Quantitative mechanisms for the Cd2+ adsorption increased by sewage-sludge biochar but decreased by rice-straw biochar after melamine modification

To explore the relative contributions of different mechanisms to Cd2+ adsorption by melamine (C3H6N6) modified biochars, the adsorption characteristic experiments including adsorption kinetics, isotherms and thermodynamics were performed, and the qualitative and quantitative characterization of adsorption mechanisms were further analyzed by SEM-EDS, XRD, FTIR, XPS and the calculation, respectively. When initial pH increased from 1.0 to 7.0, the adsorption capacities were increased, and began to decline after reaching maximum capacities at pH of 5.0 for both modified biochars. After C3H6N6-modification, the maximum adsorption capacity of sewage-sludge biochar (SSB) increased from 64.20 to 126.75 mg/g, but the maximum capacity of rice-straw biochar (RSB) decreased from 52.03 to 25.65 mg/g. For both pristine biochars, the adsorption process was better described by pseudo-second-order kinetic and Langmuir isotherm models, whereas it did not change the applicability of the adsorption models after modification. The thermodynamics confirmed that all the adsorptions were endothermic process under different temperatures of 293, 303 and 313 K, suggesting higher temperature was more conducive to the adsorption. Such an enhancement was supported by more aromatic structure in modified SSB (N-SSB) compared to SSB, which was qualitatively confirmed by FTIR and XPS observation, and the enhancement was fundamentally caused by the increases in the importance of C pi coordination mechanism, accounting for 20.30 % (SSB) to 41.12 % (N-SSB) of total adsorption. While for modified RSB (N-RSB), the reduction was primarily explained by the decreased contribution of cation-exchange mechanism to total adsorption, since the contribution proportion was reduced from 80.16 % to 55.65 % after modification. The present analyses would shed light on the mechanisms underlying the adsorption by similar modified adsorbents, and the application of rice-straw biochar by C3H6N6-modification for the Cd2+ adsorption should be investigated prudently in future.