The mechanism of complexation of Cd,Zn\|humic substance by ion\|exchange method was studied.The results showed that the stability
constans(log?K) and
coordination numbers of cadmium,zinc\|humic acids complexation increased gradually with the complicated structure of humic acids.The stability constants(log?K HA ) and coordination number of Cd,Zn\|humic acid changed with the molecular weight of graduations of humic acid.When the molecular weight was 48339,the stability constants of Cd\|humic acid and Zn\|humic acid were 5 20 and 4 81,
respectively,and their coordination numbers were 1 26 and 1 15,respectively.However,when the molecular weight was 78180,the stability constants were 5 68 and 5 37,respectively,and the coordination number were 1 32 and 1 26,respectively.When the molecular weight was 2300 the stability constants(log K FA ) of Cd\|fulvic acid and Zn\|fulvic acid were 4 44 and 4 42,respectively,and the coordination numbers were 1 30 and 1 31,respectively.It showed the heavy metal fixation by complexation with the complicated\|structure of humic acids,on the contrary,the migration of cadmium and zinc increased by the simplified\| struture of humic acids in soil.