Copper extraction and phytotoxicity of organic acid leached mine tailings

Mine tailings pose environmental hazards but can also contain economically valuable metals like copper (Cu). Organic solvents, particularly low molecular weight organic acids (LMWOAs), are natural and biodegradable acids with great potential for Cu removal and tailings remediation. We evaluated citric, malic, maleic, malonic and DL-lactic acids (1 M) in extracting Cu from legacy Cu-tailings and assessed their potential toxicity to plants. Brassica napus was grown in acid-leached tailings at three dilutions (1/4, 1/16, 1/128) to simulate solvent biodegradation or dilution by rainfall. Plant height, biomass, chlorophyll, shoot green cover and leachate metal concentrations (Al, Ca, Cu, Fe, K, Mg, Mn, V, Zn) were measured; deionised water was used as a control. At 1/4 dilution, citric, malic, and malonic acids generated the highest Cu concentrations in leachate (625–965 mg L⁻¹). This resulted in poor plant growth, likely due to Cu and Al toxicity. At 1/128 dilution, all acids had minimal impact on growth, with plant biomass similar to controls (41 g), though citric acid mobilised more elements at nutritional levels. Increased macronutrient (Ca, K, Mg) availability from diluted acids led to lower root/shoot ratios, suggesting partial alleviation of nutrient deficiencies. Multivariate analyses showed distinct metal leaching profiles among LMWOAs, which converge with dilution, except for citric acid, which retained discrete patterns even at 1/128 dilution. These findings indicate that citric acid has strong potential for Cu-tailings remediation and rehabilitation, initially extracting high Cu and Al concentrations and supplying essential nutrients as concentrations are diluted.