Assessment of Trace Metal Accumulation in Compost Produced from Water Hyacinth Biomass in Southern Togo

Abstract

Water hyacinth (Eichhornia crassipes) is an invasive aquatic plant that rapidly proliferates in the lagoons of Greater Lomé, Togo, disrupting aquatic ecosystems and human activities by obstructing waterways, reducing light penetration, and decreasing dissolved oxygen. However, this plant is of significant scientific interest due to its ability to absorb and concentrate trace metals (TMs) in its tissues, offering potential for pollution monitoring and biomass valorization. This study assessed the bioaccumulation of TMs in water hyacinth and the quality of compost produced from this biomass. Samples were collected from three lagoons heavily impacted by anthropogenic activities: Nyékonakpoè, Bè, and Hanoukopé. Water hyacinth was processed into compost in two forms: chopped biomass and whole biomass, each enriched with 5% cow manure and subjected to a 16-week aerobic composting process. Results showed higher TM accumulation in the submerged parts, with maximum concentrations of Hg up to 809.84 µg/kg at Bè, Cu up to 14.48 mg/kg, and Pb up to 12.941 mg/kg at Hanoukopé. The final compost contained TMs well below regulatory limits (Cd ≤ 3 mg/kg, Pb ≤ 180 mg/kg, Cu ≤ 300 mg/kg, Ni ≤ 60 mg/kg), with notable values for Cd (0.35 mg/kg) and Pb (7.23 mg/kg). Chopped compost exhibited higher essential nutrient contents: K = 8,453.84 mg/kg, N = 8,307.25 mg/kg, and P = 1,095 mg/kg, reflecting improved mineralization. An alkaline pH (≈ 8.0) favored nutrient stabilization and reduced TM bioavailability. These findings demonstrate that valorizing water hyacinth as compost provides a sustainable solution, combining invasive biomass management with the production of a nutrient-rich and safe organic amendment for agriculture, while allowing environmental monitoring of trace metals.