Sodium humate based double network hydrogel for Cu and Pb removal

Sodium humate (SH) is one of the derivatives humic substances, which can be utilized for heavy metal removal from water due to its containing plenty of functional groups. In this study, a double network hydrogel SH/polyacrylamide (SH/PAM) was synthesized by a simple free-radical polymerization and used for Cu2+ and Pb2+ removal from water. The adsorption process can be well described by Langmuir-Freundlich model, indicating that both physical and chemical adsorption were involved. X-ray photoelectron spectroscopy (XPS) characterization demonstrated that complexation was the main mechanism for the adsorption. Two-dimensional correlation analysis of FTIR (2D-FTIR-COS) results showed that the variation order of functional groups during Cu2+ and Pb2+ adsorption in the following order: COOH ≈ -CO > -OH > C-O and -COOH ≈ C-O > -CO > -OH, respectively. According to the density functional theory (DFT) calculation results, the O atom of SH in the COO- was the main adsorption site. Meanwhile, the adsorption energy of Pb2+ was more negative than that of Cu2+ and the orbital hybridization between O atom of SH and Pb2+ was denser than that of Cu2+, which suggested that SH/PAM had a stronger combining capacity for Pb2+ than Cu2+. Therefore, the adsorption capacity for Pb2+ was larger than Cu2+. Moreover, the removal efficiencies are 30.2% for Al, 98.79% for Cu, 99.0% for Fe, 17.2% for Mn, 93.4% for Pb, and 62.4% for Zn in actual acid mine drainage using 6 g L-1 adsorbent. Collectively, this study not only provided a new adsorbent for heavy metal removal but also explicated the mechanism of heavy metal removal by SH from molecule and electron perspective, which is helpful for the application of SH in the environmental field.