UVA-photoelectrocatalytic reactor for the treatment of wastewater effluents

: emerging pollutants degradation and bacteria inactivation

Abstract

Nowadays, there is a continuous rise in the need for new technology for the degradation of toxic low concentration contaminants of emerging concern (CECs) detected in wastewater effluent (WWE) and the inactivation of microbial pollutants. Thus, this study focuses on the development of a scalable channelled sandwich photoelectrochemical (PEC) cell employing a UV active photocatalyst under controllable UVA-LED irradiance for the degradation of CECs and inactivation of microbial pollutants in WWE.

The PEC reactor was designed based on a flow re-circulated batch sandwich-channelled configuration to increase the residence time and enhance the mass transfer. The effect of inlet flow and irradiance on the developed PEC reactor was optimized for the degradation of diclofenac as model pollutants in 10mM Na2SO4 using P25 TiO2 photoanode. The highest PEC performance for the P25 TiO2 photoanode was under 14.4 mWcm⁻² irradiance and 30 mLs-1 with a first order reaction rate constant (k) of 0.0171 ± 0.0002 min-1. Under these conditions, the PEC process did not significantly outperform the photocatalysis (PC) process (k = 0.0167 ± 0.0004 min-1) due to the limitations in the P25 TiO2 photoanode.

To enhance the P25 TiO2 photoanode diclofenac degradation performance, reduced graphene oxide composite (rGO-TiO2) photoanode was investigated. The effect of irradiance and rGO concentration was evaluated. The incorporation of rGO into TiO2 improved the degradation performance. The best-performing photoanode was 1.0 % rGO to TiO2 under 14.4 mWcm⁻2 irradiance, exhibiting a photocurrent of 122 μAcm⁻2, a k of 0.0214 min⁻1, and energy consumption (EEO) of 0.417 KWhL⁻1order⁻1. However, the composite material showed photo-instability issues, which limited its long-term effectiveness.

Subsequently, TiO2-NT TiO2 photoanodes were utilized to optimize the PEC reactor by evaluating different parameters, including the effect of applied potential, conductivity, cathode material, and irradiance for diclofenac degradation and EEO. The optimal operational parameters for the TiO2-NT PEC reactor were found to be +2.2 V cell potential, Pt as a cathode material and 14.4 mWcm⁻2 irradiance, while conductivity had limited effect on diclofenac degradation within the range of 1.0 - 2.0 mScm⁻1. Under these conditions, the photocurrent, k, and EEO were 176 ± 2 μAcm⁻2, 0.0907 ± 0.0053 min⁻1, 0.100 kWhL⁻1order⁻1, respectively.

The PEC treatment of WWE was conducted under optimized operational conditions and was compared to other control processes including photolysis, electrocatalysis (EC), and PC. The PEC exhibited the highest microbial inactivation rate. The E. coli concentration was ≤ 2 CFU/100 mL after 3.5 h of PEC treatment, achieving the WWE E. coli Class A EU standards for irrigation (≤10 CFU/100mL). While, for total coliform, the concertation was ≤ 8 CFU/100 mL after 3.5 h of treatment. The highest reduction in Turbidity, COD and DOM (Abs254) was also after the PEC treatment with 44.2%, 69 % and 40% reduction, respectively.

The degradation of 27 CECs detected in the WWE was monitored during the inactivation experiments. The PEC treatment was also found to be the most effective, achieving the highest degradation for the total CECs concentration identified (∑CECs), between 52% and 95.2% in 3.5 h of treatment time, which was highly dependent on the initial value of ∑CECs, in the first case initial concentration was 2.16 μgL-1, while the second has 85.90 μgL-1.

The PC process showed a 60.4% - 64.2% degradation of ∑CECs, which is clearly lower that the 95.2% for PEC, when ∑CECs initial values ranged between (73.0–85.9 μgL-1). In the same ∑CECs range, the degradation of individual CECs varied across the different treatment methods and the type of pollutant. The degradation of individual CECs by PEC was highly efficient, ranging between 76.8% and 99.9%, compared to PC, with degradation efficiencies from 12.4% to 99.9%. PEC treatment was demonstrated to be effective in removing by >90% of 24 CECs, and 20 CECs were reduced to below the LOD. While PC removed only 14 CECs by > 90%, and 12 below the LOD.

The EEO for the PEC was the lowest compared to other processes including photoinactivation, EC, and PC. Specifically, it was 0.146 kWh L⁻1order⁻1 for E. coli, 0.140 kWh L⁻1order⁻1 for total coliform, 1.60 kWh L⁻1order⁻1 for Abs254, 3.76 kWh L⁻1order⁻1 for COD, and 0.61 kWh L⁻1order⁻1 for CECs (CECs ≥ 75 μgL-1). These results demonstrate that PEC outperforms control processes in reducing the impact of WWE on the environment. The results show the potential of PEC treatment for CEC degradation and the inactivation of microorganisms in wastewater. Contributing to the development of more effective wastewater treatment strategies.

Date of Award	Oct 2023
Original language	English
Supervisor	John Byrne (Supervisor), Pilar Fernandez-Ibanez (Supervisor), Monsalvo Victor (Supervisor) & Zamora Patricia (Supervisor)