Decomposition of plant litter under chromium pollution and associated characteristics of chromium release

Abstract

In vegetated constructed wetlands, plants used for phytoremediation may release pollutants back into the aquatic system as a result of decomposition after senescence, lessening wastewater disposal efficiency. After treatment of wastewater containing chromium with Alternanthera philoxeroides in constructed wetland, plant litter was used to study the release characteristics of chromium with the biomass decomposition under different levels of Cr pollution. Results indicate that decomposition rates of plant litter under zero and low-level Cr pollution were larger than those under high pollution concentration. Under low Cr intensity, the total Cr concentration in the residual increased in the first 40 days, and then decreased to 67.72% of the initial concentration. In the end, the residual ratios of plant litter in different pollution intensities were 57.91%, 48.16% and 71.79% of the initial mass on average separately, and about 57.45%, 67.14% and 38.32% of Cr had been released into the aquatic environment. The changes in percentages of chemical forms in residual should be correlated with the decomposition process reflected by the interactive effects. The release of Cr could be divided into two stages, i.e. immobilization and discharge. These results were possible coming from the moderating effect of different Cr intensities on microbial decomposers.