Elsevier

Water Research

Volume 40, Issue 12, July 2006, Pages 2375-2386
Water Research

Arsenic removal from geothermal waters with zero-valent iron—Effect of temperature, phosphate and nitrate

https://doi.org/10.1016/j.watres.2006.04.006Get rights and content

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 °C). 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, Eact, and the constant temperature coefficient, θ were determined for each removal process.

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 proposed a new guideline for arsenic in water (10 μg L−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 geothermal activity is recognized to be significant. Arsenic contamination 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 (Webster and Nordstrom, 2003). Therefore, geothermal waters contain high concentration of arsenic and heavy metals.

Several studies have been conducted in order to examine the aqueous speciation of arsenic and its conservative or non-conservative behavior in geothermal systems. In many geothermal systems, arsenic exhibits a positive relation with chloride or boron indicating a conservative behavior. In these systems, thermal waters mix with cool meteoric ground waters on their way to the surface of the earth (Wilkie and Hering, 1998). In some other geothermal systems, constant ratios between a conservative element such as chloride and arsenic do not show conservative behavior of arsenic, but indicates that arsenic is highly mobile (Arnórsson, 2003). However, arsenic mobility depends on the partitioning of arsenic between the solid and liquid phases. Several geochemical conditions (pH, redox conditions, temperature, the presence of soil organic matter and Fe–Mn hydroxides) trigger important mechanisms in the cycle of arsenic in soils and sediments controlling in this way the mobility of arsenic (Guern et al., 2003). In geothermal systems, arsenic is discharged mainly as As(III) (Wilkie and Hering, 1998) and is derived from the dissolution of the host rock (Webster and Nordstrom, 2003). As(III) is oxidized to As(V) during the ascension of geothermal hot waters and their mixing with shallow oxidized ground waters. The ratio of As(V):As(III) during discharge indicates the degree of exposure of ground water to oxygenated conditions. Several studies show a rapid decrease in As(III) with distance from the geothermal spring while an increase in As(V) has been observed. The rapid oxidation of As(III) to As(V) was suggested to be primarily due to biological processes (Wilkie and Hering, 1998; Langner et al., 2001).

Greece is a country located on a geodynamically active area. Geothermal fields can be found in several volcanic islands (Milos, Santorini, Nisyros, etc.) and in mainland Greece along the Hellenic Volcanic Arc (HVA) in South Aegean. The HVA extends over a 450 km length from Korinthos in mainland Greece to the island of Kos in the east (Fytikas et al., 2000). In addition, geothermal fields are located in the areas of Aghia Paraskevi field in Kassandra Peninsula, Petralona-Triglia field in West Chalkidiki, Loutraki field in Aridea province, Lagadas and Nymhopetra-Apollonia fields in the Mygdonia basin, Therma and Sidirokastro fields in Serres Prefecture (Meladiotis et al., 2002). Geothermal fields in the region of Thrace are located in Alexandroupolis, Sappes, Mitrikou Lake (Rhodope Prefecture) as well as in Kessani, N. Erasmio, Eratino area of Xanthi Prefecture.

Zero-valent iron (ZVI) (iron filings) is an inexpensive, nontoxic material which has been used for the removal of arsenic under different conditions in laboratory and field experiments (Nikolaidis and Tyrovola, 2006). Many researchers have conducted experiments in order to elucidate the mechanism of arsenic removal, removal kinetics and the factors that influence arsenic removal (Lackovic et al., 1999; Nikolaidis et al., 2003; Su and Puls, 2001a, Su and Puls, 2001b, Su and Puls, 2003; Melitas et al., 2002; Kanel et al., 2005; Bang et al., 2005; Lien and Wilkin, 2005). In addition, several studies have examined the feasibility of nitrate removal by ZVI, evaluated the main by-products formed and predicted the removal kinetics of nitrate (Huang and Zhang, 2004; Choe et al., 2000; Huang et al., 1998; Westerhoff and James, 2003; Alowitz and Scherer, 2002).

The main objective of this study was to show the efficiency of ZVI in removing arsenic from geothermal waters located in agricultural regions with high concentrations of nitrate and phosphate due to agricultural activities. Specifically the goals of this study were: (i) to conduct a short-term field experiment in order to examine if the geochemistry of the influent geothermal origin groundwater was corrosive enough in order to trigger iron corrosion and facilitate the process of arsenic removal, (ii) to examine the influence of competing ions on the removal of arsenic by ZVI (i.e. natural geochemistry does not inhibit the process), (iii) to conduct batch experiments in order to evaluate the effect of temperature, phosphate and nitrate on As(III) and As(V) removal rates, (iv) to estimate the rates of nitrate reduction and phosphate removal by ZVI at different temperatures, and (v) to determine the activation energy, Eact, and the constant temperature coefficient, θ for each removal process.

Section snippets

Methodology

This study was focused on the geothermal field of Triglia-Petralona, northern Greece. Triglia is a town in Western Chalkidiki (Central Macedonia). The effectiveness of ZVI in removing arsenic from geothermal ground waters and the effects of nitrate and phosphate were assessed by conducting in situ field and laboratory experiments. First, a small-scale, short duration field pilot study was conducted to demonstrate the feasibility of ZVI in treating geothermal water in situ. Afterwards, batch

As distribution in Triglia's geothermal field

The region of Central Macedonia is build upon metamorphic rocks (Palaeozoic or older age) that are interrupted by granitic (of Mesozoic age) and volcanic rocks (Tertiary age). The geothermal field in Triglia is associated with active faults (of Miocene and younger age) that transect the hard rocks and the sediments (Meladiotis et al., 2002). Arsenic contamination in Triglia is caused by the Katsika area karstic geothermal water (29–41 °C). The warm water percolates into the surrounding Neogene

Conclusions

The concentrations of arsenic in geothermal waters in many places around the world (USA, Hungary, Greece, etc.) exceed the recommended drinking water standards. Many geothermal fields happen to be located in areas with agricultural activities. This study showed that arsenic removal kinetics were affected by arsenic speciation, temperature, and the presence of competing anions (phosphates and nitrates). As(V) showed faster removal rates than As(III). The presence of phosphate and nitrate

Acknowledgments

This project was funded by the General Secretariat for Research and Technology, Hellenic Ministry of Development, as part of a Greek–Hungarian Collaboration and by the Hellenic Ministry of Education under the contract number MIS88744—EPEAEK II-IRAKLITOS.

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