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Published online 27 October 2006
Published in J Environ Qual 35:2333-2341 (2006)
DOI: 10.2134/jeq2006.0076
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Waste Management

Biosolids Affect Soil Barium in a Dryland Wheat Agroecosystem

J. A. Ippolito* and K. A. Barbarick

Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO, 80523-1170

* Corresponding author (jim.ippolito{at}colostate.edu)

Received for publication February 21, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In December 2003, the USEPA released an amended list of 15 "candidate pollutants for exposure and hazard screening" with regard to biosolids land application, including Ba. Therefore, we decided to monitor soil Ba concentrations from a dryland wheat (Triticum aestivum L.)–fallow agroecosystem experiment. This experiment received 10 biennial biosolids applications (1982–2003) at rates from 0 to 26.8 dry Mg ha–1 per application year. The study was conducted on a Platner loam (Aridic Paleustoll), ~30 km east of Brighton, CO. Total soil Ba, as measured by 4 M HNO3, increased with increasing biosolids application rate. In the soil-extraction data from 1988 to 2003, however, we observed significant (P < 0.10) linear or exponential declines in ammonium bicarbonate–diethylenetriaminepentaacetic acid (AB–DTPA) extractable Ba concentrations as a function of increasing biosolids application rates. This was observed in 6 of 7 and 3 of 7 yr for the 0- to 20- and 20- to 60-cm soil depths, respectively. Results suggest that while total soil Ba increased as a result of biosolids application with time, the mineral form of Ba was present in forms not extractable with AB–DTPA. Scanning electron microscopy using energy dispersive spectroscopy verified soil Ba–S compounds in the soil surface, probably BaSO4. Wet chemistry sequential extraction suggested BaCO3 precipitation was increasing in the soil subsurface. Our research showed that biosolids application may increase total soil Ba, but soil Ba precipitates are insoluble and should not be an environmental concern in similar soils under similar climatic and management conditions.

Abbreviations: AB–DTPA, ammonium bicarbonate–diethylenetriaminepentaacetic acid • EC, electrical conductivity • ICP–AES, inductively coupled plasma–atomic emission spectroscopy • L/E WWTP, Littleton/Englewood (Colorado) Wastewater Treatment Plant • SEM–EDS, scanning electron microscopy using energy dispersive spectroscopy


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN RESPONSE to a National Research Council report on land application of biosolids (National Research Council, 2002), the USEPA amended its list of "candidate pollutants for exposure and hazard screening" with 15 additional elements and compounds (USEPA, 2003). The USEPA listed acetone (CH3COCH3), anthracene (C14H10), Ba, Be, CS2, 4-chloroaniline (C6H6ClN), diazinon (C12H21N2O3PS), fluoranthene (C16H10), Mn, methyl ethyl ketone (CH3COCH2CH3), NO2–N, NO3–N, phenol (C6H5OH), pyrene (C16H10), and Ag as potential pollutants. Human consumption of these constituents from groundwater was the pathway of concern in the risk assessment. Soils and plants may have the ability to sorb and sequester these constituents, lessening or preventing their downward movement and thus protecting against groundwater contamination. Consequently, soil accumulation and plant uptake of these components from biosolids-amended sites must be considered. Few reports on land application of biosolids have focused on Ba.

Barite (BaSO4) and witherite (BaCO3) are the most prevalent, naturally occurring forms of barium. Barium is used in the manufacture of alloys, soap, rubber, linoleum, and valves, as a loader for paper, as an extinguisher for Ra, U, and Pu fires, and has been found in coal as well as in fuel oils (Choudhury and Carey, 2001). Barium compounds are also used in cement, specialty arc welding, glass industries, electronics, roentgenography, cosmetics, pharmaceuticals, inks, and paints. These compounds have also been used as insecticides and rodenticides (e.g., barium metaborate, barium polysulfide, and barium fluorosilicate [Choudhury and Carey, 2001]).

In humans, short-term Ba exposure can result in gastrointestinal disturbances and muscular weakness (USEPA, 2005), and ingestion of small quantities of water-soluble Ba may cause difficulty breathing, heart rhythm changes, stomach irritation, changes in nerve reflexes, swelling of the brain and liver, and heart damage (Lenntech, 2005). Long-term exposure has the potential to cause high blood pressure (USEPA, 2005) and the uptake of very large quantities of water-soluble Ba may cause paralysis and in some cases even death (Lenntech, 2005). The maximum contaminant level of Ba in drinking water has been set at 2 mg L–1 by the USEPA (USEPA, 2005).

Barium sulfate can be naturally present in high concentrations in soil formed from limestone, feldspar, and biotite micas of the schists and shales (Clarke and Washington, 1924). Barium sulfate in soils is not expected to be very mobile because of the formation of water-insoluble salts and its inability to form soluble complexes with humic and fulvic materials (USEPA, 1984). The solubility of BaSO4 is 2.2 mg L–1 (Weast, 1984, p. B75–B82), indicating that only a very small amount of solid Ba will dissolve in water and release relatively small amounts of Ba to the environment.

Both specific and nonspecific adsorption of free Ba in soils has been observed, with specific sorption occurring on metal oxides (Choudhury and Carey, 2001). Liu et al. (2002) noted increased Ba concentration associated with Fe and Mn oxide nodules when they compared these to bulk soil concentrations. They suggested that weakly hydrolyzed metal ions were related to adsorption reactions occurring during intense Fe and Mn solubilization and precipitation, and ultimately nodule formation. Electrostatic forces account for a large fraction of the nonspecific sorption of Ba on soil and subsoil and, as with other alkaline earth elements, retention is largely controlled by the cation exchange capacity of the soil (Choudhury and Carey, 2001). Adsorption onto metal oxides also probably acts as a control over dissolution and thus the concentration of Ba in ground and surface waters.

We have been conducting research on biosolids in soils from dryland wheat–fallow since the early 1980s. Every other year since 1982, we have applied 0, 6.7, 13.4, and 26.8 dry Mg biosolids ha–1. Our previous research did not extensively examine the relationship between biosolids rate and soil Ba. As Ba is now listed on the USEPA's candidate pollutant list, we have reexamined our results in relation to Ba soil concentrations. Our hypothesis was that after 10 biennial applications of biosolids in increasing rates, Ba concentrations will increase within the soil at the depth of biosolids incorporation but will be in the form of a relatively insoluble precipitate. We tested this using an AB–DTPA extraction, 4 M HNO3 digestion, scanning electron microscopy in conjunction with energy dispersive spectroscopy, and a sequential extraction technique (measuring soluble and exchangeable through residual inorganic Ba fraction concentrations) at the soil layer of biosolids incorporation, 0 to 20 cm, and we monitored a soil layer underneath this, at 20 to 60 cm, for any Ba losses. Our previous research has shown decreases in winter wheat (cv. Vona or TAM107) grain and straw Ba concentrations with increasing biosolids application rates (Ippolito and Barbarick, 2005), indicating that insoluble soil Ba precipitates were forming and thus reducing plant Ba uptake. We believed that Ba in the biosolids was mostly in an insoluble mineral form such as BaSO4, and that indigenous soil Ba would react with biosolids constituents to form similar insoluble compounds.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description
This study was part of a larger experiment that was described by Barbarick et al. (1995). The field study was started in the summer of 1982 on plots ~30 km east of Brighton, CO. Briefly, mean annual maximum and minimum temperatures are 19 and 2°C, respectively, mean annual precipitation is 35 cm, and the annual growing season is ~150 d (Soil Conservation Service, 1974). We used a dryland summer fallow rotation system in which one crop is produced every other year.

The soil was a Platner loam (fine, smectitic, mesic Aridic Paleustoll). Before biosolids application, soil organic matter content was <10 g kg–1 to a depth of 60 cm, surface (0–20 cm) soil pH was 6.5, and the pH of the subsoil (20–60 cm) ranged from 6.6 to 7.5. Electrical conductivity (EC) of saturated soil extracts was <1 dS m–1 at all depths, and NO3–N plus NH4–N was <7 mg kg–1 for all depths.

Biosolids: Generation, Barium Content, Application, and Experimental Design
The L/E WWTP biosolids were generated by anaerobic digestion of domestic sewage water followed by ~2 mo of sand-bed drying. Biosolids samples were collected before application and kept refrigerated at ~3°C before they were analyzed. Barium concentrations were not determined for all of the biosolids applied from 1982 to 2002 because, before 2002, the USEPA did not consider Ba a priority pollutant. The biosolids Ba elemental composition (Table 1), on a dry-weight basis, was determined by HClO4–HNO3–HF–HCl digestion using inductively coupled plasma–atomic emission spectroscopy (ICP–AES; Soltanpour et al., 1996). Every 2 yr from 1982 to 2002, except in 1998, we applied biosolids at rates of 0, 6.7, 13.4, and 26.8 dry Mg biosolids ha–1 to 3.6 by 17.1 m plots. We did not apply biosolids in 1998 since we had been informed that the land where our plots were located might be sold. We used four replications of all biosolids application rates in a randomized complete block arrangement. We weighed the biosolids (solids content of 530–880 g kg–1) and corrected for moisture content; evenly spread them across the plots using a front-end loader; hand raked them to improve the uniformity of distribution; and incorporated the biosolids to a depth of 10 to 15 cm with a rototiller.


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  		Table 1. Total dry-weight biosolids Ba mean concentrations and standard errors for even-year applications (following a fallow rotation) from 1982 through 1996, as determined by HClO4–HNO3–HF–HCl digestion (n = 4, except 1982). Biosolids Ba concentrations were not determined in 2000 or 2002.

 
Soil Sampling
Immediately following each wheat harvest, we collected composite soil samples (2–3 cores plot–1) from the 0- to 20-cm (plow layer) and 20- to 60-cm depth from all treatments and replicates. This led to a total number of samples collected of 32 following each harvest, the result of four replications of four treatments and two depths. We obtained samples near the center of each plot to avoid the biosolids redistribution problem associated with tillage operations (Yingming and Corey, 1993). We immediately air dried the soil samples and then crushed them to pass a 2-mm sieve.

Biosolids-Borne Barium
To identify the possible mineral phase controlling the biosolids-borne Ba solubility, we obtained an L/E WWTP biosolids sample in January 2006. In duplicate, we added 20 mL of distilled, deionized water to 10 g (dry-weight equivalent) biosolids and shook for 1 wk on a reciprocating shaker at 120 oscillations per min. We then centrifuged the samples at 7000 x g for 10 min and filtered the solution through a 0.2-µm nylon membrane filter. Approximately 2.0 g NH4OAc was then added to the filtrate to precipitate organics that had passed through the filter. We recentrifuged the filtrate, refiltered it through a 0.2-µm nylon membrane filter, and analyzed it for Ba using ICP–AES.

Soil Barium Chemical Fractionation
Barium retention in soils is controlled by electrostatic forces, specific adsorption onto metal oxides, solid precipitates, and to a limited extent by soil organic material (Choudhury and Carey, 2001). The fractionation scheme in this study quantified these pools. Soil samples collected following the 2002–2003 wheat harvest were sequentially extracted to fractionate soil Ba chemical pools based on their relative binding strength (Sloan et al., 1997). One gram of soil was placed in a 50-mL centrifuge tube and sequentially extracted for soluble and exchangeable, specifically sorbed and weakly bound, easily reducible Fe and Mn oxides (i.e., noncrystalline), Fe and Mn oxides (i.e., crystalline), residual organic fraction, and residual inorganic fraction using various extractants. The solution amounts and experimental and analytical conditions used are shown in Table 2. The extraction steps at room temperature were done on a reciprocating shaker at 120 oscillations min–1; heated extractions were done on an orbital shaker in a thermostat-controlled hot water bath. Following each extraction step, samples were centrifuged at 7000 x g for 10 min and the solution decanted into glass vials, filtered through a 0.2-µm nylon membrane filter, and analyzed for Ba using ICP–AES. Recovery percentage from the sequential extraction procedure was compared with a separate 4 M HNO3 soil extraction (Bradford et al., 1975; Chang et al., 1984; Barbarick et al., 1997). The amount of Ba extracted with 4 M HNO3 is a close approximation to total soil elements. The 4 M HNO3-extractable Ba procedure consisted of 1 g of soil, predigested in 10 mL of 4 M HNO3 overnight, digested at 80°C for 6 h, with vortexing every 2 h. This was brought to a final volume of 12.5 mL and filtered through Whatman no. 5 filter paper, and the Ba concentration in the solution was determined using ICP–AES.


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  		Table 2. Sequential extraction steps and conditions used (based on Sloan et al., 1997; modified from Gibson and Farmer, 1986).

 
The addition of 0.5 M CaNO3 used to extract soluble and exchangeable Ba in the 20- to 60-cm soil samples may have caused the dissolution of BaCO3 with subsequent precipitation of CaCO3; BaCO3 and CaCO3 solubilities are 20 mg L–1 and 15.3 mg L–1, respectively (Weast, 1984, p. B75–B82). To test this possibility, we weighed 1.0 g of reagent-grade BaCO3 into 50-mL centrifuge tubes in triplicate and added either 25 mL of distilled, deionized water or 25 mL of 0.5 M CaNO3. Samples were shaken on a reciprocating shaker at 120 oscillations min–1 for 18 h at room temperature. Following the extraction, samples were centrifuged at 7000 x g for 10 min and the solution was decanted, filtered through a 0.2-µm nylon membrane filter, and analyzed for Ba using ICP–AES.

To support our Ba concentration observations in the 20- to 60-cm depth, Visual MINTEQ, Version 2.40, was used (Gustafsson, 2006). This program predicts chemical speciation of metals in soil and is based on MINTEQA2 Version 4.0, yet operates in a Windows environment.

Extractable Soil Barium
Extractable Ba in all 0- to 20-cm soil extracts was measured from 1988 onward, and from 1990 in the 20- to 60-cm soil. No soil samples were obtained in 1998–1999 since no biosolids were added (due to possible land sale). Extractable Ba was determined using the 4 M HNO3 procedure and by the AB–DTPA procedure (Barbarick and Workman, 1987). Barbarick and Workman (1987) and Barbarick et al. (1997) showed that AB–DTPA-extractable elements, not including Ba, in biosolids-amended soils significantly correlated with several total added elements from biosolids. The AB–DTPA-extractable Ba was determined by shaking 10 g of soil in 20 mL of AB–DTPA for 15 min at room temperature, and then filtering the solution through Whatman no. 5 filter paper. We determined AB–DTPA-extractable soil Ba concentrations using ICP–AES.

Scanning Electron Microscopy–Energy Dispersive Spectroscopy
In addition to the sequential extraction, the clay fraction from all soils collected following the 2002–2003 wheat harvest was separated using the pipette method with no pretreatment (NRCS, 1996, Procedure 3A1). We oven-dried the clay fraction in Al weighing boats at 105°C for 24 h, then the clays were mounted on C-coated Al stubs. Clays were then analyzed for Ba precipitates using a JEOL JSM-6500F (JEOL USA, Peabody, MA) scanning electron microscope in conjunction with energy dispersive spectroscopy (SEM–EDS) at an accelerating voltage of 15 keV and a magnification of up to 150 000x. A backscattered electron detector was used to identify elements of relatively heavier atomic weight, and a Thermo Electron EDS system (Thermo Electron Corp., Waltham, MA) was used to generate dot maps of heavier elements such as Ba.

Statistical Analysis
The experimental design consisted of four treatments (0, 6.7, 13.4, and 26.8 dry Mg biosolids ha–1) by two depths (0–20 and 20–60 cm) by four replicates, for a total of 32 soil samples collected following each wheat harvest. A total of seven biennial sampling events occurred. Analyses were performed on all data (all replicates; n = 4) using linear regression analyses and analyses of variance to evaluate the effect of biosolids application rate on AB–DTPA- and 4 M HNO3-extractable Ba, and the soil Ba chemical fractions. We tested our hypotheses at P = 0.05 and assumed the sampling population was normally distributed.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Biosolids-Borne Barium
The average concentration of soluble Ba in the biosolids was 0.32 (SE ± 0.12) mg L–1; solubility of BaSO4 and BaCO3 are 2.4 mg L–1 and 20 mg L–1, respectively (Weast, 1984, p. B75–B82). Converting BaSO4 and BaCO3 species solubility to soluble Ba only produces concentrations of 1.31 and 13.9 mg Ba L–1 (Weast, 1984, p. B75–B82), respectively. This suggests that our biosolids-borne Ba concentration is controlled by species more insoluble than BaSO4. According to Weast (1984, p. B75–B82), there are no inorganic species with solubilities lower than that of BaSO4. Therefore, we feel confident in suggesting that biosolids-borne Ba is controlled by organic phases.

Soil Barium Chemical Fractionation
Using the average biosolids Ba concentration of 407 mg kg–1 (Table 1), the highest biosolids application rate of 26.8 Mg ha–1, and a soil bulk density of 1.4 g cm–3, the 10 biosolids applications that were performed up to 2002 would have added an estimated 40 mg total Ba kg–1 to the top 20 cm of soil. The difference in 4 M HNO3-extractable soil Ba concentrations between the control and the 26.8 Mg biosolids ha–1 application–1 was 37 mg kg–1 (Table 3). Consequently, the 2002–2003 4 M HNO3-extractable Ba concentrations in the highest biosolids treatment (37 mg kg–1) accounted for up to 92% of the estimated (40 mg kg–1) Ba soil accumulation based on year-to-year variability.


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  		Table 3. Mean (n = 4) soil Ba sequential extraction mass balance compared with 4 M HNO3-extractable Ba concentration, 2002–2003 crop year, and recovery percentage. Values within parentheses represent one standard error of the mean.

 
The Ba mass balance for the sequential extraction procedure, and comparison of the sequential extraction total Ba to 2002–2003 4 M HNO3-extractable Ba concentrations, is presented in Table 3. The sum of all soil chemical fractions for all treatments and depths was greater than but usually within 20% of the 4 M HNO3-extractable concentrations. Differences may be due to the 4 M HNO3 not representing a true total Ba concentration or errors in the sequential extraction steps being additive. Sloan et al. (1997) found similar recovery results when comparing trace metals recovered from biosolids-amended soils using the sequential extraction technique vs. 4 M HNO3-extractable metals from fresh soil. Ramos et al. (1994) showed between 80 and 110% recovery using a similar sequential extraction technique compared with a total soil digest.

The results of chemical fractionation of the 0- to 20-cm depth are shown in Fig. 1. Overall, Ba fraction concentrations were in the order of soluble and exchangeable fraction > crystalline Fe and Mn oxide fraction > residual inorganic fraction > specifically sorbed and weakly bound fraction > residual organic fraction > noncrystalline Fe and Mn oxide fraction. Within this depth, the increasing rate of biosolids applications caused Ba associated with noncrystalline Fe and Mn oxides to decrease and crystalline Fe and Mn oxides to increase with application rate (P < 0.05). The increase in crystalline Ba–Fe and –Mn oxide complexation with increasing biosolids application rate suggests that this complex is either dominant in the biosolids or has enhanced the soil Ba precipitate stability. The average biosolids Fe content applied to the site was 14.1 g kg–1. The 2002–2003 average 4 M HNO3-extractable soil Fe content in the 0- to 20-cm depth was 12.6 g kg–1 and Fe content in both biosolids and soil dominated other inorganic elements present. In either biosolids or soil, specific sorption can occur onto metal oxides and hydroxides, in which adsorption onto metal oxides probably controls the concentration of Ba in surface and ground waters (Choudhury and Carey, 2001). Liu et al. (2002) studied trace element adsorption in an Alfisol and noted increased Ba concentrations associated with Fe and Mn oxide nodules compared with bulk soil. They suggested that the occurrence of metals, including Ba, could be related to adsorption reactions taking place during nodule formation. These results indicate that biosolids-borne Ba can form more stable, crystalline soil chemical complexes.


Figure 1
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  		Fig. 1. Effects of biosolids application rates on soil sequential extraction Ba concentrations in the 0- to 20-cm depth, 2002–2003 crop year. Error bars represent one standard error of the mean (n = 4). Significant (P < 0.05) regression models are presented for the noncrystalline and crystalline Fe and Mn oxide phases.

 
The Ba sequential extraction results for the 20- to 60-cm depth are illustrated in Fig. 2. The Ba fraction concentrations followed the same trends as in the 0- to 20-cm depth, although the soluble and exchangeable fraction was by far the dominant fraction. Within this depth, increasing biosolids application rate caused the soluble and exchangeable Ba to increase and noncrystalline Fe and Mn oxide-bound Ba to decrease (P < 0.05). In our system, soil soluble biosolids-borne Ba compounds are transported downward and probably precipitate as slightly insoluble BaCO3. The solubilities of BaSO4 and BaCO3 are 2.4 mg L–1 and 20 mg L–1, respectively (Weast, 1984, p. B75–B82), further suggesting soluble Ba, not Ba from the dissolution of BaSO4, was contributing to Ba accumulation in the 20- to 60-cm depth. In the 20- to 60-cm soil layer, probably BaCO3 was detected in the soluble and exchangeable fraction. The average subsurface soil pH was 7.75 (SE ± 0.16) and is calcareous. This insoluble precipitate could reduce downward Ba mobility and thus prevent groundwater contamination.


Figure 2
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  		Fig. 2. Effects of biosolids application rates on soil sequential extraction Ba concentrations in the 20- to 60-cm depth, 2002–2003 crop year. Error bars represent one standard error of the mean (n = 4). Significant (P < 0.05) regression models are presented for the soluble and exchangeable phase and the noncrystalline Fe and Mn oxide phase.

 
The addition of 0.5 M CaNO3 used to extract soluble and exchangeable Ba in the 20- to 60-cm soil samples caused the dissolution of BaCO3 with subsequent precipitation of CaCO3. The average Ba concentration, on a weight basis, in the distilled, deionized water extract equaled 24.7 mg kg–1 compared with 2491 mg kg–1 for the 0.5 M CaNO3 extractant.

We used Visual MINTEQ to further test whether BaCO3 was precipitating in the subsoil. The model parameter pH was fixed at 7.75, Ca2+ and NO3– at 0.5 M, and a finite concentration of witherite [BaCO3(s)] at 0.75 M. This was based on conversion of the average Ba concentration of 100 mg kg–1 in the soluble and exchangeable fraction of the 20- to 60-cm depth. Model saturation index results indicated dissolution of witherite and precipitation of several CaCO3 mineral species.

Although we have yet to detect BaCO3 precipitates in the 20- to 60-cm soil depth, we know that BaCO3 can crystallize in the CaCO3 structure. Chaney et al. (2003) used Raman spectroscopy and pressures of up to 45 GPa to determine the nature of bond changes associated with BaCO3 phase transitions. Barium is a larger cation than Ca, and thus changes in bonding chemistry could occur under normal soil pressures. Chaney et al. (2003), however, found that the carbonate unit remained notably stable even under increased pressures.

Extractable Soil Barium
A total of seven biennial sampling events occurred. In six of the sampling events from 1988 to 2003, AB–DTPA-extractable Ba in the top 20 cm of soil significantly (P < 0.05) decreased according to an exponential decay model; only 2002–2003 data are shown (Fig. 3A). These results indicate that despite Ba accumulation from biosolids additions, the lability of the Ba decreased. Formation of slightly soluble minerals such as BaSO4 could possibly reduce AB–DTPA extractability.


Figure 3
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  		Fig. 3. Effects of biosolids application rates on soil NH4HCO3–diethylenetriaminepentaacetic acid (AB–DTPA)-extractable Ba concentrations in the (A) 0- to 20- and (B) 20- to 60-cm depths, 2002–2003 crop year. Error bars represent one standard error of the mean (n = 4).

 
Biosolids significantly decreased AB–DTPA Ba concentrations in three out of six sampling events in the 20- to 60-cm soil depth; only 2002–2003 data are shown (Fig. 3B). If migration of biosolids-borne Ba occurred below the top 20 cm, then AB–DTPA was not sensitive enough to detect changes in Ba concentrations. The formation of insoluble minerals such as BaSO4 or BaCO3 in the 20- to 60-cm soil depth could reduce Ba mobility below this depth.

In six of the seven sampling events from 1988 to 2003, 4 M HNO3-extractable Ba in the top 20 cm of soil significantly (P < 0.05) increased according to either a linear or exponential rise to a maximum model; only 2002–2003 data are shown (Table 3). The 20- to 60-cm soil depth 4 M HNO3-extractable Ba concentrations only increased in one of the seven sampling events (data not shown). In the 0- to 20-cm depth, the observed decrease in AB–DTPA and increase in 4 M HNO3-extractable Ba further suggests the occurrence of solid-phase Ba precipitation.

Scanning Electron Microscopy–Energy Dispersive Spectroscopy
Results from SEM–EDS showed Ba–S associations in the biosolids-treated soils, indicating that Ba is probably forming a BaSO4 precipitate. Barium associations were observed in soils from the higher application rates. This was due to the increased Ba addition increasing the likelihood of observable associations in the microscope. Using the 2002–2003, 0- to 20-cm soil sample from the 26.8 Mg biosolids ha–1 rate, the dot maps in Fig. 4 clearly show a potential Ba–S precipitate. Examination of Ti dot maps also showed a potential relationship with Ba and S. The Ti spectra typically yield a strong peak at ~4.5 keV, as does Ba (see Fig. 5), and overlap of both elemental spectra was therefore observed; however, Ba spectra also yield characteristic peaks of decreasing counts at ~4.8, 5.1, and 5.5 keV. A corresponding trend for the Ti spectra was not discernable; therefore, if a Ba–Ti–S precipitate was present, Ba substantially dominated over Ti within the precipitate.


Figure 4
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  		Fig. 4. Scanning electron microscopy image and energy dispersive spectroscopy elemental dot maps from the 0- to 20-cm depth, 26.8 Mg biosolids ha–1 treatment, 2002–2003 crop year.

 

Figure 5
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  		Fig. 5. Energy dispersive spectroscopy spectral analysis of a particle, possibly BaSO4, from the 0- to 20-cm depth, 26.8 Mg biosolids ha–1 treatment, 2002–2003 crop year, compared with a reagent-grade BaSO4 standard.

 
We also analyzed the portion of the sample in the boxed area of Fig. 4, and observed the following semiquantitative atomic elemental composition: 15.0% C, 59.8% O, 4.4% Al, 7.8% Si, 0.8% K, 0.9% Ca, 0.3% Fe, 1.9% As, 4.4% Ba, and 4.9% S. Carbon and Al were probably due to the C coating for specimen mounting and the Al stub, respectively. There was also probably a mixed silicate present in the background. The atomic percentages of Ba and S were similar and warranted further investigation to verify a potential BaSO4 precipitate.

We analyzed and compared the spectra from standard reagent-grade BaSO4 to that of the presumed soil Ba–S complex (Fig. 5). We found a strong correlation between the two spectra, supporting our contention that biosolids-borne and maybe even indigenous soil Ba has reacted with the biosolids constituents to form an insoluble BaSO4 mineral. Increasing biosolids application increased soil Ba content, ultimately forming insoluble BaSO4. As shown in Fig. 1, increasing biosolids application also increased crystalline Fe and Mn oxide-associated Ba. Interpreting our SEM–EDS and fractionation findings, the Ba fraction we associated with the crystalline Fe and Mn oxide phase could possibly be precipitated BaSO4.

Environmental Implications
Our results indicated that biosolids-borne Ba solubility was controlled by species more insoluble than BaSO4, probably by Ba–organic associations. In addition, Ba may be precipitating as BaSO4 in the 0- to 20-cm soil depth and moving into the 20- to 60-cm depth and forming BaCO3. Barium movement into the subsurface may be a function of dissolved organic matter. Al-Wabel et al. (2002) noted significantly greater metal mobility in biosolids-amended soils, obtained from the same site as this study, and suggested that this could lead to significant redistribution in the subsoil. A scenario for 26.8 dry Mg biosolids ha–1 biosolids-borne Ba addition, followed by movement and precipitation, is illustrated in Fig. 6.


Figure 6
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  		Fig. 6. Schematic illustrating biosolids-Ba addition (based on 26.8 Mg biosolids-Ba ha–1 2002–2003 soils data) to soil followed by release of a Ba–dissolved organic matter (Ba–DOM) phase, Ba-DOM movement and mineralization, and mineral precipitation in the soil surface and subsurface. EC = electrical conductivity; µ = ionic strength.

 
To identify the potential of Ba to be an environmental hazard, we determined the soluble Ba concentration under our field conditions using Visual MINTEQ. The 0- to 20-cm-depth model parameter pH was fixed at 6.35 and dissolved organic C content was 0.0154 M. This was based on concentrations presented in Al-Wabel et al. (2002) for freshly applied biosolids. The ionic strength was set at 0.0343, based on converting the 2003 surface soil EC from the 26.8 dry Mg biosolids ha–1 treatment. The conversion was performed using the equation developed by Griffin and Jurinak (1973):

Formula 1[1]
where µ is the ionic strength (mol L–1) and EC is expressed in millimhos cm–1. Infinite barite as the dominant mineral species in the soil incorporation zone was assumed. Output showed that Ba associated with the dissolved organic matter (Ba–DOM) phase dominated Ba speciation but with small concentrations (5.25 x 10–5 M), Ba2+ concentration was approximately an order of magnitude smaller, and all other Ba ion pairs were <1 x 10–8 M. Adding the Ba–DOM and Ba2+ concentrations together gave a Ba concentration of 0.008 mg L–1, substantially less than the 2 mg L–1 drinking water standard.

For the 20- to 60-cm depth, Visual MINTEQ inputs were as follows: dissolved organic C content of 0.0154 M; average 2003 subsurface soil pH fixed at 7.5; ionic strength fixed at 0.0276 M based on conversion of the 2003 surface soil EC from the 26.8 dry Mg biosolids ha–1 treatment; and infinite witherite as the dominant mineral species in the soil subsurface. Output showed Ba2+ dominated Ba speciation at a calculated concentration of 1.81 x 10–3 M, Ba–DOM was approximately an order of magnitude smaller, and all other Ba ion pairs were <1 x 10–5 M. Combining Ba2+ and Ba–DOM concentrations gave a Ba concentration of 0.32 mg L–1, still less than the 2 mg L–1 drinking water standard. The modeling effort would suggest that the addition of biosolids-borne Ba should pose little environmental hazard under the climatic and management conditions prevalent at the site.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
We accept our hypothesis that biosolids additions are increasing the Ba accumulation in the 0- to 20-cm soil depth since 4 M HNO3-extractable Ba concentrations increased in six out of seven sampling events. The decline of AB–DTPA-extractable Ba in six out of seven surface-soil sampling events supports our hypothesis that insoluble Ba precipitates are forming. Our SEM–EDS analysis of soil from the 0- to 20-cm depth revealed that Ba and S were concentrated in the same area, which has led us to propose that Ba forms an insoluble Ba–S association, possibly BaSO4. The sequential extraction procedure showed an increase in surface soil Ba associated with the crystalline Fe and Mn oxide phase. This phase may actually contain BaSO4 precipitates, not Ba associated with Fe and Mn oxides, as shown by SEM–EDS analysis. Based on AB–DTPA-extractable and 4 M HNO3 total Ba concentrations, overall Ba concentrations in the 20- to 60-cm soil depth are not affected by biosolids application. Using a more sensitive sequential extraction analysis and modeling with Visual MINTEQ, soluble Ba, either as Ba2+ or Ba–DOM, appears to migrate downward and precipitate as insoluble BaCO3 beneath the plow layer. The low solubility of BaSO4 further suggests that other soluble Ba compounds are causing the increase in subsoil Ba concentration.

Even though the USEPA has listed Ba as one of the "candidate pollutants for exposure and hazard screening" (USEPA, 2003), we found that with an estimated total addition of about 110 kg Ba ha–1, labile Ba decreased because of the formation of insoluble BaSO4 in the soil surface and potentially BaCO3 in the subsurface. Modeling Ba solubility in both the soil surface and subsurface using Visual MINTEQ showed that total Ba concentrations were three and one order of magnitude lower than the 2 mg Ba L–1 drinking water standard, respectively. We therefore conclude that, in similar soils, under similar climatic and management systems, Ba might not pose a hazard.


   ACKNOWLEDGMENTS
 
We thank the cities of Littleton and Englewood, CO, the Colorado Agricultural Experiment Station (Project 15-2924), and the USEPA Region 8 (Grant CP988928-01) for their support of this project.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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