Assessment of total and mobile heavy metals in amended compost of water hyacinth obtained from Lake Victoria, Kisumu County
Towett, Kiprono Gideon
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Water hyacinth (Eichhornia Crassipes) invasion in Lake Victoria has become a matter of concern over the last two decades. There has been suggestion on the possibility of producing compost from water hyacinth as a way of managing the weed cover in the lake and at the same time help local farmers in the region boost their agricultural production by supplementing the use of commercial chemical fertilizers. However, the safety of using water hyacinth’s compost has to be investigated as studies have shown that water hyacinth accumulates heavy metals from the environment. The specific objectives of the study were to determine the total concentrations of heavy metals (Pb, Cd, Ni and Cu) in water hyacinth compost, to determine the effect of the carbonaceous materials (cattle manure and molasses), and effective microorganisms treatment of water hyacinth’s compost feedstock materials on the mobile concentrations of heavy metals and to determine the relationship between total and mobile concentrations of heavy metals in compost. In the study, water hyacinth compost was prepared using commercial effective microorganisms (EM) solution and carbonaceous materials (cattle manure and molasses) separately as starter cultures for composting. Compost samples were digested with concentrated nitric acid to extract total metals and mobile concentrations were extracted using de-ionized water. Concentration of heavy metals in the compost was determined using Atomic Absorption Spectroscopy and the data collected was analyzed for significant differences (p < 0.05) using one-way Analysis of Variance (ANOVA) with Statistical Analysis System (SAS) Version 9.2 software. Means were separated using Tukey`s test at 5 % level. Pearson correlation analysis was also carried out to check the significances of the linear relations between total and mobile concentrations. Assessment of heat released during composting of water hyacinth showed that water hyacinth compost attained stability and thus maturity after 60 days of composting. The range of total heavy metals in compost samples were 1.23-1.46 mg/kg (copper), 0.32-0.35 mg/kg (cadmium), 0.25-0.32 mg/kg (nickel), 0.95-1.41 mg/kg (lead). The mean concentrations of the mobile heavy metals in mg/kg for the control, EM, manure and molasses treatments were copper: 0.054, 0.055, 0.027 and 0.049. Cadmium: 0.023, 0.032, 0.018, and 0.035. Nickel: 0.036, 0.027, 0.033, and 0.031. Lead: 0.043, 0.061, 0.023 and 0.093. There were significant differences in the mean concentrations of total and mobile concentrations of heavy metals between treatments. The total concentrations of heavy metals in water hyacinth compost, irrespective of treatments, were within acceptable limits and the addition of carbonaceous materials significantly affected the mobility of heavy metals. Nickel had an all positive relationship between total and mobile concentrations. However, for Cu, Cd and Pb an increase in total concentrations did not necessarily correspond to an increase in mobile concentrations. The mobile concentrations of heavy metals in compost were not predictable from their total content. Manure treatment had the least fraction of mobile heavy metals. It is therefore recommended that the composting of water hyacinth for organic fertilizer and as a way of disposing harvested water hyacinth biomass be done. The study also recommends amending of water hyacinth compost feedstock with cattle manure in order to reduce mobility of heavy metals in compost and consequently help alleviate possible adverse effects of compost on the environment.