Kinetics of the Organic Compounds and Ammonium Nitrogen Electrochemical Oxidation in Landfill Leachates at Boron-Doped Diamond Anodes

At a Glance

Metadata	Details
Publication Date	2021-08-31
Journal	Materials
Authors	Barbara Wilk, Małgorzata Szopińska, Aneta Łuczkiewicz, Michał Sobaszek, Ewa Maria Siedlecka
Institutions	University of Gdańsk, Gdańsk University of Technology
Citations	10
Analysis	Full AI Review Included

Executive Summary

This study investigated the kinetics and efficiency of electrochemical oxidation (EO) of complex, refractory landfill leachates (LLs) using Boron-Doped Diamond (BDD) electrodes synthesized on Silicon (Si) substrates (BDD/Si).

High Efficiency Achieved: The BDD 0.5k electrode (lowest boron doping) operating at 100 mA·cm^-2 achieved the highest removal rates: 91% Chemical Oxygen Demand (COD) and 60% Ammonium Nitrogen (N-NH₄⁺) after 8 hours.
Material Performance Correlation: Lower boron doping (0.5k) resulted in a wider electrochemical working window (3.9 V vs. 2.23 V for 15k BDD), correlating directly with superior performance in direct oxidation processes.
Kinetic Modeling: COD removal generally followed pseudo-first-order kinetics, while N-NH₄⁺ removal primarily followed second-order kinetics, suggesting a strong dependence on electrogenerated active chlorine species.
Biodegradability Enhancement: EO treatment doubled the Biodegradability Index (BI) from 0.11 (raw LLs) to 0.22 (BDD 0.5k, 50 mA·cm^-2), confirming its potential as an effective pre-treatment step prior to biological processes.
Energy Optimization: Optimal conditions for achieving >70% COD removal were identified at 4 hours of treatment using the BDD 0.5k electrode at 100 mA·cm^-2, consuming 200 kWh·m^-3.
Process Mechanism: The study confirmed that the LL matrix, characterized by high salinity (2690 mg Cl^-·L^-1), benefits the EO process, although high current densities (j) led to energy consumption being diverted toward oxidizing non-target ions rather than solely organic compounds.

Technical Specifications

Parameter	Value	Unit	Context
Raw LL COD	3608 ± 123	mg O₂·L^-1	Initial contaminant load (Refractory)
Raw LL N-NH₄⁺	2069 ± 103	mg·L^-1	Initial contaminant load
Raw LL BOD₂₀/COD (BI)	0.12 ± 0.00	Ratio	Poor biodegradability
Optimal COD Removal	91	%	BDD 0.5k, 100 mA·cm^-2, 8h
Optimal N-NH₄⁺ Removal	60	%	BDD 0.5k, 100 mA·cm^-2, 8h
Max BI Achieved	0.22 ± 0.05	Ratio	BDD 0.5k, 50 mA·cm^-2, 8h
Lowest Boron Doping	500 (0.5k)	ppm [B]/[C]	Highest performing electrode
Highest Boron Doping	15,000 (15k)	ppm [B]/[C]	Lowest performing electrode
BDD 0.5k Grain Size	~2	µm	Average crystallite size
BDD 15k Grain Size	~0.5	µm	Average crystallite size
BDD 0.5k Window Width	3.9	V	Electrochemical working window (vs. Ag/AgCl)
BDD 15k Window Width	2.23	V	Electrochemical working window (vs. Ag/AgCl)
Optimized EC (COD > 70%)	200	kWh·m^-3	BDD 0.5k, 100 mA·cm^-2, 4h
Lowest Specific EC (COD)	122	kWh·kg^-1	BDD 0.5k, 50 mA·cm^-2, 8h
BDD 0.5k Half-Life (T_1/2) COD	147.4 ± 2.2	min	j = 100 mA·cm^-2 (pseudo-first-order)
BDD 0.5k Half-Life (T_1/2) N-NH₄⁺	292.7 ± 8.5	min	j = 100 mA·cm^-2 (second-order)

Key Methodologies

The BDD electrodes were synthesized via Microwave Plasma Assisted Chemical Vapor Deposition (MWPACVD) on two-inch Silicon wafers, followed by galvanostatic electrooxidation testing of diluted landfill leachates (1:1 V:V).

BDD Electrode Synthesis (MWPACVD)

Substrate: Two-inch Silicon wafers.
Deposition Time: 12 hours.
Process Conditions: Chamber pressure maintained at 50 Torr.
Power Input: Microwave radiation (2.45 GHz) at 1300 W.
Temperature: Induction heating stage set to 700 °C.
Gas Flow: Total flow rate of 300 sccm; Methane molar ratio equal to 1%.
Dopant: Diborane (B₂H₆) precursor used to achieve three doping levels: 500 ppm (0.5k), 10,000 ppm (10k), and 15,000 ppm (15k) [B]/[C] ratio.

Electrooxidation (EO) Reactor Setup

Reactor Type: 500-mL single-chambered reactor containing 400 mL of sample.
Anode/Cathode: BDD/Si anode (10.5 cm²) and stainless-steel mesh cathode (10.5 cm²).
Electrode Spacing: Interelectrode distance maintained at approximately 2.0 cm.
Operation Mode: Galvanostatic mode (constant current density, j).
Current Density Range: Tested at 25, 30, 50, 75, and 100 mA·cm^-2.
Mixing: Magnetic stirrer maintained at 300 rpm for homogenization.
Temperature: Maintained at 25 ± 1 °C using a cooling bath.
Sampling: Samples (15 mL) collected every 2 hours over an 8-hour test period, followed by degassing prior to physico-chemical analysis.

Commercial Applications

The findings support the use of BDD/Si anodes in Advanced Oxidation Processes (AOPs) for treating highly contaminated and refractory industrial wastewater streams.

Landfill Leachate Treatment: Direct application for post-treatment or pre-treatment of mature (old) LLs characterized by high COD, high N-NH₄⁺, and poor biodegradability.
Industrial Wastewater Remediation: Effective removal of recalcitrant organic compounds (e.g., humic/fulvic acids, halogenated compounds) from complex industrial effluents.
Nitrogen Removal: Efficient conversion of high concentrations of N-NH₄⁺, primarily through chlorine-mediated indirect oxidation, suitable for high-salinity matrices.
Biodegradability Enhancement: Use as a chemical pre-treatment step to mineralize refractory organics, making wastewater suitable for subsequent, cost-effective biological treatment.
Disinfection: The generation of hydroxyl radicals (˙OH) and active chlorine species provides a simultaneous disinfection benefit during wastewater treatment.
Electrochemical Reactor Design: The study provides optimization data (j, EC) crucial for designing compact, modular EO reactors for decentralized or variable-load wastewater treatment facilities.

View Original Abstract

Electrochemical oxidation (EO) of organic compounds and ammonium in the complex matrix of landfill leachates (LLs) was investigated using three different boron-doped diamond electrodes produced on silicon substrate (BDD/Si)(levels of boron doping [B]/[C] = 500, 10,000, and 15,000 ppm—0.5 k; 10 k, and 15 k, respectively) during 8-h tests. The LLs were collected from an old landfill in the Pomerania region (Northern Poland) and were characterized by a high concentration of N-NH4+ (2069 ± 103 mg·L−1), chemical oxygen demand (COD) (3608 ± 123 mg·L−1), high salinity (2690 ± 70 mg Cl−·L−1, 1353 ± 70 mg SO42−·L−1), and poor biodegradability. The experiments revealed that electrochemical oxidation of LLs using BDD 0.5 k and current density (j) = 100 mA·cm−2 was the most effective amongst those tested (C8h/C0: COD = 0.09 ± 0.14 mg·L−1, N-NH4+ = 0.39 ± 0.05 mg·L−1). COD removal fits the model of pseudo-first-order reactions and N-NH4+ removal in most cases follows second-order kinetics. The double increase in biodegradability index—to 0.22 ± 0.05 (BDD 0.5 k, j = 50 mA·cm−2) shows the potential application of EO prior biological treatment. Despite EO still being an energy consuming process, optimum conditions (COD removal > 70%) might be achieved after 4 h of treatment with an energy consumption of 200 kW·m−3 (BDD 0.5 k, j = 100 mA·cm−2).

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma14174971

References

2020 - Determining the effects of Class I landfill leachate on biological nutrient removal in wastewater treatment [Crossref]
2020 - Effective treatment of high-salinity landfill leachate using ultraviolet/ultrasonication/ peroxymonosulfate system [Crossref]
2019 - Landfill leachates and wastewater of maritime origin as possible sources of endocrine disruptors in municipal wastewater [Crossref]
2021 - Optimization of the process variables for landfill leachate treatment using Fenton based advanced oxidation technique
2006 - Combined biological and chemical degradation for treating a mature municipal landfill leachate [Crossref]
2021 - Performance of coupling electrocoagulation and biofiltration processes for the treatment of leachate from the largest landfill in Hanoi, Vietnam: Impact of operating conditions [Crossref]