They brighten our lives and help define countless items we use daily—from the vibrant clothes we wear to decorative paper and packaging materials, as well as our footwear, bags, and belts! What adds different colours to these things? Dyes, which bind themselves to the structure of the material they are colouring. For example, Methylene blue (MB) is a dye that is used to colour papers, leather products, silk, wool, and is also employed as a diagnostic agent and in the rubber and cosmetic industries. But what happens after these dyes have served their purpose? According to a news story on the University of Bath website, billions of tons of dye-containing wastewater enter water systems every year. 80% of such wastewater generated in low- and middle-income nations is used directly for irrigation or released untreated into water sources. These dyes eventually render streams, rivers, and lakes unfit for human and animal use. They also negatively impact the habitat of aquatic organisms. MB can also result in health issues such as vomiting, breathing problems, diarrhoea, eye burns, jaundice, and cancer.
To facilitate the removal of MB from water, strategies like membrane filtration, coagulation, and electrochemical treatment have been explored. However, some drawbacks include the expensive nature of such approaches, unwanted byproduct formation, and failure to achieve the removal of multiple dyes. In such a case, adsorption has emerged as a promising alternative due to its ease of use, lower energy investment, and reuse of adsorbent materials. Simply put, adsorption involves one substance sticking to the surface of another substance. Yet, developing materials that remain stable under different conditions and can be efficiently reused remains challenging.
A study by researchers from the Indian Institute of Technology Gandhinagar (IITGN)published in ACS Applied Polymer Materials is an effort to address this issue by developing hydrogels, which are advanced materials that act like microscopic sponges. Other than MB, the engineered materials robustly trapped other popular dyes like crystal violet and rhodamine B, even when multiple dyes were present in the same water-based solution, demonstrating their nonselective affinity. This ability is particularly important because wastewater from dye-using industries rarely contains only a single pollutant.
Hydrogels are soft materials, containing numerous tiny pores, that can absorb lots of water and biological fluids. In this study, researchers created CAPA, a hydrogel using carboxymethyl cellulose (CMC), a biodegradable material derived from cellulose (made up of many, many glucose units), and additional components, including acrylic acid.
“The key innovation lay in adjusting the amount of acrylic acid. By changing this single ingredient, we were able to fine-tune the structure, surface properties, and adsorption behaviour of the materials,” explained Dr Hitarth Patel, who recently defended his PhD thesis from IITGN and is the first author of this study.
The team tested three versions containing different amounts of acrylic acid for their ability to remove MB. The best-performing hydrogel, CAPA-2, adsorbed 99.6% of MB dye. Further, its adsorption capacity of approximately 475 mg of dye per gram (g) of hydrogel places CAPA-2 among the highest-performing CMC-based hydrogels reported for MB removal. Several studies have reported values generally between 30 mg/g to 250 mg/g.
What makes these hydrogels effective? One can think of it as an ‘opposites attract!’ situation. These hydrogels contain many negatively charged sites. Dyes like MB carry positively charged sites, which make them naturally drawn to the hydrogels. Further, the study found that increasing the proportion of acrylic acid led to an increase in the negative surface charges in the hydrogels, enhancing adsorption. Analysis also indicated the presence of hydrogen bonds as well as hydrophobic or water-repelling interactions between MB and dry pockets of the hydrogels. These materials also contain extremely tiny pores with an average pore diameter of ∼25 nanometres and suitable charged surfaces that allow them to trap, bind, and pull out the MB, crystal violet, and rhodamine B dye molecules from wastewater.
Most hydrogels tend to have a pH ‘comfort zone’ in terms of working efficiency. Interestingly, the behaviour of CAPA hydrogels engineered in this study was distinctive in this regard. While all three of them adsorbed MB most effectively at pH 7 (neutral), CAPA-2 performed this task very well in both acidic and alkaline environments (pH 3 and 10).
CAPA-2 also resembled a sponge that does not lose much of its cleaning power after being washed and reused. After four reuse cycles, it showed only a slight decline in performance (from 99.55 to 95.91 mg/g). This durability suggests that the use of such materials may help reduce wastewater treatment costs.
What is particularly encouraging about this work is that the researchers were able to achieve a substantial improvement in performance through a relatively simple design strategy. According to Dr Bhaskar Datta, “As we continue to grapple with the significant and globally concerning issue of pollution, innovations like the CAPA hydrogels with their high dye-removal capacity, stability across different pH conditions, and repeated usability represent promising solutions to industrial wastewater treatment.” Dr Datta is a Professor in the Chemistry and Biological Sciences and Engineering departments at IITGN.
On the occasion of World Environment Day, this research is a potential step towards protecting water resources and associated ecosystems for creating a sustainable future. “Future directions involve evaluating the real-world performance of the smart sponges generated in this study,” added Dr Datta.
This work resonates with the Government of India’s focus on sustainable water management and pollution control and the United Nations’ Sustainable Development Goal 6 (Clean Water and Sanitation). It also aligns with Sustainable Development Goals 9 (Industry, Innovation and Infrastructure) and 14 (Industry, Innovation and Infrastructure) by developing candidates that may contribute to more sustainable industrial practices and cleaner water resources. The researchers acknowledged support from the Central Instrumental Facility (CIF) at IIT Gandhinagar and funding from the Gujarat Council on Science and Technology (GUJCOST).