ZnOnps based photocatalytic reactor for degradation of multiple organic pollutants driven by solar light-based UV irradiation

Abstract

Organic pollutants are some of the most stubborn elements found in the environment. Their removal as well as degradation has been the topic of study for long. Their treatment in waste water is very difficult. Here nanomaterials can play a major role due to their high photostability, large surface area to volume ration, excellent tuning of size and optical band gap. [1,2] Among various nanomaterials such as TiO₂, CeO₂, etc., the problem is the use of the either co-catalyst or any acid or base. This hinders their large-scale use. To fulfil this purpose the present study has been carried out to study and develop versatile ZnO nanoparticles and an industrial photocatalytic rector prototype, which could be used to clean the waste waters contaminated with more than one organic dye efficiently under UV light irradiation. [3,4]

There is various classification of dyes, acidic dyes, basic dyes, precursor dyes, sulphur dyes, reactive dyes, etc. among which the acidic organic dyes are classified as the most toxic water pollutant because these get mixed with water easily. [5] The main constituent of these dyes are triarylmethanes, anthraquinone and azo, nitrous groups, etc. the azo dyes are characterized by the -N=N- groups which are present in different numbers, linked to phenyl, naphthyl radicles, further replaced by chlorine hydroxyl nitro, methyl, sulphonic groups etc. [6] These dyes are mainly discharged from the textile industries directly into the water bodies without treatment. These dyes are highly capable of growth of cancer cells and genetic mutations in the living body. These organic dyes also harm the aquatic life by blocking the light in waterbodies due to which the aquatic plants could not undergo photosynthesis and produce enough oxygen in the water and hence leading to the loss of the aquatic animals and ultimately death of the waterbodies. In the current study the, many of these alarming azo dyes such as methylene blue (MB), methyl orange (MO), methyl violet (MV), and Rhodamine-B (RhB) have been successfully degraded from water using the modified ZnO nps. [7]

Fig. 1 shows the UV-absorbance plot for the Photocatalytic degradation study of highly concentrated (20 ppm) a) methylene blue (MB), b) (methyl orange (MO), c) methyl violet (MV), and d) Rhodamine-B (RhB). The inset show the corresponding kinetic study for respective UV-absorbance data. The degradation percentage obtained for MB, MO, MV and RhB was 97.8%, 99.3%, 98%, and 98%, obtained in 30, 50, 50 and 30 min of UV light irradiation. The k (pseudo-first order reaction rate constant) was found to be 0.044, 0.0237, 0.035, and 0.045 for MB, MO, MV and RhB, respectively. [8]

Fig. 1: a) Photocatalytic degradation study of a) methylene blue (MB), b) methyl orange (MO), c) methyl violet (MV), and d) Rhodamine-B (RhB). Inset show the corresponding kinetic study for respective UV-absorbance data. 

Based on these results, we proposed the universal photocatalytic reactor for the degradation of organic pollutants in waste water. Figure 2 shows the proposed reactor, having three chambers fitted with ultra-sonicator, and shut-off valves. This design can be used to treat the waste water in a cost efficient and effective manner. The solar light spectrum consists of only 5%UV light, while the rest of it is either IR or visible light. Here we are proposing use of solar cells connected with batteries to power the UV lamps for the photocatalytic degradation. This ensures maximum usage of the UV light as ZnO has wide-band gap which ensures effective charge separation. This design is modular and can be scaled up depending upon the requirement.

Fig. 2: Proposed photocatalytic reactor for the UV light driven photocatalytic degradation of organic pollutants in waste water.