Ash properties of Pinus halepensis needles treated with diammonium phosphate

Abstract

The ash properties of Pinus halepensis (Aleppo pine) needles before and after treatment with diammonium phosphate (NH 4) 2HPO 4 (DAP) have been investigated, using thermogravimetric analysis (TG), differential thermal analysis (DTA), titrimetry, inductively coupled plasma-emission spectrometry (ICP-ES), X-ray diffraction (XRD) and scanning electron microscopy (SEM). DAP is extensively used as active component in wildland fire retardants. The following crystalline compounds have been identified in ashes prepared at 600 °C before treatment with DAP: KCl, Ca(OH) 2, MgO, (CaMg)CO 3, K 2CO 3·CaCO 3, K 2CO 3, K 2SO 4, CaO and CaCO 3, whereas CaO, MgO, K 2SO 4, K 2CO 3, CaCO 3, KCl and CaO, MgO, K 2SO 4 and K 2CO 3 at 800 and 1000 °C, respectively. The presence of DAP alters the composition of ashes converting, almost completely at high temperatures, the metallic oxides into phosphate salts. Thus, decreasing their alkalinity. The micrographs obtained by SEM indicate that pine needles ashes contain large porous particles of carbon compounds and several inorganic particles of irregular shape <1.0 mm, whereas after treating the needles with DAP an amorphous rigid structure was formed. To facilitate our investigation model mixtures of CaCO 3 + DAP, MgCO 3 + DAP, K 2CO 3 + DAP were heat treated under the same conditions used for preparing the ashes. The chemical transformations taken place during heating were studied by analysing the reaction products using thermal analysis and XRD. The physical, mineralogical and chemical forest ash properties determined could be used to evaluate the environmental risk of the use of fire retardants on soils, plants and aquatic systems as well as to investigate the mechanism of combustion of forest fuels in the presence of DAP.