Degradation of Agro-Industrial Wastewater Model Compound by UV-A-Fenton Process: Batch vs. Continuous Mode

Abstract

The degradation of a model agro-industrial wastewater phenolic compound (caffeic acid, CA) by a UV-A-Fenton system was investigated in this work. Experiments were carried out in order to compare batch and continuous mode. Initially, batch experiments showed that UV-A-Fenton at pH 3.0 (pH of CA solution) achieved a higher generation of HO•, leading to high CA degradation (>99.5%). The influence of different operational conditions, such as H2O2 and Fe2+ concentrations, were evaluated. The results fit a pseudo first-order (PFO) kinetic model, and a high kinetic rate of CA removal was observed, with a [CA] = 5.5 × 10−4 mol/L, [H2O2] = 2.2 × 10−3 mol/L and [Fe2+] = 1.1 × 10−4 mol/L (kCA = 0.694 min−1), with an electric energy per order (EEO) of 7.23 kWh m−3 order−1. Under the same operational conditions, experiments in continuous mode were performed under different flow rates. The results showed that CA achieved a steady state with higher space-times (θ = 0.04) in comparison to dissolved organic carbon (DOC) removal (θ = 0−0.020). The results showed that by increasing the flow rate (F) from 1 to 4 mL min−1, the CA and DOC removal rate increased significantly (kCA = 0.468 min−1; kDOC = 0.00896 min−1). It is concluded that continuous modes are advantageous systems that can be adapted to wastewater treatment plants for the treatment of real agro-industrial wastewaters.

Keywords: UV-A LEDs; caffeic acid; electric energy per order; environmental impact; photo-Fenton; winery wastewater.

Figure 1

Schematic representation of UV-A LEDs lab-scale reactor with peristaltic pump.

Figure 2

Assessment of different AOPs in (a) CA removal and ORP variation and (b) H₂O₂ consumption and Fe²⁺ concentration. Operational conditions: [CA] = 5.5 × 10⁻⁴ mol L⁻¹, [H₂O₂] = 2.2 × 10⁻³ mol L⁻¹, [Fe²⁺] = 1.1 × 10⁻⁴ mol L⁻¹, pH = 3.0, agitation 150 rpm, temperature = 298 K, radiation UV-A, $I_{\mathrm{UV}}$ = 32.7 W m⁻², t = 15 min.

Figure 3

Effect of pH in (a) CA removal and (b) H₂O₂ consumption and Fe²⁺ concentration. Operational conditions: [CA] = 5.5 × 10⁻⁴ mol L⁻¹, [H₂O₂] = 22 × 10−4 mol L⁻¹, [Fe²⁺] = 1.1 × 10−4 mol L⁻¹, agitation 150 rpm, temperature = 298 K, radiation UV-A, $I_{\mathrm{UV}}$ = 32.7 W m⁻², t = 15 min.

Figure 4

Evaluation of H₂O₂:CA molar ratio in (a) CA removal and (b) H₂O₂ consumption and Fe²⁺ concentration. Operational conditions: [CA] = 5.5 × 10⁻⁴ mol/L, [Fe²⁺] = 1.1 × 10⁻⁴ mol L⁻¹, pH = 3.0, agitation 150 rpm, temperature = 298 K, radiation UV-A, $I_{\mathrm{UV}}$ = 32.7 W m⁻², t = 15 min.

Figure 5

Outcome of several concentrations of H₂O₂ in CA degradation by the UV-A-Fenton process; (a) dependence on the kinetic constant rates; (b) electric energy per order ($E_{\mathrm{EO}}$).

Figure 6

Assessment of the H₂O₂:Fe²⁺ molar ratio in (a) CA degradation and (b) H₂O₂ consumption and Fe²⁺ concentration. Operational conditions: [CA] = 5.5 × 10⁻⁴ mol L⁻¹, [H₂O₂] = 2.2 × 10⁻³ mol L⁻¹, pH = 3.0, agitation 150 rpm, temperature = 298 K, radiation UV-A, $I_{\mathrm{UV}}$ = 32.7 W m⁻², t = 15 min.

Figure 7

Outcome of several concentrations of Fe²⁺ in CA degradation by UV-A-Fenton process; (a) dependence on the kinetic constant rates; (b) electric energy per order ($E_{\mathrm{EO}}$).

Figure 8

Assessment of different flow rates (1–4 mL min⁻¹) in (a) CA and DOC removal; (b) H₂O₂ and Fe²⁺ concentration available as function of θ. Operational conditions: [CA] = 5.5 × 10⁻⁴ mol L⁻¹, [Fe²⁺] = 1.1 × 10⁻⁴ mol L⁻¹, pH = 3.0, agitation 150 rpm, temperature = 298 K, radiation UV-A, $I_{\mathrm{UV}}$ = 32.7 W m⁻², HRT = 15 min.

Figure 9

Dependence of the kinetic constant (k) to the flow rate (F) of the continuous mode reactor.