SIMPLE METHOD FOR DETERMINATION OF CARBONATE ADSORPTION AT ELEVATED CARBON DIOXIDE CONCENTRATION

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Clay Science 12, 97-101 (2003) SIMPLE METHOD FOR DETERMINATION OF CARBONATE ADSORPTION AT ELEVATED CARBON DIOXIDE CONCENTRATION SHIN-ICHIRO WADA* and HIROYUKI ONO** * Faculty of Agriculture, Kyushu

1 Clay Science 12, (2003) SIMPLE METHOD FOR DETERMINATION OF CARBONATE ADSORPTION AT ELEVATED CARBON DIOXIDE CONCENTRATION SHIN-ICHIRO WADA* and HIROYUKI ONO** * Faculty of Agriculture, Kyushu University, Fukuoka , Japan ** Graduate School of Bioresources and Environmental Sciences, Kyushu University, Fukuoka , Japan (Received March 10, Accepted July 2, 2003) ABSTRACT We developed a simple method to measure carbonate adsorption by soils and clays at equilibrium with the air having elevated carbon dioxide concentrations. The method uses bags made of laminated polyamide-polyethylene film as experimental container. Soil or clay sample is put in the bag and sealed up. Addition of equilibrating solution and air containing CO2 and sampling of the air and solution are all made through a three-way stopcock attached to the bag. The equilibrium ph is measured with a ow-through glass electrode attached also to the stopcock. \ The carbonate adsorption by specimen boehmite was measured with the proposed method under equilibrium CO2 concentration of 3 to 3.9% and ph of 4.4 to 7.6. The carbonate adsorption increased as the ph increased, reached maximum at around ph 7.4 and then decreased. The coefficient of variation of the measured values ranged 2.0 to 4.3%, which were significantly lower than those from measurements with conventional methods. Key words: Adsorption, Carbon dioxide, Carbonate, Boehmite INTRODUCTION Soil is one of the important sources of carbon dioxide to the atmosphere and many programs are being planned and on-going to monitor the rate of emission of carbon dioxide from soils. The net carbon dioxide emission from a soil can be estimated by measuring the change in the carbon dioxide concentration in a closed chamber that covers a definite area of the soil surface. However, it is not sufficient to understand the dynamics of gaseous carbon dioxide in soils because carbon dioxide dissolves in soil solution to form various ions, complexes and sparingly soluble salts and it may be adsorbed on some type of clay minerals in soils. For aqueous reactions like dissolution, dissociation and complex formation, reliable thermodynamic databases are available and it is not difficult to calculate the concentration of carbon dioxide-related species in a soil solution from given carbon dioxide concentration in the air-phase of the soil. By combining this approach with hydrodynamic equations, it is possible to simulate the transport of carbon dioxide and related species in a soil profile (Simunek and Saurez, 1993-a,-b). To date, however, there is virtually no data on the adsorption of carbonate by soils and it is not certain whether the adsorption reaction has some significant effect on the carbon dioxide dynamics in soils. This situation is probably due to the lack of convenient methods for measuring carbonate adsorption by soils. Since the atmosphere is the huge reservoir of carbon dioxide and it is fairly soluble in water, it is difficult to carry out systematic adsorption experiments in the air. Schulthess and McCarthy (1990) and Schulthess et al. (1998) measured carbonate adsorption by an aluminum oxide in the open air but they did not measure the carbonate adsorption at elevated carbon dioxide concentrations. Van Geen et al. (1994) developed a specially designed device to measure the carbonate adsorption by colloidal minerals under controlled carbon dioxide concentrations and measured carbonate adsorption by goethite. The device was, however, fairly expensive to prepare and it is not convenient for handling soils that contains non-colloidal sand and even rock fragments. In the present study, a simple and low-cost method was developed to measure carbonate adsorption by clay minerals and soils under elevated carbon dioxide concentrations. And the performance of the method was tested by measuring the carbonate adsorption by an aluminum oxide hydoxide.

2 S.-I. Wada and H. Ono DEVELOPMENT OF METHOD Since the concentration of carbon dioxide in soil air varies from about to 5% (Brady, 1974) and the total pressure of soil air is virtually constant, a gas-tight exible bag is suitable as a container for soil fl samples, reacting solutions and gas. After tests, a bag made of laminated polyamide-polyethylene was selected. The gasbarrier property of the material was tested by mon- FIG. 1. Carbon dioxide concentration in a bag made of laminated polyethylene-polyamide as plotted against time. itoring the concentration of CO2 in the bag that was lled with the standard air containing 4.95% of CO2. fi The measured CO2 concentration was plotted against time in Fig. 1. Fig. 1 shows that there was no clea decreasing trend and the difference among the values were not statistically significant. The developed method to measure carbonate adsorption by soils and minerals in a closed system is schematically illustrated in Fig. 2. For adsorption experiment, a 25 mm-long polyethylene tube with a diameter of 5 mm was welded to the blind end of a bag and fitted with a three-way stopcock. Two weighed portions of soil or mineral sample were placed in two bags and the open end of each bag was heat-sealed. After evacuating the gas in the bags, 50.0 ml of solution containing acid or alkali was injected and equilibrated overnight. Then, 1.25 L of air enriched with carbon dioxide was injected into each bag with a syringe through the stopcock and the adsorbent, solution and air were equilibrated for more than 90 min with occasional shaking. After equilibration, carbon dioxide concentration in the equilibrium air was determined on one of the reaction bags with an infrared gas monitor connected to the stopcock. After measurement, the remaining gas was removed and a ten to twenty ml portion of the equilibrium solution was collected in a plastic syringe through the stopcock and used for determination of equilibrium carbonate concentration. The collected so- FIG. 2. Schematic presentation of whole experimental procedures. IC stands for inorganic carbon.

3 Simple Method for Determination Plastic bag containing NaOH FIG. 3. Schematic presentation of the procedure for dissolved carbonate concentration. IC stands for inorganic carbon. lution was filtered through a 0.45ƒÊm membrane filter and injected into a sealed gas-tight bag containing 10.0 ml of 15 mmol L-1 NaOH through a rubber septum that was put on the bag. Thus, the dissolved carbon dioxide in the reaction bag was turned into carbonate and the solution was fed to Shimadzu TOC-5000 inorganic carbon analyzer (Fig. 3). The stopcock of the reaction bag was then connected to a flow-through ph electrode and the ph was measured without exposure to the atmosphere. The equilibrium air in another reaction bag was removed quantitatively and 1.00 L of carbon dioxidefree air and 50.0 ml of 1 mol L-1 phosphoric acid were injected and allowed to react for 30 min to volatilize the adsorbed and dissolved carbonate. The carbon dioxide concentration in the bag was determined with an infrared gas monitor to estimate the total amount of the dissolved and adsorbed carbonate. The partial pressure of CO2 in the bag after phosphoric acid addition (P) is related to the initial volume of CO2-free air (Vair) and the volume of the evolved CO2 (Vg) through And the amount of the evolved CO2 (ng) is expressed as where R and T are the gas constant and absolute temperature. Eqs.(1) and (2) finally yield by which the amount of the evolved CO2 is calculated from the measured CO2 concentration and the volume of the injected air. With the addition of phosphoric acid, the adsorbed carbonate is desorbed and mostly volatilizes as CO2. But some CO2 still remains dissolved in equilibrium with the gas phase in the bag. The amount of the dissolved CO2 is calculated by the Henry's law where KH is the Henry's constant and Vag is the volume of the solution in the bag. The Henry's constant for CO2 in 1 mol L-1 phosphoric acid at 25 Ž was found to be The total amount of carbonate in the bag is, therefore, ng + nag. A part of this carbonate comes from the equilibrium solution and the amount of carbonate adsorption is given by where Ceq is the inorganic carbon concentration in the equilibrium solution and w is the weight of the adsorbent. TEST OF DEVELOPED METHOD The developed method was applied to measurement of carbonate adsorption by synthetic boehmite supplied by Wako Pure Chemical Industries. Two gram portions of boehmite were weighed into the prepared bags, heatsealed, and evacuated. To all these bags, 1.25 L of an air containing 4.00% of CO2 and 50.0 ml of either water, dilute HC1 or dilute NaOH solutions were injected through the stopcocks and equilibrated for 90 min with shaking. After equilibration, the CO2 concen-

4 S.-I. Wada and H. Ono TABLE 1. Summary of analytical results for boehmite tration in the gas phase, equilibrium ph and inorganic carbon concentration in the aqueous phase were determined as described above. And the amount of CO2 evolved by addition of phosphoric acid was determined. All the experiments were done in nine replications. RESULTS AND DISCUSSION The analytical results obtained for the boehmite were summarized in Table 1. The figures in the table represent the average and standard deviation of the values from nine replications. The standard deviations of the measured values are quite small for the equilibrium CO2 concentration, ph and amount of evolved CO2. The coefficient of variation for these quantities ranged from 0 to 3.3% and they were mostly around 1% or below. This excellent reproducibility suggests that the use of the laminated plastic bag worked well as a gas-tight reaction vessel. On the other hand, the coefficient of variation for the inorganic carbon concentration in the equilibrium solution ranged from 1 to 11% as calculated from the values in the fourth column of Table 1. The lower reproducibility for the inorganic carbon concentration is probably due to some error associated with the transfer of an equilibrium solution to a separate bag containing NaOH (Fig. 3). This step was adapted to avoid the volatilization of hydrated CO2 before the solution was fed to the inorganic carbon analyzer. Once the solution has been injected to the NaOH-bag, CO2 is not likely to be lost. But the filtration took some time particularly when the adsorbent was dispersed and it is probable that some CO2 might have escaped during filtration. The calculated carbonate adsorption was plotted against equilibrium ph in Fig. 4. The carbonate adsorption by the present boehmite was about 10.8 cmol kg-1 at ph 4.4 and increased as ph increased. It reached a maximum of 19.5 cmol kg-1 at around ph 7.4 and then, decreased. This trend is quite similar to that observed for goethite by van Geen et al. (1994), although the adsorption. maximum was at around ph 6 for goethite. The coefficient of variation for the calculated carbonate adsorption ranged. from 2.0 to 4.3%, suggesting that the overall reproducibility is excellent. As seen from the plots in Fig. 4, the coefficient of variation is larger at phs higher than 6. This is clearly because the error FIG. 4. Carbonate adsorption by boehmite as a function of ph. associated with the subtraction of dissolved carbonate species (Eq. 5) was larger at high phs due to the increased solubility of carbonate species (Table 1). Similar trend that the relative error increased as equilibrium ph increased was also reported by Schulthess and McCarthy (1990), Schulthes et al. (1998) and van Geen et al. (1994). The comparison of the present results (Table 1 and Fig. 4) with the results by these researchers clearly shows that the experimental errors in the method developed in the present study is much smaller than those in the previous methods irrespective of the use of less sophisticated experimental devises. This is particularly true for the adsorption measurement at high phs. In this test experiment, fixed amount of carbon dioxide was introduced in the bags and no effort was made to keep the equilibrium gas phase CO2 concentration constant. However, the gas phase CO2 concentration can be adjusted to any desired level simply by renewing the air in the bag several times after equilibration with an air having a desired CO2 concentration. The proposed method requires neither special instruments nor expertise in chemical experiment in general. But closest attention has to be paid during the transfer of sampled equilibrium solution to an NaOH-bag: Another problem in this step is that it is sometimes difficult to filter the solution when clay particles are dispersed. In this case the hydrated CO2 concentration in the equi-

5 Simple Method for Determination lies on or near the 1 : 1 line, suggesting the equilibrium carbonate concentration can be estimated by calculation. The developed method is being applied to allophanic soils to estimate the carbonate adsorption under conditions that would prevail in field soil environment. The results will be reported elsewhere. ACKNOWLEDMENT This study was supported in part by Grant-in-Aid for Scientific Research from Japanese Society for Promotion of Sciences (# and # ) REFERENCES FIG. 5. Relationship between measured and estimated carbonate concentrations. Estimation was made with the aid of the Henry's law. librium solution has to be estimated with the aid of the Henry's law. To test the accuracy of this approach, 50 ml of distilled water was equilibrated with 1.25 L of air containing 1.7 to 3.4% CO2 and the equilibrium CO2 concentration in the gas phase and the dissolved CO2 concentration was measured following the proposed method (Figs. 2, 3). The measured values are compared with the calculated ones in Fig. 5. The plots in Fig. 5 BRADY, N. C.(1974) The Nature and Properties of Soilș Macmillan Publishing Co. Inc., New York. SCHULTHESS, C. P. and MCCARTHY, J. F.(1990) Competitive adsorption of aqueous carbonic and acetic acids by an aluminum oxide. Soil Sci. Soc. Am. J., 54, SCHULTI SS, C. P., SWANSON, K. and WIJNJA, H.(1998) Proton adsorption on an aluminum oxide in the presence of bicarbonate. Soil Sci. Soc. Am. J., 62, SIMUNEK, J. and SAUREZ, D. L.(1993-a) Modeling of carbon dioxide transport and production in soil. 1. Model evelopment. Water Resour. Res., 29, SIMUNEK, J. and SAUREZ, D. L.(1993-b) Modeling of carbon dioxide transport and production in soil. 2. Parameter selection, sensitivity analysis, and comparison of model prediction to field data. Water Resour. Res., 29, VAN GEEN, A., ROBERTSON, A. P. and LECKIE, J. O.(1994) Complexation of carbonates for adsorption of metal ions in natural waters. Geochim. Cosmochim. Acta, 58,

Gain a better understanding of soil ph and how it is measured. Understand how lime requirement is determined.

LABORATORY 7 SOIL REACTION (ph) AND LIME REQUIREMENT I Objectives Gain a better understanding of soil ph and how it is measured. Understand how lime requirement is determined. II Introduction A Soil Reaction