Matej Baláž - Environmental Mechanochemistry: Recycling Waste into Materials using High-Energy Ball Milling

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Mechanochemistry is a branch of chemistry which is concerned with chemical and physico-chemical changes of substances of all states of aggregation due to the influence of mechanical energy. This definition has been formulated at the end of twentieth century and is still valid almost 150 years later. It is an outstanding branch of science applicable in wide range of fields. By some experts, it is often considered as a very narrow branch of chemistry concerned with organic reactions, but from my point of view, it offers great tools for diverse scientific areas concerning mainly chemistry, materials science and environmental science. This book aims to show this with relation to waste treatment. The studies covered in this book were written mostly by people with an experience in waste treatment, and this publication aims to provide a bridge with mechanochemistry. It is quite a difficult task to cover all waste materials treated by this method, but by showing numerous examples of its utilization for this purpose, mechanochemistry is shown here as a golden thread in the treatment of different wastes.

Mechanochemistry utilizes tools of high-energy ball milling. My initial goal was to cover all the studies dealing with ball milling (including a conventional one) of waste materials published so far, and the initial search in scientific database after typing ball milling waste has shown around 600 articles, which seemed as a reasonable number of articles to be a good basis for a monograph. However, the reviewer opened my eyes by pointing out that many important papers are missed by using just this single search and that it is mandatory to use the combination of words ball milling, waste, recycling, mechanochemical, mechanical activation and individual waste names. When I did this during the revision, I have found even larger amount of articles, and it was clear to me that my initial plan was too ambitious, as it is beyond the power of an individual to cover such a huge amount of research articles in a single-authored monograph. Moreover, the reviewer was also right that not all the studies on ball milling of waste are mechanochemical ones, as the forces of conventional ball milling are not satisfactory to bring about mechanochemical effects (e.g., mechanical activation). Nevertheless, as such works have already been included in the original manuscript, I have decided to keep the scope of the monograph so broad to possibly attract also researchers using conventional ball milling merely as a technique for particle-size reduction.

The studies reviewed in this book were published until the year 2019. As this topic is really hot and a lot of researchers devote their effort to it, new papers appear almost every day. Again, a decision needed to be made at some point, and as this book was being completed on the verge of years 2019 and 2020, this seemed like an appropriate thing to do. The earliest date from which the works devoted to each waste material was simply set by the date of the last review paper covering the mechanochemical treatment of particular waste materials (e.g., for electronic waste, a nice review was published in 2015, so with some exceptions, just studies published after this date are reviewed here).

The first chapter provides an introduction into mechanochemistry in terms of its history and basic theories, describes high-energy milling process and explains common physico-chemical phenomena observed during the treatment by this method. The rest of the book is devoted to the ball milling (mostly high-energy milling one) treatment of different types of waste. Waste materials under study are divided into three large groups (Parts) based on the character of the waste they deal with: consumer waste, technogenic waste and agricultural waste. These large groups are then subdivided into smaller groups based on the exact type of waste (e.g., electronic, coal fly ash, biomass, etc.), thus forming individual Chapters. Most often, a subchapter is created for the unique waste and the studies are arranged chronologically (when they were published in the same year, they are arranged alphabetically). At the very end, the book also contains appendix, where all the original studies covered in the book are arranged by the potential application of the recycled material in the form of table, so that application-oriented researchers might immediately find the field into which they are interested in and see what kind of waste subjected to mechanochemical treatment can be used for it.

Sometimes, it was difficult to distinguish whether the discussed material is a waste or not. In most cases, the waste materials are not directly used for the high-energy ball milling treatment in their as-received state, but are pretreated (e.g., washed, sintered at low temperature, some components might be separated, etc.). These details are usually not mentioned in the book, as the main ideas of each reviewed study could be lost in the unnecessary details then. This information can be found in each reviewed study. In order to structure this book and create a virtual borderline which studies fall within the scope of the book, subjective decision needed to be made. Nevertheless, the environmental benefits of mechanochemistry span far beyond the treatment of waste. For example, the studies of mechanochemical synthesis of powerful and cheap adsorbents of pollutants from wastewaters, remediation of polluted soils, or those showing the environmental benefit on model chemicals/solutions with a clear message related to waste treatment were not included into this book because the scope would be too broad then.