Phytoremediation of cyanide-laden mine tailings by lima beans (Phaseolus lunatus)

Abstract

• Lima beans ( Phaseolus lunatus ) reduced tailings cyanide from ∼33 to 13.4 mg/kg. • Cyanide accumulated mainly in roots, suggesting root-zone attenuation. • First-order kinetics predicted remediation (R 2 = 0.921). • Safe cyanide levels achievable in ∼130 days at 10 plants/m 2 . Cyanide contamination from gold mine tailings poses significant ecological and human health risks, particularly in artisanal and small-scale mining regions of Uganda. This study evaluated the phytoremediation potential of lima beans ( Phaseolus lunatus) for cyanide-laden tailings obtained from Greenstone Mining Limited under field conditions. Experimental plots were established at planting densities of 0, 5, 10, and 15 plants/m 2 and monitored over an 84-day growth period. Soil and plant tissue cyanide concentrations were quantified using a titrimetric method. Cyanide attenuation increased with planting density, reaching 42.6 ± 1.8%, 54.7 ± 1.4%, and 59.1 ± 0.8% at 5, 10, and 15 plants/m 2 , respectively, compared to 3.4 ± 1.2% in the unplanted control. Cyanide removal was most pronounced during the vegetative growth stage, coinciding with increased root biomass development. Tissue analysis showed cyanide distribution in the order of roots > stems > leaves > seeds, suggesting that attenuation occurred predominantly within the root zone through combined plant uptake, in situ phytodegradation, and rhizosphere-mediated processes, with limited translocation to edible tissues. First-order kinetic modeling provided the best fit to the 10 plants/m 2 treatment (R 2 = 0.921). Based on this model, the estimated indicative remediation time to reach the Canadian soil quality guideline limit for cyanide of 8 mg/kg is approximately 130 days. These findings demonstrate that P. lunatus significantly enhances cyanide attenuation beyond natural background processes and highlight its potential as a low-cost, field-applicable phytoremediation strategy for cyanide-contaminated mine tailings in tropical environments.