Immobilization of Cr in a heavily Cr(VI)-contaminated soil using alkali-activated blast furnace slag and calcium polysulfide: Remediation efficiency and mechanism

Abstract

Chromium (Cr)-contaminated soil represents a significant environmental hazard, posing substantial risks to ecological systems. This study investigated the application of Calcium Polysulfide (CPS) and Ground Granulated Blast Furnace Slag (GGBFS) for the stabilization and solidification of Cr-rich soils. The research focused on four key aspects: leachability characteristics, mechanical strength development, hexavalent chromium [Cr(VI)] reduction efficiency, and stabilization mechanisms. Experimental results demonstrated that the treated soil achieved compressive strengths exceeding 2 MPa, indicating its potential suitability as a construction material for roadbeds. Both GGBFS and CPS exhibited strong reducing capabilities, effectively converting highly mobile Cr(VI) to the less mobile trivalent chromium [Cr(III)] species, thereby enhancing Cr stabilization. A reduction ratio of nearly 100 % was achieved with the theoretical dosage of CPS and 30 wt% GGBFS after 56 days of curing. The leached total Cr decreased from 295.6 to 2.1 mg/L, while the leached Cr(VI) concentration decreased from 165.1 mg/L to below the detection limit. The sequential extraction procedure according to Tessier’s method demonstrated that chromium was predominantly transformed into more stable fractions, specifically the iron-manganese oxide-bound and residual forms. X-ray diffraction and scanning electron microscopy analyses revealed that the hydration products, predominantly calcium silicate hydrate and ettringite, effectively filled the pores and contributed to the formation of a denser microstructure. The stabilization mechanisms of Cr were identified to involve four key processes: (1) reduction of Cr(VI) to Cr(III), (2) physical encapsulation within the matrix, (3) adsorption onto hydration gels, and (4) ionic substitution of Cr(III) and Cr(VI) into the ettringite structure.