Retention of pollutants by mangrove soil and the effects of pollutants on Kandelia candel

Mangrove wetlands were effective in retaining heavy metals and nutrients from wastewater. Significantly higher concentrations of heavy metals (Cr, Cu, Ni and Zn) and total Kjeldahl nitrogen (TKN) were found in the surface soil layer (0-5 cm) of a mangrove tide-tank system after receiving synthetic wastewater for 16 weeks. The pollutant concentrations in NW (normal wastewater) and 5NW (5 times of normal wastewater) treated surface soils did not change significantly after flushing with clean seawater for 38 weeks (the recovery period), suggesting the pollutants retained in mangrove systems were not leached out. On the other hand, a significant reduction of heavy metals (except Ni) and TKN in surface soils receiving the strong wastewater (25 NW, 25 times of normal wastewater) was observed at the end of the recovery period. During the first 6 weeks of the recovery period, heavy metals and nutrients were detected in the effluent discharged from 25NW tank indicating that some initial washout of pollutants in mangrove soils with more metals accumulation, although their concentrations declined rapidly. The growth of Kandelia candel, a dominant mangrove plant, was not inhibited by wastewater treatment and best growth was observed in tide tanks treated with 25NW. These results suggested that the tide-tank system was effective in retaining wastewater-borne pollutants and the problem of releasing the pollutants back to the flushing seawater was not obvious. Indeed, the retained nutrients and trace elements in mangrove soils provided an extra nutrient source and supported better plant growth