In-situ Assembly of Bacteria-nano Metal Sulfide Bio-hybrid and Its Application for Heavy Metal Removal and Electrocatalysis

Abstract

Massive toxic heavy metals abundantly produced from industrial activities such as mining, metallurgy, electroplating, and electronic manufacturing have caused great potential threats to the natural ecological environment and human health. Thus, it’s of great importance to efficiently remove heavy metals from wastewater for ensuring the water environment and water resources security. However, traditional physical-chemistry technologies, such as absorption, ion exchange, chemical/ electrochemical precipitation and membrane filtration, are limited by the potential secondary pollution and unsatisfactory efficiency. Therefore, more efficient and environment-benign new technologies for heavy metals treatment should be developed as soon as possible.

In the last few years, biosynthesis of nanomaterials using microbial cells as “bio-factory” has attracted great attention due to the advantages of temperate reaction conditions, economical and promising potential for heavy metal removal and recovery. Previous reports have demonstrated that biogenic sulfide generated from microbial metabolism could react with heavy metal ions and form metal sulfide nanoparticles to recover heavy metals from wastewater. Meanwhile, some biogenic nanomaterials are capable of reducing high-toxicity heavy metals due to their strong reducing property and excellent dispersibility. However, the application for heavy metal-containing wastewater treatment of this existing technology is still limited by complicated extraction and purification processes. Therefore, this dissertation focuses on the controllable biosynthesis of metal sulfide nanomaterials and its application for heavy metals removal and recovery. The main contents and results of this dissertation are listed as follows:

1. Self-regenerable bio-hybrid with biogenic ferrous sulfide nanoparticles for treating high-concentration chromium-containing wastewater. Biogenic ferrous sulfide nanoparticles (bio-FeS) as green-synthesized nanomaterials are promising for heavy metals removal, but the need for complicated extraction processes and the production of iron sludge still restrict their practical application. In this chapter, we selected a typical dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1 to synthesize bio-FeS using thiosulfate and ferric iron as precursors and developed a sustainable bio-hybrid consisting of bacterial cells and self-regenerable bio-FeS to efficiently remove hexavalent chromium (Cr(VI)). A dense layer of bio-FeS was self-assembled on the cell surface of S. oneidensis MR-1, endowing the bacterium with good Cr(VI) tolerance and unusual activity for bio-FeS-mediated Cr(VI) reduction. Meanwhile, an artificial transmembrane electron channel was constituted by the conductive bio-FeS distributed in the intracellular, periplasmic and extracellular spaces to facilitate extracellular electron pumping, enabling efficient regeneration of extracellular bio-FeS for continuous Cr(VI) reduction. The bio-hybrid maintained high activity within three consecutive treatment-regeneration cycles for treating both simulated Cr(VI)-containing wastewater (50 mg/L) and real electroplating wastewater. Importantly, the bio-hybrid activity can be facilely and fully restored through bio-FeS re-synthesis or regeneration with replenished fresh bacteria. Overall, the interaction mechanisms between the bacterial cells and biogenic nanoparticles were elucidated. The bio-hybrid merges the self-regeneration ability of bacteria with the high activity of bio-FeS nanoparticles, opening a promising new route for the sustainable treatment of heavy metal-containing wastewater.

2. The pH response and regulatory mechanisms of self-assembly bio-FeS-containing bio-hybrid and its application for heavy metal-rich wastewater treatment. In the biological self-assembled bio-hybrids, bio-FeS on the cell surface play vital roles in the processes of microbial extracellular electron transfer and enhanced removal of toxic heavy metals. However, this system still suffers from slow synthesis and regeneration of bio-FeS and bacterial activity decay during continuous operation. Further optimization of the bio-FeS synthesis process and properties is of vital importance to address this challenge. In this chapter, we present a simple pH-regulation strategy to enhance bio-FeS synthesis and elucidated the underlying regulatory mechanisms. Slightly raising the pH from 7.4 to 8.3 led to a 1.5-fold higher sulfide generation rate due to upregulated expression of thiosulfate reduction-related genes, and triggered the formation of fine-sized bio-FeS. The bio-hybrid constructed at optimal pH exhibited significantly improved extracellular reduction activity and was successfully used for the treatment of higher-concentration Cr(VI)-containing wastewater (80 mg/L) at satisfactory efficiency and stability. Furthermore, the bio-augmented system coupled with this bio-hybrid and active sludge was feasible to treat real Cr(VI)-rich electroplating wastewater, showing no obvious activity decline during the 7-day operation. Overall, our work provides new insights into the environmental responses and regulatory mechanisms of biosynthesis of metal sulfide nanomaterials and may have important implications for the optimized design of bio-hybrid and its application for water pollution control.

3. Upcycling Ni from electroplating wastewater for the biosynthesis of nickel-iron sulfide and its application for electrocatalytic oxygen evolution reaction. Besides efficient performance for heavy metals removal, biosynthesis of nanomaterials also provides an alternative platform for heavy metals recovery from wastewater. In this chapter, high value-added nickel iron-sulfide electrocatalyst (bio-NiFeS) was synthesized by a typical sulfate-reducing bacteria Desulfovibrio vulgaris Hildenborough using nickel ions, iron ions and sulfate as precursors and applied for efficient oxygen evolution reaction. Results demonstrated that the conductivity and electron transfer efficiency could be efficiently improved by regulating the molar ratio of elemental Ni and Fe, while the surface properties could be optimized via annealing treatment, obviously promoting the electrocatalytic performance of the biogenic electrocatalyst. The optimal biogenic nickel-iron sulfide electrocatalyst (bio-Ni_0.70Fe_0.26S) exhibited comparable electrocatalytic performance to those synthesized using chemical methods and superior electrocatalytic performance to commercial RuO₂ with a current density of 10 mA·cm^-1 at an overpotential of 247 mV, and a Tafel slope of 60.2 mV·dec^-1. The bio-NiFeS synthesized using real electroplating wastewater containing nickel ions and sulfate as precursor also performed impressive electrocatalytic activity and stability. This work verifies the feasibility of recovering heavy metals from real electroplating wastewater and synthesizing high-performance electrocatalysts, also demonstrates the great potential of biosynthetic nanomaterials technology in the fields of heavy metal wastewater treatment, resource recovery and renewable energy production.

Date of Award	10 Mar 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wenwei Li (External Supervisor) & Xin DENG (Supervisor)