Volatile Organic Compounds

Initially, to demonstrate the selective reactivity of the sensing dyes in the presence of different volatiles, a series of tests were conducted with the following gases: water vapour, ethanol, acetic acid, and acetone. This experiment allowed us to determine potential interference due to water in our experiments, since it is ubiquitously present in all environments, and the changes in sensitivity of each dye towards different chemical functionalities. Moreover, the chosen organic volatiles (acetone, acetic acid, and ethanol) allowed us to study the impact of different functional groups on the response of the final devices. In each case, the sensing papers were subjected to 10 ppm of each gas inside a sealed pyrex borosilicate bottle, and the changes in colour were determined by a low-cost TCS34725 (Adafruit)-based spectrometer after 2 h.
After conducting this initial test using simple organic volatiles and water, the sensors were tested in the presence of trans-2-hexen-1-al, indole-3-acetic acid (auxin), and tryptophol. These molecules are involved in the growth and stress response of plants. In particular, trans-2-hexen-1-al is a volatile organic compound (VOC) emitted by wounded or stressed plants, indole-3-acetic acid is the most common naturally occurring phytohormone of the auxin class, and tryptophol is a quorum sensing molecule (QSM) found during fungal infections. We tested the sensitivity of the dyes in the presence of 10 ppm of each gas separately. The exact values were calculated using the following formula (1): $$c=\frac{22.4\times p\times d\times V_1}{M\times V_2}$$
where c denotes concentration in ppm, p denotes purity, d denotes density in g/mL, V₁ denotes the volume injected in µL, M denotes molecular weight, and V₂ denotes the volume in the bottle in L. The colorimetric sensor arrays were left in sealed 1 L glass bottles for about 2 h to react with the gases. Color changes in the drop-casted dyes were then determined. A simple and low-cost RGB colour sensor (Adafruit TCS34725) was used, connected to an Arduino board for data collection. This sensor component also had an in-built white LED next to its RGB sensor. The TCS34725 contained a 3 × 4 photodiode array, consisting of red-filtered, green-filtered, blue-filtered, and clear (unfiltered) photodiodes. This RGB sensor quantifies the level of reflected light from the white LED source and measures the following wavelengths: 465 nm (blue), 525 nm (green), and 615 nm (red).
Connections to the colour sensor were soldered to allow for stable electrical contacts and to hold the components firmly in place. During the measurement, a paper sensor was placed onto the colour sensor, so that the LED of the colour sensor evenly illuminated the sample. RGB values for each coloured spot on the filter paper were recorded. The measurements were recorded using the microcontroller Wio Terminal, which could potentially be used to automate the whole data extraction and calculation process.

The effect of Purolite^® A502PS IEX on the removal of dissolved organics (DOC) and the organic micro-pollutants was studied using an MF–IEX hybrid system (Figure 1). A hollow fibre MF membrane was submerged in the reactor tank containing 3 L of ROC. The flow of influent (ROC) and effluent (treated water) was controlled using two master flux peristaltic pumps. The TMP of the membrane filtration was measured using a pressure gauge. Different doses of Purolite^® A502PS (5 g/L, 10 g/L; 20 g/L) were added to the reactor tank. The flux of influent and effluent was 36 L/m²·h which maintained a constant water level in the reactor. The reactor tank was fed with continuous air flow at 1.5 m³/m² membrane area h (pre-determined) to keep the Purolite^® A502PS particles in suspension to enhance the removal of contaminants. The hybrid system was found to be effective for two reasons. First, because the prior removal of organics/other charged compounds before they reach the membrane surface reduced fouling/scaling effects on the membrane surface, and second, because the airflow produces shear stress across the membrane surface, and its scouring effect further reduces the deposition of organics and reduces fouling [25 (link),26 (link)].
The loss of volatile organic compounds (VOCs) due to aeration was neglected as the wastewater used in this study was previously biologically treated and stabilized. The primary purpose of hollow fibre MF was to remove tiny purolite particles, if any, from treated water. The TMP of the MF–IEX hybrid system was measured using a pressure gauge (Novus log box).
The MF membrane alone can remove less than 10% of the DOC from the wastewater due to the larger pore size, which is not small enough to retain organic molecules [26 (link)]. Purolite^®A502PS was added to enhance the removal of organics from the ROC in the MF–IEX hybrid system. The authors’ previous study [19 (link)], reported that 1 g/L of Purolite^® A502PS was optimum in removing organics from RO feed using pre-adsorption of organics. Since ROC is ~5 times more concentrated than RO feed, a 5-fold increase in Purolite^® A502PS dosage (5 g/L) was used in MF–IEX hybrid system to achieve optimum organic removal. Further, higher doses (10–20 g/L), were also trialled to enhance the removal of micro-pollutants so as to overcome the competitive effect of the organics for Purolite^® A502PS exchange sites.
In addition, the authors have performed a similar short-term experiment with GAC at varying dosages of 5 g/L, 10 g/L, and 20 g/L using the ROC as the feed for the same experimental conditions. The respective DOC removals were observed to be 20–50%, 60–80%, and 70–90% over 4 h of operation [26 (link)]. Though GAC was found to reduce the organic load in several studies, it also reduced the sites available for sorption and removal of other micro- or priority organic pollutants [27 (link)]. Further, the removal of humics with ion-exchange resin (Purolite) is excellent compared to GAC [3 (link)]. In this context, the performance of Purolite^®A502PS in the MF–IEX hybrid system was studied for the removal of organic fractions and organic micro-pollutants at the dosage of 5 g/L, 10 g/L, and 20 g/L. The ion-exchange resin was added only at the start of the experiment and no further additions were made during the experimental run.