## 1 Introduction  
The consumption of chemicals such as steroids, cosmetic products, agricultural products and pharmaceuticals have been greatly increased, their residuals flow into the receiving 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 irradiation 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 [PERSON] [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., [PERSON] et al. and [PERSON] et al. [10; 13] demonstrate the kinetics scheme of the Fenton reaction as shown in Equations (1)-(8):  
\[2\mathrm{Fe}^{2+}+\mathrm{H}_{2}\mathrm{O}_{2}+2\mathrm{H}^{+}\to 2 \mathrm{Fe}^{3+}+2\mathrm{H}_{2}\mathrm{O} \tag{1}\]\[\text{Fe}^{2+}+\text{H}_{2}\text{O}_{2}\rightarrow\text{Fe}^{3+}+ \text{{}^{\bullet}OH}+\text{OH}^{-}\text{k}_{1}=70\text{ L/mol/s} \tag{2}\] \[\text{Fe}^{3+}+\text{H}_{2}\text{O}_{2}\leftrightarrow\text{Fe} \text{-OOH}^{2+}+2\text{H}^{+}\] (3) \[\text{Fe}\text{-OOH}^{2+}\rightarrow\text{HO}_{2}\text{{}^{ \bullet}}+\text{Fe}^{2+}\text{k}_{2}=0.001-0.01\text{ L/mol}\text{-s}^{-1}\] (4) \[\text{Fe}^{2+}\text{HO}_{2}\text{{}^{\bullet}}\rightarrow\text{Fe} ^{3+}+\text{HO}^{2-}\text{k}_{3}=1.3\times 10^{6}\text{ L/mol}\text{-s}^{-1}\] (5) \[\text{Fe}^{3+}\text{HO}_{2}\text{{}^{\bullet}}\rightarrow\text{Fe} ^{2+}+\text{O}_{2}+\text{H}^{+}\text{k}_{4}=1.2\times 10^{6}\text{ L/mol}\text{-s}^{-1}\] (6) \[\text{{}^{\bullet}OH}+\text{Fe}^{2+}\rightarrow\text{OH}^{-}+ \text{Fe}^{3+}\text{k}_{5}=3.2\times 10^{8}\text{ L/mol}\text{-s}^{-1}\] (7) \[\text{{}^{\bullet}OH}+\text{H}_{2}\text{O}_{2}\rightarrow\text{H} _{2}\text{O}+\text{HO}_{2}\text{{}^{\bullet}}\text{k}_{6}=3.3\times 10^{7}\text{ L/mol}\text{-s}^{-1} \tag{8}\]  
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). Tampa 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)).