Investigation of Mercury Content in Vermicompost
Scott Harrelson, Holly Hatter, Paul Jordan, Emily Petsko, and Lara Spencer
EES 270: Environmental Chemistry
Project Advisor Dr. Elizabeth Malcolm
Methods
Samples of worms and vermicompost were collected from Dr. Rock’s compost bin located in Blocker Hall’s basement. Samples from the previous group’s experiment in 2019 were also used. They created vermicompost in eight bins, half of which had algae from campus stormwater ponds added. The 2021 worm samples were crushed with a mortar and pestle. All samples were analyzed by thermal decomposition and atomic absorption with a MA-3000. Percent recovery and percent replication of all the samples were within acceptable limits.
Discussion
The data shows no significant increase in the total mercury concentration with the addition of algae from the stormwater ponds. The samples reanalyzed show the 2019 data to be fairly consistent and demonstrate the samples were analyzed correctly. The Food Waste Only samples were higher in mercury content overall. This could be due to contamination in the base compost source. Another reason for this could be that the algae may have diluted the mercury concentrations in the compost. There were two significant outliers, one from Bin 4 as well as one from Bin 8. This could be attributed to the fact that compost is heterogeneous, so different concentrations of mercury can be found throughout the samples.
According to mercury regulations, the safe limit of mercury concentration in compost is 1 ppm (Brinton 2012). This is important to make sure compost is safe for growing crops. All of the 2021 compost samples were well below the safe limit to use for agriculture; however the study done in 2019 had many samples above the limit of which were mainly ones not containing algae. The 2021 worm samples were higher than the compost samples but below the 1 ppm safe limit. The mercury concentrations were higher in the worms than the 2021 compost samples. Panday et al. (2014) conducted a similar experiment with heavy metals found in vermicomposting, and their results showed that the worms had high levels of metals from the vermicompost. This could have happened in this experiment due to the fact earthworms can bioaccumulate mercury without affecting them (Panday et al. 2014).
References
Bernhoft, RA. 2012. Mercury toxicity and treatment: a review of the literature. J Environ. Public Health, 12:460508.
Brinton, WP. 2000. Compost Standards and Guidelines. Woods End Research Laboratory, Inc.
Environmental Protection Agency. Basic Information about Mercury. https://www.epa.gov/mercury/basic-information-about-mercury.
Kocasoy, G, Guvener, Z, 2008. Efficiency of compost in the removal of heavy metals from the industrial wastewater. Environmental Geology (57), 291-296.
Panday, R, Basnet, BB, Bhatt, SP, and Tamrakar, AS, 2014. Bioconcentration of Heavy Metals in Vermicomposting Earthworms in Nepal. Journal of Microbiology, Biotechnology and Food Science, 416-418.
Sander HA, Polasky S. 2009. Land Use Policy 26: 837-845.
Acknowledgments
We would like to thank Dr. Rock for providing the samples of compost and Dr. Malcolm for her involvement and guidance throughout this experiment. We would also like to thank the students from the previous study, James Garrett, Michael Harrison, Zach Hubbard, and Matthew Miller, for their research and compost samples.
Results
After reanalyzing the 2019 samples each twice more, it is apparent that the previous analyses were accurate. There were two outliers, one from Bin 4 and one from Bin 8. These outliers could have resulted due to the heterogeneity of the compost, the samples may have been taken from sections of the bins particularly high in mercury. The data was consistent except for two outliers, one from Bin 4 and one from Bin 8.
The 2021 compost and worm samples were all found to be below the 1 ppm total Hg threshold and lower than the 2019 compost concentrations. The 2021 compost was well below 1 ppm Hg, the highest sample being sample 2 at almost 0.25 ppm Hg. The worms were closer to 1 ppm Hg, but they were all below 0.7 ppm Hg.
Introduction
This research project investigated two questions: how does incorporation of stormwater pond algae impact the concentration of mercury in vermicompost, and are Blocker Hall vermicompost and worm mercury levels still high in 2021? For this research, compost samples from a previous study conducted in 2019 were re-analyzed, and compost and worm samples recently collected from the same source of the compost used for the previous experiment were analyzed.
The previous study evaluated the potential impact of using stormwater pond algae as vermicompost feedstock, including whether this would affect mercury concentrations in the finished compost. This is important because mercury is a heavy metal, toxic pollutant. Its sources are both natural, like volcanoes, forest fires, and anthropogenic, such as landfills, municipal waste incinerators, fossil fuel combustion, and mining (EPA). Mercury bioaccumulates in algae and plants in stormwater ponds. Algae in stormwater ponds is considered undesirable, so testing was done in order to tell whether the algae would make a suitable vermicompost feedstock. After vermicomposting, the study found the compost to be abnormally high in mercury. There were eight bins, half of which had algae added to them. The food waste only composts all ended with concentrations of higher than 1 ppm. So, further analysis of the samples was conducted.
To investigate the source of the high mercury in the 2019 compost, more samples of compost and worms were collected from the same source in Blocker Hall on Virginia Wesleyan University’s Campus. This study is important because it is necessary to check compost for unsafe levels of mercury. If the compost was to be used in agriculture, the mercury might bioaccumulate in the crops and negatively affect the health of the consumer. High levels of mercury can cause kidney damage, DNA changes, problems related to the nervous system, and death (Bernhoft 2012).
Null Hypotheses: The addition of algae from stormwater ponds to vermicompost will have no statistically significant impact on the concentration of mercury in the final compost. The mercury levels in the recently collected compost and worms will not be higher than 1 ppm.
Alternative Hypotheses: The addition of algae from stormwater ponds to vermicompost will increase the concentration of mercury in the final compost by a statistically significant amount. The mercury levels in the recently collected compost and worms will be higher than 1 ppm.
Figure 1. The concentration of total mercury from 2019 vermicompost experiment without (brown) and with addition of stormwater pond algae (green). The first three bars for each bin were analyzed in 2019 and the last two bars were analyzed during this study.
Figure 3. The concentration of total mercury from 2021 worm samples. Each sample analyzed in duplicate.
Figure 2. The concentration of total mercury from 2021 vermicompost samples. Each sample analyzed in triplicate.
Food Waste Only
Food Waste and Algae
Bin 2
Bin 4
Bin 6
Bin 8