versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.49 n.3 Concepción sep. 2004
J. Chil. Chem. Soc., 49, N 3 (2004) 219-222
DISTRIBUTION OF PHOSPHORUS FORMS IN CHILEAN SOILS AND SEWAGE SLUDGE BY CHEMICAL FRACTIONATION AND 31P-NMR
MAURICIO ESCUDEY1, GERARDO GALINDO1, KATHERINE AVENDAÑO1, DAN BORCHARDT2, ANDREW C. CHANG3, MARGARITA BRICEÑO4
1Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile, Av. B. O'Higgins 3363, Santiago, Chile.
(Received: June 18, 2003 - Accepted: May 5, 2004)
The disposal of sewage sludge from domestic wastewater treatment plants is a growing problem worldwide. Because of the total P content and other characteristics, it is highly probable that in Chile important amounts of the sewage sludge will be disposed of on agricultural lands. The distribution of P forms in Chilean soils and sewage sludge was studied by chemical fractionation and 31P-NMR spectroscopy. The 0-15 cm depth of soils derived from volcanic materials (one Ultisol and two Andisols), alluvial materials (Mollisol), and sewage sludge from a domestic wastewater treatment plant located in Santiago were considered.
The total P concentration is 6 to 18 times higher in the sewage sludge than in soils, but a similar 31P-NMR pattern was found in all samples. The most important signals correspond to inorganic orthophosphate and monoester P, with small signals assigned to pyrophosphate and diester P.
The disposal of sewage sludge might not succeed as soil P fertilizer, contributing to an increase in the accumulation of P in volcanic soils, and it might be a potential pollutant in soils and water.
Key words: sewage sludge, phosphorus, 31P-NMR, soil
Phosphorus is an essential nutrient for plants, and its addition to soil as fertilizer is a standard agricultural practice. Phosphorus is also an important component of sewage sludge from domestic wastewater treatment plants. In every city the domestic wastewater needs to be treated; in Chile, only in Santiago production of as much as 120 ton day-1 of dry sewage sludge is projected. Sewage sludge disposal is a growing problem worldwide: in Europe 38% of sewage sludge was disposed of on agricultural lands during 1992 (1), and in the USA more than 50% of wastewater sewage sludge is bound to be disposed of on land (1). Thus, it is highly probable that in Chile the sewage sludge will be disposed on agricultural lands, and consequently the total P content of the soils will increase. However, knowledge of the P forms present in the soil is more important than total P content, because of their different availability and mobility.
Chemical fractionation and 31P-NMR studies are complementary and may be used to get a good description of P distribution. Inorganic P and organic P (fulvic and humic P) forms in soil can be determined through chemical fractionation, whose methodology depends on the characteristics and properties of the soil considered (2). It is possible to identify inorganic P forms as ortho-, pyro- and polyphosphate, and also establish the presence of organic P associated to P-monoesters, P-diesters, and phosphonates through 31P-NMR, based on the chemical shift of P signals, (3-12).
Several P extraction procedures have been proposed to improve the efficiency of extraction and to minimize the impact of the procedure on the P forms originally present in the soil (13). Quantitative studies have been carried out by the application of different signal integration methods (5, 6, 14-16).
In Chile some P chemical fractionation (17, 18) and preliminary 31P-NMR (19, 20) studies have been carried out on volcanic soils. The characterization of P forms in the sewage sludge recently produced in Santiago (Chile) from domestic wastewater has not been carried out.
Thus, the two objectives of this paper are: 1) to characterize by chemical fractionation and 31P-NMR the P forms in the sewage sludge produced in Santiago, and in three acid soils derived from volcanic materials and one neutral soil formed from alluvial sediments, and 2) to discuss the potential impact of sewage sludge disposal on soils, from environmental and agricultural points of view, based on the soil characteristics and the sewage sludge P distribution.
MATERIALS AND METHODS
The sewage sludge was obtained from the La Farfana domestic wastewater treatment plant located in Santiago. Uncultivated samples of the 0-15 cm layer of one Ultisol (Collipulli, with a mineralogy dominated by kaolinite), two Andisols (Diguillín and Ralún, with a mineralogy dominated by allophane) and one Mollisol (Colina, with a mineralogy dominated by smectite) were selected. The soils and sewage sludge were passed through a 2-mm sieve.
Soil samples and sewage sludge were characterized for organic carbon content by the Walkley-Black method (21); pH using solid suspensions in double distilled water at a 1:2.5 w/v ratio (22), and mineralogy by X-ray diffraction.
Chemical fractionation of P
The distribution of organic (humic and fulvic P) and inorganic P was established by the sequential extraction procedure of Steward and Oades (23) modified by Borie and Zunino (17). Briefly, 1.5 g of soil or sewage sludge was equilibrated with 15 mL of 1 M HCl, then the suspension was centrifuged and filtered to obtain the acid extractable P fraction. Inorganic P in the fraction was determined directly in this extract. Total P of the acid extract was determined after organic matter destruction with NaBrO (24). The organic P of the acid extract was estimated by difference between the total and inorganic P contents. The remaining solids, after the acid extraction, were ultrasonically dispersed and extracted three successive times with 25, 15, and 15 mL of 0.5 M NaOH, respectively. The distribution of the inorganic and organic P of the combined alkaline extract was determined in the same manner as for the acid extract. Total inorganic P corresponds to the sum of inorganic P in the acid plus alkaline extracts. Total organic P corresponds to the sum of fulvic and humic P. Total extracted P, designated as total P, corresponds to the sum of total inorganic and total organic extracted P (18).
Extraction procedure for 31P-NMR and measurements conditions
A two step method was used; ten grams of sample (soil or sewage sludge) were extracted with 75 mL of 1 M HCl, and shaken for one hour at room temperature. The remaining solid was separated by centrifugation, sequentially treated with 125, 75, and 75 mL of 0.5 M NaOH (15), and sonicated during 3, 1, and 1 min. Acid and alkaline extracts were equilibrated with 30 g of cation exchange resin (Chelex 100, Na form, Bio-Rad 142-2832), shaked 17 hours on a reciprocal shaker, centrifuged, and filtered through 0.45 mm pore size. The final extract was freeze dried, redissolved in 3.0 mL of D2O, shaken for 2 hours, centrifuged and transferred to NMR tubes.
The 31P-NMR spectra of soil extracts were obtained at 202,459 MHz in a Varian Inova 500 MHz spectrometer, using a 45 pulse with a 1.5-s delay and acquisition time of 0.506 s. The 31P-NMR spectra were proton decoupled using the standard waltz decoupling scheme. The decoupler was gated on during the acquisition time to avoid nuclear Overhauser enhancement. Depending on the sample, 4000 to 15000 scans were accumulated; the chemical shifts were measured relative to external orthophosphoric acid (85%).
RESULTS AND DISCUSSION
Soil and sewage sludge characterization
The location and main characteristics of soils and sewage sludge are presented in Table 1. Organic carbon content of the two Andisols (Diguillín and Ralún) was higher than that of the Ultisol (Collipulli) and the Mollisol (Colina). Soils derived from volcanic materials were acid, their pH varied from 5.4 to 6.3; Colina soil presented a neutral pH (6.9). The sewage sludge showed a neutral pH similar to Colina soil, but a higher organic carbon content than the soil samples examined (Table 1).
The concentrations of inorganic P, humic P, and fulvic P in the alkaline and acid extracts were higher in the sewage sludge than in the soil samples, with the exception of fulvic P in Diguillín. The total P concentration in the sewage sludge ranged from 6 to 18 times higher than in soils. The organic P concentration, which ranged from 42 to 50% of the total P, was prominent in Diguillín and Ralún soils, both with high amounts of organic carbon; on the other hand, organic P represented only 17% and 9% of the total P in alluvial soil and sewage sludge, respectively. In terms of relative distribution, soils and sewage sludge presented similar values of humic P, ranging from 68 to 79% of the organic P in soils, and accounting for 80% in the sewage sludge.
The P chemical fractionation procedures are not exhaustive. When the total P determined by chemical fractionation is compared with the total P determined by elemental analysis (26), between 50 to 85% of the soil P was recovered (18) with the procedure used in this study.
The 31P-NMR spectra of the alkaline (NaOH) and acid (HCl) extracts of all samples are presented in Figures 1 and 2. The spectra generally exhibited good signal-to-noise ratios. The assignment of peaks in the NMR spectra for acid and alkaline extracts, was made by chemical shift measurements relative to orthophosphoric acid, and on the basis of previous reports (3-12).
The 31P-NMR spectra of the alkaline extracts of soils and sewage sludge showed only slight differences in some signal intensities (Figure 1). The signals of the 31P-NMR spectra of soils and sewage sludge were well resolved and may be assigned to inorganic orthophosphate (at 7.0 ppm) and monoester P (multiple signals in the 6.5-5.0 ppm range). Signals associated diester P from Ralún soil (1.0-0.5 ppm) and pyrophosphate at -3.5 ppm (with the exception of Colina soil) were also seen.
Usually, the acid extraction has been used as a cleaning procedure prior to the extraction of the alkali-soluble P-forms (12, 26, 27). In soils, most of the extracted P is in the alkaline extract; on the contrary, in the sewage sludge about 75% of the extracted P is found as acid-soluble inorganic P (Table 2). A tentative assignment of the P-NMR signals of the acid-soluble P-forms of the sewage sludge and soils was also considered. All 31P-NMR spectra showed a similar P distribution pattern (Figure 2) in the acid extract with the exception of Collipulli, which has a very low amount of acid extractable P (Table 2). All signals are shifted to lower ppm values because resonances are sensitive to pH (9), but are located in the same relative position as those seen in the alkaline extract. The signals found in the soil correspond to inorganic P-forms, mainly inorganic orthophosphate (at -10.0 ppm), and a signal located in the 16 to 18 ppm range, tentatively assigned to pyrophosphate. The 31P-NMR spectra of sewage sludge, whose signals are broader than those of the soil acid extracts, may be assigned to inorganic orthophosphate and pyrophosphate. Organic P-forms cannot be discarded in this extract because the broad orthophosphate peak may hide their signals.
Fig. 1.- Phosphorus-31 nuclear magnetic resonance spectra of concentrated alkaline extracts from 0-15 cm depth of soils and sewage sludge.
Fig. 2.- Phosphorus-31 nuclear magnetic resonance spectra of concentrated acid extracts from 0-15 cm depth of soils and sewage sludge.
The 31P-NMR spectrum pattern of the alkaline extract of sewage sludge was similar to those previously reported for anaerobically digested sewage sludges in the USA, where inorganic orthophosphate is the main signal seen; however, when the sewage sludge is aerobically digested, the main signal corresponds to diester P (28).
Distribution of P forms in soil and sewage sludge extracts
Usually, inorganic orthophosphate and diester P, which can be easily mineralized, are considered available P forms, but in volcanic soils inorganic orthophosphate is rapidly and strongly fixed and the organically complexed P is not readily available to plants. On the other hand, inorganic orthophosphate fixation is not a problem in Mollisols. Diester P was one of the weakest signals observed by 31P-NMR in the alkaline extracts; this signal was not present in Colina spectra, so available P in this soil must be mostly inorganic P.
The disposal of sewage sludge in Chilean soils could result in the application of important amounts of inorganic orthophosphate and limited amounts of pyrophosphate and organic P (mostly assigned to monoester P). Therefore sewage sludge disposal will contribute mainly with available inorganic orthophosphate to plants.
Some conclusions can be drawn when the P forms in sewage sludge and the soil properties are considered together. More than 90% of inorganic orthophosphate is fixed in Chilean volcanic soils over a period of time which ranges from hours to a few days (29, 30). The addition of sewage sludge will increase the availability of inorganic orthophosphate during a short period of time, followed by its fixation as inorganic P and also its accumulation as non available monoester P. The application of sewage sludge as a P fertilizer needs to be controlled in relation with plant requirements, and should probably be followed by new applications over the whole period.
The disposal of sewage sludge in Mollisols, such as Colina soil, will result in a huge increase of inorganic orthophosphate availability, higher than the P uptake by plants. Depending on the rain regime and irrigation system, most of this inorganic P will be transported and may cause water pollution and eutrophication.
Total P concentration in the sewage sludge was 6 to 18 times higher than the amount found in soils. Almost 91 % of the total P was present in sewage sludge as inorganic P.
Similar P forms were presents in soils and sewage sludge, corresponding mainly to inorganic orthophosphate and monoester P; lower amounts of pyrophosphate and diester P were also detected.
Based on soil characteristics and properties (Ultisol, Andisols, and Mollisol), and physico-chemical considerations, it might be inferred that the use of sewage sludge from anaerobically digested wastewater as P fertilizer would not be successful and could potentially contribute to soil and water pollution.
This study was supported by DICYT-USACH, FONDECYT 2003 1030778, and the University of California, Riverside. M. Briceño acknowledges the financial support provided by CONICYT to her graduate program and MECESUP.
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