An evaluation of chromium removal mechanisms in constructed wetlands treating tannery wastewaters.

摘要

Constructed wetlands have been used for polishing tannery effluents, mainly utilizing their ability to reduce organic loadings. High costs associated with conventional treatment of tannery effluents make constructed wetlands an interesting alternative, especially in developing countries. The fate of chromium, a metal with toxic effects in the environment has not been documented to date as the waste stream passes through constructed wetlands.; In order to develop an understanding of the fate and partitioning of chromium in a typical constructed wetland treating chromium bearing tannery wastes, nine experimental wetlands were constructed at the University of Memphis. The experimental design allowed for operation under a range of conditions and wetland characteristics Experimental goals were to determine where the chromium accumulated in the wetland system, and the effect of intermittent loading on system performance.; Results showed removals for 5-day biochemical oxygen demand (BOD 5) and chromium of 95-99% and 90-99%, respectively. A statistically significant difference was found in chromium removal efficiencies for all systems, with the exception of the base design against the intermittent feed systems. The removal differences in unplanted systems achieved were attributed to the change in the hydraulic behavior of these wetlands. Peat systems presented a different chemical environment resulting in lower chromium removal efficiencies with no change in BOD removals. Analysis of plant tissues and media revealed that the majority of chromium was accumulated in the media, with less than 0.1% of the chromium contained in plant tissues. Chemical speciation modeling for all systems suggests a potential Cr removal mechanism through sorption/co-precipitation with iron oxides. Scanning electron microscope analysis confirmed minimal Cr on roots and the presence of Cr in biomass and on the gravel surface. Release studies indicate that for all systems, the majority of chromium was released in the iron- and organic-bound phases, supporting both biomass sorption and iron sorption/co-precipitation. Peat systems had significantly more chromium in the organic phase, possible due to the presence of humic acids. The hypothesized chromium removal through sedimentation and sorption to media and litter matter was confirmed in this research, though the exact mechanisms require further research for clarification.