Characterization and treatment efficency of heavy metals and nutrients in a model mangrove ecosystem in constructed tide-tanks

Kandelia candel plants were grown in a model tide-tank system in a greenhouse to determine the performance of the simulated mangrove ecosystem in treating synthetic wastewater of various strengths. The system was flooded with artificial seawater intermittently to simulate the tidal regime. This study was divided into three stages: fast, recovery and second stages. The system was irrigated with synthetic wastewater at artificially elevated different strengths in the first and second stages, but not the recovery stage. The distribution of the wastewater-borne heavy metals and nutrients in various components (effluent run-off water, soil and plant) of the system was examined. Results indicate that the removal efficiencies for heavy metals and nutrients (N and P) during irrigation of wastewater at 75 times of the normal strength were over 90% and 73%, respectively. When the wastewater irrigation was terminated, less than 5% of the retained heavy metals and nutrients leached out. Soil was the most effective component in retaining wastewater-borne pollutants in the model mangrove ecosystem. Among different layers of sediment, half of the heavy metals, and total N and P were immobilized in the first layer (0-2 cm) of the soils, indicating the slow mobility of the pollutants through the soil profile. Plants did show a capacity to retain the pollutants, but the overall contribution was small when compared to soils. This might be due to the very high mass ratio of soils to plants. In terms of total N and P concentrations (on a per unit dried weight basis), higher concentrations were found in plants than in soils, suggesting the importance of the plants in retaining pollutants N and P when they are in the field, where there would be a larger biomass of mature plants instead of only the young plants present in this study. The photosynthetic rate, leaf chlorophyll concentration, stem height and diameter of plants treated with wastewater were greater than those in the control (without wastewater), indicating that the addition of wastewater did not inhibit plant growth but served as an extra nutrient source to the plants. This study has demonstrated that the mangrove ecosystem can have a very high capacity to retain or immobilize the nutrients and heavy metals from wastewater, and the loading of strong wastewater would not pose a harmful effect on the plants for a period of decades