Mechanisms of Manganese Removal from Wastewaters in Constructed Wetlands Comprising Water Hyacinth (Eichhornia crassipes (Mart.) Solms) Grown under Different Nutrient Conditions Ranil K. A. Kularatne 1 *, Jagath C. Kasturiarachchi 2 , Jagath M. A. Manatunge 2 , Suren L. J. Wijeyekoon 3 ABSTRACT: This article discusses key mechanisms involved in re- moving 1 mg/L Mn from synthetic wastewaters in constructed wetlands comprising water hyacinth (Eichhornia crassipes (Mart.) Solms) grown under different nutrient levels of 1-fold (28 mg/L and 7.7 mg/L of total nitrogen and total phosphorus, respectively), 2-fold, 1/4-fold, and 1/8-fold. A mass balance was carried out to evaluate the key removal mechanisms. Phytoremediation mainly due to phytoextraction substantially contributed to manganese removal. However, chemical precipitation was absent, suggest- ing that manganese has a higher solubility in the given average pH (6.2 to 7.1) conditions in constructed wetlands. Bacterial mediated immobilization mechanisms also did not contribute to manganese removal. Sediments constituted a minor sink to manganese, implying that manganese has a poor adsorption potential. Constructed wetlands comprising water hyacinth are effective at removing manganese from wastewaters despite the fact that the plants are grown under higher or lower nutrient conditions. Water Environ. Res., 81, 165 (2009). KEYWORDS: constructed wetlands, manganese, phytoremediation, wastewaters, water hyacinth (Eichhornia crassipes (Mart.) Solms). doi:10.2175/106143008X370403 Introduction Manganese (Mn) is a vital element to all forms of life. However, manganese is toxic to humans at very high concentrations (Gunawardhana et al., 2002; Santos-Burgoa et al., 2001); it is also toxic to plants under pH conditions less than 5 (Meagher, 2000). In Sri Lanka, contamination of groundwater and freshwater bodies with manganese and other toxic metals is a serious environmental problem in the industrial areas of Ratmalana and Moratuwa (Gunawardhana et al., 2002). This has been largely attributed to the disposal of industrial wastewaters from the textile dyeing and garment washing industries, which are predominant industries in Sri Lanka, and to open dumping of sludge produced by the wastewater treatment plants encompassing chemical precipitation (Gunawardhana et al., 2002). Furthermore, absence of a discharge standard for man- ganese has led to indiscriminate disposal of manganese-containing wastewaters leading to widespread pollution of surface waterbodies and groundwater (Gunawardhana et al., 2002). However, legislation currently under consideration by the Sri Lankan government would establish a discharge standard of at least 0.5 mg/L Mn for inland waters. Countries such as India have enacted a similar permissible limit for inland waters for toxic metals, including 0.5 mg/L Mn (Bhatia, 2005). In the United States, a higher standard of 1.1 mg/L Mn for inland waters has been enacted under the National Pollutant Discharge Elimination System (Ye et al., 2001a). Presently, aeration followed by chemical precipitation are being used to remove manganese and other toxic metals such as iron (Fe) and arsenic (As) from contaminated groundwater and wastewaters in Sri Lanka. However, chemical precipitation is costly, requires intensive management and long-term maintenance, and is some- times less efficient. Moreover, the hazardous sludge produced poses disposal problems that necessitate secure landfills. Consequently, the hazardous sludge produced is often openly dumped, resulting in recontamination of groundwater and waterways. Constructed wetlands are effective at removing manganese, iron, and other toxic metals from polluted waters such as acid mine drainage waters (Mays and Edwards, 2001; Ye et al., 2001a, 2001b). In constructed wetlands, in situ processes such as various physicochemical processes and microbial immobilization mecha- nisms govern metal removal (Gavrilescu, 2004; Jayaweera et al., 2006; Jayaweera et al., 2007, 2008; Kosolapov et al., 2004; Matagi et al., 1998; Song et al., 2001). However, plant uptake or phytoremediation has not been documented to play a key role in removing metals in the case of constructed wetlands planted with emergent aquatic macrophytes such as Typha latifolia (Mays and Edwards, 2001; Ye et al., 2001a, 2001b). Nevertheless, recent studies reported that phytoremediation plays a key role in removing metallic pollutants in the case of floating aquatic macrophyte-based treatment systems or constructed wetlands comprising free-floating macrophytes such as water hyacinth (Eichhornia crassipes (Mart.) Solms) (Jayaweera et al., 2006; Jayaweera et al., 2007, 2008; Liao and Chang, 2004). 1 * Environmental Foundation Limited, 146/34, Havelock Road, Colombo 5, Sri Lanka; e-mail: RanilKularatne@yahoo.com.au. 2 Department of Civil Engineering, University of Moratuwa, Sri Lanka. 3 Department of Chemical and Process Engineering, University of Moratuwa, Sri Lanka. February 2009 165