Another little addition to the rich tapestry of potential world-improving uses of halloysite nanotubes.
In this episode we look at the topic of HNTs, water treatment uses and related research.
A cursory google search will quickly reveal the often understated importance of water and the enormous global challenges faced in respect of this matter, such as on the United Nations site:
Hence there are enormous commercial and humanitarian opportunities within the ambit water treatment technologies and uses.
In this recent article from 2 September 2020, researchers from Surrey University (UK) looked at the use of bio-coatings for improved waste-water and other water treatment moving forward:
”They used halloysite, which consists of natural low-cost and microscopic tubes of clay, previously used as a reinforcement for plastic materials. The tiny halloysite tubes created channels in the biocoating to raise the permeability.Using a specially adapted resazurin reduction assay, the researchers found that bacteria encapsulated in halloysite biocoatings were statistically more likely to stay viable compared to bacteria in the ordinary biocoatings.
They determined that a coating made up of 29 percent halloysite had the best combination of good mechanical strength and high permeability. Importantly, fluorescence microscopy determined that the bacteria remained viable and metabolically active for extended periods of time. In the future, viable bacteria could be used to clean polluted water by removing harmful chemicals.”
As with many of these HNT subject areas, only a small amount of digging is required in order to find an abundance of research (and often recent) on the very topic.
In this research paper dated 24 January 2019 entitled “Halloysite Nanotubes as Adsorptive Material for Phosphate Removal from Aqueous Solution”, HNTs were investigated for their ability as an adsorbent to remove phosphates from agricultural water run-off (e.g chemical fertilisers, manure, etc used in farming):
Among other matters, HNTs were selected for investigation by reason of their various and superior naturally occurring characteristics:
“Nanoparticle clay mineral receives much more attention among the natural adsorbents due to the high specific surface area, high adsorption rates compared to other adsorbents, low toxicity, easy operation, and, to some extent, cost-effectiveness. The potential of clay materials (halloysite) to remove both anionic and cationic pollutants has been reported [19,32,33,34]. In recent studies, it was found that halloysite of different chemical and mineralogical composition can be used as an adsorbent and pollution remediation material”
Further, this recent publication from 17 July 2020 is entitled “Purification and Sorting of Halloysite Nanotubes into Homogeneous, Agglomeration-Free Fractions by Polydopamine Functionalization”:
While I appreciate the title is rather wordy and you have to wade through a LOT of scientific “jargon” when reading the article, there were some really interesting aspects, in particular around:
1. Purification of “unprocessed raw HNTs” into “high purity HNTs”:
”However, the three-step separation method presented in this study would now allow the use of these high-purity HNTs with significantly increased loading and functionalization capacities as well as better defined size distributions as nanocontainers in various high value-added applications such as active food packaging and drug delivery.”
2. Strength and mechanical improvements gained by using high purity HNTs in “nanocomposite films”:
”While nanocomposite films prepared with raw HNTs did not present any improvement in Young’s modulus, tensile strength, and elongation at break values compared to neat LDPE films, the mechanical properties of nanocomposite films improved as a function of the separated HNT grade quality when compared to raw HNT reinforced nanocomposite films. Compared to less uniformly dispersed, agglomerated raw HNTs and grade 1 HNTs present in the LDPE matrix as evidenced by SEM analysis, the uniform dispersion of individual, significantly less agglomerated grade 2 and grade 3 HNTs resulted in the improvement of Young’s modulus and the tensile strength, demonstrating the true nanoscale reinforcement effect.”
Additional journal references include December 2017 - “Polystyrene-halloysite nano tube membranes for water purification”:
“Membrane technologies are a sustainable solution for treatment of water and wastewater. Here, the technical feasibility of polystyrene-halloysite nanotube (PS-HNT) membranes, fabricated by an ultrasound-assisted solution casting method, was explored for water purification. To this end, the effects of various solvents on the structure, morphology, thermal, and mechanical properties of PS-HNT membranes were investigated. Introduction of HNTs (5. wt%) into the polystyrene matrix demonstrated excellent thermal and mechanical properties along with good water flux, rejection of unwanted components, recovery, and regeneration cycles. These membranes were overall useful enough to purify real wastewater collected from pulp and paper mill”
And from June 2013 - “Halloysite nanotubule clay for efficient water purification”:
“Halloysite clay has chemical structure similar to kaolinite but it is rolled in tubes with diameter of 50nm and length of ca. 1000nm. Halloysite exhibits higher adsorption capacity for both cationic and anionic dyes because it has negative SiO2 outermost and positive Al2O3 inner lumen surface; therefore, these clay nanotubes have efficient bivalent adsorbancy. An adsorption study using cationic Rhodamine 6G and anionic Chrome azurol S has shown approximately two times better dye removal for halloysite as compared to kaolin. Halloysite filters have been effectively regenerated up to 50 times by burning the adsorbed dyes.”
The water treatment and related applications of HNTs have not gained a huge amount of air-time within ADNs announcements and progress to date, but what is clear is that it represents yet another potentially vast market where high-purity HNTs could become in significant and ever-growing demand.
(20min delay)