Degradation of organic pollutants by ferrate/biochar: Enhanced formation of strong intermediate oxidative iron species

Biochar draws increasing attention as soil amendment, carbon sink, slow-release fertilizer, and adsorbent. Herein, it was interesting to find out that among 11 kinds of commercial biochar, 3 of them facilitated ferrate oxidation of sulfamethoxazole (SMX). With the addition of biochar, oxidation rates of 5 kinds of organic pollutants (including antibiotics, pharmaceuticals, and personal care product) increased by 3-14 times, and the total organic carbon (TOC) removal ratio increased by 2.4-8 times. Radical scavenging experiment, electron spin resonance (ESR) analysis, and probe compound (sulfoxide) oxidation experiment showed that no radical but intermediate iron species [Fe(IV) and Fe(V)] participated in the oxidation reactions. Redox-active moieties (phenolic hydroxyl) on biochar interact with ferrate as electron shuttle and enhance the formation of intermediate iron species through electron transfer. The intermediate iron species not only interacted with organic pollutants and accelerated their transformation, but also corrupted (oxidized) the physical structure of biochar and expanded its surface area and pore volume. Increase of surface area and pore volume of the spent biochar in turn resulted in the improved adsorption capacity. In addition to eliminating emerging organic pollutants, ferrate/biochar removed 8.7%-31.6% of TOC in authentic water and decreased the formation potential of 20 kinds of chlorinated disinfection by-products (DBPs) by 9.2%-23.9%. (c) 2020 Elsevier Ltd. All rights reserved.