Enhanced Adsorption of Anionic Dyes using Sr-Doped ZnO Nanoparticles: Nonlinear Kinetics and Isotherm Studies

Using a co-precipitation process, sr-doped ZnO was prepared and demonstrated to be effective in the removal of anionic congo red dye, since its surface positive charge was increased to 17.3 mV. To investigate the surface morphology and physicochemical characteristics of Sr-doped ZnO, FTIR spectroscopy, scanning electron microscopy (SEM), elemental analysis, and point of zero charges (pHPZC) were employed. We conducted batch adsorption experiments to investigate the effects of contact time (180 minutes), pH (3.5–11), adsorbent dosage (0.5–3.0 g L−1), and temperature (288–328 K). The kinetic study was modeled using several equations, including pseudo-firstorder (PFO), pseudo-second-order (PSO), Elovich, and intra-particle diffusion (IPD). For the purpose of describing the adsorption isotherm, Langmuir, Freundlich, and Temkin models were used. A pseudo-second-order model suggests that the adsorption process is chemisorption, which is further confirmed by the Langmuir isotherm model. The maximum capacity ( ) of Sr-doped ZnO for removing CR dye was 71.83 mg/g. At natural pH, the % removal rate of CR dye increased from 13% for pure ZnO (p-ZnO) to 82% for Sr-doped ZnO, where the change in pH significantly increased the removal rate from 7.7% at pH 11 to 92.0% at pH 3.5. The thermodynamic parameters measured at 288, 298, 318, and 328K indicate that the CR dye adsorption occurs spontaneously and endothermically. A significant role is also played by electrostatic interactions in the adsorption of anionic CR dye onto the positive surface of ZnO doped with Sr. This research study highlights the increased surface positivity of Sr-doped ZnO achieved through Sr ion doping. This resulted in a strong attraction to anionic dye molecules like Congo red dye. These findings collectively contribute to advancing the understanding of material-dye interactions and have potential implications for various applications in wastewater treatment and environmental remediation.