A New Method for Quantifying Phosphorus of Various Occurrence Forms in Solid Fuels

Quantification of phosphorus (P) and determination of the occurrence forms of P in solid fuels are important to thermochemical processing of P-containing solid fuels. This study has developed a new three-step method for separating and quantifying the total P in a solid fuel into five major P-containing fractions. These consist of three organic P-containing fractions (i.e., acid-soluble organic P, two acid-insoluble organic P including P in lipids and P in nucleic acids and other acid-insoluble organic structures) and two inorganic P-containing fractions (i.e., acid-soluble inorganic P and acid-insoluble inorganic P). The first step of this new method uses cold 0.6 M HClO4 to extract the acid-soluble P species. The extracted solution is then neutralized, followed by selectively converting the acid-soluble organic P in the solution into orthophosphate (PO4 3–), pyrophosphate (P2O7 4–), and tripolyphosphate (P3O10 5–) by UV irradiation in the presence of H2O2. The second step of this new method uses an ethanol/chloroform mixture to extract the residue from the first step to yield a solution for quantifying the contents of P in acid-insoluble phospholipids using an inductively coupled plasma optical emission spectrometer (ICP-OES). The third step of this new method uses buffered NaCl solution at 100 °C to extract P in nucleic acids and other acid-insoluble organic structures from the residue of the second step into a solution for subsequent quantification using ICP-OES. The acid-insoluble inorganic P in the residue is then quantified using ICP-OES after HF/HNO3/H2O2 acid digestion. The new method is validated with a comprehensive set of standard samples loaded with known amounts of single or mixed P-containing species with known occurrence forms. Compared to the large errors of the conventional Standards, Measurements and Testing method, complete P recovery is achieved with minimal errors. Further applications of the new method to seven different solid fuels also achieve close to 100% mass balance of P. The results demonstrate that the new method is suitable for quantifying various forms of P in solid fuels.