A parallel optimisation of adsorption and regeneration properties of activated biochars for wastewater treatment

The emergence of micropollutants, such as pharmaceuticals in wastewater, presents a potential risk for human health as well as the aquatic environment. Current wastewater treatment plants are generally not capable of removing these pollutants without additional treatment steps. Adsorption on activated carbon is an effective way to remove these contaminants, however, the use of non-renewable feedstocks as well as low regeneration efficiencies increase the environmental costs of this method¹. Biochar as a renewable carbon platform material can be specifically designed to overcome these drawbacks².

This study is aimed at designing activated mineral biochar composites with enhanced adsorption capacity for pharmaceuticals while simultaneously optimising their regeneration performance. Two standard biochars from the UK Biochar Research Centre produced at 550°C from softwood and wheat straw were activated in CO₂ at 800°C. Additionally, activated mineral biochar composites were produced by the addition of ochre – a Fe-rich mining waste – prior to pyrolysis and activation.

The activated biochars and activated mineral biochar composites were analysed for their maximum adsorption capacity for two micropollutants - caffeine and fluconazole - and compared to a commercial activated carbon as a reference material. While the activated carbon outperformed all biochar samples, the addition of ochre increased the performance of the activated biochar samples. The regeneration performance was tested in a subsequent experiment. The materials were first loaded with a mix of 10 pharmaceuticals covering antibiotics, fungicides and antidepressants. The loaded biochars were then subjected to a novel regeneration method directly utilising wet adsorbents in contrast to common methods requiring prior drying. Similar to a powerful pressure cooker, solvolytic conversion conditions of water at temperatures ranging from 160 to 320°C and elevated pressures of 15 to 120 bar were used to regenerate the biochars. Hydrothermal treatment at 320°C was found to successfully degrade the adsorbed micropollutants across all biochars. The mineral biochar composites showed increased pollutant degradation most likely due to the catalytic effects of Fe in hydrothermal conditions, lowering the necessary treatment temperature to 280°C.

The results show that while designing biochar for certain applications, a simultaneous focus on both the application as well as the regeneration of the material can give a more comprehensive picture of the overall requirements for further optimisation of biochar adsorbents.

 

1. Thompson, K. A. et al. Environmental Comparison of Biochar and Activated Carbon for Tertiary Wastewater Treatment. Environ. Sci. Technol. (2016). doi:10.1021/acs.est.6b03239
2. Liu, W. J., Jiang, H. & Yu, H. Q. Development of Biochar-Based Functional Materials: Toward a Sustainable Platform Carbon Material. Chem. Rev. 115, 12251–12285 (2015).