Fe3+-Substituted Sr6YAl(BO3)6 Red Pigments Designed Without Pb, Cd, or Hg: Colour Performance, Chemical Stability, and Elemental Release Screening

Abstract

Red inorganic pigments traditionally rely on Pb-, Cd-, Hg-, or Cr(VI)-based colourants that carry avoidable hazard and exposure risks. Here, a family of Sr6YAl1−xFex(BO3)6 (0 ≤ x ≤ 1) pigments was prepared by solid-state reaction using Fe3+ as the chromophore and no Pb-, Cd-, Hg-, Cr(VI)-, or As-containing precursors. X-ray diffraction shows that the trigonal Sr6YAl(BO3)6-type framework (space group R-3) is retained as a single phase throughout the series; on the basis of isovalent charge, six-coordinate geometry and matched ionic radii, Fe3+ substitutes for octahedral Al3+ to form a continuous solid solution, and SEM-EDS mapping confirms a broadly homogeneous, single-phase Fe-containing borate. Increasing Fe content strengthens visible absorption and moves the colour from white to a light reddish-orange (warm tan), with the x = 1 end member giving L* = 69.69, a* = 10.08, b* = 12.79 and C* = 16.28. The colour is essentially unchanged in water and dilute acetic acid, shows a moderate shift in alkaline media (ΔE* = 8.05–10.21), and changes markedly only in 0.1 mol L−1 HCl (ΔE* = 15.07). Elemental release was screened by immersing 1.00 g of powder in 50 mL of water (H1), 0.01 mol L−1 acetic acid (H2) or 0.1 mol L−1 HCl (H3) for 24 h at room temperature, with each leachate read three times by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Water and dilute acid gave similar, modest release, whereas HCl mobilized substantial B, Sr and Y (530.72, 4639.42 and 470.54 mg L−1, i.e., up to 41.87%, 45.15% and 27.08% of the nominal inventory) but very little Fe (0.156 mg L−1, only ≈0.014% of the Fe inventory, and below detection in H1 and H2). The Fe chromophore is thus effectively immobilized even when the borate framework is attacked, decoupling colouration from chromophore leaching. The pigments demonstrate source-level elimination of regulated toxic metals together with strong chromophore retention; the non-standardized extraction provides a conservative release-potential screen rather than a regulatory compliance determination.