Trimethoprim Degradation by Micro Sulfidated Zero Valent Iron Activated Persulfate System

Abstract

Pharmaceuticals are widely found in treated wastewater effluents and natural waters. Due to their potential threats to human and ecosystem health, resistance to biodegradation, and poor removal by conventional physicochemical treatment processes, advanced treatment methods provide the best opportunity for effective removal. Trimethoprim (TMP), one of the most common antibiotics used for infection treatment, illustrates this point. Previous studies found less than 10% TMP removal by traditional wastewater treatment processes. Yet, advanced oxidation processes (AOPs) that generate highly reactive species such as sulfate radical (SO4.-) and hydroxyl radical (OH.) show great promise for the complete removal of TMP from water and wastewater. As such, an iron activated persulfate (PS) system for generating sulfate and hydroxyl radicals for TMP degradation was selected for a comprehensive kinetic study to identify the rates and mechanism for this system. Initially, several different iron catalysts were tested for activating PS to remove TMP. These included ferrous salts, mackinawite (FeS), zero-valent iron (ZVI) and sulfidated-ZVI. Micro-sized sulfidated ZVI (S-mZVI)/PS system was found to be the most effective for TMP degradation and became the focus of this work. The rate and extent of TMP degradation by S-mZVI/PS system were investigated over a wide range of initial pH, dosage of PS and S-mZVI, and oxic and anoxic conditions. Results showed that S-mZVI activated PS oxidation process rapidly and effectively removed TMP over a wide range of initial pH. Higher initial pH generally resulted in a lag period for TMP degradation until sufficient protons were generated by S-mZVI oxidation to lower the pH to below 9 and ended with a final pH of ~3. For the investigation of the impact of concentration of PS and S-mZVI, under oxic or anoxic conditions, the initial pH was adjusted to pH 3. In general, higher dosages of PS and S-mZVI facilitated both the rate and extent of TMP removal. Excess S-mZVI relative to PS tended to quench TMP removal since excess iron depletes PS and scavenges sulfate and hydroxyl radicals. In addition, TMP removal was faster and more complete in the presence of oxygen than anoxic conditions due to additional hydroxyl radical production by the activation of oxygen and the reduced Fe(II) scavenging effect. Steady-state kinetic models reported in the literature for modeling and mechanistic interpretation of heterogeneous catalytic systems were found inadequate for simulating the kinetic data of TMP degradation by the S-mZVI/PS system over the range of conditions investigated in this study. As a result, a comprehensive non-steady state kinetic model, modified with new elementary surface reaction steps, was developed. The model fitted well to the time-dependent measured changes in TMP, PS and total dissolved Fe concentrations in both oxic and anoxic systems. The model indicated that the rate of the S-mZVI dissolution reaction steps, and subsequent control of the amount of dissolved and surface Fe as a function of time, controlled the rate and extent of TMP degradation. Model results predicted a dual importance of sulfate and hydroxyl radical production in the extent and rate of TMP degradation. Overall, the results of this research show that S-mZVI activated PS oxidation process is a promising method for TMP removal from water, with the developed non-steady state kinetic model giving a better understanding of the mechanism and controlling elementary reaction steps of TMP oxidation.