Interference of Nitrite with Pyrite under Acidic Conditions: Implications for Studies of Chemolithotrophic Denitrification

Pyrite-based denitrification combined with electrochemical disinfection to remove nitrate and microbial contamination from groundwater

Nitrate and microbial contamination of groundwater can occur in countries that face intense urbanization and inadequate sanitation. When groundwater is the main drinking water source, as is often the case in such countries, the need to remove these contaminants becomes acute. The combination of two technologies is proposed here, a biological step to denitrify and an electrochemical step to disinfect the groundwater, thereby aiming to reduce the chemical input and the footprint of groundwater treatment. As such, a pyrite-based fluidized bed reactor (P-FBR) was constructed to autotrophically denitrify polluted groundwater. The P-FBR effluent was disinfected in an electrochemical cell with electrogenerated Cl2. Nitrate was removed with 79% efficiency from an initial 178 mg NO3- L-1 at an average denitrification rate of 171 mg NO3- L-1 d-1, with 18 h hydraulic retention time (HRT). The electrochemical unit achieved a 3.8-log reduction in total coliforms with a 41.7 A h m-3 charge density.

Goethite Formed in the Periplasmic Space of Pseudomonas sp. JM-7 during Fe Cycling Enhances Its Denitrification in Water

Denitrification-driven Fe(II) oxidation is an important microbial metabolism that connects iron and nitrogen cycling in the environment. The formation of Fe(III) minerals in the periplasmic space has a significant effect on microbial metabolism and electron transfer, but direct evidence of iron ions entering the periplasm and resulting in periplasmic mineral precipitation and electron conduction properties has yet to be conclusively determined. Here, we investigated the pathways and amounts of iron, with different valence states and morphologies, entering the periplasmic space of the denitrifier Pseudomonas sp. JM-7 (P. JM-7), and the possible effects on the electron transfer and the denitrifying ability. When consistently provided with Fe(II) ions (from siderite (FeCO3)), the dissolved Fe(II) ions entered the periplasmic space and were oxidized to Fe(III), leading to the formation of a 25 nm thick crystalline goethite crust, which functioned as a semiconductor, accelerating the transfer of electrons from the intracellular to the extracellular matrix. This consequently doubled the denitrification rate and increased the electron transport capacity by 4-30 times (0.015-0.04 μA). However, as the Fe(II) concentration further increased to above 4 mM, the Fe(II) ions tended to preferentially nucleate, oxidize, and crystallize on the outer surface of P. JM-7, leading to the formation of a densely crystallized goethite layer, which significantly slowed down the metabolism of P. JM-7. In contrast to the Fe(II) conditions, regardless of the initial concentration of Fe(III), it was challenging for Fe(III) ions to form goethite in the periplasmic space. This work has shed light on the likely effects of iron on environmental microorganisms, improved our understanding of globally significant iron and nitrogen geochemical cycles in water, and expanded our ability to study and control these important processes.

Enhancement and mechanisms of micron-pyrite driven autotrophic denitrification with different pretreatments for treating organic-limited waters

Pyrite-driven autotrophic denitrification (PAD) represents a cheap and promising way for nitrogen removal from organic-limited wastewater, which has obtained increasing attention in recent years. However, the limited denitrification rate and unclear mechanism underlying the process have hindered the engineered application of PAD. This study aims to shed light on the impacts of different pretreatments (i.e., ultrasonication, acid-washing and calcination) on micron-pyrite surface characteristics, denitrification performance and biofilm formation during PAD in batch reactors. A series of solid-phase analyses revealed that all pretreatments could significantly promote biofilm attachment on pyrite granules, but impacted the proportion, distribution and chemical oxidation state of sulfur (S) and iron (Fe) at varying degrees. Batch tests showed that ultrasonication and acid-washing could enhance the total nitrogen reduction rate by 14% and 99%, and decrease the sulfate production rate by 51% and 42%, respectively, when compared with untreated pyrite. Microbial community analysis indicated that Thiobacillus and Rhodanobacter dominated in PAD systems. Two types of indirect mechanisms (i.e., contact and non-contact) for pyrite leaching may co-occur in PAD system, resulting in ferrous iron (Fe²⁺), thiosulfate (S₂O₃^2-) and sulfide (S^2-) as the main electron donors for denitrification. A PAD mechanism model was proposed to describe the PAD electron transfer pathway with the aim to optimize the engineered application of PAD for nitrogen removal.

Cryptic Sulfur and Oxygen Cycling Potentially Reduces N2O-Driven Greenhouse Warming: Underlying Revision Need of the Nitrogen Cycle

Increasing global deoxygenation has widely formed oxygen-limited biotopes, altering the metabolic pathways of numerous microbes and causing a large greenhouse effect of nitrous oxide (N₂O). Although there are many sources of N₂O, denitrification is the sole sink that removes N₂O from the biosphere, and the low-level oxygen in waters has been classically thought to be the key factor regulating N₂O emissions from incomplete denitrification. However, through microcosm incubations with sandy sediment, we demonstrate here for the first time that the stress from oxygenated environments does not suppress, but rather boosts the complete denitrification process when the sulfur cycle is actively ongoing. This study highlights the potential of reducing N₂O-driven greenhouse warming and fills a gap in pre-cognitions on the nitrogen cycle, which may impact our current understanding of greenhouse gas sinks. Combining molecular techniques and kinetic verification, we reveal that dominant inhibitions in oxygen-limited environments can interestingly undergo triple detoxification by cryptic sulfur and oxygen cycling, which may extensively occur in nature but have been long neglected by researchers. Furthermore, reviewing the present data and observations from natural and artificial ecosystems leads to the necessary revision needs of the global nitrogen cycle.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Application of a novel two-stage biofiltration system for simulated brackish aquaculture wastewater treatment

Biofiltration is one kind of common technology used for treating micro-polluted brackish aquaculture wastewater. Based on the characteristics of actual water quality, a novel two-stage biofiltration system was set up to reduce potential nutrient pollution brought by the frequent exchange of water in brackish pond aquaculture. Zeolite was selected as filtration media for the first stage and pyrite mixed with a small amount of sulfur for the second stage. Apart from the adsorption of nutrients exerted by these natural minerals, biofilm played a leading role in nutrient removal. The surface and internal pore of zeolite-sheltered nitrifiers and sulfur-containing compounds enhanced autotrophic denitrification. It was found that ammonia adsorption capacity of zeolite was reduced by nearly 58% when salinity was increased to 1.5%, while phosphate adsorption capacity of pyrite was hardly influenced and systematic hydraulic retention time (HRT) of 24 h was proven appropriate, 9.6 h and 14.4 h for the two stages, respectively. Meanwhile, removal efficiency of 96.5% for NH₄⁺-N and 92.1% for total inorganic nitrogen (TIN) was achieved under this condition. The analysis of microbial community of biofilm indicated that dominant genera responsible for nitritation and nitration on the surface of zeolite were Nitrosomonas and Nitrospira, respectively. Dominant genera responsible for autotrophic denitrification on the surface of pyrite and sulfur were both Thiobacillus. In addition, Ferritrophicum, related to the iron-oxidizing bacterium, also coexisted due to biological oxidation of pyrite. Long-term operation verified applicability and stability of this two-stage biofiltration system for brackish aquaculture wastewater purification.

Effect of oyster shell medium and organic substrate on the performance of a particulate pyrite autotrophic denitrification (PPAD) process

The use of pyrite as an electron donor for biological denitrification has the potential to reduce alkalinity consumption and sulfate by-product production compared with sulfur oxidizing denitrification. This research investigated the effects of oyster shell and organic substrate addition on the performance of a particulate pyrite autotrophic denitrification (PPAD) process. Side-by-side bench-scale studies were carried out in upflow packed bed bioreactors with pyrite and sand, with and without oyster shells as an alkalinity source. Organic carbon addition (10% by volume wastewater) was found to improve PPAD denitrification performance, possibly by promoting mixotrophic metabolism. After organic carbon addition and operation at a six-hour empty bed contact time, total inorganic nitrogen (TIN) removal reached 90% in the column with oyster shells compared with 70% without. SEM images and biofilm protein measurements indicated that oyster shells enhanced biofilm growth. The results indicate that PPAD is a promising technology for treatment of nitrified wastewater.