Physicochemical analysis

The physicochemical characteristics and elemental composition of the biochar samples produced at different pyrolysis temperatures are listed in Table 1. The biochar yield and heating value decreased with the increasing pyrolysis temperature. The yield of biochar dropped from 64.28 to 46.66 wt% of the dry mass when the pyrolysis temperature was increased from 300°C to 700°C. However, only an additional 4.44 wt% yield was reduced when the temperature was increased from 700°C to 900°C. The decrease in the biochar yield with the increasing pyrolysis temperature could possibly be related to the cracking and volatilization process, which was also reported8 for sludge biochar pyrolyzed at different temperatures of 450°C, 650°C and 850°C. The heating value of the biochar dropped linearly from 11054.3 to 6635.6 kJ kg−1 when the pyrolysis temperature was increased from 300°C to 600°C and then almost remained constant with the temperature increasing from 600°C to 900°C.

Table 1 The physicochemical characteristics and elemental compositions of the sewage sludge (dry basis) and sludge biochar derived (dry basis) at various pyrolysis temperatures Full size table

An increase in the pyrolysis temperature from 300°C to 900°C led to an increase in the fixed carbon (FC) content from 15.75 to 21.25 wt% and an increase in the ratio of FC to carbon from 42.4 to 84.2. The FC content represented the efficiency of the pyrolytic conversion of ash-free organic matter in the sludge to a relatively pure, ash-free carbon15. The highest ash content and lowest volatile matter content were found in biochar sample of 900°C, which was mostly due to volatilization accompanied by the accumulation of inorganic oxides such as Si, Al and Fe16. For instance, the XRF results revealed that the ash composition of the biochar samples comprised 19.1%–22.7% Si. Comparison of the Si, Al, Ca and Fe contents of the ash composition (Table 2) indicated the increase in the inorganic components with the increasing pyrolysis temperature. One of the main characteristics of sewage sludge is the presence of high amounts of inorganic ash, when compared with other materials such as wood biochar or lignocellulosic char obtained from agricultural waste1. The increase in the amount of inorganic ash in the biochar could increase its mineral composition and capacity to adsorb polar molecules.

Table 2 The Si, Al and Fe contents in the ash of raw sludge and its sludge biochar samples Full size table

Carbon, hydrogen, oxygen, nitrogen and sulfur

The variations in the results of the elemental analyses of the sludge biochar samples with temperature are shown in Table 1. The biochar samples showed the pattern of depleted content of some elements. In samples of 300–900°C, the carbon content decreased from 37.15 to 25.23 wt% of dry mass, the hydrogen content decreased from 4.35 to 0.64 wt%, the nitrogen content decreased from 6.17 to 1.24 wt%, the sulfur content decreased from 1.54 to 0.55 wt% and the oxygen content decreased from 13.37 to 1.16 wt%. Particularly, when compared with the original dried sludge, more than 67.5% of hydrogen was removed in sample at 500°C and more than 80.0% of hydrogen was lost in samples pyrolyzed at over 700°C. Moreover, the H/C and O/C atomic ratios of the biochar samples demonstrated a decreasing trend with increasing pyrolysis temperature. Lower pyrolysis temperatures (samples at 300–500°C) resulted in a higher H/C ratio (1.41–0.67) and O/C ratio (0.27–0.15), whereas higher temperatures (samples at 600–900°C) presented lower H/C ratios (0.51–0.30) and O/C ratios (0.15–0.03). When compared with the biochar samples pyrolyzed at lower temperatures, those obtained at higher temperatures were less polar and had greater aromaticity and carbonization.

Nutrients and trace metals of biochar samples in solid form

Table 3 showed the total contents of trace metals in raw sludge and sludge biochar samples measured by wet acid-extract method followed by ICP in accordance with EPA method 3050B. The contents of majority of trace metals, such as Al, Fe and Zn in the final residue were greater than those in their feedstock sludge, which showed that pyrolysis process condensed and retained these trace metals in the final residue. Furthermore, the enrichment effect became more evidently with pyrolysis temperature rising.

Table 3 Total contents of nutrients and trace metals in raw sludge and sludge biochar samples by acid-extraction method Full size table

For comparison purposes, we also determined the total amounts of heavy metals in solid form using XRF technique. The results showed that all heavy metals have their total concentration increasing with temperature (Supplemental information: Table S1). Thus, we can find a bit difference between two dataset. It should be noticed firstly that, to gain insight into the precision of the heavy metal analyses between XRF technique (sample non-destructive method) and acid-extraction method (sample destructive method), comparison is difficult when different extraction techniques are used17. Only much could be learned that most of the heavy metals are retained in the sludge biochar samples.

Secondly, incomplete recovery might happen for acid-extraction method. As pointed18, acid digestion and thermal decomposition steps can result in analyte losses, incomplete recoveries, and/or sample contamination; USEPA Method 3050 is unsatisfactory for some elements; Variations in hot plate temperatures, refluxing times and acid additions directly affect elemental recoveries; the observed relative standard deviations of 10–30% for this method are considerably greater than the expected instrumental error (<5%) for an ICP. Therefore, when samples of very trace amount were measured, the deviation will be significant. Thirdly, the total concentration of Cu and Mn increased at sludge biochar samples at 300–700°C, on the contrast, decreased at biochar samples at 800–900°C. Some researchers investigated the fate of heavy metal contents in the sewage sludge biochar samples with pyrolysis temperature and found that the total concentration of Cu and Mn increasing with pyrolysis temperature; However, the selected pyrolysis temperature was from 300–700°C10; 300–500°C11; 250–700°C19 and so on. Little references were discussed on the variations of heavy metals in the sludge biochar samples over 700°C. Therefore, the reasons of Cu and Mn concentration determined by acid-extraction method decreasing at 800–900°C in this study were explained, 1) The Cu volatilizes to a significant extend around 800°C in case of sewage sludge incineration20; 2) Guo21,22,23 observed the Mn bleeding ratio could be above 10% during coal pyrolysis. Specially, they found that the bleeding ratio of all the studied elements increased sharply when the coal pyrolysis temperature increased from 700°C to 800°C; 3) The existence of chloride could accelerate the volatile of Cu and Mn. Although the Cl concentration was not analyzed in this study, sewage sludge is generally regarded to contain Cl compounds owing to some Cl-containing conditioners during sludge dewatering or to some Cl-containing surfactant in sewage. Actually, reported17 that the Cl concentration was 3.6 mg g−1 and 4.0 mg g−1 in the studied raw sewage sludge and sludge biochar samples and it reached 5.4 mg g−1 in the studied sludge compost24. So, as a result of the existence of Cl in the biochar samples, it could be incorporated with Cu and Mn and their chlorides would be easy to be released to the gas in the form of CuCl 2 and MnCl 2 . The melting points of CuCl 2 and MnCl 2 are 498°C and 650°C in the pure samples; they would be over 700°C in the multi-composition and complicated sludge biochar system in this study.

Water-extractable fractions of the biochar samples

The agronomic availability of biochars primarily depends on the initial water-extractable nutrient contents (Table 4). Although dissolved organic matter (DOM) represents a small proportion of organic matter residue in the biochar, it is significant in the soil amendment/ecosystem owing to its mobility and reactivity25. The DOC and DN indicate the DOM contents. In the present study, the DOC contents decreased rapidly from 24.23 mg g−1 in sample at 300°C to 2.66 mg g−1 in sample at 400°C and then reduced to almost zero in samples of 600–900°C. This was due to secondary reactions, which resulted in low molecular weight acids and neutral compounds, which were dominant in the biochars at higher temperatures25. The DN and NH 4 +-N have important agronomic uses because they are the main sources of nitrogen available for plant uptake. Their contents in sample at 300°C were 6.19 and 4.39 mg g−1, respectively, which decreased to 0.57 and 2.15 mg g−1, respectively, in sample at 400°C and then reached almost zero or below the detection limit in samples of 500–900°C. Reported1 that the available nitrogen content in the form of DN and NH 4 +-N was higher in the sludge biochar samples produced at lower pyrolysis temperatures (<400°C).

Table 4 Contents of water-extractable compounds in raw sludge and sludge biochar samples Full size table

The contents of water-extractable K, Na, P and Mg reduced rapidly with the increasing pyrolysis temperature and when compared with the raw sludge, almost 90% of the water-extractable K, Na and P contents and 30% of Mg content were lost in sample at 400°C. Similarly, the water-extractable Ca content in the biochar samples also presented a downward trend, when compared with the raw sludge.

Three-dimensional fluorescence EEM was used to study the aqueous humus-like compounds generated in the sludge biochar samples. Although EEM is widely applied in compost and soil research to detect protein- or humus-like organic matters, a limited number of studies had used this technique to analyze biochar with respect to temperature. The EEM spectra, normalized to the DOC content for the primary sludge and sludge biochar samples, are presented in Fig. 1. An EEM spectrum could be divided into four excitation-emission regions: Region I (Ex < 250 nm; Em < 380 nm), protein-like organic compounds; Region II (Ex < 250 nm; Em > 380 nm), fulvic-acid-like materials; Region III (Ex > 250 nm; Em < 380 nm), soluble microbial byproduct-like materials; and Region IV (Ex > 250 nm; Em > 380 nm), humic-acid-like materials.

Figure 1 EEM spectra of the sewage sludge and sludge biochar samples at various temperatures. (a) primary sludge; (b) C300; (c) C400; (d) C500; (e) C600; (f) C700; (g) C800; (h) C900. Full size image

In the contour of raw sludge itself, organic compounds were found to be composed of aromatic proteins (Region I) and soluble microbial byproduct-like materials (Region III). The volumes of fulvic-acid-like materials (Region II) and humic-acid-like materials (Region IV) were low. The scope and intensity of the fluorescence area were the highest for the biochar sample at 300°C, followed by samples at 300–500°C and eventually became undetectable at higher temperatures (700–900°C). These results suggested that the sludge biochar produced at lower temperatures (300–500°C) had more fulvic- and humic-acid-like materials.

The pH and EC values of the biochar samples are listed in Table 1. The pH values of the biochar samples at 300–800°C ranged from 6.2 to 11.9 and then decreased to 9.4 for sample at 900°C. The EC values correspond to the concentration of total dissolved salts and could be used to describe the variation in the organic and inorganic ions. The EC value of the primary sludge was 4.7 ds m−1, which decreased to 0.3–0.4 ds m−1 with the increasing pyrolysis temperature. However, regardless of the trend of the pH and EC results, sample at 700°C presented higher values. Higher pyrolysis temperatures led to higher pH of the biochar samples, which was mainly dueto the minerals present in the biochar samples and the increase in their contents during the pyrolysis process. Thus, the accumulation of these basic cations increased the pH values of the biochar samples26,27,28. Furthermore, the concentration of Ca2+ increased with the increasing pH of the biochar, while the total content of K+ and Na+ increased with the increasing EC value of the biochar, which are in agreement with the previous results28.

The accumulation of trace heavy metals are of great concern in agricultural product due to potential threat for human and animal health. As listed in Table 3, most of trace metals in the biochar samples were greater at higher pyrolysis temperatures than those at lower temperatures. To know the bioavailability of these metals contained in solid form, we also investigated the total concentrations for trace metals in water-extractable solutions. Fortunately, no detection contents for water extractable trace metals were observed in the studied sludge biochar samples (Table 4). It meant that the metals in sludge biochars were in fixed form. The metal suppression did not only depend on the neutral to alkaline buffer properties of the biochar, but also depend on the biochar pore structure and BET surface, which enhance biochar ability to immobilize heavy metal19. Those implied that the biochar generated at 300–900°C may have minimal impact on increasing the compost/soil heavy metal contents following a single short-term application.

XRD spectra

The XRD spectra of the biochar samples are shown in Fig. S2 (Supplemental information: Fig. S2). The analysis of the XRD patterns revealed the presence of several mineral phases. Quartz, with a characteristic peak at 2θ = 26.6°, was the most recognizable crystallographic structure at all temperatures. The sharpness of the peak increased with the increasing temperature, possibly owing to the ultrastructural changes in the sludge biochar. Calcite (CaCO 3 ) and dolomite [CaMg(CO 3 ) 2 ] were detected in the biochar samples at 300–800°C, while carbonates underwent decomposition and were not present at higher temperatures (>800°C). Some amount of Ca, which was present as CaCO 3 in samples heated at 700°C, decomposed to CaO during high-temperature pyrolysis1,27. This was also a reason for the higher pH values of the samples pyrolyzed at 700–800°C and the samples' basicity was mainly linked to the presence of Ca.

BET surface area and SEM morphology

The BET surface areas of the raw sludge and biochar samples are listed in Table 1. The BET surface area of the as-received sludge was considerably low (2.88 m2 g−1). However, the surface area of the sludge biochar linearly increased with the increasing pyrolysis temperature from 4.88 m2 g−1 (at 300°C) to 19.11 m2 g−1 (at 800°C). At 900°C, the BET surface area of the biochar increased substantially up to 34.12 m2 g−1.

The SEM general morphology (Supplemental information: Fig. S3) of the biochar samples also exhibited an increased surface area with the increasing pyrolysis temperatures. The SEM images of the as-received sludge indicated plate-like layer construction and poor structure that was smoothly compacted (Fig. S3a). However, as shown in Fig. S3b, a crack appeared and tar agglomerates seemed to cover the surface of the biochar particle. Furthermore, in the biochar samples, the dense and tightly packed microstructure disintegrated, gradually forming fragments (Fig. S3c–e) and a characteristic hollow was observed (Fig. S3f–h). On the one hand, lower temperature entailed condensation of organic volatiles, which could lead to pore clogging and reduction in the total surface area. On the other hand, at higher temperature, volatilization was more subtle, making the biochar more porous and creating voids within the biochar matrix.

FT-IR and XPS spectra

Fig. S4 (Supplemental information: Fig. S4) presented the FT-IR spectra of the dried raw sludge and biochar samples. The absorption bands and peaks provided evidence of the presence of some surface functional groups. In general, the organic functional groups found in the biochar spectra decreased or even disappeared as a result of pyrolysis.

The broad band at 3400 cm−1 was assigned to hydroxyl (-OH) stretching and the peak intensity decreased rapidly at samples 300–500°C, suggesting an ignition loss of -OH10. The peaks at 2925 and 2855 cm−1 corresponded to the aliphatic CH 3 asymmetric and symmetric stretching vibration, respectively, which had been assigned to the fats and lipids of the sewage sludge29. These peak intensities decreased owing to the continuous decrease in the labile aliphatic compounds as well as demethylation and dehydration. The loss of -OH and aliphatic groups as well as a concurrent development of fused-ring structures gave rise to pore formation. These results were consistent with the SEM findings. The peaks at 1650 cm−1 were assigned to the amide I bands of protein origin. These bands gradually broadened and shifted towards lower wavenumbers as a result of pyrolysis. The decomposition of protein mainly occurred at 300–400°C30, which could be explained by a decrease in the amide groups and simultaneous increase in the amino acid functionalities. The band at 1430 cm−1 became invisible and this wavenumber had been assigned to the stretching of C in the heteroaromatic structures31. The sharp peak at 1030 cm−1 was assigned to C-O stretching of polysaccharides or polysaccharide-like substances. This peak decreased at higher pyrolysis temperature and appeared as a shoulder for the biochar samples at 400–800°C and eventually became invisible for the sample at 900°C. Meanwhile, a peak at 1080 cm−1, which was present in a similar position in the broad region, was assigned to Si-O, indicating the presence of silicate impurities and clay minerals. Si was noted to be one of the major inorganic constituents in the sludge biochar samples, which was verified by the XRF results (Table 2).

Overall, minor chemical changes occurred at lower temperature and most of the spectral features were lost and the spectrum began to resemble pure graphite over 700°C. The FT-IR results indicated that the rearrangement continued to occur at higher temperatures, resulting in the sludge biochar becoming increasingly polyaromatic in nature.

The nitrogen gradually transformed into pyridine-like structure occurring in heterocyclic compounds with the increasing temperature, which was confirmed by the peak extraction of N regions in the XPS spectra. Three binding energies of 398.7, 400.4 and 401.1 eV corresponded to pyridine nitrogen (N-6), pyrrolic nitrogen (N-5) and quaternary nitrogen (N-Q), respectively. The integrated areas of the individual components were calculated and are shown in Table 5. The fractions of N-5 and N-Q were the main components of the raw sludge, of which the integrated area of N-5 accounted for 61.1%. As the pyrolysis temperature increased, the fraction of N-6 increased. The conversion of N-5 to N-6 and N-Q under pyrolysis had already been demonstrated by Schmiers32. N-6 and N-Q were the most stable forms of nitrogen binding at higher temperatures. The pyridinic ring was preferentially incorporated into the graphitic-like carbon structure in the form of quaternary nitrogen32, which affected the characteristics of the sludge biochar, including biochar basicity and available nitrogen forms33,34.

Table 5 XPS Integrated areas (%) of N regions corresponding to their binding energies for raw sludge and sludge biochar samples Full size table

Raman spectra