Microbial Reduction of Fe(III)-Bearing Solids Recovered from Hydraulic Fracturing Flowback Water: Implications for Wastewater Treatment

Earth Sciences Environmental Engineering Chemistry Geochemistry Engineering Environmental Chemistry Life Sciences Microbiology
hydraulic fracturing flowback water microbial reduction wastewater treatment Fe(III)-bearing solids

Hydraulic fracturing is a technique used to extract natural gas from unconventional reservoirs, but it generates large volumes of flowback and produced water. These waters contain complex mixtures of organic and inorganic constituents, particularly the solids associated with these fluids, which are often rich in iron (Fe), toxic organics, heavy metals, and naturally occurring radioactive materials (NORM). Despite the potential environmental and human health risks posed by these solids, research on their composition and interactions with microbial communities remains limited. Furthermore, the long-term environmental fate of these solids is not well understood.

This study aimed to analyze the solids associated with flowback water from a hydraulically fractured well in the Bowland Shale, UK, and explore the potential for microbial reduction of these Fe(III)-rich solids under anaerobic conditions. Using the electron shuttle anthraquinone-2,6-disulfonate (AQDS), the research team identified the bioreduced mineral phases and analyzed the microbial community composition. The study provides new insights into the development of microbial-based wastewater treatment strategies, particularly how to utilize these Fe(III) solids to oxidize toxic organics and reduce the toxicity of the waste.

Source of the Paper

The research was conducted by Natali Hernandez-Becerra, Sophie L. Nixon, Christopher Boothman, and Jonathan R. Lloyd from the Williamson Research Centre for Molecular Environmental Science, Department of Earth and Environmental Sciences, University of Manchester, UK. The paper was accepted on December 2, 2024, and published in the journal Geo-Bio Interfaces with the DOI 10.1180/gbi.2024.11.

Research Process and Results

1. Recovery and Characterization of Solids

The study first recovered suspended solids from the flowback water of the Bowland Shale. Solids were separated using centrifugation and characterized using X-ray diffraction (XRD). The results showed that the solids mainly consisted of akaganeite (β-FeOOH) and barium-bearing celestine (SrSO4). Additionally, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to further confirm the morphology and elemental composition of these minerals.

2. Microbial Reduction Experiments

The research team designed a series of microbial reduction experiments using two inocula: a pure culture of Shewanella frigidimarina (a facultatively anaerobic halophilic bacterium) and an Fe(III)-reducing enrichment culture derived from the flowback water. The experiments were conducted under anaerobic conditions, with AQDS used as an electron shuttle. The production of Fe(II) was monitored, and the bioreduced mineral phases were analyzed using XRD and SEM.

The results showed that Fe(III) solids in all inoculated treatments were reduced to Fe(II), with the addition of AQDS significantly accelerating the reduction process. In the Shewanella frigidimarina and enrichment culture treatments, Fe(II) concentrations increased from 0.5 mmol/L and 2 mmol/L to 15 mmol/L and 21 mmol/L, respectively. XRD analysis indicated the formation of ankerite (Ca(Fe,Mg,Mn)(CO3)2) in the enrichment culture treatment, while highly crystalline Fe(II) minerals were not detected in the Shewanella treatment, possibly due to the formation of amorphous mineral phases.

3. Microbial Community Analysis

Using 16S rRNA gene sequencing, the research team analyzed the microbial community composition in the solids. The results showed that the dominant microbial taxa in the solids included Chromohalobacter, Caminicella, and putative Fe(III)-reducing genera. In the Shewanella treatment, sequences assigned to the genus Shewanella were dominant, while in the enrichment culture treatment, sequences most closely related to the genus Fuchsiella were most abundant. The Fe(III)-reducing capabilities of these microorganisms provide potential applications for microbial-based wastewater treatment strategies.

Conclusions and Significance

This study is the first to provide a detailed characterization of Fe(III)-bearing solids in flowback water from the Bowland Shale, UK, and demonstrates the potential for microbial reduction of these solids. The results show that Fe(III) solids can be transformed into more manageable mineral phases, such as ankerite, through microbial reduction. Additionally, the microbial reduction process can be optimized by adjusting conditions (e.g., pH and salinity) to promote the formation of magnetic minerals like magnetite, thereby improving waste recovery efficiency.

This research offers new perspectives for managing hydraulic fracturing wastewater, particularly how to utilize microbial reduction of Fe(III) solids to oxidize toxic organics and reduce waste toxicity. Furthermore, the study highlights the important role of microbial communities in wastewater treatment, providing a scientific basis for the design of future wastewater treatment strategies.

Research Highlights

  1. First characterization of flowback solids from a non-North American shale system: This study is the first to provide a detailed analysis of flowback solids from the Bowland Shale, UK, filling a geographical research gap.
  2. Potential for microbial reduction of Fe(III) solids: The study demonstrates the potential for microbial reduction of Fe(III) solids and identifies the bioreduced mineral phases.
  3. Application of electron shuttles: The addition of AQDS significantly accelerated the Fe(III) reduction process, offering new insights for optimizing microbial reduction conditions.
  4. Diversity of microbial communities: The study reveals the rich microbial communities in flowback solids, particularly the presence of Fe(III)-reducing bacteria, providing important references for wastewater treatment strategies.

Additional Valuable Information

The study also notes that barium (Ba) and strontium (Sr) (potentially including Ra-226) in the flowback water remain immobilized in the mineral phases during Fe(III) reduction, indicating the potential stability of these phases in wastewater treatment. Additionally, the research team suggests that future studies should further explore the ability of microbial communities to degrade organics and identify functional genes related to organic degradation through metagenomic approaches.

This research provides a new scientific basis for managing hydraulic fracturing wastewater and lays an important foundation for the development of microbial-based wastewater treatment strategies.