Hexachlorobenzene dechlorination as affected by nitrogen application in acidic paddy soil

Area of Science:

• Environmental Chemistry
• Soil Science
• Microbial Ecology

Background:

• Hexachlorobenzene (HCB) is a persistent organic pollutant found in acidic paddy soils.
• Understanding factors influencing HCB dechlorination is crucial for environmental remediation.
• Nitrogen fertilizers can alter soil conditions affecting contaminant degradation.

Purpose of the Study:

• To investigate the impact of different nitrogen fertilizers on HCB dechlorination in acidic paddy soil.
• To determine the optimal application rates of nitrogen fertilizers for promoting HCB dechlorination.
• To elucidate the dechlorination pathway and identify key metabolites.

Main Methods:

• Batch incubation experiments were performed using acidic paddy soil.
• Different nitrogen fertilizers (ammonium bicarbonate, urea, sodium nitrate) were applied at varying concentrations.
• Hexachlorobenzene (HCB) concentrations and its dechlorinated products were analyzed using chromatography.
• Soil pH and microbial activity were monitored.

Main Results:

• Ammonium bicarbonate and urea showed similar effects on HCB dechlorination, with optimal promotion at 0.14 g N/500 g soil.
• High application rates (0.84 g N/500 g soil) of ammonium bicarbonate and urea inhibited HCB dechlorination.
• Sodium nitrate inhibited HCB dechlorination by acting as an electron acceptor and lowering soil pH.
• The dominant HCB dechlorination pathway was HCB → PeCB → TeCB → TCB, with pentachlorobenzene (PeCB) as the main metabolite.
• The role of methanogenic bacteria was found to be uncertain and dependent on experimental conditions.

Conclusions:

• The application amount of ammonium bicarbonate and urea is critical for effective HCB reductive dechlorination in acidic paddy soils.
• Sodium nitrate is unsuitable for promoting HCB dechlorination due to its electron acceptor properties and pH-lowering effect.
• Further research is needed to clarify the specific role of methanogenic bacteria in HCB dechlorination under various soil conditions.