The composition and dissolution in citric extractants of ash from the thermal gasification of pig manure

Ash derived from the thermal gasification of anaerobically digested, dried and pelletized pig manure (GA) was characterized by standard Association of Official Agricultural Chemists (AOAC, 1990) methods of chemical analysis for phosphorus soluble in citric acid (P_CA), neutral ammonium citrate (P_NAC) and alkaline ammonium citrate (P_AAC) for several extraction times. Concentrations of other elements in the three extracts were measured by inductively coupled plasma with optical emission spectroscopy (ICP-OES). The ash had 54.4g/kg total P (TP), 28.2g/kg P_CA after 6h extraction and 48.7mg/kg water soluble phosphorus (PH2O) and is therefore suitable for use as a P-fertilizer. The very high pH (12) of the ash indicates its strong acid neutralizing ability when applied to acidic soils, although the very high lime content of the ash may reduce P bioavailability in soils. Lime-free gasifier ash (lf-GA) was obtained by treating GA with buffered sodium acetate solution (pH 5) for periods up to 120h. Removal of calcite equivalent to 70% of initial GA mass reduced pH to 6, increased TP (about 2 times), PH2O (10 times) and P_CA (1.5 times). X-ray powder diffraction (XRD) patterns of lf-GA indicate that P is mostly present as intermediate hydroxyl-apatite/carbonate-apatite. Apatite grains of various Ca (20.2-48.5%) and P (8.4-17.8%) concentrations were determined by scanning (SEM) and transmission (TEM) electron microscopy with energy dispersive X-ray spectrometry (EDS). Other elements in lf-GA are: Fe (5-15%), Si (<10%), S (<3%) and Na, Mg, Al, Cl, K, Ti (<1% each). The ash contained minor quartz, feldspar and ilmenite.