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TECHNICAL REPORTS

Wetlands and Aquatic Processes

Accumulation, Release, and Solubility of Arsenic, Molybdenum, and Vanadium in Wetland Sediments

Department of Environmental Science, Policy, and Management—Ecosystem Sciences, 151 Hilgard Hall, University of California, Berkeley, CA 94720-3110. P.M. Fox, current address: United States Geological Survey, 345 Middlefield Road, Mail Stop 465, Menlo Park, CA 94025

^* Corresponding author (doner{at}nature.berkeley.edu).

Received for publication October 7, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
This study was undertaken to determine the fate of As, Mo, and V (trace elements, TEs) in the sediments of a constructed wetland in use for the remediation of potentially toxic trace element–contaminated agricultural drainwater. After three years of wetland operation, sediment cores were collected to determine changes in TE concentrations as a function of depth and the effects of varying water column depth. All TE concentrations were highest in the top 2 to 4 cm and decreased with depth. Molybdenum accumulated in the wetland sediments, up to levels of 32.5 ± 4.6, 30.2 ± 8.9, and 59.3 ± 26.1 mg kg^-1 in the top 1 cm of sediment at water depths of 15, 30, and 60 cm, respectively. In the top 2 cm of sediment, As accumulated (28.2 ± 3.0 mg kg^-1) only at the 60-cm water depth. Below 2 cm, as much as 10 mg kg^-1 of As was lost from the sediment at all water depths. In most cases, V concentrations decreased in the sediment. In this wetland system, the lowest redox potentials were found near the sediment surface and increased with depth. Thus, in general As, Mo, and V concentrations in the sediment were highest under more reducing conditions and lowest under more oxidizing conditions. Most of the accumulated Mo (73%) became water soluble on drying of samples. This has important implications for systems undergoing changes in redox status; for instance, if these wetland sediments are dried, potentially large amounts of Mo may be solubilized.

Abbreviations: EC, electrical conductivity • TE, trace element

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
ELEVATED LEVELS of potentially toxic trace elements, including arsenic, boron, selenium, molybdenum, uranium, and vanadium are found in agricultural drainwater from many soils throughout the San Joaquin Valley (Moore et al., 1990). Agricultural drainwater is commonly impounded in evaporation ponds, where the water becomes increasingly concentrated in salts and trace elements, reaching levels that may be toxic to aquatic organisms (Bradford et al., 1990; Ong et al., 1995). Thus, a mechanism of remediation of these waters is necessary for the long-term viability of agriculture in this region. Constructed wetlands have been investigated as a remediation method for metal-contaminated water (Dunbabin and Bowmer, 1992; Hansen et al., 1998). While such systems may effectively remove some elements (e.g., Se) from the water, the environmental fate of other elements is uncertain. This study focuses on the distribution of As, Mo, and V in the sediments of a constructed wetland flooded with drainwater that contained elevated levels of these elements.

The accumulation or release of trace elements in wetland sediments is controlled largely by their geochemistry, with redox behavior playing a particularly important role. Arsenic oxidation states of -3, 0, +3, and +5 are found in nature. In reducing environments (<100 mV), As(III) is the dominant oxidation state while in oxic environments, As(V) commonly dominates. Various organic forms of As also occur, primarily methylated species formed through microbial processes. Solubilization of As under reducing conditions has been observed by numerous researchers (Amrhein et al., 1993; Masscheleyn et al., 1991; McGeehan and Naylor, 1994). McGeehan and Naylor (1994) and Masscheleyn et al. (1991) emphasized the importance of both lower adsorption of arsenite versus arsenate and the dissolution of adsorbing phases such as iron oxides in the higher solubility of As under reducing conditions. However, other researchers have shown that arsenite adsorption by Fe oxides is greater than that of arsenate at pH values above 6 or 7 (Jain and Loeppert, 2000; Sun and Doner, 1998).

Vanadium may exist in valence states ranging from +2 to +5. Under moderately reducing and aerobic conditions, V(IV) and V(V) species are dominant. Vanadium(III) may be precipitated as an oxide or oxyhydroxide, V(IV) is commonly found as the vanadyl cations [VO²⁺ and VO(OH)⁺], and V(V) is most often present as the vanadate oxyanions (H₂VO^-₄ and HVO^2-₄) (Wanty and Goldhaber, 1992). While adsorption of the anions is much lower than the cations, VO²⁺ solubility may be greatly increased through complexation with dissolved organic matter (Wanty and Goldhaber, 1992; Wehrli and Stumm, 1989). In a sediment incubation study, Amrhein et al. (1993) found that under reducing conditions V concentrations slowly decreased in the solution phase; when samples were exposed to oxidizing conditions, V concentrations in the solution phase dropped to essentially zero. They attributed the loss of V from solution under reducing conditions to the precipitation of VO(OH)₂.

Molybdenum is generally found in two oxidation states in nature, Mo(IV) and Mo(VI). While Mo(IV) is often found as MoS₂, particularly in mineral deposits, the formation of MoS₂ is often thought to require high temperatures (>200°C) such as those present during the crystallization of magma (Arutyunyan and Khurshudyan, 1966). However, Tucker et al. (1997) demonstrated that sulfate-reducing bacteria are capable of mediating the formation of MoS₂. In oxidizing environments Mo(VI) dominates and it is commonly present as the oxyanion molybdate (MoO^2-₄). In a laboratory experiment, Amrhein et al. (1993) found that Mo was lost from solution under reducing conditions and remobilized under oxidizing conditions, and hypothesized that MoS₂, a low-solubility mineral, formed in their system. These results are in agreement with a study undertaken in the field, wherein Mo exhibited similar behavior (Fox and Doner, 2002a). In this study they also found that Fe minerals were important sinks for Mo accumulation in reducing sediments. Helz et al. (1996) proposed that under reducing conditions and with the reduction of sulfate, molybdate is converted to thiomolybdate , which then binds to Fe, Al, and organic matter phases via sulfur bridges. This mechanism could also account for decreased Mo solubility under reducing conditions.

This study was undertaken to gain a greater understanding of the fate of the trace elements As, Mo, and V in the sediments of a constructed wetland and to elucidate the effects of differing redox conditions on these elements in the field. Specifically, the changes in trace element concentrations over time, depth distribution of trace elements, and the effect of water column depth were investigated through the extraction of sediment core samples.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Study Site
The study site consists of a series of flow-through constructed wetland cells located in the Tulare Lake Basin in the San Joaquin Valley (35°52'31'' N, 119°38'32'' W; Kings County, CA). In June 1996 a wetland consisting of 10 separate flow-through wetland cells, each measuring 15 x 75 m, was constructed and began operation receiving agricultural drainwater. The cell treatments consisted of different vegetative types and water depths. The study site was described in detail by Gao et al. (2000). This study focuses on two of these wetland cells, Cells 4 and 8. Cell 4 contained smooth cordgrass (Spartina alterniflora Loisel.) and duckweed (Lemna minor L.) and was covered with 15 cm of water. Cell 8 had a variable water depth ranging from 15 to 60 cm (Fig. 1) . Saltmarsh bulrush [Bolboschoenus maritimus (L.) Palla] grew only in the shallowest sections of Cell 8 (15 cm of water). The water residence times were approximately 6 and 19 d for Cells 4 and 8, respectively (Tanji, 1999). The soil in this area is characterized as a Westcamp loam (fine-silty, mixed, superactive, calcareous, thermic Fluvaquentic Endoaquept) although construction of wetland cells drastically altered the soil, creating a fairly well mixed soil profile in the top 15 cm. Some typical properties of the inlet water and sediments are shown in Tables 1 and 2. Measurements in the postflooded wetland were made in the middle of each cell in June 1998. Redox potential (Eh) was determined using a platinum combination electrode (readings taken after 15 to 20 min of equilibration time) and values were corrected to the standard hydrogen electrode by adding 200 mV to the measured values. Electrical conductivity (EC) and trace element concentrations were measured in bulk water samples taken mid-cell. A combination pH electrode was used to measure pH in both the water and sediment. Variations occur in the composition and TE content of the inlet water throughout the year.

View larger version (10K): [in this window] [in a new window]   Fig. 1. Diagram of Cell 8 showing variable water depth. Circles designate sampling locations.

Three subsamples of each core section were extracted sequentially with (i) deionized water for 1 h, (ii) 0.1 M KH₂PO₄–K₂HPO₄ (pH 8) for 24 h, (iii) 1.0 M sodium acetate (pH 5) for 5 h followed by PO₄, and (iv) 0.02 M NaOH (85°C) for 2 h followed by PO₄, at a solid to solution ratio of 1:10. The use of the phosphate extraction (Step 2, PO₄) following acetate and NaOH extractions serves to recover any trace elements released from these extracts and readsorbed by the sediment. This procedure was adapted from Lipton (1991) to target oxyanions in specific phases of the sediment: (i) water-soluble, (ii) specifically adsorbed, (iii) carbonate-bound, and (iv) organic-bound. The remaining sediment was acid-digested with nitric acid, sulfuric acid, and ammonium oxalate according to Huang and Fujii (1996) to extract residual TEs. This approach provides information on the relative availability and pools of an element in soil. The acetate-extractable samples were acidified to pH 3 with high-purity nitric acid before analysis for As, Mo, V, Fe, and Mn by inductively coupled plasma emission spectroscopy (ICP–AES; Thermo Jarrell Ash [Franklin, MA] IRIS). The concentrations are reported on a dry weight basis. The EC and pH were measured in the water extracts before analysis. The pH was also measured in slurry, with a soil to water ratio of 1:1, according to the method outlined by Thomas (1996). A National Institute of Standards and Technology (NIST; Gaithersburg, MD) standard reference material (#2709, San Joaquin Soil) was acid-digested and 77 (Fe), 82 (Mn), 109 (V), and 115% (As, Mo) of the reported total was recovered. The NIST values reported for Mo were uncertified.

Changes in trace element concentrations over time and differences in concentrations with depth and between cells were compared using analysis of variance (ANOVA). For all analyses the probability level was set to 0.05 unless otherwise noted. Concentrations measured in each of the four replicate core samples were averaged and these averages are reported with standard errors in the text and tables.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Molybdenum concentrations in sediment cores collected in June 1999 from Cells 4 and 8 are summarized in Table 3. Significant accumulations of Mo in June 1999 compared with preflooded soil were noted in almost every sample. In the top 1 cm of the sediment profile a total of 32.5 ± 4.6, 30.2 ± 8.9, and 59.3 ± 26.1 mg Mo kg^-1 was present in Cell 4, Cell 8 (shallow), and Cell 8 (deep), respectively. This compares with a total of 1.7 ± 0.1 mg Mo kg^-1 in the preflooded soil samples. Molybdenum concentrations generally were highest in the top 2 cm and decreased with depth below that. Variations in total Mo with depth were significant in the shallow and deep sections of Cell 8 at = 0.05 and in Cell 4 at = 0.10. The majority of the Mo (73% on average) was present in the water-soluble fraction.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

In this system, the greatest accumulations of Mo occurred near the sediment–water interface where a source of Mo (inflow of drainwater) and low redox status coexist. Two explanations of an accumulation of Mo under reducing conditions are possible. Several researchers have proposed the formation of MoS₂ whereby both Mo and S are reduced according to the following equation (Amrhein et al., 1993; Bertine, 1972):

Helz et al. (1996) proposed a different mechanism of Mo accumulation whereby molybdate is converted to tetrathiomolybdate (MoS^2-₄), which may form covalent bonds to Fe and Al solids and organic matter through sulfur bridges. The formation of tetrathiomolybdate requires only the reduction of S, according to the following equation:

While our data cannot distinguish between the two mechanisms, Helz et al. (1996) and Arutyunyan and Khurshudyan (1966) suggested that the formation of MoS₂ is kinetically limited. However, Tucker et al. (1997) demonstrated that the sulfate-reducing bacteria Desulfovibrio desulfuricans is capable reducing MoO^2-₄ and SO^2-₄ to form MoS₂ in pure culture. Therefore, the role of microbes may be important. Our preliminary data from laboratory studies using X-ray absorption near edge structure (XANES) spectroscopy indicated that both MoS₂ and thiomolybdate may be formed in the solid phase under conditions similar to those found at our wetland site (Fox and Doner, 2002b). In a previous study we demonstrated the importance of the Fe solid phase for Mo accumulation under reducing conditions (Fox and Doner, 2002a). Although the formation of MoS₂ cannot be discounted as a possible mechanism for Mo accumulation, the formation of thiomolybdate seems likely.

Most of the accumulation of Mo occurred in the water-extractable fraction. It is therefore tempting to assume that this accumulation was simply a result of Mo in pore water. If we assume that the Mo concentrations in the pore water were equivalent to those in the overlying water column, then estimates of 0.05 and 0.36 mg Mo kg^-1 from entrained solution for Cells 4 and 8, respectively, can be calculated based on the saturated water content of the sediment. While there is certainly some error involved in these estimates, including spatial and depth variations in Mo concentration, it is unlikely that pore water Mo enrichment could account for such high levels of Mo in the water extract, particularly in the top 4 or 9 cm of the sediment. In addition, in a separate study of these wetlands, Terry (1998) found that in the top 10 cm of soil, Mo concentrations in pore waters were low (approximately 0.5 mg L^-1) in areas of high total sediment Mo. Thus, most likely much of the Mo was transformed into a water-soluble form after air-drying, and this form comprises the bulk of the total water soluble Mo. This is in agreement with the results of Amrhein et al. (1993), who found that while Mo was lost from solution under anaerobic conditions, it was completely resolubilized under aerobic conditions, and the results of Fox and Doner (2002a), who found that water-extractable Mo accounted for the major fraction of total Mo in soil and mineral bags collected from a wetland site. One consequence of this transformation is that the newly solubilized Mo will redistribute among the other fractions, particularly the phosphate fraction and possibly the acetate and NaOH fractions as well. This may result in Mo concentrations for these fractions that are higher than they were under field conditions.

Total As and V concentrations also decreased with depth. However, unlike Mo, this trend represents a loss at depth rather than an accumulation at the surface. Losses of As and V at depth may be due to moderately reducing conditions, which result in the dissolution of adsorbing phases (i.e., Fe oxides and oxyhydroxides) as evidenced by the higher water-soluble Fe and Mn at depth than at the surface as shown in Table 6. Iron and Mn sulfides precipitated at the surface where strongly reducing conditions exist may retain some trace elements. In addition, As and V may be present in more soluble forms under the moderately reducing conditions found at depth. The pH in these sediments ranges from 7.5 to close to 10, and at these pH values arsenite adsorption is higher than arsenate adsorption. For Fe oxyhydroxides, maximum arsenite adsorption occurs around pH 9 (Jain and Loeppert, 2000; Sun and Doner, 1998). Likewise, Manning and Goldberg (1997) and Sun (1998) showed that in soil, arsenate sorption decreases with increasing pH, while arsenite adsorption increases with increasing pH. Therefore, at alkaline pH, arsenite adsorption may be greater than arsenate adsorption. Thus at greater depths where more oxidizing conditions (compared with the surface) and higher pH are present, As would be more soluble. A retention of As and V at the sediment surface may be due to reduction to less mobile phases such as arsenite, As-sulfides (FeAsS, AsS, As₂S₃), VO²⁺, and V(III) oxides. The formation of As-sulfide phases under strong reducing conditions has been observed by McCreadie et al. (2000) and Reynolds et al. (1999).

View larger version (22K): [in this window] [in a new window]   Fig. 2. Concentrations of As, Mo, and V in the surface sediments (0–2 cm) for each fraction in Cells 4 and 8.

The pH of the water extract, water-soluble Fe and Mn, and total Fe concentrations vary across cells and may provide some insight into the variation of As, Mo, and V across cells. Generally, the water extract pH in the shallow section of Cell 8 was the highest (mean = 8.43 in the water extract), followed by the deep section of Cell 8 (8.18) and Cell 4 (8.10). Both water-soluble Fe and Mn were highest in the shallow section of Cell 8, followed by Cell 4, and were much lower in the deep section of Cell 8 (Table 6). The water in these wetlands contained high levels of sulfate (approximately 22 mM). Many metals, including Fe and Mn, form insoluble metal sulfides in the presence of S^2- (e.g., amorphous FeS and pyrite, FeS₂) (Dunbabin and Bowmer, 1992; Yu, 1985). This may explain the low water-soluble Fe and Mn concentrations in the deep section of Cell 8. This interpretation suggests that sulfate reduction was greater in the deep section of Cell 8 than in Cell 4 and the shallow section of Cell 8. The total Fe concentrations were highest in the deep section of Cell 8, followed by the shallow section of Cell 8 and Cell 4 (Table 6), possibly because most of the reduced Fe was conserved in the deep section of Cell 8 through precipitation with sulfide, but lost in Cell 4 where the extent of sulfate reduction is presumably not as great. As discussed above, the higher levels of trace elements (As, Mo, and V) present in the deep section of Cell 8 may result from incorporation into Fe-sulfide phases or precipitation as discrete trace element–sulfide phases.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
This study was undertaken to determine the accumulation and solubility of trace elements in the sediments of a constructed wetland in use for the remediation of agricultural drainwater. We found that Mo accumulated in the sediments with most of the accumulation occurring in the top 2 cm and decreasing with depth. Total Mo concentrations of up to 75 mg Mo kg^-1 were found in the deep section of Cell 8 in June 1999 (after three years of operation of the wetland) at 1 to 2 cm, whereas much lower Mo concentrations (approximately 5 mg Mo kg^-1) were found at depths greater than 9 cm. Most of this accumulation (73%) was present in the water-soluble fraction, indicating that most of the Mo was resolubilized on drying. This has important implications for the long-term management of this site as well as for other systems that may undergo periodic fluctuations in redox status. If the constructed wetland is allowed to dry, then the solubility of Mo will greatly increase, possibly resulting in large fluxes to ground water, runoff, or drainwater. Unlike Mo, both As and V were lost from the sediment at depths greater than 2 cm. In the top 2 cm of sediment where conditions are most reducing, As remained at preflooded levels and even accumulated in the most reducing environment [Cell 8 (deep)], while V remained at preflooded levels only in Cell 8 (deep). It appears that Mo accumulation as well as As and V accumulation or retention in the surface sediments is dependent on the depth of the overlying water column and correspondingly on redox status. These results underscore the complexity of As and V redox behavior. It appears that under moderately reducing conditions in the field, As and V may be mobilized, while under strongly reducing conditions, As and V are accumulated or retained in sediments. Most likely, there are several competing reactions including both dissolution or desorption and precipitation or adsorption occurring simultaneously.

	ACKNOWLEDGMENTS

 
Appreciation is extended for financial support from the UC Salinity/Drainage Program, Regional Research W-184, and Hatch Project 6135-H.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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