Application of design of experiments (DoE) for optimised production of micro- and mesoporous Norway spruce bark activated carbons

In this work, Norway spruce ( Picea abies (Karst) L.) bark was employed as a precursor to prepare activated carbon using zinc chloride (ZnCl 2 ) as a chemical activator. The purpose of this study was to determine optimal activated carbon (AC) preparation variables by the response surface methodology using a Box–Behnken design (BBD) to obtain AC with high specific surface area (S BET ), mesopore surface area (S MESO ), and micropore surface area (S MICR ). Variables and levels used in the design were pyrolysis temperature (700, 800, and 900 °C), holding time (1, 2, and 3 h), and bark/ZnCl 2 impregnation ratio (1, 1.5, and 2). The optimal conditions for achieving the highest S BET were as follows: a pyrolysis temperature of 700 °C, a holding time of 1 h, and a spruce bark/ZnCl 2 ratio of 1.5, which yielded an S BET value of 1374 m 2 g −1 . For maximised mesopore area, the optimal condition was at a pyrolysis temperature of 700 °C, a holding time of 2 h, and a bark/ZnCl 2 ratio of 2, which yielded a S MESO area of 1311 m 2 g −1 , where mesopores (S MESO% ) comprised 97.4% of total S BET . Correspondingly, for micropore formation, the highest micropore area was found at a pyrolysis temperature of 800 °C, a holding time of 3 h, and a bark/ZnCl 2 ratio of 2, corresponding to 1117 m 2 g −1 , with 94.3% of the total S BET consisting of micropores (S MICRO% ). The bark/ZnCl 2 ratio and pyrolysis temperature had the strongest impact on the S BET , while the interaction between temperature and bark/ZnCl 2 ratio was the most significant factor for S MESO . For the S MICRO , holding time was the most important factor. In general, the spruce bark AC showed predominantly mesoporous structures. All activated carbons had high carbon and low ash contents. Chemical characterisation indicated that the ACs presented disordered carbon structures with oxygen functional groups on the ACs’ surfaces. Well-developed porosity and a large surface area combined with favourable chemical composition render the activated carbons from Norway spruce bark with interesting physicochemical properties. The ACs were successfully tested to adsorb sodium diclofenac from aqueous solutions showing to be attractive products to use as adsorbents to tackle polluted waters. Graphical abstract