Speciation and cysteine-simplified physiological-based extraction technique (SBET) bioaccesibility of heavy metals in biosolids

Abstract

Cysteine residues on proteins have a high affinity for metals yet formulations used to determine bioaccessibility do not contain cysteine or thiol-containing molecules. As a result, we used a cysteine-simplified physiological-based extraction technique (SBET) and, the conventional glycine-SBET to determine bioaccesibility of selected heavy metals in biosolids and compared the data. We also determined speciation of the selected metals in the biosolids to assess further the health risk posed the use of biosolids as a soil amendment in agricultural soils. Samples, including a certified reference standard were analyzed using x-ray fluorescence and flame atomic absorption. Bioaccessibility was higher in cysteine-SBET than glycine-SBET, and regression data show that the two methods give different sets of results. We proposed a bioaccessibility model that involves cysteine and the hydrogen ion complementing each other to dissolve metals. The model also includes a three mode-bioavailability mechanism: absorption of free metal ions; ligand-mediated transport of metal ions from solution; and ligand-mediated transport of metal ions directly from the biosolids into the cell. Low pH in the gut increases bioaccessibility but reduces bioavailability due to protonation of receptor ligands. With the exception of Fe, bioaccessibility was directly correlated to the sequential extraction availability which followed the order: Mn(90.3 %)>Zn(50.3 %)>Cd(26.5 %)>Cu(24.9 %)>Fe(0.367 %). We calculated bioavailability from bioaccessibility using literature estimates of percent bioavailabilities. The order of abundance of the analyzed metals in the biosolids was as follows: Fe>Mn>Zn>Cu>Pb>Cd.