*Article* **Biochar from Lemon Stalks: A Highly Active and Selective Carbocatalyst for the Oxidation of Sulfamethoxazole with Persulfate**

**Spyridon Giannakopoulos <sup>1</sup> , John Vakros 1,2,\*, Zacharias Frontistis <sup>3</sup> , Ioannis D. Manariotis <sup>4</sup> , Danae Venieri <sup>5</sup> , Stavros G. Poulopoulos <sup>6</sup> and Dionissios Mantzavinos <sup>1</sup>**


**Abstract:** Pyrolysis of lemon stalks at 850 ◦C under a limited oxygen atmosphere yields a highly active and selective biochar for the activation of persulfate ion and the oxidation of sulfamethoxazole (SMX). The biochar mainly consists of C and O atoms, with Ca and K being the most abundant minerals. It has a moderate specific surface area of 154 m<sup>2</sup> g <sup>−</sup><sup>1</sup> and carbonate species, probably in the form of calcium carbonate. Complete degradation of 0.5 mg L−<sup>1</sup> SMX can be achieved within 20 min using 500 mg L−<sup>1</sup> sodium persulfate (SPS) and 100 mg L−<sup>1</sup> biochar in ultrapure water (UPW). The acidic environment positively influences the degradation and adsorption processes, while the complexity of the water matrices usually has a negative impact on the degradation. The presence of chloride accelerates the oxidation of SMX, whose mechanism follows radical and non-radical pathways. Hydroxyl radicals seem to have the dominant contribution, while the electron transfer pathway was proven with electrochemical characterization. The biochar is stable for at least five cycles, and this makes it a good candidate for a sustainable, metal-free catalyst.

**Keywords:** biochar; sulfamethoxazole; persulfate; electron transfer
