OPTIMAL REMOVAL OF NITRATE AND PHOSPHATE IONS FROM AQUEOUS SOLUTIONS USING SYNTHESIZED ACID ACTIVATED RICE STRAW NANO-ADSORBENT AND INDUSTRIAL WASTE FROM ELECTRIC ARC FURNACE

Abstract

The removal of nitrate and phosphate ions from wastewater has a major concern nowadays. Excessive intake of nitrates by humans in drinking water and food can induce methemoglobinemia and stomach cancer. Therefore, it is necessary to remove nitrate and phosphate ions from wastewater. Adsorption using nano-adsorbents becomes a promising method for phosphate and nitrate removal from wastewater because of their availability, low cost, stability, large surface area to volume ratio and high adsorption capacity. Rice straw is one of the most common agricultural wastes in Egypt due to its large production amounts. Energy valorization is a technology used for production of silica gel from rice straw which is used for nitrate removal from wastewater. The novelty of the proposed process is that the production of Acid Activated Rice straw supported nano Zero Valent Iron (AARS-nZVI) adsorbent is carried out without the energy valorization process but, through an acid activation for rice straw followed by precipitation of nano Zero Valent Iron (nZVI) particles on its surface. Also, insitu reduction of nZVI particles on the surface of AARS-nZVI is performed at atmospheric pressure, room temperature and without presence of an inert gas. In this research two adsorbents are studied in details; synthesized AARS-nZVI adsorbent is used for removal of nitrate ion from an aqueous solution and an industrial waste from El Masryeen Steel factory which is Fume Dust from an Electric Arc Furnace (FD-EAF) is used for simultaneous removal of nitrate and phosphate ions from an aqueous solution. Detailed Surface characterization is performed before and after adsorption process for both of the mentioned adsorbents to ensure nitrate and phosphate ions removal and to predict the adsorption mechanisms for each of them. The effects of changing pH of solution, initial nitrate ion concentration, initial phosphate ion concentration, adsorbent dose and contact time on the adsorption process efficiency using AASR-nZVI synthesized adsorbent and FD-EAF waste are studied. In case of nitrate ion removal using AARS-nZVI synthesized adsorbent; the removal efficiency decreases from 97.8 to 60 % while increasing in the solution’s pH from 3 - 10 at different levels of contact time and under these fixed conditions; initial nitrate ion concentration = 6 mg/L and adsorbent dose = 3 g/L. While increasing initial nitrate ion concentration from 6 – 40 mg/L results in decreasing the removal efficiency from 91.62 to 60 % at different levels of contact time and under these fixed conditions; pH = 7.5 and adsorbent dose = 3 g/L. On the other hand, increasing the adsorbent dose from 2 – 3 g/L leads to increasing in the removal efficiency from 70 to 91.62 % at different levels of contact time and under these fixed conditions; pH = 7.5 and initial nitrate ion concentration = 6 mg/L. In case of simultaneous nitrate and phosphate ions removal using FD-EAF waste; phosphate ion removal percent decreases from 90 to 70 % while increasing initial phosphate ion concentration from 1.5 to 7.7 mg/L at different levels of time and under these fixed conditions; pH ~ 7, adsorbent dose = 3.5 g/L and initial nitrate ion concentration = 23 mg/L. While increasing the adsorbent dose from 0.63 to 3.51 g/L results in increasing the phosphate ion removal percent from 64 to 76 % at different levels of time and under these fixed conditions; pH ~ 7, initial nitrate ion concentration = 23 mg/L and initial phosphate ion concentration = 5.5 mg/L. On the other hand, nitrate ion removal percent decreases from 40 to 20 % with increasing initial nitrate ion concentration from 6 to 36 mg/L at different time levels and under these fixed conditions; pH ~ 7, adsorbent dose = 3.5 g/L, initial phosphate ion concentration = 5.5 mg/L. While increasing the adsorbent dose from 3.3 to 6 g/L leads to increasing the nitrate ion removal percent from 36 to 44 % at different levels of time and under these fixed conditions; pH ~ 7, initial nitrate ion concentration = 23 mg/L, initial phosphate ion concentration = 5.5 mg/L. The maximum simultaneous removal for phosphate and nitrate ions using FD-EAF waste are achieved at pH = 6.1 and with further increase in the solution's pH, the removal percents of phosphate and nitrate ions decrease significantly from 67.6 to 12.8 % and from 18.75 to 2.44 %, respectively. Numerical optimization using Response-Surface Methodology-Central Composite Design (RSM-CCD) statistical method is performed to reach the optimum conditions of the nitrate and phosphate ions removal percentages at neutral pH using Design Expert Software. The optimum nitrate ion removal 91.62 % using AARS-nZVI synthesized adsorbent is achieved at pH = 7.5, initial nitrate ion concentration = 6 mg/L, adsorbent dose = 3 g/L and contact time = 180 min. While the optimum nitrate and phosphate ions removal percents 52.74 and 92.38 %, respectively using FD-EAF waste are achieved under these conditions; pH~7, initial nitrate ion concentration = 6 mg/L, initial phosphate ion concentration = 1 mg/L, adsorbent dose = 6.5 g/L and contact time = 90 min.