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Molecular weight (Mw) is a fundamental property of humic acids (HAs),which considerably affect the mobility and speciation of heavy metals in the environment.In this study,soil humic acid (HA) extracted from Jinyun Mountain,Chongqing was ultra-filtered into four fractions according to the molecular weight,and their properties were characterized.Complexation of cadmium was investigated by titration experiments.For the first time,Lanuir and non-ideal competitive adsorption-Donna (NICA-Donnan) models combined with fluorescence excitation-emission matrix (EEM) quenching were employed to elucidate the binding characteristics of individual Mw fractions of HA.The results showed that the concentration of acidic functional groups decreased with increasing Mw,especially the phenolic groups.The humification degree and aliphaticity increased with increasing Mw as indicated by elemental composition analysis and FT-IR spectra.The binding capacity of Cd2+ to Mw fractions of HA followed the order UF1 (＜5 kDa)＞ UF2 (5-10 kDa)＞ UF4 (＞30 kDa) ＞ UF3 (10-30 kDa).Moreover,the distribution of cadmium speciation indicated that the phenolic groups were responsible for the variations in binding of Cd2+ among different Mw fractions.The results of fluorescence quenching illustrated that the binding capacity of Cd2+ to Mw fractions was controlled by the content of functional groups,while the binding affinity was largely influenced by structural factors.The results provide a better understanding of the roles that different HA Mw fractions play in heavy metal binding,which has important implications in the control of heavy metal migration and bio-toxicity.