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Phosphorus speciation in biosolids-amended soils : correlating phosphorus desorption, sequential chemical extractions, and phosphorus-xanes spectroscopy

Kar, Gourango. 2007. M.Sc. Thesis University of Saskatchewan

Abstract

This study was conducted to compare the speciation and behavior of P in soils receiving either different biosolids or inorganic fertilizer, as assessed by sequential chemical extractions, phosphate desorption, and synchrotron X-ray absorption near edge structure (XANES) spectroscopy. The objectives of this study were to i) measure the total amount of organic and inorganic phosphorus removed by chemical extraction method ii) investigate how P desorption kinetics are influenced in biosolids amended soils compared to inorganic fertilizer amended soils; and iii) perform solid state speciation of soil samples before and after chemical extraction and desorption with P K-edge XANES spectroscopy. Soil samples were analyzed that received three different rates of biosolids (16.8, 33.6, and 67.2 Mg ha-1 yr-1) and one inorganic fertilizer application (336 kg N, 224 kg P, and 112 kg K ha-1 yr-1) for 32 years. Both sequential chemical extraction and XANES analysis showed that total amount of P increased in biosolids amended soils (from 5292 to 10945 mg P kg-1) and that it increased with increasing application rate. Sequential chemical extractions showed that the labile portion of total P in inorganic fertilized soil (40 %) was larger than in biosolids applied soils (39 to 27 %). Results from both sequential chemical extraction and XANES analysis showed that NaOH extraction removed the highest amount of P from all biosolids applied soils (from 1857 to 2600 mg P kg-1). The amount of desorbed P decreased as the soil:solution ratio increased from 0.005 to 100 g L-1 for both soils and the desorption was typically higher in inorganic fertilizer applied soil than in biosolids applied soil. The effect of pH on P desorption was pronounced, and desorption was higher at pH 5 than pH 7.5 for both soils. A continuous flow desorption method was also used to measure cumulative P desorption over time. Cumulative P desorption in inorganic fertilizer applied soil (894.5 mg P kg-1) was higher than in the biosolids amended soils (572.9 mg P kg-1) over 20 hr period time. First-order and parabolic diffusion kinetic equations were used to model the desorption data from the continuous flow technique. This revealed that the P desorption rate was faster (and chemically-controlled) at initial stages and slower (and diffusion-limited) at later stages. The desorption rate was much faster in inorganic fertilizer applied soil than in biosolids applied soil. XANES analysis of the fractions removed in sequential chemical extractions suggested that the predominant form of P was poorly crystalline dicalcium phosphate in biosolids applied soils, and labile, sorbed forms as well as some apatite-type calcium phosphate was present in inorganic fertilizer applied soil. The combined results from sequential chemical extraction and XANES analysis indicate that P in inorganic fertilizer and biosolids-applied soils behave differently. There were larger amounts of low crystallinity phosphates in the biosolids samples, and much higher apatite content in the inorganic fertilizer amended soil.