Characteristics and mechanism of lead adsorption from aqueous solutions by oil crops straw-derived biochar

Abstract: In recent years, with the continuous development of economy, heavy metal pollution becomes increasingly serious. Biochar is often obtained from agriculture wastes through pyrolysis, in which the biomass is subjected to thermochemical conversion under an oxygen-limited condition. As a new type of cheap efficient adsorbent, because of the larger specific surface area and high surface energy, and the surface containing rich-oxygen functional groups, biochar has a good application prospect in wastewater treatment, causing great interest for scientist. Flax (Sesamum indicum) and rape (Brassica campestris L.) are the typical oil crops, which are kinds of good raw material for producing biochars. However, large amounts of flax straw (FS) and rape straw (RS) are discarded or incinerated directly and cause resource waste and environmental pollution seriously. In order to utilize the waste biomass resources, 2 types of biochars derived from FS and RS were prepared by pyrolyzing at a temperature of 700 ℃ under oxygen-limited condition, and the adsorptions of Pb2+ in aqueous solutions were evaluated. The effects of contact time, initial Pb2+ concentration and initial pH value in batch experiments were investigated by the 4 kinds of adsorption kinetics models (Pseudo first-order, Pseudo second-order, Elovich and Intra-particle diffusion model) and the 4 kinds of isothermal adsorption models (Langmuir, Freundlich, Temkin and D-R model). The structure and properties of biochars were characterized by using Brunauer-Emmett-Teller (BET) surface area and pore size analysis, scanning electron microscope (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectrometer (FTIR), which were used to preliminarily analyze adsorption mechanism. The results showed that the BET surface areas of RS and FS were 84.44 and 172.61 m2/g, respectively; SEM showed that the surface of 2 kinds of biochars was smooth, and the pore structure was compact and regular; FTIR indicated that the active functional groups such as CO32- (675-750, 835-886 cm-1), secondary alcohol (1 017 cm-1), C-C (1 437 cm-1), -COOH or C=O (1 578 cm-1), C=C (1 687 cm-1), -CH2- (2 800-3 000 cm-1), -OH (3 307 cm-1) were found to be rich on the surfaces of 2 kinds of biochars, and all of the properties were good for Pb2+ adsorption. The XRD patterns suggested that carbonate and basic carbonate containing Pb2+ were present in these biochars after adsorption. The adsorption equilibrium of RS and FS was reached in 4 and 10 h, respectively, and the adsorption capacity of RS and FS reached 307.59 and 220.07 mg/g, respectively. The adsorption kinetics were best fitted by the Pseudo second order model, while the isothermal adsorption was best described by Langmuir isotherm. This indicated that Pb2+ ions were adsorbed onto 2 kinds of biochars via monolayer. The adsorption mechanism for Pb2+ was a complex interaction of physical and chemical factors, mainly including electrostatic interaction, ion exchange/ligand exchange, cationic-π and precipitation. The results obtained show that 2 kinds of oil crop biochars are both excellent adsorbents to Pb2+ in aqueous solution and will provide important information on applying as low cost adsorbents for removal of heavy metals in contaminated water.