Adsorption of complex pollutants: Designing hydrogarnets to remove humic substances from water
[Image above] Removing humic substances from water is challenging due to the complex nature of these materials, which makes adsorbing it difficult. Credit: Pexels
Microplastics are the poster child of a polluting material, one which has universally detrimental effects on the environment wherever they are found—be it in water, soil, or air. However, for other materials, their characterization as “polluting” is more nuanced and depends on location.
Take ammonium, for example. Ammonium in soil serves not only as a major source of nitrogen for plant growth, but it is also as an important product or reactant in the nitrogen transformation processes in soil. As such, ammonium is the main form in which fertilizer nitrogen is applied in agriculture.
However, this ability to accelerate plant growth causes problems when ammonium ends up in our waterways. It can accelerate eutrophication of aquatic environments, i.e., excessive plant and algal growth due to an overadundance of minerals and nutrients. This growth leads to oxygen depletion and so-called “dead zones” in bodies of water, such as the Great Lakes.
Humic substances are another example of a material that benefits agriculture but can negatively impact water quality. These ubiquitous natural organic polyelectrolytes are formed by the breakdown of animal and vegetable matter in aqueous environments. They have positive effects on plant physiology by improving soil structure and fertility and by influencing nutrient uptake and root architecture.
In contrast, in waterways, humic substances can aggravate pollution by interacting with disinfectants such as chlorine to produce toxic byproducts. These byproducts endanger organisms in the aquatic ecosystem.
Removing humic substances from water is not a simple endeavor. Unlike discrete compounds like ammonium, which can be clearly defined in chemical terms, humic substances are complex, heterogeneous molecules that defy specific categorization.
As one review of humic substances puts it, these materials “represent a unique category of natural products in which the essence of the material appears to be heterogeneity per se.”
Because of the complex structure of humic substances, its comprehensive removal from aqueous environments through adsorbents is difficult. To succeed, an adsorbent would need to increase the number of interactions between the humic substance and the material surface.
Compared to conventional adsorbents, such as activated carbon and zeolite, hydrogarnet has shown excellent potential as an adsorbent for humic substances. Hydrogarnet refers to garnets in which hydroxide groups have partially replaced the silica units [Ca3Al2(SiO4)3−x(OH)4x]. They can be categorized as grossular (x = 0), hibschite (0.2 < x < 1.5), or katoite (1.5 < x < 3) depending on the degree of substitution.
Controlling the chemical composition of hydrogarnet is difficult, but some studies have reported that adjusting the preparation conditions can offer control over the resulting structure. With this possibility in mind, researchers from Nagoya Institute of Technology in Japan published a paper investigating the effects of heat treatments to control hydrogarnet composition and adsorption properties.
For this study, heat treatments of the hydrogarnets took place at 320°C, 350°C, and 440°C. To confirm crystallinity of the hydrogarnet before and after heat treatment, the researchers performed selected area electron diffraction.
Analysis confirmed that treatment temperature did affect the chemical composition of the hydrogarnet. Specifically, increasing the temperature tended to decrease the hydroxy group content of the hydrogarnet. Hydrogarnet content in the sample treated at 440°C was only two-thirds of that in the sample without heat treatment.
Along with a decreased hydroxy group content, heat-treated samples exhibited a partial collapse of the garnet structure, leading to the formation of a silica-rich amorphous phase around the surface.
Regarding adsorption properties, the researchers found that the heat-treated samples exhibited a much higher maximum adsorbed amount and adsorption rate for humic substances compared to the untreated sample. Specifically, the sample treated at 440°C exhibited approximately seven times the adsorption properties for humic acid.
The researchers attribute these improved adsorption properties to an increase in pH and increased exposure of the hydrogarnet surface due to dissolution of the silicon phase when samples were soaked in a solution containing the humic substance.
To enable a more in-depth discussion on the adsorption properties of the samples, the researchers are now conducting investigations with respect to the effects of the humic substance solution condition, such as the pH and ionic strength.
