Speciation of Phosphorus and Cadmium in a Contaminated Soil Amended with Bone Char: Sequential Fractionations and XANES Spectroscopy

The phosphorus (P) and cadmium (Cd) speciation was analyzed in a contaminated soil having a Cd concentration of 1,028 mg kg(-1) in order to assess the value of bone char (BC) as a Cd immobilizing agent. The soil was incubated with BC and triple superphosphate (TSP, as control) in the dark at 60 to 70 % water holding capacity for time periods between 1 and 145 days. Samples from the various incubation periods were sequentially extracted and investigated by X-ray absorption near edge structure (XANES) spectroscopy. The sequential P extraction revealed that BC increased the H2SO4 extractable P fraction, mainly consisting of Ca- and Mg-phosphates, by 14 %, whereas TSP increased the water extractable P fraction by 7 % of total P at day 1. Subsequently, the proportions of these two P fractions decreased during incubation. The increase in these two P fractions is explained by the solubility of the main components of BC (hydroxylapatite (HAP)) and TSP (Ca(H2PO4)(2)). Furthermore, this finding was confirmed for BC by the P K-edge XANES spectra using partial least square regression that provided independent evidence for increased proportion of HAP after BC application. As a result of BC dissolution, the soil pH increased resulting in Cd immobilization as indicated by significantly reduced concentrations of mobile Cd in the sequential extraction up to 23 mg Cd kg(-1). This observed immobilization was explained by the Cd L-3-edge XANES spectra revealing an immediate increase in the proportion of insoluble Cd-3(PO4)(2). By contrast, none of the speciation methods indicated a Cd immobilizing capability of TSP. Thus, by using this multi-methodological approach, we could show that besides its potential as renewable and clean P fertilizer, BC is a superior immobilization agent for Cd compared to TSP.