Highly charged swelling mica reduces free and extractable Cu levels in Cu-contaminated soils

Smelting of copper (Cu) results in the atmospheric deposition of Cu onto surrounding soils. Excess concentrations of Cu in soils can be absorbed by soil biota to toxic levels or leached into the groundwater, threatening the entire ecosystem. A means to restrict Cu mobility and uptake by plants is to remove it from the aqueous phase by applying an adsorptive material. A synthetic clay (highly charged swelling mica) was tested for its ability to decrease the levels of free and 0.1 M KNO₃-extractable Cu in 15 surface soils from three different Cu mining areas in central Chile. The soils contained excessive total Cu levels (112-2790 mg Cu (kg soil)^-1), while extractable Cu ranged from 0.3 to 22.9 mg Cu L^-1. The mica was applied to each soil at rates of 0.1%, 1%, and 2% (w/w). A 2% sodium-montmorillonite treatment and the nonamended soil served as controls. The order of treatment efficacy in reducing extractable Cu and free Cu²⁺ for low pH soils (<pH 5.5) was: 2% mica > 1% mica > 2% montmorillonite > 0.1% mica. At 120 days, the 2% mica treatment maintained reductions of up to 93% in the free Cu²⁺ activity and up to 75% in the extractable Cu concentration upon acidification to the original soil pH value. In addition, Cu retention in mica-treated soils was more resistant to acidification than in lime-treated soils. This mica has promise for the remediation of acidic soils with metal contamination at the surface.