Graphene Oxide Covalently Functionalized with 5-Methyl-1,3,4-thiadiazol-2-amine for pH-Sensitive Ga 3+ Recovery in Aqueous Solutions.

A novel graphene-based composite, 5-methyl-1,3,4-thiadiazol-2-amine (MTA) covalently functionalized graphene oxide (GO-MTA), was rationally developed and used for the selective sorption of Ga 3+ from aqueous solutions, showing a higher adsorption capacity (48.20 mg g -1 ) toward Ga 3+ than In 3+ (15.41 mg g -1 ) and Sc 3+ (~0 mg g -1 ). The adsorption experiment's parameters, such as the contact time, temperature, initial Ga 3+ concentration, solution pH, and desorption solvent, were investigated. Under optimized conditions, the GO-MTA composite displayed the highest adsorption capacity of 55.6 mg g -1 toward Ga 3+ . Moreover, a possible adsorption mechanism was proposed using various characterization methods, including scanning electron microscopy (SEM) equipped with X-ray energy-dispersive spectroscopy (EDS), elemental mapping analysis, Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Ga 3+ adsorption with the GO-MTA composite could be better described by the linear pseudo-second-order kinetic model ( R 2 = 0.962), suggesting that the rate-limiting step may be chemical sorption or chemisorption through the sharing or exchange of electrons between the adsorbent and the adsorbate. Importantly, the calculated q e value (55.066 mg g -1 ) is closer to the experimental result (55.60 mg g -1 ). The well-fitted linear Langmuir isothermal model ( R 2 = 0.972~0.997) confirmed that an interfacial monolayer and cooperative adsorption occur on a heterogeneous surface. The results showed that the GO-MTA composite might be a potential adsorbent for the enrichment and/or separation of Ga 3+ at low or ultra-low concentrations in aqueous solutions.