Changes of chemical fractions of Cd in the mixed substrates of constructed wetland

Five mixed substrates for constructed wetland were constructed to absorb Cd in water, and different Cd fractions were monitored during the operation period. The results indicated that there were significant differences in the Cd concentration of each fraction among the mixed substrates at the end of the operation. The Cd in exchangeable fraction among different substrates had significant variation. Compared to the other three fractions, the Cd concentration in organic matter and residual fractions was relatively steady and low. The Cd in exchangeable, carbonate and Fe-Mn oxide fractions was dominant in the total Cd, while the organic matter and residual fractions were only small part of it. The Cd concentration in all fractions increased with increasing operation time, and the changes of organic matter and residual Cd were relatively not obvious.