A widespread and stubborn pollutant, tetrachloroethylene (PCE), lurks in soil and groundwater across the globe. Once used heavily in dry cleaning and metal degreasing, this carcinogenic chemical now contaminates water supplies, posing a serious environmental and health risk. While we can use activated carbonActivated carbon is a form of carbon that has been processed to create a vast network of tiny pores, increasing its surface area significantly. This extensive surface area makes activated carbon exceptionally effective at trapping and holding impurities, like a molecular sponge. It is commonly More to filter the water, this method is often prohibitively expensive. This challenge led Joram Fridtjof Sobanski, in a master’s thesis for the University of Natural Resources and Life Sciences, Vienna (BOKU), to investigate a cheaper, greener alternative: biochar. The goal was to engineer the perfect filter from simple agricultural and forestry byproducts, like apricot kernels, sunflower seed shells, miscanthus, and wood chips.

The central question of the research was simple: what makes the best biochar? Sobanski focused on two key variables: the feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More (the plant matter) and the pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More temperature (the heat used to make the char in an oxygen-limited setting). The feedstocks were pyrolyzed at three different temperatures: a low 350°C, a medium 550°C, and a high 750°C. The guiding hypothesis was that hotter is better. Higher temperatures tend to burn off more volatile compounds, increasing the biochar’s total carbon content, specific surface area (SSA), and aromaticity, while decreasing its polarity. Since PCE is a non-polar compound, it should, in theory, be more attracted to the non-polar, highly carbonized surfaces created at high temperatures.

For the most part, the results proved this hypothesis correct. For wood chips, miscanthus, and sunflower seed shells, the biochars produced at 750°C were the undisputed champions, showing the highest PCE removal. The top performer of the entire study was W7, the biochar made from wood chips at 750°C. This material had all the “right” properties: the highest carbon content (94.7%), the lowest polarity (O/C ratio of 0.01), and a high specific surface area. In adsorption experiments, it consistently outperformed the others, removing nearly 65% of the PCE in one test and showing a high adsorption capacity of about 83 mg/g. This confirmed that for most feedstocks, turning the heat up to 750°C produces a more effective adsorbent for PCE.

But the study revealed two critical, unexpected twists. The first twist came from the apricot kernels (AKBC). Unlike the other feedstocks, the apricot kernel biochar broke the “hotter is better” rule. The A3 biochar, produced at the lowest temperature of 350°C, was the best performer in its group, handily beating the 550°C and 750°C versions. This baffling result suggests a completely different cleanup mechanism is at play. While the 750°C biochars rely on adsorption (PCE sticking to the hard, carbonized surface), the 350°C A3 biochar likely uses partitioning. This means the PCE is dissolving into the biochar’s softer, non-carbonized, and more polar organic parts—a quality that is lost when the char is overheated.

The second and more decisive twist was a serious safety issue. The same high-temperature process that made the biochars so effective also caused them to generate polycyclic aromatic hydrocarbons (PAHs), which are themselves persistent and toxic pollutants. When tested against the European Biochar Certificate (EBC) standards, the 750°C sunflower shell biochar (S7) failed spectacularly, with PAH levels of 48.4 mg/kg (the limit is 30 mg/kg). The star performer, W7 (wood chips at 750°C), also failed the safety test, clocking in at 32.4 mg/kg—about 8% over the legal limit. In contrast, the apricot kernel biochars were exceptionally clean, producing almost no PAHs at any temperature.

This thesis perfectly illustrates the complex trade-offs in environmental engineering. The most effective material, W7, can’t be used because it creates a new pollution problem. This leaves researchers with intriguing new paths. Should they pursue the high-performance 750°C miscanthus biochar (M7), which was effective and just passed the safety threshold? Or should they explore the peculiar 350°C apricot kernel biochar (A3), a safer, low-energy product that cleans water through a completely different chemical process? This work proves that in the quest for green solutions, the best-performing option isn’t always the best one.

Source: Sobanski, J. F. (2021). Production and characterization of biochar from residual plant matter for the sorption of organic pollutants [Master’s thesis, University of Natural Resources and Life Sciences, Vienna].