Title : 
  P release kinetics from vivianite , hydroxyapatite and bone char 
 Abstract_text_1 : 
  P release kinetics from vivianite , hydroxyapatite and bone char . 
 Abstract_text_2 : 
  The bone char used for P release experiments was produced by Bonechar Carvão Ativado Do Brasil Ltda . ( Maringá , Brazil ) in 2015 . It has been manufactured by pyrolysis of rendered ( de - fatted ) bovine bones at more than 800 ° C . A particle size analysis of the bone char was carried out , whereby 100 g of char was divided into three particle size fractions : 200 µm , 200 - 2000 µm , and 2000 µm . The P bearing minerals used were the commercially available Ca - phosphate hydroxyapatite ( Ca5[OH(PO4)3 ] ) ( Acros Organics , Geel , Belgium ) and the Fe - phosphate vivianite ( Fe32+[PO4]2 · 8H2O ) . Vivianite was prepared where 250 ml of a 0.035 M H3PO4 - solution were added to solid FeSO4 . The resulting 0.05 M FeSO4 - solution was adjusted to pH 6 with 5 M KOH . The precipitate formed was centrifuged for 5 min at 2090 × G and washed with ultrapure water . The prepared vivianite was dried at 40 ° C and ground into powder using mortar and pestle . P , Ca , and Fe were determined using ICP - AES after pressure digestion or aqua regia digestion . The composition of hydroxyapatite and vivianite was verified using X - ray diffraction ( XRD ) . The specific surface area was determined with an Autosorb-1 ( Quantachrome , Odelzhausen , Germany ) using a multi - point BET - measurement ( Brunauer - Emmett - Teller ) and N2 as adsorptive medium . The specific surface area of hydroxyapatite was with 68.4 m² g-1 1.7 fold higher than for vivianite with 39.5 m² g-1 . Bone char had the highest specific surface area with values in the range from 92.1 to 94.8 m² g-1 , increasing with decreasing particle size . P release experiments were conducted in triplicate by using a batch setup with an initial pH of 6 . For this , 2.5 g of the bone char and the phosphate minerals were weighed into PE bottles . Batch solubilization experiments were performed with CaCl2 ( Merck Millipore ) and citric acid with the formula C6H8O7 ( 99 % , Alfa Aesar ) , adjusted to pH 6 with KOH . These reaction solutions were used in concentrations 0.01 M , 0.05 M , and 0.1 M , respectively . 50 ml reaction solution was added to the bone char samples and 40 ml was added to hydroxyapatite and vivianite samples , respectively . The samples were shaken on a horizontal shaker for 24 h at 200 motions min-1 , centrifuged for 15 min at 2090 × G , and the supernatant was filtrated by using P - poor Whatman 512 1/2 filters . Following , fresh reaction solution was added to the samples . Samples for P measurement were taken after 2 , 6 , 24 , 48 , and 168 h. The cumulative P release based on 168 h with CaCl2 is very low for all used materials . While the total P release for bone char was close to the detection limit , minor P concentrations were measured for vivianite and hydroxyapatite , which increased slightly with increasing CaCl2 concentration . The use of citric acid enabled significantly more P to be solved , whereby an increase of dissolved P with increasing citric acid concentration was measured as well . At low citric acid concentration of 0.01 M , the lowest amount of P was released from bone char , where most P was released from the smallest particle fraction . In total , most P was released from hydroxyapatite after 168 hours . Research domain : Plant Nutrition Research question : Which P bindings are formed on the various mineral Fe- and Al - hydroxide surfaces and how do inorganic and organic compounds contribute to the availability of both adsorbed / precipitated and naturally bound phosphorus from phosphate minerals ? 
 Keywords : 
  Soil , Phosphate fertilizers , fertilizers , farm inputs , Phosphates , Bones , Laboratory experimentation , Vivianite , Hydroxyapatite , Bone char , Citric acid , Phosphorus , opendata , Boden