Characterization of three kinds of modified wheat straw derived biochars and their sorption capacity for Cu2+

Abstract: Copper pollution has posed a serious threat to the ecologic environment in recent years, particularly for concealment, accumulation, and irreversibility. Fortunately, biochar can serve as a new adsorption material, due to the available raw materials, environmentally friendly, and excellent adsorption performance. However, it is still limited to the effect of pristine biochar on heavy metal pollution in the water environment. The modified biochars have been prepared to improve the adsorption capacity, where the different elements can be loaded onto the pristine biochar. Among them, impregnation is one of the main approaches to modified biochars. In this study, three kinds of chemically modified wheat straw-derived biochar were prepared as the adsorbents for the remediation of Cu2+ pollution in soils and waters. A systematic investigation was implemented to determine the performance of these biochars on Cu2+ sorption in an aqueous solution. The wheat straw-derived biochar was modified with sodium silicate, magnesium chloride, and hydrogen peroxide solution. Scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS), and Fourier infrared spectroscopy (FTIR) was used to characterize the surface morphology and functional groups on the biochars before and after modification. The sorption kinetic and isotherm models were selected to explore the sorption characteristics of biochars for Cu2+ in the aqueous solution. The result showed that the sodium silicate-modified biochar (SBC) shared the largest specific surface area and micropore volume of 43.69 m2/g and 5.30 cm3/g, respectively, which increased by 6.25 and 2.79 times, compared with the unmodified biochar (6.02 m2/g and 1.40 cm3/g). The SEM-EDS showed that there was a decrease in the elemental C content in the modified biochars of SBC, whereas, an increase was observed in the elemental O content, with the largest changes in the elemental C and O content. By contrast, the Si and Mg-containing particles were loaded on the SBC surface and magnesium chloride-modified biochar (MBC). The FTIR showed that the peak value of functional groups was enhanced after modification. But there was a small enhancement degree of hydrogen peroxide modification, while the large one was in the SBC and MBC. In addition, the pseudo-first-order model was established to express the sorption kinetics of Cu2+ on the BC, MBC, SBC, and hydrogen peroxide-modified biochar (HBC). Moreover, the Langmuir model was fitted for the sorption isotherms of Cu2+ on the unmodified biochar, MBC, and SBC, whereas, the Freundlich model was for the Cu2+ on the HBC. The sorption model parameters demonstrated that the SBC shared the stronger sorption capacity for Cu2+ with a theoretical sorption amount of 230.20 mg/g, compared with the MBC, SBC, and HBC. The findings can provide a theoretical basis for the treatment of Cu2+ contaminated water using modified biochars.