In-situ and long-lasting oxidation of SMX by Mg/Al-PDS LDH in soil: Key role of solid-phase activation and CO effect.

To develop a long-term oxidative technology, peroxydisulfate intercalated magnesium aluminum layered double hydroxide (Mg/Al-PDS LDH) was prepared for in-situ oxidation of sulfamethoxazole (SMX) in soil. In the airtight system, Mg/Al-PDS LDH exhibited 99.54 % SMX removal in solution, as most PDS was retained within the LDH interlayers. However, SMX removal rate was reduced to 77.66 % in the open system, since PDS was rapidly released by the replacement of CO (formed from dissolved CO). When Mg/Al-PDS LDH was applied to the bottom layer of SMX-contaminated soil column, the effluent SMX concentration remained below 0.21 mg/L with a total removal rate of 98.72 %. In contrast, top-layer application yielded only 10.7 % removal, indicating the effectiveness of solid-phase oxidation as SMX percolated through the LDH layer. Density functional theory and Raman analyses evidenced that intercalated PDS had an elongated O-O bond with decreased bond energy, facilitating its activation by the base sites of LDH. Mechanism analysis confirmed that surface-bound SO and •OH were the dominant active species for SMX oxidation. CO derived from degradation-generated CO was intercalated into LDH, proving the carbon sequestration capability of Mg/Al-PDS LDH. This study provides new insights into the long-lasting oxidation mechanism of Mg/Al-PDS LDH for soil remediation.