Effects of Domestic Sewage on the Photosynthesis and Chromium Migration of Coix lacryma-jobi L. in Chromium-Contaminated Constructed Wetlands

To investigate the effects of domestic sewage on the photosynthesis and chromium migration of plants in chromium-contaminated constructed wetlands, small vertical flow constructed wetlands of Coix lacryma-jobi L. were set up. These wetlands were used to treat wastewater containing 0, 20, and 40 mg/L of hexavalent chromium (Cr (VI)), prepared with domestic sewage (DS), 1/2 Hoagland nutrient solution (NS), and 1/2 Hoagland nutrient solution prepared with domestic wastewater (DN), respectively. The aim was to investigate the effects of domestic sewage on indicators, such as plant growth and chromium accumulation. The results were as follows: (1) Plant heights were significantly inhibited under 20 mg/L and 40 mg/L Cr (VI) treatments, and stem diameters were not significantly affected. The use of domestic sewage in treatment alleviated the inhibition of Cr (VI) on the growth of Coix lacryma-jobi L. (2) Indicators such as root activity, photosynthetic gas exchange, and chlorophyll fluorescence properties significantly decreased with the increase in Cr (VI) concentration. The values of these photosynthetic gas exchange parameters under the DN treatment were the greatest, followed by NS and DS. On the 70th day of Cr (VI) treatment, the net photosynthetic rate ( Pn ) under the DN treatment was significantly higher than that under NS and DS treatments. (3) Glutathione (GSH) content in roots, stems, and leaves of Coix lacryma-jobi L. significantly increased with the increase in Cr concentration, and it increased more significantly under the DN and DS treatments than under the NS treatment. (4) With the same Cr treatment, the Cr content in roots, stems, and leaves of Coix lacryma-jobi L. under the NS treatment was the highest, followed by DS and DN. The total Cr content in the substrate under the DN treatment was the highest, followed by DS and NS. (5) The addition of domestic sewage reduced the Cr (VI) content in the water sample and increased the organic matter content. The Cr (VI) content in the water sample under the NS treatment was the highest, followed by DS and DN. The addition of domestic sewage increased the accumulation of chromium in the substrate, decreased the absorption of chromium by plants, increased GSH content in roots, stems, and leaves, alleviated the damage of Cr (VI) to plants, and thus benefited the growth of Coix lacryma-jobi L. in the constructed wetlands and ensured the sustainable and stable operation of the wetlands.