Biological attenuation of arsenic and nitrate in a suspended growth denitrifying-sulphidogenic bioreactor and stability check of arsenic-laden biosolids

Simultaneous removal of nitrogen and arsenite by heterotrophic nitrification and aerobic denitrification bacterium Hydrogenophaga sp. H7

Introduction

Nitrogen and arsenic contaminants often coexist in groundwater, and microbes show the potential for simultaneous removal of nitrogen and arsenic. Here, we reported that Hydrogenophaga sp. H7 was heterotrophic nitrification and aerobic denitrification (HNAD) and arsenite [As(III)] oxidation bacterium.

Methods

The appearance of nitrogen removal and As(III) oxidation of Hydrogenophaga sp. H7 in liquid culture medium was studied. The effect of carbon source, C/N ratio, temperature, pH values, and shaking speeds were analyzed. The impact of strains H7 treatment with FeCl3 on nitrogen and As(III) in wastewater was assessed. The key pathways that participate in simultaneous nitrogen removal and As(III) oxidation was analyzed by genome and proteomic analysis.

Results and discussion

Strain H7 presented efficient capacities for simultaneous NH₄ ⁺-N, NO₃ ^--N, or NO₂ ^--N removal with As(III) oxidation during aerobic cultivation. Strikingly, the bacterial ability to remove nitrogen and oxidize As(III) has remained high across a wide range of pH values, and shaking speeds, exceeding that of the most commonly reported HNAD bacteria. Additionally, the previous HNAD strains exhibited a high denitrification efficiency, but a suboptimal concentration of nitrogen remained in the wastewater. Here, strain H7 combined with FeCl3 efficiently removed 96.14% of NH₄ ⁺-N, 99.08% of NO₃ ^--N, and 94.68% of total nitrogen (TN), and it oxidized 100% of As(III), even at a low nitrogen concentration (35 mg/L). The residues in the wastewater still met the V of Surface Water Environmental Quality Standard of China after five continuous wastewater treatment cycles. Furthermore, genome and proteomic analyses led us to propose that the shortcut nitrification-denitrification pathway and As(III) oxidase AioBA are the key pathways that participate in simultaneous nitrogen removal and As(III) oxidation.

Simultaneous arsenite and nitrate removal from simulated groundwater based on pyrrhotite autotrophic denitrification

In this study, pyrrhotite is applied to remove arsenite (As(III)) and NO₃^- from groundwater simultaneously. Batch experiments find that sulfur autotrophic denitrifiers are not inhibited by As(III) with concentration up to 70 mg·L^-1, and pyrrhotite autotrophic denitrification (PAD) can effectively remove As(III), NO₃^- and PO₄^3- simultaneously. Treating water with As(III) 874.50±32.76 µg·L^-1, NO₃^--N 30 mg·L^-1, and PO₄^3--P 0.5 mg·L^-1, the pyrrhotite-sulfur-limestone autotrophic denitrification (PSLAD) biofilter can achieve effluent with total Arsenic (As) 7.84±7.29 µg·L^-1, NO₃^--N 3.78±1.14 mg·L^-1, and PO₄^3--P below detection limit at hydraulic retention time 6 h. In the PSLAD biofilter, Thiobacillus is the most abundant bacterium, and it uses pyrrhotite and sulfur as electron donor to reduce NO₃^-, and basically Fe²⁺ and As(III) are oxidized to Fe³⁺ and arsenate, respectively. As and PO₄^3- were mainly removed through precipitates FeAsO₄ and FePO₄, respectively. Technology based on the PAD is a simple, cost-effective and efficient way for remediation of As(III) and NO₃^- co-contaminated groundwater, and avoiding contaminants transference between groundwater and surface water.