Unveiling drastic influence of cross-interactions in hydrothermal carbonization of spirulina with cellulose, lignin or poplar on nature of hydrochar and activated carbon

Abstract

Spirulina platensis contains abundant nitrogen-containing organics, which might react with derivatives of cellulose/lignin during hydrothermal carbonization (HTC), probably affecting yield, property of hydrochar, and pore development in activation of hydrochar. This was investigated herein by conducting co-HTC of spirulina platensis with cellulose, lignin, and sawdust at 260 °C and subsequent activation of the resulting hydrochars with K2C2O4 at 800 °C. The results showed that cross-condensation of spirulina platensis-derived proteins with cellulose/lignin-derived ketones and phenolics did take place in the co-HTC, forming more π-conjugated heavier organics, retaining more nitrogen species in hydrochar, reducing yields of hydrochar, making the hydrochar more aromatic and increasing the thermal stability and resistivity towards activation. This enhanced the yield of activated carbon (AC) by 7 %–20 % and significantly increased specific surface area of the AC from activation of hydrochar of spirulina platensis + lignin to 2074.5 m2/g (859.3 m2/g from spirulina platensis only and 1170.1 m2/g from lignin only). Furthermore, more mesopores from activation of hydrochar of spirulina platensis + cellulose (47 %) and more micropores from activation of hydrochar of spirulina + sawdust (93 %) was generated. The AC from spirulina platensis + lignin with the developed pore structures generated sufficient sites for adsorption of tetracycline from aqueous phase and minimized steric hindrance for mass transfer with the abundant mesopores (43 %).