Availability of copper in mine tailings with humic substance addition and uptake by Atriplex halimus

The chemical characteristics of mine tailings, organic amendments (doses), and plants are the critical factors that must be evaluated and monitored to ensure the sustainability of phytostabilization. The aim of this study was to evaluate the mobility of copper (Cu) in mine tailings (MT) of the Zone Central of Chile to which commercial humic substances were added, examining their effect on the uptake of Atriplex halimus. Two commercial humic substances (HS1 and HS2) extracted from leonardite (highly oxidized lignite), of different pH and total organic carbon, were evaluated by adsorption curve for Cu. In columns, soluble Cu, pH, and electrical conductivity in leachates were evaluated for MT, MT + HS1, and MT + HS2, and HS1 and HS2 in doses of 120 mg kg^-1. In pot assay, seeds were germinated directly in MT and cultivated for 140 days with the addition of HS2 in 120 and 240 mg kg^-1. Mine tailing presents high concentration of Cu (2016 ± 223 mg kg^-1, pH 6.3 ± 0.1). The results of sequential extraction indicate that Cu is associated with the sulfide fraction of low risk of mobility. The amount of Cu sorbed by HS1 was higher than that sorbed by HS2, and both humic substances showing better fit to the Freundlich than Langmuir model. Lixiviation of Cu was significantly lower in MT + HS1 (0.166 ± 0.043 mg kg^-1) and MT + HS2 (0.157 ± 0.018 mg kg^-1) than in MT (0.251 ± 0.052 mg kg^-1). Copper concentration in plants reached 185.8 ± 37.8 mg kg^-1 in the roots and 32.6 ± 7.4 mg kg^-1 in the aerial parts cultivated in MT without effect of the humic substance addition in Cu uptake nor growth. Copper concentrations in the aerial parts were adjusted to sufficient or normal levels in plant. A good management of mine tailings through phytostabilization could consider an adequate mixture of humic substances (to avoid leaching of metals) and an organic amendment that provides essential nutrients and increases biomass generation.