A recent article published in ACS Central Science introduced a bioinspired polymer system containing carbohydrates for efficient and selective removal of heavy metals. The researchers synthesized polymers with amphiphilic glucuronate side chains using ring-opening metathesis polymerization.
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Background
Clean drinking water is essential for public health, agriculture, industry, and environmental sustainability. However, contaminants, specifically heavy metal ions, significantly threaten water quality and human health.
Current industrial methods for removing these contaminants, such as chemical precipitation, sorption, and membrane filtration, suffer from drawbacks like low efficiency, high energy use, complex regeneration, and toxic sludge generation. Therefore, new water purification methods are needed.
Carbohydrates offer high adsorption capabilities, regeneration potential, and biocompatibility, making them ideal for removing heavy metals. Although polysaccharide hydrogels have shown promise, they often lack mechanical strength and can increase water hardness.
Thus, this study focused on developing a bioinspired, carbohydrate-containing polymer that precipitates rapidly after binding with heavy metals.
Methods
The proposed heavy metal sequestering system was based on glucuronic acid, which employs charge-switching to form filterable precipitates upon capturing heavy metals. Polymers with hydrophobic poly(norbornene) backbones were synthesized using ring-opening metathesis polymerization (ROMP) to enhance cation removal efficiency.
The polymerization capabilities of different monomers (glucose-based analogs (mono-C4-Glc), glucuronic acid derivatives (mono-C4-GlcA), and methyl-protected carboxylate (mono-C4-GlcA-Me)) were assessed through 1H nuclear magnetic resonance (NMR) spectroscopy.
Subsequently, three distinct polymers were synthesized via ROMP: one from glucose (C4-Glc), one from glucuronic acid (C4-GlcA), and a 50:50 copolymer of glucuronic acid and glucose (1:1). Their performance was compared to that of commercial hyaluronic acid (HA).
All polymers were characterized via size exclusion chromatography with multiangle light scattering (SEC-MALS) to identify the degree of polymerization and dispersity.
After polymer analysis, the methyl group from C4-GlcA-Me was removed using lithium hydroxide to create a fully deprotected polymer. The zeta potential of all polymers was then measured to evaluate their charge density and colloidal dispersion stability in a neutral aqueous solution.
The ability of C4-GlcA to bind divalent heavy metal cations was initially assessed with a colorimetric assay based on 4-(2-pyridylazo)resorcinol (PAR). Additionally, inductively coupled plasma mass spectrometry (ICP-MS) was employed to quantitatively assess the cation binding of the synthesized polymers.
Finally, the effectiveness of the proposed system was demonstrated with spiked water samples from the Colorado River in Austin, Texas.
Results and Discussion
Zeta potential measurements revealed the colloidal dispersion stability in a neutral solution and the charge density of the synthesized polymers. While C4-Glc had an almost neutral zeta potential, all charged polymers (1:1, HA, and C4-GlcA) displayed significantly negative zeta potentials. Notably, C4-GlcA underwent charge switching with varying protonation states.
Transmission electron microscopy validated the nanoparticulate behavior of C4-GlcA and its complete solubility in water in the absence of Cd2+ ions, representing its ideal state for trapping ions in solutions. Alternatively, the addition of Cd2+ (100 μM) to a C4-GlcA (1.0 mg/mL) solution led to the precipitation of large aggregates, exhibiting proficient binding and trapping potential of C4-GlcA.
In initial absorption tests with the PAR colorimetric dye, C4-GlcA effectively removed most metals from the solution, while C4-Glc did not. The 1:1 polymer and HA showed similar Cd2+ removal abilities but were significantly less effective than C4-GlcA, indicating that increasing charge density is crucial for optimal heavy metal removal.
In the ICP-MS measurements using solutions with at least 5 μM K+, 40 μM Na+, and 3 μM Ca2+, C4-GlcA exhibited extraordinarily high binding affinity for several divalent ions, including Mn2+, Cu2+, Cd2+, Ni2+, Zn2+, Ba2+, and Pb2+. More than 96 % of the cations were removed from each tested solution, with Cd2+ experiencing the maximum sequestration (99.84 %).
Additionally, C4-GlcA exhibited superior binding efficiency for iron ion (Fe3+) with 99.36 % removal. Moreover, C4-GlcA demonstrated efficient sequestration of Pb2+, Cd2+, and Ca2+ from spiked water samples from the Colorado River without increasing water hardness.
Conclusion
The researchers successfully synthesized bioinspired, carbohydrate-containing polymers that demonstrated high efficiency and selectivity in capturing substantial concentrations of Cd2+, Pb2+, and other heavy metals from aqueous media. These polymer systems showed promising performance even with non-optimized parameters for polymer concentration and degree of polymerization.
The C4-GlcA system achieved over 99 % removal efficiency with just three minutes of contact time. Additionally, its pH-responsive nature allowed for easy regeneration and reuse, enabling reversible capture and release of heavy metals through pH adjustments. This approach can help reduce energy consumption and chemical sludge production during water purification, without increasing water hardness.
Journal Reference
Jeon, S., Haynie, T., Chung, S., Callmann, CE. (2024). Bioinspired, Carbohydrate-Containing Polymers Efficiently and Reversibly Sequester Heavy Metals. ACS Central Science. DOI: 10.1021/acscentsci.4c01010, https://pubs.acs.org/doi/full/10.1021/acscentsci.4c01010
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