide-like substances, whose relative intensity increases with time in LOMW–HAs and, especially, in COMW– HA. In general, the FT IR data suggest that, with increasing the time of lagooning or catalytic digestion of OMW, the aliphatic character of HA-like fractions decreases and the aromaticity and presence of O-containing groups increase. Besides confirming the trends of compositional data discussed previously, FT IR data suggest the occurrence of a partial incorporation of polysaccharide-like substances into HA-like structures. These effects are generally more marked in COMW–HAs than in LOMW–HAs. 3.2.3. Fluorescence spectra The RFI values of LOMW–HAs and, especially, COMW–HAs are lower than that of FOMW–HA, and decrease with the time of either treatment (Table 3). The emission spectra of the HA-like fractions examined consist of a broad band with the maximum centered at a wavelength that is slightly longer for COMW–HAs, especially after 60 days of catalytically digestion, with respect to the other
OMW–HAs. Further, a shoulder extending toward longer wavelengths appears in the spectra of COMW–HAs. The fluorescence excitation spectra of all HAs are characterized by three major peaks at intermediate and long wavelengths (391–396, 442–443 and 473–475 nm) and a minor peak or a shoulder at short wavelength (335 nm). The synchronous scan spectra feature four main peaks or shoulders at 328–332, 397–410, 441–443 and 478–509 nm. With increasing the time of lagooning, and especially catalytic digestion, the relative intensities of the peaks at short and intermediate wavelengths of the excitation and synchronous scan spectra tend to decrease, and that of the peak at long wavelength tends to increase and shift to longer wavelength. In agreement with literature data (Senesi et al., 1991), these results suggest a decrease of molecular heterogeneity and an increase of molecular size, aromatic polycondensation, level of conjugated chromophores and humification degree of HA-like fractions as the time of lagooning, and especially catalytic digestion, increases. Further, fluorescence results suggest a larger humification degree reached by COMW–HAs with respect to the corresponding LOMW–HAs. 3.2.4. Comparison between olive oil mill wastewater and native soil humic acids The average values of elemental and functional group composition of native soil HAs (Schnitzer, 1978) are reported in the bottom rows of Tables 2 and 3. The FOMW–HA shows larger or much larger C, H and N contents and C/N ratio, and smaller or much smaller N, O, S and acidic functional group contents and C/H and O/C ratios than native soil HAs. However, after lagooning and especially catalytic digestion, the elemental and acidic functional group compositions of the HA-like fractions of OMW tend to approach progressively the corresponding values typical of soil HA. Furthermore, after 60 days of lagooning, and especially catalytic digestion, the FT IR and fluorescence features of HA-like fractions resemble those typical of soil HA much more than those of FOMW–HA. According to currently accepted concepts (Senesi and Brunetti, 1996), the greater is the amount of the HA-like fraction in an organic amendment, and the more its compositional and structural properties approach those of soil HA, the more agronomically effi-
G. Brunetti et al. / Bioresource Technology 99 (2008) 8528–8531
cient and environmentally safe is the organic amendment. Thus, the catalytic digestion process appears able to improve the quality of OMW as an organic amendment more than simple lagooning. 4. Conclusions Manganese(IV) oxide shows able to catalyze efficiently the organic matter humification process in OMW, thus enhancing the quality of this effluent as a liquid soil amendment, while minimizing its negative environmental impact. Thus, the catalytic digestion of OMW with MnO2 proves to be a useful, valuable and promising treatment to avoid the dispersion of this organic matter-rich natural resource. References Azbar, N., Bayram, A., Filibeli, A., Muezzinoglu, A., Sengul, F., Ozer, A., 2004. A review of waste management options in olive oil production. Crit. Rev. Environ. Sci. Technol. 34, 209–247. Bas, F.J., Colinet, M.J., Lobo, J., 2001. The olive tree as an energy source in the Mediterranean area: Andalusia. In: Proceedings of the First World Conference on Biomass for Energy and Industry. James & James (Science Publishers) Ltd., London, pp. 393–395. Clesceri, L.S., Greenberg, A.E., Eaton, A.D., 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. APHA, AWWA, WEF, Washington. García-Gómez, A., Roig, A., Bernal, M.P., 2003. Composting of the solid fraction of olive mill wastewater with olive leaves: organic matter degradation and biological activity. Bioresour. Technol. 86, 59–64. Huang, P.M., 2000. Abiotic catalysis. In: Sumner, M.E. (Ed.), Handbook of Soil Science. CRC Press, Boca Raton, pp. 303–332.
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Huang, P.M., 2004. Soil mineral–organic matter–microorganism interactions: fundamentals and impacts. Adv. Agron. 82, 391–472. Jokic, A., Frenkel, A.I., Vairavamurthy, M., Huang, P.M., 2001. Birnessite catalysis of the Maillard reaction: its significance in natural humification. Geophys. Res. Lett. 28, 3899–3902. Jokic, A., Wang, M.C., Liu, C., Frenkel, A.I., Huang, P.M., 2004. Integration of the polyphenol and Maillard reactions into a unified abiotic pathway for humification in nature: the role of d-MnO2. Org. Geochem. 35, 747–762. Paredes, C., Cegarra, J., Roig, A., Sánchez-Monedero, M.A., Bernal, M.P., 1999. Characterization of olive mill wastewater (alpechin) and its sludge for agricultural purposes. Bioresour. Technol. 67, 111–115. Plaza, C., Senesi, N., Brunetti, G., Mondelli, D., 2005. Cocomposting of sludge from olive oil mill wastewater mixed with tree cuttings. Compost Sci. Util. 13, 212– 226. Schnitzer, M., 1978. Humic substances: chemistry and reactions. In: Schnizer, M., Khan, S.U. (Eds.), Soil Organic Matter. Elsevier, Amsterdam, pp. 1–64. Schnitzer, M., 1982. Organic matter characterization. In: Page, B.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties, second ed., Agronomy Monograph No. 9 SSSA, Madison, pp. 581–594. Senesi, N., Brunetti, G., 1996. Chemical and physicochemical parameters for quality evaluation of humic substances produced during composting. In: De Bertoldi, M., Sequi, P., Lemmes, B., Papi, T. (Eds.), The Science of Composting. Chapman & Hall, London, pp. 195–212. Senesi, N., Miano, T.M., Provenzano, M.R., Brunetti, G., 1991. Characterization, differentiation, and classification of humic substances by fluorescence spectroscopy. Soil Sci. 152, 259–271. Shindo, H., Huang, P.M., 1984. Catalytic effects of manganese(IV), iron(III), aluminium, and silicon oxides on formation of phenolic polymers. Appl. Clay Sci. 1, 71–81. Sierra, J., Martí, E., Montserrat, G., Cruañas, R., Garau, M.A., 2001. Characterisation and evolution of a soil affected by olive oil mill wastewater disposal. Sci. Total Environ. 279, 207–214. Stevenson, F.J., 1994. Humus Chemistry: Genesis, Composition, Reactions. WileyInterscience, New York. Wang, M.C., Huang, P.M., 2000. Ring cleavage and oxidative transformation of pyrogallol catalyzed by Mn, Fe, Al, and Si oxides. Soil Sci. 165, 934–942.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Organic matter humification in olive oil mill wastewater by abiotic catalysis with manganese(IV) oxide G. Brunetti a,*, N. Senesi a, C. Plaza b a b
Dipartimento di Biologia e Chimica Agroforestale ed Ambientale, University of Bari, Via Amendola 165/A, 70126 Bari, Italy Centro de Ciencias Medioambientales, Consejo Superior de Investigaciones Científicas, Serrano 115 dpdo., 28006 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 14 February 2007 Received in revised form 21 February 2008 Accepted 28 February 2008 Available online 11 April 2008 Keywords: Olive oil mill wastewater Humification Humic acids Abiotic catalysis Manganese(IV) oxide
a b s t r a c t The chemical changes occurring in an olive oil mill wastewater (OMW) sample digested catalytically with MnO2 for 30 and 60 days were evaluated comparatively with those occurring in the same OMW left standing for the same time in an open-air lagoon. Both treatments increased the pH and electrical conductivity and decreased the contents of dry matter, total organic C and total N, and C/N ratio of OMW. The humic acid (HA)-like fraction isolated from the fresh OMW was characterized by a marked aliphatic character, small O and acidic functional group contents, marked presence of proteinaceous materials, partially modified lignin moieties and polysaccharides-like structures, extended molecular heterogeneity, and small degrees of aromatic ring polycondensation, polymerization and humification. With increasing the time of either lagooning or catalytic digestion, a loss of aliphatic materials and an increase of extraction yield, oxygenation, acidic functional groups, carbohydrates and aromaticity occurred in the HA-like fractions. The more evident changes measured for the HA-like fractions from catalytically-digested OMW, with respect to those from lagooned OMW, indicated that MnO2 was able to catalyze organic matter humification in OMW. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The disposal of olive oil mill wastewater (OMW), which consists of the aqueous fraction of olive juice and the water added during the different phases of olive oil processing, represents a major social, economic, and environmental problem in Mediterranean olive oil producing countries (Bas et al., 2001; Plaza et al., 2005). The OMW is rich of dissolved and suspended organic substances such as sugars, phenols, nitrogenated compounds, organic acids, polyalcohols, and a residual oil emulsion (García-Gómez et al., 2003). The current technologies used to reduce the large load of potentially toxic organic substances present in OMW in order to allow its discharge in natural water bodies are expensive and/or unreliable (Azbar et al., 2004). Therefore, the need arises to adopt more economically and environmentally sustainable solutions for OMW disposal. The presence of organic matter and plant nutrients qualifies OMW as a potentially useful material for use as a soil liquid amendment (Paredes et al., 1999). The OMW is often accumulated in open-air lagoons and applied to soil without any further treatment (Sierra et al., 2001). However, this practice may cause several adverse effects on soil due to the presence of insufficiently mature and unstable organic matter in OMW. These effects include the in* Corresponding author. Tel.: +39 080 5442953; fax: +39 080 5442850. E-mail address: [email protected] (G. Brunetti). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.02.047
crease of the mineralization rate of native soil organic C through extended microbial oxidation, the induction of anaerobic conditions by mineralization of large amounts of non-stabilized organic C with associated extended O2-consumption, and the alteration of soil pH (Senesi and Brunetti, 1996). For these reasons, the transformation of fresh organic matter contained in OMW into stabilized organic compounds, the so-called humic-like substances chemically resembling native soil humic substances, is highly recommended before OMW application to soil. Humic substances are the most important components of soil organic matter. These substances are formed by secondary synthesis reactions (humification) of plant, animal, and microbial decay products (Stevenson, 1994). Manganese oxides, which are common soil mineral constituents, are proven to be able to promote the transformation in laboratory conditions of sugars, phenols, and amino compounds to humic-like substances by acting as catalysts of various condensation, oxidative polymerization, ring cleavage, decarboxylation, and dealkylation reactions (Huang, 2000; Jokic et al., 2001, 2004). In particular, MnOx are shown to be catalysts more reactive than other soil mineral constituents such as AlOx and FeOx in the polymerization of phenolic compounds by virtue of their high oxidation potential and high specific surface reactivity (Shindo and Huang, 1984; Wang and Huang, 2000). Further, MnOx may act as Lewis acids by accepting electrons from phenolic compounds and form semiquinones that, in turn, may undergo oxidative polymerization and generate humic materials (Huang, 2004).
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The objective of this work was to investigate the catalytic effect of MnO2 on the humification of organic matter contained in OMW. To reach this objective, the chemical changes occuring in an OMW sample and its humic acid (HA)-like fraction subjected to either catalytic oxidation with MnO2 or open-air lagooning were evaluated comparatively by use of chemical methods and spectroscopic techniques. 2. Methods The fresh OMW (FOMW) sample was collected from an olive oil mill located in the Bari province, Southern Italy, which employs a three-phase decanter centrifuge system for oil separation. Aliquots of 2000 L of FOMW were either left standing for 60 days in an open-air lagoon (LOMW) or catalytically treated with 2 kg of MnO2 for 60 days in a pilot reactor with continuous air bubbling from the bottom and mechanical stirring (COMW). In both treatments, the temperature ranged from 13 °C to 21 °C. Representative samples of LOMW and COMW were collected after 30 and 60 days of each treatment. The principal properties of OMW samples were determined in triplicate by conventional methods (Clesceri et al., 1998). Further, the HA-like fraction was isolated from each air-dried and 0.5mm-sieved OMW using a conventional procedure (Schnitzer, 1982), and analyzed for elemental and acidic functional group composition, and by ultraviolet/visible, Fourier transform infrared (FT IR), and fluorescence spectroscopies. The chemical methods of substrate analyses, the HA isolation procedure, and the methods of HA analyses are described in detail elsewhere (Plaza et al., 2005). 3. Results and discussion 3.1. Olive oil mill wastewaters Table 1 shows the main chemical characteristics of FOMW and LOMW and COMW after 30 and 60 days of treatment. The chemical properties of FOMW generally fall within the ranges commonly reported in the literature for this effluent (Paredes et al., 1999). With respect to FOMW, after 30 and, especially, 60 days of lagooning or catalytic treatment, the contents of dry matter, total phenols, TOC, and total N and the C/N ratio decrease, the values of pH and electrical conductivity increase, and the contents of ash, total P and total K remain almost constant. After 60 days of treatment the COMW sample, as compared to the LOMW sample, shows slightly larger values of pH, electrical conductivity, dry matter, TOC and total N, almost unvariate contents of ash, total P and total K, and smaller total phenol content and C/N ratio. The decrease of dry matter and TOC contents measured after either lagooning or catalytic digestion suggests the occurrence of a net organic matter mineralization in the OMW subjected to either treatment. The pH increase may be attributed to the miner-
alization of proteinaceous materials to yield alkaline ammonia and/or to loss of volatile acids, whereas the increase of electrical conductivity may be ascribed to formation of soluble salts during the mineralization processes. The decrease of total phenol content may be due to polymerization of low molecular weight phenols (Paredes et al., 1999). The decrease of total N content may be mostly attributed to volatilization of ammonia N due to the increased pH of treated OMW, and, secondarily, to the occurrence of nitrification and denitrification processes. Finally, the slight decrease of C/N ratio may reflect the loss of C-rich materials with mineralization. Further, the smaller total phenol content and C/N ratio achieved in COMW, especially after 60 days, with respect to LOMW, may be ascribed to a larger degree of stabilization and maturity and a more extended synthesis of organic components recalcitrant to microbial degradation (i.e., humification) possibly occurred in COMW (Paredes et al., 1999). 3.2. Humic acids 3.2.1. Yield and elemental and functional group composition The yield, elemental composition and atomic ratios and the acidic functional group composition of HA-like fractions isolated from the various OMWs examined, with respect to those of soil HA, are shown, respectively, in Tables 2 and 3. In general, with increasing the treatment time, the extraction yield, the O and acidic functional group contents and C/H and O/C ratios tend to increase, and C and H contents and C/N ratio tend to decrease, with respect to the corresponding values of the FOMW–HA. In particular, the HA-like fraction isolated from COMW, with respect to that isolated from LOMW, especially after 60 days of treatment, shows a much larger extraction yield, larger contents of O, total acidity and COOH and phenolic OH groups, smaller C and H contents and a slightly smaller C/N ratio. These results suggest that, with increasing the time of lagooning, and especially catalytic digestion, oxidation processes possibly involving methoxyl and alcoholic groups of lignin residues and oxidative polymerization of phenols would occur in HA-like fractions. The more evident effects observed for the HA-like fractions isolated from COMW than from LOMW suggest the efficient catalytic action of MnO2 in the oxidation reactions leading to organic matter humification in OMW. 3.2.2. FT IR spectra The main features of the FT IR spectra of HA-like fractions isolated from the various OMWs examined, and their corresponding assignments are the following: (a) an intense broad band centered between 3400 and 3360 cm 1, which is usually attributed to O–H stretching and, secondarily, to N–H stretching of various functional groups; (b) a faint absorption at about 3005 cm 1 generally assigned to olefinic C–H stretching; (c) two sharp bands at 2924 cm 1 and 2854 cm 1 due to aliphatic C–H group stretching,
Table 1 Main chemical properties (± standard errors of three laboratory replicates) of fresh olive oil mill wastewater (FOMW) and OMW either left standing in an open-air lagoon or catalytically treated for 30 and 60 days (LOMW30 and LOMW60, and COMW30 and COMW60, respectively) Property 1
Dry matter (g L ) Ash (g L 1) pH Electrical conductivity (dS m Total phenol (g L 1) Total organic C (TOC, g L 1) Total N (g L 1) Total P (g L 1) Total K (g L 1) C/N ratio
1
)
FOMW
LOMW30
LOMW60
COMW30
COMW60
54.5 ± 0.5 9.3 ± 0.4 4.0 ± 0.1 8.2 ± 0.1 7.0 ± 0.1 31.4 ± 0.3 0.36 ± 0.01 0.24 ± 0.01 1.91 ± 0.05 88
46.7 ± 0.3 9.2 ± 0.3 4.2 ± 0.1 9.1 ± 0.1 6.7 ± 0.1 25.9 ± 0.3 0.30 ± 0.02 0.24 ± 0.01 1.99 ± 0.08 88
43.2 ± 0.2 9.2 ± 0.2 4.8 ± 0.1 10.0 ± 0.1 6.6 ± 0.1 23.6 ± 0.2 0.28 ± 0.01 0.24 ± 0.02 1.99 ± 0.07 86
52.0 ± 0.4 9.2 ± 0.3 4.6 ± 0.1 10.1 ± 0.1 5.5 ± 0.1 28.8 ± 0.3 0.34 ± 0.02 0.23 ± 0.01 1.98 ± 0.07 85
46.4 ± 0.3 9.2 ± 0.1 5.2 ± 0.1 11.2 ± 0.1 5.0 ± 0.1 24.6 ± 0.2 0.31 ± 0.01 0.24 ± 0.01 1.96 ± 0.5 80
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Table 2 Yield, elemental composition (± standard errors of three laboratory replicates, moisture- and ash-free), and atomic ratios of humic acid-like fractions (HA) isolated from fresh olive oil mill wastewater (FOMW) and OMW either left standing in an open-air lagoon or catalytically treated for 30 and 60 days (LOMW30 and LOMW60, and COMW30 and COMW60, respectively), compared to mean values of native soil humic acids Origin of HA samples
Yield (g kg
FOMW LOMW30 LOMW60 COMW30 COMW60 Soila
0.9 2.6 5.5 7.9 16.9 –
a
1
)
C (g kg 669 ± 5 656 ± 6 647 ± 3 642 ± 4 616 ± 7 562
1
)
H (g kg 99 ± 2 93 ± 1 83 ± 2 88 ± 0 75 ± 1 48
1
)
N (g kg 20 ± 1 21 ± 1 21 ± 0 21 ± 1 21 ± 0 32
1
)
S (g kg
1
)
1±0 1±0 1±0 1±0 2±1 8
O (g kg 210 ± 6 229 ± 6 247 ± 4 248 ± 4 287 ± 5 355
1
)
C/N (atomic ratios)
C/H (atomic ratios)
O/C (atomic ratios)
38.3 36.8 35.8 36.5 34.8 20.5
0.6 0.6 0.7 0.6 0.7 1.0
0.2 0.3 0.3 0.3 0.4 0.5
From Schnitzer (1978).
Table 3 Acidic functional group contents (± standard errors of three laboratory replicates, moisture- and ash-free) and relative fluorescence intensity (RFI) of humic acid-like fractions (HA) isolated from fresh olive oil mill wastewater (FOMW) and OMW either left standing in an open-air lagoon or catalytically treated for 30 and 60 days (LOMW30 and LOMW60, and COMW30 and COMW60, respectively), compared to mean values of native soil humic acids Origin of HA samples
Total acidity (mmol g
FOMW LOMW30 LOMW60 COMW30 COMW60 Soila
5.0 ± 0.1 5.1 ± 0.2 5.3 ± 0.1 5.8 ± 0.2 6.8 ± 0.2 6.7 ± 0.1
a
1
)
COOH (mmol g 1.8 ± 0.1 1.9 ± 0.2 2.0 ± 0.1 2.4 ± 0.1 2.9 ± 0.2 3.6 ± 0.1
1
)
Phenolic OH (mmol g 3.2 ± 0.1 3.2 ± 0.1 3.3 ± 0.2 3.4 ± 0.1 3.9 ± 0.1 3.9 ± 0.1
1
)
RFI (arbitrary units) 2.4 2.0 1.7 1.5 0.9 –
From Schnitzer (1978).
whose relative intensity decreases slightly with the time of lagooning or catalytic digestion, and is slightly weaker for COMW–HAs than for LOMW–HAs; (d) an absorption at 1743 cm 1 due to C@O stretching of esters, whose relative intensity decreases in LOMW–HAs and disappears in COMW–HAs; (e) a sharp band at 1711 cm 1 due to C@O stretching of COOH and other carbonyl groups; (f) a band in the 1640–1650 cm 1 region generally attributed to absorptions of several groups including aromatic C@C, C@O stretching of amide groups (amide I band), quinonic C@O and/or C@O of H-bonded conjugated ketones, which tends to become more intense in COMW–HAs; (g) a band at about 1540 cm 1 preferentially ascribed to N–H deformation and C@N stretching of amides (amide II band), whose intensity decreases slightly in LOMW–HAs and COMW–HAs; (h) a sharp peak at about 1460 cm 1 attributed to aliphatic C–H deformation; (i) a broad band with three faint peaks centered at about 1240–1230 cm 1, generally ascribed to C–O stretching and O–H deformation of carboxyls and C–O stretching of aryl ethers, whose relative intensity increases with time in LOMW–HAs and COMW–HAs; and (j) an absorption at about 1070–1090 cm 1, generally attributed to C–O stretching of polysaccharide-like substances, whose relative intensity increases with time in LOMW–HAs and, especially, in COMW– HA. In general, the FT IR data suggest that, with increasing the time of lagooning or catalytic digestion of OMW, the aliphatic character of HA-like fractions decreases and the aromaticity and presence of O-containing groups increase. Besides confirming the trends of compositional data discussed previously, FT IR data suggest the occurrence of a partial incorporation of polysaccharide-like substances into HA-like structures. These effects are generally more marked in COMW–HAs than in LOMW–HAs. 3.2.3. Fluorescence spectra The RFI values of LOMW–HAs and, especially, COMW–HAs are lower than that of FOMW–HA, and decrease with the time of either treatment (Table 3). The emission spectra of the HA-like fractions examined consist of a broad band with the maximum centered at a wavelength that is slightly longer for COMW–HAs, especially after 60 days of catalytically digestion, with respect to the other
OMW–HAs. Further, a shoulder extending toward longer wavelengths appears in the spectra of COMW–HAs. The fluorescence excitation spectra of all HAs are characterized by three major peaks at intermediate and long wavelengths (391–396, 442–443 and 473–475 nm) and a minor peak or a shoulder at short wavelength (335 nm). The synchronous scan spectra feature four main peaks or shoulders at 328–332, 397–410, 441–443 and 478–509 nm. With increasing the time of lagooning, and especially catalytic digestion, the relative intensities of the peaks at short and intermediate wavelengths of the excitation and synchronous scan spectra tend to decrease, and that of the peak at long wavelength tends to increase and shift to longer wavelength. In agreement with literature data (Senesi et al., 1991), these results suggest a decrease of molecular heterogeneity and an increase of molecular size, aromatic polycondensation, level of conjugated chromophores and humification degree of HA-like fractions as the time of lagooning, and especially catalytic digestion, increases. Further, fluorescence results suggest a larger humification degree reached by COMW–HAs with respect to the corresponding LOMW–HAs. 3.2.4. Comparison between olive oil mill wastewater and native soil humic acids The average values of elemental and functional group composition of native soil HAs (Schnitzer, 1978) are reported in the bottom rows of Tables 2 and 3. The FOMW–HA shows larger or much larger C, H and N contents and C/N ratio, and smaller or much smaller N, O, S and acidic functional group contents and C/H and O/C ratios than native soil HAs. However, after lagooning and especially catalytic digestion, the elemental and acidic functional group compositions of the HA-like fractions of OMW tend to approach progressively the corresponding values typical of soil HA. Furthermore, after 60 days of lagooning, and especially catalytic digestion, the FT IR and fluorescence features of HA-like fractions resemble those typical of soil HA much more than those of FOMW–HA. According to currently accepted concepts (Senesi and Brunetti, 1996), the greater is the amount of the HA-like fraction in an organic amendment, and the more its compositional and structural properties approach those of soil HA, the more agronomically effi-
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