Leaching of Phosphorus from Biomass Ash and Model Chemicals

Abstract

Phosphorus is an important raw material used mainly in fertilizers, with population growth increasing the demand. Sustainable sources of phosphorus are investigated to prevent the depletion of phosphorus reservoirs. While ashes have long served as fertilizers, they often contain heavy metals, and methods to prevent contamination are crucial for environmental health. Leaching of ashes before usage can extract the useful components and separate them from the harmful ones. This thesis focuses on exploring the leaching behaviours of phosphates from both model chemicals and biomass ash. Additionally, it introduces an alternative analytical method for determining phosphorus concentrations in complex aqueous solutions. The research presented in this thesis delves into the molybdenum blue method's accuracy and its application to determine the phosphorus leaching from biomass ashes in diverse solvents. The experimental procedure investigated single phosphates and phosphate mixtures, as well as hydroxyapatites, which are present in biomass ashes, providing a fundamental understanding of phosphorus leaching. Solvents are employed to adjust pH, revealing a correlation between lower pH levels and increased phosphorus leaching, a trend also seen in equilibrium data. Finally, studying the leaching of biomass ashes stepwise, first in ultrapure water and then in either HCl or HNO3, revealed no significant difference in the choice of acid. Furthermore, the reliability of the molybdenum blue method with Sb(III) and ascorbic acid as reagents, for simple and complex samples was evaluated. All samples underwent analysis using the molybdenum blue method. The biomass ash results obtained from this method were then compared with those obtained from ICP-OES. The values were comparable but due to issues with the sensitivity of ICP-OES, some uncertainty remains. Additionally, the outcomes from ICP-OES and SEM-EDX reveal that the failure to analyze wheat bran using the molybdenum blue method was most likely due to interferences. The concentration of phosphorus in biomass ashes, determined through the molybdenum blue method, shows potential as an alternative to ICP-OES for rapid phosphorus concentration estimates, but caution is warranted due to potential interferences making the molybdenum blue method unreliable.