Effect of molecular weight of humic acid on the release and chemical fraction of cadmium in sediments

The release and chemical fraction of cadmium (Cd) in sediment have a crucial influence on the ecology of rivers. Rivers containing large amounts of humic acid (HA) with the rapid development of industry and agriculture increase the solubility of Cd enhancing the bioavailability of Cd in sediments. Therefore, the possible interactions among influencing factors for the release and chemical fraction of Cd in sediments, such as HA concentration, pH, leaching time and molecular weight (Mw) of HA were investigated by using the European Community Bureau of Reference (BCR) sequential extraction procedure, ICP-OES, UV–Vis, FTIR and XRD technology. The quasi-equilibrium leaching time was 96 h, and the release rate of Cd in sediment decreased with increased pH from 3 to 7 and stabilized as pH increased from 8 to 10. It was also found that HA inhibited the release of Cd in sediment at pH 3 and 4, whereas at pH 5–10, HA promoted the release of Cd in sediment. The change in pH mainly affected the content of F1 and F2 for Cd in sediment. For > 100 kDa HA, the content of total Cd, F1 and F2 in sediment changed very little to a certain HA concentration (700 mg-C L −1 at the initial pH 6 and 350 mg-C L −1 at the maintained pH 6). For the Mw of 30–100 kDa HA, 10–30 kDa HA, and 1–10 kDa HA, the content of total Cd and F1 in the sediment decreased with the increase of HA concentration. For Mw < 1 kDa HA, the changes in HA concentration had little effect on the content and chemical fraction of Cd in the sediment. The Mw of > 100 kDa HA had effects on F1 and F2 of Cd in sediment, and the Mw of 30–100 kDa HA, 10–30 kDa HA, 1–10 kDa HA, < 1 kDa HA contributed only to F1 of Cd in sediment. The results demonstrated that the molecular weight of humic acid affected the release and chemical fraction of Cd in sediment.