Off with Their Lead! Removal of Heavy Metals Contaminating Water

Claire Fuller
Thomas Jefferson High School for Science and Technology

Despite the town’s proximity to one of the largest freshwater reservoirs in the world, drinking water was a scarce commodity in the town of Flint, Michigan. Tap water ran a vile, soupy brown throughout people’s homes, but the water’s most malevolent feature, which could not be detected by its unsuspecting drinkers, was dissolved throughout every drop. Heavy metals, including lead, were stripped from old pipes by the harsh pollutants in the Flint River and ended up in the water that residents used for showering, cooking, and drinking, preventing the Flint community from partaking in the ordinary activities that most Americans took for granted. Lead levels in individuals soared up to three times the amount documented before the crisis, resulting in detectable delays in mental and physical development in children, among other responses suggesting that the water was unsafe to drink [4].

Water contaminated by heavy metals is a problem that has always affected communities around the globe, including developed countries like the United States, as a result of industrialization, mine waste, and fertilizer runoff in the soil [6]. These metals are detrimental to the environment and often induce adverse health effects when consumed [1]. Many efforts to remove heavy metals from water through methods such as electrochemical treatment, precipitation, filtration, and ion exchange are too complicated and expensive to use on a large scale [1]. However, decontaminating polluted water is not as complex of a process as it seems. Biosorbents, which are natural, organic materials like rice husk, algae, and eggshells, possess the ability to remove heavy metal ions from solution by adsorbing these ions onto the biosorbent’s surface. This beneficial characteristic could provide a simplistic and cost-effective solution to the issue at hand, as it utilizes abundant resources and may be more effective at purifying water than more complicated treatments [2]. But the thought of biosorbents almost seems too convenient; why aren’t algae and other aquatic plants single-handedly purifying the bodies of water we depend on as our water source? The unfortunate truth is that biosorbents are by no means completely efficient. However, their adsorbance properties can be enhanced and may inspire models for new and improved artificial adsorbents.

Through my senior research project in the Chemical Analysis Research Lab, I have spent the last couple of months creating a material that adsorbs zinc, a heavy metal that is toxic in large concentrations and causes health issues, such as gastrointestinal problems, at concentrations exceeding the United States Environmental Protection Agency’s maximum contaminant level of 5.0 mg/L [5]. The adsorbent I am making mimics the natural biosorbent structure of corals and is made of aluminum oxide. Aluminum oxide has been extensively researched as an adsorbent material because of its prevalent adsorption sites, low cost, and high surface area, making it an excellent candidate as an adsorbent for heavy metals [7].

Artificial adsorbents can be improved to promote adsorption by altering their physical and chemical properties. Most notably, materials that have a higher surface area to volume ratio can adsorb more metals with a smaller amount of adsorbent. In a recent study, aluminum oxide adsorbents removed more mercury from water when the adsorbent was created with greater amounts of a liquid called ethylene glycol (EG) [7].  Xianbiao Wang, a professor from the School of Materials and Chemical Engineering at Anhui Jianzhu University in China, mentioned that “EG helps to produce curled nanoplates, leading to [a] higher surface area,” which accounts for the curled aluminum oxide’s greater adsorbance properties (personal communication, January 31, 2017). Wang et al. also further investigated producing adsorbents at a nanoscale level to increase the surface area to volume ratio and found that, although most aluminum-based adsorbents tended to clump together, which reduces the surface area available for binding, the curled, coral-like surface of the nanoadsorbent resists aggregation, allowing the binding sites to be exposed to the metals [7]. Similarly, another study conducted by Farnane et al. found that sodium hydroxide, a corrosive and very alkaline chemical, creates more pores in carob shell biosorbents, which leads to significantly higher removal rates of the heavy metals cadmium and cobalt because of the increased surface area [1].

Huang, Fulton and Keller from the University of California, Santa Barbara have recently looked into recyclable, magnetic adsorbents for removing metal contaminants and dangerous polycyclic aromatic hydrocarbons from water in 2016 [3]. When applying a magnetic field after sufficiently mixing a solution containing the magnetic nanoparticle adsorbents, the adsorbents gather close to the source of the magnetic field. This process allows for easy separation of the adsorbents that now contain the toxic particles from the cleaned water. Because of this property, the magnetic adsorbents can be isolated and recycled for reuse.

Heavy metals contaminating water will never cease to be a universal issue until a cheap and readily available method for removing them is established. Although removing heavy metals from water on a large scale is difficult to achieve, a world where safe drinking water is universally accessible might not be far away, thanks to the impactful research that is being done on adsorbents. Through more extensive research on synthesizing adsorbents and on analyzing their abilities to adsorb a multitude of metal ions and toxic organic molecules, perhaps someday scientists can combine the properties that enhance adsorbance abilities to create a more efficient adsorbent that can remove all toxic particles. Other properties, such as recyclability and effortless separation of the adsorbent from water, are also important factors to consider as adsorbent research progresses.

Imagine a world where crystal clear water is available to everyone. With the gain in momentum of research in adsorbents, a world like that just might be possible.

References

[1] Farnane, M., Tounsadi, H., Elmoubarki, R., Mahjoubi, F. Z., Elhalil, A., Saqrane, S., . . . Barka, N. (2017). Alkaline treated carob shells as sustainable biosorbent for clean recovery of heavy metals: Kinetics, equilibrium, ions interference and process optimisation. Ecological Engineering, 101, 9-20. http://dx.doi.org/10.1016/j.ecoleng.2017.01.012

[2] Garrison, N., Cunningham, M., Varys, D., & Schauer, D. J. (2014). Discovering New Biosorbents with Atomic Absorption Spectroscopy: An Undergraduate Laboratory Experiment. Journal of Chemical Education, 91(4), 583-585. http://dx.doi.org/10.1021/ed4004029

[3] Huang, Y., Fulton, A. N., & Keller, A. A. (2016). Simultaneous removal of PAHs and metal contaminants from water using magnetic nanoparticle adsorbents. Science of the Total Environment, 571, 1029-1036. http://dx.doi.org/ 10.1016/j.scitotenv.2016.07.093

[4] It's not safe to drink the water in Flint, Michigan, a city of nearly one hundred thousand northwest of Detroit [Television episode]. (2015, October 6). In S. Pelley & D. Reynolds (Producer), CBS Evening News. Retrieved from ProQuest database. (Accession No. 1796160252)

[5] PWS parameter information zinc. (2015, February). Retrieved from http://dwqr.scot/media/12203/pws-parameter-information-zinc.pdf

[6] Stietiya, M. H., & Wang, J. J. (2014). Zinc and Cadmium Adsorption to Aluminum Oxide Nanoparticles Affected by Naturally Occurring Ligands. Journal of Environmental Quality, 43(2), 498-506. http://dx.doi.org/10.2134/jeq2013.07.0263       

[7] Wang, X., Zhan, C., Kong, B., Zhu, X., Liu, J., Zu, W., . . . Wang, H. (2015). Self-Curled Coral-Like γ-Al2O3 Nanoplates for Use as an Adsorbent. Journal of Colloid and Interface Science, 453, 244-251. http://dx.doi.org/10.1016/j.jcis.2015.03.065