Dynamics of organic and inorganic arsenic in the solution phase of an acidic fen in Germany

Abstract

Wetland soils play a key role for the transformation of heavy metals in forested watersheds, influencing their mobility, and ecotoxicity. Our goal was to investigate the mechanisms of release from solid to solution phase, the mobility, and the transformation of arsenic species in a fen soil. In methanol–water extracts, monomethylarsonic acid, dimethylarsinic acid, trimethylarsine oxide, arsenobetaine, and two unknown organic arsenic species were found with concentrations up to 14 ng As g⁻¹ at the surface horizon. Arsenate is the dominant species at the 0–30 cm depth, whereas arsenite predominated at the 30–70 cm depth. Only up to 2.2% of total arsenic in fen was extractable with methanol–water. In porewaters, depth gradient spatial variation of arsenic species, pH, redox potentials, and the other chemical parameters along the profile was observed in June together with high proportion of organic arsenic species (up to 1.2 μg As L⁻¹, 70% of total arsenic). Tetramethylarsonium ion and an unknown organic arsenic species were additionally detected in porewaters at deeper horizons. In comparison, the arsenic speciation in porewaters in April was homogeneous with depth and no organic arsenic species were found. Thus, the occurrence of microbial methylation of arsenic in fen was demonstrated for the first time. The 10 times elevated total arsenic concentrations in porewaters in June compared to April were accompanied by elevated concentrations of total iron, lower concentrations of sulfate and the presence of ammonium and phosphate. The low proportion of methanol–water extractable total arsenic suggests a generally low mobility of arsenic in fen soils. The release of arsenic from solid to solution phases in fen is dominantly controlled by dissolution of iron oxides, redox transformation, and methylation of arsenic, driven by microbial activity in the growing season. As a result, increased concentrations of total arsenic and potentially toxic arsenic species in fen porewaters were found in the growing season, suggesting an enhancing risk of arsenic transport of ground- and surface-waters under these conditions.

Introduction

Arsenic is a ubiquitous trace metalloid and is found in virtually all environmental media through natural and anthropogenic processes (Adriano, 2001). Arsenic occurs mainly inorganic in the environment with the dominance of arsenate (As(V)) under aerobic and arsenite (As(III)) under anaerobic environments. Methylation of inorganic As species by aerobic and anaerobic microorganisms produce monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), and trimethylarsine oxide (TMAO) (Cullen and Reimer, 1989, Sadiq, 1997, Bentley and Chasteen, 2002). Tetramethylarsonium ion (TETRA), arsenobetaine (AsB), arsenocholine (AsC), and arsenosugars, are thought to be originated from biosynthesis, e.g., by alga or microorganisms (Pongratz, 1998, Geiszinger et al., 2002). The toxicity and mobility of As species depends on their chemical forms: inorganic As species are more toxic and less mobile than organic As species (Wauchope, 1975, Holm et al., 1980, Chiu and Hering, 2000, Mandal and Suzuki, 2002). Thus, investigation on the total As (As_total) is not sufficient for risk assessment of As in the environment.

The release of As into ground water is thought to be linked to the redox chemistry of iron and sulfur. Under oxic conditions, the release of As may be driven by dissolution of As-rich pyrites through oxidation (Chatterjee et al., 1995, Zheng et al., 2004). The occurrence of sulfide under anoxic condition may immobilize As by its high affinity to sulfide minerals or by the formation of As-sulfide minerals (Bostick and Fendorf, 2003). Under anoxic conditions, the release of As associated with Fe and Mn oxides may be important (Nickson et al., 1998, Bhattacharya et al., 2003). Under oxic conditions, As species are immobilized by Fe and Mn oxides by adsorption and coprecipitation (Wilkie and Hering, 1996, Bednar et al., 2005). Wetland soils are characterized by high content of organic matter. The probable interactions of organic matter with As are through the mobilization and immobilization of As species due to the microbial transformation of Fe and S species as controlled by availability of organic matters (Kirk et al., 2004, O’day et al., 2004), through the probably strong affinity of As species to organic matters (Thanabalasingam and Pickering, 1986, Anawar et al., 2003) and through fueling the redox transformation and methylation of As by microorganisms (Oremland et al., 2004). However, the mechanisms of As release and accumulation in wetland soils are still unknown.

In the case of Hg, wetland soils are of special importance for transformation reactions (Weber et al., 1998, Roulet et al., 2001). Reduction or methylation resulting in the formation of methylmercury or volatile compounds like Hg⁰ and dimethylmercury influence strongly the mobility and export of organic Hg species from terrestrial catchments (Wallschläger et al., 1995, Lindberg et al., 2001). Methylation of As in soils is a strictly biological processes (Turner, 1949, Wood, 1974) and can be influenced by abiotic factors, such as pH and temperature (Cox, 1975, Huysmans and Frankenberger, 1991) and several methylated As species and AsB were detected in soils (Takamatsu et al., 1982, Tlustoš et al., 2002, Geiszinger et al., 2002). Methylation of As was demonstrated for different aerobic and anaerobic microorganisms, i.e., methanogenic and sulfate-reducing bacteria (Bentley and Chasteen, 2002, Kühnelt and Gössler, 2003). Therefore, the activity of methanogenic and sulfate reducing bacteria in wetland soils (Horn et al., 2003, Küsel and Alewell, 2004) suggests the occurrence of organic As species.

The chemistry of As in the aqueous environment is complex because of its multiple oxidation states and its association with a variety of minerals through adsorption and precipitation. The lack of knowledge about the behavior of As in wetland soils makes it difficult to estimate the transformation between organic and inorganic As species and mobility of As species in wetland soils. Therefore, this investigation focuses on As speciation in the solid phase and in porewaters in a fen soil with the objectives (i) to estimate the occurrence of As methylation, (ii) to identify the release mechanisms of As, (iii) to evaluate the mobility of As species, and (iv) to estimate the environmental relevance of As in wetland soils.