Physico-chemical transformation of bone char for soil amendment

 

^* Corresponding author. Tel: 886-62757575 ext65433; E-mail: BL93@cornell.edu; liangbq@mail.ncku.edu.tw

Abstract: Bone residue was found an important constituent of the miraculous Amazonian Dark Earth and considered as an effective supply of Ca and P nutrient to increase soil fertility. The application of bone-char to the soil as amendment is a favorable practice, yet more research is needed to understand the behavior and fate of bone hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) (HAp) in the natural environment over time. In our study, we explored Fourier Transform Infrared spectroscopy (FTIR) and X-Ray diffraction (XRD) to study the physicochemical properties of bone and bone-char (300-1200 °C) under pyrolysis temperatures and their dissolution behavior at different pH (4 and 6). We observed the structural transformation from B-type CHAp to a higher level of disorder AB or A-type CHAp with increasing temperatures, which could be explained by the reaction CO₂ with 2 OH^- in the CHAp channel. A weak band of CO₃^2- at 700 °C implied thermal decomposition of inorganic CO₃^2- at above 700 °C, which partially contributed to the increasing crystallinity and stability of bone char. As the pyrolysis temperature increased up to 1100 °C, the centroid of v_3c PO₄^3- peak shifted to a higher wavenumber (1029-1051 cm^-1), resulting from the rearrangement of P-O bonds. The loss of water and organic component contributed to an increase in vacancy.  The amide and lipids decomposition occurred within 300-600 °C, rendering a better crystal symmetry. The signal of structural OH^- band increased with increasing temperature from 300 to 500 °C, due to the reaction of inorganic CO₃^2- and H₂O [CO₃^2- + H₂O ↔ 2(OH^-) + CO₂]. There was more A-type CHAp formation due to simultaneous reverse reaction, and the OH^- band became weaker at above 500 °C. The surface consolidation of bone char was obvious at 700 °C, according to observation by the Transmission X-ray Microscopy (TXM). The P dissolution was the highest for bone at pH 4 (11.82±0.98 ppm), compared to that at pH 6 (10.93±0.39 ppm). The dissolution was relatively low in 500 °C biochar, and the amount was comparable at pH 4 (7.08±0.16 ppm) and pH 6 (6.902±0.16 ppm) after 140 hours’ incubation. The lowest P dissolution was observed at 700 °C biochar, and a higher dissolution was observed at higher pH 6(6.03±0.03 ppm) when compared to that at pH 4 (3.489 ±0.07 ppm). There was a very large increase in pH after bone char addition, which increased from 4 to 8.12 and 6 to 8.14, respectively. The solution end pH was similar after bone char addition. Surface complex reaction (≡CaOH₂⁺+HPO₄^2–⇌≡OPO₃H^-+H⁺) explained the PO₄^3- re-adsorption to bone char surface within the pH range of 4.5 to 8.2. Charring of bone would lead to a longer lifetime in the natural environment and render a stable pool of P nutrient in infertile soils. The scale-up application of bone char offers new opportunities to restore degraded soil by waste recycling and management.

Keywords: Bone char, hydroxyapatite (HAp), CHAp, FTIR, TXM, Phosphorus dissolution, soil fertility.