Cadmium adsorption on plant- and manure-derived biochar and biochar-amended sandy soils: Impact of bulk and surface properties

To investigate the role of the bulk and surface composition of both biochar and biochar-amended soils in the adsorption of Cd2+, as well as the influence of different biochars added to the soils on Cd2+ adsorption, swine-manure-derived biochars (BSs) and wheat-straw-derived biochars (BWs) were produced at 300, 450, and 600 °C. These biochars were added to a sandy soil to investigate the effect of biochars on the adsorption of Cd2+ by soil. The significantly higher surface C content of the amended soils compared to their bulk C content suggests that the minerals of the biochar-amended soils are most likely covered primarily by biochars. The maximum adsorption capacity (Qmax,total) of the BSs was 10-15 times higher than that of the BWs due to the high polarity and ash content of the BSs. The polarity ((N + O)/C) of the low-temperature biochars greatly affected their Cd2+ adsorption. The Qmax,total of the BS-amended soils increased with increasing dose, whereas the Qmax,total of the BW-amended soils showed the opposite behavior, which was attributed to the different surface composition characteristics of the two types of soil. The BSs were more effective in immobilizing Cd2+ upon application to the soil relative to the BWs. This study elucidates the spatial distribution of biochars in biochar-amended soils and highlights the importance of the surface composition of the investigated samples in Cd2+ adsorption.