Sonoelectrochemical Degradation of Propyl Paraben - An Examination of the Synergy in Different Water Matrices

At a Glance

Metadata	Details
Publication Date	2020-04-11
Journal	International Journal of Environmental Research and Public Health
Authors	Zacharias Frontistis
Institutions	University of Western Macedonia
Citations	16
Analysis	Full AI Review Included

Executive Summary

This research investigates the synergistic effect (S) of combining Anodic Oxidation (AO) using Boron-Doped Diamond (BDD) electrodes with low-frequency Ultrasound (US) (Sonoelectrochemical Oxidation, AO/US) for the degradation of Propyl Paraben (PP).

Core Value Proposition: The hybrid AO/US process consistently demonstrated true synergy (S > 0) for PP removal, providing a more efficient Advanced Oxidation Process (AOP) than the sum of the individual methods, particularly beneficial for complex water matrices.
Matrix Enhancement: Synergy was significantly enhanced by the presence of natural organic matter (Humic Acid, up to 55.8%) and Bicarbonates (up to 55.3%), suggesting the hybrid process is highly effective when high reactive oxygen species (ROS) demand exists.
Current and Power Dependence: The degree of synergy (S) decreased as current density (1.25 to 6.25 mA/cm²) or US power (20 to 60 W/L) increased. Ultrasound’s role in mass transport and radical generation is most critical when electrochemical ROS production is low.
Chloride and Wastewater Impact: The presence of Chloride ions (Cl^-) and real Wastewater Effluent (WW) reduced the observed synergy (down to 24.8%). This is attributed to the electro-generation of long-lived active chlorine species, which bypass the mass transport limitations that US typically solves.
Efficiency and Kinetics: Despite reduced synergy in WW, the hybrid process achieved 100% PP removal in 10 minutes in wastewater effluent, demonstrating superior overall performance in environmentally relevant conditions.
Material Role: The use of BDD electrodes is crucial due to their excellent stability and high potential window, enabling efficient hydroxyl radical (•OH) generation.

Technical Specifications

Parameter	Value	Unit	Context
Anode Material	Boron-Doped Diamond (BDD)	N/A	8 cm² surface area
Cathode Material	Titanium Sheet	N/A	8 cm² surface area
Reactor Volume	200	mL	Cylindrical glass, double wall
Ultrasound Frequency	20	kHz	Low-frequency operation
Standard Current Density	3.75	mA/cm²	Used for matrix and pH studies
Standard US Power Density	36	W/L	Used for matrix and current studies
Electrolyte	0.1	M	Na₂SO₄ (inert)
Operating Temperature	25	°C	Controlled via water bath
Initial PP Concentration	0.5 to 2	mg/L	Tested range
Maximum Synergy (S)	55.8	%	Achieved with 20 mg/L Humic Acid
Synergy in Ultrapure Water	37.3	%	Baseline condition (0.5 mg/L PP)
Synergy in Wastewater (WW)	24.8	%	Reduced due to high Cl^- content
Wastewater Chloride Content	68	mg/L	Secondary Effluent (WW)
Wastewater Bicarbonate Content	217	mg/L	Secondary Effluent (WW)
Max PP Removal Rate (k_app)	0.540	min^-1	Achieved in WW (AO/US)

Key Methodologies

Reactor Configuration: Experiments were conducted in a 200 mL cylindrical glass reactor using a BDD anode and a Titanium cathode, both having 8 cm² surface area.
Electrochemical Setup (AO): A programmable power supply (PeakTech 1885) controlled the current density (1.25-6.25 mA/cm²). 0.1 M Na₂SO₄ was typically used as the supporting electrolyte.
Sonochemical Setup (US): A 20 kHz Branson sonifier 450 was used. The titanium horn (1 cm²) was positioned centrally between the electrodes. Ultrasound power density (20-60 W/L) was determined calorimetrically.
Hybrid Process (AO/US): The two processes were run simultaneously under controlled temperature (25 °C).
Matrix Preparation: Tests were performed in Ultrapure Water (UPW), Bottled Water (BW), and real Wastewater Effluent (WW). Synthetic solutions were prepared by adding controlled concentrations of chloride (100-250 mg/L), bicarbonate (100-250 mg/L), and humic acid (10-20 mg/L).
pH Control: The effect of pH was tested at acidic (pH 3), neutral (pH 6), and alkaline (pH 9) conditions.
Kinetic Analysis: All processes were assumed to follow pseudo-first-order kinetics. The apparent kinetic constants (k_app) were measured, and the Synergy (S) index was calculated based on the ratio of the combined rate constant (k_AO/US) versus the theoretical sum (k_AO + k_US).
Analytical Measurement: Propyl Paraben (PP) concentration was tracked using High-Performance Liquid Chromatography (HPLC). Mineralization was assessed by measuring Total Organic Carbon (TOC) and UV₂₅₄ absorbance.

Commercial Applications

The BDD-based sonoelectrochemical technology is highly relevant for advanced water treatment applications requiring high-efficiency destruction of persistent micropollutants.

Wastewater Treatment: Tertiary treatment of municipal and industrial effluents to remove Endocrine Disrupting Chemicals (EDCs) and Pharmaceuticals and Personal Care Products (PPCPs), ensuring compliance with strict discharge limits.
Water Reclamation and Reuse: Integration into advanced water purification trains to achieve high mineralization rates (TOC removal) and eliminate oxidation by-products, crucial for safe water reuse.
BDD Electrode Manufacturing: Confirms the utility of BDD anodes (e.g., those supplied by 6ccvd.com) as robust, non-active electrode materials capable of generating high concentrations of hydroxyl radicals for AOPs, even in complex, high-ionic-strength matrices.
Hybrid AOP System Design: Provides critical data for optimizing hybrid reactor design, demonstrating that US is most effective when mass transport or radical generation is the limiting factor (i.e., at low current densities or in matrices with high radical scavenging potential like Humic Acid).
Industrial Water Management: Applicable for treating industrial streams containing high organic loads or specific ionic contaminants (like bicarbonates), where traditional AOPs might suffer from radical scavenging.

View Original Abstract

The synergistic action of anodic oxidation using boron-doped diamond and low-frequency ultrasound in different water matrices and operating conditions for the decomposition of the emerging contaminant propyl paraben was investigated. The degree of synergy was found to decrease with an increase in current in the range 1.25-6.25 mA/cm2 or the ultrasound power until 36 W/L, where a further decrease was observed. Despite the fact that the increased propyl paraben concentration decreased the observed kinetic constant for both the separated and the hybrid process, the degree of synergy was increased from 37.3 to 43.4% for 0.5 and 2 mg/L propyl paraben, respectively. Bicarbonates (100-250 mg/L) or humic acid (10-20 mg/L) enhanced the synergy significantly by up to 55.8%, due to the higher demand for reactive oxygen species. The presence of chloride ions decreased the observed synergistic action in comparison with ultrapure water, possibly due to the electro-generation of active chlorine that diffuses to the bulk solution. The same behavior was observed with the secondary effluent that contained almost 68 mg/L of chlorides. The efficiency was favored in a neutral medium, while the hybrid process was delayed in alkaline conditions.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ijerph17082621

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