Modification of ZnIn₂S₄ by anthraquinone-2-sulfonate doped polypyrrole as acceptor-donor system for enhanced photocatalytic degradation of tetracycline

Visible-light photocatalytic degradation of tetracycline was investigated by functional modified ZnIn₂S₄ via anthraquinone-2-sulfonate (AQS) doped polypyrrole (PPy). The best photocatalytic activity of AQS/PPy-ZnIn₂S₄ composite photocatalyst was obtained when the weight ratio of AQS/PPy versus ZnIn₂S₄ was 1.0% and the molar ratio of PPy to AQS was 40. After 3.0 h photocatalytic reaction, more than 98% tetracycline was removed and almost 2.0 times higher of NPOC (non-purgeable organic carbon) removal rate was obtained compared with pure ZnIn₂S₄. Active species trapping experiments demonstrated that [rad]O₂⁻ played a major role rather than the [rad]OH and photogenerated holes. The AQS doped in PPy matrix behaved as electron acceptor-donor mediator which facilitated the formation of [rad]O₂⁻. Two kinds of solid phase extraction (SPE) methods were developed before gas chromatography mass spectroscopy (GC–MS) analysis and eight intermediates were detected. The degradation of tetracycline originated from the photogenerated holes via loss of some functional groups and then aromatic ring was destroyed by the oxidation of active species [rad]O₂⁻ and [rad]OH via hydroxylation reaction.