Effects of different chromium containing wastewater on residue decomposition and chromium chemical forms in Coix lacryma-jobi constructed wetland

Abstract

To investigate the effects of two different water sources, domestic sewage and nutrient solution, on residue decomposition and the transformation of chromium chemical forms in Cr (Ⅵ) contaminated constructed wetlands, a micro Coix lacryma-jobi constructed wetland was constructed to treat chromium containing wastewater［using domestic sewage (DWS) and 1/2 Hoagland nutrient solution (HNS) as simulated solutions containing 0, 20, 40 mg·L-1 Cr (Ⅵ), respectively］. By using the buried root decomposition method, the decomposition dynamics of residual roots and the transformation pattern of chromium chemical forms were studied. The results were as follows: (1) Under 20 and 40 mg·L-1Cr (Ⅵ) stress, the growth of C. lacryma-jobi was inhibited. The plant height and stem diameter of HNS treatment were greater than those of DWS treatment, but the plant height and stem diameter of HNS treatment were more inhibited by Cr (Ⅵ) than those of DWS treatment. (2) The decomposition rate of C. lacryma-jobi residual roots decreased with the increase of Cr (Ⅵ) concentration, and the decomposition rate of HNS treatment was higher than that of DWS treatment. After 60 d of decomposition, under DWS treatment, the residual root chromium content of 20 and 40 mg·L-1 Cr (Ⅵ) treatment decreased by 11.70% and 8.09% respectively compared with that at the initial stage of root burial, while under HNS treatment, the residual root chromium content decreased by 15.80% and 18.42% respectively. The percentage of residual chromium in C. lacryma-jobi root residues at 20 and 40 mg·L-1 treatments decreased with the extension of root burial time, while the percentage of ethanol-extracted chromium and deionized water extracted chromium increased, and the percentage of acetic acid bound chromium increased significantly. (3) In the early stage of residue decomposition, the COD, TN, NH4-N, and total chromium content in the effluents treated with HNS and DWS increased, and then decreased. The trend of change was consistent with the residue decomposition process. HNS treated constructed wetlands were more efficient in removing chromium from wastewater. The research results indicate that after harvesting plants in constructed wetlands, root decomposition can quickly increase the chromium content in the effluent, and appropriately improve the nutritional status in wastewater, which can promote residue decomposition and removal of chromium by wetlands.

Keywords

• chromium
• constructed wetland
• chemical form of chromium
• residue decomposition
• water quality