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Journal of Environmental Quality 32:1516-1522 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Waste Management

Characterization of Sewage Sludge Organic Matter Using Solid-State Carbon-13 Nuclear Magnetic Resonance Spectroscopy

Ronald J. Smernik*,a, Ian W. Olivera and Graham Merringtona,b

a Dep. of Soil and Water, Waite Agricultural Research Institute, Univ. of Adelaide, Glen Osmond, South Australia 5064, Australia
b Environment Agency, National Centre for Ecotoxicology & Hazardous Substances, Wallingford, Oxfordshire, OX10 8BD, UK

* Corresponding author (ronald.smernik{at}adelaide.edu.au)

Received for publication November 14, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Six sewage sludges from five sewage treatment plants in Australia were characterized using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Spectra were acquired both before and after removal of mineral components through treatment with hydrofluoric acid (HF). Carbon mass balance indicated that little organic matter was lost on HF treatment, which significantly improved NMR sensitivity and spectral resolution, and decreased acquisition time and hence cost of NMR analysis. Two NMR techniques were used, the standard cross polarization (CP) technique and Bloch decay (BD). The BD technique had not been applied previously to the analysis of sewage sludge. For each sludge sample, both before and after HF treatment, the BD spectrum contained significantly more alkyl carbon. Spin counting, another technique applied to sewage sludge here for the first time, showed that the BD spectra of the HF-treated sludges were quantitative, while approximately 30% of the CP NMR signal went undetected. The discrepancy between CP and BD spectra was attributed to the presence of alkyl carbon with such high molecular mobility that the efficiency of cross polarization is affected. This study shows that sewage sludge organic matter is significantly different in chemistry to soil organic matter and has implications for the application of sewage sludge to agricultural land.

Abbreviations: BD, Bloch decay • Cobs, proportion of potential carbon-13 nuclear magnetic resonance signal intensity detected in nuclear magnetic resonance spectrum • CP, cross polarization • HF, hydrofluoric acid • NMR, nuclear magnetic resonance • T1{rho}H, proton spin-lattice relaxation rate in the rotating frame


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SEWAGE SLUDGE, the residual solid produced during wastewater treatment, represents both an opportunity and a problem. It is usually rich in nutrients, particularly nitrogen (Binder et al., 2002), phosphorus (Hogan et al., 2001), and trace metals, as well as organic matter, which improve soil fertility and productivity when applied to agricultural land. However, it can also contain high concentrations of potentially toxic organic chemicals (Jones and Sewart, 1997; Wang and Jones, 1994), heavy metals (Walter et al., 2002; Oudeh et al., 2002), and/or pathogens (Dumontet et al., 2001) that can affect human health, either through direct exposure, or via uptake by crops or livestock. Sludge organic matter influences both beneficial and detrimental effects of land application of sewage sludge. Moreover, it is not only the amount (carbon content) of the organic matter that is important, but also its "quality" or chemical composition. The quality of the organic matter determines its rate of decomposition, which in turn influences the rate of mineral nitrogen release (Rowell et al., 2001), as well as the release of potentially toxic organic-bound metal cations and sorbed organic molecules. Sludge organic matter also represents an additional, and chemically different, food source for soil microbes, which may affect the microbial community structure.

Solid-state 13C NMR spectroscopy is recognized as the best analytical method for determining the gross chemical composition of complex organic matrices (Kögel-Knabner, 2000). Solid-state 13C NMR spectroscopy was first used to characterize sewage sludge in 1984 (Piotrowski et al., 1984), and has been used in a number of studies since (Leinweber et al., 1996; Ayuso et al., 1997; Hsiao and Lo, 2001; Stacey et al., 2001; Rowell et al., 2001). Sewage sludge is reported to contain similar broad classes of functional groups, but is generally found to have more alkyl (unsaturated hydrocarbon) functionality than soil organic matter.

In previous studies (Piotrowski et al., 1984; Leinweber et al., 1996; Ayuso et al., 1997; Hsiao and Lo, 2001; Stacey et al., 2001; Rowell et al., 2001) the cross polarization (CP) technique was used in combination with magic angle spinning (MAS). However, it is known that this technique underrepresents some structural units in soil organic matter, in particular, fused aromatic moieties, alkyl groups with high degrees of molecular mobility, and carbonyl carbon (Hu et al., 2000; Mao et al., 2000; Smernik and Oades, 2000a, b; Preston, 2001). We have shown also, using a calibration technique known as spin counting, that the alternative Bloch decay (BD) technique, otherwise known as direct polarization (DP) or single pulse excitation (SPE), does provide accurate quantitative distributions of structural units in soil organic matter (Smernik and Oades, 2000b).

Another recent advance in solid-state 13C NMR analysis of soil organic matter is pretreatment with hydrofluoric acid (HF) (Skjemstad et al., 1994; Schmidt et al., 1997). This has two beneficial effects: (i) by removing most of the inorganic fraction, the organic matter is concentrated, improving sensitivity, and (ii) by removing paramagnetic species the observability of the organic matter 13C NMR signal is increased, resolution is improved as paramagnetic broadening is reduced, and quantitation is improved for moieties that were closely associated with paramagnetic species.

In this study, we use the spin counting technique to test the ability of CP and BD to provide quantitatively reliable solid-state 13C NMR spectra of a range of sewage sludges. We also investigate the applicability of HF pretreatment of sewage sludge before NMR analysis.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sample Collection and Description
Bolivar 95 was collected from Bolivar Sewage Treatment Works, the largest treatment plant servicing the metropolitan area of Adelaide, South Australia. The plant processes the majority of the city's domestic and industrial wastewater (approximately 130 ML/day). Sludges from primary and secondary (trickle filtration) treatment processes are combined, anaerobically digested, dried in evaporation basins, and finally stockpiled. At time of collection, Bolivar 95 sludge was 5 yr old. Due to high electrical conductivity (EC), Bolivar 95 sludge was leached to reduce salt content.

Bolivar 97 was from the same treatment plant as Bolivar 95, but was 3 yr old at time of collection. The Bolivar 97 sludge was not leached.

Chelsea 96 was collected from the Eastern Treatment Plant, one of two plants servicing metropolitan Melbourne, Victoria. The plant processes 350 to 400 ML wastewater/day, mostly of domestic origin. Sludges from primary and secondary (activated sludge) treatment are combined, anaerobically digested, dried evaporatively, and stockpiled. When collected, Chelsea 96 sludge was 3 yr old.

Werribee 97 was collected from the Western Treatment Plant, Victoria. The plant services the city of Melbourne and its surrounding region, and is the largest plant in Australia in terms of area. The plant processes domestic wastewater and 80% of Melbourne's industrial waste (500 ML/day, total). Sewage is processed by land filtration and lagoon stabilization. Drying is by evaporation.

West Hornsby was collected from West Hornsby Treatment Plant, New South Wales. The plant processes 10 ML wastewater per day, mostly of domestic origin.

Port Kembla was taken from Port Kembla Treatment Plant, New South Wales. The plant processes wastewater (13 ML/day) from heavy industry (including metal smelters) and domestic sources. Sludge undergoes primary sedimentation and anaerobic digestion before dewatering. Port Kembla sludge was less than 5 yr old when collected. To reduce electrical conductivity, Port Kembla sludge was leached with distilled water (4 pore volumes).

Chemical properties of the sludges are shown in Table 1 .


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  		Table 1. Some chemical properties of the whole sludges.

 
Sample Preparation
Whole Sludges
Air-dried samples of the sewage sludges were ground to <2 mm. The whole sludge samples underwent no further pretreatment, except for the Bolivar 95 and Port Kembla samples, which were leached with deionized water to reduce the salt content.

Hydrofluoric Acid–Treated Sludges
Hydrofluoric acid treatment was performed according to the method of Skjemstad et al. (1994). Briefly, this consisted of nine successive treatments with 2% hydrofluoric acid solution. Each treatment involved shaking 3-g portions of whole sludge in 50 mL of 2% hydrofluoric acid solution, end-over-end, for periods of 1 h (five times), 16 h (three times), and 64 h (once). Between treatments, samples were centrifuged and the supernatant was discarded and replaced with fresh 2% hydrofluoric acid solution. Following the final treatment, the residue was rinsed three times with deionized water, then freeze-dried. Note that great care must be taken when working with HF to ensure against contact with the skin or inhalation of vapors. Even relatively small exposures can result in serious injury or death.

Nuclear Magnetic Resonance Spectroscopy
Solid-state 13C magic angle spinning (MAS) NMR spectra were obtained at a 13C frequency of 50.3 MHz on a Varian (Palo Alto, CA) Unity 200 spectrometer. Samples were packed in a 7-mm-diameter cylindrical zirconia rotor with Kel-F end-caps and spun at 5000 ± 100 Hz in a Doty Scientific (Columbia, SC) MAS probe. Free induction decays were acquired with a sweep width of 40 kHz; 1216 data points were collected over an acquisition time of 15 ms. All spectra were zero-filled to 8192 data points and processed with a 50 Hz Lorentzian line broadening and a 0.005-s Gaussian broadening. Chemical shifts were externally referenced to the methyl resonance of hexamethylbenzene at 17.36 ppm.

Cross polarization (CP) spectra were acquired using a 1-ms contact time. A 0.5-s recycle delay was used for the whole sludges and a 1-s recycle delay for the HF-treated sludges. Between 5000 and 100 000 transients were collected for each spectrum. Bloch decay (BD) spectra were acquired using a 6.2-ms (90°) 13C pulse. A recycle delay of 90 s was used for all samples. Between 900 and 1000 transients were collected for each sample. The BD spectra were corrected for background signal (Smernik and Oades, 2001).

Spin counting experiments were performed using the method of Smernik and Oades (2000a)(b). Glycine (AR grade; Ajax Finechem, Seven Hills, NSW, Australia) was used as an external intensity standard (i.e., the glycine spectrum was acquired separately to those of the samples). For CP spin counting experiments, differences in spin dynamics between the sample and the glycine standard were accounted for using the method of Smernik and Oades (2000a), except that a variable spin lock (VSL) rather than a variable contact time (VCT) experiment was used to determine proton spin-lattice relaxation rate in the rotating frame (T1{rho}H) (Smernik et al., 2002). Errors in carbon NMR observabilities (Cobs values) are estimated to be ±10% in Cobs (CP) and ±15% in Cobs (BD) (Smernik and Oades, 2000a).

Elemental Analyses
Carbon contents of both whole and HF-treated sludges were measured using a LECO (St. Joseph, MI) CR12 carbon analyzer. Metal contents of whole sludges were determined by inductively coupled plasma atomic emission spectroscopy (ICP–AES) on aqua regia (1:3 HNO3 to HCl) digests.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Hydrofluoric Acid Treatment of Sludges
The removal of the mineral component of soils with HF (Skjemstad et al., 1994; Schmidt et al., 1997) has become a common pretreatment in solid-state 13C NMR studies of soil organic matter. Hydrofluoric acid treatment aids NMR spectroscopic analysis by both concentrating the organic fraction and by removing paramagnetic minerals that cause signal loss and broadening. However, for some soil samples, significant losses of carbon have been reported on HF treatment (Dai and Johnson, 1999). This is most likely to occur for soils that contain large amounts of low molecular weight organic matter associated with clay or iron and aluminium oxides. Dissolution of these mineral components may then liberate organic matter that is water soluble when not complexed. Carbon recovery on HF treatment of the six sludges is reported in Table 2 . Recoveries were in the range 79 to 87%, indicating that the majority of organic matter was recovered. Much of the unrecovered carbon was probably lost during the numerous manipulations involved in the nine successive HF treatments. Such losses are nonselective, so it can be reasonably assumed that the HF-treated residues accurately reflect the composition of the sludge organic matter. Hydrofluoric acid treatment nearly doubled the carbon content of five of the sludges, and increased the carbon content of the other sludge, Chelsea 96, by a factor of nearly eight. The HF-treated residues still contain some minerals that are particularly resistant to HF treatment (Skjemstad et al., 1994), for example, relatively large quartz grains. For this reason, differences in the carbon content of the HF-treated sludges should not be taken to reflect differences in the nature of the organic matter.


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  		Table 2. Carbon contents of whole and hydrofluoric acid (HF)-treated sludges and carbon recovery on HF treatment.

 
Solid-State Carbon-13 Nuclear Magnetic Resonance Spectra of Whole Sludges
The CP and BD 13C NMR spectra of the six whole sludges are shown in Fig. 1 . The distribution of signal intensity between four chemical shift regions is shown in Table 3 . The four chemical shift regions are: 190 to 165 ppm assigned to carbonyl carbon in carboxylic acids, esters, and amides; 165 to 110 ppm assigned to aryl carbon, including O-aryl (165–145 ppm) as well as C- and H-substituted aryl carbon (145–110 ppm); 110 to 45 ppm assigned primarily to O-substituted alkyl carbon in carbohydrates, but also including methoxyl carbon and N-substituted alkyl carbon in protein; and 45 to 0 ppm assigned to alkyl carbon.



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  		Fig. 1. Carbon-13 cross polarization (CP) and Bloch decay (BD) nuclear magnetic resonance (NMR) spectra of the whole sludges. The number of scans acquired for each CP spectrum was: Bolivar 95 = 11 000; Bolivar 97 = 18 000; Chelsea 96 = 24 000; Werribee 97 = 100 000; West Hornsby = 27 000; Port Kembla = 29 000. Each BD spectrum represents 900 to 1000 scans.

 

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  		Table 3. Percent of total nuclear magnetic resonance (NMR) signal contained in four chemical shift regions in 13C cross polarization (CP) and Bloch decay (BD) NMR spectra of whole and hydrofluoric acid (HF)–treated sludges.

 
The highest peak in each CP spectrum in Fig. 1 is at around 70 to 75 ppm and is attributed to the singly oxygen-substituted C2 through C6 carbons of sugar residues in carbohydrates. The peak at 103 to 106 ppm is attributed to the doubly oxygen-substituted anomeric (C1) carbons in carbohydrates. The O-alkyl region contains the largest proportion (45.4–49.6%) of signal in the CP spectra of the whole sludges (Table 3). O-alkyl carbon also usually represents the largest proportion of 13C signal in CP spectra of soil organic matter (Mahieu et al., 1999). The alkyl (0–45 ppm) region is also prominent in the CP spectra of the whole sludges (Fig. 1), representing 23.3 to 30.1% of total signal (Table 3). The alkyl region of each CP spectrum contains two resolved peaks, at 30 to 33 ppm and 23 to 25 ppm. Each CP spectrum in Fig. 1 contains two distinct resonances in the aromatic region, at around 130 ppm (C- and H-substituted aromatics) and 150 ppm (O-substituted aromatics) and this region contributes 14.5 to 21.2% of total signal (Table 3). The CP spectra of Werribee 97 and Port Kembla have the most prominent aromatic resonances (Fig. 1) and contain the most aromatic carbon (Table 3). Each CP spectrum contains a strong carbonyl resonance at 170 to 175 ppm (Fig. 1), and the carbonyl region contains 6.6 to 10.0% of total signal (Table 3). This resonance includes signal from carboxylic acid and carboxylate groups, the amide carbon in proteins and ester carbons.

The CP spectra in Fig. 1 also differ noticeably in signal-to-noise ratio and resolution (broadness of resonances). Signal-to-noise ratios are affected by the number of scans acquired (see Fig. 1 caption), sludge carbon content, and signal observability (see below). The resonances of Chelsea 96, Port Kembla, and particularly West Hornsby are broader than for Werribee 97 and the two Bolivar sludges. Such broadening is indicative of the presence of paramagnetic species, and Chelsea 96, Port Kembla, and West Hornsby indeed have the highest Fe contents (Table 1).

The BD technique is much less sensitive than CP (Smernik and Oades, 2000a, b; Preston, 2001), mainly due to the need for much longer recycle delays (time between acquisition of successive scans). This can be seen in Fig. 1, where the signal-to-noise ratios of the BD spectra are much lower, despite the longer acquisition times for the BD spectra. The CP spectra in Fig. 1 required 3 to 14 h, whereas each BD spectrum represents 23 to 25 h of spectrometer time. The BD spectra of the five sludges (Chelsea 96 was too low in carbon to produce a BD spectrum of adequate quality) shown in Fig. 1 appear quite similar to each other, but differ substantially from the corresponding CP spectra. The most striking difference between the CP and BD spectra is that the strongest resonance for each BD spectrum is due to alkyl rather than O-alkyl carbon. This is also apparent in the integral region data (Table 3), with the O-alkyl region 11 to 26% weaker and the alkyl region 9 to 41% stronger in the BD compared with the corresponding CP spectra.

Bloch decay spectra are more quantitatively reliable than CP spectra (Mao et al., 2000; Smernik and Oades, 2000a,b). This suggests that the discrepancy between the CP and BD spectra is caused by alkyl carbon being underestimated by the CP technique. This has also been reported for soils (Smernik and Oades, 2000b), but the degree of underestimation of alkyl carbon by CP was much smaller. Preston (2001) reported severe underestimation of alkyl carbon in CP spectra of plant litter. The alkyl signal "missing" from the CP spectra is believed to be that of alkyl structures with a high degree of molecular mobility, which results in inefficient transfer of magnetization from 1H to 13C nuclei during cross polarization (Hu et al., 2000; Smernik and Oades, 2000a, b; Preston, 2001).

Spin Counting on Whole Sludges
Spin counting provides a way to measure how quantitative an NMR spectrum is likely to be. Spin counting involves calibrating total signal intensity across the whole spectrum of a sample against a standard that is known to give a quantitative signal (glycine, in this case). The result is usually expressed in terms of carbon NMR observability (Cobs), the proportion of signal per unit carbon for the sample, compared with that for the standard. All mechanisms that affect quantitation in solid-state 13C NMR spectra of samples such as these cause the loss of potential signal. Thus, if Cobs is close to unity (Cobs = approximately 100%), signal loss is minimal and the distribution of signal between the various resonances accurately reflects the distribution of chemical structures that give rise to those resonances. In other words, a Cobs value of close to 100% ensures that the NMR spectrum is quantitative, rather than qualitative or "semiquantitative." If signal per unit carbon for the sample is less than that for the standard, then some 13C nuclei are not producing all of their potential signal intensity. Some mechanisms that cause signal loss are nonselective, affecting all 13C nuclei in a sample equally (Smernik and Oades, 2000a). Such mechanisms do not affect the ability to determine the distribution of chemical structures from the NMR spectrum. However, other mechanisms only affect certain chemical structures and so do compromise the ability to quantify chemical structures from the NMR spectrum (Pfeffer et al., 1984; Hu et al., 2000; Mao et al., 2000; Smernik and Oades, 2000a, b; Preston 2001). Thus, while a Cobs value of 100% proves that a spectrum is quantitative, the converse is not true, that is, a Cobs value of <100% only indicates that a spectrum may not be quantitative.

Results of spin counting calculations for both CP and BD spectra of the whole sludges are shown in Table 4 . The Cobs (CP) values were in the range 23 to 51%, indicating that only a quarter to a half of the potential NMR signal was detected using CP. The Cobs (CP) values include a correction for signal loss during the contact time caused by T1{rho}H relaxation (Smernik and Oades, 2000a), so the shortfall of Cobs (CP) from 100% is caused by mechanisms other than this. The Cobs (BD) values were in the range 56 to 101% and were generally approximately double those of the corresponding CP values. This indicates that the distribution of chemical structures is more likely to be accurately reflected by the BD than the CP spectra. Indeed, Cobs (BD) values for Werribee 97 (101%) and Bolivar 97 (89%) indicate that the BD spectra of these samples are quantitative. However, Cobs (BD) values of 56 and 58% for West Hornsby and Port Kembla leave open the possibility that the BD spectra of these samples are not quantitative.


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  		Table 4. Carbon-13 nuclear magnetic resonance (NMR) observabilities determined from spin counting experiments.

 
Solid-State Carbon-13 Nuclear Magnetic Resonance Spectra of Hydrofluoric Acid–Treated Sludges
The CP and BD 13C NMR spectra of the six HF-treated sludges are shown in Fig. 2 . The distribution of signal intensity between the four chemical shift regions is shown in Table 3. The benefit of HF treatment can be seen by comparing the CP spectra with those of the whole sludges (Fig. 1). Signal-to-noise ratios are generally improved despite the acquisition of fewer scans, and the resonances are better resolved, particularly for Chelsea 96, West Hornsby, and Port Kembla. The other main difference is that the alkyl region contains 13 to 42% more signal in the CP spectra of the HF-treated sludges than in the corresponding CP spectra of the whole sludges (Table 3). The increase in the proportion of alkyl carbon is compensated by decreases in the proportions of O-alkyl and aromatic carbon. Differences in the proportion of carbonyl carbon in CP spectra of whole and HF-treated sludges are small and variable.



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  		Fig. 2. Carbon-13 cross polarization (CP) and Bloch decay (BD) nuclear magnetic resonance (NMR) spectra of the hydrofluoric acid (HF)–treated sludges. The number of scans acquired for each CP spectrum was: Bolivar 95 = 10 000; Bolivar 97 = 5000; Chelsea 96 = 10 000; Werribee 97 = 6000; West Hornsby = 5000; Port Kembla = 10 000. Each BD spectrum represents 900 to 1000 scans.

 
Hydrofluoric acid treatment also clearly improved the quality of the BD spectra (Fig. 2), in terms of signal-to-noise ratios and resolution, when compared with the BD spectra of the whole sludges (Fig. 1). The improvement in signal-to-noise ratio is clearer than for the CP spectra, since approximately the same number of scans (900–1000) were acquired for all BD spectra (both whole and HF-treated sludges). Corresponding CP and BD spectra for the HF-treated sludges are again quite different in appearance (as was the case for the whole sludges), with the BD spectra containing much more alkyl signal. The BD spectra of the HF-treated sludges appear similar to the BD spectra of the corresponding whole sludges, other than the improved signal-to-noise ratios and resolution. The integral region data (Table 3) do not entirely support this conclusion, as there are substantial differences for BD spectra of corresponding whole and HF-treated sludges. As with the comparison of corresponding CP spectra, the BD spectra of the HF-treated sludges contain more (1–47%) alkyl carbon than do the BD spectra of corresponding whole sludges, but in this case the increase is mostly at the expense of aromatic and carbonyl, rather than O-alkyl carbon. This discrepancy is probably caused by the greater broadness of the resonances in the whole sludges. Broadening tends to redistribute signal to regions of lower intensity, simply because the regions with more signal (e.g., the alkyl region in these sludge spectra) lose more to adjacent regions than they gain from those same regions.

Bloch decay spectra of the six HF-treated sludges are all very similar to each other, with each dominated by the alkyl region (36.7–44.6% of total signal), with peaks at around 30 and 24 ppm, and a shoulder or weak peak at around 15 ppm. The BD spectrum of Werribee 97 contains relatively more aromatic carbon (17.6%) than those of the other sludges (9.7–15.4%). The alkyl region is particularly strong for West Hornsby and Port Kembla.

Spin Counting on Hydrofluoric Acid–Treated Sludges
The results of spin counting calculations on the HF-treated sludges are shown in Table 4. The CP observabilities were similar for all of the HF-treated sludges (67–75%). Hydrofluoric acid treatment improved CP observability for each sludge, but by differing amounts. A 35% improvement was recorded for Werribee 97, a twofold improvement for the two Bolivar sludges, and two- to threefold improvements for Chelsea 96, West Hornsby, and Port Kembla. Bloch decay observabilities for each of the HF-treated sludges were also similar and, since they were all close to 100%, indicate that the BD spectra of the HF-treated sludges are quantitative.

The spin counting results show the influence of two signal loss mechanisms. The first is the presence of paramagnetic minerals, in particular, iron minerals. Where these minerals are not closely associated with organic matter, the decrease in signal intensity is nonselective (all resonances in a spectrum are equally affected), although there may be some effect of broadening on the distribution of signal between chemical shift ranges. Both CP and BD spectra are affected in this case (Smernik and Oades, 2000a). Organic matter in close proximity (e.g., sorbed or complexed) to the paramagnetic mineral species is particularly affected, and may produce no 13C NMR signal at all. This can result in selective signal loss if the nature of the organic matter associated with the paramagnetic minerals differs from the organic matter not associated with these minerals. Hydrofluoric acid treatment removes interference caused by paramagnetic species, so, while BD observabilities of the whole sludges vary from 50 to 101%, BD observabilities of the HF-treated sludges are all close to 100%. The CP observabilities are also affected by this mechanism. For the whole sludges, the lowest CP observabilities correspond to the lowest BD observabilities (Table 4) and also the highest iron contents (Table 1).

The second signal loss mechanism is selective, affecting only the alkyl region. This mechanism is specific to the CP spectra, and so must be caused by interference to the process by which magnetization is transferred from 1H to 13C nuclei (i.e., cross polarization). Two parameters are important here, TCH, the rate of magnetization transfer, and T1{rho}H, the rate of 1H relaxation during cross polarization. Both of these parameters are affected by high degrees of molecular mobility. More detailed studies are in progress, but at this stage it is adequate to say that this second signal loss mechanism indicates the presence of alkyl carbon with a high degree of molecular mobility. It is the absence, or at least underrepresentation, of this component in the CP spectra that makes the CP and BD spectra appear so different. Interestingly, HF treatment partially alleviates this signal loss; the alkyl region is more prominent in the CP spectra of HF-treated sludges than in the corresponding whole sludges.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This work shows the clear benefits of three solid-state 13C NMR techniques (Bloch decay, spin counting, and pretreatment with HF) applied to the characterization of sewage sludge here for the first time. Without these three advances, erroneous conclusions would result. Clearly, only the BD technique provided a faithful representation of the distribution of structural groups in the sewage sludge organic matter. The CP spectra severely underestimated the proportion of alkyl carbon. The importance of this cannot be underestimated, since if the CP spectra were taken to adequately reflect the chemical structure, we would have concluded that sewage sludge and soil organic matter appear to be very similar, when in fact they are quite different. Spin counting was the vital technique for determining how quantitative the spectra were. Without employing such a technique, solid-state NMR spectra of complex and heterogeneous materials such as sewage sludge or soil organic matter can only ever be considered qualitative. The value of HF treatment was also proven. Carbon was recovered in nearly quantitative amounts, and it aided NMR spectroscopy both in terms of sensitivity and resolution, allowing better quality spectra to be acquired in less time (and with less expense). Hydrofluoric acid treatment also ensured an unbiased comparison of sludge organic matter spectra, free from interference by paramagnetic minerals, the concentration of which varied greatly between the sludges.

The highly alkyl nature (and specifically alkyl with high molecular mobility) of sewage sludge organic matter can influence the fate and behavior of sewage sludge applied to agricultural land. Leinweber et al. (1996) found that soils treated with sewage sludge were enriched in alkyl carbon. Kögel-Knabner et al. (1992a)( b) and Hu et al. (2000) have suggested that the mobile alkyl carbon is more easily decomposed than rigid alkyl carbon. Alkyl carbon (Chefetz et al., 2000; Salloum et al., 2002) and, in particular, mobile alkyl carbon (Hu et al., 2000; Mao et al., 2002) moieties have been implicated as one type of organic matter (aromatic structures being the other) that is highly sorptive of nonpolar organic contaminants. Investigations into the fate and behavior of organic matter in the sludges in this study have been initiated.


   ACKNOWLEDGMENTS
 
This work was, in part, funded by an Australian Research Council (ARC) grant.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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