Superefficient Removal of Heavy Metals from Wastewater by Mg-Loaded Biochars: Adsorption Characteristics and Removal Mechanisms.

Six types of biochar (BSB, CSB, FSB, CFSB, MSB, and TSB) were prepared from different raw materials by loading magnesium ions (Mg2+) via an impregnation process. The adsorption kinetics and thermodynamics of heavy metals at high concentrations were analyzed. The adsorption mechanisms were investigated by zeta potential, scanning electron microscopy-energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and inductively coupled plasma-atomic absorption spectroscopy analyses. The adsorption of heavy metals by BSB, CSB, FSB, CFSB, MSB, and TSB conformed to the Langmuir model and PS-order. The maximum theoretical saturation adsorption capacities for Cd(II), Cu(II), and Pb(II) were 333.33, 238.10, 75.19, 96.15, 66.23, and 185.19 mg·g-1; 370.37, 294.12, 111.11, 169.49, 84.75, and 217.39 mg·g-1; and 302.58, 200.00, 61.73, 90.91, 54.47, and 166.67 mg·g-1, respectively. According to the analysis of the contribution of adsorption, the adsorption process was mainly controlled by cation-π interactions, ion exchange, mineral precipitation, and functional group interactions. Biochars contain ash, functional groups and load a large number of Mg2+, which can form complexes with metal ions and perform strong ion exchange; therefore, mineral precipitation and cation exchange played dominant roles in the adsorption process. The prepared Mg-loaded biochars presented in this research showed excellent adsorption properties for heavy metals and have great potential for practical application; in particular, BSB had the strongest adsorption capacity for the three heavy metal ions.

Phosphate adsorption from wastewater using ZnAl-LDO-loaded modified banana straw biochar.

ZnAl-layered double hydroxide-loaded banana straw biochar (ZnAl-LDH-BSB) was prepared via the hydrothermal method, and the efficient phosphorus removal agent ZnAl-LDO-BSB was obtained by calcination at 500 °C. Based on the ZnAl-LDO-BSB adsorption characteristics, the adsorption mechanism was evaluated via TG/DTA, FTIR, XRD, SEM, HRTEM, and other characterization methods. The results showed that the ZnAl-LDO-BSB assembled into microspheres with typical hexagonal lamellar structures and presented good thermal stability. The adsorption of total phosphate (TP) by ZnAl-LDO-BSB conforms to the Langmuir model, and the theoretical maximum adsorption capacity is 185.19 mg g-1. The adsorption kinetics were in accordance with the second-order kinetic model, and the anion influence on TP adsorption followed the order CO32- > SO42- > NO3-. The combination of zeta potential measurements with the FTIR, XRD, SEM, HRTEM, and XPS results suggested that ZnAl-LDO-BSB adsorbs TP mainly by electrostatic adsorption, surface coordination, and anion intercalation. Graphical abstract.

Characteristics of nitrogen and phosphorus adsorption by Mg-loaded biochar from different feedstocks.

Herein, biochars from 6 different feedstocks (taro straw, corn straw, cassava straw, Chinese fir straw, banana straw, and Camellia oleifera shell) were produced using magnesium chloride (MgCl2) as a modifier due to their sorption behavior toward NH4+-N and phosphorus in an aqueous solution. The biochar characteristics were evaluated, including pH, pHPZC, biochar magnesium content, and total pore volume (PVtot). The experimental results in terms of the kinetics and equilibrium isotherms showed that the cassava straw and banana straw biochars exhibited the theoretical maximum saturated adsorption capacities of 24.04 mg·g-1 (NH4+-N) and 31.15 mg·g-1 (TP), respectively. Biochar produced from these feedstocks had higher magnesium contents and greater total pore volumes, reflecting the significant contributions from magnesium and steric effects. FTIR, XRD, and SEM/EDS analyses demonstrated that NH4+-N and TP sorption mechanisms predominantly involved surface electrostatic attraction, Mg2+ precipitates and complexation with surface hydroxyl functional groups.