Abstract
One of the main environmental concerns of landfills is the generation of leachate. Landfill leachate is known to be a complex and variable substance, containing pollutants such as dissolved organic matter, inorganic macro components, heavy metals, and xenobiotic organic compounds. Thus, it is potentially toxic to the environment and should be treated before being released into natural waters and lands. Affordable pre-treatment options that require low maintenance are crucial for long term leachate treatment. Phytoremediation is a sustainable, low-cost, method to pre-treat this wastewater when compared to conventional treatment such as transfer via recycling and combined treatment with domestic sewage, biodegradation, and chemical and physical methods. Among the different phytoremediation techniques, the use of artificial floating treatment wetlands (FTWs) is one of the most affordable options. However, given the complex nature of the leachate, selecting suitable plant species for phytoremediation of this substance is challenging. In this context, the use of the Florida native Rhizophora mangle for the pre-treatment of landfill leachate was proposed due to its high salinity tolerance and capacity to remove heavy metals and nutrients. This study explored the use of R. mangle in floating wetlands for landfill leachate treatment and its physiological response. The specific objectives of this study were to: (1) evaluate the physiological responses of R. mangle exposed to landfill leachate, (2) optimize plant physiology and survivability in FTWs for landfill treatment, (3) evaluate potential plant acclimation to leachate, (4) determine R. mangle uptake of landfill leachate pollutants, and (5) determine landfill leachate change after treatment. A pilot study was conducted by placing R. mangle seedlings in a hydroponic setting using floating mats. Fifteen plants were placed with no substrata and with their roots in direct contact with the leachate, while the other 15 were planted in perforated pots filled with expanded clay pebbles as substratum. Five plants from each group (with and without substratum) were exposed to three leachate treatments ranging from 100%, 50%, and 0% leachate concentrations (the leachate was diluted with tap water). A separate acclimation experiment was conducted by exposing R. mangle seedlings in perforated pots with clay pebbles to a control (25ppt saltwater), a 50% leachate treatment, and an acclimation treatment (increasing leachate concentration every two weeks from 50% to 75% to 100%). As stress responses to landfill leachate exposure, R. mangle seedlings showed wilting, chlorosis, necrosis and leaf shedding over time for both the pilot and acclimation experiment and different leachate treatments. These stress responses could have been triggered by multiple stressors, such as abrupt exposure to high salinity and heavy metals exposure. However, during the pilot experiment, R. mangle seedlings exposed to 50% leachate concentrations with substratum showed moderate stress responses and a recovery on photosynthetic functions and stomatal conductance after two weeks of exposure compared to the ones exposed to 100% leachate. This recovery in seedlings physiology could be attributed to a potential plant acclimation to the new environmental conditions. On the other hand, seedlings in the acclimation experiment showed higher stress responses, which could be explained by external factors such as greenhouse conditions (i.e., high temperatures and vapor pressure deficit).
The complex nature of landfill leachate makes plant acclimation challenging as individuals must cope with multiple stressors simultaneously. However, despite the high toxicity of landfill leachate to R. mangle seedlings evidenced by impairments of growth and other plants physiology in both experiments, new leaf growth, and recovery of stomatal functioning by the end of the experiments was observed. Therefore, these results show that, under controlled environmental conditions, R. mangle seedlings could withstand lower concentrations of landfill leachate for a longer period of time.
While the removal of nutrients and chemical oxygen demand varied, R. mangle metal removal from leachate showed promising results: In particular, the 50% leachate treatment presented the highest removal efficiency for Ag, Ni, and Zn.
Future acclimation studies under controlled temperatures should be conducted to determine if progressive exposure to increasing leachate concentrations improves R. mangle performance and leachate treatment. Additionally, the presence of a substratum in FTWs seems to have delayed the stress responses of R. mangle. Thus, its implementation should be considered in future hydroponic studies. Finally, the combined use of FTWs with other methods, including aeration, artificial biofilm, and bacterial culture, might lower leachate toxicity and grant a higher removal efficiency.
