Physico-chemical transformation of bone char for soil amendment
Authors: Biswas Partha Pratim , Weng Yi-tse , Turner-Walker Gordon , Liang* Biqing 
Published: March 2020

&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;*&lt;/sup&gt;&amp;#160;Corresponding author. Tel: 886-62757575 ext65433; E-mail: BL93@cornell.edu; liangbq@mail.ncku.edu.tw&lt;/p&gt;&lt;p&gt;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 (Ca&lt;sub&gt;10&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;6&lt;/sub&gt;(OH)&lt;sub&gt;2&lt;/sub&gt;) (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 &amp;#176;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 CO&lt;sub&gt;2&lt;/sub&gt; with 2 OH&lt;sup&gt;-&lt;/sup&gt; in the CHAp channel. A weak band of&amp;#160;CO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;&amp;#160;at 700 &amp;#176;C implied thermal decomposition of&amp;#160;inorganic CO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;&amp;#160;at above 700 &amp;#176;C, which partially contributed to the increasing&amp;#160;crystallinity&amp;#160;and stability of&amp;#160;bone char. As the pyrolysis temperature increased up to 1100 &amp;#176;C,&amp;#160;the centroid&amp;#160;of &lt;em&gt;v&lt;/em&gt;&lt;sub&gt;3c&lt;/sub&gt; PO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;3-&lt;/sup&gt; peak shifted to a higher&amp;#160;wavenumber&amp;#160;(1029-1051 cm&lt;sup&gt;-1&lt;/sup&gt;), resulting from the rearrangement of P-O bonds. The loss of water and organic component contributed to an increase in vacancy.&amp;#160; The amide and lipids decomposition occurred within 300-600 &amp;#176;C, rendering a better crystal symmetry. The signal of structural OH&lt;sup&gt;-&lt;/sup&gt; band increased with increasing temperature from 300 to 500 &amp;#176;C, due to the reaction of inorganic CO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;&amp;#160;and&amp;#160;H&lt;sub&gt;2&lt;/sub&gt;O&amp;#160;[CO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;2&lt;/sup&gt;&lt;sup&gt;- &lt;/sup&gt;+ H&lt;sub&gt;2&lt;/sub&gt;O &amp;#8596; 2(OH&lt;sup&gt;-&lt;/sup&gt;) + CO&lt;sub&gt;2&lt;/sub&gt;]. There was more A-type CHAp formation due to simultaneous reverse reaction, and the&amp;#160;OH&lt;sup&gt;-&lt;/sup&gt; band became weaker at above 500 &amp;#176;C. The surface consolidation of bone char was obvious at 700 &amp;#176;C, according to observation by the Transmission X-ray Microscopy (TXM). The P dissolution was the highest for bone at pH 4 (11.82&amp;#177;0.98 ppm), compared to that at pH 6 (10.93&amp;#177;0.39 ppm). The dissolution was relatively low in 500 &amp;#176;C biochar, and the amount was comparable at pH 4 (7.08&amp;#177;0.16 ppm) and pH 6 (6.902&amp;#177;0.16 ppm) after 140 hours&amp;#8217; incubation. The lowest P dissolution was observed at 700 &amp;#176;C biochar, and a higher dissolution was observed at higher pH 6(6.03&amp;#177;0.03 ppm) when compared to that at pH 4 (3.489 &amp;#177;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 (&amp;#8801;CaOH&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;+HPO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2&amp;#8211;&lt;/sup&gt;&amp;#8652;&amp;#8801;OPO&lt;sub&gt;3&lt;/sub&gt;H&lt;sup&gt;-&lt;/sup&gt;+H&lt;sup&gt;+&lt;/sup&gt;) explained the PO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;3-&lt;/sup&gt; 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.&lt;/p&gt;&lt;p&gt;Keywords: Bone char, hydroxyapatite (HAp), CHAp, FTIR, TXM, Phosphorus dissolution, soil fertility.&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;