Borate Buffer in Heterogeneous PMS Activation: Enhanced Stability vs. Selective Radical Suppression and Probe Distortion-A Ni‑Co LDO Case Study

The use of pH buffers, such as borate, is a common strategy to mitigate metal leaching in heterogeneous peroxymonosulfate (PMS) activation systems, but their potential interference with reactive oxygen species (ROS) and degradation mechanisms is often overlooked. This study systematically investigates the dual role of a borate buffer in a Ni‑Co layered double oxide (Ni/Co‑LDO)/PMS system using phenol and benzoic acid (BA) as model pollutants. The Ni/Co‑LDO catalyst alone achieved 100% phenol removal within 15 min and 59% BA removal within 40 min. Upon addition of the borate buffer, phenol removal remained high (94% at 40 min), whereas BA removal dropped dramatically to only 6.35%, indicating a selective suppression of radical pathways. The buffer reduced metal leaching by approximately 68% for Ni²⁺ and 65% for Co²⁺, and improved catalyst reusability (86.4% phenol removal after three cycles vs. 52.1% without buffer). Mechanistic probes revealed that the borate buffer shifts the dominant pathway from radical‑mediated to non-radical‑dominated, as evidenced by quenching tests and PMSO assays. These findings quantitatively demonstrate that while borate buffer enhances catalyst stability, it severely distorts standard mechanistic diagnostics and limits degradation of electron‑deficient pollutants. This work provides a critical, evidence‑based caution for interpreting reaction mechanisms in buffered PMS systems.