1. Introduction
The consumption of chemicals such as steroids, cosmetic products, agricultural products and pharmaceuticals have been greatly increased, their resuduals flow into the receving water courses, which might cause the adverse effects on the environment as these compounds have been classified as recalcitrant micro-pollutants [
1]. The existing biological treatment processes are found inefficient for the removal of these kind of organic compounds [
2,
3,
4,
5]. Recently, chemical oxidation methods involving hydroxyl radicals, known as advanced oxidation processes (AOPs), have been successfully applied for the pollutants’ remediation [
6,
7,
8,
9,
10]. Hydroxyl radicals oxidise persistent organic contaminants in a non-selective way thereby, providing substantial compound mineralisation [
5,
11]. The AOPs involve in situ generation of a hydroxyl radical that has great redox potential (2.80 V) and then high oxidation capacity [
12]. Both catalytic ozonation and UV irridation will enhance the hydroxyl radical production [
13] and has been researched extensively.
The classical Fenton oxidation is one of the commonly used AOPs [
13,
14,
15,
16]. The Fenton reaction consists of iron species and hydrogen peroxide (oxidant) to form free radicals that attack micro-organic contaminants [
12,
13,
14,
15]. The overall Fenton kinetics summarised by Walling [
17] suggests the need of an acidic environment to progress the Fenton reaction. Several studies [
12,
13,
14,
16,
18,
19,
20,
21] have shown the optimum Fenton performance of wastewater treatment at pH 2–3. Some reviewers, e.g., Bautista et al. and Neyens et al. [
10,
13] demonstrate the kinetics scheme of the Fenton reaction as shown in Equations (1)–(8):
The Fenton oxidation reactions are initiated by a ferrous ion acting as a catalyst and oxidising hydrogen peroxide to form hydroxyl radicals [
21]. Certainly, hydrogen peroxide donates hydroxyl radicals, which accelerate the Fenton reaction. Equation (8) demonstrates the scavenging effect of a hydroxyl radical. Thus, hydroxyl radicals play the dual role of initiation and Fenton reaction termination. However, the rate constant in Equation (7) gives 10 times higher a value than that in Equation (8), suggesting that for the given optimum iron(II) dose, the reaction tends to move forward neglecting the radical scavenging effect. The effect of iron species and hydroxyl radical is also determined by the organic contaminants [
5,
12,
22]. The organic compounds can also abstract hydrogen atoms, initiating the radical chain oxidations given in Equations (9)–(11). Tamg and Tassos suggest [
23] the fate of organic free radicals, which can further be oxidised/reduced by a ferric ion/ ferrous ion respectively or dimerised itself to form alkanes (Equations (12)–(14)).
Although good oxidation efficiency is achieved by the Fenton oxidation, a large amount of sludge production limits its use in some cases.The ferrous ions react with hydroxide ions to form ferric hydroxo solid materials [
24]. These reactions portray the coagulation ability of the Fenton’s reagent by forming iron based precipitates and then generating sludges, which are consistently observed in the Fenton oxidation steps and need a long time to settle out [
13]. However, the ferric hydroxo complexes are photo-active and can reproduce ferrous ions and hydroxyl radicals in photo-Fenton processes when Fe(0) is used as a catalyst in cooperation with the irridation via UV light (hv) [
15], and the photo Fenton process exists in homogeneous and heterogeneous phases.
In contrast to a ferrous iron, a zero-valent iron (Fe(0)) has been known to have an efficient catalytic effect and has been utilised for environmental remediation [
25]. However, its application in the Fenton oxidation is yet to be fully developed. The Fe(0) is ionised or oxidised in the presence of water, dissolved oxygen or hydrogen peroxide to form ferrous ions and thereafter proceeds the conventional Fenton oxidation procedures under its reaction mechanism [
26].
When designing experiments or reaction conditions, the Taguchi method has been used. It is to test the mean inner array and variation of each experimental run by evaluating the ratio of the signal-to-noise (SN). The signal corresponds to response or experimental yield and noise as an inevitable loss [
27]. In contrast to the standard deviation, which directly depends on the mean value of response data and thus, the error is difficult to minimise, the SN ratio is used to replace the standard deviation to obtain robust optimum reaction conditions [
27]. The Taguchi method performs two-steps optimisation at the parameter level combination, with minimum standard deviation and keeping the mean on the target [
28]. The two steps can be described as:
Set all factors that have a prominent contribution to SN ratios at the level to obtain maximum SN ratios.
Adjust the level of one or more factors that substantially affect the mean value but not SN ratios to put the response on the target.
In real experimental conditions, the targets mean values can be changed during the process development. Therefore, SN ratios are used to calculate quadratic loss function and have three approaches. The SN ratios’ condition is selected to minimise, maximise or to produce nominal results [
28]. Choosing either of the below approaches for the analysis depends on the research demand:
Smaller is better;
Nominal is better and;
Larger is better.
The dissolved organic carbon is present in a vast variety of waters and its concentration increases with the rise in pollution level [
29]. All water treatment technologies aim to completely transfer organic pollutants into carbon dioxide (mineralisation), but it is generally hard to achieve this. In most cases, organic pollutants could partially be mineralised but the formation of oxidation products is commonly observed, which could possess more toxicity than their parents’ pollutants and thus, the toxicity assessment before and after water treatment is necessary [
30,
31]. The Microtox protocol is a straightforward approach for the assessment of toxicity and provides the information on samples’ potential to inhibit/promote bioluminescence, which is influenced by the tested compound’s toxic profile [
32].
Diuron, terbutryn and terbuthylazine have been listed in the proposals of European Parliament Directives and the EU Council Amending Directives (2000/60/EC and 2008/105/EC) as hazard substances. Pharmaceuticals of gabapentin and sulfamethoxazole have also been listed as prominent emerging micropollutants (MPs) in water bodies. Thereby, these five MPs were chosen for the present study. So far, there have been a few reported studies using the Fenton oxidation to treat the above named micropollutants (MPs) and thus this study aims:
To compare the classical ferrous iron with Fe(0)-catalytic Fenton oxidation in removing and mineralising the selected MPs.
To use the Taguchi method to design experiments and to select and obtain the optimal operating conditions.
To estimate sludge production from the process and to assess the toxicity of the treated effluents.