Microalgae-derived nanoporous biochar for ammonia removal in sustainable wastewater treatment

Highlights

• NP-MBC achieved a specific surface area of 1137 m²·g⁻¹.

• Improved ammonia adsorption properties were observed at 700 °C pyrolysis temperature.

• NP-MBC has a 72% ammonia removal efficiency and adsorption capacity of 69.3 mg·g⁻¹.

• Pyrolysis up to 800 °C damages the pore structure of the NP-MBC.

• The activation of COO^- on NP-MBC assists the mechanism of ammonia adsorption.

Abstract

Ammonia is a critical pollutant in biological wastewater and is not easily separated mechanically due to similar physical properties (e.g., molecular weight, polarity, etc.) to water. Currently, an agent for ammonia removal from wastewater using zeolites must be mined, which is not environmental-friendly. The development of a sustainable and biological absorbent for ammonia-containing wastewater is therefore desirable. Since microalgae can grow rapidly by feeding on waste nutrients in wastewater and up-taking carbon dioxide flue gas, it is of high interest to be used as a bio-absorbent for secondary wastewater treatment. The problem statement of this work is to explore and understand the use of microalgae as a biological source for effective ammonia absorbent. In this work, nanoporous microalgae biochar (NP-MBC) is proposed to be synthesized by using formaldehyde-stabilized hydrothermal and KOH-activated high-temperature pyrolysis. The novelty of this work is that the mechanism for NP-MBC ammonia removal is being uncovered via spectroscopic and sorption analysis. Formaldehyde-stabilization maintains the morphology of the microalgae particles for higher quality pore formation during pyrolysis. Pyrolysis temperatures up to 700 °C improve pore structure and surface chemistry of the NP-MBC, leading to a specific surface area of 1137 m²·g⁻¹ and increased activation of COO^- groups for ammonia adsorption. Further pyrolysis at 800 °C damages the textual characteristics of the synthesized NP-MBC, causing pores agglomeration and lower ammonia adsorption. The best ammonia adsorption performance was obtained on NP-MBC prepared by pyrolysis at 700 °C with maximum ammonia removal of 72 % (within 120 min) and adsorption capacity over 69 mg·g⁻¹. Nanoporous structure by stabilized hydrothermal treatment and high-temperature pyrolysis are interesting as a paradigm to convert particulate biomass feedstock to functional material with tunable properties.