Water treatment operations to remove natural toxins from surface water

Marcel Schneider, M.Sc.

Doctoral thesis

Abstract:

Cyanobacterial toxins pose an increasing threat to drinking water supplies and thus, require adequate removal, which, especially for large-scale water treatment, must be technically and economically feasible. Conventional water treatment approaches may sufficiently remove cyanobacteria and extracellular metabolites but they are often ineffective against dissolved organic pollutants, such as extracellular, dissolved cyanotoxins. Although some advanced treatments may remove dissolved cyanotoxins effectively, they often come with various disadvantages and deficiencies. For instance, adsorption and filtration only physically remove dissolved pollutants while traditionally applied oxidants only degrade certain cyanotoxins, may require impractically high concentrations or reaction times, and may even be involved in the formation of toxic disinfection by-products. A promising approach to address this challenge and overcome these deficiencies are Advanced Oxidation Processes, which rely on the in situ formation of highly reactive species such as the hydroxyl radical. First, a review of the current state-of-the-art for cyanotoxin removal by advanced oxidation processes revealed that, although all the investigated advanced oxidation processes can degrade cyanotoxins, their degradation efficacy and efficiency depends on various factors including water quality as well as toxin- and technique-specific parameters. Therefore, the best treatment approach has to be chosen according to the prevalent water treatment challenge and with respect to the processes’ individual strengths and weaknesses. Although a single treatment step may yield promising results, due to the increasingly complex nature of surface water, a multi-barrier approach will always be best approach to provide harmless and high-quality drinking water. Based on this review, future research should focus more on the assessment of treatment processes under environmentally relevant conditions, process up-scaling and performance optimization, and the examination of the residual toxicity after the treatment as one of the most essential drinking water quality criteria. A promising but, especially with respect to cyanotoxins, underexplored advanced oxidation process is the innovative non-thermal plasma technology. Electrical discharge plasmas can be generated in the gaseous phase above the water or within the water itself. Thus, plasmas can provide a range of different reactive chemical species like radicals, electrons, ionized and excited atoms and molecules as well as photons. As they can be tailored towards a specific water treatment challenge and desired degradation mechanisms, plasma-based treatments yield a high potential. Therefore, six common but fundamentally different plasma concepts were systematically compared for the degradation of the cyanotoxin cylindrospermopsin. Based on the degradation efficacy and efficiency but also on other aspects with high practical relevance like scalability and the need for regular maintenance, a dielectric barrier discharge in air around a water mist, corona-like and spark discharges submerged in water were found to be the best, most versatile and adaptive plasma-based treatments for the degradation of cylindrospermopsin. Although the different plasmas also generated UV radiation, heat and mechanical forces, the chemically-induced toxin degradation was the dominant process. As the reaction of produced reactive species and the dissolved pollutant is defined by their interaction with each other, the active discharge volume is a crucial parameter, also with respect to the scalability of the respective plasma treatment. Based on the results from the comparison of six different discharges and with respect to other technically relevant aspects, the dielectric barrier discharge in air around a water mist and the corona-like discharge submerged in water were then investigated in more detail with respect to the effect of the most pertinent treatment pa

Abstract:

rameters on the degradation and underlying degradation mechanisms. For the dielectric barrier discharge, the degradation efficacy increased when the operating voltage was decreased, while the corona-like discharge was more effective at an increased operating voltage and at a pH-value ≥ 7.5. After optimization of the treatment parameters, the corona-like discharge yielded a removal efficacy of 0.24 ± 0.02 g/kWh/L. The dielectric barrier discharge was less effective, resulting in an efficacy of 0.03 ± 0.00 g/kWh/L. Interestingly, a pH-dependent residual oxidative effect of the plasma-treated water was observed for the corona-like discharge submerged in water, which resulted in an ongoing CYN degradation, even without further application of the plasma. For the corona-like discharge, hydroxyl radicals were identified as the main reactive species while the dielectric barrier discharge in air around a water mist mainly produced ozone. The residual toxicity of the treated water is an integral measure for the quality of drinking water. Therefore, the last study answered the question whether the degradation of cylindrospermopsin equals its detoxification. Following its degradation by hydroxyl and sulfate radicals produced in Fenton and Fenton-like reactions, the residual toxicity of the treated toxin solution was examined using a 3-dimensional in vitro human liver model. Degradation by the sulfate radicals appeared to be more effective compared with the hydroxyl radicals. However, based on the identified degradation products, the underlying degradation mechanisms were similar for both radical species. Furthermore, in both cases, the residual toxicity substantially reduced due to the treatment. Although the results indicated that some of the formed degradation products may still exhibit toxicity, the overall toxicity reduction and proposed degradation pathways allowed for the conclusion that “Yes, cylindrospermopsin degradation equals its detoxification!”.

Abstract:

Cyanobacterial toxins pose an increasing threat to drinking water supplies and thus, require adequate removal, which, especially for large-scale water treatment, must be technically and economically feasible. Conventional water treatment approaches may sufficiently remove cyanobacteria and extracellular metabolites but they are often ineffective against dissolved organic pollutants, such as extracellular, dissolved cyanotoxins. Although some advanced treatments may remove dissolved cyanotoxins effectively, they often come with various disadvantages and deficiencies. For instance, adsorption and filtration only physically remove dissolved pollutants while traditionally applied oxidants only degrade certain cyanotoxins, may require impractically high concentrations or reaction times, and may even be involved in the formation of toxic disinfection by-products. A promising approach to address this challenge and overcome these deficiencies are Advanced Oxidation Processes, which rely on the in situ formation of highly reactive species such as the hydroxyl radical. First, a review of the current state-of-the-art for cyanotoxin removal by advanced oxidation processes revealed that, although all the investigated advanced oxidation processes can degrade cyanotoxins, their degradation efficacy and efficiency depends on various factors including water quality as well as toxin- and technique-specific parameters. Therefore, the best treatment approach has to be chosen according to the prevalent water treatment challenge and with respect to the processes’ individual strengths and weaknesses. Although a single treatment step may yield promising results, due to the increasingly complex nature of surface water, a multi-barrier approach will always be best approach to provide harmless and high-quality drinking water. Based on this review, future research should focus more on the assessment of treatment processes under environmentally relevant conditions, process up-scaling and performance optimization, and the examination of the residual toxicity after the treatment as one of the most essential drinking water quality criteria. A promising but, especially with respect to cyanotoxins, underexplored advanced oxidation process is the innovative non-thermal plasma technology. Electrical discharge plasmas can be generated in the gaseous phase above the water or within the water itself. Thus, plasmas can provide a range of different reactive chemical species like radicals, electrons, ionized and excited atoms and molecules as well as photons. As they can be tailored towards a specific water treatment challenge and desired degradation mechanisms, plasma-based treatments yield a high potential. Therefore, six common but fundamentally different plasma concepts were systematically compared for the degradation of the cyanotoxin cylindrospermopsin. Based on the degradation efficacy and efficiency but also on other aspects with high practical relevance like scalability and the need for regular maintenance, a dielectric barrier discharge in air around a water mist, corona-like and spark discharges submerged in water were found to be the best, most versatile and adaptive plasma-based treatments for the degradation of cylindrospermopsin. Although the different plasmas also generated UV radiation, heat and mechanical forces, the chemically-induced toxin degradation was the dominant process. As the reaction of produced reactive species and the dissolved pollutant is defined by their interaction with each other, the active discharge volume is a crucial parameter, also with respect to the scalability of the respective plasma treatment. Based on the results from the comparison of six different discharges and with respect to other technically relevant aspects, the dielectric barrier discharge in air around a water mist and the corona-like discharge submerged in water were then investigated in more detail with respect to the effect of the most pertinent treatment pa

Abstract:

rameters on the degradation and underlying degradation mechanisms. For the dielectric barrier discharge, the degradation efficacy increased when the operating voltage was decreased, while the corona-like discharge was more effective at an increased operating voltage and at a pH-value ≥ 7.5. After optimization of the treatment parameters, the corona-like discharge yielded a removal efficacy of 0.24 ± 0.02 g/kWh/L. The dielectric barrier discharge was less effective, resulting in an efficacy of 0.03 ± 0.00 g/kWh/L. Interestingly, a pH-dependent residual oxidative effect of the plasma-treated water was observed for the corona-like discharge submerged in water, which resulted in an ongoing CYN degradation, even without further application of the plasma. For the corona-like discharge, hydroxyl radicals were identified as the main reactive species while the dielectric barrier discharge in air around a water mist mainly produced ozone. The residual toxicity of the treated water is an integral measure for the quality of drinking water. Therefore, the last study answered the question whether the degradation of cylindrospermopsin equals its detoxification. Following its degradation by hydroxyl and sulfate radicals produced in Fenton and Fenton-like reactions, the residual toxicity of the treated toxin solution was examined using a 3-dimensional in vitro human liver model. Degradation by the sulfate radicals appeared to be more effective compared with the hydroxyl radicals. However, based on the identified degradation products, the underlying degradation mechanisms were similar for both radical species. Furthermore, in both cases, the residual toxicity substantially reduced due to the treatment. Although the results indicated that some of the formed degradation products may still exhibit toxicity, the overall toxicity reduction and proposed degradation pathways allowed for the conclusion that “Yes, cylindrospermopsin degradation equals its detoxification!”.

Keywords

advanced oxidation process, cyanotoxin, cylindrospermopsin, drinking water treatment, hepatospheroids, hydroxyl radical, non-thermal plasma, sulfate radical, toxicity

Language used: English

Date on which the thesis was submitted / produced: 13. 9. 2021

Thesis defence

• Date of defence: 16. 11. 2021
• Supervisor: prof. RNDr. Luděk Bláha, Ph.D.
• Reader: prof. Ing. Blahoslav Maršálek, CSc., Maria G. Antoniou, Ph.D.

Masaryk University

Faculty of Science

Doctoral programme / field:
Environmental Health Sciences / Environmental chemistry and toxicology