E. Tocholke, L. Yampolsky, R. Mohseni, A. Vasiliev
East Tennessee State University,
United States
Keywords: organosilica, sol-gel method, nitrates, adsorption
Summary:
Agriculture is a leading source of water quality impairment in rivers, lakes and groundwater in the USA and other developed countries. World Health Organization reported that the level of nitrates (NO3-) in groundwater have grown due to intensified farming practice. Concentrations of nitrates in natural water sources above 3 mg/L (expressed as N) indicate contamination. U.S. EPA set a maximum nitrogen concentration in water of 10 mg/L. However, the level of nitrates in some of the U.S. water systems exceeds 40 mg/L. Moreover, in highly contaminated sites worldwide this value reached 600 mg/L. The most promising technologies for selective removal of nitrates are based on adsorption and ion-exchange processes. The objective of this work is the synthesis of amino-modified organosilica material and its study in adsorption of NO3- from water. Materials of this type already demonstrated high performance in adsorption of PFAS and carbon dioxide. Porous organosilica material was obtained by a sol-gel procedure at polycondensation of bis-(3-trimethoxysilylpropyl)amine with no surfactant. The load of amino groups was calculated to be 3.47 mmol/g or 12.8 molecules/nm2. Then the product was converted to surface amine hydrochloride. Its adsorption capacity on nitrates was 3.67 mmol/g. The effect of temperature on adsorption was predictable: increased temperature reduced the adsorption capacity. At low nitrate concentrations, the isotherms were linear while its increase resulted in adsorption above expected. This interesting effect might be caused by two-site adsorption. Organosilica demonstrated a good recyclability at the removal of nitrates from solution. No notable decline in adsorption capacity was detected after five adsorption/regeneration cycles. In the column tests, resulting water samples had remaining nitrate contents between 0.03 and 0.05 mmol/L that is well below maximum contaminant level. Effects of the starting nitrate concentration and flow rate were insignificant. A special test on the adsorbent stability demonstrated its stable effective work during water purification process. Considering total adsorption capacity of this material, we estimate that 0.5 g of the adsorbent may purify up to 2 L of contaminated water before regeneration. The effect of water clean-up on the eutrophication was studied on nitrogen-limited culture Tetradesmus algae demonstrating that the solutions treatment significantly decreased biological productivity and, as a result, improved water quality. Excellent adsorption properties of the amino-containing porous material make possible its use in clean-up of water contaminated from agricultural activities.