Using FTIR-photoacoustic spectroscopy for phosphorus speciation analysis of biochars

https://doi.org/10.1016/j.saa.2016.05.049Get rights and content

Highlights

  • FTIR photoacoustic spectroscopy was capable of identifying P species.

  • Pyrolysis temperature affected more the P speciation in digestate solids biochars.

  • Hydroxylapatite and calcium phosphates were the abundant species in bone meal biochars.

Abstract

In the last decade, numerous studies have evaluated the benefits of biochar for improving soil quality. The purposes of the current study were to use Fourier transform infrared-photoacoustic spectroscopy (FTIR-PAS) to analyse P species in biochar and to determine the effect of pyrolysis temperature on P speciation. The photoacoustic detector has a range of advantages for the very dark biochar samples in comparison to more traditional reflectance or transmission FTIR detectors. The spectra turned out to be more informative in the regions with P vibrations for biochar produced at temperatures above 400 °C, where most of the remaining organic compounds were aromatic and therefore not overlapping with the P vibrations. For biochars produced from the solid fraction of digestate from biogas production, an increase in the pyrolysis temperature led to the formation of a large variety of P species. Hydroxylapatite and tricalcium phosphate were the most dominant P species in the mid to high temperature range (600–900 °C), while at 1050 °C apatite, iron phosphates, variscite and calcium phosphates were identified. However, the changes in P speciation in biochars produced from bone meal at different temperatures were smaller than in the biochars from digestate. Hydroxylapatite and calcium phosphates were identified in biochar produced at all temperatures, while there was some indication of struvite formation.

Introduction

Biochar is a product of the thermal decomposition of biomass under partial or total absence of oxygen. It is a C-rich residue that can be used as a soil amendment [1]. A large number of feedstocks are used for biochar production, including crop residues, wood, olive waste, manure and sewage sludge [2], [3]. Biochar mainly consists of stable aromatic forms of organic C, which are very resistant to microbial decomposition, and as a result it increases the recalcitrant organic C pool when applied to soil [4], [5]. In addition, it improves soil quality through the increase of cation exchange capacity and pH [6], while it has been found to induce a long-term modification in soil water-holding capacity [7]. Furthermore, it affects phosphorus (P) availability in soil through its interaction with other organic or inorganic components [8]. Therefore, some of the beneficial effects of biochar addition to soil may be related to how it affects the availability of P in the soil. Nevertheless, it has been observed that even in cases where biochars contain considerable amounts of P, it can decrease P-availability in soil [9]. One obvious explanation for this is the fact that biochar usually increases soil pH, which may decrease P-bioavailability as it affects the solubility of the different phosphates [10]. However, the speciation of P within biochars seems to be of even greater importance for the availability of P in biochar-amended soils and how it interacts with the soil pH [11] e.g. Ca-phosphates are less soluble at high pH than Al-phosphates [12]. It is therefore important to acquire a better understanding of the speciation of P in biochar and how it is affected by production conditions such as production temperature and feedstock.

Various solid spectroscopic methods, such as solid-state 31P nuclear magnetic resonance (31P NMR) [13], [14], X-ray absorption near-edge structure (XANES) [15], X-ray diffraction (XRD) [16], mid-infrared [17] and Raman spectroscopy [18], have been used to analyse P speciation in a variety of environmental samples. There have only been a few attempts involving an analysis of P speciation in biochars [19], [20]. For samples that are characterised by a high content of P, mid-infrared spectroscopy may be able to detect some differences in P speciation [21], even though P peaks overlap quite significantly with other peaks in organic matter. With biochars there are some additional complications, the main one being the very dark nature of the biochar samples, which gives rise to the problem of a very low signal since most of the light is absorbed by the sample. Fourier transform mid-infrared photoacoustic spectroscopy (FTIR-PAS) is a combination of FT-IR and a photoacoustic detector (PA). A recent development in the very sensitive microphones used by the PA detectors has increased the accuracy of this technique and, as a consequence, its applicability. This technique allows the directly proportional measurement of the infrared energy absorbed by the samples [22], since the measurement remains unaffected by the redistribution of light due to scattering effects or diffraction processes. With this technique a thermal wave is produced after the interaction of the infrared radiation with the surface of the sample, which results in a thermal expansion and pressure oscillation in the surrounding gas. This is detected as an acoustic signal by the microphone and transformed into the absorption spectrum [22]. This means that it is possible to measure very dark and opaque samples without having to dilute the sample with KBr powder to increase resolution [23]. FTIR-PAS has been used in the past to follow the pyrolysis process [24], [25] as well as to determine how the surface functionality changes at higher pyrolysis temperatures [26]. Furthermore, most of the organic matter left in biochar is likely to be aromatic, which will restrict the signals to the aromatic regions. In fact, the reference that is used and subtracted from the sample for photoacoustic detection is a dark sample, e.g. black carbon or charcoal. Since the organic part of the biochar is very similar to the reference, the removal of bands corresponding to this during the subtraction of the reference allows a clearer observation of the P bonds. These facts make FTIR-PAS ideal for the study of P speciation in biochars.

The main objective of the present study was therefore to use FTIR-PAS to perform P speciation analysis in biochars produced from the solid fraction of digestate and bone meal and investigate how pyrolysis temperatures affect the speciation of P.

Section snippets

Sample set

The solid fraction of digestate (DSF) was collected from the Fangel biogas plant (Fangel Bioenergy APS, Denmark). The feedstock used by the plant consists of 75% animal slurry from local farmers and around 25% industrial organic waste, mainly originating from local food processing companies. The separation of the digestate into liquid and solid fractions was carried out using a decanting centrifuge installed at the plant (GEA Westfalia Separator A/S).

The bone meal (BM) was provided by the

Biochar yield and elemental composition

The mass yield of the biochar produced from DSF was consistently reduced with increasing pyrolysis temperature up to 900 °C (Table 1). The mass yield of BM biochar followed a similar decrease for the lower pyrolysis temperatures. Above 750 °C, the mass seemed to be more stable and the losses very small. This was caused by thermo-labile organic matter which increasingly decomposed. A consistent decrease was also observed in the N content of both biochars and the C content of the bone meal biochar.

Conclusions

In this study, there was a first attempt of using FTIR-PAS for identifying P species in biochar. It was clearly demonstrated that FTIR-PAS is a useful tool and provides another perspective which can complement the understanding that can be obtained by NMR, XRD, XANES, and other speciation techniques, while being faster and possibly less expensive. Overlapping peaks of organic and phosphate compounds could impede the use of FTIR-PAS for samples with a high content of non-aromatic carbon. The

Acknowledgements

The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007–2013/ in the ReUseWaste project under REA grant agreement n° 289887. This material reflects only the authors' views and the European Union is not liable for any use that may be made of the information contained therein. The authors would like to thank Dr. Enzo Lombi for providing the mineral variscite and apatite standards, and

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