Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres Arsenic removal from geothermal waters with zero-valent iron—Effect of temperature, phosphate and nitrate Konstantina Tyrovola a , Nikolaos P. Nikolaidis a,Ã , Nikolaos Veranis b , Nikolaos Kallithrakas-Kontos c , Pavlos E. Koulouridakis c a Department of Environmental Engineering, Technical University of Crete, 73100 Chania, Crete, Greece b IGME, Branch of Central Macedonia, 1 Frangon St., 54626 Thessaloniki, Greece c Department of Sciences, Technical University of Crete, 73100 Chania, Crete, Greece article info Article history: Received 26 October 2005 Received in revised form 7 April 2006 Accepted 12 April 2006 Keywords: Geothermal Zero-valent iron Arsenic Phosphate Nitrate ABSTRACT Field column studies and laboratory batch experiments were conducted in order to assess the performance of zero-valent iron in removing arsenic from geothermal waters in agricultural regions where phosphates and nitrates were present. A field pilot study demonstrated that iron filings could remove arsenic, phosphate and nitrate from water. In addition, batch studies were performed to evaluate the effect of temperature, phosphate and nitrate on As(III) and As(V) removal rates. All batch experiments were conducted at three temperatures (20, 30 and 40 1C). Pseudo-first-order reaction rate constants were calculated for As(III), As(V), phosphate, nitrate and ammonia for all temperatures. As(V) exhibited greater removal rates than As(III). The presence of phosphate and nitrate decreased the rates of arsenic removal. The temperature of the water played a dominant role on the kinetics of arsenic, phosphate and nitrate removal. Nitrate reduction resulted in the formation of nitrite and ammonia. In addition, the activation energy, E act , and the constant temperature coefficient, y were determined for each removal process. & 2006 Elsevier Ltd. All rights reserved. 1. Introduction Arsenic is a toxic and carcinogenic metalloid, which is ubiquitous in rock, soil and water. The adverse health effects of arsenic are well documented in the literature (Morton and Dunnette, 1994). The major pathway for human exposure to arsenic is drinking polluted ground water. In order to minimize the health effects of arsenic, the World Health Organization (WHO), the US Environmental Protection Agency (USEPA) and the European Commission have pro- posed a new guideline for arsenic in water (10 mgL 1 ). High concentrations of arsenic in groundwater have been found in many environmental conditions originating from natural processes and from anthropogenic sources. Natural occurring arsenic in ground waters associated with geother- mal activity is recognized to be significant. Arsenic contam- ination in geothermal systems has been identified in many areas of the world including the western USA, Mexico, central America, Japan, New Zealand, Papua New Guinea, Chile, Philippines, Indonesia, Kamchatka, Alaska, Iceland, France (Smedley and Kinniburgh, 2002; Webster and Nordstrom, 2003). Geothermal systems are situated in regions with normal or above normal geothermal gradients. These regions are found near plate margins where the geothermal gradients may be extremely high. In regions with high-temperature gradients subterranean faults and cracks allow meteoric water to seep underground where is heated by magma or hot rocks. Heated water can circulate back to the surface through the host rock. As geothermal water ascends to the surface reacts with the wall rocks causing mineral dissolution ARTICLE IN PRESS 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.04.006 Ã Corresponding author. Tel.: +30 28210 37785; fax: +30 28210 37847. E-mail address: nnikolai@mred.tuc.gr (N.P. Nikolaidis). WATER RESEARCH 40 (2006) 2375– 2386