Lotus carbon dots doped sodium alginate hydrogel beads for effective adsorptive removal of Cd2+ and potential application in Pb2+ removal from wastewater

A cost-effective and environmentally friendly composite hydrogel for the adsorption of Cd 2+ was fabricated by combining sodium alginate with lotus carbon dots (SA@lotus CDs). The impact of many factors, including doping amount, dose, pH, initial solution concentration, adsorption period, and temperature, on the process of Cd 2+ adsorption was studied. The adsorption isotherm was fitted well to the Langmuir isotherm model with high accuracy (R 2  = 0.9604). The adsorption kinetics conformed to the Pseudo-Second-Order kinetic model, demonstrating a high coefficient of determination (R 2  = 0.9238). The Langmuir model predicted that the adsorbent had adsorption capacity of 179.35 mg/g for Cd 2+ . After undergoing five cycles, the SA@lotus CDs continued to exhibit a removal effectiveness of 85.1 %. The SA@lotus CDs hydrogel composite was also used for the removal of Pb 2+ by 291.23 mg/g. The adsorption capability of the hydrogel was improved due to the addition of CDs. Pb 2+ was adsorbed by SA beads at 275.5 mg/g, while SA@lotus CDs had a slight increase to 291.23 mg/g (5.7 %). However, there was a 40.2 % increase in Cd 2+ adsorption from 127.89 mg/g by SA beads to 179.35 mg/g by SA@lotus CDs beads. The results of the adsorption tests showed the different removal efficiency of Pb 2+ and Cd 2+ by SA@lotus CDs. These findings suggested that SA@lotus CDs were more effective for removing Cd 2+ than Pb 2+ . The interaction mechanism between materials and metal ions was studied by using density functional theory. The interactions between CDs sites or SA sites and metal ions were calculated respectively. The results showed that the adsorption energy of CDs sites for metal ions was Cd 2+  < Cu 2+  < Pb 2+  < Fe 2+  < Co 2+  < Mg 2+ , while the adsorption energy of SA for Pb 2+ was much lower than that for Cd 2+ . Therefore, the affinity order of SA@lotus CDs for metal ions was Pb 2+  > Cd 2+  > Cu 2+  > Fe 2+  > Co 2+  > Mg 2+ , which was consistent with the experimental results.