Bone Char: A Clean and Renewable Phosphorus Fertilizer with Cadmium Immobilization Capability

Soil contamination with Cd from P fertilizer and other anthropogenic and geogenic sources is a serious problem. In situ immobilization by P application to soil is known as an applicable remediation technique leading to reduced Cd uptake by plants, and use of a Cd‐free P fertilizer from renewable sources would be most favorable. Bone char (BC) (15% P, 28% Ca, 0.7% Mg) may be used as such a quality P fertilizer, but it is unknown if its dissolution in soil provides sufficient P and immobilizes Cd in moderately contaminated soils. We incubated BC and triple superphosphate (TSP) in 11 soils that contained between 0.3 to 19.6 mg Cd kg −1 and determined the kinetics of P dissolution during a time period of 145 d. The concomitant Cd immobilization was determined by extracting the mobile Cd with 1 mol L −1 NH 4 NO 3 solution. For most soils, BC increased the concentration of labile P immediately after application, reaching a maximum after 34 d, although the solubility was below that of TSP (2.9–19.3 vs. 4.1–24.0%). Among five kinetic models, the Langmuir‐type equation provided the best description of P dissolution from BC and TSP. The Cd immobilization resulting from BC dissolution exceeded that of TSP by a factor of 1.4 to 2.7. The P dissolution from BC was negatively correlated with pH and positively with P sorption capacity, whereas Cd immobilization was positively correlated with soil pH. These causal relationships were expressed in multiple equations that enable predictions of P dissolution and Cd immobilization and thus may help to introduce BC as sustainable P fertilizer and useful soil amendment.