Characterization of bone and dung biochars for potential use as precursors for artificial fertilizers

Abstract

The Fourier Transform Infrared technique was utilized to measure the infrared spectra of bone and dung chars at ambient temperature. The spectra of the investigated char samples showed some notable similarities, suggesting that the compounds have functional groups which display bands at similar wavenumbers in IR spectrum. Additionally, correlations between the chars and the terra preta soils were found. This implies that the investigated biochars and the terra preta soils share comparable characteristics. Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy showed the existence of calcium, phosphorus, magnesium and oxygen in the bone char which are classified as macronutrients. In addition, the bone char contained two micronutrients (nickel and chlorine) and one non-essential nutrient (sodium). The dung char was constituted with all the macronutrients found in the bone char and in addition it had traces of sulfur which is also a macronutrient. Iron, manganese and chlorine were present in the dung char as micronutrients. There was also the presence of sodium and aluminum which are classified as non-essential nutrients. Zinc which is also fundamental in soil fertility was detected using Atomic Adsorption Spectroscopy in all the chars. Both the samples are alkaline with pHs ranging between 9 and 11. The results of this research will be useful in determining which biochars are best suited for production of synthetic fertilizers.

Keywords:

• AAS
• biochar
• FTIR
• SEM-EDX
• terra preta

Introduction

Biochars are carbon rich materials formed by pyrolysis of various types of organic matter (green waste, animal remains, plant offcuts etc.) locking the carbon compounds into the material.^[1,2] These compounds have been created in an attempt to replicate the highly fertile dark earths of the Amazon. The dark earths commonly known as terra preta (TP) have been found to obtain their outstanding fertility from the high concentrations of aromatic compounds.^[3–15] TP contains a range of nutrients which include phosphorus (P), potassium (K), nitrogen (N) magnesium (Mg), aluminum (Al), zinc (Zn), manganese (Mn) and calcium (Ca).^[16,17] Although successful replication of terra preta has not yet been met, researchers have established that addition of biochar into the soil raises pH, encourages greater root development, hosts more beneficial fungi and microbes because of its porosity and large surface area, and improves cation exchange capacity.^[18–20] In addition, biochar can improve the water holding capacity of the soils and has ability to hold nutrients. Furthermore, studies have shown that the existence of these materials in soil can help to improve the fertility of the soil.^[21–24] This makes it important to farmers as it reduces the use of non-organic fertilizers and hence can cut down on environmental pollution that these fertilizers can cause.

Even though biochar can be produced easily from organic material, it is crucial to know which materials when pyrolyzed will produce chars that mimic the fertility of terra preta. This would facilitate the establishment of suitable forms of biochar and therefore encouraging further research into this field. In this paper two forms of biochar were characterized using the Fourier Transform Infrared (FTIR) technique. The Atomic Absorption Spectroscopy (AAS) and Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDX) have been employed to determine the chemical composition of the different biochars. The chemical composition will reveal whether the biochars have properties which qualifies them as fertilizer additives. The pH of the biochars was also determined. The ideal soil pH for plant growth varies for different crops and it has been found that most crops grow well at the soil pH range of 5.5-7.5.^[25] The spectra of biochars will be compared to the spectra of terra preta to establish the similarities and differences which will shed light of the effectiveness of biochars as fertilizer additives.

Materials and methods

Sample preparation

The biochar samples were obtained from pyrolysis of bone and dung using the Batch Fixed Bed Pyrolysis Plant which was fabricated in the Engineering Faculty at the Botswana International University of Science and Technology. The cow bone was cleaned to remove any dirt and cut into small pieces to fit into the pyrolysis chamber. Similarly the cow dung was also reduced into small pieces which could be accommodated by the chamber. The raw materials were each pyrolyzed at 500⁰C for 1 hour. The pyrolysis temperature range for bone has been reported to be 350-900⁰C while for dung is 350-700⁰C^[26,27] and hence the pyrolysis temperature was chosen based on these ranges.

FTIR analysis

The infrared spectra of the samples were collected using a Vertex 70v Bruker spectrometer equipped with the Bruker Attenuated Total Reflection accessory. The tungsten lamp was used as the source of radiation and the detector was the deuterated triglycine sulfate (DTGS). A broadband beamsplitter was employed. The resolution at all frequencies was 4 cm⁻¹. The transmission spectrum was given by the ratio of the spectrum collected with the sample in the beam path to the reference spectrum(which was collected with nothing in the beam path).

FE-SEM and AAS analysis

Chemical composition was determined using a Jeol 7100 field emission scanning electron microscope (FE-SEM) operated at 20.00 kV fitted with an energy dispersive X-ray spectroscopy (EDS). For Atomic Absorption Spectroscopy (AAS) measurements, 0.5000 g of the bio-char samples was mixed with 10.0 mL of HCl (6.0 M). The mixture was covered with a parafilm then heated for 24 hrs at temperature ≤50 °C.The digested samples were then filtered by gravity filtration method using a filter paper of 90.0 mm diameter and then analyzed.

pH determination of the chars

Air-dried biochar sample (1 gram) ground to about 2 mm granules was mixed with 50 mL of distilled water and covered with parafilm. The solution was agitated using a reciprocating shaker for 1 hour at 25 °C.The suspension was allowed to stand for 30 minutes. The pH meter was calibrated using pH 4, pH 7 and pH 10 buffers. The pH of the biochar suspension was then measured.

Results and discussion

Figure 1 shows the infrared spectra of bone char, dung char and terra preta soil. The chars displayed an almost similar spectral profile with the possibility of common absorption bands. This is supported by the appearance of six absorption bands at approximately similar frequencies. The first band appears at 472 cm⁻¹ and 452 cm⁻¹ for bone and dung char respectively. The second band at 563 cm⁻¹ for bone char corresponds to the dung char band at 572 cm⁻¹. The third band for bone char appearing at 601 cm⁻¹ matches the band at 610 cm⁻¹ for dung char. The fourth band appears at 873 cm⁻¹ for bone char and 874 cm⁻¹ for dung char. Furthermore, the bone char band at 1024 cm⁻¹ corresponds to the dung char band at 1046 cm⁻¹. Lastly the bone char band at 1569 cm⁻¹ matches the dung char band at 1556 cm⁻¹. The observed shifts in the individual frequencies is attributed to the varying molecular structures of the components that constitute the studied samples. The appearance of common bands between these materials suggest that they contain compounds that have functional groups displaying bands at similar wavenumbers in IR spectrum. Bone and dung chars exhibited eleven absorption bands in the considered frequency range. Even though there might be common absorption bands between the spectra of these materials, the differences are also profound. Table 1 gives a summary of the absorption bands of the studied biochars.

Figure 1. Infrared spectra of bone (top spectrum), dung char (middle spectrum) and terra preta soil (bottom spectrum).

Table 1. Summary of absorption bands between terra preta, bone char and dung char.

Terra preta (cm^-1)	Bone (cm^-1)	Dung (cm^-1)
471	472	453
549	563	572
–	601	610
692	699	–
754	–	–
–	873	874
792	–	795
917	–	–
–	962	–
1038	1024	1046
1379.60	–	–
–	1414	1410
–	1456	–
–	1569	1556
1614	1607	–
–	–	1881
–	2013	–
–	–	2105
		2391
3622	–	–
3652	–	–
3698	–	–

In addition a comparison was made between the spectra of biochars with the spectrum of terra preta. This was done to establish similarities between the chars and the fertile soils of the Amazon. Six absorption bands were established for TP soils in the considered energy range which had matching absorption bands with the respective char samples. TP soil and bone char had five matching bands and there existed four common bands between the TP soil and the dung char. The TP soils bands at 692 cm⁻¹, 792 cm⁻¹, 917 cm⁻¹ and 1614 cm⁻¹ have been previously attributed to aromatic hydrocarbons.^[28] The presence of these absorption bands in the respective spectra of chars shows that the studied chars may contain aromatic compounds. The seemingly common absorption bands are also presented in Table 1.

In addition to TP soils obtaining their outstanding fertility from the high concentration of aromatic hydrocarbons, these soils are known to contain a range of elements which also contribute to their fertility. The elements include phosphorus (P), calcium (Ca), magnesium (Mg), potassium (K), zinc (Zn) and nitrogen (N). SEM-EDX was employed to determine the presence of these elements in the char samples and this is shown by Fig. 2(a,b). Both the bone and dung chars were found to contain a range of macronutrients which include calcium, phosphorus, potassium, oxygen and magnesium. Sulfur which is a macronutrient was present only in the dung char (Fig. 2(b)). In addition iron and chlorine which are classified as micronutrients were identified in the dung char, with chlorine also appearing in the bone char. Furthermore the bone char contained nickel which falls in the micronutrients category (Fig. 2(a)). Sodium which is classified as a non-essential nutrient appeared in both chars while aluminum which is also a non-essential nutrient was found only in the dung char. The analysis of the chars by AAS shows that they have a high concentration of calcium. The concentration of calcium in bone char and dung char were 9.03 mg/L and 2.79 mg/L respectively. Traces of zinc were also found in the chars with the concentration of 0.19 mg/L and 0.12 mg/L in bone and dung chars respectively. The pH of the bone char was 9.89 while the pH of the dung char was 11.31. These biochar will be good as fertilizers additives as they will raise the soil pH. The absence of heavy metals which could be toxic to living organisms in these chars qualifies them as artificial fertilizers precursors.

Figure 2. (a) Energy Dispersive Spectrometer (EDS) analysis of bone char. (b) Energy Dispersive Spectrometer (EDS) analysis of dung char.

Conclusion

Bone and dung chars were characterized using the FT-IR technique. The char samples displayed the same spectral profile indicating that they have same characteristics. Despite the presence of common bands between the two materials, there were also apparent differences in their spectra. Both the bone and dung chars were found to contain elements classified as macronutrients for plants. These elements are calcium, oxygen, phosphorus, magnesium and potassium. In addition, the dung char also contained traces of sulfur which also falls in the macronutrients category. Chlorine, manganese and iron which are micronutrients were found in the dung char in small quantities. Moreover, chlorine was also found in the bone char. Additionally, both chars contained traces of zinc which is a micronutrient. Non essential nutrients sodium was found in both of the chars while aluminum was only observed in the dung char. There were similarities in the spectra of TP and biochar understudy which highlights the potential use of these chars as additives for the manufacture of fertilizers. The bone and dung chars were found to be alkaline with pHs ranging between 9 and 11. They will be beneficial in reducing the acidity of the soils which might lead to poor plant growth. This study shows that all the investigated chars can be potential artificial fertilizer additives.

Acknowledgments

We thank Mr Lekgoba for the assistance with pyrolysis.

Disclosure statement

The authors declare that there are no conflicts of interest.

Additional information

Funding

This work was supported by the Botswana International University of Science and Technology.