Development of Metal Oxide-Based Adsorbents for the Removal of Dyes From Contaminated Water

The preservation of pure water resources, essential for life on Earth, is under severe threat due to extensive contamination from industrial activities. These activities introduce a mix of both organic and inorganic substances into water bodies. Dyes, known for their carcinogenic properties, present a substantial hazard to both human and aquatic life. Industries, particularly those in textiles and printing sectors, often discharge wastewater containing concentrated dyes, underscoring the need for effective removal strategies. This study introduces an innovative approach, utilising Strontium-doped neodymium manganite, Nd0.6Sr0.4MnO3 (NSMO) as an adsorbent for removing organic contaminants, particularly the Fast Green (FG) dye, from wastewater. Synthesized via a solid-state reaction route, NSMO displays unique orthorhombic polycrystalline properties and a dense particle growth pattern. The findings demonstrate a remarkable 99% removal efficiency of FG dye from a 100 mg/L solution using just 0.05 g of NSMO within 60 minutes. The higher removal efficiency of NSMO is due to the presence of Mn, which exists as trivalent (Mn+3) as well as tetravalent manganese ion (Mn+4). Due to mixed valence, the sites with three positive and four positive charges serve as highly efficient adsorption sites for the anionic FG dye. Thus, electrostatic interactions between adsorbent and adsorbate occur and help in the presently observed effective adsorption process. This result underscores the exceptional adsorptive potential of NSMO, marking a shift from the traditional focus on its photocatalytic properties to its effectiveness as an adsorbent. To address the low adsorption capacity of materials for FG dye, the study presents a novel adsorbent, ZnOS+C, synthesized by modifying zinc peroxide with sulfur and activated carbon. Batch adsorption experiments highlight ZnOS+C exceptional potential, achieving a maximum adsorption capacity of 238.28 mg/g for FG dye within 120 minutes over a wide pH range. The Freundlich isotherm model suggests multilayered adsorption on the outer surface of ZnOS+C, while kinetics studies align with the intraparticle diffusion model. Moreover, ZnOS+C shows good removal efficiency in up to 5 successive adsorption-desorption studies where the removal effectiveness of ZnOS+C decreased by no more than 14.2% even after five cycles relative to their initial adsorption capacity. This study also explores the removal of Crystal Violet (CV) dye, a highly toxic substance commonly found in textile industries using surface modification of zinc peroxide (ZnO2) with the sodium salt of dioctyl sulfosuccinate. ZnO2, which was inactive for the uptake of CV dye from wastewater, is made highly active by surface modification with the help of sodium dosusate. The long hydroscopic chain contains an aliphatic hydrocarbon chain of sodium docusate and a polar part of sodium docusate creates the SO3– group over the surface of ZnO2. Further, the presence of xanthan gum forms a reverse micelles system around the CV dye present in water. This micelle formation results in the uptake of CV dye from water by ZnSD. Besides, this electrostatic interaction between the SO3– group of ZnSD and the cationic nitrogen of CV dye also enhances the adsorption capacity. Also, the zeta-potential studies indicate that the potential of ZnSD decreases from −15 to −60 mV as we increase the pH from 3 to 9, which suggests a higher negative charge on adsorbent at higher pH and results in more electrostatic interaction between ZnSD and CV dye at higher pH. Surface modification significantly enhances ZnO2 adsorption efficiency for CV, achieving over 99.5% removal. The adsorption capacity reaches 123 mg/g, emphasizing the effectiveness of the modified ZnO2. Optimal physiochemical parameters, including pH, contact time, initial dye concentration and adsorbent dosage, were determined for maximal adsorption. Furthermore, the study adopts a green chemistry approach to synthesize zinc oxide (ZnO) nanoparticles using lychee peel extract for removing Congo Red (CR) dye from wastewater. The synthesized ZnO NPs could effectively remove >98% of CR dye from wastewater within 120 min of contact time at a wide pH range from 2 to 10. The primary mechanism involved in removing dye was the electrostatic interaction between ZnO adsorbent and CR dye. The antimicrobial performance of synthesized ZnO NPs was found to show 34% inhibition against Bacillus subtilis (ATCC 6538), 52% against Escherichia coli (ATCC 11103), 58% against Pseudomonas aeruginosa (ATCC 25668) and 32% against Staphylococcus aureus (ATCC 25923) using well diffusion assay. ZnO demonstrates a suitable anti-bacterial property over both gram-positive and gram-negative pathogenic bacteria. Overall, the green synthesized method for developing ZnO NPs shows promising and significant anti-bacterial performance and is a highly potential adsorbent for removing CR dye from wastewater. Lastly, the study investigates the synthesis of pure and doped zinc oxide (ZnO) nanoparticles, incorporating manganese (Mn), silver (Ag) and iron (Fe) dopants for Congo Red (CR) dye removal. The batch adsorption investigation revealed adsorption efficiencies of 99.4% for CR dye at an optimal dose of 0.03 g/30 ml for Mn-doped ZnO at a solution pH of 2. The adsorption capacity of each of the synthesized materials was found to be in order Mn-doped ZnO (232.5 mg/g) > Ag-doped ZnO (222.2 mg/g) > pure ZnO (212.7 mg/g) > Fe-doped ZnO (208.3 mg/g). Both pseudo-second-order kinetics model and the Langmuir isotherm model accurately explained the adsorption behaviors of CR dye. As such, van der Waal interactions, H-bonding and electrostatic interaction were found to be the adsorption mechanisms responsible for dye removal. In addition, the desorption–regeneration investigation indicated the successful reuse of the exhausted Mn-doped ZnO material for five cycles of CR dye adsorption with an efficiency of 83.1%. In summary, these studies collectively represent pioneering and innovative approaches to wastewater treatment, introducing novel adsorbents and methodologies as significant advancements in the efficient removal of various toxic dyes, thereby addressing a crucial aspect of environmental pollution and water resource management. Different novel metal oxide-based adsorbents are developed by doping, surface modification and functionalization by looking at the chemical structure of dyes to have better chemical interactions between developed adsorbents and targeted dyes. This results in a significant increase in the removal efficiencies and adsorption capacity of the adsorbents.