Sequential heavy metals extraction from polluted solids: influence of sulfate overconcentration

Area of Science:

• Environmental Chemistry
• Geochemistry
• Soil Science

Background:

• Understanding metal partitioning in solid phases is crucial for assessing environmental risk.
• Sulfate is a common anion in various environmental systems, potentially influencing metal behavior.
• Sequential extraction is a key technique for determining metal speciation in soils and sediments.

Purpose of the Study:

• To investigate the effect of sulfate on the chemical partitioning of copper (Cu), cadmium (Cd), and lead (Pb) in solid phases.
• To evaluate how sulfate influences metal adsorption onto goethite and natural clays.
• To elucidate metal adsorption mechanisms in sulfate-rich environmental systems.

Main Methods:

• Modified BCR sequential extraction protocol, accelerated by focused ultrasound.
• Systematic application to artificial goethite spiked with Cu, Cd, and Pb, and natural clays.
• Addition of calcium sulfate to achieve concentrations of 0.5, 1, and 1.5 g/L in extracts.

Main Results:

• Sulfate adsorption sites on goethite surfaces were found to be limited.
• Significant changes in Cd and Pb fractionation were observed in the presence of sulfate.
• Cu remained strongly adsorbed, showing minimal changes in fractionation.
• A decrease in metal amounts in the first three extraction fractions and an increase in the residual fraction were observed for Cd and Pb.
• Sulfate presence enhanced the adsorption of metals onto the studied solid phases.

Conclusions:

• Sulfate enhances the adsorption of Cd and Pb onto goethite and clay solid phases.
• The adsorption of metals onto solid phases is promoted in sulfate-rich systems.
• Hypotheses are proposed to explain the behavior and adsorption mechanisms of Cu, Cd, and Pb in sulfate-rich environments.