1 Introduction

As byproduct of wastewater treatment processes, sewage sludge is considered as one of the major environmental concerns. Because of its fertilizer and soil conditioning properties, sewage sludge has been used in agricultural application for decades which is suggested as one of the most important sludge recycling processes [1]. Landfilling, incineration, and river or sea disposal have been used as disposal alternatives [2].

The composition of sewage sludge depends on the type of wastewater and sludge treatment processes and may consist of a variety of macro- and micronutrients, inorganic and organic pollutants, and microorganisms. However, the presence of hydrocarbons, pesticides, heavy metals and further pollutants in sewage sludges and their potential accumulation in human tissues through the food chain [3], caused environmental and human health concerns. Furthermore, these concerns led to restrictions regarding the usage of sewage sludge as fertilizers. As an example, according to the German Environmental Agency (Umweltbundesamt), the usage of sewage sludge for agricultural and landscape engineering in Germany dropped from 45% in 2012 to 35% in 2016 [4].

Polyacrylamide (PAA) refers to a group of high molecular weight water-soluble polymers [4, 5]. PAA application is predetermined by the structure of chemical modified polymer derivates. Characterized by additional side groups on the polymer chain, PAA can be developed as a cationic (cPAA), anionic (aPAA), nonionic or amphoteric compound [6]. There is a wide range of applications regarding this polymer, e.g. chemical industry, flocculation processes, food processing, drilling activities, or mineral extraction [7, 8]. As mentioned, one of its main fields of application is the usage as flocculation agent in wastewater treatment and sludge dewatering processes [9, 10]. Therefore, it is a common constituent in sewage sludges.

The application of sewage sludge conditioned with polyacrylamides results in the emission of these polymers into soil. As a very rough estimation based on German regulations, assuming an agricultural area fertilized for ten years with sewage sludge conditioned with flocculants, a maximum amount of about 11 mg of polymer per kilogram of soil can be expected [11]. Due to the high adsorption capacity and the low migration tendency, PAA polymer chains stay fixed in the soil and are unlikely to leach into the groundwater [11]. PAA is not considered as a toxic compound itself. However, it can have acute toxic effects on aquatic organisms, with the cationic form being more toxic than the anionic form [12, 13]. In addition, with a rise in cPAM concentration in aqueous solution, the solution undergoes significant acidification and salinization, leading to potentially harmful effects on the early growth stages of plants [13]. Therefore, considering its wide usage range and the obvious direct release into the environment as well as the concerns about assumed indirect phytotoxic effects [14], there is a need for analysing PAA in the environment.

This study aimed to transfer a previously developed method by Kronimus and Schwarzbauer [14] comprising tetramethylammonium hydroxide thermochemolysis GC–MS analysis of modified PAA, into an offline concept. In advantage over an online approach, an offline approach allows analysis of higher sample amounts as well as the usage of surrogate standards, leading to a precise quantitative analysis of pyrolytic products also in environmental samples with very low concentration levels [15]). Applying high temperatures on polyacrylamide, several factors are influencing the thermo-degradation process e.g. molecular weight, possible presence of impurities, oxygen availability as well as the aging history of the polymer [16]. Thermal degradation is also predetermined by the applied temperature [17]. PAA is relatively stable at temperatures below 200 °C and it can undergo only a slight weight loss, due to evaporation of volatile additives and absorbed water [18, 19]. At temperatures between 200 and 300 °C, PAA undergoes irreversible intra- and intermolecular imidization reactions, releasing NH3, H2O, and CO2 as byproducts [20, 21]. The temperature region above 300 °C is characterized by releasing volatiles such as H2O and CO2 by elimination from the imide structure as well as chain scission forming glutarimide derivates [22].

According to Kronimus and Schwarzbauer, in particular glutarimide derivates are considered as specific thermal degradation products of PAA. In this study, cPAAwas used for the identification and verification of specific polymer pyrolysis products as well as for optimizing offline pyrolysis conditions. Furthermore, the developed analytical procedure was used for identifying and quantifying polyacrylamide-based flocculants in sewage sludge samples from Germany.

2 Experimental

2.1 Chemicals and reference compounds

Cationically modified polyacrylamide-based flocculant (Efaflocc 1742 ®) used for generating external calibration and for testing with spiked samples was purchased from EFA Chemie GmbH. For methylation purposes, a 0,1 M methanolic trimethylanilin hydroxide (TMAH) solution was used, purchased from CS Chromatographie Service GmbH, Langerwehe, Germany. Methanol, Acetone, Hexane and Dichlormethane (DCM) used for the preparation of sewage sludge and spiked samples and for pyrolysis experiments were purchased from Th. Geyer GmbH & Co. KG.

2.2 Samples

Soil material used for spiked samples was collected at Westpark in Aachen, Germany. Sewage sludge samples were taken from four different municipal wastewater treatment plants in Germany North-Rhine Westphalia (NRW) and Bavaria. One treatment plant from NRW handles up to 50% of industrial sewage. All samples were collected in 2015 and were stored deep frozen prior to pyrolysis.

2.3 Sewage sludge sample preparation

To decrease the amount of organic substrate, sewage sludge samples were pre-extracted. Therefore, extraction of 10 g of each sample was carried out with 40 mL of methanol followed by 40 mL of acetone and 40 mL of n-hexane and treated with a high dispersion tool at 11,000 rpm (Ultra-Turrax T25 basic, IKA Labortechnik, Staufen, GER) for 3 min respectively. This represents a common standard procedure for environmental samples. The extraction was followed by centrifugation at 1500 rpm for 10 min (Y 383, Hermle, Wehingen, GER). Afterwards, the samples were left overnight for drying and then subjected to thermochemolytic analyses.

2.4 Off-line TMAH-thermochemolysis

Off-line pyrolysis and TMAH-thermochemolysis were performed on a modified gas chromatograph Hewlett Packard instruments, GC 5890 series. The experiments were carried out in a stainless steel sample tube filled with sea sand and glass wool. On one end, the sample tube was connected with a continuous nitrogen flow, while the other end was connected to a flask tube filled with 4 mL of dichloromethane (DCM) and cooled with dry ice and ethanol. For derivatization procedures, defined amounts of TMAH methanolic solution were added. The experiments were performed under the following conditions: start at 60 °C, the heating rate of 70 °C/min, maximal temperature 400 °C, duration time 30 min, constant nitrogen flow of 120 mL/min. After the pyrolysis/TMAH-thermochemolysis, the volume of the pyrolyzates was reduced to approximately 2 mL and dried over anhydrous sodium sulfate. Finally, pyrolyzates were analysed by gas chromatography-mass spectrometry (GC/MS).

The described procedure for the TMAH-thermochemolysis was applied on pure cPAA samples, spiked samples and sewage sludge samples. Since a maximum of 4 g of the sample fits into the sample tube, to thermochemolyse 10 g of sewage sludge, three experiments were performed for each sample, with the degradation products collected in one flask tube. The amount of TMAH used for every thermochemolysis experiment varied between 500 and 700 µl, depending on the consistency of the sample.

2.5 Gas chromatography-mass spectrometry (GC–MS) analysis

Qualitative and quantitative GC–MS analysis was performed on an Thermo Quest Trace GC gas chromatograph connected to Thermo Quest Trace MS single quadrupole mass spectrometer. It was equipped with a ZB-5 capillary column (30 mm × 0.25 mm ID × 0.25 μm film). Chromatograph conditions were: splitless mode injection (injector temperature 270 °C) with a start temperature of 60 °C, isothermal time 3 min and heating up to 310 °C with a ramp of 5 °C/min, carrier gas flow of 1.5 mL/min, an electron impact ionization mode (EI + , 70 eV) with a source temperature of 200 °C and an interface temperature of 270 °C scanned from 35 to 500 m/z in full scan mode with 0.67 scans/s.

For external calibration, thermochemolysis was applied on pure cPAA dissolved in HPLC water with total amounts from 10 to 100 μg of the polymer. As methylation agent, 2 µL of a TMAH solution was added for every 100 µg of cPAA. Quantitative GC/MS analysis was corrected for inaccuracies using a surrogate standard solution (d34-hexadecane; 6.0 ng/μL).

After thermochemolysis the pyrolyzates volumes were reduced to 0.5 mL and fractionated by liquid chromatography on a microcolumn filled with 2 g of activated silica gel. The fractionation was carried out with solvents of increasing polarity, starting with 5 mL of dichloromethane (DCM), followed by 5 mL of a DCM-MeOH (90/10 v/v) mixture and 5 mL of methanol (MeOH). Degradation products were analysed in the third fraction.

The same procedure was used for spiking experiments. Prior to extraction, 10 g of sediments were spiked at a level of 500 µg/g whereas, a 1.00 µg/µL cPAA water solution was used. This experiment was carried out in triplicate.

3 Results and discussion

3.1 TMAH-thermochemolysis experiments

As an optimized form of conventional off-line pyrolysis, TMAH-thermochemolysis is a well established thermally assisted methylation method allowing analyses of organic macromolecules [23]. This process occurs under high temperatures, by replacing the protons of the functional groups with a methyl group, using TMAH as a methylation agent [24, 25]. However, the effect on the pyrolysis process is not limited to pure methylation but in comparison with conventional off-line pyrolysis, TMAH-thermochemolysis initiate also a wider range of complementary and independent pyrolysis reactions [26].

As an initial step, off-line pyrolysis without in-situ derivatization and TMAH-thermochemolysis have been compared for cPAA analysis. As illustrated in Fig. 1, off-line pyrolysis conditions revealed on the one hand less pyrolysis products in comparison to TMAH-thermochemolysis experiments indicating a better defined pyrolysis process. On the other hand, TMAH-thermochemolysis revealed a more complex chromatogram but with substantial higher intensity especially of the specific thermal degradation products. The obvious lower pyrolysis yield caused a less sensitive detection in off-line pyrolysis leading to a failed detection of specific products at lower initial amounts of cPAA of around 10 μg. Hence, for the detection of cPAA at lower concentrations as in environmental samples, the application of a TMAH thermochemolysis was identified to be a more suitable approach as compared to off-line pyrolysis without in-situ derivatisation.

Fig. 1
figure 1

a Specific thermodegradation products of cPAA with the use of TMAH and b without the use of TMAH.

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In more detail, TMAH-thermochemolysis applied on pure cPAA revealed six main degradation products from which four are already reported [14]. These products are considered as specific thermal degradation products of PAA, formed by defined intermolecular reactions between the activated nitrogen and carbonyl groups. In detail, thermal breakage of the carbon chain and cyclization reactions form methylated imide structures with characteristic substitution pattern [14] as follows (Fig. 2): N—methylglutarimide (peak A), 2 –dimethylglutarimide (peak B), 4, N–dimethylglutaconic imide (peak C) and 2, 4, N–trimethylglutaconic imide (peak D). Two further degradation products are identified as positional isomers of compounds A and B. However, a quantitative correlation between the pyrolized amount of pure cPAA and the response levels of these two structural isomers was insufficient pointing to a non-reproducible pyrolytic formation of these products. Consequently, these degradation products were not considered for further quantitative analysis and are not discussed in the following.

Fig. 2
figure 2

a Total ion chromatogram obtained by applying TMAH-thermochemolysis on pure cPAA; b Mass spectra of the specific cPAA degradation products.

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To improve the TMAH-thermochemolysis method for a quantitative determination of polyacrylamide-based flocculants in environmental samples, a systematic optimization of the pyrolysis conditions has been performed. The following parameters have been considered as sensitive for the pyrolysis yield and composition of pyrolysis products: heating rate, pyrolysis temperature, inert gas flow and pyrolysis time. By testing these different parameter conditions, no dramatic effect in the investigated ranges of parameters on the response levels of degradation products was detected. However, the conditions that revealed the highest intensity of the specific thermal degradation products, allowing a more sensitive method for cPAA detection in environmental samples have been used for further experiments. The following optimal conditions were applied: start at 60 °C, the heating rate of 70 °C/min, maximal temperature 400 °C, duration time 30 min, constant nitrogen flow of 120 mL/min.

3.2 Reproducibility, linearity and sensitivity

Basic parameters for quality assessment of quantitative analysis are linearity of calibration system, reproducibility as well as sensitivity as given by the limit of quantitation LOQ. To obtain a five-point calibration curve, 10–100 µg of pure cPAA were thermochemolysed. The calibration was based on two TMAH-thermochemolysis performed for each quantitation level, corrected with surogat standard. Experiments revealed peak A as the product with the highest response level, followed by peaks C, D, and B respectively. The peaks indicated very high correlation coefficients of 0,99 showing very high accuracy (Fig. 3) and allowing a reliable quantitative determination of polyacrylamide based flocculants in environmental samples. Based on five repetitions of TMAH-thermochemolysis performed on 60 μg of pure cPAA, the standard deviation varied between 16 and 26% (Fig. 3.).

Fig. 3
figure 3

Quantitative correlation plot of the thermal degradation products of cPAA obtained by applying the developed TMAH-thermochemolysis method on pure cPAA

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Limit of detection (LOD) and of quantitation (LOQ) were calculated based on extrapolated signal-to-noise ratios of 1:3 and 1:10, respectively. The corresponding values related to the minimum amount of pyrolyzed polymer were around 0.07 µg as LOD and 0.22 µg as LOQ. Considering maximum sample amounts of 4 g in the pyrolysis tubes, minimum limits of detection and quantitation for particulate samples are calculated to be around 18 ng/g and 55 ng/g, respectively. However, superimposition by pyrolysis products of natural organic matter and further synthetic polymers will not allow to achieve these limits in real environmental matrices.

As a final quality assessment procedure the developed TMAH-thermochemolysis concept was applied on spiked soil samples. Previous experiments on blank soil samples showed no presence of glutarimide derivates, indicating that there were no polyacrylamide based flocculants or other interfering material present in the matrix prior to spiking. As a result of the spiking experiments, all four degradation products were identified demonstrating that natural matrices do not have an impact on thermodegradation behavior of cPAA. Therefore, an unambiguous detection and quantification of these polymers in environmental samples by TMAH-thermolysis based analysis is feasible in principal.

3.3 TMAH-thermochemolysis applied on sewage sludge samples

The developed pyrolysis based analytical method was tested for suitability on four sewage sludge samples from municipal wastewater treatment plants in Germany (Northrhine-Westfalia, S1-2, and Bavaria, S3-4). All four relevant cPAA degradation products were identified unambiguously in three out of four sewage sludge samples (S1, S2 and S3). The correlation was based on the retention times and the mass spectra (Fig. 4b) compared to the ones derived from the experiments carried out on pure cPAA and spiked samples. The pyrograms revealed retention time variations in a reasonable range (Fig. 4a).

Fig. 4
figure 4

a Ion chromatograms of the specific PAA thermodegradation products obtained from pyrolysing pure cPAA, spiked samples and sewage sludge sample (S1). b Mass spectrum of the peak C obtained by TMAH-thermochemolysis-GC–MS of: a pure cPAA (first panel) and b sewage sludge sample.

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The amount of released cPAA in sewage sludge samples was calculated with a correction by a surrogate standard. Further on, to compare the isomer pattern revealed by the analyses of the different sample types, peak integrals of the specific thermal degradation products of cPAA detected in sewage sludge samples are normalized to pyrolysis product A. Comparison comprised sewage sludge samples as well as spiked and pure cPAA samples are illustrated in Fig. 5.

Fig. 5
figure 5

The corresponding pattern by peak integrals of the specific thermal degradation products of cPAA relative to compound A

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The order of compound response levels in sewage sludge samples differs from the one obtained in the experiments on pure cPAA samples (Fig. 5). Nevertheless, all experiments on sewage sludge samples revealed significantly higher concentration levels for compounds A and B. However, the calculated values for peaks C and D are in a similar concentration range, on contrary to A and B compounds. Therefore, it can be concluded that very high concentrations obtained for N-methylglutarimide can partly be derived from other compounds present in the matrix. This can be explained by the published literature data, which point to the presence of some glutarimide derivates in natural matrices. For example, by studying the chemical nature of organic sediments using flash pyrolysis-GC–MS analysis on Mangrove lake sediments, Zang and Hatcher [27] detected N-methylglutarimide as one of the degradation products. Moreover, TMAH-thermochemolysis experiments on peptide-like material in geochemical samples revealed the presence of N-methylglutarimide as well [26]. Therefore, the absence of 4, N-dimethylglutaconicimide and 2, 4, N-trimethylglutaconicimide should be taken into consideration when detecting polyacrylamide based flocculants in environmental samples. This leads to a conclusion that for an unambiguous confirmation of polyacrylamide based flocculants in environmental samples, all four specific cPAA thermal degradation products need to be detected. Nevertheless, since the presence of numerous natural matrices in environmental samples can have a significant contribution to the concentration of N-methylglutarimide, cPAA concentration in environmental samples should be calculated based on the values obtained from the glutarimide derivates methylated at positions 2 and 4. Consequently, in this study the concentrations of cPAA in sewage sludge samples are calculated based exclusively on the compounds C and D. Based on the two independent external calibrations, the obtained values vary slightly pointing to the uncertainties in this low concentration approach. As best assessment, the concentration of cPAA in sewage sludge samples was calculated as an average value of the concentrations obtained for the thermochemolysis products C and D presented in Table 1, pointing to a concentration level of cPAA in the sewage sludge samples at around 40 to 60 µg/g. The concentrations were calculated based on the obtained calibration function and corrected with the standard deviation values.

Table 1 Concentrations of cPAA in sewage sludge samples
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To our knowledge, these are the first published data on the cPAA contamination in environmental samples. However, the determined values are within an expected range for sewage sludge samples, but the relatively high concentrations (µg/g level) indicate an elevated environmental relevance of these flocculants.

4 Conclusion

This study aimed to develop an off-line TMAH-thermochemolysis method for the determination of polyacrylamide based flocculants in environmental samples. The pyrolysis products appeared in a typical pattern of homologues identified as methylated glutarimide derivates allowing not only an unambiguous identification of the polymer but also a pyrolysis based quantification. Spiking experiments on soil samples revealed the presence of all four specific thermal degradation products indicating that an unambiguous identification of PAA in environmental samples is possible. Furthermore, thermochemolysis experiments on sewage sludge samples revealed the same specific degradation products, as the experiments on pure cPAA and spiked samples.

The quantitative determination was based on these four specific thermal degradation products of PAA. However, the presence of various natural peptide-like materials in environmental samples might contribute to the level of N-methylglutarimide and, therefore, this interferences need to be considered for quantitation by calculating concentrations only with unaffected isomers. Nevertheless, for a doubtless confirmation of these polymers in environmental samples, an identification of all four degradation products is obligatory.

This study opened the possibility to detect polyacrylamide based flocculants even at low concentrations in complex matrices, hence an application to environmental samples has become feasible.