Use of Clay Minerals to Reduce Ammonium Concentration in Groundwater G. Panagopoulos 1 , D. Papoulis 2 , M. Zamparas 3 , V. Bekiari 3 , D. Panagiotaras 4 1 Technological Educational Institute of Western Greece, Department of Mechanical and Water Resources Engineering, Nea Ktiria, 30 200 Messolonghi, Greece. 2 University of Patras, Department of Geology, Rio, 26 504 Patras, Greece. 3 Technological Educational Institute of Western Greece, Department of Fisheries & Aquaculture Technology, Messolonghi, Nea Ktiria, 30 200 Messolonghi, Greece. 4 Technological Educational Institute of Western Greece, Department of Mechanical Engineering, M. Alexandrou 1, 26 334 Patras, Greece. Abstract: The results indicated that halloysite exhibited fast adsorption rates and high removal capacity from solution in respect to the NH 4 + ions concentration at pH=7.0. The removal efficiency of the treated halloysite depends on the mass/volume ratio and also on the initial concentration of NH 4 + ions in solution. High mass/volume ratios (0.4g/20mL, 0.4g/50mL) exhibit up to 30% of the halloysite’s removal efficiency, when the initial NH 4 + ions concentration is low (<40 mg/L). By contrast, low mass/volume ratios (0.4g/100mL, 0.4g/200mL) indicate lower removal capacity with respect to low NH 4 + ions concentration in solution. The Langmiur adsorprtion models were applied to describe the equilibrium isotherms. In addition, natural abundance constitutes halloysite clay mineral as a low cost effective material. Thus, it can be considered as a promising technique in large and/or industrial scale applications for water and waste water purification purposes. Key words: Ammonium ions, Halloysite, Removal efficiency, Water purification. 1. INTRODUCTION Nitrogen (N) is a very common element which is essential for almost all forms of life found on earth. Due in part to a wide range of possible valence states ranging from -3 to +5, nitrogen can form various chemical species, including nitrogen gas (N 2 ), nitrous oxide (N 2 O), ammonium (NH 4 + ), ammonia (NH 3 ), nitrate (NO 3 - ), nitrite (NO 2 - ), and organic nitrogen (N organic ). The most common N compound found in groundwater is NO 3 - , but in strongly reducing environments, NH 4 + can be the dominant form. NH 4 + levels in groundwater are normally very low but elevated concentrations are associated with both natural and anthropogenic sources. Anaerobic decomposition of organic-rich sediments is a natural source of ammonium contamination in aquifers (Böhlke et al. 2006). On the other hand, waste disposal sites and septic tanks are among the major anthropogenic point-sources that release NH 4 + in groundwater, while inorganic fertilizers and animal manure are common non-point sources responsible for NH 4 + contamination of groundwater. There are several biological processes which transform nitrogen species to ammonium. Under anoxic conditions, heterotrophic microbes break down soil organic matter and produce NH 4 + , a procedure called ammonification. The dissimilatory nitrate reduction to ammonium (DNRA) is another process that involves the reduction of NO 3 - to NH 4 + in strongly reducing environments combined with high amounts of carbon and limited available nitrate. Reversely, ammonium movement in groundwater may be retarded by sorption (including ion-exchange) and/or microbially induced transformations such as nitrification under oxic conditions or Anaerobic ammonium oxidation (Anammox) under anoxic conditions (Böhlke et al. 2006). Although NH 4 + has no known toxic effects in concentrations that can be expected to be found in drinking water, elevated levels of NH 4 + in municipal water can react with chlorine, used as a disinfectant, and lead to an increase in total coliform populations. High concentrations of NH 4 + in drinking water are not desirable, since it can be converted to nitrate in the water distribution system, cause failure of filters for removal of manganese as well as taste and odour. Additionally, the