Thermodynamic Modelling of Nitric Acid and Selected Metal Nitrate Systems on Sampled Fish Pond Waters of Kisii University

Abstract

Surface waters consist of a complex mixture of ions and solutes which interact in a complex manner under different thermodynamic conditions. This study explored thermodynamic behavior of nitric acid/nitrous acid- selected nitrate-water systems on some fish ponds using Mixed-Solvent Electrolyte, MSE, which was applied to calculate phase equilibria, speciation, and their effect on dissolved oxygen, dissolved nitrogen and concentration of nitrates in the pond waters. In particular, solubilities and chemical speciation we analyzed for various nitrogen-containing systems. The model reproduced the speciation, solubility, and Vapor–Liquid Equilibrium (VLE) data in the nitric acid + water system at ground temperature and pressure and was therefore used to predict the effects of chemical speciation, temperature, and concentrations of various acid, base, and salt components on the formation of competing solid phases over wide ranges of temperature and concentration in water. The water samples were obtained directly from four University fish ponds labeled as Migingo, Mfangano, Ringiti and Remba Islands. A sample of water was collected from each pond. The four samples were then analyzed of their concentrations of dissolved oxygen (DO), total nitrogen (TN), nitrates and nitrites, pH and electrical conductivity. The results were used to validate the model. The findings established that there is an inverse relationship between the amount of nitrates in water and the levels of dissolved oxygen. The higher the amount of nitrates the lower the amount of DO in the pond water. A pressure of about 1 atmosphere and temperature range 20-27°C favor most particle interactions resulting in higher levels of concentration of NO3- ions in pond water. This research also established that variation of water temp should not exceed ±5 within the day for optimal concentrations of competing particles in solution. The systems that were analyzed in this work include the HNO3/HNO2/NO2+ water mixtures in the full composition range that covers xHNO3 from 0 to 1 and, more generally, xNO2 from 0 to 1. Further, a model was established for the nitrate salt systems, involving the Li+, Na+, Ca2+, and Mg2+ cations, encountered in the fish pond waters. Rather than focusing on particular processes, the current work provided a comprehensive treatment on the basis of the available experimental thermodynamic data for such systems. These results provided a thermodynamic foundation to explain natural variations in salt concentrations and predict mineral equilibria in the pond waters. Validation of the model was achieved through VLE, pH, solubilities and conductivity measurements.