Removal of added nitrate in the single, binary, and ternary systems of cotton burr compost, zerovalent iron, and sediment: Implications for groundwater nitrate remediation using permeable reactive barriers

Chemosphere 67 (2007) 1653–1662 www.elsevier.com/locate/chemosphere

Removal of added nitrate in the single, binary, and ternary systems of cotton burr compost, zerovalent iron, and sediment: Implications for groundwater nitrate remediation using permeable reactive barriers Chunming Su *, Robert W. Puls Ground Water and Ecosystems Restoration Division, National Risk Management Research Laboratory, Office of Research and Development, United States Environmental Protection Agency, 919 Kerr Research Drive, Ada, OK 74820, USA Received 5 May 2006; received in revised form 21 September 2006; accepted 25 September 2006 Available online 25 January 2007

Abstract Recent research has shown that carbonaceous solid materials and zerovalent iron (Fe0) may potentially be used as media in permeable reactive barriers (PRBs) to degrade groundwater nitrate via heterotrophic denitrification in the solid carbon system, and via abiotic reduction and autotrophic denitrification in the Fe0 system. Questions arise as whether the more expensive Fe0 is more effective than the less expensive carbonaceous solid materials for groundwater nitrate remediation, and whether there is any synergistic effect of mixing the two different types of materials. We carried out batch tests to study the nature and rates of removal of added nitrate in the suspensions of single, binary, and ternary systems of cotton burr compost, Peerless Fe0, and a sediment low in organic carbon. Cotton burr compost acted as both organic carbon source and supporting material for the growth of indigenous denitrifiers. Batch tests showed that cotton burr compost alone removed added nitrate at a greater rate than did Peerless Fe0 alone on an equal mass basis with a pseudo-firstorder rate constant k = 0.0830 ± 0.0031 h1 for cotton burr compost and a k = 0.00223 ± 0.00022 h1 for Peerless Fe0; cotton burr compost also removed added nitrate at a faster rate than did cotton burr compost mixed with Peerless Fe0 and/or the sediment. Furthermore, there was no substantial accumulation of ammonium ions in the cotton burr compost system, in contrast to the systems containing Peerless Fe0 in which ammonium ions persisted as major products of nitrate reduction. It is concluded that cotton burr compost alone may be used as an excellent denitrification medium in a PRB for groundwater nitrate removal. Further study is needed to evaluate performance of its field applications. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Groundwater nitrate; Remediation; Autotrophic/heterotrophic denitrification; Cotton burr compost; Zerovalent iron; Permeable reactive barriers

1. Introduction Development of cost-effective methods is needed for groundwater nitrate remediation because nitrate is considered as the most ubiquitous chemical contaminant in the world’s aquifers and the levels of contamination are increasing (Spalding and Exner, 1993; Kapoor and Viraraghavan, *

Corresponding author. Tel.: +1 580 436 8638; fax: +1 580 436 8703. E-mail addresses: [email protected] (C. Su), [email protected] (R.W. Puls). 0045-6535/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2006.09.059

1997; Soares, 2000). Recent analyses of groundwater samples taken in the United States from approximately 1500 domestic drinking water and public supply wells show that of more than 140 contaminants measured, nitrate most frequently exceeded drinking water standard of 10 mg l1 NO 3 -N set by the US Environmental Protection Agency (Squillace et al., 2002). Nitrate degradation occurs via both abiotic and biotic pathways. In the abiotic mode, nitrate is chemically reduced by a reductant such as granular Fe0 (Siantar et al., 1996; Cheng et al., 1997; Zawaideh and Zhang, 1998; Huang

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and Zhang, 2002; Alowitz and Scherer, 2002; Su and Puls, 2004) and nanoscale Fe0 (Choe et al., 2000; Yang and Lee, 2005), H2/palladium catalysts (Siantar et al., 1996), and ferrous iron-bearing minerals such as green rust (Hansen et al., 1996). The main products of such chemical reduction are ammonium ions that are potentially toxic to aquatic organisms at high concentrations. In the biotic mode, nitrate is biologically reduced via autotrophic and heterotrophic denitrification pathways by microorganisms. Zerovalent iron can support autotrophic denitrification using hydrogen gas generated from the corrosion of iron as electron donor (Till et al., 1998; Gandhi et al., 2002). The heterotrophic denitrification is promoted by organic substrates that serve as a carbon source and electron donors for the denitrifying microbes. Organic substrates such as sucrose (Soares et al., 1988), methane (Thalasso et al., 1997), soybean oil (Hunter, 2001), molasses and yeast extract (Dutta et al., 2005) have been used to stimulate microbial heterotrophic denitrification. Other organic substrates used are the carbonaceous solid materials including crop residue compost, straw, leaf mulch, and wood byproducts (Vogan, 1993; Blowes et al., 1994; Schipper and Vojvodic-Vukovic, 1998, 2000; Soares and Abeliovich, 1998; Robertson et al., 2000, 2005). Additionally, unprocessed cotton (Volokita et al., 1996a; Soares et al., 2000; Della Rocca et al., 2005, 2006) and newspaper (Volokita et al., 1996b) have been tested as energy source for microbial heterotrophic denitrification. The reaction of complete microbial heterotrophic denitrification is commonly shown in a general equation (Eq. (1)) where microbially available carbon is simplified as carbohydrate (e.g., Alexander, 1977; Knowles, 1982):  4NO 3 þ 5CH2 O ! 2N2 þ CO2 þ 4HCO3 þ 3H2 O

ð1Þ

This equation is probably not suitable in the case of more complex substances such as cotton in that there is a significant pH decrease during microbial heterotrophic denitrification (Volokita et al., 1996a; Della Rocca et al., 2005, 2006). In a closed system where CO2 cannot be dissipated easily, Eq. (2) may be applied that shows generation of H+ from heterotrophic denitrification:  þ 4NO 3 þ 5CH2 O ! 2N2 þ H þ 5HCO3 þ 2H2 O

ð2Þ

Both Fe0 and solid organic materials are potential media in permeable reactive barriers (PRBs) that could be constructed to intercept and degrade nitrate in the groundwater plume. Barriers composed of wood byproduct materials could potentially operate for many years without the need for carbon replenishment because of the slowly soluble nature of the cellulose and hemicellulose compounds in the wood media (Robertson et al., 2000, 2005). Agricultural byproducts such as wheat straw and alfalfa (Vogan, 1993; Soares and Abeliovich, 1998), low-grade cotton (Soares, 2000) and cotton burr compost (Su and Puls, 2007) are attractive alternative PRB materials because of

their ample supply at low cost. Short fiber (low commercial value) raw cotton was used in laboratory heterotrophic denitrification bioreactors with greater than 90% nitrate removal efficiency (Della Rocca et al., 2005); it was also used in a field pilot-plant study with 80–100% nitrate removal efficiency for treating nitrate-laden groundwater with nitrate concentrations ranging from 9 to 22 mg N l1 (Soares et al., 2000). Similarly, cotton burr compost was shown in laboratory batch tests to remove added nitrate at a faster rate than did mulch compost and peat (Shiau et al., 1999). Cotton burr compost may be more advantageous than wheat straw and alfalfa because the woody cotton burr compost decays at a slower rate than the latter grassy materials so that carbon replenishment is not needed. This property would make cotton burr compostbased PRB a more lasting operation at low cost and with low maintenance. A sequential autotrophic/heterotrophic denitrification approach supported by Fe0 and cotton was proposed for nitrate removal from drinking water (Della Rocca et al., 2006). In continuous pilot-scale column reactors containing Fe0 at the bottom part and cotton at the top part of the columns (up-flow mode), these authors observed higher volumetric nitrate removal efficiencies than the column packed with cotton only. The synergistic effects of heterotrophic denitrification promoted by cotton coupled with autotrophic denitrification promoted by Fe0 are encouraging. It would be interesting to see if mixing Fe0 with cotton-like organic material (instead of placing the two materials in sequence) would produce the same positive effects. Zerovalent iron in the form of steel wool has been shown to increase nitrate removal rates and transform a greater portion of the added nitrate to innocuous gases rather than to ammonium ions in a pure culture of Paracoccus denitrificans relative to Fe0 alone treatment; whereas, Fe0 in the form of the more reactive Fe0 powder did not increase nitrate removal rate or decrease the proportion of nitrate reduced to ammonium ions (Till et al., 1998). This was attributed to a corrosion-induced increase in pH above the tolerance range of the bacteria in the Fe0 powder system (pH > 10). The possible enhancing or retarding effect of Fe0 on nitrate removal in the binary and tertiary cotton burr compost/Fe0/sediment PRB systems has not been studied. Successful implementation of a PRB requires a thorough understanding of the effects of geochemical variables such as ion composition, pH, and redox potential on the behavior of the PRB, the behavior of combinations of various PRB media, as well as the mechanisms of nitrate reduction and transformation in the subsurface environment. The objectives of this study, therefore, were to: (1) evaluate the influence of pH on nitrate reduction by Fe0; (2) compare nitrate reduction kinetics in the single, binary, and ternary systems of PRB media (cotton burr compost and Fe0) and sediment, and (3) provide information for future field applications of the media for in situ groundwater nitrate remediation.

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2. Materials and methods 2.1. Materials and chemicals Cotton burr compost previously used by Su and Puls (2007) was further tested in the present study. The pH of cotton burr compost/water suspension (1:2, w/v) was 8.7. The cotton burr compost was air-dried, ground, and then sieved to pass through a 1-mm screen before use. Peerless iron (Peerless Metal Powders & Abrasives, Detroit, MI) was selected as the representative iron. It was low-grade steel that contained 90.1% Fe and 3.31% C (data from the manufacturer). The greater than 0.5 mm fraction of Peerless iron was separated from the bulk and used in this study. This fraction had a BET N2 adsorption surface area of 0.55 m2 g1 (Su and Puls, 2004). An aquifer sediment sample collected from the US Coast Guard Support Center in Elizabeth City, North Carolina, was air-dried and passed through a 2-mm sieve. The sediment sample was collected near a Peerless iron-based PRB constructed in 1996 to intercept and treat a contaminant plume of hexavalent chromium and trichloroethene (Puls et al., 1999; Wilkin et al., 2005). The sediment sample, however, was free of chromate and trichloroethene as determined by chemical analyses. The sediment was sandy loam in texture (79% sand and 21% silt and clay) and low in organic carbon (0.06%). ACS reagent or analytical grade chemicals were used as received. Sodium nitrate was used 1 to prepare a nitrate stock solution of 2000 mg NO 3 –N l . 2.2. Kinetics tests The pH effect of nitrate reduction was tested using two acetate-acetic acid buffers. Buffer 1 was a mixture of 75 ml of a 0.2 M sodium acetate solution and 925 ml of a 0.2 M acetic acid solution (pH = 3.81); Buffer 2 was a blend of 905 ml of a 0.2 M sodium acetate solution and 95 ml of a 0.2 M acetic acid solution (pH = 5.67). Both buffers were spiked with the nitrate stock solution to yield 1 0 a final concentration of 20 mg NO 3 -N l . Peerless Fe (1.0 g) was equilibrated with 25 ml of the nitrate-laden buffers before sampling and analysis. Nitrate reduction was also tested in the following single, binary, and tertiary systems of materials (Table 1): cotton burr compost alone (1.0 g), Peerless Fe0 alone (1.0 g and 5.0 g), sediment alone (1.0 g), 1.0 g cotton burr compost + 1.0 g Fe0, 1.0 g cotton burr compost + 5.0 g Fe0, 1.0 g iron + 1.0 g sediment, 5.0 g Fe0 + 1.0 g sediment, 1.0 g cotton burr compost + 1.0 g sediment, 1.0 g cotton burr compost + 1.0 g Fe0 + 1.0 g sediment, and 1.0 g cotton burr compost + 5.0 g Fe0 + 1.0 g sediment. These tests were executed using simulated Elizabeth City groundwater [7 mM NaCl + 0.86 mM CaSO4 (pH 6.5, adjusted with 0.1 M NaOH)] that approximately represents major ionic composition of the groundwater at the Peerless Fe0 based PRB site in Elizabeth City, NC. The simulated groundwater was also spiked with 1 nitrate at 20 mg NO 3 –N l .

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Kinetic tests were generally conducted at preset time intervals of 2, 3, 5, 9, 25, 49, 73, 97, 146, 169 and 241 h of which 1 h was the centrifugation time for the various treatments of materials. Duplicate solid materials were suspended in 25 ml of appropriate solutions contained in 50-ml NalgeneÒ polypropylene copolymer centrifuge tubes and shaken at 23 ± 1 °C on a rotational shaker at 200 rpm for the predetermined time period before being centrifuged for 1 h at 2500g. After centrifugation, an aliquot of 10 ml was filtered through a 0.6 lm filter membrane and analyzed for nitrate, nitrite, and ammonium ions. The dissolved nitrate and nitrite were measured by the hydrazine reduction method (USEPA Methods 353.1, 1983), and NHþ 4 was determined by the automated phenate colorimetric method (USEPA Method 350.1, 1983). The remaining solution was then used to measure the final pH and redox potential using Orion pH and platinum redox electrodes (with daily calibrations), respectively. The measured redox potential values were corrected to a standard hydrogen electrode basis, i.e., true Eh values following procedures in the instrument manual. 2.3. Data analysis A pseudo-first-order kinetic model (Su and Puls, 2004) was used to describe the nitrate disappearance over time in various systems. The surface area-normalized kinetic parameters such as rate constants and half-lives were calculated on the basis of 1.0 m2 surface area of Fe0 per 1.0 ml of aqueous solution. Surface area normalization was not performed for treatments of organic materials alone since it has little meaning for microbially controlled nitrate reduction because the reactions do not occur exclusively on the surface of the bacteria. Furthermore, it was not practical to estimate the total surface area of the bacteria involved in nitrate reduction and transformation in our systems. 3. Results and discussion 3.1. Nitrate reduction on Fe0 in acetate buffers There was an exponential decrease in the concentrations of nitrate in both acetate buffers in contact with Fe0 with increasing time (Figs. 1a and 2a). Concomitantly, the concentrations of ammonium ions increased with increasing time. Small amounts of nitrite ions (less than 1 2:0 mg NO 2 –N l , or less than 10% of total added N) also appeared with greater nitrite concentrations observed in acetate buffer 1 (0.2 M NaAC-HAC, initial pH 3.81) relative to acetate buffer 2 (0.2 M NaAC-HAC, initial pH 5.67). Mass balance of nitrogen species (nitrate, nitrate, and ammonium ions) was good relative to the control treatment. The pseudo-first-order reaction rates, half-lives, and surface-area normalized counterparts of nitrate reduction by Fe0 are listed in Table 1. Despite our efforts to maintain a constant pH using the buffers, the pH of the acetate-acetic acid buffer 1 went up gradually from an

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Table 1 Nitrate reduction by cotton burr compost (C), Peerless iron (Fe0), Elizabeth City sediment (S), and their combinations in simulated Elizabeth City groundwater (ECGW, 7 mM NaCl + 0.86 mM CaSO4, initial pH 6.5): regression coefficients (r2), pseudo-first-order rate constants (k), surface-area normalized rate constant (KSA), calculated half-lives (t1/2), and surface-area normalized half-lives (tSA-1/2) 1.0 g 1.0 g 5.0 g 1.0 g 1.0 g 1.0 g 1.0 g 1.0 g 1.0 g 5.0 g 1.0 g 1.0 g 1.0 g

C, ECGW Fe0, ECGW Fe0, ECGW Fe0, 0.2 M NaAC-HAC (pH 3.81) Fe0, 0.2 M NaAC-HAC (pH 5.67) S, ECGW C + 1.0 g Fe0, ECGW C + 5.0 g Fe0, ECGW Fe0 + 1.0 g S, ECGW Fe0 + 1.0 g S, ECGW C + 1.0 g S, ECGW C + 1.0 g Fe0 + 1.0 g S, ECGW C + 5.0 g Fe0 + 1.0 g S, ECGW

r2

k (h1)

KSA (h1 m2 ml)

0.816 0.740 0.806 0.975 0.777

0.0830 ± 0.0031 0.00223 ± 0.00022 0.00373 ± 0.00004 0.0303 ± 0.0025 0.0553 ± 0.0001

0.759 0.942 0.845 0.849 0.904 0.928 0.943

0.0376 ± 0.0004 0.0349 ± 0.0006 0.01048 ± 0.00144 0.00904 ± 0.00077 0.0719 ± 0.0028 0.0805 ± 0.0101 0.0717 ± 0.0099

tSA-1/2 (h m2 ml)

t1/2 (h)

NA 8.35 ± 0.31 0.101 ± 0.010 313 ± 30 0.034 ± 0.004 186 ± 2 1.38 ± 0.11 23.0 ± 1.9 2.52 ± 0.01 12.5 ± 0.0 No nitrate reduction detected 1.71 ± 0.02 18.4 ± 0.2 1.59 ± 0.03 19.9 ± 0.3 0.476 ± 0.065 66.8 ± 9.2 0.411 ± 0.035 77.0 ± 6.6 NA 9.64 ± 0.37 3.66 ± 0.46 8.68 ± 1.09 3.26 ± 0.45 9.76 ± 1.35

NA 6.88 ± 0.67 20.4 ± 0.0 0.505 ± 0.041 0.276 ± 0.001 0.405 ± 0.004 2.18 ± 0.01 1.47 ± 0.20 1.69 ± 0.15 NA 0.191 ± 0.021 0.214 ± 0.031

a

Peerless Iron With Acetate Buffer 1 and Nitrate

20

Control ΣN

15 + NH4

10

NO3

5

NO2

0

10

800

b

600

Control ΣN + NH4

15

Peerless Iron With Acetate Buffer 2 and Nitrate

10 NO3

5 NO2

0

10

800

b 8

8

6

pH

pH

Eh (mV)

400

400

Eh (mV)

a 20

600

Acetate Buffer 1 = 0.2 M NaAC-HAC (pH 3.81)

200

25

pH

6

200

pH

25

Nitrogen Concentration (mg N l -1)

Nitrogen Concentration (mg N l -1)

Surface-area normalization was performed for the Fe0-containing treatments only and was based on the surface area of pristine Peerless iron.

Acetate Buffer 2 = 0.2 M NaAC-HAC (pH 5.67)

0

0

4

4

-200

-200

Eh -400

Eh -400

0

25

50

75

100

125

2 150

Time (hours) 0

Fig. 1. Nitrate reduction by Peerless Fe in a 0.2 M NaAC-HAC (pH 3.81) buffer: (a) dissolved nitrate, nitrite, ammonium, and total nitrogen, (b) Eh and pH. The control treatment was a 0.2 M NaAC-HAC (pH 3.81) 0 buffer that contained 20 mg l1 NO1 3 –N without Fe .

initial pH of 3.81 to 6.60 after contacting the Peerless Fe0 for 97 h and stayed at this value afterwards (Fig. 1b); the pH of buffer 2 ascended from an initial pH of 5.67 to 7.14 after 5 h of reaction and stayed almost constant (7.14–7.33) up to 121 h (Fig. 2b). Conversely, the Eh values dropped from 748 mV to about 330 mV in buffer 1 after 97 h (Fig. 1b) and from 655 mV to a range of 318 to 340 mV in buffer 2 after 25 h to 121 h of reaction (Fig. 2b). An internal positive pressure was developed in the centrifuge tubes containing buffer 1 of the 0.2 M

0

25

50

75

100

125

2 150

Time (hours)

Fig. 2. Nitrate reduction by Peerless Fe0 in a 0.2 M NaAC-HAC (pH 5.67) buffer: (a) dissolved nitrate, nitrite, and ammonium, and total nitrogen, (b) Eh and pH. The control treatment was a 0.2 M NaAC-HAC 0 (pH 5.67) buffer that contained 20 mg l1 NO1 3 –N without Fe .

NaAC-HAC (initial pH 3.81) solution after reaction with the Fe0 for 24 h, due to the decomposition of water to release hydrogen gas (Eq. (3)). No internal pressure buildup was observed for the other 0.2 M NaAC-HAC (initial pH 5.67) buffer. Apparently, the reduction of water (Eq. (3)) and dissolved oxygen (Eqs. (4) and (5)) by Fe0 competed more effectively with the reduction of nitrate (Eqs. (6) and (7)) and nitrite (Eq. (8)) by Fe0 in the acetate-acetic buffer at an initial pH of 3.81 than at an initial pH of 5.67. Clearly, there is an optimum pH (probably

C. Su, R.W. Puls / Chemosphere 67 (2007) 1653–1662

Fe0 þ H2 O þ Hþ ! Fe2þ þ H2 þ OH 0

2Fe þ 2H2 O þ O2 ! 2Fe 0

þ

2Fe þ O2 þ 4H ! 2Fe

2þ

2þ

þ 4OH



ð3Þ ð4Þ

þ 2H2 O

ð5Þ

þ 2þ þ NHþ 4Fe þ NO 4 þ 3H2 O 3 þ 10H ! 4Fe   0 þ 2þ Fe þ NO3 þ 2H ! Fe þ NO2 þ H2 O þ 2þ þ NHþ 3Fe0 þ NO 2 þ 8H ! 3Fe 4 þ 2H2 O 0

ð6Þ ð7Þ ð8Þ

1 g Fe0

+5 g Fe

0

5

5 g Fe0

C

10

g

A comparison of Eq. (5) with Eq. (6) indicates that for an equal molar concentration of dissolved O2 and nitrate, only half of the Fe0 and 40% of the acidity will be consumed for the reduction of dissolved O2 to form water as compared to the reduction of nitrate to form ammonium ions. In other words, an increase in acidity should favor the reduction of dissolved O2 relative to the reduction of dissolved nitrate. The effects of pH on nitrate reduction by Fe0 are twofold: (i) high pH causes Fe0 surface inactivation (little or no reduction) and (ii) low pH causes a slight decrease in nitrate reduction due to both competition of water reduction (hydrogen generation) and hydrogen entrapment on iron surface (decreased available reduction surface). A previous study has shown that the accumulation of H2 gas on the surface of Fe0 could decrease the available reactive sur-

15

0

ð10Þ

Nitrate

1

E0 ¼ 0:94 V at 25  C

a 20

Fe

þ  NO 3 þ 3H þ 2e ¼ HNO2 þ H2 O;

ð9Þ

25

1gC

E0 ¼ 1:229 V at 25  C

Figs. 3–6 show the concentrations of nitrate, nitrite, and ammonium and the solution Eh and pH with increasing time for the various single solid media treatments of cotton burr compost, Peerless Fe0, and Elizabeth City sediment. Calculated nitrate removal rate (Table 1) was a magnitude

+1g

O2 þ 4Hþ þ 4e ¼ 2H2 O;

3.2. Nitrate reduction in the single solid media in simulated groundwater

1gC

The experimental results from the two acetate-acetic buffers differ from the results from an experiment conducted on nitrate reduction by Fe0 using a pH-stat (pH 2–4.5 adjusted using HCl) reactor continuously purged with argon gas to eliminate dissolved O2 (Huang and Zhang, 2004). That experiment showed negative correlation between nitrate reduction rate and pH. This is likely due to the elimination of the competition for Fe0 by dissolved O2 (Eqs. (4) and (5)) against nitrate in the system studied by Huang and Zhang (2004). Assuming the acetate buffer was in equilibrium with ambient O2 in the air to yield a dissolved O2 concentration at about 8.6 mg O2 l1 (23 °C and 760 mm Hg atmospheric pressure), or 0.27 mM, initially dissolved O2 concentration would be about 19% of that of initially added nitrate concentration (20 mg N l1, or 1.43 mM). In addition, there was about 17 cm3 of air in the headspace of the centrifuge tube, of which 3.6 cm3 was oxygen gas. If all the oxygen gas in the headspace were dissolved into the 25 ml of solution, the total dissolved O2 concentration would have been 5.75 mM, greater than the added nitrate concentration. Dissolved O2 should be thermodyanamically more reducible than is dissolved nitrate by Fe0 as indicated by a higher standard half-cell potential of O2 reduction (Eq. (9)) than that of the nitrate reduction (Eq. (10)) (Cotton and Wilkinson, 1980):

face area for nitrate reduction by Fe0 (Choe et al., 2004). In another study (Huang and Zhang, 2004) that used argonpurged systems hydrogen entrapment on iron surface was not significant, and an increase of nitrate removal at low pH could be the result of catalyzed hydrogenolysis process (Zawaideh and Zhang, 1998). This process is characterized by the formation of nitrite as the primary intermediate in the reduction of liquid phase nitrate by hydrogen gas in the presence of a solid Pd/Cu bimetallic catalyst (Pintar et al., 1996). In the present study, nitrite was at concentrations ranging from undetectable to 10% of added nitrate (0–2 mg l1); whereas, ammonium was the predominant product. In addition, there is no known catalytic metal such as Pd found in Peerless Fe0. Consequently, the catalyzed hydrogenolysis process may not be substantial in our study.

Nitrogen Concentration (mg N l -1)

between 6 and 7) for the reduction of nitrate by the Peerless Fe0 in the aerobic acetate-acetic acid buffers:

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0 25 20

b

15

1gC

Nitrite 1 g C + 1 g Fe0

10 5

1 g C + 5 g Fe0

0 25

5 g Fe0

1 g Fe0

c

20

1 g C + 1 g Fe0

Ammonium

15 1 g C + 5 g Fe0

10 5 g Fe0

5

1 g Fe0 1gC

0 0

25

50

75

100

125

150

Time (hours)

Fig. 3. Nitrate removal in the single and binary systems of cotton burr compost (C) and Peerless iron (Fe0): (a) dissolved nitrate, (b) nitrite, and (c) ammonium.

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C. Su, R.W. Puls / Chemosphere 67 (2007) 1653–1662 600

25

a

a

Nitrate

20

400

1gS 1 g C + 5 g Fe0 + 1 g S

1 g Feo

15 1gC+1gS

o

5 g Fe

10

0

5

Nitrogen Concentration (mg N l -1)

1gC

-200

1 g C + 1 g Feo 1 g C + 5 g Feo

-400

b

10

1 g C + 5 g Feo 1 g C + 1 g Feo

5 g Feo

9

5 g Fe0 + 1 g S

o

pH

1 g Fe

8

1 g Fe0 + 1 g S

0 25

1 g C + 1 g Fe0 + 1 g S

b

20

1 g C + 1 g Fe + 1 g S

15 10 1 g C + 5 g Fe0 + 1 g S

5

1gC+1gS

0 25

c

75

100

125

150

S

50

1 g C + 1 g Fe0 + 1 g S

15 10

g

25

Ammonium 1 g C + 5 g Fe0 + 1 g S

1gC

0

5 g Fe0 + 1 g S

1 g Fe0 + 1 g S

1gS

20

7

Nitrite 0

1

Eh (mV)

200

+

Time (hours)

g

Fe

0

1 g Fe0 + 1 g S

Fig. 4. Nitrate removal in the single and binary systems of cotton burr compost (C) and Peerless iron (Fe0): (a) Eh and (b) pH.

5

5

1gC+1gS

0

1gS

0

25

50

75

100

125

150

Time (hours)

Fig. 5. Nitrate removal in the single, binary, and ternary systems of cotton burr compost (C), Peerless iron (Fe0), and Elizabeth City sediment (S): (a) dissolved nitrate, (b) nitrite, and (c) ammonium.

600

a

1gS

400

5 g Feo + 1 g S 1 g Feo + 1 g S

g C

Eh (mV)

200

1 + 1 g Fe

0

o

1gC+1gS

+ 1 g S

-200 1 g C + 5 g Feo + 1 g S

-400

10

5 g Feo + 1 g S

1 g C + 5 g Feo + 1 g S

b

1 g Feo + 1 g S

9 8

1 g C + 1 g Feo + 1 g S

pH

greater for the pristine cotton burr compost (k = 0.0830 ± 0.0031 h1 for 1.0 g of cotton burr compost) than for the Peerless Fe0-only treatment (k = 0.00223 ± 0.00022 h1 for 1.0 g of Fe0). The rate constant for cotton burr compost obtained in the present study is slightly lower than a first-order degradation constant of 0.108 h1 reported for cotton in a column test by Della Rocca et al. (2005). An earlier study also reported rapid removal of nitrate in laboratory columns packed with unprocessed cotton without the formation of nitrite or ammonium (Volokita et al., 1996a). Nitrate removal was enhanced when more Fe0 was used alone (k = 0.00373 ± 0.00004 h1 for 5.0 g of Fe0). Elizabeth City sediment alone did not change the added nitrate concentration for the whole experimental duration probably due to its low organic carbon content (0.06%) that failed to support any detectable denitrification activities (Fig. 5a). Composts are macerated waste organic materials obtained from agricultural operations (e.g., yard clippings, cotton burr waste, etc.) that are mixed with various additives, and allowed to begin humification. The process of humification eventually removes much of the carbohydrate fraction of the organic matter (including cellulose), breaks down the particles of botanical debris, and leaves a more refractory residue of lignin remnants that form the basis for humic materials. The cotton burr compost as received was alkaline (pH 8.7) probably due to mixing with alkaline additives such as fly ash although the additives were not positively identified. The concentration of nitrite reached

7 1gC+1gS

6

1gS

5 0

25

50

75

100

125

150

Time (hours)

Fig. 6. Nitrate removal in the single, binary, and ternary systems of cotton burr compost (C), Peerless iron (Fe0), and Elizabeth City sediment (S): (a) Eh and (b) pH.

C. Su, R.W. Puls / Chemosphere 67 (2007) 1653–1662

a maximum (11.4 mg l1) at 9 h in the cotton burr compost then decreased to concentrations lower than 1 mg l1 after 75 h (Fig. 3b). In systems with Peerless Fe0 (1.0 g and 5.0 g) nitrite concentrations never exceeded 0.25 mg l1 and in sediment (1.0 g) no nitrite was detected at all (Figs. 3b and 5b). In the cotton burr compost, ammonium concentrations reached a maximum value (8.83 mg l1) at 13 h and stayed at 6.91 mg l1 at 49 h before descending to levels between 1.72 mg l1 and 1.0 mg l1 afterwards. On the contrary, ammonium ions accumulated in the Peerless Fe0 (1.0 g and 5.0 g) systems with increasing time (Fig. 3b). Ammonium ions in the sediment alone system were mostly non-detectable (Fig. 5b). In the cotton burr compost the Eh value decreased from 229 mV to 117 mV at 9 h and stayed between 150 mV and 225 mV afterwards (Fig. 4a) and pH also decreased from 9.20 to 6.67 with increasing time (Fig. 4b). The decrease in pH in the cotton burr compost alone treatment as a result of microbial heterotrophic denitrification is in agreement with a similar trend in cotton as reported earlier (Volokita et al., 1996a; Della Rocca et al., 2005, 2006). The Eh values in the 1.0 g Peerless Fe0 treatment were higher than those in the 5.0 g treatment and were always positive; the pH values remained high (9.00–9.92) (Fig. 4a and b). Microbial denitrification was thought to be the mechanism for the removal of added nitrate in the cotton burr compost based on laboratory batch experiments conducted with both sterilized and non-sterilized cotton burr compost (Su and Puls, 2007). Additionally, arguments can be made from a viewpoint of thermodynamics and chemical equilibrium principles. We listed below some important reduction half-reactions involving nitrate (Lindsay, 1979) that are known to occur in natural reducing environments:  þ  NO 3 þ 2H þ 2e ¼ NO2 þ H2 O log K 298 ¼ 28:64

2NO 3

þ

ð11Þ



þ 10H þ 10e ¼ N2 OðgÞ þ 5H2 O log K 298 ¼ 151:04 ð12Þ

þ  NO 3 þ 2H þ e ¼ NO2 ðgÞ þ H2 O log K 298 ¼ 13:03

NO 3

þ



þ 10H þ 8e ¼

NHþ 4

ð13Þ

þ 3H2 O log K 298 ¼ 119:07 ð14Þ

NO 3

þ



þ 4H þ 3e ¼ NOðgÞ þ 2H2 O log K 298 ¼ 48:41 ð15Þ

2NO 3

þ



þ 12H þ 10e ¼ N2 ðgÞ þ 6H2 O log K 298 ¼ 210:30 ð16Þ

All these reactions show that chemical reduction of nitrate consumes H+, and thus in the absence of competing reactions such as reduction of dissolved O2, a decrease in pH should promote reduction and an increase in pH should inhibit reduction. This has been repeatedly observed in reactions of Fe0 with nitrate (Cheng et al., 1997; Huang et al., 1998; Chew and Zhang, 1998; Zawaideh and Zhang, 1998; Alowitz and Scherer, 2002; Hsu et al., 2004; Ruangchainikom et al., 2006). In fact, various acids have been

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used to accelerate nitrate reduction by Fe0 including acetic acid (Cheng et al., 1997), H2SO4 (Huang et al., 1998), carbonic acids through CO2 bubbling (Hsu et al., 2004; Ruangchainikom et al., 2006). If the reaction of nitrate with the organic solid materials were an abiotic reduction reaction only, then a sphagnum peat with a pH of 3.9 should show the greatest nitrate reduction, and the cotton burr compost with a pH of 8.7 should show the least nitrate reduction. This is in direct contradiction to the data presented by Shiau et al. (1999) and Su and Puls (2007). A more plausible explanation is that denitrifying bacteria have their highest activity in the pH range from 7 to 9 (Alexander, 1977) as is in the cotton burr compost and that an acidic pH inhibits biological nitrate reduction in the peat material. 3.3. Nitrate degradation in the binary and ternary media systems A mixture of sediment with Peerless Fe0 (1.0 g or 5.0 g) substantially improved nitrate reduction kinetics as compared to either sediment or Peerless Fe0 alone, and there is no substantial difference between the 1.0 g Fe0 + sediment and 5.0 g Fe0 + sediment treatments (Figs. 3a and 5a, Table 1). The presence of sediment may have helped to accelerate iron corrosion, causing increased nitrate reduction. Powell and Puls (1997) reported enhanced chromate reduction by Fe0 in the presence of aluminosilicate minerals compared to that in the absence of aluminosilicates. A combination of cotton burr compost, Elizabeth City sediment, and Fe0 offered no benefit compared to the compost-only treatment (Fig. 5a). This is likely a result of an adverse pH increase in the mixture as a result of Fe0 corrosion as indicated in Eqs. (3) and (4) (Fig. 6b). As discussed earlier, abiotic nitrate reduction consumes hydrogen ion (Lindsay, 1979), so a lower pH favors the reduction reaction. The presence of sediment to enhance nitrate reduction by Fe0 may not be dominantly the result of cooperative action of denitrifying organisms because the sediment itself did not remove any added nitrate and the sediment was expected to be low in microbes due to its low organic carbon content. A recent study has shown that inoculating Fe0 PRBs with autotrophic denitrifiers enhances nitrate removal efficiency, but the establishment of a microbial population could require provision of sufficient buffering capacity to preclude an inhibitory corrosion-induced increase in pH (Ginner et al., 2004). Cotton burr compost offers an advantage with regard to the denitrifier population and pH issues in that cotton burr compost is a natural carrier of bacteria including denitrifiers; it does not need inoculation to be able to effectively degrade added nitrate. In the treatment of 1.0 g cotton burr compost + 1.0 g Fe0, nitrite peaked at 25 h as compared to at 9 h in the treatment of 1.0 g cotton burr compost alone (Fig. 3b). An increase in the amount of Fe0 to 5.0 g with 1.0 g cotton burr compost both delayed the time from 25 h to 49 h to reach a nitrite maximum and decreased the maximum

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value from 12.7 mg l1 to 4.08 mg l1 (Fig. 3b). The highest nitrite concentration (18.0 mg l1) was observed for a ternary system (1.0 g cotton burr compost + 1.0 g Fe0 + 1.0 g sediment) at 13 h (Fig. 5b). The nitrite maximum is exhibited for all the treatments that had a component of 1.0 g cotton burr component; whereas no nitrite maximum was observed and nitrite concentrations were always below 0.5 mg l1 for treatments without the component of cotton burr compost (Figs. 3b and 5b). More ammonium ions accumulated in treatments with a component of Peerless Fe0 than without it (Figs. 3c and 5c). Since autotrophic denitrification does not produce ammonium, the accumulation of ammonium ions in the binary systems of Peerless Fe0 and sediment indicates that biological denitrification is limited without sufficient supply of organic matter such as cotton burr compost. In other words, abiotic reduction of nitrate by Peerless Fe0 is the predominant pathway of nitrate degradation. This is true even for the ternary systems where cotton burr compost was present in that more than half of the added nitrate was transformed to ammonium (Fig. 5c). It is probable that optimum conditions were not established in the systems containing Peerless Fe0 for the growth of autotrophic denitrifiers. In a recent column (up-flow mode) test (Della Rocca et al., 2006), columns packed with iron wool (supporting autotrophic denitrification) at the bottom part and cotton (supporting heterotrophic denitrification) at the top part of the columns showed better nitrate removal than the column packed with cotton only. Since the cotton and wool were not mixed, it is unknown if mixing the two would produce the same results. The Eh values in the sediment alone and sediment plus Peerless Fe0 are generally higher than the treatments that had a cotton burr compost component (Fig. 6a). The pH values are generally higher in the treatments with Peerless Fe0 as a component than without it (Fig. 6b). Two separate mechanisms of nitrate reduction and transformation seem to be operative: biotic degradation in organic rich compost and abiotic degradation in organic poor and iron-rich media. It was not advantageous to mix the cotton burr compost with Peerless Fe0 to degrade nitrate because cotton burr compost alone worked better with faster nitrate removal and no accumulation of ammonium ions. The accumulation of ammonium ions is a drawback even for laboratory porous reactive barriers containing metallic iron and hydrogenotrophic denitrifying microorganisms (Biswas and Bose, 2005). Additionally, cotton burr compost (with a price less than $200 per metric ton, information from local store) costs less than Peerless Fe0 ($667 per metric ton as of 27 April 2006, Noreen Warrens, personal communication). Research results from the present study may be useful in field applications in concentrated animal feeding operation and in the food processing industry where nitrate contamination is commonplace. Future work may be fruitful in at least two further areas before making claims as to which materials are good for nitrate removal: How can one gen-

erally distinguish analytically composts from one another using spectroscopic methods such as Fourier transform infrared spectroscopy; what differences in microbial consortia can be seen for different types of compost (e.g., peat vs. municipal compost or cotton burr compost) that influence their activity towards nitrate? The previous work (Su and Puls, 2007) presenting solid evidence for the microbial component of nitrate reduction seems to demand a much more rigorous microbial characterization of compost materials. Presumably there will be a relationship between the initial composition of the organic matter in the compost with regard to the amount of utilizable biomass and the molar amount of nitrate reduction. Pinning down what fraction of the carbon in a given type of biomass can be mineralized via humification would allow the expected amount of nitrate degradation to be estimated. Further study is needed to evaluate the limits of microbial degradation obtainable during humification from the reduction in atomic O/C ratio achievable in going from the initial value to that of the spent (non-denitrifying) compost. It may be possible to use the observed amount of CO2 evolution to monitor the course of this reaction, but only when strictly anaerobic conditions are maintained. Additionally, further study is needed to investigate performance of cotton burr compost-based PRBs at pilot and full field-scale applications for groundwater nitrate remediation. Such performance evaluation has been recently conducted for a saw dust-based PRB as a denitrification wall (Schipper et al., 2004, 2005) for removing groundwater nitrate.

4. Conclusions Cotton burr compost alone appeared to be highly effective in removing added nitrate from simulated groundwater. No removal of added nitrate was observed in the Elizabeth City sediment alone, probably due to its very low organic carbon content (0.06%) that could not support any detectable microbial denitrification. There was a positive interaction between the Elizabeth City sediment and Peerless Fe0 with respect to nitrate reduction rate probably due to the enhanced corrosion of Peerless Fe0 in the presence of sediment. Conversely, it was not advantageous to mix the cotton burr compost with Peerless Fe0 and/or sediment to degrade nitrate because cotton burr compost alone worked more efficiently without substantial accumulation of ammonium ions, than did any combination of the three materials. Nitrite reached maximum concentrations in the systems containing cotton burr compost at the earlier stages of the batch test and then decreased to low levels. No substantial nitrite was observed in Peerless Fe0/sediment systems without cotton burr compost. Further field studies would be needed to confirm and demonstrate the effectiveness and to evaluate the mechanisms of nitrate removal using cotton burr compost in the subsurface.

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