Removal of Copper (II) Ions from Contaminated Water by Encapsulation of Peppermint Tea Leaves in Alginate Beads



Lakes and rivers have always been part of our lives, but our society’s obsession with busy city life often leads us to forget and neglect the complex life under the waters. Beneath the surface, thousands of different organisms live together in an ecological harmony. This ecosystem is perhaps the least interrupted by human life in our city. Nonetheless, human activities still influence the lives of aquatic organisms, often threatening to disrupt their harmonious existence. Waste discharge and the incorrect disposal of hazardous materials cause harmful chemicals to build up, damaging the biological community. This environmental damage has dangerous effects on people, too, since we consume the fish that live in these rivers and lakes. By biological magnification, this means that the toxic chemicals that accumulate in fish are ultimately consumed by people (Liu).

Heavy metals, known for their highly toxic properties, pose an invisible but serious threat to these watery ecosystems. Human activity causes a great deal of heavy-metal water contamination. In fact, lead concentration in areas affected by humans is 20 times greater than in unpolluted regions not directly impacted by human activity (Athar). Thankfully, there are several laws and regulations designed to reduce effluence with heavy metals. The Environmental Protection Agency, a federal institution tasked with monitoring and controlling the emission of pollutants into the environment, has established the maximum allowable emissions for heavy metals as: lead 15 ppb, copper 1.3 ppm, mercury 2 ppb, cadmium 5 ppb, and chromium 100 ppb (EPA). Copper is the most prevalent metal, as it is used in industrial production, metal mechanic factories, and even food production. Although tiny amounts of copper are essential for human health, excess amounts can cause adverse health effects, including nausea and gastrointestinal problems (WebMD). Despite the laws created to limit pollution, excess copper still exists in many bodies of water, making it imperative that we find ways to safely remove it.

Copper has always been a target of remediation. Current methods for reducing copper in our lakes and rivers include activated sludge, chemical precipitation, reverse osmosis, and ion exchange, all of which can be effective in removing copper from aqueous systems (Akpor; Volesky). Unfortunately, these highly specific methods cannot be used on large bodies of water due to the high cost of these processes and the sheer volume of water involved (Liu; Chojnacka; Navarro). Moreover, some of these processes produce chemical byproducts that may threaten the organisms that inhabit these ecosystems. So far, the most promising technique is bioremediation, which uses biological systems to remove pollutants. Compared to other methods, it has a lower cost and creates minimal amounts of chemical byproducts.

I have been fortunate enough to be able to conduct bioremediation research as part of an internship with Dr. Abel Navarro, an assistant professor of chemistry at Borough of Manhattan Community College. Under Dr. Navarro’s supervision, I actively conducted research on using inexpensive biological materials like tea leaves to adsorb organic compounds from aqueous systems. Surprisingly, some common ingredients we consume daily showed high adsorption of the organic compounds. Among the different types of tea leaves tested, peppermint tea leaves showed the best rates of adsorption. The question was whether or not this type of tea leaf could also adsorb copper. During the project, Fourier transform infrared spectroscopy (FTIR) studies revealed the prevalence of different functional groups, including carboxyl groups and hydroxyl groups on the surface of the leaves. These functional groups are the main contributors of the adsorption. Because carboxyl groups have a negative charge, there is a good chance that the tea leaf will be a natural copper biosorbent. However, using peppermint tea leaves also has some drawbacks. Not only is it difficult to separate the leaves from the water due to their size, but peppermint is also easily affected by other substances present in lake and river water. To produce an ideal copper adsorbent, we would have to prepare a matrix that could immobilize the peppermint tea leaves while at the same time maintaining its adsorption capabilities.

Alginate beads (AB) are porous materials that form spontaneously when sodium alginate solution reacts with calcium chloride. AB is also known to be a great biosorbent due to its several potential adsorption sites (such as carboxyl functional groups). Because the lab I worked for was interested in the same bioremediation projects as I was, we were able to run experiments testing alginate bead and its effectiveness in adsorbing copper. Surprisingly, the alginate beads were able to adsorb even more than the peppermint tea leaves did, possibly due to their high porosity. Their structure and adsorption mechanisms meant that alginate beads were barely affected by the crowding agents that mimicked the environmental water (Navarro). Peppermint tea leaves proved to be the best adsorbent in adsorbing organic compounds, while the alginate beads proved both to adsorb well and to be usable in the peppermint tea leaf matrix.


Research Question

Can alginate-immobilized peppermint tea leaf be used as an adsorbent of copper ions from wastewaters?



Immobilization of peppermint tea leaf in alginate beads will show much higher copper adsorption at different experimental conditions.



Preparation of the Adsorbents

Peppermint teabags were purchased from a local market. Teabags were soaked in boiling distilled water to eliminate residues of color, smell, and taste and placed in an oven at no more than 50°C for overnight drying. Then the teabags were cut open and stored in plastic containers until it was time to use them in adsorption experiments. 

Peppermint mixed with sodium alginate.

Click to enlarge

Alginate beads were prepared by a well-known procedure. In brief, 4.5 g of sodium alginate were dissolved in 200 mL of deionized water with magnetic stirring. On the side, a calcium chloride solution was prepared by mixing 33 g of the salt with 1.5 L of distilled water, also under magnetic stirring. Then the alginate solution was slowly dropped into the calcium solution. Alginate beads formed immediately in the calcium solution. Lastly, the beads were thoroughly rinsed with distilled and deionized water to eliminate residues of calcium and chloride ions. The alginate beads were suspended in deionized water in a glass bottle and stored in the refrigerator (Navarro).


The encapsulation of the spent peppermint leaves in the alginate beads (ABPM) followed the same procedures as the preparation of alginate beads. For the encapsulation, different masses of the peppermint tea leaves, ranging from 1 to 4 grams, were mixed with the alginate solution. An optimum alginate/tea leaf ratio maximized the number of particles in the bead without disturbing the porosity of the alginate bead. The optimum alginate/tea leaf ratio was visually determined by the homogeneous distribution of the leaf particles in the bead.


Creating ABPM hybrids by dropping sodium alginate and peppermint tea leaves into calcium chloride (left). The peppermint leaves, alginate beads, and ABPM used for the experiments (right)


Wet-Dry Mass Conversion

Alginate beads are hydrogels and thus contain small amounts of water, which could lead to the misinterpretation of the mass of the actual materials inside of them. This was fixed by preparing a calibration curve that converts the wet mass of the beads (or hybrids) into the dry mass. Adsorption calculation was done based on the dry mass; therefore, its determination was crucial.

The mass of different sets of wet beads was recorded and then the beads were taken to the oven for drying at 50°C. Upon drying, the dry masses were recorded and a calibration curve was constructed.


Characterization of the Adsorbents by Scanning Electron Microscopy (SEM)

The texture and morphology of the adsorbents (PM, AB, and hybrids) were explored under a scanning electron microscope. This device takes pictures with high zoom and resolution without the need of gold coating. Samples were directly placed in the sample holder, and micrographs were taken for their comparison.


Adsorption Experiments

The incubator shaker.

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Batch experiments were carried out at room temperature, in duplicates, by mixing variable masses of the three adsorbents with pH-adjusted and known concentrations of copper solutions in 50 mL plastic tubes. The suspensions were placed in a shaker under orbital agitation. This procedure followed different changing parameters. These parameters included: pH (from 2 to 7), adsorbent mass (ranging from 20 to 300 mg of adsorbent), crowding agent (from 2 to 10% m/m polyethylene glycol), salinity (sodium chloride, sodium nitrate, and calcium nitrate), presence of another heavy metal (from 20 to 100 mg/L of lead), and the presence of organic pollutants (from 20 to 100 mg/l of a dye). These parameters determined the optimum conditions at which the adsorption of copper ions is maximized. These parameters were explored one at a time to avoid misinterpretation of the data.

The microplate reader used to read data.

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Upon reaching equilibrium, the solution was separated from the adsorbents by gravity, and the remaining copper concentration was quantified by spectrophotometry. 

The initial and final copper concentrations were quantified by UV-vis spectrophotometry using the Zincon Method at a 600-nm wavelength. In this procedure, a blue-colored complex is produced by mixing the copper solutions with the same number of moles of zincon (a dye). The measurements were done using an automatized microplate reader. 

A blank sample was always prepared with the same conditions as a real sample but without the adsorbent. This blank sample was used for the initial copper concentration determination and as a control for any external factor that might affect the adsorption.

Data Analysis

The amount of copper adsorbed was expressed as adsorption percentage. With the initial copper concentration (Ci) and final copper concentration (Cf), the following equation was used (Navarro): % ADS = (Ci – Cf) * 100/Ci, where % ADS is the adsorption percentage.



As expected, ABPM hybrids showed superior adsorption rates than PM or AB by themselves. 

Effect of pH

ph effects

Graph of copper adsorption percentage by PM, AB, and ABPM at different levels of pH.

The pH experiments allowed me to determine the optimum pH at which copper adsorption is maximized. The closer the pH is to the 5-7 range, the better the adsorbent, since most bodies of water have a pH within that range. In the experiment, ABPM showed the highest adsorption at pH 6 when compared to both alginate beads and peppermint leaves.

PH can easily affect adsorption, as the free hydrogen ions can attach onto the surface of the absorbent where the Cu+2 ions are supposed to bind. Therefore, pH 6 allows the adsorbents to adsorb as much Cu+2 as they can.

Effect of Mass of Adsorbent


mass effects

Graph of copper adsorption percentage of PM, AB, and ABPM with different masses.

The goal of the mass experiment was to determine the optimum ratio of mass of adsorbent and copper concentration. As I add more adsorbents, the adsorption percentage increases, but at a certain point, adding more absorbents does not increase the adsorption. The experiment showed that compared to alginate beads and especially peppermint leaves, ABPM hybrids reached the optimum mass very quickly, at 50 mg.

Effect of Crowding Agent



Graph of copper adsorption percentage of PM, AB, and ABPM under different percentages of polyethylene glycol.

In real aquatic systems, water is not as pure as distilled water. There are thousands of microorganisms as well as different compounds that might affect the adsorption of adsorbents. To mimic these external factors, polyethylene glycol was used as a “crowding agent.” Again, ABPM showed much higher adsorption compared to alginate beads alone. Peppermint, due to its structure as well as its adsorption mechanisms, was heavily affected by the polyethylene glycol and showed little adsorption.

Effect of Salinity



Graph of copper adsorption percentage of PM, AB, and ABPM under different salts.

Another difference between the distilled water used for experiments and the water in aquatic ecosystems is the difference in salinity. Most lakes and rivers contain salts. Sodium chloride (NaCl) and sodium nitrate (NaNO3) were used as salts. Calcium nitrate (Ca(No3)2) was used particularly because it disassociates into Ca+2, a positively charged ion with a +2 charge, which is the same as Cu+2. The first two salts barely affected the adsorption of copper, but calcium nitrate greatly decreased the adsorption onto ABPM. The calcium ions attach onto the surface of the adsorbents instead of the copper ions, preventing the adsorption by competition.

Effect of Heavy Metals



Graph of copper adsorption percentage of PM, AB, and ABPM under different concentrations of lead.

Although the above adsorbents have proven themselves able to remove copper, I wanted to know what would happen if there were other heavy metals present inside the aqueous solutions, as there would be in real life. The experiments were done at different concentrations of lead as a secondary metal. As lead concentration increased, the adsorption rates decreased. However, ABPM still showed its dominance compared to the other adsorbents. This shows that it remains the most effective adsorbent even when different heavy metals are present.

Effect of Organic Dyes



Graph of copper adsorption percentage of PM, AB, and ABPM under different concentrations of yellow dye.

In aquatic systems, if there are copper ions, it is likely that there are also organic pollutants such as dyes present. My prior research in the lab had shown that organic dyes adsorb onto the surfaces of alginate beads and peppermint, suggesting that the adsorbents could prefer the organic compounds to the heavy metals. However, the results seen in Figure 12 show that the ABPM still prefers copper to yellow dye because copper has a +2 charge, meaning it has a stronger affinity with the carboxyl groups on the surface.

Scanning electron microscope photos


Scanning electron microscope photos of ABPM.


As shown above, the surface of the alginate bead is highly porous. Peppermint tea leaves are also visible in the picture. Because they are on the surface in this image, it means they were also effective in the adsorption process.


My results showed that my hypothesis was supported. Throughout the experiments, copper adsorption followed the trend ABPM > AB > PM. Since ABPM’s optimum pH is 6, it can be applied to almost any natural aquatic body of water. It also has a relatively low optimum mass, allowing efficiency and low-cost methods. Even in the presence of crowding agents like polyethylene glycol, ABPM was shown to adsorb copper without being affected by it. Common salts such as NaCl slightly affected the adsorption rates, while experiments that factored in organic dyes and heavy metals still showed that the ABPM compound prefers copper to other compounds. Thus, ABPM is both a low-cost and an effective adsorbent to remediate copper from wastewaters. In the future, further research should be done on how ABPM can be modified to increase its effectiveness.



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