Chemical Speciation and Bioavailability of Heavy Metals During Water Hyacinth Composting

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Water hyacinth (Eichhornia crassipes) is the noxious weed in the whole world and it is most commonly used plant in constructed wetlands due to its fast growth rate and large uptake of nutrients and contaminants. The final disposal of water hyacinth used in wastewater treatment is still an unresolved problem. Composting and vermicomposting are the best alternative for management of this weed but final product may have high content of heavy metals due to accumulation of heavy metals in body parts of water hyacinth. The presence of non-biodegradable and high level of toxic heavy metals in the compost frequently hinders agricultural land application. Uptake of heavy metals by plants and subsequent accumulation along the food chain is a potential threat to animal and human health. The toxicity of heavy metals for plants and animals depend on the different forms of heavy metals rather than the total concentration. Thus the bioavailability and leachability of heavy metals play an important role in the toxicity of heavy metals during composting followed by land application. Therefore, studies were carried out on bioavailability, leachability and speciation of heavy metals (Zn, Cu, Mn, Fe, Ni, Pb, Cd and Cr) during agitated pile composting, rotary drum composting and vermicomposting of water hyacinth mixed with cattle manure and sawdust. Five different proportions (Trials 1, 2, 3, 4 and 5) of water hyacinth, cattle manure and sawdust were prepared for the composting process. Eisenia fetida earthworm was used for the vermicomposting process. The best proportion of waste materials during agitated pile (trial 4) and rotary drum composting (trial 4) was selected on the basis of bioavailability of heavy metals reduction. The effects of lime (1, 2 and 3%) and zeolite (5, 10 and 15%) on bioavailability, leachability and speciation of heavy metals were studied with best trial (trial 4). Total heavy metal concentration obtained after strong acid digestion of compost sample was useful as an overall pollution indicator. The bioavailability of heavy metals was investigated in the form of water soluble and diethylene triamine penta-acetic acid (DTPA) extractable. Tessier sequential extraction method was employed to investigate the changes in heavy metals speciation. Toxicity characteristics leaching procedure (TCLP) test was performed for leachability of heavy metals. This study investigated the influence of parameters such as pH, temperature and organic matter content on distribution of heavy metal bioavailability and leachability during composting of water hyacinth. Total concentrations of heavy metals were increased during agitated pile and rotary drum composting, however total concentrations of few metals (Cu, Mn, Fe, Ni and Cd) were reduced during the vermicomposting process.
Supervisor: Ajay Kalamdhad