Reduction of Mobile Heavy Metals During Composting of Water Hyacinth Weed

Abstract

Water hyacinth (Eichhornia crassipes [Mart.] Solms) invasion in Lake Victoria has become a matter of concern over the last two decades. Composting of harvested water hyacinth biomass can help protect the precious lake from the proliferation of the nuisance weed. This would also go a long way in helping local farmers boost their agricultural production by supplementing the use of commercial chemical fertilizers. However, there is a need to ensure that the final compost is environmentally safe before being applied as a soil conditioner. The study, therefore, assessed the total and mobile concentrations of heavy metals during an above ground aerobic composting of water hyacinth biomass with commercial effective microorganisms (EM), cattle manure and molasses treatments. Samples were digested with concentrated nitric acid to extract total metals and mobile concentrations were extracted with deionized water. The concentrations of heavy metals in the compost were determined using Atomic Absorption Spectrometer, and the data collected was analysed for significant differences (p < 0.05). Means were separated using Tukey's test at 5% level. The total concentrations of heavy metals were within the threshold limits prescribed for agricultural application. The range of total heavy metals in compost samples was 1.23- 1.46 mg/kg (copper), 0.32-0.35 mg/kg (cadmium), 0.25-0.32 mg/kg (nickel) and 0.95- 1.41 mg/kg (lead). However, the water-soluble forms of copper (2.3-4.0% of total Cu), cadmium (1.0-8.7% of total Cd), and lead (1.6-8.7% of total Pb) were lower than that of nickel (7.2-11.3% of total Ni) indicating that nickel in the composts had a higher toxicity potential. To alleviate the possible adverse effects of water hyacinth compost in the environment, the study recommends composting of water hyacinth compost biomass with cattle manure.