A Brief Background on Clean Water
Contamination of water streams is a growing problem, in particular as a majority of the world population is at risk of water scarcity. One contributing factor to this shortage is the presence of persistent organic molecules, such as pesticides, fungicides, and pharmaceuticals—some of which are difficult to remove using conventional water treatment methods. To combat these so-called emerging organic contaminants (EOCs), the use of adsorbents in water treatment has been tested as a way of catching challenging molecules. Commonly used adsorbents are materials such as activated carbon, clays, and zeolites. However, the choice of adsorbent heavily depends on the task at hand.
Making an Adsorbent from Plant-Based Waste
The approach presented in our paper revolves around the use of an organic molecule derived from plants – ellagic acid (Figure 1), a commonly occurring building unit of natural polyphenols. Ellagic acid and similar phenol-bearing compounds can be found in large concentrations in the exterior parts of many plants and fruits, i.e. materials otherwise considered as waste. We found that by combining ellagic acid with zirconium ions (Zr4+), a porous, crystalline, and chemically robust material can be acquired.
Figure 1: Ellagic acid, a molecule derived from plant-based waste, can be combined with zirconium cations to give a crystalline porous material, a so-called metal-organic framework. The right-most part of the figure is a scanning transmission electron microscopy (TEM) image, showing atoms as the bright parts of the image. The TEM image was acquired from a slice of a very small crystal of the material, which is obtained as a fine powder.
The exact structure of the material—how the building blocks are arranged—was investigated using advanced transmission electron microscopy techniques, showing a well-ordered material with pores that are 12 Å (1.2 nm) in diameter (right-hand part of Figure 1). Through further investigations, including the use of solid-state NMR, it was discovered that the material was negatively charged, with charge-balancing counterions residing in the pores. These charged species can readily be exchanged for simple cations, such as Na+, while the framework remains intact and porous. The material, since given the name SU-102, is chemically robust and can withstand tough chemical environments, remaining intact upon exposure to aqueous solutions across a wide pH range (3-12) as well as in contact with a broad range of organic solvents.
Sequestering Emerging Organic Contaminants from Real Water Samples
The potential of using SU-102 to sequester pharmaceutical pollutants and other EOCs was evaluated by treating the outgoing water, i.e. the effluent, of a next-generation wastewater treatment plant in Stockholm, Sweden. We knew from beforehand that a number of commonly used pharmaceuticals, such as beta-blockers, non-steroidal anti-inflammatory drugs, and anti-depressants, were present at detectable concentrations in the effluent of the treatment plant. In the studied water sample, 17 EOCs where found, some of which are expected to be either anionic, neutral or cationic at the pH of the effluent (pH ≈ 6.4).
The results, as seen in the left-hand part of Figure 2, show that many of the EOCs are removed with high efficiencies. Overall, 10 out of 17 EOCs were removed with efficiencies ~80% or higher. Also, there is a strong dependence on charge of the EOC, with cationic EOCs having an average removal efficiency (RE) of 94%, while neutral and anionic species have REs of 51% and 27%, respectively. The cationic nature of many commonly used pharmaceuticals, such as citalopram and atenolol (shown in Figure 2), originate in the presence of tertiary amine groups which are expected to be protonated under the studied conditions. In contrast, the anionic nature of certain EOCs comes from the deprotonation of carboxylic acid functionalities in their molecular structures (diclofenac, furosemide, and others).
Figure 2: Left – removal efficiencies (grouped by expected net molecular charge) of adsorption trials using SU-102 to treat wastewater treatment plant effluent, showing that cationic molecules (which atenolol would be an example of) are sequestered with high efficiencies. Right – schematic illustration of the simultaneous adsorption and photodegradation of organic contaminants using the zirconium-ellagate SU-102.
Additionally, lab-scale trials using spiked tap water solutions containing isolated EOCs showed that SU-102 can also act as a photocatalyst for the sequestration of contaminants (schematically illustrated in the right-hand part of Figure 2), showing an increase in RE when compared to pure adsorption. Considering this, the removal mechanism of SU-102 is two-fold, simultaneously acting both as an adsorbent and a photocatalyst.
As an ideal adsorbent should be reusable over several cycles, we could also show that captured EOCs could be desorbed by simply washing the loaded adsorbent with a concentrated sodium chloride solution, facilitating the reuse of SU-102 as an adsorbent. The recyclability of the material for photodegradation was also evaluated, showing that it can be used without a decrease in performance over at least seven cycles with no detectable ligand leaching.
Long Story Short
The porous material SU-102 was made by combining the plant-sourced molecule ellagic acid with non-toxic zirconium cations. The material is obtained as a fine powder and can be used to efficiently rid water of so-called emerging organic contaminants through simultaneous adsorption and photodegradation. Its potential as a water-cleaning agent was successfully evaluated using unaltered wastewater treatment plant effluent.
In the studied water sample, a mixture of 17 detectable contaminants not fully removed through conventional treatment methods were present. After treatment using SU-102, the cationic contaminants were removed with an average efficiency of 94%. Considering this, our work highlights the potential of using robust and sustainably sourced materials for making efficient adsorbents, as well as the importance of evaluating their performance for treating real and often complex water samples.
As a last but very important note, we wish to thank the reviewers for their comments which led to great improvements of the article.