Testing the Feasibility of Citric Acid-Assisted Nickel Agromining with Tropical and Temperate Hyperaccumulator Plants

The efficiency of using chelating agents in combination with hyperaccumulator species for nickel phytoextraction has yielded mixed results thus far. This study evaluated the effects of citric acid application on metal solubility in ultramafic soils and the feasibility of its use for nickel agromining by Berkheya coddii and Bornmuellera emarginata. Four citric acid application doses (0, 20, 40, and 60 mmol kg^-1) were used in a tropical and in a temperate ultramafic soil for 90 days for B. coddii and 45 days for B. emarginata, and pH and metal solubility assessments were conducted after application. Citric acid application increased the solubility of nickel, cobalt, and manganese in the soil by up to 20-fold but negatively impacted the biomass. Berkheya coddii accumulated up to dozens of times more Ni, Co, Cr, Cu, Mn, and Zn in its leaves than did B. emarginata. However, the citric acid did not enhance nickel accumulation in either of the two species. Additionally, citric acid application increased the level of impurities in the bio-ores from the plants. Synchrotron X-ray fluorescence analysis revealed that the tissue-level distribution of metals within the plants, with or without citric acid treatment, highlights manganese toxicity as the most significant limitation for Ni phytoextraction. The results of this study show that the use of citric acid is not an effective strategy for nickel agromining; however, its capacity to mobilize multiple metals suggests potential applicability in phytoremediation of multicontaminated soils.