Copper(II) removal by natural siderite (FeCO3) from surface and groundwaters

Anthropological use of land and resources releases vast amounts of waste into surface waters and aquifers. Copper(II) is one of the most widely occurring heavy metal contaminants, introduced into the environment from industrial discharge, landfill leakage, agricultural and mining sources. Common remediation strategies for redox-sensitive Cu(II) are based on adsorption or phytoremediation methods. To experimentally test the efficiency of Cu(II) retention by inorganic redox reaction processes suitable for in situ surface- and groundwater remediation applications, we used siderite (FeCO₃), which is abundant in anoxic sediments and soils and as a carbonate highly soluble in acidic environments. Its dissolution increases alkalinity and releases highly reactive, redox sensitive Fe(II). This aqueous ferrous iron can act as 1) a precursor for Fe(III) (hydr)oxides in oxic conditions, which are effective sorbents of heavy metals, and 2) a reducing agent under reducing conditions, where it can form a strong redox couple with Cu(II). We investigated the long term (1008 h) removal of aqueous Cu(II) through siderite dissolution batch experiments under oxic and anoxic conditions and monitored changes in the aqueous concentrations of Cu and Fe, pH and the reacted solids morphology over time. Cu adsorption and speciation on the reaction products was determined by X-ray absorption and photoelectron spectroscopies.

Under oxic conditions, increasing alkalinity led to a rapid increase in solution pH and the precipitation of nanoparticulate goethite and hematite from the released ferrous iron. After 1008 h of reaction, 80 % of the dissolved Cu(II) were removed from solution by sorption, whereby up to >30 % of this sorbed Cu(II) was reduced to Cu(I). Under anoxic conditions, the solution pH increased abruptly and copper uptake occurred more than twice as fast as under oxic conditions. Notably, the released Fe(II) was oxidized by Cu(II) leading to the precipitation of lepidocrocite, while all copper was removed from solution with >70 % of Cu(II) being reduced to Cu(0).

Our results suggest that 1) redox reactions between aqueous Cu(II) and Fe(II) promote coupled dissolution-precipitation and adsorption mechanisms responsible for Cu(II) removal, and that 2) siderite is a low-cost and effective material that is potentially useful for in situ remediation in either oxygenated or reduced environments.