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

•The minerals of biochar-amended soils were mainly covered by biochars.•The polarity of the biochars influenced the adsorption of Cd2+.•Plant- and manure-derived biochars had different impact on Cd2+ adsorption on soil.•Swine manure-derived biochars were effective in immobilizing Cd2+ in soil. 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.