Environment

The Adsorption of Heavy Metals and Phosphates from Stormwater Using Waste Oyster and Crab Shells

Abstract

Each year, 6-8 million tonnes of waste shells are discarded, neglecting a valuable resource. The purpose of this experiment was to tap into this resource and determine which shell would most effectively adsorb pollutants from stormwater.  The hypothesis: if different types of shells are tested to clean stormwater, the kiln-heated crab shells will adsorb the heavy metals and phosphates from the water most effectively.  Half of the oyster and crab shells were heated for one hour at 315°C. The filter groups: unheated crabs, kiln-heated crabs, unheated oysters, and kiln-heated oysters, were put into PVC pipes.  Polluted water was dripped through them. Except for the unheated crab filter group, copper levels were significantly lower in all filter groups compared to the control group. The kiln-heated oyster was most effective, removing 83% of copper.  For zinc, there were significant differences between the control group and all filter groups. The unheated oyster was most effective, removing 96% of zinc. Both oyster filter groups significantly lowered phosphates compared to the control. The unheated oyster filter removed 96% of phosphates.  In conclusion, all shell filters performed very well in adsorbing pollutants, and the oyster filters appear to be the most effective.

 


Introduction

The health of the Chesapeake Bay is steadily declining as pollutants from sources such as road salts, fertilizers, car tires, roofing materials, and car oils continue to pour into its waters.  These pollutants include heavy metals, specifically copper and zinc, and phosphates, which are examined in this experiment. Heavy metals, even at very low concentrations, have been found to accumulate in the food chain and cause serious health hazards, both for humans and the environment.[1]  Excess phosphates create dangerous algal blooms, something currently plaguing the Chesapeake Bay.  Our community is in desperate need of an environmentally friendly and economically feasible way to address this issue.  Currently, there are piles of waste oyster shells and crab shells going unused. Due to their shell structure and content of chitin and calcium carbonate, they have been shown to adsorb heavy metals and phosphates from water.[2]

Adsorption is the process in which an object, the adsorbate, adheres itself to the surface of another object, the adsorbent.  However, all of the current studies on shell adsorption flowed polluted water through the shells at very slow rates. Sometimes, the water would sit in the shells for hours.  The fastest water has flowed through 150 grams of waste shells was a rate of 200 ml per hour.[3]  Unfortunately, this speed is unrealistic for a real-life application.  Instead, this experiment chose to run the 250 ml of polluted water through about 150 grams of waste shells in less than 5 minutes, more than 14 times faster than previous experiments.  By running the water through faster, this experiment assesses the viability of using these shells in the real world. Additionally, although it previously was believed that chitin was the main aspect of shells that helps them adsorb pollutants[4], new research seems to suggest that it may be more than simply chitin such as ion exchange[5] or shell structure.[6]  Thus, this experiment chose to look at crab shells and oyster shells as crab shells contain 27% chitin[7] and oyster shells contain 0.1-1% chitin.[8]  By using these two shells with drastically different chitin content, it can then be determined whether or not chitin is the main component for improving adsorption.  Finally, most current research heats shells >750°C to convert the calcium carbonate (CaCO3) in the shells into calcium oxide (CaO).[9]  However, heating shells to such high temperatures uses a substantial amount of energy and is not as environmentally friendly.  Instead, this experiment examines whether burning shells by heating them at 315°C for one hour could improve reactivity compared to nonheated shells.  This experiment considers three different components: first, can waste shells adsorb pollutants effectively when water is run through them at faster speeds, and, if so, which filter is the most effective?  Second, does heating shells to 315°C improve reactivity? Third, is chitin the main component in shells that help them adsorb pollutants?

 

Methods and Materials

Three hundred fresh Crassostrea virginica (eastern oyster) shells and 300 fresh Callinectes sapidus (blue crab) shells were obtained from local restaurants, scrubbed and cleaned with water, and dried outdoors for three months to remove any remaining flesh or algae on the shells.  Half of the oyster shells and half of the crab shells were burned in a kiln for an hour at 315°C (Figure 1).

Figure 1: Kiln used to heat shells

All of the shells were crushed into a dust-like consistency with a sledge hammer.  Shell dust was poured into PVC pipes 1.9 centimetres in diameter and 30 centimetres in length.  Pipe dimensions were based on those of previous research.[3]  A coffee filter and landscaping fabric were attached at the bottom of the pipe with rubber bands to prevent the shell dust from falling into the collected water.  Ten empty PVC pipes with a coffee filter and landscaping fabric attached at the bottom were used as the control group for the study. The control was established this way to ensure that any removal of pollutants by the coffee filter or landscaping fabric would be the same for all groups and, thus, not impact the results of the study. Wooden test tube racks were constructed to hold the PVC pipes, and glass jars were placed beneath the pipe (Figure 2). 12.5 l of water was polluted with 1 gram of copper sulphate, 1 gram of zinc sulphate, and 1 gram of sodium phosphate.  Using a 10 ml pipette, 250ml of polluted water was dripped through each pipe and collected in a jar at the bottom for testing (Figure 3). The volume of polluted water and the concentrations of pollutants were based on recommendations from previous research.[3]

Figure 2: Experimental setup

Figure 3: Treated water dripping into jars for testing

Heavy metals were tested using an ICP-AES machine (Figure 4). An ICP-AES functions by running an aqueous sample through plasma. When exposed to plasma, the electrons in the heavy metals become excited. After the electrons relax, they return to their original state (ground state). This process of reverting back to ground state releases light in the form of photons, which the machine reads and uses to determine the type and concentration of each element. Phosphates were tested using a colorimeter, which used the phosphates (LR) test. A colorimeter functions by running wavelengths of light through a sample treated with a specific reagent. The colorimeter then assess the intensity of a specific wavelength of light that permeates the sample to determine the concentration of a pollutant. The pH levels were determined with pH test strips.  Data was analysed using means, standard deviations, and Student’s t-test (Figure 5).[10]  Test results with a p-value <0.05 were considered statistically significant.  P-values were calculated on Microsoft Excel.

Figure 4: ICP-AES used to test for heavy metals

 

Figure 5: Formula Used to calculate Student’s t-test

     

    

Results

Copper

Figure 6: Copper percent change, comparing each filter group to control

 

Table 1: ICP-AES copper measures among study groups

Filter group Sample size Mean Standard deviation
Control 4 191.875 19.2889
Unheated crab 4 663.875 390.6003
Kiln-heated crab 4 70.250 23.6661
Unheated oyster 4 62.625 28.81081
Kiln-heated oyster 4 32.875 8.7785

 

The copper values seen in the table are not the concentrations but instead values calculated by the ICP-AES from the light produced by the sample.  A linear equation can be used to estimate the concentration that is associated with the value generated by the ICP-AES, but copper concentrations were not calculated for the current study due to the fact that the study was focused on the ability of waste shells to reduce pollutants compared to the control.  The actual concentration of the pollutant was not as important. Furthermore, due to low concentrations of copper, an extrapolation of the data would provide inaccurate estimates. Instead, the numbers produced by the ICP-AES were used as accurate points of comparison when determining the effectiveness of filters, as seen by previous studies.[11]  Sample sizes were reduced to four due to the limitations of time in which the experimenter was able to use Mary Washington’s ICP-AES.

Table 2: Test results comparing treatment groups to control for copper

Group comparisons t-test statistic Degrees of freedom p-value
(1) Unheated crab v. control 2.414 6 0.0523
(2) Kiln-heated crab v. control 7.968 6 0.0021
(3) Unheated oyster v. control 7.456 6 0.0030
(4) Kiln-heated oyster v. control 15.005 6 <0.0001

 

Three of the four filter groups made significant improvements in copper levels compared to the control filter (p<0.0030).  The kiln-heated crab filter reduced copper by 63%, and the unheated oyster filter reduced it by 67%. The kiln-heated oyster shell filter was the most successful at removing copper, decreasing the concentration 83% compared to the control.  The treatment groups in comparisons 2-4 showed statistically significant reductions in copper. The large increase in copper in the unheated crab filter may be due to food handling and other conditions in the restaurant from which the crabs were obtained.

 

Zinc

Figure 7: Zinc percent change, comparing each filter group to control

 

Table 3: ICP-AES zinc measures among study groups

Filter group Sample size Mean Standard deviation
Control 4 2281.125 255.4419
Unheated crab 4 535.625 514.6946
Kiln-heated crab 4 237.875 35.87798
Unheated oyster 4 98.250 30.43709
Kiln-heated oyster 4 321.500 27.53785

 

As discussed in the copper results, zinc concentrations also were not calculated for the same reasons.  Similarly, the actual concentration of the pollutant was not important, and the numbers produced by the ICP-AES were used as accurate points of comparison when determining the effectiveness of filters. Additionally, samples sizes were reduced to four due to the experimental constraints discussed in the copper analysis section.

 

Table 4: Test results comparing treatment groups to control for zinc

Group comparisons t-test statistic Degrees of freedom p-value
(1) Unheated crab v. control 6.070 6 0.0009
(2) Kiln-heated crab v. control 15.842 6 <0.0001
(3) Unheated oyster v. control 16.971 6 <0.0001
(4) Kiln-heated oyster v. control 15.255 6 <0.0001

 

Results from the data show a significant difference in reducing the amount of zinc between the control group and each of the shell filter groups (p<0.0009).   Out of the different shell filters, the unheated oyster filter was the most successful, removing 95.69% of zinc compared to the control. The kiln-heated shells were more successful for the crab filters but less successful for the oyster filters.

 

Phosphates

Figure 8: Phosphate percent change comparing each filter group to control

 

Table 5: Phosphate among study groups

Filter group Sample size Mean (ppm) Standard deviation
Control 10 0.824 1.042
Unheated crab 10 0.768 1.28
Kiln-heated crab 10 0.230 0.35
Unheated oyster 10 0.070 0.15
Kiln-heated oyster 10 0.100 0.28

 

Table 6: Test results comparing treatment groups to control for phosphate

Group comparisons t test statistic Degrees of freedom p-value
(1) Unheated crab v. control 0.107 18 0.9160
(2) Kiln-heated crab v. control 1.707 18 0.1050
(3) Unheated oyster v. control 2.260 18 0.0365
(4) Kiln-heated oyster v. control 2.120 18 0.0482

 

Modest significant differences were found between the control and unheated oyster (p=0.0365) and between the control and the kiln-heated oyster (p=0.048).  The unheated oyster shell filter was the most successful at removing phosphates, decreasing the concentration 95.69% compared to the control. Although the crab shell findings are inconclusive, the results seem to suggest that the kiln-heated crabs were more effective than the unheated crab shells at adsorbing phosphates.  However, the unheated oyster shells were more effective than the kiln-heated oyster shells at adsorbing phosphates.

 

pH

Figure 8: pH levels for each filter group

 

Table 7: Test results comparing treatment groups to control for pH

Group Comparisons T test statistic d.f. p-value
(1) Unheated crab v. control * due to the lack of variation in the data, a T test statistic could not be calculated. This is because a zero would be in the denominator of the T test equation. EPA safe levels were achieved. n/a
(2) Kiln-heated crab v. control 4.583 18 0.0002
(3) Unheated oyster v. control 3.872 18 0.0011
(4) Kiln-heated oyster v. control 4.583 18 0.0002

 

There were significant differences in neutralising pH between the control and each of the different shell filters (p<0.0011).  The most successful shell filter for neutralising the pH was the unheated crab filter, which had a mean pH of 7 compared to the control mean pH of 6.  The unheated shell filters consistently performed better than their kiln-heated counterparts. Both crab shell filters were more successful at neutralising the water than the oyster filters.

 

 

Discussion

The oyster shells generally performed better than the crab shells in reducing copper, zinc, and phosphate.  However, both unheated crab shells and kiln-heated crab shells neutralised pH better than the oyster shells.  This finding may be due to the fact that oysters are sedentary creatures while crabs move around more. Thus, crabs may have developed the ability to handle different pH levels while oysters might have not.  Additionally, the kiln-heated crab shell filter noticeably performed better than the unheated crab shell filter in all endpoints. The unheated oyster shell filter was moderately more successful than the kiln-heated oyster filters for all endpoints except for copper.

From the observed data a few suggestions and questions emerge.  The oyster shell filter consistently performed better than the crab shell filter, except for pH.  This outcome further validifies the hypotheses made by Kim, Park[5], and Tudor[6] that chitin may not be the main component involved in adsorption. These results may be due to a few possibilities.  Calcium carbonate, which makes up around 80 – 95% of oyster shell material[12] and only about 60% of crab shell material[13], could be a more effective adsorbent that chitin.  These results do not specifically indicate if calcium carbonate was responsible for the difference any more than shell structure or ion exchange was responsible for these results as suggested by Kim, Park[5], and Tudor[6].  More research would need to be done to draw these specific conclusions.  Additionally, this study supports the conclusion that kiln-heated crab shell filters were more effective than unheated ones.  However, this conclusion cannot be made for oysters. A reason for this might be because crab shells are very thin and thus were fully burned while the oyster shells are thicker and were burned only on the surface level (Figure 10).  More research would need to be conducted to understand whether kiln-heated oyster shells are better than unheated oyster shells at adsorption of pollutants. Despite these arguments, the current study shows that heating shells up to 315°C may be a completely novel approach to greatly increase adsorption capabilities while being more environmentally friendly.

        


Figure 10: A comparison of the fully burned crab shells v. the surface level burned oyster shells

Across the world, the seafood industry discards its waste shells, which end up in landfills.  As the current research demonstrates, these shells have great potential in cleansing waste water and, specifically, stormwater.  All contaminants tested are common pollutants from urban runoff. These shell filters could be utilised to help stabilise contaminant levels. This research is beneficial to local authorities because by utilising waste shells, a contaminated watershed, such as the Anacostia River, could achieve safer levels of pollutants in water.  Additionally, these results are beneficial to industrial water filtration companies and local authorities who have access to waste shells. These shells could be put in industrial water plants or simply in storm drains. Stores and homes worldwide also could use shells instead of rocks in landscaping, helping to remove some pollutants from stormwater.

 


Conclusions

As addressed in the introduction, this experiment addressed three objectives.

  1. Can waste shells adsorb pollutants when polluted water is run through the shells at fast speeds, and, if so, which is the most effective?  
    The results of the current study indicate that waste shells, both crab and oyster, adsorb pollutants from water, even when the water was run over 14 times faster than previous studies.  This faster flow rate was more reflective of real world conditions. The data also indicates that oyster shells in general are more successful than crabs when it comes to adsorption.
  2. Does heating shells to 315°C improve reactivity?  
    The crab shells’ performance suggests that heating shells only to 315°C can greatly improve their adsorption rates.  The oyster shells do not show the same relationship as the unheated oyster shell group generally performed better than their kiln-heated counterpart in adsorbing zinc and phosphate.
  3. Is chitin that main component in shells that help the adsorb pollutants?  
    This research seems to suggest that chitin is not the main component in shells that helps them adsorb pollutants.  The oyster shell with a very low chitin content continued to perform better than the crab shell with a higher chitin content.

 

Acknowledgements

The experimenter would like to thank Sea Pearl Restaurant and Captain Pell’s Crab House for  providing 300 waste oyster shells and 300 waste crab shells, Professor Charles Sharpless at the University of Mary Washington who gave up time and resources to teach the experimenter how to operate the ICP-AES that was used for heavy metal testing, art teachers Amber Westphal and Polly McCoy who provided supervision in the heating of shells, and chemistry teacher Kayleigh Cox who supervised the use of a colorimeter for phosphate testing.

 

References

  1. An, H.K.; Park, B.Y.; Kim, D.S. “Crab shell for the removal of heavy metals from aqueous solution,” Water Research 35, no. 15 (2001): 3551-3556, https://doi.org/10.1016/S0043-1354(01)00099-9.
  2. Yan-jiao, G. “Cadmium and cobalt removal from heavy metal solution using oyster shells adsorbent.” Proceedings of the International Conference on Consumer Electronics, Communications and Networks, IEEE Foundation, pp. 1098-1101, 2011,  https://doi.org/10.1109/CECNET.2011.5769384.
  3. Craggs, R.; Cooke, J.; Mathieson,T.; Park, J. “Potential of mussel shell as a biosorbent for stormwater treatment.” NIWA Client Report, HAM, 2006.
  4. Chui, V.W.D.; Mok, K.W.; Ng, C.Y.; Luong, B.P.; Ma, K.K. “Removal and recovery of copper(II), chromium(III), and nickel(II) from solutions using crude shrimp chitin packed in small columns.” Environment International 22, no. 4 (1996): 463-468,  https://doi.org/10.1016/0160-4120(96)00034-7.
  5. Kim, D.S. & Park, B.Y. “Effects on the removal of Pb2+ from aqueous solution by crab shell.” Journal of Chemical Technology and Biotechnology 76, no. 11 (2001): 1179-1184.
  6. Tudor, H.E. “Detoxification of metal contaminated industrial effluents using shellfish processing waste.” Ph.D. Thesis. Columbia University, 1999,  https://doi.org/10.1002/jctb.505.
  7. No, H.K. & Meyers, S.P. “Preparation and characterization of chitin and chitosan.” Journal of Aquatic Food Product Technology 4, no. 2 (1995): 27–51, https://doi.org/10.1300/J030v04n02_03.
  8. Goffinet, G. & Jeuniaux, C. “Distribution et importance quantitative de al chitine dans les coquilles de mollusques [Distribution and quantitative importance of chitin in mollusk shells].” Cahiers de Biologie Marine 20(1979): 341-349.
  9. Ok, Y.S.; Oh, S.E.; Ahmad, M.; Hyun, S.; Kim, R.K.; Moon, D.H.; Lee, S.S.; Lim, K.J.; Jeon, W.T.; Yang, J.E. “Effects of natural and calcined oyster shells on Cd and Pb immobilization in contaminated soils.” Environmental Earth Sciences 61, no. 6 (2010): 1301–1308, https://doi.org/10.1007/s12665-010-0674-4.
  10. Mathematical Statistics with Application, Edited by Michael Payne.  PWS-KENT, 1981.
  11. Yavuz, O.; Guzel, R.; Aydin, F.; Tegin I.; Ziyadanogullari, R. “Removal of cadmium and lead from aqueous solution by calcite.” Polish Journal of Environmental Studies 16, no. 3 (2006): 467-471.
  12. de Alvarenga, R.A.; Galindro, B.M.; Helpa Cde  F.; Soares, S.R. “The recycling of oyster shells: an environmental analysis using Life Cycle Assessment.” Journal of Environmental Management 106 (2012): 102-109, https://doi.org/10.1016/j.jenvman.2012.04.017.
  13. Leffler, Merrill. “Treasure from Trash: Is There Profit in Crab Waste?” Maryland Sea Grant, 1997.
  14. “Lower Anacostia River Fact Sheet.” EPA, 2007.

Cover image: Paul Wilkinson, “Oyster Shells, Whitstable”, March 18, 2012,
https://www.flickr.com/photos/eepaul/7006609849.

All images by author, unless otherwise specified.

 

About the Author

Clare Sparling, USA

Clare Sparling is a junior at Oakton High School in Vienna, Virginia, USA. She has a passion for life sciences, animals, and the environment. Outside of her research, Clare enjoys volunteering in her community as well as spending time with friends and family.

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