Mn- or Zn-substituted goethite enhancing tetracycline degradation by increasing oxygen vacancies and promoting electron transfer

The metal-substituted goethite with addition of oxidants can effectively promote the degradation of antibiotics, but the usage of external agents restricts its application in in-situ remediation of antibiotic pollution. In this study, the metal-substituted goethites were prepared through co-precipitation method to investigate the effects of using manganese (Mn) or zinc (Zn) substitution alone on tetracycline (TC) degradation, combining with microscopic characterization and electrochemical methods. The results showed that the degradation rate constants of the first stage on Mn or Zn-substituted goethite were 0.299 h-1 and 0.498 h-1, respectively, which were about 2 and 4 times of the pristine goethite (0.117 h-1). In the pH range of 4.0–9.0, Mn or Zn substitution significantly promoted TC degradation. Free and non-free radical pathways existed in the process of TC degradation by Mn or Zn-substituted goethite. The active site change and enhanced electron transfer ability were the main reasons for promoting degradation. Mn or Zn substitution increased oxygen vacancy (OV) content and promoted the electron transfer between TC and goethite. In Zn-substituted goethite, the coupling of OV and electron transfer produced free radicals, and in Mn-substituted goethite, the redox reaction induced mainly by electron transfer produces free radicals. This study reveals a new mechanism of efficient degradation of TC by iron minerals without the need for external oxidants, and the materials studied can be used for in-situ remediation of antibiotic contaminated groundwater. [Display omitted] •Mn- or Zn-substituted goethite significantly enhances TC degradation.•TC degradation on Mn- or Zn-substituted goethite mainly occurs at dimethylamine and amide groups.•Besides singly-coordinated hydroxyl groups, •O2− enhances TC degradation.•Mn or Zn substitution promotes electron transfer between TC and goethite.