Adsorption of multi-heavy metals from aqueous solution by wheat straw and its biochar loaded with MnO2 nanoparticles: characteristics and mechanisms

In order to investigate the removal of Cd(II), Ni(II), Zn(II), and Pb(II) from an aqueous solution, manganese dioxide nanoparticles (MnO2) were loaded on wheat straw (WS) and its generated biochar (BC). To verify the loading of MnO2 nanoparticles on the adsorbents and examine their characteristics, SEM coupled with EDX analysis, XRD, BET, FT-IR and CHN analyses were used. The findings showed that the loading of MnO2 nanoparticles increased the specific surface of WS and BC from 6.43 m(2) g(-1) to 160.82 and 60.52 m(2) g(-1), respectively. All adsorbents performed best at pH = 6 and 1 g L-1 adsorbent dosage for heavy metal ion adsorption. The adsorption equilibrium was also attained in 120 min. The pseudo-second-order kinetic model and Langmuir isotherm provided the best fits to the batch adsorption data (R-2 > 0.999 and R-2 > 0.985, respectively), indicating that a chemical reaction occurred on a monolayer surface. According to the Langmuir model, BC@MnO2 maximum adsorption capacity for Pb(II), Ni(II), Cd(II) and Zn(II) were 76.34, 62.11, 55.55 and 43.86 mg g(-1), respectively, and those of WS@MnO2 were 68.3, 56.2, 45.45 and 33.56 mg g(-1), respectively. Based on the results of XRD, SEM-EDS, and FTIR, complexation, ion exchange and precipitation were proposed as the dominant processes for removing heavy metal ions from aqueous solution. Therefore, WS@MnO2 and BC@MnO2 can be suggested as efficient, reusable, economical, and environmentally friendly adsorbents for wastewater or water cleanup.