Organic Wastes to Increase CO2 Absorption


The objective of the study was actually the investigation of the effect of various organic wastes on the ability of urine in absorbing CO2. Urine alone or mixed with olive-oil-mill waste waters (O), poultry litter (P) or meat bone meal (M) was used on the absorption of CO2 from a gas bottle. The absorption capacity (1.35 - 2.85 gCO2/gNH4) was bigger than other solvents such as ammonia and amines. The range of CO2 absorption was significantly bigger for the organic mixtures P and PM with urine (9.1 - 11.8) g/L than urine alone 6.5 g/L. These organic wastes could be used to increase CO2 absorption in urine and reduce gas emissions.

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Aguilar, M. (2014) Organic Wastes to Increase CO2 Absorption. International Journal of Clean Coal and Energy, 3, 47-53. doi: 10.4236/ijcce.2014.34005.

1. Introduction

Global warming resulted from emission of greenhouse gases has received widespread attention. Among the greenhouse gases CO2 contributes more than 60% to global warming because of its huge emission amount. Though various CO2 capture technologies including physical absorption, chemical absorption, adsorption and membrane exist, they are not matured yet for post-combustion power plants. Among these technologies chemical absorption using aqueous alkanolamine solution is proposed to be the most applicable technology for CO2 capture. Alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), and N-methyldiethanolamine (MDEA) are widely used as absorbents for CO2 capture [1] .

However, the MEA process suffers the following disadvantages for CO2 separation from flue gases: Low carbon dioxide loading capacity (Kg CO2 absorbed per Kg absorbent); high equipment corrosion rate; amine degradation by SO2, NO2, HCL and HF, and Oxygen in flue gas; high energy consumption during high tem- perature absorbent regeneration.

An ideal absorbent should have a CO2 loading capacity of at least 1 Kg of CO2 per Kg of solution and that the regeneration energy requirement must be much lower than the MEA process. The aqua ammonia process seems to have avoided the shortcomings of the MEA process: Aqueous ammonia has high loading capacity; aqueous ammonia does not pose a corrosion problem; there is no absorbent degradation problem, thus reducing absorbent makeup rate; the energy requirements for absorbent regeneration is predicted to be at least 75% less than in the MEA process [2] .

CO2 removal efficiency and absorption capacity of NH3 solvent are better than those of MEA. The maximum CO2 removal efficiency by NH3 solvent can achieve 99% and the CO2 absorption capacity can approach 1.2 KgCO2/Kg NH3 while for MEA it is only 0.4 KgCO2/Kg MEA [3] .

Aqueous ammonia can be used to capture CO2 from flue gas of coal-fired power plant with quick reaction rate, high removal efficiency, and high loading capacity of CO2 [4] .

However, the absorbents should not be limited to alkanolamines but ionic liquid and other alkaline absorbents as well as the mixtures are also needed to test their potential. The development of suitable absorbents with high CO2 adsorption capacity is still demanded [5] .

In animals ammonia removal is performed through the CO2 by the urine. On the other hand to increase CO2 absorption we could think of using any organic residue as a source of ammonia. So, Ammonia in solution from urine has been used to capture CO2 gas and produce ammonium bicarbonate [6] .


During the hydrolysis of urea of urine or waste-water solutions pH is increased and NH4 produced. .

NH4 is in equilibrium with dissolved and gaseous ammonia. The pKa value for this equilibrium is 9.3 at 25˚C.

For other wastes, mineralization is the process by which microbes decompose organic N from manure, or- ganic matter and crop residues to ammonium.

Cumulative net N mineralized in P was fitted to a two-pool, fast and slow first-order models. The fast N pool varied from 11.6% to 56.9% of organic N and could be predicted from uric acid-N-concentration in the litter. Total mineralizable N (fast + slow N pools) range from 46.5% to 86.8% of organic N and could be predicted from uric-acid N and total N concentration. Differences that affect N mineralization rates of P include uric acid concentration (which depends on diet) and moisture content of the litter [7] . Moisture content is particularly important because it supports bacterial activity and thus the production of the enzyme urease, which breaks down uric acid molecules to simpler N forms [8] . Nitrogen mineralization (percent total organic N converted to inorganic N) rates were higher for fresh litter (range of 42% - 64%) than composted litter (range of 1% to 9%) with a relation C/N of 8:1 [9] .

M a potential organic fertiliser for agricultural crop contains considerable amounts of nutrients (on average) 8%N, 5%P, 1%K and 10%Ca [10] . The low C/N ratio of M about 4 provides great potential for N mineralization [11] . Soil amendment with M caused a noteworthy increase in both extractable NH4 and NO3 (about 50% of added N). The potential of M as effective organic fertilizer was supported by the large increase in available N [12] .

If the aqueous ammonia is the agent that can remove CO2, that may exist in the flue gas [2] . It would possible to think that organic wastes such as P [13] and M [10] like sources of organic N (1%, 8%, respectively) could be added urine and the mineralised ammonium used to increase the capture CO2. The aim of the present work was to investigate the ability of NH4 of O, P, and M to increase the CO2-absorption capacity of urine.

2. Materials and Methods

The urine used in the experiments was my own family urine, pH 5.9 ± 0.3; EC 13.8 ± 3.5; NH4-N 6 ± 2.3. The O, collected from St. Anthony oil mill of Viznar (Granada, Spain) in January 2013, had a pH of 4.2, EC of 12 dSm1. The CO2 came from an industrial gas cylinder supplied by the company Air Products Ltd.

Two samples of 200 mL of hydrolysed urine were used as controls. The different treatments for Sample 1 were: (A-urine alone; AP1-urine mixed with 0.25%P in weight; AP2-urine mixed with 0.5%P in weight; AP1O-urine mixed with 0.25%P in weight and 2.5%O in volume; AP2O-urine mixed with 0.5%P in weight and 2.5%O in volume. For the sample 2 were: (B-urine alone; BM-urine mixed with 0.25%M in weight; BMO- urine mixed with 0.25%M in weight and 1%O in volume; BMP-urine mixed with 0.125%M and 0.125%P in weight; BMPO-urine mixed with 0.125%M and 0.125%P in weight and 1%O in volume. When pH increased above 8.5, the BC half samples were kept 45 minutes in a reactor at a pressure of 6 bar CO2 (Days 15-22). In the end the pH of pressured samples decreased to around 7.5. Similarly, the AC half samples were kept 40 minutes in the reactor (Days 11, 13 and 19). All the samples were stored in stove to 25˚C for 3 months with two replicates for each treatment.

Every week, the pH, CO2, and NH4 values as well as the EC of each sample were measured. The pH was monitored using a pH/ion meter, and EC using a conductivity meter (both Crison 2002). The CO2 was analysed following the procedure reported by Lin and Chan [14] . For this, after determining the initial pH of the sample, a 2 mL urine sample was pipetted into a vial containing 10 mL of 0.1 N HCl and the mixture was placed in a boiling water bath for 10 min to expel the CO2. Allow the sample to cool to room temperature, add a magnetic stirrer and titrate with 0.1 N NaOH. Noted the volume of NaOH added to achieve the initial pH of the sample. A blank containing 2 mL of distilled water was treated in an identical way. The amount of CO2 in mEq/L was de- termined by multiplying the difference in the volumes of NaOH, required to titrate the blank and sample, by the normality of NaOH. The NH4 was analysed following Nelson [15] .

The results were subjected to an analysis of variance and comparison of means using the PC computer pro- gram Statistic 8.0 (Analytical Software, FL, USA). Also, the figures were plotted with this program.

3. Results and Discussion

The average values of pH, EC, NH4, CO2, CO2-Absorption and Absorption capacity (CO2/NH4) of each treat- ment are shown in Table 1.

In Sample 1, the carbonated treatments increased the CO2 absorption and EC in all samples, indicating that part of CO2 (6.6 - 13.9 g/L) was absorbed by the samples (Figure 1). The increase in conductivity was generated by CO2 dissolution and ionisation in water was reduced by the decomposition of urea and NH4 due to volatiliza- tion.

The treatments with the highest CO2 absorption were AP1C and AP1OC (11.8 - 11.7 g/L) (see Table 1, Fig- ure 1). This indicates that add a slight proportion of P (0.25% in weight) in urine significantly improves CO2 absorption. A bigger increase of P (0.5% in weight) does not improve CO2 absorption. O slightly reduced pH but no influence on CO2 absorption.

Table 1. Main effects of CO2 absorption on the pH, EC, CO2, NH4 and Absorption capacity for each treatment (average values for 7 measurements).

Different letters (a, b, c, d) within a column indicate significative differences between treatments at level of significance (P < 0.05) according to Tukey’s test.

Figure 1. Mean values of CO2 (Sample 1).

The carbonated treatments increased the mineralised NH4 in all samples (Figure 2). The NH4 decrease in AP1 and AP2 samples could be explained according to Buondonno et al. [16] by the fact that some organic wastes can increase immobilization of inorganic or mineralised N.

In Sample 2, the carbonated treatments increased the CO2 absorption and EC in all samples, indicating that part of CO2 (5.9 - 14.9 g/L) was absorbed by the samples.

The treatments with the highest CO2 absorption were BMOC and BMC (11.7 - 10.0 g/L) (see Table 1, Figure 3). This indicates that a slight proportion of M (0.25% in weight) or mixtures (0.125%M + 0.125%P in weight) in urine significantly improves CO2 absorption.

The carbonated treatments increased the mineralised NH4 in all samples (Figure 4). The effect is not as clear as in Sample 1. However, in all cases the absorption of CO2 causes an increase in free NH4 with its implications at the agricultural level.

Liu et al. [4] study the CO2 removal efficiencies at room temperature and different concentrations of aqueous ammonia. The best concentration of aqueous ammonia should be selected from 5% - 10%. The CO2 removal ef- ficiency was 60% in 50 minutes, for a concentration of aqueous ammonia 1%.

In our samples with a concentration of aqueous ammonia smaller than 1%, we should think in an absorption time of about 2 hours to get an efficient removal of 100% CO2. The absorption time was 120 minutes for sample 1 and 90 minutes for Sample 2. The shorter absorption time (Sample 2 compared to 1), could explain a smaller increase NH4 and insufficient removal of CO2.

Theoretically according Equation (1), absorption capacity of ammonia was 2.5 (44gCO2/17gNH3). Previous research shows that aqueous ammonia has a higher absorption capacity than that of monoethanolamina (MEA) at same temperature and pressures. Absorption capacity of aqueous ammonia can approach 1.2 KgCO2/KgNH3 while for MEA it is only 0.4 KgCO2/Kg MEA [3] .

Our research shows that the absorption capacity of urine was similar to aqueous ammonia (between 0.8 - 1.3 gCO2/gNH4). However, the absorption capacity of urine-organic samples mixtures (Table 1) is twice (2 - 2.9 gCO2/gNH4). These results indicate that the organic samples can reach almost the theoretical value, namely the maximum possible absorption of CO2 for a given concentration of NH4 (2.5gCO2/gNH4).

The absorbed CO2 lowered the pH of the urine mixtures due to the formation of carbonic acid. For all the samples, the pH variations could be explained in function of the CO2 and NH4 increases. So, the pH, CO2, and NH4 showed a statistically significant relationship (P < 0.0001) (correlation coefficient 0.624) at the 95% confi- dence level (see Figure 5); NH4 contributed nearly 2-fold more than CO2 to the pH variation.

The addition of a low percentage of O (acid pH) could reduce urine pH and increase buffering capacity due to fatty acids. To more buffering capacity there is a smaller variation in pH for the addition of CO2 with a better stabilization of the CO2 absorbed [6] .

NH4 volatilization increased with the pH, so that all the factors that tended to lower the pH reduced NH4 losses [17] . Therefore all treatments to reduce the pH proved useful to NH4 conservation. The addition of a low percentage of O (acid pH) could reduce urine pH and avoid ammonia volatilization [6] .

Figure 2. Mean values of NH4 (Sample 1).

Figure 3. Mean values of CO2 (Sample 2).

Figure 4. Mean values of NH4 (Sample 2).

The fact that pH of the carbonated samples reaches a next value to 8.0 could be explain because the minerali- zation is still producing new NH4 which is not neutralised by new contributions from CO2.

Ammonia availability rate has a fast decreasing after 30 days [7] . In our laboratory, CO2 and NH4 contents of all samples treated with CO2 remained conservative for more than three months.

With the use of organic waste power consumption is minimized because it is not necessary to recover the ab- sorbent again. The CO2 absorbed by the urine mixtures is fixed to the soil as carbonate or organic carbon through fertilization [18] .

Figure 5. Correlation between pH, CO2, NH4.

The advantages of this technology are undeniable: A process for removal of CO2 at room temperature, with- out catalyst, with relative low-cost and low-energy requirements. The urine and other wastes produced by a family could be stored and be useful to reduce emissions into towns.

Fertilization with urine and O has previously been demonstrated to be a feasible method for reducing the en- vironmental impact of O and CO2 [19] .

P and M could provide N and other plant nutrients when used as fertilizers. The effects of M as N fertiliser were evaluated by Salomonsson (1994) [20] . In case of spring wheat found no significant differences in grain yields and nitrogen uptake efficiency between M and urea. Diaz, et al., 2008, [13] indicate P like a source of ammonia 11 g/Kg. Thus, urine mixed with the above-mentioned wastes could provide all plant nutrients and be recycled by the fertilizer industry.

However, new wastes, manures, and compost need to be tested with added urine as a means for increasing NH4 contents and CO2 adsorption. The proposed strategy requires further research to reduce the risks associated with the waste-water reuse [21] .

4. Conclusions

In conclusion, hydrolysed urine mixed with a small percentage (0.25% in weight of P, M, and 1% - 2% in vol- ume of O could be considered a stable long-term system for greenhouse-gas absorption. The absorption coeffi- cient (1.35 - 2.85 gCO2/gNH4) was bigger than other solvents such as ammonia and amines. The range of CO2 absorption was significantly bigger for the organic mixtures P and PM with urine (9.1 - 11.8) g/L than urine alone 6.5 g/L.

Some organic wastes similar to urine should be tested as CO2 sinks. In addition, the reduction of CO2 emis- sions requires further research to increase the NH4 contents and CO2 absorption.


This study was funded by the Institute of Agricultural Research and Training, Andalusian Government. The au- thor is grateful to the anonymous reviewers for insightful comments which greatly improved the quality of the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.


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