Simultaneous Removal of Nitrate and Phosphate Ions from Aqueous Solution Using Solid Waste from Alum and Sulphuric Acid Manufacturing Process

Abstract

Nitrogen and Phosphorous are limiting nutrients for plant growth. Contrary to this, phosphorous between 20 to 100 µg/L and nitrogen between 500 to 1000 µg/L can cause eutrophication in most aquatic systems. Therefore, developing low cost and efficient method of removing these nutrients to protect the aquatic environment is mandatory. The objective of this study was to investigate simultaneous nitrate and phosphate adsorption capacity of solid waste residue collected from Awash Melkassa Aluminium Sulphate and Sulphuric Acid Factory. Batch adsorption experiments were conducted to evaluate the effect of contact time, initial solution pH, initial nitrate and phosphate ion concentration, adsorbent dose and effect of coexisting ions on simultaneous removal of nitrate and phosphate ions. Besides, adsorption isotherms and kinetics were analysed to determine the mechanism and rate determining steps during the adsorption process. After conducting batch experiment, the equilibrium time was achieved within 90 minutes while the optimum pH and adsorbent dose were found to be 7 and 20 gm/L respectively. The adsorption kinetics of nitrate and phosphate ions best fitted to the pseudo-second order equation with R2 values 0.992 and 1 respectively. Moreover, isotherm studies showed adsorption of nitrate and phosphate fitted better to Freundlich isotherm and Langmuir isotherm with R2 0.988 and 0.984 for phosphate and 0.991 and 0.955 for nitrate respectively. Studies on reusability of the spent adsorbent showed nearly 50% the adsorbed phosphate can be desorbed with 0.1M NaOH while that of nitrate is 99.75% showing there is a possibility of reusing. Last but not least, application on wastewater showed promising result though there is decrease in percent removal. At the optimums found during the batch experiment, AW residue collected from Awash Melkassa Aluminium sulphate and Sulphuric acid factory can potentially remove phosphate from wastewater. But, the untreated AW was not suitable to remove nitrate ion from wastewater since its percent removal was from 27-30%.