Adsorption and peroxydisulfate-induced catalytic degradation of synthetic dyes based on green Cu (II) alginate hydrogel and its composite with CuO

Alginate hydrogel is a promising green adsorbent, but it has limited capacity to adsorb organic pollutants. Therefore, modification is needed to enhance its adsorption capacity and add catalytic activity, such as activating peroxydisulfate (PDS). This study used Cu2+ as a cross-linker to create Cu2+ alginate hydrogel (CuAlg) and synthesized its composite with CuO. These materials were tested for catalyzing PDS activation to efficiently degrade organic pollutants. The materials were thoroughly characterized by FTIR (Fourier transform infrared spectroscopy), XRD (X-ray diffraction), TGA (thermogravimetric analysis), zeta potential, EDS (energy-dispersive X-ray spectroscopy), BET (Brunauer–Emmett–Teller) analysis, and SEM (scanning electron microscopy). The results showed characteristic peaks in FTIR spectra and XRD patterns. The CuAlg mainly consisted of O, C, and Cu elements, with a surface area of 4.15 m2/g and a pore size of 38.5 nm. The zeta potential was −10.0 mV for CuAlg and −27.8 mV for Methyl orange (MO), both more negative than Congo red (CR) at −0.9 mV. The main reactive oxygen species (ROS) involved in CR degradation are active free radicals. The adsorption and degradation efficiency of CR were significantly higher than those for MO. Notably, CuAlg showed better CR catalytic activity compared to pure CuO. The MO degradation followed the pseudo-second-order (PSO) kinetics, while CR degradation conformed to the pseudo-first-order (PFO) model. Additionally, both CR and MO adsorption processes matched the Langmuir isotherm model. These findings emphasize the importance of surface chemistry and charge interactions in determining adsorption and catalytic performance.