Qualitative and quantitative characterization of Cd2+ adsorption mechanisms was performed with rice-straw and sewage-sludge biochars produced at different temperature (300-700 degrees C), respectively. The pH effect, adsorption kinetics and isotherms were investigated, and chemical analyses of Cd2+-loaded biochars were conducted by SEM-EDS, XRD, FTIR and Boehm titration. This demonstrated that rice-straw biochars (RSBs) have greater adsorption capacities for Cd2+ than sewage-sludge biochars (SSBs), which was mainly due to precipitation and cation exchange mechanisms, with their contribution proportion to total adsorption from 76.1% to 80.8%. While in SSBs, both mechanisms were overshadowed by coordination with p electrons mechanism accounting for 59.2%-62.9% of total adsorption, even the role of cation exchange was negligible in the adsorption mechanisms accounting for 2.3%-6.7%. The relationship of each mechanism with biochar's properties were discussed, which further deepen our understanding of adsorption on biochars. These results suggest RSBs have great potential for removing Cd2+ from aqueous solutions.