Hierarchically porous biochar preparation and simultaneous nutrient recovery from sewage sludge via three steps of alkali-activated pyrolysis, water leaching and acid leaching

• Hierarchically porous biochar was derived from sludge via KHCO 3 -activated pyrolysis. • Porous biochar prepared at 900 °C has a high specific surface area of 1476.0 m 2 /g. • Water-soluble potassium salts were formed during KHCO 3 -activated sludge pyrolysis. • Porous biochar showed a high adsorption rate of 14.4 mg/(g∙min). • Water leaching solution rich in nutrient elements could be reused for plant growth . In this study, preparation of hierarchically porous biochar (PB) and simultaneous nutrient recovery from sewage sludge were realized through three steps of KHCO 3 -activated pyrolysis, water leaching and acid leaching. The prepared PB samples were applied as adsorbents for the removal of 4-chlorophenol (4-CP) from aqueous solutions. Effect of pyrolysis temperature on the physicochemical properties of PB samples was investigated. The PB prepared at 900 °C (PB-900) had the highest surface area (1476.0 m 2 /g) and total pore volume (1.138 cm 3 /g). The formation mechanism of porous structure of PB-900 was revealed by investigating the phase transformation, element distribution and pore structure evolution during its whole preparation process. PB-900 showed a high 4-CP adsorption rate of 14.4 mg/(g∙min), and its maximum adsorption capacity for 4-CP was up to 192.4 mg/g. The mechanism of 4-CP removal by PB-900 was mainly physical adsorption, and its strong aromaticity strengthened its interaction with 4-CP through π-π bonding. The water leaching solution in the preparation process of PB-900 contained a lot of nutrient elements (K, Si, P), which was recovered for grass seeds germination. Compared with the samples cultured with ultrapure water only, the seed germination rate of samples cultured with ultrapure water added with 20% of pretreated water leaching solution increased from 63% to 80%, and the average shoot length of grass grown for 7 days increased by 51.4%. The findings of this study suggest that KHCO 3 is an alternative activator to prepare hierarchically porous biochar adsorbent from sewage sludge and realize simultaneous nutrient recovery.