A novel process to recover cadmium and zinc from the hyperaccumulator plant Noccaea caerulescens
Résumé
Soils contaminated with heavy metals represent a serious threat for humans and all ecosystems. In some cases, they can be remediated by metal phytoextraction, using accumulator or hyperaccumulator plants, however the fate of harvested metal-enriched biomass has to be addressed. Cadmium (Cd) and, zinc (Zn) are very common contaminants of urban, industrial and agricultural soils. To date, no process has been developed to recover these metals from plant biomass.This contribution presents a novel hydrometallurgical process designed to recover Cd and Zn from the biomass of a Zn/Cd hyperaccumulator plant Noccaea caerulescens. Dried plants are first ashed at 620 degrees C, a process which has been carefully investigated to avoid metal loss, and then the ash is acid leached. Following this, the process comprises two main steps: one to extract Cd and Zn and the other for Cd cementation using Zn powder and selective precipitation of Zn. Optimal cementation conditions have been determined with synthetic solutions using a Box-Behnken experimental design. In this context, a mole ratio Zn:Cd of 2:1, a temperature of 25 C and a duration of 50 min proved optimal. The full process was tested on a sample of plant biomass. It demonstrated that Cd and Zn recovery was possible by cementation and precipitation. This new process still has to be optimized and up-scaled but it paves the way to Cd and Zn recovery from diverse types of soils, and even to 'metal cultivation', an emergent phytoteclmology now referred to as agromining.