Anaerobic manganese- or iron-mediated pharmaceutical degradation in water

Pharmaceutical compounds, originating mainly from industrial production and public consumption, are detected at extremely low levels (ng·L-1 –µg·L-1) in groundwater, surface water, and wastewater. So far, the adverse effects of pharmaceuticals and their intermediates have been widely reported, and include toxicity to humans and ecosystem, and enhancement of antimicrobial resistance. These effects call for the elimination of pharmaceuticals from water. This can be done by both abiotic and biotic degradation in the presence of oxygen (aerobic conditions) or in the absence of oxygen (anaerobic conditions). The technologies under anaerobic conditions are generally more sustainable and attractive because they require less energy and produce less pollutants, such as greenhouse gas, compared to technologies under aerobic conditions. Anaerobic degradation with metal oxides such as manganese (Mn) or iron (Fe) oxides has clear advantages in both drinking water treatment and wastewater treatment. Therefore, anaerobic degradation of pharmaceuticals in water with Mn or Fe is promising to study and develop into applicable techniques. This thesis investigates the feasibility of anaerobic degradation of pharmaceuticals in Mn- and Fe-mediated systems via both abiotic removal processes and by biodegradation. In Chapter 1, the scientific and technological motivation of the thesis is proposed.

Applications and scientific developments of Mn- or Fe-based technologies to remove pharmaceuticals from water are reviewed and discussed in Chapter 2. Based on the removal mechanisms found in nature and technical systems, these Mn- or Fe-based technologies can be classified into 3 groups – physico-chemical removal, chemical removal, and biologically-related removal. A review of previous research indicates that pharmaceutical removal with Mn- or Fe-based technologies from water is efficient, and the removal efficiency varies whit the different technologies applied. Positive and negative aspects of these processes, such as (non-)specificity, treatment conditions, formation of and effects of intermediates and by-products, and effects of Mn or Fe compounds were evaluated. Based on that, new and promising Mn- or Fe-based technologies are proposed as future potential effective and sustainable pharmaceutical removal technologies. Among these proposed technologies, the dissimilatory Mn or Fe reduction is identified as a most attractive, sustainable, and low-cost technology because this novel technology requires neutral conditions and the bacteria involved are able to completely mineralize the pharmaceuticals.

The anaerobic biodegradation of pharmaceuticals coupled to dissimilatory Mn(IV) or Fe(III) reduction is tested with different types of Mn(IV) and Fe(III) (Chapter 3). With a mixture of adapted sediment to metoprolol and chemically synthesized Mn(IV), anaerobic biodegradation with amorphous, chemically synthesized Mn(IV) can effectively remove caffeine (26%) and naproxen (52%) after 42 days of incubation. Further experiments with Mn(IV) obtained from drinking water treatment plants show that this type of Mn(IV) can be used to remove metoprolol and propranolol, with respectively 96% and 31% after 72 days of incubation. The inoculum can also use Fe(III) as alternative electron acceptor to degrade metoprolol. Results show that metoprolol degradation with insoluble chemically synthesized Fe(III) and soluble Fe(III)-citrate reaches 57% and 52%, respectively. No significant removal is observed in all the abiotic controls, showing that the biodegradation is the main removal mechanism in pharmaceutical removal with Mn(IV) or Fe(III).

Abiotic removal of selected pharmaceuticals with MnO2 is compared under aerobic conditions and anaerobic conditions (Chapter 4). Results show that anaerobic conditions promote diclofenac removal, while it inhibits removal of metoprolol and propranolol. In demineralized water (demiwater), diclofenac removal under anaerobic conditions is 78%, and higher than the 59% found under aerobic conditions. In 50 mM phosphate buffer, and under aerobic conditions, the diclofenac removal achieves complete removal. Under anaerobic conditions the observed removal is similar as in demiwater. Preliminary investigation shows that diclofenac removal with MnO2 under anaerobic condition is better at acidic pH (pH 4 – 5) and the removal is higher when applying amorphous MnO2 compared to applying crystalline MnO2. The key factors determining the extent of pharmaceutical removal with MnO2 under anaerobic conditions are the following: the chemical structure and molecular properties of the pharmaceuticals, and the properties and activity of reactive sites on the MnO2 surface.

Applying MnO2 under anaerobic conditions to remove diclofenac from water is further investigated (Chapter 5). Results show that increasing the temperature from 10 to 30°C leads to an increase in the diclofenac removal, whereas further increase of temperature to 40°C results in a decrease in the removal. The latter effect is possibly due to Ostwald ripening and/or aging processes. Increasing the amount of MnO2 increases the diclofenac degradation, as this provides more reactive sites for diclofenac conversions. Further shifting the molar ratio of MnO2 and diclofenac from 2200:1 to 8900:1, however, does not further increase diclofenac removal, probably due to limited oxidation capacity of MnO2. The presence of metal ions strongly inhibits the diclofenac removal following the order of Mn2+> Ca2+ ≈ Mg2+ >Fe3+. The metal ions appear to adsorb onto the MnO2 surface and compete with diclofenac for reactive sites. Phosphate has a diverse effect on diclofenac degradation: low concentrations inhibit and high concentrations promote the removal. The humic acids significantly promotes diclofenac removal, probably caused by affecting MnO2 reactive surface sites.

To reuse the Mn or Fe during pharmaceutical removal under anaerobic conditions, biological production of Mn(IV) or Fe(III) is investigated under oxygen-limiting conditions, or with nitrate as electron acceptor (Chapter 6). Mn(IV) is successfully produced with Mn(II)-oxidizing bacteria under O2-limiting conditions, and the produced Mn(IV) is amorphous. Pharmaceutical removal with the Mn(II)-oxidizing bacteria is not observed. In abiotic pharmaceutical removal, using Mn(IV) from a drinking water production plant, is effective to remove metoprolol and propranolol. The successful production of Fe(III) is also observed under NO3--reducing conditions via biological processes. The biologically produced Fe(III) is also amorphous. There is no significant removal of pharmaceuticals coupled to the biological Fe(III) production. When comparing the biologically produced Fe(III) and other types of Fe(III), only Fe(III) from a drinking water production plant and one Fe(III)-based sorbent can remove propranolol.

Finally, the outcomes of this thesis are discussed and provide insights into the application of anaerobic degradation of pharmaceuticals with mediation of Mn and Fe oxides (Chapter 7). The removal mechanisms include adsorption, chemical oxidation, and biodegradation and are identified to contribute to the different removal processes. The anaerobic Mn(IV)- and Fe(III)-mediated pharmaceutical degradation processes are evaluated on the basis of removal performance, environmental and operational conditions, sustainability of the processes, as well as the Mn and Fe types involved. Results described in this thesis provide a proof of principal for anaerobic Mn(IV)- or Fe(III)-mediated degradation in removing pharmaceuticals from water. To translate the process into a pharmaceutical removal technology for water treatment, three steps are proposed including (1) exploring the limits of anaerobic Mn- or Fe-mediated pharmaceutical degradation processes; (2) simulating the process in practice with a controlled systems, and (3) translating the processes to a pilot-scale system before a full-scale application. In addition, research topics are identified that can help to meet these challenges in the future. In summary, anaerobic Mn(IV)- or Fe(III)-mediated systems can remove pharmaceuticals from water through both abiotic removal and biotic degradation. These are promising processes which can be developed into a robust, sustainable, affordable, and environmentally friendly technology to remove pharmaceuticals from water.