Gilbert Appah carried out his Master’s thesis research at RPTU in Kaiserslautern. His experimental work was supervised by Dr. Allan Philippe.
According to Dr. Philippe, Gilbert worked independently and responsibly. He quickly learned the experimental methods and was able to find suitable solutions when problems occurred.
I consider the topic of this Master’s thesis current and important for environmental chemistry and analysis. Titanium dioxide nanoparticles are widely used and can enter the environment. However, their analysis in real environmental samples is difficult because of the complex sample matrix and the natural presence of titanium.
The thesis therefore deals with an important analytical problem – the separation of TiO₂ nanoparticles from environmental samples using magnetic microextraction. In a water system, the method achieved a TiO₂ recovery of about 95 %. The application to soil samples was much more difficult. The analysis was affected by naturally occurring titanium-containing minerals, especially ilmenite. The results therefore show both the potential of the method and the challenges of its application to complex soil samples.
As Gilbert’s supervisor at the Faculty of Chemistry, Brno University of Technology, I was very satisfied with his communication. We had regular online consultations. Gilbert responded well to my comments and discussed his results and their interpretation with me.
The submitted Master’s thesis is of a very good standard. The results are clearly presented and discussed. The conclusions are supported by the results, and relevant scientific literature was used. The requirements of the Master’s thesis assignment were fulfilled.
Overall, my assessment of Gilbert’s Master’s thesis is very positive. I recommend the thesis for defence and propose the overall grade A – Excellent.

This Master's thesis focuses on the separation of TiO₂ particles from environmental matrices using the magnetic PAA-Fe₃O₄@YPO₄ composite. I consider this a highly relevant topic, given the widespread use of TiO₂, the potential release of anthropogenic TiO₂ particles into the environment and the analytical challenges associated with separating and determining them in complex environmental matrices.

One of the thesis's major strengths is the large number of experiments performed and their logical sequence. The experimental work begins with the synthesis of PAA-Fe₃O₄, followed by the preparation of PAA-Fe₃O₄@YPO₄. It then moves on to the characterisation of the prepared materials and investigation of their properties. Finally, it proceeds to the optimisation of magnetic separation and evaluation of the applicability of the synthesised material for TiO₂ separation, first from an aqueous medium and then from a considerably more complex soil matrix. Thus, the thesis presents a coherent experimental workflow ranging from the preparation of the separation material to the assessment of its potential practical applicability.

I would also like to express my appreciation for the wide range of instrumental techniques employed. ESEM-EDX, FTIR spectroscopy, TGA-QMS, zeta-potential measurements and DLS were used to characterise the prepared materials and investigate their properties, while ICP-OES and ICP-MS were employed for elemental analysis. Thus, the author combines techniques that provide information on particle morphology, elemental composition, the presence of functional groups, thermal behaviour, surface charge and the concentrations of the elements of interest.

The thesis follows the standard structure expected of a Master’s thesis, and the individual chapters are logically organized. Nevertheless, the text contains several formal and terminological inaccuracies. Examples include the incorrect formula of ilmenite, FeTiO₂ instead of FeTiO₃, inconsistent rounding of reported results, and the use of ppm without clearly specifying the quantity being expressed. Greater attention should also be paid to defining abbreviations when they first appear in the text. In some sections, better interconnection between individual passages would improve the overall coherence of the thesis.

A more substantial weakness of the theoretical part is that some of the factors affecting the behaviour of TiO₂ in soil are listed rather than explained in terms of the mechanisms by which they act. For example, the author discusses the effects of ionic strength, pH, dissolved organic matter, soil texture, and organic matter content on the mobility and aggregation of TiO₂; however, a more detailed explanation of these relationships would provide a stronger theoretical basis for the subsequent interpretation of the experimental results. Considering the focus of the thesis, I would also have appreciated a broader critical comparison of the proposed approach with other methods available for the separation of TiO₂ from environmental matrices, including a comparison of their efficiency, selectivity, and, where relevant, experimental and economic demands.

The experimental section raises some methodological questions that merit further explanation. These include the formation mechanism of PAA-Fe₃O₄ and how PAA binds to the surface of forming particles. Other issues include the interpretation of FTIR bands around 1000 cm⁻¹ where different functional groups may overlap and the limitations of EDX analysis for assessing light elements, particularly carbon.

In my opinion, the most significant limitation of the study is the high natural Ti background in the investigated soil, as this substantially complicates the evaluation of the separation efficiency of the added TiO₂ particles. The author correctly identifies and discusses this issue. In the soil experiment, similar extraction efficiencies were obtained with and without the addition of the composite (approximately 3 % and 3.6%, respectively) and the author attributes these results primarily to the separation of naturally occurring Ti-bearing phases, particularly ilmenite. However, this finding also highlights a fundamental analytical problem: determining total Ti alone does not provide sufficient information to distinguish the added TiO₂ particles unambiguously from the naturally occurring Ti forms in the soil. Therefore, I would have appreciated a proposal for an alternative or follow-up experimental approach that could eliminate or minimise this interference, or at least provide more conclusive evidence of the efficiency of the synthesised material in separating the added TiO₂ particles.

Despite the above comments, I consider the thesis to be extensive in terms of experiments, diverse in terms of methodology, and scientifically valuable. Its main weakness is the limited depth of the critical discussion of the author’s own results, and the insufficient comparison of these results with those reported in the scientific literature. This is particularly noticeable given the extensive experimental dataset, which provides considerable scope for a more in-depth interpretation. Nevertheless, the author has demonstrated their ability to synthesise and characterise the investigated material, work with a broad range of instrumental techniques, and apply the proposed approach to a real, complex environmental matrix. The thesis also highlights the limitations of the proposed approach when it is applied to soil samples. Topics for thesis defence:

1. During the synthesis of PAA-Fe₃O₄, the starting reagents (including sodium acetate and sodium acrylate) are added to the reaction mixture together. Could you please explain the mechanism of Fe₃O₄ nanoparticle formation in more detail, and describe how PAA binds to the surface of the forming nanoparticles? Which of the results obtained in your thesis provide experimental evidence for this binding?
2. In your thesis, you state that Ca²⁺ can promote 'bridging and destabilisation' of TiO₂ nanoparticles. Could you please explain what particle bridging means in this context, and how this leads to the destabilisation of the suspension? From this perspective, what was the purpose of adding CaCl₂ in your separation experiments?
3. In the FTIR spectra, the band around 1049 cm⁻¹ is assigned to a C–O vibration. After phosphorylation, however, the enhanced absorption around 1026 cm⁻¹ is attributed to a vibration of the phosphate group. Given the potential overlap of absorption bands in this region, how can the individual contributions be distinguished, and how can the presence of the phosphate-containing layer be demonstrated unambiguously? Could another analytical technique be used to support this interpretation?
4. A major limitation of the soil experiments is the high natural Ti content and the simultaneous magnetic separation of naturally occurring Ti-bearing phases, particularly ilmenite. How would you design a follow-up experiment that would allow the added TiO₂ particles to be clearly distinguished from the naturally occurring Ti, thus enabling the actual efficiency of PAA-Fe₃O₄@YPO₄ for separating them to be reliably determined?