Title. The Retention of Pb and Cd Cations in Organic, Urban, and Topsoil. Author. Sushmitha Ravikumar

Transcription

1 Title The Retention of Pb and Cd Cations in Organic, Urban, and Topsoil Author Sushmitha Ravikumar TA: Raffeal Bennett Chemistry 1220, Fall 2013, The Ohio State University

2 Abstract The problem question of which soil type organic, topsoil, and urban soil retained the most cations of Pb and Cd was examined. It was hypothesized that urban soil or topsoil would retain the most ions of both cations because urban soil had the highest clay content percentage and topsoil had the highest organic soil content. High clay content and high organic matter content both correlate to high retention. To prepare the three types of soils for a total of six solutions, all three soils were contaminated with 100 ppm of Pb + (aq) and 10 ppm of Cd 2+ (aq) in a neutral ph of 7. The absorbance of the two cations remaining in the filtered solutions of the three soils was measured using a flame atomic absorption spectrophotometer (FAAS). The absorbance readings were used to find the concentration of cations in solution, which were then used to find the concentration of cations in the soil. The organic soil retained 95.95% of Pb + ions and 91.82% of Cd 2+ ions. The topsoil retained 99.5% of Pb + ions and 89.8% of Cd 2+ ions. Finally, the urban soil retained 98.59% of Pb + ions and 81.4% of Cd 2+ ions. This suggests that under a neutral ph, topsoil retained the most lead cations and organic soil retained the most cadmium ions. Research Hypothesis It was hypothesized that based on clay content, urban soil would retain ions better since urban soil had the most clay content percentage, and based on organic matter percentage, topsoil would retain the most ions since it had the higher SOM percentage. Since urban soil had a much larger clay content compared to the other two soils, it was hypothesized that urban soil would retain ions better. Therefore, in this experiment it was believed that organic soil would continue to have the highest retention of Pb + and Cd 2+ ions. To test this hypothesis, an experiment was conducted to determine which soil type organic, topsoil, and urban soil would retain the most cations of Pb and Cd. In the experiment, all three soils were contaminated with lead and cadmium in a neutral ph of 7. Then, these three contaminated soils were filtered using filter paper. The absorbance of the two cations remaining in the filtered solutions of the three

3 soils were measured using the FAAS. These absorbance readings were used to find the concentration of the two cations in the solution. Then, the concentration of cations in the soil was calculated. In previous experiments, it was learned that the more basic or higher the ph, the better the soil was at retaining cations. Therefore, for the experiment conducted, the ph was held constant at the optimal level of a neutral ph since a basic ph was not available. By holding the ph constant, the retention of the two ions would only be based on the soil properties. Procedure and Methods In the previous experiments, the clay content percentage was determined using X-Ray Florence spectrometer. A bag of sample soil was placed in the XRF spectrometer and information about the element and concentrations in ppm were recorded. Using those numbers, the clay content was calculated. Using the given equation y = 0.244x 4.52, the rubidium (Rb) concentration from the XRF was used as the x value and the y value of clay content percentage was calculated for each soil. The clay content correlates with the soil s ability to retain ions. In the experiment where the clay content determinations were found, it was found that urban soil had the highest clay content percentage of 14.3%, making it the better soil at retaining cations, compared to organic and topsoil, both of which had a clay content percentage of 3.3%. To test for soil organic matter, the soil color was compared to a known sample. Because EDTA binds to metal cations, the organic matter percentage was found by comparing the color intensity of the extract to the standard samples with known SOM. The darker the color of the extract, the higher the SOM content and a higher SOM content corresponds to a better retention ability of the soil. To test a sample, 0.5 g of the soil was placed in a vial and was added with 10 ml of 0.25 M NaOH and 10 ml of 0.05 M EDTA disodium salt. The vial was then shaken vigorously, filtered, and then the color intensity was compared to the known standard samples. In the experiment, it was found that topsoil had the higher SOM percentage of 4% compared to organic soil with 3% and urban soil

4 with 2%. Hence, it would be predicted that topsoil would be better at retaining metal cations since it had the higher SOM percentage. Pre-prepared solutions of urban, topsoil, and organic soil were created with a concentration of 10 ppm of Pb + and 10 ppm of Cd 2+ ions for each soil. The contaminated samples were prepared by combining 1.00 g of soil with ml of a Pb + (aq) solution under neutral (ph=6-7) conditions. The same process was repeated for Cd 2+ (aq) solution. The total volume of a contaminated sample was ml. Solutions were shaken for 1-hour on a reciprocating shaker, then left to equilibrate for 12-hours. The six contaminated solutions were then filtered using filter paper and the filtered solutions were transferred to a vial. The filtered solution was then tested using the FAAS machine to calculate the absorbance. The sample in the vial was taken to a room with the FAAS machine. In the FAAS, the samples were decomposed into atoms by heating them at high temperatures. As the light passed through the atoms and the flame, the absorbance of light was calculated (Guihua 1997). The machine gave out an absorbance reading, which was then compared to the calibration data. The amount of lead in the sample was determined using a Pb lamp and the amount of cadmium was determined using the Cd lamp. The calibration data was given and a plot was formed by plotting the ppm concentration on the x axis and absorbance on the y axis. Then, using the experimental absorbance readings from the FAAS, the two points from the calibration plot that the absorbance reading was between was used to plot a trend line. The equation for this trend line was used to calculate the concentration of cations in solution (ppm), where x represented ppm and y represented absorbance. This procedure was repeated for each sample. The cations in the soil were then calculated by subtracting the calculated concentration from the initial concentration that was used to prepare the solution.

5 Results Using the XRF instrument, the element and concentration in different soils was given. Using these concentrations, the clay content percentage was calculated for each soil type. The results were shown in Table 1 below. The SOM instrument was used to calculate the soil organic matter percentage. The results were shown in Table 1. The contamination summary for the pre-prepared solutions was also recorded in Table 1 below. Table 1: Descriptions of the 6 Sample Solutions known information Sampl e Soil Type 1 Organi c Contamination Summary Pb + medium 2 Urban Pb + medium 3 Organi c Cd 2+ medium 4 Urban Cd 2+ medium 5 Topsoil Cd 2+ medium 6 Topsoil Pb + medium Clay content % 3.3 % 3% % 2% 3.3 % 3% % 2% 3.3 % 4% 3.3 % 4% Soil Organic Matter % During the experiment, quantitative data was collected using the FAAS machine. The FAAS machine provided the absorbance readings. The calibration data for the FAAS machine was provided and shown in Table 2 and 3 below.

6 Table 2: Calibration Data for Pb Concentration (ppm) Absorbance reading Absorbance FAAS calibration data graph for lead Pb Concentration (ppm) Figure 1: FAAS calibration data graph for lead Pb Table 3: Calibration Data for Cd Concentration (ppm) Absorbance reading

7 FAAS calibration data graph for cadmium Cd Absorbance Concentration (ppm) Figure 2: FAAS Calibration data graph for cadmium Cd The FAAS machine was used to find the absorbance reading for each soil. These absorbance readings were analyzed by comparing the values to the calibration values by seeing how close it was to the absorbance values calibrated. The calibration data was given and a plot was formed by plotting the ppm concentration on the x axis and absorbance on the y axis as shown above in Figure 1 and Figure 2. Then, using the experimental absorbance readings from the FAAS, the two points from the calibration plot that the absorbance reading was between was used to plot a trend line. The equation for this trend line was used to calculate the concentration of cations in solution (ppm), where x represented ppm and y represented absorbance. This procedure was repeated for each sample. The cations in the soil were then calculated by subtracting the calculated concentration from the initial concentration that was used to prepare the solution. These results were shown in Table 4 below.

8 Table 4: The Calculated Values of Absorbance and Retention of Cations Sampl e Soil Type Absorbance Cations in filtrate solution Cations on soil (ppm) % retained by soil (ppm) 1 Organic % Pb + 2 Urban % Pb + 3 Organic % Cd 2+ 4 Urban % Cd 2+ 5 Topsoil % Cd 2+ 6 Topsoil Pb % Sample Calculations: The equations that were used in this experiment were found from graphing the absorbance readings and calibrated ppm for the certain cations. For lead the equation was found to be y=.0205x, which then was used to calculate the ppm in solution from the absorbance. So, sample 1 had an absorbance of.083 which when plugged into the equation gave us the ppm in solution, so.083=.0205x which gives sample 4.05 ppm in solution. The same process was used for cadmium except the equation was found using the values specific to cadmium which resulted in the equation y=.088x. Using the absorbance reading of.072 for sample 3 the ppm was calculated as.072=.088x, which equals.818 ppm in solution for sample 3. To find the cations on soil we simply took the original

9 concentration, which was 100 ppm for lead and 10 ppm for cadmium and subtracted the ppm in solution. For sample 1, it originally had 100 ppm lead and 4.05 ppm were in solution so the cations on soil were ppm. Discussion In this experiment topsoil, organic soil, and urban soil are all tested to see which one has the highest retention of the metal cations lead and cadmium. Topsoil is the best at retaining lead. However, urban soil was close with 98.6% retention of lead cations compared to the 99.5% retention of topsoil. Organic soil had the highest retention of cadminum ions with 91.82% with topsoil having the second best retention of 89.9%. Moreover, lead cations were retained better in all the soils compared to cadmium cations. Overall, topsoil would be the best choice to use to line the rivers to prevent the harmful cations lead and cadmium from entering the water sources since it was the best and second best at retaining lead and cadmium ions respectively. Urban soil would also be a good choice because it has a 98.6% retention of lead and a 81.4% retention of cadmium. From a cost standpoint urban soil would be a cheaper alternative that is almost as equal in retention to the topsoil. After this experiment, it would make the most logical sense to start employing urban soil in riverbanks to retain the harmful cations from the water supply. Lead and cadmium are highly toxic metals, therefore it is important to know how these two cations are retained in soils (Halttunen T, 2007 ). This data is supported by information found out during REEL days 1 and 2. In REEL day 1 it was found that soils with different sand or clay contents have very different properties for retaining cations and filtering liquids. It was found that soils with higher clay content retained metal cations better. The experiment showed that urban soil had the higher clay content percentage, thus making it more likely to retain more cations than the other soil samples. Urban soil had 14.3% clay content, while topsoil and organic potting soil only had 3.3% clay content. The results from REEL day 1 did and did not support the results of the

10 experiment. The results showed that topsoil had the best retention percentage of the three soils for lead cations. However, urban soil was close with 98.6% retention of lead cations compared to the 99.5% retention of topsoil. REEL day 2 focused on the soil organic matter aspect of the different soils with their retention properties. The test for soil organic matter used EDTA and the color intensity of the extract, after shaking vigorously and filtered, was compared to the intensity of known standard samples. The results showed that topsoil had the highest SOM percentage, which suggests that it had the highest organic matter and would be the best at retaining cations. Organic potting soil followed with 3% and urban soil with 2%. The results from the experiment of REEL day 3 supported these findings in that topsoil did have the highest retention of the lead cation. But, organic soil had the highest retention of cadminum ions with 91.82%. Overall, the group s hypothesis was supported and refuted. For lead, topsoil was better at retaining the ions, but for cadmium, organic soil was better at retaining the ions. The discrepancies can be attributed to the fact that clay content and organic soil are both factors that can increase or decrease the retention percentage of the different soils and one factor cannot be attributed completely to the results. Both have a role and as shown by the results from the three days, multiple factor play a role in the retention and each factor can cause different soils to be better at retaining cations. Conclusions It was definitively determined that lead was better retained in the soil than cadmium; the lowest ppm of lead in the soil, 95.95% in organic soils, was still higher than the largest ppm of cadmium, 91.82% in organic soils. Management decisions should be made based off the knowledge that a proportionally larger amount of cadmium will enter waterways. The possible high positive charge of

11 lead, 4+, would be more attracted to negative clay particles, helping to explain the phenomenon high soil retention. The retention rate based on soil was less conclusive. Lead was best retained in topsoil, while cadmium was best retained in organic soils. It was predicted that a high CEC, being the measure of the maximum amount of ions a soil can hold, would equate to a high amount of metal-ion retention. Based on this measure, topsoil is the most attractive soil. A high clay content was predicted to be equated with higher metal ion retention, due to its negative charge and the soil s positive charge. However, urban soil, with % clay, was relatively poor in retaining ions. Between the two measures it seems CEC is the most predictive of ion retention. Overall, it was learned that no single factor completely determines ion retention, and that management decisions should be made based on the dominant ion present as well as soil properties. References Halttunen T T., Salminen S., and Tahvonen R. "Result Filters." National Center for Biotechnology Information. U.S. National Library of Medicine, 28 Feb Web. 02 Dec Ma, Guihua, and Georgina Wilson Gonzalez. "Flame Atomic Absorption Spectrometry."Flame Atomic Absorption Spectrometry. Andrea Dietrich, 10 Sept Web. 02 Dec