Study On Transfer And Accumulation Of Chromium (Cr) In Constructed Wetland Systems Wih Coix Aquatica Roxb

The experiment was carried out to investigate the transfer and accumulation of chromium (Cr) in constructed wetland systems with or without Coix aquatica Roxb. Cr content in the effluent of the system and chemical forms of Cr in different substrate layer of the constructed wetland systems were analyzed. The results were as follows:(1) Planting C. aquatic in constructed wetland system decreased the Cr6+flux of wetland systems. The Cr6+flux increased with the increase of Cr treatment concentration in all four kinds of wetland systems. Compared to the system using sewage as nutrient source, the Cr flux from the system using Hoagland nutrient solution as nutrient source was lower. Compared to the systems without planting C. aquatic, the Cr flux from the systems with C. aquatic was lower.(2) Planting C. aquatic in constructed wetland system changed the Cr content in substrate of systems in different months. With the low Cr (10mg/L) treatment, the total Cr content in substrate of wetland systems with or without planting C. aquatic decreased with the increase of treatment time. With the medium Cr (30mg/L) treatment, the total Cr content in substrate of wetland systems with planting C. aquatic decreased with the increase of treatment time, while the total Cr content in substrate of wetland systems without planting C. aquatic showed an opposite trend. With the severe Cr (50mg/L) treatment, the total Cr content in substrate of wetland systems increased with the increase of treatment time, whether planting C. aquatic or not.(3) Planting C. aquatic in constructed wetland system changed the Cr content in different substrate layers of wetland systems. In the wetland systems with planting C. aquatic, the Cr distribution in substrate was:upper layer> middle layer> lower layer. In the wetland systems without planting C. aquatic, the Cr distribution in substrate showed an opposite trend. It is suggested that planting C. aquatic promote the Cr accumulated at upper layer of substrate in the wetland systems.(4) Planting C. aquatic in constructed wetland system changed the Cr forms in different substrate layers of wetland systems. With the increase of Cr treatment concentration and treatment time, all of five Cr forms in the substrate of the all wetland systems increased, regardless of planting C. aquatic and nutrient source. Compared to other Cr forms, the residual form of Cr showed the biggest increase. The Cr content of exchangeable and carbonate-bounding forms in the substrate of wetland systems with planting C. aquatic were higher than that without planting C. aquatic, while the Cr content of iron-manganese-oxide bounding, organic bounding and residual forms in the substrate of wetland systems with planting C. aquatic were lower than that without planting C. aquatic. In the wetland systems with planting C. aquatic, the Cr content of exchangeable and carbonate-bounding forms in upper layer of substrate were higher than that in lower layer of substrate, while the Cr content of iron-manganese-oxide bounding, organic bounding and residual forms in upper layer of substrate were lower than that in lower layer of substrate. The distribution of Cr forms in different layer of substrate in the wetland systems without planting C. aquatic showed an opposite trend.