Phytoextraction of Ni from a toxic industrial sludge amended with biochar

Abstract

Agromining is a technology based on the phytoextraction of metals by hyperaccumulator plants, combining agronomic and hydrometallurgical processes, to produce metal-based compounds. So far, it has been primarily developed to recover Ni from ultramafic soils, but secondary materials, namely industrial wastes containing Ni, may offer new opportunities for agromining. However, because of the toxicity of such materials, plants cannot be grown without formulating suitable substrates. The aim here was to assess the feasibility of growing Ni-hyperaccumulating plants on a Technosol containing a toxic industrial sludge and to test the influence of a biochar amendment on plant growth and Ni uptake. A constructed soil was prepared by mixing a decontaminated soil with an industrial sludge containing high concentrations of Fe, Ni, P and Zn, and amending it with biochar at different rates (0 to 5 wt%, dry matter). An ultramafic, Ni-rich soil was used as a reference material. Pot experiments were conducted with the hyperaccumulator Alyssum murale and the non-accumulating plant Lolium multiflorum used as a reference plant. After twelve weeks of growth, plant shoots and roots and soil samples were collected and analysed. Soil pore water was also collected over the experiment and analysed.Results showed that the growth of both plants was higher on the constructed soil than on the ultramafic soil, and increased with biochar amendments. The highest amounts of phytoextracted Ni were reached by A. murale on the ultramafic soil in the presence of biochar, whereas they remained low on the constructed soil. Contrary to the ultramafic soil, the constructed soil contained high amounts of Zn which was shown to impair Ni uptake as a result of the strong competition between Ni and Zn. Further investigations should therefore focus on practical solutions for decreasing this competition in order to maximize Ni uptake. In conclusion, agromining was proven feasible on soils constructed from industrial waste containing metals, providing that such soils are carefully designed to meet hyperaccumulator requirements.