Alternative natural dyes in water purification: Anthocyanin as TiO₂-sensitizer in methyl orange photo-degradation

Abstract

Natural molecular dye, anthocyanin, is described here as safe sensitizer for TiO₂ particles in photo-degradation of organic contaminants in water. The dye is a promising replacement for the more costly and hazardous heavy metal based systems, such as CdS particles and Ru-compounds. TiO₂/anthocyanin effectively catalyzed the photo-degradation of methyl orange contaminant under solar simulator radiation. The new TiO₂/anthocyanin catalyst showed comparable efficiency to earlier systems, while avoiding their hazardous nature. When supported onto activated carbon (AC) particles, the resulting AC/TiO₂/anthocyanin system showed enhanced efficiency and ease of recovery from the catalytic reaction mixture. The natural dye molecules showed the tendency to degrade under photo-degradation conditions, just like earlier hazardous sensitizers. However, complete mineralization of anthocyanin occurred leaving no traces of organic species in solution. Sensitizer degradation caused deactivation of the supported catalyst on recovery. Such a shortcoming was overcome by re-treatment of the recovered catalysts with fresh dye.

Effects of different reaction parameters on the catalyst efficiency were studied. A mechanism, similar to earlier CdS-sensitized catalyst systems, is proposed for the TiO₂/anthocyanin catalyst.

Introduction

In our search for safe processes to purify contaminated waters, complete mineralization of organic contaminants has been approached [1], [2], [3], [4]. This strategy could be achieved by a number of techniques, the most economic of which is solar-based degradation. The simplest technique is to mix powders of suitable catalysts in the contaminated water and expose the system to direct solar light. Different catalyst systems were suggested. Due to its chemical stability, robustness, non-toxic nature and low cost, the TiO₂ is the best photo-degradation catalyst system. However, its wide band gap (∼3.1 eV) limits its use to UV radiations only. Therefore, only small portions of direct solar light can be used in case of pristine TiO₂ catalysts. In order to function under solar radiations, TiO₂ can be sensitized with attached dye molecules or particles. Different types of dyes are reported as sensitizers for TiO₂ system, such as ruthenium complex molecules, CdS particles and others [5], [6], [7], [8]. Such sensitizers were effectively used in Graetzel type solar cells and in water purification.

Natural dyes extracted from plants were widely reported as sensitizers for TiO₂ particles in Graetzel type solar cells. Examples of such dyes are chlorophyll derivatives, natural porphyrines and anthocyanins which are molecular in nature [9], [10], [11], [12], [13]. Unlike the case with solar cells, natural dyes have limited use as sensitizers in water purification studies [14].

In a recent study [1] we critically assessed using heavy metal based sensitizers such as CdS particles, and demonstrated that such hazardous systems should be avoided in water purification strategies. We also highlighted the need to either stabilize such dyes or replace them with safer sensitizers. A well-known natural dye, anthocyanin (also spelled as anthocaynin), will be investigated here as a safe alternative sensitizer in photo-degradation of organic water contaminant catalyzed by TiO₂ particles. Methyl orange (MO), I, is an irritant hazardous contaminant, as observed from its MSDS, and was chosen here as a model contaminant. Anthocyanin dye is responsible for several colors in the red–blue range depending on pH value. The red anthocyanin absorbs at 530 nm (band gap 2.3 eV). This pigment occurs in fruits, flowers and plant leaves. The structural formula for anthocyanin, II, is shown below. The pigment was extracted from flowers of a plant called Karkade (Hibiscus). Karkade is commonly abundant at market-places, and is used as a safe low cost beverage in different Middle East regions. As sensitizers themselves may degrade under photo-degradation experiments yielding hazardous species, CdS is one example, the tendency of anthocyanin to degrade will also be investigated here.

Sensitization with natural molecular dyes follows similar rationale to sensitization by CdS semiconducting particles [15], [16]. The valence band – conduction band formalism used in CdS particles is paralleled with HOMO/LUMO formalism in molecular dye sensitization. When a dye molecule is excited by a visible light photon, one electron jumps from HOMO to LUMO, leaving a positive charge (hole) in the HOMO. The resulting hole is responsible for contaminant molecule oxidation. On the other hand, the excited electron travels from the dye molecule LUMO to the TiO₂ particle conduction band, and consequently reduces other species, such as O₂ molecules.

It is believed that the electron transfer from anthocyanin to TiO₂ particles occurs through chemical bonds that bridge them together, as shown in III below. Such bonds occur by virtue of surface hydroxyl groups on TiO₂ particles and on dye molecules [10], [11], [15], [17], [18], [19], [20].

Photo-catalytic efficiency of the new TiO₂/anthocyanin system will be assessed here as compared to earlier CdS system. A number of parameters will be studied including pH effect, reactant concentration, contaminant concentration and catalyst recovery. For recovery purposes, the catalyst system was supported onto activated carbon, giving AC/TiO₂/anthocyanin.