Kinetics and mechanism of ultrasound-enhanced citric acid leaching of chromium from steel slag.

Steel slag is a promising eco-material, yet its utilization remains constrained by chromium (Cr) levels exceeding regulatory limits in many countries. This study investigated the selective leaching of Cr from steel slag using an ultrasound-enhanced organic acid system. Among six carboxylic acids screened, including several bio-based options, citric acid exhibited the highest Cr extraction (63.3%) and Cr/Ca selectivity (2.86), attributed to its multidentate chelation capability. Under optimized ultrasonic conditions (0.8 mol L-1 citric acid, 240 W, 30 °C, 10 min), Cr extraction reached 84.3%, reducing residual Cr from approximately 2189 mg kg-1 to 281 mg kg-1. Kinetic analysis indicated that conventional leaching followed a chemical reaction-controlled shrinking core model (Ea = 48.10 kJ mol-1), whereas ultrasonic leaching exhibited mixed-control behavior with a markedly lower apparent activation energy (Ea = 18.02 kJ mol-1), suggesting that cavitation reduced interfacial mass transfer resistance. Multi-scale residue characterization supported that ultrasound enhanced Cr accessibility through particle refinement, pore development, and disruption of the Ca-Si interlocking layer. A stability window was identified: prolonged reaction or elevated temperature promoted Si(OH)4 accumulation and probable gel formation, impairing solid-liquid separation. Confining the process within this window allowed the primary Cr extraction to proceed before gel-dominated effects prevailed. Leaching toxicity tests (GB 5086.1-1997) suggested that treated residues met environmental safety standards. This work clarifies the apparent enhancement mechanism of ultrasound-assisted citric acid leaching and provides a feasible basis for the green utilization of Cr-bearing metallurgical slags.