Heavy metal speciation in solid-phase materials from a bacterial sulfate reducing bioreactor using sequential...

Area of Science:

• Environmental Chemistry
• Geochemistry
• Microbiology

Background:

• Heavy metal mobility, bioavailability, and toxicity are dictated by their chemical forms, influencing environmental pollution potential.
• Understanding metal speciation in sludges from sulfate-reducing bioreactors is crucial for assessing environmental risks.

Purpose of the Study:

• To determine the partitioning and chemical forms of heavy metals (Fe, Ni, Zn, Cu) and a metalloid (As) in anoxic solid-phase material (ASM).
• To evaluate the mobility and bioavailability of these elements using sequential extraction and sulfide measurements.

Main Methods:

• A modified Tessier sequential extraction procedure (SEP) was employed.
• Acid volatile sulfide (AVS) and simultaneous extracted metals (SEM) measurements were conducted.
• Partitioning into six operational fractions: water soluble, exchangeable, carbonate-bound, Fe-Mn oxide-bound, organic matter/sulfide-bound, and residual.

Main Results:

• The organic matter and sulfide fraction was the primary sink for As, Fe, Ni, Zn, and Cu (59.8-86.7%).
• SEM/AVS ratios were consistently below one, suggesting low toxicity and bioavailability.
• Apparent mobility increased in the order Cu < As < Ni < Fe < Zn.

Conclusions:

• Heavy metals in ASM are predominantly associated with organic matter and sulfides, forming stable metal sulfides.
• The low SEM/AVS ratio indicates a low probability of bioavailable metals in pore water, suggesting the sludge is non-toxic.
• The study provides critical insights into metal speciation for environmental risk assessment of bioreactor sludges.