Application of soluble bio-organic substances (SBO) as photocatalysts for wastewater treatment: Sensitizing effect and photo-Fenton-like process

Abstract

Soluble bio-organic substances (SBO) isolated from urban biorefuses have been investigated in photocatalytic processes for wastewater treatment using a cationic dye, namely crystal violet (CV) as target compound. The SBO have been found to enhance the photobleaching of CV solutions with an optimal SBO concentration of ca. 20 mg L⁻¹. Mechanistic investigation based on chemical probes and changes in the absorption spectrum of CV in the presence of SBO seems to indicate that a complex formed between sensitizer and substrate plays a major role in the process. SBO have also been tested for the implementation of photo-Fenton processes at circumneutral media: the observed pseudo first order rate constants for CV decoloration was higher for UV/SBO/Fe(II)/H₂O₂ (5.5 × 10⁻³ min⁻¹) than for UV/Fe(II)/H₂O₂ (1.5 × 10⁻³ min⁻¹). A modified mechanism, in which HO are less relevant that at the acidic medium, is in agreement with results of experiments carried out with chemical probes.

Introduction

Dissolved organic matter (DOM) has received much attention from researchers in recent years because of its photochemical properties. In particular, humic and fulvic acids have been shown to generate, upon solar irradiation, oxidizing species such as hydroxyl radical [1], [2], singlet oxygen [3], superoxide radical ion [4] or reactive triplet excited states [5], [6]. These species can react with chemicals present in surface waters and participate in the most important abiotic processes for xenobiotics removal [7]. A review on the use of organic compounds with photochemical activity (namely dyes) for water treatment has been very recently published [8]. Unfortunately, DOM cannot be considered at commercial level as viable agent for the detoxification of wastewater due to the low concentration in water and soil.

Recent work reports how urban bio-wastes (UBW) have become a sustainable source of soluble bio-organic substances (SBO) having similar origin, chemical nature and photosensitizing properties as DOM [9]. It is interesting to note, that bio-wastes may have a potential beneficial fall out for environment. The potential use of SBO as photocatalyst for water detoxification of aqueous effluents may be considered a green process since it valorizes solid waste as a material of technological application.

The SBO are described as mixtures of macromolecules with weighted average molecular weight (M_w) ranging from 67 to 463 kg mol⁻¹ and polydispersity indexes (M_w/M_n) in the 6–53 range. When analyzing chemical composition data these macromolecules appear to be formed by long aliphatic C chains substitued by aromatic rings and several functional groups as COOH, CON, CO, PhOH, O-alkyl, OAr, OCO, OMe, and NRR′, with R and R′ being alkyl substituens. These organic moieties are the likely memory of the main constituents of the sourcing bio-organic waste which are not completely mineralized during aging under aerobic fermentation conditions. For this reason, SBO may be considered to be the pristine material of DOM formed under longer aging conditions. The former, together with easier and continuous availability, seem to have some other interesting features. They can be obtained over a wide range of chemical composition depending on the type, location and treatment of the sourcing UBW [10]. Due to the presence of functional groups with strong metals’ chelating power, the SBO contain a considerable fraction of minerals which are hard to separate from the organic matter. In principle some of these minerals, such as iron, could contribute to determine the SBO photochemical activity. Therefore, for their capacity to hold iron ions in solution at neutral–alkaline pH, SBO offer intriguing scope to investigate possible photo-Fenton-like processes occurring under pH conditions where iron ions are normally not soluble.

To fully understand the rationale motivating the present work, it should be considered that the Fenton reagent consists of a mixture of iron salts and hydrogen peroxide which is able to generate highly oxidizing species, such as hydroxyl radical [11]. The process is highly enhanced by irradiation and sunlight can be employed for this purpose with ecological and economical advantages [12]. Although this method has proven to be efficient, it is limited by the acidic pH that is required, partly due to the low solubility of iron at neutral media. In order to overcome this inconvenience, complexing agents such as humic acids have been employed [13], [14], [15], [16]. Although previous papers have reported interesting photosensitizing behavior [17], [18], the applicability of SBO for photo-Fenton remains unexplored. The present paper is meant to contribute further insight into the applicability of SBO in photochemical processes for wastewater treatment. For this purpose, crystal violet (CV), a cationic dye, was used as model compound. The role of SBO in the photobleaching of CV solutions was then studied in two different directions, namely as photosensitizer and as complexing agent to drive photo-Fenton processes under non acidic conditions.