Enhanced adsorption and co-adsorption of heavy metals using highly hydrophilicity amine-functionalized magnetic hydrochar supported MIL-53(Fe)-NH2: performance, kinetics and mechanism studies

Abstract

Abstract It is a "kill two birds with one stone" method to convert invasive plants into hydrochar via hydrothermal carbonization as well as coinciding with 3R rules (reduction, recycling and reuse). In this work, a series of hydrochars (pristine, modified and composite) derived from invasive plants Alternanthera Philoxeroides(AP) were prepared and applied to the adsorption and co-adsorption of heavy metals (HMs) such as Pb(II), Cr(VI), Cu(II), Cd(II), Zn(II) and Ni(II). The results show MIL-53(Fe)-NH2- magnetic hydrochar composite (M-HBAP) displayed a strong affinity for HMs, which the maximum adsorption capacity for HMs were 261.74 (Pb(II)), 252.50 (Cr(VI)), 180.92 (Cd(II)), 163.76 (Cu(II)) and 77.84 (Zn(II)) mg/g calculated by Langmuir model. This may be because the doping of MIL-53(Fe)-NH2 enhanced the surface hydrophilicity of hydrochar, which allows hydrochar to disperse in the water within 0.12 s and possessed excellent dispersibility compared with pristine hydrochar (BAP) and amine-functionalized magnetic modified hydrochar (HBAP). Furthermore, the BET surface area of BAP was improved from 5.63 to 64.10 m2/g after doing MIL-53(Fe)-NH2. M-HBAP shows a strong adsorption effect on the single HMs system (52-153 mg/g), while it decreased significantly (17-62 mg/g) in the mixed HMs system due to the competitive adsorption. Cr(VI) can produce strong electrostatic interaction with M-HBAP, Pb(II) can react with CaC2O4 on the surface of M-HBAP for chemical precipitation, and other HMs can react with functional groups on the surface of M-HBAP for complexation and ion exchange. In addition, five adsorption-desorption cycle experiments and vibrating sample magnetometry (VSM) curves also proved the feasibility of the M-HBAP application.