The Use of Anhydrous Ammonia for Bioventing

R.G. Zytnera, M. Hallmanb, B.Fernández Giménezc, R. Jenningsc and K. Leekc

aProfessor and Associate Director, bGraduate Student and cUndergraduate StudentSchool of Engineering, University of Guelph, Guelph, Ontario, Canada, N1G 2W1

ABSTRACTBioventing is becoming a popular in situ soil remediation technology for the treatment ofhydrocarbon contaminated soil. It is ideal for the removal of residual contamination leftby soil vapour extraction (SVE) using low or intermittent air flow rates to produceoxygen-rich conditions in the vadose zone. This promotes the growth of indigenousmicroorganisms which can mineralize the contaminant in the presence of sufficientnutrients. Although bioventing is currently being used as a remediation technology, thereis concern regarding the effective application of nutrients (nitrogen) to the subsurface ascurrent application methods are not effective in uniformly dispersing the nitrogenthroughout the subsurface. Accordingly, two research phases were completed usinggasoline contaminated soil.

The first phase involved a study on how best to deliver nitrogen to the subsurface.Review of the literature indicated that a lot of work has been completed in theagricultural field using anhydrous ammonia as the nitrogen source. This suggested thepossibility of injecting anhydrous ammonia into the contaminated subsurface andallowing it to disperse into the contaminated zone. The resulting gaseous form sorbs tothe soil and reacts with the soil-water in the subsurface to form ammonium, which isideal as ammonium is effective in stimulating the growth of hydrocarbon degraders.Model simulations showed that anhydrous ammonia can effectively be dispersed in thesubsurface with satisfactory degradation of hydrocarbons. Based on the results of thefirst phase, a laboratory phase was initiated to test the findings. Respirometers were setup to compare the decay rates of the direct addition of ammonium or the indirect additionvia anhydrous ammonia. The results indicate that the application of anhydrous ammoniais effective in promoting bioventing.

Key words: bioventing, nutrient addition, anhydrous ammonia, gasoline remediation,modelling, respirometer,

INTRODUCTION

Hydrocarbon contamination of soil is a global environmental problem, with a widevariety of contributing sources. One of the major sources in North America is leakingunderground storage tanks (USTs). Franzmann et al. (1999) reported that about 35% ofall existing USTs are currently leaking, amounting to about 500 000 leaking gasolinetanks in the United States alone. A similar trend is expected in Canada. Contamination ofsubsurface soil with gasoline and other hydrocarbons threatens groundwater supplies,diminishes agricultural capacity, endangers human health and can prevent further use ordevelopment of the contaminated land. The individual clean up costs for many of these

sites are currently prohibitive so that they are not remediated, and the problem remainsfor future generations.

There are several methods of gasoline remediation in use, including excavation anddisposal, soil flushing, air sparging, soil vapour extraction (SVE) and bioventing. Ofthese, SVE is currently the most widely used remediation method. Despite its popularity,there are several problems associated with SVE. These include high operating costs dueto the required high air flow rates, poor removal of high molecular weight and non-volatile hydrocarbons, the need for post-extraction treatment of the air stream, and thephenomenon of �tailing� that prevents the achievement of clean up goals (Malina et al.,1998 and Harper et al., 1998).

Bioventing was first developed in the 1970�s to address some of the problems associatedwith SVE (Leeson and Hinchee, 1997). Like SVE, it is an in situ remediation technologythat uses air extraction to remove contamination. However, where the intent of SVE is tovolatilize as much of the contaminant as possible, bioventing uses low or intermittent airflow rates to produce oxygen-rich conditions in the vadose zone (Hickey, 1995) andstimulate indigenous microbial degradation of the hydrocarbon contaminant. Bioventinghas several advantages over SVE including reduced operating costs, elimination of theneed for post-treatment of the air stream, and degradation of residual contamination leftby SVE so that clean up criteria can be met. In addition, bioventing is a true remediationtechnology in that the contaminant is degraded rather than removed.

There are several environmental parameters that influence bioventing performance. Theseinclude soil moisture content, pH, nutrient content and availability, oxygen content,temperature, toxicity and bioavailability of the contaminant, and physical characteristicsof the soil matrix. Although the design of bioventing systems is necessarily site-specificto some extent, researchers have reached a general consensus on the optimum values ofsome of the important environmental conditions. For example, there is general agreementin the literature that many bioventing sites are nutrient limited, especially in terms ofnitrogen and phosphorous. Reported carbon-nitrogen-phosphorous (CNP) ratiospresented in many papers vary widely, from 100:10:1 to 1000:10:1. However, a majorquestion that exists is how to deliver the appropriate amount of nutrient to produceoptimum biodegradation conditions in the field.

Several nitrogen sources such as ammonium nitrate, ammonium sulphate, potassiumnitrate and urea oligimers have been investigated. Ammonia based nitrogen has a shorterlag time and higher degradation rate because the microorganisms require less energy formicrobial metabolism (Walworth and Reynolds, 1995; Jorio, 2000). Unfortunately,ammonium salts cause the soil pH to decrease in poorly buffered systems (Wrenn et al.,1994; Jackson and Perdue, 1999; Foght et al., 1999). The benefit of nitrate compounds isno pH change, while more nitrate is required to achieve the same degradation with alonger lag time (Wrenn et al., 1994). However, the addition of excess nitrate-nitrogencan be inhibitory in some cases (Brook et al., 2001). All these sources of nitrogen canbe solubilised for subsurface injection. Unfortunately, practice has shown that the

application is non-uniform, especially when the contaminated site is kept operational.One of the options for providing the required nitrogen is the use of anhydrous ammonia.

Anhydrous ammonia (AA) is popular in agricultural applications and can be added ineither gaseous or liquid form (Cronce and Cagnetta, 1996). Of the two forms, gaseoushas the greatest potential as it could be applied through an existing SVE/bioventingsystem and should easily disperse through the soil. Then upon contact with the water, theAA dissolves and becomes ammonium as shown in Equation 1, the ideal form formicroorganisms (Shewfelt and Zytner, 2001) .

(1) −+ +∩+ OHNHOHNH 423

The disadvantages of using AA are the potential rise in soil pH as the reaction causes aninitial alkaline environment in the ammonia retention zone, where the pH of the soil cantemporarily rise above 9 at the point of highest concentration. Since ammonia has a pka

value of 9.3 (McVickar et al, 1966), at a pH of 6 the equilibrium of ammonia (NH3) toammonium (NH4

+) is 0.1% to 99.9% respectively, while at a pH of 9 the equilibrium is50% to 50% (Whitman, 2002).

One additional concern is the safety of using AA in terms of handling because ofpotential dangers associated with the gaseous form (MSDS, 2001). However, AA is thesecond highest form of nitrogen fertilizer sold in Canada as 635,388 metric tonnes of AAwere sold in Canada in 2000 (Agriculture and Agri-food Canada, 2001). Thus, propersafety procedures are well established and widely available (Johnston et al, 2002) andshould not hinder the use of AA.

Accordingly, model and laboratory tests were completed to determine if AA could beapplied to the soil and whether the resulting ammonium concentration is sufficient toinduce biodegradation. The model simulations completed consisted of a finite differencesolution of an advective gaseous transport model. The model would assist in evaluatingthe suggested injection rate and frequency of injection based on simple first order decayrate for gasoline. The laboratory tests consisted of using respirometers to measure thegasoline degradation rate of AA when compared to the addition of ammonium in saltform. The results of the comparison are presented and discussed in the paper.

METHOD

Modelling

A variety of prepackaged models (Coupmodel, LeachN, SoilN and Modflow) wereinvestigated to determine their applicability to predict the movement/transport of AA insoil. Unfortunately all were more complex than needed, plus they were all based onvapour extraction and not injection, so a radial model based on the work of Guiguer et al.(1995) was developed. This radial model is two-dimensional as shown in Figure 1, anddescribes forced advective gaseous transport in unsaturated soil based on basic soil and

chemical properties. See Guiguer et al. (1995) for the governing equations as themodelling structure was not the intent of the paper. The constructed model was solvedusing explict finite difference.

The major assumptions applied to the model include:� AA behaves like an ideal gas� aqueous and organic phases are stagnant� negligible biological and chemical degradation of AA during the injection phase� gas flow is described by modified Darcy�s law� mechanical mixing in the gaseous phase is negligible when compared to

diffusive and advective processes

As shown in Figure 1, ∆r and ∆z represent the difference in radius and height respectivelybetween each node. For this model, ∆r and ∆z were 0.05m. The nodes were recognizedby the subscripts j and k, where j represents the position in the radial direction and k theposition in the vertical direction. The concentration of ammonia was calculated for eachnodal point in the system. The applicable boundary conditions of the system are shown inFigure 2.

Figure 1: Schematic of Model

Based on the soil to be used in the laboratory phase of the research, a sandy loam soilsubsurface was simulated, with an assumed density of 2500 kg/m3, having a porosity of0.4 m3, a water content of 11% (wt) and a hydraulic conductivity of 3.77x10-4 m/s. Thedepth of the soil profile was 4 m, with the screened portion of the well 2 m, having aradius of 0.075 m. Based on safety considerations, the initial input concentration was 0.5kg/m3 at a pressure of 3 atm. The application temperature was 20 ΕC.

Asphalt Covered Ground Surface

No Screen (0.5m)

No Screen (1.5m)

Screen (2m)ROI

Impermeable Layer

dP/dZ = 0 dCi/dz = 0 q=0

dP/dZ = 0 dCi/dz = 0 q=0

dP/dZ = 0dCi/dz = 0q=0

dP/dZ = 0dCi/dz = 0q=0

P=PatmCi=0

P=PwellCi=Capp

Figure 2: Model Boundary Conditions

Laboratory

The soil tested in the laboratory was obtained from a gasoline-contaminated site inOntario, Canada through the co-operation of the consultant and the client. Standardparticle size and nutrient analyses were conducted, and the soil was identified as a finesand with low nitrogen (<0.05%) and phosphorous (1 mg/L soil) content, and 10% watercontent (mass of water/total mass of sample, on a dry weight basis). Analysis of the soilshowed a concentration of 730 mg/kg of TPH as gasoline.

Upon receiving the soil, it was sieved to remove larger soil particles, then mixed toensure homogeneity. The water content of the soil was adjusted to 15% by adding theappropriate amount of ultra-pure water. The nitrogen content of the soil was then adjustedby adding the appropriate amount of NH4Cl or AA to attain a C:N ratio of 10:1. Thislevel was based on the previous work of Shewfelt and Zytner (2001), where it wasdetermined that ammonium worked best at a 10:1 ratio for a different soil.

To add the NH4Cl to the soil, calculations were performed to determine the amount ofNH4Cl powder required to obtain a C:N ratio of 10:1. Once calculated, the powder wasdissolved in the water to be added to bring the soil water content to 15%. Calculation ofthe required amount of AA assumed that it would all be converted to NH4

+ once in thesoil based on the work of Cronce and Cagnetta (1996) on TCE contaminated soil.Addition of the AA to the soil was accomplished through extraction of AA from a Tedlarbag using a 50mL gas-tight syringe, and subsequent injection beneath the soil layer. Thesoil was then gently mixed to distribute the AA.

After the addition of the nitrogen, the soil was divided into approximately 150 g samplesfor use in the respirometers. At each stage of sample preparation, two 30 g samples ofsoil were removed to provide information on the initial conditions. The samples weresealed and stored at 4oC if intended for microbial analysis, and at �15oC if intended forTPH analysis.

Biodegradation rates for the conditions tested were measured in respirometers originallydesigned and developed by Law (1996). The respirometers (Figure 3) function bymeasuring oxygen consumed and carbon dioxide produced by the aerobic respiration ofthe indigenous soil microbial population. The respirometers consist of Teflon-sealed 1Lglass jars equipped with pressure transducers, which took readings every 30 minutes thatwere recorded via a data-logging program. These readings were later converted to anamount of oxygen remaining in the headspace of the jar. All respirometers were placedin an incubator set at 25oC.

Figure 3: Schematic of Respirometer

The carbon dioxide evolved during microbial metabolism of the gasoline is trapped in avial of KOH solution. Oxygen consumption as measured by the calibrated transducers,which measure the decrease in pressure inside the respirometer over a specified length oftime. Bioventing conditions were maintained through an aeration tube, which could beopened to the atmosphere by a plug valve once oxygen concentration inside therespirometer dropped below 18%.

In total, 27 respirometers were available to run simultaneously. Each of the completedexperiments employed 11 bioreactors. Ten of these reactors contained soil samples withidentical treatments and were incubated in duplicate for 2, 5, 10, 15 or 30 d. One reactorcontained no soil, and acted as a thermobarometer to account for fluctuations of internalpressure due to atmospheric changes. The completed experiments including appliedtreatments, controls and replicates are summarized in Table 1.

Table 1 : Experiments Completed

Run # C:N %NH4-N(% of total N)

%H2O(w/w)

IncubationPeriods (d)

1 10:1 100% (NH4Clas NH4 source)

15 2, 5, 10, 15, 30

2 10:1 100% (AA asNH4 source)

15 2, 5, 10, 15, 30

Plug ValvePressure Transducer

Cord to Datalogger

Teflon Stopper

1L Glass Bo tt le

Aerat ion Tube

KOH Solu tion

Contaminated Soil

Normally the amount of carbon dioxide evolved through the degradation process wouldbe determined by measuring the pH shift of the KOH solution (Shewfelt and Zytner,2001). Similarly, the amount of oxygen consumed during degradation could be related toTPH degradation through the stoichiometric reaction of a representative hydrocarbon.However, for this study, the intent was only to measure the TPH decay and compare theresults of AA and NH4Cl. Accordingly, the only purpose of the KOH solution was toensure the proper functioning of the respirometers by preventing the build-up of CO2.

The extent of TPH degradation was determined by measuring the difference between theinitial and final TPH content of the soil in each reactor. Soil samples were extracted usingmethylene chloride, and the TPH content was determined using a gas chromatographequipped with a flame ionization detector (GC-FID). The GC-FID was calibrated byconstructing a five-point calibration curve using known concentrations of commercialgasoline (aged for 24 h) in methylene chloride.

In order to observe any acidification of the soil, the soil pH was measured before andafter each incubation as well. Soil pH was measured in a 1:1 slurry of soil and ultra-purewater using a pH meter.

The microbial population of the soil samples before and after incubation was measuredby plating serial dilutions of sodium pyrophosphate-extracted soil. R2A growth mediumwas used for heterotrophic plate counts, and Bushnell-Haas (BH) media was used forcounts of hydrocarbon degrading bacteria. The BH plates were incubated in the presenceof commercial gasoline as the sole carbon source. No attempt was made to isolate orcharacterize the bacteria; only population data was obtained for total heterotrophic andpetroleum hydrocarbon degraders.

RESULTS and DISCUSSION

Modelling

The use of the mathematical model confirmed that AA could easily be injected into thesoil through the bioventing well. The results indicated that sparge times of approximately30 minutes at 2 atm (absolute) were required to attain an AA concentration of 0.15 kg/m3

at the outer edge of the radius of influence (7.5 m), having a pressure of 1 atm. Theradius of influence for the subsurface condition being simulated was based on fieldmeasurements for SVE. Increasing the inlet pressure increased the resultingconcentration, but safety concerns and ammonium needs were exceeded. Consequently,the choices of inlet concentration should be based on the contaminant concentration inthe system, while the pressure should be based on the available equipment and/or timeconstraints. It should be noted that laboratory experiments will determine the idealconcentration of AA based on microbial needs.

The degradation component of the model worked very well. The time to consume theAA in the system depends on the ammonia and contaminant concentrations in the system.Based on an assumed contaminant level of 1500 mgcontaminant/kgsoil and an AA applicationof 0.15 kg/m3, the levels of AA in the system drop to approximately 80% of the startingvalue in 30 days, indicating a reasonable residual for effective degradation.

Laboratory

Table 2 displays the respirometry results of this study. The NH4Cl experiment showsvery little decrease in pH with time, with the biggest pH drop being to 7.7. Similar trendswere observed by Shewfelt and Zytner (2001) who studied the effectiveness ofammonium-nitrogen vs. nitrate-nitrogen, and combinations of the two. All the values arewithin the pH range considered ideal for bacterial growth (Huesmann, 1994). However,the AA experiment developed some acidification, with the pH getting as high as 9.2,which is not an uncommon result when dealing with the application of this particularcompound (Whitman, 2002).

Table 2: Soil pH for NH4Cl and AA respirometry

Incubation Period (d) 10:1 C:N, in form of NH4Cl 10:1 C:N, in form of AA2 8.2 9.15 7.9 9.2

10 8.0 9.215 7.7 9.030 8.0 9.0

Figure 4 compares the TPH results of the two experiments. It is evident that AA servesas a good stimulant for TPH degradation. It is also observed that for the NH4Clexperiment, the TPH concentration actually increases from Day 0 to Day 2; this resultcan be attributed to the spatial heterogeneity of the contaminant, and this occurrence isnot uncommon (Shewfelt and Zytner, 2001). As shown in Figure 4, there is initially afaster decrease with AA when compared to NH4Cl. However, by day 15 both treatmentsare at the same level, which is still above the clean-up criteria. These trends suggest thatfor this soil incubation time should be longer. Further studies are required.

Table 3 gives the first-order biodegradation rates calculated from the TPH analysis. Bothtreatments have identical rates, showing that AA has potential. Table 3 also gives acomparison with the earlier work of Shewfelt and Zytner (2001). Comparison of thevalues shows that the current study had a lower degradation rate. However, it must benoted that the two soils were from separate sites with dissimilar contamination levels;current study approximately half the contamination. It is also highly likely that the twosoils have differing ages of contamination. Soil that has been contaminated for a longerperiod of time makes bioremediation more difficult, thus affecting the overall degradationrate.

0

100

200

300

400

500

600

700

800

Day 0 Day 2 Day 5 Day 10 Day 15 Day 30

Co

nta

min

atio

n L

evel

m

g T

PH

/kg

so

il

NH4Cl

AA

Figure 4: Graph of TPH Concentrations for NH4Cl and AA

Table 3: First-order degradation rates

First order degradation rate (d-1)Treatment Shewfelt and Zytner (2001) Current

NH4Cl, 15% wc 0.081 0.028AA, 15% wc - 0.023

Figure 5 shows the increase in microbial activity of the total heterotrophs (R2A growthmedia) and hydrocarbon degraders (BH growth media) for the AA experiments. Thegrowth is slow for both tests, which was also observed by Shewfelt and Zytner (2001).Extending the period of incubation should increase the microbial activity. Figure 5 alsodemonstrates the consistency of the duplicate reactors.

As noted in the Methods section, the amount of AA added was controlled. However,analytical results of the amended soil showed that approximately half the amount of thedesired ammonium was present. Review of the application procedure suggests thatsufficient AA gas escaped from the treated soil, reducing the amount of ammonium in thesoil. Future tests will need to adjust for this in order to provide satisfactory nutrientconditions in the respirometers. However, it must be noted that the AA test attainedsimilar degradation results when compared with the current NH4Cl, with only half asmuch ammonium present in the soil. This suggests that AA is a promising candidate forthe use in bioventing.

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

0 5 10 15 20 25 30 35

Time (d)

Mic

rob

ial P

op

ula

tio

n (

cfu

/g)

R2A-AREACTORS

R2A-BREACTORS

BH-AREACTORS

BH-BREACTORS

Figure 5: Microbial activity for AA respirometry experiments

CONCLUSIONS

The modelling results showed that AA could easily be injected into the soil at areasonable pressure of 2 atm to supply nitrogen to the indigenous microorganisms. Thesimulations also showed that a sparge time of 30 minutes would suffice and provide areasonable AA concentration of 0.15 kg/m3 in the soil. Using a conservativecontamination level, the simulations showed that the AA would decrease to 80% of theinitial value after 30 d.

Laboratory results demonstrate that AA is successful in acting as a primary nitrogensource, with a degradation rate equal to that of NH4Cl, even with half as muchammonium. Two challenges seem to be the level of acidification that develops and theamount of AA that needs to be added to the soil to obtain the desired concentration. Theloss of AA from the soil, which is associated with easy migration needs to be overcomefor enhanced success in the future.

ACKNOWLEDGEMENTS

Funding for this project was provided by CRESTech (Centre for Research in Earth andSpace Technology) and NSERC of Canada through a Discovery Grant. The support ofCushman-Ball Environmental Ltd., Tecumseh, ON, Canada is also greatly appreciated.The authors also appreciate the lab work completed by Jennifer Webb, Stella Bezerra andSalma Dharsee.

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