Research

Emerging pollutants and anthropogenic micropollutants in the water cycle are increasingly becoming the focus of scientific and social interest. Compounds found in water in concentrations ranging from a few ng per litre to µg per litre include, for example, residues of pharmaceuticals, pesticides, ingredients in personal care products and industrial chemicals. Detecting and quantifying these compounds is challenging because they occur in a very complex matrix. Determining the transformation products resulting from oxidative/reductive treatment processes is part of our research. The composition and temporal development of the concentrations of these compounds is important for ecotoxicity as well as for the design of appropriate processes. Knowledge of the degradation mechanisms and kinetics is relevant for this purpose.

In addition to dissolved water pollutants, undissolved organic pollutants such as microplastics are also relevant. Microplastics enter the water cycle either directly (primary microplastics) or indirectly (secondary microplastics). Secondary microplastics are created, for example, by various degradation mechanisms of macroscopic plastics (including in nature, e.g. photochemical ageing). Microplastics comprise plastic particles smaller than 5 mm. Our working group's research focuses on the influence of various ageing effects (photochemical, mechanical) on the sorption behaviour of (aged) polymers with regard to different micropollutants (polarity, size) and the release of different polymer components.

Cavitation describes the formation, growth and collapse of gas- or vapour-filled bubbles in liquids. The physical (microjets, hydrodynamic shear forces, shock waves, microstreaming) and chemical (homolytic bond breaks, hydroxyl radicals) associated with bubble collapse can be exploited in various ways for water technology. Cavitation effects can be generated by acoustic waves (ultrasound) or hydrodynamic flows, either individually or in combination.

Cavitation can initiate, activate or intensify various processes. Cavitation field analysis is carried out to further develop cavitation processes or cavitation-assisted processes. This analysis uses optical or acoustic information and provides insight into the qualitative and quantitative distribution of cavitation fields in a reactor. Furthermore, methods for determining the temporally and spatially resolved bubble fields are used. With this information, the bubble field and cavitation field can be correlated with the corresponding result (degradation kinetics of micropollutants, mineralisation, etc.) and used for reactor design as well as process engineering design.

The working group designs, builds, tests, optimises and further develops cavitation reactors for various areas of application. This also includes the integration of sensor technology and, where possible, basic control and regulation of the systems. Furthermore, based on laboratory tests, industrial systems are also designed and adapted in collaboration with relevant companies.

Our working group researches and develops various advanced oxidation processes. Advanced oxidation processes produce very strong oxidants (reactive oxygen species, e.g. hydroxyl radicals) directly from water and on site (usually) without the addition of external chemicals. These oxidants are capable of removing almost all organic water pollutants.

The advanced oxidation processes investigated by our working group include the use of cavitation, electrochemical processes, photo(kata)lysis, Fenton reaction, hydrogen peroxide, ozonation and non-thermal plasma, as well as their targeted combination. Among other things, the working group is also researching the very new processes of pyro- and piezoelectrocatalysis. The aim is to use waste heat and/or sound as an initiator for the oxidative removal of organic pollutants in water using pyro- or piezoelectric materials. The detection and quantification of the reactive oxygen species formed (e.g. hydroxyl radicals) is essential for the development of new and optimisation of conventional advanced oxidation processes. In this field of work, ESR and various chemical dosimetries are used and further developed for specific applications.

A targeted combination of cavitation processes with other methods results in synergy effects based on physical-chemical effects during bubble collapse. Within this field of work, sonoelectrochemical or sonophotocatalytic processes and combinations of cavitation and ozonation are being further developed for various applications. In addition to developing and optimising appropriate reactor designs for various (combined) processes, aspects such as scale-up and control are being addressed and corresponding analyses are being carried out.

Our working group also investigates and develops sorption processes for the removal of anthropogenic (micro)pollutants. The research focuses on the synthesis and characterisation of carbon-based adsorbent materials (e.g. graphene oxide) and the (further) development of the sorption process (e.g. ultrasound-assisted).

n addition, surface-modified (switchable) ceramic adsorbers are being developed. These are materials that (mostly) consist of an inorganic carrier (e.g. Al2O3) to whose surface organic molecules are bound via various anchor groups, which perform the actual function (removal of individual substances, substance groups according to polarity, for example). The function can be switched on or off via various external mechanisms (light, pH, temperature). The main advantages of these systems are that they can be regenerated using electricity and tailored to different applications. The adsorber materials developed will then be immobilised on various carrier materials and integrated into corresponding modules.

At TUC, micro-, ultra- and nanofiltration are used with ceramic membranes, e.g. to separate (mineral) oil-water emulsions such as cooling lubricant emulsions in the metalworking industry or to remove (micro)pollutants from water.

In addition to process optimisation, surface modification of the separating layer plays a key role. Typically, perfluorinated silanes are used for this purpose, but these present various environmental, functional and economic problems. Our working group is developing new environmentally friendly coatings that are more stable and cheaper than the state of the art and can be obtained from renewable raw materials. In addition, various functionalities such as surface polarity or heavy metal complexation can be integrated into these coatings, making them suitable for various processes.

Water quality is currently determined centrally using instrumental analytical methods. Our working group aims to research and develop decentralized, digitally networked, and cost-effective sensors that can determine overall pollution levels as well as individual substances or groups of substances (inorganic and organic) in real time in a complex matrix. 
Chemical oxygen demand (COD) is a summary parameter and is found in numerous German and European laws and directives (AbwV, AbwAG, GrwV, OGewV, WRRL). It is suitable for the assessment and regulation of all organic water pollutants. In addition, COD plays an important role as a control parameter in process optimization (energy savings, cost reduction) in municipal and industrial wastewater treatment plants as well as in the bioeconomy.
Due to the many shortcomings of the dichromate method, the Bräutigam working group is developing a novel electrochemical measurement system that links the (sono-)electrochemically generated hydroxyl radicals and the amount of degraded organic (harmful) substances in an in situ measurement signal (e.g., current, voltage). In addition to the further development of the continuously operating, chemical-free, and sustainable measurement method and design, the focus is particularly on expanding the linear working range and reducing the detection limit. Further approaches can be found in the continuous determination of organic trace substances using electrochemical and optical methods.

Various technological solutions employ artificial intelligence methods. In forecasting, research is being conducted into substance-specific prediction models for the removability of pollutants in technical systems, among other things. In technical degradation processes, the degradation rate and mineralizability depend heavily on the chemical structure and the process used. Previous approaches have tended to follow a trial-and-error approach to remove micropollutants as quickly and efficiently as possible.
Our working group is pursuing a new approach—linking chemical structure to the respective standardized degradation process and developing a predictive model. In particular, this approach makes it possible to predict the degradability of a compound using various methods, e.g., from the field of advanced oxidation processes, based on its known chemical structure (structural formula or specific molecular structures or functional groups). To this end, structural elements and molecular descriptors are linked using machine learning methods to the actual measured chemical behavior and reaction kinetics of a large number of actual measured samples (test data set).
With the help of such a (universal) predictive model, it is possible to determine, for example, which removal process is most suitable for a pollutant, how quickly this can be achieved, at which operating points, and at what cost. This can also be done for future substances that are not yet commercially available (e.g., newly developed drugs). A retroactive calculation of chemical structures from the models can also be used to develop “optimal” chemicals that can be removed with minimal effort. In addition, the model also allows environmental authorities, for example, to recommend alternatives to chemicals previously used that have a low environmental impact under the cleaning techniques prevalent in the region. A high-throughput system for generating data on the degradation of (micro)pollutants using various technical processes is also available.