Phyto-extraction of zinc, lead, nickel, and cadmium from a zinc leach residue

Abstract

Zinc leach residue is one of the substantial industrial wastes containing large quantities of heavy metals, releasing toxic elements into the environment. Phyto-extraction was applied to extract zinc, lead, nickel, and cadmium by Amaranthus retroflexus plant. Through three stages of screening, soil amendment, and modification, the bio-concentration factor, weight of biomass, and lifetime of Amaranthus retroflexus were enhanced. Due to the high salinity of the residue, following screening tests, mixture of 25 wt% zinc leach residue +75 wt% soil, was determined as the optimum condition. For further reduction of the salinity, the soil amendment was carried out both by draining and leaching of the residue with distilled water. In the leached condition, the plant illustrated better results. Finally, soil modification was performed by adding two levels of citric acid (0.005 M and 0.1 M) to the previous optimum conditions. Using the lower level of citric acid, bio-concentration factor of zinc, lead, nickel, and cadmium were obtained as 0.6838, 0.3042, 1.1027, and 2.3621, respectively and the plant survived 5 weeks. Phyto metal extraction index (ratio of concentrations of metal in dried plant to concentration of metal in tailing) for zinc, lead, nickel, and cadmium at the final optimum condition were obtained as 17.09%, 7.60%, 27.57%, and 59.05%, respectively. The separation factors showed that Amaranthus retroflexus’s tendency to extract heavy metals was in ascending order for lead, zinc, nickel, and cadmium.