Single Extraction Study using Oxalic Acid, Citric Acid, and Calcium Chloride on the Bioavailable Fraction of Vanadium, Scandium, and Chromium in Red Mud

Department of Environmental Engineering, Institut Teknologi Sepuluh Nopember, Surabaya 60111, Indonesia

^* Corresponding author: voijant@its.ac.id

Abstract

Red mud, a by-product of alumina production, is a valuable secondary resource containing critical metals such as vanadium (V), scandium (Sc), and chromium (Cr). Identifying the bioavailable fraction of these metals is essential for assess their potential for sustainable recovery. However, single extraction methods often face limitations in efficiency and kinetics when identifying a universal reagent suitable for multiple metals. This study investigated the bioavailable fraction of V, Sc, and Cr in red mud using single extractions with oxalic acid, citric acid, and calcium chloride (CaCl₂) over a 28-day period. Few studies have directly compared oxalate, citrate, and CaCl₂ for red mud metal extraction, but existing evidence shows that organic acids enhance solubility via complexation, while CaCl₂ promotes ionic exchange of loosely bound metals. The results revealed that each extractant exhibited distinct effectiveness. For scandium, citric acid was the most effective, mobilizing (1.96%–8.21%), significantly outperforming oxalic acid (0.01%–0.03%) and CaCl₂ (0.1%–0.3%). These findings underscore that the extractant choice profoundly influences the perceived bioavailable fraction. The results provide insights for environmentally responsible metal recovery strategies, supporting the circular economy, and critical mineral policies. Therefore, the extractant must be tailored to each target metal’s geochemical characteristics.