EFFECTS OF PH ON UREASE INHIBITION

Transcription

1 EFFECTS OF PH ON UREASE INHIBITION IN UREA SOLUTION AND URINE ALISON V. DEVINEY NORTH CAROLINA STATE UNIVERSITY BIOLOGICAL AND AGRICULTURAL ENGINEERING DEPARTMENT ANTICIPATED GRADUATION DATE: DECEMBER 2018 DR. JOHN J. CLASSEN FACULTY ADVISOR 1

2 Purpose of Paper This paper is a report of preliminary experimental work conducted at the request of Waste to Green, LLC to establish the effectiveness of inhibiting urease activity in swine urine as a way to preserve urea nitrogen as a fertilizer. The original goal of the procedure was to determine whether application of an inhibitor treatment could effectively preserve urea in swine urine, what dosing rate is required for effectiveness and how long does the effect last. While several other urease inhibitors were also tested this paper focuses only on the effects of high and low ph. The author would like to acknowledge her co-authors, Dr. John Classen and Extension Specialist J. Mark Rice who led the study and guided her through the experimental procedures and evaluation of data. 2

3 ABSTRACT Urea has the highest fraction of nitrogen among common fertilizer materials. Swine urine has potential as a source of urea if the urine can be collected before urea is hydrolyzed by urease enzymes. However urease from fecal bacteria quickly catalyzes the conversion of urea to ammonia and carbon dioxide. The ammonia then volatilizes to the atmosphere or oxidizes to nitrite and nitrate and may move into groundwater. Thus urea hydrolysis in manure can be a major source of nitrogen pollution of the environment and loss of a valuable plant nutrient. This study consists of four lab experiments to test the effectiveness of urease inhibition on fresh swine urine by adjusting ph. A jack bean urease enzyme preparation was added to solutions of reagent-grade urea and to urine collected directly from gestating sows; Sulfuric acid and sodium hydroxide were added to the solutions to either lower or raise the ph and allowed to incubate at room temperature. The release of ammonia and preservation of organic nitrogen from these solutions were quantified periodically over several weeks. Results indicate that a solution ph below 3 or above 12 is most effective at inhibiting urease activity. Keywords: swine urine, urea preservation, urease inhibition Introduction One of the largest sources of environmental ammonia pollution comes from livestock operations when urea nitrogen in animal manure is converted to ammonia (Aneja et al. 2008; Zhao, Manuzon, and Hadlocon. 2014). When the urease enzyme released by fecal or soil bacteria comes in contact with the urea excreted in urine it catalyzes a hydrolysis reaction that breaks down the urea into ammonia and carbon dioxide. At higher ph levels ammonia volatilizes as a gas that is irritating to the airways of animals in confined areas such as barns and has a detrimental effect on livestock health (Jones, Wathes, and Webster 1998). At more acidic ph levels the ammonia remains in solution as ammonium which can then be converted to nitrate by nitrifying bacteria. Excess nitrate can also have a negative environmental impact by leaching into groundwater during wastewater storage or after land application (Karr et al. 2001). The concept of controlling ammonia emissions by either the inhibition of urease enzymes or preservation of urine urea has been documented through studies for multiple reasons. In one livestock application, commercially available soil urease inhibitors Thymol and Agrotain (NBPT) were applied to swine manure pits (Varel and Wells 2007). These treatments exhibited temporary inhibition of urea hydrolysis from 6 to 10 days. A lab study suggested a single application of urease inhibitor directly to dairy barn floors reduced ammonia emissions an average of 48% over the test period of 4 to 6 days (Hagenkamp-Korth et al. 2015). Most agricultural applications focus on limiting urease activity using soil amendments to reduce ammonia volatilization in fields when urea fertilizer is applied (Ni, Pacholski and Kage 2014; Soares et al, 2012; Watson et al. 2008). However, while many of these studies focus on reducing ammonia emissions few consider the additional benefit of nitrogen recovery. If the urea in fresh urine can be preserved at the source it would prevent the volatilization of ammonia that causes adverse effects on livestock, humans and the environment and can then be applied to crops as a readily available nitrogen source. Several studies have suggested that urease activity is limited to a range of ph values. Fidaleo and Lavecchia (2003) conducted a kinetic study of enzymatic urea hydrolysis from ph 4 to 9 and observed that the rate of urea hydrolysis is ph dependent. Frankenberger and Johanson (1982) observed that urease stability in soil varied with ph with activity ranging across 3 soil types from ph 4 to 10. Hellström, Johansson, and Grennberg (1999) were 3

4 able to stabilize urea in source separated urine for more than 100 days at ph < 5 with a one-time does of 60 meq H 2SO 4. Randall et al. (2016) were able to stabilize fresh urine by addition of Ca(OH) 2 to a saturation ph of Our study investigated methods of using ph adjustment to inhibit urease activity in swine urine to preserve the urea content during storage, processing and transport. Based on the cited studies, this project proposed to determine whether ph < 3.0 and ph > 12.0 would be effective at inhibiting urea hydrolysis by urease enzyme. If successful, further tests would be conducted to determine if a higher low ph or lower high ph would also be effective at urease inhibition. Materials and Methods The jack bean urease used in this experiment was purchased from Sigma-Aldrich Corporation (St. Louis, MO), catalog no. U1500 with labeled activity of 40,318 units/gram. Per the manufacturer s data sheet, one unit of urease will liberate 1.0 mmol of NH 3 from urea per minute at ph 7.0 at 25 C. Urease enzyme was prepared fresh for each experiment by mixing urease with 0.4M phosphate buffer solution at ph 7.0 (Dai and Karring 2014) for a unit activity of 32.2U urease per ml of phosphate buffer. All other chemicals used were reagent grade purchased through Fisher Scientific (Hampton, NH). Experiment 1: swine urine vs. urea solution A urease enzyme solution was prepared by mixing 100mg of jack bean urease with 125 ml 0.4M phosphate buffer solution. A 1M urea solution was prepared by mixing 60.06g crystalized urea with deionized water for a total volume of 1L and then 300 ml of the urea solution was divided into 100 ml aliquots and placed into three 250 ml containers. Frozen urine collected from three gestating sows was thawed, mixed and then 300 ml portioned into 100 ml aliquots and placed into three 250 ml containers. Each of the 100 ml urine and urea solution replicates was mixed with 9 ml of the urease phosphate buffer and split into 6 samples of >10 ml each in 20 ml lidded glass jars. The urease dosing level was 3U urease per 1 ml urine or urea solution. One sample jar of each replicate of the urine with urease or urea solution with urease was immediately acidified with H 2SO 4 to ph < 2.0 for initial TKN and TAN analysis. A subsequent sample from each replicate was acidified to ph < 2 at 1, 2, 3, 14 and 22 hours and submitted to the lab for TKN and TAN analysis. Experiment 2: effect of high and low ph on urease activity in urea solution High ph (> 12) and low ph (< 3) nominal values were selected as outside the range of urease activity in soil per Frankenberger and Johanson (1982). Three replicates of each were created by dividing 600 ml of 1M urea solution prepared as before into six 100 ml aliquots and dispensed into 250 ml glass containers. Three of the glass containers were each titrated with 1N H 2SO 4 to ph < 3 while the other three were each titrated with 0.5N NaOH to ph > 12. Urease was mixed with phosphate buffer in the same proportions as in experiment 1 and 9 ml of this urease buffer solution was mixed into each of the 100 ml urea solution in the glass containers. The contents of each container was then split into six >10 ml samples dispensed into 20 ml lidded glass jars for 3 weekly replicates each of the high and low ph treatments. One set of replicates was ph tested each week, acidified to ph < 2.0 and submitted for lab analysis of TKN and TAN. This experiment was conducted over a six week period. All treatments were stored at room temperature. Ammonia formation was determined weekly for six weeks. 4

5 Experiment 3: ph range test Hellstrom et al.(1999) were able to reduce urease activity at ph < 5 while Randall et al. determined ph 11 to be the upper limit of urease activity, therefore the third experiment was conducted to determine if urease activity could be effectively stopped at a ph value below 12 or above 3. Three replicates each of urea solution mixed with urease at one of eight nominal ph values from 4 to 11 were allowed to sit for one week at room temperature and then tested for ph, TKN and TAN. Actual ph values varied at each level because of buffer effect but this effect was determined to be small (data not shown). For this experiment 2.5L of 1M urea solution was divided into 100 ml aliquots and dispensed into 24 glass jars with lids for storage. 100 mg urease was mixed with 125 ml 0.4M phosphate buffer. 5 ml of urease phosphate buffer solution was dispensed to each of 24 ml lidded glass containers. ph in each container was adjusted by titration with 0.5N NaOH or 1N H 2SO 4 to produce three replicates each of nominal ph values 4, 5, 6, 7, 8, 9, 10, and 11. One container of ph adjusted urease solution was mixed into each jar of 100 ml urea solution for a urease dosing level of 1.61U urease per ml urea solution. The final ph value of each replicate was noted. After incubating for 7 days at room temperature, the ph in each glass jar was tested before a 10 ml sample was taken for lab analysis of TKN and TAN. The ph was checked again at 14 days but no samples taken for lab analysis. Experiment 4: effect of low and high ph on urease activity in urine The final experiment tested the inhibitory effect of ph < 3 and ph > 12 on urease activity in actual swine urine. Fresh urine was collected from 3 sows and a sample of the urine was acidified to ph 2.93 with H 2SO 4 and submitted for lab analysis of TKN and TAN. Urease was mixed in 250 ml 0.4M phosphate buffer for A urease buffer solution was prepared by mixing g urease with 250 ml 0.4M phosphate buffer. 150 ml urine was added to each of 2 jars. 7.5 ml of the urease buffer solution was mixed into each jar for a urease dosing level of 1.62 U urease per ml urine. The solution in each jar was then divided into three 50 ml aliquots in lidded jars. 3 of the lidded jars were titrated with 1N H 2SO 4 to below ph 3 and 3 jars were titrated with 0.5N NaOH to above ph ml samples were taken from each of the jars at 7 days and 14 days, tested for ph and submitted for lab analysis of TKN and TAN. Results and Discussion Experiment 1: Swine urine vs. urea solution Figure 1 shows the change in ammonia concentration of fresh sow urine and a 1M aqueous urea solution over a 22-hour test period. Possible contamination of the urine with fecal bacteria may have resulted in higher concentrations of urease enzyme in the urine, increasing the initial rate of urea hydrolysis. However, overall the formation of ammonia trended similarly between urine and urea solution over time suggesting that the urea solution could be an acceptable substitute for urine with regard to urease activity. Experiment 2: effect of high and low ph on urease activity in urea solution Experimental results showed that although the initial average concentration of TAN was higher for the high ph treatment than in the low ph treatment, both were still very low relative to TKN indicating a significant inhibitory effect on urease activity (Figures 2a and 2b). The low ph samples were initially titrated < 3 and at week 6 the ph still averaged The high ph samples were initially titrated > 12 and by week 6 the ph had dropped to an average of ph This likely occurred as a result of urea hydrolysis in an alkaline solution; As the ammonium ions form they quickly give up a proton to form stable NH 3, thereby lowering ph. TKN also 5

6 TAN, mg/l declined over time. At ph 12 most ammonia is in the NH 3 form, therefore some ammonia volatilization would be expected with the high ph treatment over time. The difference in TAN and the loss of TKN in the high ph treatment suggests that some ammonia gas in the headspace of the jar was released each time a sample was taken. 5,000 4,000 3,000 2,000 1,000 Urine Urea Time, hours Figure 1: Solution TAN after urease enzyme added to sow urine and 1M urea solution. Experiment 3: ph range test Testing a range of nominal ph values in urea solution between 4 and 11 it was observed that urease enzyme remained active over a 2 week period across all values, with the highest activity observed at ph 7.0. Enzyme activity declined with higher or lower ph but was not stopped at any ph tested between 4 and 11 (Figure 3). These results are consistent with previously mentioned studies. Because urease activity was not stopped at any of the nominal ph values after one week, TKN and TAN were not tested the second week. Ammonia has an equilibrium ph of around 9.3 where half of the ammonia exists as ammonium ions. Two phenomena were observed with this experiment. First, regardless of the initial ph of the sample, the ph of all samples averaged 9.72 after one week and 9.68 at the end of the second week (Figure 3). The pka of ammonia is about 9.3 so this is likely due to increase of ammonia from urea hydrolsis driving the solutions to an equilibrium ph. The second observation was with regard to a particular sample, UAP41 (the first rep at a nominal ph value 4). The initial ph of this sample was titrated < 3 and brought back to ph 3.56 with 2 drops of NaOH in the urease buffer solution with a final ph of 3.93 after the urea solution was added. This sample had a ph of 4.42 at the end of the first week and 5.33 at week 2. This is the only sample that demonstrated significant inhibition of urease activity in the ph range test with a TAN concentration at one week of only 5.8 mg/l. This result suggests that while a 4.0 < ph < 11 variably in... urease activity, ph < 3.0 denatured the enzyme such that it cannot regain its function. 6

7 TAN, mg/l TKN, mg/l TAN, mg/l TKN, mg/l (a) Average of TKN, mg/l Average of TAN, mg/l Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 ph < 3 (b) 35,000 30,000 25,000 20,000 15,000 10,000 5,000 0 Average of TKN, mg/l Average of TAN, mg/l Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 ph > 12 35,000 30,000 25,000 20,000 15,000 10,000 5,000 0 Figure 2: Comparison between the concentrations of TKN and TAN over a 6-week test period for urease inhibition treatment of ph < 3 (a) and ph > 12 (b). Note: For ph >12, TKN was not tested for week 1. 7

8 TAN, mg/l TAN, mg/l 6,000 5,000 4,000 3,000 2,000 1, Nominal ph Figure 3: TAN released from ph adjusted urea solutions treated with urease at one week. Initial ph of unadjusted urea solution was 7.05, TAN 2.4 mg/l and TKN 28,729 mg/l. 7-Day TAN 14_Day TAN initial sample ph Figure 4: TAN analysis at 7 and 14 days after urease solution adjusted to ph < 3 or ph > 12 added to urine. Initial urine TAN 133 mg/l. 8

9 Experiment 4: effect of low and high ph on urease activity in urine The final experiment in this preliminary analysis tested the inhibitory effect of ph > 12 and ph < 3 on urease in actual urine. Figure 4 illustrates the change in TAN concentration over a 2 week test period. An ANOVA test between the 7-day and 14-day TAN for the low ph test yielded a p-value of suggesting there is not enough evidence to assume a difference in the inhibitory effect of ph < 3 between a 7 day and 14 day holding time. The results of the individual reps are shown however, because the data appear to suggest a difference in inhibitory effect between ph 2.9 and 2.5 but additional work would be required to confirm this. Statistical analysis was not done on data for the high ph values because some loss of TAN likely occurred due to volatilization of ammonia and this could not be measured. All three urine samples at high ph exhibited precipitation shortly after treatment while none of the low samples showed any precipitation. This precipitate is likely formed from calcium, magnesium and phosphate ions present in the urine and represents a possible method for recovering additional nutrients (Randall et al. 2016). Analysis to determine the content of the precipitate was outside the scope of the current study but will be included in future work. (a) (b) Conclusion Figure 5: Photos of a urine sample treated with NaOH to ph > 12.0 (a) and a sample treated with H 2SO 4 to ph < 3.0 (b) approximately 1 hour after treatment. Preliminary experiments demonstrated that reducing urine ph to below 3 or raising ph above 12 exhibits an inhibitory effect on the hydrolysis of urea by urease enzyme in fresh swine urine. These results lay the groundwork for future experiments using a pilot scale scraper system for solid/liquid separation from grow/finish pigs. These future studies will help determine if the same inhibitory effects on urea hydrolysis will be produced when some manure solids come in contact with the liquid waste stream. Acknowledgements The authors would like to thank Waste to Green, LLC of Cocoa, FL for their contribution to this study. 9

10 Literature Cited Aneja, V. P., Blunden, J., James, K., Schlesinger, W. H., Knighton, R., Gilliam, W., Cole, S. (2008). Ammonia assessment from agriculture: U.S. status and needs. Journal of Environmental Quality, 37(2), Dai, Xiaorong and Henrik Karring "A Determination and Comparison of Urease Activity in Feces and Fresh Manure from Pig and Cattle in Relation to Ammonia Production and ph Changes." Plos One 9 (11): e Frankenberger, W. T. and J. B. Johanson Effect of ph on Enzyme Stability in Soils. Vol. 14. doi://dx.doi.org/ / (82) Hagenkamp-Korth, Frauke, Angelika Haeussermann, Eberhard Hartung, and Annett Reinhardt-Hanisch "Reduction of Ammonia Emissions from Dairy Manure using Novel Urease Inhibitor Formulations Under Laboratory Conditions." Biosystems Engineering 130: Hellström, Daniel, Erica Johansson, and Kerstin Grennberg "Storage of Human Urine: Acidification as a Method to Inhibit Decomposition of Urea." Ecological Engineering 12 (3 4): Jones, J. B., C. M. Wathes, and A. J. F. Webster "Operant Responses of Pigs to Atmospheric Ammonia." Applied Animal Behaviour Science 58 (1 2): Karr, Jonathan D., William J. Showers, J. Wendell Gilliam, and A. Scott Andres "Tracing Nitrate Transport and Environmental Impact from Intensive Swine Farming using Delta Nitrogen-15." 30 (4): Ni, Kang, Andreas Pacholski, and Henning Kage "Ammonia Volatilization After Application of Urea to Winter Wheat Over 3 Years Affected by Novel Urease and Nitrification Inhibitors." Agriculture, Ecosystems & Environment 197: Randall, Dyllon G., Manuel Krähenbühl, Isabell Köpping, Tove A. Larsen, and Kai M. Udert "A Novel Approach for Stabilizing Fresh Urine by Calcium Hydroxide Addition." Water Research 95: Soares, Johnny Rodrigues, Heitor Cantarella, and Menegale, Marcella Leite de Campos "Ammonia Volatilization Losses from Surface-Applied Urea with Urease and Nitrification Inhibitors." Soil Biology and Biochemistry 52: Varel, Vincent H. and James E. Wells "Influence of Thymol and a Urease Inhibitor on Coliform Bacteria, Odor, Urea, and Methane from a Swine Production Manure Pit." 36 (3): Watson, C. J., N. A. Akhonzada, J. T. G. Hamilton, and D. I. Matthews "Rate and Mode of Application of the Urease Inhibitor N-(N-Butyl) Thiophosphoric Triamide on Ammonia Volatilization from Surface-Applied Urea." Soil use and Management 24 (3): Zhao, Lingying, Manuzon, Roderick and Hadlocon, Lara J. "Ammonia Emission from Animal Feeding Operations and its Impacts." Ohio State University Extension., last modified May , accessed 5/14/, 2017, 10