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Acid-Base Properties of Fulvic Acids Extracted From An Untreated Sewage Sludge and From Composted Sludge

Acid-Base Properties of Fulvic Acids Extracted From An Untreated Sewage Sludge and From Composted Sludge
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  See discussions, stats, and author profiles for this publication at: Acid–Base Properties of Fulvic AcidsExtracted from an Untreated SewageSludge and from Composted Sludge  Article   in  Water Research · February 1998 DOI: 10.1016/S0043-1354(97)00205-4 CITATIONS 22 READS 48 3 authors:Some of the authors of this publication are also working on these related projects: Photodynamic therapy of cancer based on chemiluminescence and bioluminescence   ViewprojectSynthesis of Nanoparticles   View projectJoaquim C G Esteves da SilvaUniversity of Porto 324   PUBLICATIONS   3,484   CITATIONS   SEE PROFILE Adelio A S C MachadoUniversity of Porto, Faculty of Science 191   PUBLICATIONS   1,881   CITATIONS   SEE PROFILE Monica SilvaUniversity of Coimbra 9   PUBLICATIONS   84   CITATIONS   SEE PROFILE All content following this page was uploaded by Joaquim C G Esteves da Silva on 29 December 2016. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the srcinal documentand are linked to publications on ResearchGate, letting you access and read them immediately.  Pergamon Plh S0043-1354(97)00205-4 War. Res. Vol. 32, No. 2, pp. 441--449, 1998 © 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 19.00 + 0.00 ACID-BASE PROPERTIES OF FULVIC ACIDS EXTRACTED FROM AN UNTREATED SEWAGE SLUDGE AND FROM COMPOSTED SLUDGE JOAQUIM C. G. ESTEVES DA SILVA, ADikLIO A. S. C. MACHADO* and MONICA A. B. A. SILVA LAQUIPAI, Faculdade de Ci6ncias, R. Campo Alegre 687, P4150 Porto, Portugal (First received April 1996; accepted in revised orm June 1997) Abstract--The acid-base properties of two samples of anthropogenic fulvic acids (FA) extracted from the untreated sewage sludge (rsFA) of a municipal wastewater treatment plant and from the compost (csFA) obtained in the aerobical digestion of this sludge were studied. Potentiometric titrimetry, infra- red spectroscopy and synchronous molecular fluorescence (SyF) spectroscopy were used to obtain ex- perimental information about the acid-base properties of the samples. Moreover, the SyF spectral data which were collected as function of the pH were analysed by a self-modelling curve resolution technique, evolving factor analysis with a gradient concentration window (EFA-GCW), to calculate the number of acid-base systems, their SyF spectra and the corresponding distribution diagrams, from which the pKa values were estimated. Two acid-base systems with pKa values about 6.2 and 8.9 were detected for rsFA and three systems with pKa values about 3.8, 6.6 and 9.4 for csFA. The amounts of acid groups directly titrated in water were (standard deviations in brackets) about 6.8(1) for rsFA and 6.73(3) for csFA. This information for rsFA and csFA was compared with similar information for natural soil FA samples extracted from a pinewood soil. Beyond different structural composition, this comparison showed that the anthropogenic samples have a higher amount of acid structures, but the structures of natural samples have stronger acid properties. © 1998 Elsevier Science Ltd. All fights reserved Key words--sewage sludge, compost, fulvic acids, soil, acid-base properties INTRODUCTION One of the alternatives for the recycling of large volumes of sludge produced at municipal waste- water treatment plants is their use as soil fertilizers, directly as the untreated sewage sludge or as the treated product obtained upon aerobical digestion of the sludge (compost). The macroscopic elemental composition of these products is adequate for agri- cultural applications and they introduce humified organic matter in the soil. The characterization of these anthropogenic humic substances (artS) has been investigated (Pfeffer et al., 1984; Senesi and Sposito, 1985, 1987; Giusquiani et al., 1994) with the objective of clarifying their nature and compar- ing their characteristics with those of natural soil humic substances (sHS). The final purpose of such research is to obtain information about the fate of the arts in soils and the possible impact in their physico-chemical characteristics. An environmentally important role of sHS is their contribution to the acid-base buffer capacity of soils. In the present work, to obtain information to help to predict the effect of arts on the acid- *Author to whom all correspondence should be addresssed. base behaviour of soils, two samples of fulvic acids (FA), which are the most soluble fraction of humic substances, were extracted from the untreated sew- age sludge (rsFA) and compost (csFA) produced in a wastewater treatment plant, and their acid-base properties were studied using a previously devel- oped methodology (Machado and Esteves da Silva, 1993; Esteves da Silva and Machado, 1995a,b; Esteves da Silva et al., 1996). This is based on the chemometric analysis of synchronous fluorescence (SyF) sets of spectra obtained at varying pH, and it proved to be useful for the characterization of the acid-base properties of FA extracted from materials obtained from a pinewood (leaf litters and soil) (Esteves da Silva et al., 1996) and from a coastal seawater (Esteves da Silva and Machado, 1995a) and for the study of mixtures of fluorescent acids (Estcves da Silva and Machado, 1995b). Indeed, the SyF spectra of humic substances contain much in- formation about these complex mixtures (Silva et al., 1994; Cook and Langford, 1995; Esteves da  Silva et al., 1995; Pullin and Cabaniss, 1995), which can be made useful if suitable data analysis tech- niques are employed to transform the untreated data into physico-chemical properties. The experimental basis of the methodology used in this work is constituted by potentiometric ti- 441  442 Joaquim C. G. Esteves da Silva et aL trations and the collection of the SyF spectra of the samples as the pH is changed. The spectral vari- ations are analysed by a chemometric self-modelling curve resolution technique, namely evolving factor analysis with a gradient concentration window (EFA-GCW), to reduce the untreated sets of spec- tra to a small set of components that characterize the acid-base systems of the sample, allowing the calculation of their SyF spectra and an estimation of their ionization constants. Moreover, by combin- ing the potentiometric titration data with this infor- mation about the acid-base systems, an estimation of the concentration of the acid group of each acid- -base couple can be obtained by a non-linear least squares procedure (Machado and Esteves da Silva, 1993; Esteves da Silva and Machado, 1995a,b; Esteves da Silva et al., 1996). After characterization of the acid-base properties of the rsFA and csFA samples, a comparison was made with previously studied FA samples extracted from materials collected in a natural maritime pine- wood (pine needles, upper and lower soil horizons). (Esteves da Silva et al., 1996). This type of pine- wood, composed of the Pinus pinaster pine species, is common to the whole Portuguese Atlantic coast, and the soils are usually weakly developed sandy soils (arenosols). This work is a first contribution to a project with the objective of studying the impact of using recycled materials as organic correctives of this type of soils. The comparison was made by a similarity analysis of the SyF spectra, which are re- lated to the structure of the acid-base species pre- sent in the samples, using multidimensional scaling, which is a pattern recognition method, to detect the existence and extent of affinities between the anthropogenic and the maritime pinewood FA samples. EXPERIMENTAL Materials The samples of municipal sewage sludge and its com- post, respectively, were collected at the Parada's waste- water processing plant and its composting station (Main, Porto). Composting is made by mixing (about 1:1 pro- portion) the sludge with a bulking agent (pinewood saw- dust) and letting to stabilize (aerobic stabilization and maturation steps) for about 20 days. The composition of the compost provided by the manufacturer is 65% organic matter, 1.5% total nitrogen, 0.8% phosphorous (P2Os), 0.13% potassium (K20), 0.19% magnesium (MgO), 0.10% sodium, 0.02% chlorides (NaCI) and a C/N ratio of 25. FA were isolated by a procedure recommended by the IHSS for the extraction of these materials from soils. (Thurman, 1988). Briefly, this procedure consists on the following sequence of operations: (i) acidification of the untreated materials to pH 1 with hydrogen chloride; (ii) after filtration of the acid mixture, elution of the filtrate through a column containing XAD-8 resin; (iii) extraction of the adsorbed material by elution with 0.1 M sodium hy- droxide and collection of the alkaline extract in a mixture of hydrogen chloride and hydrogen fluoride; (iv) re-elution of this acidified (pH 1) extract through a column contain- ing XAD-8 resin; (v) re-extraction of the adsorbed ma- terial by elution with 0.1 M sodium hydroxide; (vi) elution of the alkaline extract through a column containing a cation exchanger (Amberlite IR-120) in the acid form; and (vii) freeze-drying. FA solutions (120 and 200mg/litre) were prepared in 0.1 M potassium nitrate, for, respectivdy, the SyF varying pH experiments and for the potentiometric titrations. In the SyF varying pH experiments, nitric acid was added to the titrated solution to adjust the initial pH to about 2. A solution of decarbonated potassium hydroxide was used as titrant (in potentiometric titrations the titrant increment was always 0.016 ml). Equipment Potentiometric titration with pH measurement and flu- orescence measurements were made as described pre- viously (Esteves da Silva and Machado, 1995a; Esteves da Silva et aL, 1996). SyF measurements were made with a Perkin-Elmer, Beaconsfield, Bucks, U.K. LS-50 luminescence spec- trometer with a flow cell. SyF spectra were recorded with excitation between 270 and 550 nm, with the following set- tings; 7.5 um excitation and emission slits width; wave- length difference between the excitation and emission monocromators (offset) of 25 rim; scan rate of 200 nm/ min. When the effect of increasing the wavelength offset was analysed, it was found that it produces a decrease of spectral resolution, i.e. the SyF bands become more over- lapped. Because with higher spectral resolution more in- formation is obtained on the sample, the offset of 25 nm was used. A lower offset would give srcin to a higher spectral resolution, but it would require more narrow monocromators slits and, consequently, would decrease instrumental sensitivity. The FT-IR spectra of solid FA samples (5% in KBr) were recorded between 600 and 4000 cm l with a Nicolet (Madison, WI, U.S,A.) Magna 550 FT-IR spectropho- 2 cm- resolution), using a micro ometer (200 scans and 1 sampling diffuse reflectance accessory (DRIFT Baseline Nicole0. Elemental analysis were performed with a Carlo- Erba (Milan, Italy) 1106 Elemental Analyzer. Data analysis All data analysis software, except for pattern recog- nition calculations, was developed in this laboratory and written and compiled with Turbo Pascal 5.0 (Borland International, Scolts Valley, USA) (Machado and Esteves da Silva, 1993; Esteves da Silva and Maehado, 1995a,b; Esteves da Silva et al. 1996). Multidimensional sealing software was a part of SPSS (Statistical Package for the Social Sciences). RESULTS AND DISCUSSION Elemental analysis The elemental composition (N, C and H) and the atomic ratios (H/C and C/N) of the rsFA and csFA samples are presented in Table 1, together with similar data for the three natural FA extracted from a pinewood. The elemental compositions for all the samples are in the range of the correspond- ing average values for soil FA (Steelink, 1985). The comparison between the two anthropogenic FA shows that they have similar elemental composition except for nitrogen. The rsFA sample contains more nitrogen than csFA, and this decrease may be  443 due to two factors: (i) mixture of untreated sewage sludge with pinewood sawdust; and (ii) loss of nitrogen containing functional groups from the humic fraction during the digestion of the mixture. The comparison of the anthropogenic and natural FA atomic ratios shows that: (i) the H/C ratios for the two anthropogenic FA (both about 1.4) are similar to those for the two samples that corre- spond to the first stage of the natural soil humifica- tion process (pine needles 1.31, and upper horizon soil 1.30), but higher than that for the more mature soil sample (1.09); and (ii) the C/N ratio for the natural FA is much higher than for the anthropo- genic samples. Sample N C H H/C C/N Ref. rsFA 3.71 44.09 5.35 1.46 13.8 This work csFA 2.36 46.35 5.38 1.39 22.8 This work Pine needle FA 0.46 52.94 5.75 1.3l 175 Esteves da Silva et al. (1996) Upper soil FA 1.30 49.20 5.30 1.30 57.1 Estevas da Silva et al, (1996) Soil FA 0.60 44.65 4.08 1.09 95.1 Esteves da Silva et al. (1996) FT-IR spectra The FT-IR spectra of the rsFA and csFA samples are shown in Fig. 1 (only the 60{)- 2000 cm I range). In the 2000--4000 cm I range, the spectra of the two samples are similar, and they are similar to other samples of FA from different ori- 0.~- Abl 0.3 gins (Stevenson, 1982a; MacCarthy and Rice, 1985; Bloom and Leenheer 1989), and characterized by three bands: a broad band centred at about 3300 cm ~, which is attributed to the O--H stretch, and two narrower bands in the range 2950- 3070cm -l, which are attributed to the C--H stretch. However, these bands are always expected for humic substances because these are constituted mainly by carbon, hydrogen and oxygen. At lower wavenumbers, the spectra of the two samples (Fig. 1) also show a high level of similarity, but a band appears at about 1543 cm -1 in the spec- trum of the rsFA which is lacking for csFA. This band, due to N--H structures, suggests the exist- ence of a larger amount of such structures in rsFA than in csFA, in agreement with the elemental analysis results. Other bands appear in both spec- tra, as well as for natural FA, and are due to oxy- gen-containing structures as reported in Table 2. In conclusion, although the FT-IR spectra pro- vided scarce information about the structural differ- 0 1 6OO I '~ I 1000 1400 1800 Wlwenull~r Acid-base properties of fulvic acids Table 1. Elemental composition (mass %) and atomic ratios for the FA samples 0.7- 0.5. 0.3 o.1 600 b. | ' I 4 1000 1400 1800 Wa~mumb~ Fig. l. FT-IR spectra of (a) rsFA and (b) csFA.  444 Joaquim C. G. Esteves da Silva et al. Table 2. Main characteristics of the FT-IR spectra (1900-600 cm ~) of the FA samples Band Assignment b 1720 CO stretch (carboxylic groups) 1620 CC stretch 1534 c N--H bend 1446 CH2 and CH3 bend 1415 O---H bend (carboxylic groups) C---O stretch (phenolic OH) 1224 1174 1037 C--O stretch (ether and carbohydrates) Wavenumber of the maximum of the most important bands (cm-~). bPossible assignments according to the literature (Bloom and Leenheer, 1989; MacCarthy and Rice, 1985; Stevenson, 1982a). ~Only present in the rsFA sample ences of the two anthropogenic FA samples, they confirmed that nitrogen containing structures are lost during the composting process. Potentiometric titration curves The shapes of the titration curves with a strong base of aqueous solutions of rsFA and csFA and of their first derivatives are similar to those for other FA samples described in the literature (Gamble, 1970; Stevenson, 1982b) (Fig. 2 shows the curves corresponding to rsFA as example). The titration curves are characterized by a plateau in the acid pH region, showing a relatively strong buffer capacity, and by a end-point at neutral pH (more clearly detected in the first derivative curves). However, the amount of acid groups titrated up to the equivalent point at about pH 7 (the so-called total acidity directly titrated in water, Ntec0, is higher than for natural samples (Table 3). While Nteq is about 6,7 mmol/g for the two anthropogenic FA, it is in the range between 4,2 and 5.6 mmol/g for the natu- ral FA. These results confirm those of a previous report about anthropogenic FA (Giusquiani, 1994). Synchronous fluorescence spectra The SyF spectra of rsFA and csFA as a function of pH arc shown in Fig. 3. A comparison of the two spectral sets shows that the two samples have fluorescent structures with different acid-base char- acters. Indeed, the main SyF bands are in the 380- 11. 8. 5.. 2. I I I I O.O 0.4 0.8 1.1 1.6 Vlnd. FD 10. O. O.O 0.4 0.8 1.2 1.8 ViraL Fig. 2. Titration curve of rsFA with (a) a strong base and (b) its first derivative curve. 450 nm range for rsFA but at less than 350 nm for csFA. However, a visual interpretation of the acid-base reactions underlying the spectral variations pro- voked by pH variation is difficult, if not impossible, because multispectral sets are under analysis. Therefore, a chemometric procedure is needed to obtain useful information. The previously described EFA-GCW procedure (Machado and Esteves da Silva, 1993; Esteves da Silva and Machado, 1995a,b; Esteves da Silva et al., 1996) allows (i) the determination of the number of acid-base systems in the sample, which correspond to the acid-base structures that show different SyF spectra upon pH variation; (ii) the reconstitution of their spectra; and (iii) the plot of the distribution diagram of the species of the acid-bas~ couples, from which the acid-base constants (pKa values) can be obtained. The EFA-GCW of the SyF spectral data sets detected two and three acid-base systems for the rsFA and csFA samples, respectively. The calcu- Table 3. Total concentration of acid groups directly titrated (Ntcq) and concentrations (CJ of the groups that correspond to the macro- scopic acid-base systems of FA 'b Non-linear adjustment c Sample Nteq Ci C2 C3 C4 N AD Ref. rsFA 6.78(11) This work csFA 6.73(3) 4.96(6) 2,76(8) 7.72 0.55(7) 8.27 3 14 This work Pine needle FA 4.24(6) 4.05(7) 0.7(2) 4.8 1.5(1) 6.2 Estevcs da Silva et al. (1996) Upper soil FA 4.70(7) 2.40(5) 2.12(1) 4.51 0.7(1) 5.68 1,11(4) 6.39 Esteves da Silva et al. (1996) Soil FA 5.56(4) 3.54(6) 1.07(2) .60 1.08(5) 5.68 0.68(4) 6.34 Esteves da Silva et al. (1996) In mmol H+/g. The values of Ct in bold are cumulative. bAverage of four independent experiments (standard deviations in brackets). CAn average of 73 I~oints n the 3.8 to 10.9 pH range were used in calculations, N - number of independent experiments; AD - average de- viation (mix 10 °)
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