In this study, a pristine biochar (BC) and Fe-Mn binary oxide-biochar (FMBC) were prepared using Pennisetum sp. straw as the feedstock for Cd(II) removal from aqueous solutions. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and specific surface area (SSA) analyses revealed the physico-chemical characteristics of the pristine and designer adsorbents, suggesting that an ultrasonic treatment during synthesis enhanced the SSA and pore volume of the BC, and assisted successful loading of Fe-Mn binary oxide particles on the BC surface. The Cd(II) adsorption data of the adsorbents were fitted to the Langmuir isothermal and pseudo-second-order kinetic models. At a system temperature of 25 ℃ and pH 5, the maximum Cd(II) adsorption capacities of BC (30.58 mg/g) and FMBC (95.23 mg/g) were obtained. Multiple Cd(II) adsorption mechanisms by FMBC were identified, including precipitation with minerals, complexation with surface functional groups, Cd(II)-π interactions，and cation exchange. As the most dominant adsorption mechanism, Cd-O bonds were formed on the FMBC surfaces precipitating Cd(OH)2 (63.9 wt%) and CdO (36.1 wt%). The FMBC thus could be potentially used as an effective adsorbent for Cd(II) removal from aqueous solutions.

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Carbon sequestration of organic materials and biochar in soil is a vital way to increase soil organic carbon (SOC), but the stability of SOC especially the humic substances (HS) is different with these amendments. In this study, cotton straw, compost derived from cotton straw (compost) and biochar derived from cotton straw (biochar) were added with equivalent carbon content into soil and incubated for 180 days. The structural characteristics of humic acid (HA), fulvic acid (FA) and humin (Hu) was studied by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The results showed that the aryl C of Hu and FA increased by biochar treatment or biochar incorporated straw or compost and decreased with low ratio of biochar incorporated straw or compost. In addition, phenolic C of Hu and HA increased by straw treatment or biochar incorporated straw and increased with low ratio of biochar incorporated straw or compost. However, O-alkyl C of Hu and HA increased by compost treatment or biochar incorporated compost and increased with low ratio of biochar incorporated straw or compost. These results suggest that the stability of FA amended with straw or compost can be enhanced by incorporating biochar through increasing aryl functional groups, but the stability of HA amended with straw or compost can be enhanced by incorporating biochar through increasing alkyl groups. Biochar was in favor of aryl C formation of FA and Hu, while straw or compost was beneficial to alkyl C formation of HA and Hu.