The purification performance of the LaVie system: effect of free chlorine and dissolved oxygen concentration
Below is a summary in French of the English report, produced by the Environmental Chemistry Laboratory of theAix Marseille University and the CNRS : “The purification performance of LaVie system: effect of free chlorine and dissolved oxygen concentration“
Notes : We have deliberately placed the conclusion below the introduction so that the results of this study can be quickly understood without having to go through the more technical part.
Introduction and objectives
The LAVIE system aims to provide pure, high-quality water that is both pleasant to drink and beneficial to health, while offering a drinking water solution for developing countries. The system purifies mains water containing chlorine while eliminating bacteria and viruses. The report focuses on demonstrating the effectiveness of the LAVIE system in degrading organic compounds by photolysis (i.e. direct absorption of light) and explores the effectiveness of *free chlorine in this process.
Conclusion
The LAVIE system has clearly demonstrated its effectiveness in degrading organic compounds in the presence of free chlorine and oxygen. The photolysis of chlorine and the production of reactive oxygen species play a crucial role in this process, making the LAVIE system an innovative method for purifying tap water by removing traces of pharmaceutical compounds and other organic pollutants.
Summary of the study
Materials and analytical methods
- Products and reagents : Use of *Diclofenac (DCF) as a reference compound, bleach and other specific chemicals for analysis.
- Preparation of solutions: DCF solutions prepared with ultrapure water and mixed with tap water to obtain different concentrations of DCF and free chlorine.
- LaVie purification system: Equipped with UV-A LEDs for a programmed exposure time of 15 minutes.
Analytical methods
- Measurement of free chlorine and dissolved oxygen: Use of photometers and specific electrodes for these measurements.
- Abatement of organic compounds: Monitoring by high-performance liquid chromatography (HPLC).
Results
- Experimental parameters: Adjustment of DCF and free chlorine concentrations to observe significant degradation of organic compounds.
- Effect of free chlorine concentration on the degradation of DCF :
- High DCF concentrations: DCF degradation improved in the presence of free chlorine, with a linear relationship observed between chlorine concentration and the percentage of DCF elimination.
- Low DCF concentrations: DCF elimination improved as the concentration of free chlorine increased, confirming the involvement of chlorine in the degradation process.
- Effect of oxygen concentration: Experiments carried out under aerobic and anaerobic conditions show that eliminating oxygen significantly reduces DCF degradation, suggesting the involvement of reactive oxygen species in the process.
References
One of the figures illustrating the study
Legend DCF degradation as a function of free chlorine concentration in tap water enriched with bleach after irradiation in the LAVIE system under *aerobic (Aerated) and anaerobic (Dearated) conditions, [DCF] = 2.5 mg/L and [free chlorine] approximately 2 mg/L.
Questions / Answers
*What is free chlorine?
Free chlorine refers to the chlorine available in water in the form of active chlorine, capable of disinfecting and destroying micro-organisms. It exists in two main forms:
- Hypochlorous acid (HOCl) Disinfection: This is the most effective form of disinfection because it easily penetrates the cell walls of micro-organisms.
- Hypochlorite ion (OCl-) Chlorine: This is a less effective form of free chlorine due to its ionic charge, which limits its ability to penetrate cells.
Free chlorine is often used in drinking water treatment to kill bacteria, viruses and other pathogens, as well as to oxidise chemical contaminants. Its concentration is measured to ensure effective disinfection while minimising the formation of undesirable by-products such as trihalomethanes (THMs).
What is the relationship between bleach and free chlorine?
Bleach and free chlorine are related but are not exactly the same thing.
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Bleach :
- Bleach is an aqueous solution containing sodium hypochlorite (NaClO). It is a chemical used mainly as a disinfectant and bleach.
- When dissolved in water, sodium hypochlorite dissociates into hypochlorite ions (OCl-) and hypochlorous acid (HOCl), contributing to its disinfectant action.
Free chlorine :
- Free chlorine in water refers to the active forms of chlorine, mainly hypochlorous acid (HOCl) and hypochlorite ion (OCl-).
- It is free chlorine that is responsible for the disinfectant effect in treated water. The term "free chlorine" is used to measure the amount of chlorine available for disinfection in drinking or swimming pool water.
Relationship between the two :
- When bleach is added to water, it releases free chlorine in the form of hypochlorous acid (HOCl) and hypochlorite ions (OCl-).
- In this sense, bleach is a source of free chlorine when added to water.
Summary:
– Bleach = source of free chlorine (sodium hypochlorite solution).
– Free chlorine = active forms (HOCl and OCl-) of chlorine in water, derived from bleach or other sources of chlorine.
Why was diclofenac (DCF), considered toxic, included in this experiment?
- Toxicity Diclofenac, a non-steroidal anti-inflammatory drug (NSAID) commonly used to treat pain and inflammation, is often detected in surface water and wastewater due to its widespread use and excretion by the body. Even at low concentrations, it can have toxic effects on aquatic life, particularly fish and aquatic invertebrates.
- Presence in tap water Diclofenac: Although generally in very low concentrations, diclofenac can be found in tap water, as a result of the inability of water treatment plants to completely eliminate pharmaceutical residues.
- Inclusion in the experiment Diclofenac was used in this study as a reference compound to assess the effectiveness of the LAVIE system in degrading organic contaminants present in tap water. This demonstrates the system's ability to eliminate traces of pharmaceutical products, thereby improving the quality of drinking water.
Why were the experiments carried out in aerated and dearated conditions?
The experiments were carried out under aerobic and anaerobic conditions to assess the impact of dissolved oxygen on the efficiency of the LAVIE purification system.
- Aerobic (aerated) Water contains dissolved oxygen, which encourages the formation of reactive oxygen species (ROS) such as hydroxyl radicals, which effectively break down contaminants.
- Anaerobic (dearated) The water is devoid of dissolved oxygen, which limits the formation of ROS and reduces the efficiency of contaminant degradation.
Conclusion The presence of dissolved oxygen significantly improves the degradation of diclofenac by the LaVie system, demonstrating the importance of aerobic conditions (i.e. with the normal level of oxygen found in tap water) for optimal purification.
