In-situ pyrolysis of Undaria pinnatifida as a green carbo-catalyst for degradation of organic contaminants: Role of inherent N and P in the degradation pathway

Tetracycline (TC) is one of the most used antibiotics for human, veterinary medicine, and food additives. The consumption of TC is not thoroughly utilized, and the residual TCs are released from the human and animal bodies by renal excretion. It leads to the accumulation of TC in the environment, which causes adverse effects on the ecosystem. Thus, it is necessary to remove TC from water bodies. The present study demonstrates a single-step biochar preparation method from seaweed (Undaria pinnatifida) which can be applied for groundwater treatment. Physicochemical characterization of the seaweed-derived biochar (SWBC) reveals that pyrolysis temperature plays a crucial role in peroxymonosulfate (PMS) activation and tetracycline (TC) degradation performance. Complete degradation of TC (10 mg L⁻¹) was achieved in 60 min with 0.5 mM of PMS and 0.1 g L⁻¹ of SWBC pyrolyzed at 800 °C (SWBC₈₀₀). The TC removal trends were further verified by the pseudo- (PFOKM) and two-compartment (TCFOKM) first-order kinetic models. Chemical scavenger experiments and electron spin resonance spectra (ESR) confirmed that both radical (hydroxyl radical, HO[rad], and sulfate radical, SO₄[rad]⁻) and non-radical (singlet oxygen, ¹O₂) reactive oxygen species (ROS) formed in the SWBC₈₀₀/PMS system. The quantification of ROS was calculated to be 2.45 × 10⁻², 1.97 × 10⁻², and 4.05 × 10⁻² mg min⁻¹ for HO[rad], SO₄[rad]⁻, and ¹O₂, respectively, indicating singlet oxygen is more dominant in the catalytic process. Furthermore, an electrochemical analysis also verified that a non-radical reaction mechanism and an electron-transfer pathway occurred in the SWBC₈₀₀/PMS system. The SWBC₈₀₀ catalyst with the PMS system successfully demonstrated reusability in TC degradation. Based on the toxicity analysis SWBC₈₀₀ is found to be an eco-friendly catalyst for groundwater treatment.