Mangrove soils in removing pollutants from municipal wastewater of different salinities

Soil leaching experiments were conducted to assess the capacity of mangrove soils in purifying synthetic wastewater containing pollutant concentrations four times of that found in local municipal sewage and of two salinities (fresh vs, saline mater). Results on leachate nutrient and heavy metal concentrations reveal that the mangrove soils were capable of removing certain amount of pollutants from wastewater, and the removal efficiency varied between pollutants. The soils were most effective in retaining heavy metals such as Cu but were less effective for Mn and Zn. Similarly, the wastewater-borne NH4+ was more easily leached than P. The soil data show that most pollutants were accumulated in the top layer (0-1.5 cm) of the soil tray, with little downward migration. Differences between treated and control soil nutrient and heavy metal concentrations were not found in the soil masses below the surface 1.5 cm. In the surface layer, the mangrove soils treated with wastewater had significantly higher concentrations of NH4+-N, total and extractable P, total and extractable Cu, Cd, Zn and Mn. On the other hand, there was no significant elevation in total nitrogen content in mangrove soils treated with wastewater when compared with the control. Soils receiving wastewater prepared in deionized water (fresh) had slightly higher pollutant concentrations, and larger enrichment factors than that treated with saline wastewater (containing 1.5% salinity). These results suggest that mangrove soils could retain pollutants from wastewater but its efficiency would slightly be affected by salinity.