Optimization of Lignite Particle Size for Stabilization of Trivalent Chromium in Soils

Abstract

Stabilization capacity of 5% by weight lignite added in three particle size ranges (<0.5; 0.5–1.0 and 1.0–2.0 mm) to acidic (pH = 5.0) or calcareous (pH = 7.7) sandy soil samples of <2 mm grain size artificially contaminated with 375 mg/kg Cr(III) was assessed through fit-for-purpose leaching tests performed with CH3COOH, NH2OH·HCl and EDTA. About 90% and 60% of Cr(III) were immobilized in calcareous and acidic soils per se, respectively. Although the point of zero charge measurements (pHpZC≈4.5) indicated that lignite could effectively bind positively charged Cr hydroxo complexes at the pH of the calcareous soil, the application of lignite is unnecessary due to the low solubility of these species. Although the acidic soil mobilizes Cr, the addition of lignite has been proven to stabilize it. The phytoavailable Cr fraction in the soils per se was about 10–25%. In spite of the mobilization of Cr by acidic soil, the phytoavailable Cr concentration decreased to about half when the lignite particle size was either 0.5–1.0 or 1.0–2.0 mm. Lignite was suitable for Cr stabilization in acidic soil in a close particle size range, ensuring the dispersibility needed to enhance efficient Cr transfer from the contaminated soil to the stabilizer.