Groundwater contamination by nitrates and pathogens poses significant risks to public health and water security, particularly in developing regions with limited access to safe drinking water. Current treatment methods often rely heavily on chemicals, raising environmental and economic concerns. This study explores a sustainable and cost-effective alternative: combining autotrophic denitrification using pyrite (FeS2) as an electron donor with electrochemical disinfection. Autotrophic denitrification offers a promising approach to nitrate removal, harnessing microbial activity to convert nitrates to harmless nitrogen gas. Pyrite, an abundant and relatively inexpensive material, serves as an excellent electron donor in this process. Electrochemical disinfection, on the other hand, uses electricity to generate chlorine in situ, eliminating the need for chemical disinfectants and providing on-site treatment. This combined system aims to provide a decentralized, renewable, and chemical-independent solution for groundwater remediation, addressing the need for safe drinking water while minimizing environmental impact and operational costs. The research investigates the performance of this integrated system under both synthetic and real groundwater conditions, evaluating nitrate removal efficiency, disinfection effectiveness, and energy consumption.

Extensive research explores various methods for nitrate removal from groundwater, including biological denitrification using various electron donors such as organic carbon sources, hydrogen, or sulfur compounds. However, reliance on organic carbon is unsustainable, and hydrogen-based systems can be expensive. Pyrite (FeS2) emerges as a cost-effective and environmentally friendly alternative, offering autotrophic denitrification without the need for external carbon sources. Several studies demonstrate the feasibility of pyrite-based denitrification, although optimal operational parameters and the underlying mechanisms remain areas of active research. Electrochemical disinfection also shows promise, with various electrode materials being investigated for efficiency and cost-effectiveness. The use of Pt/Ti electrodes, while not as widely studied as other options, offers a potentially attractive balance between performance and cost. This study builds on previous research, aiming to optimize the combination of these two technologies for a comprehensive groundwater treatment solution.

The study employed a two-stage treatment process. First, a pyrite-based fluidized bed reactor (P-FBR) was used for denitrification. The P-FBR, a 1 L glass column containing 120 g of FeS2, was inoculated with a mixed microbial culture obtained from a previous study. The reactor was operated in batch mode initially, then switched to continuous mode with varying hydraulic retention times (HRTs) of 24, 12, and 18 hours using both synthetic and real groundwater. Synthetic groundwater (SGW) was prepared according to a standard recipe, while real groundwater (GW) was sampled from two locations in Ireland. Real groundwater samples were supplemented with varying concentrations of nitrate, chloride, and total coliforms (TC) to simulate different contamination levels across three cycles (GW-Cycle I, II, and III). The denitrification efficiency was evaluated by measuring nitrate removal rates and effluent concentrations. The second stage involved electrochemical disinfection using a two-compartment electrochemical cell with a Pt/Ti anode and cathode separated by a cation exchange membrane. The denitrified effluent from the P-FBR was treated in the anodic compartment with varying current densities (2, 4, and 6 mA cm⁻²) and electrolysis times. Free and total chlorine concentrations, pH, alkalinity, and conductivity were monitored. Total coliform counts (MPN 100 mL⁻¹) were measured using the Colilert-18® test before and after disinfection to assess its effectiveness. Charge density and specific energy consumption (SECw) were calculated to assess the energy efficiency of the electrochemical disinfection process. A range of analytical methods were used to measure various parameters, including ion chromatography for anions, ICP-OES for cations, colorimetric methods for iron and ammonia, and a TOC analyzer for total organic carbon.

The P-FBR consistently achieved high nitrate removal rates (average 171 mg NO3⁻ L⁻¹ d⁻¹) at ambient temperature (22 ± 2 °C), exceeding rates reported in previous studies by 1-2 orders of magnitude. The system maintained a stable performance with real groundwater, achieving an average nitrate removal efficiency of 79% and consistently meeting drinking water standards (<50 mg NO3⁻ L⁻¹). Electrochlorination effectively disinfected the denitrified effluent, eliminating total coliforms. A charge density of 41.7 Ah m⁻³ and a specific energy consumption (SECw) of 0.4 kWh m⁻³ were sufficient for complete TC removal in most cases. Increased chloride concentrations marginally decreased denitrification rates but did not inhibit the process. The study observed a marginal increase in effluent sulfate and potassium concentrations, suggesting potential release of impurities from pyrite. The electrochemical disinfection process was influenced by the organic matter content in the real groundwater, resulting in some differences in free and total chlorine production rates compared to synthetic groundwater. However, consistent disinfection was achieved regardless.

The results demonstrate the feasibility and efficiency of the combined autotrophic FeS2-based denitrification and electrochemical disinfection system for groundwater remediation. The high denitrification rates achieved at ambient temperature offer a significant advantage, reducing energy requirements compared to systems operating at higher temperatures. The successful application of the system with real groundwater highlights its potential for practical implementation. The low energy consumption of the electrochemical disinfection process is notable, suggesting its cost-effectiveness. The marginal impact of increased chloride and organic content on treatment effectiveness underscores the robustness of the combined system. While further investigation is warranted into the exact mechanism of pyrite utilization in denitrification and the impact of specific organic components, the overall findings offer a promising pathway towards sustainable groundwater treatment.

This study successfully demonstrates a sustainable and cost-effective integrated system for groundwater remediation. The combined use of autotrophic FeS2-based denitrification and electrochemical disinfection offers a promising alternative to traditional methods. The high denitrification efficiency at ambient temperature, coupled with the efficient and energy-saving disinfection process, positions this technology as a viable solution for decentralized water treatment, particularly in areas with limited resources. Future research should focus on optimizing the reactor design, further investigating pyrite utilization mechanisms, and exploring the long-term performance and potential for scaling up this integrated system. Further studies on a wider range of pathogens including viruses are also important.

The study's duration was limited to 100 days, and longer-term experiments are needed to assess the long-term performance and stability of the system. While total coliforms were effectively removed, further investigation is needed on the inactivation efficiency for other microorganisms, such as viruses and more persistent pathogens, to determine the robustness of the combined treatment system for complete pathogen elimination. Additionally, a detailed analysis of the composition of the pyrite used and a thorough investigation into the by-products generated during the denitrification and disinfection processes would provide more comprehensive insights into the system's overall environmental impact and sustainability.