Abstract
Cadmium (Cd) is a toxic element that can be consumed by humans through food, posing a public health problem. Application to soil of zinc chloride (ZnCl2), and amendments, such as compost and biochar, have been proposed as solutions to prevent Cd from entering the food chain. Biochar is a stable carbon-rich charcoal that is produced by an eco-friendly method. The objective of this research was to compare the effectiveness of the treatments as remediation materials for an agricultural soil enriched with Cd. Pot experiments were conducted with spinach and soil material collected in the Salinas Valley of California, for three consecutive life-cycles of spinach. We utilized five biochars (pre-selected in laboratory) in their unoxidized and oxidized (with hydrogen peroxide - H2O2) versions, at application rates of 1% and 5% by volume. Cd uptake by spinach was compared to uptake in soil alone (control), soil + 25 mg/kg Zn, and soil + 5% poultry litter compost by volume. Compared to the control, some biochars significantly reduced Cd uptake and increased yield. Compost was also a successful amendment and may be the most cost-beneficial for farmers, although biochar produces carbon credits. The Zn treatment generally reduced Cd uptake, but also reduced yield. To achieve uptake values close to the CODEX limit (4 mg/kg dry wt.) it is necessary to lime all treatments to approximately pH 7. The oxidation of biochars with H2O2 did not decrease Cd uptake.